U-Pb Dating

Chemical Geology 205 (2004) 115 – 140
www.elsevier.com/locate/chemgeo
Improved 206Pb/238U microprobe geochronology by the monitoring
of a trace-element-related matrix effect; SHRIMP, ID–TIMS,
ELA–ICP–MS and oxygen isotope documentation for a
series of zircon standards
Lance P. Black a,b,*, Sandra L. Kamo c,1, Charlotte M. Allen b, Donald W. Davis c,1,
John N. Aleinikoff d, John W. Valley e, Roland Mundil f, Ian H. Campbell b,
Russell J. Korsch a, Ian S. Williams b, Chris Foudoulis a
a
Minerals and Geohazards Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
c
Jack Satterly Geochronology Laboratory, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario, Canada M5S2C6
d
U.S. Geological Survey, Mail Stop 963, Denver, CO 80225, USA
e
Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison, WI 53706, USA
f
Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA
b
Received 31 March 2003; accepted 13 January 2004
Abstract
Precise isotope dilution – thermal ionisation mass spectrometry (ID – TIMS) documentation is given for two new Palaeozoic
zircon standards (TEMORA 2 and R33). These data, in combination with results for previously documented standards (AS3,
SL13, QGNG and TEMORA 1), provide the basis for a detailed investigation of inconsistencies in 206Pb/238U ages measured
by microprobe. Although these ages are normally consistent between any two standards, their relative age offsets are often
different from those established by ID – TIMS. This is true for both sensitive high-resolution ion-microprobe (SHRIMP) and
excimer laser ablation – inductively coupled plasma – mass spectrometry (ELA – ICP – MS) dating, although the age offsets are
in the opposite sense for the two techniques. Various factors have been investigated for possible correlations with age bias, in
an attempt to resolve why the accuracy of the method is worse than the indicated precision. Crystallographic orientation,
position on the grain-mount and oxygen isotopic composition are unrelated to the bias. There are, however, striking
correlations between the 206Pb/238U age offsets and P, Sm and, most particularly, Nd abundances in the zircons. Although
these are not believed to be the primary cause of this apparent matrix effect, they indicate that ionisation of 206Pb/238U is
* Corresponding author. Minerals and Geohazards Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia. Fax:
+61-2-6249-9971.
E-mail address: Lance.Black@ga.gov.au (L.P. Black).
1
Jack Satterly Geochronology Laboratory, Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario, MSS3B1,
Canada.
0009-2541/$ - see front matter. Crown Copyright D 2004 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.chemgeo.2004.01.003
116
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
influenced, at least in part, by a combination of trace elements. Nd is sufficiently representative of the controlling trace
elements that it provides a quantitative means of correcting for the microprobe age bias. This approach has the potential to
reduce age biases associated with different techniques, different instrumentation and different standards within and between
laboratories.
Crown Copyright D 2004 Published by Elsevier B.V. All rights reserved.
Keywords: Zircon; U – Pb geochronology; SHRIMP; ELA – ICP – MS; Oxygen isotopes; Reference standards; Matrix effect
1. Introduction
Over the past several decades, U –Pb dating of
zircon has played a pivotal role in geochronology.
Dating of relatively young (less than about 1000 Ma)
zircon has mostly been based on the 238U – 206Pb
decay scheme, because the relatively small amounts
of 207Pb produced during those times render the
207
Pb/206Pb technique less effective. Derivation of
those ages by microprobe analysis is significantly
complicated by U+ and Pb+ ions being emitted in
different proportions from the atomic abundances in
their host mineral (Compston et al., 1984). In the case
of secondary ion mass spectrometry (SIMS), of
which the sensitive high-resolution ion-microprobe
(SHRIMP) is an example, the bias of 206Pb/238U for
zircon can be as much as a factor of three. This
enrichment of Pb over U must be corrected through
concurrent measurement of a standard, which is a
well-dated and well-behaved representative of the
same (i.e., matrix-matched) mineral. Because the
age of a standard is independently established (usually by isotope dilution – thermal ionisation mass
spectrometry (ID – TIMS)), the degree of inter-element fractionation that occurs during microprobe
analysis of the standard can be determined. The same
fractionation factor is then applied to concurrently
analysed zircon of unknown age using the (critical)
assumption that its Pb and U emission was fractionated to the same extent as in the standard. Deviation
from that assumption will result in an aberrant age.
206
Pb/238U fractionation during excimer laser ablation –inductively coupled plasma – mass spectrometry
(ELA – ICP – MS) is also corrected using the assumption that standard and unknowns will have been
similarly affected.
Another important role for the standard is to assess
the overall quality of the analytical session. Unlike
ID – TIMS dating, during which micrograms or more
of zircon are consumed, the nanograms of zircon
expended during an individual microprobe analysis
yield considerably lower precision. This can be improved by performing a large number of replicate
analyses, which are then statistically combined. It
has been established (e.g., Black et al., 2003a) for
one SIMS instrument—SHRIMP II at the Research
School of Earth Sciences, Australian National University (RSES, ANU)—that the 206Pb/238U calibration is
less well defined during some analytical sessions than
in others. By comparing the quality of concurrent
analyses of standards and unknowns, it is possible to
discriminate between components of imprecision arising from the analytical process and those that reflect
206
Pb/238U heterogeneity within the unknown zircons.
This makes it simpler to identify outliers of a geological nature, which could be a consequence of either
open-system behaviour (e.g., Pb loss, producing younger ages) or the presence of older zircon (due to, for
example, to the incorporation of pre-existing zircon
into a magma).
An ideal zircon standard should have uniform
206
Pb/238U on all scales at which it is analysed. The
standard should also be both accurately and precisely
dated by ID– TIMS. It is a distinct advantage if the
standard is sufficiently abundant to be utilised indefinitely by different laboratories.
This article introduces two new zircon standards,
TEMORA 2 and R33. In combination with two
standards that have recently been documented
(TEMORA 1, Black et al., 2003b; QGNG, Black et
al., 2003a), they are used in a series of SHRIMP and
ELA – ICP –MS comparisons to test their consistency,
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
Table 1
Chemical compositions of host rocks of three Palaeozoic zircon
standards
TEMORA 1a
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
47.84
2.71
16.59
3.35
10.48
0.28
4.07
9.31
3.38
0.42
0.52
0.52
99.59
Trace elements (ppm)
As
2.8
Ba
278
Be
0.9
Ce
53.7
Cr
<1
Cs
2.0
Cu
31
Dy
9.2
Er
5.2
Eu (ppb)
2500
F
1101
Ga
22.2
Gd
10.5
Ge
1.8
Ho
1.8
La
22
Li
3.5
Lu
0.6
Mo
1.3
Nb
27
Nd
35
Ni
8
Pb
4
Pr
7.5
Rb
10
Sb
< 0.2
Sc
35
Sm
8.9
Sn
< 1.0
Sr
362
Ta
1.5
Tb
1.7
Th
1.5
U
0.5
V
274
Y
51.7
Yb
4.3
TEMORA 2a
49.45
2.26
16.87
1.86
9.98
0.21
3.80
8.39
3.34
0.61
0.48
2.63
99.99
1.7
260
1.3
53.4
<1
1.7
20
5.4
3.2
2057
583
21.8
6.3
1.8
1.1
23.9
10.6
0.4
0.5
14
27
11
4
6.5
21
0.3
31
5.7
1.1
369
1.1
1.0
2.3
0.4
270
33.4
3.0
R33b
57.20
2.25
14.20
3.75
6.34
0.17
2.77
4.99
4.19
1.62
0.58
0.83
98.89
3.1
180
75.5
2.35
3.9
2710
12.9
3.03
29.1
1.04
117
Table 1 (continued )
TEMORA 1a
Trace elements (ppm)
Zn
Zr
CIPW norms
Quartz
Orthoclase
Albite
Anorthite
Diopside
Diopside (CaMg)
Hedenbergite
Hypersthene
Enstatite
Ferrosilite
Olivine
Forsterite
Fayalite
Magnetite
Ilmenite
Apatite
120
130
0.00
2.48
28.60
28.85
11.61
5.59
6.02
14.28
6.39
7.89
1.91
0.81
1.10
4.86
5.15
1.23
TEMORA 2a
110
136
1.27
3.60
28.26
29.24
7.74
3.46
4.29
19.03
7.86
11.17
0.00
0.00
0.00
2.70
4.29
1.14
R33b
96
467
12.49
9.68
35.85
15.32
4.82
2.92
1.90
9.82
5.62
4.19
0.00
0.00
0.00
5.55
4.35
1.37
a
Analysis by XRF, total Fe as Fe2O3 by XRF, FeO by titrimetry,
rest by ICP – MS.
b
Analysis of major elements by XRF, minor and trace elements
by INAA; the assumptions of Irvine and Barragar (1971) for the
allocation of total Fe between Fe2O3 and FeO have been used for
the calculation of the norm of R33.
with the aim of investigating the limits to precision
and accuracy that can be achieved with microprobe
dating. These results are then compared with previously acquired data for two other standards (AS3 and
SL13).
48
2. Geological background of the new standards
71
0.54
21
12
168
2.1
4.3
1.57
7.46
2.1. TEMORA 2
In common with TEMORA 1 (Black et al., 2003b),
TEMORA 2 zircon crystallised within the Middledale
Gabbroic Diorite. This forms a small, high-level stock
within the Palaeozoic Lachlan Orogen of Eastern
Australia (Wormald, 1993). Both TEMORA samples
come from the northern margin of the stock, but recent
farming activity has disturbed the positions of the
exposed boulders, and it is not possible to determine
original field relationships. Dominant primary minerals are labradoritic plagioclase, pargasitic hornblende,
118
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
pyroxene and opaque minerals. The main difference
between the two host rocks, which is described in more
detail by Black et al. (2003b), is that the TEMORA 2
host is coarser-grained and more deuterically altered,
although it too is extremely fresh. All original augite
and hypersthene appears to have been replaced by
secondary amphibole, and there has been extensive
alteration of hornblende to chlorite. Secondary epidote
is common. Opaque minerals consist of ilmenite and
haematite.
The TEMORA 2 host rock is somewhat more
enriched in SiO2 (Table 1), is slightly quartz-normative, and is less oxidised than that of TEMORA 1.
Despite their mafic nature, however, both rocks contain respectable concentrations of zircon. A distinct
advantage of TEMORA 2 is that not only are its host
boulders much more common, but they contain an
order of magnitude more zircon (about 0.1 g/kg,
compared with about 0.01 g/kg zircon in the
TEMORA 1 host).
2.2. R33
R33 zircon is derived from coarse-grained, biotitehornblende monzodiorite in the interior of a 60 mthick dioritic dyke of the Braintree Complex, VT,
northeastern USA. The complex intrudes biotite-grade
sedimentary formations of Ordovician age in the
Connecticut Trough, near Randolph, VT (Ratcliffe
and Aleinikoff, 2000). The monzodiorite consists of
phenocrysts of brown hornblende (25%), biotite (5%)
and plagioclase (60%) in a diabasic interlocking
texture. Late-stage interstitial overgrowths of quartzalbite and microcline are also present.
3. ID – TIMS documentation of the new standards
It is crucial that the age of any potential microprobe standard be both accurately and precisely
established by an independent technique, and not
by an alternative form of microprobe dating. This
section presents the ID – TIMS documentation for the
age and homogeneity of the two new zircon standards (TEMORA 2 and R33). The data for R33 were
obtained at two different laboratories, the Berkeley
Geochronological Center (BGC), USA and the Royal
Ontario Museum (ROM), Canada. Most of the
details of the analytical procedures can be found in
Black et al. (2003a); some salient points are listed in
the footnotes to Table 2.
3.1. TEMORA 2
TEMORA 2 was only analysed at the ROM,
during four sessions from September 2000 until
November 2001 (Fig. 1, Table 2). Twenty-one analyses were performed, giving a weighted mean 206Pb/
238
U age of 416.50 F 0.22 Ma (MSWD = 0.78). The
initial 12 analyses were obtained during the first two
sessions shortly after a new ion-counting Daly detector system was installed. During this period, the dead
time correction varied linearly with intensity, based on
measurements of the SRM982 Pb standard. In July
2001, the magnet position was adjusted and the dead
time correction was found to have changed to a
constant value of 23.0 ns, independent of count rate.
The detector configuration and performance characteristics are considered to have been stable from this
point on, when the remaining nine analyses were
undertaken. In August 2001, Faraday-cup data were
acquired for a large, multigrain fraction and Dalydetector data from an aliquot derived therefrom.
During the same session period, Faraday and Daly
data were taken on a second large, multigrain fraction
using a VG M354 mass spectrometer. This instrument
showed the same mass discrimination for both
Faraday and Daly detectors and a deadtime correction of 18.5 ns for Pb and U on the Daly detector.
Data for five single grains were obtained in November 2001 concurrently with two other standards,
TEMORA 1 and R33, in a ‘‘round robin’’ session
(see below). The weighted mean age from the latter
nine TEMORA 2 analyses, unaffected by detector
bias, of 416.78 F0.33 Ma (MSWD = 0.56, probability of equivalence = 0.81) is considered to provide
the best estimate of the age of TEMORA 2.
In common with TEMORA 1 (Black et al., 2003b),
when measurement errors alone are considered, the
nine preferred TEMORA 2 analyses plot significantly
below concordia (probability of concordance = 0.000).
However, the probability of concordance increases to
an acceptable value (0.54) if the uncertainties proposed by Mattinson (1987) for the U decay constants,
and the 0.13% (1j) ROM Pb/U spike calibration
uncertainty, are taken into account.
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
Because the main use of a standard for dating
relatively young zircons by microprobe is to correct
for analytical fractionation of 206Pb/238U, this isotopic
ratio (and its associated age) should serve as the
reference for that dating. The 206Pb/238U age of
416.78 F 0.33 Ma, based on the preferred nine ID –
TIMS measurements, is chosen in this study as a
benchmark for inter-comparison between the various
standards that have been analysed in the ROM laboratory, because their relative ages are independent of
spike-calibration (and also U decay constant) uncertainty). However, both spike-calibration and U decayconstant uncertainties need to be taken into account if
the TEMORA 2 age is to be meaningfully compared
with ages derived from other isotopic systems. In
those instances, the reference age for TEMORA 2
becomes 416.8 F 1.3 Ma.
3.2. R33
3.2.1. BGC results
R33 has been analysed in two different ID –TIMS
laboratories, using different sample preparation procedures. At the BGC, individual grains were first set
in resin and polished to almost half thickness, so that
internal structures could be imaged using cathodoluminescence (CL). Unzoned or weakly zoned grains
were chosen for analysis. After extraction from the
resin, some grains were abraded only (Krogh, 1982),
others were leached in NaOH and abraded (A/L), and
the remainder were not treated by either process
(Table 2). The polishing to half thickness before
abrasion produced residual grains that were about
one seventh the size of those analysed at the ROM,
which has resulted in significantly worse precision for
207
Pb/206Pb and 207Pb/235U in the BGC data (Table 2).
In addition, the sectioning of the crystals along the
crystallographic c-axis has made it difficult to remove
all of the surface material, as the following results
show.
Fig. 1C shows that A/L was the most successful
procedure for limiting data spread. Perhaps surprisingly, data from the abraded grains disperse more widely
than data from the two grains that were not treated at
all. Even the eight A/L analyses show a significant
spread in 206Pb/238U (MSWD = 3.2, probability of
equivalence = 0.002). Based on the trend of these
results, the oldest grain (f 422 Ma) possibly reflects
119
an inherited component; the youngest four ages were
affected by Pb loss. A preferred 206Pb/238U age of
418.9 F 0.4 Ma (2j, MSWD = 1.0, probability = 0.43)
is derived for zircon crystallisation using data from the
other seven A/L grains, one of the untreated grains and
one grain that was abraded only (Fig. 1D).
3.2.2. ROM results
The R33 grains dated at the ROM were carefully
selected under a binocular microscope and abraded
only, prior to dissolution. The results (Table 2) were
obtained over a few days (in November 2001) rather
than over 1 or 2 years, as was the case with TEMORA
1 and TEMORA 2 data, respectively. In common with
TEMORA 2 (and TEMORA 1—(Black et al., 2003b),
the group of six R33 analyses appears to lie below
concordia until spike-calibration and U decay-constant
uncertainties are considered. When this is done, the
probability of concordance of the dataset is high
(0.75). These analyses produce mutually indistinguishable 206Pb/238U ages (MSWD = 0.31, probability
of equivalence = 0.91) that combine to yield a weighted mean age of 419.26 F 0.39 Ma for R33. This age is
indistinguishable (probability of equivalence = 0.21)
from the BGC age derived above for R33.
3.3. Detailed ID – TIMS comparison of the three
Palaeozoic standards
The comparisons of ID – TIMS results for QGNG
in Black et al. (2003a) and those reported above for
R33 show that reasonable agreement can be obtained
between different laboratories. However, these studies
have also highlighted differences in detail within the
data. Moreover, the TEMORA 2 results indicate the
limits to accuracy from ion-counting ID –TIMS analyses using the Daly detector. Thus, TEMORA 2 yields
a suggestion of variation of mean 206Pb/238U ages
from 416.1 F 0.6 through 416.4 F 0.3 – 416.6 F 0.5
Ma for the three TEMORA 2 dating sessions prior
to the round-robin experiment. As reported above, the
first two of those results are believed to reflect slight
biases due to deadtime instability. It is probably not
possible to obtain accuracy of better than about one
part per thousand because of limitations imposed by
detector characteristics.
In order to minimise any influences that might
prohibit the maximum level of scrutiny, a single
120
Analysis
No. of
grains
Weight
(mg)
U
(ppm)
Th/U
PbCom
(pg)
207
Pb/
Pb
204
206
Pb/238U F 2j
207
Pb/235U F 2j
207
Pb/206Pb F 2j
206
Pb/238U F
2j age (Ma)
TEMORA 2
ROM analysed September 2000 ion-counting Daly; F/D = 0.0007 per AMU; deadtime correction 17 ns for Pb and 15 ns for U (linearly varying)
sk13p84a
1
17
163
0.39
1.3
534.7
0.06669 F 0.00023 0.5071 F 0.0021
0.05515 F 0.00011 416.18 F 1.40
sk13p85a
1
17
271
0.35
1.3
877.2
0.06661 F 0.00020 0.5071 F 0.0017
0.05522 F 0.00013 415.71 F 1.19
sk13p86a
1
9
146
0.43
1.8
190.8
0.06671 F 0.00016 0.5075 F 0.0024
0.05518 F 0.00023 416.28 F 0.96
1
4
94
0.34
2.6
49.5
0.06667 F 0.00020 0.5067 F 0.0098
0.05512 F 0.00105 416.07 F 1.21
sk13p87a
ROM analysed January 2001 ion-counting Daly; F/D = 0.0007 per AMU; deadtime correction 17 ns for Pb and 15 ns for U (linearly varying)
sk14p07a
1
30
136
0.50
0.5
1821.8
0.06668 F 0.00019 0.5072 F 0.0017 0.055171 F 0.00013 416.09 F 1.15
sk14p08a
1
27
201
0.37
0.6
2225.7
0.06671 F 0.00016 0.5074 F 0.0014 0.055165 F 0.00008 416.28 F 0.98
1
14
83
0.45
0.7
390.5
0.06673 F 0.00015 0.5071 F 0.0020 0.055118 F 0.00019 416.39 F 0.93
sk14p09a
a
sk14p10
1
10
309
0.41
0.5
1514.8
0.06660 F 0.00016 0.5069 F 0.0014 0.055204 F 0.00011 415.61 F 0.96
a
sk14p11
1
19
133
0.42
1.2
524.3
0.06675 F 0.00015 0.5080 F 0.0015 0.055198 F 0.00013 416.55 F 0.91
1
9
82
0.44
0.8
239.0
0.06665 F 0.00022 0.5062 F 0.0025 0.055086 F 0.00022 415.96 F 1.30
sk14p12a
sk14p13a
1
11
97
0.44
0.8
348.5
0.06679 F 0.00014 0.5092 F 0.0017 0.055288 F 0.00013 416.80 F 0.85
a
1
11
160
0.50
1.5
296.3
0.06678 F 0.00016 0.5077 F 0.0021 0.055146 F 0.00019 416.71 F 0.95
sk14p14
ROM analysed August 2001; F/D = 0.0007 per AMU; deadtime correction 23 ns for Pb and 20.5 ns for U (constant)
sk14p96a
m
770
173
0.40
23.6
1349.8
0.06667 F 0.00017 0.5079 F 0.0013
0.05525 F 0.00011 416.08 F 1.03
m
50b
173
0.40
0.4
4818.3
0.06681 F 0.00015 0.5078 F 0.0013
0.05513 F 0.00009 416.89 F 0.88
sk14p98a
ROM analysed August 2001 on M354 ion-counting Daly; no F/D; deadtime correction 18.5 ns (constant)
a
m
650
157
0.40
6.5
3740.7
0.06674 F 0.00016 0.5082 F 0.0013
0.05523 F 0.00010 416.50 F 0.96
sk14p97
sk14p99a
m
40c
157
0.40
0.5
3624.6
0.06676 F 0.00013 0.5084 F 0.0012
0.05523 F 0.00007 416.58 F 0.80
ROM analysed November 2001 ion-counting Daly; F/D = 0.0007 per AMU; deadtime correction 23 ns for Pb and 20.5 ns for U (constant)
sk15p07a
1
34
130
0.51
0.5
2341.0
0.06681 F 0.00014 0.5078 F 0.0014
0.05513 F 0.00009 416.88 F 0.85
sk15p08a
1
9
232
0.40
0.9
566.6
0.06687 F 0.00017 0.5083 F 0.0015
0.05513 F 0.00012 417.27 F 1.01
1
6
320
0.43
0.7
670.3
0.06685 F 0.00017 0.5083 F 0.0016
0.05515 F 0.00015 417.16 F 1.00
sk15p09a
4
270
0.33
0.5
544.7
0.06677 F 0.00063 0.5066 F 0.0048
0.05503 F 0.00044 416.66 F 3.82
sk15p11a,d 1
sk15p12a,d 1
3
187
0.40
0.4
339.8
0.06687 F 0.00028 0.5088 F 0.0027
0.05518 F 0.00019 417.30 F 1.66
207
Pb/235U F
2j age (Ma)
207
Pb/206Pb F
2j age (Ma)
416.5 F 1.4
416.5 F 1.2
416.8 F 1.6
416.2 F 6.6
418.4 F 4.6
420.9 F 5.1
419.4 F 9.5
417.0 F 42.9
416.6 F 1.2
416.7 F 1.0
416.5 F 1.3
416.4 F 1.0
417.1 F 1.0
415.9 F 1.7
417.9 F 1.2
416.9 F 1.4
%
Disc
q
0.5
1.3
0.8
0.2
0.861
0.745
0.459
0.150
419.2 F 5.3
418.9 F 3.1
417.0 F 7.8
420.5 F 4.3
420.3 F 5.1
415.7 F 8.9
423.9 F 5.4
418.2 F 7.7
0.8
0.6
0.2
1.2
0.9
0.1
1.7
0.4
0.733
0.874
0.467
0.736
0.663
0.615
0.701
0.576
417.0 F 0.8
417.0 F 0.9
422.1 F 4.4
417.4 F 3.7
1.5
0.1
0.698
0.757
417.3 F 0.9
417.4 F 0.8
421.4 F 4.2
421.7 F 2.7
1.2
1.3
0.724
0.859
417.0 F 0.9
417.3 F 1.0
417.3 F 1.1
416.2 F 3.2
417.6 F 1.8
417.5 F 3.8
417.5 F 4.7
418.1 F 5.9
413.6 F 18.1
419.4 F 7.8
0.2
0.1
0.2
0.8
0.5
0.779
0.708
0.588
0.634
0.748
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
Table 2
ID – TIMS U – Pb isotopic compositions for Palaeozoic zircon standards TEMORA 2 and R33
AMU; deadtime correction 23 ns for Pb and 20.5 ns for U (constant)
0.06724 F 0.00014 0.5121 F 0.0012 0.05524 F 0.00006
419.5 F 0.8
0.06717 F 0.00018 0.5115 F 0.0015 0.05523 F 0.00012
419.1 F 1.1
0.06716 F 0.00016 0.5113 F 0.0015 0.05522 F 0.00010
419.0 F 1.0
0.06716 F 0.00015 0.5111 F 0.0013 0.05520 F 0.00009
419.0 F 0.9
0.06721 F 0.00016 0.5112 F 0.0015 0.05517 F 0.00010
419.3 F 0.9
0.06726 F 0.00017 0.5117 F 0.0015 0.05518 F 0.00011
419.6 F 1.0
0.06774 F 0.00028
0.06726 F 0.00015
0.06725 F 0.00030
0.06721 F 0.00015
0.06714 F 0.00029
0.06704 F 0.00015
0.06710 F 0.00020
0.06708 F 0.00027
0.06699 F 0.00026
0.06700 F 0.00027
0.06628 F 0.00024
0.06656 F 0.00048
0.06586 F 0.00026
0.06447 F 0.00025
0.5178 F 0.0167
0.5105 F 0.0093
0.5162 F 0.0218
0.5120 F 0.0022
0.5095 F 0.0033
0.5118 F 0.0045
0.5110 F 0.0140
0.5194 F 0.0195
0.5093 F 0.0050
0.5120 F 0.0061
0.5041 F 0.0035
0.5014 F 0.0079
0.5011 F 0.0032
0.4859 F 0.0044
0.05544 F 0.00166
0.05505 F 0.00094
0.05567 F 0.00223
0.05525 F 0.00017
0.05504 F 0.00028
0.05537 F 0.00044
0.05522 F 0.00144
0.05616 F 0.00197
0.05514 F 0.00050
0.05542 F 0.00061
0.05516 F 0.00033
0.05464 F 0.00071
0.05518 F 0.00028
0.05466 F 0.00044
422.5 F 1.7
419.6 F 0.9
419.6 F 1.9
419.3 F 1.0
418.9 F 1.8
418.3 F 1.0
418.7 F 1.2
418.5 F 1.3
418.0 F 1.6
418.1 F 1.7
413.7 F 1.5
415.4 F 3.0
411.2 F 1.6
402.7 F 1.6
419.8 F 0.8
419.5 F 1.0
419.3 F 1.0
419.2 F 0.9
419.3 F 1.0
419.6 F 1.0
421.8 F 2.2
421.6 F 4.8
421.1 F 3.9
420.2 F 3.8
419.2 F 4.2
419.4 F 4.3
0.6
0.6
0.5
0.3
0.0
0.1
0.909
0.701
0.811
0.777
0.764
0.763
423.7 F 13.6
418.8 F 7.6
422.6 F 17.9
419.8 F 1.8
418.1 F 2.8
419.7 F 3.7
419.1 F 11.4
424.8 F 16.0
418.0 F 4.1
419.8 F 5.0
414.5 F 2.9
412.6 F 6.5
412.4 F 2.7
402.1 F 3.6
429.9 F 67.5
414.2 F 38.5
439.2 F 89.4
422.3 F 7.7
414.0 F 10.7
427.2 F 18.3
421.3 F 57.9
458.8 F 78.7
418.0 F 19.1
429.1 F 24.0
418.7 F 12.6
397.4 F 29.5
419.6 F 10.7
398.5 F 17.1
1.8
1.3
4.7
0.7
1.2
2.1
0.6
9.6
0.0
2.6
1.2
4.3
2.0
1.0
0.520
0.470
0.510
0.620
0.700
0.380
0.500
0.560
0.490
0.440
0.590
0.560
0.670
0.530
Model Th/U calculated from radiogenic 208Pb/206Pb ratio and 207Pb/206Pb age assuming concordance.
207
Pb/204Pb corrected for fractionation and spike; Pb/U ratios corrected additionally for blank and initial Pb (Stacey and Kramers, 1975).
% disc is percent discordance for the given 207Pb/206Pb age.
U concentrations calculated from crystal weight (estimated from crystal dimensions).
Rho is the correlation coefficient of radiogenic 207Pb/235U vs. 206Pb/238U.
Uncertainties of individual ratios and ages do not include decay constant uncertainties.
Decay constants from Jaffey et al. (1971).
ROM laboratory: PbCom is total common Pb (assuming blank isotopic composition for all of 206Pb/204Pb = 18.221, 207Pb/204Pb = 15.612, 208Pb/204Pb = 39.36.
m signifies multiple grains.
BGC laboratory: PbCom is total common Pb (analytical Pb blank is 1.3 F 0.8 pg, with 206Pb/204Pb = 18.57 F 0.55, 207Pb/204Pb = 15.52 F 0.3, 208Pb/204Pb = 37.96 F 0.89.
a
Abraded only.
b
1/15th aliquot of sk14p96.
c
1/15th aliquot of sk14p97.
d
No chemical processing.
e
Abraded and leached.
f
No abrasion or leaching.
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
R33
ROM analysed November 2001 ion-counting Daly; F/D = 0.0007 per
sk15p13a
1
36
237
0.61
0.5
4180
sk15p14a
1
32
398
0.60
0.8
3648
1
20
353
0.64
0.6
2980
sk15p15a
1
20
160
0.75
0.7
1128
sk15p16a
sk15p17a
1
14
189
0.56
0.6
1058
a
1
12
312
0.64
0.7
1298
sk15p18
BGC analysed March, April and May 2000
RS33Z07e 1
2.3
73
0.94
1.9
36.10
RS33Z03e 1
0.6
149
0.66
0.8
43.04
e
1
1.8
162
0.90
6.4
26.36
RS33Z06
e
RS33Z01
1
3.9
126
0.81
0.7
182.7
9.0
123
0.66
1.4
201.5
RS33Z19f 1
RS33Z04e 1
2.8
107
0.66
0.9
91.40
RS33Z08e 1
2.8
61
0.82
2.3
32.90
e
RS33Z05
1
0.8
170
0.90
2.1
30.89
4.2
134
0.94
1.2
125.3
RS33Z14a 1
3.6
102
0.85
0.9
115.3
RS33Z02e 1
R33Z18f
1
15.0
129
0.87
1.7
282.2
a
1
4.5
103
0.96
1.8
76.00
R33Z16
R33Z17a
1
3.8
273
0.80
1.8
146.3
4.8
138
0.44
1.8
95.98
RS33Z13a 1
121
122
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
Fig. 1. ID – TIMS analytical data for two Palaeozoic zircon standards. The ellipses and error bars represent F 2j precision. (A)
207
Pb/235U – 206Pb/238U concordia diagram for TEMORA 2 showing all 21 ROM analyses. The dark ellipse at the centre represents the weighted
mean. (B) All of the individual 206Pb/238U ages (in time sequence) for TEMORA 2. (C) 207Pb/235U – 206Pb/238U concordia diagram for the BGC
analyses of R33, depicting the range of results obtained following different pre-treatment procedures. (D) 206Pb/238U ages from the BGC data.
Nine ages with F 2j uncertainties represented by the white bars, and encompassing a combination of untreated, abraded only and abraded/
leached grains, yield a preferred age of 418.9 F 0.4 Ma. The black bar appears to represent a slightly older inherited component, whereas the
four grey bars reflect Pb loss. (E) 207Pb/235U – 206Pb/238U concordia diagram for the six ROM analyses of R33. (F) Individual 206Pb/238U ages as
determined at the ROM for R33.
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
operator (SLK) dating session was set up at the ROM
in November 2001, during which TEMORA 1,
TEMORA 2 and R33 were analysed. This approach
removes inter-laboratory and operator differences, and
minimises any instrument-generated effects.
The TEMORA 1 results from this ‘‘round robin’’
session are the last six analyses reported by Black et al.
(2003b), the TEMORA 2 results are the last five
analyses listed in Table 2 of this article, and the R33
data are those discussed above. The TEMORA 1
206
Pb/238U age of 416.64 F 0.45 Ma is in excellent
agreement with that derived from the previous 15
analyses (416.78 F 0.28 Ma), so all of these analyses
have been averaged. It is proposed that the most
appropriate 206Pb/238U reference ages for the standards, especially when they are to be used for detailed
inter-comparison, are 416.75 F 0.24 Ma (TEMORA
1, 21 analyses), 416.78 F 0.33 Ma (TEMORA 2, 9
analyses) and 419.26 F 0.39 Ma (R33, 6 analyses).
4. SHRIMP documentation of the new standards
Analytical procedures are summarised and referenced in Black et al. (2003a). The data reside in
OZCHRON, Geoscience Australia’s (GA) geochronological database (www.ga.gov.au/oracle/ozchron), or
are available on request from the senior author. Data
were reduced using SQUID software (Ludwig, 2002).
For three of the analytical sessions, the exponent in the
algorithm used to correct for fractionation in Pb/U has
been set at 2.0 (Claoué-Long et al., 1995). However,
the observed exponent (1.6 for Z3627—November
2000, 1.8 for Z3673— July 2001 and Z3627—September 2001) has been used in the four instances where
it was found to differ significantly from the commonly
accepted value of 2.0 (see caption of Fig. 2 for
explanation of the Z-prefixed numbers). Very few of
the individual analyses that comprise an analytical
session have been discarded. When such culling has
occurred, it has rarely been on a purely statistical basis.
Independent criteria used to identify anomalous analyses include dramatically changing, atypically low
and/or unstable secondary ion emission, abnormally
high common Pb and/or U, a poorly focussed analytical
spot, and movement of the sample stage during analysis. Rejected analyses are identified on Fig. 2, which
shows each of the individual ages for every analytical
123
session. All uncertainties are cited at the 2j level,
unless stated otherwise.
Whereas Black et al. (2003a) used data calibrated
against QGNG for their examination of zircon standards, TEMORA 1 has now been included in SHRIMP
inter-comparisons often enough (e.g., in seven of the
eight sessions reported herein) for it to be used as the
primary reference standard in this study. The major
advantage of this approach is that whereas the ID –
TIMS analyses of QGNG are significantly dispersed
(Black et al., 2003a), those of TEMORA 1 are not
(Black et al., 2003b). Six different multi-day sessions
(Fig. 2, Table 3) are newly reported. The RSES-GA
SHRIMP II in Canberra was used for five of these,
with the other session being undertaken on the Perth
SHRIMP II. Data from two of the sessions reported
by Black et al. (2003a) are also presented here, but
these are now referenced to TEMORA 1 instead of
QGNG. Only one of the eight sessions (Z 3627,
September 2001, Fig. 2A) includes all four of the
main zircon standards under discussion (TEMORA 1,
TEMORA 2, R33 and QGNG), with the remaining
seven sessions including either two or three of these
standards.
4.1. TEMORA 2
TEMORA 2 has been jointly analysed with
TEMORA 1 on two occasions, with essentially identical results for both sessions (Fig. 2, Table 3). It is
possible to derive two slightly different weighted mean
ages from the first of those sessions (Z3407, April
2000, Fig. 2E), depending on whether one or both of
the two TEMORA 2 analyses identified by SQUID
as statistical outliers are rejected. One of those
analyses can be independently identified as anomalous because it was obtained from an abnormally
deep crater within the zircon (following an episode
of instrumental instability). Rejection of only that
analysis produces a grouping that is at the limits of
being significantly scattered (MSWD = 1.34, probability of equivalence = 0.05). Rejection of both outlying analyses increases the probability of equivalence
to the considerably more acceptable value of 0.28
(MSWD = 1.10 from 51 analyses). The resultant age
is increased by 0.4 to 418.1 F 2.2 Ma, which is
regarded as the best estimate from this session for
the age of TEMORA 2.
124
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
125
Fig. 2. Calibrated ages for individual analyses during each of the six SHRIMP sessions (time sequence is from left to right). All error bars
represent F 2j precision. TEMORA 1 has been used as the calibration standard for all but one of the sessions. Errors shown for the other zircon
suites have been augmented to take account of the uncertainty of the TEMORA 1 calibration. The latter is represented by the calculated 2j spotto-spot error of Ludwig (2002). Numbers prefixed by the letter Z are catalogue references for individual grain-mounts, which usually contain
grains from more than one zircon standard (e.g., Z3673 contains grains from the TEMORA 1, R33 and QGNG standards). Very few analyses
have been excluded from the weighted mean ages (see bottom right of each diagram). Those few outliers are either terminated by small circles,
or lie beyond the confines of the diagrams. The information included in this diagram and data from Black et al. (2003a) is used to construct Figs.
3 and 4. (A) Z3627 (September 2001), (B) Z3673 (February 2001), (C) Z3673 (July 2001), (D) Z3673 (June 2001), (E) Z3407 (April 2000), (F)
Z3627 (November 2000).
The second direct comparison between the two
TEMORA standards was made on grain-mount
Z3627 in September 2001 (Fig. 2A). Only one
TEMORA 2 analysis (yielding a very young age of
f 340 Ma, and being independently identifiable by
an abnormally low secondary beam intensity) is
rejected, resulting in a weighted mean age of
418.1 F 2.4 Ma. The estimates from the two sessions
are clearly within error of each other and can therefore
be combined to give a combined TEMORA 1-calibrated age of 418.1 F 1.6 Ma for TEMORA 2. That value
and the age derived from ID – TIMS dating
126
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
Table 3
Summary of SHRIMP
206
Pb/238U ages for the zircon standards (calibrated to the ID – TIMS age of 416.75 Maa for TEMORA 1)
Session
QGNG
Z3406
Z3219
Z3673
Z3407
Z3627
Z3673
Z3673
1841.2 F 5.5 Ma
1844.8 F 6.6 Ma
1839.8 F 12.9 Ma
(August 2000)
(May 1999)
(June 2001)
(April 2000)
(September 2001)
(July 2001)
(February 2001)
1841.3 F 9.5 Ma
1838.6 F 12.3 Ma
1844.3 F 10.3 Ma
Weighted mean
1842.2 F 3.3 Ma
TIMS age
1842.0 F 3.1 Mab
Probability of equivalence
0.93
Calibrated against TEMORA 2 at 416.78 Mac
Z3627 (November 2000)
TEMORA 2
418.1 F 2.2 Ma
418.1 F 2.4 Ma
418.1 F 1.6 Ma
416.78 F 0.33 Ma
0.11
R33
418.6 F 2.4 Ma
421.7 F 3.1 Ma
420.2 F 2.6 Ma
419.9 F 1.5 Ma
419.26 F 0.39 Ma
0.41
(417.1 F 3.0 Ma)
a
Black et al. (2003b).
b
Black et al. (2003a).
c
The TEMORA 2-calibrated age for R33 cannot be directly compared with the TEMORA 1-calibrated ages because, this study when
combined with that of Black et al. (2003a), demonstrates that the standards do not necessarily have equivalent Pb/U ionisation on SHRIMP.
(416.78 F 0.33 Ma) are not significantly different at
the 95% confidence level, although the agreement
between them is marginal (probability of equivalence = 0.11). Possible reasons for this discrepancy
will be discussed below.
4.2. R33
R33 can be calibrated against TEMORA 1 for three
different sessions. Interpretation of the February 2001
session on Z3673 (Fig. 2B) is straightforward, with the
removal of all of the statistical outliers (for both the
TEMORA 1 standard and R33) being independently
justifiable. An age of 420.2 F 2.6 Ma is calculated
from 47 of the 48 analyses of R33 (MSWD = 1.13,
probability of equivalence = 0.25). Although SQUID
identifies three statistical outliers within the TEMORA
1 analyses from the July 2001 session on Z3673 (Fig.
2C), there is no independent evidence to support their
rejection, and they have all been retained within the
dataset. All 42 of the individual R33 ages are within
error of each other (MSWD = 0.6, probability of
equivalence = 0.98), and yield a weighted mean age
of 421.7 F 3.1 Ma from this session. Just one statistical
outlier is rejected from the 47 TEMORA 1 analyses
from the September 2001 session on Z3627 (Fig. 2A).
All 50 of the R33 ages are within error of each other
(MSWD = 1.15, probability of equivalence = 0.22) and
produce a weighted mean age of 418.6 F 2.4 Ma. As
was the case with TEMORA 2, the three weighted
mean ages reported above for R33 are within error of
each other (MSWD = 1.3, probability of equivalence = 0.28). They yield a combined SHRIMP age
for R33 of 419.9 F 1.5 Ma, which is well within error
of the ID – TIMS age of 419.26 F 0.39 Ma
(MSWD = 0.68, probability of equivalence = 0.41).
4.3. QGNG
Black et al. (2003a) reported two SHRIMP comparisons between TEMORA 1 and QGNG, using the
latter as the reference standard. The same data are
reported here, but with TEMORA 1 as the reference
(Table 3). Together with the four newly reported
sessions that incorporate both of those standards, this
now yields a total of six comparisons between
TEMORA 1 and QGNG. There has been little culling
of data for derivation of QGNG ages. Only 5 of the
entire 299 analyses of the associated TEMORA 1
standard have been rejected, and only 1 of those
rejections is on purely statistical grounds. In addition,
only one of the 290 QGNG analyses has been
rejected, because of its very low UO/U. Individual
analyses are graphically represented in Fig. 2(A – D)
and in Black et al. (2003a). A summary of the results
is given in Table 3. The six sessions produce a set of
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
mutually indistinguishable ages (MSWD = 0.29, probability of equivalence = 0.93) and a weighted mean
age of 1842.2 F 3.3 Ma for QGNG. This is in excellent agreement (MSWD = 0.008, probability of equivalence = 0.93) with the ID – TIMS 206Pb/238U age of
1842.0 F 3.1 Ma, an agreement that was noted by
127
Black et al. (2003a) for their much smaller dataset.
The new data therefore support the contention of those
authors that QGNG and TEMORA 1 behave consistently during SHRIMP analysis. Perhaps surprisingly,
that consistency occurs despite the slight (f 0.5%)
discordance and heterogeneity of the QGNG ID –
TIMS analyses (Black et al., 2003a).
4.4. Calibration of TEMORA 1 and TEMORA 2
against R33
The ages reported above have been derived by
calibration against TEMORA 1. An intriguing aspect
of those results is that they yield a 206Pb/ 238U
SHRIMP age for TEMORA 2 that is only marginally
within error of that given by ID – TIMS dating
(418.1 F 1.6 and 416.8 F 0.3 Ma, respectively). This
possible anomaly can be further examined by using
R33 as the reference standard, partly because that
option permits a different combination of SHRIMP
inter-comparisons. For example, the use of R33 as
reference, eliminates the Z3407 (April 2000) session
as a source of information, but allows the November
2000 session on Z3627 (during which TEMORA 1
was not analysed) to be utilised (Fig. 2F, Table 3).
Weighted mean ages are derived for TEMORA 1
(415.3 F 1.4 Ma from three sessions) and TEMORA
2 (418.3 F 1.8 Ma from two sessions). These results
do not agree within error (probability of equivalence = 0.009) even though the ID – TIMS data demonstrate that TEMORA 1 and TEMORA 2 are of
indistinguishable age.
4.5. Summary of SHRIMP results
Fig. 3. Diagram showing the relative deviations of the 206Pb/238U
ages derived by micro-beam analysis for each of the individual
standards from their respective 206Pb/238U ID – TIMS ages.
TEMORA 1 has been used as the calibration standard for both
SHRIMP and ELA – ICP – MS analyses (it therefore plots at zero
offset). Any rectangle to the right of TEMORA 1 represents a
microprobe age that exceeds its corresponding ID – TIMS age, and
those to the left of TEMORA 1 indicate the reverse. Uncertainties
represent F 2 standard errors of the mean. The dispersion of the
ages is significant for both microprobe techniques (see text).
The SHRIMP dating has produced both expected
and unexpected results. There is a fixed 206Pb/238U
age-relationship between any two of the four standards within the limits of analytical error. However, the
derived SHRIMP ages are not always consistent with
those determined by ID –TIMS. QGNG yields ages
that are compatible with those obtained for TEMORA
1, but the TEMORA 2 age appears to be offset by
about 0.3% from its ID – TIMS age. When previously
reported results (Black et al., 2003a) for the SL13 and
AS3 standards are also taken into account, the range
of this variation is considerably enlarged (Fig. 3).
Possible reasons for these biases are discussed below.
128
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
5. ELA –ICP –MS dating
Laser ablation was conducted by a pulsed ArF
LambdaPhysik LPX 120I UV Excimer Laser operated at a constant energy of 100 mJ, at 5 Hz, with a
spot diameter of 29 Am. The excellent resulting tophat cross-sectional shape of the ablated hole suggests
that fluence generated by this laser (k = 193 nm) and
optical system was constant during drilling to a
depth of approximately 20 Am. The ablated material
was carried by Ar –He –H gas via a custom-built
flow homogeniser to an Agilent 7500s quadrupole
ICP –MS. The ICP –MS and laser conditions were
those listed in Eggins and Shelley (2002), with RF
power of 1250 W.
Initial conditions for the analytical session (RF
power, gas flow and torch position) were set such
that counts on ThO/Th < 0.3%. This parameter was
chosen because ThO is extremely stable in a plasma
relative to other molecules due to a very high metaloxide bond strength. Because of this, it can be
confidently assumed that production of any other
molecule entering the lens system and detector is at
this rate or less. At the beginning of the main
analytical session, a 29-Am spot scanned across the
NIST 612 standard (38.5 ppm Pbtot) yielded 40,000
cps on 208Pb with background counts of 400 cps, as
compared with 20 cps on the isotope-absent mass of
220. Whereas Pb backgrounds are low, Hg backgrounds are high with calculated 204Hg/204Pb = 49.9,
making Hg-corrections to 204Pb untenable.
Raw count rates for 17 masses were recorded in
time-resolved mode by peak hopping. The integration
time for the isotopes was 50 ms for 206Pb, 207Pb and
208
Pb, 25 ms for 232Th, 235U and 238U, and 5 ms for
29
Si, 31P 91Zr, 178Hf and 7 REEs, resulting in a total
mass sweep of 315 ms. Counts were collected in both
pulse counting and analogue mode with the transition
at 1 106 cps. Data were acquired for 20 s with the
laser off and 40 s with the laser on, giving approximately 120 mass scans for a penetration depth of ca.
20 Am. Pairs of the primary zircon standard and a
single NIST 610 were analysed in a round robin of
pairs of the of the three zircon standards studied in the
main analytical session. Washout time was 12 s.
Corrections were made for background, instrumental mass-bias drift, depth-dependent elemental
fractionation and common Pb. The three to four
mass scans required for the counts to reach a
maximum after laser triggering were discarded.
Depth-dependent inter-element fractionations of Pb,
Th and U, documented by previous workers (i.e.,
Hirata and Nesbitt, 1995; Horn et al., 2000), were
corrected by reference to standard zircon Temora 1
(206Pb/238U = 0.066809, 208Pb/232U = 0.0020840 and
207
Pb/206Pb = 0.055115; Black et al., 2003b) and
232
Th/238U to NIST 610 (1.01874). Using the most
critical of these ratios as an example, the average
factor required to raise the 206Pb/238U of the standard
TEMORA zircon from the measured value (the
average of 35 ablations over the analytical session)
to the accepted value of 0.066809 for each mass scan
was calculated from multiple measurements of the
standard. This factor was then applied to the appropriate mass scan for each of the unknowns, making
the assumption that there is no variation in the
correction factor among zircons. As an indication
of the significance of down hole fractionation, the
206
Pb/238U in the average of NIST610 (n = 19) increased by about 8.5% from the top to the bottom of
the ablated hole, whereas in TEMORA 1 (n = 35), the
ratio increased by 34%. Thus, the down-hole mass
fractionation factor for TEMORA 1 is four times that
of NIST 610. Comparatively, there is little
207
Pb/206Pb fractionation with decrease of 0.18% in
TEMORA 1 and 0.13% in NIST 610. 232Th/238U
increased 1.3% in NIST 610 and 2.3% in TEMORA
1, although the Th/U in TEMORA is not expected to
be constant. The magnitude of these down hole
changes re-emphasises the importance of the mass
scan-by-mass scan data reduction method which was
applied to all appropriate U, Th and Pb ratios.
Co-aspiration of standard solution was not used to
measure mass fractionation. First, as the particle size
distribution of this nebulised material must differ
significantly from that of the ablated material, a mass
fractionation factor obtained by this method cannot
represent mass fractionation in ablated zircon. Second,
difficulties in consistent nebulisation, and third, the
complications of making a dry-plasma wet (O and OH
interferences and changes in ion energy distribution
even after desolvation) far exceed that of repeatedly
measuring the down hole fractionation in a standard
zircon (Huang et al., 2000; Horn et al., 2000).
During the main session the standard error for
the NIST 610 glass monitor was 0.44% for
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
206
Pb/238U and 0.11% for 207Pb/206Pb. Other isotope
ratios of interest had uncertainties lying between
these limits.
Because the ID – TIMS and SHRIMP analyses
reveal a very high degree of isotopic homogeneity,
virtually all of the dispersion between individual
ELA – ICP – MS analyses is attributed to analytical
effects rather than real sample heterogeneity. Consequently, in common with the SHRIMP results
reported above, the favoured data treatment is to reject
only those ELA –ICP – MS analyses that can be independently identified as being anomalous. The ages
have not been corrected for common Pb because of
the isobaric interference at mass 204 noted above. The
extremely low levels of common Pb identified within
these standards by ID – TIMS analysis (see above and
in Black et al., 2003a,b) justify the decision to use
uncorrected data.
The centrepiece of the ELA – ICP –MS study is a
single 6-h analytical session in November 2001 that
comprised about 36 alternating analyses each of
TEMORA 1, TEMORA 2, R33 and QGNG. This
analytical format enables the latter three standards to
be directly referenced to TEMORA 1, as was done
for most of the SHRIMP comparisons. A link to the
AS3 standard (the SHRIMP 206Pb/238U characteristics of which were reported by Black et al., 2003a)
is provided by an earlier session that included 15
129
analyses each of AS3 and TEMORA 2. The latter is
now the primary standard of choice, with TEMORA
1 being used in the round-robin session for historical reasons.
5.1.
207
Pb/206Pb
Although the primary focus of this study is
Pb/238U dating, the accompanying 207Pb/206Pb information is useful as a guide to the feasibility of
applying ELA – ICP –MS to the dating of old rocks.
Even though this parameter can be strongly influenced
by common Pb, the latter is effectively absent in the
zircons under review (Table 2 and Black et al.,
2003a,b) when compared with the precision of the
individual analyses.
Because of its enhanced radiogenic Pb levels,
QGNG provides more precise 207Pb/206Pb ages than
do any of the other standards being reviewed here. All
36 analyses of QGNG yield a combined age of
1843.2 F 4.1 Ma (MSWD = 13), which is younger
than the corresponding ID – TIMS age of 1851.6 F
0.6 Ma (Black et al., 2003a). In contrast, 15 analyses
of AS3 produce an average age of 1100 F 8 Ma
(MSWD = 21), which is within error of the ID – TIMS
age of 1099.1 F 0.5 Ma (Paces and Miller, 1993).
Considerably reduced precision for 207Pb/206Pb dating
of young zircon does not permit a comparable level of
206
Table 4
Summary of the ELA – ICP – MS dating of the zircon standards (calibrated against the TEMORA 1 ID – TIMS age of 416.75 Maa)
Standard
Age F 2j (Ma)
MSWD
Number of analyses
ID – TIMS age (Ma)
Pb/206Pb ages
QGNG
AS3
1843.2 F 4.1
1100 F 8
13
21
36 of 36
15 of 15
1851.6 F 0.6b
1099.1 F 0.5c
Pb/238U ages
QGNG
TEMORA 2
R33
AS3
1870 F 10
412.8 F 2.7
411.9 F 2.6
1123 F 12d
36
35
36
15
1842.0 F 3.1b
416.78 F 0.33
419.26 F 0.39
1099.0 F 0.7c
207
206
1.14
1.15
0.45
0.80
of
of
of
of
36
36
36
15
The MSWD values for the 207Pb/206Pb ages are based on measurement errors alone, whereas those reported for the 206Pb/238U ages have been
referenced to the concurrent analyses of the TEMORA 1 standard (see text).
a
Black et al. (2003b).
b
Black et al. (2003a).
c
Paces and Miller (1993).
d
Based on the results of this study, zircons calibrated against TEMORA 2 cannot be directly compared with those calibrated against other
standards, including TEMORA 1. A conversion factor of approximately + 0.95% is required for more meaningful comparison of this result with
the TEMORA 1-calibrated analyses (see text).
130
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
scrutiny for the Palaeozoic standards (Table 4). Nevertheless, the preferred ELA –ICP –MS 207Pb/206Pb
age for TEMORA 2 of 420 F 16 Ma (MSWD = 60)
agrees with its ID –TIMS age of 416.78 F 0.33 Ma.
R33 behaves similarly, with its preferred ELA – ICP –
MS age of 421 F 17 Ma (MSWD = 78) agreeing with
the ID –TIMS age of 419.26 F 0.39 Ma. Overall, the
data are consistent with no major fractionation
between Pb isotopes during ELA – ICP –MS analysis,
although the certainty of this conclusion is limited
by relatively large errors and low probabilities of
equivalence.
5.2.
206
Pb/238U
5.2.1. TEMORA 1
All but one of the 35 analyses of TEMORA 1,
which has been used as the reference standard, define
a moderately coherent distribution. The exception is
an analysis that yields a considerably older age
(f 470 Ma), which is three times as far from the
mean as any of the other individual ages. This analysis
also has by far the highest common Pb component, as
determined by the 208Pb method. The data are significantly scattered (MSWD = 24), even in the absence of
that analysis, and this scatter is taken to provide an
indication of instrumental uncertainties beyond those
contributing to normal counting statistics. In common
with the SHRIMP data-processing procedure, this
component of excess scatter, which equates with a
coefficient of variation of 1.88%, is added in quadrature to the counting error uncertainties of the individual analyses of the other concurrently analysed
standards. If that procedure is appropriate, it is to be
expected that with few if any exceptions, the data for
each of those standards will exhibit no degree of
excess scatter.
5.2.2. QGNG
The grouping of all 36 individual analyses
of QGNG, which is not significantly scattered
(MSWD = 1.14, probability of equivalence = 0.27),
yields a 206Pb/238U age of 1870 F 10 Ma. This age is
significantly older (probability of equivalence =0.000)
than its corresponding 206Pb/238U ID –TIMS age of
1842.0 F 3.1 Ma. Another important aspect of the
ELA – ICP – MS data is that, as a group, they
are reversely discordant when calibrated against
TEMORA 1 (see Table 4; the mean 206Pb/238U age
exceeds the mean 207Pb/206Pb age). Application of any
common Pb correction would only exacerbate this
anomaly.
5.2.3. TEMORA 2
As a group, all 36 of the TEMORA 2 analyses
fulfil the requirements of a single population at the
95% confidence level, but barely so (MSWD = 1.36,
probability of equivalence = 0.075). There are no
dramatic statistical outliers, and these data produce a
mean age of 413.4 F 2.6 Ma, which is significantly
different from the ID – TIMS age (416.78 F 0.33 Ma,
probability of equivalence = 0.022). An alternative
option is to reject an analysis with very high P (more
than 700 ppm in excess of the next highest value),
which presumably reflects the accidental analysis of an
apatite inclusion. This data combination is preferred
because not only is it independently justifiable, but it
also reduces the scatter to a more acceptable level
(MSWD = 1.15, probability of equivalence = 0.25).
The resultant age of 412.8 F 2.7 Ma (MSWD = 1.15)
is even further from the ID –TIMS age (probability of
equivalence = 0.003).
5.2.4. R33
None of the 36 analyses of R33 are statistical outliers, which is confirmed by an MSWD of 0.45 (probability of equivalence = 0.99). The resultant age is
411.9 F 2.6 Ma, which is clearly not in agreement with
the ID –TIMS value of 419.26 F 0.39 Ma (probability
of equivalence = 0.000).
5.2.5. AS3
AS3 was analysed well before the round-robin
experiment was performed, and before the complications observed above were identified. Consequently, the AS3 data have an additional component of
uncertainty, because they were calibrated against
TEMORA 2, rather than TEMORA 1. The
TEMORA 2-calibrated age for the 15 individual
analyses is 1123 F 12 Ma (MSWD = 0.80, probability of equivalence = 0.67, after the individual ages
have been augmented by 1.88%). This age is
significantly older (probability of equivalence =
0.000) than the 206Pb/238U age as determined by
ID – TIMS (1099.0 F 0.7 Ma; Paces and Miller,
1993). However, the difference between the ELA –
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
131
Table 5
Offset of the SHRIMP and ELA – ICP – MS 206Pb/238U ages (calibrated against the 416.75 Maa TEMORA 1 reference standard) from their ID –
TIMS ages
Standard
ID – TIMS age (Ma)
SHRIMP age (Ma)
SHRIMP offset (%)
ELA – ICP – MS age (Ma)
ELA – ICP – MS offset (%)
TEMORA 2
R33
QGNG
AS3
SL13
416.78 F 0.33
419.26 F 0.39
1842.0 F 3.1b
1099.0 F 0.7c
572.1 F 0.4d
418.1 F 1.6
419.9 F 1.5
1842.2 F 3.3b
1087.5 F 2.5c
577.4 F 2.0d
0.31 F 0.39
0.15 F 0.37
0.01 F 0.25
1.05 F 0.24
0.92 F 0.36
411.8 F 2.1
412.5 F 1.5
1873 F 11
1140 + 12*
0.95 F 0.65
1.75 F 0.63
1.50 F 0.57
3.18 F 1.27
A positive offset indicates that a SHRIMP or ELA – ICP – MS age is greater than that derived by ID – TIMS, and a negative offset indicates the
reverse.
a
Black et al. (2003b).
b
Black et al. (2003a).
c
Paces and Miller (1993).
d
Claoué-Long et al. (1995).
* Adjusted upwards by 0.95% to compensate for calibration of AS3 against TEMORA 2, rather than TEMORA 1 (see text).
ICP – MS and ID – TIMS results is even more
marked than this. It is shown below that the
ELA –ICP– MS technique yields inconsistent results
for TEMORA 1 and TEMORA 2, which necessi-
tates an adjustment if ages calibrated against those
standards are to be inter-compared. An upward
adjustment of about 0.8% is required to convert
TEMORA 2- to TEMORA 1-calibrated ages, which
Table 6
Oxygen isotopic data for the three Palaeozoic zircon standards
Run sample
Weight (mg)
Amol CO2
Yield (Amol/mg)
d13C
d18O raw
d18O corrected
Reference standard
1 UWG-2
2 UWG-2
3 UWG-2
4 UWG-2
5 UWG-2
1.89
1.71
1.31
1.69
1.50
26.5
22.9
17.3
21.1
19.6
14.0
13.4
13.2
12.5
13.1
27.91
27.93
27.90
27.93
27.90
5.60
5.64
5.67
5.66
5.70
5.80 F 03
R33
16 R33
17 R33
2.64
2.33
23.2
20.5
8.8
8.8
27.93
27.90
5.37
5.44
5.50
5.57
TEMORA 2
19 2000-84-4522
20 2000-84-4522
21 2000-84-4522
2.71
2.43
3.41
21.0
18.8
29.4
7.7
7.7
8.6
27.89
27.94
27.90
8.07
8.06
8.08
8.20
8.19
8.21
TEMORA 1
22 9884-4520
23 9884-4520
24 9884-4520
2.94
3.10
3.16
22.7
27.0
26.9
7.7
8.7
8.5
27.88
27.91
27.87
7.83
7.76
7.81
7.96
7.89
7.94
Reference standard
25 UWG-2
26 UWG-2
1.75
1.60
24.1
20.9
13.8
13.1
27.89
27.94
5.67
5.73
1 UWG-2 was not used for the calculation of the weighted mean composition of the reference standard at the beginning of the session. The other
four analyses at that time combine to yield a d18O value of 5.67 F 0.03. Analyses were then corrected by 0.13x(Valley et al., 1995). d13C
provides a secondary check on the quality of the analyses.
results in a revised
for AS3.
140 F 17
79 F 15
131 F 25
103 F 14
188 F 35
3.21 F 0.07
5.7 F 0.6
3.5 F 0.6
4.55 F 0.86
6.4 F 1.3
11.4 F 2.9
nm
3.20 F 0.29
1.98 F 0.39
2.17 F 0.41
3.9 F 1.1
7.5 F 2.5
0.39 F 0.02
Pb/238U age of 1134 F 14 Ma
6. Oxygen isotopes
The age anomalies recorded above (Fig. 3, Table
5) are a significant impediment to precise microprobe analysis, unless their causes can be identified
and a method of compensation established. Of particular concern are the different microprobe
206
Pb/ 238 U results for the two closely related
All uncertainties are F 2 standard errors of the mean. nm = not measured.
3.57 F 0.32
3.12 F 0.20
4.44 F 0.44
20.3 F 1.4
13.2 F 3
0.26 F 0.01
f 0.13
< 0.013
< 0.11
f 0.16
0.38
< 0.05
196 F 17
165 F 9
219 F 21
288 F 23
363 F 38
25 F 5
0.92 F 0.01
0.98 F 0.01
1.12 F 0.02
1.25 F 0.04
1.20 F 0.11
0.900 F 0.005
132 F 36
54 F 10
100 F 20
188 F 26
233 F 54
18.3 F 0.3
228 F 56
130 F 21
148 F 20
212 F 23
360 F 80
210 F 3
TEMORA 1
TEMORA 2
R33
QGNG
AS3
SL13
206
5.2.6. Summary of 206Pb/238U ELA –ICP –MS results
The ELA – ICP – MS 206Pb/238U ages in their
entirety are inconsistent with the ID – TIMS ages.
The two Proterozoic standards (QGNG and AS3)
give ages that are too old, and TEMORA 2 and
R33 yield ages that are too young (Fig. 3, Table
5). Notably, there is a f 1.5% difference in the
offset of AS3 from QGNG. This offset from the
expected age (against a common reference—
TEMORA 1) is in the opposite sense from the
offset found for SHRIMP (Fig. 3, Table 5). The
offset is larger than the measurement error for the
ELA – ICP –MS technique.
1.12 F 0.12
0.71 F 0.13
1.04 F 0.19
0.81 F 0.16
0.40 F 0.10
0.12 F 0.01
Dy (ppm)
Eu (ppm)
Sm (ppm)
Nd (ppm)
Ce (ppm)
La (ppm)
P (ppm)
HfO2 (wt.%)
Th (ppm)
U (ppm)
Standard
Table 7
Average trace element concentrations for the zircon standards (determined by ELA – ICP – MS)
93 F 8
61 F 7
97 F 13
50 F 5
84 F 12
2.90 F 0.04
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
Lu (ppm)
132
Table 8
Correlation between ion-probe age offsets and trace element
concentrations
Species
U
Th
P
HfO2
Ce
Nd
Sm
Eu
Eu#
Dy
Lu
SHRIMP
ELA – ICP – MS
Slope
Correlation
coefficient
Slope
Correlation
coefficient
negative
negative
negative
negative
negative
negative
negative
negative
negative
negative
negative
0.74
0.91
0.93
0.65
0.59
0.98
0.99
0.14
0.94
0.92
0.65
positive
positive
positive
positive
positive
positive
positive
negative
negative
positive
negative
0.91
0.93
0.89
0.57
0.74
0.94
0.91
0.72
0.21
0.60
0.23
Age offset is taken as the ordinate and elemental abundance as the
abscissa, for the definition of the slope of the regression. La
concentrations are mostly too low to provide convincing data arrays.
Eu# excludes the data for AS3, which has a prominent negative Eu
anomaly.
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
133
6
3
A
SHRIMP
B
ELA-ICP-MS
2
4
R33
1
QGNG
0
TEMORA 2
AS3
-1
Offset (%)
Offset (%)
SL13
TEMORA 1
2
AS3
QGNG
0
TEMORA 2
TEMORA 1
-2
-2
R33
-4
-3
0
100
200
300
0
400
100
200
C
SHRIMP
4
QGNG
R33
1
QGNG
Offset (%)
Offset (%)
D
ELA-ICP-MS
2
0
TEMORA 2
-1
-2
2
AS3
TEMORA 2
0
TEMORA 1
-2
AS3
TEMORA 1
R33
-4
-3
0
2
4
6
8
10
12
0
14
2
4
6
8
10
12
14
Sm (ppm)
Sm (ppm)
6
3
E
SHRIMP
F
ELA-ICP-MS
4
SL13
TEMORA 2
1
Offset (%)
Offset (%)
400
6
3
2
300
P (ppm)
P (ppm)
QGNG
0
R33
2
0
AS3
QGNG
TEMORA 2
-1
TEMORA 1
AS3
-2
-2
TEMORA 1
R33
-4
-3
0
2
4
6
Nd (ppm)
8
10
12
0
2
4
6
8
10
12
Nd (ppm)
Fig. 4. Correlations between P, Sm and Nd abundances, and the 206Pb/238U age difference between ID – TIMS and microprobe analysis.
Uncertainty in the age offset is dominated by the component arising from microprobe analysis, with the ID – TIMS component being essentially
negligible. The high precision for the age offset of TEMORA 1 reflects the ID – TIMS age alone for this reference standard. The ellipses represent
F 2 standard errors of the mean. Dashes define lines of best fit. (A) Age offset vs. P for SHRIMP, (B) age offset vs. P for ELA – ICP – MS, (C) age
offset vs. Sm for SHRIMP, (D) age offset vs. Sm for ELA – ICP – MS, (E) age offset vs. Nd for SHRIMP, (F) age offset vs. Nd for ELA – ICP – MS.
134
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
TEMORA standards. Oxygen isotope studies were
undertaken at the University of Wisconsin, partly to
see if there is any relationship with age bias (and
also to investigate its potential as an oxygen-isotope
standard). Fractions of the TEMORA 1, TEMORA 2
and R33 standards were soaked in HF overnight to
remove radiation-damaged zones (if present) and
silicate impurities. They were subsequently ground
to fine powder, divided into f 2 – 3 mg aliquots, and
analysed by a CO2 laser using BrF5 reagent, as
discussed by Valley et al. (1995). A round-robin
format of analysis minimises some sources of analytical uncertainty. d18 O data are corrected to
VSMOW via the UWG-2 standard.
The difference between TEMORA 1 and TEMORA
2 (Table 6), although small, is significant. However, it
is most unlikely to be of sufficient magnitude to
provide an explanation for the microprobe age anomalies, especially as R33 has a very different oxygen
isotopic composition, but appears to have an intermediate microprobe age offset.
The oxygen isotopes record a typical mantle signature (5.5x) for R33. In contrast, even though their
host rocks are of considerably more mafic composition (Table 1), the elevated d18O (f 8.1x) of the
TEMORA zircons is indicative of a crustal contribution (Valley, 2003), either by the incorporation of
sedimentary rocks into the host magma, or via hydrothermal alteration.
The precision of the analyses indicates TEMORA 1,
TEMORA 2 and R33 have potential as oxygen-isotope
standards, but micro-beam analysis will first be required to determine if they have any small-scale ( < 50
Am) heterogeneity.
7. Trace-element compositions
ELA – ICP – MS trace-element documentation of
the standards was undertaken to determine whether
the age offsets for both ELA – ICP – MS and
SHRIMP might result from a matrix effect. Concentration data for R33, QGNG, TEMORA 1,
TEMORA 2 and AS3 were derived using NIST
610 as the reference standard (results previously
based on NIST 612 were used for SL13) and by
using Zr as an internal standard. This means that the
ratio of element/zirconium background-corrected
counts from the unknown were divided by that in
the standard for the same depth interval, multiplied
by the known element/zirconium in the standard
glass, and by the assumed stoichiometric abundance
of Zr in unknown zircon. The latter value was
assumed to be 67.22% minus the HfO2 abundance
(estimated initially at 65% ZrO2). Use of an internal
standard in this manner generally negates the problems of using a different matrix (glass) for the
standard than the target material (zircon). NIST
610 concentrations are taken from Pearce et al.
(1997).
Seven rare earth elements (REE), as well as Si, Zr,
P and Hf (Table 7), were measured during the same
Table 9
Regression parameters for the microprobe age offsets, and P, Nd and Sm abundances
Regression
Slope
ELA – ICP – MS
Age offset vs. Nd
Age offset vs. Sm
Age offset vs. P
Age offset vs. Th
SHRIMP
Age offset
Age offset
Age offset
Age offset
vs.
vs.
vs.
vs.
Nd
Sm
P
Th
1.17 F 0.58
0.79 F 0.86
0.024 F 0.032
0.023 F 0.023
0.277 F 0.095
0.170 F 0.077
0.0052 F 0.0033
0.0072 F 0.0047
Intercept
MSWD
Probability of fit
3.8 F 1.8
4.4 F 4.6
5.2 F 7.1
3.0 F 2.9
2.0
3.5
9.0
4.6
0.110
0.002
0.000
0.003
0.90 F 30
0.97 F 45
1.08 F 75
0.97 F 66
0.60
0.28
3.7
2.7
0.660
0.840
0.005
0.027
All uncertainties are 95% confidence limits (t j). Age offset is taken as the ordinate, and elemental abundance as the abscissa, for the definition
of the slope of the regression. Where the probability of fit is less than 0.05, the uncertainties on the slope and the intercept have been augmented
by MMSWD.
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
analyses as the U, Th and Pb isotopes. If a single trace
element is responsible for the 206Pb/238U anomalies,
Hf might be expected to be the suspect, because of its
relative abundance in zircon. But the correlation of
age offset with HfO2 content is very weak (Table 8).
In contrast, there is a much better linear correlation
between the 206Pb/238U anomalies and P abundance
(Fig. 4, Tables 8 and 9). For the ELA –ICP– MS data,
the MSWD of 9 for the whole group is reduced to 0.4
(probability of fit = 0.69) if R33 is omitted. In the case
of the equivalent SHRIMP comparison, the MSWD of
3.7 for the whole group is reduced to 1.2 (probability
of fit = 0.31) if QGNG is not considered. A striking
feature of the two comparisons is that the ELA –ICP –
MS trend is positively correlated, whereas the
SHRIMP trend is negative. More striking examples
of the same relationship will be demonstrated below
for Sm and Nd.
Two of the seven analysed REE, Sm and Nd,
correlate well with the age offsets observed for both
forms of microprobe dating (Table 8). Although the
correlation of ELA – ICP – MS age bias with Sm
abundance is not within error of a straight line, it is
reasonably close to being so (MSWD = 3.5). All five
analyses from the equivalent (i.e., Sm) SHRIMP
comparison are well aligned (MSWD = 0.28, probability of fit = 0.84).
Neither set of Nd data requires culling to achieve
an acceptable measure of linearity at the 95% confidence level. For the ELA –ICP – MS data, all five
analyses yield an MSWD of 2.0 (probability of
fit = 0.11). In the case of the SHRIMP data, the
corresponding parameters are 0.60 and 0.66. Therefore, Nd is the only element to provide a quantitative
match of the analysed elements with the ion-probe age
biases on both instruments.
Other than the Pb isotopes themselves, Th and U
are the only other trace elements to have been routinely measured in the past as part of the SHRIMP U/
Pb dating process. Consequently, it is important to see
if the concentrations of U and Th might also correlate
with the age biases, particularly if efforts are to be
made to correct for them with historical data. However, although both elements are reasonably well
correlated with the age biases (as demonstrated by
their correlation coefficients in Table 8), those correlations are not as well defined (Table 9) as those for
Nd or Sm. That additional degree of uncertainty
135
considerably reduces the worth of a matrix correction
based on either U or Th.
8. Discussion
The new ID –TIMS and SHRIMP dating together
with results previously reported by Black et al.
(2003a,b) provide a sound basis for the assessment
of 206Pb/238U dating by microprobe. It has been
shown (e.g., Compston, 1999; Black et al., 2003a)
that the SL13 standard yields variable data, consistent
with it being heterogeneous on the 30-Am scale of
SHRIMP analysis. In contrast, SHRIMP analyses of
each of the QGNG, AS3, TEMORA 1, TEMORA, 2
and R33 zircon standards show no signs of chronic
heterogeneity. Despite this consistency, their relative
ages as determined by SHRIMP analysis in some
cases deviate from ages determined by ID – TIMS.
The same is true for ages derived by ELA – ICP –MS
analysis. Moreover, both methods commonly yield
nominal 95% age uncertainties of about 0.5%, which
permit the age offsets to be assessed with reasonable
rigour. The data even reveal that zircon from adjacent
rocks of very similar composition and age, such as the
mafic hosts of the TEMORA 1 and TEMORA 2
zircon, can have different microprobe age offsets.
Several different processes have been cited as
possible causes of 206Pb/238U bias in SHRIMP ages.
Although Black and Jagodzinski (2003) have shown
that actual errors almost always exceed those predicted
from counting statistics, these are of a relatively
random nature, and cannot be responsible for systematic offsets. Wingate and Compston (2000) demonstrated that 206Pb/238U emission on SHRIMP varies
with crystal orientation for baddeleyite, another Zrrich mineral. They were unable, however, to detect any
orientation-related differences in zircon. The results of
the current study are even more unlikely to be a
consequence of crystallographic orientation, because
the grains (many of which were fragments) were
oriented randomly on the analytical mount. Stern and
Amelin (2003) presented a strong case from studies on
the Ottawa SHRIMP II for another source of analytical
error. Those authors have identified a significant
variation, especially approaching the edges of the
grain-mount, in 206Pb/238U ionisation in the same
(horizontal) plane that the primary beam impacts the
136
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
target. This effect is not thought to be important for the
current study, because of the analytical sequence used
for seven of the eight sessions. The zircon standards on
all of the grain-mounts were set in parallel rows,
which, for the seven Canberra sessions, were oriented
horizontally. For each of the standards, analysis commenced at the same edge of the mount, and proceeded
at roughly the same rate to the opposite edge, with
random infilling thereafter. Even if the effect reported
by Stern and Amelin (2003) is present in our data, this
sampling strategy should eliminate any systematic bias
arising from it, because all standards will have been
similarly affected.
Williams and Hergt (2000) report another effect
that can produce a systematic offset of zircon ages
during SHRIMP analysis. They established a threshold value of about 2500 ppm U, above which
206
Pb/238U ages increase at a rate of between 1.5%
and 2.0% for every additional 1000 ppm of U.
Fletcher et al. (2000) and Rasmussen and Fletcher
(2002) also report a correlation between apparent
SHRIMP 206Pb/238U ages and U content for xenotime
and monazite, respectively. However, U contents are
considerably below 2500 ppm for all the standards
studied above. The new results document a matrix
effect of a different kind, one that is correlated with
the abundance of a variety of elements, including P,
Sm and particularly Nd. These elements, particularly
Nd and Sm, are unlikely to be primarily responsible
for this effect because of their low concentrations. It is
therefore proposed that P, Sm and Nd are only proxy
indicators of the matrix effect, contributing to it, but
far from being its sole cause.
The correlation noted above would be expected
both within and between standards whether Nd is
the primary cause of bias or whether it is a proxy
Fig. 5. Diagrams showing the concentration of Nd for each of the individual ELA – ICP – MS spot ages used in the main inter-comparative
experiment. (A) QGNG, (B) R33, (C) TEMORA 2, (D) TEMORA 1. Unlike the strong correlation that exists between Nd concentration and
206
Pb/238U age between the different standards (Fig. 4), there is no such correlation within any of those zircon standards using individual spot
ages. The significance of this is discussed in the text.
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
element. However, Fig. 5 shows that the age variation within each standard is decoupled from Nd
concentrations for individual ELA– ICP – MS analyses (there are also no correlations between age and
the other analysed trace elements). This would be
most surprising if the measured individual Nd and/or
age variations are real. The lack of correlation on
Fig. 5 is far more likely to indicate that either (or
both) age or trace-element variations at the 30– 50Am level are due to random instrumental factors (of
about F 5%, if only the age is varying). However,
the correlations obtained between standards (Fig. 4
and Table 9) indicate that a sufficiently large number of analyses can be made to obtain a meaningful
average.
Based on the data summarised in Tables 8 and 9,
it seems that the microprobe age offsets are governed by a combination of trace elements. From our
limited dataset, Nd is shown to be the most representative of the analysed trace elements for delineating this apparent matrix effect. Apart from Hf, the
most abundant trace elements in zircon are the heavy
REE and Y (Hoskin and Schaltegger, 2003), and
these are charge-balanced, at least in part, by P
substitution into the Si-site; the so-called ‘‘xenotime’’ substitution mechanism. The correlation of
age biases with P abundance (Table 9) indicates that
the observed matrix effect is a function of REE + Y
and P substitution. It is possible that the lattice strain
induced by such ‘‘xenotime’’ coupled substitution
(Finch et al., 2001) affects secondary ion formation
and mass bias.
For the SHRIMP comparison, all 11 of the analysed trace elements are negatively correlated with age
offset, though to different extents (Table 8). For the
ELA –ICP– MS comparison, only two of the trends
are not positively correlated. One exception is the Eu
trend, where the regression is controlled by the AS3
analysis. The latter can be shown from the data in
Table 7 to be anomalously low in Eu, and therefore
unrepresentative of the REE as a group. Omitting the
AS3 analysis produces a slope that is too poorly
defined (R = 0.02) to determine its sense. This might
be a consequence of the variability of the Eu anomaly
for the different zircon standards, making it a particularly unreliable index of total REE content. The other
exception is Lu, although its negative slope is also
poorly defined (R = 0.15).
137
Trace-element abundances are therefore considered to be the primary cause of the age offsets
between the different zircon standards. It is interesting to speculate whether they might also play a role
in the reported heterogeneity of 206Pb/238U within
SL13. Black et al. (2003a) have proposed that the
variation of SHRIMP ages for SL13 might be due to
an incomplete isotopic resetting during metamorphism of an originally much older zircon. An alternative mechanism is also possible: the SHRIMP age
spread for SL13 might be more apparent than real,
resulting from a different manifestation of the matrix
effect documented above. Perhaps, all of that zircon
is of one age (572 Ma) on the volume of SHRIMP
analysis, but near stoichiometric Zr(Hf)SiO4 crystals
might be particularly sensitive to matrix effects
induced by variation of trace-element concentrations.
Rare areas of extreme 206Pb/238U enrichment in
SL13 are genuine, and are not an aberration resulting
from a matrix effect.
A matrix effect of the type shown here offers a
possible explanation for the common dispersion of
SHRIMP 206Pb/238U ages beyond ranges expected
from counting statistics alone. Compston (2001) has
attributed all such variation to genuine heterogeneity
in 206Pb/238U, whereas it might reflect, at least in
part, previously unaccounted for matrix effects.
This and previous studies (e.g., Black et al.,
2003a) have shown that under normal operating
conditions, the Canberra SHRIMP II and ELA –
ICP –MS are capable of producing weighted mean
206
Pb/238U ages at a precision of about 0.5% (95%
confidence level), although the individual analyses
used to derive those means are typically significantly
scattered. These same studies have also demonstrated
that some zircon standards yield SHRIMP ages up to
1.0% deviant from their ID – TIMS ages, while
average ELA –ICP – MS age biases can be up to
several percent deviant (Fig. 3). The accuracy of
the ages is, therefore, much worse than the quoted
precision, which incorporates all known sources of
random error. The occurrence of systematic error
related to trace-element substitution has now been
demonstrated. Although the adoption of a single
standard should reduce this effect, without making
appropriate correction it will not be possible to
compare the ages of different zircons calibrated
against that standard at the f 0.5% level, because
138
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
those unknown zircons themselves could have substantially different trace-element compositions. Even
the TEMORA 1 and TEMORA 2 zircons have been
shown to behave differently in this respect, although
they are petrographically, spatially and temporally
very closely related.
Based on the observations reported above, it is
believed that an accuracy of no better than f F 1%
can be achieved for SHRIMP and f F 3.0% for
ELA –ICP – MS dating of zircon if there is no correction for matrix effects. Current data indicate that
correction for matrix mismatch between a standard
and unknowns is best defined from Nd concentrations
according to the following formulae (where the uncertainties represent 95% confidence limits):
SHRIMP II:
206
Pb=238 U corrected age
¼ ð206 Pb=238 U measured ageÞ ð1 Cs =100Þ
where C S =[ 0.277 F 0.095] * [ppm Nd]+[0.90 F
0.30] [SOS].
ELA –ICP – MS:
206
Pb=238 U corrected age
¼ ð206 Pb=238 U measured ageÞ ð1 CICP 100Þ
where C I C P =[1.17 F 0.58] * [ppm Nd] [3.8 F
1.8] [SOICP].
SOS and SOICP are the age offsets in percent of
the standard used relative to TEMORA 1 for
SHRIMP II and ELA – ICP –MS, respectively. The
error in the correction factors (CS and CICP) is
about 30% for SHRIMP, and will result in a loss
of age precision proportional to the difference
between Nd concentration in the sample versus
TEMORA 1. However, the corrected values will
be more accurate.
An intriguing aspect of this study is the different
senses of the trace element vs. age-offset trends for the
ELA –ICP – MS (positive) and the SHRIMP (negative) results. The most trace-element-enriched crystal
lattices (e.g. AS3) lead to anomalously high measurements of 206Pb/238U by ELA – ICP – MS, but low
206
Pb/238U by SHRIMP.
It should be emphasised that the results obtained
above are not necessarily representative of other
microprobes, especially those produced by different
manufacturers or those with different primary ion
sources. It is also possible that different results will
be achieved from the use of different primary ion
species, for example O, rather than the O2 that was
used to produce the results reported in this study.
9. Conclusions
1. A series of zircon standards yield micro-beam
206
Pb/238U data that are in some cases deviant from
precise ID – TIMS age determinations. This applies
to both SHRIMP and ELA – ICP –MS data.
2. With the exception of the single metamorphic
zircon (SL13), any one of the standards shows a
consistent relationship with any other, but that
relationship can be different for different pairs of
standards. The sense of the relationships is
reversed for the two microprobe techniques; that
is, a standard yielding younger SHRIMP ages
commonly yields older ELA – ICP –MS ages.
3. Unless the reasons for those differences are identified and quantified, the nominal f 0.5% precision
generated by the datasets will not truly reflect the
accuracy of microprobe dating which, without such
correction, is no better than F 1% for SHRIMP and
about F 3% for ELA– ICP – MS dating.
4. Although the oxygen isotopic compositions for
three of the standards are significantly different
from each other, they do not correlate with bias
in206Pb/238U. Neither do crystal orientation effects
nor the position of analysed zircon on the grainmount.
5. There are distinct linear correlations between the
bias in206Pb/238U, and P, Sm and especially Nd
abundances. The sense of those correlations is also
mirrored, but generally not as definitively, by other
trace elements.
6. The bias in 206Pb/238U is attributed to a matrix
effect resulting from compositional differences in
the host zircon. It is believed that the age bias is
probably caused by the ‘‘xenotime’’ substitution
mechanism, and consequent lattice strain.
7. Although Nd, Sm and P are not considered to be
the primary cause of this effect, they are among the
trace elements contributing to it, and Nd in
particular provides a quantitative means of correct-
L.P. Black et al. / Chemical Geology 205 (2004) 115–140
ing for the bias generated by both SHRIMP and
ELA –ICP– MS.
8. This measurable matrix-dependence provides a
means of unifying the microprobe and ID –TIMS
data for the standards without having to resort to
explanations involving isotopic disturbance.
9. Such a matrix effect will not be confined to the
zircon standards, but will be present to an
independent extent in the unknown zircons. This
effect can be accounted for in ELA– ICP – MS
analysis by corrections based on the intensity of a
Nd+ peak. Future work is required to identify any
Nd-bearing molecular ions in the vicinity of the
SHRIMP mass spectrum used for U – Pb isotopic
dating (204 – 254 amu), so that this correction can
also be quantitatively applied to that technique.
Acknowledgements
K.A. Armstrong, S. Ridgway and G. Kuehlich
provided invaluable laboratory support in the separation of the zircon separates. A host of students helped
in the acquisition of overnight SHRIMP data. Ian
Fletcher is thanked for supplying data from the
experiment performed by University of Western
Australia staff on the Perth SHRIMP II. The chemical
analyses were performed by D. Siems, J. Budahn, W.
Pappas, E. Webber and J. Pyke. Oxygen isotope
analyses are made by Mike Spicuzza at the University
of Wisconsin. The ELA – ICP –MS data reduction
procedure was developed by J.M. Palin. Useful
discussions on the content of this article have been
held with many colleagues, including Helen Degeling,
John Sheraton, Geoff Fraser, Jon Claoué-Long and
Andrew Cross. Nick Ratcliffe (USGS) introduced
JNA to the Braintree Complex monzodiorite (source
of R33), and Morrie Duggan (ex GA) introduced LPB
and CF to the Middledale Gabbroic Diorite (source of
the TEMORA zircons). Chris Pigram, Chief of
Minerals and Geohazards Division, Geoscience Australia, has continued to be an enthusiastic supporter of
this research. The authors are grateful for critical
reviews of an early version of the manuscript Trevor
Ireland, Mark Harrison and Steve Eggins. L.P.B., C.F.
and R.J.K. publish with the permission of the Chief
Executive Officer, Geoscience Australia. [PD]
139
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