2 School of Earth and Environmental Sciences, University of Adelaide. Introduction. High-precision geochronology is an essential component of any geological.
LA-ICPMS U–Pb dating
A new geochronological capability for South Australia: U–Pb zircon dating via LA-ICPMS Anthony J Reid1, Justin L Payne2 and Ben P Wade2 1 Geological Survey Branch, PIRSA
2 School of Earth and Environmental Sciences, University of Adelaide
Introduction High-precision geochronology is an essential component of any geological investigation into crustal evolution, sedimentary provenance, or metallogenic process. Geochronology is based on the natural decay of unstable isotopes to daughter products, and our ability to measure these isotopic abundances in a wide variety of minerals. The U–Pb isotopic decay scheme is one of the most widely used by geochronologists, largely because of the chemical and physical properties of accessory minerals such as zircon, monazite and rutile. These minerals are able to partition trace amounts of U (and Th) into their crystal structure during formation and the presence of these minerals in a variety of geological environments makes them suitable for a wide range of geochronological applications. The Geological Survey Branch of PIRSA is able to access U–Pb geochronology on a routine basis through two methodologies — sensitive highresolution ion microprobe (SHRIMP) and laser ablation - inductively coupled plasma mass spectrometry (LA-ICPMS). The SHRIMP is accessed through an agreement with Geoscience Australia and is overseen by PIRSA’s senior geochronologist, Dr Liz Jagodzinski, in conjunction with PIRSA geologists. The LA-ICPMS instrumentation has recently been installed at Adelaide Microscopy within the University of Adelaide and can now be accessed by PIRSA staff through an agreement with the University of Adelaide and the University of South Australia. PIRSA has made a commitment to the technique through joint funding installation of the instrument. The SHRIMP methodology is the industry standard for zircon geochronology due to its high-isotopic precision and ability to analyse very small portions of individual accessory mineral grains. The LA-ICPMS technique was developed during the mid 1990s and has often been overlooked as a geochronological tool by geologists, who tend to perceive SHRIMP
data as being more reliable. However, a number of studies have shown that the LA-ICPMS technique is able to generate accurate and precise geochronological results (e.g. Jackson et al., 2004; Chang et al., 2006). The LA-ICPMS also has the advantage of being relatively low cost and because the analysis of the isotopic composition of a single zircon grain can be completed in ~5 minutes, large data sets can be accumulated rapidly. The latter feature makes the LA-ICPMS highly useful for studies of detrital zircons, which is perhaps the best application for WKLVWHFKQRORJ\ HJ*ULI¿QHWDO %HORXVRYDHWDO *ULI¿QHWDO This article presents the results of U–Pb dating of South Australian igneous zircons of known age using the instrumentation installed at Adelaide Microscopy. The LA-ICPMS is able to reproduce previous U–Pb ages obtained by both isotope dilution - thermal ionisation mass spectrometry (ID-TIMS) and SHRIMP techniques, which should JLYHLQFUHDVHGFRQ¿GHQFHLQIXWXUHVWXGLHV with this instrumentation.
LA-ICPMS instrumentation and method The LA-ICPMS system consists of a laser, a transport medium, an ion source and a mass spectrometer (Fig. 1). The laser heats the sample surface causing it to evaporate (‘ablate’). This ablation process creates a pit within the sample, which in the case of zircon dating is typically ~40 µm in diameter and up to ~200 µm deep (Fig. 2). The ablated zircon is then transported in a combined He–Ar carrier gas into a plasma, which operates at a temperature of ~6725 °C. The high temperatures encountered in the plasma ionise the zircon particles enabling them to be transported through an electrostatic ¿HOGDQGWKHQLQWRWKHTXDGUXSROHPDVV spectrometer, which registers the ions as counts per second at a given mass. The mass spectrometer is able to rapidly switch between different masses, in order to provide a quasi-simultaneous measurement of the isotopic composition
of the zircon. A time resolved output of the isotopic composition is thus generated by the mass spectrometer, from which a representative signal is selected and integrated to enable calculation of the isotopic ratios 207Pb/206Pb, 206Pb/238U, and 208 Pb/232Th (Fig. 3). Because of the low abundance of 235U, 207Pb/235U is calculated assuming the naturally occurring abundance ratio of these isotopes: 235U = 238 U/137.88. Data reduction is conducted using the online software GLITTER (van Achterbergh et al., 1999), which then calculates the 207Pb/206Pb and 206 Pb/238U age and error for each analysis. GLITTER calculates isotopic ratios from background-subtracted signals for the relevant isotopes. Uncertainties from counting statistics for the signal and background for the standard and unknown analyses are added in quadrature. One of the limitations of the laser ablation method is the differential fractionation of U and Pb (and other elemental species) during an individual analysis. Fractionation occurs at a number of stages during the analytical procedure, including at the site of ablation, during transport and during plasma ionisation. The ICPMS methodology aims to ensure that fractionation is consistent between analyses, as it uses a zircon standard of known fractionation characteristics and known age in order to apply a correction to the unknown analyses for instrument-induced mass fractionation. Since the standard has a precisely known age, GLITTER is able to determine the fractionation trends between U and Pb during an individual analysis of the standard. The fractionation characteristics of the standard zircon are then assumed to mimic those of the unknown zircons and a correction algorithm is then applied to each unknown analysis. 7KLVFRUUHFWLRQPRGL¿HVWKHUHODWLYH abundances of U and Pb that are fed into the age equation in order to generate the age of the unknown zircon. In practice, each analytical ‘run’ of ten unknowns is bracketed by three or four analyses of a standard, the gem quality zircon, GJ, which has a 207Pb/206Pb age of 608.5 ±
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Geochronology
presence of elevated common Pb within the sample, in which case doubt is cast on the reliability of such an analysis. In the present study, analyses included within age calculations recorded no elevated 204 Pb.
Test cases: analysis of the Yardea Dacite and Curramulka Gabbronorite
Figure 1 LA-ICPMS installed at Adelaide Microscopy. The sample is loaded into the sample stage and ablated using the New Wave Research Nd–YAG laser operating at an output wavelength of 213 nm. Ablated material is transported into the ICPMS within the clear plastic tubing via the action of an Ar–He carrier gas medium. The ICPMS is an Agilent 7500 quadrupole plasma mass spectrometer. (Photo 405156)
the calculation of errors for individual analyses (Jackson et al., 2004). A second limitation of the LA-ICPMS technique is the inability to provide a direct correction for the presence of non-radiogenic, or ‘common’ Pb within a given analysis, which causes an overestimation in the resulting age calculation. SHRIMP and ID-TIMS instrumentation overcome Figure 2 Scanning electron microscope image of laser this problem by measuring ablation pits within a single monazite crystal. For comparison, the non-radiogenic isotope the circular spot between the two ablation pits is the site of a 204 Pb with each analysis SHRIMP analytical spot. (Photo 405158) and assuming a natural ratio of this to other Pb 0.4 Ma, 206Pb/238U age of 600.7 Ma and 207 235 isotopes to calculate the non-radiogenic a Pb/ U age of 602.2 Ma (Jackson component. The ICPMS is unable et al., 2004). Additionally, an analytical to measure 204Pb due to an isobaric run also includes regular analyses of a interference with mercury (204Hg), which second zircon standard of known age. is present in trace amounts within the This second standard is analysed as if it argon carrier gas. Some workers have were an unknown and the fractionation developed mathematical approaches to correction calculated from the GJ the correction for common Pb in ICPMS standard zircon is applied to this standard data (Andersen, 2002). In this study, to ensure the instrument setup and and in others (Chang et al., 2006), no fractionation corrections are producing correction for potential common Pb has meaningful data. GLITTER also been applied. Nevertheless, in order to combines uncertainties estimated for the provide an indicator of potential common standard ratios with uncertainties in the Pb contamination 204Pb is also monitored unknown ratios in quadrature, and adds a as part of the analytical process. 1% uncertainty to the ID-TIMS values of Any increase in 204Pb value beyond the isotope ratios for the GJ standard in background levels is taken to indicate the
28
MESA Journal 42
August 2006
In order to provide an independent test for the reliability of the LA-ICPMS installed at Adelaide Microscopy, two Mesoproterozoic rocks from the Gawler Craton — the Yardea Dacite and the Curramulka Gabbronorite — were sampled for zircon U–Pb analysis. Zircons from these two rocks have both been previously dated via the ID-TIMS and SHRIMP techniques respectively. Zircon concentrates were prepared from crushed samples at the University of Adelaide, using standard density and magnetic separation techniques. Hand-picked zircons were then mounted in epoxy discs and polished so as to expose the grains. Grains were imaged using transmitted light, and backscattered electron and cathodoluminescence techniques to reveal the internal structure of the grains and assist in targeting analytical spot sites. The Yardea Dacite of the Gawler Range Volcanics (Allen et al., 2003), returned an ID-TIMS upper intercept concordia age of 1592 ± 3 Ma, interpreted to be the crystallisation age of the porphyritic dacite (Fanning et al., 1988). Material remaining from the same sample (6132 RS56) was prepared for the LA-ICPMS study (new PIRSA sample number R698670). Zircons from this sample are euhedral, clear, inclusion free and show zonation typical of igneous zircons (Fig. 4a). The Curramulka Gabbronorite forms a pluton of high magnetic intensity in central Yorke Peninsula, the type section for which occurs in drillhole MCD 1 over the interval 181.2–344.3 m (Zang, 2002). SHRIMP analysis of zircons from the Curramulka Gabbronorite provides a cluster of concordant data with a weighted mean 207Pb/206Pb age of 1583 ± 6.6 Ma (Zang et al., in press). The gabbronorite sample analysed in this study (R786133) was taken from the same drillhole over the interval 315.4–315.8 m. The gabbronorite consists of plagioclase, pyroxene, hornblende, biotite plus minor quartz. Zircons from this sample are clear, irregularly shaped and show some evidence for magmatic zonation (Fig. 4b).
LA-ICPMS U–Pb dating
Results
Figure 3 Time-resolved output of isotopic analysis obtained from the mass spectrometer, with time on the horizontal axis and abundance of each measured isotope on the vertical. At ~60 s the laser is turned on resulting in the increase in signal intensity beyond background values, which are recorded prior to 60 s. After exporting these data to GLITTER, the user is able to VHOHFWWKHSRUWLRQRIWKHDQDO\WLFDOWUDFHWKDWEHVWUHÀHFWVWKHFRPSRVLWLRQRIWKHDQDO\VHGJUDLQ for calculation of U–Pb ages. Coloured traces correspond to the measured isotopes: 204Pb, 206 Pb, 207Pb, 208Pb, 232Th and 238U.
Yardea Dacite. Nineteen analyses of zircons from the Yardea Dacite gave a cluster of concordant to near concordant data with 207Pb/206Pb ages clustering around 1595 Ma (Table 1; Figs 5a, b). Only one analysis (13) is discordant, the rest remaining within ±5% concordance, indicating the zircons have not H[SHULHQFHGDQ\VLJQL¿FDQWOHDGORVV7KH 18 concordant analyses yield a weighted mean 207Pb/206Pb age of 1598 ± 10 Ma (2σ error; mean squared weighted deviates (MSWD) = 0.84, probability = 0.64), which is within error of the ID-TIMS age of 1592 ± 3 Ma obtained by Fanning et al. (1988). Curramulka Gabbronorite. Twentyeight analyses of zircons from the Curramulka Gabbronorite gave dominantly concordant data with only seven analyses showing discordance greater than 5% (Table 1; Fig. 5c). A GLVFRUGLDOLQHFDQEH¿WWHGWRWKHVHGDWD to produce an upper intercept age of 1584 0D KRZHYHUWKLVDJHLVGH¿QHGE\D model 2 solution, which ignores errors for individual data points and is potentially unreliable. The 21 concordant analyses GH¿QHDZHLJKWHGPHDQ207Pb/206Pb age of 1579 ± 8 Ma (2σ error; MSWD = 0.20, probability = 1.0; Fig. 5d), which is within error of the upper intercept concordia age indicating that in this case the former is likely to be a reliable age estimate. Both ages are within error of the 1583 ± 6.6 Ma age reported by Zang et al. (in press).
Geochronology via LAICPMS at Adelaide Microscopy: implications
(a)
(b)
Figure 4 Transmitted light images of representative zircon from analysed samples. (a) R698670, Yardea Dacite, and (b) R756133, Curramulka Gabbronorite. (Photos 405159,
The results presented here demonstrate that U–Pb zircon dating obtained through the LA-ICPMS instrumentation installed at Adelaide Microscopy can produce precise and accurate age data, replicating within error previously acquired ages from both ID-TIMS and SHRIMP methodologies from simple igneous zircons. The individual spot uncertainties obtained via LA-ICPMS are larger than those obtained by ID-TIMS and SHRIMP, as a result of the lower sensitivity of the mass spectrometer and counting statistics used in the laser ablation technique. It is worthwhile to note, however, that the uncertainties on the weighted mean 207Pb/206Pb ages in some instances approach those obtainable via the SHRIMP; a fact that should
405160)
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29
Geochronology
argue for a greater acceptance of zircon U–Pb data obtained via the LA-ICPMS. Thus, as demonstrated here accurate age estimates for simple igneous zircons can be obtained through careful application of the LA-ICPMS technique. Two important points must be understood when considering the LAICPMS technique for a geochronology program. Firstly, the ~40 µm spatial resolution obtained by the laser is considerably larger than the routine 15 to 25 µm spot size of SHRIMP. Consequently zircons that are complexly zoned or contain narrow rims of
geochronological interest will not be able to be accurately dated via LAICPMS. In these cases, the spatial resolution of the ion probe technology is superior. Secondly, as is the case with all geochronological methods, the quality of the mineral subject to investigation will determine the quality of the resultant data and hence the understanding that can be gained from the geochronology. Metamict, high-U, altered or weathered minerals will not yield useful age information — using any technique — and therefore careful sample selection is critical to the success of any dating program.
(a)
Acknowledgements Angus Netting, John Terlett and Peter Self are acknowledged for their efforts towards the set up of the U–Pb dating method within the Adelaide Microscopy facility. John Foden and Martin Hand are also acknowledged for their role in setting up the facility and for ongoing helpful discussions. Careful reviews by Richard Stern (Geoscience Australia) and Liz Jagodzinski (PIRSA) are gratefully acknowledged.
(b) Data-point error ellipses are 68.3% confident
0.30
1595 ± 10 Ma
1650
1550 0.26
206
1350 0.22
Relative probability
Number
Pb/ 238 U
1450
1250 1150
0.18
0.14 1.6
2.0
2.4
2.6
2.8
3.0
3.2
3.4
1500
1540
207
Pb/235 U
1580 207
1620
1660
1700
1660
1700
206
Pb/ Pb Age (Ma)
(c)
(d) Data-point error ellipses are 68.3% confident
20
0.30
1579 ± 8 Ma
1650 1550 15
0.26
1350 0.22
1250
Relative probability
Number
206
Pb/ 238 U
1450
10
1150 0.18
0.14 1.6
5
Intercepts at 306 ± 83 and 1584 ± 8 Ma MSWD = 15
2.0
2.4
2.6
2.8
207
235
Pb/
U
3.0
3.2
3.4
0 1500
1540
1580 207
1620
206
Pb/ Pb Age (Ma) 203399_013
Figure 5 Results of LA-ICPMS analysis of zircons from the two samples. (a) Concordia plot of analyses from Yardea Dacite sample R698670. (b) Probability density distribution for sample R698670. (c) Concordia plot for Curramulka Gabbronorite sample R786133. (d) Probability density distribution excluding >5% discordant analyses for sample R698670. Note that the concordia upper intercept age calculated for R786133 is calculated using a model 2 solution ignoring data point errors.
30
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LA-ICPMS U–Pb dating
Table 1 Summary of LA-ICPMS U–Pb zircon results for samples R698670, Yardea Dacite, and R756133, Curramulka Gabbronorite Isotopic ratio
Analysis 206
Pb/238U
±
207
% disc.
Age (Ma)
Pb/235U
±
207
Pb/206Pb
±
206
Pb/238U ±
207
Pb/206Pb ±
R698670, Yardea Dacite 1
0.2850
0.0034
3.883
0.047
0.0989
0.0011
1617 17
1603
20
1
2
0.2721
0.0034
3.644
0.047
0.0972
0.0011
1551 17
1570
20
–1
3
0.2806
0.0033
3.846
0.047
0.0972
0.0011
1594 17
1614
20
–1
4
0.2755
0.0035
3.719
0.050
0.0972
0.0011
1569 18
1585
21
–1
5
0.2705
0.0035
3.642
0.049
0.0972
0.0011
1543 18
1580
21
–2
6
0.2807
0.0034
3.824
0.047
0.0972
0.0011
1595 17
1603
20
0
7
0.2675
0.0034
3.604
0.047
0.0972
0.0011
1528 17
1581
20
–3
8
0.2668
0.0032
3.628
0.046
0.0972
0.0011
1524 16
1599
21
–5
9
0.2764
0.0036
3.712
0.053
0.0972
0.0012
1573 18
1575
23
0
10
0.2742
0.0036
3.744
0.052
0.0972
0.0012
1562 18
1606
22
–3
11
0.2802
0.0035
3.821
0.051
0.0972
0.0011
1593 18
1604
21
–1
12
0.2803
0.0035
3.796
0.048
0.0972
0.0010
1593 17
1591
20
0
13
0.2414
0.0032
3.304
0.046
0.0972
0.0012
1394 16
1611
22 –13
14
0.2687
0.0036
3.663
0.052
0.0972
0.0012
1534 18
1604
22
15
0.2647
0.0034
3.567
0.047
0.0972
0.0011
1514 18
1582
20
–4
16
0.2774
0.0033
3.814
0.049
0.0972
0.0012
1578 17
1619
22
–3
17
0.2831
0.0034
3.898
0.052
0.0972
0.0013
1607 17
1621
23
–1
18
0.2864
0.0034
4.008
0.053
0.0972
0.0012
1623 17
1652
22
–2
19
0.2827
0.0035
3.830
0.049
0.0972
0.0010
1605 18
1592
20
1
–4
R756133, Curramulka Gabbronorite 1
0.2240
0.0026
3.047
0.036
0.0987
0.0011
1303 13
1599
20
19
2
0.2751
0.0031
3.694
0.041
0.0974
0.0010
1567 16
1575
19
1
3
0.2771
0.0031
3.716
0.042
0.0973
0.0010
1577 16
1572
19
0
4
0.2806
0.0032
3.774
0.042
0.0976
0.0010
1594 16
1578
19
–1
5
0.2820
0.0032
3.825
0.043
0.0984
0.0010
1601 16
1594
19
0
6
0.2600
0.0036
5.016
0.090
0.1400
0.0026
1490 18
2227
31
33
7
0.2599
0.0030
3.449
0.039
0.0963
0.0010
1489 15
1553
19
4
8
0.2650
0.0030
3.545
0.040
0.0970
0.0010
1515 15
1568
19
3
9
0.1738
0.0020
2.171
0.025
0.0906
0.0009
1033 11
1439
19
28
10
0.2752
0.0032
3.719
0.042
0.0980
0.0010
1567 16
1587
19
1
11
0.2722
0.0031
3.667
0.042
0.0977
0.0010
1552 16
1581
19
2
12
0.2745
0.0032
3.693
0.042
0.0976
0.0010
1563 16
1579
19
1
13
0.2772
0.0032
3.723
0.042
0.0974
0.0010
1577 16
1575
19
0
14
0.2789
0.0032
3.750
0.043
0.0975
0.0010
1586 16
1578
19
–1
15
0.2793
0.0032
3.753
0.043
0.0975
0.0010
1588 16
1576
19
–1
16
0.2785
0.0032
3.760
0.043
0.0979
0.0010
1584 16
1585
19
0
17
0.2793
0.0032
3.770
0.043
0.0979
0.0010
1588 16
1584
19
0
18
0.2785
0.0032
3.755
0.043
0.0978
0.0010
1584 16
1583
19
0
19
0.2800
0.0033
3.761
0.044
0.0974
0.0010
1591 16
1576
19
–1
20
0.1733
0.0020
2.156
0.025
0.0902
0.0009
1030 11
1430
19
28
21
0.2720
0.0032
3.672
0.043
0.0979
0.0010
1551 16
1585
19
2
22
0.2285
0.0031
3.091
0.044
0.0982
0.0011
1589 21
1327
16 –17
23
0.2492
0.0034
3.317
0.044
0.0966
0.0010
1559 19
1434
17
–8
24
0.2747
0.0037
3.701
0.049
0.0977
0.0010
1581 18
1565
19
–1
25
0.2769
0.0037
3.744
0.050
0.0981
0.0010
1588 19
1576
19
–1
26
0.2742
0.0037
3.685
0.049
0.0975
0.0010
1577 18
1562
19
–1
28
0.2520
0.0034
3.401
0.046
0.0979
0.0010
1585 19
1449
18
–9
27
0.2709
0.0037
3.647
0.049
0.0977
0.0010
1580 19
1545
19
–2
Note:
References Allen, S.R., Simpson, C.J., McPhie, J. and Daly, S.J., 2003. Stratigraphy, distribution and geochemistry of widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics, South Australia. Australian Journal of Earth Sciences, 50:97-112. Andersen, T., 2002. Correction of common lead in U-Pb analyses that do not report 204Pb. Chemical Geology, 192:59-79. %HORXVRYD($5HLG$6FKZDU]0*ULI¿Q W.L. and Fairclough, M., 2006. Crustal evolution of the Gawler Craton, South Australia: Application of the TerraneChron® technique to detrital zircon from modern stream sediments. South Australia. Department of Primary Industries and Resources. Report Book, 2006/4. Chang, Z., Vervoort, J.D., McClelland, W.C. and Knaack, C., 2006. U-Pb dating of zircon by LA-ICP-MS. Geochemistry, Geophysics, Geosystems, 7:Q05009, doi:10.1029/ 2005GC001100. Fanning, C.M., Flint, R.B., Parker, A.J., Ludwig, .5DQG%OLVVHWW$+ 5H¿QHG Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology. Precambrian Research, 4041:363-386. *ULI¿Q:/%HORXVRYD($6KHH65 Pearson, N.J. and O’Reilly, S.Y., 2004. Archean crustal evolution in the northern Yilgarn Craton: U-Pb and Hf-isotope evidence from detrital zircons. Precambrian Research, 131:231-282. *ULI¿Q:/%HORXVRYD($:DOWHUV6*DQG O’Reilly, S.Y., 2006. Archean and Proterozoic crustal evolution in the Eastern Succession of the Mt Isa district, Australia: U-Pb and Hfisotope studies of detrital zircons. Australian Journal of Earth Sciences, 53:125-149. -DFNVRQ6(3HDUVRQ1-*ULI¿Q:/DQG Belousova, E.A., 2004. The application of laser ablation-inductively coupled plasmamass spectrometry to in situ U-Pb zircon geochronology. Chemical Geology, 211:4769. YDQ$FKWHUEHUJK(5\DQP&**ULI¿Q:/ 1999. GLITTER: On-line interactive data reduction for the laser ablation ICP-MS microprobe. In: Proceedings of the 9th V.M. Goldschmidt Conference, Cambridge, Massachusetts, pp. 305-306. Zang, W., 2002. Late Palaeoproterozoic Wallaroo Group and early Mesoproterozoic mineralisation in the Moonta Subdomain, eastern Gawler Craton, South Australia. South Australia. Department of Primary Industries and Resources. Report Book, 2002/001. Zang, W., Fanning, C.M., Purvis, A.C., Raymond, O.L. and Both, R.A. (in press). Early Mesoproterozoic bimodal plutonism in the southeastern Gawler Craton, South Australia. Australian Journal of Earth Sciences.
For further information contact Anthony Reid, phone +61 8 8463 3039, email .
For % disc., 0% denotes a concordant analysis.
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