GEMOC ARC National Key CentreO.Alard, N.J. Pearson, W.L. Griffin, S. Graham and S.E. Jackson
GEMOC, Macquarie
Introduction:
The Re-Os isotopic system is now widely used to study the timing and nature of melting and metasomatism in the lithospheric mantle. Petrographic and microanalytical data show that essentially all of the Os, and most of the Re, in mantle-derived peridotites is concentrated in sulfide phases. These sulfides occur as two basic types: (1) residual, Ni-rich monosulfide solid solutions (now largely exsolved), that are generally enclosed inside primary silicates, oxides and diamonds; (2) interstitial sulfides, interpreted as deposited from metasomatic fluids or melts. The enclosed sulfides typically are enriched in Os+Ir and relatively depleted in Pt+Pd, while the interstitial sulfides typically are depleted in Os+Ir and relatively enriched in Pd±Pt (Alard et al., 1999a).
Many, and perhaps most, mantle-derived peridotites contain a mixture of these two sulfide types, and this mixing has major implications for the interpretation of whole-rock PGE patterns. It also raises serious questions about the interpretation of whole-rock Re-Os data. Burton et al. (1999) recently demonstrated a significant difference in the γOs of interstitial and enclosed sulfides in a peridotite xenolith from Kilbourne Hole, suggesting that a meaningful depletion age of the rock may be given only by the sulfide inclusions. The sulfide phases are small, rare and difficult to separate, and separation obliterates the microstructural context needed for interpretation. We therefore have investigated the potential of laser-ablation analysis of Os isotopes in such sulfides, using multi-collector ICPMS techniques.
Analytical Methods
We have used a Merchantek LUV266 nm laser microprobe, attached to a Nu Plasma MC-ICPMS. The Nu instrument's geometry features a variable dispersion ion optics and a fixed collector array with 12 Faraday cups and 3 ETP ion counters. Two configurations have been tested thus far:
· Masses 194, 192, 191, 190, 189, 188, 186 in Faraday cups; 187, 185 in ion counters.
Table 1 shows the accuracy and precision of the instrument, as determined by replicate analysis of several Os standards. The accuracy of the Re correction is demonstrated by repeated analysis of a mixed Re-Os standard supplied by V. Bennett (ANU-RSES).
Table 1. Solution Analyses of Os Standards
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Note: All analyses carried out using Cetac MCN6000.
The LAM analysis has been tested using a synthetic NiS bead (PGE-A) with 200 ppm Os of known isotopic composition and PGE pattern. Repeated analyses of PGE-A (Table 2) showed the precision obtainable as a function of signal intensity, and demonstrated that under the conditions (small spot size, short runs) expected for real samples with similar levels of Os, we could achieve precision equivalent to N-TIMS results using the Faraday cup array; better precision was achieved using the combined Faraday plus ion-counting setup. In both cases, a major limitation on the precision obtainable is the need for a signal high enough and long enough to get a precise measurement of the mass bias in the Faraday cups.
Table 2. LAM-MC-ICPMS analyses of PGE-A
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Results
The results of sample analyses are shown in Table 3 and Figure 1.
Table 3. Results on Natural Samples
| Sample |
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(ppm) |
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(Ga) |
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(Ga) |
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| Siberia: Udachnaya | ||||||||||
| UD-Ol61 |
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| UD-Ol60 |
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| UD-Ol67 |
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| * data obtained by LAM-ICP-MS, confirmed by Proton probe analyses (Os=892±126 ppm, Pers. Comm. C.G. Ryan) | ||||||||||
| Slave Craton | ||||||||||
| A-481 |
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| Kaapvaal Craton | ||||||||||
| FS-8 1 |
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| FS-8 2 |
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| *LAM-ICP-MS analyses of FS-8 sulfides range from 7 to 20 ppm | ||||||||||
| Mt Gambier, S. Australia, Newer Volcanics Province. | ||||||||||
| GAM-VL9-S3 |
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| GAM-VL9-S1(b) |
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| Mt Quincan, (Queensland), Atherton Volcanic Province. | ||||||||||
| 94-A4-37 S1 |
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| 94-A4-39-s1(a) |
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| Montferrier, Southern France (Montpellier Volcanic Province) | ||||||||||
| Pg2-s3a |
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| PG 5 s1 |
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| MTF 37a |
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| MTF 37a |
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| MTF 37a |
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| MtfF37C S3 |
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Siberia: Three sulfide inclusions in olivine macrocrysts, derived from disaggregated peridotites, were extracted from the Udachnaya kimberlite. The largest has been analysed for PGEs by LAM-ICPMS and has a high Os content. A similar precision and a slightly older age was obtained from a smaller inclusion. A still smaller inclusion gave a lower precision, and a slightly younger model age. These ages are equivalent to the oldest TRD model ages (3.0-3.2 Ga) obtained by conventional analysis of Udachnaya peridotite xenoliths (Pearson et al., 1995), and to the TRD ages of three sulfide inclusions in a single diamond from the same pipe (3.1±0.3 to 3.5±0.3 Ga; Pearson et al., 1999).
Slave Craton: A single large inclusion was extracted from an olivine macrocryst (Fo92) in the A841 kimberlite, Lac de Gras area. The age is equivalent to the oldest model ages (3.1 Ga) reported from xenoliths in the Jericho pipe to the north (Irvine et al., 1999), and is significantly older than the oldest event (2.8 Ga) known in the overlying crust (Yamashita and Creaser, 1999).
Kaapvaal Craton: Two interstitial sulfide inclusions in a high-temperature sheared peridotite xenolith from the Frank Smith kimberlite (100 Ma eruption age) have modest Os contents and the analyses have relatively low precision. Other geochemical evidence (low mg#, high Ti, Zr and Y in rims of strongly zoned garnets; Griffin et al., 1989) suggests that these xenoliths were infiltrated by asthenosphere-derived melts close to the time of kimberlite eruption, and the sulfides may contain a mixture of old and young components.
Mt. Gambier (S. Australia): One enclosed sulfide gives a model age of 1.8±0.17 Ga, within uncertainty of the 1.9 Ga model ages obtained by Handler (1998) on three xenoliths from the same volcano. An interstitial sulfide from another Mt. Gambier xenolith gives a 187Os/188Os equivalent to modern asthenosphere, emphasising that these rocks contain two generations of sulfides with different Os isotopic compositions and Re/Os ratios.
Massif Central: MTF37 is a fertile peridotite (Al2O3 = 4.2%) with a
PUM-like 187Os/188Os (0.1277; Meisel et al., 1996,1999). Individual grains
of interstitial sulfides have 187Os/188Os ranging from 0.1209-0.1361, indicating
that the whole-rock 187Os/188Os value represents mixing of older (Proterozoic)
components and younger radiogenic Os (Fig. 1). Other xenoliths from the
same locality display even more radiogenic Os, with 187Os/188Os = 0.1272-0.1756,
and whole-rock 187Os/188Os increases with S contents and Pd/Ir ratios (Alard
et al., 1999). The data suggest that the radiogenic Os and high S contents
of this suite reflect high proportions of metasomatic sulfide. This sample
suite highlights the resolving power of the in-situ analytical method.
Figure 1. Os data for Montferrier sulfides. PUM and CI values from Meisel et al. (1996).
Conclusions
LAM-MC-ICPMS analysis of single sulfide inclusions in mantle-derived peridotites can give useful data on the Re-Os system. With current technology, the best results are obtained from sulfide grains 350 µm in diameter, with Os contents 340 ppm. The data obtained from such grains have a precision equivalent to N-TIMS data on whole rock samples. The lower-precision data obtained on low-Os interstitial sulfides are still very useful in understanding the movement of Os within the lithosphere, as demonstrated by the Massif Central samples. The major advantage of the LAM-MC-ICPMS technique lies in providing in-situ analyses with the complete spatial and microstructural context required for meaningful interpretation.
Further improvements in sensitivity are expected to increase precision; these developments include multicycle ion-counter analysis, a shield torch for the MC-ICPMS and the use of He as the carrier gas in the ablation cell.
The application of this technique will make Re-Os analysis of mantle-derived rocks more rapid and less expensive, and enable larger-scale surveys of the age structure of the continental lithosphere.
References
Alard, O., Griffin, W.L., Lorand, J.P., Jackson, S.E. and O'Reilly, S.Y. 1999a. Highly siderophile elements in mantle sulfide by LAM-ICPMS. Ext. Abst. 9th Goldschmidt Conf.
Alard O., Lorand, J.P., Reisberg, L., Bodinier. J.L., O'Reilly, S.Y., Griffin, W.L. and Dautria, J.M., 1999b. Sulfur enrichment via volatile-richmetasomatism in montferrier xenoliths (Southern France): Consequences for highly siderophile elements in the sub-continental mantle. Orogenic lherzolites and mantle processes conference, Pavia, Italia, Ofioliti, 24:48-49.
Burton, K.W., Chiano, P, Birck, J.-L. and Allegre, C.J. 1999. Osmium isotope disequilibrium between minerals in a spinel-lherzolite. Earth Planet. Sci. Lett. 172, 311-322.
Griffin, W.L., Smith, D., Boyd, F.R., Cousens, D.R., Ryan, C.G., Sie, S.H. and Suter, G.F. 1989: Trace element zoning in garnets from sheared mantle xenoliths. Geochim. Cosmochim. Acta. 53, 561-567
Handler, M.R., Bennett, V.C. and Esat, T.M. 1997. The persistence of off-cratonic lithospheric mantle: Os isotopic systematics of variably metasomatised southeast Australian xenoliths. Earth Planet. Sci. Lett. 151, 61-75.
Irvine, G.J., Kopylova, M.G., Carlson, R.W., Pearson, D.G.,Shirey, S.H. and Kjarsgaard, B.A. 1999. Age of the lithospheric mantle beneath and around the Slave Craton: A Re-Os isotope study of peridotite xenoliths from the Jericho and Somerset Island kimberlites. Ext. Abst. 9th Goldschmidt Conf.
Meisel, T., Walker, R.J., Morgan, J.W., 1996, The osmium isotopic composition of the Earth's primitive mantle. Nature, 383, 517-520.
Meisel T., Walker R.J., Irving, A.J. and Lorand J.-P. 1999: Osmium isotopic composition of mantle xenoliths: a global perspective (submitted to GCA).
Pearson, D.G., Shirey, S.B., Carlson, R.W., Boyd, F.R., Pokhilenko, N.P. and Shimizu, N. 1995. Re-Os, Sm-Nd and Rb-Sr isotope evidence for thick Archaean lithospheric mantle beneath the Siberian craton modified by multistage metasomatism. Geochim. Cosmochim. Acta 59, 959-977.
Pearson, D.G., Shirey, S.B., Bulanova, G.P., Carlson, R.W. and Milledge, H.J. 1999. Re-Os isotope measurements of single sulfide inclusions in a Siberian diamond and its nitrogen aggregation systematics. Geochim. Cosmochim. Acta 63, 703-711.
Yamashita, K. and Creaser, R.A. 1999. Geochemical and isotopic constraints
for the evolution of the Slave Province. Ext. Abst. 9th Goldschmidt Conf.