GEMOC ARC National Key Centre

N.J. Pearson¹, O. Alard^1,2, W.L. Griffin¹ and S.Y. O'Reilly¹

1 GEMOC Key Centre, Dept of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia,
2 Dept of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK.
 

 Sulfide is the dominant host for Os and the other plati-num group elements (PGE) in mantle-derived peridotites, contributing between 80-100% of the whole-rock budget of these elements.  In-situ LAM-ICPMS analysis of PGEs shows that multiple generations of sulfide occur within many peridotites as evidenced by the variations in PGE patterns with microstructural context [1].  Inclusions of sulfide in primary silicates are characterised by high Os (20-1000 ppm) but low Pd/Ir (0.001-1), whereas interstitial sulfides and those associated with metasomatic phases typically have low Os (<30 ppm), Ir but high Pd/Ir ratios (up to 1000).

The development of in-situ techniques for Re-Os isotopes using LAM-MC-ICPMS enables the determination of ¹⁸⁷Os/¹⁸⁸Os in single sulfide grains [2]. Analyses of enclosed and interstitial sulfides in peridotite xenoliths show that the two types of sulfide differ significantly in their Re-Os systematics.

Enclosed sulfides, such as those occurring in macrocrystic olivine in kimberlites, typically have unradiogenic Os compositions.  In-situ LAM-ICP-MS showed that the sulfides from Udachnaya kimberlite (Siberian Craton, Russia) may have very high PGE contents (up to = 900 ppm) and very low Pd/Ir (0.0002-0.0093). ¹⁸⁷Os/¹⁸⁸Os ratios of these sulfides range between 0.1033 ± 0.0002 and 0.1118 ± 0.0013, while the low ¹⁸⁷Os/¹⁸⁸Os ratios (0.006 to 0.081) are consistent with their low Pd/Ir.  Unradiogenic Os isotopic ratios and low Re/Os ratios indicate that these sul-fides have evolved in a low Re/Os environment since roughly 3 Ga.  T_RD ages vary from 3.6 to 2.3 Ga and T_MA range from 2.9 to 3.6 Ga.  The in-situ results are in good agreement with data for diamond-enclosed sulfides from the same locality [3] and demonstrates the accuracy and precision of the in-situ method relative to the micro-chemistry N-TIMS method.

Interstitial sulfides typically have ¹⁸⁷Os/¹⁸⁸Os ranging from asthenospheric (0.127) to highly radiogenic (0.175) values.  Analyses of multiple sulfide grains from single samples of spinel peridotites define mixing trends that may have either positive or negative slopes.  The results from 2 spinel lherzolites from the Massif Central, France have been selected to highlight the power of the in-situ technique in understanding processes within the mantle. In both samples it is clear that the whole-rock Re-Os sig-nature reflects a mixing of several sulfide populations.  Sample Mtf37 is a low-temperature spinel lherzolite from the Montferrier alkali basalt (Languedoc, southern France) and its sulfides, all of which are interstitial, produce an inverse correlation between ¹⁸⁷Os/¹⁸⁸Os and Re/Os ratio. The low ¹⁸⁷Os/¹⁸⁸Os of some of these sulfides yields T_RD ages down to = 1.8 Ga. The relationship between sulfide and whole-rock data [4] indicates that the whole-rock Os isotopic composition represents the mixing of older (possibly Proterozoic) components and younger radio-genic Os.  A negative ¹⁸⁷Os/¹⁸⁸Os - Re/Os correlation shows that the radiogenic signature has not been developed by in-situ Re-decay and suggests that the radiogenic Os, but not Re, has been transported from a high Re/Os reservoir via a metasomatic fluid.  MBR11 is a coarse, granular spinel-lherzolite from Mont Briancon, Massif Central, France. The whole-rock Os isotopic composition [4] indicates a ratio more radiogenic than the primitive mantle as is the ¹⁸⁷Os/¹⁸⁸Os ratio. Therefore both T_RD and T_MA yield unrealistic ages (future or unreasonably old, respectively). Given that the Re/Os is higher than estimated for the PUM, the common interpretation would call upon host lava contamination.  In-situ Os data reveal that olivine-enclosed sulfides show ¹⁸⁷Os/¹⁸⁸Os ranging from 0.1145 to 0.1210 and ¹⁸⁷Os/¹⁸⁸Os between 0.055 to 0.240.  A clinopyroxene-hosted sulfide has more radiogenic ¹⁸⁷Os/¹⁸⁸Os (0.1272±0.0027) and ¹⁸⁷Os/¹⁸⁸Os ca. 0.223.  The analysed interstitial sulfide shows an even more radiogenic Os signature (¹⁸⁷Os/¹⁸⁸Os = 0.1354 ± 0.0014) and higher Re/Os ratio (¹⁸⁷Os/¹⁸⁸Os = 0.791), and is considered to be the product of metasomatic processes.  It is thus clear that the whole-rock Re-Os signature reflects a mixing of several sulfide populations.

Meaningful interpretation of Re-Os data in terms of mantle events requires understanding of the occurrence and mobility of sulfides in mantle peridotites.  Unless these are considered then whole-rock derived depletion ages will have little significance, especially if there are multiple generations of sulfide. Os isotope ratios in low-Os sulfides may be less precise than conventional techniques but the in-situ analyses provide the spatial context to recognise different sulfide generations and provide more readily interpretable depletion ages.
 

References:

[1] Alard O.A. et al. (2000) Nature 407, 891-894.
[2] Pearson N.J. et al. (2000) Abstract 10th Goldschmidt Conference, p777.
[3] Pearson D.G. et al. (1999) Geochim. Cosmochim. Acta, 63, 703-711.
[4] Meisel T. et al. (2001) Submitted to Geochim. Cosmochim. Acta