GEMOC - Department of EPS - Faculty of Science

S-CONTENT IN AUSTRALIAN SUB-CONTINENTAL LITHOSPHERE: PRELIMINARY RESULTS

Olivier Alard¹, Jean-Pierre Lorand², William J. Powell¹, Suzanne Y. O'Reilly¹, William L. Griffin¹

1. GEMOC Maquarie,
2. Museum National d'Histoire Naturelle de Paris, Lab de Minéralogie, ESA CNRS 7058; 61 rue Buffon, 75005 Paris, France

Sulfides are minor but common phases in mantle rocks. They are thought to be the main mineral host in mantle rocks for highly siderophile elements (HSE). Therefore, sulfide mineralogy and S concentration are key factors for understanding the behaviour of PGEs and the Re/Os isotopic system in the mantle. Previous studies have shown drastically different S contents for the two main types of upper mantle samples, i.e. orogenic massifs and xenoliths. The massifs have S contents in the same order of magnitude as estimated for the MORB source (=200 ppm) and correlated with major element contents. In contrast, peridotite xenoliths brough up by alkali basalts generally have low S contents (¾50 ppm), poorly correlated with fertility indexes (FI#; e.g., Al2O3, CaO). Some authors have postulated that these features are due to an overprint by magma transport and weathering processes. However this interpretation is still strongly debated.
Four mantle xenolith suites from Eastern Australia have been investigated: Mt Quincan (Atherton Province, Qld), Wallabadah Rock (Liverpool Province, NSW), Allyn River (Barrington Province, NSW), and Mt Gambier (Newer Volcanics Province, SA). Mt Quincan xenoliths are mainly lherzolite (7*cpx%*19), but a few are ol-rich (75>Ol%*97). Mt Gambier peridotites display more depleted characteristics, with occurrence of harzbugite and cpx-poor lherzolites (0>cpx%>9); two are cpx-rich with 14 and 25% of cpx. Xenoliths from Wallabadah Rocks range from harzbugites to lherzolites (4>cpx%>15) and Allyn River samples are mainly lherzolitic (cpx%~16).

Mt Gambier xenoliths have higher sulfide modal abundances than Mt Quincan samples. Isolated blebs fully enclosed in olivine, spheroidal bodies up to 400 µm in diameter at spinel-silicate junctions, small droplets along fluid inclusion trails or in vermicular intergrowths at the silicate grain boundaries and sulifide veins penetrating the silicates are the main sulfide occurrences observed in Mt Gambier xenoliths; weathering is not uncommon but of very limited extent (0-30 %). In contrast, Mt Quincan xenoliths contain only a few sulphide blebs (50-300 µm), generally highly weathered (30 to 90 %) per polished thin section. Wallabadah peridotites have very variable sulfide habits and abundances, and the degree of weathering is variable (up to 50%). Allyn River peridotites show very unusual numerous large interstitial sulfides with angular shapes and 'jagged' rims. The common sulfide assemblage is typical of mantle rocks, i.e. pentlandite, monosulfide solid solution (mss) - both Ni poor (=10%) or Ni rich (=30%) and chalchopyrite/isocubanite solid solutions. This assemblage is well preserved in Mt Gambier xenoliths. On the other hand, the sulfides recognizeable in Mt Quincan xenoliths are Ni-poor/Fe-rich mss and pyrrhotite. This difference may reflect both the Ni-poor olivine chemistries of host peridotites and the fact that Ni-rich MSS are less weathering -resistant.

Sulfur contents were measured by iodometric titration at the Museun National d'Histoire Naturelle de Paris. The results conform with the petrographic study. S content for Mt Quincan xenoliths lies in narrow range between 10 to 28 ppm. Wallabadah peridotites show S contents ranging from 22 to 130 ppm and some contents as high as 344 ppm were found in Allyn River peridotites. In Mt Gambier peridotites the S concentration varies from 10 to 131 ppm. Mt Quincan S contents do not show any correlation with FI# in contrast for Mt Gambier and Walabadah samples that display a good correlation with FI#. Cu content ranges from 5 to 15 ppm and in all the xenolith suites Cu is well correlated with FI#; Cu contents are higher in Mt Quincan samples than in Mt Gambier xenoliths.

The low S contents and the lack of correlation between S and FI# in Mt Quincan peridotite are typical of sub-continental xenoliths. As Cu is still correlated with FI#, and given the weathering features observed, the low and scattered S distribution is ascribed to post-entrainment processes. However, the good correlation between S content and FI# for the Mt Gambier and Wallabadah suites allow us to estimate the S content Primitive Upper Mantle (PUM), using estimated content of Al2O3 and CaO contents in PUM. This method yields a [S]PUM around 150 ppm, which is 50 to 100 ppm lower that the other estimates. The unusually high S of Allyn River samples is more problematic, as their S contents are even higher than earlier estimates for the PUM value; therefore this feature may suggest that S has been added by metasomatism. This hypothesis is supported by the trace element pattern of the cpx, which suggests metasomatism by a volatile-rich small-volume melt. These four xenolith suites give us the opportunity to look at the effect of several processes (melting, metasomatism, weathering) upon the HSE and to try to answer question such as whether the lower and more variable PGE contents of mantle xenoliths are due to the perturbed S contents of the xenoliths, or do they reflect differences within the subcontinental mantle? Preliminary results show that the Mt Gambier xenoliths, where S contents appear undisturbed, have higher Au and Ir contents than those of Pyrenean peridotites and PUM, even though their S contents are lower.