Further Insights on S Content and Behaviour in the Lithospheric Mantle

Abstract

Sulphides 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, sulphide 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 extremelly 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 MORB source (~200 ppm, e.g. O'Neil, 1991) and correlated with major element contents. In contrast, peridotite xenoliths brought up by alkali basalts generally have low S contents (~ cpx%~>19) , bu t a few are o l r i c h (75>O1%~>97). Mt Gambier peridotites are more depleted, including 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). Montferr ier xenol i ths are fert i le lherzol i te (1 l>cpx%>15) displaying highly deformed porphyroclastic microtexture and rare disseminated pargasite. Among the Montferrier xenoliths two populations can be distinguished one consist of fresh dark-green xenoliths in basaltic lava flow, the second one of altered yellowish xenoliths within a breccias. Sulphur contents were measured using iodometric method at the Museum National d'Histoire Naturelle, Paris, Cu and trace element analysis were performed on ICP-MS at Macquarie Universi ty or at Montpellier II University. Mt Gambier xenoliths have higher sulphide modal abundances than Mt Quincan samples. Sulphides in Mt Gambier xenoliths occur as Isolated blebs fully enclosed in olivine, spheroidal bodies up to 400 gm in diameter at spinel-silicate junctions, small droplets along fluid inclusion trails, in vermicular intergrowths at the silicate grain boundaries or in sulphide veins penetrating the silicates. Weathering is not uncommon but of very limited extent (0-30%). In contrast, Mt Quincan xenoliths contain only a few sulphide blebs (50-300 gin), generally highly weathered (30 to 90%) per polished thin section. Wallabadah peridotites have very variable sulphide habits and abundances, and the degree of weathering is variable (up to 50%). Allyn River peridotites show very unusual numerous large interstitial sulphides with angular shapes and 'jagged' rims. Montferrier xenoliths have large sulphides (up to 300 gm) with polygonal shapes in microtextural equilibrium with the neoblastic matrix. These sulphides are highly altered (50 to 80%) in breccia-hosted periotite and almost unaltered (<15%) in lava hosted-xenoliths. The common sulphide assemblage in all the xenolith suites is pentlandite, pyrrhotite, monosulphide solid solution (mss) and chalcopyrite.