Other
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.v51b..04p&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #V51B-04
Other
1025 Composition Of The Mantle, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1065 Trace Elements (3670)
Scientific paper
Seven basic and ultrabasic xenoliths from the Kerguelen Archipelago (KA), previously characterized for Sr, Nd, Pb isotopic and lithophile trace elements have been analyzed for the chalcophile and siderophile elements (incl. Os, Ir, Ru and Pt). The xenoliths include one dunite, two websterites, one lherzolite and three metagabbros. Previous work has shown that the dunite (91-114) has distinctive isotopic and geochemical features, reflecting incorporation of isolated fragments of continental lithosphere in the Kerguelen plume source. Although depleted in magmaphile elements such as Al2O3 and CaO and enriched in MgO and FeO, it is more fertile than depleted peridotites derived from cratonic lithopshere. Its high Ru/Pt (3.88) and low Pt/Ir (0.64) whole rock ratios however overlap those of cratonic (depleted) peridotites from the Kaapvaal Craton, and are similar to ratios found in residual sulfides enclosed in silicates from refractory cratonic peridotites high degree partial melting. Its anomalous Os/Ir ratio of 4.52 may be related to metasomatic modification. However, it is comparatively LREE depleted (both whole rock, or in clinopyroxene). This suggests that metasomatic modification was not related to LREE-Pt-enriched basaltic (or carbonatitic) melts. The bulk of the remaining ultrabasic (lherzolites, websterites) and basic (metagabbros) xenoliths represent tholeiitic cumulates or basaltic segregates which contributed to lithospheric growth beneath the KA via magmatic underplating mainly in the granulite facies. Iridium contents in metagabbros range from 0.46 to 1.47 ppb, whereas Pt ranges from 0.11-5.95 ppb. Only one metagabbro has chondritic PGE ratios (Os/Ir, Pt/Ir, Ru/Ir, ARC 835) and PGE contents <1 ppb. The remainder display a large range of Pt/Ir ratios from 0.24 to 9.25 resulting from varying Pt concentrations at relatively constant Ir contents. The average Ru/Ir ratio for all these xenoliths (including dunite 91-114) is suprachondritic and relatively constant at 2.36 ± 0.36 (1sd). This suggests that Ru and Ir are hosted in the same mineral phase and that these two elements are more compatible during mantle melting and segregation. The mantle source beneath Kerguelen appears to be enriched in Ru relative to Ir, a feature observed in other samples of abyssal peridotites, basalt-borne xenoliths and orogenic peridotites. The basic and ultrabasic xenoliths form a continuous trend in Ru/Pt vs. Pt/Ir space from highly suprachondritic Ru/Pt and subchondritic Pt/Ir to highly subchondritic Ru/Pt and suprachondritic Pt/Ir. Similar PGE are found in sulfides from depleted and fertile mantle samples, respectively. This trend in the metagabbros is interpreted to reflect increasing degrees of tholeiitic/basaltic magma at depth during silicate mineral fractionation/sulfide segregation. The positive correlation between Ru content and wt% MgO for all the xenoliths (including 91-114), suggests that olivine fractionation may play a role in the distribution of Ru.. Ultrabasic and basic xenoliths have Os/Ir ratios close to chondritic (0.82-1.22), except for one lherzolite (0.46, MG91-33) and websterite (2.43, GM92-252). Extensive re-equilibration and possibly melt infiltration at depth may have contributed to fractionation of Os from Ir. The combined chalcophile, siderophile and isotopic trends displayed by the xenoliths in this study argue for a combination of mixing and fractionation accompanying melting of different sources prior to magmatic underplating beneath the KA.
Mattielli Nadine
Pretorius W.
Scoates James
Weis Dominique
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