Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v44b..07b&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #V44B-07
Physics
8450 Planetary Volcanism (5480), 8121 Dynamics, Convection Currents And Mantle Plumes, 7207 Core And Mantle, 3640 Igneous Petrology, 1025 Composition Of The Mantle
Scientific paper
If zero-age basalts are enriched with respect to 187Os/188Os relative to present-day primitive mantle, one may assume that they tap reservoirs in the mantle that are superchondritic with respect to Re/Os and/or 187Os/188Os. Most interesting are coupled enrichments in 186Os/188Os and 187Os/188Os; if these signatures could only be derived from the outer core, they would testify that some mantle plumes indeed originate at the core-mantle boundary. We report experiments with (Fe,Ni,Cu)1-xS monosulfide in silicate mantle matrix that quantify noble metal fractionation during partial silicate melting. Our model permits the derivation of isotopically enriched melts from primitive mantle sources with time-integrated chondritic Os-isotope ratios. Melting experiments of (Fe,Ni,Cu)1-xS monosulfide in fertile mantle matrix to 1400°C and 3.5 GPa show that two sulfide phases are stable at the dry silicate solidus, a crystalline FeS-rich monosulfide and a Cu2S-enriched sulfide melt. The noble metals fractionate between the sulfide phases: Os, Ir and Ru into crystalline monosulfide, and Re, Rh, Pt, and Pt into the sulfide melt. During silicate melt segregation, crystalline monosulfide remains with silicate minerals in the mantle, concentrating Os, Ir, and Ru in the residue. The molten sulfide fraction is entrained in the silicate melt as immiscible droplets and is drained from the mantle along with the silicate melt, defining the noble metal inventory of the basaltic component. Physical processes are more important in fractionating the noble metals than chemical partitioning laws. The noble metal contribution to a basaltic melt by sulfide-silicate partitioning is small. Principally, it is possible to produce basalts with superchondritic Os-isotope ratios from chondritic mantle sources, as long as there is compositional heterogeneity among the mantle sulfides. Partial melting preferentially mobilizes and selectively entrains in the silicate melt sulfide compositions with low melting points, i.e., compositions rich in Cu, Re, Pt, and Pd. If sulfide heterogeneity is an ancient signature of the mantle, old enough to allow sufficient ingrowth of radiogenic 186Os and 187Os, the entrained sulfide component will on average be more radiogenic than bulk mantle, and so will be the basalt. There is indeed evidence for grain-scale heterogeneity among mantle sulfides both with respect to major elements, noble metals, and Os-isotopes. The key question is whether such heterogeneities may survive partial melting and may be passed on to a segregating silicate melt.
Ballhaus Chris
Bockrath Conny
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