Is a 'Late Veneer' Necessary? Answers From Metal-Silicate Partitioning of Te, Se, and S

Details Is a 'Late Veneer' Necessary? Answers From Metal-Silicate Partitioning of Te, Se, and S Is a 'Late Veneer' Necessary? Answers From Metal-Silicate Partitioning of Te, Se, and S

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.v31a..06r&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #V31A-06

Other

3630 Experimental Mineralogy And Petrology, 1025 Composition Of The Mantle, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1212 Earth'S Interior: Composition And State (7207, 7208, 8105, 8124), 1213 Earth'S Interior: Dynamics (1507, 7207, 7208, 8115, 8120)

Scientific paper

Chondrite-normalized mantle abundances of Te, Se, and S show depletions from the Planetary Volatility Trend (McDonough and Sun, 1995). This is likely due to their siderophile behavior under reducing conditions (Kilburn and Wood, 1997), resulting in strong partitioning into the core-forming Fe-Ni liquid. A mass balance calculation using Te, Se, and S concentrations in C1-chondrites and a modeled primitive mantle shows similar depletions for each element, with core/mantle "partitioning" of about 100 (McDonough and Sun, 1995). Values for Se and Te relative to S used in this estimate are consistent with those measured in sulfide globules from the upper mantle (Hattori et al., 2001). Experiments have been done to investigate whether this depletion could be achieved solely by equilibrium metal-silicate partitioning at the P-T-fO2 conditions relevant to core formation. If experimental results match the estimated Dcore-mantle, this could reflect either a single P-T condition at the base of a magma ocean where metallic liquid ponded, or an average P-T condition covering a depth range within which metallic liquid droplets rained through semi-crystalline silicate. If the results are discrepant, the abundance of Te, Se, and S were disrupted after initial equilibrium partitioning. Higher experimental DTe,Se,S necessitates the addition of chalcophile-rich material to the mantle after core formation (i.e. a 'late veneer') and lower experimental DTe,Se,S requires removal of chalcophiles from the mantle, perhaps in a late 'Hadean sulfide matte' (Wood and Halliday, 2003). Liquid metal-liquid silicate partitioning experiments were performed using both piston-cylinder and multi-anvil presses over a range of pressure (1-12 GPa) and temperature (1835-2135°C). Powdered oxide and metal added in 50:50 proportion was contained within MgO crucibles. Oxygen fugacity was varied by adding different amounts of Si to the starting material. Experiments ranged from -7 to -0.5 log units below the iron-wüstite buffer (IW). Major elements in both quenched metal and silicate were determined by EMPA while Te and Se abundance in the quenched silicate portion was measured using LA-ICP-MS at the University of Toronto. At constant P and T, all three metal-silicate partition coefficients increase with increasing fO2. Te and S are the most siderophile, followed by Se. The presence of 5-10 wt% S increased both DTe and DSe by a factor of three. At 2.9 GPa, 1960°C and log fO2 between IW-4 and IW-1.5, metal- silicate partitioning of the three elements converge to 100, matching the observed core-mantle partitioning. Previously, it was shown that both DS and DSe are positively correlated with P while increased T lowers DS (Li and Agee, 1996; Li, 2000). After applying these corrections and extrapolating our data to the estimated conditions of core formation (P=25-40 GPa; T=2000-3500°C; e.g. Li and Agee, 1996; Righter et al., 1997), the point of equal metal- silicate partitioning of Te, Se, and S is only raised by a factor of 3. Consequently, the observed core-mantle partitioning of Te, Se, and S is consistent with high temperature equilibrium within a magma ocean, obviating the need for a 'late veneer'. Chalcophile element (including Pb) loss to the core with the separation of a Hadean sulfide 'matte' would have further depleted the mantle. It would be quite fortuitous if a 'late veneer' refertilized the mantle to match the expected equilibrium abundances prescribed by metal-silicate partitioning. This result is consistent with conclusions based on the metal-silicate partitioning behavior of other elements, for instance Ni, Co, and Pt.

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