Chlorine Partitioning Between Mantle and Core: Implications for Early Earth Processes

Details Chlorine Partitioning Between Mantle and Core: Implications for Early Earth Processes Chlorine Partitioning Between Mantle and Core: Implications for Early Earth Processes

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr41a1778s&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #MR41A-1778

Physics

1015 Composition Of The Core, 1025 Composition Of The Mantle, 1027 Composition Of The Planets, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5205 Formation Of Stars And Planets

Scientific paper

The ratio of chlorine in the Bulk Silicate Earth (BSE) to primitive carbonaceous chondrites (or solar abundance) is an order of magnitude below that expected for a moderately volatile element. Given that crustal and mantle concentrations of Cl are fairly well constrained, the enormous Cl depletion can be attributed to any of three possible explanations: 1) a missing sink on Earth, 2) a ~500K overestimate of the 50% condensation temperature of Cl from the solar nebula or 3) an early volatile loss of Cl during Earth formation. Accepting the Cl concentrations of mantle and crust, McDonough (Treatise Geochem. 2003) proposed a core sink, with a Cl concentration of 200 ppm. We tested this hypothesis by conducting high pressure Cl partitioning experiments. Two experiments, one (A503) with equal proportions of primitive basalt (10.4 wt% MgO, Mg# 67) and pure Fe with 0.4 wt% FeCl2, and the second (A505) with equal proportions of basalt and Fe95.5S4.5 with 1.56 wt% FeCl2, were run in graphite capsules at 5 GPa and 1800° C in a Walker-style multianvil press. Both experiments produced homogeneous quenched silicate and metallic liquid. Average Cl contents in the quenched silicate melts were 5300±700 ppm for A503 and 10500±1000 ppm for A505 (2σ, n=6). Cl contents in metal were below detection (60 ppm) in A503 and in three of the six analyses of metal in A505; the average value for the above-detection points in that run was 140±50 ppm. The maximum metal-silicate DCl values are 0.011 and 0.018 for A503 and A505, respectively, three orders of magnitude lower than required for significant sequestration of Cl in the core. Unless there are strong effects of pressure and/or silicate liquid composition on DCl, our experimental results imply that Cl does not reside in the core. Explaining the low Cl concentrations on Earth via a lower 50% condensation temperature requires a decrease of >500K from previous estimates. This is inconsistent with all thermodynamic data for sodalite formation or the crystallization of NaCl(s) from vapor. The third possibility is that Cl was lost during expulsion of a primary atmosphere. Cl is classified as lithophile, but it is uniquely partitioned into water, making it 'hydrophile'. If Cl was strongly concentrated in an early ocean or atmosphere as HCl(g) or dissolved chloride, a large impact could volatilize the early ocean, causing Cl loss to space (e.g., Genda and Abe, Nature 2005), explaining the low Cl content on Earth.

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