Noble Gases in the Earth's Core?

Details Noble Gases in the Earth's Core? Noble Gases in the Earth's Core?

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.u33b..06j&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #U33B-06

Mathematics

Logic

8125 Evolution Of The Earth, 8147 Planetary Interiors (5430, 5724), 3924 High-Pressure Behavior, 1025 Composition Of The Mantle, 1015 Composition Of The Core

Scientific paper

Chemical inertness, surface volatility and low abundance have made the noble gases a unique trace elemental and isotopic system for constraining the formation and evolution of the solid Earth and its atmosphere. This geochemical role parallels extensive physical-property measurements on the condensed rare gases alone at the pressures equivalent to those of the Earth's deep mantle and core from diamond-anvil cell (DAC) experiments. Traditional geochemical approaches to the processes of planetary evolution have involved crystal-melt partitioning at low pressures relevant more to near-surface degassing. The degree of compatibility has fluctuated among different studies and largely rests with the conclusion that, for common upper mantle phases, the noble gases are highly incompatible. But the long-known high 3He/4He ratios for some ocean-island basalts and more recent observations for some of the rare gases (Ne, Ar and possibly Xe) that there is a solar component emanating from the Earth, continue to raise questions on the source reservoir as well as on accretionary and incorporation processes. Changes in models of mantle convection style have made it harder to rely on the deep mantle as a reservoir, and the core has remained a particularly unfavourable location either because of difficulty in constructing a retention mechanism during planetary accretion or simply because of lack of data: Partitioning studies at pressure are rare and complicated by the difficulty in reproducing not only absolute concentrations, but confinement of gas in high-pressure apparatus and post-run analysis. We have investigated noble gas solubility in silicate liquids at high pressures in a DAC (relevant to a magma-ocean model of the early Earth) that suggests that the detailed composition and structure of silicate liquids may act as an important control on the level of incompatibility. The long-held idea of partial melting as a single-stage, efficient process for extracting noble gases from the Earth's mantle at all depths, may well be oversimplified. For molten metal compositions interacting with silicate melt, Matsuda et al. (1993) defined the near-zero limits of noble gas solubility expected in metal with increasing pressure. We re-visit the phenomenological aspect of (saturated) noble gas solubility in metals with new experiments in noble gas pressure-transmitting media in the laser-heated DAC. First results with argon analysed with SEM methods suggest up to an order of magnitude higher partition coefficient (D(Ar)Fe/sil ˜ 0.1) for liquids in the DAC at 5 GPa. We have also recovered samples for analysis with more sensitive UV laser-ablation mass spectroscopic techniques that provide additional, depth-resolved constraints on noble gas solubility at moderate pressures.

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