Computer Science – Databases
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p12a..07t&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P12A-07
Computer Science
Databases
[1060] Geochemistry / Planetary Geochemistry
Scientific paper
The bulk compositions of the terrestrial planets are fundamentally important in testing models for planetary accretion. This is particularly true for the abundances of volatile elements. In the absence of direct samples of the mantle, we must rely on samples of surface materials obtained from orbit (specifically from the Mars Odyssey Gamma-Ray Spectrometer, GRS), Martian meteorites, and in situ analyses. Use of these databases requires understanding the processes that formed and modified the igneous rocks composing the crust; aqueous processes are particularly important. Halogens are useful elements for understanding Martian bulk composition and surface aqueous alteration. Here, we focus on Cl, which is an incompatible element during partial melting. Cosmochemically, Cl is a moderately volatile element with a condensation temperature of 948 Kelvin, only slightly below that of K (1006 Kelvin), another incompatible lithophile element. Cl is substantially lost during magma degassing at or near the surface, making it difficult to determine its abundances in the interior through analyses of rocks, leading to an underestimate of Cl abundance in bulk silicate Mars. GRS data for Mars between approximately 52 degrees north and south show that K and Cl are uncorrelated. This is not surprising as they fractionate easily by release of Cl-bearing gases from magmas near the surface and during eruptions, by aqueous alteration of surface materials, and by the large solubility of Cl salts in water. A positive correlation of Cl with H supports the role of water in Cl redistribution. In spite of the lack of correlation between K and Cl, the mean Cl/K ratio is roughly chondritic: 1.5 ±0.1 compared to 1.28 in CI chondrites. However, Cl appears to be enriched at least in the uppermost few tens of cm analyzed by the GRS: Cl correlates with both H and S, but a linear fit to the data shows a positive Cl intercept of about 0.3, which suggests a decoupling of Cl from S and H. Adjusting the mean Cl abundance at the surface by subtracting 0.3 yields a global near-surface Cl/K of 0.6, about half the chondritic value. These data provide one framework for understanding the details of aqueous alteration on Mars. A reasonable interpretation of the Cl/K ratio at the surface is that the moderately-volatile elements K and Cl are present in chondritic proportions in bulk Mars, but are redistributed by volcanic and aqueous processes. Considering their similar incompatible igneous behavior, any deviations in Cl/K are likely caused in part by aqueous processes on or near the surface. Thus, Cl/K might be a useful index for identifying regions enriched or depleted in deposits modified or formed by aqueous alteration. Enhancement of Cl/K above chondritic could also be caused by additions from volcanic outgassing and variations in the K concentration of igneous rocks, so additional parameters are clearly necessary (e.g., H and S concentrations). Although complicated, these compositional relationships emphasize the need to understand aqueous processing on Mars quantitatively.
Boynton Willam V.
McLennan Scott M.
Taylor Jacob
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