Other
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p21a..04t&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P21A-04
Other
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5410 Composition, 5415 Erosion And Weathering, 5455 Origin And Evolution, 5464 Remote Sensing
Scientific paper
Concentrations of K and Th on planetary surfaces record crustal evolution. Both are incompatible elements, so they concentrate in magma. During igneous processing, the ratio of K to Th is approximately constant, so K/Th in igneous rocks reflects that ratio in the bulk silicate planet. However, aqueous processes can fractionate K from Th, in principle giving us a way to investigate the extent of aqueous alteration of a planetary surface. The Mars Odyssey Gamma Ray Spectrometer has obtained global data for K and Th, allowing a fresh look at the evolution of the Martian crust. K and Th are not uniformly distributed on Mars. Some regions are richer in one or both of these elements. K ranges from 1000 to 6000 ppm; Th ranges from 0.2 to 1.1 ppm. The K/Th ratio varies from 2000 to 7000. These variations probably reflect a combination of the variety of surface igneous rocks and the effects of aqueous processes. The GRS and other data suggest: (1) Concentrations of K and Th are higher than those in basaltic Martian meteorites (K = 200-2600 ppm; Th = 0.1-0.7 ppm), suggesting different mantle sources for the meteorites compared to the bulk of the crust: Martian meteorites from depleted sources, the bulk of the crust from undepleted mantle sources. (2) The concentration of Th on Mars does not vary as much as it does on the Moon (where it ranges from 0.1 ppm to 12 ppm), suggesting that the primary differentiation of Mars differed from that of the Moon. This implies a magma ocean did not form on Mars, its characteristics (e.g., formation of garnet at its base, presence of water) differed significantly from those of the lunar magma ocean, or its products are not exposed at the surface. (3) If the average Th concentration (about 0.7 ppm) of the surface is equal to the average of the entire crust, the crust cannot be thicker than about 100 km. If the crust is about 50 km thick, as suggested by geophysical studies, then about half the Th is concentrated in the crust. (4) The variations in the K/Th ratio suggest that aqueous processes might have affected surface deposits. K and Th concentrate in different phases (e.g., K in feldspars and residual glass; Th in apatite) so differential dissolution of phases can cause fractionation of K from Th.
Boynton Willam V.
Taylor Geoff
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