Geochemical Evidence for the Volcanic Origin of Home Plate in the Inner Basin of the Columbia Hills, Gusev Crater

Details Geochemical Evidence for the Volcanic Origin of Home Plate in the Inner Basin of the Columbia Hills, Gusev Crater Geochemical Evidence for the Volcanic Origin of Home Plate in the Inner Basin of the Columbia Hills, Gusev Crater

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p44a..07s&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #P44A-07

Other

1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5480 Volcanism (6063, 8148, 8450)

Scientific paper

From Sol 743 to 778, the Mars Exploration Rover Spirit has examined Home Plate, a ~4.5 to 1 m tall, ~80 m diameter light-toned outcrop, made up of layered to cross-bedded, clastic rock in the Inner Basin of the Columbia Hills. Three sites at Home Plate were targeted for analysis by the Alpha particle X-Ray Spectrometer and Mossbauer for composition and Fe-bearing mineralogy. The five resulting Home Plate analyses are nearly identical in most elements to Masada, a vesicular basalt that was analyzed 350 m north of Home Plate. Compositional as well as mini-TES spectral similarities between Home Plate and nearby basalts suggest they were derived from a common magmatic source and have not been mechanically mixed with other rock types. Excess volatile elements at Home Plate relative to Masada, including Cl, Br, Zn, and Ge cannot be derived from a meteoritic source and are most likely from a volcanic origin. In particular, the volatile siderophile element Ge has been detected in a few rocks in Gusev Crater, yet ranges from 30-70 ppm at Home Plate, comparable to Ge in Fe meteorites. Home Plate Ni ranges from 300 to 500 ppm, significantly less than 5 wt% in Fe meteorites. The Ge corroborates with rock textures including a bomb sag at Home Plate that indicate it formed as the result of a phreatomagmatic volcanic eruption. Home Plate's excess halogens relative to basalts do not correlate with major elements, such as S, Al, K or Zn. This indicates that igneous fractionation and fumarolic enrustation are likely not the source of Cl and Br. While isochemical alteration or salt coatings are difficult to rule out at Home Plate, Mossbauer observations indicate that Home Plate's Fe3+/Fe^{total} (0.51-0.53) is not particularly oxidized and but high halogen Cl has not been found in nearby basalts or soils. Our preferred model is that Home Plate incorporated a Cl- rich briny aquifer during its formation during a phreatomagmatic eruption. The fluid likely consists of a dissolved salts that include the cations Na and Zn and my include Ca, Mg, and Fe. The ultimate source of the brine is unknown, but high Cl/SO3 suggests that a S-rich acid vapor phase has been extracted from the brine by boiling. White sulfate soils near Home Plate may be fumarolic or acid spring deposits derived from this vapor-brine fractionation.

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