Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31c1717s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31C-1717
Mathematics
Logic
[1039] Geochemistry / Alteration And Weathering Processes, [1060] Geochemistry / Planetary Geochemistry, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [9310] Geographic Location / Antarctica
Scientific paper
The hyper-arid and hypo-thermal conditions that are pervasive throughout the McMurdo Dry Valleys (MDV) result in slow, immature chemical alteration that is often shadowed by physical erosion and mass wasting processes. Beacon Valley is an exception to this model in that its surface is extremely stable and has been for at least several million years. This stability allows for chemical alteration to progress in the absence of extensive physical erosion. We have collected fourteen Ferrar Dolerite samples from the floor of central Beacon Valley and have analyzed them to assess the range and extent of chemical and mineralogical variations between the unaltered interiors and their altered surfaces. Surface rocks from Beacon Valley, Antarctica, exhibit a distinct red/brown weathering/alteration rind relative to their dark grey interiors as a result of in situ chemical alteration. The rind is readily visible in broken and cut rocks and is typically 0.5-10 mm in thickness. It shows a gradational transition to the unaltered interiors. Reflectance and emission spectroscopy both show distinct disparity between unaltered rock interiors and their corresponding surfaces related to the alteration rind. Interestingly, spectroscopic signatures of primary minerals are clearly present showing that the alteration process does not completely mask the primary rock constituents from remote sensing. Visible-near infrared spectra show distinct ferric absorptions between 0.3 and 1.0 μm indicating a significant increase in ferric iron. Emission spectroscopy shows distinct absorptions related to poorly crystalline alteration products suggesting a breakdown in mineral structure in the optically active zone. EPMA analyses and BSE imagery of the dolerite surfaces and the transition between altered and unaltered regions, however, do not show any obvious chemical or mineralogical variability. This result is supported by flux fusion and ICP-AES analyses that show no substantial chemical variations between rock interiors and their surfaces. While initial XRD measurements may show some subtle mineralogical variations, further work is currently underway to quantitatively assess the extent and nature of these differences. The subtle chemical and mineralogical variability between rock interiors and their respective surfaces indicate the juvenile nature of chemical alteration in Beacon Valley. The presence of nanocrystalline and amorphous phases and the absence of crystalline and evolved alteration products suggest a scenario that is both water- and energy-limited. When considering the exposure ages of these rock surfaces, which is likely on the order of 10^5-10^6 years, the rate of chemical alteration is likely one of the slowest on Earth. As a result, the physics, rate, and requirements of chemical alteration can be effectively explored without the added contributions from of biological activity or the worry of reaching a plateau in alteration. Additionally, this helps to support the utility of the MDV as a relevant and appropriate terrestrial analog to studies of the martian surface.
Cooper Reid F.
Head James W.
Marchant David R.
Mustard John F.
Salvatore Mark R.
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