Alteration of fresh rock surfaces proceeds very rapidly in most terrestrial environments so that initial stages of modification of newly exposed surfaces are quickly masked by subsequent aqueous weathering processes. The hyper-arid and hypo-thermal environment of Beacon Valley, Antarctica, is limited in terms of available liquid water and energy available for alteration, which severely slows weathering processes so that the initial stages of alteration can be studied in detail. We report on the nature of initial chemical alteration of the Ferrar Dolerite in Beacon Valley, Antarctica, using a multiplicity of approaches to characterize the process. We suggest that initial chemical alteration is primarily driven by cation migration in response to the oxidizing environment. Morphological studies of altered rock surfaces reveal evidence of small-scale leaching and dissolution patterns as well as physical erosion due to surface weakening. Within the alteration front, mineral structures are largely preserved and alteration is only indicated by discrete zones of discoloration. Mineralogical investigations expose the complexity of the alteration process; visible/near-infrared reflectance and mid-infrared emission spectroscopy reveal significant variations in mineralogical contributions that are consistent with the introduction of oxide and amorphous phases at the surfaces of the rocks, while X-ray diffraction analyses reveal no definitive changes in mineralogy or material properties. Chemical analyses reveal large-scale trends that are consistent with cation migration and leaching, while small-scale electron microprobe analyses indicate that chemical variations associated with magmatic processes are still largely preserved within the alteration rind. This work confirms the incomplete and immature chemical alteration processes at work in the McMurdo Dry Valleys. Liquid water is not a significant contributor to the alteration process at this early stage of rind development, but assists in the removal of alteration products and their local accumulation in the surrounding sediments. These results also suggest that the McMurdo Dry Valleys (and Beacon Valley, in particular) are relevant terrestrial analogs to hyper-arid and hypo-thermal alteration processes that may be dominant on the martian surface.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (ANT-0739702). The authors would like to thank several people and institutions for their support and assistance with data collection, analyses, and interpretations: The National Science Foundation, the United States Antarctic Program, Raytheon Polar Services Company, the staff of McMurdo Station, PHI Inc., Cherie Achilles, Nilanjan Chatterjee and the Electron Microprobe Facility at the Massachusetts Institute of Technology, M. Darby Dyar, Timothy Glotch and the Vibrational Spectroscopy Laboratory at Stony Brook University, Takahiro Hiroi and the Brown University RELAB Facility, Colin Jackson, the LacCore Facility at the University of Minnesota, Anthony McCormick, Douglas Ming, Richard Morris, David Murray, Joseph Orchardo, Stephen Parman, A. Deanne Rogers, Alberto Saal, and Paul Waltz and the School of Engineering at Brown University. We are also extremely grateful for the helpful reviews from Steve Ruff, Penelope King, and an anonymous reviewer.
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