The history of ALH 84001 revised: Multiple shock events

Details The history of ALH 84001 revised: Multiple shock events The history of ALH 84001 revised: Multiple shock events

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33..753t&link_type=abstract

Meteoritics & Planetary Science, vol. 33, no. 4, pp. 753-764.

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Scientific paper

The geologic history of martian meteorite ALH 84001 is more complex than previously recognized, with evidence for four or five crater-forming impacts onto Mars. This history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient martian life in ALH 84001. ALH 84001 formed originally from basaltic magma. Its first impact event, I1, is inferred from the deformation D1, which produced the granular-textured bands ("crush zones") that transect the original igneous fabric. D1 is characterized by intense shear, and may represent excavation or rebound flow of rock beneath a large impact crater. An intense thermal metamorphism followed D1, and may be related to it. The next impact, I2, produced the Fr2 fractures in which carbonate 'pancakes' were deposited, and produced feldspathic glass from some of the igneous feldspars and silica. After I2, carbonate pancakes and globules were deposited in Fr2 fractures and replaced feldspathic glass and possibly crystalline silicates. Next, feldspars, feldspathic glass, and possibly some carbonates were mobilized and melted in impact 3 (I3). Microfaulting, intense fracturing, and shear are also associated with I3. In impact I4, the rock was fractured and deformed the rock without significant heating, permitting remnant magnetization directions to vary across fracture surfaces. Finally, ALH 84001 was ejected from Mars in event I5, which could be identical to I4. This history of multiple impacts is consistent with the photogeology of the martian highlands, and may help resolve some apparent contradictions among recent results on ALH 84001. For instance, the sub-micron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition, while the elongate magnetite grains, epitaxial on carbonates, could be ascribed to vapor-phase deposition during I3.

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