Shock-Induced Melting of Martian Basalts: Insights on Subducting Oceanic Crust Melting Processes

Details Shock-Induced Melting of Martian Basalts: Insights on Subducting Oceanic Crust Melting Processes Shock-Induced Melting of Martian Basalts: Insights on Subducting Oceanic Crust Melting Processes

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....5124b&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #5124

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Experiments carried out on rocks at upper and lower mantle P-T conditions have produced series of candidate minerals for the Earth mantle mineralogical model. Basaltic compositions can also suffer ultra high-pressure and temperature when subducting in the mantle. The phase diagram of basalts has been studied to characterize potential chemical and mineralogical heterogeneities produced by partial melting and phase transformations of the oceanic crust. Shergottites that represent the most important sub-class of Martian meteorite have compositions close to terrestrial basalts and gabbros. During their extraction from Mars, they were severely shocked with pressures up to 50 GPa. These shocks induced partial melting. These melt pocket are an opportunity to study melting phenomena of basaltic compositions (i.e. oceanic crust) under high-pressure. We have performed a Raman spectroscopy investigation to determine the mineralogy of the melt pockets. Four shergottites were studied, NWA 480, NWA 856, NWA 1068 and Zagami. In each meteorite, abundant "large" minerals in melt pockets are hollandite (both Ca-Na and K-Na hollandite), stishovite, amorphous pyroxene and high-pressure phosphate. Meltpocket matrix seems to have a similar mineralogy as "megacrysts". In NWA 856 we observed at a melt pocket rim that maskelynite successively transforms into hollandite, and a polycrystalline aggregate. This aggregate was identified as a mixture of stishovite and a calcium aluminosilicate phase (CAS), a phase previously described in high-pressure experiments, but never observed in natural samples. The Raman spectra identifies unambiguously this silicate of composition CaAl_4Si_2O11 and of Ba-ferrite type structure. Such a phase is supposed to be present in basalt subsolidus melting experiments for pressures above 25 GPa and temperatures between 2500 and 2700 K. Its discovery reinforces the proposition that this CAS phase is a valuable candidate for hosting Al in subducting oceanic crust.

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