Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p13e..08b&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P13E-08
Physics
1519 Magnetic Mineralogy And Petrology, 1595 Planetary Magnetism: All Frequencies And Wavelengths, 3630 Experimental Mineralogy And Petrology, 3672 Planetary Mineralogy And Petrology (5410), 5440 Magnetic Fields And Magnetism
Scientific paper
A suite of synthetic Martian basalts is generated with the objective of providing fundamental material properties data for use in modeling and interpretation of mission data. We systematically evaluate the effects of major element composition, oxygen fugacity (fO2), and cooling rate on phase chemistry and magnetic mineralogy, grain size, and intensity of remanent magnetization. The range of experimental compositions and fO2 are chosen to bracket the range expected in the Martian crust; our results should therefore bracket the range of possible mineralogies, textures, and magnetic properties in rapidly-cooled Mars crustal materials. Two basic starting compositions are used for the sample synthesis. The first is Fe-rich, Al-poor and is patterned after SNC basaltic meteorites. The second has a much lower Fe/Al ratio and is based on satellite thermal emission spectrometer observations of the southern highlands that suggest a more terrestrial-like composition. fO2 varies between the iron-wustite (IW) and quartz-fayalite-magnetite (QFM) buffers. The resulting magnetic carrying phase in QFM samples is a Mg- and Al-bearing Fe-Ti-Cr oxide, with increasing Cr substitution over Ti during early, rapid crystallization. Under more reducing conditions, the meteorite-based samples show evidence for increased impurity substitution, while the terrestrial-type samples appear to have a phase closer to pure magnetite. Magnetic grain size is controlled by fO2, cooling rate, and sample composition; the smallest grains form under reducing conditions, while the largest grains form under QFM conditions and at slow cooling rates. Magnetic intensity is most strongly influenced by fO2, with more subtle composition and cooling-rate effects. Moderately oxidizing QFM conditions reliably result in an intense magnetization, especially in the meteorite-derived basalts. However, an increase of grain size into the multi- domain range (meteorite-type) and/or low unblocking temperatures resulting from increased Cr- and Al- substitution (terrestrial-type) may affect the long-term stability of the remanence in QFM samples. A (significantly weaker) remanence acquired under reducing conditions is more likely to persist in samples of terrestrial composition, which are characterized by higher unblocking temperatures.
Bowles Julie A.
Brachfeld Stefanie A.
Hammer Julia E.
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