Mathematics – Logic
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.v23b..02s&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #V23B-02
Mathematics
Logic
3630 Experimental Mineralogy And Petrology, 3640 Igneous Petrology, 5410 Composition (1060, 3672)
Scientific paper
Several recent studies have demonstrated that the fO2 in martian basalts varies by 2 to 3 log units and is correlated with geochemical parameters such as LREE/HREE, initial 87Sr/86Sr, and initial d. These correlations have been interpreted to indicate the presence of reduced, incompatible element-depleted and oxidized, incompatible element-enriched reservoirs that were produced during early stages of martian differentiation. Olivine-phyric basalts represent the closest approximation of primary martian basalts in the meteorite collection. Therefore, gaining a better understanding of the fO2 and incompatible element characteristics of the earliest phase (olivine) in these basalts may provide a clearer view into the martian mantle. Using the bulk composition of Yamato 980459 (an olivine-phyric basaltic melt composition), we conducted a series of near-liquidus experiments to determine the partitioning of V between olivine-melt at fO2 conditions between QFM and IW-1. XANES analyses of the glass indicate that the V4+/V3+ varies systematically with redox conditions of the experiments. SIMS analyses of V in glass and olivine indicated a systematic increase in DVol/melt from approximately 0.055 at QFM to approximately 0.5 at IW-1. This is consistent with the observed variation in V4+/V3+ measured in the glass by XANES and the crystal chemical preference of the olivine octahedral site for V3+ rather than V4+. Applying this oxybarometer calibration to well-defined lunar (IW-1) and terrestrial basaltic systems (QFM+0.2, Makaopuhi lava lake, Hawaii) indicate that it is vary robust over a wide range of planetary fO2 conditions. The determination of DVol/melt and incompatible element abundance using SIMS from martian olivine-phyric basalts indicate that they crystallized over a rather broad range in fO2 (IW+0.2 to IW+2) and from basaltic magmas with a range of incompatible element enrichments. These data indicate that correlations among these geochemical parameters are fundamental characteristics of the closest approximations of primary martian magmas. These signatures were not incorporated into the basaltic magmas during crystallization or involved late-stage processes. This suggests that if assimilation of martian crust by reduced, mantle derived magmas was an important mechanism for producing this array, assimilation occurred prior to the crystallization of any of the olivine now in these olivine-phyric shergottites. Thus the assimilation must have occurred at the base of the martian crust and all mineralogical evidence of assimilation was removed during transport to the martian surface. It appears more-likely that rather than assimilation, these arrays represent different mantle sources. This has substantial implications for the dynamics of a martian mantle that maintained two distinctly different reservoirs that were formed during initial martian differentiation at 4.5 Ga.
Karner J.
McKay Gordon
Papike James J.
Shearer C.
Sutton Sean
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