Petrogenetic linkages among fO_2, isotopic enrichments-depletions and crystallization history in Martian basalts. Evidence from the distribution of phosphorus in olivine megacrysts

Abstract

An important geochemical characteristic of the shergottites is the large range in initial Sr isotopic ratios and initial ε^(Nd) values. The depleted olivine-phyric shergottites are commonly reduced (∼IW+1) and exhibit limited variation in fO_2 during crystallization, whereas the enriched olivine-phyric shergottites are more oxidized and exhibit a substantial variation in fO_2 during crystallization (∼IW+1 (1 log unit above the iron-wüstite buffer) to FMQ+0.7 (0.7 log units above the fayalite-magnetite-quartz buffer). Using a reduced, depleted shergottite Y980459 (Y98) and an oxidized, enriched shergottite (NWA1183), we reconstructed the crystallization history of the olivine megacrysts using phosphorus zoning and placed trace element variation and indices of oxygen fugacity into the context of that crystallization history. The olivine megacrysts in Y98 and NWA1183 exhibited various episodes of growth defined by the P zoning. Common to both samples were continuous and discontinuous oscillatory chemical zoning in the outer portions of the olivine megacrysts. However, the morphologies of the cores suggest a xenocrystic origin for the core of Y98 and a phenocrystic origin for the core of NWA1183. Phosphorus in the olivine ranges from less than 0.02 to up to 0.70 wt.% P_2O_5. The P in the olivine megacrysts manifests subtle, but real differences, in the P content of different shergottite melts, incompatible element behavior of P during olivine crystallization, and solute trapping during periods of rapid olivine growth. Although P should behave incompatibly in basaltic systems, there is little relationship between the isotopic characteristics and P content in the enriched-depleted array of shergottites. In the olivine, Al is generally correlated with P enrichment and reflects a coupled substitution of P–Al. On the other hand, V is not correlated with P in Y98, but correlated with P in the oscillatory zoned regions of the olivine in NWA1183. The changes in correlation to P may reflect differences in V valance state during olivine crystallization. Within the crystallization histories defined by the P zoning in olivine, the calculated fO_2 remains fairly constant at ∼IW+1 for Y98, but changes dramatically for NWA1183 from olivine core to rim (∼IW+1.8 to FMQ+.07). Numerous lines of evidence suggest that the changes in fO_2 during olivine crystallization may reflect auto-oxidation that may be at least partially due to degassing of the NWA1183 magma prior to eruption. These observations suggest a limited variation in the fO_2 of the Martian mantle (IW to IW+1), that enriched and depleted sources for the shergottites reside in the mantle, and that magmas produced in these sources have different capabilities for changing their redox state during crystallization.

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