Crystallization Kinetics of Olivine-Phyric Shergottites

Petrographic and geochemical techniques applied to Martian meteorites can reveal their petrogenesis, and ultimately the magmatic evolution and thermal history of Mars. Two quantitative textural tools, crystal size distribution (CSD) and spatial distribution pattern (SDP) analysis, are utilized to describe the early crystallizing phases, olivine and pyroxene, of one olivine-phyric shergottite (EET 79001A, DaG 476, and Dho 019) from each sampling locality as inferred from Mars ejection ages. Qualitative observations of trace element zonation patterns (P, Cr, and Al) in olivine are used in conjunction with CSD and SDP to characterize the crystallization history of these samples. Previously reported CSDs are re-evaluated using a newer stereographically corrected methodology. The linear CSDs for pyroxene in DaG 476 and EET 79001A indicate single populations that grew under steady-state conditions. The CSDs for olivine in the three meteorites analyzed are more complex. Kinks in the olivine CSD plot for DaG 476 suggest up to three populations crystallized at different cooling rates. The olivines CSD for both EET 79001A and Dho 019 are dominated by single linear trends representing one crystal population; however, elemental X-ray mapping suggests two populations in Dho 019. A variety of P zonation patterns are present in olivine grains in these samples. Oscillatory P zonation suggests variation in growth rate, where high-P olivine experienced rapid growth compared to low-P olivine. Residence times are calculated using CSD slopes and an assumed constant growth rate. As implied by the observed P zonation in olivines, ranges of residence times are presented using different growth rates inferred from previous experimental data. SPD analysis classifies the pyroxene populations in DaG 476 and EET 79001A as clustered touching frameworks, indicating that the crystals grew or accumulated in clusters rather than as randomly distributed grains. The olivines in DaG 476 and Dho 019 are also clustered but are classified as non-touching frameworks. Quantitative textural analyses, used in conjunction with quantitative elemental mapping, also provide constraints on the origin of the olivine megacrysts in these martian meteorites. The data indicate that most of the olivines are phenocrysts or antecrysts, although small amounts of xenocrysts may also be included.