TERRESTRIAL IMPLICATIONS FROM THE PETROGENIC HISTORY OF OLIVINE MEGACRYSTS IN MARTIAN BASALTS

The distribution of P in olivine records the state of a magmatic system. Formation and preservation of P zoning contrasts with zoning of divalent cations. These differences have been attributed to diffusion rates. We explore the origin of olivine megacrysts in Martian shergottites by combining WDS mapping with EPMA spot analyses of olivines from two Martian basalts: NWA 1183, an “enriched” shergottite, and Y 98, a “depleted” shergottite. This study reconstructs the thermal history in these two shergottites by examining P zoning in large olivine megacrysts. The data will be used to examine relationships between geochemical characteristics in these two basalts and to understand the phenomenon of P zoning in terrestrial rocks.

Concentric P zoning in these Martian basalts is shown by bands of higher P concentrations and is tied to the diffusivity of P. The incompatibility of P results in higher concentration of P within the melt while slow diffusion leads to the inability to “outrun” the growing crystal faces. Visible P zones are the result of rapid crystal growth due to increases in cooling rate. Fabbrizio et al. (2010) and Milman-Barris et al. (2006) found P zoning in olivines from Hawaii and explain the phenomenon as undercooling and/or rapid crystal growth. Oscillatory zoning is preserved by high P cores in the megacrysts because of the low diffusivity of P. Phosphorous substitution into the tetrahedral site requires a coupled mechanism to maintain charge balance. The correlation of P and Al in NWA 1183 points to a possible substitution mechanism:

2Si_T⁴⁺ ↔ P_T⁵⁺ + Al_T³⁺

Part of the incompatible element signature is fingerprinted in the P₂O₅ concentration with bulk Y 98 having 0.29% P₂O₅ while enriched shergottites commonly have higher P₂O₅. The difference in P₂O₅ in these megacrysts indicates that the first silicate phase (olivine) crystallized from enriched and depleted basalts, respectively. Thus olivine megacrysts most likely represent phenocrysts and the enriched and depleted signatures were added to the basalts before crystallization. This indicates that the enriched signature and fO₂ are decoupled and that an enriched and oxidized Martian mantle may not exist.