Petrogenesis of Enriched and Intermediate Poikilitic Shergottites: From Magmatic Source to Emplacement

Rachel Rahib


Poikilitic shergottites, making up >20% of martian meteorites, likely represent a significant composition of the martian crust, as intrusive gabbroic rocks. To further constrain petrogenetic relationships amongst enriched and intermediate poikilitic shergottites, we utilize bulk rock trace element compositions, mineral major element compositions, phosphorus maps of olivine grains, oxygen fugacity (ƒO2) values, subsolidus equilibration temperatures, and quantitative textural analyses, of the most comprehensive suite of poikilitic shergottites yet (11 samples), including three newly recovered samples (Northwest Africa [NWA] 11065, NWA 11043, NWA 10961). Although petrographically light rare earth element (LREE) enriched and intermediate poikilitic shergottites are similar, distinct LREE abundances suggest derivation from at least two unique magmatic sources. The characteristic bimodal textures (poikilitic and non-poikilitic textures) of poikilitic shergottites record evolving magmatic conditions at different stages of crystallization. Hotter, more reducing conditions, during early-stage crystallization, are recorded in the poikilitic textures and, cooler, more oxidizing conditions, at late-stage crystallization, are recorded in the non-poikilitic textures. Oxygen fugacity estimates for early-stage olivine-pyroxene-spinel assemblages of enriched and intermediate poikilitic shergottites suggest decoupling of ƒO2 and degree of LREE-enrichment (i.e., [La/Yb]CI), and increases in ƒO2 exceeding 1 log unit from poikilitic to non-poikilitic textures implies auto-oxidation and degassing. Phosphorus maps of poikilitic and non-poikilitic olivine grains within NWA 10618, NWA 7755, and ALHA 77005 demonstrate transition from equilibrium growth in a magma staging chamber, followed by disequilibrium growth in transit to the surface, whereas phosphorus maps within LEW 88516 and NWA 11065 show disequilibrium growth during poikilitic olivine crystallization. Quantitative textural analyses of both enriched and intermediate poikilitic shergottites supports emplacement as various cumulate piles or shallow intrusive bodies, as well as a potentially widespread emplacement mechanism responsible for a major lithology of the martian crust.