Two‐stage polybaric formation of the new enriched, pyroxene‐oikocrystic, lherzolitic shergottite, NWA 7397

Abstract

Northwest Africa (NWA) 7397 is a newly discovered, enriched, lherzolitic shergottite, the third described example of this group. This meteorite consists of two distinct textural lithologies (1) poikilitic—comprised of zoned pyroxene oikocrysts, with chadacrysts of chromite and olivine, and (2) nonpoikilitic—comprised of olivine, low‐Ca and high‐Ca pyroxene, maskelynite, and minor abundances of merrillite, spinel, ilmenite, and pyrrhotite. The constant Ti/Al ratios of pyroxene oikocrysts suggests initial crystallization of the poikilitic lithology at depth (equivalent to pressures of approximately 10 kbar), followed by crystallization of the nonpoikilitic lithology at shallower levels. Oxygen fugacity conditions become more oxidizing during crystallization ranging from fO2 conditions of approximately QFM‐2 to QFM‐0.7. Magma calculated to be in equilibrium with the major rock‐forming minerals is LREE‐enriched relative to depleted or intermediate shergottites and has flat overall profiles. Therefore, we suggest that the parental magma for NWA 7397 had sampled an enriched, oxidized, Martian geochemical source, similar to that of other enriched basaltic and olivine‐phyric shergottites. We present a polybaric formation model for the lherzolitic shergottite NWA 7397, to account for the petrologic constraints. Three successive stages in the development of NWA 7397 are discussed (1) formation of a REE‐enriched parental magma from a distinct Martian mantle reservoir; (2) magma ponding and development of a staging chamber concomitant with initial crystallization of the poikilitic lithology; and (3) magma ascent to the near surface, with entrainment of cumulates from the staging chamber and subsequent crystallization of the nonpoikilitic lithology en route to the surface.

Trace elements in olivine and the petrogenesis of the intermediate, olivine‐phyric shergottite NWA 10170

“…; Howarth et al. ; Treiman and Filiberto ). These plumbing‐systems, much like terrestrial plumbing‐systems associated with plume/rift‐related continental settings, comprise magmatic crystal mush composed of olivine and pyroxene phenocrysts and possibly plagioclase.…”

“…Illustration modified after Howarth et al. (). (Color figure can be viewed at wileyonlinelibrary.com.…”

Detailed mineralogy and petrology of highly shocked poikilitic shergottite Northwest Africa 6342

“…Detailed analysis of newly discovered shergottites is needed to provide constraints on these source reservoirs and mixing dynamics on Mars, as well as advance our understanding of Martian igneous processes as a whole (Howarth et al. ).…”

“…Analysis of new poikilitic shergottites like NWA 6342 is particularly important as poikilitic shergottites are thought to have formed from two stages of crystallization occurring at different depths in the Martian interior, making them good candidates for analyzing the evolution of Martian magmas (Howarth et al. ; Rahib et al. ).…”

“…Alternatively, the ITE‐enriched poikilitic shergottites are thought to have undergone a polybaric crystallization sequence based on differences in both oxygen fugacity and chemical composition between the poikilitic and nonpoikilitic domains (Howarth et al. ). The polybaric model has not yet been proposed for any intermediate poikilitic shergottites, and its applicability to NWA 6342's petrogenesis will be examined in this paper.…”