PHOSPHORUS ZONATION OF OLIVINE: A MAJOR KEY TO THE PETROGENESIS OF MARTIAN METEORITES

Within the last decade, the proliferation of applications of P-zoning in olivine has been extended from terrestrial igneous rocks to now include Martian meteorites. This work adds olivine shergottites Northwest Africa (NWA) 10170 and Dhofar 019 to the growing list of Martian shergottites that display olivine phosphorus zonation. An unique feature observed in many igneous olivines (Millman-Barris, 2008; Watson et al., 2015). Both meteorites contain olivine megacrysts, set in a fine-grained groundmass of olivine, pyroxene and maskelynite (shocked plagioclase). Broad compositional ranges are observed in olivine: Fo_45-78 in NWA 10170 and Fo_25-73 in Dhofar 019. Pyroxenes in NWA 10170 are normally zoned, with pigeonite (Wo_4-20En_44-68) and augite (Wo_20-30En_44-52), similar to Dhofar 019 pigeonite (Wo_9-15En_40-70) and augite (Wo_30-40En_40-50) compositions.

Elemental maps of phosphorus reveal two primary patterns: Type-1 olivines, typically the megacrysts, have P-free cores with strong oscillatory P-zoning near the rims. Type-2 olivines, typically in the groundmass, contain a phosphorus-rich core (<0.4 wt %), followed by oscillatory zoning. The P-rich core is observed as either a nucleus that matches current olivine edges or as an hour-glass-shaped “spine.” Zoning patterns also indicate glomero-crystic olivines (e.g., Ennis et al., 2010). The crystallization sequence of these meteorites is summarized from petrography and mineral chemical relationships: 1) olivine megacrysts (P-free), with Fo₇₂, ~500 ppm Ni, and chondrite-normalized V/Sc=16; 2) megacryst rims and groundmass olivine (P-rich) with ≤Fo₆₅, ~600 ppm Ni, and (V/Sc)_CI=4, with coeval pyroxene crystallization (~Mg# 70 and (V/Sc)_CI=69); and 3) continued crystallization produces evolved pyroxenes (Mg# 62) and Fe-rich olivine (~Fo₅₂).

Petrogenesis of the olivine shergottites has been hypothesized as the products of mixing of two end-members, namely basaltic and lherzolitic shergottites (Mittlefehldt et al., 1999; Barrat et al., 2002a), or alternatively they represent re-entrained cumulates (Usui et al., 2008, 2009). Our work enhances three-stage models previously proposed by Usui et al. (2009) and Howarth et al. (2014), which can now be re-examined using the P-zoning record of olivine, combined with their minor- and trace-element contents.