Petrogenesis of olivine-phyric shergottites Sayh al Uhaymir 005 and Elephant Moraine A79001 lithology A

Abstract

Martian meteorites Sayh al Uhaymir (SaU) 005 and lithology A of EETA79001 (EET-A) belong to a newly emerging group of olivine-phyric shergottites. Previous models for the origin of such shergottites have focused on mixing between basaltic shergottite-like magmas and lherzolitic shergottite-like material. Results of this work, however, suggest that SaU 005 and EET-A formed from olivine-saturated magmas that may have been parental to basaltic shergottites. SaU 005 and EET-A have porphyritic textures of large (up to ∼3 mm) olivine crystals (∼25% in SaU 005; ∼13% in EET-A) in finer-grained groundmasses consisting principally of pigeonite (∼50% in SaU 005; ∼60% in EET-A), plagioclase (maskelynite) and < 7% augite. Low-Ti chromite occurs as inclusions in the more magnesian olivine, and with chromian ulvöspinel rims in the more ferroan olivine and the groundmass. Crystallization histories for both rocks were determined from petrographic features (textures, crystal shapes and size distributions, phase associations, and modal abundances), mineral compositions, and melt compositions reconstructed from magmatic inclusions in olivine and chromite. The following observations indicate that the chromite and most magnesian olivine (Fo 74–70 in SaU 005; Fo 81–77 in EET-A) and pyroxenes (low-Ca pyroxene [Wo 4–6] of mg 77–74 and augite of mg 78 in SaU 005; orthopyroxene [Wo 3–5] of mg 84–80 in EET-A) in these rocks are xenocrystic. (1) Olivine crystal size distribution (CSD) functions show excesses of the largest crystals (whose cores comprise the most magnesian compositions), indicating addition of phenocrysts or xenocrysts. (2) The most magnesian low-Ca pyroxenes show near-vertical trends of mg vs. Al 2O 3 and Cr 2O 3, which suggest reaction with a magma. (3) In SaU 005, there is a gap in augite composition between mg 78 and 73. (4) Chromite cores of composite spinel grains are riddled with cracks, indicating that they experienced some physical stress before being overgrown with ulvöspinel. (5) Magmatic inclusions are absent in the most magnesian olivine, but abundant in the more ferroan, indicating slower growth rates for the former. (6) The predicted early crystallization sequence of the melt trapped in chromite (the earliest phase) in each rock produces its most magnesian olivine-pyroxene assemblage. However, in neither case is the total crystallization sequence of this melt consistent with the overall crystallization history of the rock or its bulk modal mineralogy. Further, the following observations indicate that in both SaU 005 and EET-A the fraction of solid xenocrystic or xenolithic material is small (in contrast to previous models for EET-A), and most of the material in the rock formed by continuous crystallization of a single magma (possibly mixed). (1) CSD functions and correlations of crystal size with composition show that most of the olivine (Fo 69–62 in SaU 005; Fo 76–53 in EET-A) formed by continuous nucleation and growth. (2) Groundmass pigeonites are in equilibrium with this olivine, and show continuous compositional trends that are typical for basalts. (3) The CSD function for groundmass pigeonite in EET-A indicates continuous nucleation and growth (Lentz and McSween, 2000). (4) The melt trapped in olivine of Fo 76 to 67 in EET-A has a predicted crystallization sequence similar to that inferred for most of the rock and produces an assemblage similar to its modal mineralogy. (5) Melt trapped in late olivine (Fo ∼ 64) in SaU 005 has a composition consistent with the inferred late crystallization history of the rock. The conclusion that only a small fraction of either SaU 005 or EET-A is xenocrystic or xenolithic implies that both rocks lost fractionated liquids in the late stages of crystallization. This is supported by: (1) high pigeonite/plagioclase ratios; (2) low augite contents; and (3) olivine CSD functions, which show a drop in nucleation rate at high degrees of crystallization, consistent with loss of liquid. For EET-A, this fractionated liquid may be represented by EET-B.