Martian Dunite NWA 2737: Petrographic constraints on geological history, shock events, and olivine color

Abstract

[1] Meteorite Northwest Africa (NWA) 2737 is the second known chassignite, an olivine-rich igneous rock with mineral compositions and isotopic ratios that suggest it formed on Mars. NWA 2737 consists of $85% vol. olivine (Mg#, molar Mg/(Mg + Fe), of 78.3 ± 0.4%), which is notable because it is black in hand sample and brown in thin section. Other minerals include chromite, pyroxenes (augite, pigeonite, orthopyroxene), and diaplectic glass of alkali-feldspar composition. Aqueous alteration is minimal and appears only as slight dissolution of glass. NWA 2737 formed by accumulation of olivine and chromite from a basaltic magma; the other minerals represent magma trapped among the cumulus grains. Minerals are compositionally homogeneous, consistent with chemical equilibration in late and postigneous cooling. Two-pyroxene thermometry gives equilibration temperatures $1150°C, implying a significant time spent at the basalt solidus. Olivine-spinel-pyroxene equilibria give $825°C (possibly the T of mesostasis crystallization) at an oxidation state of $QMF-1. This oxidation state is consistent with low Fe 3+ in olivine (determined by EMP, Mössbauer spectra, and synchrotron micro-XANES spectroscopy) and with $10% of the iron in pyroxene being Fe 3+ . NWA 2737 experienced two shock events. The first shock, to stage S5-S6, affected the olivine by producing in it planar deformation features, intense mosaicism and lattice strain, and abundant droplets of iron-nickel metal, 5-15 nm in diameter. At this stage the olivine became deeply colored, i.e., strongly absorbing at visible and near-infrared (NIR) wavelengths. This shock event and its thermal pulse probably occurred at $170 Ma, the Ar-Ar age of NWA 2737. The colored olivine is cut by ribbons of coarser, uncolored olivine with long axes along [100] and shorter axes on {021} planes: These are consistent with the easy slip law for olivine [100]{021}, which is activated at moderate strain rate at high temperature. Within these ribbons the olivine was coarsened and the iron metal globules coalesced to micron-sized grains. The ribbons also are mosaicized and cut by planar fractures, which bespeak a second shock event, possibly that of ejection from Mars. The deeply colored olivine in NWA 2737 is unusual and represents a new ''ground truth'' type for remote sensing of Mars. Understanding the occurrence of the brown color in olivine in NWA 2737 places important constraints on interpretation of optical measurements.

Petrology of Martian meteorite Northwest Africa 998

“…The high oxidation state of QFM + 1.8 inferred for the magmatic inclusion in NWA 998 augite, which also was observed for magmatic inclusions in Chassigny (Delaney and Dyar 2001;Treiman et al 2007), presents a puzzle. The absence of olivine in the NWA 998 inclusions may be important, since in the presence of olivine, titanomagnetite and low-Ca pyroxene buffer fO 2 by Reaction 1; in the absence of olivine, this reaction does not constrain fO 2 , which can then rise as the inclusion magma evolves.…”

Oxygen fugacity in the Martian mantle controlled by carbon: New constraints from the nakhlite MIL 03346

“…It seems that an origin near FMQ satisfies the bulk of the data currently available, and the reduced results are still being assessed. Estimates for Chassigny (Delaney and Dyar 2001) and Northwest africa (NWA) 2737 (Treiman et al 2007) indicate similarly oxidizing conditions (Fig. 4).…”

The Tissint Martian meteorite as evidence for the largest impact excavation

“…Shock features in Tissint are abundant and are suggestive of strong shock metamorphism. All olivines display unusual reddish-brown to black colour imparted by submicron Fe or Fe-O grains formed by shock 7 . Some olivines display undulatory extinction, mosaicism and planar deformation features (PDFs), especially as planar cleavages.…”