Petrogenesis and Provenance of Ungrouped Achondrite Northwest Africa 7325 from Petrology, Trace Elements, Oxygen, Chromium and Titanium Isotopes, and Mid-IR Spectroscopy

1,2Cyrena A. Goodrich, 3Noriko T. Kita, 4Qing-Zhu Yin, 4Matthew E. Sanborn, 4Curtis D. Williams, 3Daisuke Nakashima, 2Melissa D. Lane, 1,5Shannon Boyle
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.021]
1Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, TX 77058, USA
2Planetary Science Institute, 1700 E. Ft. Lowell Drive, Tucson, AZ 85719, USA
3WiscSIMS, University of Wisconsin-Madison, Madison, WI 53706, USA
4Department of Earth and Planetary Sciences, University of California at Davis, Davis, CA 95616, USA
5Department of Earth and Planetary Sciences, Rutgers University, Busch Campus, 610 Taylor Road, Piscataway, NJ 08854, USA
Copyright Elsevier

Northwest Africa (NWA) 7325 is an ungrouped achondrite that has recently been recognized as a sample of ancient differentiated crust from either Mercury or a previously unknown asteroid. In this work we augment data from previous investigations on petrography and mineral compositions, mid-IR spectroscopy, and oxygen isotope compositions of NWA 7325, and add constraints from Cr and Ti isotope compositions on the provenance of its parent body. In addition, we identify and discuss notable similarities between NWA 7325 and clasts of a rare xenolithic lithology found in polymict ureilites.

NWA 7325 has a medium grained, protogranular to poikilitic texture, and consists of 10-15 vol.% Mg-rich olivine (Fo 98), 25-30 vol.% diopside (Wo 45, Mg# 98), 55-60 vol.% Ca-rich plagioclase (An 90), and trace Cr-rich sulfide and Fe,Ni metal. We interpret this meteorite to be a cumulate that crystallized at ⩾1200 °C and very low oxygen fugacity (similar to the most reduced ureilites) from a refractory, incompatible element-depleted melt. Modeling of trace elements in plagioclase suggests that this melt formed by fractional melting or multi-stage igneous evolution. A subsequent event (likely impact) resulted in plagioclase being substantially remelted, reacting with a small amount of pyroxene, and recrystallizing with a distinctive texture.

The bulk oxygen isotope composition of NWA 7325 plots in the range of ureilites on the CCAM line, and also on a mass-dependent fractionation line extended from acapulcoites. The ε54Cr and ε50Ti values of NWA 7325 exhibit deficits relative to terrestrial composition, as do ordinary chondrites and most achondrites. Its ε54Cr value is distinct from that of any analyzed ureilite, but is not resolved from that of acapulcoites (as represented by Acapulco).

In terms of all these properties, NWA 7325 is unlike any known achondrite. However, a rare population of clasts found in polymict ureilites (“the magnesian anorthitic lithology”) are strikingly similar to NWA 7325 in mineralogy and mineral compositions, oxygen isotope compositions, and internal textures in plagioclase. These clasts are probably xenolithic in polymict ureilites, and could be pieces of NWA 7325-like meteorites.

Using constraints from chromium, titanium and oxygen isotopes, we discuss two possible models for the provenance of the NWA 7325 parent body: 1) accretion in the inner solar system from a reservoir similar to that of acapulcoites in Δ17O, ε54Cr and ε50Ti; or 2) early (< 1 Ma after CAI formation) accretion in the outer solar system (beyond the snow line), before 54Cr and 50Ti anomalies were introduced to this region of the solar system. The mid-IR emission spectrum of NWA 7325 obtained in this work matches its modal mineralogy, and so can be compared with spectra of new meteorites or asteroids/planets to help identify similar materials and/or the parent body of NWA 7325

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