^1,2S. Benaroya,^1,3,4,5J. Gross,¹P. Burger,⁶M. Righter,⁶T.J. Lapen,⁷S. Eckley
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.02.004]
¹Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, USA
²Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton Alberta, Canada
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, USA
⁴Lunar and Planetary Institute, Houston, TX, USA
⁵Astromaterials Research and Exploration Science Division, NASA JSC, Houston, TX, USA
⁶Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA
⁷Jacobs – JETS, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX, USA
Copyright Elsevier

Petrologic investigations of martian meteorite Northwest Africa (NWA) 13227 indicate it is an olivine-gabbroic shergottite, a relatively new shergottite group, which differs from previously described gabbroic shergottites due to relatively high quantities of olivine. NWA 13227 is comprised of phenocrystic, oscillatory-zoned pyroxene and olivine, set in a matrix of maskelynite, Fe-Cr-Ti oxides, phosphates, and sulfides. It displays gabbroic and poikilitic textures in 2D from back-scattered electron images, and in 3D from X-ray Computed Tomography (XCT) imaging, suggesting affinities to both poikilitic and gabbroic shergottites. Measured εHf and εNd values of bulk rock (-19.7 and −5.9, respectively) and its chondrite-normalized La/Yb ratio of 1.13 indicate the specimen is derived from a mantle reservoir relatively enriched in incompatible trace elements and is similar to that which produced most ‘enriched shergottites.’ Based on the Ti/Al ratio of pyroxene, phosphorous zoning in olivine, and minor components in phosphates and oxides, we infer that NWA 13227 began crystallizing under reducing conditions of QFM–2.6 and temperatures of ∼ 1100 °C, consistent with conditions in Mars’ lower crust/upper mantle. The sample finished crystallizing at or near the surface under redox conditions between QFM–0.5 to QFM–0.1 and temperatures of ∼ 850 °C. The volatile element compositions in apatite indicate that NWA 13227 experienced degassing during the last stages of crystallization. The timing of crystallization is estimated at 225 Ma ± 50 Ma using a Lu-Hf and Sm-Nd source versus age model.