¹Z. Vaci,²Z. Tian,²P. Koefoed,³M. Habermann,⁴M. Humayun,³K. Ziegler,⁵H. Busemann,⁵D. Krietsch,⁶J. M. D. Day,²K. Wang
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70199]
¹Institute of Petrology and Structural Geology, Charles University, Prague, Czech Republic
²Department of Earth, Environmental, and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri, USA
³Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
⁴National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida, USA
⁵Institute of Geochemistry and Petrology, ETH Z€urich, Z€urich, Switzerland
⁶Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA
Published by arrangement with John Wiley and Sons

Some of the oldest igneous rocks in the Solar System include evolved and silica-rich achondrites that originate from parent bodies less than 1000 km in diameter, referred to as planetesimals. While Earth was initially in a molten state and required continental crust formation and plate tectonics to generate andesite bulk compositions, evolved and silica-rich achondrites likely formed from smaller degrees of melting and differentiation on initially chondritic parent bodies. Petrographic descriptions, bulk and in situ chemical analyses, oxygen and potassium isotope measurements, and noble gas analyses are presented to constrain the petrogenesis and possible associations of a suite of evolved and silica-rich achondrites including a trachyandesitic clast from the Almahata Sitta fall (ALM-A), Northwest Africa (NWA) 6698, NWA 11119, its launch pair NWA 11558, NWA 11575, and Graves Nunataks 06128 and 06129. In addition, leaching experiments were conducted that included terrestrial samples to examine the effects potential weathering-induced alteration might have on potassium isotope compositions. The measured potassium isotopic compositions do not covary with volatile depletion, as found when comparing samples from Earth, the Moon, Mars, and the asteroid Vesta, indicating that the planetary depletion trend observed in larger bodies does not apply in the absence of complete planetary differentiation. Modeled noble gas retention ages confirm the ancient formation times of several of these achondrites, while cosmic ray exposure ages suggest separation from their parent bodies in the past ~25 million years.

^1,2Robert W. Nicklas,²Dylan M. Seal,²Melody Z. Chen,²Kyra L. Schroeder,³James M. D. Day,⁴Ben G. Rider-Stokes,⁵Anthony B. Love,⁴James Malley
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70202]
¹Lunar and Planetary Institute, USRA, Houston, Texas, USA
²Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, Massachusetts, USA
³Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA
⁴School of Physical Sciences, The Open University, Milton Keynes, UK
⁵Department of Geological and Environmental Sciences, Appalachian State University, Boone, North Carolina, USA
Published by arrangement with John Wiley & Sons

As the most common samples available from Mars, shergottites offer important constraints on the igneous history of the planet into the Amazonian epoch. The newly recognized shergottite Northwest Africa (NWA) 13440 is here classified as a gabbroic shergottite and is likely launched-paired with NWA 6963, exhibiting many of the unusual textural features of that sample. The Sm-Nd isotope systematics of NWA 13440 yielded an errorchron age of 206 ± 34, with an εNdi = −7.1. This age and εNdi, coupled with a bulk rock (La/Yb)N of 1.02, allow for its classification as an enriched shergottite. The presence of unusual augite inclusions in pigeonite laths testifies to the importance of undercooling and nonequilibrium crystallization early in the history of the parental magma of the meteorite. Additionally, Si-rich mesostasis consisting of fine-grained irregular quartz-alkaline feldspar intergrowths suggests extreme fractional crystallization of the final few percent of melt. Shock textures indicate a moderate shock stage of approximately M-S4. The discovery of NWA 13440 supports the model that many of the enriched shergottites are the martian equivalent of a continental flood basalt province.