^1,2,3Benjamin E. Cohen,^1,4Darren F. Mark,⁵William S. Cassata,³Lara M. Kalnins,²Martin R. Lee,^2,6Caroline L. Smith,^7,8David L. Shuster
Earth and Planetary Science Letters 621, 118373 Open Access Link to Article [https://doi.org/10.1016/j.epsl.2023.118373]
¹Scottish Universities Environmental Research Centre (SUERC), East Kilbride, UK
²School of Geographical and Earth Sciences, University of Glasgow, UK
³School of GeoSciences, University of Edinburgh, UK
⁴Department of Earth and Environmental Sciences, University of St Andrews, UK
⁵Lawrence Livermore National Laboratory, CA, USA
⁶Department of Earth Sciences, The Natural History Museum, London, UK
⁷Department of Earth and Planetary Science, University of California, Berkeley, CA, USA
⁸Berkeley Geochronology Center, Berkeley, CA, USA
Copyright Elsevier

The shergottites are the most abundant and diverse group of Martian meteorites and provide unique insights into the mafic volcanic and igneous history of Mars. Their ages, however, remain a source of debate. Different radioisotopic chronometers, including 40Ar/39Ar, have yielded discordant ages, leading to conflicting interpretations on whether the shergottites originate from young (mostly <700 Ma) or ancient (>4,000 Ma) Martian volcanoes. To address this issue, we have undertaken an 40Ar/39Ar investigation of seven shergottite meteorites utilizing an innovative approach to correcting data for cosmogenic isotope production and resolution of initial trapped components which, crucially, do not require assumptions concerning the sample’s geologic context. Our data yield statistically robust 40Ar/39Ar isochron ages ranging from 161 ± 9 Ma to 540 ± 63 Ma (2σ), synchronous with the U-Pb, Rb-Sr, and Sm-Nd ages for the respective meteorites. These data indicate that, despite experiencing shock metamorphism, the shergottites were sourced from the youngest volcanoes on Mars.

^1,2L. Folco,³P. Rochette,^1,2M. D’Orazio,^1,2M. Masotta
Geochimica et Cosmochimica Acta (in Press) Open Aceess Link to Article
[https://doi.org/10.1016/j.gca.2023.09.018]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, Pisa, Italy
²CISUP, Centro per la Integrazione della Strumentazione dell’Università di Pisa, Lungarno Pacinotti, Pisa, Italy
³Aix-Marseille Université, CNRS, IRD, INRAE, UM 34 CEREGE, Aix-en-Provence, France
Copyright Elsevier

In the Earth’s crust, Ni is generally concentrated in mafic and ultramafic rocks and is coupled with Mg in Mg-olivine, Mg-pyroxene and spinel. Whether the Ni-rich, and in general, the mafic component of Australasian tektites and microtektites is terrestrial or meteoritic is still debated. To test the origin of the Ni-rich component, we studied the Ni versus Mg distribution in a large geochemical database of Australasian tektites (n = 208) and microtektites (n = 238) from the literature. Nickel contents of up to 428 µg/g in tektites and 678 µg/g in microtektites covary with Mg in tektites and in most (∼85%) of the microtektites defining a mixing trend between crustal and chondritic values, thereby documenting the chondritic origin of the Ni-rich component in Australasian tektites/microtektites. Mixing calculations indicate up to 4% and up to 6% by weight chondritic component in tektites and microtektites, respectively. A possible mafic component of terrestrial origin is observed in a minority of tektite and microtektite specimens. This finding is consistent with previous works suggesting a possible occurrence of a chondritic signature in high-Ni tektites, based on the study of highly siderophile elements and Os isotopes, and high-Ni microtektites, based on Ni, Co, and Cr ratios. The combined geochemical and isotopic analysis of high-Ni tektites and microtektites in collections worldwide may thus reveal the chondritic impactor type that generated one of the presumably largest impacts in the Cenozoic.