¹Ninna K. Jensen, ²Alexander A. Nemchin, ³Gavin Kenny, ³Martin J. Whitehouse, ¹James N. Connelly, ⁴Takashi Mikouchi, ¹Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.11.014]
¹Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, 1350 Copenhagen, Denmark
²School of Earth and Planetary Sciences (EPS), Curtin University, GPO Box U1987, Perth, WA 6845, Australia
³Swedish Museum of Natural History, SE-104 Stockholm, Sweden
⁴The University Museum, The University of Tokyo, 118-0033 Tokyo, Japan
Copyright Elsevier

Impact events were frequent in the early history of our Solar System, and the dynamics of planetary crust formation were, consequently, substantially different from the processes that dominate today. Mars, a planet with stagnant lid tectonics and a unique preservation of ancient surface terrains, provides an outstanding opportunity to investigate the early processes related to the formation and reshaping of the first crust. Northwest Africa (NWA) 7034 and paired meteorites (such as NWA 7533) are fragments of polymict, regolith breccia that provide a tangible record of the ancient, brecciated crust on Mars. Zircon and baddeleyite from NWA 7034/7533 record evidence for two events of intense crustal reworking at 4442 ± 17 and 4474 ± 10 million years ago (Ma) triggered by impacts, placing important constraints on the timing and the dynamics of early crust formation on Mars. To date, only few studies have focussed on the geochronology of the igneous clasts present within NWA 7034 and its pairs. Although these studies consistently report ancient ages (∼4.4 Ga) for basaltic, basaltic andesitic and monzonitic clasts, the associated precisions are generally too low to link the different lithologies with the two age peaks inferred from NWA 7034/7533 zircon and baddeleyite. Here, we conduct an isotopic and petrographic study of igneous clasts from NWA 7533 to shed further light on the timing and nature of crustal reworking in the early history of Mars. We show that six out of seven investigated igneous clasts, representing at least four distinct types, record undisturbed Lu-Hf isotope systematics that indicate contemporaneous formation. Together with two zircons hosted in basalt and basaltic andesite clasts, these igneous clasts yield an isochron age of 4440 ± 41 Ma (2SE, MSWD = 2.1). This isochron age is consistent with clast ages inferred from zircon U-Pb geochronology, and altogether the available age constraints for the lithic components in NWA 7533 indicate that they derive from the younger of the two peaks of intense crustal reworking on early Mars (4442 ± 17 Ma). The initial εHf values (the 176Hf/177Hf ratio in the sample normalised to that of the chondritic uniform reservoir at the time of crystallisation in parts per ten thousand) of the igneous clasts range between −2.07 and −0.74, consistent with crystallisation from enriched source melts deriving from impact-induced reworking of the crust. The mean Lu-Hf isotope composition of the igneous clasts constrains the timing of primordial crust formation and reveal planet formation and differentiation within the first 10 Myr of the history of the Solar System, in consistence with the conclusions in earlier reports. The results presented here suggest a 176Lu/177Hf ratio of ∼ 0.0135 or higher in the primordial martian crust.

^1,2,3Damanveer S. Grewal, ³Surjyendu Bhattacharje, ³Gabriel-Darius Mardaru,
³Paul D. Asimow
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.11.011]
¹School of Molecular Sciences, Arizona State University, Tempe, AZ 85281, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
Copyright Elsevier

Understanding the relationships between the nitrogen (N) isotope ratios of early solar system planetesimals and terrestrial reservoirs is crucial for tracing the origin of volatiles on Earth. The Earth primarily grew from planetesimals and planetary embryos that accreted rapidly (within ∼1–2 Ma after CAIs) in the inner solar system, also known as the non-carbonaceous (NC) reservoir. Magmatic iron meteorites, which sample the metallic cores of the earliest solar system planetesimals, have emerged as a promising proxy in this exercise. NC irons are distinctly ¹⁵N-poor compared to their CC (carbonaceous or outer solar system) counterparts. However, the utility of this proxy is limited by the lack of knowledge of N isotope fractionation during core crystallization. Using high pressure-high temperature experiments, we show that equilibrium N isotopic fractionation between solid and liquid metal (Δ¹⁵N^{solid–liquid} = δ¹⁵N^solid − δ¹⁵N^liquid) is limited (≤1.2 ‰) under conditions relevant for core crystallization. This, combined with the siderophile character of N and limited equilibrium N isotope fractionation during core-mantle differentiation, suggests that the δ¹⁵N values of iron meteorites accurately represent the N isotopic composition of their parent bodies. Unlike the variation in the N isotope ratios of NC and CC chondrites, which can be attributed to the effects of parent-body processes acting on organic precursors, the ¹⁵N-poor nature of NC irons relative to CC irons likely offers the most definitive evidence for the distinct N isotopic compositions of the earliest inner and outer solar system planetesimals. The N isotopic composition of Earth’s primordial mantle (δ¹⁵N = <−40 ‰) suggests that it retains the memory of the early accretion of ¹⁵N-poor NC iron meteorite parent body-like planetesimals. The early accreted ¹⁵N-poor nitrogen may be stored in the deep mantle, segregated into the core, or lost to space during atmospheric loss caused by impacts. This signature was overprinted by the subsequent accretion and admixing of CC materials, which is reflected in the relatively ¹⁵N-rich nature of Earth’s atmosphere (δ¹⁵N = 0) and convecting mantle (δ¹⁵N = −5 ‰).