Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 302 (2021) 120–140 www.elsevier.com/locate/gca Exploring relationships between shock-induced microstructures and H2O and Cl in apatite grains from eucrite meteorites T.J. Barrett a,⇑,A.Cˇ ernok a, G. Degli-Alessandrini a, X. Zhao a, M. Anand a,b, I.A. Franchi a, J.R. Darling c a The Open University, School of Physical Sciences, Walton Hall, Milton Keynes MK7 6AA, UK b Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK c University of Portsmouth, School of the Environment, Geography and Geosciences, Burnaby Road, Portsmouth PO1 3QL, UK Received 21 November 2020; accepted in revised form 15 March 2021; Available online 26 March 2021 Abstract The abundance and isotopic composition of volatile elements in meteorites is critical for understanding planetary evolution, given their importance in a variety of geochemical processes. There has been significant interest in the mineral apatite, which occurs as a minor phase in most meteorites and is known to contain appreciable amounts of volatiles (up to wt. % F, Cl, and OH). Impact-driven shock metamorphism, pervasive within many meteorites, can potentially modify the original signatures of volatiles through processes such as devolatilization and diffusion. In this study, we investigate the microstructures of apatite grains from six eucrites across a broad range of shock stages (S1–S5) using electron backscatter diffraction (EBSD) to explore shock-induced crystallographic features in apatite. New Cl and H abundance and isotopic composition data were collected on moderate to highly shocked samples (S3-S5) by Nano Secondary Ion Mass Spectrometry (NanoSIMS). Previously reported vola- tile data for S1 and S2 eucrites were integrated with EBSD findings in this study. Our findings indicate that apatite microstructures become increasingly more complex at higher shock stages. At low shock stages (S1–S2) samples display brecciation and fracturing of apatite. Samples in S3 and S4 display increasing crystal plastic deformation indicated by increasing spread in pole figures. At the higher shock stages (S4/S5) there is potential recrystallisation demonstrated by an increased density of subgrain boundaries. The Cl content and d37Cl values of highly-shocked apatite grains range from 940 to 1410 ppm and À3.38 to +7.70‰, respec- tively, within the range observed in less-shocked eucrites. In contrast, H2O abundances are more variable (from 186 to 4010 ppm), however, the measured water content still falls within the range previously reported for low-shock eucrites. The measured dD values range from À157 to +163‰, also within the range of values from known low-shock basaltic eucrites. Weighted averages for both isotopic systems (dD À122 ± 20‰, d37Cl + 1.76 ± 0.66‰) are consistent with the range displayed in other inner Solar System bodies. NanoSIMS isotope images of apatite grains display heterogeneity in their Cl abundance at the nanoscale which increases in complexity with shock stage. This increasing complexity, however, does not correlate with defor- mation microstructures observed in EBSD or with the Cl isotopic composition at either an inter-grain or intra-grain scale. These findings are similar to analyses of variably shocked lunar apatite and, therefore, apatite appears to be a robust recorder of Cl and H (at least at spatial resolution and precision currently achievable by NanoSIMS) on airless bodies, despite intensive shock. Ó 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/). Keywords: Apatite; Volatiles; Hydrogen; Chlorine; Shock; EBSD; NanoSIMS; Eucrites ⇑ Corresponding author. E-mail address: [email protected] (T.J. Barrett). https://doi.org/10.1016/j.gca.2021.03.018 0016-7037/Ó 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). T.J. Barrett et al. / Geochimica et Cosmochimica Acta 302 (2021) 120–140 121 1. INTRODUCTION techniques such as Secondary Ion Mass Spectrometry (SIMS) (e.g. Hallis, 2017; Shearer et al., 2018; McCubbin The Howardite-Eucrite-Diogenite (HED) meteorites and Barnes, 2019). Whilst previous works considered the comprise the largest suite of crustal and sub-crustal rocks geological context of the apatite grain and the surrounding available from a differentiated basaltic asteroid (likely 4 mineralogy (e.g. Potts et al., 2016), less attention has been Vesta) (e.g. McSween et al., 2013). The eucrites, which are given to understanding as to how the internal structure of composed primarily of plagioclase and pigeonite, are also apatite and its volatile inventory may be influenced by ther- some of the oldest basaltic igneous rocks in the Solar System mal metamorphism (Sarafian et al., 2019; Stephant et al., (typical ages range from 4547 to 4559 million years (Ma) with 2021) and shock deformation (Cˇ ernok et al., 2019, 2020; 2r uncertainties usually ranging from 5 to 13 Ma; Misawa Darling et al., 2021). et al., 2005; Zhou et al., 2013; Touboul et al., 2015). As such, The HEDs can be subdivided petrologically into basaltic understanding the evolution of volatile elements (e.g. hydro- eucrites (basalts), cumulate eucrites (gabbros), polymict gen and chlorine), their abundance, and isotopic composition eucrites (breccias of basaltic and sometimes cumulate within these samples can provide insight into the origin of eucrites), diogenites (orthopyroxenites, occasional harzbur- volatiles, the material accreted into differentiated bodies, gites, and rare dunites and norites), and howardites (poly- and primary/secondary processes that operated on asteroid mict breccias composed of eucrites and diogenites some of parent bodies early in Solar System history (e.g. Barrett which contain elevated noble gas contents, owing to solar et al., 2016, 2019; Sarafian et al., 2017a, 2017c; Stephant wind implantation, indicative of being a regolith; Warren et al., 2021). These elements, however, can undergo fraction- et al., 2009; McSween et al., 2019). The basaltic eucrites, ation during the evolution of the parent body (e.g. magma which are the most common of all eucrites, are typically fine ocean degassing, eruption, large impact-melt sheet degas- to medium-grained igneous rocks, composed mainly of sing; Roszjar et al., 2011; Sarafian et al., 2017b; Alexander pigeonite and calcic plagioclase (ranging from bytownite et al., 2018; McCubbin and Barnes, 2019) potentially mask- to anorthite; Mittlefehldt, 2015). Cumulate eucrites are ing their original signature. Despite this, each isotope system thought to be mixtures of cumulus minerals (pyroxene can provide unique and complementary perspectives on these and plagioclase) and trapped melt whose parent magmas important processes. were similar to basaltic eucrites (Treiman, 1997; Barrat Owing to the large difference in relative mass between et al., 2000; Mittlefehldt and Lindstrom, 2003; Barrat, hydrogen (1H) and deuterium (2H or D), it is possible for 2004). significant fractionation to occur. It is because of this signif- Basaltic eucrites can be further characterised based on icant fractionation that H isotopes can be used to ‘finger- their bulk chemical compositions into a Main Group- print’ source(s) of water and/or any processes that may Nuevo Laredo (hereafter referred to as Main Group), the have affected the sample (e.g. Hallis, 2017). The hydrogen incompatible element rich Stannern trend, and the incom- isotopic composition of the inner Solar System is highly patible element depleted residual eucrites (e.g., Stolper, variable, ranging over several orders of magnitude, 1977; Mayne et al., 2009; Yamaguchi et al., 2013). The from near protosolar in composition (dD À700‰, Main Group comprises the majority of the basaltic eucrites D/H 4.67 Â 10À5, please see methods Section 3.3.2 for and is widely accepted as a fractional crystallisation trend derivation of the standard delta ‘d’ notation in per mil) (Stolper, 1977; Warren and Jerde, 1987; Mittlefehldt and to several thousand per mil in the martian atmosphere Lindstrom, 2003). The Stannern trend eucrites have similar (dD >+3000‰, D/H > 6.23 Â 10À4)(McCubbin and major element abundances to Main Group eucrites but Barnes, 2019 and references therein). have higher abundances of Ti and incompatible elements, The chlorine isotopic composition, on the other hand, is enrichments that are not linked to Mg# (Barrat et al., relatively restricted for terrestrial rocks, martian meteorites, 2000; Hutchison, 2004; Barrat et al., 2007). Barrat et al. and all chondritic meteorites groups (d37Cl À5.6 to (2007) suggested that the contamination of Main Group +3.8‰, although one data point for a martian meteorite eucrites by crustal melts could explain both the elevated does reach +8.6‰, please see Section 3.3.1 for derivation incompatible element concentrations and the distinctive of d37Cl) (e.g. Sharp et al., 2013, 2016; Bellucci et al., Eu, Sr, and Be anomalies shown by Stannern trend eucrites. 2017). Samples from both the Moon and eucrites (likely This crustal partial melt would leave behind a residuum from 4 Vesta), however, display a significant range in Cl with LREE depletion, which Yamaguchi et al. (2009) isotopic composition potentially related to degassing at var- observed in several eucrites and believed to be a third geo- ious points within their histories (d37Cl À4 to +81.1‰ chemical trend dubbed the ‘residual eucrites’. and À4 to +39‰, respectively)(e.g. Boyce et al., 2015; When classifying eucrites for thermal metamorphism, Barnes et al., 2016; Sarafian et al., 2017b; Barrett et al., Takeda and Graham (1991) separated the eucrites into six 2019; Wang et al., 2019). types (1–6) based on the characteristics of pyroxene. Type Apatite [Ca5(PO4)3(F, Cl, OH)] is a common phosphate one represents the lowest level of thermal metamorphism mineral in planetary materials, known to contain apprecia- indicated by extensively zoned pyroxene; conversely, type ble amounts of volatiles (up to wt.% levels F, Cl, and OH; six pyroxene contain easily resolvable exsolution lamellae e.g.