The shocking state of apatite and merrillite in shergottite Northwest Africa 5298 and extreme nanoscale chlorine isotope variability revealed by atom probe tomography

J.R Darling,L.F White,T Kizovski,A Černok,D.E Moser,K.T Tait,J Dunlop,B Langelier,J.O Douglas,X Zhao,I.A Franchi,M Anand

doi:10.1016/j.gca.2020.11.007

Abstract

The elemental and chlorine isotope compositions of calcium-phosphate minerals are key recorders of the volatile inventory of Mars, as well as the planet’s endogenous magmatic and hydrothermal history. Most martian meteorites have clear evidence for exogenous impact-generated deformation and metamorphism, yet the effects of these shock metamorphic processes on chlorine isotopic records contained within calcium phosphates have not been evaluated. Here we test the effects of a single shock metamorphic cycle on chlorine isotope systematics in apatite from the highly shocked, enriched shergottite Northwest Africa (NWA) 5298. Detailed nanostructural (EBSD, Raman and TEM) data reveals a wide range of distributed shock features. These are principally the result of intensive plastic deformation, recrystallization and/or impact melting. These shock features are directly linked with chemical heterogeneities, including crosscutting microscale chlorine-enriched features that are associated with shock melt and iron-rich veins. NanoSIMS chlorine isotope measurements of NWA 5298 apatite reveal a range of δ37Cl values (−3 to 1‰; 2σ uncertainties <0.9‰) that is almost as large as all previous measurements of basaltic shergottites, and the measured δ37Cl values can be readily linked with different nanostructural states of targeted apatite. High spatial resolution atom probe tomography (APT) data reveal that chlorine-enriched and defect-rich nanoscale boundaries have highly negative δ37Cl values (mean of −15 ± 8‰). Our results show that shock metamorphism can have significant effects on chemical and chlorine isotopic records in calcium phosphates, principally as a result of chlorine mobilization during shock melting and recrystallization. Despite this, low-strain apatite domains have been identified by EBSD, and yield a mean δ37Cl value of −0.3 ± 0.6‰ that is taken as the best estimate of the primary chlorine isotopic composition of NWA 5298. The combined nanostructural, microscale-chemical and nanoscale APT isotopic approach gives the ability to better isolate and identify endogenous volatile-element records of magmatic and near-surface processes as well as exogenous, shock-related effects.

Highlights

The calcium phosphate minerals apatite and merrillite have become very important to our understanding of planetary evolution and geological processes
Our results demonstrate that shock metamorphism has a major effect on the nanostructural state of Ca-phosphates, and that shock-induced structures lead to chemical and isotopic heterogeneities in apatite at the micro- to nano-scale
Detailed Scanning electron microscopy (SEM) imaging was undertaken on six apatite and ten merrillites, from which four of each were selected for electron backscatter diffraction (EBSD) analysis

Summary

Introduction

The calcium phosphate minerals apatite and merrillite have become very important to our understanding of planetary evolution and geological processes. McCubbin et al, 2015; McCubbin and Jones, 2015; Barnes et al, 2016; Williams et al, 2016; Bellucci et al, 2017), provide thermochronological constraints on endogenic geological processes and meteorite impact events (Grange et al, 2013; Snape et al, 2016; Min et al, 2017), and track magmatic evolution through trace element and isotopic studies (Bruand et al, 2017) This is due to the incorporation of volatile species in apatite group minerals, hexagonal Ca5[PO4]3[F,Cl,OH], and the fact that both apatite and merrillite, trigonal Ca9Na(Fe,Mg)(PO4), are the principal carriers of a wide-range of trace elements such as rare earth elements, uranium and thorium in many planetary crustal rocks. This has led to extensive efforts to characterize the water content, D/H ratio and chlorine isotopic composition of the martian mantle (e.g. Hallis et al, 2012; Sharp et al, 2016; Weis et al, 2017; Bellucci et al, 2017; Barnes et al, 2020)

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Discussion

Conclusion