Clast populations within the cm2.2 brecciated carbonaceous chondrite aguas zarcas: implications for understanding aqueous alteration on ryugu

Introduction: With the successful return of the JAXA Hayabusa2 sample collection from the Ryugu asteroid, the study of extra-terrestrial carbonaceous samples is now, more than ever, relevant in answering one of planetary science’s most prominent question: the origin of Earth’s water [1, 2]. Carbonaceous chondrites have a relatively high water content (~ 9 wt%; [3]) and show signs of having undergone various degrees of aqueous alteration. Such alteration is thought to have stemmed from a heating event that occurred the CM parent-body [4]. To this day, the intensity and duration of this heating process remain poorly constrained. Aguas Zarcas was originally classified as CM2 (Mighei-like) following the petrographic description by L. Garvie (Arizona State University, USA) and oxygen isotope analysis by laser fluorination by K. Ziegler (University of New Mexico, USA) [5]. Since then, Aguas Zarcas was further categorized as a CM2.2 brecciated carbonaceous chondrite (according to the classification provided by [3, 6]), using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) and Raman Spectroscopy [7]. In this study, we further investigate the brecciated nature of Aguas Zarcas by characterising the different grades of aqueous alteration of its larger clasts. Materials and Methods: This investigation compiles the study of three polished thin sections and two polished blocks from Aguas Zarcas. Each sample was fully mapped by EDS using a Zeiss Sigma Variable Pressure Analytical Scanning Electron Microscope (SEM; accelerating voltage of 20 keV; carbon coated ~10 nm thickness). Clasts within the samples were identified using BSE (Backscattered Electron) images in conjunction with the geochemical maps. Size and shape measurements (i.e. surface area, major and minor axis length, circularity) of these objects were made using ImageJ. Several locations of interest were then further mineralogically analysed by Raman spectroscopy using a Renishaw InVia coupled with a 512 nm laser source (power up to 45W and 2400 grating). Results: Aguas Zarcas’s clastic matrix is fine grained, dominated by phyllosilicates, Ca-carbonates and iron sulphides (cf. Fig. 1). The extent of aqueous alteration throughout the samples is relatively homogeneous and due to the low clast density, the chondrite verifies its CM2.2 classification [3, 6, 7].