Primary and secondary features of chondrules and refractory inclusions in the anomalous CM chondrite Northwest Africa (NWA) 11346: insights into the early Solar System alteration of C-chondrite asteroids

Abstract

Carbonaceous chondrites (CC) are a group of primitive meteorites that have remained nearly unchanged since the Solar System’s formation. As such, CCs are extensively studied for the insights that they give into the earliest time of Solar System history. CM chondrites, the most abundant type of CC recovered, have water-bearing minerals and organic compounds that could provide insight into Earth’s formation and, possibly, the origin of life. CM chondrites typically have abundant matrix, small chondrules (~30vol%) of various types, and refractory inclusions such as calcium-aluminum rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs). The CM chondrite group is primarily characterized by having experienced extensive aqueous alteration and different degrees of brecciation in their parent asteroids shortly after they formed, ~4.56Gyrs ago. In fewer cases, some CMs have undergone thermal metamorphism as well. There are >160 known CMs but only a small number of these (around 20) underwent thermal metamorphism. Thermal metamorphism on CMs may have occurred before, after or before and after aqueous alteration; it is still unclear how the effects of heating superimpose those of aqueous alteration. Understanding how these two processes affected CMs is important for our understanding of the early Solar System and the origins of the inner planets. Northwest Africa (NWA) 11346 is an anomalous CM carbonaceous chondrite that was previously found to have no hydrous minerals or phyllosilicates that are characteristic of the CM chondrite group. Preliminary research on NWA 11346 suggests it may be a thermally metamorphosed CM chondrite. This study is an investigation of NWA 11346, primarily using a scanning electron microprobe (SEM) and an electron probe micro-analyzer (EPMA), to look at the primary and secondary features of its chondrules, CAIs and AOAs. The purpose of the study is to gain insight into the extent of aqueous alteration and heating NWA 11346 experienced; and hence, to investigate the processes that took place on the CM parent asteroid. CM chondrites are currently receiving much attention in light of the Hayabusa-2 and OSIRIS-REx sample return missions to asteroids Ryugu and Bennu respectively. The payload or Hayabusa-2 returned to Earth in December 2020 and that of OSIRIS-REx will return in September 2023. The missions aim to understand the formation of the early Solar System, the formation of Earth and the origin of life on Earth. Asteroids Ryugu and Bennu are thought to resemble CM and CI chondrites based on spectroscopic evidence that indicates they have hydrated and partially heated surfaces. Seeing as NWA 11346 is a potential analogue for the returned samples, understanding its origin and evolution is of particular importance as it could give context for the research that will be conducted on the returned samples.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Rhian Jones (Supervisor) & Romain Tartese (Supervisor)