The aqueous and thermal history of volatile-rich asteroids: Insights from sulfide and phosphate minerals in carbonaceous chondrites and asteroid return samples

Abstract

Volatile-rich asteroids played an important role in the formation of the solar system, including the delivery of water and organics to Earth, but their evolution remains poorly known. This thesis aims to constrain the aqueous and thermal alteration history of volatile-rich asteroids by investigating coarse (>10 μm) Fe-sulfide and phosphate grains in CI, CM, and CY carbonaceous chondrites and samples returned from asteroid (162173) Ryugu using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). We experimentally heated and cooled samples of CI and CM chondrites to better understand how Fe-sulfide grains evolve with heating. We show that Fe-sulfides respond rapidly to heating, with changes in mineralogy and composition occurring over timescales on the order of hours. By combining our observations of Fe-sulfides in naturally heated meteorites with those in experimental studies, we use Fe-sulfide characteristics to constrain the thermal histories of the CM and CY chondrites. We suggest the atomic Fe/(Fe+Ni) ratio of pentlandite is a good indicator of the relative degree of heating between carbonaceous chondrite samples. Furthermore, we show that Fe-sulfides in Stage IV (>750 oC) CM and CY chondrites are consistent with cooling rates >0.1 oC/min, indicating a short-duration heating event (hours to days). Overall, it appears that impact heating likely occurred on the CM and CY chondrite parent body(ies), however, solar radiation cannot be ruled out. To explore the relationship between samples returned from asteroid Ryugu and the CI and CY chondrites, we compared their coarse Fe-sulfide grains. Our observations support a relationship between these materials; however, we identify subtle differences in alteration conditions (e.g. temperature of aqueous alteration and degree of oxidation) between Ryugu particles and the CI chondrites. The CI and CY chondrites also contain F-rich apatite with compositions that are unique in comparison to other analysed chondrites, and Mg-phosphate, which is a rare extraterrestrial mineral. Likewise, we find evidence for the rare Cu-sulfide cubanite in the CY chondrites, which is also in the CI chondrites and Ryugu. The shared presence of rare minerals that formed during aqueous alteration in the CIs, CYs, and Ryugu suggest comparable fluid compositions and environmental conditions on their respective bodies and supports a genetic link between these materials.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Rhian Jones (Supervisor)