Tracking Solar Nebula Evolution with the Analysis of Single Chondrules

Abstract

Knowledge of the timing, order, and duration of events in the nascent Solar System is the key to understanding the mechanisms that led to its formation. Two cornerstones of cosmochronology are the extinct 26Al→26Mg and extant 238,235U→206,207Pb decay systems. A key assumption of the 26Al→26Mg chronometer is spatial (26Al/27Al) homogeneity in the protoplanetary disc. Recent measurements have thrown this assumption into question. The canonical model of spatial (26Al/27Al) homogeneity predicts the initial Solar System 26Mg/24Mg to be ~34 ppm less radiogenic than modern- Earth (Δ′26Mg ~ −34 ppm), while a recently proposed alternative suggests major silicate formation in reservoirs with initial Δ′26Mg of −16 ppm. To test these models, I measured the magnesium isotope composition of early-formed refractory forsterite grains. Alongside developing techniques to characterise chondrules with minimal preparation, I petrographically and chemically characterised refractory forsterite grains using electron- beam techniques, and measured complementary oxygen isotope compositions using secondary ionisation mass spectrometry. I developed novel techniques to extract single ~100 μm crystals from sections, and new ways of measuring the isotopic composition of small amounts (<5 μg) of magnesium to high-precision (±2-17 ppm) using multi-collector plasma-source mass spectrometry. My data are consistent with homogenous, ‘canonical’ 26Al/27Al across diverse Solar System reservoirs. My refractory forsterite model ages place their formation alongside CAIs, consistent with chondrule Pb-Pb chronometry, suggesting that conflicting 26Al→27Al and 238,235U→206,207Pb ages are due to late-stage resetting of internal Al-Mg isochrons. In parallel, I made significant progress towards establishing a Pb-Pb dating procedure at the NERC Isotope Geoscience Facility. I removed adhering chondrite matrix from chondrules using a diamond-coated micro-abrader, and isolated radiogenic lead from chondrules using fluid mineral separation and progressive dissolution. I also constrained the zirconium-rich nature of the host-phase of uranium in chondrules by measuring the trace element composition of acid leaches, providing fertile ground for future research.

Date of Award	1 Oct 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Tim Elliott (Supervisor), Christopher D Coath (Supervisor), Sara S. Russell (Supervisor) & Stephen R Noble (Supervisor)