Aqueous Alteration in the Tarda Meteorite: Isotopic and Geochemical Analyses of Secondary Minerals

Abstract

Water appeared on some carbonaceous asteroids within the first few million years of Solar System history and modified the other accreted components in a process known as aqueous alteration. Recovered meteorites from such bodies, termed carbonaceous chondrites, are composed of diverse assemblages of secondary minerals and organic compounds that differ depending on the fluid environment the meteorite experienced. Since carbonaceous asteroids likely seeded the early planets with prebiotic organic matter, understanding the conditions that governed aqueous alteration is critical for evaluating their role in the origin of life.

This dissertation investigates the aqueous alteration history of Tarda, a new and unusual C2-ungrouped carbonaceous chondrite that fell in Morocco in 2020. The secondary mineralogy of Tarda implies exposure to significant aqueous alteration, and their investigation enables the fluid environment of Tarda to be constrained. The goal of this dissertation is to constrain the temperature, timing, evolution, and chemistry of the fluid responsible for altering the Tarda meteorite. Chapter 1 first establishes a novel, non-polar sample preparation procedure capable of polishing Tarda and other clay-rich samples for sensitive microanalytical techniques. Hexane, toluene, and mineral oil were identified as effective polishing liquids for such samples, which prevent clay minerals from swelling. Chapter 2 uses this polished surface and employs in situ secondary ion mass spectrometry (SIMS) on dolomite and magnetite to analyze oxygen, carbon, and manganese-chromium isotopes to constrain the timing, temperature, and evolution. Here, dolomite and magnetite were found to have precipitated approximately ~4.563 billion years ago at ~90˚C, from a relatively evolved fluid after significant water-rock interaction. Chapter 3 employs transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), and atom probe tomography (APT) to investigate the nanoscale chemistry of magnetite framboids in Tarda. These analyses reveal discrete boundary enrichments in elements such as Ti, Si, Na, Mg, Ca, and Mn, capturing a chemical record of the altering fluid from which the framboids precipitated. The observed element distributions and inferred surface charge conditions constrain the fluid to alkaline pH (>5.4) and support a chemically diverse, cation-rich environment. Together, these chapters present a multi-technique, multi-scale investigation into the aqueous alteration history of Tarda. Beyond providing new insights into Tarda itself, this work contributes to broader efforts to reconstruct parent body fluid histories and evaluate the potential for carbonaceous asteroids to host environments favorable to prebiotic organic synthesis.