Reconstructing Impact and Fluid Histories on Earth and Mars by Nanoscale Measurements

Gabriel A. Arcuri, The University of Western Ontario

Abstract

The U-Pb geochronology minerals zircon (ZrSiO₄) and baddeleyite (ZrO₂) occur as microscopic grains in many planetary rocks, are among the oldest known solids, and preserve robust records of chemical, isotopic and orientation microstructures useful for reconstructing the evolution of early planetary lithospheres and hydrosphere. Less well known are the nanoscale characteristics of these long-lived minerals, and their connections, if any, to processes that operated at larger length scales in the crust important to habitability such as global planetary bombardment and fluid flow. The aim of this thesis was therefore to develop and apply nanoscale analytical techniques, primarily atom probe tomography (APT), to expand our knowledge of the chronology and chemical records in zircon and baddeleyite grains exposed to impact bombarded and fluid-altered crust. Rare samples from Earth and ancient Mars were investigated in order to explore signatures of high temperature metamorphism and intervening periods of low temperature fluid alteration. Shock metamorphosed zircon grains from deep (15 km) beneath the center of the giant Vredefort impact structure show two styles of impact-related nanoscale Pb mobility. U-Pb and Pb-Pb ratios measured by APT for nanodomains of 100% Pb loss and Pb retention as clusters are combined to identify the interval between zircon crystallization and shock metamorphism. These results point to unusually rapid, multi-path diffusion processes within sub-micrometre volumes which, when averaged, yield discordant U-Pb dates. Zircon and baddeleyite grains from the Martian polymict breccia meteorite NWA 7475 that are now known to have crystallized from the earliest known, pre-Noachian crust of Mars (>4.4 billion years old), have primary compositions similar to igneous grains from Earth, yet exhibit two classes of nanoscale features unlike those in the Earth sample. Secondary, nanoscale enrichments of alkalis and metals are seen in crystalline zircon in the form of curvilinear features interpreted as healed, fluid altered fractures. Cl-bearing domains in metamict zircon, and a thin reaction rim of zircon around baddeleyite are attributed to later episodes, and processes, of fluid alteration of the Martian crust during or after breccia lithification at 1.4 Ga. A clast of igneous baddeleyite partly replaced by granular metamorphic zircon rim from a melt-laden clast in the same meteorite signifies an earlier, >1.4-billion-year, high temperature episode and, attests to the rich, more than 3-billion-year, polycyclic record of processes embedded in samples derived from the southern Highlands of Mars. This work contributes methodological developments in U-Pb geochronology and planetary science applicable to Earth and future Mars work on meteorites and returned samples.