Abstracts Earth and Environmental Sciences

Phosphorus Pathways In Deep Time

by Craig Walton

Some of the most fundamental questions in natural science ask about the nature of early Earth. The conditions under which Earth formed and life emerged on its surface are especially uncertain. However, we are left with precious little evidence to study: most sufficiently ancient terrestrial rocks have long since been destroyed, and our sampling of the wider Solar System remains largely incomplete. This deficit may be reduced by combining insights from planetary science, geochemistry, and biology. The element phosphorus (P) is limiting for life in many environments on the modern Earth. Changes in global P availability may have played a large role in shaping biogeochemical evolution. Moreover, the baseline availability of P in planetary crusts is determined by processes of accretion, core formation, and late bombardment. Phosphorus is therefore of biological, cosmochemical, and astrophysical interest, providing a focal point from which to explore these diverse yet inter-related topics. Most of the P in our Solar System is stored in the form of minerals. Phosphorus-bearing minerals preserve information on pressures and temperatures experienced both during their initial formation and across the subsequent reaches of geological time. These minerals act as useful tools for probing the geological history of rocky objects, including the collisional processes through which asteroids and planets may be assembled, or indeed destroyed. However, the mechanisms by which P-bearing minerals form and by which they record collisions are uncertain, compromising interpretation of shocked meteorites as a record of Solar System history. The highly shocked Chelyabinsk meteorite exemplifies this point, containing a suite of variably deformed phosphate minerals of uncertain origin that have been used to infer several mutually exclusive scenarios for the collision history of the parental asteroid. Chelyabinsk preserves three lithologies: light (host rock), dark (containing a higher proportion of melted phases), and shock-melt (fully melted and quench crystallised material). Here, a comprehensive analysis of P mineral distribution and associated microtextures in each lithology is presented. I observe continuously strained as well as recrystallized strain-free merrillite populations. Grains with strain-free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock-metamorphic features. Complete melting of portions of the meteorite is recorded by the shock-melt lithology, which contains a population of phosphorus-rich olivine grains. The response of phosphorus-bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. I propose a paragenetic history for P-bearing phases in Chelyabinsk involving initial phosphate growth via P-rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and…