Phosphorus Pathways In Deep Time
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 instead contains P-enriched olivine. I generalise these findings to propose a new classification scheme for Phosphorus-Olivine-Assemblages (Type I-III POAs). I highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of primitive asteroids.
Whilst phosphate mineral microtextures help to determine a relative series of geological events in the history of an asteroid, absolute dating methods allow a temporal sequence to be more exactly defined. Such information is crucial for gaining confidence in our understanding of how primitive asteroids may record the long term collisional evolution of the Solar System. For example, at 4.5-4.4 billion years ago, the final orbital architecture of our Solar System was established by the migration of Giant Planets and the Earth-Moon forming giant impact event. An invaluable record of this period may be written in the phosphate minerals of asteroids, which should have experienced enhanced collisional activity during such events. However, there is long-standing uncertainty in the interpretation of phosphate mineral ages which, via meteorites, can otherwise be used to trace ancient asteroid collisions. Here, again studying the Chelyabinsk meteorite, it is shown that phosphate U-Pb systematics should be filtered by observed mineral textural features. Damaged phosphate domains record a recent minor collision, which liberated Chelyabinsk from its parent asteroid. Pristine phosphate domains record an early high-energy collision at the expected time of Earth-Moon formation and Solar System reorganisation.
Phosphorus-bearing minerals are not just useful tools for tracing ancient events in our Solar System. Phosphorus is a key ingredient for the chemistry that likely gave rise to life on Earth. Lacking a major gas phase at ambient conditions, the concentration of P in early aqueous environments will have been governed by the mineral sources of P present at Earth's surface. A knowledge of early Earth P mineralogy and prevailing global and local environmental conditions is therefore needed to understand which scenarios for prebiotic chemistry are most plausible. Here, I reassess the diversity of P-bearing phases at Earth’s surface during the emergence of life. I consider phases that were delivered by meteorites (exogenous phases), as well as those that developed solely as a result of Earth system processes (endogenous phases). I take into account the known formation conditions of individual phases, as well as the observed temporal distributions of P-bearing minerals found at Earth’s surface today. Our approach allows us to leverage what is known about changes in the Earth system in order to rule out the prebiotic relevance of many P-bearing phases. Meanwhile, I highlight a small number of phases that are of possible prebiotic relevance; specifically, exogenous schreibersite, merrillite, and apatite, and endogenous apatite, olivine, and glass. Prebiotic mineral-chemical scenarios can be formulated for each phase, with distinct requirements for the environmental and tectonic state of early Earth. We can therefore relate the plausibility of mineral-chemical scenarios to the nature of early Earth, bridging the fields of geoscience and prebiotic chemistry.
If P is considered limiting for life, then the possible total mass of a P-dependent biosphere will be set by the composition of crustal rocks. However, the chemical composition and relative abundance of rock types within Earth’s crust over time remains uncertain. Here, Macrostrat – a database of rock age, volume, and chemistry – is used to reconstruct the evolution of Earth’s weatherable continental crust. I identify a long-term increase in the relative abundance of sedimentary rock, which reshaped crustal nutrient inventories whilst leaving the bulk composition largely unchanged. Rapid compositional change occurred across the Neoproterozoic-Phanerozoic boundary (600-400 Ma) as elevated erosion replaced Precambrian rocks with young, nutrient-rich sediments. Plate tectonics may have acted to increase global nutrient supply coincident with the rise of animal life.
Focusing on P offers one particular perspective on mechanisms that may have in part governed the emergence and evolution of life on Earth. However, the chemical and geological origins of life currently remain a mystery. This is no small part owing to the lack of accepted tests that a plausible scenario for prebiotic chemistry must pass. Here, a conceptual framework is presented that allows for the formulation and application of one such test: interference chemistry. In interference chemistry, a prebiotic reaction, or reaction system, is placed into a geochemical context (environment), creating a prebiotic scenario. The interaction between reaction efficacy and environmental conditions may be neutral, or alternatively result in constructive or destructive interferences with the pathway. Systematically exploring environmental interference chemistry for given reactions provides a common language with which to evaluate the plausibility of different scenarios for the origin of life: a test of environmental resilience which goes beyond asking whether the minimum conditions for a pathway are reached. Instead, interference chemistry provides a means to identify where on the early Earth prebiotic pathways may have been most favoured. A truly interdisciplinary approach to interference chemistry would incorporate constraints on early Earth environments from the study of astrophysics, meteorites, and preserved crustal rocks.