Planetary Stoichiometry, Mineral Ecology, and the Rise of Habitability
Monday, 15 December 2014
The near-surface mineralogy of terrestrial planets and moons, and the corresponding evolution of habitable environments, is shaped by physical, chemical, and biological processes. Four factors contribute to a planet’s mineral distribution and diversity: (1) planetary stoichiometry; (2) crystal chemical characteristics; (3) mineral stability ranges; and (4) the probability of occurrence for rare minerals. Measurements of stellar stoichiometry reveal that stars can differ significantly from the Sun in relative abundances of rock-forming elements, which implies that bulk compositions of some extrasolar Earth-like planets differ significantly from those of Earth, particularly if fractionation processes in evolving stellar nebulas and planetary differentiation are factored in. Comparison of Earth’s upper continental crust and the Moon shows that differences in element ratios are reflected in ratios of mineral species containing these elements. More abundant elements generally have larger numbers of mineral species, though elements that mimic other more abundant elements are less likely to form their own species. Total mineral diversity for different elements is not appreciably influenced by the relative stabilities of individual phases, e.g., the broad pressure-temperature-composition stability range of zircon (ZrSiO4) does not significantly diminish the total number of observed Zr minerals. To the extent that origins and evolution of life depend on key mineral species, habitability is dependent on the emergence of a minimal mineral diversity. Evolving life has, in turn, exerted the largest single influence on mineral diversity. Notably, changes in near-surface redox conditions on Earth through the evolution of oxygenic photosynthesis tripled the available “composition space” of Earth’s near-surface environment, and resulted in a corresponding tripling of mineral diversity subsequent to atmospheric oxidation. Statistical analysis of mineral frequency distributions suggests that thousands of plausible rare mineral species await discovery or could have occurred at some point in Earth’s history, only to be subsequently lost by burial, erosion, or subduction—i.e., much of Earth’s mineral diversity associated with rare species results from stochastic processes.