Anorthite in Magmatic Systems

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Meeting our ever increasing need for a steady supply of natural resources requires an increasingly sophisticated understanding of processes that form and store such resources. Gaining insights into these processes requires research that spans all aspects of the transformation of geologic material into exploitable ground-based resources, from what is originally brought up from the Earth's interior, to the nature of the final mechanisms of concentration of resources on the surface. This project focuses on furthering our fundamental understanding of what lies in the Earth's interior, deeper than humans can directly access, and what portion of this mantle material is brought to the Earth's surface during quiescent volcanic activity beneath the sea at mid-ocean ridges and during violent volcanic eruptions on land. New insights into the mantle in these regions and the mechanisms of moving magma to the surface will be gleaned by studying a mineral found in both of these geologic environments, the mineral anorthite, whose origin remains enigmatic and cannot be explained by our current understanding of the geologic processes at work in the Earth's mantle. This research capitalizes upon a set of new observations obtained through research on a synthetic version of the mineral that suggests very different behavior in magmas than has been broadly assumed and has the potential to dramatically change our view on how magmas are produced in the mantle and the changes they undergo as they ascend. This research involves a combination of experiments simulating conditions within the Earth, modeling, and looking at natural anorthite crystals in rock samples. This multi-faceted approach is vital for training the next generation of geoscientists at all levels and will be used to introduce college freshmen to interdisciplinary science, provide research experiences for undergraduates, and finally to provide Ph.D. level research opportunities. In specific, the proposed work uses experimental and thermodynamic modelling approaches to investigate a little-recognized deviation from the classic melting loop topology accepted as dictating plagioclase compositional behavior in igneous rocks since the time of Bowen. This deviation is in the form of a pseudo-azeotrope in the anorthite-rich region of plagioclase and plagioclase-olivine compositional space at elevated pressure that destabilizes plagioclase and inhibits anorthitic plagioclase from evolving normally towards Na-enrichment as the magma cools. The proposed research will constrain this behavior in the compositional space of upwelling mantle and by doing so will provide new insights into the origin of highly anorthitic plagioclase crystals in mid-ocean ridge and subduction zone magmas. Furthermore, the change in plagioclase and melt stability relations dictated by the pressure effect on this topology implies an elevated pressure region of high melt production and an important role for a peritectic reaction involving aluminous spinel. As neither of these implications can be predicted by current models of magma production in these environments, this research has the potential to open the door to a variety of new studies on the origin and evolution of magmas at compressional and extensional plate margins that can integrate such behavior into a next-generation mechanistic model of magma formation in the Earth's mantle.