Upper amphibolite- and granulite-facies metabasites: the natural record

Upper amphibolite- and granulite-facies metabasites, ubiquitous in Archaean and Proterozoic terranes, represent a critical source of information on the lower continental crust. Elucidating the metamorphism of these rocks provides insights into the processes occurring at depth throughout Earth’s history. However, despite a century of geochemical studies examining high-temperature metabasites in the rock record, several aspects of their phase equilibria remain enigmatic, complicating the development of reliable thermodynamic equations of state for key phases.

This problem exists in part because studies of natural rocks primarily aim to establish the pressure-temperature (P-T) conditions of their respective region. Whilst researchers typically examine the relationship between the geochemical composition of different phases in pursuit of these P-T estimates, observations are necessarily restricted to the bulk composition of their rock and the P-T conditions at which the minerals equilibrated. In contrast, when developing equations of state, a rigorous understanding of how mineral compositions vary for a wide range of pressures, temperatures, and bulk compositions is required. This study lays the groundwork for such an understanding of metabasic phase equilibria by compiling a database of geochemical analyses from studies of natural samples worldwide.

Whole rock compositions, phase assemblages, modal abundances, and mineral compositions have been extracted from over 100 studies in the literature to construct a database containing >1000 samples. First, we determine the median value for and examine the range in the major- and minor-element whole rock composition of metabasites. These values are then compared to several average basalt compositions from the literature. Second, we examine the range and variability in the composition of individual minerals and the partitioning of elements between them. Third, using wet chemical analyses from the database, we test the ability of current ferric iron estimation techniques to reproduce that measured in garnet, orthopyroxene, clinopyroxene, and amphibole. Future work will use this database to test the capability of new thermodynamic equations of state and make it available as a community resource.

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 850530).