Causes, effects, and implications of the relationships amongst fluids, serpentinisation, and alloys

Evans, Katy; Frost, B. Ronald; Reddy, Steven M.; Brown, T.C.

doi:10.1016/j.lithos.2023.107132

Cited by 6 publications

(1 citation statement)

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“…Kohn, 2020;Osborne et al, 2022), although we can expect another generation of activity-composition models, likely with improvements to Fe 3+ partitioning and the solubility of species in fluids and melt as a function of P -T -X, where X is the bulk chemical composition of the equilibration volume. Cases such as the study of serpentinite-hosted alloys will require new thermodynamic data and activity-composition models for many of the alloys of interest (Evans et al, 2023). By contrast, elastic barometry is a fast-developing method with scope for much wider use in the future and ongoing expansion of the range of minerals to which the method can be applied (e.g.…”

Section: Thermobarometrymentioning

confidence: 99%

Some thoughts about eclogites and related rocks

Brown

2023

Eur. J. Mineral.

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Abstract. The past 40 years have been a golden age for eclogite studies, supported by an ever wider range of instrumentation and enhanced computational capabilities, linked with ongoing developments in thermobarometry and geochronology. During this time, we have made robust estimates of pressure–temperature (P–T) conditions; determined ages related to the prograde, metamorphic peak and retrograde stages; and calculated time-integrated rates of cooling and exhumation for eclogites and related rocks, including blueschists, from orogenic belts worldwide. Improvements to single mineral thermometers and new developments in elastic barometry using inclusions of one mineral in another (e.g. quartz and/or zircon in garnet), coupled with ongoing innovations in petrochronology and diffusion modelling, presage a new age for eclogite studies in which detailed quantification of metamorphic conditions and timescales will be linked to an improved understanding of processes at all scales. Since the turn of the century, numerical modelling of subduction zone and rock exhumation processes has become increasingly important. As a result, subduction and exhumation are quite well understood, but the volume of continental crust subducted to and returned from mantle conditions and the amount lost to the mantle are largely unknown. We have generated sufficient data to investigate the spatiotemporal distribution of metamorphism and secular change but not without controversy in relation to the rare occurrence of orogenic eclogites and the absence of blueschists prior to the late Neoproterozoic and the emergence of plate tectonics on Earth. Since the turn of the century, the assumption that metamorphic pressure is lithostatic has come under increasing scrutiny. Whether local variations in stress extrapolate to the crustal scale and, if so, whether the magnitude of the calculated deviations from lithostatic pressure can be generated and sustained in mechanically heterogeneous rock units remains contentious. Could the paradigm of subduction of continental lithosphere to mantle depths be simply an artefact of the lithostatic assumption? Fluid cycling in subduction zones and understanding the role of fluids in the generation of intermediate-depth earthquakes remain important topics of current research. Dry (H2O-absent) conditions are unlikely around the peak of ultrahigh-pressure (UHP) metamorphism or during exhumation, due to dehydroxylation of nominally anhydrous minerals and breakdown of hydrous minerals at P–T conditions in the realm of supercritical fluid and hydrous melt. Indeed, the presence of melt may be necessary to facilitate the exhumation of HP and UHP tectonometamorphic rock units. Finally, our ability to interrogate inclusions in superdeep diamonds should lead to a better understanding of how the deep interior and surface are linked in the context of Earth as a fully coupled system.

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Section: Thermobarometrymentioning

confidence: 99%