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.
