The release of water (and other volatiles) from subducted rocks represents an important part of the global geological water (and CO2) cycle. More specifically, it is a crucial triggering process for arc volcanism, seismicity, and the generation of ore forming fluids. Knowledge of the primary dehydration flux from subducted rocks as well as the timing and location (e.g. depth and temperature range) of this release is fundamental to understanding such broad tectonic processes, but rates of fluid production are currently generally poorly understood. The proposed research will employ novel methods to reconstruct dehydration fluxes from a 45 million year old subduction complex now exposed at the surface as well-preserved blueschist and eclogite rocks on the islands of Syros and Sifnos, Greece. With this unique information, geodynamic model predictions of subduction zone dehydration processes will be tested and refined.
A key question is whether the primary dehydration flux from subducted rocks is continuous or pulsed. While many past predictions suggest relatively continuous release of water from descending rocks, recent work (including preliminary work on Sifnos) has suggested that metamorphic reactions during subduction can occur in brief bursts that rapidly liberate water. Concentrically zoned garnet crystals, which grow during subduction zone dehydration, will be dated like tree rings with the Samarium-Neodymium decay system. Crucially, the dating resolution will be sufficient to resolve changes in the rate of garnet growth (i.e. pulses) as short as a few hundred thousand years. Thermodynamic analysis will provide constraints on the specific depth and temperature interval of garnet growth, which will be linked directly to concomitant water release for each rock. Constraints from thermodynamic analysis will be integrated with direct rate & age information from zoned garnet geochronology to provide a unique dehydration chronology. These novel methods and resulting datasets will provide scientists a new window into subduction zone processes occurring deep within the Earth.
