Using garnet to constrain the duration and rate of water-releasing metamorphic reactions during subduction: An example from Sifnos, Greece

摘要

We present a method to reconstruct the dehydration flux associated with garnet-forming reactions during subduction. Garnet-bearing blueschists from the island of Sifnos, Greece, in the Attic-Cycladic Blueschist Belt are used as a test case to extract information on the timescales of dehydration during subduction. We use garnet growth as a proxy for the net dehydration reaction. Thermodynamic (pseudosection) analysis of a mafic blueschist in this unit (representative of an altered basalt protolith) indicates that garnet grew via net reaction(s) of the form chlorite+chloritoid+glaucophane+phengite=garnet+pyroxene+lawsonite+paragonite+quartz+H _2O. Garnet core and rim chemistry indicates that growth began at 2.0GPa and 460°C and ended at 2.2GPa and 560°C. The stabilization of matrix lawsonite (promoted by the bulk chemical shift of the matrix due to fractionation of the garnet and its inclusions from the system) throughout garnet growth limits the amount of water liberated from the rock over this P-T span. The average stoichiometry of the garnet-forming reaction(s) indicates an average molar production ratio of garnet to water of ~1.0:0.7. Given the 11vol.% garnet observed in the rock, this analysis indicates a loss of 0.3 to 0.4wt.% H _2O from the bulk rock during garnet growth. Zoned garnet geochronology from these rocks provides a constraint on the dehydration rate from this lithology during the time span of garnet growth. Two garnet grains, roughly 1.5cm each in diameter, were microdrilled based on major element zoning contours. Three concentric growth zones were extracted from each garnet for Sm-Nd geochronology. Very low Nd concentrations in acid-cleansed garnet (0.03 to 0.09ppm) yielded very small samples (~1ng Nd) that were analyzed with TIMS using a NdO ~+ with Ta _2O _5 activator method. Untreated "garnet" powders rich in mineral inclusions from each garnet zone were also analyzed. The powders fall off of the garnet-matrix isochron, likely indicating age inheritance in the inclusions. Combining acid-cleansed garnet data from each garnet, multi-point garnet-matrix isochron ages of 46.50±0.80Ma for the core, 46.49±0.53Ma for the intermediate zone, and 46.46±0.59Ma for the rim were determined, indicating a brief growth duration of 0.04Ma with an upper bound (2 SD) on growth duration of 1.0Myr. This equates to a release of 0.3-0.4wt.% of water due to this reaction in this lithology, and heating of 100°C, in less than 1.0Myr. The short time interval represents a focused, rapid pulse of dehydration and heating within the context of the overall subduction descent timescale of ~10-20Myr.