T254 Consequences of Fluids in Subduction Zones: From Grain Scale to Plate Boundary Shear Zone (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

PRESSURE AND FLUID PULSES DURING SUBDUCTION: RECORDS FROM GARNET

Paper number: T254-6
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

High pressure/low temperature (HP/LT) garnets from subduction mélanges of the Franciscan Complex, California, Syros, Greek Cyclades and Puerto Cabello, Venezuela (among other locations worldwide) display fine-scale, rhythmic major-element zoning. Textural observations suggest the rhythmic zoning resulted from garnet growth–dissolution cycles that mark fluctuating garnet stability during subduction.

Synchrotron Fourier transform infrared (FTIR) microspectroscopy was performed on garnets from California and Venezuela in order to examine core–rim variation in (structural) OH and (non-structural) H2O content. The garnets were found to record a decrease in OH immediately rim-side of garnet dissolution–precipitation surfaces (‘petrologic unconformities’ marked by major-element compositional breaks). At mantle pressures, OH in pyrope is positively correlated with crystallization pressure (Lu & Keppler 1997, Contrib Miner Petrol 129, 35; Withers et al. 1998, Chem Geol 147, 151). Thus, the association between rhythmic zoning and OH suggests garnet growth–dissolution cycles may have been driven by pressure fluctuations during subduction. The relationship between rhythmic zoning and (free) H2O in the garnets (and fluids during subduction) is less clear.

Rhythmic major-element zoning in the HP/LT garnets can be accounted for by models of periodic pore-fluid overpressure development then dissipation, linked to passing porosity waves and/or seismic cycles within the subduction environment. These models each predict clear association between (over)pressure pulses and fluids. In order to test the origin of the (over)pressure pulses and rhythmic garnet zoning, oxygen isotope analyses were performed by secondary ion mass spectrometry on multiple garnets from California, Greece and Venezuela. Associations among rhythmic zoning, OH content and oxygen isotope composition of garnet track fluid-source evolution during subduction and elucidate the role of fluids in (over)pressure pulses.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-305304

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