INVESTIGATING METAMORPHIC DRIVERS IN SUBDUCTION ZONES USING RAMAN AND FTIR SPECTROSCOPY

The utility of spectroscopic techniques in petrology, combined with the fine spatial resolution they offer, makes them particularly suited for investigating metamorphic episodicity and its drivers.

Major-element zoning in high pressure/low temperature (HP/LT) garnets from Ring Mtn, California, Pto Cabello, Venezuela and Syros, Greece are suggestive of multiple growth–resorption cycles. Such features may mark episodes of relative garnet stability then instability during subduction, presumably driven by rapidly changing P, T and/or chemistry. Quartz-in-garnet barometry by Raman spectroscopy was performed in combination with FTIR microspectroscopy to investigate potential associations between the garnet growth–resorption cycles and fluctuating P and fluids during subduction.

Quartz-in-garnet Raman work performed on the Californian samples demonstrates ~1 cm^-1 variation in the position of the 464 cm^-1 quartz peak, over radial distances of 10s of µm, in association with the garnet growth–resorption zoning. This result indicates rapid P fluctuations of 100–300 MPa—values similar to the expected strength of the rock—during subduction.

Zoning in structural OH and molecular H₂O in the Californian and Venezuelan garnets was probed by transmission FTIR microspectroscopy. Integrated intensities of the FTIR spectra in the 3450–3700 cm^-1 range demonstrate local ‘lows’ in structural OH, which occur in association with the local Mn ‘highs’ that define the garnet growth–resorption zoning. Incorporation of OH in pyrope displays a positive P dependence at the range of P encountered in subduction zones [Lu & Keppler 1997, Contrib Miner Petrol 129, 35; Withers et al. 1998, Chem Geol 147, 151], suggesting that variation in OH over 10s of µm may reflect fluctuating P during subduction. Integrated intensities of the FTIR spectra in the 3200–3450 cm^-1 range indicate an increase in molecular H₂O in association with the OH ‘lows’, suggesting greater fluid availability during putative episodes of low P and garnet dissolution.

Garnet growth–resorption features in the subduction-zone garnets may represent multiple cycles of overpressure development then release relating to fluctuating permeability. These features may be a metamorphic record of porosity waves and/or seismic cycles within the subduction environment.