OSCILLATORY MAJOR ELEMENT AND ISOTOPIC ZONING IN HIGH-PRESSURE LOW-TEMPERATURE GARNETS: RECORDS OF NON-UNIFORM FLUID TRANSFER PROCESSES? (Invited Presentation)

In subduction zones, the fate of fluids derived from devolatization remains enigmatic: they may either stagnate such that the system remains undrained, or fluids may be mobilized over large length scales, draining rock volumes. Prograde eclogitic veins greater than cm-scale are volumetrically scarce in the high-pressure low-temperature (HP-LT) rock record, suggesting either transient channelized flow is efficient, necessitating negligible grain boundary transfer and low intact rock permeability, or a large proportion of fluid migration through the slab occurs via more elusive grain boundary mechanisms. Oscillatory zoning is a ubiquitous feature in HP garnet porphyroblasts and is interpreted to reflect fundamental processes in subduction zones. Here, we present results from an integrated suite of geochemical approaches applied to HP metabasites from As Sifa (Oman), Syros (Greece), Puerto Cabello (Venezuela) and the Samana Complex (Dominican Republic) to assess whether such zoning may be causally linked to heterogeneous fluid fluxing.

Rim zones of garnets from all samples exhibit short-wavelength major element oscillatory zoning coupled with spatially coincident trace element oscillations and complementary near-rim zoning in matrix minerals. Diffusion modelling is consistent with their preservation over rapid metamorphic cycles lasting on the order of < 200 Kyr. Oscillatory zoning is most often accompanied by homogeneous core-to-rim Al content, and < 0.4 ‰ vacillations in δ¹⁸O across rhythmic zoning in such samples are consistent with rock-buffered histories and, potentially, a relatively stagnant pore-fluid reservoir. However, the onset of oscillatory zoning in some garnets from As Sifa and Syros is concomitant with a rimwards increase in Al content and a decrease in calculated Fe³⁺. O-isotope analyses across this change in garnet composition are used to assess the hypothesis that it marks a transient period of localized open system fluid-fluxing superimposed on a broadly stagnant/rock-buffered fluid history. In such a case, this work points to the predominance of impermeable grain boundaries, with temporally transient and stochastic open system fluid fluxing yielding a change in the composition and potentially redox state of the reactive grain boundary.