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Paper No. 56-8
Presentation Time: 3:40 PM
FRONTIERS IN THE ANALYSIS AND INTERPRETATION OF ZONED GARNET CRYSTALS: FE-ISOTOPES AS A RECORDER OF REDOX FLUID PROCESSES (Invited Presentation)
About 30 years ago, a renaissance in the analysis and interpretation of chemically zoned garnets drove study beyond the foundational major element zonation first described in the 1960’s [1], to include trace element zonation [2], age zonation [3], and isotopic zonation [4]. Since then, these zoned garnet records, often spanning millions of years, have been expanded and interpreted to reflect evolving pressure, temperature, strain, dehydration, hydration, and fluid chemistry. More recent developments have involved garnets from high-pressure blueschists and eclogites that record rapid pulses of dehydration during subduction [5]. These fluids may then participate in the fertilization of the mantle wedge leading to arc volcanism. Numerous studies have shown that arc magmas are oxidized relative to typical MORB, therefore requiring an oxidizing mechanism [6]. Fe-isotopes have been shown to respond to dehydration processes during subduction [7]. Here we show that Fe-isotope zonation in single garnet crystals coupled with thermodynamic analysis and garnet-epidote oxybarometry in subduction zone garnets from Sifnos, Greece document the release and export of an oxidizing fluid. The fluids are produced by the dehydration of lawsonite. The oxidizing nature of the fluids likely derives from sulfate species liberated from the breakdown of anhydrite [8]. Fe-isotope zonation in garnet appears to be a recorder of fluid and redox processes within subduction zones, and likely in other metamorphic environments.
[1] Hollister (1966) Science 154, 1647–1651; [2] Hickmott et al. (1987) Geology, 15, 573–576; [3] Christensen et al. (1989) Science, 244, 1465–1469; [4] Chamberlain and Conrad (1991) Science, 254, 403-406; [5] Dragovic et al. (2015) EPSL, 413, 111–122; [6] Kelley and Cottrell (2009) Science, 325, 605-607; [7] Debret et al. (2016) Geology, 44, 215-218; [8] Tomkins and Evans (2015) EPSL, 428, 73–83.