HR: 1330h
AN: T32A-0919    [PDF]
TI: Mineral Chemical Records of Subduction Factory Metamorphism and Devolatilization in UHP Eclogites From the Italian Alps
AU: * King, R L
EM: robbie@lehigh.edu
AF: EES, Lehigh Univ, 31 Williams Dr, Bethlehem, PA 18015 United States
AU: * King, R L
EM: robbie@lehigh.edu
AF: PML, ISEI, Okayama Univ, Misasa, Tottori-ken, Misasa, 682-0193 Japan
AU: Bebout, G E
AF: EES, Lehigh Univ, 31 Williams Dr, Bethlehem, PA 18015 United States
AU: Bebout, G E
AF: PML, ISEI, Okayama Univ, Misasa, Tottori-ken, Misasa, 682-0193 Japan
AU: Kobayashi, K
AF: PML, ISEI, Okayama Univ, Misasa, Tottori-ken, Misasa, 682-0193 Japan
AU: Nakamura, E
AF: PML, ISEI, Okayama Univ, Misasa, Tottori-ken, Misasa, 682-0193 Japan
AB: Across-arc geochemical studies indicate progressive changes in the magnitude and character of slab-derived fluids with depth. However, the precise nature of fluid chemistry as a function of metamorphic reaction history in subducting slabs is poorly constrained. Blueschists, eclogites, and other metamorphic products exhumed from ancient subduction zones have been the subject of mostly whole-rock geochemical studies to evaluate the results of fluid processing during subduction, but such studies suffer from an inability to provide the continuous geochemical records necessary to evaluate progressive changes in the metamorphic fluid flux to the mantle wedge. Furthermore, many of these records are partly (to completely) obscured by overprinting during exhumation or are limited to samples that reflect forearc depths ($<$50 km) in subduction zones and do not appear to record fluid histories occurring beneath the sub-arc mantle. To partly address the shortcomings of the present SubFac metamorphic record, we have applied {\it in-situ} high spatial resolution electron- and ion-microprobe methods to HP eclogites from Monviso and coesite-bearing UHP eclogites from Lago di Cignana, both located in the western Italian Alps and together record peak {\it P-T} conditions of up to $630\deg$C and 2.9GPa (Reinecke, 1998). Garnets from Cignana are generally $>$3mm and record initial growth in the blueschist facies from inclusion mineral assemblages of Na-amphibole, paragonite and clinozoisite in garnet cores. Cignana garnets preserve prograde growth zoning for major elements, HREEs, and Li and are highly LREE-depleted (Chondrite-normalized, La$_{N}$/Yb$_{N}$ $<$10$^{-3}$) and, in some metamafic eclogites, coesite occurs as inclusions near garnet rims. Reactions liberating REEs within the eclogite facies appear to be recorded in garnet as abrupt positive deviations from growth zoning profiles for Gd to Lu; the relative magnitude of enrichment is stronger for the MREEs than for the HREEs. This suggests decomposition of a L- and MREE-enriched phase such as clinozoisite, but incompatibility of the LREEs in garnet precludes a record of changes in LREEs directly from garnet. Depletions in the LREE and HREE about MREE such as Gd in matrix CPX reflect the influence of growth coeval with garnet and the general compatibility of REEs in the CPX structure. Corroborating such inferences, CPX inclusions in garnet generally display higher HREE contents than matrix CPX. Fluid-mobile elements such as the LILEs and B are generally hosted by white micas in these eclogites. Both phengite and paragonite are present in the eclogites and record distinct TE partitioning. Phengite is the primary host for Cs and Ba, with Ba/Sr ratios $>$20, while Sr is more strongly partitioned into paragonite, with Ba/Sr ratios $<$0.1. While Li contents for paragonite and phengite are indistinguishable, B preferentially resides in paragonite, creating B/Li ratios of $\sim$10 in paragonite versus $\sim$1 in phengite. These data indicate that mobility of individual LILEs during devolatilization in the eclogite facies will be strongly controlled by the stability of distinct phases. Our results indicate eclogite-facies minerals can provide detailed geochemical information regarding trace element behavior during metamorphism and, when combined with traditional thermobarometric data, will provide a powerful record of devolatilization in subduction zones. Our approach also allows characterization of overprinting mineral parageneses and trace element redistribution related to the complex exhumation histories of these rocks.
DE: 1010 Chemical evolution
DE: 1065 Trace elements (3670)
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: 2003 Fall Meeting