Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists, Dora Maira Massif, western Alps

Abstract

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The δ 18 O( smow ) values of the quartz (after coesite) (δ 18 O=8.1 to 8.6‰, n =6), phengite (6.2 to 6.4‰, n =3), kyanite (6.1‰, n =2), garnet (5.5 to 5.8‰, n =9), ellenbergerite (6.3‰, n =1) and rutile (3.3 to 3.6‰, n =3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H 2 O, the a (H 2 O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a (H 2 O) cannot be due to dilution by CO 2 , as pyrope is not stable at X (CO 2 )>0.02 ( T =750 °C; P =30 kb). In the absence of a more exotic fluid diluent (e.g. CH 4 or N 2 ), a melt phase is required. Granite solidus temperatures are ∼680 °C/30 kb at a (H 2 O)=1.0 and are calculated to be ∼70°C higher at a (H 2 O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f (H 2 O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (δ 18 O=11.6‰) and garnet (δ 18 O=8.7‰) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (δD( SMOW )=-27 to-32‰), on secondary talc and chlorite rite after pyrope (δD=-39 to -44‰) and on the surrounding biotite (δD=-64‰) and phengite (δD=-44‰) gneiss. All phases appear to be in nearequilibrium. The very high δD values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar δD, but dissimilar δ 18 O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.