Stable isotope geochemistry and phase equilibria whiteschists, Dora Maira Massif, western Alps

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen iso- tope thermometry. The 818 O(svow) values of the quartz (after coesite) (~18 0 = 8.1 to 8.6%0, n = 6), phengite (6.2 to 6.4%o, n = 3), kyanite (6.1%o, n = 2), garnet (5.5 to 5.8%~ n = 9), ellenbergerite (6.3%0, n = 1) and futile (3.3. to 3.6%0, n = 3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-ruffle fractionation are 700- 750 ~ 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 + H20, the a(H20) must be reduced to 0.4-0.75 at 700-750 ~ The reduced a(H20) cannot be due to dilution by CO2, as pyrope is not stable at X(CO2) > 0.02 (T = 750 ~ P = 30 kb), In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are ~680 ~ kb at a(HzO) = 1.0 and are calculated to be 70 ~ higher at a(H20) = 0.7, consistent with this hy- pothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f (H20) estimates for the whiteschist are 34 __+ 2 kb, 700-750 ~ and 0.4-0.75. The oxygen isotope fractionation between quartz (51sO = 11.6%o) and garnet (5180 = 8.7%0) in the sur- rounding orthognesiss is identical to that in the coesite- bearing unit, suggesting that the two units shared a com- mon, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (SD~s~ow~ = -27 to -32%o), on secondary talc and chlo- rite after pyrope (SD = -39 to -44%~ and on the sur- rounding biotite (SD = -64%o) and phengite (SD = -44%0) gneiss. All phases appear to be in near- equilibrium. The very high 8D values for the primary hydrous phases is consistent with an initial oceanic- derived/connate fluid source. The fluid source for the retrograde talc + chlorite after pyrope may be fluids evol- ved locally during retrograde melt crystallization. The similar 6D, but dissimilar 8180 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.