Peak metamorphic temperatures for the coesite-pyrope-bearing
whiteschists from the Dora Maira Massif, western Alps were determined
with oxygen isotope thermometry. The deltaO-18(SMOW) values of the
quartz (after coesite) (delta O-18 = 8.1 to 8.6 parts per thousand, n =
6), phengite (6.2 to 6.4 parts per thousand, n = 3), kyanite (6.1 parts
per thousand, n = 2), garnet (5.5 to 5.8 parts per thousand, n = 9),
ellenbergerite (6.3 parts per thousand, n = 1) and rutile (3.3. to 3.6
parts per thousand, n = 3) reflect isotopic equilibrium. Temperature
estimates based on quartz-garnet-rutile fractionation are
700-750-degrees-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 + H2O, the a(H2O) must be reduced to 0.4-0.75 at 700
750-degrees-C. The reduced a(H2O) cannot be due to dilution by CO2, as
pyrope is not stable at X (CO2) > 0.02 (T = 750-degrees-C; 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 approximately
680-degrees-C/30 kb at a(H2O) = 1.0 and are calculated to be
approximately 70-degrees-C higher at a(H2O) = 0.7, consistent with this
hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt
phase. Peak P-T-f(H2O) estimates for the whiteschist are 34 +/- 2 kb,
700-750-degrees-C and 0.4-0.75. The oxygen isotope
fractionation between quartz (deltaO-18 = 11.6%.) and
garnet (deltaO-18 = 8.7 parts per thousand) in the surrounding
orthognesiss is identical to that in the coesite-bearing unit,
suggesting that the two units shared a common, final metamorphic
history. Hydrogen isotope measurements were made on primary talc and
phengite (deltaD(smow) = -27 to -32 parts per thousand), on secondary
talc and chlorite after pyrope (deltaD = - 39 to - 44 parts per
thousand) and on the surrounding biotite (deltaD = -64 parts per
thousand) and phengite (deltaD = -44 parts per thousand)
gneiss. All phases appear to be in near-equilibrium. The
very high deltaD 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 evolved
locally during retrograde melt crystallization. The similar deltaD, but
dissimilar deltaO-18 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.