Properties of CO2-induced Dehydration of Amphibolite

An amphibole-bearing gneiss from the Seward Peninsula, Alaska, underwent local dehydration to two-pyroxene gneiss. Dehydration was driven isothermally and isobarically, close to the metamorphic maximum, by a small amount of CO2-rich fluid evolved from an underlying impure marble layer. Stable isotope evidence indicates that the CO2 diffused 85 cm into the gneiss through a stationary pore fluid that was seldom fully connected. This created a gradient in H2O activity from 0.20 to 0.24 within the two-pyroxene alteration zone. Whole-rock analyses and mineral mass balances suggest that, apart from loss of H2O, the change proceeded isochemically, a conclusion that differs from some of the more recent detailed chemical studies of outcrop-scale charnockitic alteration. Hornblende reacted out according to the reaction Hbl+1.86 Qtz=1.26 Cpx+1.36 Opx+0.96 An65+0.29 Kfs+0.23 Ilm+H2O. Biotite was largely conserved. Clinopyroxene, orthopyroxene, and new-formed plagioclase occur in relatively fine-grained granular aggregates. New-formed K-feldspar occurs as replacement antiperthite. It is argued that the reaction did not involve the production of anatectic melt. We suggest that CO2-induced dehydration in its simplest form is isochemical. We further suggest that the sequence of disappearance of biotite and hornblende in prograde granulite terranes may be indicative of the process of simple dehydration (at low aH2O) or vapor-absent dehydration melting (at higher T and aH2O). Our observations may be helpful in interpreting cases where the evidence for introduced CO2 is more ambiguous.