The Distribution of H2O between Cordierite and Granitic Melt: H2O Incorporation in Cordierite and its Application to High-grade Metamorphism and Crustal Anatexis

Abstract

Experiments defining the distribution of H2O [Dw = wt % H2O(melt)/wt % H2O(crd)]) between granitic melt and coexisting cordierite over a range of melt H2O contents from saturated (i.e. coexisting cordierite + melt + vapour) to highly undersaturated (cordierite + melt) have been conducted at 3–7 kbar and 800–1000°C. H2O contents in cordierites and granitic melts were determined using secondary ion mass spectrometry (SIMS). For H2O vapour-saturated conditions Dw ranges from 4·3 to 7 and increases with rising temperature. When the system is volatile undersaturated Dw decreases to minimum values of 2·6–5·0 at moderate to low cordierite H2O contents (0·6–1·1 wt %). At very low aH2O, cordierite contains less than 0·2–0·3 wt % H2O and Dw increases sharply. The Dw results are consistent with melt H2O solubility models in which aH2O is proportional to Xw2 (where Xw is the mole fraction of H2O in eight-oxygen unit melt) at Xw ≤ 0·5 and 0·25kw{exp[(6·52 − (2667/T)) × Xw]} at Xw > 0·5, coupled with cordierite hydration models in which aH2O is proportional to n/(1 − n), where n is the number of molecules of H2O per 18-oxygen anhydrous cordierite formula unit (n < 1). Combination of our 800–1000°C cordierite H2O saturation results with previous cordierite hydration data leads to the following geohydrometer relation, applicable for temperatures in the range 500–1000°C: \[ \ln K_{\rm eq} = [4203(\pm 320)/\hbox{T}]- 11\cdot75 (\pm 0\cdot33) \] where Keq = [nsat/(1 − nsat)]/fH2O(P,T), nsat is the saturation value of n for the P–T condition of interest, and T is in Kelvin. Moderate to high aH2O (0·4–0·9) are calculated for H2O-rich cordierites in several pegmatites and zones of hydrous fluid infiltration in high-grade terrains in Antarctica and central Australia, whereas aH2O calculated from the measured H2O contents in cordierites from several granulite migmatites are lower and in the range 0·1–0·4. Calculated H2O contents of melts that equilibrated with low-H2O (0·6–1·2 wt %) cordierites in several migmatite terrains are in the range 2·8–4·4 wt %, consistent with dehydration-melting reactions involving biotite (± sillimanite). Calculated melt H2O contents that in other studied migmatites are unrealistically low for the specified temperature conditions of melting probably reflect post-equilibrium H2O loss from the cordierites.