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 [D-w = 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 degreesC. H2O contents in cordierites and granitic melts were determined using secondary ion mass spectrometry (SIMS). For H2O vapour-saturated conditions D-w ranges from 4.3 to 7 and increases with rising temperature. When the system is volatile undersaturated D-w decreases to minimum values of 2.6-5.0 at moderate to low cordierite H2O contents (0.6-1.1 wt %). At very low aH(2)O, cordierite contains less than 0.2-0.3 wt % H2O and D-w increases sharply. The D-w results are consistent with melt H2O solubility models in which aH(2)O is proportional to X-w(2) (X-w is the mole fraction of H2O in eight-oxygen unit melt) at X-w less than or equal to 0.5 and 0.25k(w){exp[(6.52 - (2667/T)) x X-w]} at X-w > 0.5, coupled with cordierite hydration models in which aH(2)O is proportional to (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 degreesC cordierite H2O saturation results with previous cordierite hydration data leads to the following geohydrometer relation, applicable for temperatures in the range 500-1000 degreesC: ln K-eq=[4203(+/- 320)/T]-11.74(+/-0.33) where K-eq = [n(sat)/(1-n(sat))]/ fH(2)O((P,T)), n(sat) is the saturation value of n for the P-T condition of interest, and T is in Kelvin. Moderate to high aH(2)O (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 aH(2)O 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.