The distribution of H2O between cordierite and granitic melt: H2O incorporation in cordierite and its application to high-grade metamorphism and crustal anatexis

Experiments defining the distribution of H2O [D-w = wt % H2O(melt)/wt % H2O (crd)]) between granitic melt and coexisting cordierite over a range of mel t H2O contents from saturated (i.e. coexisting cordierite + melt + vapour) to highly undersaturated (cordierite + melt) have been conducted at 3-7 kba r and 800-1000 degreesC. H2O contents in cordierites and granitic melts wer e determined using secondary ion mass spectrometry (SIMS). For H2O vapour-s aturated conditions D-w ranges from 4.3 to 7 and increases with rising temp erature. When the system is volatile undersaturated D-w decreases to minimu m 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 solubil ity models in which aH(2)O is proportional to X-w(2) (X-w is the mole fract ion 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 cordierit e hydration models in which aH(2)O is proportional to (1 -n), where n is th e number of molecules of H2O per 18-oxygen anhydrous cordierite formula uni t (n < 1). Combination of our 800-1000 degreesC cordierite H2O saturation r esults with previous cordierite hydration data leads to the following geohy drometer relation, applicable for temperatures in the range 500-1000 degree sC: 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 v alue of n for the P-T condition of interest, and T is in Kelvin. Moderate t o 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 i n Antarctica and central Australia, whereas aH(2)O calculated from the meas ured H2O contents in cordierites from several granulite migmatites are lowe r and in the range 0.1-0.4. Calculated H2O contents of melts that equilibra ted with low-H2O (0.6-1.2 wt %) cordierites in several migmatite terrains a re in the range 2.8-4.4 wt %, consistent with dehydration-melting reactions involving biotite (sillimanite). Calculated melt H2O contents that in othe r studied migmatites are unrealistically low for the specified temperature conditions of melting probably reflect post-equilibrium H2O loss from the c ordierites.