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Cordierite as a sensor of fluid conditions in high‐grade metamorphism and crustal anatexis
Author(s) -
Harley S. L.,
Thompson P.,
Hensen B. J.,
Buick I. S.
Publication year - 2002
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.0263-4929.2001.00344.x
Subject(s) - anatexis , granulite , cordierite , migmatite , geology , partial melting , biotite , metamorphism , geochemistry , saturation (graph theory) , mineralogy , chemistry , metamorphic rock , mantle (geology) , facies , ceramic , geomorphology , gneiss , organic chemistry , paleontology , quartz , mathematics , structural basin , combinatorics
Cordierite H 2 O and CO 2 volatile saturation surfaces derived from recent experimental studies are presented for P–T conditions relevant to high‐grade metamorphism and used to evaluate fluid conditions attending partial melting and granulite formation. The volatile saturation surfaces and saturation isopleths for both H 2 O and CO 2 in cordierite are strongly pressure dependent. In contrast, the uptake of H 2 O by cordierite in equilibrium with melts formed through biotite dehydration melting, controlled by the distribution of H 2 O between granitic melt and cordierite, D w [ D w = wt% H 2 O (melt)/wt% H 2 O(Crd)], is mainly temperature dependent. D w = 2.5–6.0 for the H 2 O contents (0.4–1.6 wt percentage) typical of cordierite formed through biotite dehydration melting at 3–7 kbar and 725–900 °C. This range in D w causes a 15–30% relative decrease in the total wt% of melt produced from pelites. Cordierite in S‐type granites are H 2 O‐rich (1.3–1.9 wt%) and close to or saturated in total volatiles, signifying equilibration with crystallizing melts that achieved saturation in H 2 O. In contrast, the lower H 2 O contents (0.6–1.2 wt percentage) preserved in cordierite from many granulite and contact migmatite terranes are consistent with fluid‐absent conditions during anatexis. In several cases, including the Cooma migmatites and Broken Hill granulites, the cordierite volatile compositions yield a H 2 O values (0.15–0.4) and melt H 2 O contents (2.2–4.4 wt%) compatible with model dehydration melting reactions. In contrast, H 2 O leakage is indicated for cordierite from Prydz Bay, Antarctica that preserve H 2 O contents (0.5–0.3 wt%) which are significantly less than those required (1.0–0.8 wt%) for equilibrium with melt at conditions of 6 kbar and 860 °C. The CO 2 contents of cordierite in migmatites range from negligible (< 0.1 wt%) to high (0.5–1.0 wt%), and bear no simple relationship to preserved cordierite H 2 O contents and a H 2 O . In most cases the cordierite volatile contents yield total calculated fluid activities ( a H 2 O + a CO 2 ) that are significantly less than those required for fluid saturation at the P–T conditions of their formation. Whether this reflects fluid absence, dilution of H 2 O and CO 2 by other components, or leakage of H 2 O from cordierite is an issue that must be evaluated on a case‐by‐case basis.