Oxygen isotope geothermometers for metamorphic rocks

The Chicago mineral‐carbonate oxygen isotope fractionation curves have been combined with mineral‐water fractionation data for jadeite, zoisite and rutile and new data for grossular‐water to provide a set of self‐consistent mineral‐pair calibrations. The A coefficients in the equation 1000 In α=A× 10⁶T^‐2 of the new mineral‐pair fractionations are Jadeite Zoisite Grossular Rutile Quartz 1.69 2.00 3.03 5.02 Jadeite 0.31 1.34 3.33 Zoisite 1.03 3.02 Grossular 1.99 The isotopic fractionation properties of natural pyralspite garnet [(Ca, Fe, Mg, Mn)₃Al₂Si₃O₁₂] can be approximated by those of the grossular end‐member. Appropriate substitutions also yield coefficients for the solid‐solution minerals: sodic pyroxene and epidote, e.g. A _{quartz‐sodic pyroxene}= 2.75 ‐ 1.06X_jd, A _{quartz‐epidote}= 2.00 + 0.75X_ps where X_Jd and X_Ps are the mole fractions of the jadeite and pistacite components, respectively. The new data set is particularly suitable for the geothermometry of metamorphic rocks. δ¹⁸O data from minerals of the high‐pressure metamorphic rocks of the Sesia Zone of Italy and Cyclades Complex of Greece yield well‐constrained mean temperatures of 572 and 478d̀ C, respectively. Type III blueschist metabasalts of the Franciscan Formation of California give mean quartz‐garnet temperatures of 354d̀ C.