OXYGEN-ISOTOPE GEOCHEMISTRY OF THE AL2SIO5 POLYMORPHS

The development of laser-based extraction methods for oxygen isotope analysis allows for even the most refractory oxides and silicates to be analyzed. The fluorine-resistant minerals found in metapelites (kyanite, sillimanite, andalusite, corundum, garnet, spinel, sapphirine) also tend to have extremely slow oxygen diffusion rates, so that they should retain their delta(18)O values of crystallization. Whereas cation-exchange thermometers (for example, garnet-biotite) may reset during slow cooling, oxygen isotope thermometry made using refractory phases should preserve peak temperatures. In this study, oxygen isotope data for natural samples were used to derive equilibrium fractionation factors for Al2SiO5 polymorphs, with the ultimate goal of calibrating new phases for stable isotope thermometry. The delta(18)O values of coexisting quartz (qz) (coesite) + garnet (gt) + Al2SiO5 (AS) +/- rutile, biotite, plagioclase, spinel, pyroxene were measured in metapelites, eclogites, and mantle nodules with formation temperatures ranging from 535 to similar to 1300 degrees C. On the basis of natural isotopic variations, the 1000 In alpha (qz-AS) is equal to 0.73 (+/-0.02) x 1000 In alpha (qz-gt) (R(2) = 0.87). Assuming a temperature coefficient of fractionation = 3.1 for qz-gt, the following equation is obtained: 1000 In alpha (qz-AS) = 2.25 x 10(6)/T-2. There are only slight differences among the fractionations of the different AS polymorphs; the kyanite (ky) and sillimanite (sil) data are best fit by the expressions 1000 In alpha (qz-ky) = 2.17 x 10(6)/T-2, and 1000 In alpha (qz-sil) = 2.36 x 10(6)/T-2. These results are opposite to qualitative theoretical predications, although, within the precision limits, 1000 In alpha (ky-sil) is probably close to zero at metamorphic temperatures. The Delta(18)O (ky-and) values from a coarse-grained amphibolite and a finer grained three-aluminum silicate pelite are 0.2 and 1.0 per ml, respectively. The polymorphs for the first sample are in isotopic equilibrium, while those for the second are clearly out of isotopic equilibrium and crystallized at different times. The slight scatter in the entire data set can be ascribed to minor differences in the crystallization temperatures for garnet and kyanite. Agreement between oxygen isotope and conventional thermometers Is generally very good. The oxygen isotope temperatures for several samples are far higher than the regional metamorphic temperatures, possibly reflecting early, high temperature contact metamorphic effects that are preserved only in the most refractory phases. In contrast to the Al2SiO5-garnet data, delta(18)O values of rutile and biotite are generally modified during retrogression and cannot be used for oxygen isotope thermometry in high-temperature metamorphic rocks.