OXYGEN-ISOTOPE GEOCHEMISTRY OF THE AMPHIBOLES - ISOTOPE EFFECTS OF CATION SUBSTITUTIONS IN MINERALS

The occurrence of coexisting amphiboles in rocks and the likelihood of concurrent isotope closure allows equilibrium oxygen isotope fraction ations among the amphiboles to be recovered from natural samples. Oxyg en isotope analyses of mineral separates using laser fluorination show that coexisting amphiboles increasingly partition O-18 in the order: hornblende much less than gedrite < cummingtonite less than or equal t o anthophyllite. The observed fractionations at similar to 575 degrees C are: Delta(Ged-Hbl) = 0.8 parts per thousand, Delta(Cum-Hbl) = 0.9, Delta(Cum-Ged) = 0.2, Delta(Ath-Ged) = 0.3, and Delta(Ath-Hbl) > 0.9. Previously published data for hornblende, actinolite, glaucophane, an d garnet show that Delta(Act-Hbl) similar to 0.2, Delta(Gln-Grt) much greater than 1, and Delta(Hbl-Crt) similar to 0. Thus, glaucophane str ongly partitions O-18 relative to the calcic amphiboles. The fractiona tion between two amphiboles of arbitrary composition can be predicted from the known fractionations for mica endmembers, pyroxene endmembers , acid exchange components such as CaAl(NaSi)(-1), NaAl(CaMg)(-1), CaM g-1, MgFe-1, FeMn-1, KNa-1, KAl(Si)(-1), and Fe3+ Al-1. Applications o f the exchange component method reproduce measured amphibole fractiona tions to within +/-0.1 to +/-0.2 parts per thousand, whereas other pre dictive methods cause misfit for typical metamorphic hornblende of gre ater than or equal to 0.5 parts per thousand at 575 degrees C. Althoug h the isotope effects of cation exchanges may be small at high-T, they magnify dramatically for minerals formed in surficial, diagenetic, an d low-T metamorphic environments. Different composition clays are pred icted to have equilibrium delta(18)O differences of 2-9 parts per thou sand. If the: isotope fractionation can be determined for one mineral endmember, then calibrated exchanges allow accurate prediction of the isotope fractionations for intermediate compositions of most ortho-, r ing-, chain-, and sheet-silicates. Copyright (C) 1998 Elsevier Science Ltd.