OXYGEN-ISOTOPE GEOCHEMISTRY OF ZIRCON

The high-temperature and small sample size of an I.R. laser system has allowed the first detailed study of oxygen isotope ratios in zircon. Low-magnetism zircons that have grown during metamorphism in the Adiro ndack Mts., N.Y. preserve primary delta(18)O values and low-magnetism igneous zircons are likewise primary, showing no significant affect du e to subsequent granulite facies metamorphism. The measured fractionat ion between zircon and garnet is Delta(Gt-Zrc)=0.0+/ 0.2 parts per tho usand(1 sigma) for most low-magnetism zircons in meta-igneous rocks. T he consistency of this value indicates equilibration at temperatures o f 700-1100 degrees C and little or no change in the equilibrium fracti onation over this temperature range. In contrast, detrital low-magneti sm zircons in quartzite preserve igneous compositions, up to 4 parts p er thousand out of equilibrium with host quartz, in spite of granulite facies metamorphism. The oxygen isotope composition of zircon can be linked to U-Pb ages and can 'see through' metamorphism, providing a ne w tool for deciphering complex igneous, metamorphic and hydrothermal h istories. Zircons separated by magnetic susceptibility show a consiste nt correlation. Low-magnetism zircons have the lowest uranium contents , the most concordant U-Pb isotopic compositions, and primary delta(18 )O values. In contrast, high-magnetism zircons are up to 2 parts per t housand lower in delta(18)O than low-magnetism zircons from the same r ock. The resetting of oxygen isotope ratios in high-magnetism zircons is caused by radiation damage which creates microfractures and enhance s isotopic exchange. Zircons from the metamorphosed anorthosite-manger ite-charnockite-granite (AMCG) suite of the Adirondacks have previousl y been dated (1125-1157 Ma) and classified as igneous, metamorphic or disturbed based on their physical and U-Pb isotopic characteristics. L ow-magnetism zircons from the AMCG suite have high, nearly constant va lues of delta(18)O that average 8.1+/-0.4 parts per thousand(1 sigma) for samples ranging from 39 to 75 wt% SiO2. Only olivine metagabbros h ave lower average values (6.4 parts per thousand), consistent with the hypothesis that they represent nearly pristine samples of the anortho site's parent magma. Whole-rock values of delta(18)O are also high in the AMCG suite and increase with SiO2 content, as predicted for a proc ess of assimilation and fractional crystallization. Taken together, th ese data suggest that the elevated values of oxygen isotope ratios res ult from partial melting and contamination involving metasediments in the deep crust, before the crystallization of zircon. More normal valu es elsewhere in the Grenville Province record deep-seated, pre-1150 Ma regional differences.