Oxygen Isotope Geochemistry of Zircon

Valley, John W., Chiarenzelli, Jeffrey R., and McLelland, James M. (1994) Oxygen isotope geochemistry of zircon. Earth and Planetary Science Letters, v. 126, p. 187-206


Abstract

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 Adirondack Mts., N.Y. preserve primary d18O values and low-magnetism igneous zircons are likewise primary, showing no significant affect due to subsequent granulite facies metamorphism. The measured fractionation between zircon and garnet is D(GtZrc)=0.0±0.2‰(1s) for most low-magnetism zircons in meta-igneous rocks. The consistency of this value indicates equilibration at temperatures of 700-1100ºC and little or no change in the equilibrium fractionation over this temperature range. In contrast, detrital low-magnetism zircons in quartzite preserve igneous compositions, up to 4‰ 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 though’ metamorphism, providing a new tool for deciphering complex igneous, metamorphic and hydrothermal histories. Zircons separated by magnetic susceptibility show a consistent correlation. Low-magnetism zircons have the lowest uranium contents, the most concordant U-Pb isotopic compositions, and primary d18O values. In contrast, high-magnetism zircons are up to 2‰ lower in d18O than low-magnetism zircons from the same rock. The resetting of oxygen isotope ratios in high-magnetism zircons is caused by radiation damage which creates microfractures and enhances isotopic exchange.

Zircons from the metamorphosed anorthosite-mangerite-charnockite-granite (AMCG) suite of the Adirondacks have previously been dated (1125-1157 Ma) and classified as igneous, metamorphic or disturbed based on their physical and U-Pb isotopic characteristics. Low-magnetism zircons from the AMCG suite have high, nearly constant values of d18O that average 8.1 0.4‰(1s ) for samples ranging from 39 to 75 wt% SiO2. Only olivine metagabbros have lower average values (6.4‰), consistent with the hypothesis that they represent nearly pristine samples of the anorthosite’s parent magma. Whole-rock values of d18O are also high in the AMCG suite and increase with SiO2 content, as predicted for a process of assimilation and fractional crystallization. Taken together, these data suggest that the elevated values of oxygen isotope ratios result from partial melting and contamination involving metasediments in the deep crust, before the crystallization of zircon. More normal values elsewhere in the Grenville Province record deep-seated, pre 1150 Ma regional differences.

Figure 3. Plot of d18O values of high-magnetism and low-magnetism zircons for meta-igneous samples from the Adirondacks. In all cases except one, the low-magnetism zircons preserve the highest d18O value and more magnetic zircons are increasingly retrogressed with values decreasing towards 6‰ (see Appendix 1 for magnetism and d18O). Sample #2 is the exception, showing the reverse trend: high-magnetism zircons are shifted upwards towards 6‰, suggesting that low-magnetism zircons preserve primary values while high-magnetism zircons have been retrogressed by late fluids with d18O = 6-7‰ (see text)

 

Figure 4. Plot of zircon concordance vs. the shift in d18O. Concordance is calculated from the position between the upper and lower intercepts on a U-Pb concordia diagram. The d18O shift is the difference in d18O for a zircon compared to low-magnetism zircons in the same rock. All samples show the same trend: low-magnetism zircons preserve the most primary d18O and the most concordant U-Pb values. The high-magnetism zircons have been retrogressed, probably due to radiation damage after cooling and uplift from granulite facies conditions. Sample #2 has reverse concordance and is not plotted (see text).
Figure 5. Plot of d18O (garnet) vs. d18O (low-magnetism zircon) in meta-igneous rocks of the Adirondack Mts. All samples except #19 (see text) show a very consistent fractionation of 0.0‰ for zircons that vary in d18O by almost 3‰. No difference is observed among igneous and metamorphic zircons or with rock type. The value of D (Gt-Zrc) = 0.0 is in excellent agreement with theoretical calculations for equilibrium at 800-1000° C.
Figure 6. Plot of 18O (quartz) vs. d18O (low-magnetism zircon) for detrital zircons in amphibolite and granulite facies quartzites of the Frontenac Arch, adjacent to the Adirondacks. The zircons closely preserve igneous values, up to 4‰ out of equilibrium with host quartz, and little or no metamorphic exchange has occurred.
Figure 8. Plot of D (WR-Zrc) vs. the whole-rock weight percent SiO2 for all Adirondack meta-igneous samples. The four triangles represent calculated model rock compositions (see text). The majority of the samples fall within 0.5‰ of the trend shown by the model rocks, consistent with fractional crystallization in the absence of fluid infiltration. Seven samples have whole-rock vs. zircon fractionations that are more than 0.5‰ larger than predicted; these samples have experienced at least two distinct episodes of 18O enrichment (see text).