OXYGEN ISOTOPE DATA FROM HYDROTHERMALLY ALTERED ARCHEAN PLUTONS: CONSTRAINTS ON THE ISOTOPIC EVOLUTION OF THE HYDROSPHERE

Coexisting quartz and feldspar oxygen isotope data of Archean plutons from South Africa, Australia and Canada were compared in order to calculate the oxygen isotope composition of Archean hydrothermal fluids. These data place limits on the ¹⁸O variation of Archean surface water (seawater and meteoric water). In Canada and South Africa, plutons exhibit non-equilibrium quartz-feldspar arrays on "δ-δ"plots indicative of rapid cooling while other areas exhibit equilibrium arrays (e.g. in the Pilbara, sheared granitoids exhibit depletions of several per mil from primary δ¹⁸O values). The minimum δ¹⁸O value of exchanged feldspar constrains the isotopic composition of the fluid, provided that the alteration occurs under conditions analogous to Phanerozoic examples. In all plutons, the δ¹⁸O of feldspar ranges between 11.7‰ and 4‰. Assuming an average temperature of water-rock interaction of 350° C and typical water-rocks ratio(~1) for these systems, the calculated fluid δ¹⁸O values are between -1 ‰ and +5 ‰. If ocean water was depleted in δ¹⁸O by ≤ -6 ‰,the calculated δ¹⁸O feldspar in equilibrium with those fluids would exhibit more negative δ¹⁸O values, ≤ -4‰. Low-¹⁸O, altered igneous rocks appear to be absent in the Archean. In a seafloor spreading dominated regime, the average δ¹⁸O of sea water and meteoric water are approximately 0‰ and -4 ‰, respectively so that in the Phanerozoic, hydrothermally-exchanged, low-¹⁸O rocks are confined to 1) convergent margins where there is significant uplift or 2) to high latitudes in Tertiary rift zones. These environments are currently the only place where meteoric fluids are sufficiently low to mimic the dominant exchange mode that would be observed in a low-¹⁸O ocean. The consequence of an hypothesized low-¹⁸O Archean hydrosphere is the prevalence of low-¹⁸O altered igneous rocks, the product of hydrothermal processes. Instead, the most ¹⁸O depleted Archean rocks are metasedimentary rocks. Their low-¹⁸O character only defines low-¹⁸O oceans if their isotopic values are primary and the same fluids produce the expected ¹⁸O depletions in altered igneous rocks. The oxygen isotope ratios of Archean metabasalts and granitic rocks are therefore inconsistent with the existence of low-¹⁸O oceans.