HYDROGEN ISOTOPE COMPOSITION OF EARLY PROTEROZOIC SEAWATER

The hydrogen isotope composition of Early Proterozoic seawater has bee n indirectly determined on the basis of delta D values measured in maf ic-ultramafic samples from a hydrothermally altered layered flow from the Chukotat Group of the Lower Proterozoic (2.0-1.9 Ga) Cape Smith fo ld belt (northern Quebec). This flow and other mafic-ultramafic units of the fold belt represent some of the oldest relict oceanic crust ide ntified on Earth. Measured delta D values of whole-rock samples range from -53 parts per thousand to -40 parts per thousand, matching closel y values found in modern metavolcanic rocks and Phanerozoic ophiolites . A model of water-rock interaction for conditions of metamorphism ded uced from mineral parageneses and Sr and O isotope data (T = 200-250 d egrees C; water/rock ratio close to 1.0) indicates that the delta D va lues of Chukotat how samples reflect interaction,vith seawater that ha d a delta D = 0 parts per thousand +/- 20 parts per thousand. This res ult implies that the hydrogen isotope composition of Early Proterozoic seawater was certainly similar to the modern value and that subductio n and ridge processes that control the global water cycle have not cha nged significantly since the Early Proterozoic.