OPHIOLITES AND THE HISTORY OF THE OCEANS: PLATE TECTONICS, OCEAN VOLUME AND CONTINENTAL EVOLUTION

Ophiolites are pieces of allochthonous ancient oceanic lithosphere. Their original tectonic setting and the mechanism of their emplacement remains controversial most likely because, in plate tectonics, the fate of most ocean lithosphere is to be recycled back into the mantle. No single mechanism accounts for the diversity of ophiolite complexes. Ophiolites are important because of the insights they provide into processes that occur at the oceanic spreading centers. The existence of hydrothermal systems at spreading boundaries solved the discrepancy between the age of the Earth and the much younger age of the Earth inferred from river fluxes and oceanic salinity. The competition between continental weathering and sub-seafloor hydrothermal exchange controls the chemical and isotopic composition of seawater. Plate tectonics and the geometry of accretion of new crust buffers the oxygen isotope composition of seawater. The isotopic composition of seawater becomes a proxy for plate tectonics. The meteoric water cycle anchored by the isotopic composition of the ocean indicates the existence of a plate tectonic regime back through the Archean. The hydrogen isotope composition of the ocean is a proxy for ocean volume because processes that shift the D/H ratio of the ocean tend to enrich the oceans over time. The constancy of the ranges of hydrogen isotope ratios of igneous, sedimentary and metamorphic rocks of any age suggests that oceanic volume has remained relatively constant over post Hadean geologic time. Stable isotopes lend support to the link the between ocean volume and continental thickness. The rapidity of mechanical weathering, 10 Myr timescales and sea level changes afforded by plate tectonic rates tend to stabilize continental crustal thicknesses at 35-40 km thick over geologic time.