FOUR BILLION YEARS OF OCEAN CRUST FORMATION

We have used geological, geochemical and tectonic datasets of 116 ophiolites from the major global Phanerozoic orogenic belts, combined with 111 Precambrian greenstone belts, in order to construct an overview of oceanic crust formation through four billion-years of Earth history. Geochemical discrimination analysis reveals that basaltic units of the Phanerozoic ophiolites are dominantly subduction-related (75%), specifically associated with backarc processes, and that they are characterized by a strong MORB component, similar to ophiolites in Precambrian greenstone sequences (85%). The remaining 25% of the Phanerozoic and 15% of the Precambrian subduction-unrelated ophiolites are represented by Mid-Ocean-Ridge (MOR), Rift/Continental Margin, and Plume types. Throughout the Phanerozoic ophiolite record there are large geochemical variations in major and trace element patterns with no obvious secular trends. By contrast, basaltic units in the ophiolites of the Precambrian greenstones, starting in the late Paleo- to early Meso-proterozoic (ca. 2.0-1.8 Ga), exhibit an apparent decrease in the average values of incompatible elements such as Ti, P, Zr, Y and Nb, and an increase in the compatible elements Ni and Cr, with deeper time to the end of the Archean and into the Hadean. These changes can be attributed to the decreasing degrees of partial melting of the upper mantle from Hadean/Archean to the Present. The onset of these geochemical changes coincide with the timing of detectible developments in the structural architecture of ophiolites, such as the occurrence of greater volumes of gabbro and more common sheeted dyke complexes, and lesser occurrences of ocelli (varioles) in the pillow lavas in ophiolites younger than 2 Ga. The global data from the Precambrian ophiolites, representative of nearly 50% of all known worldwide greenstone belts, provide significant clues for the operation of plate tectonic processes in the Archean.