THE ROLE OF OPHIOLITE STUDIES IN CONSTRAINING SEAWATER CIRCULATION AND ALTERATION OF MODERN OCEANIC CRUST

Ophiolites provide a critical three-dimensional view into oceanic crust to address questions of the depth and distribution of seawater penetration and the cumulative mineralogical and geochemical impact of seawater alteration. Systematic mapping of the lower crustal sections of ophiolites (e.g. Oman and Cyprus) and lower crustal sections of modern oceanic crust (e.g. Atlantis Bank (ODP Site 735B) and Hess Deep) document the presence of high-temperature seawater interactions. Large variations in oxygen isotopic variations within complete crustal sections (3 to 12 permil d18O) and secondary mineral chemistry reveal water-rock interactions to variable depths within the lower dikes and uppermost gabbros. Studies from Oman, Atlantis Bank, and the Hess Deep provide evidence for high temperature reactions (above 500C), seawater penetration early in cooling history, shift of magnetic signal from extrusives to intrusives by formation of secondary magnetite. Secondary mineral assemblages typical of dikes and gabbros from ophiolites (e.g. chlorite and epidote after plagioclase and pyroxene, titanite after magnetite; talc after olivine) can explain the chemistry of the hydrothermal fluids.

Ophiolite studies may still prove to be powerful analogues in looking at more complex processes in modern oceanic crustal formation. Multiple hydrothermal systems are created by diachronous intrusive events. The presence of high temperature shear zones and late magmatic fluids, examined in detail at the ultra-slow spreading Atlantis Bank, has extended models for seafloor structures. The paradigm of propagating rifts and overlapping spreading centers has been built primarily upon arguments from geophysics and basalt chemistry. Yet the lower crust formed in such an environment would show much complexity as a new magmatic system is superimposed upon slightly older intact crust. Late gabbro dikes crosscutting deep deformed gabbros with extensive high temperature alteration indicate that this complexity may be evident in the northern portion of the Oman ophiolite. The "non-ideality" of this ophiolite may in fact be a window into the subsurface processes related to non-linear behavior of mid-ocean ridges.