Significance of Melt-Wall Rock Reaction: A Comparative Anatomy of Three Ophiolites

The ascent of basaltic melts through the upper mantle results in chemical disequilibrium between the melts and the pyroxene and plagioclase of the wall-rock peridotite. Phase and cryptic variations in ophiolitic peridotites demonstrate that the resulting reactions deplete the mantle in magmatophile components and enrich ascending melts in Ca, Na, Al, and incompatible trace elements while buffering their Mg/Mg + Fe ratios at primitive values (>0.6). A comparative anatomy of the Trinity, Oman, and Darb Zubaydah ophiolites illustrates both the significance of this process in shaping the composition of the shallow lithospheric mantle and how the integrated effects may reflect tectonic setting. A first-order correlation between crustal thickness and degree of mantle depletion exists, but multiple rifting events may remove part of the crustal record so that melt/rock ratios are difficult to quantify. Most impressive in ophiolitic peridotites is the abundance of melt crystallization and reaction products in zones of focused porous flow and(or) conduits indicating that melt segregation occurs at depths >20 km in extensional tectonic settings. Within these zones of melt transport, melt/rock reaction ratios will vary as the composition of the wall rocks evolves with melts becoming chemically insulated from the wall rocks when reaction zones of dunite develop. This may help explain why many MORBs retain trace-element signatures of a deep garnet-bearing source.