Composition, Temperature, and Focused Melting Beneath Ocean Ridges

Abstract:

Global variations in mid-ocean ridge basalt (MORB) composition, the most abundant magma type on the planet, variously correlate with ridge depth. This has been attributed to either variations in mantle temperature, resulting in varying crustal thickness and thermal buoyancy, or to varying mantle composition and density. MORB compositional variations over ocean rises, large regional highs in ridge depth, however, are not uniform, and require more than one primary variable to explain them. A key consideration is that while mantle residue is continuously removed from the melting column as it narrows upward, due to focusing, melt is conserved, progressively interacting with the mantle to the base of the lithosphere where it begins to segregate. This is consistent with field observations that most dunites representing melt segregation and transport are late-crosscutting features in abyssal and ophiolitic mantle sections. We use pMELTS, a thermodynamic modeling program, to illustrate the relative effects of source composition and potential temperature on MORB during focused melting and show that both mantle temperature and composition variations can explain global MORB geochemistry. Thus estimates of lateral temperature variations in the upper mantle that ignore the effects of source composition represent only a maximum – and likely lead to an overestimate.