Fractionation of mid-ocean ridge basalt (MORB)

This paper presents new experimental data on MORB lavas obtained over the pressure range 0.001 to 10 kbar, discusses the evidence for the operation of fractional crystallization in MORB suites and offers a method for calculating the fractional crystallization history experienced by a MORB. Two important saturation boundaries that control MORB compositional evolution during fractional crystallization are the olivine (oliv)+plagioclase (plag) and oliv+plag+high-Ca pyroxene (augite) boundaries. A method is presented for locating these two saturation boundaries as functions of liquid composition and pressure of crystallization. Expressions also are presented for predicting the compositions of oliv, plag and augite. With these tools the effect of differentiation processes on MORB magma evolution can be deduced over the pressure range of 0.001 to 10 kbar. The compositional variations exhibited by suites of MORB from the mid-Atlantic near the Kane Fracture Zone at 23°N, the AMAR region at 36°47'N, the Reykjanes Peninsula in Iceland and the mid-Cayman rise in the Caribbean are consistent with fractional crystallization at pressures of 3 to 6 kbar (oceanic mantle fractionation). At 10-12 °N on the East Pacific Rise MORB compositional variation is consistent with fractional crystallization at pressures of 0.001 to 2 kbar (ocean crustal fractionation). The occurrence of fractional crystallization in the oceanic upper mantle indicates cooler upper mantle temperatures beneath some spreading centers, which allow magmas derived by partial melting of mantle peridotite to crystallize and differentiate as they rise. A part of the positive correlation between Na₂O and FeO at 8 wt % MgO observed in spatially associated MORB suites [Klein and Langmuir, 1989] can be ascribed to fractional crystallization of MORB magmas over a depth range of 0 to 18 km. The compositional variation in MORB suites and the mineralogical evidence preserved in plagioclase phenocrysts indicate that MORB magma production and evolution involves fractionation of some mantle-derived partial melts prior to aggregation (or mixing).