Diversity and spatial zonation of volcanic rocks from the East Pacific Rise near 21° N

A substantial range of petrologic rock types has erupted on the accreting plate boundary near 21° N on the East Pacific Rise (EPR). Young olivine basalts have Fo_89-86 phenocrysts, low bulk TiO₂ (1.1–1.3%), Ba (7–10 ppm), and high Ni contents (>100 ppm). Older plagioclase-olivine-pyroxene (POP) basalts have Fo_86-81 phenocrysts, high TiO₂ (1.4–1.7%), Ba (9–40 ppm), and low Ni (<100 ppm). The youngest olivine basalts erupt immediately around a segmented axial fissure system. Progressively older, more fractionated POP basalts have spread farther from the same fissure system, producing a stratigraphically-controlled zonal pattern of basalt type distribution around the eruptive fissures. A topographic and morphologic en echelon displacement of the ridge axis fissure of 1.7 km to the NW near 20°54N offsets this zonal distribution pattern. Low pressure crystal fractionation of olivine, plagioclase, and clinopyroxene (251) from an olivine basalt parent would yield POP basalts of the observed Zr, Ti, Y, P and major element chemistry. However very incompatible elements Ba and K are too variably enriched in POP basalts for this model to be viable. Small, variable degrees of mantle partial melting is not a viable model either because of the substantial depletion of Ni which correlates with incompatible element enrichment and because of the precise low pressure cotectic character of POP basalts. The in situ fractionation model of Langmuir (1987) can explain these features. The relative abundance of fractionated lavas, their small-scale areal chemical zonation, the petrochemical correlation between types, and geophysical evidence point to the existence of shallow fractionating magma reservoirs beneath the EPR at 21° N.