Axial and off‐axial heterogeneity of basaltic rocks from the East Pacific Rise at 12°35′N–12°51′N and 11°26′N–11°30′N

Abstract

Surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the EPR at 12°35′N–12°51′N and at 11°26′N–11°30′N and from four adjacent seamounts centered at less than 18 km from the rise axis. The basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition. There is no correlation between lava flow morphology, geological settings, and compositions. Based on mineral and compatible element composition, these basalts were classified into three types. The least evolved olivine‐basalts and highly phyric plagioclase‐basalts (type A) have high Fo87–89, high Mg # (66–70), and high Ni (>100) contents. The most evolved plagioclase‐olivine basalts with or without pyroxene (type B and C) have relatively low Fo78–86, low Mg # (55–65), and low Ni (60–100 ppm) contents. Simple crystal fractionation (3–18%) accounts for some of the observed compositional range. However, based on their incompatible element contents, the above basalts are divided into three groups (the various types A to C are found in each group): depleted basalts with low K/Ti (0.04–0.15) ratios, low Zr (<100 ppm) and low Nb (<0.4 ppm) contents, undepleted basalts with the highest K/Ti (0.25–0.46), Zr (150–170 ppm), and Nb (8–16 ppm) contents and the transitional basalts having compositions between these two extremes. These three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3–4 km in width near 12°43′N–12°51′N on the EPR. Similar volcanic diversity in a more limited area (<3 km2) is observed on off‐axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis. Depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle. The transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation. Two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting; we suggest that a lherzolitic mantle, with minerals having a variable behavior under partial melting, constitutes this type of composite mantle. Indeed, clinopyroxene, enriched in incompatible elements, will be consumed at an earlier stage of melting than the olivine plus orthopyroxene. Eruption of compositionally distinct magma batches in close proximity on axial and off‐axial structures implies periods of quiescence between successive magmatic stages and cycles.