Discrete plumbing systems and heterogeneous magma sources of a 24 km3 off-axis lava field on the western flank of East Pacific Rise, 14 °S.

The largest known mid-ocean-ridge off-axis lava flow field occurs slightly off-axis near 14 degrees S along the East Pacific Rise (EPR). It comprises at least four volcanologically discrete units. We collected lava samples from 24 sites within the off-axis lava field and 7 sites on the adjacent ridge crest using the Shinkai-6500 submersible. The lava field comprises at least three distinct MORB compositions, all of which are different from the lavas collected from the nearby ridge axis. The east and west cones and the northern lobe of the lava field consist of normal MORB (N-MORB) lavas with a low concentration of incompatible elements and low LILE/HFSE and LREE/HR_EE ratios. By contrast, the samples from the west plain of the field have a higher concentration of incompatible elements and higher LILE/HFSE and LREE/HREE ratios indicating T-MORB character. The lava samples collected from the summit of the east cone show the highest concentration of LILE elements and LREEs among the lava field (E-MORB lava). The N-MORB of the off-axis lava field are more depleted in incompatible elements than the adjacent EPR axis lavas, possibly reflecting the re-melting of the residual mantle in the off-axis region. The E-MORB lava was probably derived from fertile mantle that did not undergo melting beneath the spreading center. T-MORB, which occupies the main part of the 14 degrees S lava field, is the product of magma mixing between N-MORB and E-MORB magmas. (C) 2007 Elsevier B.V All rights reserved.