EPSL zyxwvutsrq ELSEVIER Earth and Planetary Science Letters I42 ( 1996) 24 I-25 1 Geochemical diversity of the large lava field on the flank of the East Pacific Rise at 8’17’s Linda S. Hall *, John M. Sinton zyxwvutsrqponmlkjihgfedcbaZYXWV Department of Geology and Geophysics, School of Ocean and Earth Science and Technology. University of Hawaii, Honolulu, HI 96822. USA Received 30 May 1995; accepted 15 April 1996 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR Abstract We recovered samples by dredge and glass corer from ten sites on the large lava flow field near 8”17’S on the East Pacific Rise and the nearby ridge crest. The lava field comprises at least four distinct normal MORB compositions, different from the lavas collected from the ridge axis. All of the samples appear relatively young, and are indistinguishable in apparent age from the axis samples. Incompatible element variations suggest that the flow field had at least two distinct parental magmas, one of which had greater and one smaller trace element concentrations at the same MgO than the parent of the axis samples. The three parental magmas can be related by a magma mixing model. Major and trace element modeling shows that the three parental magmas could not have been produced by different degrees of melting of a homogeneous mantle source, but that they are consistent with melting of a generally depleted mantle containing variable volumes of embedded enriched heterogeneities. The presence of seamounts on the flanks of the axis in this area and the fact that the samples from one dredge appear to have come from an off-axis vent, along with the compositional bracketing of the axis parental magma by the lava field parents, suggests that this lava field may be an off-axis flow field similar to seamount fields and other smaller lava fields common along the East Pacific Rise. Keywords: East Pacific Rise; mid-ocean ridge basal&; vents; geochemistry 1. Introduction The large lava field on the east flank of the East Pacific Rise CEPR) at 8”17’S (called the 8% lava field in this paper) was discovered by Macdonald et al. [l] in 1987. From its high reflectivity on sidescan sonar they inferred that it is younger than the seafloor around it. They estimated that the lava field covers 220 km* in area, with an estimated total volume of * Corresponding author. Fax: 808 956 2538. E-mail: lindah@soest.hawaii.edu 15 lcm3, comparable to the volumes of small seamounts (e.g. [2]). The flow field lies next to a magmatically robust ridge segment with a high spreading rate of approximately 144 mm/yr [3]. The ridge segment between 7”12’S and 8’38’s is uni- formly shallow ( < 2800 m> and has a large cross- sectional area and a summit graben [4,5]. In a bathy- metric study of this segment, Cochran et al. [6] found that the cross-sectional area of the crestal high is greatest between 7”50’ and 8”15’S, near and slightly south of the center of the segment, and just north of the apparent origin of the lava field at 8”17’S. The 0012-821X/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SOOIZ-821X(96)00089-1