RECENT VOLCANISM IN THE SIQUEIROS TRANSFORM-FAULT - PICRITIC BASALTS AND IMPLICATIONS FOR MORB MAGMA GENESIS

Small constructional volcanic landforms and very fresh-looking lava fl ows are present along one of the inferred active strike-slip faults th at connect two small spreading centers (A and B) in the western portio n of the Siqueiros transform domain. The most primitive lavas (picriti c and olivine-phyric basalts), exclusively recovered from the young-lo oking flows within the A-B strike-slip fault, contain millimeter-sized olivine phenocrysts (up to 20 modal%) that have a limited composition al range (Fo(91.5)-Fo(89.5)) and complexly zoned Cr-Al spinels. High-M gO (9.5-10.6 wt%) glasses sampled from the young lava flows contain 1- 7% olivine phenocrysts (Fo(90.5)-Fo(89)) that could have formed by equ ilibrium crystallization from basaltic melts with Mg# values between 7 1 and 74. These high MgO (and high Al2O3) glasses may be near-primary melts from incompatible-element depleted oceanic mantle and little mod ified by crustal mixing and/or fractionation processes. Phase chemistr y and major element systematics indicate that the picritic basalts are not primary liquids and formed by the accumulation of olivine and min or spinel from high-MgO melts (10% < MgO < 14%). Compared to typical N -MORB from the East Pacific Rise, the Siqueiros lavas are more primiti ve and depleted in incompatible elements. Phase equilibria calculation s and comparisons with experimental data and trace element modeling su pport this hypothesis. They indicate such primary mid-ocean ridge basa lt magmas formed by 10-18% accumulative decompression melting in the s pinel peridotite field (but small amounts of melting in the garnet per idotite field are not precluded). The compositional variations of the primitive magmas may result from the accumulation of different small b atch melt fractions from a polybaric melting column.