Rhyolite, dacite, andesite, basaltic andesite, and basalt volcanism on the Alarcon Rise spreading-center, Gulf of California

The Alarcon Rise is a ~50 km long intermediate-rate (~50mm/a) spreading segment at the southern end of the Gulf of California. The Rise is bounded by the Tamayo and Pescadero transforms to the south and north. In Spring 2012, an MBARI-led expedition mapped a ~1.5- 3km wide swath of the ridge axis at 1-m resolution and completed 9 ROV dives (Clague et al., this session). Sampling during the ROV dives was supplemented by use of a wax-tip corer to recover volcanic glass: 194 glassy lava samples were recovered from the Rise. The vast majority of lava flows along the axis are basalt and rare basaltic andesite. More than half the basalts are plagioclase-phyric to ultraphyric (Martin et al., this session), and the rest are aphyric. Rare samples also include olivine or olivine and clinopyroxene phenocrysts. Analyses of half of the recovered glass basalt rinds range in MgO from 4.3 to 8.5 wt.% and those with MgO > 6 wt % have K2O/TiO2 = 0.07-0.11. The basalts are broadly characterized as normal mid-ocean ridge basalts (N-MORB). E-MORB is also present near the center of the ridge segment, but has been found only as pyroclasts in sediment cores. A much greater range in lava composition is associated with an unusual volcanic dome-like edifice that lies ~9 km south of the Pescadero transform. Two dives in the vicinity of the dome collected lava and volcaniclastic samples consisting of moderately to sparsely phyric light brown to colorless volcanic glass. Feldspar is the dominant phase, but magnetite, fayalitic olivine, light tan and light green clinopyroxene, orthopyroxene, zircon, and rare pyrite blebs also occur. Melt-inclusions are common in many phenocrysts, especially of plagioclase. Hydrous mineral phases are not observed. These samples have rhyolitic glass compositions (75.8- 77.4 SiO2 wt %), but their whole-rock compositions will be somewhat less silicic. Pillow flows to the immediate west have dacitic glass compositions (67.4- 68.8 wt % SiO2). Basaltic andesitic glasses (~56% SiO2), and basaltic glasses, more typical of the rest of the ridge, occur within 100m of the dome. Flow(s) with andesite glass compositions (~62 wt % SiO2) are exposed in fault scarps ~1km SW of the dome. Minor seawater contamination in evolved lavas (> 53 wt % SiO2) is indicated by generally increasing Cl-/K2O with decreasing MgO and increasing SiO2. Three preliminary Sr-isotopic analyses indicate that crustal assimilation and assimilation of altered crustal rocks has been minimal. Major element trends, and a preliminary subset of laser-ablation ICP-MS data, are consistent with extended fractional crystallization from a multiply-saturated parental liquid(s) of limited compositional range. Rare earth element abundances range from 15-150x chondritic, and patterns are coarsely described as flat with moderate LREE-depletion (LaN/CeN ~ 0.8- 0.9). Differentiated lavas have distinct negative Eu-anomalies documenting extensive crystal fractionation of plagioclase in the generation of the more evolved lavas.