Effects of variable magma supply on magma reservoirs and eruption characteristics along the Galapagos Spreading Center

This dissertation examines the effects of magma supply on mid-ocean ridge magma reservoirs and eruption characteristics, using mapped lava flow fields erupted at two locations along the Galapagos Spreading Center. Low- and high-magma-supply study areas at 95° and 92°W have similar spreading rates, but differ by 30% in the rate of magma supply due to varying proximity to the Galapagos hotspot.;Detailed geologic maps of each study area incorporate observations of flow contacts and sediment thickness, in addition to sample petrology, geomagnetic paleointensity, and bathymetry data. Mapped lava flow fields are used to compare characteristics of eruptions at the two locations. At the 92°W study area, lava morphologies characteristic of higher effusion rates are more common, and eruptions typically occur along elongated fissures with total eruptive volumes ranging from 0.002--0.13 km3. In contrast, at the 95°W study area, eruptions typically produce axial seamounts or more irregular clusters of pillow mounds, and eruptive volumes are an order of magnitude larger (0.09 to 1.3 km3).;Whereas a seismically imaged melt lens exists ∼1.7 km below the seafloor at the 92°W study area, no melt lens has been detected at the 95°W study area. Measurements of CO2 and H2O in olivine-hosted melt inclusions are used to constrain depths of magma residence at the low-magma-supply study area. Results indicate that prior to three recent eruptions, magma resided at depths of 3.0-3.4 km below the seafloor.;Compositional data were used to compare the characteristics of magmatic systems at the two locations. At 92°W, the rate of magma supply is sufficient to sustain a shallow melt lens (Blacic et al., 2004), within which the competing effects of fractional crystallization and frequent magmatic recharge result in eruption of lavas with low crystal contents and highly variable MgO. In contrast, melt-rich bodies at 95°W are likely only present intermittently; magmatic evolution at this location appears to be dominated by processes involving crystal-rich mush, which is mixed in varying proportions with hotter, more melt-rich magma during recharge events that closely precede eruptions. Limited residence within melt-rich lenses allows mixing trends to be preserved in erupted lavas.