Mid-Crustal Magma Chambers at Intermediate Magma Supply along the Galápagos Spreading Center

Along the Galápagos Spreading Center (GSC), a decrease in the rate of magma supply to the crust with increasing distance from the Galápagos hotspot is associated with deepening of a seismically imaged melt lens from 1-2.5 km below the seafloor near the hotspot to 3-4 km at 94.4°W; west of this location, no melt lens has been detected [Blacic et al., 2004]. Here, we present olivine-hosted melt inclusion compositions from a sequence of three mapped lava flow fields with known relative ages at 95°W on the GSC. Vapor-saturation pressures derived from CO₂ and H₂O concentrations in glassy melt inclusions constrain entrapment depths. These depths, combined with major element compositions of melt inclusions, host olivines, and carrier magmas, reveal magmatic conditions and processes prior to eruption. The entrapment depths of melt inclusions from the two older eruptions cluster at 2.8 km below the seafloor. After correcting for post-entrapment crystallization, major element compositions of these melt inclusions follow trends predicted by petrologic models of equilibrium crystallization at depths corresponding to the entrapment pressure. Olivines from these eruptions are essentially unzoned and in equilibrium with their carrier magmas (Fo of olivine cores: 84-85 and 84-89 for the first and second eruptions, respectively). Although melt inclusions have more variable K/Ti and range to higher MgO, the range in melt inclusion K/Ti brackets that of the carrier magmas. Differences in K/Ti between the two flow fields at roughly the same MgO content require that they were derived from different parental magmas. The entrapment depths of melt inclusions from the most recent eruption are slightly deeper, roughly 3.2 km below the seafloor. Although the compositions of melt inclusions within this flow field approximate trends expected from equilibrium crystallization, bimodal distribution of K/Ti indicates that two different parental magmas were involved. Furthermore, the carrier magma composition is inconsistent with equilibrium or fractional crystallization of a single parental magma, with variable incompatible element ratios, and olivine core compositions ranging from Fo 76-85, including both normally and reversely zoned crystals. At least two recharge events must have occurred between the second and third mapped eruptions at this location; one likely occurred shortly before, and may have triggered, the most recent eruption. These data indicate that prior to eruption, magmas at 95°W resided, at least intermittently, at a narrow range of mid-crustal depths generally consistent with the global trend of increasing melt lens depth with decreasing rates of magma supply along the mid-ocean ridge system.