Magmatic Processes at Variable Magma Supply along the Galápagos Spreading Center: Constraints from Single Eruptive Units

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Petrological study of eruptive units in two locations along the Galápagos Spreading Center provides insight into how the rate of magma supply affects mid-ocean ridge magmatic systems. Study areas with lower magma supply (95°W) and higher magma supply (92°W) have similar spreading rates (53 and 55 mm a–1), but differ by 30% in the time-averaged rate of magma supply (0·3 × 106 and 0·4 × 106 m3 km–1 a–1) as a result of varying proximity to the Galápagos hotspot. We use major and trace element compositions of glass and whole-rock samples, chemistry of mineral phases, and petrography to characterize parental magma variability, fractional crystallization and magma mixing in crustal reservoirs, and timescales of magmatic recharge relative to eruption. At the low magma supply study area, eruptible magma appears to be present only intermittently within the crust; magma recharge is probably infrequent, occurring with a periodicity of several hundred to one thousand years. The shallowest magma body in the crust is thought to be at ∼3 km below the seafloor, and lavas are restricted to a relatively limited compositional range (6·2–9·1 wt % MgO). Magmatic evolution at this location is probably dominated by processes occurring within a crystal-rich mush, with limited subsequent residence in melt-dominated magma reservoirs. Eruptions here appear to be closely coupled to magmatic recharge events; lower MgO lavas have compositional trends controlled by mixing of low- and high-MgO magmas from compositionally distinct parents, and commonly contain both normally and reversely zoned crystals. In contrast, at the high magma supply study area, where a seismically imaged melt lens is located ∼1·7 km below the seafloor, fractional crystallization within a melt-rich magma reservoir results in a larger range in major element compositions of the erupted magmas (2·7–8·2 wt % MgO) with less variation in trace element concentrations or ratios. Temperatures within the melt lens over the last several hundred years have varied by at least 100°C (1070–1170°C); cooling rates within the melt lens are estimated to be greater than 0·5°C per year. Relatively low-MgO lavas have over-enrichments in Cl that are best explained by assimilation of brine associated with hydrothermal circulation within the overlying crust. Between magmatic recharge events, resident magma fractionates and feeds one or more low-volume fissure eruptions. Small bodies of magma may become isolated from the larger magmatic system in the crust, allowing more extreme degrees of fractionation, locally reaching basaltic andesite. This study demonstrates that persistent melt lenses at intermediate rates of magma supply need not be ‘steady state’. The variations in magma composition among eruptive episodes at each location allow us to assess the temporal variability in magma reservoir properties at ridge segments along the Galápagos Spreading Center, in the context of regional variations in magma supply.

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