Effects of Magma Supply on Mid-Ocean Ridge Magma Reservoirs, as Sampled by Individual Eruptions on the Galápagos Spreading Center

Abstract:

Petrologic study of eight individual eruptive units in each of two locations along the Galápagos Spreading Center allows spatial and temporal variability in magma reservoir properties and processes to be constrained, providing insight into how the rate of magma supply affects crustal magma reservoirs at mid-ocean ridges. Low- and high-magma supply study areas at 95°W and 92°W have similar spreading rates (53 and 55 mm/yr), but differ by 30% in the time-averaged rate of magma supply (0.3×10⁶ and 0.4×10⁶ m³/km/yr) as a result of varying proximity to the Galápagos hotspot. We use major and trace element analyses of glass and whole rock samples, chemical analyses of mineral phases, and observations of rock microstructure to characterize fractional crystallization, assimilation, magma mixing, and the timescales of magmatic recharge events relative to eruptions. At 92°W, high magma supply sustains a shallow melt lens ~1.7 km below the seafloor, within which the competing effects of fractional crystallization, assimilation, and frequent magmatic recharge result in eruption of lavas with low crystal contents and highly variable MgO (2.7-8.2 wt. %). Resident magma can be repeatedly tapped by low-volume fissure eruptions between magmatic recharge events. In contrast, at 95°W melt-rich bodies are likely only present intermittently, and at greater depths (3.0-3.4 km) below the seafloor. Magmatic evolution at this location is dominated by processes involving crystal-rich mush, which is co-mingled in varying proportions with hotter, more melt-rich magma during recharge events that closely precede volumetrically larger and probably less frequent eruptions. Erupted lavas are generally more phyric than those at 92°W, with higher and less variable MgO (6.2-9.1 wt. %). Limited residence within melt-rich reservoirs allows mixing trends to be preserved in erupted lavas.