Depth, transience and eruptibility of magma-mush reservoirs modulated by varying magma supply along the Galápagos Spreading Center

The Galápagos Spreading Center (GSC) is characterized by an intermediate spreading rate, and influenced by the nearby Galápagos hotspot, resulting in a pronounced along-axis gradient in magma supply that decreases by ~40% from east to west. Between 92°W and 97°W, the axial morphology shifts from a high to a valley, as the seismic crustal thickness decreases from 7.5 to 5.6 km, and  the seismically-imaged axial melt lens (AML) deepens from 1.4 km at 92°W to 3 km at 94°W, beyond which it becomes undetectable, e.g., at 97°W (Blacic et al., 2004, doi: 10.1029/2004jb003066). However, a P-wave low-velocity anomaly persists along the GSC between 92°W and 97°W, suggesting the widespread presence of an axial crystal-rich mush zone (Canales et al., 2014, doi: 10.1002/9781118852538.ch17). These along-GSC variations provide an ideal laboratory to explore the impact of melt flux on the dynamics (e.g., depth, transience, and eruptibility) of magma (crystal-poor) – mush (crystal-rich) systems at the axis of mid-ocean ridges.

We use a 2-D numerical thermal model, multiporo-magma, which couples repeated, instantaneous melt emplacement events in the lower crust, parameterized magma convection within individual magma bodies, and hydrothermal circulation (porous flow) in the uppermost crust. Our reference model predicts that, from 92°W to 97°W, decreasing melt flux leads to a deepening of the crystal mush zone (from 1.5 to 3.5 km), and to the formation of increasingly smaller and more transient melt-rich magma bodies within the mush zone. These results highlight that higher melt fluxes (e.g., 92°W) support nearly steady-state magma bodies capable of sustaining frequent eruptions, whereas lower melt fluxes (e.g., 97°W) result in deeper, short-lived magma bodyies with reduced eruptive potential. Importantly, we show that the absence of a seismically-imaged AML at any given time can reflect increased transience in the thermal state of the axis, and does not require that the 1000ºC isotherm lies below the Moho, as previous thermal models had postulated. Our simulations further reveal how the behaviour of the crystal mush zone is modulated by the efficiency of hydrothermal cooling and the size of individual melt sills.