Patterns in Galápagos Magmatism Arising from the Upper Mantle Dynamics of Plume-Ridge Interaction

Abstract

The origin of various patterns seen in Galápagos magmatism is investigated using numerical simulations of mantle plume-ridge interaction with the realistic geometry and evolution of the Galapágos Spreading Center (GSC). Models predict magma generation and composition from a mantle composed of fusible veins of material enriched in incompatible elements, and a more refractory depleted matrix. Model 1 simulates a low-viscosity plume, owing to a temperature-dependent mantle rheology; Model 2 includes the added dependence on water content, which leads to high viscosities in the dehydrated, shallow upper mantle. Model 1 produces the most favorable results. It shows how a modest crustal thickness anomaly observed along the Western GSC can arise from a plume with large excess temperatures (greater than 100°C). Model 1 also predicts geographic patterns in magma isotopic compositions broadly resembling those observed along the GSC as well as around the Galapágos Archipelago. These patterns are predicted to arise out of the differences in melting depths between the enriched veins and depleted matrix, coupled with spatial variations in the rate of mantle upwelling and decompression melting. The results provide an alternative to traditional explanations involving the plume mixing with or entraining the ambient mantle. The models are still missing some essential factors, as indicated by the predicted increases, rather than the observed decrease in incompatible element concentration away from the hotspot along the GSC. Possible factors include a regional-scale zoning in incompatible element and water content within the plume, or melt migration that delivers a larger flux of incompatible-element-rich melts to the GSC.