High-Mg andesite genesis by upper crustal differentiation

Whereas arc magmas typically undergo early degassing-induced crystallization and viscous stagnation at mid-crustal levels, hotter and less hydrous melts that are associated with elevated surface heat flux may experience delayed crystallization at shallower levels. Using MELTS modelling, we demonstrate here that high-Mg andesites, which have been regarded as particularly hydrous primary melts generated in equilibrium with mantle peridotite, can form by crystal fractionation from low-H2O primitive arc basalts in the upper crust. This is consistent with many characteristics previously attributed to their primary origin, including forsteritic olivines that contain chromite inclusions and lack significant reaction rims, and Cr-rich pyroxenes.