Author name: Jonathan D. Blundy

Crustal Magmatic Systems from the Perspective of Heat Transfer

Crustal magmatic systems are giant heat engines, fed from below by pulses of hot magma, and depleted by loss of heat to their surroundings via conduction or convection. Heat loss drives crystallization and degassing, which change the physical state of the system from relatively low-viscosity, eruptible melt, to high-viscosity, immobile, partially molten rock. We explore the temporal evolution of incrementally grown magmatic systems using numerical models of heat transfer. We show that their physical characteristics depend on magma emplacement rates and that the majority of a magma system’s lifetime is spent in a highly crystalline state. We speculate about what we can, and cannot, learn about magmatic systems from their volcanic output.

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Superhydrous Arc Magmas in the Alpine Context

Magmatic rocks in the Alps are scarce. What little arc magmatism there was pre-dates the Eurasia–Adria collision at 43–34 Ma but ends at 30–29 Ma. Conversely, geochemical data for magmatic rocks from the Alps resemble that of subduction-related magmatic arcs. A characteristic of Alpine magmatism is the occurrence of relatively deep (80–100 km) superhydrous (>8 wt% H2O) low-K primary magmas in the east and shoshonitic K-rich magmas in the west. These features are likely related to the absence of vigorous mantle wedge convection. Superhydrous primary magmas undergo extensive crystallization and fluid saturation at depth, producing high ratios of plutonic to volcanic rocks. We speculate that superhydrous primary arc magmas are a consequence of slow convergence and the initial architecture of subducting crust.

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