Wet and Deep vs. Dry and Shallow: Global Fractionation Conditions in Volcanic Arcs
Abstract
In volcanic arcs, primitive basalts differentiate to form erupted products ranging from basalts to high-silica rhyolites, and this magmatic differentiation is dominantly produced by crystal fractionation within the arc crust. During crystal fractionation, melt compositions evolve along liquid lines of descent (LLD) toward cotectic lines controlled by the simultaneous fractionation of multiple phases. The locations of these cotectics are not a function of parental magma composition, but of intensive parameters, primarily fractionation pressure and H2O activity. These parameters can therefore be inferred directly from the compositional trends of erupted arc lavas. In this study, we present a global compilation of volcanic arc lava whole rock compositions coupled with their phenocryst assemblages. Using this dataset, we show that lava compositions that fall on cotectic trends dominantly contain the predicted cotectic phenocryst assemblage, verifying our model for LLD-controlled magmatic differentiation. Further, using these cotectic trends, we introduce proxies for magmatic water activity and differentiation pressure based on major element cotectic trends: the cpx+pl cotectic, best observed in Ca-Al systematics of fractionating magma series, can be used to infer water activity; the ol+cpx cotectic, best observed Ca-Mg systematics, can be used to infer relative pressures of early fractionation. These proxies correlate with previously developed arc magma hygrometers, and also with both crust and mantle wedge thickness. We find that water activities of fractionating arc magma series vary from relatively dry along the entire fractionation trend to early fluid saturation. Island arcs dominantly preserve water-poor fractionation trends, while continental arcs preserve both water-poor and water-rich fractionation trends, often within a single arc system. This variation in H2O-content primarily reflects variations in the primitive magmas, which, in water-rich arcs, subsequently controls the depth at which fractionating magmas reach water saturation. The initial water content in primitive melts appears to be controlled by the thickness of the melting column in the mantle wedge, suggesting that variations in fluid fluxes from the slab to the melting region are of secondary importance.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T21B..07K
- Keywords:
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- 8145 Physics of magma and magma bodies;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY