Water in Arc Magmas

The water content of arc magmas is arguably the most important chemical component controlling their formation, evolution, and eruption. Yet, few direct measurements exist because magmas nearly completely degas upon ascent and eruption. Volcanic rocks are thus largely devoid of water, with the exception of melt inclusions - bits of undegassed melt trapped at pressure inside crystals. Such melt inclusions demonstrate that H2O is actually a major species (2-7 wt%) dissolved in mafic arc magmas at depth. Although an enormous amount has been learned from melt inclusions, they are nonetheless rare in many volcanic deposits, and so other methods are needed. One promising technique is based on the water content in nominally-anhydrous clinopyroxene phenocrysts in well-quenched tephra, using recent models for water partitioning between melt and pyroxene. We have demonstrated the fidelity of clinopyroxene in accurately recording the maximum water content and degassing trends of melt inclusions from several volcanic deposits.

New estimates of the water content of arc magmas provide new constraints on where magmas stall in the crust and crystallize.  Melt inclusion entrapment pressures for different Aleutians volcanoes vary from 2 - 20 km depth, largely as a function of their water content, and relate well with magma inflation/deflation depths inferred from  geodetic data. Such relationships are consistent with models whereby magmas stall near their depths of water saturation, due to the large increase in magma viscosity and crystallinity.  Finally, we also find H2O strongly affects how magmas differentiate, from classic Fe-enrichment (tholeiitic trend) occurring only in the driest magmas (< 2 wt% H2O), to strong-Fe depletion (calc-alkaline trend) occurring in the wettest magmas (> 4 wt%). A new tholeiitic index (THI) both relates strongly to H2O content and provides a new tool for predicting H2O in ancient magmas.