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Squeezing Water from a Stone (Water in Arc Magmas)

Terry Plank, Boston University

2007 F. Earl Ingerson Lecture Abstract
GSA Annual Meeting
Colorado Convention Center Room 503
Monday 29 October 2007 - 12:15p

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 the conditions of mantle melting and magma differentiation in the crust. Unlike at mid-ocean ridges, the water content of the mantle correlates positively with the degree of melting beneath back-arc basins and volcanic arcs, demonstrating the important of water in driving melting at subduction zones. Mantle temperatures beneath back-arc basins are nonetheless similar to those beneath global mid-ocean ridges (Tp 1300-1500°C). Beneath the Central American arc, olivine-liquid temperatures are also high (~ 1300°C), even when taking into account magmatic H2O, and in agreement with seismic wave attenuation. Thus, the combination of high temperatures and high H2O may lead to large degrees of melting (> 20%) in the mantle wedge, enough in some cases to generate boninites (within the active Tonga arc). Finally, 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.