Frontiers PaperWhy do mafic arc magmas contain ∼4 wt% water on average?

The last 15 yr have seen an explosion of data on the volatile contents of magmas parental to arc volcanoes. This has occurred due to the intense study of melt inclusions trapped in volcanic phenocrysts, aliquots of magma that have presumably escaped degassing during eruption. The surprising first-order result is the narrow range in H2O concentrations in the least degassed melt inclusions from each volcano. Nearly all arc volcanoes are sourced with mafic magmas that contain 2-6 wt% H2O. The average for each arc varies even less, from 3.2 (for the Cascades) to 4.5 (for the Marianas), with a global average of 3.9±0.4 wt% H2O. Significant variations occur from volcano to volcano within each arc, but the means are indistinguishable within one s.d. The narrow range and common average value for H2O are in stark contrast to the concentrations of most other subduction tracers, such as Nb or Ba, which vary by orders of magnitude. A modulating process, either in the crust or mantle, is likely responsible for the restricted range in the H2O contents of arc melt inclusions. One possibility is that melt inclusion H2O values reflect vapor saturation at the last storage depth in the crust prior to eruption. In this scenario, magmas rise from the mantle with variable H2O contents (>4 wt%), become vapor-saturated and start degassing, and continue to degas up until the depth at which they stall. If the stalling depths are ∼6 km, which is common for storage depths beneath volcanoes, magmas would be saturated at ∼4 wt% H2O, and melt inclusions, most of which become closed during further ascent, would thus record ≤4 wt% H2O. Another possibility is that the mantle melting process modulates water content in the melt such that magmas rise out of the mantle with ∼4 wt% H2O. A strong relationship between the water content of the source, H2O(o) and the degree of melting (F) maintains nearly constant water contents in the melt for a restricted range in mantle temperature. Magmas with 3-4 wt% H2O can be generated at ∼50° below the dry solidus for a wide range in F and H2O(o). The narrow range in wedge temperatures may be another manifestation of a planet with average upper mantle of 1400 °C potential temperature. The characteristic mean and range of H2O contents of arc magmas has implications for both the volatile fuel for explosive eruptions and the mass balance of H2O recycled through subduction zones.

► Melt inclusions in arc volcanic rocks vary surprisingly little in their maximum water content. ► This mean must have meaning, and reveals a modulating process in either the crust or mantle. ► Crustal magma storage may enforce an average H2O concentration via vapor saturation. ► Wet melting in the mantle includes a negative feedback on the water concentration in melts.