High water contents in basaltic magmas from Irazú Volcano, Costa Rica

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Irazú volcano, in Costa Rica, erupts magmas unusually enriched in incompatible trace elements (e.g., K, REE) relative to most other arc volcanoes worldwide. Previous studies place this enrichment in the mantle, with minimal inputs from the subducting slab. In order to test the subduction vs. mantle hypotheses, we present here the first published measurements of the pre-eruptive volatile content of Irazú magmas. Olivine-hosted melt inclusions from basaltic-andesite scoria from the 1723 eruption are volatile-rich, containing > 3 wt.% H2O, > 200 ppm CO2, > 2500 ppm S, > 2200 ppm Cl and > 1800 ppm F. The average composition of the 1723 melt inclusions is very similar to that of the host scoria (SiO2 = 54% SiO2), although inclusions include more mafic (48% SiO2) and felsic (57% SiO2) compositions. The 1723 melt inclusions have the same trace element characteristics (e.g., Ba/La) as the host scoria, ruling out exotic crustal or mantle sources. Together, the melt inclusions and their host olivines (Fo87-79) define a closed-system ascent path (150-20 MPa) of coupled degassing, crystallization, and cooling (1075-1045 °C). The maximum H2O measured in the melt inclusions and the shape of the degassing path together constrain the pre-eruptive H2O content to 3.2-3.5 wt.%, significantly higher than in ocean island basalts, but typical of arc magmas. The high H2O in Irazú melts, coupled with their high Cl/K2O, are inconsistent with enriched mantle with minimal slab fluid addition. We propose instead that subducting input is the dominant contributor to Irazú's geochemical compositions. Galapagos-derived seamounts and volcaniclastics are currently entering the trench near Irazú, and provide to the Irazú source both volatiles (from seafloor hydration and chlorination) and ocean-island-type trace elements and isotopes. A few percent of subducted Galapagos volcanics added to MORB mantle can create Irazú compositions quantitatively, provided elements are further fractionated according to solute-rich liquid or melt-eclogite partition coefficients. Subduction of seamount chains may create high-K arc volcanism elsewhere, such as in the northern Marianas. © 2007 Elsevier B.V. All rights reserved.

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Journal of Volcanology and Geothermal Research