Water in Aleutian magmas: Its origins in the subduction zone and its effects on magma evolution.

摘要

Water is the most important chemical species in subduction zones because it facilitates the transfer of elements from the subducting slab to the mantle wedge, causes flux melting that ultimately leads to volcanism, and provides a first order control on crystal fractionation and the depth at which magmas stall in the crust. While much work has been dedicated to modeling these processes, this study provides the first comprehensive study of water measurements in Aleutian magmas. Melt inclusions were analyzed for major, trace, and volatile (H2O, CO2, S, Cl, F) elements in volcanic deposits from eleven volcanoes along the Aleutian volcanic arc. The Aleutians span the global range in magmatic water contents, from 2-7 wt.%. Augustine volcano records the highest water contents (7 wt.%) globally measured in any arc basalt to date. Trace element and volatile contents are used to identify and quantify the slab components contributing to Augustine, and to determine if high water contents result from a H2O-rich slab component, or from a high flux of a lower-H2O slab component. Consideration of Augustine within the spectrum of other Aleutian volcanoes supports the latter scenario. Magmatic water contents are also combined with whole rock data to develop an index of iron enrichment (the THI, or tholeiitic index), that demonstrate the dominant control by water on major element fractionation of magmas in the crust. Water-induced suppression of silicate minerals, especially plagioclase, has been demonstrated experimentally, but this is the first time the resulting effect on iron has been demonstrated with natural samples. Lastly, the pressure-dependent solubility of H2O-CO2 in magmatic liquids is used to show that the depths of melt inclusion entrapment relates to geodetically-determined depths of magma intrusion and storage. This relationship suggests magmatic water contents also influence where magmas stall in the crust, possibly due to decompression-induced crystallization and the subsequent increase in magma viscosity. In addition, the conditions required for melt inclusion formation occur near the top of magma storage zones and in the conduit during ascent to the surface, rather than in slowly-cooling magma reservoirs.