HIGH H2O IN WESTERN VS EASTERN ALEUTIAN BASALTS FROM CALCIUM-IN-OLIVINE HYGROMETRY

The Buldir and East Cape Volcanoes of Buldir Island are the westernmost emergent volcanic systems in the Aleutian island arc. Criteria of Miyashiro (1974 – Am J Sci) and Zimmer et al. (2010 – J Petrol) indicate that Buldir volcanic rocks define an igneous series that is more highly calc-alkaline than observed in any volcano of the central or eastern Aleutians. Whole-rock data indicate this is because FeO* is low and SiO₂ is high at Buldir relative to MgO and Mg#. Buldir rocks also carry relatively low CaO and high Na₂O, and low ratios of CaO and FeO* to Al₂O₃. These characteristics are common among primitive seafloor basalts of the Ingenstrem Depression (located west of Buldir Island), but they are rare in central and eastern Aleutian volcanoes, where the dominant calc-alkaline series originates in primitive basalts with somewhat higher CaO and FeO* relative to MgO and Al₂O₃. Thus, whole-rock major element data indicate that primitive basalts in the western Aleutians are more hydrous and drive more highly calc-alkaline volcanism than is common in the central and eastern Aleutians. This conclusion can be tested by application of Ca-in-olivine hygrometry (Gavrilenko et al., 2016 – J Petrol). Primitive eastern Aleutian basalts carry olivine phenocrysts (FO80-91) with 1000-1600 ppm Ca. This is low compared to similar-FO olivine in MORB, which most commonly have 1600-2700 ppm Ca (Sobolev et al., 2007 – Science). Similarly forsteritic olivine in primitive western Aleutian basalts contain <1000 ppm Ca. These sharp differences in olivine Ca cannot reflect the CaO contents of the magmas in which the olivine crystals grew. Instead, Ca in olivine is interpreted to reflect (primarily) the H₂O present in the magmas that produced the olivine. Application of the hygrometer indicates that eastern Aleutian olivine grew in magmas with 2.2-5.0% H₂O. The same calculation for western Aleutian basalts indicates H₂O from 4.2-6.2%. The modest increase in Aleutian H₂O east-to-west is similar to the increase south-to-north in Kamchatka (Gavrilenko et al.). In both cases, H₂O increases toward the hot edge of the subducting plate. From this we infer that H₂O available to drive calc-alkalinity in hot-slab settings originates predominantly within the subducting plate and not in sediment or hydrated mantle peridotite that lie above the subducting plate.