Origin and implications of the low-Fe igneous series at Buldir Island, western Aleutian arc

Volcanic whole-rock compositions from Buldir Island define a highly calc-alkaline (low-Fe) igneous series, with molar Mg/Mg + Fe* (total iron Mg#) mostly > 0.60; SiO₂ from 50–64 wt.%; and distinctly low FeO*, CaO, CaO/Al₂O₃, and FeO*/Al₂O₃ relative to MgO. Many Buldir samples have trace element characteristics similar to western Aleutian seafloor lavas along upward curving trends of increasing Sr and Sr/Y with increasing SiO₂ and decreasing MgO. Least radiogenic Sr in andesites only (57–62 wt.% SiO₂ with ⁸⁷Sr/⁸⁶Sr ~ 0.7029) requires a key role for source mixing in the origin of the low-Fe series at Buldir. Andesites and dacites falling outside these source mixing trends show evidence of fractional crystallization interpreted to be a late-stage (over-printing) effect. Partitioning data indicate that heavy REEs will behave compatibly in amphibole-saturated melts evolving at temperatures < 1000 °C. However, recent hydrous and high fO₂ experiments show that Buldir melts with more than 4–5 wt.% MgO were unlikely to have reached amphibole saturation and cannot explain the decline in Yb concentrations starting at >9 wt.% MgO. Distinctive reverse sigmoidal (backward-S) REE pattern shapes in Buldir samples are also unlikely to be products of amphibole removal from evolving liquids and are instead interpreted to reflect melting of a light REE-depleted MORB source under eclogite conditions followed by interaction of the resulting hydrous-felsic slab melt with mantle peridotite upon ascent. Published pMELTS simulations and experimental observations indicate this style of melt-rock reaction will also create low FeO* and CaO in primitive (high-Mg#) melts over a wide range of SiO₂ and MgO, as observed in Buldir and western Aleutian seafloor datasets. The implication is that distinctive aspects of low-Fe calc-alkalinity in most Buldir samples with SiO₂ from 50 to 62% are created at sub-MOHO depths where cool-hydrous-siliceous partial melts of subducted-eclogitized MORB rise, decompress, and undergo thermal-chemical exchange with depleted peridotite that is hotter by hundreds of °C.