115-4 Why Buldir is the Most Calc-Alkaline Volcanic System in the Aleutian Island Arc

Session: Petrology, Volcanology, and Mantle Plumes across the Solar System (Posters)

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Abstract:

The Buldir Island volcanic system has produced the most highly calc-alkaline igneous series in the Aleutian Island Arc (Kay & Kay, 1986 – Contrib Min Petr). With a tholeiitic index of 0.68 (formulation of Zimmer et al., 2010 J. Petrol), Buldir whole-rock data define a trend of iron depletion that may be described as a “low-Fe series” (Arculus, 2003 – J. Petrol). A recent melt inclusion and experimental study concludes that Buldir is among the most oxidized and wettest arc volcanoes globally (Andrys et al., 2024 – J. Petrol). What sources and processes create these distinctive aspects of the Buldir volcanic system? The Fe depletion trend coupled to spoon-shaped rare-earth element patterns suggest a role for fractional crystallization of an amphibole-bearing mineral assemblage. However, this process is inconsistent with Sr greater than 500 ppm in Buldir andesites with 87Sr/86Sr (87/86) that is linked to relative Sr enrichments. Key relationships are illustrated in a plot of 87/86 versus Zr/Sr which produces a steep positive correlation for Buldir samples with 87/86 from 0.7029 to 0.7032 superimposed over western Aleutian seafloor lavas, which define a longer but otherwise identical trend with 87/86 from <0.7026. Whole-rock data thus require an origin for the Buldir series via source mixing that differs only in degree from that of western Aleutian seafloor lavas, which define a continuum from high-MgO basalts to adakitic rhyodacites that are isotopically more depleted (87/86<0.7029), reflecting a source component produced by partial melting of subducting Pacific MORB. Petrographic observations indicate that (like andesites and dacites of the Trans-Mexican volcanic Belt) pre-eruptive H2O in key western Aleutian samples was high enough to suppress the onset of plagioclase crystallization in magmas with >60% SiO2. Combined petrographic, whole-rock, and experimental datasets indicate that high-Mg# andesites, dacites, and rhyodacites had pre-eruptive H2O>5-6% and may have been H2O-saturated (Blatter & Carmichael, 2001 – GCA; Andrys et al., 2024 J. Petrol). Trace element and isotopic compositions indicate further that the hydrous and oxidized character of western Aleutian volcanism was derived largely or entirely via melting of subducting oceanic crust. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10799

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