241-10 Accessory Mineral Saturation and Dissolution Revealed in Primitive Rhyodacites of the Western Aleutian Arc

Session: Petrology, Volcanology, and Mantle Plumes across the Solar System, Part I

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Accessory mineral saturation and dissolution are widely observed in hydrous melting experiments but are rarely documented in products of island-arc volcanism. This is because temperatures of last equilibration are too high (>1100 °C) and key elemental concentrations are too low to produce rutile, apatite, monazite, or allanite saturation in primitive (high-Mg#) arc magmas, which are most often basalts. Primitive (high Mg#) rhyodacites from the western Aleutians are an exception in this context because their compositions and geologic context require an origin linked to hydrous melting of subducting Pacific MORB, and because they have siliceous-aluminous compositions that may be saturated in accessory minerals below 1000 °C. All western Aleutian rhyodacites are similar with respect to SiO2 (67-70%), Mg# (>0.65), and Pb-Sr-Nd isotopes (which in some cases are indistinguishable from Pacific MORB). However, 10X differences in Th (0.52-5.2 ppm) lead to the recognition of low-Th and high-Th subgroups with similarly large differences in light rare-earth elements (Ce=17-103 ppm) but only modest differences for K and Sr. A simple explanation for these differences is by saturation in a residual phase rich in Th and Ce during formation of the low-Th group followed by dissolution of that phase to create the high-Th group which has somewhat higher MgO and is presumably hotter. This implies that the prominent and unambiguously arc-type trace element patterns of western Aleutian rhyodacites are produced by residual garnet, rutile, and an accessory phase rich in Th and light rare-earth elements. These results may be interpreted to indicate that enrichments of K and Sr over La and Ce in products of arc volcanism are not created by a hydrous fluid flux from the subducting plate, but instead, reflect relative depletions of La and Ce held in accessory minerals produced during hydrous melting of subducted sediment and oceanic crust.

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

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