EFFECTS OF CRUSTAL ASSIMILATION ON 238U-230TH DISEQUILIBRIA IN CONTINENTAL ARC SETTINGS: EVIDENCE FROM REVENTADOR VOLCANO, ECUADOR

Compositions of arc magmas depend on several factors and are often thought to reflect conditions in the mantle wedge and at the slab-mantle interface. However, in continental arc settings, magmas are also influenced by interaction with and assimilation of continental crust. Here, we present measurements and modeling of ²³⁸U-²³⁰Th activity ratios, Sr, Nd, Hf, and Pb isotopic compositions, and major and trace element concentrations in young, historic lavas erupted from Reventador, an active stratovolcano in the Ecuadorian Andes. In arc lavas, ²³⁸U-²³⁰Th disequilibria are often assumed to reflect processes occurring in the mantle wedge, as U and Th behave differently in this relatively oxidized and fluid-rich environment. Enhanced mobility of hexavalent U in aqueous fluids results in (²³⁰Th/²³⁸U) < 1 and elevated (²³⁸U/²³²Th), which are common in arc lavas. However, the majority of Reventador lavas have (²³⁰Th/²³⁸U) = 1.0–1.1 and (²³⁸U/²³²Th) = 0.94–1.12, the latter of which is considerably lower than the depleted mantle ((²³⁸U/²³²Th) ~1.5, Sims and Hart, 2006). While this Th enrichment could be due to melting of subducted oceanic crust, trends between (²³⁰Th/²³²Th), wt. % SiO₂, and radiogenic isotope ratios indicate otherwise. We argue that crustal assimilation lowers long-term, or time-integrated, (²³⁸U/²³²Th) in Reventador magmas. To quantify the effects of assimilation we modeled stepwise assimilation and fractional crystallization. Observed trends between (²³⁰Th/²³²Th), ⁸⁷Sr/⁸⁶Sr, ε_Nd, ε_Hf, and ²⁰⁸Pb/²⁰⁶Pb can be reproduced by up to 10% assimilation, consistent with previous regional studies. However, reproducing the full spectrum of isotopic diversity among Reventador lavas requires heterogeneous basalt compositions. In a broader context, this study emphasizes the need to consider crustal processes when examining continental arcs. While ²³⁸U-²³⁰Th disequilibria develop in the mantle wedge, they can be overwritten by subsequent interaction with continental crust.