Journal article
Effects of crustal assimilation on U-238-Th-230 disequilibria in continental arc settings
Geochimica et cosmochimica acta, Vol.354, pp.165-185
08/01/2023
DOI: 10.1016/j.gca.2023.05.022
Abstract
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 assimilation of continental crust. Here, we present measurements and modeling of U-238-Th-230 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, U-238-Th-230 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 (Th-230/U-238) < 1 and elevated (U-238/Th-232), which are common in arc lavas. However, the majority of Reventador lavas have (Th-230/U-238) = 1.0-1.1 and (U-238/Th-232) = 0.94-1.12, the latter of which is considerably lower than the depleted mantle ((U-238/Th-232) similar to 1.5, Sims and Hart, 2006). While this Th enrichment could be due to melting of subducted oceanic crust, approximately linear trends between (Th-230/Th-232), wt. % SiO2, and radiogenic isotope ratios indicate otherwise. We argue that crustal assimilation lowers long-term, or timeintegrated, (U-238/Th-232) in Reventador magmas. To quantify the effects of assimilation we modeled stepwise assimilation and fractional crystallization. Observed trends between (Th-230/Th-232), Sr-87/Sr-86, eNd, eHf, and Pb-208/Pb-206 can be reproduced by up to 10% assimilation, which is 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 U-238-Th-230 disequilibria develop in the mantle wedge, they can be overwritten by subsequent interaction with continental crust.
Details
- Title: Subtitle
- Effects of crustal assimilation on U-238-Th-230 disequilibria in continental arc settings
- Creators
- L. B. Kant - University of WyomingK. W. W. Sims - University of WyomingG. M. Yogodzinski - University of South Carolina SumterJ. M. Garrison - California State University Los AngelesJ. Blichert-Toft - École Normale Supérieure de LyonM. Reagan - University of IowaC. L. Waters - University of WyomingT. P. Mathews - University of WyomingS. R. Scott - University of WyomingP. A. Mothes - Instituto Geofísico de la Escuela Politécnica NacionalM. L. Hall - Instituto Geofísico de la Escuela Politécnica NacionalP. Ramon - Instituto Geofísico de la Escuela Politécnica NacionalE. Gaunt - Instituto Geofísico de la Escuela Politécnica NacionalM. Almeida - Inst Geofisico Escuela Politecn Nacl, Fac Ingn Civil & Ambiental, Ladron Guevara E11-253,Piso 6to, Quito, EcuadorS. Hidalgo - Instituto Geofísico de la Escuela Politécnica Nacional
- Resource Type
- Journal article
- Publication Details
- Geochimica et cosmochimica acta, Vol.354, pp.165-185
- DOI
- 10.1016/j.gca.2023.05.022
- ISSN
- 0016-7037
- eISSN
- 1872-9533
- Publisher
- Elsevier
- Number of pages
- 21
- Grant note
- Center for Global Studies Excellence Fund at the University of Wyoming EAR 1019545; EAR 1019546 / NSF; National Science Foundation (NSF) National Geographic Society Richard B. and Lynne V. Cheney Study-Abroad Scholarship Fund ANR-10-BLAN-0603M / French Agence Nationale de la Recherche; Agence Nationale de la Recherche (ANR) US and Ecuadorian Fulbright Commissions
- Language
- English
- Date published
- 08/01/2023
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984455611702771
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