Abstract
Effects of crustal assimilation on (super 238) U- (super 230) Th disequilibria in continental arc settings; evidence from Reventador Volcano, Ecuador
Abstracts with programs - Geological Society of America, Vol.55(5)
Geological Society of America, Rocky Mountain Section, 73rd annual meeting
05/2023
DOI: 10.1130/abs/2023RM-388089
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 interaction with and assimilation of continental crust. Here, we present measurements and modeling of (super 238) U- (super 230) Th 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, (super 238) U- (super 230) Th 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 ( (super 230) Th/ (super 238) U) < 1 and elevated ( (super 238) U/ (super 232) Th), which are common in arc lavas. However, the majority of Reventador lavas have ( (super 230) Th/ (super 238) U) = 1.0-1.1 and ( (super 238) U/ (super 232) Th) = 0.94-1.12, the latter of which is considerably lower than the depleted mantle (( (super 238) U/ (super 232) Th) nearly equal 1.5, Sims and Hart, 2006). While this Th enrichment could be due to melting of subducted oceanic crust, trends between ( (super 230) Th/ (super 232) Th), wt. % SiO (sub 2) , and radiogenic isotope ratios indicate otherwise. We argue that crustal assimilation lowers long-term, or time-integrated, ( (super 238) U/ (super 232) Th) in Reventador magmas. To quantify the effects of assimilation we modeled stepwise assimilation and fractional crystallization. Observed trends between ( (super 230) Th/ (super 232) Th), (super 87) Sr/ (super 86) Sr, epsilon Nd, epsilon Hf, and (super 208) Pb/ (super 206) Pb 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 (super 238) U- (super 230) Th disequilibria develop in the mantle wedge, they can be overwritten by subsequent interaction with continental crust.
Details
- Title: Subtitle
- Effects of crustal assimilation on (super 238) U- (super 230) Th disequilibria in continental arc settings; evidence from Reventador Volcano, Ecuador
- Creators
- Lisa B. Kant - University of WyomingKenneth Sims - University of WyomingGene YogodzinskiJennifer GarrisonJanne Blichert-ToftSean R. ScottMark K. ReaganPatricia Mothes
- Resource Type
- Abstract
- Publication Details
- Abstracts with programs - Geological Society of America, Vol.55(5)
- Conference
- Geological Society of America, Rocky Mountain Section, 73rd annual meeting
- DOI
- 10.1130/abs/2023RM-388089
- ISSN
- 0016-7592
- Publisher
- Geological Society of America (GSA)
- Alternative title
- Geological Society of America, Rocky Mountain Section, 73rd annual meeting
- Language
- English
- Date published
- 05/2023
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984962544102771
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