Date of Award

Summer 2025

Document Type

Open Access Dissertation

Department

Geography

First Advisor

Michael Bizimis

Abstract

Chemical and physical transfer between the surface, lithosphere and asthenosphere at subduction zones leads to arc magmatism and continental crust (CC) production. Subduction zones magmas and associated volcanic products sample a variety of magmatic sources that contribute to their chemical compositions in various proportions. This dissertation contributes to the study of subduction zones magmatism by assessing the relative role of subduction inputs (sediments, altered oceanic crust – AOC – and serpentinite) as flux agents in the arc magmatism and in CC formation in subduction zones under the scope of Mo isotope systematics.

I present the first Mo isotope analyses form input to output in the Aleutian arc and the first Mo isotope data for the Middle American Trench in Costa Rican. The Aleutian arc is an active system that allows investigation of arc magmatism under presence of various sources (marine sediments with different redox conditions and various lithologies, AOC, serpentinite, and slab eclogitic melts). In turn, Costa Rica represents the best example of arc continentalization in which CC-like volcanic rocks are formed as the result of partial melting of the subducting oceanic crust overprinted by mantle-plume magmatism, thus providing a unique opportunity to test if slab melting is a viable mechanism producing the characteristic heavy Mo isotopes of the CC.

I demonstrate that Aleutian arc lavas from Piip to Frosty Peak (n = 77) have light, MORB-like, Mo isotope compositions, excepting in volcanoes adjacent to the subducting Amlia Fracture Zone (AFZ), where the Mo isotopes are particularly heavy. Mo isotope analyses in abyssal serpentinites (as proxies for serpentinites in subduction settings) show that these have the heaviest Mo isotopes of fluid-rich inputs. I also present Mo isotope analyses on North Pacific sediments outboard the Aleutian arc (n = 53) from drill cores ODP 886, DSDP 183, DSDP 178 and IODP 1417 and show that subducting sediments input predominantly light, mantle-like or lighter, Mo isotopes. Light Mo isotopes of open-ocean pelagic sediments (ODP 886, DSDP 183) are consistent with an oxide control (e.g., Fe-Mn nodules). Heavier Mo isotopes are observed in IODP 1417, where local reducing conditions are prevalent and where associated high S suggest a sulfide vs. oxide control.

From Mo isotopes and trace element systematics of Aleutian lavas, as well as Mo isotopes in sediments outboard the Aleutian arc, the heavy Mo isotopes in AFZ lavas can be explained by three non-mutually exclusive scenarios: (1) melting of trench turbidites with high organic carbon and diagenetic pyrite carrying heavy Mo isotopes, (2) an enhanced fluid-flux in shallower magmas such as Seguam with low Dy/Yb or, (3) presence of fluid-rich sources (e.g., serpentinites) associated with the AFZ.

Finally, I test if slab melting in Costa Rica can produce heavy CC-like Mo isotopes. I show that < 10 Ma old Costa Rican volcanic rocks, representing juvenile CC formation in subduction zones have light, MORB-like, Mo isotopes. From this data and comparisons with global arc settings, I conclude that slab melting cannot produce the heavy Mo isotopes of the CC and that additional crust-specific processes (e.g., CC differentiation and magmatic fluid exsolution) may be required to generate the heavy Mo isotopes of the CC. This work sheds light on the factors controlling Mo isotope fractionation across various systems (e.g., fluids, open-ocean sediments, volcanic rocks) and contributes to the current understanding of the global Mo cycle in subduction zones.

Rights

© 2025, Ekaterina Rojas Kolomiets

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