Date of Award
Fall 2025
Document Type
Open Access Dissertation
Department
Geography
First Advisor
Besim Dragovic
Abstract
Molybdenum, a fluid-mobile and redox sensitive element, has been characterized in many of Earth’s reservoirs, such as seawater, mid-ocean ridge basalt, sediments, altered oceanic crust, and arc lavas; however, little is understood regarding the behavior of Mo during subduction metamorphism. During subduction of these lithologies, fluids can be released by dehydration reactions caused by metamorphic phase transitions. It is suggested that these fluids transport isotopically heavy Mo, while isotopically light Mo is retained in Ti-oxide-bearing minerals. This study characterizes Mo behavior within subducted lithologies and unsubducted equivalents from the Voltri Massif, Ligurian Alps, Italy to constrain oceanic inheritance versus subduction-related processes.
Mylonitic and static Fe-Ti metagabbros from the Vara area of the Voltri Massif have been characterized to retain isotopically light Mo (-0.18 ± 0.22‰) and has variable Mo concentrations. Combined with fluid mobile element concentrations and a Rayleigh distillation model, I infer that these Vara metagabbros reflect dehydration signatures. Mg-Al metagabbros derived from the Erro Tobbio area of the Voltri Massif retains an overall isotopically light Mo signature (-0.01 ± 0.25‰), overlapping with the unsubducted equivalent from the Northern Apennines (0.01 ± 0.11‰). I infer that Erro Tobbio Mg-Al metagabbros obtained these signatures during external fluid infiltration from the Vara area lithologies when the area was accreted to the overhanging plate during subduction.
Schists from the Voltri Massif display isotopically light Mo (-0.51 ± 0.39‰) and unsubducted Apennines serpentinites retain highly variable Mo isotope compositions (0.02 ± 0.85‰). Serpentinites from the Vara area are characterized by isotopically light Mo (-0.08 ± 0.68‰), coupled with highly variable concentrations. Erro Tobbio area serpentinites record an isotopically heavy Mo signature (0.08 ± 0.49‰) overlapping with Apennines serpentinites with greater Mo concentrations. Based on fluid mobile elements and Rayleigh distillation models, it is inferred that Vara serpentinites likely obtained their Mo composition via interactions with crustal-derived fluids. Meanwhile, Erro Tobbio was accreted to the overhanging plate during subduction and likely obtained a Mo composition resulting from both serpentinite and crustal derived fluids.
A metasomatic reaction zone between juxtaposed serpentinite and metagabbro from the Voltri Massif formed by HP fluid-mediated mass transfer in a subduction zone was also characterized to assess Mo as a fluid tracer. Serpentinites from the reaction zone are defined by largely variable Mo compositions (-0.05 ± 0.71‰) with an average concentration of 0.37μg/g. Unaltered metagabbros largely unaffected by the metasomatic fluid retain Mo isotope signatures comparable to MORB (-0.16 ± 0.21‰). The single isotopically heavy sample (0.001 ± 0.02‰) is inferred to be where the fluid, characterized by heavy Mo, high fluid element concentrations, and high Fe3+ contents, stopped. This corresponds with an overall hydrous assemblage of epidote, ilmenite, omphacite, garnet, and Na-Ca amphibole. Therefore, if the fluid progressed through the slab-mantle interface, it would have delivered heavy Mo and fluid mobile elements to the source region of arc magmas. Overall, Mo systematics are a promising proxy for defining source characteristics within metamorphosed lithologies and as a fluid tracer at depth.
Rights
© 2025, Allison Noelle Brown
Recommended Citation
Brown, A. N.(2025). Molybdenum Systematics of Subducted Lithologies from the Western Alps. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/8607