03/30/2022
By Samuel Marshall

The Kennedy College of Sciences, Department of Environmental, Earth, and Atmospheric Sciences, invites you to attend a master’s thesis defense by Samuel Marshall on “molybdenum behavior in Western Alps metasedimentary rocks and Catalina mélange material: Investigation of partitioning and isotopic fractionation during subduction metamorphism.”

Defense Date: Wednesday, April 13, 2022
Time: 2 to 3 p.m.
Location: Room 312, Olney Hall, North Campus
Thesis/Dissertation Title: molybdenum behavior in Western Alps metasedimentary rocks and Catalina mélange material: Investigation of partitioning and isotopic fractionation during subduction metamorphism

Advisor: Richard Gaschnig, Environmental, Earth, and Atmospheric Sciences, University of Massachusetts Lowell

Committee Members:

  • Gray Bebout, Earth and Environmental Sciences, Lehigh University
  • Nelson Eby, Environment, Earth, and Atmospheric Sciences, University of Massachusetts Lowell

Brief Abstract:
In recent years, new non-traditional stable isotope tracers, such as the redox-sensitive transition metal molybdenum (Mo), have been employed to further understand subduction zone dynamics contributing to arc magmatism. Disparate Mo isotope compositions between subducting material and resultant arc rocks has engendered a model where Mo is isotopically fractionated during dehydration of the subducting slab, leading to the retention of isotopically light Mo and partitioning of heavy Mo into fluids. To further understand this isotope system and corresponding mass transfer processes from the subducting slab to the volcanic arc, we examine 25 exhumed metasedimentary rock samples from the Schistes Lustrés and Lago di Cignana as well as 11 mélange matrix samples from the Catalina Schist.

Western Alps samples vary in molybdenum concentrations from 0.074 to 1.014ppm and tend to decrease with increasing grade. δ98Mo values range from -2.45 to +0.66‰ and do not illustrate a systematic pattern throughout metamorphism. In lower grade Schistes Lustrés samples, molybdenum is primarily held in carbonate, Fe-Mn oxides, and carbonate-oxide matrix phases. At the highest grade, rutile is the dominant Mo host. Western Alps samples can be most effectively explained through a three-component mixing process with endmembers of continental detrital material (SiO2-rich), carbonate (CaO -rich), and shale/clay (Al2O3-rich), with additional isotopic fractionation and loss due to variable fluid-rock interactions. Results support the existing hypothesis that molybdenum is mobilized and fractionated in preference of heavy Mo loss during subduction zone metamorphism.

In Catalina mélange material, Mo concentrations range from 0.061 to 0.664ppm and compositions range from -1.75 to -0.19‰. Lawsonite-albite facies mélange material can be explained through mixing between mafic and ultramafic components, with mafic material serving as the dominant constituent. Lawsonite-blueschist facies samples behave similarly, though Mo appears mobilized and redistributed from the introduction of metasomatic fluids. At both grades, δ98Mo appears related to the relative abundance of Ti-bearing mineral phases. At the amphibolite grade, no such relationship exists. Depletions in Mo compared to the lower-grade mélange reflect Mo mobilization and loss as mineral host phases are lost through the combined effects of metasomatic alteration and mechanical mixing between predominantly ultramafic and mafic components.