02/10/2022
By Erica Gavin

The Kennedy College of Sciences, Department of Environmental, Earth, and Atmospheric Sciences, invites you to attend a Master’s thesis defense by Cassidy Stegner on “Molybdenum behavior during subduction zone metamorphism in the Catalina Schist, California."

Defense Date: Tuesday, March 1, 2022
Time: 3:30 to 4:30 p.m.
Location: Room 212, Olney Hall, North Campus
Thesis/Dissertation Title: Molybdenum behavior during subduction zone metamorphism in the Catalina Schist, California

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

Committee Members:

  • Kate Swanger, Environmental, Earth, and Atmospheric Sciences, University of Massachusetts Lowell
  • Gray Bebout, Earth and Environmental Sciences, Lehigh University

Brief Abstract:
Molybdenum (Mo), a redox-sensitive transition metal, experiences low-temperature isotopic fractionation in ocean basins which produces distinct isotope signatures within sediments and altered oceanic crust. Contrasting Mo isotope compositions observed between subducting slabs and resultant arc rocks encourages research that aims to understand Mo cycling and isotopic behavior during subduction-related metamorphism. Molybdenum isotope compositions and concentrations of 17 metasedimentary samples and 10 block-rind mélange samples from the well-characterized Catalina Schist (California, USA) were examined to determine Mo behavior and mineral residency during HPLT subduction zone metamorphism. The Catalina Schist encompasses a metamorphic sequence that ranges from lawsonite-albite to amphibolite grade facies and the higher-grade units of the complex formed during anomalously warm subduction, displaying open system behavior and a significant degree of devolatilization.

Bulk-rock Mo isotope and concentration data, in situ thin section analysis, and mineral budgets were used to establish elemental behavior and mineral residency, providing insight into Mo behavior during HPLT metamorphism. The metasedimentary suite displays a trend of decreasing Mo concentrations and a shift to more negative δ98Mo values with increasing metamorphic grade. The Mo concentrations range from 0.20 to 1.41 ppm and δ98Mo values range from -0.50 to +0.09‰ (relative to NIST-3134). The observed trends in concentration and δ98Mo are consistent with isotopic fractionation and loss of Mo during progressive metamorphism. Based on trace and major element concentrations, other processes, including rock source heterogeneity, diagenesis, hydrothermal alteration prior to subduction, and chemical weathering are unlikely to cause the observed δ98Mo values. Samples from the block-rind mélange traverse display higher Mo concentration at the outer rind and lower concentrations nearer the core, ranging from 0.26 to 0.59 ppm. The δ98Mo values across the transect are variable and most rind values overlap with core values, ranging from -1.59 to -0.49‰. These results demonstrate the behavior of Mo in HPLT metamorphic systems, suggesting that Mo is isotopically fractionated when released from subducting slabs, retaining isotopically light Mo.