Graduate Student & Post-Doc Seminar

Date: 

Thursday, February 14, 2019, 12:00pm to 1:00pm

Location: 

Faculty Lounge, Hoffman 4th floor

Speaker: Patrick Beaudry (MIT)

"Tales of magmatic sulfur: origins, redox, degassing, and isotopic fractionation"

 

Abstract: The behavior of sulfur (S) in magmas has critical effects on a wide array of processes such as volcanic degassing, climatic impacts of volcanic eruptions, or the formation of ore deposits. It also holds clues to the S isotopic signatures associated with different tectonic settings: mid-ocean ridge basalts (MORB), ocean island basalts (OIB), and subduction zone magmas. The speciation of S in magmas has a first-order control on S solubility, degassing, sulphide saturation, and isotopic fractionation: the solubility of oxidized S6+ is an order of magnitude higher than that of reduced S 2-, and oxidized S is generally isotopically heavier. Yet, knowledge of S speciation (as a function of melt composition, P, T, fO2) remains relatively limited for upper mantle conditions where primitive magmas originate. In this talk, I will first describe experiments aimed at understanding the redox transition from S2- to S6+ bearing magmas in the upper mantle. I will then present a case study of a hotspot volcano at El Hierro, Canary Islands, where degassing of S from an initially S-rich oxidized magma was accompanied by a decrease in oxidation state, sulphide saturation and isotopic fractionation. The S isotope composition of the evolving silicate and sulphide melts paints an unusually clear picture of the mechanisms by which S isotopes fractionate during degassing and sulphide saturation. Discriminating between these effects finally allows us to trace the isotopic composition of the mantle source, highlighting the geodynamic implications of S isotopes at volcanic hotspots.

Bio: Patrick is presently a PhD candidate at MIT, working jointly with Tim Grove and Shuhei Ono to combine experimental petrology and stable isotope geochemistry to probe volatile cycling. Prior to arriving at MIT, he completed undergraduate studies at McGill University, and a master's degree at Queens College of the City University of New York. His master's work was recently published in Nature Communications: Degassing-induced fractionation of multiple sulphur isotopes unveils post-Archaean recycled oceanic crust signal in hotspot lava.

Lunch will be provided. As always, please plan to bring reusable plates and cutlery to reduce waste.