Metal stable isotope research
Molybdenum and uranium are redox sensitive elements. When in oxidized 6+ valence states, they are soluble, and their isotopes are prone to fractionation when dissolved Mo and U are removed from seawater. Consequently, sediments and altered basaltic oceanic crust can develop unique Mo (98Mo/95Mo) and U (238U/235U) isotope signatures depending on the redox state of the ocean. The two isotopes of thallium (203 and 205) also experience isotope fractionation when being scavenged from ocean water by sediments. These isotope signals should be carried into the Earth's interior when the sediments and oceanic crust are subducted. This begs the question: what happens to these Earth surface signals in subduction zones? Is nearly all Mo, U, and Tl transferred from the subducting slab into the mantle wedge, ending up in arc lavas? Are some of these elements carried deeper and ultimately mixed back into the convecting mantle, and if so, do additional isotope fractionations occur?
Research attempting to answer these and related questions has being conducted in collaboration Chris Reinhard (Georgia Tech), Noah Planavsky (Yale), and Jeremy Owens (Florida State). We've worked on the Mo, U and Tl isotope chemistry of modern basalts from different tectonic settings. Some results:
Gaschnig, R.M., Rader, S., Reinhard, C.T., Owens, J., Planavsky, N., Wang, X., Asael, D.,
Greaney, A., Helz, R. (2021) Behavior of the Mo, Tl, and U isotope systems during 1 differentiation in the Kilauea Iki lava lake. Chemical Geology, v. 574, 12039. https://doi.org/10.1016/j.chemgeo.2021.120239
Gaschnig, R.M., Reinhard, C.T., Planavsky, N., Wang, X., Asael, D., Jackson, M.
(accepted) The impact of primary processes and secondary alteration on the stable isotope composition of ocean island basalts: Chemical Geology, v. 81, 120416. https://doi.org/10.1016/j.chemgeo.2021.120416
Gaschnig, R.M., Reinhard, C., Planavsky, N., Wang, X., Asael, D., and Chauvel, C. (2017) Mo isotopes as a tracer of slab input in subduction zones: an example from Martinique, Lesser Antilles arc: Geochemistry, Geophysics, and Geosystems. Link
More recently, I have expanded this work to the metamorphic realm and just received NSF support for a study of the Mo and Tl isotope systems in rocks that have experienced subduction-related metamorphism (i.e., HPLT rocks). This work is in collaboration with Shelby Rader, who began it as a postdoc with me and is now a research scientist at Indiana University-Bloomington. Using samples of eclogite-facies metasedimentary and melange rocks from the western Alps and Catalina Island in California, we seek to determine what effect HPTL metamorphism has on the Mo and Tl isotope systems. In other words, does this metamorphism cause isotope fractionation that then influences the isotope composition of the slab component added to arc magmas? What minerals are the primary reservoirs for these elements and how do they change with progressive metamorphism? What is the nature of inter-mineral isotope fractionation under these conditions?
Some initial results:
Rader, S.T., Gaschnig, R.M., Newby, S.M., Bebout, G.E., Mirakian, M.J., Owens, J.D.
The Mo and Tl isotope compositions of igneous rocks and continental crust are increasingly well constrained but comparatively little work has been done on metamorphic rocks. HPLT rocks are of particular importance because processes that occur under those conditions have been invoked to explain offsets between the Mo isotope compositions of the down-going slab package and the overlying arc and continental crust.
The main focus of this project is on two well characterized suites of HPLT rocks provided by Gray Bebout of Lehigh University. The Schistes Lustres and related Lago di Cignana rocks are metasedimentary rocks from the Alps that show a progressive increase in peak PT conditions from SW to NE. The Catalina Schist from California represents a "warm" subduction environment and shows greater loss of fluid-mobile elements. Figure from Gray.