Global, Regional, Micro

Convergent margin processes are central to some of the most important unsolved problems in the Earth sciences:

  • Subduction zones have been the loci of continental crustal growth since at least the Archean, though compositional differences between arc magmas and the bulk continental crust are not well explained.
  • Most mantle heterogeneity is produced at subduction zones, though existing models designed to account for these heterogeneities either fail in detail or do not provide unique results.
  • Variations in volatile cycling through arc volcanoes are clearly linked to past climatic conditions, though constraints on these processes are poor, and they are rarely incorporated into paleoclimate models.

The solutions to these problems hinge on understanding subduction zone processes, the proportions of slab materials that are recycled into the deep mantle, and the net contributions to volcanic arcs. My research applies a global-systems approach to better understand subduction and arc volcanism. This approach is advancing our knowledge of core questions regarding the formation of continents, mantle heterogeneity, and connections between the solid Earth and climate over long timescales.

Global Arc Systematics

One major goal of my program is to highlight and remediate inconsistencies in the commonly accepted model for the generation of arc magmas. In this model, metamorphic reactions release water from the subducting oceanic crust, leading to water-fluxed melting of the overlying subducting sediment. Melts from sediment and fluids from the crust then ascend into the mantle beneath the arc, depress the mantle solidus, and generate arc magmas. If the trace element and isotopic compositions of arc magmas reflect a simple mixture of sediment melts, ocean crust fluids, and peridotite, however, then why is it that certain arc magma trace element abundances span up to two orders of magnitude, with no apparent correspondence with subducting sediment compositions? In fact, this textbook conception of arc magmatism often leads to basic violations of mass balance. Inconsistencies such as these often go overlooked by studies restricted to a regional or local frame of reference.

Regional Arc Systematics

Another major goal of my ongoing and future research is to evaluate our new global framework for volcanic arcs using targeted regional and volcano-scale studies case studies and collaborations with investigators from diverse backgrounds. Combining geochemical interpretations with numerical modeling results, for example, I have used data from Central Chile to test models relating lithospheric structure and mantle temperature in South America (Turner et al., 2016). I am now expanding this approach to regional studies target the Pacific Northwest, where I led UMass Amherst students on a research expedition in August 2021, and the Alaskan Wrangell Arc, the target of an interdisciplinary project incorporating both seismology and igneous geochemistry to better understand how terrane accretion and subsequent arc volcanism contribute to the growth of the continental crust.

Melt Inclusions, Volatile Elements, And Their Proxies

I am also working to develop tracers of water and carbon during subduction. For example, I have used measurements of boron isotope ratios in olivine hosted ‘melt inclusions’ (see example image above) to better understand the deep water cycle. Boron isotopes are unique because B is fluid-mobile and becomes sequestered into the same minerals that carry water back into the mantle during subduction. Expanding the global database of high-quality boron isotopic analyses thus provides one of the best ways to understand the provenance and pathways of water during subduction. I have applied this approach to studies of Kamchatka (Iveson et al., 2021), Chile and Argentina (Turner et al., Goldschmidt 2017; and Turner et al., in prep for Journal of Petrology) and Nicaragua (Barickman et al., in prep for Earth and Planetary Science Letters), with ongoing work in Costa Rica being carried out by students at UMass Amherst.

I am using a similar approach to track subducting carbon, in this case using strontium as a proxy element via newly developed techniques to measure stable Sr isotope ratios (88Sr/86Sr) in in volcanic rocks. These measurements can be used to track the pathways of subducting carbonate and to better understand long-term carbon cycling on Earth.

Other work on water and carbon cycling involves ongoing collaborations with gas geochemists at the Woods Hole Oceanographic Institute (e.g., Barry et al., Nature, 2018; Bekaert et al., 2021; Bekaert et al., in review at PNAS).