Research
I am interested in crust-mantle recycling processes at convergent margins; therefore, I spent my graduate life above the "hot" Cascadia subduction zone! I use non-traditional stable isotopes to trace the recycling of subducted crustal materials to arc magmas and deep mantle, as these crustal materials carry a highly-fractionated isotopic signatures imparted during low-temperature. fluid-rock interactions. My research involves field sampling, chromatography separation of target element, and high-precision isotopic analysis, as well as development of new analytical techniques (e.g., stable K isotopes) and isotopic standards.
1. Magnesium isotope geochemistry in the subduction zone factory
My graduate research uses Mg isotopes to constrain the sources and processes leading to mantle compositional heterogeneity because Mg is a major element in the crust and mantle. I approach this topic by analyzing samples that represent subduction inputs (subducting sediments and altered oceanic crust), subduction output (arc lavas), slab-mantle interface (sub-arc peridotites), and reaction products between mantle peridotite and slab residues (mantle pyroxenites). These studies reveal that subducting slabs have highly heterogeneous Mg isotopic compositions compared to normal mantle and slab contributions are identified in both arc lavas and metasomatically formed mantle pyroxenites.
1. What processes control the compositional variation in subducting slab?
Subducting sediments and altered oceanic crust are primary crustal input to subduction zones
Subducting slab is compositionally highly heterogeneous when compared to the mantle
Weathering forms clay, preferentially retaining heavy Mg isotopes
Carbonate preferentially incorporate light Mg isotopes
2. Incoming plate hydration: A multidisciplinary approach (Poster presentation at 2017 AGU)
Student and Postdoc Participants: Meghan Guild (Arizona State Univ.), Owen Evans (Columbia Univ.), Katherine Fornash (Univ. of Minnesota), Yan Hu (Univ. of Washington), Samer Naif (Columbia Univ.), Foteini Vervelidou (GFZ-Potsdam)
Senior Participants: Terry Plank (Columbia Univ.); Donna Shillington, (Columbia Univ.); Jessica Warren (Univ. of Delaware); Douglas Wiens (Washington Univ. in St. Louis)
3. Development of an olivine standard for Mg isotopic analyses
There are substantial analytical challenges for measurements of non-traditional stable isotopes due to the limited isotopic variation in nature compared to precision, which necessitates the need for well-established geostandards for data quality control. In a study conducted with a previous ESS undergraduate (Melissa Harrington), we demonstrated that the previously reported inter-laboratory discrepancies on Mg isotope ratios measured for San Carlos olivine, which is ten times of current analytical uncertainty, reflect analytical artifacts instead of sample heterogeneity. Our study confirms the isotopic homogeneity of San Carlos olivine and provides a recommended value for this standard. For instrumental optimization, we have also systematically investigated the potential factors deteriorating the analytical accuracy and precision of Li, Mg, Fe, Cu, and Zn isotopes. These results provide a guideline for researchers that are interested in using non-traditional isotope tools to solve their geological problems.
4. Development of new isotopic tracers
I have developed a new method for high-precision K isotope ratios on MC-ICPMS, using dry and cold plasma to suppress the formation of argon hydrides and high-resolution to resolve the residual 40Ar1H beam from 41K. This method can achieve a routine precision of better than 0.06‰ (95% confidence interval), which makes K isotopes a potential tracer for a variety of terrestrial and planetary processes, such as formation of the Moon.
5. Potassium isotope geochemistry in the subduction zone factory and beyond
(Oral presentation at 2018 Goldschmidt)
Different from Mg that is a major constituent of the mantle, K is highly enriched in the crust and to a lesser degree in the ocean. The fractionated crustal and seawater K isotopic signature can be transferred to the mantle wedge, which then may be returned to the surface through arc volcanism. Alternatively, these crustal signatures may be delivered beyond sub-arc depth and contribute to deep mantle heterogeneity. When coupled, K isotopes may provide an archive of recent slab addition whereas Mg isotopes preserve a fingerprint of comparatively longer-term modification event and mantle heterogeneity.