My overarching research goal is to further our understanding of how planets formed and became habitable. Specifically, I focus on two integral aspects of this fundamental question, which are (1) how planets acquired their volatile elements and (2) how subduction zone processes regulate the chemical evolution of Earth's crust-mantle system and help stabilize Earth's climate. My research involves field sampling, chromatography separation of the target element, high-precision isotopic analysis, and developing isotopic standards and novel isotopic tracers.
Research at Institut de Physique du Globe de Paris
My current research utilizes the substantially improved K sensitivity offered by Nu Sapphire Collision Cell-MC-ICP-MS (> 2000 V/ppm, Moynier, Hu, et al., Chem. Geol., 2021), which makes it possible to analyze K isotopes in low-K meteorites and mission return samples that are limited in size (e.g., Ryugu particles retrieved by the Hayabusa2 mission). The improved collision cell system on Sapphire provides high-precision K isotope analyses in low-resolution mode, and distinguishes isotopic variations that were not resolvable on older CC-MC-ICP-MS (i.e., the IsoProbe).
I use meteorites as time capsules of the early Solar System and study how K their isotopic compositions vary during evaporation and condensation. I am particularly interested in K isotopes because K is the most abundant moderately volatile element. To infer the δ41K value of a bulk planetary body from its basalts erupted in the crust, we need to quantify the degree of isotope fractionation during mantle partial melting and igneous differentiation. I found these processes do not affect resolvable K isotope fractionation; therefore, planetary basalts are isotopically representative of the bulk planets (Hu et al., JGR, 2021).
1. Volatile-depletion processing of the building blocks of Earth and Mars recorded by potassium isotopes
The unsettled origin of δ41K variation in chondrites leads to orthogonal implications for the nature of building blocks of terrestrial planets. I analyzed 34 compositionally diverse meteorites and found an unprecedented K isotopic variation (over 5‰), which provides unequivocal evidence that planetary variability in K isotopes reflects volatility-related fractionations rather than presolar nucleosynthetic isotope anomalies. Furthermore, volatile depletion on Earth and Mars is most likely due to incomplete accretion of volatiles by their precursors, and possibly less so for the precursors of Mars than those of Earth (in review with EPSL).
2. Potassium isotope heterogeneity in the early Solar System controlled by extensive evaporation and partial recondensation
Angrites are rare basaltic (mostly) meteorites that are extremely depleted in K (> 99.8%). They sample the most volatile depleted planetesimal in the Solar System. I found that they are strikingly depleted in the heavier K isotopes, which is best explained by partial recondensation of vaporized K following extensive evaporation. These findings represent the first observation of isotope fractionation controlled by condensation, rather than evaporation, at a planetary scale.
Research at University of Washington
- Mg and K cycling at subduction zones
Earth is the only planet in the Solar System that has active plate tectonics, which provides long-term regulation of heat and chemical cycling in the ocean-crust-mantle system, a stable climate, and a habitable surface environment for the development of life. To study these processes, I use a combination of mineralogical, geochemical, and isotopic analyses on samples that represent slab input (trench sediments and altered oceanic crust), slab-mantle interface (mantle wedge peridotites), subduction output (arc magmas), and subducted residual slabs (mantle pyroxenites).
(1) What processes control the compositional variation in subducting slab?
Oral presentations at 2015 AGU and 2018 Goldschmidt Conference
Subducting marine sediments and the underlying altered oceanic crust are the two main inputs to the subduction factory. My studies on subducting sediments drilled along the world's major subduction zones revealed substantial variation in Mg and K isotopic compositions of sedimentary columns being delivered to different trenches, which is controlled by sediment mineralogy (calcareous vs. silicate) and the extent of chemical weathering (for detrital silicate sediments), and clay authigenesis (reverse weathering). These studies indicate that Mg and K isotopes can be used as tracers of subducted materials in the mantle (Hu et al., Chem. Geol., 2017; Hu et al., Science Adv., 2020). In particular, carbonate-rich sediments display considerably lighter Mg isotopic compositions than siliciclastic sediments, making Mg isotopes a sensitive tracer of the carbon cycle. In addition, low-temperature hydrothermal alteration of oceanic crust and authigenic clay formation are the two primary sinks for the light K isotopes in the oceans. Therefore, isotopic variation of seawater over geologic time may provide critical insights into continental weathering intensity and climate change.
(2) How does slab signature transfer to mantle wedge and arc magmas?
Poster presentation at 2016 AGU & 2017 GSA
The mantle wedge is the primary sink for slab-derived components and the main source of arc magmas. My Mg isotope studies of arc peridotites and arc lavas highlight that the thermal structure of a subduction zone plays a vital role in controlling the dehydration path of subducting slabs. (Hu et al., GCA, 2020; Teng, Hu, Chauvel, PNAS, 2016). I also conducted the first K isotopic study in arc lavas, which clearly distinguished crustal inputs of slab-derived fluids from those of subducted sedimentary melts. Furthermore, it suggests that recycled old crustal materials contribute significantly to the formation of juvenile crust (Hu et al., GCA, 2021).
(3) How do subducted slabs contribute to mantle heterogeneity?
Oral presentation at 2014 GSA
The residual subducted slabs transport their fractionated isotope signatures to the deeper mantle. I conducted a comprehensive Mg isotopic study on various types of mantle pyroxenites (Hu et al., GCA, 2016). This study provides direct evidence for the first time that mantle metasomatism can create local Mg isotopic heterogeneity over 1‰, which is linked to different types of slab-derived melts (silicate vs. carbonatite). These findings support that melt-rock interaction significantly destabilizes the thick, refractory, and buoyant roots of continents.
2. Method development and improvement
An essential part of my research involves the development of isotopic standards and new isotopic tracers for identifying various types of recycled crustal materials. I developed a new method for K isotope analyses on Nu Plasma II MC-ICP-MS, using a cold and dry plasma in pseudo-high resolution mode. This method improves the analytical precision by an order of magnitude relative to previous measurements using SIMS or TIMS (Hu et al., CG, 2018; Xu, Hu, et al., CG, 2019).
I also worked to optimize analytical protocols for Mg, Li, Cu, Fe, and Zn isotopes (Hu & Teng, JAAS, 2019).
I worked with a former UW undergraduate to establish the K isotopic values of global oceans (Hille, Hu et al., Sci Bull, 2019).
I worked with a former UW undergraduate to confirm the Mg isotopic homogeneity of the San Carlos olivine standard (Hu, Harrington et al., RCM, 2016).
3. Incoming plate hydration: A multidisciplinary approach
Poster presentation at 2017 AGU
Student & 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)
Constraining the source, distribution, and flux of slab-released water is critical for understanding arc magmatism and seismicity along plate interface. In addition to hydrothermal alteration, bend faults at the trench-outer rise provide another pathway to further hydrate the downgoing plate, yet its impact is poorly known. I have participated in a multidisciplinary collaboration that involves linking geophysical and geochemical observations with petrological and geodynamic modeling to quantify both the hydration state of the incoming plate and the dehydration flux during subduction. Our results from the Northern Central America margin suggest that, in addition to mantle serpentinization, the incoming oceanic crust also experiences a high degree of bending-induced hydration and transports a substantial flux of water to the mantle wedge. We propose to extend this study to subduction zones with different thermal structures, such as Cascadia, Alaska, and N. Japan.
2017AGU_Hydration of the incoming plate_Dec8.pdf