Volatile contents and oxygen isotopic composition of zircon-hosted melt inclusions

Summary diagram of the Monitoring Earth Evolution Through Time (MEET) project (drafted by Andrée Valley, https://www.isterre.fr/english/research-observation/research-projects/european-projects/article/monitoring-earth-evolution-through-time-meet.html)

Melt inclusions in zircons (MIZs) could be “magmatic time capsules” that can record the composition as well as the P-T conditions of magmas at a given time. I analyzed  the chemical composition of MIZs as a postdoc working with Professor John Valley at UW-Madison, as part of the Monitoring Earth Evolution Through Time (or MEET) project (funded by the European Research Council, corresponding P.I. Professor Alexander Sobolev Université Grenoble Alpes, P.I. Professor Stephen Sobolev GFZ Potsdam, P.I. Professor John Valley, UW-Madison). 

Young volcanic MIZs (naturally quenched into glass) from the Laguna del Maule volcanic field, Southern Andes were analyzed for H2O contents, oxygen isotopic composition, and major element composition, which were correlated with the host zircon 238U-230Th disequilibrium ages to provide a unique perspective on the evolution of magma storage and composition of the crystal mush over time (Shimizu et al. CMP 2024). 

Archean age zircon-hosted MIZs were homogenized in an internally heated gas pressure vessel (Dr. Renat Almeev, Leibniz University of Hannover), and measured for their major and trace element chemistry, oxygen isotopic composition, H2O contents. This will allow reconstruction of conditions of formation of Archean magmatic rocks, and to constrain the source of their H2O with potential implications for the geodynamic setting of Archean magma formation. 

Highlighted in red is a chondrule (in meteorite DOM 08006) composed of silicate glass (light gray), olivine (dark gray) and metal (white) embedded in matrix material. Chondrules are thought to be the oldest Solar System 'rocks', and it is a constituent of meteorites and asteroids such as Itokawa, Bennu, and Ryugu. Schematic of Solar System credit to Maximilien Verdie-Paoletti.

Chondrites are undifferentiated meteorites that are thought be one of the major building blocks of terrestrial planets.  Except for CI chondrites, chondrites are comprised of between 30 vol.% and 80 vol.% chondrules, which are small igneous spheres (0.1-1 mm diameter) that formed at peak temperatures of ~1700-2100 K. Chondrules are some of the oldest Solar System ‘rocks’ having formed ~1-4 millions years after the formation of the very oldest rocks, CAIs in CV chondrites. While they evidently formed as molten droplets from predominantly silicate precursor materials, their formation conditions and heating mechanism(s) remain very poorly constrained.

I measured the volatile contents (C, H, F, Cl, S) as well as hydrogen isotopic composition (D/H) in chondrule glasses to better constrain the formation conditions and alteration processes. Some chondrule glasses showed significant effect of aqueous alteration and thermal metamorphism  (up to 1 wt.% H2O and D/H up to 10,000 permil).  After filtering for these effects, I constrained the maximum pressure under which the chondrules formed (Shimizu et al. GCA 2021). 

Schematic diagram of part of the deep Carbon cycle. My study so far focuses on estimating the CO2 content in the mantle and the flux of CO2 at mid-ocean ridges.

Concentrations of volatiles such as water and carbon in Earth’s mantle are essential information for understanding processes such as mantle melting and constraining its physical properties such as viscosity and seismic velocity. Flux of volatiles from Earth's mantle into the atmosphere is also critical for maintaining Earth's long-term climate and habitability.

I study volatiles in mid-ocean ridge basalts to better constrain the volatile contents in Earth's mantle (Shimizu et al. GCA 2016 & Shimizu et al. GCA 2019). My recent work has focused on studying highly volatile depleted mid-ocean ridge basalts, which are less degassed in volatiles such as CO2. This provides a more reliable estimate of CO2 content in Earth's mantle. My latest work suggests that the CO2 content of parts of Earth's upper mantle (enriched parts of the depleted upper mantle) may be significantly higher than previous estimates made by similar techniques (Shimizu et al. GCA 2023).

Amphibole structure (Uvarova et al., 2007). REEs reside in the M4 site.

Differentiation of arc magmas in subduction zones through fractional crystallization and assimilation of crustal material is an important process to understand the origin of the continental crust. Amphibole is a mineral that has a potentially strong effect on the REE pattern of the differentiating arc magma, but its importance has been ambiguous due to the similarity with clinopryoxene in terms of the REE partitioning behavior.

I developed a model for amphibole-melt REE partitioning that is based on the lattice strain model (Shimizu et al. Am. Min. 2017), to accurately model the effect of amphibole fractional crystallization. I have shown that fractional crystallization of amphibole significantly affects the rare earth element (REE) concentration in arc magmas, and it is an essential crystallizing phase to generating arc magmas with REE patterns that resembles the continental crust. This model is also useful in estimating the REE concentration in melt in equilibrium with amphiboles in cumulates to better understanding the origin of the amphibole bearing cumulates.

Cumulates perspective on the tholeiitic vs. calcalkaline trends

Comparison between mid-ocean ridge, island arc, and continental arc cumulates as well as experimentally generated cumulates (Chin et al., 2018).

To better understand the origin of the tholeiitic and calc-alkaline trends observed in MORBs and arc magmas, I have examined the composition of mid-ocean ridge and arc cumulates respectively, in collaboration with Prof. Emily Chin at Scripps Institution of Oceanography and Prof. Grant Bybee at Wits University. In our study (Chin et al. EPSL 2018), we have shown that mid-ocean ridge and arc cumulates are Fe depleted and enriched respectively, providing a complimentary cumulate perspective on the tholeiitic and calc-alkaline trends observed in melts. Further, we have shown that arc cumulates become more Fe enriched with increasing crustal thickness, which is consistent and complementary to observations that arc magmas become more Fe depleted with increasing crustal thickness. Our result supports the idea that arc crustal thickness plays an important role in generating calc-alkaline trends observed in arc magmas.