Research Summary
I study planetary materials (both meteorites and planetary analogs) to understand chemical processes at depth and at the surface, with a particular focus on the role magmatic volatile elements (S, C, F, Cl, H) play in the redox, volatile, and thermal evolution of planetary bodies. I obtain high-fidelity geochemical data using a combination of several analytical instruments, including analytical microscopy (EPMA, SEM), secondary-ion mass spectrometry (SIMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), x-ray absorption near edge structure (XANES) spectroscopy, infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and evolved gas analysis (EGA). In addition, I am a regular user of the synchrotron facility Advanced Photon Source (APS) at Argonne National Lab (8 successful beam time sessions). Because astromaterials are often the products of exotic chemical environments, laboratory experiments are critical for putting such analyses into the proper context. My experimental repertoire spans a range of pressures and temperatures and includes my own developed experimental setups. These include meteorite vapor deposition experiments, piston-cylinder, multi-anvil, in-situ X-ray microtomography, and falling sphere viscometry experiments.
I recently co-developed a revised Goldschmidt classification table for Mercury-relevant conditions that illustrates how elemental geochemical affinity, compatibility, and volatility changes in highly reduced systems (McCubbin & Anzures, Mercury in Treatises of Geochemistry 3rd Edition, under review). There is a growing recognition that low oxygen fugacity (fO2 < IW-2) conditions prevailed during the formation of planetary bodies including the parent bodies of enstatite chondrite and aubrite meteorites, Mercury, early Earth, and exoplanets around C-rich stars. To compare outgassing and condensate formation across a redox range (IW-7 < fO2 < IW+3), I investigate the outgassing mass and species of heated chondritic material (carbonaceous chondrites (CC), ordinary chondrites (OC), enstatite chondrites (EC), and Rumuruti chondrites (RC)). Specifically, quantification of outgassing from carbonaceous chondrites improves remote water content estimates of hydrous asteroids and the Moon, as well as the development of primordial atmospheres in the early Solar System. Current orbital and sample-return missions to near-Earth C-class asteroids, including Hayabusa2 and OSIRIS-Rex, make it particularly timely to advance our ability to remotely quantify water on asteroid surfaces.
Redox Evolution: Melt Speciation and Oxybarometry in Reduced, Sulfur-Rich Systems
What happens if a planet forms in an oxygen-poor environment? What is the initial chemistry and pressure of atmospheres formed during accretion of rocky planets?
Effect of sulfur speciation on the chemical properties of magmas: MESSENGER revealed that lavas on Mercury are enriched in sulfur (1.5-4 wt.%) compared with other terrestrial planets (<0.1 wt.%) due to high S solubility under its very low oxygen fugacity (ƒO2 between IW-3 and IW-7). Meteorites with these characteristics (e.g., enstatite chondrites and aubrites) present additional evidence for oxygen and sulfur-rich planetary bodies that are unique in our Solar System. Sulfur speciation in magmas has substantial impacts on their physicochemical properties such as viscosity, melting temperature, volatility, and mineral stability, all of which led to Mercury’s distinct evolution. To better understand S solubility and overall melt speciation in reduced magmas, I collect S, Cl, Cr, and Si XANES spectra in synthetic magmas that span a range of P (0.1 to 5 GPa), T (1225 to 1850 °C), and ƒO2 (IW-0.8 to IW-8.6). For this I have developed new XANES standards of sulfides stable under reducing conditions and XANES unmixing technique. I have found that oxygen fugacity and pressure control whether FeS, CaS, or MgS is the dominant S species in silicate melt, and that temperature and composition have a negligible effect. This S bonding environment influences phase equilibria by decreasing the stability of forsterite and anorthite relative to other silicate minerals in S-rich magmas.
1 inch mount with meteorite chips embedded in indium (aubrites top right, EH chondrites top left, EL chondrites, bottom) for SIMS volatile analyses.
Check out our recent LPSC abstract on volatile partitioning in reduced meteorites.
S-in-enstatite oxybarometer reveals incomplete reduction of enstatite chondrites: The early solar system experienced a wide range of oxygen fugacity as recorded in different meteorites. Enstatite chondrites (EC) and aubrites are the most reduced meteorites. Mercury and perhaps some of the Earth may also have formed at such low fO2. However, enstatite and other components in EC’s are not uniformly reduced, and implying ƒO2 varied in space and/or time. Quantifying these variations requires multiple oxybarometers. Toward this end, we have developed an oxybarometer based on the amount of S dissolved in enstatite at sulfide saturation. Oxygen fugacity estimates from our new sulfur-in-enstatite oxybarometer do not correlate with Si-in-kamacite, but do correlate with metamorphic degree. We suggest that the two oxybarometers record evolving ƒO2 conditions in the solar nebula.
This work is funded by my NASA Earth and Space Sciences Fellowship grant (2018-2021), GSA Graduate Student Research Grant (2019-2020), and NSF/GSA Graduate Student Geoscience Grant (2020-2021).
Volatile Evolution: Volatile Reservoirs and Outgassing in the Solar System
What is the abundance and distribution of water, fluorine, and chlorine in the early solar system? How, where, and when did aqueous alteration occur?
Continuum removed reflectance spectra of one CM chondrite for various grain sizes that should all have the same bulk water content. The 3 μm water absorption deepens and narrows with decreasing particle size especially comparing chip to powder; the inset quantifies the increase in absorption strength as measured by band depth (y-axis scale: 0.35-0.70).
Check out our recent LPSC abstract assessing the effect of particle size on water content estimates of carbonaceous chondrites and their parent bodies!
Improving water content estimates for hydrous asteroids: The presence of water- and hydroxyl-bearing minerals in meteorites and asteroids has long been recognized, but quantifying the absolute amount of water in these objects from remote observations remains difficult. Constraining the water content of near-Earth asteroids (NEOs) is important in understanding the early Solar System and assessing NEO potential for in-situ resources in space exploration and industrialization. High water contents (>10 wt.% H2O) have been measured in carbonaceous chondrites and inferred for C-class asteroids based on their spectra. Mission data from recent sample return missions Hayabusa2 and OSIRIS-REx for C-class asteroids 162173 Ryugu and 101955 Bennu revealed that these objects are rubble pile asteroids with surfaces dominated by boulders, and that Ryugu grains analyzed on Earth were more water-rich compared to remote estimates based on a weak 3 µm water absorption. My research in this theme has determined that particle size strongly affects reflectance spectral parameters related to OH/H2O, and grain size can influence bulk H2O estimates by up to 6 wt.% (Anzures et al., 2021a). Additional studies of C chondrite powders with grain size <100 μm have shown that certain spectral parameters are highly correlated with water content. My research highlights the need to properly account for particle size in order to accurately estimate water content. In the future I will continue to improve the methods for extracting quantitative information from the reflectance spectra of regolith-poor (rubble pile) asteroids, as well as areas of the Moon.
Thermal Evolution: Thermometry of Meteorites and Differentiation of Mercury
How does redox influence the distribution of major, minor, and trace elements, especially heat producing elements during planetary differentiation? To what extent is the frequency of catastrophic collisions in the early Solar System recorded in the cooling histories of meteorites?
Major and trace element geothermometry: winonaite/IAB iron meteorite and enstatite chondrite/aubrite parent bodies: Meteorites record a complex history of heating, brecciation, fragmentation, and metamorphism. While primitive achondrite winonaites and IAB irons exhibit geochemical trends consistent with differentiation and partial melting, there are questions about whether they reflect precursor chondrites or parent body differentiation processes. A key path forward in distinguishing these two hypotheses is to develop multiple stage cooling histories using geothermometers sensitive to different temperature intervals. In collaboration with Nick Dygert’s lab (UT Knoxville), I have applied REE-in-two-pyroxene and major element-based thermometers to enstatite chondrites, aubrites, and winonaite-IAB iron meteorites. Our results suggest that these parent bodies cooled quickly through high temperature intervals and demonstrate that catastrophic collisions in the early Solar System were more frequent than previously thought (LPSC abstract on the thermochemical evolution of the winonaite and IAB iron meteorite parent body)
Enstatite chondrites and aubrites experienced a complex history of thermal metamorphism (petrologic type 3-7) as well as possible fragmentation and reassembly. Interestingly, the parent bodies of enstatite chondrites seem to have experienced more severe thermal and shock metamorphism than other meteorites, however the peak temperatures and high temperature cooling rates are poorly constrained. To place constraints on the thermochemical evolution of enstatite chondrite and aubrite parent bodies, we apply different silicate geothermometers, which suggest the parent bodies underwent a fragmentation-reassembly event (LPSC abstract by Brendan’s 2022 LPI intern Emily Etheridge on the thermochemical evolution of the enstatite chondrite and aubrite meteorite parent bodies)