^1,2Robert W. Nicklas,¹James M. D. Day,³Zoltán Váci,⁴Minghua Ren,⁵Kathryn G. Gardner-Vandy,⁶Kimberly T. Tait
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.04.019]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093, USA
²Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA, 02467, USA
³Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO, 63130, USA
⁴Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV, 89154, USA
⁵Aviation and Space, Oklahoma State University, Stillwater, OK, 74078, USA
⁶Department of Natural History, Royal Ontario Museum, Toronto, ON, M5S 2C6, Canada
Copyright Elsevier

Metal-silicate segregation is one of the most fundamental mechanisms in planetary differentiation, with primitive achondrites offering important constraints on this process. Brachinites and brachinite-like achondrites (BLA) are olivine-dominated primitive achondrites that experienced up to ∼20% partial melt removal under relatively oxidized (ΔIW∼-1) conditions within an initially chondritic parent body and represent residues with inefficient metal-loss. We present bulk rock and in situ lithophile and highly siderophile element (HSE) abundance systematics as well as ¹⁸⁷Re-¹⁸⁷Os data for five olivine-rich primitive achondrites. These new data confirm classification of Reid 013 as a brachinite, three of the samples as BLA (Northwest Africa [NWA] 6874, NWA 7499, and Miller Range 090805), and the final sample as an ungrouped primitive olivine-rich achondrite (NWA 7680). An aliquot of MIL 090805 shows amongst the highest total HSE contents (>35 ppm) and the highest Pt content (∼23 ppm) of any primitive achondrite. Compiled HSE data for brachinites and BLA show correlations between total HSE abundance, Pt enrichment, and average olivine Fo. This correlation can be explained by variable melting (∼10-20%) of an H ordinary chondrite-like protolith, with retention of both Fe-metal and a Pt-rich alloy phase distinct from the observed Fe-metal phases in more depleted residues. Such a Pt-alloy phase is likely stabilized by elevated abundances of the HSE during chondrite melting and the low solubility of HSE in melts. Rhenium-Os isotope data in the studied samples has been modified by recent mobilization of Re during terrestrial weathering, with the limited range of measured ¹⁸⁷Os/¹⁸⁸Os in brachinites and BLA supporting minor fractionation of Re/Os during melting and an ancient (∼4.5 Ga) partial melting event to explain their compositions. These results indicate that models of planetary differentiation should consider the low solubility of Pt in chondrite melts and the potential for alloy formation to modify HSE abundances of silicate mantles.

¹Pierre-Marie Zanetta,^1,2Venkateswara Rao Manga,¹Yao-Jen Chang,²Tarunika Ramprasad,¹Juliane Weber,³John R. Beckett,^1,2Thomas J. Zega
American Mineralogist 108, 881-902 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P0881.pdf]
¹Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721, U.S.A.
²Materials Science and Engineering, The University of Arizona, Tucson, Arizona 85721, U.S.A.
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
Copyright: The Mineralogical Society of America

Calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites are composed of refractory
minerals thought to be the first solids to have formed in the solar nebula. Among them, hibonite,
nominally CaAl12O19, holds particular interest because it can incorporate significant amounts of Ti
into its crystal structure in both Ti3+ and Ti4+ oxidation states. The relative amounts of these cations
that are incorporated reflect the redox conditions under which the grain formed or last equilibrated and
their measurement can provide insight into the thermodynamic landscape of the early solar nebula.
Here we develop a new method for the quantification of Ti oxidation states using electron energy-loss
spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscope (STEM)
to apply it to hibonite.
Using a series of Ti-bearing oxides, we find that the onset intensity of the Ti L2,3 edge decreases with
increasing Ti-oxidation state, which is corroborated by simulated Ti-oxide spectra using first-principles
density-functional theory. We test the relationship on a set of synthetic hibonite grains with known
Ti4+/ΣTi values and apply the developed method on a hibonite grain from a compact type A inclusion
in the Northwest Africa (NWA) 5028 CR2 carbonaceous chondrite. The STEM-EELS data show that
the chondritic hibonite grain is zoned with a Ti4+/ΣTi ratio ranging from 0.78 ± 0.04 to 0.93 ± 0.04
over a scale of 100 nm between the core and edge of the grain, respectively. The Ti substitution sites
are characterized by experimental and calculated high-angle annular-dark-field (HAADF) images and
atomic-level EEL spectrum imaging. Simulated HAADF images reveal that Ti is distributed between
the M2 and M4 sites while Mg sits on the M3 site. Quantitative energy-dispersive X-ray spectroscopy
shows that this grain is also zoned in Al and Ti. The Mg distribution is not well correlated with that
of Ti and Ti4+/ΣTi at the nanoscale.
The spatial decoupling of the element composition and Ti-oxidation states suggests a multistage
evolution for this hibonite grain. We hypothesize that Ti and Mg were incorporated into the structure
during condensation at high temperature through multiple reactions. Transient heating, presumably
in the solar nebula, adds complexity to the crystal chemistry and potentially redistributed Ti and Mg.
Concurrently, the formation of oxygen vacancies as a result of a reducing gas, led to the reduction of
Ti4+ to Ti3+. The multiple defect reactions occurring in this single hibonite crystal preclude a simple
relationship between the Ti4+/ΣTi and the fO2 of formation. However, moving forward, these measurements are fundamental inputs for modeling of the thermodynamic conditions under which hibonite
formed in the early solar nebula.