^1,2Gokce Ustunisik,^2,3Denton S.Ebel,^3,2David Walker,^4,1Roger L.Nielsen,^3,2Marina Gemma
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.038]
¹Department of Geology and Geological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, 57701
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, 10024-5192
³Department of Earth and Environmental Sciences, Lamont Doherty Earth Observatory of Columbia University, Palisades, NY, 10964-8000
⁴104 CEOAS Admin, College of Earth Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331
Copyright Elsevier

We determined the mineral-melt partition coefficients (D_i’s) and the compositional and/or temperature dependency between grossite, melilite, hibonite, olivine and Ca-, Al-inclusion (CAI)-type liquids for a number of light (LE), high field strength (HFSE), large ion lithophile (LILE), and rare earth (REE) elements including Li, Be, B, Sr, Zr, Nb, Ba, La, Ce, Eu, Dy, Ho, Yb, Hf, Ta, Th. A series of isothermal crystallization experiments was conducted at 5 kbar pressure and IW+1 in graphite capsules. The starting compositions were selected based on the calculated and experimentally confirmed phase relations during condensation in CI dust-enriched systems (Ebel and Grossman, 2000, Ebel, 2006, Ustunisik et al., 2014).

The partition coefficients between melt and gehlenite, hibonite, and grossite show that the trace element budget of igneous CAIs is controlled by these three major Al-bearing phases in addition to pyroxene. In general, LE, LILE, REE, and HFSE partition coefficients (by mass) decrease in the order of ^{Gehlenite-Melt}D_i > ^{Hibonite-Melt}D_i > ^{Grossite-Melt}D_i. The partition coefficients between gehlenitic melilite and CAI melt are approximately Be:0.14, B:0.07, Sr:0.79, Zr:0.02, Nb:0.01-0.02, Ba:0.05, La:1.03-3.44, Ce:1.2-3.86, Eu:1.19-2.88, Dy:1.14-3.23, Ho:1.04-2.91, Yb:0.7-1.70, Hf:0.02, Ta:0.01-0.02, Th:0.31-1.71. These results suggest that ^{Gehlenite-Melt}D_i vary by a factor of 2-3 in different melt compositions at the same T (∼1500 ^oC). ^{Hibonite-Melt}D_i exhibit a range as Be:0.02-0.04, B:0.01, Sr:0.21-0.66, Zr:0.02-0.18, Nb:0.03-0.05, Ba:0.02-0.06, La:0.56-4.38, Ce:0.52-3.54, Eu: 0.33-0.84, Dy: 0.25-0.32, Ho:0.17-0.29, Yb:0.05-0.19, Hf:0.05-0.38, Ta:0.02, Th:0.31-1.71. Increased Al and Ca, relative to earlier work, increases the compatibility of ^{Gehlenite-Melt}D_i , and also the compatibility of ^{Hibonite-Melt}D_i, especially for La and Ce.^{Grossite-Melt}D_i of individual mineral-melt pairs are Be:0.43, Sr:0.31, Zr:0.09, Nb:0.01, Ba:0.03, La:0.06, Ce:0.07, Eu:0.13, Dy:0.04, Ho:0.04, Yb:0.03, Hf:0.01, Ta:0.01, Th:0.01 for #18 at 1550 ^oC and as Sr:0.29, Nb:0.03, La:0.07, Ce:0.09, Eu:0.10, Dy:0.05, Ho:0.04, Yb:0.02, Hf:0.003, Ta:0.02, Th:0.02 for #7 at 1490 ^oC.

Olivine partitioning experiments confirm that olivine contribution to the trace element budget of CAIs is small due to the low ^Olivine-MeltD_i at a range of temperatures while ^Olivine-MeltD_{Eu, Yb} are sensitive to changes in T and fO₂. The development of a predictive model for partitioning in CAI-type systems would require more experimental data and use of analytical instruments capable of obtaining single phase analyses for crystals < 5µm.

¹Timothy J.McCoy,¹Catherine M.Corrigan,²Tamara L.Dickinson,³Gretchen K.Benedix,⁴Devin L.Schrader,⁴Jemma Davidson
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125536https://doi.org/10.1016/j.chemer.2019.125536]
¹Dept. of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, 20560-0119, USA
²Science Matters Consulting, LLC, Washington, DC, 20016, USA
³School of Earth and Planetary Sciences, Curtin University, Bentley, WA, 6102, Australia
⁴Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287-1404, USA
Copyright Elsevier

Although acapulcoites and lodranites played a key role in understanding partial differentiation of asteroids, the lack of samples of the chondritic precursor limits our understanding of the processes that formed these meteorites. Grove Mountains (GRV) 020043 is a type 4 chondrite, with abundant, well-delineated, pyroxene-rich chondrules with an average diameter of 690 μm, microcrystalline mesostasis, polysynthetically striated low-Ca pyroxene, and slightly heterogeneous plagioclase compositions. GRV 020043 shows that evidence of partial melting is not an essential feature for classification within the acapulcoite-lodranite clan. GRV 020043 suggests a range of peak temperatures on the acapulcoite-lodranite parent body similar to that of ordinary chondrites, but shifted to higher temperatures, perhaps consistent with earlier accretion. Similarities in mineralogy, mineral composition, and oxygen isotopic composition link GRV 020043 to the acapulcoite-lodranite clan. These features include a high low-Ca pyroxene to olivine ratio, high kamacite to taenite ratio, and relatively FeO-poor mafic silicates (Fa_10.3, Fs_10.4) relative to ordinary chondrites, as well as the presence of ubiquitous metal and sulfide inclusions in low-Ca pyroxene and ƒO₂ typical of acapulcoites. GRV 020043 experienced modest thermal metamorphism similar to type 4 ordinary chondrites. The mineralogy and mineral compositions of GRV 020043, despite modest thermal metamorphism, suggests that most features of acapulcoites previously attributed to reduction were, instead, inherited from the precursor chondrite. Although partial melting was widespread on the acapulcoite-lodranite parent body, ubiquitous Fe,Ni-FeS blebs in the cores of silicates were not implanted by shock or trapped during silicate melting, but were inherited from the precursor chondrite with subsequent overgrowths during metamorphism.