¹Dian Ji,¹Nicholas Dygert
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.11.004]
¹Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, United States of America
Copyright Elsevier

Apatite, as an accessory phase in igneous and metamorphic rocks, has important petrological significance due to its capacity to accommodate appreciable amounts of many trace elements in its mineral structure. To better constrain trace element partitioning between apatite and silicate melts, we conducted experiments that produced apatites approaching fluorapatite (FlAp), hydroxylapatite (OHAp) and chlorapatite (ClAp) endmembers separately at 1050 and 1100 °C, 1 GPa pressure, under oxygen fugacity (fO₂) about one log unit below iron-wüstite buffer to four log unit above fayalite-magnetite-quartz buffer. We report the results of 12 experiments which demonstrate that ClAp exhibits lower trace element partition coefficients compared with FlAp and OHAp, especially for Rare Earth Elements (REEs) under all run conditions explored, suggesting trace element partitioning is sensitive to anion site occupancy. Divalent cations are less sensitive to anion occupancy. Positive Eu partitioning anomalies (DEu/DEu*, where Eu is the chondrite normalized abundance and Eu* is the interpolated value from neighboring elements ordered by atomic number) are observed in ClAp experiments under the relatively low fO₂, whereas negative Eu anomalies are exhibited by FlAp and OHAp under the same fO₂ conditions. We infer that anionic occupancies have a direct impact on the substitution mechanisms of trace elements in apatite, thereby influencing their partition coefficients. Beyond the anions, correlations of apatite compositional components (�Ca, �Na, �� and �Si) with partition coefficients suggest they exert crystal chemical controls on trace element partitioning. Based on these observations, we developed parameterized lattice strain models to predict the partitioning of divalent and trivalent elements as a function of temperature and apatite composition, and an fO₂-dependent apatite-melt Eu partitioning model and oxybarometer. We further developed a Eu in apatite-plagioclase oxybarometer that enables us to calculate the fO₂ of apatite and plagioclase-bearing magmatic and subsolidus systems, and evaluated the influence of subsolidus reequilibration on the new oxybarometer. Applied to one of our experiments, winonaite HaH193, and samples from Sept-Iles layered intrusion, the oxybarometer recovers their anticipated fO₂s, ranging from about two log units below the iron-wüstite buffer to the fayalite-magnetite-quartz buffer. Using the new REE and fO₂-dependent Eu partitioning models, we constrained the petrogenesis of lunar KREEP basalt and estimated the relative volatile content in the late lunar magma ocean (LMO) cumulates. The model suggests a relative depletion of Cl in the LMO cumulates, consistent with Cl isotopic analyses and volatile abundance measurements in previous work, suggesting that differential loss of volatiles occurred before or during the late-stage evolution of the LMO.

¹Emily M. Chiappe,¹Richard D. Ash,²Arto Luttinen,³Sari Lukkari,³Jukka Kuva,⁴Connor D. Hilton,¹Richard J. Walker
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14095]
¹Department of Geology, University of Maryland, College Park, Maryland, USA
²Finnish Museum of Natural History, University of Helsinki, Helsinki, Finland
³Geological Survey of Finland, Espoo, Finland
⁴Pacific Northwest National Laboratory, Richland, Washington, USA
Published by arrangement with John Wiley & Sons

The meteorite Lieksa was found in 2017 in Löpönvaara, Finland, and later donated to the Finnish Museum of Natural History. Here, we report siderophile element concentrations, genetic isotopic data, and a metal–silicate segregation age for the meteorite. The ~280 g Lieksa is ~80% metal and ~20% silicate and oxide inclusions by volume, with the inclusions consisting primarily of Fe-rich olivine. Due to Lieksa’s silicate content, coupled with a texture characterized by metal enclosing the silicates, it has been classified as a pallasite. Lieksa’s olivine and bulk chemical characteristics are distinct from those of the known pallasite and iron meteorite groups, consistent with its classification as ungrouped. The meteorite exhibits a flat, chondrite-normalized highly siderophile element pattern, consistent with an origin as an early crystallization product from a metallic melt with chondritic relative abundances. Molybdenum, Ru, and 183W isotopic data indicate that Lieksa formed in the non-carbonaceous (NC) domain of the solar nebula. Radiogenic 182W abundances for Lieksa yield a model metal–silicate segregation age of 1.5 ± 0.8 Myr after calcium-aluminum-rich inclusion formation, which is within the range established for other NC-type pallasite and iron meteorite parent bodies.