^1,2Mingming Zhang,^1,2Yangting Lin,³Guoqiang Tang,³Yu Liu,⁴Ingo Leya
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.12.011]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
²University of Chinese Academy of Sciences, Beijing100049, China
³State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁴Physical Institute, Space Sciences and Planetology, University of Bern, 3012 Bern, Switzerland
Copyright Elsevier

Aluminum-rich (Al₂O₃ > 10 wt%) chondrules (ARCs) are important chondritic components that petrologically link two other major chondritic components, ferromagnesian chondrules (FMCs) and calcium-aluminum-rich inclusions (CAIs), which formed in different regions of the protoplanetary disk. They are closely related to FMCs as indicated by their similar igneous textures, mineral assemblages, and Al-Mg isotope systematics; meanwhile, they have genetic relationship with CAIs as indicated by their distinctly Al₂O₃-rich compositions and occasional occurrences of relict CAI minerals. In order to further understand their formation mechanism and genetic relationships to FMCs and CAIs, nine ARCs and three ARC-related objects from Allende (CV3 oxidized), Leoville (CV3 reduced), and the ungroup Ningqiang carbonaceous chondrites were studied for petrology, mineralogy, bulk compositions, rare earth element (REE) abundances, and in situ oxygen isotopic compositions. Our results suggest that (i) ARCs crystallized from incompletely molten droplets with crystallization sequences mainly determined based on their bulk compositions. Projection of their bulk compositions onto the forsterite-saturated tridymite-diopside-spinel diagram allows us to classify them into Al-rich [Sp], Al-rich [En], and Al-rich [Plag]; (ii) ARC precursors are mixtures of refractory materials and the precursors of FMCs, in which the refractory materials have diverse sources rather than a single type of CAI/AOA (amoeboid olivine aggregate); this is inferred from the bulk compositions, relict minerals (both coarse- and fine-grained spinel, olivine, and Al-Ti-diopside), and various CAI-like REE patterns (unfractionated Group I/ III and highly fractionated Group II/ II-like) of ARCs. The source include AOAs and igneous Type B/C CAIs; (iii) ARCs were melted in the FMC-forming region, possibly by the same heating mechanism or during the same transient heating event, which is consistent with the similar oxygen isotopic compositions of their phenocrysts (Δ¹⁷O = -5.2 ± 1.7‰, 2SD). Thus, we consider that ARCs formed by melting of mixtures of diverse refractory components with the FMC precursors in the FMC-forming region.

¹Alessandro Maltese,^1,2Klaus Mezger
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.12.021]
¹Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
²Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
Copyright Elsevier

Constraining the evolution of Pb isotopes in the bulk silicate Earth (BSE) is hampered due to the lack of a direct determination of Earth’s U/Pb and initial Pb isotope composition. All estimates of these parameters are strongly model dependent and most Pb evolution models start with a meteoritic source, i.e., the primordial Pb composition determined in troilite from the Canyon Diablo iron meteorite. During the condensation of the elements in the solar nebula, accretion of the Earth, and its subsequent chemical evolution, the U/Pb was modified. Different models make different assumptions about the timing and extent of this U-Pb fractionation during Earth’s chemical evolution that cannot always be related to known global geological processes at the time of this modification. This study explores geochemical constraints that can be related to known geological processes to derive an internally consistent model for the evolution of the U-Th-Pb systematics of the silicate Earth.

Lead is chalcophile, moderately volatile, and as a result strongly depleted in the BSE compared to primitive meteorites. Any process affecting the abundance and isotope composition of Pb in Earth throughout its early history has to be consistent with the abundance of elements with similar chemical and physical properties in the same reservoir. The abundances of refractory to moderately and highly volatile elements in the BSE imply that the proto Earth was highly depleted in volatile elements and therefore evolved with a very high U/Pb (²³⁸U/²⁰⁴Pb = µ ≥100) prior to collision with the Moon-forming giant impactor. This impactor had close to chondritic abundances of moderately to highly volatile elements and delivered most of Earth’s volatile elements, including the Pb budget. Addition of this volatile rich component caused oxidation of Earth’s mantle and allowed effective transfer of Pb into the core via sulfide melt segregation. Sequestration of Pb into the core therefore accounts for the high µ_BSE, which has affected ca. 53 % of Earth’s Pb budget. In order to account for the present-day Pb isotope composition of BSE, the giant impact must have occurred at 69 ±10 Myr after the beginning of the solar system. Using this point in time, a model-derived µ-value, and the corresponding initial Pb isotope composition of BSE, a single stage Pb isotope evolution curve can be derived. The result is a model evolution curve for BSE in ²⁰⁸Pb-²⁰⁷Pb-²⁰⁶Pb-²⁰⁴Pb-isotope space that is fully consistent with geochemical constraints on Earth’s accretionary sequence and differentiation history. This Pb-evolution model may act as a reference frame to trace the silicate Earth’s differentiation into crust and mantle reservoirs, similar to the CHUR reference line used for other radio-isotope systems. It also highlights the long-standing Th/U paradox of the ancient Earth.

¹Markus Patzek,²Peter Hoppe,¹Addi Bischoff,³Robbin Visser,³Timm John
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.12.017]
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
²Max Planck Institute for Chemistry, Particle Chemistry Department, P.O. Box 3060, D-55020 Mainz, Germany
³Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstr. 74-100, D-12249 Berlin
Copyright Elsevier

Volatile-rich, CI- and CM-like clasts occur in different brecciated achondrite and chondrite groups. The CI-like clasts in HEDs, polymict ureilites, as well as ordinary, CR, and CB chondrites have a similar mineralogy, indicating a similar alteration history. However, when viewed in detail, their mineral chemistry shows some minor differences between the clasts from different meteorite groups. For CM-like clasts found in HED meteorites, the clasts are, based on their mineralogy, clearly fragments of CM chondrites. To be able to decipher whether CI- (or CM-)like clasts from different meteorite groups are related to certain meteorite classes known to contain volatiles, we obtained D/H ratios of several clasts from the meteorite groups mentioned above and compared them with those of CI and CM chondrites as well as to unique carbonaceous chondrites such as Bells, Essebi, and Tagish Lake. Considering the δD-values, CM-like clasts in HEDs span a similar range compared to bulk values of CM chondrites, further indicating that CM-like clasts are fragments of CM chondrites. For CI-like clasts a clear distinction can be made: While CI-like clasts in HEDs and ordinary chondrites show a very similar range in their δD-signatures compared to “common” CI chondrites, meaning that these clasts are likely related to CI chondrites, the CI-like clasts in polymict ureilites are enriched in D up to 3000 ‰; a similarly high enrichment is found for the CI-like clasts in CR chondrites. Thus, although the CI-like clasts in ureilites and CR chondrites likely experienced similar alteration histories as the CI-like clasts found in the other meteorite types, these clasts probably formed in a different region than the CI chondrites and, thus, are more accurately referred to as C1 clasts. Overall, the existence and isotopic signatures of the C1 clasts in several meteorite groups proves the existence of additional primitive, volatile-rich material in the (early) Solar System besides the matter we study as the CI, CM, and CR chondrites. This material was distributed throughout the Solar System very early and might have played an important role for the volatile inventory of the terrestrial planets.