¹Marina A.Ivanova,^2,3Ruslan A.Mendybaev,¹Sergei I.Shornikov,¹Cyril A.Lorenz,⁴Glenn J.MacPherson
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.023]
¹Vernadsky Institute of Geochemistry of the Russian Academy of Sciences, Moscow, Russia
²Department of the Geophysical Sciences, University of Chicago, Chicago, IL, United States
³Chicago Center for Cosmochemistry, University of Chicago, Chicago, IL, United States
⁴National Museum of Natural History, Smithsonian Institution, Washington DC, USA
Copyright Elsevier

To address the bulk compositions of CAIs from CH-CB chondrites we have used a new thermodynamic method to model the evaporation of CAI-like melts. The model calculations agree closely with the results of evaporation experiments on individual bulk compositions, and thus could provide a general means of predicting the evaporation trajectory of any CAI bulk composition melt. The model calculations and evaporation experiments show that the initial stages of CAI melt evaporation are controlled by the relative evaporation rates of MgO and SiO2, whereas the late stages are dominated by the initial CaO/Al2O3 ratio of the melt. Application of the model to the puzzling bulk compositions of very refractory CAIs from CH-CB chondrites, many of which are grossite-, hibonite-, and spinel-rich, shows that such compositions can be derived via evaporation of precursors unusually enriched in Al2O3 with CaO/Al2O3 ratios (weight %) < 0.3. This rules out most silicate-rich CAI varieties. Only spinel- and spinel-hibonite-rich fine-grained inclusions with group II REE patterns (common in CV3 chondrites), which may have been present in the region where CH CAIs formed, could be a precursor for the grossite- and hibonite-rich igneous CAIs.

¹Sin-iti Sirono,¹Takuma Shibata²Nagayoshi Katsuta,³Hidekazu Yoshida
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.017]
¹Graduate School of Earth and Environmental Sciences, Nagoya University, Furo-tyo, Tikusa, Nagoya 464-8601, Japan
²Fuculty of Education, Gifu University, Yanagito 1-1, Gifu 501-1193 Japan
³Nagoya University Museum, Nagoya University, Furo-tyo, Tikusa, Nagoya 464-8601, Japan
Copyright Elsevier

Iron oxide concretions are found in sedimentary rocks on both Earth and Mars. On Earth, concretions are common in eolian formations, such as the Jurassic Navajo Sandstone in Utah, USA and those found in the Cretaceous Djadokhta Formation, Gobi Desert, Mongolia. Although it is known that the formation conditions of the iron oxide concretions were affected by the paleoclimate of these regions, quantitative modeling of such formations still requires development, especially concerning initial and diagenetic conditions. A 1-D diffusion-reaction simulation was conducted by assuming that a calcite concretion was initially located in a homogeneous layer of sandgrains. Favorable conditions for the formation of iron oxide concretions have been found to be 4.5⩽pH⩽6, and 10-7⩽[Fe2+]fO2⩽10-5, where [Fe2+] and fO2 are the concentration of ferrous Fe2+ ions and dissolved oxygen relative to the atmospheric value, respectively. An iron-rinded concretion from ferric Fe3+ ions is not possible. For the case of Fe2+ ions, the flow speed of the groundwater should be faster than 2×10-5mms-1. The formation timescale is determined by the diffusion flux of the hydrogen ion, and varies between 2.7×102 and 1.5×104 years for a calcite concretion with an initial radius of 15 mm. Formation conditions of iron-rinded concretions on Earth and Mars are discussed.