¹Michael P.Lucas,¹Nick Dygert,²Jialong Ren,^2,3Marc A.Hesse,²Nathaniel R.Miller,¹Harry Y.McSween
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.09.010]
¹Department of Earth & Planetary Sciences, University of Tennessee, 1621 Cumberland Ave., 602 Strong Hall, Knoxville, TN 37996
²Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX 78712
³Oden Institute of Computational Sciences and Engineering, University of Texas at Austin, 201 E 24th St., Austin, TX 78712
Copyright Elsevier

Ordinary chondrites (OCs) are variably thermally metamorphosed meteorites thought to originate from at least three different parent bodies (H, L, and LL) in the Main Belt of asteroids. The thermal evolutions of OC parent bodies are frequently explained by the onion shell model; however, a competing hypothesis is the fragmentation-reassembly model. The onion shell model proposes undisrupted, internally heated parent bodies with concentrically stratified thermal structure, and posits that OC petrologic types (i.e., 3 to 6) develop with increasing temperature and burial depth. In this model, petrologic types are inversely correlated with depth in the parent body, and cooling rate. The alternative fragmentation-reassembly model invokes catastrophic collisional disruption of parent bodies that initially possessed onion shell structures, followed by rapid reaccretion of hot fragments, forming rubble pile bodies. Fragmentation would result in fast cooling (quenching) of collisional fragments from the temperature experienced by the parent body at the time of collision. Discrimination between these two models may be possible via investigation of the thermal histories of OCs by application of geothermometry and geospeedometry, which are used to constrain the temperatures and rates through which igneous and metamorphic rock samples cool. Most published cooling rate data for OC parent bodies are based on methods that record rates through low closure temperatures (∼500-200 °C) rather than from peak metamorphic temperatures. Recently, a rare earth element (REE)-in-two-pyroxene thermometer has been shown to establish peak or magmatic temperatures (TREE; Liang et al. [2013]. GCA 102, 246-260) for rocks that cooled at moderate to fast geologic rates. We applied the REE-in-two-pyroxene method to determine peak temperatures for 18 OC samples (mostly type 6), in conjunction with conventional two-pyroxene thermometry (TBKN; Brey and Köhler [1990]. J. Pet. 31, 1353-1378) and Ca-in-olivine thermometry (TCa-Ol; Köhler and Brey [1990]. GCA 54, 2375-2388), to determine closure temperatures and estimate cooling rates for OC parent bodies. Inconsistent with slow cooling rates expected in an onion shell scenario, we obtain fast cooling at rates ≳0.5 °C/y from peak temperatures of ∼900 °C. Corroborating the TREE and TBKN measurements, TCa-Ol suggests that the OCs cooled through TCa-Ol closure temperatures (∼700 to 800 °C) at ∼10-2 to 10-1 °C/y. These cooling rates are three to six orders of magnitude faster than rates determined using methods sensitive to low temperature (≤500 °C) cooling (e.g., metallography, 40Ar–39Ar ages, 244Pu fission track). We developed a novel numerical thermal model that incorporates fragmentation of an initial onion shell body and reassembly into a rubble pile body that reproduces both the fast cooling from high temperatures and the slow cooling through low temperatures observed in chondritic meteorites. We hypothesize that OC parent bodies initially possessed onion shell thermal structures, but later experienced collisional breakup, then reaccreted rapidly to form thermally stable rubble-pile asteroids.

^1,2Daisuke Nakashima,^3,4Makoto Kimura,⁴Kouichi Yamada,⁵Takaaki Noguchi,^2,6Takayuki Ushikubo,²NorikoKita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.09.003]
¹Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
²Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
³National Institute of Polar Research, Tokyo 190-8518, Japan
⁴Faculty of Science, Ibaraki University, Mito, Ibaraki, 310-8512, Japan
⁵Faculty of Arts and Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁶Kochi Institute for Core Sample Research, JAMSTEC, Monobe-otsu 200, Nankoku, Kochi 783-8502, Japan
Copyright Elsevier

We measured oxygen isotope ratios and major elemental compositions of non-porphyritic chondrules and lithic fragments with various textures and chemical compositions in the Asuka-881020 CH chondrite. The oxygen isotope ratios plot along the primitive chondrule mineral line with Δ17O (= δ17O – 0.52 × δ18O) values from ∼ –21‰ to +5‰. The Δ17O values increase with decreasing Mg# (= molar [MgO]/[MgO+FeO]%) from 99.6 to 58.5, similarly to the Δ17O-Mg# trends for the chondrules in other carbonaceous chondrites.
Most of the measured objects (non-porphyritic chondrules and lithic fragments) including chondrules analyzed in the previous studies are classified into three groups based on the Δ17O values and chemistry; the –2.3‰ group with FeO-poor compositions (the most abundant group), the +1.4‰ group with FeO-rich compositions, and the –6.3‰ group with FeO-poor compositions. Skeletal olivine and magnesian cryptocrystalline (MgCC) chondrules and MgCC chondrule fragments, which are the –2.3‰ group objects, may have formed via fractional condensation in the isotopically uniform gaseous environment with Δ17O of –2.3‰. When silica-normative materials condensed from gas at ∼ 1200 K, 16O-rich refractory solids, similar to Ca-Al-rich inclusions, were incorporated into the environment. The silica-normative materials that condensed onto the 16O-rich refractory solids were reheated at 1743 – 1968 K and formed cristobalite-bearing chondrules with Δ17O of ∼ –6‰. This scenario can explain the absence of silica-bearing chondrules in the –2.3‰ group and refractory element abundances in the cristobalite-bearing chondrules as high as those in the MgCC chondrules.
Refractory element abundances of the +1.4‰ group objects decrease from FeO-Al-rich and ferroan CC (FeCC) chondrules to FeCC chondrule fragments to FeNi metal-bearing to silica-bearing chondrules. This suggests the formation via fractional condensation in the isotopically uniform gaseous environment. The Δ17O values and FeO-rich compositions of this group could be explained by an addition of 16O-poor water ice as an oxidant to the relatively 16O-rich solids with Δ17O of –2.3‰, which may also explain existence of some MgCC chondrules and fragments with intermediate Δ17O values between –2.3‰ and +1.4‰. The immiscibility textures in the silica-bearing chondrules suggest a reheating event at a temperature of > 1968 K after condensation of silica-normative materials. Thus, the non-porphyritic chondrules and fragments in CH and CB chondrites, which are classified into three distinct Δ17O groups, require multiple chondrule-forming environments and heating events. Energy source for the heating events could be either impact plume and/or other dynamical processes in the protoplanetary disk, though a single heating event would not fully explain observed chemical and isotope signatures in these non-porphyritic chondrules.

¹Ruslan A.Mendybaev,^2,3Michiru Kamibayashi,⁴Fang-Zhen Teng,⁵Paul S.Savage,⁶R.Bastian Georg,¹Frank M.Richter,^2,3,7ShogoTachibana
Geochimica et Cosmochimca Acta (inPress) Link to Article [https://doi.org/10.1016/j.gca.2020.09.005]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
³Department of Earth and Planetary Sciences, University of Tokyo, Tokyo 113-0033, Japan
⁴Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195
⁵School of Earth and Environmental Sciences, University of St. Andrews, St. Andrews, KY16 9AL, Scotland
⁶Water Quality Center, Trent University, Peterborough, ON K9L0G2, Canada
⁷Institute of Space and Astronautical Science, JAXA, Tokyo 252-0222, Japan
Copyright Elsevier

It is widely believed that the precursors of coarse-grained CAIs in chondrites are solar nebula condensates that were later reheated and melted to a high degree. Such melting under low-pressure conditions is expected to result in evaporation of moderately volatile magnesium and silicon and their mass-dependent isotopic fractionation. The evaporation of silicate melts has been extensively studied in vacuum laboratory experiments and a large experimental database on chemical and isotopic fractionations now exists. Nevertheless, it remains unclear if vacuum evaporation of CAI-like melts adequately describes the evaporation in the hydrogen-rich gas of the solar nebula. Here we report the results of a detailed experimental study on evaporation of a such melt at 1600°C in both vacuum and low-pressure hydrogen gas, using 1.5- and 2.5-mm diameter samples. The experiments show that although at 2×10−4 bar H2 magnesium and silicon evaporate ∼2.8 times faster than at 2×10−5 bar H2 and ∼45 times faster than in vacuum, their relative evaporation rates and isotopic fractionation factors remain the same. This means that the chemical and isotopic evolutions of all evaporation residues plot along a single evaporation trajectory regardless of experimental conditions (vacuum or low-PH2) and sample size. The independence of chemical and isotopic evaporation trajectories on PH2 of the surrounding gas imply that the existing extensive experimental database on vacuum evaporation of CAI-like materials can be safely used to model the evaporation under solar nebula conditions, taking into account the dependence of evaporation kinetics on PH2.
The experimental data suggest that it would take less than 25 minutes at 1600°C to evaporate 15–50% of magnesium and 5–20% of silicon from a 2.5-mm diameter sample in a solar nebula with PH2∼2×10−4 bar and to enrich the residual melt in heavy magnesium and silicon isotopes up to δ25Mg ∼ 5–10‰ and δ29Si ∼ 2–4‰. The expected chemical and isotopic features are compatible to those typically observed in coarse-grained Type A and B CAIs. Evaporation for ∼1 hour will produce δ25Mg ∼30–35‰ and δ29Si ∼10–15‰, close to the values in highly fractionated Type F and FUN CAIs. These very short timescales suggest melting and evaporation of CAI precursors in very short dynamic heating events. The experimental results reported here provide a stringent test of proposed astrophysical models for the origin and evolution of CAIs.