Fractionation of highly siderophile elements in metal grains from unequilibrated ordinary chondrites: Implications for the origin of chondritic metals

Okabayashi, Satoki; Yokoyama, T.; Nakanishi, Nao; Iwamori, Hikaru

doi:10.1016/j.gca.2018.10.003

Cited by 9 publications

(15 citation statements)

References 89 publications

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“…After the ejection of metal grains from molten chondrule precursors, metal grains may collide and merge with other metal grains. Okabayashi et al (2019) measured the abundances of highly siderophile elements on metal grains from type 3 ordinary chondrites and found that larger metal grains have relatively homogeneous abundances of highly siderophile elements that are close to the bulk metal composition. This observed trend is consistent with the idea that some of the metal grains collided and merged with other metal grains (Okabayashi et al 2019).…”

Section: Metal Grainsmentioning

confidence: 99%

“…Okabayashi et al (2019) measured the abundances of highly siderophile elements on metal grains from type 3 ordinary chondrites and found that larger metal grains have relatively homogeneous abundances of highly siderophile elements that are close to the bulk metal composition. This observed trend is consistent with the idea that some of the metal grains collided and merged with other metal grains (Okabayashi et al 2019). For iron and nickel, Leliwa-Kopystynski et al (1984) performed collision experiments by using 8 mm-sized projectiles.…”

Section: Metal Grainsmentioning

confidence: 99%

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Compound Chondrule Formation in Optically Thin Shock Waves

Arakawa

Nakamoto

2019

ApJ

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Shock-wave heating within the solar nebula is one of the leading candidates for the source of chondruleforming events. Here, we examine the possibility of compound chondrule formation via optically thin shock waves. Several features of compound chondrules indicate that compound chondrules are formed via the collisions of supercooled precursors. We evaluate whether compound chondrules can be formed via the collision of supercooled chondrule precursors in the framework of the shock-wave heating model by using semi-analytical methods and discuss whether most of the crystallized chondrules can avoid destruction upon collision in the post-shock region. We find that chondrule precursors immediately turn into supercooled droplets when the shock waves are optically thin and they can maintain supercooling until the condensation of evaporated fine dust grains. Owing to the large viscosity of supercooled melts, supercooled chondrule precursors can survive high-speed collisions on the order of 1 km s −1 when the temperature is below ∼ 1400 K. From the perspective of the survivability of crystallized chondrules, shock waves with a spatial scale of ∼ 10 4 km may be potent candidates for the chondrule formation mechanism. Based on our results from one-dimensional calculations, a fraction of compound chondrules can be reproduced when the chondrule-to-gas mass ratio in the pre-shock region is ∼ 2 × 10 −3 , which is approximately half of the solar metallicity.

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Section: Metal Grainsmentioning

confidence: 99%

Section: Metal Grainsmentioning

confidence: 99%

Compound Chondrule Formation in Optically Thin Shock Waves

Arakawa

Nakamoto

2019

ApJ

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“…Although this hypothesis is difficult to test, it raises the question of the uniqueness of CB a and CB b chondrites in terms of the elemental and isotopic zonations in their small metal grains. To our knowledge, these specific patterns of condensation in CB metal grains have not been described in the metal components of other chondrites, likely because of their reprocessing by complex heating-cooling cycles (e.g., Okabayashi et al 2019) (Gilmour et al 2009;Pravdivtseva et al 2014) is 1.48 AE 0.9 Myr younger, attributable to inhomogeneous 129 I/ 127 I ratios within the formation environment (Bollard et al 2015).…”

Section: Disk Versus Plume Model For Cb Metal Formationmentioning

confidence: 79%

“…To our knowledge, these specific patterns of condensation in CB metal grains have not been described in the metal components of other chondrites, likely because of their reprocessing by complex heating–cooling cycles (e.g., Okabayashi et al. 2019) in the hot protoplanetary disk. Additionally, CB a and CB b components (metal and silicates) formed in a short period of time (4562.2 ± 2.4 Ma Hf‐W, Kleine et al.…”

Section: Disk Versus Plume Model For Cb Metal Formationmentioning

confidence: 80%

Condensation and evaporation processes during CB chondrite formation: Insights from Ge isotopes and highly siderophile element abundances

Florin

Luais

Alard

et al. 2021

Meteorit & Planetary Scien

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We analyzed the highly siderophile element (HSE) contents and bulk Ge isotopic compositions of large metal grains in the CB chondrites Bencubbin (CB a ), Gujba (CB a ), and HaH 237 (CB b ). Our results suggest that the large grains were formed by the aggregation of smaller condensed grains, and the two Benccubinite groups are distinguishable based on their bulk metal δ 74/70 Ge mass-dependent isotopic values of 0.99 AE 0.30‰ (CB a ) and −0.65 AE 0.10‰ (CB b ). Based on our observations of these three samples, the isotopic compositions of metal in CB a chondrites are best explained by condensation at slow cooling rates in the center of an impact plume, whereas the metal in CB b chondrites formed under fast cooling rates along the plume edges. We also analyzed the Ge contents and isotopic compositions of the core, intermediate, and rim fractions of two Gujba metal grains, which were separated by sequential digestion. These results show a gradual decrease in δ 74/70 Ge and [Ge] from core to rim. We suggest that these δ 74 Ge zonations result from nearequilibrium condensation and evaporation processes in a heterogeneous plume. We propose a model for their formation in which (1) small grains (to become grain cores) condensed at equilibrium; (2) these grains were transported to a warmer region of the plume where they reached temperatures lower than that of Fe-Ni condensation, but high enough for the rapid evaporation of Ge; (3) Ge evaporation followed by slow cooling enriched the grains in heavy Ge isotopes and the surrounding gas in light Ge isotopes; and (4) equilibrium recondensation of metal from the gas and around the small grains formed the light Ge isotopic zonations observed in grain rims.

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“…In contrast, the similarity of the metal-silicate partitioning coefficients of siderophile elements measured in OCs (Kong and Ebihara, 1997) to those in measured melting experiments (Brenan and McDonough, 2009) suggests metal formation by the reduction of FeO from silicate precursors during chondrule and chondritic metal formation (Kong and Ebihara, 1997). Despite this apparent consistency, recent high-precision in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) measurements highlight large variations in highly siderophile element (HSE) concentrations from one metal grain to another within the same sample (Campbell and Humayun, 2003;Okabayashi et al, 2019). Additionally, the lack of correlation between the concentrations of refractory HSEs (Re, Os, Ir, Ru, Pt) and moderately refractory siderophile elements (Pd, Au) (Campbell and Humayun, 2003), as well as the solar Fe/Si ratio (Palme et al, 2014) makes it difficult to explain metal formation in H chondrites exclusively by the reduction of FeO in silicates.…”

Section: Introductionmentioning

confidence: 99%