OS3-11
原始太陽系星雲の高温過程で形成されたコンドリュールの再現実験
Reproduction experiments of chondrules formed at hightemperature processes in the protoplanetary disk
中村智樹 1，森田朋代 1，小山千尋 2，木村勇気 3，土山明 4，織田裕久 2，下西里奈 2
Tomoki NAKAMURA1, Tomoyo MORITA1, Chihiro KOYAMA2, Yuki KIMURA3, Akira
TSUCHIYAMA4, Hirohisa ODA2, and Rina SHIMONISHI2
東北大，Tohoku Univ.,
宇宙航空研究開発機構, JAXA,
3 北海道大, Hokkaido Univ.,
4 立命館大, Ritsumeikan Univ.
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1. Introduction
Chondrules are solid spherical materials with a diameter of about 1 mm contained in primitive meteorites(Figure 1) and are the major component of solid particles that were suspended in the solar nebula before theformation of planets and asteroids. Chondrules are thought to have been formed by high-temperature
heating (approximately 1200°C or higher) of the precursor material in the no gravity conditions of the nebula,followed by rapid cooling, but the specific formation mechanism has not been clarified. The main reason isthat the structure and the elemental distribution of chondrules cannot be reproduced by ground-based
experiments. In this study, we aim to reproduce chondrules in microgravity melting experiments for the firsttime in the world.

Figure 1. Barred olivine chondrule in meteorites and corresponding structure.

Chondrules are classified into several categories based on their structure (Grossman et al. 1988), and thechondrule that we will attempt to reproduce in this experiment is barred-olivine (BO) chondrule (Figure 1).
We chose this type of chondrule because (i) it was formed by total melting of the precursor material, whichclearly constrains the melting and cooling conditions in the experiment, and thus the high-temperature
1 of 4phenomena that occurred in the nebula, (ii) it has a characteristic texture (Figure 1: barred olivine and glass
between them, with an olivine rim surrounding the whole structure). (iii) because it is easy to determine thesuccess or failure of reproducible experiments.
In this study, we would like to focus on the olivine rim (Figure 1) in particular. The reason is that pastground-based experiments have reproduced barred-shaped olivine (in 3D, plate-like crystal), but not olivine
rims.
2.

Past experiments for BO chondrule reproduction

Over the past 30 years, experiments have been conducted to reproduce BO chondrules. Most of theexperiments have been performed using high-temperature furnaces1, 2). Our research group also attempted
ground-based experiments using (1) gas-jet levitation and laser melting (the equipment is detailed in aprevious paper 3) and (2) electrostatic levitation and laser melting, but failed to reproduce the olivine rim
structure (Figure 2).
The reason for the failure is that in (1) the gas-jet levitation method, the sample is levitated by gas, whichcauses strong vibrations on the surface of the molten sample and strong convection currents in the melt, whichinhibits crystallization of the rim (Figure 2). Therefore, (2) the electrostatic levitation method was tried, but
this method also levitated the melt under high voltage against gravity, which caused strong microvibration of
the melt surface, and the crystals that were first crystallized go down to the bottom of the melt, resulting in norim formation. In addition to the surface vibration and the convection of the melt, another possible reason forthe lack of successful rim formation in the ground experiments was the difficulty in attaching temperatureand compositional gradients due to evaporation on the melt surface in the ground experiment. We believe thatcrystallization experiments under microgravity can avoid all of these causes.

Figure 2. Experimentally produced BO chondrule without rim structure. This sample was produced by
gas-jet levitation system using Ar gas. Cooling rate is 74.6℃/s.

3.

Experiments at ELF

In this study, starting materials with the actual chemical composition of BO chondrules in meteorites (type Iwith high Mg/Fe and type II with low Mg/Fe: Table 1) will be used, and the samples are heated to a temperature(about 1800°C) above the melting point (about 1650°C) and totally melted. After confirming melting, the2 of 4

sample is cooled to approximately 700°C by various cooling rates (0.5 K/s to 100 K/s in the crystallization
temperature range). The detailed cooling profile will be determined basis on the results of ground experiments.
In developing the experimental conditions, we would like to conduct the experiment under conditions thatcan clarify the reason why the BO chondrule could not be achieved in the ground experiment. Table 2 shows
the success criteria for our experiment.

Table 1. Chemical compositions of starting materials.

Type I

Type II

Na2O

0.15

1.50

mol %

MgO

50.59

40.16

mol %

Al2O3

3.15

1.99

mol %

SiO2

38.82

39.82

mol %

CaO

5.59

3.66

mol %

FeO

1.26

12.50

mol %

NiO

0.45

0.38

mol %

Table 2. Success criteria of our experiment.

SuccessCriteria
To obtain the temperature history of the sample from melting tosolidification.

Minimum SuccessTo recover the solidified samples to the earth.
To obtain information on the internal structure and composition ofsamples solidified at different rates of cooling.
In addition to the above conditions, the following must be fulfilled:

Full Success

Formation of olivine bar structures as seen in natural chondrules. In
addition, to obtain information on the elemental fractionation between
olivine bars and the surrounding glass.

4.

Extra

In addition to the above conditions, the following must be fulfilled:

Success

To reproduce chondrules with, at least parts of, olivine rim structures.

SummarySince chondrules are the raw material of asteroids and the Earth, they are the most important solid particles
in the formation of solar system bodies 4). Therefore, understanding chondrule formation is a first-class issue
in planetary science. Currently, the most supported model is that nebular gas is accelerated by shock waves
and impacts the progenitor, causing melting and chondrule formation5, 6). In this case, crystallization occurs
3 of 4when the nebular gas is blowing against the chondrule (strongly surface oscillating). If our ELF experiment
can reproduce the structure of BO chondrules by crystallization in a state of suppressed surface oscillation, it
will greatly constrain the model described above. Clearly, this result will have a major impact on the modelof the solar system formation.

References
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6)

Lofgren, G., and Lanier, A. B. (1990). Dynamic crystallization study of barred olivine chondrules. Geochimica et
Cosmochimica Acta, 54(12), 3537-3551. doi.org/10.1016/0016-7037(90)90303-3
Tsuchiyama, A., Osada, Y., Nakano, T., and Uesugi, K. (2004). Experimental reproduction of classic barred olivinechondrules: Open-system behavior of chondrule formation. Geochimica et cosmochimica acta, 68(3), 653-672.
doi.org/10.1016/S0016-7037(03)00448-4
Nagashima, K., Moriuchi, Y., Tsukamoto, K., Tanaka, K. K., and Kobatake, H. (2008). Critical cooling rates for glass
formation in levitated Mg2SiO4-MgSiO3 chondrule melts. Journal of Mineralogical and Petrological Sciences, 103(3),204-208. doi.org/10.2465/jmps.070620c
Alexander, C. M. D. (2022). An exploration of whether Earth can be built from chondritic components, not bulk
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Hood, L. L., and Horanyi, M. (1991). Gas dynamic heating of chondrule precursor grains in the solar nebula. Icarus,
93(2), 259-269. doi.org/10.1016/0019-1035(91)90211-B
Connolly Jr, H. C., & Love, S. G. (1998). The formation of chondrules: Petrologic tests of the shock wave model.
Science, 280(5360), 62-67. DOI: 10.1126/science.280.5360.6

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