The reproduction experiment of radial pyroxene chondrules using aerodynamic levitator furnace
in a reduced condition
Tomoyo Morita1, Tomoki Nakamura1 and Kana Watanabe1
1Tohoku University, Japan
We performed reproduction experiments of radial pyroxene chondrules (RP chondrules) using the aerodynamic
levitator furnace in a reduced condition that simulates the environment in the protoplanetary disk [1]. Chondrules were formedby flash heating of solid precursors and crystallization during subsequent cooling. They show characteristic texture thatrefracts thermal histories and the local environment of the protoplanetary disk [2]. In this study, we attempt to constrain thethermal history of the RP chondrules, which formed at high temperature by total melting of the precursor material.
The experiments were performed using the aerodynamic levitator furnace at Tohoku University [3]. The sample
spheres were levitated by Ar-H2 gas (Ar = 97 %, H2 = 3 %), heated to melt by a CO2 laser (LC-100NV from DEOS), andcooled to crystallize in a non-contact state. The cooling rates were from 1 to ~1000 °C/s in the temperature range from 1600 °Cto 500 °C. During cooling small glass particles called "seed" impacted the molten sample to crystallize pyroxene. The furnace
was enclosed in an acrylic container and the inside of the container was replaced with Ar-H2 gas to achieve a reducing
atmosphere of ΔIW from -1.5 to -0.5. The starting material was a mixture of oxide powders (SiO2, MgO, FeO, CaO, Al2O3,and Na2O) with three different compositions similar to natural RP chondrules [4-6]. No metallic Fe was used. During theexperiment, images of the samples were taken with a CCD camera (ELMO) and a high-speed monochrome camera (Photron
FASTCAM-Net Max). The temperature of the samples was measured with a radiation thermometer (LumaSense Technologies
ImpacIN140/5-H). After the heating experiments, the samples were then buried in an epoxy resin and polished. The
microstructure was observed and chemical composition was determined using SEM/EDS (Hitachi S-3400N) and FESEM/EDS (JOEL JSM-7001F).
The experiment reproduced RP chondrules: radial pyroxene crystals growing from one or more points, mesostasis glass
filling in between the pyroxene crystals, and two types of metallic iron with different morphological characteristics. Pyroxene
crystals have dendritic shapes with linear surface contours, or are an aggregate of fine crystals similar to that in
cryptocrystalline chondrules, indicating rapid crystallization from a supercooled melt. The Mg# of the pyroxene crystals reflect
Mg/(Mg+Fe) ratios of the starting materials. Some samples have Fe-richovergrowth on large (> 1 μm) Mg-rich pyroxene crystals, suggesting that therapid crystal growth was followed by continued slow crystal growth due toslow cooling (1-10 °C/s).
The metallic iron in the samples was classified into two types basedon the texture: one was primary metal which is variable in size from µm tomm, crossing the radial pyroxene crystals. SEM observations showed that
relatively small (< 10 μm) primary metals are incorporated into the pyroxene
crystals and that radial crystals of pyroxene were aligned along the surfaceof the larger primary metals. The other was submicron-size metallicinclusions (hereafter secondary metal) that occurred only in mesostasis glass(Fig.1). The secondary metal is spherical or dendritic in shape, sometimes
clinging to the surface of pyroxene crystals. In addition, they were found ina portion of the sample, not in the entire sample. Both primary andsecondary metals were produced by reduction of Fe oxides due to low
oxygen fugacity around the molten samples (Ar-H2 ambient gas), because nometallic iron was added to the starting material. However, the timing offormation was different between the two inferred from their texture andFigure 1. (a) Back-scattered electron (BSE)
observation.
image of a RP chondrule produced by theThe primary metal was formed during total melting of the starting
experiment. Dendritic radial pyroxene crystals
material. We succeeded in-situ imaging of the formation of the primary
spread over the sample. (b) Enlargement view ofmetal using high-speed camera: the metal sphere forms in the melt when thethe white box in (a). The spherical secondary
sample was heated for about 10 or more seconds. In addition, the textural
metals are abundant in mesostasis glasses. Px;
observation indicates that the metal solidified in a silicate melt duringpyroxene and gl; glasses.
cooling, prior to the pyroxene crystallization. On the other hand, thesecondary metal is considered to have formed at temperature between the pyroxene crystallization and the glass transition(mesostasis glass formation). Secondary metal was observed in some samples cooled slowly (cooling rate is 1-10 °C/s) fromthe crystallization temperature to 500°C, while it was not observed in the glass of samples cooled rapidly after crystallizationof pyroxene. The secondary metal formation only by slow cooling is interpreted as the time required for the reduction reaction
and for the diffusion of Fe in the glass.
Similar primary type-metal inclusions are found in the natural RP chondrules, suggesting that the large metallic
spheres were formed immediately after total melting by the reduction of iron oxide during chondrule formation, or they werepresent in the precursors. On the other hand, the metallic spheres of submicron size are universally found in natural RP
chondrules, and their microstructure is similar to that of secondary metal produced in this experiment. This indicates that RPchondrules cooled at < 10 °C/s after crystallization of pyroxene and before glass solidification in a reduced condition to makesecondary metallic inclusions.
References[1] Villeneuve, J. et al. 2015. Geochim. Cosmochim. Acta 160:277. [2] Nagahara. H. 1981. Nature 292:135. [3] Nagashima. Ket al. 2006. Journal of Crystal Growth 293:193. [4] Dodd. R.T. 1973. Earth and Planetary Science Letters 40:71. [5]Nagahara. H. 1981. Memoirs of National Institute of Polar Research 20:145. [6] Kita. N.T. et al. 2010. Geochim. Cosmochim.
Acta 74:6610.