Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca-Al-rich inclusion of Efremovka CV3 chondrite

Authors

  • Noriyuki KAWASAKI,

    Corresponding author
    1. Department of Natural History Sciences, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, Japan
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  • Naoya SAKAMOTO,

    1. Isotope Imaging Laboratory, Creative Research Institution “Sousei,” Hokkaido University, N21W10, Kita-ku, Sapporo 001-0021, Japan
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  • Hisayoshi YURIMOTO

    1. Department of Natural History Sciences, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, Japan
    2. Isotope Imaging Laboratory, Creative Research Institution “Sousei,” Hokkaido University, N21W10, Kita-ku, Sapporo 001-0021, Japan
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Corresponding author. E-mail: kawasaki@ep.sci.hokudai.ac.jp

Abstract

Abstract– Different oxygen isotopic reservoirs have been recognized in the early solar system. Fluffy type A Ca-Al-rich inclusions (CAIs) are believed to be direct condensates from a solar nebular gas, and therefore, have acquired oxygen from the solar nebula. Oxygen isotopic and chemical compositions of melilite crystals in a type A CAI from Efremovka CV3 chondrite were measured to reveal the temporal variation in oxygen isotopic composition of surrounding nebular gas during CAI formation. The CAI is constructed of two domains, each of which has a core-mantle structure. Reversely zoned melilite crystals were observed in both domains. Melilite crystals in one domain have a homogeneous 16O-poor composition on the carbonaceous chondrite anhydrous mineral (CCAM) line of δ18O = 5–10‰, which suggests that the domain was formed in a 16O-poor oxygen isotope reservoir of the solar nebula. In contrast, melilite crystals in the other domain have continuous variations in oxygen isotopic composition from 16O-rich (δ18O = −40‰) to 16O-poor (δ18O = 0‰) along the CCAM line. The oxygen isotopic composition tends to be more 16O-rich toward the domain rim, which suggests that the domain was formed in a variable oxygen isotope reservoir of the solar nebula. Each domain of the type A CAI has grown in distinct oxygen isotope reservoir of the solar nebula. After the domain formation, domains were accumulated together in the solar nebula to form a type A CAI.

Introduction

Oxygen isotopic compositions of solar system materials are widely distributed with mass-independent isotope fractionation that maintains almost constant 17O/18O ratios (e.g., Clayton et al. 1973; Kobayashi et al. 2003; Sakamoto et al. 2007). Coarse-grained Ca-Al-rich inclusions (CAIs) have heterogeneous distributions of oxygen isotopic compositions along the carbonaceous chondrite anhydrous mineral (CCAM) line of their constituent minerals (Clayton et al. 1977; Clayton 1993; Yurimoto et al. 1994). In coarse-grained CAIs, spinel and fassaite are usually 16O-rich, while melilite and anorthite are 16O-poor (Clayton 1993). The oxygen isotopic distribution among the minerals in coarse-grained CAIs cannot be simply explained by the crystallization sequences of the minerals from either liquid (Stolper 1982) or gas (Wood and Hashimoto 1993).

The oxygen isotopic distributions are interpreted to be the result of multiple heating processes in the solar nebula with different oxygen isotopic environments. (Yurimoto et al. 1998; Ito et al. 2004; Yoshitake et al. 2005; Yurimoto et al. 2008). Both 16O-rich and 16O-poor gaseous reservoirs were considered present in the solar nebula (Yurimoto and Kuramoto 2004) from the oxygen isotopic distributions of CAIs, and the mixing of two gaseous reservoirs could frequently occur in the CAI-forming region (Kim et al. 2002; Itoh and Yurimoto 2003; Fagan et al. 2004a, 2004b;Yoshitake et al. 2005; Aléon et al. 2007; Yurimoto et al. 2008; Simon et al. 2011). Mechanisms of oxygen isotope exchange are considered between gas and melt (e.g., Yurimoto et al. 1998; Aléon et al. 2007), and between gas and solid (e.g., Fagan et al. 2004b; Simon et al. 2011). The possibility that CAIs were transferred radially across and/or through the disk is also suggested (Ciesla 2007, 2009).

Type A CAIs are mainly composed of melilite (Grossman 1975) and subdivided into compact and fluffy CAIs (Grossman 1980). Compact type A CAIs are believed to experience partial or total melting, according to the major and trace element chemistry of the coexisting minerals and experimental petrology (Beckett and Stolper 1994; Simon et al. 1999). In contrast, fluffy type A CAIs are considered to be formed by gas–solid condensation and not extensively molten, because of their irregular shapes and the presence of reversely zoned melilite crystals that have magnesium-rich cores and aluminum-rich mantles (MacPherson and Grossman 1984).

The reverse zoning of melilite is readily explained by direct condensation from a solar nebular gas with decreasing pressure (MacPherson and Grossman 1984) or by crystallization from melt with melt evaporation (Grossman et al. 2002). Therefore, the oxygen isotopic compositions of reversely zoned melilite in fluffy type A CAIs are identical to those of the surrounding solar nebular gas. The reversely zoned melilite crystals of a fluffy type A CAI from Vigarano CV3 chondrite have heterogeneous oxygen isotopic distributions correlated with the chemical compositions (Harazono and Yurimoto 2003; Yurimoto et al. 2008; Katayama et al. 2012), where magnesium-rich melilite was 16O-poor and aluminum-rich melilite was 16O-rich. It is considered that the oxygen isotopic compositions of the solar nebular gas that formed the reversely zoned melilite in type A CAI were changed from 16O-poor to 16O-rich during condensation of the melilite, and each fluffy domain in the CAI may have been formed separately in the nebula and then accumulated together to form the CAI. However, there are very few studies focused on the relationship between oxygen isotopic and chemical compositions of condensed melilite.

In this paper, we study the chemical compositions and oxygen isotopic distributions of melilite in a type A CAI from Efremovka CV3 chondrite to elucidate the formation processes and variations in oxygen isotopic composition of the solar nebular gas of type A CAI-forming area.

Experimental

A polished thin section of Efremovka CV3 chondrite coated with carbon thin film was examined using backscattered electron (BSE) imaging, X-ray elemental mapping, electron back scattered diffraction (EBSD) mapping, and secondary ion mass-spectrometry (SIMS). The BSE images were obtained using a field emission type secondary electron microscope (FE-SEM; JEOL JSM-7000F) of Hokkaido University. X-ray elemental maps were obtained with an energy dispersive X-ray spectrometer (EDS; Oxford INCA) installed with the FE-SEM. A 15 keV electron beam probe with a current of 10 nA was used. Quantitative calculations for the X-ray elemental map were conducted using the Oxford INCA Quantmap software. The error of åkermanite contents of quantitative maps is approximately 3% (1σ), and spatial resolution is approximately 2 μm.

Crystal orientation mapping of melilite was obtained using an EBSD system (HKL Channel 5) equipped with the FE-SEM to determine the grain boundaries of melilite crystals using a 20 keV electron beam probe with a current of 4 nA.

Oxygen isotopic compositions of the melilite crystals were measured using a SIMS instrument (Cameca ims-1270) of Hokkaido University. A Cs+ primary beam (20 keV, 60–110 pA) with a diameter of 3–10 μm was used. Negative secondary ions (16O, 17O, and 18O) were measured. A normal incident electron flood gun was used for charge compensation of sample surface charging during the measurement. 16O was measured using a Faraday cup while 17O and 18O were measured using an electron multiplier with the peak jumping mode of a sector magnet. Each measurement was conducted for 30 cycles of a counting sequence with the background of faraday cup for 1s, 16O for 1s, 17O for 2s, and 18O for 1 s. Synthetic åkermanite with a known oxygen isotopic composition was used as a standard to correct the instrumental mass fractionation. The reproducibility of δ17, 18O was about 2‰ (2σ). Other conditions used for the oxygen isotopic measurements are described elsewhere (Wakaki et al. 2012).

Results

Mineralogy and Petrology

Figure 1 shows a BSE image of a type A CAI from Efremovka CV3 chondrite, named HKE 01. The CAI is 10 × 4 mm and has an irregular texture. The CAI is mainly composed of melilite (62%), spinel (19%), and fassaite (13%), in addition to diopside, anorthite, forsterite, hibonite, and metal grains as accessory minerals. Nepheline is present as a secondary alteration mineral, although the amounts are very small. The CAI is surrounded by mineral layers of gehlenitic melilite, spinel, and diopside, referred to as a Wark-Lovering (W-L) rim (Wark and Lovering 1977). The W-L rim divides the CAI into two domains. Figure 2 shows that the W-L rim, which is composed of spinel and diopside layers, separates the CAI into two domains: domain-1 and domain-2. A matrix is observed between the W-L rims of both domains.

Figure 1.

 Backscattered electron (BSE) image of a type A CAI, HKE 01, from Efremovka CV3 chondrite. White boxes numbered 2a, 3a, 3c, 3e, and 4a indicate the areas shown in Figs. 2a, 3a, 3c, 3e, and 4a, respectively.

Figure 2.

 BSE images of the W-L rim that separates the CAI into domains 1 and 2. a) Region outlined as 2a in Fig. 1. b) Magnified image of the rectangular area indicated in (a). di, diopside; mel, melilite; mtx, matrix; sp, spinel.

The area of domain-1 is dominant in the CAI and it has a core-mantle structure enclosed by a W-L rim. The core consists of many euhedral spinel crystals that are poikilitically enclosed within anorthite, fassaite, and melilite (Figs. 3a and 3b). The size of the spinel crystals is less than several tens of micrometers. The mantle consists of mainly melilite (Figs. 3c–f) with typical grain sizes of several tens of micrometers, and fassaite as the second most abundant phase. Spinel is not abundant in the mantle. The mantle is subdivided into inner and outer mantles according to the melilite composition. The boundary of the inner and outer mantles locates approximately 50 μm in depth from the W-L rim (Fig. 3g). The chemical compositions of the melilite are åkermanite-rich (Åk30–60) in the inner mantle and åkermanite-poor (Åk15–30) in the outer mantle. Each åkermanite-poor crystal of the outer mantle exhibits reverse zoning in the composition (Fig. 3h). The reverse zoning is typically Åk30 in the center of the crystal and Åk15 at the grain boundary. Melilite with Åk10 rarely appears at the grain boundary.

Figure 3.

 Representative textures of domain-1. a) BSE image of the core part indicated as 3a in Fig. 1. b) Combined X-ray elemental map of the area in (a) with Mg (red), Ca (green), and Al (blue). c) BSE image of the mantle part indicated as 3c in Fig. 1. d) Combined X-ray elemental map of the area in (c) showing Mg (red), Ca (green), and Al (blue). e) BSE image of the mantle part indicated as 3e in Fig. 1. f) Combined X-ray elemental map of the area in (e) showing Mg (red), Ca (green), and Al (blue). g) X-ray elemental map corresponding to the åkermanite contents of melilite in the region shown in (e). h) Magnified image of the area indicated as (h) in (g) showing the grain boundaries of melilite crystals. i) Magnified image of the area indicated as (i) in (g) showing the grain boundaries of melilite crystals. Dashed lines indicate the grain boundaries. j) EBSD map of the area in (h). k) EBSD map of the area in (i). an, anorthite; fas, fassaite; mel, melilite; sp, spinel.

Melilite crystals of the inner mantle exhibit complex zoning, alternated as normal zoning, reverse zoning, and then changing to normal zoning again (Fig. 3i), although some melilite crystals exhibit zoning patterns that start as reverse zoning and change to normal zoning. The åkermanite content of the melilite crystals typically changed from Åk30 to Åk35, Åk35 to Åk30, and Åk30 to Åk40 from the grain center to the grain boundary. Highly åkermanite-rich (Åk40–60) melilite was partially observed at the inner mantle. The typical width of the normal zoning part at the grain boundary was 2 μm. The typical grain size of fassaite was several tens to hundreds of micrometers. Fassaite is also present interstitially among åkermanite-rich melilite crystals.

Domain-2 has a core-mantle and W-L rim structure, although the texture of the core-mantle is different from that observed in domain-1 (Figs. 4a and 4b). In the core part, spinel is enclosed by anorthite, melilite, and fassaite, but in the core of domain-2, there are lower amounts of spinel than in the core of domain-1. The mantle part is mainly composed of melilite and contains fassaite, spinel, and perovskite (Figs. 4c and 4d). The chemical composition of the melilite crystals are Åk3–35 (Fig. 4e), which is a narrower range than that for the melilite present in domain-1 (Åk10–60). The melilite crystals locate within approximately 150 μm from the W-L rim exhibit reverse zoning (Figs. 4f and 4g), which is typically Åk15 in the center and Åk5 at the grain boundary.

Figure 4.

 a) BSE images of domain-2 indicated as 4a in Fig. 1. b) Combined X-ray elemental map of the area in (a) showing Mg (red), Ca (green), and Al (blue). c) Representative BSE image of the mantle part area indicated as (c) in (a). d) Combined X-ray elemental map of the area in (c) showing Mg (red), Ca (green), and Al (blue). e) X-ray elemental map corresponding to the åkermanite contents of melilite in the region shown in (c). f) Magnified image of the area indicated as (f) in (e) showing the grain boundaries of melilite crystals. g) Magnified image of the area indicated as (g) in (e) showing the grain boundaries of melilite crystals. Dashed lines indicate the grain boundaries. h) EBSD map of the area in (f). i) EBSD map of the area in (g). an, anorthite; fas, fassaite; mel, melilite; pv, perovskite; sp, spinel.

Oxygen Isotopic Compositions of Melilite Crystals

The oxygen isotopic compositions of melilite crystals in domain-1 and domain-2 were measured (Table 1). Figure 5 shows the oxygen isotopic compositions plotted on a three oxygen isotope diagram. The spatial distribution of oxygen isotopic compositions in the CAI is shown in Fig. 6.

Table 1. Oxygen isotopic compositions and åkermanite contents (Åk) of melilite crystals. Each entry number corresponds to the spot numbers in Fig. 6.
Spotδ17Oδ18OΔ17OÅkSpotδ17Oδ18OΔ17OÅk
Domain-1
10.32.18.91.9−4.32.11820−17.72.0−15.31.6−9.71.712
2−0.61.54.31.2−2.81.63121−23.81.8−19.31.5−13.82.08
3−0.61.97.81.8−4.71.92122−16.11.8−12.01.4−9.91.711
4−0.61.75.91.2−3.71.85023−25.81.8−21.41.4−14.61.610
52.91.76.71.1−0.61.71724−20.52.0−18.71.5−10.72.09
60.71.96.01.0−2.41.82725−24.32.0−23.51.8−12.11.96
70.81.67.31.3−3.01.82426−9.11.6−7.31.5−5.31.513
8−1.41.79.01.2−6.11.82027−29.91.6−31.21.0−13.71.59
9−1.61.54.81.3−4.11.73728−16.12.0−15.31.5−8.12.011
101.71.96.81.2−1.92.01729−9.22.0−5.51.6−6.32.012
110.71.75.71.2−2.21.61830−17.32.5−11.61.6−11.32.410
12−0.11.84.11.4−2.31.74331−27.72.0−28.91.6−12.62.27
13−2.32.04.31.3−4.52.03632−32.42.0−31.31.2−16.11.98
14−1.51.73.01.2−3.11.73533−28.62.0−27.21.2−14.51.86
152.92.110.21.5−2.32.43234−19.92.2−13.81.3−12.62.119
161.32.46.91.5−2.32.53535−10.41.7−3.31.6−8.61.715
173.62.48.71.5−1.02.43336−10.02.3−6.41.4−6.72.29
181.62.49.01.8−3.02.85437−22.91.9−16.51.2−14.31.811
193.02.68.32.0−1.32.63338−23.92.2−21.01.3−12.92.011
201.92.46.11.9−1.32.71839−18.72.3−13.11.5−11.92.015
211.42.08.12.0−2.82.02240−7.92.2−4.51.6−5.61.914
        41−15.62.2−11.01.2−9.91.911
Domain-242−31.02.1−28.61.7−16.11.610
1−29.82.0−29.01.7−14.82.0943−17.32.0−14.61.3−9.72.210
2−34.32.0−32.71.4−17.22.0944−19.52.2−13.11.5−12.62.511
3−40.32.3−40.51.5−19.22.2745−10.22.0−10.31.3−4.92.08
4−32.41.8−31.71.9−15.91.71346−17.72.1−16.41.4−9.22.27
5−33.02.1−33.11.4−15.81.91347−9.42.0−5.21.5−6.82.46
6−29.71.9−30.41.6−13.91.7948−12.21.9−7.11.4−8.52.113
7−33.62.0−33.11.1−16.41.81149−13.31.8−11.91.5−7.02.09
8−39.61.8−36.31.4−20.81.7650−14.91.7−12.91.5−8.22.011
9−9.92.1−5.01.5−7.31.81251−16.91.6−14.31.5−9.52.19
10−6.62.30.11.5−6.62.11652−23.52.1−22.31.1−11.92.310
11−4.51.8−1.21.4−3.91.61253−27.41.7−25.01.3−14.41.99
12−12.41.8−5.81.4−9.41.61554−25.12.0−21.91.4−13.72.411
13−10.61.9−7.21.6−6.91.82155−27.21.9−26.01.5−13.72.09
14−10.32.1−6.71.7−6.82.01656−30.52.3−27.41.3−16.32.57
15−11.21.8−5.41.3−8.41.72257−13.51.8−8.21.5−9.22.117
16−11.12.5−6.21.2−7.92.31258−6.81.9−0.21.2−6.72.018
17−11.21.9−5.51.4−8.31.91259−5.11.8−0.21.5−5.02.018
18−16.61.6−13.31.6−9.61.31260−19.02.3−17.21.4−10.02.58
19−11.72.4−8.41.5−7.32.41361−23.02.3−22.31.4−11.42.37
Figure 5.

 Oxygen isotopic compositions of the melilite crystals in domain-1 and domain-2. Error bars are 2σ.

Figure 6.

 Measurement positions and oxygen isotopic compositions plotted on BSE images of the regions shown in Figs. 3e and 4c. The spot numbers of each plot correspond to the entries listed in Table 1. The symbol colors correspond to the oxygen isotopic composition scale.

The oxygen isotopic compositions of melilite crystals in domain-1 are distributed homogeneously at δ18O = 5–10‰, despite the different chemical compositions and complex growth zoning patterns observed from the CAI interior to the W-L rim (Fig. 3g). In contrast, the oxygen isotopic compositions of the melilite crystals in domain-2 are distributed along the CCAM line ranging between δ18O = −40 and 0‰ (Fig. 5). The melilite crystals positioned near the W-L rim are 16O-rich (δ18O = −40‰), while the melilite becomes 16O-poor (δ18O = 0‰) with increasing distance from the W-L rim (Fig. 6).

The relationship between the oxygen isotopic compositions and åkermanite contents of the melilite crystals in domain-1 and domain-2 is shown in Fig. 7. The oxygen isotopic composition of melilite in domain-1 is distributed homogeneously, despite the wide range of chemical composition from Åk17 to Åk54. In contrast, the oxygen isotopic compositions of melilite crystals in domain-2 are heterogeneously distributed between Δ17O = −21 and −4‰ within a relatively smaller range of the åkermanite content (Åk6 to Åk22) and åkermanite poor compositions than those of domain-1. The oxygen isotopic composition of domain-2 tends to become 16O-rich with decreasing åkermanite content.

Figure 7.

 Relationship between the oxygen isotopic compositions and chemical compositions of melilite crystals in domain-1 and domain-2. Error bars are 2σ. The chemical compositions of melilite crystals are obtained from X-ray quantitative maps.

Discussion

Oxygen Isotopic Compositions and Chemical Zoning of Melilite Crystals in Domain-1

The melilite crystals in domain-1 have complex chemical growth zoning patterns (Fig. 3). The melilite crystals positioned shallower than approximately 50 μm from the W-L rim (outer mantle) exhibit reverse zoning (Fig. 3h). Reversely zoned melilite crystals found in fluffy type A CAIs are explained by condensation from a solar nebular gas during a period of decreasing pressure (MacPherson and Grossman 1984) or by crystallization from incomplete melting with melt evaporation (Grossman et al. 2002). Significant mass dependent isotope fractionation is predicted for the melt evaporation origin for the reverse zoning. However, mass dependent fractionation of oxygen isotopes is not clear (Fig. 5) and it supports a condensation origin for the reverse zoning.

On the other hand, the åkermanite-rich melilite crystals in the inner mantle show oscillatory zoning, where the crystals are initially grown with normal zoning, change to reverse zoning, and then change to normal zoning (Fig. 3i). The oscillatory zoned melilite crystals could be formed by direct condensation in a solar nebular gas with decreasing temperature under variable pressure change. In this case, the formation processes could be described using the melilite condensation diagram by Yoneda and Grossman (1995) (Fig. 8) as follows: (1) Melilite is formed by condensation from a gas with decreasing temperature under a constant or decreasing pressure to form normal zoning. (2) The pressure is decreased enough to form reverse zoning. (3) The temperature decrease continues to form normal zoning. After condensation of individual melilite crystals with oscillatory zoning, the melilite crystals accumulate to form the inner mantle.

Figure 8.

 Relationship between the melilite composition, condensation temperature, and pressure in domain-1 and domain-2. The equilibrium condensation diagram is from Yoneda and Grossman (1995). The eutectic melting temperature of melilite and fassaite is from Stolper (1982) and Onuma and Moridaira (1991) and is indicated by the gray area.

Åkermanite-poor reversely zoned melilite crystals are observed in the outer mantle of domain-1. Therefore, if the outer mantle melilite grew following the condensation of oscillatory zoned melilite crystals in the inner mantle, then the inner mantle melilite should form åkermanite-poor reverse zoning at the outermost edge. Furthermore, if the outer zoned melilite crystals condense before the end of the oscillatory zoned melilite crystal accumulation, then the reversely zoned melilite crystals should be incorporated among the oscillatory zoned melilite crystals. The åkermanite-poor reversely zoned melilite crystals observed in the outer mantle are not observed in the inner mantle; therefore, the melilite crystals in the outer mantle must be formed after the end of the inner mantle formation and accumulation onto the inner mantle.

In the mantle, melilite and fassaite coexist (Figs. 3c–f). The mantle melilite was never largely molten because the inner mantle melilite preserves the oscillatory growth zoning pattern and the outer mantle melilite preserves the reverse zoning pattern. Therefore, the mantle melilite was condensed below the eutectic melting temperature of melilite and fassaite. The eutectic temperature has been experimentally determined as 1520 K (Stolper 1982) or 1568 K (Onuma and Moridaira 1991); therefore, the melilite crystals in domain-1 were condensed below 1520–1568 K and at pressures lower than 10−2 atm (Fig. 8).

The chemical zoning of the inner mantle melilite ranges from Åk30 to Åk60 (typically from Åk30 to Åk40). Therefore, the condensation conditions for the formation of the inner mantle melilite are within the green area shown in Fig. 8. On the other hand, the reverse zoning of melilite crystals in the outer mantle was typically Åk30 in the center and Åk15 at the grain boundary, and thus, condensation would occur under the conditions indicated by the red area shown in Fig. 8. If the condensation temperature was constant or decreased during crystallization of the inner and outer mantle melilite, then the condensation pressure was lower during outer mantle melilite crystallization than that during inner mantle melilite crystallization.

The melilite crystals in domain-1 inherited the oxygen isotopic compositions from the solar nebular gas according to the condensation origin. Therefore, the homogeneous 16O-poor composition (δ18O = 5–10‰) of melilite in domain-1 (Fig. 5) indicates that the oxygen isotopic composition of the nebular gas was maintained at a 16O-poor state during the formation of melilite in the inner mantle and outer mantle.

Oxygen Isotopic Compositions and Chemical Zoning of Melilite Crystals in Domain-2

The melilite crystals positioned shallower than approximately 150 μm from the W-L rim in domain-2 exhibit reverse zoning (Fig. 4). These reversely zoned melilite crystals could have crystallized by direct condensation from solar nebular gas with decreasing pressure and then accumulate onto the CAI, similar to the process discussed in the previous section.

The melilite in domain-2 was condensed below the eutectic temperature of melilite and fassaite for the same reason as that discussed for domain-1 (i.e., they are zoned). These crystals were condensed under the conditions indicated by the blue area shown in Fig. 8. The reversely zoned melilite in domain-2 could condense under a pressure 1.5–2 times lower than the condensation pressure of the outer mantle melilite of domain-1 if the condensation temperature was the same. For example, if the temperature was 1400 K, then the pressure was changed from 3.2 × 10−4 to 2.2 × 10−4 atm for the formation of the outer mantle of domain-1, and from 2.2 × 10−4 to 9.7 × 10−5 atm for that of domain-2.

The oxygen isotopic compositions of the melilite crystals in domain-2 are heterogeneously distributed along the CCAM line ranging between δ18O = −40 and 0‰ (Fig. 5). The oxygen isotopic compositions of the reversely zoned melilite crystals tend to change from 16O-poor (δ18O = 0‰) at the CAI interior to 16O-rich (δ18O = −40‰) near the W-L rim (Fig. 6). The width of the heterogeneous oxygen isotope layer is approximately 100 μm of the mantle. The distinct oxygen isotopic compositions between melilite of both domains show that they were formed at distinct nebular environments.

The oxygen isotopic variation in melilite from the CAI interior to the W-L rim has been observed in a compact type A CAI (Simon et al. 2011), which was interpreted as high-temperature gas–solid diffusion in the solar nebula after the CAI formation. In their study, the oxygen isotopic compositions of melilite changed within a range of about 75 μm from 16O-rich at the CAI interior to 16O-poor near the W-L rim, which is opposite to the trend determined in this study.

The oxygen isotopic variation from 16O-poor at the CAI interior to 16O-rich near the W-L rim determined in this study is not monotonic as described by Simon et al. (2011). 16O-poor and 16O-rich melilites are often mixed in the CAI interior in this study. For example, spots 30 and 34 in Fig. 6b are more 16O-rich than spots 28, 36, and 46. Therefore, the oxygen isotopic variation in domain-2 cannot simply be discussed according to gas–solid diffusion with 16O-rich gas of the solar nebula after CAI solidification. This oxygen variation pattern also cannot simply be achieved by parent body processes because oxygen isotopic compositions around the CAI are 16O-rich. The oxygen isotopic variation observed in domain-2 is described as reversely zoned melilite crystals condensed from gas where the oxygen isotopic composition gradually changed toward 16O-rich during formation of the mantle layer.

Domain-2 appears to have formed by the accumulation of single melilite crystals that condensed separately. Therefore, the melilite crystals at the CAI interior are older than those near the W-L rim. The general oxygen isotopic variation in the melilite crystals from the CAI interior to the W-L rim indicates that the oxygen isotopic composition of the solar nebula gas from which the melilite crystals were condensed was changed from 16O-poor to 16O-rich. Some irregularity of the general oxygen isotopic variation observed may be due to the fluffy nature of the CAI caused by the accumulation process. The chemical and oxygen isotopic distribution of the melilite indicates that the oxygen isotopic composition of the nebular gas around the CAI was temporally changed from 16O-poor to 16O-rich with decreasing pressure, or the CAI was transported toward 16O-richer and lower-pressure regions of the solar nebular.

Formation Sequence of HKE 01

The reversely zoned melilite crystals in the outer mantle of domain-1 and mantle of domain-2 condensed from solar nebular gas with decreasing pressure and accreted together to form the separate CAI domain. However, the melilite crystals in domain-1 and domain-2 were formed independently from different oxygen isotopic and chemical compositions (Fig. 9). Melilite of domain-1 was condensed in an 16O-poor nebular environment and accumulated on the core. The environment changed, first with decreasing temperature and then with decreasing pressure. These changes may correspond to the heliocentric and vertical motion of the CAI.

Figure 9.

 Schematic scenario of melilite formation in domain-1 and domain-2. The inner mantle melilite of domain-1 condensed from 16O-poor gas with decreasing temperature and accreted onto the core. The reversely zoned melilite then condensed from the gas with decreasing pressure and accreted on the inner mantle. The oxygen isotopic compositions of the gas was always 16O-poor. The reversely zoned melilite of domain-2 condensed from 16O-poor gas with decreasing pressure. The environment where domain-2 was formed is different from that where domain-1 was formed, because the oxygen isotopic composition of the nebular gas surrounding domain-2 was varied from 16O-poor to 16O-rich. Domain-1 and domain-2 were aggregated together to form the entire CAI, HKE 01.

On the other hand, the melilite of domain-2 was condensed on the core in a nebular environment with variable oxygen isotopic composition. The environment changed from 16O-poor to 16O-rich with decreasing pressure. The environmental changes resemble those for the formation of a fluffy type A CAI from Vigarano chondrite (Katayama et al. 2012). This situation may correspond to the inner edge region of the solar nebula where 16O-rich solar and 16O-poor planetary gases are encountered and mixed (Yurimoto et al. 2008). The environmental changes around domain-2 occurred by the fluctuation of the inner edge of the solar nebula (Yurimoto et al. 2008) or the radial transportation of grains (Ciesla 2007, 2009).

After the formation of domain-1 and -2, the domains aggregated to form HKE 01. HKE 01 is a compound object like several CAIs previously described (e.g., Kim et al. 2002; Aléon et al. 2005). The W-L rims surrounding the domains would be formed in the environment where the domains encountered each other.

Conclusions

We have measured the oxygen isotopic and chemical compositions of melilite crystals in the type A CAI, HKE 01, from Efremovka CV3 chondrite. The W-L rim divides the CAI into two domains, denoted domain-1 and domain-2. Both domains show a core-mantle structure and are surrounded by the Wark-Lovering rims. The mantle parts of each domain were mainly composed of melilite.

The mantle part of domain-1 was subdivided into inner and outer mantles according to the melilite composition. The chemical compositions of the melilite are åkermanite-rich (typically Åk30–40) in the inner mantle and åkermanite-poor (typically Åk15–30) in the outer mantle. Each åkermanite-poor crystal of the outer mantle exhibits reverse zoning in the composition. On the other hand, the melilite crystals of the inner mantle exhibit complex zoning, initially started as normal zoning that changes to reverse zoning and then changes to normal zoning. Melilite crystals in the mantle part of domain-2 exhibit reverse zoning. The chemical compositions of these crystals typically ranged from Åk5–15.

The oxygen isotopic compositions of the melilite crystals in domain-1 are homogeneously distributed at δ18O = 5–10‰ on the CCAM line, despite the differences in the chemical compositions and complex growth zoning patterns. In contrast, the oxygen isotopic compositions of the melilite crystals in domain-2 are distributed along the CCAM line ranging between δ18O = −40 and 0‰. The melilite crystals positioned near the W-L rim are 16O-rich (δ18O = −40‰) and the melilite crystals become 16O-poor (δ18O = 0‰) with increasing distance from the W-L rim.

The reversely zoned melilite crystals in the outer mantle of domain-1 and domain-2 could be crystallized by direct condensation from solar nebular gas during a period of decreasing pressure. However, the reversely zoned melilite crystals in each domain were formed independently, because they have different oxygen isotopic and chemical compositions. The oscillatory zoned melilite crystals in the inner mantle of domain-1 could be formed by direct condensation from the solar nebular gas with decreasing temperature under variable pressure change.

The melilite crystals in the mantle part of each domain inherited the oxygen isotopic compositions from the solar nebular gas because of the condensation origin. The homogeneous 16O-poor composition of melilite in domain-1 indicates that the oxygen isotopic composition of the nebular gas was maintained at the 16O-poor condition during the formation of melilite in the inner and outer mantles. The environment was first changed with decreasing temperature and then with decreasing pressure, while the domain was formed in an oxygen isotopic reservoir of the solar nebula. The changes in temperature and pressure may correspond to the heliocentric and vertical motion of the CAI.

On the other hand, the oxygen isotopic variation in the reversely zoned melilite crystals in domain-2 indicates that the domain was formed in a variable oxygen isotopic reservoir of the solar nebula, and the oxygen isotopic composition of the region of solar nebular gas from which the melilite condensed was changed from 16O-poor to 16O-rich. The environment changed from 16O-poor to 16O-rich with decreasing pressure. This situation may correspond to the inner edge region of the solar nebular where 16O-rich solar and 16O-poor planetary gases are encountered and mixed (Yurimoto et al. 2008) or radial transport of the grains (Ciesla 2007, 2009). After the formation of each domain, the domains aggregated to form HKE 01.

Acknowledgments— We thank Shoichi Itoh for helpful discussions. Extensive reviews by Justine Simon and an anonymous reviewer significantly improved the quality of the article. We are also grateful to associate editor Alexander Krot for his constructive comments and suggestions. This study was supported by the Monka-sho grant.

Editorial Handling— Dr. Alexander Krot

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