Heavily metamorphosed clasts from the CV chondrite breccias Mokoia and Yamato-86009

Authors

  • Kaori JOGO,

    Corresponding author
    1. School of Ocean, Earth Science and Technology, Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822, USA
    2. NASA Astrobiology Institute, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822, USA
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  • Kazuhide NAGASHIMA,

    1. School of Ocean, Earth Science and Technology, Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822, USA
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  • Ian D. HUTCHEON,

    1. Glenn Seaborg Institute, Lawrence Livermore National Laboratory, Livermore, California 94551, USA
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  • Alexander N. KROT,

    1. School of Ocean, Earth Science and Technology, Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822, USA
    2. NASA Astrobiology Institute, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822, USA
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  • Tomoki NAKAMURA

    1. Department of Earth and Planetary Materials Science, Faculty of Science, Laboratory for Early Solar System Evolution, Tohoku University, Sendai, Miyagi 980-8578, Japan
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* Corresponding author. E-mail: kaori@higp.hawaii.edu

Abstract

Abstract–  Metamorphosed clasts in the CV carbonaceous chondrite breccias Mokoia and Yamato-86009 (Y-86009) are coarse-grained, granular, polymineralic rocks composed of Ca-bearing (up to 0.6 wt% CaO) ferroan olivine (Fa34–39), ferroan Al-diopside (Fs9–13Wo47–50, approximately 2–7 wt% Al2O3), plagioclase (An37–84Ab63–17), Cr-spinel (Cr/(Cr + Al) =  0.19–0.45, Fe/(Fe + Mg) = 0.60–0.79), nepheline, pyrrhotite, pentlandite, Ca-phosphate, and rare grains of Ni-rich taenite; low-Ca pyroxene is absent. Most clasts have triple junctions between silicate grains, indicative of prolonged thermal annealing. Based on the olivine-spinel and pyroxene thermometry, the estimated metamorphic temperature recorded by the clasts is approximately 1100 K. Few clasts experienced thermal metamorphism to a lower degree and preserved chondrule-like textures. The Mokoia and Y-86009 clasts are mineralogically unique and different from metamorphosed chondrites of known groups (H, L, LL, R, EH, EL, CO, CK) and primitive achondrites (acapulcoites, brachinites, lodranites). On a three-isotope oxygen diagram, compositions of olivine in the clasts plot along carbonaceous chondrite anhydrous mineral line and the Allende mass-fractionation line, and overlap with those of the CV chondrule olivines; the Δ17O values of the clasts range from about −4.3‰ to −3.0‰. We suggest that the clasts represent fragments of the CV-like material that experienced metasomatic alteration, high-temperature metamorphism, and possibly melting in the interior of the CV parent asteroid. The lack of low-Ca pyroxene in the clasts could be due to its replacement by ferroan olivine during iron-alkali metasomatic alteration or by high-Ca ferroan pyroxene during melting under oxidizing conditions.

Introduction

CV chondrites are a highly diverse group of meteorites. Neither the size nor the thermal history of the CV chondrite parent asteroid is well known. Based on the mineralogy, petrography, bulk chemical and oxygen-isotope compositions, CV chondrites are subdivided into the oxidized Allende-like (CVOxA), oxidized Bali-like (CVOxB), and reduced (CVRed) subgroups (McSween 1977; Weisberg et al. 1997). These subgroups experienced different styles and degrees of aqueous/metasomatic alteration and thermal metamorphism. The CVOxB chondrites experienced aqueous alteration that resulted in formation of secondary phyllosilicates, magnetite, Fe,Ni-sulfides, Fe,Ni-carbides, fayalite, salite-hedenbergite pyroxenes, and andradite (Krot et al. 2004). In contrast, the CVOxA chondrites recorded intense Fe-alkali-halogen metasomatic alteration resulting in the formation of secondary nepheline, sodalite, andradite, grossular, wollastonite, kirschsteinite, and salite-hedenbergite pyroxenes (Kimura and Ikeda 1995; Ikeda and Kimura 1995; Krot et al. 1995, 1998ab, 2004). The reduced CVs experienced metasomatic alteration similar to that of the CVOxA chondrites, but to a smaller degree. The CV subgroups may represent different lithologies of a single CV asteroid (e.g., Krot et al. 1995, 1998a; Bonal et al. 2006).

Based on the unidirectional paleomagnetic record of the Allende meteorite, it has been hypothesized that the CV parent asteroid experienced melting, igneous differentiation, and core formation (Weiss et al. 2009, 2010; Carporzen et al. 2010; Elkins-Tanton et al. 2011). However, no CV chondrites heated above 850 K have been reported yet (Bonal et al. 2006; Brearley 1997, 1999).

On the basis of observed similarities in the mineralogy, bulk chemical compositions and oxygen-isotope compositions of the CV and CK chondrites, Greenwood et al. (2010) and Isa et al. (2012) argue that both chondrite groups originated on the same parent asteroid and that CK4–6 chondrites may represent strongly metamorphosed CV chondrites. In addition, Irving et al. (2004) and Schoenbeck et al. (2006) described two North West Africa (NWA) meteorites, NWA 3133 and NWA 1839, which show a metamorphic texture with triple junctions and consist of olivine, pyroxene, plagioclase, diopside, chromite, merrilite, troilite, and Fe,Ni-metal. Because these meteorites have the similar bulk chemical and oxygen-isotope compositions and 182Hf-182W age to those of CV chondrites, these authors concluded that NWA 3133 and NWA 1839 could have been derived from the interior of the CV chondrite parent asteroid and named them CV metachondrites.

Here, we report on the mineralogy, petrography, aluminum–magnesium and oxygen-isotope systematics of metamorphosed clasts in the CV chondrite breccias Mokoia and Yamato-86009 (Y-86009). These clasts, originally described in Mokoia by Cohen et al. (1983), may be genetically related to CV chondrites and provide important constraints on the thermal history of the CV asteroid. The preliminary results of this study were reported by Krot and Hutcheon (1997), Jogo and Krot (2010), and Jogo et al. (2011).

Samples and Methods

Mineralogy and Petrography

We studied seven polished thin sections of Mokoia (Mokoia-0, -1, -2, -3, and -25 from the Johnson Space Center, -c1 from the Smithsonian Observatory collection, and -UH from the University of Hawai’i (UH), and -USNM2166 from the Smithsonian Institution) and one polished thick section of Y-86009 from National Institute of Polar Research, Japan, using optical microscopy, scanning electron microscopy (SEM), and field emission electron microprobe analyzer (FE-EPMA). The sections were studied in secondary and backscattered electrons using the Kyushu University (KU) JEOL-5800LV and the University of Hawai’i (UH) JEOL-5900LV SEMs equipped with energy dispersive X-ray spectrometer. Chemical compositions of silicates and opaque minerals in clasts were measured using the KU JEOL JXA-8530F and the UH JEOL JXA-8500F FE-EPMA. Quantitative chemical analyses were performed at 15 keV accelerating voltage and 10–20 nA beam current with spot beam. Bulk chemical compositions of the clasts were measured by broad-beam (30–100 μm in diameter) electron microprobe analysis; bulk compositional data were corrected using the broad-beam correction factors proposed by Ikeda (1980). Matrix effects were corrected using ZAF correction procedure.

Oxygen Isotopic Measurements

Oxygen-isotope compositions of olivine in the Mokoia and Y-86009 clasts were measured in situ with the UH Cameca ims-1280 ion microprobe using analytical procedure described in detail by Makide et al. (2009). An approximately 2 nA primary Cs+ ion beam focused to approximately 10 μm was first rastered over 25 × 25 μm2 area for 250 s of presputtering to remove the carbon coating. Then, the raster size was reduced to 10 × 10 μm2 for automated centering of secondary beam to the mass spectrometer, followed by data collection. An energy window of 50 eV was used. A normal incident electron flood gun was used for charge compensation. Secondary ions of 16O, 17O, and 18O were measured simultaneously in multicollection mode with the magnetic field controlled by a nuclear magnetic resonance (NMR) probe. 16O and 18O were measured by multicollector Faraday cups (FCs) with low mass resolving power (MRP approximately 2000), while 17O was measured using the axial monocollector electron multiplier (EM) in pulse counting mode with MRP of approximately 5600, sufficient to separate the interfering 16OH signal. As olivines in metamorphosed clasts show compositional variations, we investigated the instrumental mass-fractionation effects (IMF) depending on olivine compositions. Our measurements on San Carlos olivine (Fa∼11), NWA 4872 olivine (Fa∼35), and LEW 86010 olivine (Fa∼63) showed no systematic differences in IMF (Fig. 1). Therefore, San Carlos olivine standard was used for the IMF corrections of all measurements. Errors represent 2σ analytical uncertainty including both the internal measurement precision and the external reproducibility for San Carlos olivine measurements during a given analytical session. Ogliore et al. (2011) reported that the mean of isotopic ratios calculated from isotopic ratios of individual measurement cycles introduces an additive positive bias to the results when measurement count rates are low. A method of reducing the data with much less bias is to total the counts for each of the isotopes over all cycles and then calculate the ratios. We used this method for our data reduction.

Figure 1.

 Instrumental mass-fractionation (IMF) effects for olivine standards of different compositions (San Carlos, NWA 4872 brachinite, and LEW 86010 angrite) measured during three analytical sessions. The IMF is defined as follows: IMF = δ18Omeasured–δ18Otrue. Errors are 2σ.

Results

Mineralogy and Petrography

Heavily Metamorphosed Clasts

Twenty-nine heavily metamorphosed clasts in Mokoia and three clasts in Y-86009 were found in the sections studied. The clasts are fragmented and embedded in the matrix of Mokoia and Y-86009 breccias. The representative clasts are shown in Fig. 2; their mineralogy and mineral chemistry are summarized in Tables 1 and 2, respectively. The clasts are coarse-grained, granular, polymineralic rocks with sizes ranging from 30 to 250 μm. The clasts are composed of Ca-bearing ferroan olivine (Fa34–39, up to 0.6 wt% CaO), ferroan Al-diopside (Fs9–13Wo47–50; approximately 2–7 wt% Al2O3), plagioclase (An37–84Ab63–17), Cr-spinel (Cr/(Cr + Al) = 0.19–0.45, Fe/(Fe + Mg) = 0.60–0.79), nepheline, pyrrhotite, pentlandite, Ca-phosphate, and rare grains of Ni-rich taenite; low-Ca pyroxene is absent. Olivines has low nickel contents (<0.1 wt% NiO; Table 2). Ferroan Al-diopside contains 1–2 wt% TiO2, 0.6–0.8 wt% Cr2O3, and approximately 0.6 wt% Na2O (Table 2). Spinel contains 1.1–2.5 wt% TiO2 (Table 2).

Figure 2.

 Backscattered electron (BSE) images of heavily metamorphosed clasts MUSNM2166#1 (a, b), M25#4 (c, d), and Mc1#3 (e, f) embedded in matrices of the CV chondrite breccias Mokoia (a–d) and Y-86009 (e–f). Regions outlined in “a,”“c,” and “e” are shown in detail in “b,”“d,” and “f,” respectively. The clasts are fragmented and have equilibrated textures with triple junctions as indicated by arrows in “a” and “d.” met = Fe,Ni-metal; nph = nepheline; ol = olivine; pl = plagioclase; pn = pentlandite; px = pyroxene; py = pyrrhotite; sp = spinel.

Table 1. Mineralogy of metamorphosed clasts with measured oxygen-isotope compositions from Mokoia and Y-86009.
SectionClast#Mineralogy and chemical compositions
olFaavr ± 2σpxFs/Woavr ± 2σplOthers
  1. met = metal; nph = nepheline; ol = olivine; pl = plagioclase; px = pyroxene; sf = sulfide; sp = spinel; n.a. = not analyzed due to small grain sizes.

Heavily metamorphosed clasts
Mokoia–25 M25#4 Fa35–3635.6 ± 0.6Fs11Wo49–5010.9 ± 0.4/49.0 ± 0.8An59–72Ab28–41sp, sf, met, nph
Mokoia–25 M25#2 Fa36–3736.6 ± 1.2Fs12Wo49–5011.9 ± 0.4/49.4 ± 1.8Absentsp, sf, nph
Y-86009–UH Y86UH#1 Fa37–3837.1 ± 0.6AbsentAn74Ab26sp, sf
Y-86009–UH Y86UH#2 Fa3736.7 ± 0.2AbsentAn59–64Ab32–41sp, sf, met
Y-86009–UH Y86UH#3 Fa36–3736.4 ± 0.4AbsentAn67–68Ab32–33sp, sf, met
Less metamorphosed clasts
Mokoia–3 M3#4 Fa3736.9 ± 0.4Fs10Wo50–519.8 ± 0.6/50.7 ± 1.8An54–64Ab36–45sp, sf
Mokoia–25 M25#8 (core)Fa25-3027.8 ± 4.4n.a.n.a.sp, sf
Mokoia–25 M25#8 (mantle)Fa38–3938.1 ± 1.0Fs10–12Wo49–5110.6 ± 0.6/50.4 ± 2.8An70–72Ab28–30sp, sf
Table 2. Representative electron microprobe analyses (in wt%) of the major minerals in metamorphosed clasts with measured oxygen-isotope compositions from the CV chondrite breccias Mokoia and Y-86009.
Clast#MineralSiO2TiO2Al2O3Cr2O3FeOMnOMgOCaONa2OK2ONiOTotalol-spcpx
Heavily metamorphosed clasts
M25#4 cpx51.70.963.40.816.60.0313.923.10.53<0.01<0.05101.0 1062
                
 ol36.6<0.02<0.01<0.0232.70.1932.50.29<0.01<0.010.07102.3  
 sp0.091.930.926.330.80.356.2<0.01<0.01<0.01<0.0596.7929 
  pl53.40.0829.1<0.020.56<0.030.0212.04.6<0.01<0.0599.8  
M25#2 cpx50.91.24.50.817.20.1213.523.20.520.020.07102.1 1027
  ol36.20.060.020.0733.70.2232.10.50<0.010.030.06102.9  
  sp0.092.529.824.333.00.206.4<0.01<0.01<0.01<0.0596.2999 
Y86UH#1 ol36.6<0.02<0.010.0931.90.2530.20.35<0.01<0.01<0.0599.4  
  sp0.332.931.225.332.50.215.50.01<0.01<0.01<0.0597.9864 
  pl49.0<0.0233.50.031.30.030.214.62.9<0.01<0.05101.5  
Y86UH#2 ol36.4<0.020.080.0731.50.2230.50.310.010.01<0.0599.1  
  sp0.342.629.627.831.40.175.60.040.02<0.01<0.0597.5879 
  pl51.0<0.0231.60.021.0<0.030.112.84.0<0.01<0.05100.5  
Y86UH#3 ol36.2<0.020.020.0631.50.2430.70.250.01<0.01<0.0598.9  
 sp0.262.130.527.030.30.235.90.03<0.01<0.01<0.0596.3894 
 pl50.0<0.0231.8<0.021.1<0.030.1613.63.5<0.01<0.05100.2  
Less metamorphosed clasts
M3#4 cpx49.81.24.60.805.70.0613.123.50.59<0.01<0.0599.4  
  ol36.6<0.020.02<0.0232.70.1831.50.29<0.01<0.010.08101.4  
  sp0.751.137.221.128.30.148.00.650.05<0.010.1497.3  
  pl49.90.0630.00.050.77<0.03<0.0113.94.4<0.01<0.0599.2  
M25#8 (core) ol37.4<0.020.620.0726.30.1334.60.260.20<0.01<0.0599.5  
M25#8 (mantle) cpx50.31.95.40.596.2<0.0312.923.50.48<0.01<0.05101.3 1031
  ol36.2<0.020.03<0.0233.60.2731.00.20<0.01<0.010.14101.4  
  pl50.4<0.0230.7<0.021.1<0.030.0614.03.4<0.010.15100.0  
Clast#MineralNiFeSPCrMnCoTotal
  1. cpx = high-Ca pyroxene; met = Fe,Ni-metal; ol = olivine; pl = plagioclase; pnt = pentlandite; py = pyrrhotite; sp = spinel; cpx = clinopyroxene thermometer of Kretz (1982); ol-sp = olivine-spinel thermometer of Sack and Ghiorso (1991).

Heavily metamorphosed clasts
M25#4 met66.530.8<0.05<0.05<0.050.061.398.7
  pnt20.844.433.4<0.050.10<0.050.9799.7
  py0.0661.436.7<0.050.08<0.050.1098.3
M25#2 pnt23.240.033.1<0.05<0.050.050.9797.2
  py<0.0559.835.7<0.05<0.05<0.050.0995.6
Less metamorphosed clasts
M3#4 met68.527.3<0.05<0.050.05<0.051.297.2
  pnt20.942.332.4<0.05<0.05<0.050.9296.5
  py<0.0560.735.6<0.050.20<0.050.1696.7
M25#8 (mantle)pnt26.935.932.7<0.050.18<0.051.096.8
  py0.0761.736.6<0.05<0.05<0.05<0.0598.5

Olivine, ferroan Al-diopside, and plagioclase—the major minerals in the clasts—commonly form triple junctions, indicative of sintering and annealing at high temperature. Plagioclase does not occur interstitially to olivine and ferroan Al-diopside, unlike in most chondrules, but forms separate crystals with sizes up to 40 μm. The plagioclase grain sizes are similar to those in type 5–6 ordinary (approximately 2–10 μm in type 5 and ≥50 μm in type 6) and CK chondrites (≥4–50 μm) (e.g., Van Schmus and Wood 1967; Huss et al. 2006). Olivine and ferroan Al-diopside have rather uniform fayalite and ferrosilite contents within an individual clast. There are, however, compositional variations of these minerals between the clasts (Table 1; Figs. 3a–d). Similar equilibration of olivine and pyroxene compositions is observed in type 5–6 ordinary and CK chondrites (e.g., Huss et al. 2006). Plagioclase is compositionally variable within an individual clast (Table 1; Figs. 3e and 3f). The significant compositional variations of plagioclase are commonly observed in metamorphosed basaltic eucrites (e.g., Mittlefehldt 2006) and are consistent with slow diffusion rates of CaAl-NaSi in plagioclase (e.g., Grove et al. 1984; Liu and Yund 1992). Sulfides (up to approximately 100 μm in size) consist of anhedral pentlandite and pyrrhotite; they occupy interstitial regions between silicates (Figs. 2 and 4). Such occurrences of sulfides are similar to those in type 5–6 ordinary chondrites (e.g., Huss et al. 2006).

Figure 3.

 Chemical compositions of olivine, pyroxene, and plagioclase in metamorphosed clasts from Mokoia and Yamato-86009. a) Fayalite contents (Fa, mol%) in olivine. b) Differences in Fa contents between olivine grains within individual clasts. c) Ferrosilite (Fs) and wollastonite (Wo) contents (mol%) in pyroxenes. d) Differences of Fs and Wo contents between the pyroxene grains within an individual clast. e) Albite (Ab) and anorthite (An) contents (mol%) in plagioclase. f) Histogram of An content (mol%) in plagioclase. Compositions of pyroxenes in brachinite (Rumble et al. 2008) and plagioclase in type 4–6 H, L, LL, CK, lodranite, acapulcoites, brachinites (Brearley and Jones 1998) and CV metachondrites (Schoenbeck et al. 2006) are shown for references. The heavily metamorphosed clasts have the narrower ranges of Fa, Fs, and Wo contents than the less metamorphosed clasts. Compositions of plagioclase are variable in both types of clasts.

Figure 4.

 Chemical compositions of opaques in the clasts. a) Fe-Ni-S compositions of pyrrhotite, pentlandite, and Fe,Ni-metal. b) Chemical compositions of pyrrhotite and pentlandite are plotted in the Fe-Ni-S isothermal section at 880 K (Raghavan 2004). Pentlandite was exsolved from monosulfide solid solution (mss) below approximately 880 K.

Based on the olivine-spinel (Sack and Ghiorso 1991) and high-Ca pyroxene (Kretz 1982) thermometry, the estimated metamorphic temperatures recorded by the clasts range from approximately 900 to 1100 K (Table 2). The high-Ca pyroxene equilibration temperature is estimated from equation 19b in Kretz (1982) using only the high-Ca pyroxene data.

The representative and average bulk chemical compositions of the clasts are shown in Fig. 5a and Table 3. The observed variations in bulk chemical compositions of the clasts are mainly due to their coarse-grained textures and fragmentation resulting in variations in modal mineralogy (Table 1; Figs. 2 and 5a). The average bulk chemical composition of the clasts is similar, except depletion in Al and enrichment in Cr, to those of carbonaceous chondrites (e.g., CV, CK, and CO) and different from those of ordinary (H, L, LL) and R chondrites and primitive achondrites (acapulcoites, brachinites, lodranites) (Figs. 5a–c).

Figure 5.

 Bulk chemical composition of metamorphosed clasts from Mokoia normalized to CI chondrite elemental ratio and silicon (Lodders 2003). Elements are arranged in order of increasing volatility from left to right. a) Averaged and representative compositions of metamorphosed clasts. Averaged compositions of heavily metamorphosed clasts compared with those of (b) metamorphosed chondrites (H, L, LL, R, and CK) and primitive achondrites (acapulcoites, lodranites, and brachinites) (Hutchison 2007; Brearley and Jones 1998) in “b,” and (c) of carbonaceous chondrites (CM, CR, CH, CO, CK, CV, Allende, and Allende dark inclusions [DI]) (Hutchison 2007; Brearley and Jones 1998; Buchanan et al. 1997). d) Averaged compositions of a mantle in metamorphosed clast M25#8 and those of coarse-grained igneous rims (Rubin 1984) and matrices of CV chondrites Kaba, Vigarano, and Allende (Hutchison 2007).

Table 3. Bulk chemical compositions (normalized to 100%) of metamorphosed clasts from the CV chondrite breccias Mokoia and Yamato-86009.
ClastSiO2TiO2Al2O3Cr2O3FeOMnOMgOCaONa2OK2O
Heavily metamorphosed clasts
M25#4 38.10.243.70.5826.50.2024.94.51.10.12
M25#2 27.50.110.840.4345.40.1724.70.830.050.01
Y86UH#1 31.40.312.03.433.80.2528.50.30<0.01<0.01
Y86UH#2 35.10.193.8<0.0231.20.2827.91.40.24<0.01
Y86UH#3 34.20.111.81.131.80.2029.90.720.11<0.01
Average 35.90.162.31.030.20.2026.83.10.280.17
Less metamorphosed clasts
M3#4 31.70.153.51.333.20.2228.41.30.26<0.01
M25#8 35.60.173.90.6329.80.1926.33.20.22<0.01
M25#8 (core)37.10.143.40.3726.10.1730.02.60.20<0.01
M25#8 (mantle)33.10.173.50.8234.60.2024.92.60.17<0.01
Average 37.70.254.50.8226.20.1924.35.7<0.010.82

Less Metamorphosed Clasts

Fourteen clasts with sizes 20–1000 μm in Mokoia appear to be less metamorphosed than those described above (Fig. 6). Although they have similar mineralogy and bulk chemical compositions to those of the heavily metamorphosed clasts (Tables 1, 2, and 3; Figs. 2 and 5a), they have nongranular textures (Fig. 6) and show some variations in chemical compositions of olivine and pyroxene (Table 1; Fig. 3). In some clasts, plagioclase, pyroxene, and Fe,Ni-sulfides fill interstitial regions between coarser olivine grains (Figs. 6a and 6b). The grain sizes range from a few microns to 100 μm within an individual clast (Fig. 6). Several clasts preserved chondrule-like textures—barred olivine, porphyritic olivine, and porphyritic olivine-pyroxene (Figs. 6c–f).

Figure 6.

 BSE images of the less metamorphosed clasts M3#4 (a, b), MUH#100 (c, d), and M25#8 (e, f) from Mokoia than those shown in Fig. 2. Regions outlined in “a,”“c,” and “e” are shown in detail in “b,”“d,” and “f,” respectively. The clasts consist of the same minerals as the heavily metamorphosed clasts, but have unequilibrated textures. MUH#100 has a barred-olivine chondrule-like texture. M25#8 has a coarse-grained porphyritic-olivine chondrule-like core surrounded by a finer-grained mantle with plagioclase and/or pyroxene filling interstitial regions between olivine crystals. Olivine in the core shows Fe-Mg zoning (Fa25–30); olivine grains in the mantle are compositionally uniform (Fa38–39). met = metal; ol = olivine; pl = plagioclase; pn = pentlandite; px = pyroxene; py = pyrrhotite; sp = spinel.

Clast M25#8 with a porphyritic olivine-pyroxene texture is surrounded by an irregularly shaped igneous olivine-pyroxene-plagioclase mantle (Figs. 6e and 6f). Olivine grains in the core-chondrule of this clast are chemically zoned (Fa25–30), whereas those in the mantle are compositionally uniform (Fa38–39). The anhedral grains of pentlandite and pyrrhotite associate with olivine, pyroxene, and plagioclase (Figs. 4 and 6). Using the high-Ca pyroxene thermometer of Kretz (1982), the metamorphic temperature recorded by the mantle is estimated to be approximately 1100 K (Table 2). Bulk chemical composition of the mantle in M25#8 is similar to those of the coarse-grained igneous rims around Allende chondrules (Fig. 5d and Table 3).

Oxygen-Isotope Compositions of Olivine in Metamorphosed Clasts

Oxygen-isotope compositions of olivine grains in two types of metamorphosed clasts from Mokoia and Y-86009 described above are shown in Fig. 7 and listed in Table 4. On a three-isotope oxygen diagram (Figs. 7a and 7b), the olivine compositions plot below the terrestrial fractionation line (Δ17O ranges from −3.0‰ to −4.3‰, 2σ approximately 1‰; Fig. 7c), along or near carbonaceous chondrite anhydrous mineral (CCAM) line and the Allende mass-fractionation (AMF) line. The data obtained overlap with oxygen-isotope compositions of chondrule olivines from CV (Jones et al. 2004; Chaussidon et al. 2008; Cosarinsky et al. 2008) and CK chondrites (Clayton and Mayeda 1999), and of olivines from the CV metachondrite NWA 3133, which is believed to have formed by annealing of CV chondrites (Irving et al. 2004).

Figure 7.

 Oxygen-isotope compositions of olivine grains in metamorphosed clasts from Mokoia and Yamato-86009. In “a” and “b,” the data are plotted as δ17O versus δ18O. In “c,” the same data are plotted as Δ17O (=δ17O – 0.52 × δ18O). Errors are 2σ. Based on the Δ17O values, the data are divided into 3 groups: high Δ17O (light-purple), low Δ17O (dark-purple), and intermediate Δ17O (purple). The locations of ion probe spots with different filling colors indicating the high, low, and intermediate Δ17O values in clast M25#8 are shown in “d.” Carbonaceous chondrite anhydrous mineral (CCAM) line, the terrestrial fractionation line (TFL), and Allende mass fractionation line (AMF) (Young et al. 1999) are shown for reference. The heavily metamorphosed clasts have uniform O-isotope compositions within an individual clast; there are, however, compositional differences between the different clasts. The less metamorphosed clasts show a bimodal distribution of oxygen isotopes within individual clasts. The δ17O values of less metamorphosed clasts are more variable than those of heavily metamorphosed clasts. O-isotope compositions of the olivines from clasts are similar to those from CV chondrules, CV metachondrite, and CK chondrules. ol = olivine; WR = whole rock. Literature data are from [1] Jogo et al. (2008); [2] Greenwood et al. (2000); [3] Clayton and Mayeda (1999); [4] Irving et al. (2004); [5] McSween (1999a); [6–8] McSween (1999b); [9] Chaussidon et al. (2008); [10] Jones et al. (2004); [11] Cosarinsky et al. (2008).

Table 4. Oxygen-isotope compositions of olvine in metamorphosed clasts from CV chondrite breccias Mokoia and Yamato-86009.
ChondriteClastAnalysis#δ18Oδ17OΔ17O
Heavily metamorphosed clasts
Mokoia M25#4 12.80.6−2.40.6−3.80.7
-”--”-22.40.6−2.20.6−3.40.7
-”--”-32.30.6−2.70.6−3.90.7
Average±2σ 2.50.4−2.40.6−3.70.4
Mokoia M25#2 1−0.50.6−3.50.6−3.30.7
-”--”-2−0.70.6−3.90.6−3.60.7
-”--”-3−0.90.6−4.70.6−4.30.7
Average±2σ −0.70.4−4.11.2−3.71.0
Y-86009 Y86UH#1 19.10.81.30.8−3.40.9
-”--”-28.60.80.90.8−3.60.9
-”--”-39.70.81.30.8−3.70.9
-”--”-48.80.80.80.8−3.70.9
-”--”-59.00.31.10.8−3.60.8
Average±2σ 9.10.81.10.4−3.60.2
Y-86009 Y86UH#2 10.60.3−3.40.8−3.70.8
-”--”-2−1.00.3−3.50.8−3.00.8
-”--”-30.10.3−3.40.8−3.40.8
-”--”-4−0.30.3−3.50.8−3.30.8
-”--”-50.50.3−3.20.8−3.50.8
Average±2σ 0.21.2−3.50.6−3.60.8
Y86009Y86UH#310.60.4−3.61.0−3.91.0
-”--”-2−0.10.4−3.90.9−3.91.0
-”--”-31.00.5−3.51.0−4.01.0
-”--”-40.20.5−3.91.0−4.01.0
Average±2σ 0.40.9−3.70.4−3.90.2
Less metamorphosed clasts
Mokoia M3#4 1−3.30.5−6.70.5−5.00.6
-”--”-2−6.80.6−7.50.6−4.00.6
-”--”-3−4.70.5−7.80.5−5.40.6
-”--”-43.20.5−1.70.6−3.40.6
-”--”-5−4.60.7−7.90.6−5.50.7
-”--”-6−4.90.7−7.80.6−5.30.7
-”--”-72.80.7−1.90.6−3.40.7
-”--”-82.70.7−2.50.6−3.90.7
-”--”-9−3.40.7−6.60.6−4.80.7
Average±2σ −3.47.8−5.65.4−4.51.7
Mokoia M25#8 1−4.30.4−7.40.7−5.20.7
-”--”-2−5.50.4−7.30.7−4.50.8
-”--”-3−4.00.4−6.80.7−4.70.7
-”--”-4−3.80.4−6.30.7−4.40.7
-”--”-5−3.80.4−6.20.7−4.30.7
-”--”-6−4.60.4−7.80.7−5.40.7
-”--”-7−5.90.4−7.60.7−4.60.7
-”--”-8−3.40.4−7.10.7−5.40.7
-”--”-93.40.7−1.60.7−3.40.7
-”--”-10−2.20.7−5.50.6−4.40.7
-”--”-11−3.70.7−7.20.6−5.30.7
Average±2σ    −3.44.9−6.43.5−4.71.2

In the heavily metamorphosed clasts, oxygen-isotope compositions of olivine grains are uniform within an individual clast (Figs. 7a and 7b); the Δ17O values are similar within uncertainty of our measurements (Δ17O = −3.7 ± 0.6‰ (2SD)) (Fig. 7c). However, there are significant differences (up to approximately 10‰) in δ18O values between different clasts (Figs. 7a and 7b; Table 4). In the less metamorphosed clasts, oxygen-isotope compositions of olivine grains show a bimodal distribution and plot along CCAM line (Figs. 7a–c); the Δ17O values are more variable than those in the heavily metamorphosed clasts (Figs. 7c and 7d).

Discussion

Origin of the Metamorphosed Clasts in Mokoia and Yamato-86009

The equilibrated textures, occasional preservation of chondrule-like textures, and nearly uniform chemical compositions of olivine and pyroxene in the Mokoia and Y-86009 metamorphosed clasts (Table 1 and 2; Figs. 2 and 3) indicate that the clasts experienced sintering and high-temperature annealing that postdated chondrule formation. The metamorphic temperatures recorded by the clasts (approximately 1100 K) provide a lower limit on peak metamorphic temperature they experienced (Kessel et al. 2007). These observations preclude a nebular origin of the clasts advocated by Cohen et al. (1983) and suggest instead that the clasts formed by prolonged annealing of chondritic materials on a parent asteroid.

In the clasts, sulfides occur as anhedral grains of pyrrhotite and pentlandite, which occupy interstitial regions between silicates (Fig. 2), suggesting that the sulfides may have experienced melting, melt migration, and solidification as monosulfide solid solution during subsequent cooling (e.g., Kelly and Vaughan 1983). Below approximately 880 K, pyrrhotite and pentlandite exsolved from monosulfide solid solution (Fig. 4b; Raghavan 2004). The coarse-grained, rather than lamellae, occurrences of pentlandite suggest that the growth of pentlandite occurred during slow cooling, probably in the interior of an asteroid.

Olivine in the clasts has high CaO contents (up to 0.6 wt%). High CaO in olivine (0.3–0.45 wt%) is observed in ultramafic achondrites—ureilites (Mittlefehldt 2006). In contrast, in type 4–6 ordinary chondrites, CaO abundance in olivine decreases with an increasing of petrologic type and is typically <0.1 wt% (e.g., Dodd 1969). These observations suggest that the clast may have experienced partial melting prior to excavation from the deep interior of their parent asteroid.

Comparison of Clasts With Known Groups of Chondrites and Achondrites

Equilibrated textures and uniform chemical compositions of olivines and pyroxenes similar to those in the Mokoia and Y-86009 metamorphosed clasts are observed in meteorites of high petrologic types (5–6) of known chondrite groups of high petrologic types (CK, H, L, LL, and R), CV metachondrites NWA 3133 and NWA 1839, and primitive achondrites (e.g., acapulcoites, lodranites, and brachinites). In this section, we explore a genetic relationship between the clasts and these meteorites (Table 5).

Table 5. Comparison of metamorphosed clasts from Mokoia and Y-86009 with metamorphosed chondrites of known groups and primitive achondrites.
Characteritics of metamorphosed clastsMetamorphosed chondritesCV metachondritesPrimitive achondritesReferences
 H4–6L4–6LL4–6R4–6CK4–6 AcapulcoitesLodranitesBrachinites 
  1. “+” & “−” = characteristics of metamorphosed clasts which are consistent or inconsitent with those of metamorphosed meteorites of known chondrite groups and primitive achondrites. b.d. = not detected.

  2. References: [1] Hutchison (2004); [2] Brearley and Jones (1998); [3] Schoenbeck et al. (2006); [4] Huss et al. (2006); [5] Rubin and Kallemeyn (1994); [6] Rumble et al. (2008); [7] McSween (1999); [8] Greenwood et al. (2000); [9] Clayton and Mayeda (1999); [10] Irving et al. (2004); [11] Goodrich et al. (2011); [12] Kessel et al. (2007); [13] Noguchi (1993); [14] Patzer et al. (2004); [15] Benedix et al. (2009); [16] Schulze et al. (1994).

Bulk chemical compositions+No data+[1,2]
Lack of low-Ca pyroxene+[2,3]
Grain sizes (up to 50 μm) ≤300≤300≤300≤1000≤300≤200150–230540–700≤1000[2–6]
 
O-isotope compositions of olivine++No dataNo data[7–11]
Fa content in olivine (31–39)16–2022–2626–3237–4029–3322–234–127–1330–35[2,3]
+++ 
NiO content in olivine (up to 0.3 wt%)b.d.b.d.b.d.≤0.30.3–0.7No datab.d.b.d.≤0.5[2,8]
+  
Cr/(Cr+Al) ratio in spinel (0.19–0.45)0.84–0.850.860.86–0.870.84–0.87≤0.10.730.85–0.930.86–0.990.81–0.83[2,12–15]
+ 
Plagioclase (An37–84Ab63–17)++[2,3,14,16]
Lack of magnetite++++++++[2]
  • 1CK5–6 chondrites. Pyrrhotite and pentlandite are the major sulfides in the clasts and in CK chondrites of petrologic types 5–6 (Brearley and Jones 1998). In addition, oxygen-isotope compositions of olivine in the clasts are similar to those in CK chondrites (Clayton and Mayeda 1999) (Fig. 7b). However, the absence of low-Ca pyroxene and magnetite in the clasts, the lower abundance of NiO (<0.3 versus 0.3–0.7 wt%; Fig. 8) and the higher fayalite contents (31–39 versus 29–33) in olivine of the clasts compared with those of CK chondrites, and the differences in chemical compositions of plagioclase (Noguchi 1993; Brearley and Jones 1998; Dunn 2011) do not support a genetic relationship between these materials (Table 5).
  • 2H5–6, L5–6, LL5–6 chondrites. A common presence of low-Ca pyroxene, Fe,Ni-metal, and troilite in metamorphosed ordinary chondrites (Brearley and Jones 1998); the larger grain sizes of silicates (up to >300 μm; Huss et al. 2006); and the differences in chemical and oxygen isotopic compositions of olivine (Δ17O: approximately 1–2‰ versus approximately −3‰ to approximately −5‰) and spinel (Cr/(Cr + Al) = 0.84–0.87 versus 0.19–0.45) in metamorphosed ordinary chondrites and the clasts (Table 5) preclude their genetic relationship.
  • 3R5–6 chondrites. The high NiO (up to 0.3 wt%) and fayalite contents (37–40) in olivine and the lack of magnetite in R5–6 chondrites (Brearley and Jones 1998; Greenwood et al. 2000) are consistent with the mineralogy of metamorphosed clasts (Table 5). However, the differences in oxygen isotopic compositions of olivine (Δ17O: approximately 2–4‰ versus approximately −3‰ to approximately −5‰) and chemical compositions of spinel (Cr/(Cr + Al) = 0.84–0.87 versus 0.19–0.45), the common presence of low-Ca pyroxene, and the larger grain sizes of silicates (up to 1 mm versus 50 μm) in R5–6 chondrites (Rubin and Kallemeyn 1994; Brearley and Jones 1998; Greenwood et al. 2000) do not support a genetic relationship between the clasts and R chondrites.
  • 4CV metachondrites. The oxygen-isotope compositions of olivine (Irving et al. 2004) and the lack of magnetite (Schoenbeck et al. 2006) in CV metachondrites are similar to those of clasts (Table 5). However, the presence of low-Ca pyroxene in CV metachondrites and the differences in grain sizes (up to 200 μm versus 50 μm) and in chemical compositions of olivine (Fa22–23 versus Fa31–39) and spinel (Cr/(Cr + Al) = 0.73 versus 0.19–0.45) in CV metachondrites and the clasts preclude their genetic relationship.
  • 5Primitive achondrites. The absence of low-Ca pyroxene in the clasts is inconsistent with mineralogy of acapulcoites and lodranites (e.g., Brearley and Jones 1998; Mittlefehldt et al. 1998). Although brachinites are known to contain high-Ca pyroxene and no or rare low-Ca pyroxene (e.g., Rumble et al. 2008), they are much coarser grained (up to approximately 1 mm in size) than the clasts (up to approximately 50 μm in size). In addition, there are significant differences in oxygen-isotope compositions of brachinites and olivines in the clasts (Fig. 7b). In addition, the average bulk chemical compositions of metamorphosed clasts are inconsistent with those of primitive achondrites: the latter are highly depleted in Al, Na, and K relative to those of the clasts (Fig. 5b).
Figure 8.

 NiO (wt%) versus fayalite (Fa, mol%) content in olivine of metamorphosed clasts from Mokoia and Y-86009. Outlined regions represent compositions of olivine in CK4–6 (Brearley and Jones 1998; Noguchi 1993) and CV chondrites (Weinbruch et al. 1990; Kimura and Ikeda 1998).

To summarize, metamorphosed clasts in Mokoia and Y-86009 are not related to metamorphosed ordinary, R, and CK chondrites and to primitive achondrites.

Genetic Relationship Between Metamorphosed Clasts and CV Chondrites

The lack of low-Ca pyroxene in metamorphosed clasts suggests that it was either originally absent in their precursors or was destroyed during thermal metamorphism or partial melting. The chondritic texture preserved in some of the clasts and the common presence of low-Ca pyroxene in all known chondrite groups (e.g., Brearley and Jones 1998) are difficult to reconcile with the first interpretation. The preferential replacement of low-Ca pyroxene by ferroan olivine was inferred from modal mineralogy of metamorphosed ordinary chondrites (McSween and Labotka 1993), and is commonly observed in metamorphosed CO chondrites (Kerridge 1972; Tomeoka and Itoh 2004; Brearley and Krot 2012), Allende-like oxidized CV chondrites and Allende dark inclusions (Kojima and Tomeoka 1996; Krot et al. 1995, 1998ab, 2000; Brearley and Krot 2012). In the Allende dark inclusions, this replacement is often nearly complete (e.g., Krot et al. 1998b, 2000).

Although bulk chemical (Fig. 5c) and O-isotope compositions (Krot et al. 2006) of CO chondrites are similar to those of the clasts, chondrules in CO chondrites have smaller sizes (148 +132/−70 micrometer in diameter; Rubin 1989) than the clasts with chondrule-like texture, probably excluding CO chondrites from the precursor of the clasts.

We suggest that the heavily altered Allende-like CV chondrites could be a possible precursor material for the metamorphosed clasts based on several similarities between these materials. (1) Bulk chemical compositions of the clasts, including high calcium contents, are similar to those of CVOxA chondrites and Allende dark inclusions (Fig. 5c). (2) Oxygen-isotope compositions of olivine in the clasts overlap with those in the CV chondrules and matrices (Fig. 7; Jones et al. 2004; Cosarinsky et al. 2008; Chaussidon et al. 2008). (3) Clasts with chondrule-like textures have similar sizes (200–1000 μm in diameter) to those of chondrules in CV chondrites (e.g., mean diameter of chondrules is 240 μm in Efremovka [King and King 1978] and 970 μm in ALH 84028 [Paque and Cuzzi 1997]). One of the clasts is surrounded by a mantle, which is texturally and compositionally (Figs. 5d, 6e, and 6f) similar to coarse-grained igneous rims around Allende chondrules (Rubin 1984). (4) Ferroan olivine, ferroan high-Ca pyroxene, nepheline, Fe,Ni-sulfides, and Ni-rich metal—the major minerals in the clasts—are the common secondary minerals in the oxidized CV chondrites.

However, there are also several chemical and mineralogical differences between the clasts and the oxidized CV chondrites. Secondary magnetite, andradite, grossular, wollastonite, and sodalite, commonly observed in chondrules in the Allende-like CVs (e.g., Kimura and Ikeda 1995; Ikeda and Kimura 1995), are not found in the clasts. In addition, bulk chemical compositions of the clasts are Al-depleted and Cr-enriched compared with CV chondrites. Thermal metamorphism and possible melting experienced by the clasts might be responsible for these differences. For example, grossular is unstable above 1100 K and decomposes to wollastonite, gehloenite, and anorthite (Huckenholz et al. 1974). Andradite decomposes to pseudowollastonite and hematite at 1400 K (Suwa et al. 1976). Magnetite and hematite could be replaced by ferroan olivine at high temperature or low oxygen fugacity (Garrels and Christ 1965).

Replacement of magnesian low-Ca pyroxene by igneous ferroan high-Ca pyroxene is also observed in some Type II chondrules in unmetamorphosed (CO3.0, CR2, and CM2) carbonaceous chondrites. These chondrules, composed of ferroan olivine, ferroan high-Ca pyroxene, Fe,Ni-sulfides, Cr-spinel, and Na-rich feldspathic mesostasis, contain relict Type I porphyritic olivine-pyroxene chondrules or their fragments (Fig. 9). The relict magnesian low-Ca pyroxene (Fs∼1–2Wo∼1–2) is replaced by the ferroan high-Ca pyroxene (Fs∼10–20Wo∼30–50) that contains approximately 1–2 wt% Al2O3, TiO2, and Cr2O3, and approximately 0.3–0.8 wt% Na2O. Some relict grain-free type II chondrules lack low-Ca pyroxene and consist of ferroan olivine, ferroan high-Ca pyroxene phenocrysts, and feldspathic mesostasis. These chondrules are mineralogically similar to metamorphosed clasts with chondrule-like textures in Mokoia and Y-86009. It seems to be possible that melting under oxidizing conditions and high partial pressure of sodium during chondrule formation and/or on the parent asteroid could also explain the lack of low-Ca pyroxene in the clasts.

Figure 9.

 Backscattered electron images of relict Type I chondrule fragments, composed of magnesian olivine (fo), magnesian low-Ca pyroxene (px), and Fe,Ni-metal (met), in Type II chondrules from Y-81020 (CO3.0). Region outlined in “c” is shown in detail in “d.” Relict magnesian low-Ca pyroxenes are replaced by ferroan high-Ca pyroxene (cpx) that co-crystallized with ferroan olivine (fa) and Na-Ca-rich feldspathic mesostasis (mes) of the host Type II chondrules.

Implications for Constraining the Accretion Time of the CV Chondrite Parent Asteroid and Understanding its Alteration and Thermal Metamorphism

Accretion Time of CV Parent Body

High metamorphic temperature, approximately 1100 K, recorded by the clasts provides a lower limit on a peak metamorphic temperature they experienced. Assuming uniform distribution of 26Al in the protoplanetary disk with the canonical initial 26Al/27Al ratio of 5 × 10−5, Wakita et al. (2011) performed numerical calculations of thermal evolution of a CV-like asteroid with various initial parameters—accretion time, size, and water/rock mass ratio. The results of this modeling are summarized in Fig. 10. According to this model, to reach 1100 K, the CV asteroid with >50 km radius should accrete within approximately 2 Myr after formation of calcium-aluminum-rich inclusions (CAIs) with the canonical 26Al/27Al ratio.

Figure 10.

 Temperature in the center of asteroids 50 km in radius and initial water/rock mass ratio of 0.1 that accreted 1.9 and 2.0 Myr after CAI formation (from Wakita et al. 2011). Assuming a uniform distribution of 26Al in the inner solar system with the canonical 26Al/27Al ratio of approximately 5 × 10–5, an asteroid should have accreted earlier than 2 Myr after formation of CAIs to reach metamorphic temperature of 1100 K as inferred from the heavily metamorphosed clasts in Mokoia and Yamato-86009.

Alteration and Thermal Metamorphism on the CV Parent Asteroid

Oxygen-isotope compositions of olivine in heavily metamorphosed clasts are nearly uniform within individual clasts (Figs. 7a and 7b). They represent either the original compositions of precursor materials or the compositions that were modified by oxygen self-diffusion during thermal metamorphism at ≥1100 K. Because no chemical zoning is observed in 50 μm-sized olivine grains of the heavily metamorphosed clasts (Fig. 2), these grains were probably chemically homogenized during thermal metamorphism. Based on Fe-Mg diffusion coefficient in olivine at 1100 K, approximately 10−19 m2 s−1 (Chakraborty 1997), thermal metamorphism at 1100 K for approximately 200 yr is required to complete Fe-Mg exchange in a 50 μm-sized olivine grain. Using oxygen self-diffusion coefficients at 1100 K, approximately 5 × 10−21 m2 s−1 (Gérard and Jaoul 1989) and approximately 6 × 10−23 m2 s−1 (Ryerson et al. 1989), the estimated diffusion distances of oxygen in olivine during this time are approximately 10 μm and approximately 1 μm, respectively. Because these values are obtained using the minimum temperature and duration of thermal metamorphism recorded by the clasts, they provide a lower limit of the oxygen-diffusion distance. The actual duration time of metamorphism caused by decay of 26Al is certainly longer than 200 yr. For example, in Fig. 10, a central part of an asteroid 50 km in radius and initial water/rock mass ratio of 0.1 that accreted 2.0 Myr after CAI formation is heated above 1100 K for more than 10 Myr. Under these conditions, the oxygen-diffusion distances are 3000 and 300 mm (using oxygen self-diffusion coefficients of Gérard and Jaoul [1989] and Ryerson et al. [1989], respectively), which are much larger than grain sizes of the clasts (≤50 μm). Thus, we infer that the narrow range in oxygen-isotope compositions of heavily metamorphosed clasts could be due to oxygen-isotope equilibration during thermal metamorphism.

In less metamorphosed clasts, oxygen-isotope compositions of olivine show a bimodal distribution (Figs. 7b and 7c); occasionally, the adjacent olivine grains show different Δ17O values (Fig. 7d). In addition, a 50 μm sized olivine grain preserved Fe-Mg zoning (Fig. 6e). As these clasts experienced weaker degrees of thermal metamorphism than the heavily metamorphosed clasts, a diffusion distance of oxygen isotopes in the less metamorphosed clasts should be also shorter, i.e., <10 μm (Gérard and Jaoul 1989) and <1 μm (Ryerson et al. 1989). Because the distances between ion probe spots in olivine grains are larger than approximately 10 μm, we infer that oxygen-isotope compositions of olivine in less metamorphosed clasts measured were not significantly modified during thermal metamorphism and could have preserved the original compositions of the precursor materials.

Olivine grains in the heavily metamorphosed clast M25#4 have highly fractionated oxygen-isotope compositions which on a three-isotope oxygen diagram are displaced from CCAM line to the right by approximately 10‰ (Figs. 7a and 7b). They plot along the Allende mass-fractionation (AMF) line defined by the whole-rock oxygen-isotope compositions of the metasomatically altered chondrules from Allende (Jabeen et al. 1998, 1999; Ash et al. 1999; Young et al. 1999). These observations may indicate that oxygen-isotope compositions of the M25#4 olivines recorded metasomatic alteration experienced by the clast prior to the thermal metamorphism. This interpretation is supported by oxygen-isotope compositions of secondary minerals in CV chondrites that resulted from metasomatic alteration—fayalite, magnetite, salite-hedenbergite pyroxenes, and andradite: on a three-isotope oxygen diagram, compositions of these mineral plot along the AMF line and show a spread in δ18O values up to 20‰ (Choi et al. 2000; Hua et al. 2005; Krot et al. 2006; Cosarinsky et al. 2008; Wasserburg et al. 2011).

We suggest that the precursor materials of the metamorphosed clasts from Mokoia and Y-86009 experienced various degrees of hydrothermal alteration prior to thermal metamorphism. The similar sequence of alteration and thermal metamorphism has been previously inferred for the oxidized Allende-like CV chondrites and Allende dark inclusions (e.g., Kojima and Tomeoka 1996; Krot et al. 1995, 1998ab, 2000).

Conclusions

Metamorphosed clasts composed of Ca-bearing ferroan olivine, ferroan Al-diopside, plagioclase, Cr-spinel, nepheline, pyrrhotite, pentlandite, Ca-phosphate, and rare grains of Ni-rich taenite are commonly observed in the CV carbonaceous chondrite breccias Mokoia and Y-86009. Most clasts have triple junctions between silicate grains, indicative of thermal annealing. Occasionally, clasts preserved chondrule-like textures.

Based on the mineralogy, petrography, bulk chemical compositions, and oxygen-isotope compositions, we infer that the clasts represent fragments of the CV-like material that experienced hydrothermal alteration followed by high-temperature (>1100 K) metamorphism and possibly small degree of melting in the interior of the CV parent asteroid, and were subsequently excavated and added to the Mokoia and Y-86009 breccias.

The inferred thermal history of the clasts provide constraints on the accretion time of the CV parent asteroid: it must have accreted within the first 2 Myr after formation of CV CAIs with the canonical 26Al/27Al ratio.

Acknowledgments—  We thank M. Zolensky (Johnson Space Center), M. Petaev (Harvard University), and G. MacPherson (Smithsonian Institution) for providing polished sections of Mokoia. Constructive reviews by M. Petaev, M. Kimura, and E. Scott are highly appreciated. This work was supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement No. NNA09DA77A issued through the Office of Space Science.

Editorial Handling—  Dr. Edward Scott

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