Fayalite in the Vigarano CV3 carbonaceous chondrite: Occurrences, formation age and conditions

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Abstract

We have performed petrographic characterization, 53Mn–53Cr age determination and thermodynamic stability evaluations of fayalite in Vigarano meteorite that belongs to the reduced subgroup of CV3 chondrites. Vigarano is a breccia consisting of clasts which are separate chondrules surrounded by olivine-rich fine-grained materials. Four out of twenty three explored clasts contain fayalites that represent materials of the Bali-like oxidized subgroup of CV3 chondrites. The fayalites (Fa> 80) with grain sizes typically < 20 µm occurs in veins that extend from chondrules into the fine-grained materials. The fayalite commonly coexists with troilite and/or magnetite. The fayalite-bearing veins terminate at the boundaries of clasts. No evidence of strong impact enough to make melt veins is found in materials adjacent to the veins. These observations suggest that the fayalite-bearing veins in the Bali-like clasts formed through aqueous alteration in an asteroid prior to fragmentation and re-accretion to the Vigarano parent body. In saponite-rich fine-grained materials, we also found troilite–magnetite veins, which are similar to the fayalite-bearing veins in morphology. Morphological evidences and thermochemical equilibrium calculations suggest that fayalite replaced magnetite, and that replacement occurred at temperatures < 200 °C and low water/rock mass ratios from 0.07 to 0.18, which represent aqueous to metamorphic transition. Fayalite grains typically show iron-magnesium zoning (fayalite content decreases towards the grain edges). Based on equilibrium models, this zoning may have occurred at increasing temperature. The observed initial ratio of (53Mn/55Mn)0 = (2.3 ± 0.5) × 10 6 suggests that fayalite formed ~ 5 Ma before the timing when the Mn–Cr system was closed in angrite NWA 4801 and has an absolute age of ~ 4563 ± 1 Ma. The age of fayalite is identical within errors to that in Mokoia and Kaba CV3 chondrites, which belong to the Bali-like oxidized subgroup. The identical age implies that aqueous alteration occurred at the same time in parent asteroids of Bali-like subgroup materials. These fayalite-bearing materials may have been derived from a single CV3 asteroid or from separate CV3 asteroids where aqueous alteration simultaneously occurred.

Introduction

CV3 carbonaceous chondrites are subdivided into the reduced (CV3Red) and two oxidized subgroups, Allende-like (CV30xA) and Bali-like (CV30xB) (McSween, 1977, Weisberg et al., 1997). These subgroups are considered to be affected by different degrees of aqueous alteration and subsequent metamorphism, which resulted in different secondary mineralization (Lee et al., 1996, Krot et al., 1995, Krot et al., 1998a, Krot et al., 1998b, Krot et al., 2004). For example, olivine in the CV3Red and CV30xA chondrites is Fa30–60, but that of CV30xB chondrite is Fa10–100 (Weisberg et al., 1997, Krot et al., 2004). Generally, only CV30xB chondrites contain near-pure fayalite (Fa> 90) (Hua and Buseck, 1995, Krot et al., 1995, Krot et al., 1998a, Krot et al., 1998b, Krot et al., 2000, Krot et al., 2004; Weisberg et al., 1997, Hutcheon et al., 1998, Choi et al., 2000, Hua et al., 2005).

The near-pure fayalite in the CV30xB chondrites occurs as discrete grains (up to 600 µm in size) or as a component of veins. The fayalite commonly associates with troilite and/or magnetite, and exists in chondrules and in matrix. Several formation models of the CV30xB fayalite have been proposed (Nagahara et al., 1988, Nagahara et al., 1994, Hua and Buseck, 1995, Krot et al., 1998a, Krot et al., 1998b, Krot et al., 2000, Krot et al., 2004, Choi et al., 2000, Ohnishi and Tomeoka, 2002, Krot et al., 2004, Choi et al., 2000; Zolotov et al., 2006, Jogo et al., 2008). According to the asteroidal models, fayalite may have formed through aqueous alteration (e.g., Krot et al., 1998a, Krot et al., 1998b, Krot et al., 2000, Krot et al., 2004, Choi et al., 2000, Jogo et al., 2008), or dehydration of Fe-rich phyllosilicates (e.g., Krot et al., 1995, Ohnishi and Tomeoka, 2002). According to the nebular model, fayalite may have formed by condensation of a gas (e.g., Nagahara et al., 1988, Nagahara et al., 1994), or by reaction of SiO gas and magnetite (e.g., Hua and Buseck, 1995). The asteroidal origin of fayalite is consistent with the oxygen isotopic data for fayalite and magnetite (Choi et al., 2000; Hua et al., 2005; Jogo et al., 2008). Thermodynamic analysis of fayalite stability is also consistent with asteroidal formation at temperatures below 300–350 °C (Krot et al., 1998a, Krot et al., 1998b, Zolotov et al., 2006).

Mn–Cr dating were performed on the large fayalite grains (100 to 600 µm in size) in the CV30xB Mokoia and Kaba meteorites. The fayalite grains in both meteorites show excesses of 53Cr corresponding to the initial (53Mn/55Mn)0 ratios of (2.32 ± 0.18) × 10 6 (Mokoia, Hutcheon et al., 1998) and of (2.07 ± 0.17) × 10 6 (Kaba, Hua et al., 2005). These ratios indicate that both of the Mokoia and Kaba fayalite have formed at the same time of 4562–4563 Ma, which is calculated using the angrite NWA 4801 as a time marker; Pb–Pb age of this angrite is 4558.0 ± 0.13 Ma (Amelin and Irving, 2007) and the corresponding (53Mn/55Mn) ratio is (0.96 ± 0.04) × 10 6 (Shukolyukov et al., 2009).

Although Vigarano meteorite is classified to the CV3Red subgroup, it also contains fayalite (Fa> 80) (Krot and Todd, 1998; Meibom and Krot, 1998; Tomeoka and Tanimura, 2000, Krot et al., 2000, Noguchi et al., 2003). The fayalite has similar occurrence to the CV30xB fayalite. It coexists with troilite and/or magnetite, and occurs as discrete grains (up to 50 µm in size) or as a component of veins. The Vigarano fayalite exists in chondrules, and in fine-grained materials in “clasts” (not in the host Vigarano). The clasts are considered to have formed at a CV3 asteroid(s) prior to formation of Vigarano breccia (Tomeoka and Tanimura, 2000, Krot et al., 2000). Noguchi (1997) reported magnetite–troilite veins in Vigarano with similar morphology to the fayalite-bearing veins. This may suggest that fayalite-bearing veins replaced the magnetite–troilite veins (Noguchi, 1998, Noguchi, 1999, Noguchi and Nakamura, 2000, Noguchi et al., 2003).

In this study, we describe new fayalite-bearing veins in Vigarano CV3Red chondrite. We determined the age of fayalite grains in a vein using the 53Mn–53Cr dating method. This is the first report of the age of CV3Red fayalite formation that is important to constrain the time of formation of the Vigarano breccia. We also proposed a formation mechanism of the fayalite-bearing veins based on detailed mineralogical observations and thermochemical equilibrium calculations in a water–rock system.

Section snippets

Mineralogical characterization

We examined two polished thin sections of CV3Red Vigarano with areas of ~ 2 cm2 and ~ 3 cm2 using an optical microscope, a scanning electron microscope (SEM), a field emission scanning electron microscope (FE-SEM) and an electron probe micro analyzer (EPMA). Carbon thin film (25 nm) was applied on the surface prior to SEM, FE-SEM and EPMA analyses in order to eliminate the electrostatic charge. The SEMs (JEOL-5300 at Tokyo Institute of Technology, JEOL-5600LV at Ibaraki University, and

Mineralogy and petrology

In both of CV3 Vigarano sections, areal ratios of coarse-grained objects (chondrules and chondrule fragments) to fine-grained matrix are about 1. In total, 23 chondrules were identified. They range in diameter from 0.3 to 1 mm and have porphyritic textures; among them 9 porphyritic olivine pyroxene chondrules (POP), 11 porphyritic olivine chondrules (PO), and 3 porphyritic pyroxene chondrules (PP) can be recognized. Metal and minor iron sulfides are present in the interiors of chondrules. In

Discussions

Observed clastic structures of Vigarano CV3Red chondrite (Fig. 1, Fig. 2, Fig. 3, Fig. 4) are similar to previous studies (Noguchi, 1997, Noguchi, 1998, Noguchi, 1999, Noguchi and Nakamura, 2000, Krot et al., 2000, Tomeoka and Tanimura, 2000, Noguchi et al., 2003). Occurrences of secondary minerals in the 4 clasts containing fayalites indicate that mineralogy of the clasts is similar to that of CV30xB chondrites (Weisberg et al., 1997). Thus, the 4 clasts would be CV30xB clasts that formed via

Summary

We found four CV30xB clasts containing fayalite in the CV3Red chondrite Vigarano. Mineralogical observations indicate that these clasts represent Bali-type CV30xB chondritic material formed in a CV3 asteroid(s) prior to formation of the Vigarano breccia. The identical formation age of fayalites in these CV30xB clasts and in the CV30xB Mokoia and Kaba chondrites implies that all CV30xB materials derived from the same CV3 asteroid. Chemical equilibrium models for water–rock interactions indicate

Acknowledgements

The paper is benefited from the very contractive reviews from Dr. Ito and Dr. Krot. We thank Dr. Miyamoto for measuring Mn/Cr ratio of San Calros olivine by ICP AES. We also thank Dr. Ikeda for the support to use JEOL-5800LV SEM and Mr. Shimada for technical assistant in the use of JEM-733 EPMA at Kyushu University. We also thank Drs. Nakamura and Yurimoto for the support to use JSM-5300 SEM at Tokyo Institute of Technology; Dr. Nagahara for the support, and Mr. Yoshida and Dr. Yamamoto for

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