Devgaon (H3) chondrite: Classification and complex cosmic ray exposure history

Abstract

Abstract-The Devgaon meteorite fell in India on February 12, 2001 and was immediately collected. It is an ordinary chondrite having a number of SiO 2 -rich objects and some Ca, Al-rich inclusions. Olivines ) are fairly equilibrated, while pyroxenes ) are unequilibrated. Occasionally, shock veins are visible, but the bulk rock sample is very weakly shocked (S2). Chondrules and chondrule fragments are abundant. Based on chemical and petrological features, Devgaon is classified as an H3.8 group chondrite. Several cosmogenic radionuclides ranging in half-lives from 5.6 d ( 52 Mn) to 7.3 × 10 5 yr ( 26 Al), noble gases (He, Ne, Ar, Kr, and Xe), and particle track density have been measured. The track density in olivines from five spot samples varies between (4.6 to 9) × 10 6 cm −2 showing a small gradient within the meteorite.The light noble gases are dominated by cosmogenic and radiogenic components. Large amounts of trapped gases (Ar, Kr, and Xe) are present. In addition, (n, γ) products from Br and I are found in Kr and Xe, respectively. The average cosmic ray exposure age of 101 ± 8 Ma is derived based on cosmogenic 38 Ar, 83 Kr, and 126 Xe. The track production rates correspond to shielding depths of about 4.9 to 7.8 cm, indicating that the stone suffered type IV ablation. Low 60 Co, high ( 22 Ne/ 21 Ne) c , and large neutron produced excesses at 80 Kr, 82 Kr, and 128 Xe indicate a complex exposure history of the meteoroid. In the first stage, a meter-sized body was exposed for nearly 10 8 yr in the interplanetary space that broke up in ~50 cm-sized fragments about a million years ago (stage 2), before it was captured by the Earth. FALL, PETROGRAPHY, AND MINERALOGYA single stone weighing about 12 kg fell in the village of Devgaon, Bastar district, Chattisgarh, Central India on February 12, 2001 at about 16:00 hours IST. The meteorite was fully covered with fusion crust (Fig. 1) at the time of recovery. The meteorite was quickly recovered and delivered to the district Collectorate from whom one of us (G. Srinivasan) got a piece weighing about 3.8 kg.Devgaon is an ordinary chondrite consisting of abundant chondrules and chondrule fragments embedded within a moderately recrystallized fine-grained matrix (Fig. 2). Studies of thin sections show that most constituents of the meteorite are fragmented. In contrast, the hand specimen of Devgaon looks quite homogeneous without any indication of brecciation. Occasionally, shock veins are visible (Fig. 3), although the bulk rock is very weakly shocked and is classified as shock grade S2 (Stöffler et al. 1991). The Olivines show undulatory extinction and irregular fractures but no planar deformation. The multiple degrees of shock effects indicate that the event responsible for the formation of shock veins is not responsible for the fragmentation of the meteorite components. Therefore, we conclude that the process of fragmentation must have occurred before accumulation of the material on the parent body from which the Devgaon meteoroid is derived. In thin section, many type...

Cosmic-Ray Exposure Ages of Meteorites

“…The CRE age distribution for all H chondrites spans an apparent range from less than 1 to $100 Ma (Figure 2 and see Murty et al, 2004). For stones at the low end of the distribution, the possibility of complex irradiation looms large, and indeed, that possibility has been confirmed in several cases (Herzog et al, 1997).…”

Spectacular fall of the Kendrapara H5 chondrite

“…Using these depths of 9 and 30 cm, we have computed the production rates for 3 He and 21 Ne using the chemical composition of Kendrapara and the obtained exposure ages T 3 and T 21 for sample A (3.35 and 4.40 Ma) and for sample B (3.38 and 4.63 Ma) as given in Table 6a. Realizing the possible presence of nucleogenic 36 Ar n from a 35 Cl (n, γ) reaction and its effect on T 38 (Bogard et al 1995;Wieler et al 1996;Bhandari et al 2002;Murty et al 2004), we did not use 38 Ar c for exposure age calculation. Both samples, thus, give consistent ages, with T 3 being about 25% lower.…”

Origin of SiO2-rich components in ordinary chondrites

“…The mineralogy of SRCs is quite diverse. In decreasing order of occurrence, reported mineral assemblages are: (1) SiO 2 -pyroxene (e.g., Binns, 1967;Olsen et al, 1981;Planner, 1983;Brigham et al, 1986;Bridges et al, 1995;Kring et al, 2000;Murty et al, 2004), (2) SiO 2 -pyroxene-fayalite (Brigham et al, 1986), (3) SiO 2 -fayalite (Brigham et al, 1986;Wasson and Krot, 1994;Newton et al, 1995), (4) SiO 2 -pyroxene-feldspar (Bridges et al, 1995;Ruzicka et al, 1995), (5) SiO 2 -merrihueite/roedderite , (6) SiO 2 -K-feldspar-albite-Cl-apatite-whitlockite-ilmenite-zirconpyroxene-Na, Ti-bearing silicate (Bischoff et al, 1993), (7) SiO 2 -metal (Lauretta and Buseck, 2003; see Table 1). The discussion about the origin of SRCs is an ongoing debate since the first and so far only comprehensive paper about these objects by Brigham et al (1986).…”