SAU 290

CH3 chondrite
standby for sau 290 photo
Found February 13, 2004
21° 04′ 31.6′ N., 57° 08′ 49.3′ E. During an expedition in Adam County, Oman, Rainer and Claudia Bartoschewitz found 64 fragments of a single meteorite lying on a limestone gravel plateau all within a distance of 10 meters (Bartoschewitz et al., 2005). The total weight of the constituted meteorite was 1.796 kg. The classification of SaU 290 was initially considered to be an anomalous E chondrite microbreccia with a weathering grade of W2, but the O-isotope composition of Sayh al Uhaymir 290 plots within the CH chondrite field, distinguishing it as a member of this very rare carbonaceous group.

The microstructure and microchemistry of a variety of FeNi-metal condensate particles from CH chondrites were examined by Goldstein et al. (2005). A portion of the metal particles were found to be zoned, with cores that are enriched in Ni and Co and depleted in P, Cr, and Fe. Other particles are unzoned and homogeneous, possibly reflecting diffusive equilibration during an extended period within the hot gas, a period of longer duration than that experienced by zoned particles. Ni-rich precipitates occur in a portion of both of these condensate types. In addition, a high-Ni metal grain identified as tetrataenite has been found. A silica-rich component similar to that observed in other CH chondrites was also identified (Zhang et al., 2007). CH-chondrite components originated from both highly reducing and highly oxidizing environments, constituting more than a single reservoir (Kimura et al., 2011).

A large N gas content has been found in SaU 290, residing in at least four components (Murty et al., 2007). The heavy N component (δ15N) is present in the same abundance as in other CH chondrites. Several carrier phases for the δ15N have been identified, including C–silicate aggregates, FeNi-metal+Fe–Cr-sulfide, Si-rich metal+FeS, and in hydrated areas; however, it is believed that a single carrier, the C–silicate aggregates, was the original δ15N source prior to its redistribution into metallic phases during a quick nebular heating event (Sugiura and Zashu, 2001). In their in-depth study of Bencubbin, Perron et al. (2007) proposed that water and 15N-bearing organics were degassed from the hydrated clasts during the impact of one or more chondritic objects. These hydrated clasts were agglomerated onto the Bencubbin parent body during its initial accretionary stages. The lower abundance of δ15N located within the hydrated areas is thought by some investigators to be the product of mixing of normal N and δ15N on the parent body. The presence of a carrier phase having a low abundance of δ15N has been seen in experiments, but has not yet been identified (Murty et al., 2007).

Based on a noble gas studies at the University of Tokyo (J. Park, K. Nagao, and R. Okazaki), SaU 290 was shown to contain the highest solar noble gas abundances among CH group members. The He and Ne isotopic ratios in SaU 290 are consistent with solar values. A 21Ne-based CRE age has been established at 1.2 m.y. The average Δ17O value of –2.2 (±0.6‰) is nearly identical to the values in most CH, CB, and CH/CB magnesian cryptocrystalline chondrules, indicating their likely formation from a common reservoir resulting from a single impact event (Nakashima et al., 2011). Given the range of O-isotopic values in those cryptocrystalline chondrules having anomalous values, it is proposed that they might have formed along with the type-I porphyritic chondrules.

A complete mineralogical inventory of the refractory inclusions and Al-rich chondrules present in SaU 290 has been prepared by Zhang and Hsu (2009). The CAI content of SaU 290 reflects the low abundance (0.1 vol%) and relatively small size (ave. ~40 µm) typical for other CH chondrites, but unlike those from other carbonaceous chondrite groups. The small CAI sizes are thought to be the result of either a nebular sorting mechanism or a mechanism which controlled the growth rates. The CH group CAIs are unique in composition from those of other carbonaceous chondrite groups and exhibit a highly refractory nature experiencing higher solar nebular peak temperatures of 1386–1074°C. They consist primarily of hibonite and grossite, along with melilite, spinel, fassaite, and anorthite, and various combinations thereof. While some CAIs exhibit igneous features consistent with crystallization from melt droplets, others have a layered texture indicative of nebular gas–solid condensation. An ultrarefractory inclusion was identified in SaU 290 by Zhang et al. (2015), which is composed of such ultrarefractory phases as panguite, davisite, and Sc-rich anosovite, together with the more typical refractory phases spinel, anorthite, and perovskite. This ultrarefractory CAI is depleted in 16O, which attests to its formation in the outer region of the protoplanetary disk where isotopic self-shielding during UV photolysis of CO preferentially produces an 16O-poor gas (Krot et al., 2009).

In addition, rare amoeboid olivine aggregates (AOAs) composed of olivine, Al-diopside, and anorthite have been identified in CH chondrites (Krot et al., 2014). These are texturally and mineralogically similar to AOAs in other carbonaceous chondrite groups that were formed as nebular condensates during the earliest stages of Solar System history. A suite of Al-rich chondrules compositionally unique to those of other carbonaceous chondrite groups have also been documented. These are likely products of melted precursor material derived from both CAIs and ferromagnesian chondrules. The study presented evidence for low degrees of both thermal metamorphism and aqueous alteration on the CH parent body, leaving the inclusions in a pristine state and reflecting the nebular conditions in which they were formed.

Further information about the CH group can be found on the Acfer 214 page. The specimen of SaU 290 pictured above is a 2.88 g partial end section with an unpolished cut face. The photo below shows the ~200 g fragment from which the above piece was cut.

standby for sau 290 photo
Photo courtesy of S. Turecki

For additional information on CH chondrite petrogenesis, read the PSRD article by G. Jeffrey Taylor: ‘The Oldest Metal in the Solar System‘, Sep 2000.

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