DaG 298

standby for dar al gani 298 photo
Found 1997
26° 58.20′ N., 16° 42.28′ E. A mass totaling 2,459 g was found on the Dar al Gani plateau in the eastern Sarir al Qattusah in central Libya, an area that covers ~12,000 km², of which ~8,000 km² are favorable for meteorite recovery (Schlüter et al., 2002). The area of meteorite recovery lies on Tertiary limestone marine sediments.

Dar al Gani 298 has a very fresh appearance with a weathering grade of W0, and it is rich in noble gases suggesting a very recent fall. The meteorite has been shocked to stage S3, with a 21Ne-based CRE age of 52.6 m.y. In contrast to many LL chondrites with Ar–Ar ages reflecting late impact resetting, some events as recent as ~1 b.y. ago (Weirich et al., 2009), the gas retention age of DaG 298 was calculated to be 4.48 b.y. This age likely preserves its original crystallization age; a similar Ar–Ar age of 4.471 (±0.009) b.y. was measured for a sample of the LL5 chondrite Tuxtuac (Lindsay et al., 2009). The specimen of DaG 298 shown above is a 28.9 g partial slice.

It is known that the large Flora family of asteroids, as well as the compositionally linked Baptistina asteroid family, is located near the ν6 secular resonance at the inner edge of the asteroid belt (2.1 AU) and might be the source of LL chondrites (Bottke et al., 2009; Binzel et al., 2016). In a study of LL6 chondrite orbits, Ustinova et al. (2008) propose that the ~7 km-diameter asteroid 3628 Božněmcová, which has a reflectance spectrum almost identical to the LL-group, is a good match to this chondrite group. In their study they determined a probable orbit for two 2002 LL6 falls, Bensour and Kilabo, which fell within five months of each other in northern Africa and which show striking similarities to each other. Based on their measured content of cosmogenic 26Al, the calculated orbits for these meteorites coincidentally intersect that of 3628 Božněmcová at the same point near the inner asteroid belt. They proposed that Bensour and Kilabo were ejected in a common event 19 m.y. ago, and that any differences in their CRE ages reflect differences in their shielding depth while on the parent asteroid. Spectral and compositional studies of the LL5 Chelyabinsk meteorite by Reddy et al. (2014) have led them to conclude that the Baptistina asteroid family is also consistent with the Flora asteroid family, the LL chondrites, and the asteroid Itokawa. They determined that the spectral variations observed in members of the Baptistina family are consistent with incorporation of an impact melt component (associated with the presence of fine-grained opaques) similar to that observed in portions of the Chelyabinsk meteorite. standby for resonance diagram
Diagram credit: Binzel et al., 47th LPSC, #1352 (2016) In their near-IR spectrographic study of asteroids located near the 3:1 orbital resonance (a Kirkwood gap associated with Jupiter), Fieber-Beyer et al. (2009) found that close similarities exist in the absorption spectra of the S(IV) asteroid 974 Lioba and that of LL chondrites. Importantly, the recent touchdown of the Hayabusa spacecraft onto the surface of the sub-km-sized, S-type asteroid Itokawa has provided a wealth of knowledge about this most abundant type of asteroid. Itokawa’s reflectance spectrum reveals an olivine and pyroxene composition very similar to that of known LL chondrite meteorites, such as DaG 298 (Abe et al., 2006). The mean Δ17O for the mineralogy of 25143 Itokawa is the same as that for LL or L chondrites (Yurimoto et al., 2011). standby for itokawa photo The photo shown above is a view of the southern hemisphere of asteroid Itokawa taken by the Japanese Space Agency (JAXA) Hayabusa spacecraft. It shows a surface largely strewn with angular boulders, many too large to have originated from impact craters present on Itokawa (e.g., the boulder ‘Yoshinodai’ measures 50 × 30 × 20 m in size; Demura et al., 2006) (see top photo below). By contrast, the lowest topographic areas, such as the touchdown/sample collection location in Muses Sea, are smoothly surfaced with cm-sized gravel (see bottom photo below), probably produced by grain-size sorting due to seismically-driven gravitational settling (Zolensky et al., 2013). The unusual double-lobed shape of Itokawa (~540 m in largest dimension), which is characterized by a constricted neck 20 m deep and up to 120 m wide, demonstrates that it is a low-density (1.90 ±0.13 g/cm³) contact binary formed by gravitational coalescence of separate portions of an impact-disrupted parent body—its structure is a rubble-pile with a macro-porosity of ~40%. New geothermometry results for LL meteorite samples obtained by Dygert et al. (2018) show that rapid cooling/quenching (≥1°C/yr) occurred near peak temperatures, which is consistent with a catastrophic breakup event. Other temperature data indicate that LL chondrites cooled much slower at lower temperatures (≤500°C), which is consistent with conditions at some depth in a reassembled rubble-pile.

Sub-mm-sized particles of Itokawa returned by the Hayabusa spacecraft have been analyzed, and the mineralogy is consistent with that of an LL5–6 chondrite (Busemann et al., 2013). Evidence of space weathering is present in the form of nano-sized Fe and S phases. Noble gas studies based on 21Ne indicate a CRE age of 3–8 m.y., based on a sample residence either on the surface or buried half a meter deep, respectively (Nagao et al., University of Tokyo). No melting was observed on any of the returned particles, which is consistent with low impact velocities on the parent asteroid of ~5 km per second (Tsuchiyama et al., 2011), but it is estimated by Busemann et al. (2013) that the rate of loss from the asteroid is relatively high. Continued analyses of the returned particles is ongoing.

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standby for itokawa photo
Images presented courtesy of JAXA—Japan Aerospace Exploration Agency.

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Particles from asteroid Itokawa have compositions similar to LL-group ordinary chondrites that have experienced space weathering. Coarsened particles of plagioclase and diopside are indicative of a high-temperature metamorphic period (~800°C) that established a petrologic type of 5 to 6 (Nakamura et al., 2011). See also the article from PSRD ‘Samples from Asteroid Itokawa’, by G. J. Taylor and L. M. V. Martel, August, 2011.

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