Thermal and fragmentation history of ureilitic asteroids: Insights from the Almahata Sitta fall

Abstract

Abstract-The Almahata Sitta fall event provides a unique opportunity to gain insight into the nature of ureilitic objects in space and the delivery of ureilite meteorites to Earth. From thermal events recorded in the mineralogy, petrology, and chemistry of ureilites recovered from the fall area, we reconstruct a timeline of events that led to their genesis. This history is similar to that of other known ureilites and supportive of a disrupted ureilite parent body hypothesis. Temperatures of final mantle equilibrium were 1200-1300°C, but this hightemperature history was abruptly terminated by rapid cooling and reduction associated with pressure loss. The onset of late reduction reactions and onset of rapid cooling must have been essentially simultaneous, most likely engendered by the same event. Cooling rates of 0.05-2°C h )1 determined from reversely zoned olivines and pyroxenes in Almahata Sitta imply rapid disassembly into fragments tens meters in size or smaller. This phenomenon seems to have affected all known portions of the ureilite parent body mantle, implying an event of global significance rather than localized unroofing. Reaccretion of one or more daughter asteroids occurred only after significant heat loss at minimum time scales of weeks to months, during which time the debris cloud surrounding the disrupted parent was inefficient at retaining heat. Fragments initially dislodged from the ureilite parent body mantle underwent subsequent size reduction and mixed with various chondritic bodies, giving rise to polylithologic aggregate objects such as asteroid 2008 TC 3 .

Stable isotopes and the noncarbonaceous derivation of ureilites, in common with nearly all differentiated planetary materials

“…Thus, 21 (or slightly fewer) separate chondritic clasts are exclusively noncarbonaceous, albeit a wide variety of lithologies are represented. If, as suggested by Bischoff et al (2010), this mixing occurred early, during the reaccretion of the ureilites into a second-generation parent body after the catastrophic disruption of the original ureilite body (e.g., Warren and Kallemeyn, 1992;Goodrich et al, 2004;Herrin et al, 2010), then the acquisition of diverse yet consistently noncarbonaceous chondritic materials may be another indication that the original body was within a portion of the solar system dominated by noncarbonaceous materials.…”

“…A highly localized, disequilibrium, abortive form of impact "smelting" undoubtedly occurred to varying degrees in all ureilites, usually localized in olivine rims; in rare cases concentrated in pyroxenes (Warren and Rubin, 2010). But this "smelting" occurred in conjunction with an impact disruption of the parent asteroid that caused sudden pressure reduction and rapid cooling, which in turn terminated the anatexis Herrin et al, 2010). The stable-isotopic evidence discussed above militates against a carbonaceous-chondritic precursor material, and among the known noncarbonaceous chondrites, only the R chondrites have the high FeO consistent with the anatectic smelting model.…”

“…In contrast, the mafic-silicate cores in any individual monomict ureilite are remarkably uniform in composition. Nearly all ureilites exhibit a remarkably step-wise cooling history: slow at first, but very rapid later (Miyamoto et al, 1985;Takeda et al, 1989;Herrin et al, 2010).…”

In situ observation, X–ray diffraction and Raman analyses of carbon minerals in ureilites: Origin and formation mechanisms of diamond in ureilites

“…Mineralogical studies of ureilites (Takeda, 1989;Weber et al, 2003;Goodrich et al, 2004;Herrin et al, 2010;Mikouchi et al, 2010) show that ureilites undergo a two-stage cooling history: slow cooling at temperatures above 1200-1250°C, followed by very rapid cooling at rates possibly as high as 20°C h −1…”

Mineralogy and crystallography of some Itokawa particles returned by the Hayabusa asteroidal sample return mission

“…For example, Almahata Sitta meteorite, which fell in October 2008 in Sudan, is a polymict ureilite consisting of centimeter-to decimeter-sized fragments of different types of ureilites and various chondrites (Jenniskens et al 2009;Zolensky et al 2010;Bischoff et al 2010). The detailed studies of Almahata Sitta lead to a scenario involving break-up of a large ureilite parent body whose original diameter was larger than 200 km and reaccumulation into smaller daughter bodies (e.g., Bischoff et al 2010;Herrin et al 2010;Mikouchi et al 2010;Zolensky et al 2010;Goodrich et al 2014;Horstmann and Bischoff 2014). The parent asteroid of Almahata Sitta, asteroid 2008 TC3, is considered to have been a smaller rubble-pile block of these daughter bodies in which fragments were loosely packed, as evidenced by the fact that no recovered stones consisted of more than one lithology.…”