After cutting a field of grass located 36 km southeast of the settlement of Divnoe, in the Stavropol region of Russia, a single mass of 12.7 kg was found. The meteorite had a rusty brown fusion crust indicating a significant terrestrial residence time. Divnoe is an olivine-rich (~70 vol%) achondrite with subchondritic chemistry and mineralogy. It contains an opaque-rich, fine-grained lithology (ORL) along with patches of pyroxene and plagioclase (PP), within a coarse-grained olivine groundmass (CGL). Veins of troilite and rare metal occur throughout.
Formation of this meteorite began as a chondritic body (trapped xenon isotopic patterns are the same as those of ordinary chondrites) that experienced 20 wt% partial melting at 1300°C (Petaev et al., 1994). The measured HSE abundances in Divnoe are consistent with a partially melted parent body in which heating from short-lived radionuclides came to a halt before a core was fully formed. Terrestrial contamination makes an accurate KAr gas retention age difficult to determine. After crystallization of 60 wt% of the partial melt, the remaining 40 wt% Na and K-rich liquid portion of the melt was segregated. The CGL and metal components are consistent with the residue after extraction of the melt, while the PP component represents a partial melt phase that was trapped and crystallized within the rock. The ORL component was formed late in the partial melt phase by reaction between sulfur vapor and residual olivine. All of this material experienced extensive recrystallization during slow cooling from 1000°C to 500°C, after which a secondary reheating event increased the temperature to 700°C, perhaps as a result of impact ejection from the parent body 17.2 m.y. ago. This was followed by low-temperature annealing, which erased most of the shock features and produced the unique olivine lamellar structure.
An alternative explanation for the observed crystallographic preferred orientation (CPO) of Divnoe olivine grains was proposed by Hasegawa et al. (2014, 2015), who argued that this fabric texture was the result of crystal accumulation at the bottom of a convecting magma chamber. However, in recognition of the mutually consistent description of Divnoe as a residue of partial melting, they concluded that the rock must have experienced a complex petrogenetic history probably involving melt flow. Notably, the brachinite-like achondrite NWA 6112 and brachinite EET 99407 have a similar O-isotopic compositions and CPO patterns as Divnoe.
Divnoe is similar to the brachinite group in chemical composition and in oxygen and xenon isotopic ratios. Similarities also exist in O-isotopic and bulk chemical composition between Divnoe and the HED suite (particularly diogenites), although some major differences exclude a common origin. On the other hand, the bulk chemical composition of Divnoe and a related anomalous achondrite, RBT 04239, matches that of the brachinites Brachina and ALH 84025 very closely, consistent with a derivation from a common precursor (Weigel et al.,1996). Moreover, primordial trapped noble gases indicate that both similarities and differences exist between Divnoe and the brachinites, while at the same time revealing a ~100 m.y. difference in their crystallization ages. The paired ungrouped Antarctic meteorites RBT 04255 and RBT 04239 also show some similarities to Divnoe.
On a newly compiled O-isotope diagram for brachinites and other planetary achondrites, published by Rumble III et al. (2008), Divnoe has a Δ17O value that plots with Brachina, and these investigators believe that Divnoe should probably be lumped with the brachinites. However, through their studies of highly siderophile element (HSE) abundances, and upon examinination of the metal-sulfide segregation processes, Day et al. (2012) determined that Divnoe and similar brachinite-like achondrites were not likely genetically related (i.e. from the same parent body) to brachinites, but instead, they argued that these meteorites originated on similar volatile-rich, oxidized, chondritic precursor asteroids that experienced similar petrologic processes during their formation history. Goodrich et al. (2017) determined that brachinites and brachinite-like achondrites have a distinct redox trend and a higher Fe/Mg ratio compared to all other primitive achondrites, consistent with formation in a similar nebula reservoir, and they suggest that brachinites and brachinite-like achondrites be called the brachinite clan.
Evidence for differences in redox conditions between brachintes and brachinite-like achondrites during formation is demonstrated by Crossley et al. (2018) on a coupled Δ17O vs. Fe/Mg diagram. It is apparent that Divnoe and other brachinite-like achondrites contain olivine that is less ferroan than that in brachinites, which supports the contention that they derive from distinct parent bodies (see diagram below).
Diagram credit: Crossley et al., 49th LPSC, #2540 (2018)
Recent advanced spectrographic techniques were applied to a set of 1,478 meteorite spectra and to members of the Eos family of asteroids, traditionally considered to be a good match to the CO/CV chondrites (Mothé-Diniz and Carvano, 2005). It was concluded that Divnoe was actually a much better, and very close spectral match to these asteroids, especially as compared to asteroids 221 Eos and 653 Berenike (see diagram below). Although the CRE age, or transport time, of the Divnoe meteorite to Earth of 17.2 m.y. is considerably less than that predicted for transport from the 9:4 resonance near the Eos family region (50 m.y. minimum; di Martino et al., 1997), a collisional cascade process could explain the discrepency. In support of this theory, the calculated inefficiency of the delivery process from the Eos region to Earth (~2%) might be considered commensurate with the rarity of the brachinites and brachinite-like meteorites in our collections. Furthermore, the breakup of the partially differentiated parent body of Divnoe would be expected to produce a highly diverse group of fragment lithologies, and this is exactly what is observed throughout the Eos family. The specimen of Divnoe shown above is a 0.19 g thin cut fragment.
Diagram credit: M. M. M. Meier et al., Earth and Planetary Science Letters, vol. 490 (2018)
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