DIVNOE

After cutting the grass in a field, which was located 36 km southeast of the settlement of Divnoe, in the Stavropol region of Russia, a single mass of 12.7 kg was found. It had a rusty brown fusion crust indicating a significant terrestrial residence. Divnoe is an olivine-rich primitive 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). Terrestrial contamination makes an accurate K–Ar gas retention age difficult to determine. Crystallization of 60 wt% of the partial melt was followed by the removal of the remaining 40 wt% Na and K-rich liquid portion of the melt. The CGL component, as well as the metal component, represents the residue of the melt. 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 was extensively recrystallized during slow cooling from 1000°C to 500°C with a secondary reheating event to 700°C, perhaps caused by separation from the parent body 17.2 m.y. ago. This was followed by low-temperature annealing, which erased all shock features and produced the unique olivine lamellar structure.

Divnoe is similar to the brachinites in chemical composition and oxygen and xenon isotopic ratios. Similarities also exist in oxygen-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 both similarities and differences exist between Divnoe and the brachinites, while revealing a discrepency in their crystallization age of ~100 m.y.

On a newly compiled O-isotope diagram for brachinites and other planetary achondrites published by D. 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 studies of highly siderophile element (HSE) abundances, and upon examining the metal-sulfide segregation processes, it was determined by Day et al. (2012) that Divnoe and similar brachinite-like achondrites were not likely genetically related (i.e. from the same parent body) to brachinites, but rather, originated on similar volatile-rich, oxidized, chondritic precursor asteroids while experiencing similar petrologic processes during their formation history. The measured HSE abundances 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. The paired ungrouped Antarctic meteorites RBT 04255 and RBT 04239 also show some similarities to Divnoe. The specimen of Divnoe shown above is a 0.19 g thin cut fragment.

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