BORISKINO


CM2
standby for boriskino photo
Fell April 20, 1930
54° 14' N., 52° 29' E.

At 1:30 P.M., a carbonaceous chondrite fell in the Federated SSR, USSR. Only two stones with a combined weight of 1,165.6 g were recovered, while three other small stones were reportedly destroyed. This CM chondrite is unusual in exhibiting a brecciated texture over a large field of view. Olivine and pyroxene in Boriskino are essentially Fe-free, occurring as forsterite and enstatite. According to Ohnishi and Kazushige (2003), enstatite has been aqueously altered to serpentine by relatively low pH fluids that are enriched in Fe and depleted in Na and Si (smectite is produced at higher pH values and higher Na contents as in the CV group). Iron is present mainly within the serpentine-like phyllosilicates, with a minor amount present in magnetite. It is thought that magnetite was produced by the oxidation of FeS during low-temperature aqueous alteration processes, although some could have a nebular origin (Hyman and Rowe, 1983). Since Boriskino has one of the lowest magnetite contents of any CM chondrite, it probably experienced a relatively moderate degree of parent body alteration.

Boriskino contains several FeNi-sulfide/phosphide phases, some of them unknown before. A significant proportion of these phases are P-rich, but they can also contain K and Cr, each of these condensing in a non-typical chalcophile manner. These phases are thought to have formed by the sulfidation of kamacite in the nebula under reducing conditions at temperatures below 427°C, or alternatively, during sulfidation of presolar FeNi-carbide grains (Nazarov et al., 1997, 1999). A secondary mineral assemblage consisting of mackinawite (FeS), carbonate, magnetite, and FeNi-metal is associated with low-temperature aqueous alteration processes on the CM parent body (Boctor et al., 2002, 2004).

Similar to other CM chondrites, a presolar nanodiamond phase is present in Boriskino that contains the noble gases Xe and Ar, which were implanted at various energies corresponding to different interstellar events. A new noble gas component with intermediate characteristics has recently been identified in nanodiamond fractions from Boriskino (Fisenko et al., 2002).

Current studies suggest that both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets which originate in the Kuiper belt. Studies have shown that Antarctic micrometeorites have a similar carbonaceous chondrite:ordinary chondrite ratio ((~7:1) as the composition of zodiacal dust (M.M.M. Meier, 2014). Based on observational evidence and current modeling, it is thought that comets should be dark in color and have a low density and strength, a high porosity, a solar ratio of elements, an elevated ratio of C, H, O, and N, a high interstellar grain content, anhydrous and highly unequilibrated silicates, few to no chondrules, and a low cosmic-ray exposure age (<10 m.y.). Both the CI and CM groups of meteorites exhibit characteristics which are consistent with the above descriptions.

Orbital data obtained from several carbonaceous chondrites (e.g., CI Orgueil [eyewitness plotting]; CMs Maribo and Sutter's Mill [instrument recording]) are a good match to the orbits expected from the disruption of Jupiter-family comets, but are unlike the orbits of ordinary chondrites and most other asteroidal objects (M.M.M. Meier, 2014). Both the orbital eccentricity and semimajor axis for Maribo is nearly identical to those of Comet Encke and the associated Taurid swarm of objects (Haack et al., 2011). On the other hand, a CRE age study of CM chondrites conducted by Meier et al. (2016) shows a possible relationship exists to the asteroid breakup event ~8.3 m.y. ago that formed the Ch/C/Cg-type members of the Veritas family. In addition to the large abundance of 3He-enriched interplanetary dust discovered in 8.2 m.y.-old deep-sea drill cores, ~1/6 of all CM meteorites have 21Ne-based CRE ages that are consistent with derivation from this catastrophic breakup, while others with significantly younger CRE ages could represent secondary collisions among the Veritas fragments.

The main mass of Boroskino is curated at the Academy of Sciences in Moscow, while no more than 3.8 g is kept at other institutions (Macke et al., 2011). The photo above shows the interior of a 0.146 g fragment of Boriskino.