KRYMKA


LL3.2
standby for krymka photo
standby for krymka photo
Fell January 21, 1946
47° 50' N., 30° 46' E.

Following a shower of stones that fell over the Nicholayev region of the Ukraine, USSR, about 50 kg of stones were collected; they turned out to be one of the most primitive ordinary chondrites ever discovered. This is a highly unequilibrated chondrite, poor in both metallic and oxidized Fe. Due to the widely varying results obtained when utilizing the ratio of metallic to total Fe for distinguishing between an L3 and LL3 classification, less ambiguous criteria were employed (total Fe/Mg and Ni/Mg ratios, and Fe–S relationships) to establish a classification for Krymka of LL3. Early analyses of Krymka based on texture, TL sensitivity, highly volatile element content, and silicate heterogeneity, were consistent with a sub-classification of LL3.0. However, subsequent studies led to its re-classification as LL3.1, consistent with a very mild metamorphic history and retention of primary features.

Most recently, a new decimal scheme which is more discriminating at the lowest petrologic types associated with highly unequlibrated chondrites (3.0–3.1–3.2) was proposed by J. Grossman (2004), and J. Grossman and A. Brearley (2005). The new classification scheme, based on a sensitive analytical technique utilizing the variation in the distribution of Cr in ferroan olivine, is virtually unaffected by the processes of terrestrial weathering and aqueous alteration. The scale of the new decimal system has been extended as follows:

3.00–3.05–3.10–3.15–3.2

They have identified several additional parameters, which, when used in combination, are instrumental in determining an accurate classification at the lowest petrologic grades:

At the onset of thermal metamorphism, 1) Cr is exsolved from ferroan olivine forming fine Cr-rich precipitates, which, with progressive metamorphism, become coarser within the olivine cores and form rims on the olivine surfaces; 2) very fine-grained FeS grains in chondrule rims and in fine-grained matrix become coarser, and secondary sulfides form within chondrules; 3) Fe and Mg in olivine are homogenized and metal grains are equilibrated; 4) abundances of presolar grains are diminished; 5) Na and other alkalis are initially lost from the matrix and enter type-I chondrules, causing zonation, only to reverse direction with progressive metamorphism; 6) albite crystallizes from type-II chondrules causing blue CL and increased TL sensitivity.

In subsequent studies of chromite zoning profiles along with the chromite content of individual ferroan olivine grains, Grossman (2008) was able to further resolve the petrologic type for chondrites at the lowest metamorphic stages. These two petrographic features provide a reference for a sequencial history of increasing thermal metamorphism that is consistent among olivine grains within each meteorite. For metamorphic types 3.00–3.03, chromite zoning profiles are smooth and correlate with igneous FeO zoning profiles. In addition, at this lowest metamorphic stage chromite contents account for 0.3–0.5 wt% in the chondrite groups studied. While chromite contents in type 3.05–3.10 chondrites still reflect the lowest degrees of metamorphism, chromite now exhibits igneous zoning profiles which are no longer smooth. Upon reaching a degree of metamorphism equivalent to type 3.15, chromite zoning has diminished considerably, and chromite abundance is now only 0.1–0.2 wt%. With metamorphic types of at least 3.2, no zoning is observed and chromite abundance is mostly less than 0.1 wt%.

Based on details of this new scheme, Krymka was found to have a low Cr content in olivine, a diminished abundance of presolar grains with no Xe–P3 noble gas release, and other characteristics most consistent with a petrologic type 3.2. In a study by Bonal et al. (2006), utilizing Raman spectroscopy and other petrologic tracers (i.e., noble gas content, presolar grain abundance, and zoning of olivine phenocrysts), all results supported a petrologic type of 3.2 for Krymka.

An intense shock event corresponding to pressures of at least 30 GPa is recorded in mm- to cm-sized melt pockets in Krymka, reflecting closed system behavior of an in situ melting event. Temperatures reached ~1500°C in the melt zones producing igneous textures, which was followed by a high cooling rate, greater than 100°C/hour, resulting in non-equilibrium crystallization (Semenenko and Perron, 2005). Metal–troilite intergrowths were formed, with chromite and Fe–Na phosphate glass crystallizing within troilite. Following crystallization, a less intense shock caused mineral deformation features. In a separate study of two LL chondrites, NWA 1701 and LAR 06298, Weirich et al. (2009) determined an Ar–Ar age of ~1 b.y., possibly reflecting the last major impact on the LL chondrite parent body.

Fayalitic olivine of heterogeneous composition is the most common matrix component. It occurs in several morphologies, and can be found as fine-grained rims on chondrules and lithic clasts. It is thought to represent a primary condensate derived from the vaporization and recondensation of olivine-rich dust in an oxidizing nebula (Weisberg et al., 1997). This fayalitic olivine was then accreted and incorporated into the Krymka parent body, and thereafter it experienced repeated episodes of lithification and fragmentation. The clastic matrix of Krymka comprises many diverse mineral and lithic components, including various silicates, oxides, and metals. In addition, xenoliths of Kakangari-like chondrite material have been observed (Funk et al., 2011). Metzler and Chaussidon (2014) identified a lithic clast in the Krymka meteorite that consists of an assemblage of plastically deformed chondrules; this type of clast was termed a "cluster chondrite clast". Utilizing Al systematics, the crystallization age of several chondrules from this clast was determined to be 0.44 (±0.18) m.y. after CAI formation (one chondrule might be slightly older still). It was conjectured that this data represents the earliest known crystallization of chondrules in solar system history, and could mark the earliest stage of accretion of this chondrite parent body.

Metal occurs in chondrule interiors and rims, as well as in the matrix. The metal in chondrule interiors and matrix is mostly spheroidal and Cr-rich, and is located between olivine grains. Large (0.2–1.0 mm), rimmed (an inner silicate and an outer sulfide layer), metal-sulfide nodules occur in the matrix. In those cases where the metal nodules contain Si and Cr, the associated sulfide contains inclusions of Si, Cr, and Ca. In addition, sulfide–metal–phosphate assemblages incorporating merrillite and chlorapatite are present. These phosphates were formed in a multi-stage process, initially involving the exsolution of schreibersite from FeNi-metal. This was followed by sulfidation of schreibersite by nebular hydrogen-sulfide gas to produce sulfide–metal–schreibersite assemblages, and finally, oxidation and reaction with Ca and Cl derived from matrix silicates. These metal nodules appear to be primary nebular condensates, with a Co in kamacite content that plots within the LL group range.

Notably, porous, dusty spherules, which are primarily composed of fine-grained, cryptocrystalline silicate dust, and which are rimmed by sub-µm- to µm-sized sulfide and metal grains, were identified within dark lithic fragments in Krymka (Semenenko and Girich, 1999). These porous spherules are thought to represent chondrule precursors that formed by direct accretion of fine-grained nebular dust onto coarse-grained nuclei. A large abundance of tiny CAI components were also found within a dark lithic fragment (Semenenko et al., 2001).

In another study, a rare olivine microchondrule-bearing clast was identified within a lithic fragment in Krymka (Rubin,1989). It is composed of 0.003–0.031 mm-sized or less, BO and G chondrules, embedded within a recrystallized silicate matrix incorporating tiny sub-µm-sized FeNi grains. The microchondrules, which are zoned in FeO (increasing from core to rim), are found among Mg-rich constituents within the clast, indicating that oxidation occurred before accretion.

Additional exotic components were accreted during the formation of the Krymka host, including rare black carbonaceous xenoliths, commonly referred to as "mysterite". This material is enriched in volatile siderophiles such as Ag, Tl, Zn, and Bi, and represents a late condensate from a metal-depleted region of the solar nebula, possibly related to cometary material (Campins and Swindle, 1998). These xenoliths contain abundant troilite in the matrix and they lack evidence of relict chondrules. They also contain graphite microcrystals, considered to be a metamorphic product from organic compounds, which are distributed between silicate grains (Semenenko, 1996). The metamorphic temperature is constrained at ~300°C and ~500°C (Weber et al., 2006). Evidence of shock melting is manifest in the alteration effects of the rim and matrix.

Another exotic component is the fine-grained, graphite-bearing fragments that may share a common organic-based precursor with the black carbonaceous xenoliths. These graphite-bearing fragments are friable, fine-grained objects with recrystallized textures, containing carbon in the form of organic compounds, C-rich material, and graphite (Semenenko et al., 2005). The morphologically unique graphite crystals in these xenolithic fragments were formed either by shock metamorphism of C-bearing material on the Krymka parent body, or by metamorphism on the primary carbonaceous parent body (Semenenko et al., 2004, 2005). Like the carbonaceous xenoliths, these fragments represent previously unknown unequilibrated carbonaceous material which contains a high abundance of troilite within the silicate mesostasis, and which contains a low abundance of relict chondrules. Interestingly, these exotic fragments have O-isotope ratios and other chemical and isotopic values that are similar to the graphite in CR chondrites.

Other noteworthy xenolithic material has been documented from Krymka. Small, pre-accretionary, fine-grained, C-rich clasts are scattered throughout this meteorite, possibly having a distant relationship to the graphite-bearing and carbonaceous inclusions. Furthermore, some C-rich material has been mobilized by shock to form irregular areas and veins that penetrate into cracks and boundaries of silicate and feldspathic grains. A zoned macrochondrule was studied that consists of two metal–troilite mantles with irregular graphite grains (crystallized during igneous processes), along with a fine-grained rim containing regular graphite crystals (formed by shock metamorphism) (Semenenko and Girich, 2011). This macrochondrule was likely formed by flash melting of a huge silicate precursor dust aggregate containing metal–troilite and organic compounds.

Most recently, dark, porous clasts containing magnetite spherules and framboids have been identified (Girich and Semenenko, 2001). These clasts exhibit chemical and mineralogical similarities to the CI chondrites, and may represent precursor material from that carbonaceous group. Another fine-grained, dark xenolith that was identified contains rare chondrules and FeNi-metal, and has a composition and a SiO/MgO ratio which suggests that it may be a primitive precursor of H chondrite material (Semenenko and Girich, 2005).

Also present are presolar nanodiamonds that contain a new high-temperature Xe phase. Xenon isotope data suggest that Krymka was irradiated within a regolith on a large parent body at a significant shielding depth. Corundum grains constituting ~5 ppm have also been identified within the matrix (Strebel et al., 2000); however, none of them exhibit an anomalous presolar signature. On the other hand, one presolar spinel grain identified in a combined residue of Krymka, Semarkona, and Bishunpur has anomalous isotopic compositions of Mg, O, and Al—all consistent with an origin in an intermediate-mass AGB star of ~5 Msun (Lugaro et al., 2007).

In a study by Tomomura et al. (2004), the Si/Mg-derived formation ages of ferromagnesian chondrules from Krymka were measured. The results indicate that chondrules were formed continuously from 1.4 to 2.6 m.y. after CAI formation, with a peak at 1.9–2.0 m.y. Two views of a 4.5 g slice of Krymka are shown above—photography is courtesy of S. Vasiliev.