Mineralogy of fine‐grained material in the Krymka (LL3.1) chondrite

Abstract

Abstract-Two dark lithic fragments and matrix of the Krymka LL3.1 chondrite were mineralogically and chemically studied in detail. These objects are characterised by the following chemical and mineralogical characteristics, which distinguish them from the host chondrite Krymka: (1) bulk chemical analyses revealed low totals (systematically lower than 94 wt%) due to high porosity; (2) enrichment in FeO and depletion in S, MgO and Si02 due to a high abundance of Fe-rich silicates and low sulfide abundance; (3) fine-grained, almost chondrule-free texture with predominance of a porous, cryptocrystalline groundmass and fine grains; (4) occurrence of a small amount of oncemolten material (microchondrules) enclosed in fine-grained materials; ( 5 ) occurrence of accretionary features, especially unique accretionary spherules; (6) high abundance of small calcium-aluminiumrich inclusions (CAIs) in one of the fine-grained fragments. It is suggested that the abundance of CAIs in this fragment is one of the highest ever found in an ordinary chondrite.Accretionary, fine-grained spherules within one of the fragments bear fundamental information about the initial stages of accretion as well as on the evolution of the clast, its incorporation, and history within the bulk rock of Krymka. The differences in porosity, bulk composition, and mineralogy of cores and rims of the fine-grained spherulitic objects allow us to speculate on the following processes: (1) Low velocity accretion of tiny silicate grains onto the surface of coarse metal or silicate grains in a dusty region of the nebula is the beginning of the formation of accretionary, porous (fluffy) silicate spherules. (2) Within a dusty environment with decreasing silicate/(metal + sulfide) ratio the porous spherules collected abundant metal and sulfide particles together with silicate dust, which formed an accretionary rim. Variations of the silicate/(sulfide + metal) ratio in the dusty nebular environment result in the formation of multi-layered rims on the surface of the silicate-rich spherules. (3) Soft accretion and lithification of rimmed, fluffy spherules, fine-grained, silicate-rich dust, metal-sulfide particles, CAIs, silicate-rich microchondrules, and coarse silicate grains and fragments followed. (4) After low-temperature processing of the primary, accretionary rock collisional fragmentation occurred, the fragments were subsequently coated by fine-grained material, which was highly oxidized and depleted in sulfides. ( 5 ) In a final stage this accretionary "dusty" rock was incorporated as a fragment within the Krymka host.

Microdistribution of primordial Ne and Ar in fine‐grained rims, matrices, and dark inclusions of unequilibrated chondrites—Clues on nebular processes

“…Compared to the DI of Lewis et al (1979), ours has probably experienced a slightly higher degree of thermal or hydrothermal alteration lowering the 20 Ne prim concentrations in particular and, thus, increasing the ( 36 Ar/ 20 Ne) prim ratio. Low temperature processing of our DI before its incorporation to the Krymka parent body was also concluded by Semenenko et al (2001). Despite slightly different alteration states, the extraordinarily high gas concentrations in both DIs argue for a common origin.…”

“…Most formation scenarios for DIs include the accretion of nebular dust to small or larger precursor bodies. Subsequently, these bodies are destroyed or single fragments of them are released by impacts to the nebular reservoir, from where the final accretion of the mmto cm-sized objects to the present-day host meteorites took place (Bischoff 1989;Bischoff et al 1988;Grady et al 1999;Johnson et al 1990;Kurat et al 1989;Semenenko et al 2001). The degrees of precursor body, nebular, and parent body alteration of the DIs are highly variable in these scenarios (e.g., Buchanan et al 1996;Kojima and Tomeoka 1997;Tomeoka and Kojima 1998).…”

“…Based on mineralogical observations, Semenenko et al (2001) describe in detail the evolution of the Krymka dark inclusion "fragment BK13," which is from the same DI as the Krymka DI studied here: dust particles first accreted to small porous spherules and then to fine-grained "accretionary rocks." After mild thermal metamorphism, these were destroyed by impacts, and the fragments were transported into the nebular formation region of Krymka where they were admixed to the Krymka host meteorite.…”

“…The Ne and Ar isotopic and elemental noble gas compositions for Q and HL gases, as well as for other reservoirs relevant here, are given in Table 1. Based on textural criteria, matrix-like material can be further subdivided (e.g., Brearley and Jones 1998;Metzler et al 1992;Scott et al 1988;Semenenko et al 2001;Weisberg and Prinz 1998). Aside from its most common occurrence as an opaque groundmass (termed "matrix" in the following), fine-grained concentric rims ("rims") around larger objects such as chondrules and inclusions are common (e.g., MacPherson et al 1985;Metzler and Bischoff 1989;Metzler et al 1992).…”

Reclassification of Villalbeto de la Peña—Occurrence of a winonaite‐related fragment in a hydrothermally metamorphosed polymict L‐chondritic breccia

“…) and exotic fragments in Dhajala (H3) and Krymka (LL3) were described as foreign fine‐grained, accretionary objects (Semenenko et al. ; Funk et al. ).…”