Impact plume‐formed and protoplanetary disk high‐temperature components in CB and CH metal‐rich carbonaceous chondrites

Abstract

AbstractWe report on the mineralogy, petrology, and oxygen isotopic compositions of ferroan olivine–pyroxene‐normative cryptocrystalline chondrules (Fe‐CCs) in CH chondrites and discuss their origin and the origin of other components in the genetically related CH and CB chondrites. There are two kinds of Fe‐CCs: (1) compositionally uniform (Fe/[Fe+Mg] = 0.17–0.34) chondrules with euhedral Fe, Ni‐metal grains and (2) metal‐free chemically zoned (Fe/[Fe+Mg] = 0.05–0.4) chondrules surrounded by ferroan olivine (Fa44−62) rims; the Fe/(Fe+Mg) ratio increases toward the rims. Both types contain low CaO and Al2O3 (<0.04 wt%), but relatively high contents of MnO and Cr2O3 (up to 1 wt%). Compositionally uniform euhedral Fe, Ni‐metal grains are Ni‐rich (9–20 wt%) and have subsolar Co/Ni ratio. There is a positive correlation between iron content in the metal grains and Fe/(Fe+Mg) ratio in silicate portion of their host chondrules. Some Fe‐CCs experienced postcrystallization solid‐state reduction of ferroan silicates to metallic iron. Ferroan cryptocrystalline chondrules and olivine rims have similar oxygen isotopic compositions (interchondrule Δ17O ranges from ~ −2‰ to 2‰), which are slightly 16O‐depleted relative to those of magnesian olivine–pyroxene‐normative cryptocrystalline chondrules (Mg‐CCs; Δ17O ~ −2‰) commonly observed in CBs and CHs. We suggest that the Fe‐CCs and Mg‐CCs formed in the impact plume under different redox conditions (~IW−1 and ~IW−3, respectively), which may have been controlled by heterogeneous distribution of water‐bearing phases (water ice, hydrated materials) in the collided bodies and/or in the disk. We propose the following impact plume scenario for the origin of Fe‐CCs: (1) condensation of ferromagnesian silicate melt around Fe, Ni‐metal melt droplets from a highly oxidized portion of the plume; (2) crystallization of euhedral metal grains from the supercooled ferromagnesian silicate melt followed by its solidification; (3) condensation of ferroan olivine rims around solidified Fe‐CCs; (4) high‐temperature annealing of Fe‐CCs and their rims in the plume accompanied by Fe‐Mg interdiffusion between ferroan olivine rims and their host chondrules. Subsequently, some Fe‐CC experienced solid‐state reduction to various degrees, possibly in the reduced portions of gaseous plume. The impact plume‐produced or reprocessed components in CBs and CHs include Ca,Al‐poor magnesian and ferroan cryptocrystalline chondrules; Ca,Al‐rich skeletal olivine chondrules; isotopically uniform, 26Al‐poor 16O‐depleted (Δ17O ~ −15 to −5‰) igneous CAIs surrounded by igneous forsterite rims; chemically zoned and unzoned Fe,Ni‐metal grains; and metal‐sulfide nodules. These objects are dominant in CBs and abundant in CHs. The CH chondrites also contain other high‐temperature chondritic components, which avoided processing in the plume and most likely predate the plume event: 26Al‐poor and 26Al‐rich, mostly 16O‐rich CAIs (Δ17O ~ −40 to −10‰) surrounded by Wark–Lovering rims, and porphyritic chondrules (magnesian [type I], ferroan [type II], and Al‐rich) showing a range of Δ17O (from ~ −10 to ~ +5‰). Some of these components appear to have been melted in the plume. We conclude that CH and CB chondrites contain multiple generations of chondrules and refractory inclusions formed by different mechanisms at different times and different regions of the protoplanetary disk, consistent with the hypothesis of Wasson and Kallemeyn (1990).