Correlated petrologic and geochemical characteristics of CO3 chondrites

Abstract

Abstract-Many properties of C03 chondrites have been shown previously to have resulted from thermal metamorphism; petrologic subtypes 3.0-3.7 have been assigned to members of the group. Additional properties that correlate with the metamorphic sequence but seem to have resulted from hydrothermal alteration include the modal abundance of amoeboid olivine inclusions (AOI), chondrule size, the types of refractory inclusions and whole rock 0 isotopic composition. The percentage of rimmed A01 increases with petrologic subtype. The rims most likely formed during hydrothermal alteration. The previously reported correlation between A01 abundance and chondrite subtype is probably an artifact due to the difficulty in recognizing small unrimmed A01 in the least metamorphosed C03 chondrites. Because large (2200 pm size) porphyritic chondrules have nearly the same mean size in all C03 chondrites, it seems likely that the correlation between chondrule size and subtype is due to alteration of the smallest chondrules to the point of unrecogizability as complete objects in the more metamorphosed C03 chondrites. The previously reported decrease in the proportion of melilite-rich refractory inclusions with increasing petrologic subtype may have resulted from more extensive hydrothermal alteration in C03.4-3.7 chondrites that converted primary melilite into Ca-pyroxene, andradite and nepheline. Alteration probably caused the preferential occurrence of 160-poor oxygen isotopes in the more metamorphosed whole rock samples.

Amino acid compositions in heated carbonaceous chondrites and their compound-specific nitrogen isotopic ratios

“…It also contains evidence of a minor extent of shock metamorphism, which indicates that the asteroid has been exposed to elevated temperatures during the accretion process (Scott et al 1992). Studies of geochemical features such as FeO enrichment in olivine equilibration of ALHA77003 have revealed a peak metamorphic temperature of about 500°C (Rubin 1998), which agrees with spectroscopic studies of the chondritic IOM (Kebukawa et al 2011). The high peak metamorphic temperature leads to the subtype 3.5 given to this meteorite.…”

“…Contrarily, CO parent body was exposed to a prolonged heating during or post accretion with heat sources derived from in situ decay of radionuclides and hydration reactions. The duration of the thermal alteration is a function of the size of the asteroid body (Fujiya et al 2013) but can generally last 10 6 -10 8 years (Rubin 1998). While half-lives for gas-phase thermal decarboxylation reactions of amino acids are on the order of 1-100 years at 500°C, the reactions will take 10 6 -10 9 years at 300°C (see Burton et al 2012a and references therein).…”

Northwest Africa 1232—a CO3 carbonaceous chondrite with two lithologies

“…The mineralogy of the chondrules, CAIs, AOIs, and matrix are also consistent with this classification (e.g., Scott and Jones, 1990;Kojima et al, 1995;Rubin, 1998;Tomeoka and Itoh, 2004). However, lithology A and lithology B show minor, but significant differences in mineralogy.…”

Petrography of refractory inclusions in CM2.6 QUE 97990 and the origin of melilite‐free spinel inclusions in CM chondrites

“…Under these conditions, melilite altered directly to phyllosilicate. The CM-chondrite style of alteration is different than that undergone by CO chondrites (which experienced fluid-assisted thermal metamorphism; e.g., McSween 1977; Scott and Jones 1990;Rubin 1998;Brearley 2006) or CV chondrites (which experienced oxidation and aqueous metasomatism; e.g., Krot et al 1995Krot et al , 1998aKrot et al , 1998bKrot et al , 2000Brearley 1999). …”

“…In those CO3 chondrites (e.g., CO3.5 Lancé) that have experienced intermediate degrees of fluidassisted metamorphism or hydrothermal-alteration/thermalmetamorphism (e.g., McSween 1977;Rubin 1998), melilite is replaced by feldspathoids, diopside, and sulfide (Russell et al 1998). In a survey of refractory inclusions in the Moss CO3.5/ 3.6 chondrite fall, Bischoff and Schmale (2007) found only a single (altered) melilite-bearing inclusion.…”