Secondary calcium‐iron‐rich minerals in the Bali‐like and Allende‐like oxidized CV3 chondrites and Allende dark inclusions

Self Cite

Abstract-The CV (Vigarano-type) chondrites are a petrologically diverse group of meteorites that are divided into the reduced and the Bali-like and Allende-like oxidized subgroups largely based on secondary mineralogy (Weisberg et al., 1997;Krot et al., 1998b). Some chondrules and calcium-aluminum-rich inclusions (CAIs) in the reduced CV chondrite Vigarano show alteration features similar to those in Allende: metal is oxidized to magnetite; low-Ca pyroxene, forsterite, and magnetite are rimmed and veined by ferrous olivine (Fs40-50); and plagioclase mesostases and melilite are replaced by nepheline and sodalite (Sylvester et af., Kimura and Ikeda, 1996, 1998.Our petrographic observations indicate that Vigarano also contains individual chondrules, chondrule fragments, and lithic clasts of the Bali-like oxidized CV materials. The largest lithic clast (about 1 x 2 cm in size) is composed of opaque matrix, type-I chondrules (400-2000 p m in apparent diameter) surrounded by coarse-grained and fine-grained rims, and rare CAIs. The matrix-chondrule ratio is about 1 . 1 . Opaque nodules in chondrules in the clast consist of Cr-poor and Cr-rich magnetite, Ni-and Co-rich metal, Ni-poor and Ni-rich sulfide; low-Ni metal nodules occur only inside chondrule phenocrysts. Chromium-poor magnetite is preferentially replaced by fayalite. Chondrule mesostases are replaced by phyllosilicates; low-Ca pyroxene and olivine phenocrysts appear to be unaltered. Matrix in the clast consists of very fine-grained (4 pm) ferrous olivine, anhedral fayalite grains (FasGloo), rounded objects of porous Ca-Fe-rich pyroxenes (Fs~(s_~oWO~O), Ni-poor sulfide, Ni-and Co-rich metal, and phyllosilicates; magnetite is rare. On the basis of the presence of the Bali-like lithified chondritic clast-in addition to individual chondrules and CAIs of both Bali-like and Allende-like materials-in the reduced CV chondrite Vigarano, we infer that (1) all three types of materials were mixed during regolith gardening on the CV asteroidal body, and (2) the reduced and oxidized CV materials may have originated from a single, heterogeneously altered asteroid.

Section: Discussionsupporting

confidence: 77%

A clast of Bali‐like oxidized CV material in the reduced CV chondrite breccia Vigarano

Krot

Meibom

Keil

2000

Self Cite

“…Itoh and Tomeoka (2003) revealed that CO3 chondrites contain abundant dark inclusions (DIs), although they are smaller in size than those in CV3 chondrites. The DIs in the CO3 chondrites were found to be lithic clasts similar in mineralogy and chemistry to their host chondrites and that show evidence for aqueous alteration and subsequent dehydration, like DIs in CV3 chondrites (e.g., Kojima et al 1993;Kojima and Tomeoka 1996;Krot et al 1997Krot et al , 1998Ohnishi and Tomeoka 2002). This implies that the CO parent body once contained aqueous fluids and that there was a region (or regions) in the parent body in which aqueous alteration and subsequent dehydration took place.…”

Section: Nephelinization By Aqueous Alteration and Dehydrationmentioning

confidence: 86%

Sodium‐metasomatism in chondrules in CO3 chondrites: Relationship to parent body thermal metamorphism

Tomeoka

Itoh

2004

Sodium-metasomatism in chondrules in CO3 chondrites:Relationship to parent body thermal metamorphism Abstract-We have studied the mineralogy and petrology of mesostases of 783 type I chondrules in seven CO3 chondrites that range in petrologic subtype from 3.0 to 3.7. Chondrule mesostases in the CO chondrite of subtype 3.0 consist mainly of primary glass and plagioclase, while chondrule mesostases in the CO chondrites of higher subtypes (3.2-3.7) contain various amounts of nepheline in addition to glass and plagioclase. Nepheline has replaced glass and plagioclase, forming finegrained aggregates and thin parallel lamellar intergrowths with plagioclase. The nephelinization has proceeded preferentially from the outer margins of chondrules toward the inside. Although the degree of nephelinization differs widely among chondrules in each of the metamorphosed chondrites, our modal analyses and bulk chemical analyses of individual mesostases indicate that the amounts of nepheline in chondrules systematically increase with the increasing petrologic subtype of the host chondrites. Nepheline also has a tendency to increase in grain size with increasing petrologic subtype. We conclude that nepheline in chondrules in the CO3 chondrites has formed largely as a result of effects related to heating on the meteorite parent body. We suggest that nepheline initially formed as hydrous nepheline under the presence of aqueous fluids and subsequently was dehydrated after exhaustion of aqueous fluids. The degree of hydrothermal activity must have increased with increasing degree of heating, and thus, chondrules in more thermally metamorphosed chondrites produced larger amounts of nepheline. The results imply that CO3 chondrites have gone through lowgrade aqueous alteration and subsequent dehydration at the early stage of heating on the meteorite parent body.

“…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). …”

Section: Origin Of Spinel Inclusionsmentioning

confidence: 99%

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

Rubin

2007

Abstract-Queen Alexandra Range (QUE) 97990 (CM2.6) is among the least-altered CM chondrites known. It contains 1.8 vol% refractory inclusions; 40 were studied from a single thin section. Inclusion varieties include simple, banded and nodular structures as well as simple and complex distended objects. The inclusions range in mean size from 30 to 530 μm and average 130 ± 90 μm. Many inclusions contain 25 ± 15 vol% phyllosilicate (predominantly Mg-Fe serpentine); several contain small grains of perovskite. In addition to phyllosilicate, the most abundant inclusions in QUE 97990 consist mainly of spinel-pyroxene (35%), followed by spinel (20%), spinel-pyroxene-olivine (18%), pyroxene (12%), pyroxene-olivine (8%) and hibonite ± spinel (8%). Four pyroxene phases occur: diopside, Al-rich diopside (with ≥8.0 wt% Al 2 O 3 ), Al-Ti diopside (i.e., fassaite), and (in two inclusions) enstatite. No inclusions contain melilite. Aqueous alteration of refractory inclusions transforms some phases (particularly melilite) into phyllosilicate; some inclusions broke apart during alteration. Melilite-free, phyllosilicate-bearing, spinel inclusions probably formed from pristine, phyllosilicate-free inclusions containing both melilite and spinel. Sixty-five percent of the refractory inclusions in QUE 97990 appear to be largely intact; the major exception is the group of spinel inclusions, all of which are fragments. Whereas QUE 97990 contains about 50 largely intact refractory inclusions/cm 2 , estimates from literature data imply that more-altered CM chondrites have lower modal abundances (and lower number densities) of refractory inclusions: Mighei (CM ~ 2.3) contains roughly 0.3-0.6 vol% inclusions (~10 largely intact inclusions/cm 2 ); Cold Bokkeveld (CM2.2) contains ~0.01 vol% inclusions (on the order of 6 largely intact inclusions/cm 2 ).