Chemical and physical studies of type 3 chondrites XII: The metamorphic history of CV chondrites and their components

Abstract

Cosmochemistry Group, Department of Chemistry and Biochemistry, The University of Arkansas, Fayetteville, Arkansas 72701, USAIPresent address: Natural Science Division, Missouri Baptist College, St. Louis, Missouri 63142, USA*To whom correspondence should be addressed(Received 1994 December 8; accepted in revised form 1995 August I4)Abstract The induced thermoluminescence (TL) properties of 16 CV and CV-related chondrites, four CKchondrites and Renazzo (CR2) have been measured in order to investigate their metamorphic history. Thepetrographic, mineralogical and bulk compositional differences among the CV chondrites indicate that theTL sensitivity of the -130 °C TL peak is reflecting the abundance of ordered feldspar, especially inchondrule mesostasis, which in turn reflects parent-body metamorphism. The TL properties of 18 samples ofhomogenized Allende powder heated at a variety of times and temperatures, and cathodoluminescencemosaics of Axtell and Coolidge, showed results consistent with this conclusion. Five refractory inclusionsfrom Allende, and separates from those inclusions, were also examined and yielded trends reflectingvariations in mineralogy indicative of high peak temperatures (either metamorphic or igneous) and fairlyrapid cooling. The CK chondrites are unique among metamorphosed chondrites in showing no detectableinduced TL, which is consistent with literature data that suggests very unusual feldspar in these meteorites.Using TL sensitivity and several mineral systems and allowing for the differences in the oxidized andreduced subgroups, the CV and CV-related meteorites can be divided into petrologic types analogous tothose of the ordinary and CO type 3 chondrites. Axtell, Kaba, Leoville, Bali, Arch and ALHA81003 are type3.0-3.1, while ALH84018, Efremovka, Grosnaja, Allende and Vigarano are type 3.2-3.3 and Coolidge andLoongana 001 are type 3.8. Mokoia is probably a breccia with regions ranging in petrologic type from 3.0 to3.2. Renazzo often plots at the end of the reduced and oxidized CV chondrite trends, even when those trendsdiverge, suggesting that in many respects it resembles the unmetamorphosed precursors of the CVchondrites. The low-petrographic types and low-TL peak temperatures of all samples, including the CV3.8chondrites, indicates metamorphism in the stability field of low feldspar (i.e., <800 °C) and a metamorphichistory similar to that of the CO chondrites but unlike that of the ordinary chondrites.INTRODUCTIONVirtually all classes of chondrite have experienced some level ofparent body metamorphism; although in the case of type 1 and 2carbonaceous chondrites the metamorphism involved considerableaqueous alteration. Both the type 3 ordinary chondrites and the COchondrites display mineralogical and petrographic evidence formetamorphic alteration that can be evaluated with a high degree ofprecision using induced thermoluminescence (TL) measurements,although the time-temperature histories of the ordinary and COchondrites are quite different (Dodd et aL, 1967; McSween, 1977a;Keck and Sears, 1987; Scott and Jones, 1990; Sears et al., 1991a, b).The present paper extends our studies of metamorphism of type 3chondrites to the CV and the possibly related CK chondrites(Kallemeyn et al., 1991 ).Compositional equilibration between refractory inclusions andthe ferromagnesian components, and variations in the homogen-ization of matrix olivines, suggests that the CV chondrites havesuffered various levels of parent-body metamorphism (McSween,1977b; Peck, 1984; Scott et aL, 1988). It has been proposed thatmetamorphism increased along the series Kaba, Mokoia, Vigarano,Grosnaja and Allende. Since the CV chondrites consist of bothoxidized and reduced subgroups, a single metamorphic series isprecluded although two parallel series are possible (McSween,1977b). Recently, Weinbruch et al. (1993) estimated spinel-olivineequilibration temperatures for Allende -625 °C and equilibrationtemperatures based on olivine profiles of ~325 °C. Guimon andSears (1986) suggested <600 °C based on induced TL data. Ofparticular interest are the refractory inclusions (or, calcium- andaluminum-rich inclusions, CAIs) in CV chondrites, which exhibit anumber of properties that suggest a complex history (MacPherson etal., 1988). it has been argued that some Allende inclusions weremetamorphosed prior to emplacement in the meteorites (Meeker etat., 1983), although MacPherson et aL (1988) argue that the featuresin question are igneous in origin.Here we report induced TL measurements for 16 CV and CV-related chondrites and Renazzo, a CR chondrite, and five of therefractory inclusions and their mineral separates from the Meeker etal. (1983) study of Allende. We also prepared cathodoluminescence(CL) images of polished sections of selected CV chondrites. Weheated samples of homogenized Allende powder for 1-100 h at500-1000 °C (Guimon and Sears, 1986), since such experimentshave proved essential in understanding the TL data of other classes.EXPERIMENTALSamplesThe samples we studied are listed in Tables 1 and 2. They consist ofboth reduced and oxidized CV chondrites, as defined by McSween (1977b).Coolidge and Loongana 001 have been descibed as a new "grouplet" relatedto CV chondrites by Kallemeyn and Rubin (1995), who argued that thesetwo meteorites had different volatile element abundances and could nothave been formed by closed-system metamorphism of the other CVchondrites. Since the CV chondrites are highly heterogeneous, and small innumber, we think that such a conclusion may be premature. In mostrespects, these meteorites have the properties expected of meteorites closelyresembling the CV chondrites prior to metamorphism. Nianqiang wasdescribed as an anomalous CV chondrite by Kallemeyn and Wasson (1982)and as an anomalous CK chondrite by Kallemeyn et al. (1991). The CVand CK chondrites have very similar properties; the most distinctive to dateis that the CV chondrites have measurable TI, sensitivities, while the CKchondrites do not. In this respect, Ninqiang is more closely related to theCV chondrites, ttowever, we stress that these are subtle nuances inclassification, and the matter of whether it is better to stress similarities704