Dark Inclusions in CV3 Carbonaceous Chondrites

Dark Inclusions (DIs)

Dark inclusions (DIs) have been found and reported in numerous carbonaceous chondrite groups but have been most prevalent and distinctive in the CV3 chondrites. The first meteorite to have this feature studied in considerable detail was the Allende (CV3) fall of 1969 which prompted them to be focused on more thoroughly in subsequent years. The information presented below is generally derived from studies on various CV3 chondrites.

DIs have often been described as fine-grained xenoliths or lithic clasts and range in size from one millimetre to a few centimetres. They often appear to be considerably different to the host meteorite and their origin/formation has been the cause of some controversy.

There are three kinds of Dark Inclusions that are found in the CV3 chondrites.

Type A: contains chondrules in a fine-grained matrix and is similar to the host meteorite.

Type B: consists entirely of fine grains of iron-rich olivine, lacks chondrules and instead contains rounded porous aggregates of iron-rich olivine.
Type C: consists mostly of fine grains of iron-rich olivine and are devoid of chondrules and porous aggregates.

There are a few references towards Type A/B DIs which some regard as an intermediate between Types A and B. Despite the textural variations, DIs are similar in bulk chemical, oxygen isotopic and noble gas isotopic composition to their host CV3 chondrites. (Ohnishi/Tomeoka, 2002, Palme et al, 1989; Johnson et al, 1990; Bunch et al, 1980). However, there have been two main divisions in the origin of DIs. The first model infers that they are primary aggregates from the Solar Nebula while the second model suggests that they are actually fragments of the CV parent-body that were aqueously altered and subsequently dehydrated on the parent body (Ohnishi/Tomeoka, 2002, Kojima et al, 1993; Kojima & Tomeoka, 1996; Buchanan et al, 1997; Krot et al, 1997, 1998a,b, 1999). The most recent studies lean towards the second formation model.

It is possible that the CV3 parent-bodies may not be as homogenous as previously thought and that they are rather a heterogeneous conglomerate of rocks, each of which has suffered different degrees of thermal and shock metamorphism.

"It is rather surprising that the degree of shock and thermal metamorphism ranges from almost none, as represented by the host meteorite, to those shocked to shock stage S4 and heated to such high temperatures to produce local melts, as represented by the DIs. This implies that the events that caused thermal and shock metamorphism occurred locally on the surface of the parent body." - (Ohnishi & Tomeoka, 2002)

"There is also growing evidence that most of the CV3 chondrites were involved in various degrees of aqueous alteration (Tomeoka and Buseck, 1990; Keller and Buseck, 1990; Keller et al, 1994; Lee et al, 1996; Brearley 1997). Many DIs contain chondrules, Calcium-Aluminium-rich inclusions (CAI's) and/or pseudomorphs of these objects. The pseudomorphs formed by aqueous alteration and subsequent dehydration. Thus, originally the DIs were probably lithic clasts of chondritic material, most likely the host CV3 chondrites (Kojima et al, 1993; Kojima and Tomeoka, 1996). This implies that there was a local region (or regions) in the CV parent body that at one time was involved in extensive aqueous activity." - (Tomeoka & Kojima, 1998)

"DIs in Mokoia (CV3) are probably clasts that are genetically related to the host meteorite, and they have been once involved in an aqueous alteration process similar to that for the host meteorite. However, it is evident that the DIs have experienced additional metamorphic events that the host meteorite has not. The more homogeneous, Fe-rich compositions of olivines and the scarcity of phyllosilicates in the DIs suggest that they experienced thermal metamorphism and dehydration. These characteristics are consistent with the interpretation previously proposed for the DIs in other CV3 chondrites." - (Ohnishi & Tomeoka)

"Most authors have concluded that these clasts are petrogenetically related to the meteorites in which they occur, even though textures can be strikingly different. These inclusions apparently represent materials that have been affected by different processes from those that affected typical CV3 meteorites, or by the same processes either in a different sequence or to different degrees." - (Buchanan, Zolensky, Wentworth & Reid, 1996)

While there are still various points of view regarding the formation of the somewhat enigmatic Dark Inclusions, it seems most recent studies have a relatively common formation model where the DIs are clasts which are thought to be genetically related to the host meteorite. DIs have been involved in aqueous alteration similar to the host meteorite but have undergone additional metamorphic events that the host meteorite did not. The DIs experienced thermal metamorphism and then later dehydration. The heat source for the thermal metamorphism is thought to be primarily from impacts at the CV parent-body surface although some heat from the decay of radionuclides can not be ruled out.

Surface impacts also help to explain a couple of other points. The first being the large variation in shock and thermal metamorphism. As impacts occurred locally on the parent-body, it resulted in various extents of shock and thermal metamorphism. Consequently, the degree of shock and thermal metamorphism varied widely from location to location on the surface. Secondly, the impacts would also simultaneously cause brecciation and the intermixing of rocks from various locations in the parent-body. This would help explain the introduction of the DI's into the less metamorphosed host meteorite.

Dark inclusions in the Mokoia CV3 chondrite: Evidence for aqueous alteration and subsequent thermal and shock metamorphism. Ohnishi & Tomeoka (2.73MB)

Arcuate band texture in a dark inclusion from the Vigarano CV3 chondrite: Possible evidence for early sedimentary processes. Tomeoka & Kojima (1.11MB)

Analysis of Dark Inclusions from Allende. Buchanan, Zolensky, Wentworth & Reid (366kb)

Dark Inclusions in the Mokoia CV3 Chondrite: Record of aqueous alteration, thermal metamorphism and shock metamorphism. Ohnishi & Tomeoka (12kb)

Evidence for a high temperature episode during multistage alteration of Allende Dark Inclusions. Brenker & Krot (13kb)

Two-stage asteroidal alteration of the Allende Dark Inclusions. Krot et al. (12kb)

NWA 3118 (CV3) 7.7g Complete Slice with Dark Inclusion.
NWA 3118 (CV3) 7.7g Complete Slice with Dark Inclusion.
Enlargement ---> 1000 x 481 (188KB)

NWA 3118 (CV3) 7.7g Complete Slice with Dark Inclusion.
NWA 3118 (CV3) 7.7g Complete Slice with Dark Inclusion.
Enlargement ---> 1000 x 638 (249KB)

NWA 3118 (CV3) 1.55g Complete Slice with Dark Inclusion.
NWA 3118 (CV3) 1.55g Complete Slice with Dark Inclusion.

Allende (CV3) with Dark Inclusion.
Allende (CV3) with Dark Inclusion. (�2007 John Kashuba)
Enlargement ---> 1122 x 1034 (275KB)

Allende (CV3) with Dark Inclusion.
Allende (CV3) Thin Section image of the above specimen showing a finer-grained Dark Inclusion (left). (�2007 John Kashuba)
Enlargement ---> 1280 x 944 (505KB)

NWA 3118 (CV3) with Dark Inclusion.
NWA 3118 (CV3) with Dark Inclusion. (�2007 John Kashuba)
Enlargement ---> 1280 x 570 (374KB)

NWA 3118 (CV3) with Dark Inclusion.
NWA 3118 (CV3) with Dark Inclusion. (�2007 John Kashuba)
Enlargement ---> 1280 x 818 (623KB)

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