Chondrites unaltered

In recent years, a new source of information about stellar nucleosynthesis and the history of the elements between their ejection from stars and their incorporation into the solar system has become available. This source is the tiny dust grains that condensed from gas ejected from stars at the end of their lives and that survived unaltered to be incorporated into solar system materials. These presolar grains (Fig. 5.1) originated before the solar system formed and were part of the raw materials for the Sun, the planets, and other solar-system objects. They survived the collapse of the Sun s parent molecular cloud and the formation of the accretion disk and were incorporated essentially unchanged into the parent bodies of the chondritic meteorites. They are found in the fine-grained matrix of the least metamorphosed chondrites and in interplanetary dust particles (IDPs), materials that were not processed by high-temperature events in the solar system. 

Until recently, it was generally believed that the chondrites, which consist of relatively unaltered nebular material and thus are primitive objects, accreted early, whereas the differentiated meteorites (achondrites, irons, pallasites), which have had a history of melting... 

The aqueous fluids formed by melting of ices in asteroids reacted with minerals to produce a host of secondary phases. Laboratory studies provide information on the identities of these phases. They include hydrated minerals such as serpentines and clays, as well as a variety of carbonates, sulfates, oxides, sulfides, halides, and oxy-hydroxides, some of which are pictured in Figure 12.15. The alteration minerals in carbonaceous chondrites have been discussed extensively in the literature (Zolensky and McSween, 1988 Buseck and Hua, 1993 Brearley, 2004) and were most recently reviewed by Brearley (2006). In the case of Cl chondrites, the alteration is pervasive and almost no unaltered minerals remain. CM chondrites contain mixtures of heavily altered and partially altered materials. In CR2 and CV3oxb chondrites, matrix minerals have been moderately altered and chondrules show some effects of aqueous alteration. For other chondrite groups such as CO and LL3.0-3.1, the alteration is subtle and secondary minerals are uncommon. In some CV chondrites, a later thermal metamorphic overprint has dehydrated serpentine to form olivine. 

Carbonaceous chondrite A rare type of stony meteorite that is rich in carbon compounds and is thought to be relatively unaltered since the beginning of the solar system its spectrum (and probably also its composition) closely resembles that of the C-type asteroids... 

The Solar System, in comparison, offers two lines of evidence to constrain the timescale for the lifetime of the proto-solar nebula and the epoch of planetesimal formation. On one side, relatively unaltered chondritic components preserve traces of their chemical and thermal history on the other side, dynamical information is imprinted on the hierarchy of the Solar System. 

Below we review the geochemical properties and possible origins of chondritic components focusing on what appear to be primary features in the least altered type 2-3 chondrites. The rich diversity of chondritic components even in unaltered chondrites and the vast differences... 

The mineralogy of CV3 chondrite matrices prior to alteration is not clear. The two reduced CV3 chondrites that have been studied by TEM are not ideal samples. In Leoville, which is moderately shocked, Nakamura et al (1992) found aggregates of rounded, 10-100 nm olivine grains with interstitial amorphous material, but attributed this to shock. In Vigarano, which is a breccia of altered and unaltered material... 

Many different processes were involved in making each chondritic component. Unaltered chondrite matrices may contain at least six different types of micrometer-to-nanometer-sized components, which formed in diverse environments amorphous FeO-rich silicate, forsterite and enstatite grains, refractory grains, presolar grains, carbonaceous material, and iron-rich olivine. Chondrules formed by several nebular processes (closed-system melting, condensation, and possibly evaporation) and at least one asteroidal process (impact melting in regoliths). CAIs may be condensates, residues or processed versions of both. An exception to this preference for complexity is provided by the amoeboid olivine inclusions all AO As could have formed by the same basic process nebular condensation. Aluminum-rich chondrules may provide a second exception, at least within carbonaceous chondrites. 

Figure 21 Backscattered electron image of an unaltered type A inclusion in the unequihbrated ordinary chondrite Quinyamhie. Abbreviations as used previously (photo modified from Russell et al, 1996) (reproduced by permission of the American Association for the Advancement of Science from Science 1996, 273, 757-762).

Four principal models have been proposed to explain the presence of alteration phases in chondritic meteorites. These models can be summarized as follows (i) reaction of anhydrous, high-temperature condensate phases with water vapor as the solar nebula cooled to temperatures below —375 K (e.g., Grossman and Larimer, 1974]) (ii) hydration of anhydrous dust in icy regions of the nebula during the passage of shock waves (Ciesla et al., 2003) (iii) alteration within small (tens of meters), ephemeral parent bodies that were subsequently disrupted and their altered components accreted with unaltered materials into the final asteroidal parent bodies (pre-accretionary alteration) (Metzler et al., 1992 Bischoff, 1998) and (iv) alteration within asteroidal parent bodies (Kerridge and Bunch, 1979 Zolensky and McSween, 1988). 

Evidence to constrain the location of alteration for the CO chondrites is limited. However, most data suggest a late-stage event that postdated metamorphism, implying that it was probably a parent-body process. Development of alteration phases in CO matrices has occurred interstitially to matrix phases and there is no evidence of distinct hydrous phases intermixed with unaltered phases that would support pre-accretionary alteration. However, formation of phyllosilicates in chondmles in ALH77307 may have occurred prior to accretion, based on the fact that the ALH77307 matrix shows essentially no evidence of aqueous alteration (Brearley, 1993). 

While these data do not rule out alteration in the solar nebula, they indicate that the alteration history of CV chondrites and their dark inclusions was a process that extended over several million years. Maintaining separation of altered and unaltered objects within the nebula for this period of time is problematic. In models for asteroidal alteration, spatial separation of unaltered and altered regions appears to be much less problematical and also provides an environment where alteration can occur for extended periods of time. 

Oxidation effects in CO chondrites are variably developed. Many CO chondrites contain unaltered kamacite, whereas in ALH77307, Ornans, and several Antarctic CO chondrites, magnetite is common, replacing metal (McSween, 1977b Scott and Jones, 1990 Shibata, 1996). Pentlandite appears to have replaced troilite in ALH77307, whereas troilite still coexists with magnetite in Ornans. 

The main evidence for nebular metasomatism comes from a few CAls in low petrologic type CO chondrites. Although Russell et al. (1998) found that alteration in most CAls correlated with petrologic type, they observed rare inclusions in low petrologic type CO chondrites (e.g.. Colony, 3.0) that show evidence of alteration and enrichment in alkalis, that was inconsistent with the unaltered nature of other similar inclusions in the same chondrite. The advanced degree of alteration of these inclusions does not correlate with the low degree of alteration experienced by the host... 

A clast from Mount Padbury has a mg of 36, a molar FeO/MnO of 36, and a flat REE pattern at 9-10 X Cl chondrite abundances (Mittlefehldt, 1979)—all within the ranges for basaltic eucrites. However, many basaltic clasts are distinct in major element composition, with higher mg s and lower molar FeO/MnO ratios than those of basaltic eucrites, and have LREE-depleted patterns and (Eu/Sm)ci > 1— patterns unknown among unaltered basaltic eucrites. Some gabbro clasts are similar to cumulate eucrites in major-and trace-element contents, but many are distinct in having extreme depletions in the most incompatible elements (Mittlefehldt, 1979 Rubin and Mittlefehldt, 1992). In extreme cases, samarium abundances are only 0.02-0.03 X Cl chondrites (Rubin and Jerde, 1987 Rubin and Mittlefehldt, 1992), much less than the 1-2 X Cl typical of cumulate eucrites. These clasts have (Eu/Sm)ci of 220-260, the most extreme ratios known among solar system igneous rocks (Mittlefehldt et al., 1992). 

Unaltered chondrites are characterized by extreme mineralogical parsimony, with no more than about twenty different mineral species (4). As planetesimal accretion progressed and chondrite parent bodies became larger, aqueous and thermal alteration led to new suites of minerals (5,6). Yet the total mineralogical... 

Chondrite Meteorite from an undifferentiated planetary body, most likely an asteroid. Unaltered varieties generally contain chondrules or chondrule fragments. 

Ashworth, and Hutchison, 1975 [11] made electron microscopic observations of the hydrous alteration products of olivine in an achondrite and in an ordinary chondrite. Their conclusion was that the Nakhla achondrite, and possibly the Weston chondrite, contain water of extraterrestrial origin which was mobilized by mild shock deformation. Carbonaceous chondrites are believed to be unaltered material left over from the formation of the solar system. They contain substantial amounts of reduced carbon and of water in the form of hydroxyl ions. The oxidation state of iron in some carbonaceous chondrites has been determined by means of Moess-bauer spectroscopy, and it is demonstrated that there is a correlation between the oxidation state of iron and the content of water and reduced carbon in the meteorites (Roy-Poulsen et al., 1981 [284]). 

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