Chondrites finds

Many properties of ordinary chondrites demonstrate that each group has its own special history, even in something so simple as the numbers of each chemical-petrographic type (Table I). For example, proportions of H3 or L3 are low, 1-2%, while 13% of LL falls are LL3. Proportions of more evolved chondrites also differ (Table I). The plurality, 44% of H falls are H5, while type 6 dominates L and LL chondrites - 68% and 49%, respectively. Non-desert-cluster chondrite finds generally exhibit similar trends. This hints at smaller H chondrite parent(s) than L or LL. 

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

The mean 143Nd/144Nd of a sample has been found to be 0.513114 with a standard deviation of0.000 007. Given a present-day 143Nd/144Nd ratio in chondrites of 0.512638 (an arbitrarily precise estimate), find the standard deviation on the mean eNd(0) value. 

The question as to the potential availability of the requisite amphiphilic precursors in the prebiotic environment has been addressed experimentally by Deamer and coworkers, [143,145] who looked into the uncontaminated Murchison chondrite for the presence of such amphiphilic constituents. Samples of the meteorite were extracted with chloroform-methanol and the extracts were fractionated by thin-layer chromatography, with the finding that some of the fractions afforded components that formed monomolecular films at air-water interfaces, and that were also able to self-assemble into membranous vesicles able to encapsulate polar solutes. These observations dearly demonstrated that amphiphiles plausibly available on the primitive Earth by meteoritic infall have the ability to self-assemble into the membranous vesides of minimum protocells. ... 

Equation 11.118 finds practical application in cosmological studies and in geology (dating of sulfide deposits and sediments). Figure 11.27A shows, for instance, the Re-Os isochron for iron meteorites and the metallic phase of chondrites, obtained by Luck and Allegre (1983). The fact that all samples fit the same isochron within analytical uncertainty has three important cosmological implications ... 

The discovery happened by accident. Lewis and Anders were frustrated by their failure to find the carrier of anomalous xenon in carbonaceous chondrites. They decided to try an extreme treatment to see if they could dissolve the carrier. They treated a sample of the colloidal fraction of an Allende residue with the harshest chemical oxidant known, hot perchloric acid. The black residue turned white, and to their surprise, when they measured it, the anomalous xenon was still there The residue consisted entirely of carbon, and when they performed electron diffraction measurements on it, they found that it consisted of tiny (nanometer sized) diamonds. After a detailed characterization that included chemical, structural, and isotopic studies, they reported the discovery of presolar diamond in early 1987 (Lewis et al., 1987). The 23-year search for the carrier of CCFXe (Xe-HL) was over, and the study of presolar grains had begun. 

C60 has not yet been detected in primitive meteorites, a finding that could demonstrate its existence in the early solar nebular or as a component of presolar dust. However, other allotropes of carbon, diamond and graphite, have been isolated from numerous chondritic samples. Studies of the isotopic composition and trace element content and these forms of carbon suggest that they condensed in circumstellar environments. Diamond may also have been produced in the early solar nebula and meteorite parent bodies by both low-temperature-low-pressure processes and shock events. Evidence for the occurrence of another carbon allotrope, with sp hybridized bonding, commonly known as carbyne, is presented. 

People interested in CO carbonaceous chondrites will find information in Ref. 5. 

What is important in the context of chondrite chemistry is the fact that the probable presence in the protosolar nebula of dust particles and complex organic molecules is evidenced. This does not mean that all the organic matter detected in carbonaceous chondrites is necessarily molecules still present in the protosolar nebula. Readers interested in details of the formation of the solar system and the accretion phenomena will find a lot of information in the papers by Larimer 14) and Cameron 1S). 

At the end of this section, two final points merit some attention. The first one concerns the location of the parent bodies of the chondrites. The asteroid belt between Mars and Jupiter is one possibility favoured by many authors, but some other possibilities exist (such as the families of asteroids crossing the earth s orbit). Asteroids themselves are of many different types and some of them are probably extinct comets (after too many passages at the perihelion). Some chondrites could be fragments of these extinct comets, but this hypothesis is not the most probable one. People interested in this problem will find information in Refs. 2, 5 and 9. 

It is conceivable that chemical reactions may have taken place during metamorphism, and it is difficult to find any reasons for excluding organic reactions. In other words, some of the organic molecules detected today in carbonaceous chondrites could be the products of reactions which happened after accretion in the parent body. What kind of reactions At this level, it is extremely difficult to suggest any reasonable answer. It is tempting to consider reactions essentially involving hydrolysis... 

An interesting consequence of this model is that bodies from the terrestrial-planet region may be scattered outwards while the planets are forming and implanted in the Asteroid Belt. With dynamical and collisional modeling of this process, Bottke et al. (2006) find that this may be the origin of most iron meteorites. This would explain the diversity of iron-meteorite types, why there is little observational or meteoritical evidence of mantle material from differentiated bodies in the Asteroid Belt, and the fact that most iron-meteorite parent bodies appear to have formed >1 Myr before the parent bodies of the chondritic meteorites (Kleine et al. 2005). 

The C03 meteorites have been subdivided into metamorphic grades from 3.0 to 3.7, with peak temperatures in the range 450-600 °C (Rubin, 1998). In contrast to the metamorphism of the ordinary chondrites, the CO metamorphism probably occurred in the presence of water, and the system was not closed with respect to oxygen. The least metamorphosed CO meteorites, ALH 77307 and Y 81020, are the most 0-rich Loongana 001 and HH 073, classified as 3.8 or 4, are the most 0-poor. There is not, however, a simple one-to-one correspondence between metamorphic grade and isotopic composition. Since most of the CO chondrites are finds, some may have been altered by terrestrial weathering. 

Chondrites contain a small fraction of chondrules that may have formed on asteroids. One rare type of chondrule (—0.1%) in type 4-6 ordinary chondrite breccias is composed largely of plagioclase (or mesostasis of plagioclase composition) and chromite (Krot et al, 1993). Krot and Rubin (1993) suggest that these chondrules may have formed by impact melting as they find impact melts with similar compositions inside shocked ordinary chondrites and such chondrules are absent in type 3 chondrites. In addition, some chromite-rich chondrules contain chromite-rich aggregates, which appear to be fragments of equilibrated chondrites. Other possible impact products were described in LL chondrites, which are mostly... 

Figure 4 Ne exposure ages of LL chondrites. Ne ages recalculated with the formulas of Eugster (1988). Multiple analyses for meteorites were averaged. Meteorite finds from Antarctica and northern Africa with similar exposure ages and collected at the same site were treated as paired (source L. Schultz).

These results require that the mantle source of the 3.8 Ga basalts and related sediments experienced Sm/Nd fractionation within 50-200 million of years of planetary accretion and that this fractionation was preserved from that time until at least 3.8 Ga. This finding was strikingly confirmed by Boyet and Carlson in 2005, who redetermined the e142Nd value of chondritic meteorites. These new ultrahigh precision measurements showed that the chondritic value is 20 ppm lower than that for terrestrial rocks, implying an ubiquitous 142Nd anomaly in the Earth s mantle. 

Studies of the short-lived isotope 142Nd in primitive chondritic meteorites have recently shown that the Earth s mantle does not have a chondritic 142Nd-isotope ratio (Boyet Carlson, 2005). The implication of this finding is that the Earth experienced a major differentiation event in which an Fe-rich, trace element-enriched basaltic crust formed on a magma ocean. This crust has subsequently been removed and isolated from the convecting mantle and may now be represented by the D" layer (Tolstikhin Hofmann, 2005). Similar 142Nd results on lunar samples suggest that this differentiation took place before the formation of the Moon, that is, within 30 Ma of the formation of the solar system (Boyet Si Carlson, 2005). 

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