Chondrite Classes Research Articles

Organic constituents of meteorites—a review

All meteoritic organic chemical analyses published since 1900 are critically reviewed. Inactive during the first half of the century, this field has been revived by the modern realities of spaceflight and exploration, but work has been concentrated on searching the organic constituents of the carbonaceous chondrites for compounds possibly indicative of extraterrestrial life. It is one theme of this paper that this is a poorly focussed effort. A second theme is that many compound identifications reported in the literature are, by present standards, insufficiently supported. The characteristics and thermal histories of the various classes of chondrites are very briefly reviewed. It is shown that certain classes other than carbonaceous chondrites might reasonably be expected to contain organic material. The distribution of carbon among these classes is discussed, and the forms which carbon takes in each case are noted. It is shown that significant amounts of extractable organic material may be expected not only in the carbonaceous chondrites but also in the unequilibrated ordinary chondrites and in the ureilites. Evidence indicating that volatile materials, possibly including carbon and its compounds, are heterogeneously distributed in chondrite specimens is considered. Amounts of organic material extracted or volatilized from various meteorite specimens are tabulated. Various crude but informative studies undertaken using unfractionated extracts are discussed. Data indicating the elemental composition and general chemical nature of meteorite extracts are tabulated and the infrared and ultraviolet absorption spectra of meteorite extracts and crude fractions are described and discussed. Isotope ratio analyses of extracts, volatilized and combusted materials, and whole stones are discussed and the carbon and hydrogen isotope ratios found in carbonaceous chondrite analyses are tabulated. Analytical studies proving or claiming to prove the presence in meteorites of particular organic compounds are discussed critically and in detail. Evidence indicating that certain analytical studies of hydrocarbons and of amino acids have been crucially affected by the presence of contaminating materials is considered. It is concluded that optically active compounds have not been proven to be present in meteorites. Possible origins of meteorite organic compounds and the relations of these origins to theories of meteorite origin are briefly discussed.

Olivine composition in chondrites—a supplement

Olivine compositions are given for 122 olivine-bronzite and olivine-hypersthene chondrites. From these data and those of an earlier paper ( Mason, 1963) the numbers of the different classes of chondrites are: enstatite, 16; olivine-bronzite, 430; olivine-hypersthene, 496: carbonaceous 33. It is suggested that the term olivine-pigeonite chondrite be abandoned, and those meteorites included with the carbonaceous chondrites.

Mercury in chondrites

The Hg abundance has been determined in chondritic meteorites by neutron activation and separation by volatilization over a series of temperatures ranging from 110° to 1200°C. Hg is highly fractionated between classes of chondrites and within the classes themselves. Most hypersthene chondrites have Hg concentrations between 0.01 and 1.0 ppm; bronzite chondrites, between 0.24 and 7 ppm. Enstatite chondrites have low Hg contents similar to hypersthene chondrites. Carbonaceous chondrites have Hg contents as high as, or higher than, the bronzite chondrites. There appears to be a correlation between the total Hg content and the Hg released at temperatures above 450°C. There is also a parallel between the K-Ar ages and the Hg/Arp36, ratios. Both these observations, if substantiated by additional measurements, should prove relevant to the thermal history of meteorites.

The abundance of mercury in meteorites and rocks by neutron activation analysis

Mercury abundances have been determined in all classes of chondritic meteorites, the principle classes of achondritic meteorites, alkali and tholeiitic basalts, differentiated tholeiitic basalt intrusions, possible lower crustal and upper mantle rock inclusions, and a collection of six standard crustal rocks. The analytical method used was based on thermal neutron activation. Relatively high mercury abundances in the carbonaceous chondrites, previously reported by Reed, Kigoshi and Turkevich, have been confirmed. Abundance data among the various classes of chondrites and achondrites suggest a more complex behavior for mercury in the evolution of the meteorites than had been previously proposed. Possible lower crustal or upper mantle materials exhibit high mercury abundances in the range of the basic achondrites while six standard crustal rocks have abundances in the range of only 0·004–0·04 ppm. Implications of the data with respect to the estimation of cosmic abundances, and the evolution of the meteorites and the earth are discussed.

The distribution of trace elements in chondritic meteorites. 1. Gallium, germanium and indium

A rapid neutron activation technique, using the nuclides 70Ga + 72Ga, 75Ge and 116mIn, was developed for the simultaneous determination of gallium, germanium and indium in geological materials. This method was employed to examine the fractionation of gallium, germanium and indium between ordinary, enstatite and carbonaceous chondrites and to investigate the distribution of gallium and germanium between the magnetic and non-magnetic fractions of ordinary and enstatite chondrites. A comparison of the gallium and germanium contents of the metal phases of different chondrite groups was also made. Some indium determinations on separated ordinary chondrite phases show that indium is concentrated mainly in the non-magnetic phases. Germanium is strongly siderophile in both ordinary and enstatite chondrites; gallium is strongly siderophile in enstatite chondrites, but less strongly in ordinary chondrites. Germanium results for metal phases belonging to the same chondrite groups are very similar, but there exist distinct differences between the germanium values found in the metal phases of enstatite, bronzite and hypersthene chondrites. The gallium and germanium contents of chondritic metal phases are not so variable as gallium and germanium results for iron meteorites. All three elements are shown to be fractionated among the different chondrite classes with depletion ratios in the order Ga< Ge⪡ In. Germanium, but not gallium, is enriched in bronzite relative to hypersthene chondrites. The results obtained here are in support of previous suggestions that enstatite chondrites should be subdivided into two groups. The fractionation patterns of gallium and germanium between types I, II and III carbonaceous chondrites are similar, but indium is more strongly fractionated.

Halogens in chondrites

Cl, Br and I contents in samples of the several classes of chondrites have been simultaneously measured by activation analysis. Auxiliary information derived from this work confirms U and Te concentrations and I/Te ratios reported in the literature. For the ordinary (bronzite and hypersthene), pigeonite, enstatite and carbonaceous chondrites the respective Cl contents are 4–90, 120, 516 and 280 ppm; the Br contents are 0·05–0·2, 2·0, 1·5–6·5 and 2·0–11·0 ppm and the I contents are 0·05–0·08, 0·16, 0·07–0·47 and 0·23–1·21 ppm. Cl and Br are positively correlated; most chondrites fall on correlation lines with Cl/Br weight ratios between 20 and 100. Br and I exhibit a simple correlation; most meteorites fall near a line with a Br/I weight ratio of about 1. Leaching experiments yielded results that supported these interhalogen relations, and also gave the degree of water solubility of the halogens: Cl < Br ~ I ~ Te. Abundances derived from the data support current thinking that the Type I carbonaceous chondrites have best retained a primordial complement of solar system matter. The astrophysical and meteoritic abundances of chlorine based on oxygen agree reasonably well, 22 vs. ~9 atom Cl 10 4 atom 0; whereas the abundance based on Si differ greatly, 1·5 × 10 5 vs. 2800 atom Cl 10 6 atom Si.

Genesis of chondritic meteorites

Genesis of chondritic meteorites

Chemical evolution of the terrestrial planets

Chemical evolution of the terrestrial planets

Fluorine in stone meteorites

The fluorine content of stony meteorites was measured by F 18 produced by an activation technique. Different classes of chondrites, enstatite, ‘ordinary’ and pigeonite-Wiik Class III, contain amounts of fluorine, 238, 121 and 66 ppm respectively that in a sense reflect their state of oxidation. In general, the fluorine concentration within a class of stone meteorites is fairly constant. The enstatite chondrite, Hvittis, seems to be an exception. Two Ca-rich and one Ca-poor achondrites contain about 63 and 11 ppm F, respectively. An atomic abundance of 1.6 × 10 3atoms F/10 6 atoms Si postulated by Stress and Urey (1956) was confirmed.

Chondritic Earth Model

GEOCHEMISTS have frequently used the abundances of elements in meteorites as a guide to the overall composition of the Earth. The abundances in the chondritic meteorites have been commonly used on the assumption that they represent the closest approach to the composition, for the non-volatile elements, of the primitive unfractionated material in the solar system. Recent work on trace element abundances in chondrites has shown many complications in this picture, and fractionation of the elements, by processes not yet understood, appears to have occurred among the various classes of chondrites. Thus the ordinary chondrites are depleted in the chalco-phile elements such as lead, thallium and bismuth1, compared with the carbonaceous chondrites, which are accordingly thought to be more primitive. Attempts to use the composition of the carbonaceous chondrites are hampered by a lack of data for some trace elements, but the overall similarity in composition of the lithophile elements with that of the high iron group of chondrites enables use to be made of the larger quantity of analytical data existing for that group. The abundances and general geochemistry of the chalcophile elements are too imperfectly known to enable useful calculations to be made at present. In this communication, attention is directed to some problems and consequences of the chondritic Earth model. A fuller discussion, for a total of forty-four elements, will be presented elsewhere2.

On the origin of chondrules and chondrites

The calculated ferrous iron content of a silicate liquid at equilibrium with solar gas approximately equals the Fe 2+ content of chondrules in a primitive chondrite (Renazzo). This points to a relationship between chondrules and the primordial solar nebula, and supports the proposition that chondrules are original (liquid) condensations from the nebula, predating the planets. Basic silicate liquids would be stable in an undifferentiated solar gas only at 10 2−10 4 atmospheres total pressure and ∼2000°K. Recent theoretical studies of the temperature-hydrostatic pressure history of a contracting model protosun do not yield such a combination of high pressure and moderate temperature. However, chemical fractionation of the nebular gas or transient pressures associated with T Tauri-like mass ejection could make the condensation of liquid droplets (chondrules) possible. The postulated sequence of events by which chondrules and the various classes of chondrites were formed is summarized in Section 12. By the time planets accreted, a number of processes would probably have acted to fractionate Fe, O, and S with respect to Si: chemical fractionation prior to condensation; chondrule-dust, metal-silicate, and solid-gas fractionations after condensation; and incomplete reaction of solid with gas phase at the time of accretion. These effects can account for Prior's law and for chemical differences between the various chondrite classes.