Undifferentiated meteorites

There are two different groups of meteorites— undifferentiated and differentiated. The undifferentiated meteorites are pieces of planetesimals that have never been heated to melting temperatures. Their chemical and isotopic composition should... 

Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

Undifferentiated meteorites these are derived from asteroids which never underwent the heating which leads to fusion. They consist of millimetre-sized spherules (chondrules) embedded in a matrix. 

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

Chondrites. Over 909, of the meteorites lhat are observed lo fall out of the sky are classified as chondrites, samples lhat are distinguished from terrestrial rocks in many ways. One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Chondrites also have basically undifferentiated elemental compositions lor most nonvolatile elements and match solar abundances except for moderately volatile elements. The imtsl cunipositionally primitive chondrites are members for the type I carbonaceous f Cl I class. 

Figure 3.5 Radically anomalous noble gas isotopic compositions in extrasolar materials isolated from undifferentiated meteorites (from Anders Zinner, 1993). Stepwise heating of whole-rock meteorites liberates slightly more or less of components such as Xe-HL and Ne-E, relative to other noble gas reservoirs in the rock, leading to the modest isotopic variations (e.g., Xe compositions as illustrtated in Figure 3.4, or Ne compositions to the lower-left of the air-spallation-solar wind triangle in Figure 3.3) from which the presence of anomalies was originally inferred.

Undifferentiated and differentiated meteorites. 03.1.3.2 Cosmochemical classification of elements Cl Chondrites as Standard for Solar Abundances... 

Figure 3 Element/Si mass ratios of characteristic elements in various groups of chondritic (undifferentiated) meteorites. Meteorite groups are arranged according to decreasing oxygen content. The best match between solar abundances and meteoritic abundances is with Cl meteorites (see text for details).

The achondritic meteorites can be subdivided into the differentiated achondrites igneous rocks from parent bodies that were extensively melted, and the undifferentiated, or primitive, achondrites from parent bodies that underwent little melting. 

The two elements calcium and aluminum are RLEs. The assumption is usually made that aU RLEs are present in the primitive mantle of the Earth in chondritic proportions. Chondritic (undifferentiated) meteorites show significant variations in the absolute abundances of refractory elements but have, with few exceptions discussed below, the same relative abundances of lithophile and siderophile refractory elements. By analogy, the Earth s mantle abundances of refractory lithophile elements are assumed to occur in chondritic relative proportions in the primitive mantle, which is thus characterized by a single RLE/Mg ratio. This ratio is often normalized to the Cl-chondrite ratio and the resulting ratio, written as (RLE/Mg)N, is a measure of the concentration level of the refractory component in the Earth. A single factor of (RLE/Mg) valid for all RLEs is a basic assumption in this procedure and will be calculated from mass balance considerations. 

There is a wide range of meteorite types, which are readily divided into three main groups the irons, the stony irons and the stones (see also Volume 1 of the Treatise). With this simple classification, we obtain our first insights into planetary dilferentiation. All stony irons and irons are differentiated meteorites. Most stony meteorites are chondrites, undifferentiated meteorites, although lesser amounts are achondrites, differentiated stony meteorites. The achondrites make up —4% of all meteorites, and <5% of the stony meteorites. A planetary bulk composition is analogous to that of a chondrite, and the differentiated portions of a planet—the core, mantle, and crust— have compositional analogues in the irons, stony irons (for core-mantle boundary regions), and achondrites (for mantle and crust). 

The rather constant fractionation of Sm/Nd ratios in upper continental cmstal rock reservoirs is the basis for the widely applied neodymium model age that is illustrated in Figure 3. The Sm-Nd systematics of chondritic meteorites serve as a reference for the parent/daughter ratio of the undifferentiated Earth (Jacobsen and Wasserburg, 1984), labeled as CHUR for chondritic uniform reservoir. The evolution of this undifferentiated Earth is the basis for calculation of CHUR model ages (McCulloch and Wasserburg, 1978), while the neodymium isotopic evolution of the depleted upper part of the mantle is a more valid reference for most cmstal materials, resulting in the DM model age (DePaolo, 1981). Neodymium isotopic compositions are usually given by s d, where the deviations of Nd/ Nd above or below... 

Figure 7 shows the abundances of the four refractory lithophile elements—aluminum, calcium, scandium, and vanadium—in several groups of undifferentiated meteorites, the Earth s upper mantle and the Sun. The RLE abundances are divided by magnesium and this ratio is then normalized to the same ratio in Cl-chondrites. These (RLE/Mg)N ratios are plotted in Figure 7 (see also Figure 1). The level of refractory element abundances in bulk chondritic meteorites varies by less than a factor of 2. Carbonaceous chondrites have either Cl-chondritic or higher Al/Mg ratios (and other RLE/Mg ratios), while rumurutiites (highly oxidized chondritic meteorites), ordinary chondrites, acapulcoites, and enstatite chondrites are depleted in refractory elements. The (RLE/Mg)N ratio in the mantle of the Earth is within the range of carbonaceous chondrites.

Chondrites. Chondrites are stony meteorites and are the most abundant meteorite type (87% of all meteorites). Their radiometric ages are around 4.56 Ga and these ages are thought to define the time when the solar system formed. Chemically their element abundance patterns, apart from the very light and/or volatile elements, are the same as that of the sun and other stars, and for this reason they are thought to represent undifferentiated cosmic matter. Chondrites therefore are thought to represent the most primitive material in the solar system. They are the "stuff" from which all other rocky materials were built. 

Evidence for condensation is seen in the meteorites, fragments of the asteroids that formed in the region between Mars and Jupiter. The stony meteorites that have elemental compositions that closely match those of the Sun (except for volatile elements such as H, He, N, etc.) are called chondrites after the presence of small spherical particles called chondrules. Although the chondrites generally contain close to undifferentiated solar composition, there is elemental fractionation in these objects that is related to condensation processes. Different chondrite groups are distinguished by Fe/Si ratios that vary by 50%, ratios of Ca, Al, and Ti to Si that vary by about 40%, and abundances of volatile elements, such as Cd, Bi, In, and Pb, that vary by orders of magnitude. The depletion of volatile elements is believed to be due to incomplete condensation. The correlated depletion of Ca, Al, and... 

Besides long gas retention ages, three lines of evidence link all undifferentiated and most differentiated meteorites to asteroids mineralogy spectral reflectance and the orbits of nine meteorite falls. 

This result is inconsistent with the standard model for asteroid formation, in which chondrites represent the precursor material from which asteroids accreted and then differentiated. The Hf-W ages for iron meteorites reveal that the opposite is the case, i.e., differentiated asteroids are the oldest planetesimals and undifferentiated asteroids (the chondrite parent bodies) formed later. This most likely reflects different abundances of Al at the time of parent body accretion. Early formed bodies were heated by the decay of abundant Al and could melt and differentiate into core and mantle, whereas later formed bodies remained undifferentiated because they contained too little Al to cause melting and differentiation. 

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

Chondrule Spherical millimeter-sized silicate droplet formed by melting and quenching prior to incorporation into undifferentiated meteorites. 

There are three popular ways of normalizing trace element data for presentation as a spider diagram. These include an estimated primitive mande composidon and chondridc meteorites — two views of the primitive undifferentiated earth. Others normalize their data to primitive MOKE. Each version of the spider diagram has a slightly different array of elements with a slightly different order. In detail there are innumerable variations on each particular theme, usually dictated by the number of trace elements and the quality of their determinations in a particular data-set. This state of affairs is not satisfactory and some standardization is desirable. First, however, we consider the present state of the art . 

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