Chondrites composition

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

The only in situ chemical data for asteroids are from the NEAR Shoemaker spacecraft, which orbited 433 Eros in 2000-1, and from the Japanese Hayabusa spacecraft, which visited 25143 Itokawa in 2003. NEAR obtained numerous measurements of the surface composition using X-ray fluorescence and gamma-ray spectrometers, and Hayabusa carried an XRF. The magnesium/silicon and aluminum/siUcon ratios for both asteroids are consistent with the compositions of chondrites. However, sulfur is depleted in Eros relative to chondritic compositions, possibly due to devolatilization by impacts or small degrees of melting. 

The winonaites are compositionally similar to silicate inclusions in some IAB irons (described below). They have chondritic compositions, and relict chondrules have been found in some meteorites. They consist of olivine, pyroxenes, plagioclase, metal, troilite, and other minor minerals (Benedix et al., 1998), and most have been recrystallized. Like the acapulcoites, they have experienced only small degrees of melting. 

Another important chemical difference between chondrite groups is in their oxidation states. In Chapter 7, we learned that differences in chondrite compositions cannot be simply explained by fractionation of metallic iron, but also must involve oxidation or reduction of iron (see Fig. 7.14). 

Elemental abundances, normalized to Mg and Cl chondrites, for two groups of primitive achondrites (acapulcoites and winonaites) that experienced low degrees of partial melting. These abundances are similar to chondritic abundances (average H-chondrite composition is illustrated). Modified from Mittlefehldt (2004). 

Huss, G. R., Lewis, R. S., Hemkin, S. (1996) The normal planetary noble gas component in primitive chondrites Compositions, carrier, and metamorphic history. Geochim. Cos-mochim. Acta, 60, 3311-40. 

Figure 7 Urey-Craig diagram showing relative iron contents and oxidation states of the chondrite groups. Iron present in metal and sulfide phases is plotted versus iron present in silicate and oxide phases, for bulk chondrite compositions (after Brearley and Jones, 1998) (reproduced by permission of the Mineralogical Society of America from Reviews in Mineralogy 1998, 36, 1-398).

Based on the bulk chemistry, IDPs are divided into two groups (i) micrometer-sized chondritic particles and (ii) micrometer-sized nonchondritic particles. A particle is defined as chondritic when magnesium, aluminum, silicon, sulfur, calcium, titanium, chromium, manganese, iron, and nickel occur in relative proportions similar (within a factor of 2) to their solar element abundances, as represented by the Cl carbonaceous chondrite composition (Brownlee et al., 1976). Chondritic IDPs differ significantly in form and texture from the components of known carbonaceous chondrite groups and are highly enriched in carbon relative to the most carbon-rich Cl carbonaceous chondrites (Rietmeijer, 1992 Thomas et al., 1996 Rietmeijer, 1998, 2002). 

Figure 27 Elemental abundances in matrix material in C03 chondrites normalized to bulk chondrite data for rims on chondrules in ALHA77307 (type 3.0) obtained by synchrotron X-ray fluorescence microprobe analysis (Brearley et aL, 1995) and for bulk matrix in Omans (type 3.3) analyzed by instrumental neutron activation (data from Rubin and Wasson, 1988). Chondrule rims in ALHA77307 are uniform in composition but deviate significantly from the bulk chondrite composition. Matrix in Ornans is relatively unfractionated (reproduced by permission of Elsevier from...

A very minor population of CAIs have ultra-refractory compositions, with enrichments in the most refractory trace elements of over a factor 1,000 times Cl chondritic composition. The most spectacular of these is OSCAR, which consists of... 

Tests. The compositional tests are similar to those for the refractory-volatile mixtures, though the metal-rich chondrite composition has lower concentrations of volatiles. However, the partial melt composition is olivine normative, so the surface mineralogy will consist of olivine (—20%), high-calcium pyroxene (—40%), and plagioclase (35%), plus minor phases (5%). 

Both, the uncertainty over ( Hf/ Hf)Bssi and the fact that the tungsten isotopic composition of the silicate Earth is now unequivocally resolvable from a now well-defined chondritic composition (Kleine et al, 2002 Lee and Halliday, 2000a Schoenberg et al., 2002 Yin et al, 2002), affect the calculated timescales for terrestrial accretion. It had been argued that accretion and core formation were fairly protracted and characterized by equilibration between accreting materials and the silicate Earth (Halliday, 2000 Halliday et al,... 

From the budgets of potassium, silicon, carbon, and sulfur extrapolated from carbonaceous chondrite compositions, one can evaluate the amounts of various light elements possibly incorporated in... 

The major-element compositions of 200 chondritic IDPs were measured by EDS (Table 1 and Figure 12). All of the particles were identified as extraterrestrial because they have approximately chondritic compositions or consist predominantly of a single mineral grain like forsterite or pyrrhotite (commonly found within chondritic IDPs) 37% of the particles are CSIDPs, 45% are CP IDPs, and 18% IDPs composed predominantly of a single mineral. Table 1 summarizes the compositions of the IDPs. Within a factor of 2 the abundances of oxygen, magnesium, aluminum, sulfur, calcium, chromium, manganese, iron, and nickel are approximately chondritic. CP IDPs are a closer match to Cl carbonaceous chondrites than CS IDPs, and they are closer to Cl bulk than to Cl... 

For this review the Earth s composition will be considered to be more similar to carbonaceous chondrites and somewhat less like the high-iron end-members of the ordinary or enstatite chondrites, especially with regard to the most abundant elements (iron, oxygen, silicon, and magnesium) and their ratios. However, before reaching any firm conclusions about this assumption, we need to develop a compositional model for the Earth that can be compared with different chondritic compositions. To do this we need to (i) classify the elements in terms of their properties in the nebula and the Earth and (2) establish the absolute abundances of the refractory and volatile elements in the mantle and bulk Earth. 

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