Achondrites parent bodies

Goodrich, C. A. and Delaney, J. S. (2000) Fe/Mg-Fe/Mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies. Geochimica et Cosmochimica Acta, 64, 149-160. 

Delaney J. S., Nehru C. E., and Prinz. M. (1980) Olivine clasts from mesosiderites and howardites clues to the nature of achondritic parent bodies. Proc. 11th Lunar Planet. Sci. Conf 1073-1087. 

Mittlefehldt D. W. (1987) Volatile degassing of basaltic achondrite parent bodies evidence from alkali elements and phosphorus. Geochim. Cosmochim. Acta 51, 267-278. 

Irons are mostly fragments of asteroid cores. As with achondrites, their compositional variations reflect differences in parent body chemistry as well as changes wrought by crystallization. Pallasites may represent samples of core-mantle boundaries, and meso-siderites indicate poorly understood mixing of crust and core materials, probably by impact. 

The peak temperatures that asteroids experienced can be estimated from chemical exchange reactions between minerals - so-called geothermometers. For example, the exchange of calcium between coexisting orthopyroxene and clinopyroxene in highly metamorphosed chondrites has been used to estimate their equilibration temperatures (Slater-Reynolds and McSween, 2005). For ordinary chondrites, these temperatures range up to -1175 K. The experimental conditions at which achondrites melt provide minimum temperatures for their parent bodies. Melting of achondrites typically requires temperatures of>1200 K. 

Ceres 2.5-3 264 1,020 175 close resemblances between some asteroid and meteorite spectra suggest parent asteroid bodies Vesta and basaltic achondrites indicate a differentiated parent body. 

The W isotopic compositions of various terrestrial samples, chondrites, iron meteorites, basaltic achondrites, lunar samples, and Martian meteorites are expressed as deviations in parts per 104 from the value for the silicate earth (such as the W in a drill bit or chisel), which are the same as those of average solar system materials, represented by carbonaceous chondrites. These values are summarized in Fig. 8.9, from which it can be seen that early segregated metals such as the iron meteorites and metals from ordinary chondrites have only unradiogenic W because they formed early with low Hf/W. The time differences between metal objects segregated from parents with chondritic Hf/W are revealed by the differences in W isotopic compositions between each of the metal objects and chondrites. The Hf-W model ages of all these metals indicate that all of their parent bodies formed within a few million years, implying rapid accretion in the early history of the solar system. 

Therefore these data for the short-lived chronometer Hf-W provide a consistent picture of rapid accretion, equilibration, and planetesimal differentiation in the early solar system with only small (106-year) time differences resolvable between some events for the parent bodies of chondrites, basaltic achondrites, and iron meteorites. 

The differentiated meteorites were derived from parent bodies that experienced large-scale partial melting, isotopic homogenization (ureilites are the only exception), and subsequent differentiation. Based on abundance of FeNi-metal, these meteorites are commonly divided into three types achondrites (metal-poor), stony irons, and irons each of the types contains several meteorite groups and ungrouped members. 

Binzel R. P. and Xu S. (1993) Chips off of asteroid 4 Vesta evidence for the parent body of basaltic achondrite meteorites. Science 260, 186-191. 

Goodrich C. A. and Righter K. (2000) Petrology of unique achondrite Queen Alexandra Range 93148 a piece of the pallasite (howardite-eucrite-diogenite ) parent body Meteorit. Planet. Sci. 35, 521—535. 

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. 

In contrast to the small number of differentiated parent bodies represented by evolved achondritic meteorites, the number of parent bodies inferred from the chemical compositions of iron meteorites may be as large as 50 (Wasson, 1990). Of the 13 major iron meteorite groups, 10 appear to be from cores of differentiated meteorites. Many additional cores are inferred from the ungrouped irons, which make up —15% of iron meteorites. It is a puzzle why we appear to sample many more cores than mantles of these asteroids (see Chapter 1.12 for further discussion). 

Many questions remain unanswered. What was the anhydrous precursor for the CR-CH-CB group What were the precursors of the metamorphosed CM and Cl meteorites What was the relationship between ureilites and carbonaceous chondrites Why are so few differentiated parent bodies represented by achondrites Why is the isotopic composition of the Earth identical to that of the Moon, but different from that of Mars What is the relationship (if any) between the Earth and the enstatite meteorites Some of these questions may be successfully addressed once we have accurate fine-scaled chronology so as to put solar nebular events into the correct time sequence. 

Palme H., Baddenhausen H., Blum K., Cendales M., Dreibus G., Hofmeister H., Kruse H., Palme C., Spettel B., Vilcsek E., and Wanke H. (1978) New data on lunar samples and achondrites and a comparison of the least fractionated samples from the Earth, the Moon and the eucrite parent body. Proc. 9th Lunar. Sci. Conf. 25—57. 

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