Petrology and Geochemistry of Enstatite Chondrites and Achondrites
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The enstatite chondrites and achondrites (aubrites) are the most oxygen-poor objects in the solar system. The enstatite or E chondrites are divided mainly into two groups, the EH (high iron) and EL (low iron) chondrites. Some of these chondrites are unequilibrated mineralogically (type 3), and therefore have a unique record of the events and processes that preceded their accretion to a planetary body. These EH3 and EL3 chondrites have only been recognized in recent years so there is much yet to learn about their characteristics and significance. The aubrites have been considered to be achondrites that were formed by igneous processes from an enstatite chondrite parent which may differ in some ways from the known E chondrites. They record a complex history and may sometimes be polymict breccias, although most have been considered to be monomict breccias. Some aubrite-like meteorites, such as Happy Canyon and Ilafegh 009, may be impact melts. One of the major debates concerns the origin of the calcium sulfide mineral oldhamite which carries most of the rare-earth elements (REE) in the aubrites, as well as in the E chondrites. The REE patterns in the aubrite oldhamites are not equilibrated and have a variety of patterns, many of which are similar to those in the E chondrites. If the aubrites were truly igneous the REE patterns should have been equilibrated. The REE patterns in the E chondrites are generally considered to be of nebular origin, but they would not be expected to have survived in the aubrites if they are igneous. Experimental studies may help resolve some of these questions.
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