The Omolon pallasite: Chemical composition,mineralogy, and genetic implications |
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Authors: | Z A Lavrentjeva A Yu Lyul G M Kolesov |
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Institution: | 1.Vernadsky Institute of Geochemistry and Analytical Chemistry,Russian Academy of Sciences,Moscow,Russia |
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Abstract: | The chemical composition of mineral components of the Omolon pallasite was determined by neutron-activation. Six types of
olivines were distinguished. Four types differ in the abundance of Co relative to Ni of CI chondrites. The fifth and sixth
types were distinguished on the basis of REE distribution in them. Both last types are variably enriched in LREE relative
to CI chondrites. In terms of Ca content relative to CI chondrite, these six types are subdivided into two groups: low-calcium
and high-calcium. The difference in Ca contents can be caused by different cooling rate of the precursor of these olivines.
The distribution pattern of siderophile elements in the pallasite metal indicates that a metallic phase experienced chemical
transformations since the time of its formation. The analysis of chemical composition of accessory minerals showed that: (1)
HREE are accumulated in tridymite; (2) troilite and daubreelite were formed under different temperature conditions; (3) magnetite
is the mineral of the outer zone of melting crust. Four fragments with anomalous contents of lithophile elements were found
in the pallasites and studied. The unusual chemical composition of phases and high degree of HREE fractionation in the fragments
suggest their formation at high temperatures at the early stage of the Solar system evolution. It is assumed that the Omolon
pallasite was formed as impact-brecciated mixture of the asteroid core (with composition close to IIIAB group of iron meteorites)
and mantle olivine from incompletely differentiated parent body of chondrite composition. |
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