Elemental fractionations among enstatite chondrites

Abstract

Neutron activation data on 14 elements in eight enstatite-chondrite falls are reported. These and literature data on an additional 28 elements show that intragroup elemental fractionations generally fall into one of three basic patterns: 1. (1) siderophilic- and chalcophilic-element abundances are about 1.5 times greater in E4-5 than in E6 chondrites; 2. (2) non-volatile lithophile-element abundances in E4-5 chondrites are generally about 1.0–1.2 times those in E6 chondrites; 3. (3) highly volatile elements are higher in E4-5 chondrites than in E6 chondrites by factors of 6–50. In addition, abundances (relative to Si) of most refractory and volatile elements are lower (by factors of 0.5–0.9) in E4 chondrites than in C1 chondrites. Because of the compositional hiatus often observed between E4-5 and E6 chondrites, there exists the distinct possibility that they are separate groups which were stored in different parent bodies. However, because of their close similarity in oxidation state, it seems likely that they originated at the same nebular location, far removed from the formation locations of the other, much more oxidized groups of chondrites. The E-group fractionation patterns can be plausibly explained in terms of four fractionation processes: 1. (1) loss of oxidizing agents (i.e. H 2O) and refractory materials from starting materials of solar composition: 2. (2) partial loss of moderately volatile elements, perhaps as a result of gradual loss of nebular gas during condensation; 3. (3) more efficient agglomeration of metal particles than silicate particles; and 4. (4) increase of nebular temperatures during agglomeration-accretion resulting in the loss of volatile-rich late condensates from E6 chondrites. The low degree of oxidation of enstatite chondrite materials is best understood in terms of a fractionated nebula. At a pressure of 10 −4 atm the Si content of E4 metal can be produced at 1350°K if the H 2O H 2 ratio is 5 times lower than that in unfractionated solar-system material. A nebula-wide fractionation process involving radial transport of refractories and H 2O is indicated, and a suitable model in which the nebula-wide mixing of the gas phase continues during condensation is proposed.