Unequilibrated Enstatite Chondrites Research Articles

Heterogeneous REE patterns in oldhamite from aubrites: Their nature and origin

A comprehensive study of the REEs in oldhamite from three aubrites confirms that heterogeneous REE patterns are a general feature of CaS in these meteorites. Ten distinct patterns have been observed in 109 grains analyzed. Four patterns are directly comparable to REE patterns seen in oldhamite from the unequilibrated (E3-4) enstatite chondrites, and many of these exhibit positive Eu and/or Yb anomalies that are best understood in terms of condensation from the solar nebula. Other REE patterns, however, show evidence of igneous fractionation. More than one process is thus required to explain the variety of REE patterns seen in oldhamite from the aubrites. It is proposed that oldhamite was close to the liquidus under the conditions of aubrite formation and that some grains melted while others did not. Thus, some oldhamites have REE patterns that reflect the fractionations of igneous processing, while others have retained REE characteristics established during formation in the solar nebula. In contrast to oldhamite from the aubrites and E3-4 chondrites, CaS from two equilibrated (EL6) enstatite chondrites shows high and uniform REE abundances that result from equilibration of the REEs with neighboring phases during a prolonged metamorphic episode.

Gel synthesis of magnesium silicates: A 29Si magic angle spinning NMR study

The formation of the magnesium silicate minerals forsterite, enstatite, and roedderite by heating of amorphous “protosilicate” gels precipitated from aqueous solution has been studied by 29Si MAS nmr. Gentle drying of the hydrogels at 110° C gives materials with broad nmr signals that do not differ appreciably with preparation conditions, but the minerals formed by heating at 750° C or higher are greatly dependent on the precipitation and washing conditions of the original gel. The rare mineral roedderite, best known from studies of unequilibrated enstatite chondrite meteorites, becomes a major species along with forsterite when the hydrogels are washed with sodium hydroxide solution before drying and heating to 750° C.

Compositional Trends in Chondrules from Unequilibrated Enstatite Chondrite, Parsa

Multielemental analysis of chondrules from unequilibrated E3 chondrite Parsa show that the lithophile elements, both refractory and volatile, are enriched and siderophiles are depleted relative to C1 abundances. This behaviour is generally similar to chondrules from unequilibrated ordinary chondrites (UOC). A detailed comparison of compositional trends in chondrules from E3 chondrites (Parsa and Quingzhen) with UOC's indicates that some of the precursor components of chondrules of E3's are different from those of UOC's.

ROEDDERITE IN THE QINGZHEN (EH3) CHONDRITE

The rare mineral roedderite, (Na1.09 K0.89 Ca0.02)2.00 (Mg4.71 Fe0.27)4.98 (Si11.80 Al0.09)11.89 O30 has been found in accessory amounts in the highly unequilibrated enstatite chondrite, Qingzhen. It occurs in association with minor amounts of albite and SiO2 as inclusions within the metal or sulfide phases of metal‐sulfide assemblages. The roedderite crystals are connected through oxide and silicate veins to the surrounding matrix. The presence of glass coated vesicles on the surface of the assemblages strongly suggests that roedderite originated in the presence of a fluid phase, presumably during post‐accretional planetary processes.

Chemical and physical studies of type 3 chondrites—V: The enstatite chondrites

We report instrumental neutron activation analysis determinations of 19 major, minor and trace elements in three enstatite chondrites. Based on these, and literature data on the bulk and mineral composition of enstatite chondrites, we discuss the history of the type 3 or unequilibrated enstatite chondrites, and their relationship with the other enstatite chondrites. The type 3 enstatite chondrites have E chondrite lithophile element abundances and their siderophile element abundances place them with the EH chondrites, well resolved from the EL chondrites. Moderately volatile chalcophile elements are at the low end of the EH range and Cr appears to be intermediate between EH and EL. We suggest that the type 3 enstatite chondrites are EH chondrites which have suffered small depletions of certain chalcophile elements through the loss of shock-produced sulfurous liquids. The oxygen isotope differences between type 3 and other enstatite chondrites is consistent with equilibration with the nebula gas ~30° higher than the others, or with the loss of a plagioclase-rich liquid. The mineral chemistry of the type 3 chondrites is consistent with either low temperature equilibration, or, in some instances, with shock effects.

Evidence for relict grains in chondrules of Qingzhen, an E3 type enstatite chondrite

Petrographic and chemical studies of the Qingzhen chondrite strongly suggest that it is the most highly unequilibrated (type 3) enstatite chondrite recognized so far. Qingzhen contains abundant, well-defined chondrules, some of which were incompletely molten during the chondrule formation process. The relict olivine grains within these chondrules contain dusty inclusions of almost pure metallic Fe, which appear to be the in-situ reduction product of the fayalitic component of the olivine. The reduction process presumably took place at the time of chondrule formation and the chondrule precursor material must have been more oxidized than average enstatite chondrite material. We believe that this oxidized material may have formed at the enstatite chondrite formation location in the solar nebula, provided fluctuations in the degree of oxidation of the nebular gas existed at such locations. Reheating of this material under more reducing conditions would lead to the observed reduction of the olivine. Igneous olivines within chondrules always contain detectable amounts of CaO, while relict olivines are essentially CaO-free. This seems to suggest that the relict olivines did not originate during a previous igneous process of chondrule formation and might represent condensation products from the early solar nebula.