A Chondritic Inclusion of Unique Type in the Cumberland Falls Meteorite

Abstract

New chemical, mineralogical and electron-probe data reveal one of the black chondrite inclusions in Cumberland Falls to be a variety bridging the apparent gap between enstatite and olivine-bronzite chondrites. Transitional properties include oxidized iron content, abundance of olivine and of the minor constituents schreibersite, daubreelite and alabandite, and average composition of olivine and pyroxene. The inclusion contains no taenite, but its kamacite lacks the alloyed silicon characteristic of enstatite chondrites. Although its overall state of oxidation is low (FeO≑2%), the Mg:Si ratio (0.94) of this inclusion suggests affinities with ordinary chondrites rather than enstatite chondrites, and its total iron content (19.5%) resembles that of LL group chondrites.The inclusion is texturally primitive, in that clinoenstatite is the dominant pyroxene and some chondrules contain fresh or finely devitrified glass. It is also unequilibrated. Pyroxene ranges up to 17 mol. % in ferrosilite content, and the olivine up to 5% in fayalite content, but in each case highly magnesian varieties predominate. Post-consolidation shock or reheating is implied by the blackened character of the inclusion.The white host material in Cumberland Falls is more thoroughly brecciated, and its pyroxene more extensively disordered, than most other enstatite achondrites. Petrographic relationships indicate that the chondritic and achondritic portions of the stone accumulated together after having been shocked, but do not reveal any fundamental genetic relationship between the two components. Indeed, the unrecrystallized character of the dark fragments, and the contrasted oxidation states of the chondritic and achondritic portions, suggest that they were not closely associated on the same parent body prior to fragmentation.The unique composition of the Cumberland Falls inclusions implies that chondritic stones do not display the complete range of possible oxidation states. A biased sampling process may be responsible for the restricted compositional variation of chondrites with earth-intersecting orbits.KeywordsParent BodyOlivine PhenocrystOrdinary ChondriteTotal Iron ContentOxidize Iron ContentThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.