STALKING THE LREE-ENRICHED COMPONENT IN UREILITES

Abstract

The results of an experiment using progressively more aggressive acid leaches on a LREE-enriched whole-rock sample of Kenna show that the vast majority of the leachable LREE-enriched component in this ureilite is removed by weak (0.04 M and 0.2 M) HNO3, and suggest that this component resides principally on grain surfaces of interstitial phases, rather than in a minor, LREE-enriched mineral. Our best estimate of the Sm-Nd composition of this component is 147Sm/144Nd = 0.112, εNd = -8.5, [Nd] = 18 ppm, but neodymium isotopes are incompletely equilibrated. Strontium and Ba are also enriched in this component, but other incompatible elements are not. The olivine + pyroxene residue in this experiment is over-leached relative to what is necessary to remove the leachable LREE-enriched component, and has 147Sm/144Nd = ~0.33, similar to the most LREE-depleted whole-rock samples of Kenna. This result strengthens the argument that the 3.79 Ga 147Sm/144Nd#143Nd/144Nd line defined by whole-rock samples and mineral separates of Kenna, Novo Urei, ALHA 77257 and Goalpara is an isochron, because if it were a line of mixing between 4.55 Ga ultramafic rocks and leachable LREE-enriched component then samples devoid of this component would have 147Sm/144Nd = 0.51 (the composition at which the 3.79 Ga line intersects the 4.55 Ga chondritic evolution line). The 3.79 Ga isochron may record the time of reequilibration of highly LREE-depleted (147Sm/144Nd = 0.51) 4.55 Ga proto-ureilite material with LREE-enriched material. The best candidate for this LREE-enriched material is a CO#CO2 fluid and the most likely time for its generation was during the event that led to formation of reduction rims on ureilite olivine and pyroxene grains. Although this model has serious difficulties which prevent us from embracing it, we suggest that it offers the most promising lines along which to think about the origin of the LREE-enriched component in ureilites. Acid-leaching experiments have failed to fully reveal the nature of this component and some other method of investigation is needed.