Oxygen isotope measurements of individual unmelted Antarctic micrometeorites

We present oxygen isotope measurements of 28 unmelted Antarctic micrometeorites measuring 150-250 mu m (long axis) collected in the South Pole water well. The micrometeorites were all unmelted and classified as either fine-grained, scoriaceous, coarse-grained or composite (a mix of two other classes). Spot analyses were made of each micrometeorite type using an ion microprobe. The oxygen isotope values were measured relative to standard mean ocean water (SMOW) and range from delta O-18 = 3 parts per thousand to 60 parts per thousand and delta O-17 = -1 parts per thousand to 32 parts per thousand falling along the terrestrial fractionation line (TFL) within 2 sigma errors. Several analytical spots (comprising multiple phases) were made on each particle. Variability in the oxygen isotope ratios was observed among micrometeorite types, between micrometeorites of the same type and between analytical spots on a single micrometeorite indicating that micrometeorites are isotopically heterogeneous. In general, the lowest isotope values are associated with the coarse-grained micrometeorites whereas most of the fine-grained and scoriaceous micrometeorites have an average delta O-18 >= 22 parts per thousand, suggesting that the matrix in micrometeorites is isotopically heavier than the anhydrous silicate phases. The oxygen isotope values for the coarse-grained micrometeorites, composed mainly of anhydrous phases, do not lie along the carbonaceous chondrite anhydrous mineral (CCAM) line, as observed for olivines, pyroxenes and some kinds of chondrules in carbonaceous chondrites, suggesting that coarse-grained MMs are not related to chondrules, as previously thought. Our measurements span the same range as values found for melted micrometeorites in other studies. Although four of the micrometeorites have oxygen isotope values lying along the TFL, close to the region where the bulk Cl carbonaceous chondrites are found, 21 particles have very enriched O-17 and O-18 values that have not been reported in previous analyses of chondrite matrix material, suggesting that they could be a new type of Solar System object. The parent bodies of the micrometeorites with higher O-18 values may be thermal metamorphosed carbonaceous asteroids that have not been found as meteorites either because they are friable asteroids that produce small particles rather than rocks upon collision with other bodies, or because the rocks they produce are too friable to survive atmospheric entry. (c) 2006 Elsevier Inc. All rights reserved.