Micro-distribution of oxygen isotopes in unequilibrated enstatite chondrites

We report petrology and high precision, in situ oxygen isotope analyses of silicates in chondrules, fragments, metal-rich nodules and refractory inclusions from the ALH 81189 (EH3), ALH 85159 (paired with ALH 81189) and from the MAC 88136 (EL3) chondrite. This is the first report of oxygen isotope ratios for individual objects in an EL3 and for the silicates associated with the metal-rich nodules that are characteristic of unequilibrated enstatite (E3) chondrites. The oxygen isotopic data from the chondrules and other objects form a trend, on a 3-isotope plot, that coincides with the slope similar to 1 primitive chondrule mineral (PCM) line (initially defined by chondrules from the Acfer 094 primitive carbonaceous chondrite), with most objects clustering at the intersection of the PCM line with the terrestrial fractionation (TF) line, near whole rock E3. The data from EH3 and EL3 overlap and show a similar distribution, suggesting they formed from a similar pool of precursors or in similar gaseous environments, but their mineral compositions suggest differences in their nebular environments and/or parent bodies. Silicates in the metal-rich nodules we analyzed (in both EH3 and EL3) have oxygen isotope ratios (as well as mineral compositions) similar to the silicate (metal-free) chondrules. This is consistent with formation of the metal-rich nodules prior to chondrite accretion, in an environment and from a process similar to that which formed the coexisting chondrules, but from more metal-rich mixtures of precursors. Olivine in an AOA from ALH 81189 is O-16-rich with delta O-18=-46.5 parts per thousand, delta O-17=-48.0 parts per thousand, similar to the AOAs and refractory inclusions previously reported in E3 and in all other chondrite groups. There is a clear distinction in oxygen isotopic compositions between the chondrules in the E3 chondrites and those in the LL and R as well as those in CV and CM chondrite groups. Chondrules from CR and E chondrites plot closer to the PCM line than all other chondrite groups with E3 chondrules having a different distribution toward more O-16-poor compositions. Chondrules in other chondrite groups form trends above and below the PCM. From the distribution of EC chondrules along the PCM line, we propose that similar pools of chondrule precursors were present in the different (carbonaceous, CR and Acfer 094 and non-carbonaceous, E) chondrule forming regions in the protoplanetary disk but with different amounts of O-16-rich refractory materials, prior to development of the postulated Jupiter divide that potentially separated inner (non-carbonaceous) from outer (carbonaceous chondrite) Solar System materials or Jupiter was inefficient in completely separating these materials. (C) 2021 Elsevier Ltd. All rights reserved.