Oxygen-isotope compositions of chondrule phenocrysts and matrix grains in Kakangari K-grouplet chondrite: Implication to a chondrule-matrix genetic relationship

摘要

To investigate a possible relationship between chondrules and matrix, we studied mineralogy, mineral chemistry, and in situ O-isotope compositions of chondrules, clastic matrix grains, and amoeboid olivine aggregates (AOAs) in the Kakangari K-grouplet chondrite. Most olivines and low-Ca pyroxenes in the Kakangari chondrules, matrix, and AOAs have similar magnesium-rich compositions, Fo(similar to 95-97)(similar to 0.3-0.5 wt% MnO) and En(similar to 90-96), respectively. These rather uniform chemical compositions of the different chondritic components are likely due to partial Fe-Mg-Mn equilibration during thermal metamorphism experienced by the host meteorite. Oxygen-isotope compositions of olivine and low-Ca pyroxene grains in chondrules and matrix plot along a slope-1 line on a three O-isotope diagram and show a range from O-16-enriched composition similar to that of the Sun to O-16-depleted composition similar to the terrestrial O-isotope composition. Most olivines and low-Ca pyroxenes in chondrules are O-16-poor and plot on or close to the terrestrial mass-fractionation line (mean Delta O-17 values +2 standard deviations: 0.0 +/- 0.8 parts per thousand and +0.2 +/- 0.9 parts per thousand for olivine and pyroxene, respectively), consistent with the previously reported compositions of bulk chondrules (Delta O-17 = -0.16 +/- 0.70 parts per thousand). In addition to these O-16-poor grains, a coarse-grained igneous rim surrounding a porphyritic chondrule contains abundant O-16-rich relict olivines (Delta O-17 similar to -24 parts per thousand). Oxygen-isotope compositions of olivines and low-Ca pyroxenes in matrix show a bimodal distribution: 12 out of 13 olivine and 4 out of 17 pyroxene grains measured are similarly O-16-rich (Delta O-17 similar to -23.5 +/- 2.9 parts per thousand), others are similarly O-16-poor (Delta O-17 similar to -0.1 +/- 1.7 parts per thousand). Due to slow oxygen self-diffusion, olivines and low-Ca pyroxenes largely retained their original oxygen-isotope compositions. The nearly identical O-isotope compositions between the chondrule phenocrysts and the O-16-poor matrix grains suggest both chondrules and matrix of Kakangari sampled isotopically the same reservoirs. In addition, the presence of abundant O-16-rich grains in matrix and the chondrule igneous rim suggests both components acquired similar precursor inventories. These observations imply that chondrules and matrix in Kakangari are genetically related in the sense that material that formed matrix was one of the precursors of chondrules and chondrules and some fraction of matrix experienced the same thermal processing event. The O-16-enriched bulk matrix value compared to the bulk chondrules reported previously is likely due to presence of abundant O-16-rich grains in the Kakangari matrix. (C) 2014 Elsevier Ltd. All rights reserved.