Microprobe Analyses of Rare Earth Elements in Minerals of Jeju Peridotite and Jinju Chondrite

Abstract

Analytical protocols to measure rare earth element (REE) in silicates and phosphates using secondary ion mass spectrometry (SIMS) were developed and applied to measure REEs in clinopyexene of Jeju peridotite and in merrillite of Jinju chondrite. Cameca IMS-6f at the Meteorite Research Lab of Seoul National University was used in this study. Relative sensitivity factors (RSFs) and oxide forming ratios (OFRs) were obtained at a high mass resolution (mass resolving power ~ 6700) from standards of known REE concentrations. Traditional energy filtering method, employed with -60 V offset and 40 eV window, was applied to measure REEs in clinopyexene in Jeju peridotite. The high mass resolution without energy offset was used to measure REEs of merrillite in Jinju chondrite. Fourteen REE measurements of clinopyroxene in three Jeju peridotite xenoliths were carried out. Each of them has distinct REE patterns

(1) strongly depleted light REEs (LREEs

e.g., La: ~0.05 × CI chondrite) which are exponentially increased with increasing atomic numbers and followed by flat middle to heavy REEs (~3-10 × CI chondrite), (2) flat REEs (3-10 × CI chondrite) with slightly depleted LREEs and (3) flat REEs with enriched LREEs. They are consistent with previously measured bulk data (Choi et al., 2005): comparing REE abundances of spot data to those of corresponding bulk data, the latter is relatively enriched in La. Considering the analytical errors, pattern (1) and (2) are internally homogeneous, whereas pattern (3) shows heterogeneity in LREEs even in a single grain. Prior to this study, there were only bulk REE data of clinopyroxene in Jeju peridotite. The in-situ analyses show that heterogeneous LREE enrichment is found near the alteration veins. This observation implies that the heterogeneity of LREEs might be the result of secondary alteration process. Since La is the most mobile during weathering (Ludden and Thompson, 1979), the discrepancy between spot and bulk data in La might also be due to the secondary alteration. However, differences of LREEs in three distinctive patterns are too large to be explained by alteration only. There is no internal heterogeneity in pattern (1) and (2). And any alteration product is not found in samples having them. Thus these REE patterns might have formed by different origins or geological processes at the primary stage. Seven REE measurements on four merrillite grains in Jinju chondrite gave more or less homogeneous and flat REEs (100-300 × CI chondrite) with negative Eu anomaly. The pattern is similar to and the composition falls within the previously reported REEs from other equilibrated ordinary chondrites (Terada and Sano, 2002

Jones et al., 2014). Jinju is classified as a H5 ordinary chondrite and its shock stage was reported higher than S3 (Choi et al., 2015). It seems that the thermal and shock metamorphism, the parent body of Jinju had experienced, made its REEs equilibrated. It has been suggested that Jinju chondrite had been neither equilibrated nor compacted prior to the impact events (Choi et al., 2015). The Pb-Pb ages of merrillite in Jinju measured using Laser MC-ICP at Korea Basic Science Institute and gave 4505 ±12 Ma (Goh et al., 2016). It falls near the younger end of age distributions of phosphate minerals in equilibrated ordinary chondrites (Gӧpel et al., 1994). Considering relatively young age of Jinju chondrite, short–lived radioactive nuclides could not have been the major heat source. It is more plausible that post shock annealing has contributed to the thermal process of Jinju. Since REE abundances in merrrillite grains of Jinju are homogeneous within the analytical uncertainty, I suspect that the thermal metamorphism, possible due to impact annealing, was efficient enough to equilibrate not only major elements but also trace elements including REEs in Jinju chondrite.