PGA data of Antarctic carbonaceous chondrites with CI and CM affinity
Description
At present, carbonaceous chondrites are classified into nine groups based on their chemical characteristics. Among carbonaceous chondrites, CI and CM chondrites are considered the more primitive meteorites. Currently, there are five known non-Antarctic CI chondrites, and four have been identified among Antarctic meteorites. In this study, elemental analysis was conducted on a total of twelve meteorites, focusing primarily on the major element composition, including four Antarctic CI chondrites and eight others having affinities with CI or CM but insufficiently chemically clarified. The analytical method employed in this study was neutron-induced prompt gamma-ray analysis (PGA), which shares analytical advantages similar to wet chemical analysis. Quantitative values were obtained for 15 elements: Fe, Mg, Si, S, Ni, H, Al, Ca, Cr, Mn, Co, Ti, chlorine, B, and Gd, and almost all of the main constituent elementals were quantified. Comparison with literature values, including those obtained by the wet chemical method, confirmed that the values obtained by the PGA method in this study are sufficiently reliable with the exception of B. Oxygen isotopic compositions of five of the 12 meteorites analyzed, including the Y-86029 meteorite classified as CI, have been reported, and based on the results, a new group name CY was proposed for these meteorites. PGA analysis showed that, in common with the 12 meteorites, H and chlorine are more deplete than those in non-Antarctic CI chondrites. Notably, the degree of this depletion varies significantly between meteorites and exhibits a correlation between the two elements. Petrological and mineralogical observations suggest that these meteorites underwent thermal metamorphism after aqueous alteration on their parent body, being consistent with the observed depletion of chlorine and H. Based on these findings, it was deemed appropriate to classify the analyzed twelve meteorites as CY chondrites. Based on the abundance of Mn and S, it was revealed that these twelve meteorites are classified into two distinct groups. Since the abundance of Mn and S is expected to remain unchanged through aqueous alterations and thermal metamorphism on the parent body, the differences in their abundance likely originated during the formation processes of the parent body, reflecting elemental fractionation during condensation and subsequent accumulation. The Mn and S compositions suggest that the CY chondrites originated from two parent bodies (or parent materials), one formed under conditions similar to non-Antarctic CI, and the other formed through a process intermediate between CM and CI. For facilitating further discussion on CY chondrites, it is proposed to refer to these two groups as CYi and CYm, representing those with compositions closer to CI and CM, respectively.
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In this study, neutron-induced prompt gamma-ray analysis (PGA) was used to determine the elemental composition of Antarctic chondrite meteorites. PGA involves measuring prompt gamma rays emitted immediately (within 10-14 seconds) after neutron capture. This entails conducting the measurement of prompt gamma rays while irradiating the sample with neutrons. A notable characteristic of the PGA method is its use of a neutron flux several orders of magnitude lower than that of INAA, resulting in practically negligible effects of neutron irradiation, such as residual radioactivity and transmutation by neutron capture reactions. Consequently, samples analyzed by PGA can be reused, and the less restrictive size requirements make PGA advantageous for the analysis of samples that should remain physically intact, such as archaeological and meteorite samples. The PGA experiment was executed using the PGA measurement system of the JRR-3 research reactor at the Japan Atomic Energy Agency (JAEA), situated approximately 50 m from the core of JRR-3. During irradiation with thermal neutrons, the emitted prompt gamma rays were measured using a Ge semiconductor detector. The thermal neutron flux at the sample position was approximately 1 x 108 n/cm2/s. The measurements were conducted under a carbon dioxide atmosphere, and a robot facilitated the automated exchange of samples. Meteorite samples were heat-sealed in 1.5 cm x 1.5 cm bags made of fluorinated ethylene-propylene (FEP) film. For each meteorite sample, the allotted measurement time ranged from 6,000 to 18,000 s. The quantification of elemental compositions was carried out through comparative methods. In this process, the analyzed target samples (meteorite samples) and comparison standard samples with known content of the element to be quantified measured for their emitting prompt gamma-rays under the same conditions. By comparing the gamma-ray counting rates per unit time, the content of the respective element in the meteorite samples was determined. Chemical reagents and rock standard samples were employed as comparison standard samples. Among them, the quantitative values for 9 elements, namely Ca, Si, Co, Ni, Mg, Cr, chlorine, S, and H, were determined based on chemical reagents of CaCO3 powder, Si metal, Co metal, Ni metal, Mg metal, Cr metal, NH4Cl (for H and chlorine) and elemental S. For the remaining 5 elements (Al, Ti, Gd, Mn, B), a basaltic geochemical reference material JB-1 prepared by the Geological Survey of Japan was used as the comparison standard sample. Additionally, for the remaining element, Fe, the Allende meteorite powder sample prepared and supplied by the Smithsonian Institution, USA was employed as the comparison standard. The measurement time allocated for the reference standard samples was set within the range of 600 to 6,000 s.