2017 年 51 巻 3 号 p. 119-133
Radiochemical neutron activation analysis (RNAA) has great analytical advantages in elemental determination over any other analytical method, even over sophisticated modern instrumental analytical methods like inductively coupled plasma (ICP) atomic emission spectrometry (ICP-AES) and ICP mass spectrometry (ICP-MS). Examples are described for rare earth elements (REEs), Zr, Hf, In and halogens (Br and I) in geochemical reference samples. There appear systematic differences between our RNAA data and literature/recommended/certified values, for which we believe that RNAA values are correct from the methodological view point of RNAA, where chemical loss and contamination of analytes during chemical procedures are protected. RNAA was applied to olivine separated from pallasite meteorites. The olivine separates were washed in HCl and both leachate (solution) and residue (solid) samples were subjected to RNAA for determination of REEs, Ir and Au contents. REEs, especially light REEs, were extremely depleted in well-washed olivine. No V-shaped CI-normalized abundance patters of REEs were always reproduced although preceding studies reported such abundance patters. V-shaped abundance patters were found for leachate samples, suggesting such patters are not real but superficial for pallasite olivine. Similar abundance gradients were observed for Ir and Au; both elements are extremely depleted in well-washed olivine and are enriched in the leachate. Literature values of Ir and Au contents in pallasite olivine are similarly high. I am confident that genuine contents of REEs, Ir and Au in pallasite olivine could be determined by applying RNAAe. Based on these data, the formation scheme of pallasite meteorites was constracted. Our data support that pallasites formed in the boundary region of metallic core and silicate mantle. REEs must have been partitioned into olivine under equilibrium, but light REE abundances are even lower than those expected from experimental partition coefficient between olivine and silicate melt. Either the cumulative olivine accommodated REEs from the light REE-depleted silicate melt or the light REEs were excluded from the olivine crystal to the olivine-metal boundary through diffusion.