A seafloor hydrothermal fluid circulation system develops beneath the seafloor where a magma as heat source and fault systems as fluid conduits are equipped. Seafloor hydrothermal systems act as important intermedia among the lithosphere, hydrosphere and biosphere. Seawater entrained into the seafloor evolves to the vent fluid that exit smokers or chimneys, through the processes during the fluid circulation such as elemental exchanges by water-rock interactions and involvement of magmatic volatiles. These interactions cause hydrothermal alteration of the lithosphere, looking from the opposite perspective. Chemical species dissolved in the vent fluids provide energy sources for biosphere around and beneath the seafloor of hydrothermal fields. Geochemical flux transported into the deep ocean by hydrothermal plumes affects chemical and biological processes in the hydrosphere. Formation of mineral deposits from the hydrothermal fluids leads concentration of specific metal elements resulting from long duration of hydrothermal activity. As hydrothermal vent sites have been discovered ubiquitously in various geologic settings, their diversity especially in geochemical aspects becomes more noticeable and interesting.

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.

This paper reviews the principles and applications of the compound-specific radiocarbon dating developed mainly during the last two decades. This methodology requires two technical challenges: 1) purification of target compounds from natural samples, and 2) small-scale radiocarbon dating with accelerator mass spectrometry. Since its establishment, the compound-specific radiocarbon dating has been successfully applied not only to geochronology of marine and lacustrine sediments, but also to various fields including biogeochemistry, environmental science, and archaeology. Here I describe some of these applications of this approach that provided unique information and revolutionized our knowledge. Variable ages of the organic compounds from the marine sediments have clearly suggested that the transport and deposition processes of the organic matter in the ocean are more complex than that had been expected. Source apportionment has also successfully conducted for organic pollutants from various environmental samples. I end this review with a call for continued efforts including technical improvements for evaluating the purity of the target compounds and extension of the target compounds to variable compounds like amino acids.