[SGC40-P06] Mineralogical and geochemical characteristics of cuttings from Taketomi hot spring
Keywords:Taketomi Island, Hot spring, cuttings, vitrinite reflectance, carbon cycle, helium isotope
In 2017, the hot spring was drilled up to depth of 940 m in Taketomi Island. The cuttings with muddy water were sampled every 10 m. The thin sections were prepared from cuttings to be observed using polarization microscope, and the chemical compositions of grains were analyzed by SEM-EDX. The ground samples were analyzed for mineral and chemical compositions by XRD and EDX. The clay minerals gathered by levitation were measured for half-value width of illite using XRD. He and CO2 contained in samples that were crashed in vacuum were measured for isotope ratios by mass spectrometer. Calcite and dolomite collected from the cuttings were measured for the carbon and oxygen isotope ratios by mass spectrometer. The vitrinite in the cuttings was separated by a heavy liquid, and the reflectance was measured by a reflecting microscope.
In the microscopy, the stratum was roughly divided two formations; cryptocrystalline quartz layer and sand stone layer. Carbonaceous materials were observed in most of the stratum. Dolomite was limitedly observed in the vicinity of 800 m. In the XRD analysis, Quartz was detected at all depth. Calcite was detected at shallower depth than 30 m, and dolomite was detected only around 800 m. As a result of the EDX analysis, silicon was the most dominant in all layers, and especially, it was contained around 0-100 m, 300 m and around 900 m over 80%. There was a layer that contained much calcium. Around 0-30 m, 400-500 m and around 800 m, calcite was contained 15 % or more. The half-value width of illite was large, 0.8 Δ°2θ, in only 10 m surface, and as low as 0.2 to 0.4 Δ°2θ below 50 m. Vitrinite reflectance was around 5% in the whole layer, close to measurement limit. The helium content was 10-14-10-11 mol per 1g of cuttings, and tended to increase with increasing depth. The highest content was detected at 800 m depth. The helium isotope ratio(3He/4He)have value of 0.4-3.6 times that of atmosphere, and tended to increase with increasing depth. Particularly, it was higher at 100 m, 300-400 m and 700-800 m. The CO2 content per gram of cuttings, was in the range of 10-8-10-6 mol in most layers, but it showed higher values at 400 m and 800 m depth. the water tended to decrease with increasing depth, and showed especially high values at 400 m and 900 m. The carbon isotope ratio (δ13C) of calcite was -4.7‰VPDB, and the oxygen isotope ratio (δ18O) is -16.3‰VPDB. The δ13C value of dolomite was -1‰VPDB, and the δ18O value was -10.9‰VPDB. The δ13C value of CO2 from cuttings at 800 m was -7.3‰VPDB, which contained sufficiently amount of carbon to measure carbon isotope ratio.