Carbonates precipitated from the cold seepage are mainly composed of calcite, aragonite and dolomite. These minerals frequently coexist in same deposit. This occurrence suggests the different chemical condition during each mineral precipitation because simultaneous precipitation of carbonate minerals is unlikely from a single solution at any given physico-chemical condition. Therefore, isotopic analyses based on mineral separation or based on mineral composition in carbonate deposits is key for understanding their formation process. Isotopic analyses were carried out to carbonate deposits which were composed of calcite and aragonite collected from a mud volcano (Conical Seamount) at Mariana Forearc. Mineral identification and separation was carried out by using staining method. Calcite-aragonite content ratio in carbonate fragments was estimated by X-ray diffractometer. Stable carbon isotopic value falls in a range from –0.48‰ to –2.12‰, whereas stable oxygen isotopic value falls in a range from +4.20‰ to +6.71‰. Oxygen isotopic values have larger than that of other marine carbonates precipitated from the seawater, and calcite values show a pronounced propensity to deplete 18O compared with coexisting aragonite. This result was suggested that calcite and aragonite in carbonate deposits was largely affected by cold seepage. Different isotopic value between carbonate minerals was also observed in strontium isotope and radiocarbon measurements. Strontium isotopic value of aragonite was 0.70914 (avg.) and this value was close to present ocean value. Whereas, strontium isotopic value of calcite was 0.70674 (avg.) and this value was completely different in present ocean value. The difference in radiocarbon activity (Δ14C) between calcite and aragonite, being from –976‰ (avg.) and –831‰ (avg.) for aragonite were observed. These results suggest that calcite and aragonite precipitation was result from mixing of seawater and cold seepage. Aragonite was precipitated from the mixing fluid of about 60% of seawater and about 40% of cold seepage, and calcite was about 90% of seawater and about 10% of cold seepage. These two mixing ratio was estimated by using radiocarbon results. Since all isotopic results, except for stable carbon isotope, show isotopic variation depend on carbonate mineral species, I have interest in iodine isotopic value of calcite and aragonite. If possible to measure iodine isotope of aragonite and calcite, as with radiocarbon, iodine isotope will be a tool for understanding of carbonate formation process and will be able to estimate mixing ratio between cold-seepage and seawater. The results of iodine isotope of carbonates will be presented on the day.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)