Genesis and diagenesis of travertine, Futamata hot spring, Japan

Genetic and diagenetic processes of travertine were examined at Futamata hot spring, where both active and inactive (subfossil and fossil) travertines are present. Geochemical and geomicrobiological analyses of active travertines reveal that abiotic process of CaCO3 precipitation is predominant. Photosynthetic inhibition of CaCO3 precipitation occurs at the upstream, while the cyanobacterial population is low in the downstream, resulting in photosynthesis-induced CaCO3 precipitation only in the midstream. Nonetheless, microorganisms have an effect upon depositional fabrics, and filamentous cyanobacteria possessing non-acidic sheaths contribute to the particle trapping/binding and the generation of pore space. Both active and inactive travertines are mainly composed of calcite; however, active travertine in the up-/midstream also contains some aragonite due to high rates of CO2 degassing. Active and subfossil travertines are characterized by domal topography, and relatively high flow rates are assumed due to the dominance of slope facies. On the other hand, fossil travertines are characterized by fissure-ridge topography and relatively low flow rates are assumed due to the dominance of crest and marsh facies. The elemental composition of travertines primarily reflects their constituent minerals, and the active travertines consisting of both aragonite and calcite show higher Sr, Ba, Na, and K contents, whereas travertines consisting solely of calcite show higher Mg and Mn contents. Due to elevated Mn content, bright cathode luminescence is exhibited by the primary calcite of active travertines. Most fossil travertines have experienced prominent diagenetic alteration in the vadose zone and are commonly cemented and recrystallized. In addition to meteoric water, percolating hot spring water was also involved in the diagenetic process. Combined with the results of U–Th dating, our results suggest that the formation of fissure-ridge travertines at Futamata hot spring began approximately 20 thousand years ago (ka) via fault activity, which terminated at around 7 ka and the domal travertines were formed by hot spring water discharging from several vents.