Review of Japanese Taiga project 1, Indian Ocean

InterRidge-Japan community conducted an interdisciplinary research project TAIGA, Trans-crustal Advection and In-situ biogeochemical processes of Global sub-seafloor Aquifer, from 2009 to 2013. The flows and chemical compositions of subseafloor fluid are regulated by tectonics, and are modified by the host rocks. During 5 years project, we selected three integrated study sites of different tectonic settings and carried out multi-disciplinary studies. The Central Indian Ridge (CIR) is one of our intensive survey areas, where four active hydrothermal fields show remarkable diversity in the chemical compositions of fluids and associated ecosystems. 

The southernmost CIR (segment CIR-S1 to S4) was completely mapped. Two active hydrothermal fields and a new inactive field were reported in this area. Kairei Hydrothermal Field (KHF) is unique hydrothermal system, located at the northeastern rift valley wall of CIR-S1. The fluids venting from the KHF are characterized by its high concentration of hydrogen with low methane/hydrogen ratio [Gamo et al., 2001; Takai et al., 2004]. A newly discovered Yokoniwa Hydrothermal Field (YHF) is inactive site with dead chimneys, located on the non-transform offset massif just north of KHF. We discovered that the KHF itself is located above basaltic lava field but gabbro and ultramafic rocks are widely exhumed around the KHF. Besides a previously known oceanic core complex [Sato et al., 2009], small oceanic core complexes and several small hills where we collect deep crustal and mantle rocks are distributed between segments S1 and S2 [Kumagai et al., 2008; Nakamura et al., 2009; Morishita et al., 2015]. On the non-transform offset massif where the YHF is located, we also sampled peridotites and gabbros. These hills suggesting melt-limited environment extend mainly along 2nd order segment boundary from the axial valley to 30km off-axis, i.e. ~1.7 Ma. The regional surface geophysical mapping and deep-tow magnetic profiling show high mantle Bouguer anomaly and prominent asymmetric spreading in the southernmost CIR segment [Okino et al., 2015]. These observations are consistent with limited melt supply and possible implication of detachment faults. Although the location of Edmond hydrothermal field at 23°52’S is similar to the KHF, i.e. near the top of axial valley wall, no evidence of ultramafic exposure is recognized and the fluid shows a typical sulfur-rich content. 

Besides the southernmost CIR, we also surveyed the Rodriguez Segment (CIR-S15 and S16) between 18°S and 20°S. Two new hydrothermal filed were confirmed in this area. The Solitaire field is located at the western flank of the CIR-S15B segment center. It consists mainly of clear smokers with high pH (4.8 measured at 25°C and 1 atm) fluids and a rich faunal community. The axial valley shows a highly asymmetric structure. The western rift wall shows a gentle step-like morphology and the Solitaire field has developed on the step. We also identified a number of volcanic cones on the step, and these are clearly different from the series of hummocks forming the neo-volcanic zone of the CIR axis. These volcanic cones on the step are undeformed and basalt samples with fresh glass were recovered from one of these cones, supporting the idea that the volcanism has recently occurred. Basalts from the volcanic cones on the step show higher Na2O and total alkali compositions, suggesting that the cones are part of volcanic knolls extending from the Gasitao Ridge [Machida et al., 2014]. These observations likely suggest that the Solitaire field is influenced by intra-plate volcanism related to the mantle plume or mantle heterogeneity. The Dodo field, on the other hand, is located just on the neo-volcanic zone of CIR-S16, where smooth sheet-flow lava covers an area of about 70 km2. We observed high temperature (356°C at a maximum), and H2 rich fluids (> 2mmol/L) spout directly from cracks in the basalt sheet lava in 2009 [Nakamura et al., 2012], but the fluid venting has already stopped in 2013 when we revisited the filed. There are no signs of surface/shallow ultramafic rocks, nor deep-rooted faults, that are usually considered to provide rich hydrogen in hydrothermal fluid. Kawagucci et al. [2008] proposed that serpentinization extensively occurs due to the enormous amount of water supplied from the Marie Celeste fracture zone. However, his idea contradicts the shallow, small-scale circulation at the sheeted basalt sheet-flow, suggested by the geological setting. We have not yet understood this type of hydrogen-rich systems, and coming cruise in early 2016 will focus on the fracture zone and the related alteration process.