SWIR (Southwest Indian Ridge; MODE)
Subject: Hydrothermal activity and massive sulfide formation along the Southwest Indian Ridge; InterRidge Program
Research Management: Ute Münch, Bernhard Pracejus, Bibhuranjan Nayak
Financing Institution: BMBF, 03 G 538A; also supported by the Humboldt Foundation in the frame of a Scholarship succeeding the initial funding
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Abstract: Tectonic deformation especially in zones of extensive dynamic regimes is, generally, considered to be a prerequisite for the convective heat transport and circulation of hydrothermal fluids and the associated formation of sulfide deposits. So far, exploration and investigations on hydrothermal seafloor mineralization have mainly been focused on spreading ridges in the Pacific and in the Atlantic. Considering that only fast spreading ridges may generate such high temperature events, the studies were extended to the (very fast) South Pacific Rise. On the slow spreading Mid Atlantic Ridge (MAR), extensive manganese oxide deposits of hydrothermal origin were found at 26°N (TAG field) followed by the discovery of black smoker activity in the same area two years later proving that high temperature massive sulfide chimneys may also develop even at slow spreading ridges. Compared to axial deposits on fast spreading ridges in the northern Pacific hydrothermal activity is better focused on slow spreading ridges, which also explains the bigger size of deposits e.g. in the Atlantic. For slow spreading rates, the topography of the ridge is characterized by a very rough and deep valley morphology with very pronounced walls. The slow to intermediate spreading ridges in the Indian Ocean are still poorly investigated, except for few low temperature hydrothermal deposits. Indications for recent high temperature hydrothermal activity are still unknown in the entire Indian Ocean, however, only one fossil Cu-rich sulfide deposit is known today, located at the Central Indian Ridge (for further information see "Hydrock"). Based on these perceptions, the INDOYO (INdian ocean Diving Observation by YOkosuka) diving cruise was carried out in 1998 by using the submersible Shinkai 6500 and the R/V Yokosuka with the following two major objectives: 1) To characterize crustal accretionary processes in the coldest section of the ridge; 2) To locate active hydrothermal fields in the Indian Ocean.

Regional morphology of the SWIR

A well-defined neovolcanic ridge, referred to as Mt. Jourdanne, is located in the 11th Segment (central co-ordinates: 27°51ÕS / 63°51ÕE) of the SWIR west of the Rodriguez Triple Junction. The area was investigated to determine the results of swath bathymetry and TOBI data, because the patterns indicated tectonic and volcanic activity at the summit of Mt. Jourdanne. To examine those interrelationships as well as to define the variety of extrusive rock assemblages, two geological traverses were carried out at the top of Mt. Jourdanne.

The seafloor displays several types of basaltic lava. Whereas the shallower slopes are dominated by sheet flow (lobate, folded) often covered by a thin sediment layer, the summit is characterized by pillows and tubes. Furthermore an approx. 100m wide E-W trending graben was identified at the summit. This prominent tectonic structure is accompanied by several smaller fissures running both parallel and perpendicular to the graben. Favored by both volcanic (heat source) and tectonic activity (pathways for fluid convection) extinct hydrothermal activity was detected at several places in the vicinity of the main graben at a water depth of about 2941m.

Hydrothermal products, which were recovered comprise sulfide impregnated basalts, massive sulfides and chimney-edifices. The newly discovered hydrothermal site at Mt. Jourdanne is the first known sulfide deposit along the ultra-slow spreading SWIR and proves now that high temperature hydrothermal activity is also possible along the ultra-slow SWIR.