Cruise GIMNAUT (Geochronology, ridge-hotspot Interaction, Magnetics with NAUTile) started on May 14^th, 2000 in Mauritius Island and ended on June 7^th, 2000 in Reunion Island. The onboard scientific team was led and mainly constituted by members of Unité Mixte de Recherche 6538 "Domaines Océaniques" of CNRS, a research laboratory of Institut Universitaire Européen de la Mer (Université de Bretagne Occidentale) in Brest. It also included scientists from Toulouse, Paris and Le Mans, France, as well as colleagues from Japan, India and Mauritius. As a major topic, cruise GIMNAUT addressed the problem of accurate dating methods for recent processes at mid-ocean ridges, with a particular attention to

- the combined use of existing geochemical absolute dating methods for recent Mid-Ocean Ridge Basalt and their intercalibration for the overlapping period ; producing reliable ages is essential to study the magmatic and tectonic processes at mid-ocean ridges; and

- the observation of a dated and continuous sequence of the recent geomagnetic intensity variations through the acquisition of near-bottom anomalies and measurement of paleointensity on rock samples ; if the oceanic crust accurately records the relatively well known sequence of these variations, deep sea magnetics may ultimately be used to date the ocean floor at small scale as classically done at the scale of million years with surface magnetics.

These two major themes could be efficiently studied on the Central Indian Ridge at the latitude of the Rodrigues Ridge (19°S), where basalt are enriched in trace elements required for an optimal quality of the ages. We have realized two transects of 9 and 8 dives, respectively, which allow investigation of the last million year with a special emphasis on the youngest 400 ky. In addition, this target provides the opportunity to study a slow to intermediate spreading center overlying a hot mantle, and more specifically a recent ridge-hotspot interaction, through detailed investigations of the intersection area (2 dives), and surface and sea-bottom gravity measurements to evaluate possible crustal thickening.

The cruise has used the following investigation techniques during the 20 deep sea dives of submersible Nautile at depths ranging between 2000 and 3200 m :

• direct visual geological observation of the tectonic and volcanic structures and of the different types of lithologies, together with the acquisition of videos and photographs;
• continuous acquisition of vector magnetic field data (the three component magnetometer of the Ocean Research Institute of the University of Tokyo was successfully used on 17 dives) and of sea water turbidity (the nephelometer of IFREMER DRO/GM was successfully used on the first dive);
• acquisition of gravity data at 29 stations on the seafloor, on 13 dives;
• collection of rock samples to be analyzed in terms of petrology, geochemistry, and magnetic properties; about 150 sites have been sampled, and a first petrological and petromagnetic analysis have been carried out onboard; later on, radiochronologic dating and magnetic paleointensity determination will play a crucial role to fulfill the objectives of the project.

• acquisition of 2 deep tow magnetic profiles (deep tow magnetometers of CNRS and of the Ocean Research Institute of the University of Tokyo) several hundred meters above seafloor ; this program have been interupted following the loss of the Japanese instrument and of several other devices, due to a failure of the acoustic positioning system
• collection of rock samples by dredging at 12 locations, aimed to enlarge the area covered by Nautile sampling
• acquisition of multibeam bathymetry and imagery, surface magnetic and gravity profiles to be compared to deep sea data to constrain the mid-ocean ridge structure

Last but not least, it should be mentioned that cruise GIMNAUT was the first cruise to operate Nautile in the Indian Ocean and only the second one to operate a deep sea submersible in this ocean.  It has given the chance to strengthen existing scientific collaborations with the Ocean Research Institute of the University of Tokyo and the National Institute of Oceanography of Goa, India, two Japanese and one Indian colleagues having dived onboard Nautile. The participation of two Mauritian scientists, required to work in the Exclusive Economic Zone of the Republic of Mauritius, has provided the opportunity to establish links with the newly established Mauritius Oceanographic Institute.