(2) Rift failure stage, which is responsible for decreasing magmatic eruption and
enhancing the cooling of the sub-axial magma lens, which leads to the eruption of more evolved lava and increases the crystal-liquid fractionation process
in magma reservoirs. This stage is observed on the ridge segments at 18°30
0 S
at 2640 m depth with a small graben 50 m wide and 60–100 m depth, showing
tectonic activity with fissured and faulted crust associated with declining
hydrothermal events. Also, it is shortly after each eruption that low temperature (\60 °C) fluids emanated from the interstices of fractured fresh lava
flows. Prior to the main eruption, bacteria and tube worms live on the warm
pillow crusts associated with diffuse venting.
(3) Amagmatic stage occurs when magma upwelling is replaced by tectonic
activity responsible for faulting and fissuring with the formation of a larger
graben 150–200 m deep and 1 km wide as shown on the 21°26
0 S EPR segment at 2,830 m depth. This ridge segment is in a stage of tectonic extension
and characterized by intense hydrothermal activity with black smokers and
several chimneys up to 20 m high. Both low (30–60 °C) and high temperature
hydrothermal vents ([200 °C) are observed. This is due to the retrieval of
deeper lying dykes marked by the decrease/or absence of magmatic input.
Because of the large amount of crustal fissuring, the concentration of larger
hydrothermal fields due to more intense fluid circulation is increased. Lobated
flow and sporadic pillow lavas form the floor of the valley.
In summary, during crystal-liquid fractionation a zoned magma chamber will
favor episodic volcanic eruptions underneath a spreading ridge. The domed
structure and the fresh aspect of the lava flows suggested an intense magmatism.
When this magmatic activity declines, hydrothermalism takes over and becomes
the prominent event. The reduction of magmatic activity is related to a cooling of
the magma reservoir underlying the ridge segment. The domed shape structure
along the ridge will eventually collapse and form a linear graben or rift valley. It is
at this stage that the faulting and fissuring increases and opens the path for
hydrothermal fluid circulation giving rise to active hydrothermal chimneys. This
will last until new magma upwelling occurs, to replenish the magma reservoir.
Another expedition called NAUDUR (anachronism for NAUtile Dive Ultrafast
Ridge) took place between the 2nd and 30th of December 1993, in the same area of
the East pacific Rise. The cruise was headed by Jean-Marie Auzende from the
Geoscience department of IFREMER and included petrologists and geochemists
(Daniel Bideau, Yves Fouquet, Rodey Batiza, John Sinton), structural geologists
(Yves Lagabrielle, Jean-Marie Auzende, Marie-Helene Cormier, Valerie Ballu and
Piera Spadea) and a biologist (Patrick Geistdoerfer). The invited American scientists were from the Universities of Hawaii (Batiza and Sinton) and from the
University of California in Santa Barbara (Cormier) who had previously worked in
the area. The other scientists were from different French institutions such as
IFREMER in Brest (Auzende, Bideau, Fouquet), the University of Bretagne Occidentale in Brest (Lagabrielle), University of Paris 6 (Ballu), the Museum of
Magmatic and Hydrothermal Cyclicity
243
enhancing the cooling of the sub-axial magma lens, which leads to the eruption of more evolved lava and increases the crystal-liquid fractionation process
in magma reservoirs. This stage is observed on the ridge segments at 18°30
0 S
at 2640 m depth with a small graben 50 m wide and 60–100 m depth, showing
tectonic activity with fissured and faulted crust associated with declining
hydrothermal events. Also, it is shortly after each eruption that low temperature (\60 °C) fluids emanated from the interstices of fractured fresh lava
flows. Prior to the main eruption, bacteria and tube worms live on the warm
pillow crusts associated with diffuse venting.
(3) Amagmatic stage occurs when magma upwelling is replaced by tectonic
activity responsible for faulting and fissuring with the formation of a larger
graben 150–200 m deep and 1 km wide as shown on the 21°26
0 S EPR segment at 2,830 m depth. This ridge segment is in a stage of tectonic extension
and characterized by intense hydrothermal activity with black smokers and
several chimneys up to 20 m high. Both low (30–60 °C) and high temperature
hydrothermal vents ([200 °C) are observed. This is due to the retrieval of
deeper lying dykes marked by the decrease/or absence of magmatic input.
Because of the large amount of crustal fissuring, the concentration of larger
hydrothermal fields due to more intense fluid circulation is increased. Lobated
flow and sporadic pillow lavas form the floor of the valley.
In summary, during crystal-liquid fractionation a zoned magma chamber will
favor episodic volcanic eruptions underneath a spreading ridge. The domed
structure and the fresh aspect of the lava flows suggested an intense magmatism.
When this magmatic activity declines, hydrothermalism takes over and becomes
the prominent event. The reduction of magmatic activity is related to a cooling of
the magma reservoir underlying the ridge segment. The domed shape structure
along the ridge will eventually collapse and form a linear graben or rift valley. It is
at this stage that the faulting and fissuring increases and opens the path for
hydrothermal fluid circulation giving rise to active hydrothermal chimneys. This
will last until new magma upwelling occurs, to replenish the magma reservoir.
Another expedition called NAUDUR (anachronism for NAUtile Dive Ultrafast
Ridge) took place between the 2nd and 30th of December 1993, in the same area of
the East pacific Rise. The cruise was headed by Jean-Marie Auzende from the
Geoscience department of IFREMER and included petrologists and geochemists
(Daniel Bideau, Yves Fouquet, Rodey Batiza, John Sinton), structural geologists
(Yves Lagabrielle, Jean-Marie Auzende, Marie-Helene Cormier, Valerie Ballu and
Piera Spadea) and a biologist (Patrick Geistdoerfer). The invited American scientists were from the Universities of Hawaii (Batiza and Sinton) and from the
University of California in Santa Barbara (Cormier) who had previously worked in
the area. The other scientists were from different French institutions such as
IFREMER in Brest (Auzende, Bideau, Fouquet), the University of Bretagne Occidentale in Brest (Lagabrielle), University of Paris 6 (Ballu), the Museum of
Magmatic and Hydrothermal Cyclicity
243