The Pitcairn hotspot in the South Pacific: distribution and composition of submarine volcanic sequences

The Pitcairn hotspot in the South Pacific: distribution and composition of submarine volcanic... Multibeam bathymetry and bottom imaging (Simrad EM12D) studies on an area of about 9500 km 2 were conducted over the Pitcairn hotspot near 25°10′S, 129° 20′W. In addition, 15 dives with the Nautile submersible enabled us to obtain ground-true observations and to sample volcanic structures on the ancient ocean crust of the Farallon Plate at 3500–4300 m depths. More than 100 submarine volcanoes overprint the ancient crust and are divided according to their size into large (>2000 m in height), intermediate (500–2000 m high) and small (<500 m high) edifices. The interpretation of seafloor backscatter imagery accompanied by submersible observations and sampling enabled us to infer that the total volume of submarine lava erupted during hotspot activity is about 5900 km 3 within a radius of about 110 km. The most recent volcanic activities occur on both small and large edifices composed of a great variety of lava flows. These flows vary in composition, following a succession from picritic basalt to alkali basalt, trachybasalt, trachy-andesite and to trachyte. Their large range of SiO 2 (48–62%), Na 2 O+K 2 O (2–11%), Ba (300–1300 ppm), MgO (1–11%), Nb (19–130 ppm), Ni (4–400 ppm) and rare earth elements suggests that crystal–liquid fractionation from basanite and/or picritic melt sources was a major process. The variation in composition between the least evolved basaltic rocks and the other more evolved silicic lava is marked by a difference in their flow morphology (pillow, giant tubes, tabular to blocky flows). The lava composition and field observation indicate that several magmatic pulses giving rise to cyclic eruptions are responsible for the construction of the edifices. The two larger edifices (>2000 m high) show more extensive eruptive events and a wider range in compositional variability than the smaller (<500 m high) ones. Several (five) submersible transects made along the slope of one of the largest edifices (Bounty) enabled us to observe at least nine successive eruptive cycles progressing from pillow and giant tubular basalt to tabular/blocky trachy-andesite and trachyte flows. Pyroclasts and hyaloclastites are often found with these eruptive sequences. The smaller edifices, forming individualized cones, are built mainly of evolved silicic (SiO 2 >53%) flows consisting essentially of alternating sequences of trachy-andesite and trachyte. The distribution and composition of the small edifices suggest that they are the result of sub-crustal forceful magma injection and channeling supplied from reservoirs associated with the large volcanoes. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Volcanology and Geothermal Research Elsevier

The Pitcairn hotspot in the South Pacific: distribution and composition of submarine volcanic sequences

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Publisher
Elsevier
Copyright
Copyright © 2002 Elsevier Science B.V.
ISSN
0377-0273
eISSN
1872-6097
D.O.I.
10.1016/S0377-0273(02)00427-4
Publisher site
See Article on Publisher Site

Abstract

Multibeam bathymetry and bottom imaging (Simrad EM12D) studies on an area of about 9500 km 2 were conducted over the Pitcairn hotspot near 25°10′S, 129° 20′W. In addition, 15 dives with the Nautile submersible enabled us to obtain ground-true observations and to sample volcanic structures on the ancient ocean crust of the Farallon Plate at 3500–4300 m depths. More than 100 submarine volcanoes overprint the ancient crust and are divided according to their size into large (>2000 m in height), intermediate (500–2000 m high) and small (<500 m high) edifices. The interpretation of seafloor backscatter imagery accompanied by submersible observations and sampling enabled us to infer that the total volume of submarine lava erupted during hotspot activity is about 5900 km 3 within a radius of about 110 km. The most recent volcanic activities occur on both small and large edifices composed of a great variety of lava flows. These flows vary in composition, following a succession from picritic basalt to alkali basalt, trachybasalt, trachy-andesite and to trachyte. Their large range of SiO 2 (48–62%), Na 2 O+K 2 O (2–11%), Ba (300–1300 ppm), MgO (1–11%), Nb (19–130 ppm), Ni (4–400 ppm) and rare earth elements suggests that crystal–liquid fractionation from basanite and/or picritic melt sources was a major process. The variation in composition between the least evolved basaltic rocks and the other more evolved silicic lava is marked by a difference in their flow morphology (pillow, giant tubes, tabular to blocky flows). The lava composition and field observation indicate that several magmatic pulses giving rise to cyclic eruptions are responsible for the construction of the edifices. The two larger edifices (>2000 m high) show more extensive eruptive events and a wider range in compositional variability than the smaller (<500 m high) ones. Several (five) submersible transects made along the slope of one of the largest edifices (Bounty) enabled us to observe at least nine successive eruptive cycles progressing from pillow and giant tubular basalt to tabular/blocky trachy-andesite and trachyte flows. Pyroclasts and hyaloclastites are often found with these eruptive sequences. The smaller edifices, forming individualized cones, are built mainly of evolved silicic (SiO 2 >53%) flows consisting essentially of alternating sequences of trachy-andesite and trachyte. The distribution and composition of the small edifices suggest that they are the result of sub-crustal forceful magma injection and channeling supplied from reservoirs associated with the large volcanoes.

Journal

Journal of Volcanology and Geothermal ResearchElsevier

Published: Mar 1, 2003

References

  • Magmatic sulfides and oxides in volcanic rocks from the Pitcairn hotspot (South Pacific)
    Ackermand, D.; Hekinian, R.; Stoffers, P.
  • SeaBeam depth changes associated with recent flows, CoAxial segment, Juan de Fuca Ridge: evidence for multiple eruptions between 1981-1993
    Chadwick, W.W.; Embley, R.W.; Fox, C.G.
  • The June-July 1993 seismo-acoustic event at CoAxial segment, Juan de Fuca ridge: evidence for a lateral dike injection
    Dziak, R.P.; Fox, C.G.; Schneideret, A.E.
  • Unspiked K-Ar dating of young volcanic rocks from Loihi and Pitcairn hot spot seamounts
    Guillou, H.; Garcia, M.O.; Turpin, L.
  • Ridge-hotspot interaction: the Pacific-Antarctic Ridge and the Foundation seamounts
    Hekinian, R.; Stoffers, P.; Ackermand, D.; Révillon, S.; Maia, M.; Bohn, M.
  • Magma migration
    Turcotte

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