ALBERT

Description: Multibeam bathymetry and bottom imaging (Simrad EM12D) studies on an area of about 9500 km(2) were conducted over the Pitcairn hotspot near 25degrees10'S, 129degrees 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 in depths. More than 100 submarine volcanoes overprint the ancient crust and are divided according to their size into large ( 〉 2000 in in height), intermediate (500-2000 in high) and small ( 〈 500 in 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 SiO2 (48-62%), Na2O+K2O (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 in high) show more extensive eruptive events and a wider range in compositional variability than the smaller (〈 500 in 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 (SiO2 〉 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.

Description: Apart from being popular holiday destinations, oceanic-island volcanoes such as Hawaii, Tahiti, or the Canaries provide magmas that yield valuable information about the interior of our planet. Until recently, studies have concentrated on the easily accessible, subaerial parts of the volcanoes, largely ignoring their earlier-formed, submarine parts. These submarine parts, however, provide critical information about how the mantle begins to melt and about the lowest-melting-point mantle components—information not available from the subaerial volcanoes but highly relevant for the chemical evolution of the whole mantle. We present here compositional information from small (〈500 m) volcanoes on the seafloor near Tahiti and Pitcairn Islands and show that these small volcanoes erupt only highly differentiated magmas. These early melts are derived exclusively from the most trace element–enriched, isotopically extreme mantle component, evidence that this component has the lowest melting temperature and is the first product of melting of a new batch of mantle. The geochemical mantle components (enriched mantle EM-I, EM-II) proposed in the 1980s to explain the compositional variations among oceanic volcanoes worldwide appear in reality to represent distinct rock masses in the mantle.