No abstract

No abstract

No abstract

No abstract

Samples of basalt were collected during the Rapid Response cruise to Loihi seamount from a breccia that was probably created by the July to August 1996 Loihi earthquake swarm, the largest swarm ever recorded from a Hawaiian volcano. 210 Po-210 Pb dating of two fresh lava blocks from this breccia indicates that they were erupted during the first half of 1996, making this the first documented historical eruption of Loihi. Sonobuoys deployed during the August 1996 cruise recorded popping noises north of the breccia site, indicating that the eruption may have been continuing during the swarm. All of the breccia lava fragments are tholeiitic, like the vast majority of Loihi's most recent lavas. Reverse zoning at the rim of clinopyroxene phenocrysts, and the presence of two chemically distinct olivine phenocryst populations, indicate that the magma for the lavas was mixed just prior to eruption. The trace element geochemistry of these lavas indicates there has been a reversal in Loihi's temporal geochemical trend. Although the new Loihi lavas are similar isotopically and geochemically to recent Kilauea lavas and the mantle conduits for these two volcanoes appear to converge at depth, distinct trace element ratios for their recent lavas preclude common parental magmas for these two active volcanoes. The mineralogy of Loihi's recent tholeiitic lavas signify that they crystallized at moderate depths (F8-9 km) within the volcano, which is approximately 1 km below the hypocenters for earthquakes from the 1996 swarm. Taken together, the petrological and seismic evidence indicates that Loihi's current magma chamber is considerably deeper than the shallow magma chamber (F3-4 km) in the adjoining active shield volcanoes.

Basement outcrops sampled by submersible and dredge from the inner slope of the Izu^Bonin trench at 32³N and 6200^6700 m water depth have a distinct mid-ocean ridge basalt (MORB) chemistry unlike any other rocks previously sampled in the Izu^Bonin arc. They are low K tholeiites with moderate TiO 2 (0.7^1.8 wt%), extremely low Ba (1.5^7 parts per million), low Ba/La (1.2^3) and are depleted in light rare-earth elements. These samples could represent either an accreted piece of subducting Pacific plate or a trapped remnant of Philippine Sea plate on which the Izu^Bonin arc was built. Although their major and trace element chemistry do not help to distinguish their source, the Sr, Nd and Pb isotopes clearly support a Philippine Sea plate origin. The isotopic signature of the inner trench slope samples matches that of Philippine Sea plate lavas, with 87 2) values compared to the Northern Hemisphere reference line (NHRL) and their isotopic signature is distinct from the Mesozoic Pacific MORB being subducted. These are the first samples of trapped Philippine Sea oceanic crust discovered in the IzuB onin^Mariana arc. They require that models for the formation of intra-oceanic arc crust account for pre-existing oceanic crust and that estimates of arc magma production rates are lowered accordingly. ß 1999 Elsevier Science B.V. All rights reserved.

No abstract

No abstract