Active submarine volcanism on the Society hotspot swell (west pacific): A geochemical study

The present work deals with the petrography and geochemistry of lavas dredged from five active submarine volcanoes (named Mehetia, Moua Pihaa, Rocard, Teahitia, and Cyana) from the southeast end of the Society Islands hotspot trace. Most samples are basic and alkaline, ranging from 16 to 5 wt % MgO, with about 5% normative nepheline. Fractionation modelling based on major and minor compatible element variations suggests that olivine and minor clinopyroxene were the major fractionating phases and implies a maximum range of fractionation of 30–35%. Rocard and Cyana have yielded more evolved, trachy-phonolitic, glassy samples. These evolved samples are thought to be derived by removal of 70% cumulate from the basalts. Both basaltic and phonolitic samples are incompatible-element enriched, with La/YbN ≈ 15 in most of the basalts. The trachy-phonolite patterns show middle rare earth element (REE) depletion and negative Eu anomalies. The Moua Pihaa basalts have flatter patterns than the other basalts (La/YbN = 7.5–12.4). All samples, with the exception of a sample from Moua Pihaa which has elevated 206Pb/204Pb, fall on linear Sr-Nd-Pb isotopic arrays, suggesting two end-member mixing. The most depleted end-member is shown to be a pristine ocean island basalt magma with no detectable contribution from a depleted, mid-ocean ridge basalt (MORB) upper mantle. The flatter REE patterns and higher 206Pb/204Pb of the Moua Pihaa sample are taken to indicate a more depleted, U-enriched (high μ) component in its source. This component may be altered oceanic crust. The Sr isotopic variations in the samples excluding Moua Pihaa correlate positively with Rb/Nb, Pb/Ce, and SiO2 variations, indicating a component of mantle enriched by injection of material from a subducted oceanic slab. Correlation of 207Pb/204Pb with 87Sr/86Sr suggests that the subducted material is geochemically old. Mapping the geochemical variations shows that the contribution to the lavas from the subduction component is greater over the north of the hotspot than in the south. The absence of a MORB component in the Society magmatism, the small volumes of the Polynesian hotspot volcanoes, and the lack of more intense volcanic activity near the center of the Pacific Supers well, all lead us to conclude that the latter is unlikely to be caused by a large convective plume. The Superswell is more probably located above a region in the asthenospheric mantle which, due to its higher content of recycled continental debris, is anomalously hot

Geology of an active hot spot: Teahitia-Mehetia region in the South Central Pacific

The Teahitia-Mehetia hot spot region located in the southeastern extension of the Society Islands chain, near 18° S–148° W consists of several active volcanoes. The distribution of recent volcanic activity correlates with seismic epicenters, and covers an area of more than 1000 km2. Intermittent volcanic activity has given rise to large (>1000 m high) and small (<500 m high) edifices composed of various types of flows. Several recent volcanic events have produced a suite of alkalic rocks ranging from ankaramites, through alkali basalts to trachy-phonolites. The presence of altered MORB-like tholeiites on one small seamount suggests that a different mantle source material was involved in forming some of the crust in this hot spot region

Campagne theahitia 1: Geologie d'un point chaud actif: Theahitia-Mehetia, iles de la Societe (Pacifique Central Sud)

The Teahitia-Mehetia hot spot region located in the southeastern extension of the Society Islands chain, near 18 degree S-148 degree W, consists of several active volcanoes. The distribution of recent volcanic activity correlates with seismic epicenters, and covers an area of more than 1,000 km super(2). Intermittent volcanic activity has given rise to large (> 1,000 m high) and small (< 500 m high) edifices composed of various types of flows. Several recent volcanic events have produced a suite of alkalic rocks ranging from ankaramites, through alkali basalts to trachy-phonolites. The presence of altered MORB-like tholeiites on one small seamount suggests that a different mantle source material was involved in forming some of the crust in this hot spot region

Campagne Mesopac: Leve de sites de forages ODP en sismique multitraces dans le bassin de Nauru

The Mesopac Cruise was the first multichannel seismic study of the western basins of the Pacific Plate. It was concentrated in the Nauru Basin and in the western area of the Central Pacific Basin. Profiles were calibrated with drilling results from DSDP Sites 462 and 169. For the first time one could observe reflectors within the Cretaceous volcanic complex down to approximately equals 8.5 seconds. Profiles did not allow direct observation of the top of the oceanic crust. After correcting for the load on top of the oceanic crust, results suggest that it cannot lie much deeper than those reflectors. If the oceanic crust lies only a few hundred meters beneath the 2.4 km thick complex, it would be right on the normal thermal subsidence curve corresponding to a Jurassic age

Emergence of abrupt gravel to sand transitions along rivers through sorting processes

Gradual downstream fining along gravel-bed rivers is often followed by a relatively abrupt change to a sand bed. This has usually been explained by the breakdown of pebbles of certain lithologies to sand, but it is not restricted to particular rock types and can occur over distances too short for significant abrasion. An alternative explanation is that as shear stress declines downstream, size sorting is enhanced through nonlinearities and thresholds in bedload transport and deposition mechanisms. This hypothesis is tested by numerical modeling of an idealized channel with a mixed gravel and sand bed. Abrupt and persistent gravel fronts with associated breaks of slope develop from a range of smooth initial states when a new initial-motion equation is used, but not with a conventional equation. The results suggest an emergent phenomenon, but one that is sensitive to process specification rather than initial or boundary conditions