Robert Brousse

Origin of anomalous rare-earth element and yttrium enrichments in subaerially exposed basalts: Evidence from French Polynesia

Abstract Basalts from French Polynesian islands occasionally display extremely high abundances and anomalous distributions of rare-earth elements (REE) and yttrium, whereas other incompatible element concentrations and O, Sr, Nd and Pb isotopic ratios do not differ from those of “normal” basalts from the same area. The REE- and Y-enriched basalts contribute up to 15% of the sample set, suggesting that this feature is more widespread than previously thought. REE-Y enrichment is related to the presence of rhabdophane-type REE-Y-phosphate identified through electron microprobe analyses in the most enriched sample and inferred from leaching experiments in the others. This phenomenon is confined to subaerially exposed basaltic sequences, indicating a close relationship to supergene processes. This is supported by negative Ce anomalies in these basalts, since decoupling of Ce from the other REE is restricted to oxidizing, low-temperature, aqueous environments. Similar Nd isotopic ratios for enriched and normal basalts allow us to exclude the possibility that additional REE and Y are derived from marine sediments or guano, but rather suggest an origin from the local basalts. Moreover, light REE enrichment in the REE-Y-phosphates suggests short migration distances of the fluids, supporting the conclusion that additional REE and Y were mobilized from weathered basalts and transported by descending meteoric waters.

Events preceding the great eruption of 8 May, 1902 at Mount Pelée, Martinique

Abstract A chronological reconstruction of the volcanic events that preceded the catastrophic eruption of Mount Pelee on May 8, 1902 has been compiled from all the available written documents. A reinterpretation based on this record shows that the crisis, which took place over three and a half months, had three stages of unequal length: (1) intensification of fumarolic activity over a period of more than three and a half months (January to mid-April); (2) thirteen days of phreatic eruptions (23 April to 5 May); and (3) magmatic eruptions during the last two days before the cataclysmic nuee ardente. Geophysical phenomena were not reliably recorded and provide poor evidence of the nature of events. The responsibilities of authorities must be re-evaluated in terms of the volcanic risks as they were perceived during the course of these events. Although no order for evacuation was issued, those who wished to leave Saint Pierre were free to do so.

Phases d'érosion—comblement de la vallée de la Papenoo et volcanisme subrécent à Tahiti, en relation avec l'évolution des iles de la Société (Pacifique Sud)

Abstract A detailed mapping and data from borings support the conclusion that there is a late lava flow inside the Papenoo valley, the main valley of Tahiti island. This last volcanic manifestation which took place 400,000 years ago came after the phase of erosion corresponding to the Illinoian glacial period. It is one of the four phases of erosion followed by filling observed in the cross section of the Papenoo valley.

Données géochronologiques KAr sur quelques gisements à plantes du massif volcanique néogène du cantal (Massif Central, France)

Abstract Most deposits of fossil leaves, in volcanic-sedimentary layers of the strato-volcano of Cantal (Massif Central, France), usually ascribed to Pliocene-Villafranchian, are attributed to the Upper Miocene by KAr dating. In addition, a strong cooling of the climate of the Cantal is noted between 10 and 6.5 m.y.