El esfuerzo de restituir la pluralidad del fenómeno comunista a través de la búsqueda colectiva. Reseña del libro Le siécle des communismes, bajo la dirección de Michel Dreyfus, Bruno Groppo, Claudio Ingerflom, Roland Lew, Claude Pennetier, Bernard Pudal y Serge Wolikow; París, les editions de ratelier, 2000, 542 páginas

Fil: Bisso, Andrés. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Element contents and isotope composition of sediments and basalts from DSDP Site 78-543

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb (206Pb/204Pb up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and 143Nd/144Nd increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas. Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of "normal" depleted MORB.

Geochemistry and trace elements at DSDP Hole 78-543A

The upper part of the basaltic substratum of the Atlantic abyssal plain, approaching subduction beneath the Barbados Ridge and thus presumably beneath the Lesser Antilles island arc, is made of typical LREE-depleted oceanic tholeiites. Mineralogical (microprobe) and geochemical (X-ray fluorescence, neutron activation analyses) data are given for 12 samples from the bottom of Hole 543A, which is 3.5 km seaward of the deformation front of the Barbados Ridge complex. These basalts are overlain by a Quaternary to Maestrichtian-Campanian sedimentary sequence. Most of the basalts are relatively fresh (in spite of the alteration of olivine and development of some celadonite, clays, and chlorite in their groundmass), and their mineralogical and geochemical compositions are similar to those of LREE-depleted recent basalts from the Mid-Atlantic Ridge. The most altered samples occur at the top of the basaltic sequence, and show trends of enrichment in alkali metals typical of altered oceanic tholeiites.

El esfuerzo de restituir la pluralidad del fenómeno comunista a través de la búsqueda colectiva. Reseña del libro Le siécle des communismes, bajo la dirección de Michel Dreyfus, Bruno Groppo, Claudio Ingerflom, Roland Lew, Claude Pennetier, Bernard Pudal y Serge Wolikow; París, les editions de ratelier, 2000, 542 páginas

Fil: Bisso, Andrés. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.