Christophe Innocent

Sr isotopic evidence for ion-exchange buffering in tropical laterites from the Paraná, Brazil

Sr isotopes have been measured in laterite sequences and in associated waters from the region of Ribeirao Preto (Parana basin, Brasil), in order to characterize the water-rock interactions corresponding to the successive stages of weathering of these continental flood basalts, We analyzed detailed sections of concentric rims of alteration in ball-shaped spheroids developed around cores of fresh basalts. Waters were also analyzed on the different types of facies in this region: unaltered basalts, thick weathered profiles of lateritic material, and surrounding sandstones, The results show that a major part of the initial Sr is removed from the basalt during the earliest stages of weathering, within the narrow grey rims surrounding the unweathered core. Radiogenic Sr is also leached from hydrothermal minerals located in basalt fractures or disseminated in the basalt groundmass at this early stage. Very high water/rock ratios (of at least 100,000) are calculated for this process, from the Sr-87/Sr-86 in the related waters. A general trend of increasing Sr-87/Sr-86 ratios is observed in the increasingly weathered facies (inner yellow cortexes, red kaolinitic soils and matrices) and in their related waters. The very high Sr-87/Sr-86 ratios of the red kaolinitic horizons probably result from isotopic exchange with radiogenic rainwaters or sandstone waters, and also from the presence of residual or neoformed radiogenic minerals. Budget calculations, as well as experimental leaching of kaolinite samples, suggest that significant amounts of radiogenic Sr can be leached from these facies during weathering, Thus, the kaolinitic facies of tropical laterite profiles are able first to trap strontium from surficial waters, then to provide radiogenic Sr to the waters that weather the fresh basalts at the weathering front. Therefore, the thick weathered horizons must be considered as a natural cation-exchange layer, that play an active role in the weathering process.

U-Pb zircon age of 548 Ma for the leptynites (high-grade felsic rocks) of the central part of the Maures Massif. Geodynamic significance of the so-called leptyno-amphibolitic complexes of the Variscan belt of western Europe

U-Pb zircon and Rb-Sr geochronological, and Sm-Nd isotopic studies have been carried out on the so-called leptyno-amphibolitic complex of the central part of the Maures Massif. The emplacement of the protolith of the felsic end-member (« leptynites ») has been dated at 548 Ma, an age much older than those (lower Ordovician) previously obtained on other leptyno-amphibolitic complexes. Rb-Sr data obtained on whole rocks and on mineral separates give an age of 348 Ma for the amphibolite-facies metamorphism. Nd isotopes indicate that the amphibolites display clear-cut mantle-derived signatures, whereas a significant crustal contribution is recorded in the three analyzed felsic facies. One of these acidic terms can be interpreted in terms of a simple mixing between two components, respectively similar to the amphibolites and to the two other felsic samples. These latter involve another mantle source, distinct from that of the amphibolites, and comparable to that of continental alkali basalts. These data indicate that the central part of the Maures Massif and the southern Massif Central were possibly part of the same pre-Variscan structural unit. The lack of evidence for a clear genetic relationship between the respective protoliths of the two end-members of the leptyno-amphibolitic complex raises once again the problem of the geodynamic significance of these formations.

Rb/Sr chronology and crystal chemistry of celadonites from the Parana continental tholeiites, Brazil

Abstract Celadonites from the Parana flood basalts have been dated by internal Rb/Sr isochrons, using the ammonium acetate leaching technique previously used by Hart and Staudigel (1986) for celadonites from DSDP holes. The residues after leaching have very high 87Rb/86Sr ratios (18.7–1900), and very radiogenic 87Sr/86Sr (0.73–2.96). Electron microprobe analyses show that potassium, and thus also presumably rubidium, became enriched in these minerals during the transformation of a proto-celadonite smectite phase into celadonite. Our leaching experiment suggests that radiogenic 17Sr and rubidium are very resistant to chemical fractionation during leaching and thus may have remained in closed-system for Rb/Sr chronology. Ages varying from 107 ± 1 to 81 ± 1 Ma have been obtained at two different sites, one on high-Ti basalts in the northern Parana basin and the other on low-Ti basalts in the southern basin. These ages are significantly younger than the eruption of the flood basalts (127–137 Ma), and in this respect are in good agreement with results obtained on celadonites from oceanic ridges. Initial 87Sr/86Sr ratios are distinctly higher in the celadonites than in the host basalts, probably due to contamination by radiogenic strontium transported by the hydrothermal fluids. Our results suggest that mineralogy and isotopic composition are different in celadonites from the northern and southern Parana basins, although this needs to be confirmed on a larger dataset.