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 87 Rb/ 86 Sr ratios (18.7–1900), and very radiogenic 87 Sr/ 86 Sr (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 17 Sr 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 87 Sr/ 86 Sr 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.