Abstract

The Rb-Sr system of phillipsites from an indurated volcano-sedimentary mass sampled in the Southern Pacific Ocean became closed long after the deposition of the host rock: Middle Miocene (14.7 + or - 3.3 Ma) and Middle Eocene, respectively. This isotopic age of about 15 Ma probably represents the end of an induration phenomenon which stopped the chemical exchanges between the zeolites and their environment. These minerals inherited Sr primarily from volcanic material, as demonstrated by their initial 87 Sr/ 86 Sr ratio at 0.70441 + or - 0.00036 (2sigma ) which plots within the values of the known 87 Sr/ 86 Sr ratios of the nearby basalts. The leaching technique with diluted acid was checked by recalculation of the Rb and Sr contents of the leachates and residues according to their percentage in the untreated sample and comparison with their amounts in the corresponding untreated sample. Any discrepancy is, in most cases, smaller than 3.5 percent for both Rb and Sr amounts. This technique may, therefore, be used to obtain a correct amount of the distribution of these elements among the soluble and insoluble fractions. The examination of the leachates also showed that the volcano-sedimentary material forming the cores of nodules from the water-sediment interface experienced a more complex history than the underlying analogous volcano sedimentary level.

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