Silicate diagenesis in deep-sea sediments from the Tonga forearc (SW Pacific): a strontium and Rare Earth Elements signature

Sr-87/Sr-86 isotopic ratios, strontium and Rare Earth Element concentration s obtained on volcano-sedimentary rocks and separated clay mineral and zeol ite fractions reveal a formation by pore water-volcanic rock interaction fo r most of the hydrous silicate minerals of the Site 841 ODP collected from the Tonga forearc. Unusual strontium concentrations and isotopic ratios rec orded in the Miocene tuffs associated with specific REE patterns indicate t hat the formation of these hydrous silicates does not follow a simple buria l diagenesis model, but was related to the cooling of intruding basaltic si lls in the Miocene volcano-sedimentary series. Migration of strontium into the pore water in response to the heat flow induced the formation of Sr-bea ring zeolites such as clinoptilolite, heulandite and chabazite. No evidence of any influence of a further thermal pulse in the Eocene rhyolitic tuffs could be found. As recorded by the chemistry of their clay mineral fraction , the rhyolitic tuffs developed a polyphasic diagenetic process, which migh t have been influenced by a possible circulation of a fluid into structural ly weak areas. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicale s Elsevier SAS.