The depth variations in the major chemical components dissolved in interstitial waters from the Tonga margin (ODP Site 841) are much more pronounced than those usually observed in deep-sea sediments. The extensive alteration of volcanic Miocene sediments to secondary minerals such as analcime, clays, and thaumasite forms a CaCl2-rich brine. The brine results from a high exchange of Ca to Na, K, and Mg and an increase in Cl concentrations due to removal of H2O from the fluid during the authigenesis of hydrous minerals. The formation of thaumasite could have partly controlled the concentration of dissolved SO4, HCO3, and Ca in the Miocene sediments. The strontium isotopic signature of the interstitial water suggests that alteration of the volcanic Miocene sediments occurred a long time after sedimentation. A transient diffusion model indicates that molecular diffusion was not prevented by lithologic barriers and that the formation of secondary minerals in the Miocene sediment occurred over a short period of time (e.g.,</=1000 years). The extensive diagenetic processes in the Tonga margin were mostly caused by the recent intrusion of andesite sills and dikes into the Miocene sediments.

Supplement to: Blanc, Gérard; Vitali, Frédéric; Stille, Peter (1995): Unusual diagenetic alteration of volcanoclastic sediments in the Tonga fore-arc: Evidence for chemical and strontium isotopic compositions of interstitial waters. Geochimica et Cosmochimica Acta, 59(22), 4633-4644