DEEP SEDIMENTARY BASINS AS DIAGENETIC REACTORS - WHAT THE FLUIDS TELL US

Ray Siever's many major contributions to the field of diagenesis include: 1) pioneering work on the use of interstitial pore water chemistry to deduce incipient mineral transformations during early diagenesis; 2) a long term interest in the silica cycle and the diagenesis of silica minerals; 3) the investigation of the global cycling of major solutes; and 4) an early analysis of plate tectonic controls on diagenesis. A large body of information on the composition of fluids in deep sedimentary basins now exists which permits further development of each of these concepts and themes to problems of burial diagenesis.

Chloride is the dominant anion in deep brines in sedimentary basins and appears to have originated primarily as a result of the infiltration of subaerially-evaporated marine waters and/or the subsurface dissolution of halite. The major cation composition of most subsurface brines, however, has been highly modified from the composition of either evaporated seawater or of simple NaCl waters formed by dissolving halite. The systematic progression from Na-Cl dominated fluid compositions at low salinity to Na-Ca-Cl and Ca-Na-Cl compositions with increasing salinity can be explained by the attempt of these fluids to maintain metastable equilibrium with a multiphase assemblage of ambient silicate and carbonate minerals. In the process, a large mass of reactive components, including silica, is transferred between mineral phases via the pore fluids. It can be shown, at least on theoretical grounds, that a large volume of silica can be precipitated as quartz cement by mixing of modest volumes of rock-buffered fluids of differing salinity.

Deep water-rock reactions enrich subsurface brines in dissolved B, Li, Rb, Cs, Sr, Ba, and I. Chloride in high concentrations complexes and solubilizes significant levels of Pb and Zn. Less well understood are the rates and mechanisms by which these diagenetically-derived solutes and major dissolved salts are cycled back into shallow environments. The expulsion of solutes can be geologically rapid in sedimentary basins located along convergent continental margins but may be prolonged and much reduced in intensity in intracratonic basins and basins located along passive continental margins.