Abstract:
Systematic investigations on authigenic zeolites and other authigenic silicate minerals were performed in sediments from the Barbados Accretionary Prism, Exmouth Plateau and Yamato Basin to assess the occurrence, distribution and modes of clinoptilolite (a heulandite-group zeolite) formation. These investigations provide insight into the physical and chemical controls on the formation of authigenic silicate minerals in marine sediments. The determination of the oxygen isotope fractionation between clinoptilolite and pore water was conducted to investigate the application of clinoptilolite as a geothermometer in low temperature environments. Based on XRD and SEM data it was possible to reconstruct the diagenetic sequence as follows: 1- smectite; 2- clinoptilolite; 3- opal-CT; 4- quartz. The absence of vulcanogenic material and growth of clinoptilolite crystals on the inner walls of foraminifer tests, radiolarians and diatom frustules demonstrate the association of biogenic opal dissolution and clinoptilolite formation. Elevated pore water Si concentrations seem to trigger the formation of clinoptilolite inside foraminifer tests where a distinctive geochemical environment had developed. The morphology of the zeolites indicates that clinoptilolite formation is, unlike the opal-A/opal-CT transformation, not a replacement reaction. Furthermore, a dissolution-precipitation step is always involved in the formation of clinoptilolite. There is no evidence for an in situ transformation of an e.g. volcanic precursor. The mineral chemistry of the zeolites is very similar to the composition of clinoptilolite from other marine environments. In combination with pore water and sediment data it can be concluded that the pore water Si concentration controls the Si content of the clinoptilolites. The mono- and bivalent extraframework cations of the clinoptilolite structure exhibit very variable reactions to the pore water concentrations of these elements. Whereas the influence of pore water on the Na and Ca incorporation into the zeolite structure is clearly perceptible, no such influence can be made out for K and Mg. The formation of Ba-rich clinoptilolite near the base of the sulfate reduction zone provides a sink for elevated Ba pore water concentrations caused by dissolution of biogenic barite in ODP-Site 797. On the other hand, formation of Ba-rich clinoptilolites from Ba-poor pore water solutions is also possible as exemplified at Site 762. One potential explanation is the selectivity of clinoptilolite for elements of higher charge, like Ba (super 2+) , from diluted solutions (concentration-valency effect). The oxygen isotope signal of clinoptilolite can be reproduced with a precision of + or -0.20 per mil (SMOW). The reproducibility is mainly controlled by sample inhomogeneities. Secondary isotope exchange processes between channel water and framework oxygen during the dehydration of the samples do not affect the delta (super 18) O-signal of the zeolites. For the first time it is possible to estimate formation temperatures of clinoptilolite by determining the oxygen isotope fractionation between clinoptilolite and pore water from 25 degrees to 50 degrees C. These results suggest that the formation of authigenic clinoptilolite took place at temperatures as low as 20 degrees C with the conclusion that the previously postulated minimum temperature for clinoptilolite formation of 40 degrees -50 degrees C is no longer valid.