ALBERT

Beschreibung: The environmental conditions and reaction paths of shallow-water glauconititization (〈500 m water depth, ~15ºC) close to the sediment–seawater interface are generally considered to be well understood. In contrast, the key factors controlling deep-sea glauconite formation are still poorly constrained. In the present study, green grains formed in the recent deep-sea environment of the ODP Site 959, Ivory Coast–Ghana Marginal Ridge, (~2100 m water depth, 3–6ºC) were investigated by X-ray diffraction and electron microscopic methods in order to determine the rate and mechanism of glauconitization. Green clay authigenesis at Hole 959C occurred mainly in the tests of calcareous foraminifera which provided post-depositional conditions ideal for glauconitization. Within this organic-rich micro-environment, Fe-smectite developed 〈10 ky after deposition of the sediments by precipitation from precursor gels containing Fe, Mg, Al, and silica. This gel formation was supported by microbial activity and cation supply from the interstitial solution by diffusion. At a later stage of early marine diagenesis (900 ky), the Fe-smectites reacted to form mixed-layer glauconite-smectite. Further down (~2500 ky), almost pure glauconite with no compositional gaps between the Fe-smectite and glauconite end members formed. This burial-related Fe-smectite-to-glauconite reaction indicates that the glauconitization process was controlled mainly by the chemistry of the interstitial solutions. The composition of the interstitial solution depends heavily on micro-environmental changes related to early diagenetic oxidation of biodegradable (marine) organic matter, microbial sulfate reduction, silicate mineral alteration, carbonate dissolution, and Fe redox reactions. The availability of Fe is suggested as the probable limiting factor for glauconitization, explaining the various states of green-grain maturity within the samples, and this cation may be the most important rate-determining element. The rate of glauconite formation at ODP Site 959 is given by %Gl Sed = 22.6·log(age Sed ) + 1.6 (R 2 = 0.97) where %Gl Sed is the state of glauconitization in the sediment and age Sed is the sediment age (in ky). This glauconitization rate depends mainly on continuous cation supply (in particular Fe) and is about five times less than that in shallow-shelf regions, suggesting significantly slower reaction at the lower temperature of deep-sea environments.

Beschreibung: An X-ray diffraction and electron microscopy study of very-low-grade metamorphic clay mineral assemblages of Upper Carboniferous (Stephanian) mudrocks from the coal-bearing Ciñera-Matallana basin of NW Spain reveals a complex mineralization history related to localized igneous and hydrothermal activity associated with strike-slip-faulting. This thermally active, pull-apart basin experienced peak temperatures (up to ca . 296 C) that reached anchizonal grades in areas of high heat and fluid flow, with hot hydrothermal growth of well crystallized 2 M 1 illite–muscovite, chlorite, and pyrophyllite occurring pre- and synchronous with Late Carboniferous/Early Permian folding and faulting of the sedimentary sequence. No burial pattern could be recognized in the clay mineral reactions with stratigraphic depth due to the complexities of hydrothermal alteration. Clay mineral growth was controlled by the location of igneous activity and the circulation of CH 4 -bearing fluids released during the maturation of coal seams and dispersed organic matter. Extensive retrograde reaction to illite-smectite (1 M d polytype) with abundant R1 rectorite (50–50) in altered rocks adjacent to igneous intrusions and along faults is attributed to the circulation of cooler (〈140 °C) hydrothermal fluids. These and younger fluids were also probably responsible for the extensive crystallization of kaolinite in pore spaces within the mudrocks and related quartz-carbonate veins.

Beschreibung: The up-dip limit of seismogenesis in subduction zone forearcs depends on the lithological composition of the incoming sediment and its subsequent modification during compaction and diagenesis. Here we present results of a multimethodological approach to characterize the smectite-to-illite diagenesis in the accretionary prism of the Nankai Trough subduction zone offshore SW Japan. Our X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy clay mineralogical analysis reveal that advanced states of smectite-to-illlite diagenesis occur in samples recovered from down to 3 km subseafloor at Integrated Ocean Drilling Program Site C0002, which sampled the inner accretionary prism. Our temperature- and time-dependent reaction kinetics models require elevated temperatures in the prism to explain the illitization, which is consistent with revised thermal models based on recent plate reconstructions. Biostratigraphic data suggest that the inner prism sediment was deposited during a period of slow or inactive subduction and buried in the accretionary prism after the Philippine Sea plate resumed subduction. Rapid burial in the past 6 Ma led to the formation of two authigenic smectite-illite phases consistent with a broad 1–9.3 Ma age determined by K-Ar dating. The low K + smectite-illite reflects the long-term burial history, whereas the K + -rich smectite-illite was inherited from the younger accretion event. Our study predicts the illitization of smectite approaches completion at ~5 km below the seafloor in the hanging wall of the plate boundary and the megasplay fault zone, respectively, which coincides with the proposed seaward extent of coseismic slip in the 1944 M w  = 8.1 Tonankai earthquake along those faults. ©2019. American Geophysical Union. All Rights Reserved.

Beschreibung: No abstract available. doi:10.2204/iodp.sd.s01.33.2007