Chemical composition, characteristics and age of glauconites at DSDP Leg 66 Holes

During the analysis of "glaucony" recovered during Leg 66, off Mexico, we reviewed the data on previously studied glaucony layers in active margin areas. We found the depth of Leg 66 glaucony sediments to be significantly greater than conventionally assumed appropriate to their genesis (100-500 m). Accordingly, we hypothesize their occurrence at unusual depth to be due to (1) transport of shallow sediments and redeposition at greater depths, (2) margin subsidence, or (3) genesis at greater depth than is generally assumed. For the area off Mexico, we reject (1). (2) has already been verified in Japan and is possible as an explanation for the present phenomenon without excluding (3), which we investigate in this chapter.

Versiones de la Matrona (Catulo 66-68)

Fil: Galán, Lía Margarita. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Versiones de la Matrona (Catulo 66-68)

Fil: Galán, Lía Margarita. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Petrography, minerals and isotopic composition at DSDP Leg 66 Holes

We collected 20 carbonate nodules from the inner trench slope deposits of the Middle America Trench area off Mexico. Carbonate nodules are found only within the methane-rich layer beneath the mixed layer of methane and hydrogen sulfide. They have been investigated by microscopic, scanning electron microscopic (SEM), X-ray diffraction, and stable isotopic analytical methods. Calcite, magnesian calcite, dolomite, and rhodochrosite were recognized as carbonate minerals. Each carbonate nodule is usually represented by single species of carbonate minerals. Carbonate nodules are subdivided into micrite nodules and recrystallized nodules according to textural features. The carbonate crystallites in each micrite nodule are equidimensional. Their sizes range from several to 30 µm, as revealed by SEM micrographs. The chemical composition of calcite is changed from pure calcite to high magnesian calcite, as shown by the shift of the (104) reflection in X-ray diffraction patterns. Fe substitution for Ca in dolomite was also observed. Carbon isotopic composition shows an unusually wide range - from -42.9 to +13.5 per mil - in PDB scale, whereas oxygen isotopic compositions of almost all the carbonate nodules are constantly enriched in 18O from +3.4 to +7.60 per mil in PDB scale. These wide variations in carbon isotopic composition indicate several sources for the carbon in carbonate nodules. Carbon with a negative d13C value was derived from biochemical oxidation of methane with a negative d13C value. On the other hand, carbon with positive d13C value was probably formed during methane production in an anoxic condition.