Resumen

SCASSO, Roberto A  y  BAUSCH, Walter M. Geoquímica y transformaciones mineralógicas en concreciones carbonáticas durante la diagénesis temprana (Formación Ameghino, Península Antártica). Rev. Asoc. Argent. Sedimentol. [online]. 1995, vol.2, n.1-2, pp.1-18. ISSN 1853-6360.

Carbonate concretions developed in the Ameghino Formation, an Upper Jurassic sequence of tuffs and radiolaria rich mudstones deposited in a suboxic to anoxic environment, are exposed in several localities along the north-eastern tip of the Antarctic Peninsula (Fig. 1). These localities underwent different burial histories and grade of diagenesis, and the rocks offered a good opportunity to study mineralogical and geochemical changes on early-diagenetic concretions, as well as their stable isotope composition. Tabular concretions were preferably sampled. The main techniques for chemical and semiquantitative mineralogical determinations were X-ray fluorescence and X-ray diffraction (Table 1). The original mineralogical composition of the sediments was inferred (Table 2) from the early and late diagenetic mineral associations. Stable isotopes of C, O and S in concretions and veins were also studied (Table 3). Concretions showed well preserved, delicate structures like radiolarians, peloids or shards (Fig. 2) mostly replaced by calcite, chlorite, pyrite and analcite, without any evidence of compaction. This points to an early diagenetic origin. Thickness reduction after diagenesis was about 1/3 of the concretion thickness. According to their high contents of Mn2+, these concretions showed no zonation and strong orange luminescence under the cathodoluminescence microscope. A positive correlation between MnO and CaCO3 was verified for the less-diagenized concretions (Fig, 3). Calcite not only filled passively the porosity but also replaced some silicates, specially quartz and feldspar. Silica was preferably removed from the concretions (Fig. 4) and many ions were rearranged according to their ionic radius in relation to Ca2+. Some minerals like pyrite and chlorite showed positive correlation with S, MgO and Fe2O3 contents (Figs. 5, 6 and 7). The stable isotope composition: d13C -19 a -4 ‰ (PDB); d18O -16 a -2 ‰ (PDB); d34S 13,73 a 24,56 ‰ (CDT) together with textural evidence arising from petrographic studies, indicated an early diagenetic origin for the concretions, with authigenic pyrite formed from sulfate reduction of sea water by anaerobic bacteria. Concretion growth was mainly from bicarbonates originated by organic matter decaying in the area of sulfate reduction. The sulphur isotope composition pointed to an origin in the reduction sulfate zone (Fig. 10) with little communication to the sea water. d13C indicated an origin due to anaerobic oxidation of organic matter in the sulfate reduction zone together with a heavier isotope source, probably from fossil dissolution or from the fermentation zone (Fig. 10). Low values of d18O could be due to recrystallization together with change in the pore water composition due to crystallization of clays. In other cases (Fig. 8) recrystallization of carbonate about 100°C (Table 3), beyond the fermentation zone, would be the explanation for the oxygen and carbon isotope composition. A mineralogical association composed of pyrite, chlorite, calcite and analcite was developed during the early diagenesis independently of the original lithology. Pyrite would have been the first mineral formed by precipitation or replacement of mafìcs, ores and glass (sources of iron), followed by chlorite (formed from the remaining iron) calcite and analcite. Calcite was the most abundant component, and it mostly replaced quartz, opal, feldspar and glass. Analcite was mainly an alteration product of plagioclase and glass and its change to albite was verified only in the samples with higher diagenetic grade. Smectite, interlayered smectite-illite/chlorite and illite were other of the main minerals present in the rocks. Two main mineralogical-geochemical groups arose from the diagenetic processes. The first group. called the "ferromagnesian group" was composed of pyrite and chlorite formed from the oxides, mafics and glass containing iron, magnesium and titanium, which were dissolved and mainly replaced by pyrite and chlorite (and anatase). This process was independent from the carbonate replacement observed in the concretions. The second group was the "sodium-calcium group": the distribution of these elements is mainly controlled by the replacement of quartz (and opal) and plagioclase by calcite and analcite. A third, non-independent group, was the "alumina-potassium group", in which these are controlled by the formation smectite/illite and dissolution replacement of K-feldspar. The last group might also act as a temporary repository of some elements (silica, alumina, Mg, Fe. K, Na) for the first two.

Palabras clave : Geochemistry; Stable Isotopes; Upper Jurassic; Antarctic Peninsula; Early Diagenesis; Concretions.

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