Different preservation modes of Cambrian trilobites from the precordillera of Mendoza, Argentina

D`ANGELO, JOSÉ A; BORDONARO, O; ACOSTA, A

Neuquén

Congreso; XVIII CONGRESO GEOLOGICO ARGENTINO; 2011

Scanning electron microscopy in combination with energy-dispersive X-ray spectrometry (SEM-EDX) is one of the most powerful and versatile techniques for the determination of the inorganic chemical composition of different materials (e.g., metal, alloy, ceramic, etc.). The increasing use of SEM-EDX technique relies on its nondestructive character and the relatively low-cost of analysis. SEM-EDX is a suitable technique for the study of small sample areas (a few milligrams), including fragile samples (e.g., archaeological materials; Craig et al, 2007; Iordanidis et al, 2009; Prieto Olavarría and D’Angelo, 2009). Recently, SEM-EDX has proved useful in the study of the chemistry associated with different preservation modes in fossil remains (e.g., Lin and Briggs, 2010). For the first time, the exoskeleton of representative trilobite specimens from Argentina is studied by SEM-EDX. Fossilized pygidia of Clavaspidella digesta Leanza, 1947 reasigned to Athabaskia anax (Walcott, 1916) by Bordonaro and Banchig (1995) exhibit at least three main preservation modes as shown by the samples analyzed (QO1, QO2, QO3 and QO4, Fig. 1). The objective of this study is the determination of the different pathways of transformation of the original matter making up the carapaces occurring in a single stratigraphic level.Middle Cambrian (Series 3) pygidia carapaces of A. anax from one olistholith of San Isidro, Precordillera of Mendoza are analyzed. The samples (QO1, QO2, QO3 and QO4) are thin-bedded to laminated calcareous siltstones obtained from the same stratigraphic level cropping out in the Quebrada Oblicua, situated to the North of the Estancia San Isidro (western Mendoza).SEM-EDX spectra of samples QO1 to QO4 (Fig. 1) indicate a similar qualitative composition including the presence of O, Na, Mg, Al, Si, K, Ca, Fe and traces of some other elements. However, estimated concentrations of chemical elements (semi-quantitative determinations) show differences among the studied specimens. Table 1 shows some estimated values (w/w %) of the main elements found in four pygidia of A. anax. These results are qualitative to semi-quantitative due to the inconsistency of take-off angles between measured areas and between samples (see Goldstein et al., 1992). Nevertheless, our chemical data clearly correlate with the macroscopic characteristics of the samples as indicated below:QO1: pygidium fossilized as calcium carbonate (calcite?) mineralization. This result is supported by the highest contents of calcium recorded for this sample (see Table 1). The marginal border of the exoskeleton is exfoliated showing the internal mold (dark gray color). QO2: pygidium fossilized as iron oxides mineralization (ocher-colored). This is reflected by the highest content of iron (Table 1).QO3: cast of pygidium (greenish gray color) preserved on a calcareous siltstone matrix (see the higher contents of silicon and calcium in Table 1). The composition of QO3 is similar to that of the sediment.QO4: black color pygidium preserved as silicate / clacite (?). The presence of organic matter in this specimen cannot be ruled out.Despite the specimens analyzed belong to a single taxon occurring in the same stratigraphic level, our results clearly demonstrate the differences in their preservation modes. This would indicate different pathways of transformation of the organic/inorganic matter originally present in the once living organisms and its subsequent mineralization.