Localization of minor elements by EDS microanalysis in aragonitic sponges from the St. Cassian Beds, Italian Dolomites



The minor elements Sr, S, K, Mg, Fe, and Mn were detected by election microprobe EDS (electron dispersive spectrometer) in small regions of the skeletal tissue of the Triassic sponges Sestrostomella robusta (Zittel), Hartmanina involuta (Klipstein), Atrochetetes medius Cuif & Fischer, Ceratoporella sp., and Eudea polymorpha (Klipstein) from Alpe di Specie (St. Cassian Beds, Dolomites, Italy). Data were compared with analyses of the modern sponges Astrosclera willeyana Lister from the Mozambique Channel and Ceratoporella nicholsoni (Hickson) from the Bahamas. Sr content in Triassic sponges (mean value 9,300 ± 600 ppm) is similar to the Sr content of Recent samples (mean value 8,500 ± 1,500 ppm). This concentration of Sr shows very slight biochemical fractionation like oolitic aragonite. It is therefore possible to infer that the Triassic sea of the St. Cassian Beds had the same Sr/Ca ratio and equal aragonitic depositional conditions as those presently found in the Bahamas and the Channel of Mozambique, i.e. warm shallow waters. Generally, we did not observe a preferential distribution of minor elements with respect to structure. Only when a lower concentration in the center of spherules is observed does a preferential distribution pattern seem to exist. This could mean an initial stage of Sr leaking indicative of an incipient diagenetic process (excluding experimental errors or morphological effects). Excluding these exceptions, the Alpe di Specie spongial fauna is surely diagenetically unaltered. The Sr content shows that the micritic microstructure of Eudea polymorpha is not due to a diagenetic process either. The constant, clearly detectable occurrence of sulphur (mean value 1,000 ppm) was observed. In addition, the S content was found to be linearly correlated with the strontium content. Sulphur is probably of primary organic origin.