科研之友
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摘要
There exist few data on the geochemical signatures and occurrence of silica-rich coatings developed on siliceous carbonate substrates. The rock coatings in the Maran-e-Galu area in southeast-central Iran are composed of four layers; a lower thick layer (1 mm) of silica, a lower thin film of Fe-Mn oxide, an upper thick (1 mm) silica-rich layer, and an upper thin film of Fe-Mn oxide. Energy dispersive X-ray spectrometer (EDS), X-ray diffraction and bulk coating geochemistry data obtained by XRF and ICP-MS highlight that the 1-3 mm thick silica-rich coatings occur as a silica glaze of opal composition on the siliceous carbonate substrate. The coatings were probably formed by the interaction of rainfall, water vapor or dew with silicate and carbonate-rich detrital atmospheric dust, releasing H(4)SiO(4)with a pH of 5-10 in this semiarid-arid desert environment. This led to the formation of silica gels as well as minor Fe-Mn oxide gels by evaporation and supersaturation, and finally changed to the black brown silica glaze probably at ambient temperatures of >40 degrees C Major and minor element signatures are consistent with the known silica glazes, displaying enrichment in Ba, Sr, P, and LREEs with little to no Eu and Ce anomalies and a weak Tm anomaly. However, the very low content of aluminum oxide is well correlated with the known classified silica glaze of genetic type 1, reflecting the absence of detrital clay minerals. The low Fe and Mn contents in bulk coatings (3.6 and 10 magnitudes lower than crustal abundance, respectively) and lack of biochemical processes in the atmospheric dust highlight the semiarid-arid climate setting. We thereby conclude that a new variant of silica glaze of 1-3 mm thickness could form on siliceous carbonate substrates under an alkaline pH in semiarid-arid regions.
- 出版日期2018-10
