Elemental composition of lime mud recovered from coral reef Lagoonal basins, the Yaeyama Islands, Okinawa, Japan

概要

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Surface unconsolidated carbonates were sampled from lagoonal basins in the Shiraho and Sekisei Coral Reef areas, the Yaeyama Islands. Lime mud fractions were isolated from these carbonates by sieving and sedimentation methods to collect particles between 62.5μ and 2μ. The isolated lime mud fractions were analyzed for carbonate-bound trace elements, CO3 contents, mineralogical compositions by the X-ray diffraction analysis (XRD), and petrographic observations including scanning electron microscope (SEM) observations. The carbonatebound trace element concentrations were determined by subtracting the 0.75 N LiCl-0.25N CsCl in 60% ethanol leachable fraction from the 1N ammonium acetate-acetic acid (pH adjusted to 5.2) leachable fraction. Lime mud is composed mineralogically by HMC (high-Mg calcite), HSA (high-Sr aragonite), LSA (low-Sr aragonite), and LMC (low-Mg calcite). However, the carbonate-bound Mg/Ca and Sr/Ca ratios are considerably lower in lime mud than the carbonate sand and gravels which contain the same amount of carbonate minerals with the lime mud. This suggests that Mg and Sr were leached without changing mineralogy during the formation of lime mud from coarse carbonate grains. Diagenetic changes in lime mud might have already started before its lithification processes to limestone.
日本堆積学会の論文