Studies on the mechanisms and environments of formation of sedimentary dolostones by means of inorganic minor and trace elements

采集于日本东南部冲绳岛以东约350公里、北纬25ﾟ50‘、东经131ﾟ14’的北台岛岛，用无机微量元素，特别是氟和钠研究了其新生代沉积白云岩的形成机制和环境。露头样品(n=62)主要由原白云岩组成，钻孔岩心样品(n=47)由原白云岩(约100m)、原白云石与方解石的混合物(约100m-约200m)和方解石(约200m-430m)组成。(2)碳酸盐岩样品中氟和镁的含量呈良好的正相关，表明氟在白云岩中比在灰岩中更富集。此外，该岛白云岩样品的氟含量比较恒定(约500ppm)，与南北岛…白云岩样品的氟含量基本相同北大岛岛以北约10公里处有更多陆地。由于白云石化流体中F~(2+)/MgF~(2+)的比值取决于流体中的Mg~(2+)和Gt~(2+)的浓度，F的恒定含量表明这些岛屿的所有原白云岩都是在几乎相同的Mg~(2+)和Gt~(2+)浓度的白云石化流体作用下形成的。北大岛岛白云岩样品的Na含量约为300 pm，表明岛上的原白云岩是由海水和淡水的混合水形成的。(4)同一太平洋上的埃尼威克和纽埃群岛的白云岩是由正常海水组成的白云岩化流体形成的。但由于北大岛岛的碳酸盐地层与这些岛屿的碳酸盐地层不同，且没有观察到重金属在北大岛岛白云岩中的富集，因此产生北大道岛原白云岩的白云石化流体与正常海水不同。较少

The mechanism and environment of formation of Cenozoic sedimentary dolostones collected from Kitadaitojima Island located at 25ﾟ50'N, 131ﾟ14'E, ca350km east of Okinawa Island, southewstern Japan, were investigated by using inorganic minor and trace elements, particularly fluorine and sodium.(1) According to X-ray diffractometry for bulk samples, constituent minerals are low-Mg calcite and protodolomite, and no silicate minerals are present. Most of outcrop samples (n=62) consist of protodolomite, whereas boring core samples (n=47) consist of protodolomite (>ca.100m), mixtures of protodolomite and calcite (ca.100m-ca.200m) and calcite (<ca.200m-430m).(2) A good positive correlation can be seen between the F and Mg contents of carbonate rock samples, indicating that F is more enriched in dolostones than in limestones. In addition, the F content of dolostone samples of this isle is comparatively constant (ca.500ppm) and is almost the same as that of dolostone samples of Minamidaitojima Is … More land ca.10km north of Kitadaitojima Island. Since ratio of F^-/MgF^+ in a dolomitizing fluid depends upon the concentration of Mg^<2+> in the fluid, the constant F content suggests all the protodolomites of these isles had been formed by the action of dolomitizing fluids with almost the same Mg^<2+> concentration.(3) Sodium is also enriched in dolostones than in limestones. The Na content of dolostone samples from Kitadaitojima Island is about 300pm, which value suggests that the protodolomite in the isle had been formed by mixed waters of seawater and freshwater.(4) Dolostones of Eneweak and Niue Islands in the same Pacific Ocean were estimated to have been formed by dolomitizing fluids consisting of normal seawater. However, the dolomitizing fluid producing protodoloites of Kitadaitojima Island is different from normal seawater, since carbonate stratigraphy of Kitadaitojima Island is different from those of these islands and no enrichment of heavy metals in Kitadaitojima dolostones is observed. Less