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

1993 Fiscal Year Final Research Report Summary

• Project/Area Number: 04640540
• Research Category: Grant-in-Aid for General Scientific Research (C)
• Allocation Type: Single-year Grants
• Research Field: 分析・地球化学
• Research Institution: Gunma University
• Principal Investigator: AIZAWA Shoichi Gunma Univ., Dept. of Engineering, Assistant Professor, 工学部, 助手 (20008527)
• Project Period (FY): 1992 – 1993
• Keywords: Dolomitization / Kitadaitojima Island / Minamidaitojima Island / Sodium / Protodolomite / Dolomitizing fluid / Mixing zone dolomite / Fluorine

Research Abstract

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 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.