冲绳海槽东坡表层沉积物中自生铁方解石的发现与研究

The Discovery and Study of Authigenic Ferrocalcite in the Surface Sediments from the Eastern Slope of the Okinawa Trough

在冲绳海槽东坡水深５３０ｍ的海底表层沉积物中发现有大量致密、坚硬的自生方解石。它呈褐色生物状，并以微体生物壳内充填物的形式产出，大多数为有孔虫和腹足类，少数为瓣腮类，有的呈粪粒状。其ＦｅＯ含量在０．１９％～１３．４３％之间，平均为７．１８％，大部分在９％以上，远大于ＭｇＯ的含量，即为铁方解石，铁的多寡与自生非晶质粘土物质的含量关系密切。铁方解石以固溶体的形式与镁方解石共生，特有的六方晶系、较高的Ｓｒ含量及碳、氧同位素特征说明，生物化学过程对铁方解石的形成起着重要作用。

Compact and hard authigenic calcite was discovered in the surface sediments at the water depth of 530 m on the eastern slope of the Okinawa Trough. It has brown color and biological shapes, and occurs as infilling materials in microbiological crusts, most of which were formed in foraminifera and gastropoda, and a few of which were formed in lamellibranch. Less calcite occurs as fecal pellet. Analyses of mineral and chemical composition as well as carbon and oxygen isotopes were carried out with X_ray diffractometer, electron microprobe, neutron activation and mass spectrograpgh respectively. The analytical results indicate that the iron content of calcite ranges from 0.19%～13.43%(mostly over 9%), with an average of 7.18%, much higher than the content of magnesium, suggesting ferrocalcite calcite; magnesium content is from 3.36% to 5.56%, averagely 4.9%. The calcite contains relatively high Si, Al, K, Na and Sr (1800 μg/g). Ferrocalcite coexists with magnesian calcite as solid solution. There are obvious correlations between most elements. It is worth mentioning that Fe is correlated not only with Si, but also with Si, Al, K and Na. The facts that the Al/Si ratio of ferrocalcite is similar to that of clay minerals and that no clay peaks have been found on the X_ray spectral lines indicate that Al, Si, K and Na in the calcite exist in the form of amorphous clay, which plays a role of filter in the enrichment of iron. δ 13 C(2.3‰) and δ 18 O(1.4‰) of the calcite fall between shallow water calcite and deep sea calcite, whereas δ 13 C(1.5‰) and δ 18 O(0.3‰) of the shells fall within the range of shallow water mollusk and foraminfera. Crystal and chemical characteristics of ferrocalcite, such as hexagonal system, relatively high content of Sr and isotopic composition of carbon and oxygen, show that the biochemical process play an important role in the formation of ferrocalcite. In addition to physical and chemical conditions in the sedimentary environment, the formation of ferrocalcite presumably has close relation to shells in which beneficial microenvironment has been formed.