Occurrence and type of dolomite in ODP Site 124-768 samples (Table 1)

Occurrence of deep-sea dolomites has been reported from numerous settings (for discussion see Lumsden, 1988). Different authors agree that dolomite formation in the pelagic realm is a relatively early diagenetic process (e.g., Jorgensen, 1983; Shimmield and Price, 1984; Kablanow et al., 1984; Kulm et al., 1984). Baker and Burns (1985) suggest that most of the pelagic dolomites formed within a few tens of meters below the seafloor within the zone of microbial sulfate reduction. According to Fuechtbauer and Richter (1988), dolomite can form in the deep-sea at a minimum temperature of 10°C. Other deep-sea dolomites are products of fluids derived from underlying evaporites or submarine weathering of basalts (Garrison, 1981). In some cases (Mullins et al., 1985; Dix and Mullins, 1988; Mullins et al., 1988), the existence of dolomite is linked to disconformities and its formation may have resulted from circulation of seawater through the sediment during prolonged exposure (Dix and Mullins, 1988, p. 287). At Site 768 (Fig. 1), lithified carbonate layers, some containing variable amounts of dolomite, occur below 201 mbsf (Miocene). These beds alternate with unconsolidated or semi-lithified marl layers interbedded in clays and siliciclastic turbidites. The irregular depth distribution of the limestone beds and the variation in preservation and recrystallization of the calcareous microfaunas suggest that lithification of carbonates at Site 768 not only reflects burial diagenesis as described by Garrison (1981) and others, but in part may be a selective, early diagenetic process. The different types and distribution of the dolomite additionally seem to support this assumption. The purpose of this report is to document the occurrence and textural nature of the dolomite at Site 768. Methods used were analyses of stained thin sections (Alizarin S and Ferrocyanide) and studies with the scanning electron microscope. No geochemical analyses (e.g., stable isotopes) were carried out; they will be the subject of further investigations.深海白云岩的产出已在多种地质背景中被报道（相关讨论参见Lumsden, 1988）。不同学者均认为，远洋环境中的白云岩形成属于相对早期的成岩作用过程（例如Jorgensen, 1983; Shimmield与Price, 1984; Kablanow等, 1984; Kulm等, 1984）。Baker与Burns (1985)提出，绝大多数远洋白云岩形成于海底以下数十米范围内的微生物硫酸盐还原带中。根据Fuechtbauer与Richter (1988)的研究，深海环境中白云岩形成的最低温度为10℃。其余深海白云岩则源自下伏蒸发岩析出流体或玄武岩海底风化作用（Garrison, 1981）。在部分案例中（Mullins等, 1985; Dix与Mullins, 1988; Mullins等, 1988），白云岩的产出与沉积不整合面相关，其形成可能源于沉积物在长期暴露期间的海水循环作用（Dix与Mullins, 1988, 第287页）。 在768站位（图1），岩化碳酸盐岩层（部分白云石含量可变）赋存于中新统201米海底以下（meters below seafloor, mbsf）深度处。这些岩层与未固结或半岩化的泥灰岩层互层，后者夹于黏土与硅质碎屑浊积岩中。灰岩床层的深度分布不均，以及钙质微化石群的保存与重结晶程度存在差异，表明768站位的碳酸盐岩岩化作用不仅反映了Garrison (1981)等学者所描述的埋藏成岩作用，还在一定程度上属于选择性早期成岩作用过程。白云岩的不同类型与分布特征进一步佐证了这一推论。 本报告旨在记录768站位白云岩的产出特征与组构性质。本次研究采用的方法包括染色薄片分析（茜素红S与亚铁氰化钾染色法）以及扫描电子显微镜观测。本次未开展地球化学分析（如稳定同位素分析），相关工作将作为后续研究的内容。