On the basis of the textural relationships and petrographic characteristics, two types of micrite have been distinguished: micrite 1 and micrite 2. Micrites, as defined by Folk (1962), are characterized by grains smaller than 4 µm, whereas microspar ranges from 4 to 31 µm in size. Because these grain-size classes may not be clearly divided, some authors use the term to include both micrite and microspar, as I do in this paper.
This micrite is the first type of carbonate material observed in the basalt sequence and was found only in interval 165-1001A-54R-5, 13-18 cm. It appears to have been infiltered and deposited from the seafloor into the newly formed cavities within the basalts. Physical sedimentation of micrite as laminated and graded deposits took place either during or soon after the last episodes of basalt formation (Fig. 4A). Because of the lack of orientation of the rock fragments containing this phase, it is not possible to assess whether this sediment had geopetal orientation or not. The micrite consists of grains 5-15 µm in size (Fig. 6A, B). Microfossils are not clearly observable in thin section within the micrite, but ghosts of recrystallized nannoplankton can be seen under the SEM (see Fig. 6B), suggesting this sediment has a pelagic origin and consists of nannoplankton ooze. The fact that the sediment is now primarily characterized by grains larger than 5 µm may be explained by recrystallization. Microsparites (see definition above) can be formed by hydrothermal alteration of pelagic ooze (e.g., Easton et al., 1982; in the Indian Ocean). Hydrothermal alteration and overprint has always been considered a reason for why it is difficult to distinguish between recrystallized pelagic and diagenetic internal sediment (cf. Bernoulli et al., 1978; Bernoulli and Weissert, 1985).
In summary, the characteristics of this micritic sediment suggest that it has a pelagic origin and was infiltered down from the seafloor into available pore space. Its deposition took place either between episodes of basalt formation or immediately afterward at an early stage, indicating connection of the pore space with the seafloor.
The second type of micrite constitutes an important phase of infilling of the fractures and cavities by carbonate and is particularly well developed throughout Core 165-1001A-54R (Fig. 4B, 4C). Micrite 2 forms with a geopetal orientation within the horizontal fractures and shows some thickness variation at the millimeter scale (Fig. 4C, Fig. 5A, 5C). Within subvertical fractures, micrite 2 forms in an asymmetrical fashion on the downward side of the fracture wall (Fig. 4B, Fig. 5C, 5D). Textural relationships indicate that this phase was lithified before further reopening of the fractures and precipitation of the later phases of infilling. In fact, the micrite forms coatings on the fracture wall with a texture that would not be gravitationally stable unless it were lithified. Clearly, infiltration cannot explain the observed textures, and the micrite must have been generated as a cement by in situ precipitation. Under the optical microscope, the micrite shows a laminated structure (Fig. 5A). Laminae are characterized by undulating upper surfaces, which appear to mantle pre-existing topography, and contain micritic lumps of distinctively homogenous grain size (6-10 µm in size) for each lamina (Fig. 5A, 5B, Fig. 6C). No evidence is observed for redeposition, such as grading of grains, or evidence for dissolution and reprecipitation at the scale of the laminae. Within the laminae, the micrite are in lumps forming pseudopeloids that appear to be almost "floating" in the later calcite (Fig. 5B). Typically, these types of structures are considered to result from the dismantling of microbial filamentous mats (Monty, 1995). At the SEM scale, the micrite crystals are euhedral to anhedral, and crystal surfaces reveal the presence of small spherules (<1 µm in size), which impart a blobby appearance to the crystals. These spherules are caused by the presence of fossilized bacteria entombed in the micritic calcite crystals (Fig. 6D-H), supporting a microbial origin for micrite 2 (Morita, 1980; Novitski, 1981). In addition, calcified "bridges" exist between grains (Fig. 6E-F), which point to the former presence of bacteria. These crystals, unlike micrite 1, do not show any evidence of recrystallization.