This study has identified a paragenetic sequence of the major phases of carbonate deposition forming in fractures and cavities within the basalts recovered at Site 1001 and their infilling by carbonate sediment and cements. More specifically, two types of micrite (micrite 1 and micrite 2) have been identified on the basis of textural characteristics, optical microscopy, and SEM petrography. The first type, micrite 1, is a pelagic sediment infiltered from above seafloor and deposited either during or immediately after the emplacement of the basalt sequences. The second type, micrite 2, clearly postdates the formation of the breccias and the alteration of the basalt in seawater and has been generated by precipitation in situ. Its irregularly laminated character suggests that it is the product of dismantling of microbial filamentous mats. SEM images reveal the traces of fossilized bacteria entombed in the micritic calcite crystals that generate blobby crystal surfaces.
The petrographic data presented in this paper indicate that micrite 1 was recrystallized, whereas micrite 2 was not. This suggests that the formation of micrite 2 postdated the major phase of hyrdothermal alteration of the basalts. On the basis of this observation, we can exclude the hypothesis that all the calcite is of hydrothermal origin. This would imply that the lavas had a rapid cooling history and lost their heat to the ocean without any significant precipitation in the voids.
More generally, the data of this study present evidence for a new mechanism for the origin of micrite, which is commonly found within basalt sequences. In accordance with results from previous studies (e.g., Bernoulli and Weissert, 1985) this cement is formed in situ, but what is new is that precipitation was controlled by processes related to microbial activity. At this stage it is not known whether this precipitation process took place in truly marine pore waters or in pore water of a modified composition. Future geochemical data are needed to better constrain the composition of the fluids from which micrite 2 precipitated and the conditions under which these bacteria lived.