PRESERVATION OF MICROBORINGS AS FLUID INCLUSIONS

Abstract

In this study, we identify endolithic microbial borings preserved as fluid inclusions in Pennsylvanian and Permian brachiopods from the subsurface of western Kansas. Endolithic microbial boring is a new biologically controlled mode of formation for inclusion vacuoles. The microborings are preserved as three types of linear, curved, and branched arrays of tubular fluid inclusions, each with a distinct diameter. Each of the thousands of microborings observed in 60 brachiopod fragments has changed shape by forming calcite partitions that have divided the original tubules into separate aligned fluid inclusions. On average, only 35% of the original lengths of the microborings are preserved as fluid inclusions; values range from less than 5% to about 85%. Enlarged diameters of inclusions, variable vapor-to-liquid ratio in paired inclusions, and low salinities of inclusions indicate that closed-system necking down is one of the mechanisms responsible for alteration of the microborings. Preservation of parallel walls, fillings with luminescent calcite and high salinity of included fluid indicate that open-system cementation is the other mechanism responsible for alteration of the microborings. As most inclusions have leaked and refilled or stretched during thermal re-equilibration, these fluid inclusions are not particularly reliable as records of diagenetic history. The linear arrays of fluid inclusions that remain after alteration of the original microboring are excellent as trace fossils of microbial activity. The record preserved here indicates how fluid-inclusion techniques can be applied to identify and determine the timing of entrapment of microbial trace fossils in ancient Earth and extraterrestrial materials.