INTEGRATED SEM-BASED CATHODOLUMINESCENCE IMAGING AND FLUID INCLUSION ANALYSIS OF CRACK-SEAL QUARTZ CEMENT IN SANDSTONE MACROFRACTURES

Fluid inclusions trapped in crack-seal texture during fracture opening may record thermal history of fracture growth in sandstones. To better characterize fluid inclusion data from quartz in macrofractures, we superposed SEM-based cathodoluminescence images (SEM-CL) and transmitted light images of fluid inclusions from which homogenization temperature and salinity values had previously been acquired. Samples include quartz-lined fractures from several sandstone units. High-magnification color SEM-CL images were made of the exact areas containing fluid inclusions on which homogeniza-tion temperatures and other data were available. Previous SEM-CL imaging has shown that repeated episodes of fracturing and quartz cementation are common in sandstone fractures. SEM-CL imaging allows fluid inclusion data to be placed in the context of the crack-seal history.

Three-dimensional geometry is critical in discovering the relationship between the fluid inclusions, which occur at varying levels inside samples, and the CL patterns, which are generated from a volume at and just below the sample surface. We used optical microscopy to determine depths of fluid inclusions within samples and dips of fluid inclusion planes relative to the sample surface. Without this orientation data, fluid inclusions could be assigned to the wrong microfractures within the crack-seal texture.

SEM-CL imaging enhances the interpretation of fluid inclusion arrays in quartz in several ways. The traditional classification of fluid inclusions as primary, secondary, or pseudo-secondary typically is inadequate to describe the complicated opening history of most macrofractures, which typically require palinspastic reconstruction. A series of microfractures make up the crack-seal structure, and most would be considered pseudo-secondary, but these microfractures represent different steps in the opening of the fracture, and thus a range of crystallization conditions. In some cases, a group of fluid inclusions originally interpreted to be part of one fluid inclusion plane have turned out to be parts of two separate planes that can only be delineated using SEM-CL. Without SEM-CL, inherited fluid inclusions in quartz grains along edges of fractures might be mistaken for fluid inclusions occurring in quartz cement.