From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
39th Annual Meeting, South-Central Section of the Geological Society of America, San Antonio, Texas, April 1-2, 2005
Occurrence of Mineral-filled Microfractures in Igneous and Metamorphic Rocks as Determined by SEM-based cathodoluminescence Imaging
Microfractures lined with substrate-controlled mineral fill (primarily quartz but also feldspar and calcite) are common, if not ubiquitous, features in igneous and metamorphic rocks. SEM-based cathodoluminescence systems (scanned CL) allow improved visualization of these microfractures, which are typically filled with minerals similar to their host grains and are therefore obscure to invisible using light microscopy. Microfractures in these rocks tend to cut across multiple grains and form in sets of similar orientation. Multiple generations of microfractures are not uncommon. Temperatures of formation for various microfracture generations vary from metamorphic to diagenetic conditions. Previous studies had recognized some high-temperature microfractures in igneous and metamorphic rocks, but lower temperature microfractures are present as well. Microfractures may provide important fluid pathways during metamorphism or diagenetic zone alteration.
Microfracture-filling minerals observed
in this study appear to form by sealing (crystallization of minerals from
a fluid), rather than healing (fill by intracrystalline diffusion). Fluid
inclusion planes, which are commonly associated with these fractures,
are narrower than the fracture aperture visible in CL images. In some
larger fractures, multiple fluid inclusion planes in the fill result from
progressive opening (crack-seal texture).
Determination of the temperature at
which the microfractures formed is nontrivial. Clues include fracture
morphology, minerals cut by the microfractures, cross-cutting relationships
with structural elements in the rocks, the type of mineral fill, and the
CL characteristics of the fill. In metamorphic rocks, microfractures run
the gamut from those relict from the protolith (premetamorphic) to those
formed under submetamorphic conditions during uplift. Fluid inclusions
in the microfracture fill provide an opportunity for qualitative determination
of conditions during deformation.