The role of water in neocrystallization of feldspar during shear deformation: water distributions revealed by FT-IR mapping

It is well known that fluid water, during shearing of a rock, participates in deformation and subsequent neocrystallization caused by dissolution-precipitation and deformation-induced reactions. Evidence of this involvement is frequently observed around feldspar, one of the main minerals in deformed granitic rocks, and grain-size reduction by neocrystallization may be one of the controlling factors of earth’s rheology. However, the distribution of fluid water, and the ways in which it is transported around the feldspars, is poorly understood. To address this shortcoming, we used infrared (IR) spectroscopy to map and study the distribution of water in granite mylonites deformed under greenschist condition. In localized parts of a K-feldspar porphyroclast, <200 μm from where it is in contact with biotite, the water, trapped as fluid inclusions, measures more than five times the average water content of 350-450 ppm. This finding indicates that water was introduced into the K-feldspar, possibly as a result of the release of water from biotite as it underwent decomposition during shearing. The supply of water from a nearby decomposing biotite can produce a heterogeneous water distribution within a feldspar porphyroclast. Dissolution-precipitation and myrmekite-forming reaction cause grain-size reduction by neocrystallization to <30 μm in size. Water in rounded fine-grained feldspars, formed by neocrystallization, is homogeneously distributed, and its concentration is less than half the average amount in the analyzed feldspar porphyroclasts. This finding indicates that a significant amount of water is released in response to concentration gradients during newly creating feldspar grains. As a consequence, the entire process would result in a positive feedback to promote neocrystallization, other reactions and deformation of which fluid water is involved.