Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan

Abstract

Infrared microspectroscopy reveals that liquid-like water is always present in polycrystalline quartz grains from low-grade metamorphic cherts and shale in the Chichibu Group and the Mikabu Greenrock Complex of the Kanto Mountains area. The water distribution in these rocks is heterogeneous and is related to rock textures. In the microcrystalline quartz, O–H species due possibly to Si–OH are generally present. The liquid-like water is also present at grain boundaries and/or in sub-micron fluid inclusions in the microcrystalline parts. On the other hand, in quartz grains larger than 20 μm in diameter within veins, only the liquid-like water is present, possibly as fluid inclusions trapped during quartz vein formation. In the microcrystalline cherty matrix of the metamorphic rocks, a general decrease in liquid-like water content is observed with increasing metamorphic grade, associated with an increase in grain size. Two representative grain shapes, a cube and a regular tetradecahedron (having 14 planes), are used to estimate the surface area between grains per unit volume with the grain size D (μm): 3/ D for cubic and 2.37/ D for tetradecahedron. Grain boundary volumes were then calculated assuming grain boundary widths from 0.5 to 20 nm and normalized by unit volume of the rock. The measured IR data on low-grade metamorphic cherty rocks fall closely on these curves with grain boundary width of around 10 nm for the both models, assuming saturation of H 2O at grain boundary. The observed decrease in water content with increase in grain size can be rigorously explained by the decrease in grain boundary volume per unit volume. These results suggest that liquid-like water occurs mainly between grain boundaries in the microcrystalline quartz of low-grade metamorphic rocks. Although this grain boundary water model is the first approximation and requires further details of quantitative grain boundary textures, the present model can provide a new approach to understand water distribution in polycrystalline rocks.