Three-dimensional miarolitic cavity distribution in the Kakkonda granite from borehole WD-1a using X-ray computerized tomography

Three-dimensional imaging of a granite core from the borehole WD-1a, drilled at the Kakkonda geothermal field, NE Japan, was performed by X-ray computerized tomography (CT). The CT images displayed miarolitic cavities (possible geothermal reservoir), felsic minerals, mafic minerals, and enclaves in the granite. Voxels with CT numbers less than 1500 were estimated to be the miarolitic cavities on the basis of (i) the CT imaging of the synthetic glass tubes, and (ii) the detailed comparison between the CT image and the cut surface of the rock sample. A three-dimensional distribution of miarolitic cavities was reconstructed from a set of contiguous two-dimensional CT images and showed a spatially heterogeneous distribution with a boundary that strikes N 20° E. This trend is probably important as the regional distribution of the geothermal reservoir, which demonstrates that X-ray CT is a powerful tool in geothermal studies. Chemical fluctuations of Na and K were found around the miarolitic cavity zone. This may be useful for finding miarolitic cavity zones without coring.