Takayuki Sawaki

Petrographic features of a high-temperature granite just newly solidified magma at the Kakkonda geothermal field, Japan

Abstract A 3729-m-deep geothermal research well, WD-1a, provides us with a unique opportunity to study initial petrographic features of a high-temperature granite just after solidification of magma. The well succeeded in collecting three spot-cores of the Kakkonda Granite that is a pluton emplaced at a shallow depth and regarded as a heat source of the active Kakkonda geothermal system. The core samples were collected at the present formation temperatures of 370, 410 and over 500°C. These samples are granodiorite to tonalite consisting mainly of plagioclase, quartz, hornblende, biotite and K-feldspar. A sample collected at a formation temperature of over 500°C possesses the following remarkable petrographic features compared to the other two samples. Interstitial spaces are not completely sealed. K-feldspar exhibits no perthite by the exsolution of albite lamella. Quartz includes glassy melt inclusions without devitrification. Hornblende is less intensively altered to actinolite, and biotite is not altered. This study directly confirmed that perthite in K-feldspar is a recrystallization texture formed at 410–500°C based on a comparison of the in situ temperatures of the samples. Chemical compositions of minerals were analyzed to compare temperatures determined from geothermometers in several publications to the in situ temperatures of the samples.

Near-Infrared Spectroscopic Determination of Salinity and Internal Pressure of Fluid Inclusions in Minerals

A near-infrared (NIR) spectroscopic method is proposed to achieve the simultaneous determination of salinity and internal pressure of fluid inclusions in natural minerals. A combination band between the antisymmetric stretching and bending vibrations of molecular water at approximately 5180 cm−1 was observed for standard salt solutions and natural minerals containing fluid inclusions with known salinities. A curve-fitting procedure was used to analyze the change in the band shape of the combination. Justification of the calibration was confirmed by observation of fluid inclusions in natural minerals whose salinities had already been determined using microthermometry. The detection limit of the present method is 1 NaCl-eq wt. %. The minimum size of fluid inclusions that produced well-resolved spectra was approximately 30 μm. This method was applied to assess micro fluid inclusions in a natural diamond with cubic growth habit (cuboid). The salinity and residual pressure of those fluid inclusions were estimated respectively as 4.4 wt. % NaCl-eq and 0.6–0.8 GPa. The present method is complementary to Raman microscopy and microthermometry for the determination of salinity in fluid inclusions of geological samples.

Fluid inclusion study of the Kirishima geothermal system, Japan

Abstract Gases from fluid inclusions in quartz and anhydrite were analyzed with a quadrupole mass spectrometer and a capacitance manometer. The quartz and anhydrite occur in hydrothermal veins in volcanic and pelitic rocks collected from geothermal wells in the Kirishima area, southwest Japan. The geothermal wells are located in a graben made up of Quaternary volcanic rocks underlain by sedimentary rocks of the Shimanto Group. Results of individual fluid inclusion analyses show that the fluid inclusions comprise mainly H 2 O and a variable but small amount of CO 2 . CH 4 and other hydrocarbons are also detected in inclusions in a hydrothermal sample from the pelitic Shimanto Group. Peak ratios of CO 2 /H 2 0 in individual fluid inclusions are variable in some samples. This indicates that there is a difference in gas compositions of the fluid inclusions, and suggests that the inclusions were formed in multistages or trapped heterogeneous boiling fluids. Results of bulk analyses show that the inclusions are mainly composed of H 2 O (98–99 mol%) with small amounts of non-condensable gases, mainly C0 2 and N 2 , CH 4 and Ar. The proportion of N 2 is about one order of magnitude lower than C0 2 , CH 4 is generally two orders of magnitude lower than C0 2 and Ar is just above the detection limit of the mass spectrometer. The gas concentration in the fluid inclusions is much higher than that in the present-day discharge fluids in this area. CO 2 /N 2 and C0 2 /CH 4 ratios of the fluid inclusions from the volcanic rocks are lower than those of the present-day discharge fluids. CO 2 /N 2 and CO 2 /CH 4 ratios in residual fluids increase with progressive degassing, because N 2 and CH 4 are released from the residual fluids more easily than CO 2 . Thus, the difference in the CO 2 /N 2 and CO 2 /CH 4 ratios between the fluid inclusions and the present-day discharge fluids in the Kirishima area may be ascribed to the degree of degassing, and the fluid inclusions in the area were probably formed by trapping fluids that were weakly influenced by degassing. P co 2, values calculated from the gas compositions of the fluid inclusions are higher than that of buffer systems involving alteration minerals in the area. This suggests that the fluid inclusions might be trapped fluids which were not in equilibrium with the alteration mineral assemblages, that is, fluids prior to considerable degassing and alteration.

Synthetic fluid inclusion logging to measure temperatures and sample fluids in the Kakkonda geothermal field, Japan

Synthetic fluid inclusion logging is a new tool to measure temperatures and sample fluids in high-temperature geothermal wells. Fluid in the microcracks of a crystal can be trapped in inclusions through healing. Fluid inclusions in quartz, for example, can be synthesized easily in geothermal boreholes and can be used as long as the host crystal is stable (e.g. α-quartz is stable up to 573°C). This technique can be applied to high-temperature geothermal wells where conventional temperature measurement methods are not feasible. Cracked crystals of quartz, soaked in silica-saturated solutions in gold or platinum capsules mounted on containers, are placed in a geothermal borehole. Geothermal fluid enters the microcracks in the crystals at the selected sampling depths, and inclusions containing ambient fluid are formed through crack healing. Trapping temperatures of fluid inclusions in quartz are determined by microthermometry using a heating stage with pressure corrections. Other cracked crystals mounted in containers with rupture disks are used for fluid sampling. The first borehole experiment was conducted at WD-1, a deep research hole drilled in the Kakkonda geothermal field, northeast Japan, from September to October 1994 (24 days). Results from the experiment confirmed that temperatures measured from fluid inclusions are consistent with borehole temperatures measured by conventional logging tools.