09:45 〜 10:00
[SMP28-04] In-situ metamorphism of arc crust revealed by petrological analyses of super-hot geothermal drill samples
キーワード：長石の変質、超高温地熱、岩石ー流体相互作用、葛根田地熱地域
Fluids in the crust play an essential role in mass and energy transport, leading to ore deposit formation, magma generation, and earthquake triggering[e.g., 1]. Traces of crustal fluids are geologically recognized as fluid-rock reaction zones such as hydrothermal alteration, metamorphic reactions, and ore-deposits exposed on the surface[e.g., 2, 3]. However, due to limited in-situ access to deep crustal rocks, the distribution of “on-going” high-temperature fluid-rock interactions in the crust remains largely unknown. Here we report mineralogical analyses of drilling samples from in-situ super-hot granite (> 500°C), and demonstrate their in-situ recrystallization by supercritical geothermal fluids.
The study area is the Kakkonda geothermal field, NE Japan, where borehole WD-1a penetrated the Quaternary Kakkonda granite (2985–3725 m depth) and recorded >500°C at 3500 m depth[4]. Kakkonda granite intruded into pre-Tertiary basement rocks and overlying Tertiary dacitic-andesitic tuffs, showing biotite, cordierite, and andalusite isograds[e.g., 5]. Despite little fluid production from its bottom, a recent magnetotelluric survey detected nearby conductive zones below 3 km, indicating a supercritical geothermal reservoir above a partial melting zone[6]. To investigate the alteration processes within the super-hot granite, 24 samples (2895–3725 m depth) from WD-1 were analyzed for petrography and mineral chemistry.
The Kakkonda granite comprises quartz, plagioclase, alkali feldspar, biotite, hornblende, and actinolite, with minor amounts of clinopyroxene, magnetite, ilmenite, sphene, and zircon. Feldspars (50–400 µm) show distinct alterations with sharp Na-rich rim/Na-poor core boundaries in plagioclase and <10 µm pores in feldspars. Chemical compositions of Na-rich rims of plagioclase and adjacent rims of alkali feldspar were analyzed for 463 pairs.
Plagioclase albite content (XAb = Na/(Na+K+Ca)) systematically increases from 0.73–0.92 at 2895 m to 0.49–0.70 to 3725 m depth. Alkali feldspar orthoclase content (XOr = K/(Na+K+Ca)) decreases from 0.88–0.93 to 0.81–0.86 over the same depth. The two feldspar thermometers (e.g., Benisek et al., 2010) resulted in a systematic temperature increase from 375–468°C at 2895 m to 532–608°C at 3725 m depth, closely matching borehole temperatures (378°C at 2895 m, 579°C at 3725 m).
These alteration textures of feldspars, compositional trends, and thermometry results, evidence alteration via interface-coupled dissolution-reprecipitation under in-situ borehole conditions. Experimental alteration of feldspars at 400–600°C and 200 MPa with supercritical NaCl-CaCl2 fluids further confirms these results.
Considering the wet solidus of granite (~760°C) and that chlorite and biotite records temperatures 260–370°C and 630–760°C in the contact aureole[7], the Kakkonda granite solidified ~760°C, heated the aureole for ~630–760°C, and gradually cooled to ~380°C near the granite boundary. While biotite records the peak metamorphism, the chlorite records the borehole temperature[7]. Contrary, feldspars rims indicate continuous alteration from magmatic solidification to present borehole conditions.
Such feldspar dissolution-reprecipitation alteration has been commonly recognized as “subsolidus reactions/alterations” in granitic bodies worldwide[e.g., 2, 3]. These reaction textures, feldspar temperatures, and corresponding conductive zones in the Kakkonda granite suggest that such feldspar alteration is ongoing processes driven by deep geothermal fluids, and would be ubiquitously occurring in the arc crust.
1: Weiss et al., 2012 Science, 338, 1613–1616.
2: Plümper et al., 2017 Nature Geoscience, 10, 685–690.
3: Nurdiana et al., 2021 Lithos, 388–389, 106096.
4: Ikeuchi et al., 1998 Geothermics, 27, 591–607.
5: Doi et al., 1998 Geothermics, 27, 663–690.
6: Yamaya et al., 2022 Geothermics, 103, 102412.
7: Uno et al., 2023 Geothermics, 115, 102806.
The study area is the Kakkonda geothermal field, NE Japan, where borehole WD-1a penetrated the Quaternary Kakkonda granite (2985–3725 m depth) and recorded >500°C at 3500 m depth[4]. Kakkonda granite intruded into pre-Tertiary basement rocks and overlying Tertiary dacitic-andesitic tuffs, showing biotite, cordierite, and andalusite isograds[e.g., 5]. Despite little fluid production from its bottom, a recent magnetotelluric survey detected nearby conductive zones below 3 km, indicating a supercritical geothermal reservoir above a partial melting zone[6]. To investigate the alteration processes within the super-hot granite, 24 samples (2895–3725 m depth) from WD-1 were analyzed for petrography and mineral chemistry.
The Kakkonda granite comprises quartz, plagioclase, alkali feldspar, biotite, hornblende, and actinolite, with minor amounts of clinopyroxene, magnetite, ilmenite, sphene, and zircon. Feldspars (50–400 µm) show distinct alterations with sharp Na-rich rim/Na-poor core boundaries in plagioclase and <10 µm pores in feldspars. Chemical compositions of Na-rich rims of plagioclase and adjacent rims of alkali feldspar were analyzed for 463 pairs.
Plagioclase albite content (XAb = Na/(Na+K+Ca)) systematically increases from 0.73–0.92 at 2895 m to 0.49–0.70 to 3725 m depth. Alkali feldspar orthoclase content (XOr = K/(Na+K+Ca)) decreases from 0.88–0.93 to 0.81–0.86 over the same depth. The two feldspar thermometers (e.g., Benisek et al., 2010) resulted in a systematic temperature increase from 375–468°C at 2895 m to 532–608°C at 3725 m depth, closely matching borehole temperatures (378°C at 2895 m, 579°C at 3725 m).
These alteration textures of feldspars, compositional trends, and thermometry results, evidence alteration via interface-coupled dissolution-reprecipitation under in-situ borehole conditions. Experimental alteration of feldspars at 400–600°C and 200 MPa with supercritical NaCl-CaCl2 fluids further confirms these results.
Considering the wet solidus of granite (~760°C) and that chlorite and biotite records temperatures 260–370°C and 630–760°C in the contact aureole[7], the Kakkonda granite solidified ~760°C, heated the aureole for ~630–760°C, and gradually cooled to ~380°C near the granite boundary. While biotite records the peak metamorphism, the chlorite records the borehole temperature[7]. Contrary, feldspars rims indicate continuous alteration from magmatic solidification to present borehole conditions.
Such feldspar dissolution-reprecipitation alteration has been commonly recognized as “subsolidus reactions/alterations” in granitic bodies worldwide[e.g., 2, 3]. These reaction textures, feldspar temperatures, and corresponding conductive zones in the Kakkonda granite suggest that such feldspar alteration is ongoing processes driven by deep geothermal fluids, and would be ubiquitously occurring in the arc crust.
1: Weiss et al., 2012 Science, 338, 1613–1616.
2: Plümper et al., 2017 Nature Geoscience, 10, 685–690.
3: Nurdiana et al., 2021 Lithos, 388–389, 106096.
4: Ikeuchi et al., 1998 Geothermics, 27, 591–607.
5: Doi et al., 1998 Geothermics, 27, 663–690.
6: Yamaya et al., 2022 Geothermics, 103, 102412.
7: Uno et al., 2023 Geothermics, 115, 102806.