Introduction

Feldspars (plagioclase and alkali feldspars) are the dominant constituents of the Earth’s crust. During the circulation of hydrothermal fluids through the crust, feldspar minerals are intensely altered to other feldspar minerals and/or by clay minerals, such as sericite, to produce characteristic replacement textures (e.g., Plümper and Putnis 2009; Yuguchi et al. 2019). Some feldspar alterations, such as the albitization of granitic rocks, also occur in relation to regional-scale ore mineralization (e.g., Pollard 2001; Watanabe et al. 2018).

The composition of feldspars is expressed in a ternary system of anorthite (An, CaAl2Si2O8)–albite (Ab, NaAlSi3O8)–orthoclase (Or, KAlSi3O8), and feldspar usually comprises solid solutions along the anorthite–albite (plagioclase) and orthoclase–albite (alkali feldspar) joins. Therefore, a variety of replacement reactions of feldspars have been reported, depending on the host rock and geological settings. The types of replacement reactions are defined by the combinations of parent feldspar compositions and the compositions of saline fluids; including albitization of oligoclase and labradorite (Sasaki et al. 2003; Engvik et al. 2008; Friðleifsson et al. 2020; Nurdiana et al. 2021), albitization of K-feldspar (Norberg et al. 2011), and K-feldspar replaced Na-rich feldspar (Gruner 1944; O’Neill 1948; Wyart and Sabatier 1956; Orville 1962; O’Neil and Taylor 1967; Nishimoto and Yoshida 2010; Anders et al. 2014; Jonas et al. 2014; Plümper et al. 2017; Yuguchi et al. 2019). Feldspar replacement is commonly characterized by (1) pseudomorphism, which preserves the original outline of the original crystal, and (2) the development of nano- to microporosities that were formed during alteration (Putnis 2009; Plümper and Putnis 2009). The generation of reaction-induced micropores significantly increases the porosity of bulk rock and potentially enhances the pervasive fluid flow within the crust (e.g., Plümper et al. 2017). In volcanic regions, the formation of micropores in altered feldspars could play an important role in the development of supercritical geothermal reservoirs (Friðleifsson et al. 2020; Nurdiana et al. 2021).

Several hydrothermal experiments have been conducted to investigate the replacement of feldspar. Representative studies include the albitization of oligoclase and labradorite (Hövelmann et al. 2010), the albitization of K-feldspar (Norberg et al. 2011; Schäffer et al. 2014; Duan et al. 2021), K-feldspar replaced Na-rich feldspar (Labotka et al. 2004; Niedermeier et al. 2009; Duan et al. 2021), and the deuteric coarsening of perthite (Norberg et al. 2013). For example, the replacement of K-feldspar by albite and albite by K-feldspar can be expressed as follows:

$$\mathop {{\text{KAlSi}}_{3} {\text{O}}_{8} + {\text{Na}}^{ + } }\limits_{{\text{K - feldspar}}} \mathop { \to {\text{NaAlSi}}_{3} {\text{O}}_{8} + {\text{K}}^{ + } }\limits_{{{\text{Albite}}}}$$
(1)
$$\mathop {{\text{NaAlSi}}_{3} {\text{O}}_{8} + {\text{K}}^{ + } }\limits_{{{\text{Albite}}}} \to \mathop {{\text{KAlSi}}_{3} {\text{O}}_{8} + {\text{Na}}^{ + } }\limits_{{\text{K - feldspar}}}$$
(2)

The above reactions are expressed as exchanges of Na+ and K+ ions. However, based on the experiments with \(\delta\) 18O labeled H2O, Niedermeier et al. (2009) show that the feldspar replacement in hydrothermal solution does not involve an ion exchange, but proceeds via coupled processes of the dissolution of parent mineral and reprecipitation of product mineral. Reaction 1 is characterized by a solid volume decrease (∆V =8.13%) and reaction 2 by a solid volume increase (∆V =  + 8.85%). Interestingly, porosity generation occurs not only in reactions with a solid volume decrease but also in reactions with a solid volume increase (Putnis 2009). The time evolution of micropores during replacement reactions has been experimentally investigated in relatively simple systems, such as the replacement of KCl by KBr, where the pores are generated and migrated with evolving fingering structures owing to a volume decrease and subsequently pinched off (Putnis et al. 2005; Beaudoin et al. 2018).

In contrast to replacement reactions 1 and 2, which involve the overall exchange of Na+ and K+, some replacement reactions of feldspars involve Si and Al. For example, the replacement of anorthite by albite is expressed in an Al-fixed reference frame as follows:

$${\text{CaAl}}_{2} {\text{Si}}_{2} {\text{O}}_{8} + 2{\text{Na}}^{ + } + 4{\text{SiO}}_{2} \left( {{\text{aq}}} \right) \to 2{\text{NaAlSi}}_{3} {\text{O}}_{8} + {\text{Ca}}^{2 + }$$
(3)

Reaction 3 suggests that in addition to Na+ ions, a supply of SiO2(aq) is required at the reaction sites for the progress of the alteration. In addition, Al mobility strongly controls the stoichiometry of the overall reaction. As such, a variation in compositions of parent plagioclase defined by Na+Si4+ ⇆  Ca2+Al3+ substitution strongly affects the progress of later alteration by saline fluids and replacement textures (pseudomorphism, porosity generation). However, systematic studies on the effects of plagioclase and fluid composition on plagioclase replacement are limited.

In this study, we conducted a series of hydrothermal experiments of plagioclase replacement in aqueous solutions of KCl (KCl(aq)) and NaCl (NaCl(aq)) with a range of plagioclase compositions. In addition, based on comparative experiments in quartz-present and quartz-absent systems, we showed that the concentration of silica in the aqueous solution significantly controls porosity generation and pseudomorphism during plagioclase replacement. We also conducted experiments on the replacement of polycrystalline plagioclase-bearing rocks to reveal the spatial relationship between dissolution and reprecipitation of feldspars and porosity in polycrystalline rocks. The effects of fluid composition on porosity generation within a rock and its impact on hydrological properties during hydrothermal alteration are also discussed in the paper.

Methods

Starting materials

For the hydrothermal experiment, plagioclase crystals with three different compositions, albite (Ab), anorthite (An), and labradorite (Lab), were used as starting materials (Table 1). Compositions of the starting plagioclase are summarized in Table 2. Albite has a composition of An1-2Ab99-98, and anorthite has a composition of An96-100Ab4-0. Labradorite has a slightly different composition of An64-77Ab35-19Or4-1.

Table 1 Experimental condition for each mineral-solution pair conducted in this study
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Table 2 Major compositions of starting material and the product in all experimental pairs
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In addition, biotite schist and hornblende schist were also used as starting materials. These metamorphic rocks were obtained from Kinkasan Island, NE Japan, and metamorphosed in response to granitoid intrusions during the Cretaceous age (Ehiro et al. 2016; Nurdiana et al. 2021, 2022). The biotite schist consists mainly of quartz (24 area%), andesine (An21-41Ab79-59; 40 area%), and biotite (33 area%), with minor garnet and apatite. The hornblende schist consists mainly of amphibole (40 area%) and labradorite (An67-78Ab33-28; 50 area%), with minor titanite. Notably, the hornblende schist does not contain quartz (Nurdiana et al. 2021).

Hydrothermal experiments

Individual mineral or rock samples were crushed and sieved to prepare mineral grains of 125–250 µm size. The mineral particles were then washed twice using distilled water or HCl of pH 4.5 in ultrasonic waves for 2 min and dried in a heating stage of 120 °C for over 3 h. Aqueous solutions of NaCl (2 M, 11.70 wt%) and KCl (2 M, 15 wt %) were used in the experiment. pH values at room temperature were 5.5 and 6.0 for the NaCl and KCl aqueous solutions, respectively. These somewhat low pH values suggest that the atmospheric CO2 dissolved into the starting solution.

The hydrothermal experiments were conducted using a cold-seal apparatus. The starting mineral or rock sample (25 mg) and solution (50 mg) were enclosed in a gold tube with a length of 25 mm and an inner diameter of 2.7 mm. The fluid-to-rock (mineral) mass ratio was 2:1. For each pair of feldspar and fluids, quartz-absent and quartz-present experiments were conducted. For the quartz-bearing experiments, 50 mg of quartz powder was added to 25 mg of feldspar. After placing the mineral powders and fluids into the gold tube, both sides of the tube were welded, and the tube was placed in a cold-seal pressure vessel (Fig. 1). The confining pressure was maintained at a constant value with water, using a syringe pump. The experimental P–T condition was 600 °C and 150 MPa; the duration was 1, 2, 4, or 8 days (Table 1). The temperature of the vessel exhibited a gradient from 570 °C at the top to 610 °C at the bottom. After each run, the vessel was cooled to room temperature within 1 h. In the quartz-bearing experiments, the feldspar replacement reactions proceed nearly under the quartz-saturated conditions, as even at 400 °C, quartz powder was dissolved to reach near its solubility within 12 h (Okamoto et al. 2021).

Fig. 1
figure 1

Schematic illustration of the experimental apparatus as vertical sample vessel. Water is used for the external pressure medium

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Eleven runs were performed for different combinations of feldspar (Ab, An, Lab)–solution (KCl(aq), NaCl(aq)) with or without quartz (Table 1): albite–KCl(aq) experiments with quartz (Ab-KCl-Qz) or without quartz (Ab-KCl), anorthite–NaCl(aq) experiment with quartz (An-NaCl-Qz), anorthite–KCl(aq) experiment with quartz (An-KCl-Qz) or without quartz (An-KCl), labradorite–NaCl(aq) and labradorite–KCl(aq) experiments with quartz (Lab-NaCl-Qz, Lab-KCl-Qz) or without quartz (Lab-NaCl, Lab-KCl). For the two-day labradorite–KCl(aq) experiment (run Lab-KCl_2d), two different runs were conducted in H2O with different oxygen-isotope compositions: one with pure water (Milli-Q water; H216O) and the other with oxygen isotope labeled H2O (18O-enriched water; 18O = 97%; Sigma-Aldrich Co., USA). The experiments with biotite schist and hornblende schist in aqueous KCl solutions are denoted as BtLabQz-KCl and HblLab-KCl, respectively.

Analyses

After the experiment, the solid samples were taken from the gold tube, washed using distilled water in an ultrasonic wave, and dried for > 12 h inside an oven at 120 °C. The surface morphologies of the products were observed using field-emission scanning electron microscopy (FE-SEM; SU-8000, Hitachi) at Tohoku University. Polished sections were prepared to obtain backscattered electron (BSE) images, and the chemical compositions were analyzed using an electron probe microanalyzer (EPMA; JEOL JXA-8200) at Tohoku University. Quantitative and elemental mapping analyses of the major minerals were conducted at an accelerating voltage of 15 kV at 12 nA and 120 nA, respectively. Areas of the product minerals and porosities were measured from the BSE images or elemental maps using ImageJ software.

To determine the variation in the oxygen isotope compositions of the feldspars in the oxygen-isotope labeled experiments, micro-Raman spectroscopy (Horiba XploRA PLUS) equipped with an optical microscope at Tohoku University was used. A green laser (532 nm) was used for excitation, with 10 accumulations and a 5 s integration time. The scattered light was collected through a 100 μm entrance slit, dispersed by 1800 grooves/mm grating. A 100 × objective with a 100 μm confocal hole was utilized.

Geochemical calculations on mineral–fluid equilibria

The composition of the fluids in the gold tube could not be analyzed because of the small amounts of solutions. In this study, to infer the in-situ solution chemistry during the experiments, we conducted geochemical calculations on mineral–fluid equilibria at 600 °C and 150 MPa in the system of Na–K–Ca–Al–Si–O–Cl using SOLVEQ-XPT and CHIM-XPT software (Reed 1982; Reed et al. 2010; 2016). First, we calculated the speciation of the aqueous species at in situ experimental P–T conditions using SOLVEQ-XPT. Then, CHIM-XPT was used to simulate the fluid chemistry and product minerals by titrating plagioclase (Ab, An, or Lab; up to 1000 g) into 1000 g of KCl or NaCl aqueous solution, corresponding to an increase in the fluid/rock ratio from 0.001 to 1. The thermodynamic dataset of minerals was obtained from Holland and Powell (2011) and the dataset of aqueous species from Shock et al. (1997) and Stefánsson (2001). We did not treat the solid solution of feldspar in the thermodynamic calculation but created thermodynamic data for labradorite as the mechanical mixing of An70Ab30. The output of these simulations was the abundance of each species (molal) and precipitated minerals (mole), along with the pH during the reactions.

Results

The mineral compositions of the products of individual experiments are summarized in Fig. 2 and Table 2. Because the initial feldspar grains or rock fragments showed an irregular shape and the product minerals were often formed not only as replacements but also as overgrowths, the original grain outline was difficult to identify in some experimental products. In addition, the thickness of the reaction rim is highly dependent on the crystal faces of the particles. Therefore, as an indicator of the progress of the replacement reaction, we used the reacted area%, which is defined as the area of the product feldspar \(\times\) 100/((area of the product feldspar) + (area of the parent feldspar)) (Fig. 3a). We defined two types of porosity: the “maximum porosity” is defined as the area of the porosity \(\times\) 100/area of inferred original feldspar grain (Fig. S1), whereas the “minimum porosity” also called “total porosity,” is defined as the area of the porosity \(\times\) 100/((area of the porosity) + (area of total feldspar after the experiment)) (Fig. 3b, S1). The area of the total feldspar after the experiment includes the relics of parent feldspar, the secondary feldspar replacing the original grain, and overgrowth. It is noted that the starting minerals of albite, anorthite, and labradorite have initial porosities of 3%, 2%, and 2%, respectively (Fig. 3b). Note that in quartz-present reaction, the dissolution of quartz grains was evident as the change from angular to rounded grain shapes through the experiments.

Fig. 2
figure 2

The compositional range of original starting material and mineral-fluid pairs. N.D. not detected, Ab albite, An anorthite, Lab labradorite. XAn = Ca/[Ca + Na + K]; XAb = Na/[Ca + Na + K]; XOr = K/[Ca + Na + K]

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Fig. 3
figure 3

Plot of reacted area (%) and total porosity (%) of all mineral-fluid pairs after 4 days replacement (except for Lab-NaCl-Qz, which was for 1 day). N.D. indicates the reacted area and porosity is not detected after replacement. The gray highlight in (b) suggest the total porosity range of the pristine starting material

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The BSE images and elemental maps of the individual experimental products are shown in Figs. 4 and 5: albite-bearing (Fig. 4a, b), anorthite-bearing (Fig. 4c–e), and labradorite-bearing experiments (Figs. 4f–i and 5).

Fig. 4
figure 4

Representative plagioclase replacements in quartz-present and quartz-absent system of anorthite, labradorite, and albite. All replacements were four days, except for f Lab-NaCl-Qz, which was reacted for 1 day. Ab albite, An anorthite, Kfs K-feldspar, Lab labradorite

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Fig. 5
figure 5

BSE images and Ca-K elemental maps of labradorite replacement in quartz-absent KCl fluid after a 1 day, b 2 days, and c 8 days of experiment. Note that four-day experimental result is in Fig. 4i. Scanning electron microscopy (SEM) photomicrograph of the surface of labradorite d unreacted labradorite, e after eight-day experiment. The porous anorthite indicates the complete replacement of labradorite. f The growth of K-feldspar outside of the labradorite grain, with the common and widespread occurrence of garnet. An anorthite, Grt garnet, Kfs K-feldspar, Lab labradorite

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Replacement texture of albite and anorthite

Replacement of albite \(\pm\) quartz with KCl(aq)

The albite reaction with KCl(aq) produced K-feldspar with a slight variation in the compositions of Ab6-14Or94-86 in the quartz-present experiment and Ab10-20Or90-80 in the quartz-absent experiment (Fig. 2; Table 2). The Or component gradually increased from the reaction front to the marginal part of the grain. No significant textural differences were observed between the Ab-KCl-Qz (Fig. 4a) and Ab-KCl (Fig. 4b) experiments. The replacement proceeded from the surface of the Ab grains along the cleavage plane (Fig. 4a, b). The outline of the original albite grains appeared to be largely preserved (Fig. 4a, b); however, K-feldspar showed euhedral overgrowth in some parts (Fig. 4b). In both experiments, distinct pores of size 0.1 to 100 μm were generated preferentially at the reaction front, whereas porosity was almost absent at the outer parts of the replacement in the BSE images (Fig. 4a, b).

The progress of the replacement reaction (Fig. 3a) did not vary between the experiments with quartz (Ab-KCl; reacted area of 20–45% in 4 days) and those without quartz (Ab-KCl-Qz; reacted area of 15–86% in 4 days). Although the variations are large grain by grain, the total porosities were similar in both experiments and were between 3 and 19% in the 4-day experiment (Figs. 3b, 4a, b).

Replacement of anorthite \(\pm\) quartz with NaCl(aq) and KCl(aq)

Anorthite replacement experiments were conducted using NaCl(aq) (Fig. 4c) and KCl(aq) (Fig. 4d, e) solutions. The progress of the replacement reactions differed between the quartz-present and quartz-absent systems (Fig. 4c–e).

For the anorthite replacement with quartz experiments (An-NaCl-Qz; An-KCl-Qz), Fig. 4c, d shows the progress of the replacement reactions and the generation of micropores along the reaction front.

In the An-NaCl-Qz run, anorthite grains were replaced by oligoclase with a composition of An15-26Ab85-74 (Fig. 2; Table 2). Secondary oligoclase occurred at the margins of the anorthite grains and along the fractures or preexisting pores (Fig. 4c). The Ab component of the secondary oligoclase increased from the reaction front to the outermost rim. The reacted area% of An-NaCl-Qz experiment was 1–7% over 4 days, which was lower than that of An-KCl-Qz experiment (1–42% over 4 days; Fig. 3a). The total porosity formed by the An-NaCl-Qz experiment was also low (1–4% in 4 days; Fig. 3b), and it is not always clear whether the pores found in the secondary feldspar were formed by the alteration or pre-existing porosity.

The An-KCl-Qz experiment produced K-feldspar with an endmember composition of Ab0-1Or100-99 at the rim of the anorthite grains (Fig. 2; Table 2). Some magnetite inclusions in anorthite remained unreacted, even in the reaction rims (Fig. 4d). The reacted area% in 4 days varied in the range of 1–42%. Micropores were also formed near the reaction front, with a total porosity of 1–6% per 4 days (Fig. 3). In contrast, in the quartz-absent system (An-KCl), the formation of secondary feldspar and porosity generation were not observed in 4 days (Figs. 2, 4e). Small amounts of pores were observed in the central part of the anorthite, but they probably represent the initial porosity of anorthite.

Replacement of labradorite \(\pm\) quartz with NaCl(aq) and KCl(aq)

The replacement of labradorite (An64-77Ab35-19Or1-4) using NaCl(aq) or KCl(aq) with or without quartz yielded contrasting occurrences of secondary minerals and micropores.

Lab \(\pm\) quartz with NaCl (aq)

Labradorite replacement using NaCl(aq) with quartz in 1 day exhibited no reaction rims or micropores (Figs. 2, 3, 4f; Table 2). The original angular outline of the labradorite grains was clearly preserved (Fig. 4f). In the Lab-NaCl experiment, there was no replacement texture or porosity generation, although the experimental duration was 4 days (Figs. 2, 3, 4g).

Lab + quartz with KCl (aq)

Labradorite replacement using KCl(aq) with quartz generated a secondary K-feldspar with an almost pure K-feldspar composition of Ab1-2Or99-98 (Fig. 2; Table 2). The K-feldspar commonly formed at the margins of the grains or along the cleavage (Fig. 4h). The replacement texture appeared largely pseudomorphic; however, euhedral K-feldspar overgrowth was also observed at some edges of the grains (Fig. 4h). The reacted area% reached 40–79% over 4 days. Micropores were generated preferentially at the reaction front, and the total porosity was 1–5% for 4 days (Fig. 3).

Lab with KCl (aq)

The replacement of original labradorite grains (Fig. 5d) in Lab-KCl without quartz produced two types of secondary feldspars: anorthite and K-feldspar. Anorthite showed a variety of compositions (An83-93Ab9-4Or8-3; Fig. 2; Table 2) and high porosities (Figs. 4i, 5a–c, e). In contrast, K-feldspar occurred as an overgrowth of the labradorite grains and showed relatively homogenous compositions (Ab0-6Or94-100) (Figs. 2, 5a–c, e–f; Table 2). A distinct interface existed between K-feldspar and anorthite (Fig. 5c). Large number of pores were generated, preferentially at the replacement front between the labradorite and anorthite or within the anorthite regions (Figs. 4i, 5a–c, e), whereas there were almost no pores within the K-feldspar overgrowth. The replacement significantly proceeded, with a reacted area of 50–90% in 4 days. Micropores (1–44%), some existing as holes, were generated in 4 days (Fig. 3). Anisotropic replacement occurred where overgrowth developed at the two sides of the labradorite grain, and anorthite and porosity formation occurred at different sides of the overgrowth (Fig. 5a–c). Fine-grained garnet aggregate (4–7 µm in size; Figs. 5f, 7a) with grossular-rich composition (Sps0.2Alm24Prp8Grs60Adr7.8) partly formed around the boundary of secondary anorthite and K-feldspar overgrowth.

The BSE images of the experimental products suggest that labradorite was completely replaced in less than 8 days (Figs. 5, 6). The average produced anorthite area% increased from 16.5% in 1 day to 25% in 2 days, 44% in 4 days, and 70% in 8 days (Fig. 6a). However, K-feldspar overgrowth did not always increase in correlation with anorthite formation: 20.5% for 1 day to 38% for 2 days, 37% for 4 days, and 23% for 8 days. With the increasing anorthite area%, maximum porosity also increased by 12.3% for 1 day, 15.5% for 2 days, 30% for 4 days, and 31% for 8 days (Fig. 6b). The total porosity is slightly lower than maximum porosity, with values of 10.4% for 1 day, 11.4% for 2 days, 23.5% for 4 days, and 26% for 8 days (Fig. 6b).

Fig. 6
figure 6

Quantitative comparison of labradorite replacement for 1, 2, 4, and 8 days by the plot of anorthite area growth (%) and a K-feldspar overgrowth (%) and b maximum (orange; %) and total porosities (gray; %), respectively. The maximum porosity is defined as the area of the porosity \(\times\) 100/area of inferred original feldspar grain, and total porosity is defined as the area of the porosity \(\times\) 100/((area of the porosity) + (area of total feldspar after the experiment)) (Fig. 3b, S1)

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Lab-KCl experiments were conducted with oxygen isotope-labeled water to understand the mechanism of alteration. The Raman spectra of anorthite, K-feldspar, and labradorite were obtained in Lab-KCl experiments with 18O-labeled water (Lab-KCl_18O) and compared with those of the secondary minerals in the experiments with 16O water (normal reaction; Fig. 7a). The Raman spectrum peak of the secondary anorthite systematically changed from 503.73 cm−1 (16O water experiment) to 491.46 cm−1 (18O-labeled water experiment), and that of the secondary K-feldspar changed from 512.48 cm−1 (16O water) to 498.47 cm−1 (18O-labeled water) (Fig. 7b). The peaks of the secondary feldspars in 18O-labeled water experiment were shifted to the lower wave numbers than those in 16O water experiment, respectively, which is consistent with the results reported by Niedermeier et al. (2009).

Fig. 7
figure 7

a BSE images and Ca-K elemental maps of labradorite replacement in quartz-absent KCl fluid after 2 days experiment in 18O enriched water. Blue and red open circles indicate the spot for Raman analysis in 16O water and 18O enriched water reaction sample, respectively. b Representative Raman spectra of anorthite, labradorite, and K-feldspar after Lab-KCl in 2-days reaction. The blue line represents the assigned tetrahedral framework in common 16O water and the red line that in 18O water. The observed spots for 16O water are indicated in Fig. 5b

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Replacement texture of feldspar-bearing schists

Biotite schist with KCl(aq)

The biotite schist used in the experiments contained plagioclase (andesine: An21-41Ab79-59) and quartz (Table 2). In the 4-day experiment, andesine was replaced by K-feldspar with a uniform composition (Or99Ab1) at the margin of the rock sample or the grain boundaries of plagioclase (Fig. 8a). The reacted area of plagioclase (\(\left( {{\text{area of K - feldspar}} \times 100} \right)/\left( {{\text{area of K - feldspar}} + {\text{area of plagioclase}}} \right)\)) was ~ 30% for 4 days. The replacement occurred mostly along the grain boundary with microscale pore occurrences with a total porosity of 2% in 4 days (with respect to the plagioclase region; Fig. 8b). No secondary minerals, other than K-feldspar, were present during the experiment (Fig. 8b).

Fig. 8
figure 8

BSE image of unreacted biotite schist (a) and elemental mapping of reacted biotite schist (b) after 4 days reaction in KCl (BtLabQz-KCl) showing the generation of K-feldspar as secondary minerals and micropores following grain boundaries. c The BSE image of quartz-absent mafic schist (HblLab). d–f BSE images and X-ray mapping of mafic schist reacted with KCl for 2 days in quartz-absent fluid (HblLab-KCl). d BSE image of the wider view of mafic schist and its map of overlap Ca-K. It shows the distribution of K-feldspar which is more frequent along the aggregate rim. e BSE image showing K-feldspar overgrowth and labradorite replacement by anorthite along with micropores generation preference in anorthite. Hornblende is partly reacted, which shows a patchy texture with a gap between the reaction interface and the pristine hornblende (bottom right). f Closer view of d (yellow box) and the elemental mapping of overlap Ca-K. An anorthite, Bt biotite, Kfs K-feldspar, Hbl hornblende, Lab labradorite, Pl plagioclase, Qz quartz

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Hornblende schist with KCl (aq)

In the experiment with hornblende schist, which contained labradorite and was quartz-absent (Fig. 8c), anorthite and K-feldspar were formed as secondary minerals of labradorite, whereas hornblende was generally unreacted (Fig. 8d–f). Anorthite is commonly formed through grain boundaries or microcracks (Fig. 8e–f), replacing pre-existing labradorite grains. In contrast, K-feldspar occurred as an overgrowth of plagioclase grains. Thin K-feldspars were found along the grain boundaries in the inner part of the hornblende schist; however, the most distinct overgrowth preferentially occurred at the margin of the hornblende schist sample. The reacted areas of anorthite and K-feldspar for 4 days were 24% and 31%, respectively. The compositions of secondary anorthite varied (An83-93Ab9-4Or8-3), whereas the secondary K-feldspar showed a near-end-member composition (Ab1-2Or99-98). The reaction interface appeared sharp (Fig. 8e, f), and micropores were preferentially formed in the anorthite regions, whereas porosity was absent in the K-feldspar overgrowth. The total porosity was 6–10% for 4 days. It is noted that the wider K-feldspar overgrowth and anorthite replacement were formed along the marginal part of the hornblende schist blocks (Fig. 8d).

Discussion

Effect of fluid compositions and plagioclase compositions on micropore generation

During most replacement reactions in the systems of Ab-KCl-Qz, Ab-KCl, An-KCl-Qz, and Lab-KCl-Qz, the outlines of the original grains were largely preserved (pseudomorphic replacement; Fig. 4a, b, d, h), although euhedral overgrowth was partly observed (Fig. 4b). Pseudomorphic replacement with a sharp reaction front represents a coupled dissolution and reprecipitation (Putnis 2009). Although the replacement of albite by K-feldspar resulted in a solid volume increase (Reaction 1), distinct micropores were generated near the reaction front (Fig. 4a, b), as reported in previous studies (e.g., Labotka et al. 2004).

The total porosity (%) and the reacted area (%) obtained from BSE images showed some uncertainties because of the lack of information on the original grain shape and cutting effects of the grains (Fig. 3). Nevertheless, by measuring more than ten grains in each run, several distinct features were observed, which are discussed below.

First, in the quartz-present system, the replacement of plagioclase with KCl(aq) (An-KCl-Qz, Lab-KCl-Qz; Fig. 4d, h) proceeded more than that in the experiments with NaCl(aq) (An-NaCl-Qz; Lab-NaCl-Qz, Fig. 4c, f). This is probably because the driving force of dissolution (degree of undersaturation) of the parent plagioclase is larger in KCl(aq) than in NaCl(aq), as discussed later. The porosity increased with an increase in the reacted area% (Fig. 3).

The second feature is the effect of quartz on the replacement. The replacement rate of Ab in KCl(aq) was similar in both the quartz-absent (Ab-KCl) and quartz-present (Ab-KCl-Qz) systems (Fig. 2a, b). In contrast, the reacted area% in An-KCl-Qz experiments (1–42% per 4 days) was greater than that in An-KCl without quartz, in which no replacement was detected (Fig. 3a). In the cases of Lab-KCl-Qz and Lab-KCl, the replacement proceeded significantly, but the latter (reacted area% of 50–90% after 4 days) proceeded slightly more than the former (40–80%; Fig. 3a). Interestingly, the generated total porosities in the Lab-KCl (15–86%) were much larger than those in the Lab-KCl-Qz (1–5%; Fig. 3b).

In the quartz-absent experiments with KCl(aq), the reacted area and the total porosity for 4 days significantly varied with plagioclase compositions: 50–90% and 1–44% for Lab-KCl, 15–86% and 2–18 for Ab-KCl, and 1% and 1% for An-KCl, respectively (Fig. 3). These results suggest that the plagioclase composition (c.f. An component) significantly influences the progress of the replacement and porosity generation.

Variation in porosity generations and reaction mechanism

The occurrence of microscale porosity is grouped into two distinct categories.

The first category of micropores was predominantly found along the reaction front between the newly formed product and parent minerals, whereas the rest of the product rim was generally porosity-poor or absent (Fig. 4). These micropores were observed in the quartz-present KCl(aq) reactions with anorthite, labradorite, and albite (Fig. 4a, d, h) and the quartz-absent albite reaction (Fig. 4b). This type of pores formed during feldspar replacement is consistent with that found in previous experimental studies (Labotka et al. 2004; Niedermeier et al. 2009; Hövelmann et al. 2010). In the BSE images, porosities at the reaction fronts appeared isolated from the fluid reservoirs outside; however, observations with transmitted electron microscopy (TEM) of altered feldspars (e.g., Hövelmann et al. 2010) and other minerals (i.e., replacement of leucite by analcime; Putnis et al. 2007) revealed the nanoscale porosities of the product minerals. The connectivity of nano- to micropores is strongly influenced by the crystal structure of minerals, such as cleavage planes and twin boundaries (Altree-Williams et al. 2015).

Second, large pores were generated, coupled with the overgrowth of secondary minerals. In our experiments, this type of pores was formed only in the labradorite replacement with the quartz-absent fluid system (Lab-KCl), in which large voids (10–100 µm) were formed in the anorthite region replacing original labradorite grains (Figs. 4i, 5) along with K-feldspar overgrowth. In such replacements, pseudomorphism is not maintained. A similar texture was reported for the replacement of pentlandite by violarite (Xia et al. 2009), and aragonite replacement by calcite (Perdikouri et al. 2011, 2013). Although crystallographic orientation of parent and secondary minerals was not measured in this study, a topotactic relation is expected between labradorite and secondary minerals (anorthite and K-feldspars), as observed in plagioclase replacement by albite + K-feldspar in natural samples (Nurdiana et al. 2021).

In general, during the replacement of minerals, pores are produced when the volume of the product mineral is smaller than that of the parent mineral at the reaction site (Putnis 2009). For the simplest cases, the molar volume of the product mineral was smaller than the parent mineral, as shown in the K-feldspar replacement by albite (\({\Delta }\)V = − 8.13%), in which Na and K were exchanged with each other and the other elements (Si, Al, O) were fixed in the overall reaction. Oxygen isotope labeled experiments reveal that the replacement reaction does not proceed by the exchange of ions in the solid state, but it proceeds as the dissolution of whole K-feldspar in solution and precipitation of albite at the same site (Niedermeier et al. 2009). In contrast, porosity was generated even in the albite replacement by K-feldspar (\({\Delta }\)V = + 8.85%), as shown in Fig. 4a, which is consistent with previous studies (Labotka et al. 2004; Niedermeier et al. 2009). This porosity was likely caused by the transport of the dissolved albite components (Na, Al, and Si that were not used for the precipitation of K-feldspar) to the outside of the original grains. In our experiments (Ab-KCl and Ab-KCl-Qz), the starting fluid did not contain Al; therefore, the dissolution of Ab proceeded until the Al concentration in the fluids increased to the saturation level with K-feldspar. The volume of the pores increased with an increasing fluid-to-rock volume ratio.

Thermodynamic constraints on the mobility of elements in quartz-present and quartz-absent systems

The replacement rate, texture, and porosity generation differed between albite, anorthite, and labradorite. Geochemical modelling of the titration of albite, anorthite (Fig. S2, S3), and labradorite (Fig. 9) was conducted to understand the effects of plagioclase composition on the fluid composition. Geochemical modelling indicates that the starting saline solution (KCl(aq), NaCl(aq)) was pH ~ 7 at room temperature, which is higher than the initial pH (5.5–6.0) in our experiments. However, even when the initial pH was adjusted to 6 in the modeling, the results of titration of feldspars did not change.

Fig. 9
figure 9

Reaction path of labradorite reaction with KCl simulated by CHIM-XPT and SOLVEQ-XPT using labradorite (An70Ab30) as the reactant for titration. Calculations were performed with two different fluid compositions: quartz-present (a, b) and quartz-absent (c, d). pH change in response to the fluid/rock ratio was also determined. The Lab-KCl-Qz reaction produces the precipitation of K-feldspar, while the Lab-KCl suggests anorthite and K-feldspar precipitation. Please note that the x-axis scale varies

Full size image

During the titration of albite in KCl(aq) in quartz-present and quartz-absent systems (Fig. S2), K-feldspars were always formed in a fluid/rock ratio of 102–100, but albite appeared only at fluid/rock ratios < 4. In both cases, Na concentration increased with the addition of albite, K concentration decreased, and both concentrations overlapped at a fluid/rock ratio of approximately 4. The Si concentration was higher in the quartz-bearing system (1.1 × 10–2–2.5 × 10–1 mol kg−1H2O) than in the quartz-absent system (2.1 × 10–3–1.5 × 10–2 mol kg−1H2O), whereas Al concentration was lower (5.2 × 10–6–1.4 × 10–3 mol kg−1H2O) in the former than in the latter (7.1 × 10–4–4.0 × 10–3 mol kg−1H2O). The pH increased from 6.7 to 6.8 with increasing Na concentration for both quartz-present and quartz-absent systems. The amount of K-feldspar produced did not change regardless of the presence of quartz. This is consistent with the experimental results that the presence of silica does not influence the albite alteration by KCl(aq) (reaction 2; Fig. 4a, b).

The solution chemistry and product minerals obtained by the titration of anorthite in KCl(aq) were significantly different between the quartz-present and quartz-absent systems (Fig. S3). In the An-KCl-Qz system, K-feldspar precipitation occurred, and the concentrations of Ca (1.1 × 10–4–1.5 × 10–1 mol kg−1H2O), Si (1.2 × 10–3–8.7 × 10–2 mol kg−1H2O), and K (1.6–1.9 mol kg−1H2O) were substantial to the extent of K-feldspar at fluid/rock > 100. After Al became constant, replacement proceeded with an increase in Ca and a decrease in Al and K concentrations, as shown in reaction 3, and became almost constant when anorthite appeared (the solution is in equilibrium with K-feldspar-anorthite-quartz). In the solution, Al showed the lowest concentration (5.2 × 10–6–1.1 × 10–3 mol kg−1H2O). In contrast, the An titration in KCl(aq) showed no product minerals except for anorthite (Fig. 4d). There was no change in the element concentration with every additional rock to the solution: Si = 6.7 × 10–4–2.4 × 10–3 mol kg−1H2O, Al = 6.7 × 10–4–2.4 × 10–3 mol kg−1H2O, K = 1.8 mol kg−1H2O, and Ca = 3.4 × 10–4–1.2 × 10–3 mol kg−1H2O. These results are consistent with the formation of K-feldspar in the An-KCl-Qz (Figs. 3a, 4c) and the absence of secondary minerals in the An-KCl experiments (Fig. 3a), indicating that anorthite replacement by K-feldspar cannot proceed without the addition of Si (reaction 3).

Modelling the titration of labradorite (An70Ab30) in KCl(aq) reveals how the presence of quartz has a pronounced impact on the mineralogy of secondary feldspars and fluid compositions. When labradorite was added into the quartz-present KCl(aq), K-feldspars were produced (Fig. 9b), and pH decreased from 6.7 at a fluid/rock ratio of 100 to 6.3 at a fluid/rock of 1. An70Ab30 also became saturated at a fluid/rock ratio of 3.3. With decreasing fluid/rock, the concentration of K (1.2–1.8 mol kg−1H2O) slightly decreased, whereas the concentrations of Na (3.4 × 10–3–1 × 10–1 mol kg−1H2O) and Ca (8 × 10–3–2.5 × 10–1 mol kg−1H2O) increased. The concentration of Al was the lowest (5.2 × 10–6–9.1 × 10–6 mol kg−1H2O; Fig. 9a); therefore, the overall reaction can be expressed as an Al-fixed reference flame as follows:

$${\text{Ca}}_{0.7} {\text{Na}}_{0.3} {\text{Al}}_{1.7} {\text{Si}}_{2} \cdot 3{\text{O}}_{8} + 1.7{\text{K}}^{ + } + 2.8{\text{SiO}}_{2} ({\text{aq}}) \to \;1.7{\text{KAlSi}}_{3} {\text{O}}_{8} + 0.7{\text{Ca}}^{ + } + 0.3{\text{Na}}^{ + }$$
(4)

According to geochemical models, in the absence of quartz, K-feldspars and anorthite are produced as a result of the dissolution of labradorite (An70Ab30; Fig. 9d), which is consistent with experimental observations (Fig. 5). The element with the lowest concentration in the aqueous solution was not Al (~ 3.4 × 10–3 mol kg−1H2O) but Ca (1.5 × 10–5–3.6 × 10–5 mol kg−1H2O; Fig. 9c). Based on the results of geochemical modeling (Fig. 9) and the occurrence of secondary feldspars during the alteration of labradorite, the distribution of product minerals and the generation of large porosity in Lab-KCl experiments are expected by a two-step reaction. The first step is the dissolution of labradorite and precipitation of anorthite at the reaction front, which can be expressed in a Ca-fixed reference frame.

$$\mathop {{\text{Ca}}_{0.7} {\text{Na}}_{0.3} {\text{Al}}_{1.7} {\text{Si}}_{2.3} {\text{O}}_{8} + 1.2 {\text{H}}^{ + } }\limits_{{{\text{Labradorite}}}} \to \mathop { 0.7 {\text{CaAl}}_{2} {\text{Si}}_{2} {\text{O}}_{8} + 0.9{\text{ SiO}}_{2} \left( {{\text{aq}}} \right) + 0.3{\text{Na}}^{ + } + 0.3{\text{Al}}^{3 + } + 0.6 {\text{H}}_{2} {\text{O}}}\limits_{{{\text{anorthite}}}}$$
(5)

The reaction coefficient is slightly different when anorthite is not a pure endmember, as observed in the experiments (Fig. 1; Table 2). This sub-reaction 5 causes significant solid volume reduction (dV = − 29.9%).

During the progress of reaction 5, the released SiO2(aq) and Al3+ diffuse toward the outer parts of the feldspar particles and encounter K+ ions to cause K-feldspar overgrowth, following the reaction below:

$$0.3 {\text{K}}^{ + } + 0.9 {\text{SiO}}_{2} \left( {{\text{aq}}} \right) + 0.3{\text{Al}}^{3 + } + 0.6{\text{H}}_{2} {\text{O}} \to \mathop {0.3{\text{KAlSi}}_{3} {\text{O}}_{8} + 1.2{\text{H}}^{ + } }\limits_{{{\text{K}} - {\text{feldspar}}}}$$
(6)

The overall reaction that combines reactions 5 and 6 can be written as follows:

$$\mathop {{\text{Ca}}_{0.7} {\text{Na}}_{0.3} {\text{Al}}_{1.7} {\text{Si}}_{2.3} {\text{O}}_{8} + 0.3{\text{K}}^{ + } }\limits_{{{\text{Labradorite}}}} \to \mathop {0.7{\text{CaAl}}_{2} {\text{Si}}_{2} {\text{O}}_{8} }\limits_{{{\text{Anorthite}}}} \mathop { + 0.3{\text{KAlSi}}_{3} {\text{O}}_{8} + 0.3{\text{Na}}^{ + } }\limits_{{\text{K - feldspar}}}$$
(7)

This overall reaction was a 2.6% volume increase, but high porosity was produced in the anorthite following reaction (5), as K-feldspar grew on the outside of the particle. When 100% replacement proceeded, a maximum porosity of ~ 30% is formed.

Figure 10 shows a schematic illustration of replacement and porosity generation, local fluid compositions and saturation index, Q/K, where Q is the ion activity product and K is the equilibrium constant of the mineral dissolution experiments. In the Lab-KCl, the concentration of K increases from the reaction front to outside, whereas the concentrations of Al, Si, Na and Ca decrease (Fig. 10a). As the result, albite is always undersaturated, resulting in the removal of the albite component. Anorthite is saturated near the reaction front within the altered labradorite particles, and K-feldspar is saturated in the overgrowth region owing to the transport of Al and SiO2. In contrast, in the quartz-present system, Al concentration is low, and anorthite and albite are always undersaturated, and only K-feldspar is saturated to proceed with the replacement of labradorite by K-feldspar (Fig. 10b). In summary, the differences in the solubility of plagioclase in KCl(aq) and the local transport of elements such as Al and SiO2(aq) cause significant differences in the replacement texture (pseudomorphism) and porosity generation.

Fig. 10
figure 10

Interpretative illustration of the concentration of elements in fluid, saturation state of feldspar component at the reaction front growth in (a) non pseudomorphism replacement of Lab-KCl reaction and in (b) pseudomorphism replacement of Lab-KCl-Qz

Full size image

The extent of spatial coupling of dissolution and reprecipitation (pseudomorphism) in mineral replacement is controlled by the relative rates of dissolution, precipitation and element transport (Putnis 2009; Xia et al. 2009). In the case of Lab-KCl experiment, the elementary steps of the replacement reactions are (i) the dissolution of labradorite (reaction 5), (ii) transport of K+ to the precipitation site, (iii) transport of Ca2+, Al3+ and SiO2(aq) from the reaction front to outside of the labradorite grain, (iv) precipitation of K-feldspar (reaction 6). If the precipitation rates of anorthite and K-feldspar are much faster than those of element transport and labradorite dissolution, anorthite and K-feldspar co-precipitate at the same site with forming a texture of pseudomorphic replacement. However, this is not the case of our experiment. The observed texture of labradorite replacement (Fig. 5a–c) indicates that (1) anorthite precipitation occurred much faster than Ca2+ and Al3+ diffusion in solution at the reaction front to develop the labradorite replacement texture by anorthite, and (2) Al3+ and SiO2(aq) transport toward the outside was faster than the precipitation of K-feldspar inside of the labradorite region (as K+ concentration was low), resulting in K-feldspar overgrowth. In addition, the temporal evolution of area% of anorthite and K-feldspar overgrowth (Fig. 6a) indicates the relative rate of anorthite precipitation and K-feldspar overgrowth varied with time. The overall reaction (7) suggests that the 1 mol of labradorite produces 0.7 mol of anorthite with 30 area% of maximum porosity, then precipitated 0.3 mol of K-feldspar (Vkfs/Van \(\approx\) volume of K-feldspar/volume of anorthite = 0.46). This is largely consistent with the result of 8-days experiments (Vkfs/Van \(\approx\) Akfs/Aan = 0.22–0.42, maximum porosity of 30%; Fig. 6a). In contrast, in 1–4 days experiments, the Akfs/Aan exceeds 0.84, suggesting that at the initial stage of the replacement of labradorite, K-feldspar precipitation at the outside of the grain occurred faster than anorthite precipitation at the interior of the labradorite grains.

Comparison of experimental results of mineral grains and polycrystalline rocks

Experiments with biotite schist and hornblende schist represent the hydrothermal alteration of plagioclase (labradorite composition) in quartz-present and quartz-absent crustal rocks, respectively. The experiments with mineral grains and polycrystalline rocks largely showed consistent mineralogical features in both quartz-present (Lab-KCl-Qz; BtLabQz-KCl) and quartz-absent systems (Lab-KCl, HblLab-KCl). In quartz-present systems, secondary minerals are solely K-feldspar (Figs. 4h, 8b). In contrast, plagioclase alteration in quartz-absent systems resulted in the formation of two secondary feldspars: anorthite, which replaced the original grains, and K-feldspar overgrowth (Figs. 4i, 8d–f). The replacement reactions of labradorite with KCl(aq) did not change, regardless of the presence of other minerals in the rock.

In both the experiments with polycrystalline rocks, plagioclase alteration proceeded into the rock interior along the grain boundaries (Fig. 8b, d). This suggests the importance of grain boundaries as the primary fast fluid pathway in rocks, as suggested by a numerical modelling study (Koehn et al. 2021) and a previous experimental study on marble replacement by calcium phosphate (Jonas et al. 2014). The replacement reactions in polycrystalline rocks produced abundant pores along the reaction fronts in the biotite schist and in the secondary anorthite region in the hornblende schist (Fig. 8b, f). One of the large differences between the experiments with mineral grains and polycrystalline rocks is the growth sites of the K-feldspar rim. The K-feldspar rim grown in the former experiment occurred at sites adjacent to the reacted mineral grains (Figs. 4i, 5), whereas that in the latter occurred not along the grain boundaries adjacent to the reaction front, but in the outermost rim of the rock sample (Fig. 8d). If K-feldspar grains grew only around the pre-existing feldspars, the total volume increasing reaction (including pores) probably induced radial cracking around the altered feldspars, as observed in the radial cracking around the serpentinized olivine grains in troctolite (e.g., Jamtveit et al. 2009; Yoshida et al. 2020). This is not the case for feldspar alteration, probably because the elemental diffusion was faster than the precipitation of K-feldspar within the interior of the rock. Elements can be transported over a relatively long distance and are precipitated in open pores such as fractures in nature. This is consistent with the formation of K-feldspar veins related to the hydrothermal alteration of plagioclase in the middle crust (Nurdiana et al. 2021).

Implication for fluid flow at deep crust

Deep-seated saline fluids have been reported in volcanic areas under high-temperature geothermal systems (Kasai et al. 1998; Chambefort and Stefánsson 2020; Heřmanská et al. 2020). Under such conditions, plagioclase replacements can pervasively occur. Our experiments using polycrystalline rocks (Fig. 8) suggest that the grain boundaries act as dominant fluid pathways and fast elemental transport by diffusion. However, the grain boundaries of the metamorphic rocks analyzed in this study could have opened during exhumation; thus, if the grain boundaries were narrower or closed at deep crustal conditions (Hiraga et al. 2001), the roles of the reaction-induced porosities would be more significant than expected.

The Kakkonda geothermal field In Japan possesses a granite-hosted supercritical geothermal reservoir (e.g., Doi et al. 1998; Okamoto et al. 2019). In a deep drilling project at > 500 °C, the fluids extracted from the well bottom showed hypersaline compositions with ~ 55% NaCl equivalent (Kasai et al. 1998). Previous studies suggest that the dissolution and reprecipitation of quartz have played significant roles in the development and maintenance of supercritical geothermal reservoirs (e.g., Saishu et al. 2014; Ishizu et al. 2022; Watanabe et al. 2021a). However, it remains questionable whether exploitable supercritical resources with sufficient porosity and permeability exist in high-temperature granites. In deep reservoirs, the fluid is saturated with quartz, albite, and K-feldspar (Okamoto et al. 2021). In this study, we showed that in the quartz-present system, the replacement of all types of feldspars can occur and generate porosity, which potentially enhances the porosity of supercritical geothermal reservoirs. In the Kakkonda, micropores have been reported in alkali feldspar placed in granodiorite of 410–500 °C at ~ 3 km (Sasaki 2003; Nakano et al. 2014). Therefore, reaction-induced porosity may play an essential role in the creation of supercritical geothermal reservoir.

This study reveals that the absence of quartz (low silica concentration in fluids) is essential for generating high porosity during plagioclase (especially intermediate compositions) alteration with saline fluids. Such quartz-absent systems can be plausible in hydrothermal systems hosted by basaltic or andesitic bulk rock compositions (Chambefort and Stefánsson 2020; Heřmanská et al. 2020). For example, the Reykjanes geothermal system is hosted by the basalt diabase-dolerite, which contains plagioclase with the composition of andesine to anorthite (Friðleifsson et al. 2014, 2020). The Reykjanes geothermal system is an analog of mid-oceanic ridge hydrothermal systems (Friðleifsson et al. 2020). Understanding the depths of hydrothermal circulation is essential to understand not only the potential of geothermal energy (Chambefort and Stefánsson 2020) but also metal extraction to form mineral resources such as volcanic massive sulfide deposits (Robb 2005). Despite the porous and extrusive nature of the upper oceanic crust, porosity can decrease in the lower gabbroic crust. In such cases, plagioclase alteration with large porosity can play an important role in mass and energy transport during hydrothermal circulation within the lower crust, although direct evidence in large porosities in the rocks of the lower crust has not yet been reported.

Fluid flow enhances the removal of specific species faster than reprecipitation, which may be preferable for pore formation. Preferential porosity generation in deep geothermal reservoirs can be a useful method for chemical stimulation (Luo et al. 2018; Watanabe et al. 2021b) for enhanced geothermal systems (EGSs). The use of specific saline fluids to plagioclase-rich crusts may be more environmentally friendly than conventional chemical stimulation using acids or chelating agents.

Conclusions

We conducted hydrothermal experiments on plagioclase replacement at 600 °C and 150 MPa using various combinations of plagioclase compositions (anorthite, albite, and labradorite) and fluid chemistry (KCl(aq) and NaCl(aq)) in quartz-bearing and quartz-absent systems. The experiments revealed the following features regarding the progress of the replacement and porosity generation.

  1. 1.

    In all runs, the replacement proceeded from the margin or along the cleavage or pre-existing fractures, and porosity was preferentially produced at the reaction front. Pores were formed along the grain boundaries, cracks, and cleavages of plagioclase. The presence or absence of quartz did not affect the replacement of albite. In contrast, the replacement of anorthite proceeded preferentially with the presence of quartz.

  2. 2.

    Labradorite-KCl(aq) experiments, with and without quartz, produced contrasting textures and products. The Lab–KCl run without quartz produced two types of secondary feldspar: anorthite, which replaced the original labradorite grain, and K-feldspar as overgrowth of the original grain. Large porosities were generated (total porosity of 10–30%) preferentially in the anorthite region.

  3. 3.

    The analyses of the products of Lab-KCl experiment with \(\delta\) 18O-doped H2O revealed that labradorites were dissolved totally and then precipitated again as anorthite inside and K-feldspar at different sites. Geochemical modeling explains that labradorite dissolution and anorthite precipitation occur in the inner part, causing a large volume reduction, and Na and Al move towards that, causing the precipitation of K-feldspar in the outer region because of the high K+ concentration around the grains.

  4. 4.

    The experiments with plagioclase-bearing polycrystalline rocks in KCl(aq) were essentially similar to those with labradorite-bearing rocks; Only the K-feldspar replacing plagioclase process occurred in quartz-bearing rocks, whereas plagioclase replacement by anorthite and overgrowth of K-feldspar occurred in quartz-absent rocks. In polycrystalline experiments, the grain boundaries acted as important fluid pathways to enhance the alteration of plagioclase, and high porosity was formed near the reaction front within individual plagioclase grains. K-feldspar precipitation in quartz-absent rocks occurred preferentially, not at grain boundaries adjacent to the reaction sites, but in the open spaces of the samples.

  5. 5.

    The results of this study suggest that the infiltration of K-rich fluids into plagioclase-bearing rocks, especially in quartz-absent crustal rocks (e.g., gabbro), favors the uncoupled dissolution and reprecipitation mechanism and increases the porosity of crustal rocks.