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Proceedings of the National Academy of Sciences of the United States of America logoLink to Proceedings of the National Academy of Sciences of the United States of America
. 2012 Oct 29;109(46):18695–18700. doi: 10.1073/pnas.1207687109

Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism

Tatsuhiko Kawamoto a,1, Masami Kanzaki b, Kenji Mibe c, Kyoko N Matsukage d,2,✉, Shigeaki Ono e
PMCID: PMC3503209  PMID: 23112158

Abstract

Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry.

Keywords: water, second critical point, synchrotron X-ray, pelite, adakite

Addition of aqueous fluids to the mantle wedge plays an important role in reducing the melting temperature and in producing partial melts (1–4). Alternatively, partial melts of downgoing sediment or oceanic basalt themselves are added to the mantle wedge, resulting in subduction-zone magmatism (5), especially in hot environments occurring with subduction of a young oceanic plate (6, 7). Arc basalts are characterized by their higher concentrations of H2O and large ion lithophile elements than those of mid-ocean ridge basalts (3, 4, 8–12). Such subduction-zone signatures are attributed to the addition of slab-derived components into a mid-ocean ridge basalt source mantle. Whether the slab-derived component is an aqueous fluid, a partial melt, or a supercritical fluid persists as an open question (13–15). In general, with increasing pressure, aqueous fluids dissolve more silicate components and silicate melts dissolve more H2O. Under low-pressure conditions, those aqueous fluids and hydrous silicate melts are divided by an immiscibility gap. Its highest temperature is designated as a critical point. The temperature of the critical point decreases concomitantly with increasing pressure, finally meeting the H2O-saturated solidus temperature. At this pressure and temperature (P–T) condition, no difference exists between hydrous silicate melts and aqueous fluids. This point is designated as a critical endpoint (14, 16).

At a third-generation synchrotron facility (SPring-8) in Japan, we have been conducting a series of in situ observations using synchrotron X-ray radiography under high-pressure and high-temperature conditions to elucidate mixing and unmixing behaviors that occur between aqueous fluids and various silicate melts (17, 18). In the present study, we examined unmixing and mixing behavior of a sediment (pelite; Table S1 and Fig. S1) melt and aqueous fluids, and of a high-Mg andesite (HMA; Table S1) melt and aqueous fluids under high-pressure and high-temperature conditions. Sediment, a chemically distinct component of a downgoing slab (19), can be flushed by aqueous fluids from dehydrating hydrous minerals in the underlying basalt and peridotite (12, 15, 20). Beyond a critical endpoint between sediment and aqueous fluids, a silica-rich sediment-derived supercritical fluid can be fed to the mantle wedge and can react with peridotite. The chemical composition of silicate components dissolved in the slab-derived fluid can change to HMA because silica-rich melts can react with peridotite to form high-Mg felsic melts (21). Ayers et al. (22) report that solutes dissolved in aqueous fluids coexisting with mantle peridotite have high-Mg dacite–andesite compositions at 2 GPa, although they have much less silica at 3 GPa. Therefore, the mixing and unmixing relation between sediment/HMA and aqueous fluids is important to elucidate the behaviors of the slab-derived fluids in the subducting sediment layer and in the mantle wedge beneath volcanic arcs.

Results

X-ray radiography experiments (17) were conducted using the SPEED 1500 Kawai-type multianvil apparatus installed at BL04B1 of SPring-8. The experimental procedures are described in Materials and Methods. We added 5% (wt) gold powders, which can facilitate visualization of the melt–fluid phase boundary (Movie S1). The gold powders fall uniformly in a supercritical fluid during heating (Movie S2). The experimental conditions and results are presented in Table 1. When we heated the sample at lower pressures with variable H2O concentrations, we observed one round material in the other material with different X-ray absorbance, which we interpreted as coexisting silicate melt and aqueous fluid (Fig. 1 A and C and Movie S1). At higher pressures, we observed no such coexisting phase, but found gold powders falling during heating (Fig. 1F and Movie S2). After the experiment, we opened the diamond lid and observed the sample surface under a stereomicroscope and a Raman microscope to identify some quench phases. In run products in which we had observed round material surrounded by a matrix on X-ray radiography, we found round aggregates of glass globules, typically larger than 10 μm, surrounded by a void space (Fig. 1B), or much larger glassy globules with a void space (Fig. 1D) or a crescent-shaped glassy portion along the capsule wall with a round void space (Fig. 1E). Mibe et al. (23, 24) reported similar observations in the experimental products of peridotite/ basalt–H2O system. We interpret the large glass globules and portions as quench products from hydrous melts, and the void space as a quench product from aqueous fluids. Tiny glass globules were present in the interface between the void space and glass portion (Fig. 1 D and E). We interpret these as quenched products from silicate components dissolved in aqueous fluids. In contrast, we observed uniform aggregates of tiny glass globules, accompanied in some cases by crystals, from other experiments in which we had observed no round material surrounded by a matrix on X-ray radiography (Fig. 1G). We regard them as quench products from a single supercritical fluid.

Table 1.

Summary of experimental conditions and results

Run no. P, GPa H2O in starting materials, % (wt) Max/quench temperature, °C Two fluids temperature, °C Quenched product
HMA–H2O system
 1,469 1.4 50 1140/1140 750–1140 —
 2,234 1.5 55 1014/1014 1000–1014 Large void space, large/small glass globules, crystals
 1,470 2.0 50 1080/800 780–1080 Large void space, large/small glass globules, crystals
 1,478 2.0 70 1160/900 750–1160 Large void space, large/small glass globules, crystals
 1,483 2.4 30 1000/800 737–1000 Large void space, large/small glass globules, crystals
 1,472 2.4 50 1105/800 800–1050 Large void space, large/small glass globules, crystals
 1,482 2.4 70 1043/750 740–930 Large void space, large/small glass globules, crystals
 1,602 2.6 30 1040/1000 * —
 1,541 2.6 50 982/800 * —
 1,601 2.6 60 1050/1000 * —
 2,593 2.6 63 800/800 * Homogeneously distributed small glass globules, crystals
 1,533 2.6 70 977/900 841–977 Large void space, large/small glass globules, crystals
 1,471 2.8 50 1075/950 * Homogeneously distributed small glass globules, crystals
 1,534 2.8 60 1000/700 * —
 1,542 2.8 60 970/800 * Homogeneously distributed small glass globules, crystals
 1,481 2.8 70 1135/740 700–900 Large void space, large/small glass globules, crystals
 1,532 2.8 70 904/740 * Homogeneously distributed small glass globules, crystals
 1,531 3.0 70 964/740 * Homogeneously distributed small glass globules, crystals
Sediment–H2O system
 1,474 1.4 50 1129/1050 760–1129 Large void space, large/small glass globules
 2,238 1.5 50 952/952 880–952 Large void space, large/small glass globules
 2,236 1.5 60 958/958 850–958 Large void space, large/small glass globules
 1,475 2.0 50 1106/800 748–926 Large void space, large/small glass globules, crystals
 1,486 2.4 30 1050/850 810–990 Large void space, large/small glass globules, crystals
 1,476 2.4 50 1084/650 600–780 Large void space, large/small glass globules, crystals
 1,484 2.4 62 1030/720 660–840 Large void space, large/small glass globules, crystals
 1,539 2.6 30 850/750 * —
 1,536 2.6 50 1010/750 * Homogeneously distributed small glass globules, crystals
 1,537 2.6 66 960/960 * Homogeneously distributed small glass globules
 1,540 2.6 62 948/750 * —
 1,477 2.8 50 1000/746 * Homogeneously distributed small glass globules, crystals
Albite–H2O system
 1,932 1.0 64 1010/700 700–810 —
 2,222 1.2 38 1016/750 750–1016 —
 2,220 1.4 38 995/800 * —
 1,900 1.4 49 1020/700 540–720 Large void space, large/small glass globules
 2,130 1.4 61 1000/635 640–710 Large void space, large/small glass globules
 1,930 1.4 64 815/800 * Homogeneously distributed small glass globules
 2,132 1.7 50 942/700 * —
 1,901 2.0 50 1015/700 * Homogeneously distributed small glass globules, crystals
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Two fluids temperature, temperature ranges in which two-fluid phases were observed by using X-ray radiography.

*No observation of two fluids using X-ray radiography.

Fig. 1.

Fig. 1.

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Snapshots of radiographs and photographs of quenched samples. (A) X-ray radiograph shows unmixing between HMA (dark circle) and aqueous fluid (matrix) at 1011 °C and 1.5 GPa (run 2,234). The sample was surrounded by a AuPd tube (black) in the 1.2-mm-diameter sample. (B) Quench product of the experimental charge shown in A. Aggregates of glassy globules and a void space, respectively, represent portions quenched from melt and aqueous fluid. (C) X-ray radiograph shows unmixing between HMA melt (dark circle) and aqueous fluid (matrix) at 884 °C and 2.6 GPa (run 1,533). The sample image is 1.5 mm wide (Movie S1). (D) Quench product of the experimental charge in the sediment with 50% (wt) H2O, quenched at 1050 °C and 1.4 GPa. Three larger globules and a thin glassy wall are quenched from melt, and there is void space from aqueous fluids (run 1,474). Tiny glassy globules can be quenched from aqueous fluids. The sample room is 1.5 mm wide. (E) Quench product of experimental charge in the sediment with 50% (wt) H2O, quenched at 800 °C and 2 GPa. The round void is surrounded by a glassy wall quenched from a melt (run 1,470). Tiny glassy globules are observed on the surfaces of glassy wall. They can be quenched from aqueous fluids. The sample room is 1.5 mm wide. (F) X-ray radiograph shows the gold powder-rich portion (lower dark half) falling in a uniform single supercritical fluid in the sediment with 50% (wt) H2O at 953 °C and 2.6 GPa (run 1,536). The sample image is 1.5 mm wide (Movie S2). (G) The quench product of the experimental charge in the HMA with 63% (wt) H2O, quenched at 800 °C and 2.6 GPa. Garnet crystals of 50-μm diameter are scattered in a matrix with tiny (<10 μm) glass globules (run 2,593). The sample is approximately 1.2 mm in diameter. (H) Backscattered images of polished section cut parallel to the X-ray path of the run product in albite–49% (wt) H2O quenched at 700 °C and 1.4 GPa show single large glass globules with many bubbles. The X-ray path was vertical (run 1,900). The sample is 1.2 mm wide. (I) Backscattered images of a polished section cut parallel to X ray of run product of albite–50% (wt) H2O quenched at 700 °C and 2 GPa show uniform appearance and no large void such as that seen in H (run 1,901). The sample is 1.2 mm wide.

Two phases were observed during heating and cooling in radiography images at pressures lower than 2.8 GPa in HMA–H2O and 2.5 GPa in sediment–H2O (Fig. 2 A and B). At pressures higher than each of them, only a single phase was recognized. The radiography results are also shown in P–T diagrams at constant bulk H2O, in which critical curves and H2O-saturated solidus temperature (25, 26) meet at critical endpoints in the HMA–H2O and sediment–H2O (Fig. 2 D and E). These results show that the pressures of the critical endpoint can be at 2.8 GPa and 750 °C for HMA–H2O and 2.5 GPa and 700 °C for sediment–H2O.

Fig. 2.

Fig. 2.

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(A–C) Pressure vs. bulk H2O composition phase diagrams shows single-fluid and two-fluid regions along heating and cooling at constant pressure (Table 1). (A) Phase diagram shows single-fluid and two-fluid regions in HMA–H2O. At 2.8 GPa and 70% (wt) H2O, two experiments (Table 1) show one fluid (run 1,532) and two fluids (run 1,481). We infer that this inconsistency exists because of the small pressure difference between them. (B) Phase diagram shows single-fluid and two-fluid regions in sediment–H2O. Pressure values of intersection between these two regions in sediment–H2O system are constant with respect to H2O concentration in this diagram. This feature is not seen in the other systems. (C) Phase diagram shows single-fluid and two-fluid regions in albite–H2O. (D–F) P–T diagrams showing experimentally obtained results and phase relations in the HMA–H2O, sediment–H2O, and albite–H2O system. Open bars represent the P–T conditions at which two fluid phases (aqueous fluid and silicate melt) were observed, whereas filled bars represent the P–T conditions at which two fluids were not observed in radiographic images. These results show that the pressure of the critical endpoint can be at 2.8 GPa and 750 °C for HMA–H2O, 2.5 GPa and 700 °C for sediment–H2O system, and 1.55 GPa and 700 °C for albite–H2O system. The amount of H2O in the starting materials of data shown here is limited as explained in each diagram (Table 1). Red curves represent hydrous solidus in the andesite–H2O (25), the pelagic red clay–H2O (26), and albite–H2O (28). The blue dotted lines represent the estimated critical curve in each system.

To assess the precision and accuracy of the present method for estimation of critical endpoints between aqueous fluids and silicate melts, we conducted experiments using the albite–H2O system, which has a critical endpoint at approximately 1.5 GPa based on visual observation of the solvus using a Bassett-type hydrothermal diamond anvil cell, as described by Shen and Keppler (27) and quench experiments (28). We conducted a series of experiments using albite with 38%, 50%, and 64% (wt) H2O at SPring-8: we observed two fluids with 38% (wt) H2O at 1.2 GPa, 50% (wt) H2O at 1.4 GPa, and 60% (wt) H2O at 1.4 GPa, and a single fluid with 38% (wt) H2O at 1.4 GPa, 50% (wt) H2O at 1.7 GPa, and 64% (wt) H2O at 1.4 GPa (Fig. 2 C and F and Table 1). These observations show that the albite melt and aqueous fluids are closing the solvus between 1.4 GPa and 1.7 GPa at 50% (wt) H2O, which is consistent with results of those previous studies. This simple test demonstrates the ability of the present method to determine critical endpoints in silicate melts and aqueous fluids within uncertainty of, at worst, ±10% relative or ±0.15 GPa for this case.

Discussion

Formation of Slab-Derived Supercritical Fluids.

The newly determined critical endpoints (sediment and HMA in Fig. 3A) suggest that slab-derived fluids are expected to be under supercritical conditions in the slab sediment layer and at the base of the mantle wedge, where HMA-bearing supercritical fluid is presumably formed by a reaction between sediment-derived supercritical fluids and peridotite beneath the volcanic arcs. Therefore, no isolated fluid/melt phase exists, but a single supercritical fluid phase with a continuum characteristic between aqueous fluid and hydrous melt in the downgoing sediments and the base of the mantle wedge underneath volcanic arcs at least >83 km and >92 km, respectively. Whether slab-derived supercritical fluids have chemical characteristics resembling a partial melt or an aqueous fluid depends largely on the temperature (11, 12, 14, 15, 29). If the slab-derived supercritical fluids are in warm conditions, more silicate components can be dissolved than in cold conditions. Under sufficiently warm conditions, a melt-like supercritical fluid can be formed in the downgoing sediment layer, where aqueous fluids are formed through dehydration reactions and also where they are supplied continuously from the underlying basalt/peridotite layers (12, 15, 20, 30) (Fig. 3B).

Fig. 3.

Fig. 3.

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P–T diagram shows critical endpoints and an inferred schematic illustration in a mantle wedge with thermal structure suggested by Peacock and Wang (37). (A) P–T diagram shows critical endpoints and critical curves (13, 23, 24, 28). (Ab, albite; Ca-Hgr, CaO bearing haplogranite; Hgr, haplogranite; Jd, jadeite; Ne, nepheline.) The P–T paths beneath the volcanic front in southwestern Japan (green) and the northeastern Japan arc (blue) are shown with solidus temperature of H2O-saturated mantle peridotite (61). The observed critical endpoints between more felsic rocks and H2O are located at lower pressures (sediment < HMA < basalt < peridotite). (B) Schematic illustration shows separation of supercritical fluids into aqueous fluid and hydrous melt with thermal structure suggested by Peacock and Wang (37) for the southwestern Japan arc. Under sufficiently warm conditions, a melt-like supercritical fluid can be formed in the downgoing sediment layer, where aqueous fluids are formed through dehydration reactions and where they are also supplied continuously from the underlying basalt/peridotite layers (12, 15, 20, 30). When such a sediment-derived supercritical fluid enters the overlying mantle, the fluid will react with the peridotite to become HMA or basalt-bearing supercritical fluid (22). This supercritical HMA or basalt-bearing fluid migrates upward and meets the critical endpoint (2.8 GPa for HMA, 3.4 GPa for basalt). It will then separate into a melt phase and a fluid phase. The HMA or basalt melt will continue to react with mantle to form a melt-derived magma or an Mg-rich andesitic magma (21). The fluid will trigger hydrous partial melting of the ambient mantle peridotite to form a fluid-derived magma. Two slab-derived components (melt and fluid) have been recognized in many arc basalts (9, 10, 42). Separation of slab-derived supercritical fluids into melts and aqueous fluids can occur commonly in most subduction zones.

Separation of Supercritical Fluids to Form Two Slab Components at Base of Mantle Wedge.

The thermal structure in the mantle wedge is a key to infer the present experimentally obtained results for the magma genesis in subduction zones. Since Tatsumi et al. (31) suggested super-dry solidus conditions based on an assumption of almost dry features of frontal tholeiitic basalts in northeastern Japan and the Izu arc, 1400 °C has been a popular temperature for the mantle wedge center in numerical models (30, 32). In contrast, Hamada et al. (33) reexamined the H2O contents in the tholeiite basalt in the Izu arc and found that the tholeiitic basalt contains more than 6% (wt) H2O in the lower crust conditions. It successively degasses during ascent to shallower magma chambers. Tatsumi et al. (31) assumed almost dry tholeiite based on its phenocryst assemblage (34), which can represent reequilibrated conditions at the shallower magma chambers. Recent numerical models suggesting a high-temperature mantle wedge apparently fail to explain the lack of a volcano above their supersolidus mantle wedge under wet conditions (30, 32). In contrast, seismic observations suggest moderate mantle temperature (35, 36). To safeguard consistency with the distributions of volcanoes above H2O-saturated solidus temperature and the seismic observations, we chose the thermal structure suggested by Peacock and Wang (37) with temperatures of 1150 to 1200 °C at the hot core of the asthenosphere beneath the volcanic fronts (Fig. 3).

When a sediment-derived supercritical fluid enters the overlying mantle, the fluid will react with the peridotite to dissolve olivine and precipitate orthopyroxene (21), consequently becoming an HMA-bearing fluid (22). The HMA bearing fluid reacts successively with the surrounding mantle. At higher temperature conditions, HMA might change its major element composition to basaltic (38). Only if the reaction with the surrounding mantle is localized or such a hydration reaction occurs at much shallower pressure conditions, the HMA-bearing fluids can retain their major element composition as andesitic. Whether or not they do so, a critical endpoint between basalt or HMA and H2O exists in 3.4 GPa (110 km) and 800 °C (24) or in 2.9 GPa (92 km) and 750 °C. Therefore, such a basalt/HMA-bearing fluid can separate into a melt and an aqueous fluid at the base of mantle wedge (Fig. 3). The melt will continue to react with mantle to form a melt-derived magma, and the fluid will trigger hydrous partial melting of mantle to form a fluid-derived magma (Fig. 3B). The melt-derived magma can be HMA or basalt depending on the degree of reaction with mantle peridotite and on pressure conditions. However, the melt-derived magma inherits trace element signatures as if it were sedimentary melt, whereas the fluid-derived magma inherits only fluid-preferred elements from the sediment-derived supercritical fluids (39–41).

In the Mariana arc, Elliott et al. (42) recognized two slab-derived components in basalt chemistry: a partial melt from the subducting sedimentary layer and an aqueous fluid from the subducting basaltic layer. They suggest that those two components can be mixed by mantle flow delivering sedimentary partial melts toward a place beneath the volcanic front, where aqueous fluids are supplied through dehydrating hydrous minerals in basalt. In our hypothesis (Fig. 3B), the melt and the fluid components can coexist in the mantle wedge if the supercritical slab-fluids supplied from the downgoing slabs separate into a melt and an aqueous fluid. They have identical isotope contents at the time of separation. Further reactions with the mantle result in isotope contents that are different between a fluid-derived and a melt-derived magma because of the partition behavior of trace elements between a melt and an aqueous fluid and variable degrees of reaction with mantle. These modification processes can explain the trace element ratios and isotopic contents observed in the Mariana basalts (10, 42). Two such distinct slab-derived components have also been suggested to explain the chemistry of many arc basalts (9, 10). Separation of slab-derived supercritical fluids into melts and aqueous fluids can commonly occur in most subduction zones. This can suggest that subducting slabs are warm enough to feed sediment-derived supercritical fluids to the overlying mantle wedge in most subduction zones.

Coexistence of Basalts and HMAs.

Extraordinary magmas are produced in extremely hot subduction zones: adakite (6, 7, 43) and Mg-rich sanukitoids (44). These rocks are also classified as HMAs (45). Experimental studies have duplicated major-element abundances of HMAs through partial melting of hydrous mantle peridotite leaving a harzburgite residue (46–50). Isotopic compositions and trace-element abundances of some HMAs suggest the addition of enriched components from downgoing slabs (43, 45, 51, 52). Adakite is characterized by high ratios of light rare earth element (REE) to heavy REE and low concentrations of heavy REEs. These features have been proposed to represent a partial melt of downgoing basaltic crust with a garnet residue (6). The Mg-rich sanukitoids, which have no such conspicuous REE features, are inferred as forming by partial melting of downgoing sediment layers at shallower depths in the amphibolite facies (51, 52). The silica-rich feature of adakite and Mg-rich sanukitoids suggests that silica-rich material plays an important role in their respective geneses: through partial melting of subducting basaltic layer for adakites or sedimentary layer for Mg-rich sanukitoids. An enigma hindering the understanding of magma generation in adakite and Mg-rich sanukitoid suites is how to produce the basaltic magmas accompanying them: variable degrees of reaction between slab melts and mantle (53) and different temperature and water fugacity conditions (54) have been proposed. Here we suggest an alternative hypothesis: HMAs (adakite and Mg-rich sanukitoids) are producible by reaction of mantle and melt derived from slab-derived supercritical fluid. Basaltic magmas are producible through partial melting of the mantle wedge induced by fluid from it. In hotter subduction zones, dehydration of downgoing slab and subsequent hydration of mantle wedge occur at shallower depths. The silicate component dissolved in the aqueous fluids equilibrated with mantle is more silica-rich (22, 55) and can have a critical endpoint with H2O at lower pressure (Fig. 3A). Such a silica-rich supercritical fluid can separate into an HMA-bearing melt and aqueous fluid producing a basaltic magma. Under low pressures, the HMA-bearing melt reacts with mantle to become an HMA magma. For HMA magma genesis, the shallow processes associated with hot subduction can be necessary (45).

Materials and Methods

The chemical compositions of sediment and HMA were adopted for starting materials, respectively, from Ono (56) and Tatsumi (54) (Table S1). The sediment composition was a pelite used to determine the stability of hydrous phases under high-pressure and high-temperature conditions. The pelite is similar to pelites described as used experimentally in the literature (Fig. S1) (57, 58). H2O-saturated solidus temperatures described by Hermann and Spandler (57) are similar to those determined by Nichols et al. (26). Schmidt et al. (58) proposed temperatures 100 °C higher than those. According to Hermann and Spandler (57), the critical endpoint in pelite-H2O can be located at 3.5 GPa (57), which is shallower than that proposed by Schmidt et al. (58), at 5.5 GPa. The HMA composition [Teraga-Ike andesite (TGI)] (54) is an extremely primitive HMA produced through equilibrium between mantle peridotite and partial melts of a downgoing sediment layer (45, 51). Sample powders prepared from mixtures of SiO2⋅0.3 H2O, Al(OH)3, FeOOH, TiO2, Mg(OH)2, Ca(OH)2, and synthetic Na2O–K2O–SiO2 glass were analyzed by Takeshi Sugimoto (Institute for Geothermal Sciences, Kyoto University, Beppu, Japan) using XRF (anhydrous compositions are presented in Table S1). These powders respectively have 16.5% (wt) and 14.9% (wt) H2O; Milli-Q water was added to the sample by using a microsyringe immediately before each experiment. The total water contents of the starting materials are 30–70% (wt) H2O.

The present experimental assembly is similar to that used in our previous studies and is of slightly different size (18). We put Milli-Q water with sample powders inside an Au75Pd25 tube with a single-crystal diamond plug (2.5 mm in diameter, 2 mm long) and sealed them with another single-crystal diamond plug. This capsule is surrounded by a MgO insulator sleeve, a graphite sleeve heater, a CaO-doped Zirconia sleeve (Mino Ceramic) as a thermal insulator, and a Cr2O3-doped MgO octahedron (18M; Mino Ceramic). After the addition of water and sealing with a second diamond plug, we pressurized the assembly to the desired value. Then we raised the temperature. The commercially available single-crystal diamond plugs (Sumitomo Electric Industries) offer advantages of low X-ray absorbance, low scattering, a good seal for water, and inertness with aqueous fluids and silicate melts under high-pressure and high-temperature conditions.

X rays pass through a graphite gasket, the Cr2O3-doped MgO octahedron, MgO plug, one diamond, the sample; and then the other diamond, MgO plug, Cr2O3-doped MgO octahedron, and a graphite gasket to an X-ray camera. Eight cubes of WC with 11-mm truncations are used to pressurize the assembly. R-type thermocouples are attached to the outer surface of the graphite heater. As a quench experiment, we heated a Ca-rich and Ca-poor pyroxene mixture in the same assemblage for 24 h. Results show that the thermocouple temperature equals that estimated using a pyroxene geothermometer (59) within an uncertainty of 25 °C. The pressure is calibrated by using equations of state of gold (60) with X-ray diffraction data at SPring-8 (Fig. S2). Repeated calibration experiments were conducted with identical assemblages with gold and MgO powders used instead of sample powders and water. We observed X-ray radiography at high-pressure and high-temperature conditions for a few tens of minutes and then quenched experiments by switching off the electric power.

Supplementary Material

Supporting Information
supp_109_46_18695__index.html (1.2KB, html)

Acknowledgments

The authors thank Drs. Ken-ichi Funakoshi and Yuji Higo [Japan Synchrotron Radiation Research Institute (JASRI), SPring-8] for longtime support in X-ray radiography experiments. We thank Drs. Jun-Ichi Kimura, Ken Koga, and Jun Korenaga for helpful discussions. This work was supported by Kakenhi through the Japan Society for the Promotion of Science (JSPS), the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), and the Institute for Study of the Earth's Interior (ISEI), Okayama University.

Footnotes

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1207687109/-/DCSupplemental.

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