Separation of Supercritical Slab-Fluids to Form Aqueous Fluid and Melt

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

Tatsuhiko Kawamotoa,1, Masami Kanzakib, Kenji Mibec, Kyoko N. Matsukaged,2, and Shigeaki Onoe

aInstitute for Geothermal Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan; bInstitute for Study of the Earth’s Interior, Okayama University, Misasa 682-0193, Japan; cEarthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan; dDepartment of Environmental Science, Ibaraki University, Mito 310-0056, Japan; and eInstitute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan

Edited by Bruce Watson, Rensselaer Polytechnic Institute, Troy, NY, and approved October 4, 2012 (received for review May 7, 2012)

Subduction-zone magmatism is triggered by the addition of H2O- pressure and temperature (P–T) condition, no difference exists rich slab-derived components: aqueous fluid, hydrous partial melts, between hydrous silicate melts and aqueous fluids. This point is or supercritical fluids from the subducting slab. Geochemical analy- designated as a critical endpoint (14, 16). ses of island arc basalts suggest two slab-derived signatures of At a third-generation synchrotron facility (SPring-8) in Japan, ameltandafluid. These two liquids unite to a supercritical fluid we have been conducting a series of in situ observations using under pressure and temperature conditions beyond a critical end- synchrotron X-ray radiography under high-pressure and high- point. We ascertain critical endpoints between aqueous fluids and temperature conditions to elucidate mixing and unmixing be- sediment or high-Mg andesite (HMA) melts located, respectively, haviors that occur between aqueous fluids and various silicate at 83-km and 92-km depths by using an in situ observation tech- melts (17, 18). In the present study, we examined unmixing and nique. These depths are within the mantle wedge underlying mixing behavior of a sediment (pelite; Table S1 and Fig. S1) volcanic fronts, which are formed 90 to 200 km above subduct- melt and aqueous fluids, and of a high-Mg andesite (HMA; ing slabs. These data suggest that sediment-derived supercritical Table S1) melt and aqueous fluids under high-pressure and high- fl uids, which are fed to the mantle wedge from the subducting temperature conditions. Sediment, a chemically distinct com- EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES slab, react with mantle peridotite to form HMA supercritical ponent of a downgoing slab (19), can be flushed by aqueous fl fl fl uids. Such HMA supercritical uids separate into aqueous u- fluids from dehydrating hydrous minerals in the underlying basalt ids and HMA melts at 92 km depth during ascent. The aqueous and peridotite (12, 15, 20). Beyond a critical endpoint between fl fl uids are uxed into the asthenospheric mantle to form arc sediment and aqueous fluids, a silica-rich sediment-derived su- basalts, which are locally associated with HMAs in hot subduction percritical fluid can be fed to the mantle wedge and can react zones. The separated HMA melts retain their composition in lim- with peridotite. The chemical composition of silicate compo- ited equilibrium with the surrounding mantle. Alternatively, they nents dissolved in the slab-derived fluid can change to HMA equilibrate with the surrounding mantle and change the major because silica-rich melts can react with peridotite to form element chemistry to basaltic composition. However, trace ele- fl high-Mg felsic melts (21). Ayers et al. (22) report that solutes ment signatures of sediment-derived supercritical uids remain fl fl dissolved in aqueous uids coexisting with mantle peridotite more in the melt-derived magma than in the uid-induced magma, – which inherits only fluid-mobile elements from the sediment- have high-Mg dacite andesite compositions at 2 GPa, although derived supercritical fluids. Separation of slab-derived supercrit- they have much less silica at 3 GPa. Therefore, the mixing and fl fl unmixing relation between sediment/HMA and aqueous fluids ical uids into melts and aqueous uids can elucidate the two fl slab-derived components observed in subduction zone magma is important to elucidate the behaviors of the slab-derived uids chemistry. in the subducting sediment layer and in the mantle wedge beneath volcanic arcs. water | second critical point | synchrotron X-ray | pelite | adakite Results fl X-ray radiography experiments (17) were conducted using ddition of aqueous uids to the mantle wedge plays an im- the SPEED 1500 Kawai-type multianvil apparatus installed at Aportant role in reducing the melting temperature and in pro- – BL04B1 of SPring-8. The experimental procedures are described ducing partial melts (1 4). Alternatively, partial melts of downgoing in Materials and Methods. We added 5% (wt) gold powders, sediment or oceanic basalt themselves are added to the mantle which can facilitate visualization of the melt–fluid phase boundary wedge, resulting in subduction-zone magmatism (5), especially (Movie S1). The gold powders fall uniformly in a supercritical in hot environments occurring with subduction of a young fluid during heating (Movie S2). The experimental conditions oceanic plate (6, 7). Arc basalts are characterized by their higher and results are presented in Table 1. When we heated the sample concentrations of H2O and large ion lithophile elements than – at lower pressures with variable H2O concentrations, we ob- those of mid-ocean ridge basalts (3, 4, 8 12). Such subduction- served one round material in the other material with different zone signatures are attributed to the addition of slab-derived components into a mid-ocean ridge basalt source mantle. Whether fl the slab-derived component is an aqueous uid, a partial melt, Author contributions: T.K. and S.O. designed research; T.K., M.K., K.M., K.N.M., and S.O. or a supercritical fluid persists as an open question (13–15). In performed research; T.K. and K.M. analyzed data; and T.K. wrote the paper. fl general, with increasing pressure, aqueous uids dissolve more The authors declare no conflict of interest. silicate components and silicate melts dissolve more H2O. Under This article is a PNAS Direct Submission. fl low-pressure conditions, those aqueous uids and hydrous sili- 1To whom correspondence should be addressed. E-mail: [email protected]. cate melts are divided by an immiscibility gap. Its highest tem- 2Present address: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, perature is designated as a critical point. The temperature of the Japan. critical point decreases concomitantly with increasing pressure, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. finally meeting the H2O-saturated solidus temperature. At this 1073/pnas.1207687109/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1207687109 PNAS Early Edition | 1of6 Downloaded by guest on October 1, 2021 Table 1. Summary of experimental conditions and results

H2O in starting Max/quench Two ﬂuids Run no. P, GPa materials, % (wt) temperature, °C 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

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.

X-ray absorbance, which we interpreted as coexisting silicate melt and E). We interpret these as quenched products from silicate and aqueous fluid (Fig. 1 A and C and Movie S1). At higher components dissolved in aqueous fluids. In contrast, we observed pressures, we observed no such coexisting phase, but found gold uniform aggregates of tiny glass globules, accompanied in some powders falling during heating (Fig. 1F and Movie S2). After the cases by crystals, from other experiments in which we had observed experiment, we opened the diamond lid and observed the sample no round material surrounded by a matrix on X-ray radiography surface under a stereomicroscope and a Raman microscope to (Fig. 1G). We regard them as quench products from a single identify some quench phases. In run products in which we had supercritical fluid. observed round material surrounded by a matrix on X-ray radi- Two phases were observed during heating and cooling in ra- ography, we found round aggregates of glass globules, typically diography images at pressures lower than 2.8 GPa in HMA–H2O larger than 10 μm, surrounded by a void space (Fig. 1B), or much and 2.5 GPa in sediment–H2O(Fig.2A and B). At pressures larger glassy globules with a void space (Fig. 1D) or a crescent- higher than each of them, only a single phase was recognized. shaped glassy portion along the capsule wall with a round void The radiography results are also shown in P–T diagrams at con- space (Fig. 1E). Mibe et al. (23, 24) reported similar observations stant bulk H2O, in which critical curves and H2O-saturated solidus – in the experimental products of peridotite/ basalt–H2O system. temperature (25, 26) meet at critical endpoints in the HMA We interpret the large glass globules and portions as quench H2O and sediment–H2O(Fig.2D and E). These results show products from hydrous melts, and the void space as a quench that the pressures of the critical endpoint can be at 2.8 GPa product from aqueous fluids. Tiny glass globules were present in and 750 °C for HMA–H2O and 2.5 GPa and 700 °C for the interface between the void space and glass portion (Fig. 1 D sediment–H2O.

2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1207687109 Kawamoto et al. Downloaded by guest on October 1, 2021 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. Fig. 1. Snapshots of radiographs and photographs of quenched samples. Under sufficiently warm conditions, a melt-like supercritical fluid (A) X-ray radiograph shows unmixing between HMA (dark circle) and aqueous can be formed in the downgoing sediment layer, where aqueous fluid (matrix) at 1011 °C and 1.5 GPa (run 2,234). The sample was surrounded fl by a AuPd tube (black) in the 1.2-mm-diameter sample. (B) Quench product uids are formed through dehydration reactions and also where of the experimental charge shown in A. Aggregates of glassy globules they are supplied continuously from the underlying basalt/peridotite and a void space, respectively, represent portions quenched from melt and layers (12, 15, 20, 30) (Fig. 3B). 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 Separation of Supercritical Fluids to Form Two Slab Components at sample image is 1.5 mm wide (Movie S1). (D) Quench product of the ex- Base of Mantle Wedge. The thermal structure in the mantle wedge EARTH, ATMOSPHERIC,

perimental charge in the sediment with 50% (wt) H2O, quenched at 1050 °C is a key to infer the present experimentally obtained results for AND PLANETARY SCIENCES and 1.4 GPa. Three larger globules and a thin glassy wall are quenched from the magma genesis in subduction zones. Since Tatsumi et al. (31) melt, and there is void space from aqueous fluids (run 1,474). Tiny glassy fl suggested super-dry solidus conditions based on an assumption globules can be quenched from aqueous uids. The sample room is 1.5 mm of almost dry features of frontal tholeiitic basalts in northeastern wide. (E) Quench product of experimental charge in the sediment with 50% Japan and the Izu arc, 1400 °C has been a popular temperature (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 for the mantle wedge center in numerical models (30, 32). In observed on the surfaces of glassy wall. They can be quenched from aqueous contrast, Hamada et al. (33) reexamined the H2O contents in fluids. The sample room is 1.5 mm wide. (F) X-ray radiograph shows the gold the tholeiite basalt in the Izu arc and found that the tholeiitic powder-rich portion (lower dark half) falling in a uniform single supercritical basalt contains more than 6% (wt) H2O in the lower crust con- fl uid in the sediment with 50% (wt) H2O at 953 °C and 2.6 GPa (run 1,536). ditions. It successively degasses during ascent to shallower magma The sample image is 1.5 mm wide (Movie S2). (G) The quench product of the chambers. Tatsumi et al. (31) assumed almost dry tholeiite based experimental charge in the HMA with 63% (wt) H2O, quenched at 800 °C μ on its phenocryst assemblage (34), which can represent re- and 2.6 GPa. Garnet crystals of 50- m diameter are scattered in a matrix equilibrated conditions at the shallower magma chambers. Re- 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 cent numerical models suggesting a high-temperature mantle wedge apparently fail to explain the lack of a volcano above 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. their supersolidus mantle wedge under wet conditions (30, 32). The X-ray path was vertical (run 1,900). The sample is 1.2 mm wide. (I) In contrast, seismic observations suggest moderate mantle tem- Backscattered images of a polished section cut parallel to X ray of run product perature (35, 36). To safeguard consistency with the distributions – of albite 50% (wt) H2O quenched at 700 °C and 2 GPa show uniform ap- of volcanoes above H2O-saturated solidus temperature and the pearance and no large void such as that seen in H (run 1,901). The sample seismic observations, we chose the thermal structure suggested is 1.2 mm wide. by Peacock and Wang (37) with temperatures of 1150 to 1200 °C at the hot core of the asthenosphere beneath the volcanic fronts To assess the precision and accuracy of the present method (Fig. 3). When a sediment-derived supercritical fluid enters the over- for estimation of critical endpoints between aqueous fluids and lying mantle, the fluid will react with the peridotite to dissolve silicate melts, we conducted experiments using the albite–H O 2 olivine and precipitate orthopyroxene (21), consequently be- system, which has a critical endpoint at approximately 1.5 GPa coming an HMA-bearing fluid (22). The HMA bearing fluid based on visual observation of the solvus using a Bassett-type reacts successively with the surrounding mantle. At higher tem- hydrothermal diamond anvil cell, as described by Shen and Keppler perature conditions, HMA might change its major element com- (27) and quench experiments (28). We conducted a series of position to basaltic (38). Only if the reaction with the surrounding experiments using albite with 38%, 50%, and 64% (wt) H2Oat mantle is localized or such a hydration reaction occurs at much fl SPring-8: we observed two uids with 38% (wt) H2O at 1.2 GPa, shallower pressure conditions, the HMA-bearing fluids can re- 50% (wt) H2O at 1.4 GPa, and 60% (wt) H2O at 1.4 GPa, and tain their major element composition as andesitic. Whether or fl a single uid with 38% (wt) H2O at 1.4 GPa, 50% (wt) H2Oat not they do so, a critical endpoint between basalt or HMA and C F 1.7 GPa, and 64% (wt) H2O at 1.4 GPa (Fig. 2 and and H2O exists in 3.4 GPa (110 km) and 800 °C (24) or in 2.9 GPa Table 1). These observations show that the albite melt and aqueous (92 km) and 750 °C. Therefore, such a basalt/HMA-bearing fluids are closing the solvus between 1.4 GPa and 1.7 GPa at fluid can separate into a melt and an aqueous fluid at the base 50% (wt) H2O, which is consistent with results of those previous of mantle wedge (Fig. 3). The melt will continue to react with studies. This simple test demonstrates the ability of the present mantle to form a melt-derived magma, and the fluid will trigger method to determine critical endpoints in silicate melts and aqueous hydrous partial melting of mantle to form a fluid-derived

Kawamoto et al. PNAS Early Edition | 3of6 Downloaded by guest on October 1, 2021 A 3 B 3 C 3 Albite-H 2 O one fluid 2.5 2.5 2.5 two fluids

2 2 2 Pressure (GPa) Pressure (GPa) 1.5 1.5 Sediment-H O Pressure (GPa) 1.5 HMA-H2 O 2 one fluids one fluid two fluids two fluids 1 1 1 20 30 40 50 60 70 80 20 30 40 50 60 70 80 20 30 40 50 60 70 80 wt. % H 2 O wt. % H 2 O wt. % H 2 O D E F 1200 1200 1100

HMA-70%H O 1100 Sediment- 1000 1100 2 50%H O 2 1000 900 1000 900 800 900 800 700 800 700 600 Temperature (°C) Temperature (°C) Temperature (°C) 700 500 600 Albite-50%H O 2 600 500 400 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 Pressure (GPa) Pressure (GPa) Pressure (GPa)

Fig. 2. (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(run1,532)andtwofluids (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.

magma (Fig. 3B). The melt-derived magma can be HMA or slab-derived components have also been suggested to explain basalt depending on the degree of reaction with mantle pe- the chemistry of many arc basalts (9, 10). Separation of slab- ridotite and on pressure conditions. However, the melt-de- derived supercritical fluids into melts and aqueous fluids can rived magma inherits trace element signatures as if it were commonly occur in most subduction zones. This can suggest sedimentary melt, whereas the fluid-derived magma inherits that subducting slabs are warm enough to feed sediment-derived only fluid-preferred elements from the sediment-derived su- supercritical fluids to the overlying mantle wedge in most percritical fluids (39–41). subduction zones. In the Mariana arc, Elliott et al. (42) recognized two slab- derived components in basalt chemistry: a partial melt from the Coexistence of Basalts and HMAs. Extraordinary magmas are pro- subducting sedimentary layer and an aqueous fluid from the duced in extremely hot subduction zones: adakite (6, 7, 43) and subducting basaltic layer. They suggest that those two compo- Mg-rich sanukitoids (44). These rocks are also classified as HMAs nents can be mixed by mantle flow delivering sedimentary partial (45). Experimental studies have duplicated major-element abun- melts toward a place beneath the volcanic front, where aqueous dances of HMAs through partial melting of hydrous mantle pe- fluids are supplied through dehydrating hydrous minerals in basalt. ridotite leaving a harzburgite residue (46–50). Isotopic compositions In our hypothesis (Fig. 3B), the melt and the fluid components and trace-element abundances of some HMAs suggest the ad- can coexist in the mantle wedge if the supercritical slab-fluids dition of enriched components from downgoing slabs (43, 45, supplied from the downgoing slabs separate into a melt and an 51, 52). Adakite is characterized by high ratios of light rare earth aqueous fluid. They have identical isotope contents at the time element (REE) to heavy REE and low concentrations of heavy of separation. Further reactions with the mantle result in iso- REEs. These features have been proposed to represent a partial tope contents that are different between a fluid-derived and a melt of downgoing basaltic crust with a garnet residue (6). The melt-derived magma because of the partition behavior of trace Mg-rich sanukitoids, which have no such conspicuous REE fea- elements between a melt and an aqueous fluid and variable tures, are inferred as forming by partial melting of downgoing degrees of reaction with mantle. These modification processes sediment layers at shallower depths in the amphibolite facies can explain the trace element ratios and isotopic contents (51, 52). The silica-rich feature of adakite and Mg-rich sanukitoids observed in the Mariana basalts (10, 42). Two such distinct suggests that silica-rich material plays an important role in their

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1207687109 Kawamoto et al. Downloaded by guest on October 1, 2021 A B Temperature (°C) fluid-derived magma melt-derived magma 500 600 700 800 900 1000 1100 1200 0 crust sediment mantle wedge basalt 600°C 0.5 beneath mantle peridotite volcanic front wet-solidus 800°C 1000°C 1.0 1200°C 83 km 92 km 1.5 mantle Ne Jd 2.0 Ab Hgr Ca-Hgr

Pressure (GPa) 2.5 1000°C aqueous fluid Sediment 1200°C 3.0 supercritical sediment-derived fluid HMA supercritical HMA/basalt fluid 3.5 Basalt hydrous melt Peridotite 4.0

Fig. 3. 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 EARTH, ATMOSPHERIC,

subduction zones. AND PLANETARY SCIENCES

respective geneses: through partial melting of subducting basaltic Geothermal Sciences, Kyoto University, Beppu, Japan) using XRF (anhydrous layer for adakites or sedimentary layer for Mg-rich sanukitoids. compositions are presented in Table S1). These powders respectively have An enigma hindering the understanding of magma generation 16.5% (wt) and 14.9% (wt) H2O; Milli-Q water was added to the sample by in adakite and Mg-rich sanukitoid suites is how to produce the using a microsyringe immediately before each experiment. The total water contents of the starting materials are 30–70% (wt) H O. basaltic magmas accompanying them: variable degrees of re- 2 The present experimental assembly is similar to that used in our previous action between slab melts and mantle (53) and different tem- studies and is of slightly different size (18). We put Milli-Q water with sample perature and water fugacity conditions (54) have been proposed. powders inside an Au75Pd25 tube with a single-crystal diamond plug (2.5 mm Here we suggest an alternative hypothesis: HMAs (adakite in diameter, 2 mm long) and sealed them with another single-crystal diamond and Mg-rich sanukitoids) are producible by reaction of mantle plug. This capsule is surrounded by a MgO insulator sleeve, a graphite sleeve and melt derived from slab-derived supercritical fluid. Basaltic heater, a CaO-doped Zirconia sleeve (Mino Ceramic) as a thermal insulator, magmas are producible through partial melting of the mantle and a Cr2O3-doped MgO octahedron (18M; Mino Ceramic). After the addi- wedge induced by fluid from it. In hotter subduction zones, tion of water and sealing with a second diamond plug, we pressurized the dehydration of downgoing slab and subsequent hydration of assembly to the desired value. Then we raised the temperature. The com- mantle wedge occur at shallower depths. The silicate component mercially available single-crystal diamond plugs (Sumitomo Electric Industries) dissolved in the aqueous fluids equilibrated with mantle is more offer advantages of low X-ray absorbance, low scattering, a good seal for water, and inertness with aqueous fluids and silicate melts under high- silica-rich (22, 55) and can have a critical endpoint with H2Oat pressure and high-temperature conditions. lower pressure (Fig. 3A). Such a silica-rich supercritical fluid can fl X rays pass through a graphite gasket, the Cr2O3-doped MgO octahedron, separate into an HMA-bearing melt and aqueous uid producing MgO plug, one diamond, the sample; and then the other diamond, MgO

a basaltic magma. Under low pressures, the HMA-bearing melt plug, Cr2O3-doped MgO octahedron, and a graphite gasket to an X-ray reacts with mantle to become an HMA magma. For HMA magma camera. Eight cubes of WC with 11-mm truncations are used to pressurize genesis, the shallow processes associated with hot subduction can the assembly. R-type thermocouples are attached to the outer surface of be necessary (45). 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 Materials and Methods that the thermocouple temperature equals that estimated using a pyroxene The chemical compositions of sediment and HMA were adopted for starting geothermometer (59) within an uncertainty of 25 °C. The pressure is cali- materials, respectively, from Ono (56) and Tatsumi (54) (Table S1). The sedi- brated by using equations of state of gold (60) with X-ray diffraction data ment composition was a pelite used to determine the stability of hydrous at SPring-8 (Fig. S2). Repeated calibration experiments were conducted with phases under high-pressure and high-temperature conditions. The pelite is identical assemblages with gold and MgO powders used instead of sample similar to pelites described as used experimentally in the literature (Fig. S1)(57, powders and water. We observed X-ray radiography at high-pressure and high-temperature conditions for a few tens of minutes and then quenched 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) experiments by switching off the electric power. proposed temperatures 100 °C higher than those. According to Hermann and ACKNOWLEDGMENTS. The authors thank Drs. Ken-ichi Funakoshi and Yuji 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 Higo [Japan Synchrotron Radiation Research Institute (JASRI), SPring-8] for longtime support in X-ray radiography experiments. We thank Drs. Jun-Ichi HMA composition [Teraga-Ike andesite (TGI)] (54) is an extremely primitive Kimura, Ken Koga, and Jun Korenaga for helpful discussions. This work was HMA produced through equilibrium between mantle peridotite and partial supported by Kakenhi through the Japan Society for the Promotion of melts of a downgoing sediment layer (45, 51). Sample powders prepared from Science (JSPS), the Ministry of Education, Culture, Sports, Science, and Tech- · mixtures of SiO2 0.3 H2O, Al(OH)3,FeOOH,TiO2,Mg(OH)2,Ca(OH)2,andsyn- nology (MEXT), and the Institute for Study of the Earth’sInterior(ISEI), thetic Na2O–K2O–SiO2 glass were analyzed by Takeshi Sugimoto (Institute for Okayama University.

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