RECEIVED MARCH 25, 2000; REVISED TYPESCRIPT ACCEPTED AUGUST 16, 2001

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1 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 PAGES The Fluid-absent Partial Melting of a Zoisite-bearing Quartz Eclogite from 1 0 to 3 2 GPa; Implications for Melting in Thickened Continental Crust and for Subduction-zone Processes KJELL P. SKJERLIE 1 AND ALBERTO E. PATIÑO DOUCE 2 1 DEPARTMENT OF GEOLOGY, UNIVERSITY OF TROMSØ, 9037 TROMSØ, NORWAY 2 DEPARTMENT OF GEOLOGY, UNIVERSITY OF GEORGIA, ATHENS, GA 30602, USA RECEIVED MARCH 25, 2000; REVISED TYPESCRIPT ACCEPTED AUGUST 16, 2001 Fluid-absent melting experiments on a zoisite- and phengite-bearing INTRODUCTION eclogite (omphacite, garnet, quartz, kyanite, zoisite, phengite and Zoisite or clinozoisite is present in many high-pressure rutile) were performed to constrain the melting relations of these eclogites, and many occurrences are known from the hydrous phases in natural assemblages, as well as the melt and Scandinavian Caledonides (e.g. Holsnøy: Austrheim & mineral compositions produced by their breakdown. From 1 0 to Mørk, 1988; Jamtveit et al., 1990; Western gneiss region: 3 2 GPa the solidus slopes positively from 1 5 GPa at 850 C to Griffin et al., 1985; Seve Nappe: Kullerud et al., 1990). 2 7 GPa at 1025 C, but bends back at higher pressures to 975 C Indeed, most eclogites worldwide contain minor amounts at 3 2 GPa. The melt fraction is always low and the melt of zoisite or other hydrous phases, according to the compositions always felsic and become increasingly so with increasing compilations of Smith (1988) and Carswell (1990). During pressure. The normative Ab An Or compositions of the initial prograde eclogitization, zoisite forms by breakdown of melts vary from tonalites at 1 0 GPa to tonalite trondhjemites at the anorthite component of plagioclase in the presence 1 5 GPa, adamellites at 2 1 and 2 7 GPa, and to true granites of a hydrous fluid phase. The origin of these hydrous at 3 2 GPa. At pressures <>2 5 GPa zoisite and phengite break fluids is controversial. One possibility is that they are down more or less simultaneously. At 3 2 GPa and 1000 C zoisite introduced from below during continental collision, for is unreacted whereas phengite is absent, so that the first formed melt example, if wet continental sedimentary rocks are deeply at these conditions is granitic. Our experiments show that if subducted. During subduction of oceanic crust zoisite sufficiently high temperatures (of the order of 1000 C) are attained, forms by prograde metamorphism of hydrothermally zoisite- and phengite-bearing eclogites can produce small fractions altered oceanic crust, which in its upper levels contains of silicic melts of a wide range of compositions. These melts are low-temperature hydrous Ca-rich phases (e.g. Poli & rich in water and, probably, in Sr and other incompatible elements, Schmidt, 1997). At the gabbro to dyke transition zone so that they can act as metasomatic agents in the mantle wedge. the temperature and pressure are high enough to allow formation of clinozoisite and epidote as a result of hydrothermal circulation (see Skjerlie & Furnes, 1996, and references therein). Because zoisite and epidote are hydrous phases that KEY WORDS: zoisite; dehydration-melting; orogenic thickening; subduction; have been experimentally shown to coexist with melt felsic melt; metasomatism (e.g. Naney, 1983; Thompson & Ellis, 1994; Schmidt & Corresponding author. kjell@ibg.uit.no. Oxford University Press 2002

2 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002 Thompson, 1996) the epidote group minerals could in systems at water-saturated conditions from 2 2 to 7 7 principle be involved in fluid-absent dehydration-melting GPa, to determine the stability of hydrous phases in reactions. Zoisite may thus be a potentially important subducting oceanic crust, and to constrain reactions that phase for melt generation at high pressures in thickened result in the release of H 2 O to the mantle wedge. Their continental crust and in subduction zones. Experimental experiments showed a large stability field for zoisitebearing studies of zoisite stability, including its melting relations, assemblages. They also demonstrated that zoisite have been performed mostly in model systems and under may occur as a stable phase under water-undersaturated fluid-present conditions (both water-saturated and waterundersaturated conditions in eclogites. Their study suggested an upper conditions). In contrast, fluid-absent zois- pressure stability for zoisite at >3 2 GPa under water- ite melting experiments on natural starting materials are saturated conditions. At higher pressures zoisite is re- scarce, so that very little is known about the fluid-absent placed by lawsonite-bearing eclogites at low temperatures phase relationships of this mineral in natural rocks at (T < >700 C) and dry eclogites at higher elevated pressures and temperatures. temperatures. Poli & Schmidt (1997) further showed that amphibole has a non-temperature sensitive upper pressure stability of >2 3 GPa. They thus argued that zoisite may be the most important hydrous mineral in REVIEW OF PREVIOUS the GPa pressure range under water-saturated EXPERIMENTAL WORK ON conditions in basaltic to andesitic bulk compositions. EPIDOTE AND ZOISITE AND APPLICATION TO NATURAL ECLOGITES Fluid-absent melting experiments Fluid-bearing experiments Boettcher (1970) performed experiments under fluid- Several sub-solidus and super-solidus experimental studabsent and water-saturated conditions in the CASH ies have been performed on synthetic and natural epidotezoisite goes through dehydration-melting at pressures (CaO, Al 2 O 3, SiO 2,H 2 O) model system and showed that and zoisite-bearing assemblages under fluid-present, water-saturated and water-undersaturated conditions. <>800 MPa. He also argued that the thermal stability Super-solidus experiments have demonstrated beyond of zoisite is strongly reduced in the presence of H 2 O, any doubt that epidote and zoisite are magmatic phases and that the dehydration-melting reactions have positive at high pressures in both mafic and felsic systems (i.e. dp/dt slopes to 3 5 GPa (the highest pressure in- Naney, 1983; Schmidt, 1993; Thompson & Ellis, 1994; vestigated). Above 2 0 GPa, the CASH dehydration- Schmidt & Thompson, 1996). Schmidt & Thompson melting reaction proposed by Boettcher, (1996) showed that magmatic epidote has a wide stability field in the tonalite system at water-saturated conditions zoisite + quartz = anorthite + kyanite + liquid and f O 2 buffered at NNO (nickel nickel oxide). Ac- takes place at temperatures higher than 1050 C. cording to their experimental results, epidote dehydration Thompson & Ellis (1994) performed water-saturated intersects the H 2 O-saturated solidus at approximately experiments on the CMASH model system (CaO, MgO, 500 MPa and 660 C. At higher pressures epidote exists Al 2 O 3, SiO 2,H 2 O), and showed that zoisite is stable to as a magmatic phase and its upper temperature limit temperatures above the water-saturated solidus at high increases with pressure until the plagioclase to garnet pressures. Although they did not perform any debreakdown reaction is intersected at >1 3 GPa. At 1 3 hydration-melting experiments, they calculated that zois- GPa, Schmidt & Thompson (1996) determined that the ite could be involved in dehydration-melting reactions epidote stability field extends from the water-saturated with amphibole and quartz to yield anorthite (low P) or solidus at >630 C to >790 C. In the presence of garnet, garnet (high P) in addition to clinopyroxene and water- above 1 4 GPa, the upper temperature stability limit for undersaturated melt. Their analysis suggested that zoisite epidote has a steep negative Clapeyron slope. Schmidt undergoes dehydration-melting in the presence of am- & Thompson (1996) also performed experiments at more phibole and quartz from >1 0 to >2 5 GPa at temoxidizing conditions [hematite magnetite (HM) buffer] peratures from >780 C to >820 C in the CMASH and showed that the epidote stability field was enlarged system. In the absence of zoisite, amphibole undergoes down to 300 MPa. Similar experiments on a granodiorite dehydration-melting at considerably higher tem- located the epidote-out reaction at 100 MPa, but the peratures. These calculations suggest that the presence maximum thermal stability is about 50 C lower toward of epidote may cause fluid-absent melting in some bulk compositions to occur at temperatures close to the water- saturated solidus. Skjerlie & Johnston (1996) performed fluid-absent melting experiments on a crustal rock that higher pressures. Poli & Schmidt (1997) determined the sub-solidus phase relations in natural andesitic and synthetic basaltic 292

3 SKJERLIE AND PATIÑO DOUCE ZOISITE DEHYDRATION-MELTING contained the hydrous phases biotite (16 vol. %), amphibole (15 vol. %) and epidote (13 vol. %) in addition to plagioclase and quartz. Their results strongly suggested that the thermal stability for biotite is lowered in the presence of epidote through a dehydration-melting reaction such as biotite + epidote + quartz = amphibole + garnet + alkali feldspar + melt. material that may represent the crystallized products of high-pressure melts. Melting in the eclogites of western Norway has always been discussed in terms of anatexis in the presence of a water-rich fluid phase (e.g. Jamtveit et al., 1990), but, in the light of the experimental results discussed above, H 2 O-undersaturated melting should not be excluded. If high-p zoisite dehydration-melting occurs in nature, it is likely to produce small amounts of melt owing to the low H 2 O content of zoisite and the high solubility of water in silicate melts at high pressures. Small-scale zoisite dehydration-melting in the lower levels of thickened continental crust is potentially im- portant. It is a well-known fact that the rheological strength of a molten rock is dramatically reduced at high melt fractions when the rock changes from matrix- to melt-supported (i.e. van der Molen & Paterson, 1979). However, as shown by Stevenson (1989), any partially melted rock undergoing deformation is texturally unstable because of small-scale redistribution of melt relative to solid. Further, in crust undergoing deformation, melt will inevitably localize in veins and the strength of the crust will consequently be significantly reduced because strain will tend to localize where melt is present (e.g. Davidson et al., 1994; Tommasi et al., 1994; Rushmer, 1995). Thus, the generation of small amounts of melt that segregate into veins could potentially help to initiate and/or accelerate orogenic collapse. At pressures higher than the amphibole stability field, zoisite is likely to be the most important water-carrier in mafic bulk compositions and its melting may be important in promoting or accelerating de- stabilization of overthickened crust with following collapse and exhumation of deep-seated rocks. Zoisite melting may also be envisioned to occur during exhumation of deep-seated rocks, in particular if exhumation is characterized by heating. Small-scale zoisite melting can also be envisaged to occur in subduction zones. Because zoisite is the main carrier of Sr in plagioclase-free rocks and also contains much of the light rare-earth elements (LREE; e.g. Hickmott et al., 1992; Nagasaki & Enami, 1998), the melts are likely to be strongly enriched in these elements and can be added to the mantle wedge and the melts may thus act as a metazomatizing agent. Because of the discussion above, it is most important that we understand the melting behaviour of zoisite-bearing high-pressure rocks. The purpose of this study is to determine under which P T conditions zoisite undergoes dehydration-melting in eclogitic assemblages, and to study the compositions of the experimentally produced melts and solid phases. Our main goal is to determine if zoisite dehydration-melting is to be expected under those P T conditions that can be reached during overthickening of continental crust and the following exhumation, and during subduction of oceanic crust. At 1 0 GPa this reaction produces 5 10 vol. % melt at 850 C and amphibole dehydration-melting produces an additional 25 vol. % melt from 875 C to925 C. Evidence for the involvement of epidote in the melting reaction was the observation that more amphibole formed in the melting reaction than could be explained if all the anorthite component of plagioclase broke down, and that the glasses were fairly rich in CaO. The experimental results of Skjerlie & Johnston (1996) therefore support the work of Thompson & Ellis (1994) in that the presence of epidote may lower the dehydration-melting tem- perature to those of amphibole and biotite at high pres- sure. In the study by Singh & Johannes (1996) of de- hydration-melting of tonalitic rocks, zoisite formed inside plagioclase crystals at 1 2 GPa in a Fe-free phlogopite + plagioclase + quartz assemblage. When using an assemblage with biotite composition in the range ann- ite ( f O 2 close to Co CoO buffer) epidote formed above 0 8 GPa. Epidote also formed inside the plagioclase crystals and these were surrounded by alkali feldspar. Singh & Johannes (1996) concluded that the chemical conditions inside and outside the plagioclase crystals were different. Melting in natural eclogites The high-temperature phase relations of epidote zoisite under fluid-absent conditions may have important bearings on deep crustal processes and on subduction-zone processes. Several eclogites in Norway record temperatures close to and above the water-saturated solidus (see Fig. 8, below) and many of these eclogites contain zoisite. In the western Gneiss Complex of Norway eclogites probably formed in response to subduction of the continent Baltica below Laurentia [see Austrheim & Mørk (1988) for a discussion and other references], and maximum P T conditions are as high as 800 C and 3 0 GPa ( Fig. 8; e.g. Cuthbert, 1995, E. K. Ravna, personal communication, 1999). The P T estimates increase towards the coast, and along the coastline high-p migmatites have been described (Cuthbert, 1995). Possible high-p migmatites have also been described from the eclogites north of Bergen (Andersen et al., 1991). The eclogites of the Tromsø area also contain small amounts of felsic 293

4 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002 Table 1: Characterization of starting material (Verpenesset eclogite) Bulk rock a Cpx Garnet Garnet Zoisite Quartz Kyanite Phengite Rutile cores rims Mode b c <1 SiO TiO Al 2 O d Fe 2 O FeO e MgO MnO CaO Na 2 O K 2 O Total a Analysed by X-ray fluorescence. b Modal composition is calculated by a combination of mass balance and point counting thin sections. c Mode for garnet is the sum of cores + rims. d Fe 2 O 3 determined by titration. Fe 3+ is present in zoisite and in garnet. e Total Fe calculated as FeO. All values are in weight percent;, no analyses. Probe analyses of minerals are averages of 5 10 different analyses. run for a considerable length of time (Table 2), to achieve as much reaction as possible. The capsules were weighed after each run and discarded if weight loss was detected. Glass analyses of selected capsules with tears always yielded Cl-bearing glasses, so that the absence of Cl from capsules with similar post- and pre-run weights is a reliable indicator that there was no mass exchange with the pressure medium during the run. The oil pressure was monitored by Heise gauges at Georgia and by a Heise-type gauge at Tromsø, and was converted to sample pressure by the ratio of ram to piston areas. Pressures are assumed to be accurate to within 50 MPa. Tem- perature was measured with type C thermocouples (W5Re/W26Re) relative to an external electronic ice- point (OMEGA MCJ) and controlled by Eurotherm 808 regulators. Successful runs were polished for scanning electron microscope ( Tromsø) and electron microprobe (Georgia) studies. Glass analysis were performed with the JEOL JXA 8600 superprobe at the Department of Geology, University of Georgia. Alkali loss during probing of hydrous silicate glasses has been investigated in other experimental studies in which pools of glass large enough to allow multiple analyses with different analysed areas and counting times were available (e.g. Patiño Douce & Johnston, 1991; Patiño Douce & Harris, 1998). Such large glass pools were not produced in any of the experimental products reported here. Because of this, the results obtained in those earlier studies were assumed to be valid for this CHARACTERIZATION OF THE STARTING MATERIAL AND EXPERIMENTAL PROCEDURES To understand the melting behaviour of zoisite-bearing high-pressure rocks, we have chosen as starting material a non-retrograded zoisite-bearing eclogite (Table 1) from Verpenesset in West Norway kindly provided by Professor E. K. Ravna. This starting material was chosen because it contains primary high-pressure zoisite in addition to kyanite and quartz. It is also important that there are no signs of retrogression in the sample so that no water is tied up in secondary phases. The starting material contains minor amounts of phengite, and it experienced Caledonian eclogite-facies metamorphism at about 700 C and 2 5 GPa (E. K. Ravna, personal com- munication, 1998), probably related to continental subduction of Baltica below Laurentia at >420 Ma (e.g. Austrheim & Mørk, 1988). The Al 2 O 3 -rich nature of the starting material ( Table 1) suggests that it might represent a plagioclase-rich layer in a layered mafic intrusion. Experiments on the Verpenesset eclogite were per- formed in piston cylinder apparatus at the University of Tromsø (1, 1 5, 1 8, 2 1 GPa) and the University of Georgia (2 7 and 3 2 GPa). The rock was crushed and loaded into 1 3 mm Au capsules that were welded shut after drying for 24 h in a 110 C oven. The capsules were enclosed in NaCl that acted as the pressure-transmitting medium in NaCl MgO graphite cells. Experiments were 294

5 SKJERLIE AND PATIÑO DOUCE ZOISITE DEHYDRATION-MELTING Table 2: Phase assemblages and experimental run conditions Sample T P Duration Phase assemblage ( C) (GPa) (h) KALB Cpx Opx Pl Gt Gl KALB Cpx Opx Pl Gt Gl KALB Cpx Opx Pl Gt Gl KALB Cpx Opx Pl Gt Gl KALB Cpx Zo Qtz Pl Ky Gt Gl KALB Cpx Zo Qtz Pl Ky Gt Gl KALB Cpx Zo Qtz Pl Ky Gt Gl KALB Cpx Qtz Pl Ky Gt Gl KALB Cpx Qtz Pl Ky Gt Gl KALB Cpx Zo Qtz Pl Ky Gt Gl KALB Cpx Zo Qtz Pl Ky Gt Gl KALB Cpx Qtz Pl Ky Gt Gl KALB Cpx Zo Ph Qtz Ky Gt KALB Cpx Zo Ph Qtz Ky Gt KALB Cpx Zo Pl Ksp Qtz Ky Gt KALB Cpx Zo Pl Ksp Qtz Ky Gt Gl KALB Cpx Zo Pl Qtz Ky Gt Gl KALB Cpx Zo Pl Qtz Ky Gt Gl KALB Cpx Zo Pl Qtz Ky Gt Gl KALB Cpx Zo Pl Qtz Ky Gt Gl KALB Cpx Zo Pl Qtz Ky Gt Gl APD Cpx Zo Ph Qtz Ky Gt FCH Cpx Zo Ph Qtz Ky Gt APD Cpx Zo Ph Ksp Qtz Ky Gt Gl APD Cpx Zo Qtz Ky Gt Gl FCH Cpx Zo Qtz Ky Gt Gl APD Cpx Zo Qtz Ky Gt Gl APD Cpx Zo Qtz Ky Gt Gl APD Cpx Zo Ph Qtz Ky Gt APD Cpx Zo Qtz Ky Gt Gl APD Cpx Zo Qtz Ky Gt Gl APD Cpx Zo Qtz Ky Gt Gl Mineral symbols from Kretz (1983). study too. Decay of K count rates has never been It is well known that increasing f O 2 increases the observed, so that K values reported here are uncorrected. stability field of Fe-bearing zoisite and epidote (e.g. For Na, Patiño Douce & Harris (1998) observed that Schmidt & Thompson, 1996), and it is therefore important count-rate decay is a function of glass H 2 O content (as to know the redox conditions operative during inferred from difference from 100%) and calibrated a experiments designed to study zoisite epidote stability. Na correction factor that ranged from >20% to >50% Unfortunately, f O 2 cannot be buffered to specific values for glasses in which the uncorrected probe totals ranged by the use of solid buffers in H 2 O-undersaturated experiments. from 97 to 91%, respectively. These same correction However, because the cell assembly is very factors have been applied in this study (the correction much larger than the sample, the f O 2 conditions generated factor was not extrapolated to glasses with totals lower by the cell assembly will also be acting on the than 91%, however, but instead a uniform correction of sample during the experiment. Experiments with the 50% was applied to Na values in all such glasses). same cell assembly (NaCl MgO C) as employed in this 295

6 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002 euhedral neoformed plagioclase, clinopyroxene and ky- anite form (Fig. 2f ). Relict cores of zoisite are present even at 1100 C. Quartz is absent at 1 GPa ( Fig. 2a). It is present, but corroded in all higher-pressure supersolidus runs. Phengite is never present inside the plagioclase stability field ( P <2 7 GPa), within which plagioclase becomes more abundant with decreasing pressure. Kyanite, which is present in the starting material (Table 1), forms above 1 5 GPa (Figs 1 and 2e h), but is corroded at 1 5 GPa (Fig. 2b) and absent at 1 0 GPa (Fig. 2a). At 2 7 GPa the starting material is unreacted at 1000 C. At 1025 C phengite is absent but pseudomorphed by potassium feldspar ( Fig. 2g) whereas zoisite is slightly corroded. Our highest temperature experiment at 2 7 GPa (1125 C) contains corroded zoisite included in glass pools that also contain crystals of euhedral kyanite and clinopyroxene (Fig. 2h). At 3 2 GPa the starting material is unreacted at 925 C. At 1000 C phengite is absent and zoisite appears unreacted ( Fig. 2i). Small pools of melt are associated with neoformed garnet and kyanite, replacing phengite. At 3 2 GPa and higher temperatures zoisite is corroded or absent and pools of melt contain kyanite and clinopyroxene. Garnet forms at 2 7 and 3 2 GPa ( Fig. 2j) and becomes more abundant with increasing pressure. Garnet appears to behave as an inert phase at 2 1, 1 8 and 1 5 GPa, but its abundance decreases from 1 5 to 1 0 GPa. study showed that the cell assembly imposes on the sample an f O 2 that is 1 2 log units less reducing than that generated by the quartz fayalite magnetite (QFM) solid buffer (Patiño Douce & Beard, 1994, 1995). Approach to equilibrium Descriptions of many previous fluid-absent melting ex- periments clearly show that bulk equilibrium is generally not reached [see summary by Skjerlie & Johnston (1996) and references therein]. Non-equilibrium features in de- hydration-melting experiments include neoformed plagioclase and garnet mantling residual cores of these phases. However, the neoformed phases are generally of homogeneous composition, both within single crystals and among different crystals, suggesting that they have approached equilibrium. Disequilibrium features in the present study include growth of new garnet and clino- pyroxene on relict crystals, and persistence of corroded cores of zoisite and kyanite surrounded by neoformed plagioclase mantles. These mantles may have prevented the complete breakdown of zoisite and kyanite, as will be discussed below. The neoformed rims are generally euhedral and homogeneous in composition, and the glasses and neoformed phases show systematic com- positional variations with pressure and temperature (see Figs 3, 5 and 6 below). Thus, despite the presence of disequilibrium features, we argue that the neoformed phases represent near-equilibrium assemblages. Experimental glass compositions High-pressure melts produced experimentally from the eclogite have SiO 2 contents generally >70 wt % on an H 2 O-free basis (Table 3, Fig. 3). Less silicic melts (SiO wt %) are produced only at 1 0 GPa ( Table 3, Fig. 3). The compositions of the initial melts vary systematically with pressure ( Fig. 3, Fig. 4a), from tonalites at 1 0 GPa to tonalite trondhjemites at 1 5 GPa, ad- amellites at 2 1 and 2 7 GPa, and to true granites at 3 2 GPa. This trend reflects the changing roles of zoisite and phengite in the initial melting of the eclogite (Fig. 1). As temperature rises above the solidus and both hydrous phases break down, melts converge towards tonalitic granodioritic compositions at all pressures. Concentrations of ferromagnesian components decrease with increasing pressure (Figs 3 and 4b) This is in agreement with the behaviour observed in dehydration- melting experiments of other bulk compositions, and is a consequence of the fixed H 2 O budget of dehydrationmelting coupled to the increase in water solubility with increasing pressure (e.g. Patiño Douce & Beard, 1995, 1996; Patiño Douce & McCarthy, 1998; Patiño Douce, 1999). Melts with >5 wt % FeO + MgO + TiO 2 are produced only at 1 0 GPa ( Fig. 3). At greater pressures EXPERIMENTAL RESULTS Experimental conditions and phase assemblages in the experimental products are listed in Table 2 and shown together with the approximate location of the solidus and other phase boundaries in Fig. 1. Zoisite appears unreacted on the high-p, low-t side of the shaded area, and phengite is unreacted on the high-p, low-t side of the dotted line (Fig. 1). The solidus is located on the basis of the absence of glass in lower-temperature ex- periments, but minute amounts of glass could have gone undetected in some of these low-temperature experiments (see below). Within the shaded area zoisite is corroded and mantled by various phases that change with P and T. At temperatures lower than the inferred solidus and P <2 1 GPa, corroded zoisite crystals are surrounded by thin mantles consisting of plagioclase, with minor amounts of potassium feldspar and tiny needles of kyanite ( Fig. 2c). If subsolidus dehydration of zoisite occurred at these conditions then some melting should have taken place, but, if so, the glasses have remained undetected in the experimental products. At 2 1 GPa and 975 C the mantles include minor amounts of glass (Fig. 2d). As temperature rises the abundance of glass increases and 296

7 SKJERLIE AND PATIÑO DOUCE ZOISITE DEHYDRATION-MELTING Fig. 1. Pressure temperature diagram showing the experimental results on the Verpenesset eclogite. Χ, experiments with glass present; Β, charges where glass could not be confirmed. Zoisite is unreacted to the left of the shaded field, corroded in the shaded field, and absent to the right of the shaded field. The phengite-out boundary is represented by the bold dotted line, which coincides with the solidus at pressures >2 1 GPa. Kyanite is absent at pressures lower than those limited by the line denoted kyanite out, and plagioclase is absent at pressures higher than those limited by the line denoted plagioclase out. FeO + MgO + TiO 2 contents are generally <3 wt %, generally increase with increasing pressure, but they are and as low as 1 2 wt % at 2 1, 2 7 and 3 2 GPa (Table almost always lower than in cpx in the starting material, 3, Fig. 3). except at 3 2 GPa (Fig. 5b). Na depletion relative to cpx Incipient melting at GPa produces potassic in the starting material is particularly strong at 1 and 1 5 leucogranite melts ( Fig. 4a, Table 3), reflecting the fact GPa, reflecting abundant plagioclase crystallization. Al that phengite is the dominant hydrous phase involved in contents in cpx in 1 and 1 5 GPa experiments are lower their production. However, only traces of these K-rich than in cpx in the starting material, whereas Al contents melts are formed, owing to the low phengite content of at 2 7 and 3 2 GPa are generally higher than in the starting the starting material. Higher-temperature melts at P = material ( Fig. 5b). Al contents in 1 8 and 2 1 GPa runs 2 7 GPa become enriched in Ca, reflecting zoisite breakdown, are comparable with those in the starting material. In most but they remain leucocratic (<3 wt % FeO + cases, Na and Al contents in cpx decrease with rising MgO + TiO 2 ) even up to the maximum temperatures temperature, probably reflecting progressive incorporation investigated (1150 C, Fig. 3). It is interesting to note that of normative jadeite into the melts. This behaviour was also these remarkably leucocratic high-temperature melts are observed by Skjerlie & Johnston (1996) in their dehydration- produced from a protolith of basaltic bulk composition melting experiments on a greywacke. The behaviour of ( Table 1). This observation has implications for processes cpx at 2 7 GPa does not follow this general trend, however, at subduction zones, which are discussed below. as at this pressure Na decreases but Al increases with rising temperature ( Fig. 5b). The reason for this different behaviour is not clear. Clinopyroxene Clinopyroxene is present in all run products as euhedral and generally unzoned tabular crystals. Cpx present in 1 Orthopyroxene and 1 5 GPa runs is augite whereas at all higher pressures Orthopyroxene is not present in the starting material but it is omphacite (Table 4, Fig. 5a). Na contents in cpx is found as small acicular crystals in experiments at 1 297

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9 SKJERLIE AND PATIÑO DOUCE ZOISITE DEHYDRATION-MELTING GPa, C. Opx in these experiments is an En Fs experiments at all pressures are generally indistinguishable solid solution (Table 4, Fig. 5a), with Al 2 O 3 contents from those of garnet in the starting masolid increasing regularly from >1 wt% at950 C to >6 wt % terial ( Table 7, Fig. 7). These textural and compositional at 1050 C. characteristics suggest that garnet behaves as a largely inert phase during partial melting of eclogites. Sub-solidus and melting reactions Figure 8 shows the P T locations of various dehydration reactions for zoisite and muscovite in the end-member KCASH system [plotted with the software package TWQ, which employs the thermodynamic database of Berman (1988) and various updates]. Also shown is the water-saturated basalt solidus determined experimentally by Lambert & Wyllie (1972). The melt-absent reactions have been plotted to attempt to better understand the phase relationships. The dehydration reactions are meta- stable relative to melting reactions at temperatures higher than the H 2 O-saturated solidus. However, in a fluid- absent situation, melt will be the most water-rich phase, and will occupy a chemographic position analogous to water [see also Vielzeuf & Montel (1994) and Skjerlie & Johnston (1996)]. Na is not considered in the sub-solidus reactions. The presence of Na in our natural starting material (chiefly as jadeite in omphacitic clinopyroxene) is likely to lower the melting temperatures and increase the melt fraction relative to the Na-free system, but should not affect the phase relations in any major way. The dehydration reaction Plagioclase Plagioclase is not present in the starting material but is formed in experiments at 1, 1 5, 1 8 and 2 1 GPa. It occurs as subhedral unzoned grains, commonly larger than 10 μm across. At constant temperature plagioclase generally becomes more sodic with increasing pressure and at all pressures it becomes more calcic with rising temperature ( Table 5, Fig. 6). These compositional trends are consistent with the greater pressure stability of albite relative to anorthite and with the preferential dissolution in the melts of albite relative to anorthite as temperature rises and melt fraction increases. Orthoclase contents in plagioclase are always low (<10 mol %), reflecting the low bulk K 2 O content. Zoisite Zoisite is present at GPa but not at 1 GPa. Its behaviour and textural relationships are complex and are discussed in detail below, within the context of the melting relations. It is always near end-member zoisite, containing <2 wt % Fe 2 O 3 (Table 6). Its composition is essentially indistinguishable from that of zoisite present in the starting material. Garnet Garnet is present in all melting experiments, but shows clear signs of recrystallization only at 2 7 and 3 2 GPa. Even at these pressures, recrystallization is limited to the formation of narrow euhedral rims, generally <5 μm thick. These rims do not yield consistent chemical compositions, probably owing to inclusion in the electron beam excitation volume of original garnet underlying reaction stable (see Fig. 8): the neoformed rims. At 1 GPa garnet appears notably Qz + 4Zo = Gr + 5An + 2H 2 O intersects the water-saturated solidus at >650 C at >700 MPa ( Fig. 8). This intersection constrains the minimum pressure at which dehydration-melting of zoisite is possible in quartz-bearing eclogites such as our natural starting material, because dilution of grossular and anorthite by other garnet and plagioclase components, respectively, shifts the dehydration reaction to higher pressure. The presence of muscovite in the starting ma- terial has the same effect, by rendering the following corroded, emphasizing its role in orthopyroxene-forming 2 zoisite + 2 quartz + muscovite = 4 anorthite + reactions (see below). Garnet compositions in the melting sanidine + 2H 2 O. (1) Fig. 2. Back-scattered electron micrographs of selected experiments: (a) 950 C and 1 GPa (note absence of zoisite and quartz and presence of orthopyroxene and abundant plagioclase); (b) 950 C and 1 5 GPa (note corroded kyanite mantled by plagioclase); (c) 900 C and 2 1 GPa (note the very thin mantles surrounding the zoisite in the middle of the photograph; these rims consist of plagioclase, potassium-rich alkali feldspar and kyanite in decreasing amount); (d) 975 C and 2 1 GPa (note that the largest zoisite crystal is surrounded by glass and plagioclase); (e) 1050 C and 2 1 GPa, corroded zoisite is still present; (f ) 1100 C and 2 1 GPa (note that glass pools contain plagioclase and euhedral neoformed kyanite; zoisite is absent); (g) 1025 C and 2 7 GPa, long pseudomorph of K-spar with kyanite crystals after phengite [also note recrystallization of clinopyroxene (lighter colour) around the pseudomorph]; (h) 1125 C and 2 7 GPa, corroded zoisite surrounded by glass (note kyanite and neoformed clinopyroxene); (i) 1000 C and 3 2 GPa, zoisite is unreacted; ( j) 1050 C and 3 2 GPa (note new garnet growing on the old garnet grains). 299

10 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002 Table 3: Glass analyses P (GPa): T ( C): SiO TiO Al 2 O FeO MgO MnO CaO Na 2 O K 2 O Total Total Q C Or Ab An Reported analyses are averages of 5 10 analyses of different glass pools. Q, C, Or, Ab, An normative amount of quartz, corundum, orthoclase, albite, anorthite. Fe reported as FeO. Original probe total. 300

11 SKJERLIE AND PATIÑO DOUCE ZOISITE DEHYDRATION-MELTING Fig. 3. Temperature oxide diagram for all analysed experimental glasses. Χ, 1 GPa; Β, 1 5 GPa; Ε, 1 8 GPa; Η, 2 1 GPa; Μ, 2 7 GPa; Ο, 3 2 GPa. In any event, Fig. 8 shows that zoisite dehydrationmelting is possible at pressures >>1 0 GPa, which are relevant to our experimental study as well as to melting of eclogites in nature. The phase relations of the natural starting material are complex owing to the presence of two hydrous phases with very different properties (zoisite and phengite). Zoisite appears corroded over a wide temperature interval, shown by the shaded area in Fig. 1. The composition of the corroded zoisite crystals is indistinguishable between experiments at different pressures and temperatures, and is also similar to the composition of zoisite in the starting material ( Tables 1 and 6). Breakdown of zoisite over a finite P T interval, as observed in our experiments, is probably not a result of solid solution in this phase as the composition of zoisite in the various experiments is indistinguishable. Thus, zoisite may be metastably preserved, aided by the formation of mantles of reaction products. Whereas plagioclase mantles are always present at P <2 7 GPa, at 2 7 and 3 2 GPa corroded zoisite is always in contact with glass pools of homogeneous composition. This may suggest that, at least at high pressure, reaction of zoisite over a wide temperature range may be an equilibrium process. Initial breakdown of zoisite at 2 1 GPa is manifested by mantling of corroded zoisite by thin rims dominated by plagioclase and potassium feldspar and tiny needles of kyanite (Fig. 2c). No glass was detected at this pressure and T <975 C, but glass becomes readily observable at higher T. At the same time the size of the plagioclase rims increases and euhedral plagioclase forms in the glass pools, whereas potassium feldspar disappears, quartz becomes corroded and clinopyroxene recrystallizes to a less sodic variety. As the pressure decreases to 1 5 GPa, the abundance of plagioclase increases and kyanite changes from being a product of the incongruent melting reaction to becoming a reactant (manifested by its corroded appearance and the absence of neoformed euhedral crystals). At 2 7 and 3 2 GPa, plagioclase is absent and garnet is a product of the incongruent melting reaction. These features of the experimental products constrain the nature of the melting reactions and their changes with pressure. Kyanite appears corroded in all of the 1 5 GPa run products, and is absent from all of the 1 0 GPa runs. Neoformed plagioclase is abundant at 1 0 and 1 5 GPa. These observations suggest that at P <1 8 GPa zoisite breakdown follows the reaction (see Fig. 8) 301

12 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002 Fig. 4. (a) Normative Ab An Or diagram after O Connor (1965). The dashed line traces the initial melts that form at each of the studied pressures. The shaded field encompasses glasses experimentally produced by amphibolite dehydration-melting. Symbols as in Fig. 3. (b) All glass analyses plotted in a ternary (Na 2 O + K 2 O) CaO (FeO + MgO + TiO 2 ) diagram. The dashed line traces the initial melts that form at each of the studied pressures. (Note that the melts become poorer in ferromagnesian components with increasing pressure.) 2 zoisite+kyanite+quartz=4 anorthite+h 2 O. (2) where CaO and H 2 O represent components that are either dissolved in the melt phase or, in the case of CaO, During zoisite dehydration-melting at these relatively low could also in part go to forming other Ca-bearing phases, pressures some of the anorthite dissolves in the melt and such as plagioclase, garnet and diopside component of the rest crystallizes as plagioclase. clinopyroxene (see below). The observation that quartz The melt pools contain kyanite in the experimental in the run products always appears corroded shows that charges produced at P >1 5 GPa, suggesting that the the SiO 2 liberated by zoisite breakdown reaction (3) zoisite dehydration-melting reaction at these high-presalso enters the melt. Both plagioclase and kyanite are sure conditions produces kyanite as a peritectic phase. neoformed phases at 1 8 and 2 1 GPa, suggesting that No kyanite-forming dehydration reactions are stable in in the bulk composition that we studied zoisite breakdown Fig. 8, suggesting that incongruent breakdown of zoisite at high pressure could be modelled by means of the reactions (2) and (3) overlap over this pressure range. reaction Clinopyroxene and garnet are present in all the run products. Garnet shows virtually no textural indications 2 zoisite=3 kyanite+3 quartz+4 CaO+H 2 O (3) of reaction from 1 5 to2 1 GPa. At greater pressures 302

13 SKJERLIE AND PATIÑO DOUCE ZOISITE DEHYDRATION-MELTING Table 4: Clinopyroxene compositions (no added H 2 O) and orthopyroxene compositions Clinopyroxene P (GPa): T ( C): SiO TiO Al 2 O MgO FeO CaO Na 2 O Total Si Ti Al Mg Fe Ca Na Total Clinopyroxene Orthopyroxene P (GPa): T ( C): SiO TiO Al 2 O MgO FeO CaO Na 2 O Total Si Ti Al Mg Fe Ca Na Total Oxide values in weight percent. All values are averages of 4 5 analyses. Fe reported as FeO. 303

14 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002 Fig. 5. (a) Pyroxene compositions in eclogite melting experiments [classification scheme after Morimoto (1988)]. Composition of clinopyroxene in the starting material is shown with a large filled diamond. The arrows show general compositional trends with rising temperature at , 2 1 and 2 7 GPa. Orthopyroxene (present only at 1 0 GPa) plots in the enstatite ferrosilite (En Fs) field. (b) Al and Na contents in clinopyroxene as a function of temperature and pressure. Dashed lines show composition of clinopyroxene in the starting material. ferromagnesian content of phengite forms garnet by an incongruent dehydration-melting reaction such as garnet is clearly formed in the experiments, and its abundance increases from 2 7 to 3 2 GPa. Garnet formation probably results from the following de- hydration-melting reaction suggested by Boettcher (1970): zoisite + quartz = grossular + kyanite + melt. (4) The experimental charge at 1000 C and 3 2 GPa contains unreacted zoisite whereas phengite has broken down to garnet, kyanite and glass. This, together with the notable backbending of the phengite reaction boundary between 2 7 and 3 2 GPa, suggests that at high pressures the phengite + quartz = kyanite + garnet + melt. (5) Except at 3 2 GPa, clinopyroxene in all glass-bearing run products is less sodic than that in the starting material, showing that the albite component of the melts is largely supplied by breakdown of the jadeite component of clinopyroxene. Formation of a less sodic clinopyroxene can be represented by the coupled substitution NaAl cpx CaMg cpx, which suggests that Ca liberated by zoisite 304

15 SKJERLIE AND PATIÑO DOUCE ZOISITE DEHYDRATION-MELTING Table 5: Plagioclase compositions, no added H 2 O P (GPa): T ( C): SiO Al 2 O FeO CaO Na 2 O K 2 O Total Si Al Fe Ca Na K Total Oxide values in weight percent. All values are averages of 4 5 analyses. Fe reported as FeO. 305

16 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 2 FEBRUARY 2002 Fig. 6. Plagioclase compositions in eclogite melting experiments as a function of temperature and pressure. The starting material contains no plagioclase. breakdown also enters clinopyroxene in addition to entering the melt and forming anorthite or grossularite. Orthopyroxene is present only at 1 0 GPa. Because the equilibration pressure of the natural starting material is greater than this (>2 5 GPa), orthopyroxene in the run products probably formed by the decompression reaction clinopyroxene + garnet + quartz = orthopyroxene + plagioclase (6) (see also Patiño Douce & McCarthy, 1998). It should be noted that quartz is always absent at 1 0 GPa, but is present in all higher-pressure experimental products. Reaction (6) is also compatible with the fact that there is less garnet at 1 GPa than in higher-pressure experiments. Summarizing and integrating all of these observations, we conclude that dehydration-melting of zoisite-bearing eclogites at pressures lower than >1 8 GPa takes place by the following melting reaction: zoisite + NaAl clinopyroxene + kyanite + quartz ± garnet = melt + plagioclase + CaMg clinopyroxene ± orthopyroxene. (7) At high pressures zoisite dehydration-melting probably takes place by a reaction such as zoisite + NaAl clinopyrexene + quartz = melt + kyanite + CaMg clinopyroxene ± garnet. (8) Table 6: Zoisite compositions, no added H 2 O P (GPa): T ( C): SiO Al 2 O FeO CaO Total Si Al Fe Ca Total Oxide values in weight percent. All values are averages of 4 5 analyses. Fe reported as FeO. 306