The importance of blueschist —» eclogite dehydration reactions in subducting oceanic crust SIMON M. PEACOCK Department of Geology, Arizona State University, Tempe, Arizona 85287-1404 and Institut für Mineralogie und Pétrographie, ETH Zentrum, CH-8092 Zürich, Switzerland ABSTRACT that may be stable in subducting oceanic crust to depths of—100 km, including chlorite (Delany and Helgeson, 1978; Bebout, 1991), phlog- The metamorphic evolution and dehydration of subducting oceanic opite (Wyllie, 1973; Fyfe and McBirney, 1975), phengite (Nicolls and crust may be predicted by combining calculated pressure-temperature others, 1980), and amphibole (Tatsumi, 1989; Peacock, 1990b). (P-T) paths with a model of metabasalt phase equilibria. In steady-state The research presented in this paper differs from most previous subduction zones with high rates of shear heating, the upper parts of the investigations in two important respects. (1) Rather than using end- subducting oceanic crust progress through the greenschist —» amphib- member dehydration reactions, I use the metamorphic-facies concept olite —» granulite —»eclogite facies, whereas lower parts of the subducting to determine the important dehydration reactions that occur in a rock oceanic crust progress through the blueschist —» eclogite facies. In of oceanic-basalt composition. In P-T space, metamorphic-facies steady-state subduction zones with moderate rates of shear heating, most boundaries are broader than end-member univariant reaction curves, of the subducting oceanic crust passes through the blueschist —» eclogite but I believe that the metamorphic-facies concept provides a more transition. In steady-state subduction -zones with low rates of shear realistic picture of the continuous dehydration reactions that occur in heating, the entire subducting oceanic crust lies within the blueschist subducting oceanic crust. (2) In addition to presenting cross sections facies to depths greater than 70 km. For oceanic crust containing 1-2 through subducting oceanic crust, I superpose calculated P-T paths on wt% H20, dehydration will not begin until the onset of eclogite- or a metamorphic facies diagram to illustrate metamorphism and dehy- amphibolite-facies metamorphism, depending on the P-T path. For dration more clearly in subducting oceanic crust as a function of time, many subduction zones, the most important dehydration reactions in the position in the slab, and subduction-zone parameters. subducting oceanic crust occur at the blueschist —> eclogite facies tran- I begin by presenting a metamorphic-facies diagram and an es- sition associated with the breakdown of lawsonite (or clinozoisite), glau- timate derived from the literature of the bulk composition of oceanic cophane, and chlorite. Large amounts of H20 released by blueschist -» crust. For each metamorphic facies, I calculate a modal mineralogy eclogite dehydration reactions could trigger partial melting in the over- from which the maximum amount of bound H20 may be estimated. lying mantle wedge and may play a crucial role in the generation of arc Using Molnar and England's (1990) analytical expressions, I calculate magmas. a range of possible P-T paths for subducting oceanic crust for different rates of shear heating. The overlaying of calculated P-T paths on the INTRODUCTION metamorphic facies diagram limits the maximum H20 content of sub- ducting oceanic crust, and important dehydration reactions that occur The geochemistry of most arc magmas indicate that partial melt- in metabasaltic compositions may be identified. As is shown below, ing occurs in the mantle wedge above the subducting slab as a result most P-T paths calculated for subducting oceanic crust pass through of the infiltration of slab-derived, H20-rich fluids (Gill, 1981). How is the blueschist -» eclogite facies in contrast to higher-temperature H20 transported in the subducted slab to depths of 100-150 km? What greenschist —» amphibolite —» eclogite paths proposed by previous hydrous minerals are stable in subduction zones? The answers to these workers (for example, Wyllie and Sekine, 1982; Wyllie, 1988). questions lie in determining the position of dehydration reactions in subducting oceanic crust. Determining the amount and position of DEVELOPMENT OF THE THERMAL-PETROLOGIC MODEL HzO released from subducting oceanic crust as a function of the P-T path and other variables is an important step toward understanding Bulk Composition of the Oceanic Crust subduction-zone processes, arc magma genesis, and the recycling of H20 into the mantle. In this contribution, calculated P-T paths are Average oceanic crust consists of —7 km of basaltic and gabbroic combined with metabasalt phase equilibria in order to depict the met- rocks. The composition of the oceanic crust is dominated by tholeiitic amorphic evolution and dehydration of subducting oceanic crust. basalt emplaced at mid-ocean ridges, with lesser amounts of alkali and Global estimates of subduction-zone volatile budgets suggest that tholeiitic basalt emplaced at hot spots (oceanic-island basalts). The as much as —90% of the bound H20 subducted past the accretionary circulation of hydrothermal fluids through the oceanic crust may prism is contained in the mafic oceanic crust (Ito and others, 1983; change the bulk composition of the oceanic crust, primarily through Peacock, 1990a); the remaining subducted HzO is contained in oceanic the addition of volatiles (H20 and C02) and alkali elements (Na and sediments and hydrated uppermost mantle. During subduction, in- K). Basalt consists of eleven major oxides plus H20 (Table 1). creases in pressure (P) and temperature (T) tend to liberate H20 from In order to calculate metamorphic mineral modes, the bulk com- subducting oceanic crust as hydrous silicates become progressively positions in Table 1 were reduced to the five component Na20-Ca0- unstable. Previous research has suggested a variety of hydrous phases Mg0-Al203-Si02 (NCMAS) system, based on the following crystal Geological Society of America Bulletin, v. 105, p. 684-694, 7 figs., 4 tables, May 1993. 684 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/5/684/3381769/i0016-7606-105-5-684.pdf by guest on 29 September 2021 DEHYDRATION REACTIONS IN SUBDUCTING OCEANIC CRUST TABLE 1. ESTIMATES OF THE BULK COMPOSITION OF OCEANIC CRUST BASED ON A DRILL CORES, DREDGE HAULS, AND OPHIOLITES Oceanic Oceanic Oceanic Oceanic Oceanic Alkaline Spilite basalt tholeiite basalt basalt basalt olivine basalt Reference 1 2 3 4 5 6 7 No. of analyses (460) (161) (101) n.r. n.r. (199) (124) Si02 48.5 49.3 50.53 49.5 49.6 47.1 48.8 TiOj 2.6 1.8 1.56 1.5 1.5 2.7 1.3 A1A) 15.0 15.2 15.27 16.0 16.8 15.3 15.7 Fe203 3.1 2.4 n.r. n.r. n.r. 4.3 3.8 FeO 8.5 8.0 10.46 10.5 8.8 8.3 6.6 MnO 0.17 0.17 n.r. n.r. 0.2 0.17 0.15 MgO 7.2 8.3 7.47 7.7 7.2 7.0 6.1 CaO 10.5 10.8 11.49 11.3 11.8 9.0 7.1 NazO 2.5 2.6 2.62 2.8 2.7 3.4 4.4 K,0 0.8 0.24 0.16 0.15 0.2 1.2 1.0 P2O5 0.31 0.21 0.13 n.r. 0.2 0.41 0.34 Total 99.18 99.02 99.69 99.45 99.00 99.88 95.29 References: 1. Manson (1967); chemical analyses compiled from literature; Pacific, Atlantic, and Indian Oceans. 2. Hyndman (Tables 4-8, 1972); oceanic tholeiite analyses compiled from literature. 3. Melson and others (1976); oceanic basalt glasses from Mid-Atlantic Ridge, East Pacific Rise, Figure 1. Ternary ACF diagram with composition of mineral and Indian Ocean. 4. Taylor and McLennan (1985); estimated composition of basaltic layer 3. phases in the NCMASH system. Oceanic basalt compositions (Table 1), 5. Condie (Tabic 4.5, 1989); miscellaneous sources, including dredge samples, core samples, and selected ophiolites. labeled 1 through 7, plot in a narrow field shown in trapezoidal inset. 6. Hyndman (Tables 4-8, 1972); alkali olivine basalt, analyses compiled from literature. A = AI0l s + FeOj 5-(NaO0 s + KO0 S), C = CaO, F = FeO + MgO 7. Hyndman (Tables 4-1, 1972); spilite analyses compiled from literature, n.r., not reported + MnO. Projected through Si02, H20, and either albite (NaAlSi3Os) or jadeite (NaAlSi206). Grt, garnet solid solution; Amph, amphibole solid solution. See Table 2 for mineral abbreviations. chemical similarities: (1) Na20 = Na20 + K20; (2) CaO = CaO; (3) sent a subdivision of metamorphic conditions based on mineral as- MgO = MgO + FeO + MnO; (4) A1203 = A1203 + Fe203; and (5) semblages (or mineral reactions) (Fig. 2). For a given bulk composi- Si02 = Si02. The oxides Ti02 and P205 were ignored, and the cal- tion, the metamorphic mineral assemblage depends only on the culations were performed on a molar basis. For those chemical anal- metamorphic conditions (for example, P, T, oxygen fugacity, fluid yses that did not report Fe203, 20% of the iron reported as FeO was composition). Solid solutions, particularly Fe-Mg solutions in am- assumed to be ferric iron. The different oceanic-basalt compositions phibole, chlorite, and pyroxene, cause the boundaries between met- plot in a narrow field on the ACF diagram (Fig. 1); the small differences amorphic facies to be transitional in nature and to be generally much result in part from the uncertainty in Fe203 content of bulk compo- broader than shown in Figure 2. I consider the metamorphic-facies sitions 3, 4, and 5. For this paper, I used Hyndman's (1972) average boundaries depicted in Figure 2 to have widths of at least 50-100 °C oceanic tholeiite as a model for oceanic crust, because he differenti- and 0.1-0.2 GPa. In addition, oxygen fugacity and fluid composition ated Fe203 from FeO, and because he also presented average bulk strongly influence the position of metamorphic facies; for example, the compositions for olivine alkali basalt and spilite.