The Open Mineralogy Journal, 2007, 1, 1-4 1 The Lehmann Discontinuity Due to Dehydration of Subducted Sediment Shigeaki Ono*,1,2 1Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japan 2Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK Abstract: Recent high-pressure experiments have led to the conclusion that water release from subducted sediments lying under continents in subduction regions occurs at about 220 km depth. This dehydration reaction is in good agreement with the seismological signature of the discontinuity indicating that sediment dehydration causes the Lehmann discontinuity in the upper mantle. Keywords: Lehmann discontinuity, high pressure, water, dehydration, 220 km, sediment. 1. INTRODUCTION the upper mantle, whereas most observations are under con- tinents. It is therefore impossible to explain the local occur- The Lehmann discontinuity was first observed in Europe rence of the discontinuity using this concept. and North America from seismic refraction studies beneath continents [1, 2]. The discontinuity is characterized by the In the case of the silica phase transition, if the coesite- following features: (1) The depth of the seismic discontinuity stishovite transformation occurs at 220 km depth, the tem- is around 220 km depth. (2) A regionally varying negative perature of the upper mantle would be about 800 K; far too seismological Clapeyron slope (dP/dT) of the discontinuity low a typical temperature for this depth. Although both depth has been estimated [3]. (3) The discontinuity has not phase transitions have a positive Clapeyron slope, such a been detected everywhere at around 220 km depth [4]. It is negative seismological Clapeyron slope has been reported observed under continents more than twice as often as under [3]. It is, therefore, difficult to explain the origin of the dis- oceans [5], and the largest amplitudes of this discontinuity continuity simply by mineral transition. appear beneath the continents [6]. (4) An increase in the The relationship between the seismic discontinuity and compressional or shear wave velocity and in seismic reflec- chemical boundaries in the mantle has often been discussed tions from around 220 km depth has been reported [7, 8]. (5) [15]. Chemical change induces a different mineral assem- A seismic transition from anisotropic to a more isotropic blage. As the physical properties of rock are influenced by state occurs at depths corresponding to the Lehmann discon- each constituent mineral, the seismic discontinuities can be a tinuity [9, 10]. reflection of a chemical boundary. There is no clear evi- The phase transitions of (Mg,Fe)SiO3 pyroxene [11, 12] dence, however, that such a boundary exists at 220 km depth and silica (SiO2) [13, 14] were reported at high pressures in the upper mantle [16, 17]. corresponding to the depth of the Lehmann discontinuity. Karato [18] proposed that the change of deformation These transitions seemed to be candidates for the origin of mechanism from dislocation creep to diffusion creep caused the discontinuity. However, both models have serious flaws, the Lehmann discontinuity. The diffusion creep is due to and from the view point of the mineralogy, those characteris- transport of matter by self-diffusion through the grains of a tic features of this discontinuity could not be explained by polycrystal. In the case of the dislocation creep, dislocations previous models. are carriers of plastic deformation in solids. Olivine shows a The dP/dT slope of the mineralogical phase transition is pressure-induced change in its deformation mechanism. This one of the important features to understand the structure of seems to occur between 240 and 380 km depth. This varia- the mantle. This slope can be expressed using differences of tion is due to differing parameters of grain size, temperature, entropy and volume at the transition (dP/dT = dS/dV). The water fugacity, and strain rate. This model can explain a ve- seismological Clapeyron slope can be estimated using the locity jump at the depth of the deformation mechanism depth of the discontinuity and the relative seismic velocity change. However, if the mechanism change causes the seis- perturbation taken from a tomographic model [3]. mic discontinuity, this would be observed everywhere, be- cause olivine is a major mineral present in the upper mantle. (Mg,Fe)SiO pyroxene is one of the major minerals pre- 3 As most observations of the Lehmann discontinuity are lo- sent in the pyrolite mantle, which has a typical upper mantle cal, this hypothesis cannot be accepted. composition. If the phase transition of pyroxene causes the seismic discontinuity, this would be observed everywhere in In this paper, it is proposed that the Lehmann discontinu- ity is a consequence of the dehydration of subducted sedi- ment. The proposal is made on the basis of high-pressure *Address correspondence to this author at the Department of Earth Sci- experiments using a multianvil press. This new model can ences, University College London, Gower Street, London WC1E 6BT, UK; explain all the characteristic features of the discontinuity Tel: +44-(0)20-7679-3424; Fax: +44-(0)20-7679-4166; such as depth, seismological Clapeyron slope, locality of E-mail: [email protected] observation and the velocity jump. 1874-4567/07 2007 Bentham Science Publishers Ltd. 2 The Open Mineralogy Journal, 2007, Volume 1 Shigeaki Ono 2. EXPERIMENTAL WORKS OH. The volume of fluid released in low temperature loca- tions typified by the subduction zone is therefore less than in We conducted experiments with hydrous sediment. Sam- a normal mantle geothermal region. ples were contained in Au75Pd25 capsules and the tempera- ture was varied between 1073 and 1673 K, while a pressure of 6-15 GPa was applied using the multi-anvil press. A syn- Temperature (K) 800 1000 1200 1400 1600 1800 2000 thetic gel was used to produce a reactive and homogeneous 100 starting material. We confirmed that the gel starting material Oceanic 4 quickly achieved a nearly equilibrium state in the multi-anvil Continental experiments [19]. The typical pelite composition was used as 6 the sediment [20]. Sediment with 6 wt% H2O was produced 200 G+C+Co+P+f from synthetic dry gel and reagent-grade Al(OH)3 as the G+C+Co+f Pressure (GPa) maximum H2O content stored in the sediment composition 8 G+C+S+P+f was less than 6 wt%, our experiment was a water saturated condition. It is known that the natural subducted sediment 300 10 has a considerable variation of chemical composition. Com- G+C+S+H+F+T+f G+C+S+H+F+K+f positional changes of experimental starting material can Depth (km) 12 change the mineral proportions of hydrous minerals. The influence of compositional change in sediments was dis- 400 14 cussed in our previous study [21]. The average composition G+C+S+H+F+E+f G+C+S+H+F+f of continental crust is sufficiently close to the sediment composition so that the phase relationships of the sediment Geotherm 16 composition may be generally applicable to that of continen- 500 tal crust. Details of the experimental procedure and results are described elsewhere [21]. Fig. (1). Experimental constraints on the dehydration of subducted Garnet ((Ca,Fe,Mg)3(Al,Fe)2Si3O12), clinopyroxene ((Ca, sediment. Symbols represent observed phase assemblages. Thin Fe,Mg)2-x(NaAl)xSi2O6), and silica (SiO2) phases were pre- lines represent the inferred boundaries of phase relation of sediment sent in all of the experiments. Three hydrous phases were [21]. Abbreviations of phases are as follows: G, garnet; C, clinopy- observed at temperatures below 1573 K. The stable crystal- roxene; Co, coesite; S, stishovite; K, kyanite; H, K-hollandite; F, line hydrous minerals consisted of phengite ((K,Na)Al2(Si3 Fe-Ti oxide; P, phengite; T, topaz-OH; E, phase egg; f, fluid. The Al)O10(OH)2) below 8 GPa, topaz-OH (Al2SiO4(OH)2) from thick line represents the dehydration boundary of phengite. The 9-12 GPa, and phase egg (AlSiO3(OH)) above 12 GPa (Fig. dashed lines represent typical geotherms under oceanic and conti- 1). The breakdown boundaries of topaz-OH and phase egg nental crust. The geotherms intersect with the phengite dehydration show a positive Clapeyron slope. In contrast, the breakdown boundary at ~220 km depth. reaction of phengite gave a negative slope at about 7 GPa 3. ORIGIN OF LEHMANN DISCONTINUITY corresponding to 220 km depth. In the case of most high- pressure hydrous mineral, such as alphabet phases, the stabil- The physical properties of water in the upper mantle are ity limit in response to temperature is not high [22]. How- distinctly different to those of silicate minerals. It seems ever, the upper temperature limit for phengite is greater than nevertheless difficult to explain the origin of the Lehmann 1473 K. This phase is thus likely to be stable within average discontinuity by water alone. As the volume of the subducted adiabatic mantle conditions. sediment is likely to be very small and the volume of the released water from the sediment is also negligible, the in- Water-saturated experiments show that the maximum fluence of water itself on the discontinuity is not considered water content in subducted sediment is ~2 wt% H O, be- 2 important. One interesting feature of water is its percolation cause phengite, which includes ~4 wt% H O, constitutes ~50 2 into the surrounding rocks. When the dehydration occurs, wt% of the assemblage. In the case of the continental com- water can migrate upwards by permeable flow because of the position, the water content is estimated to be a half of the large density difference between the fluid and the surround- sediment composition [21].