mantle flow in regions of complex tectonics insights from indonesia 1 indonesia is arguably one of the tectonically most complex regions on earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion it is thus an ideal location to study the interaction between subducting plates and mantle convection seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes such as mantle flow for example here we present novel shear wave splitting results across the indonesian region using three different shear phases local s sks and downgoing s to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features for example the block rotation history of borneo is reflected in coast parallel fast directions which we attribute to fossil anisotropy furthermore we are able to unravel the mantle flow pattern in the sulawesi and banda region we detect toroidal flow around the celebes sea slab oblique corner flow in the banda wedge and sub slab mantle flow around the arcuate banda slab we present evidence for deep sub 520 km anisotropy at the java subduction zone in the sumatran backarc we measure trench perpendicular fast orientations which we assume to be due to mantle flow beneath the overriding eurasian plate these observations will allow to test ideas of for example slab mantle coupling in subduction regions

the effect of fluorine boron and excess sodium on the critical curve in the albite h 2 o system experiments to define the critical curve for a series of silicate melts in equilibrium with a hydrous fluid were carried out in a hydrothermal diamond anvil cell silicate compositions studied were albite with several wt excess na2o b2o3 and f2o 1 complete miscibility between melt and water was observed at lower pressure and temperature conditions compared to pure albite for all compositions for albite excess na2o the critical curve had been lowered by 143 and 247 c at 10 kbar for 5 and 10 wt excess na for albite 5 and 10 wt f the difference at 10 kbar was 147 and 246 c respectively and for albite 5 and 10 wt b differences of 168 and 262 c were found these results are likely to be additive with the presence of more than one of the components depressing the critical curve to even lower temperatures and pressures the results suggest that in complex pegmatitic systems complete miscibility between melt and fluid may be important in the final stages of crystallisation the unusual properties of fluid phases under conditions close to the critical curve in a silicate melt water system may be essential for the enrichment of trace elements in pegmatites as well as for the formation of typical pegmatite textures

composition of aqueous fluid coexisting with mantle minerals at high pressure and its bearing on the differentiation of the earth s mantle in order to understand the role of aqueous fluid on the differentiation of the mantle the compositions of aqueous fluids coexisting with mantle minerals were investigated in the system mgo sio2 h2o at pressures of 3 to 10 gpa and temperatures of 1000 to 1500 c with an ma8 type multianvil apparatus phase boundaries between the stability fields of forsterite aqueous fluid forsterite enstatite aqueous fluid and enstatite aqueous fluid were determined by varying the bulk composition at constant temperature and pressure the composition of aqueous fluid coexisting with forsterite and enstatite can be defined by the intersection of these two phase boundaries the solubility of silicate components in aqueous fluid coexisting with forsterite and enstatite increases with increasing pressure up to 8 gpa from about 30 wt at 3 gpa to about 70 wt at 8 gpa it becomes almost constant above 8 gpa the mg si weight ratio of these aqueous fluids is much higher than at low pressure 0 2 at 1 5 gpa and almost constant 1 2 at pressures between 3 and 8 gpa at 10 gpa it becomes about 1 4 aqueous fluid migrating upward through the mantle can therefore dissolve large amounts of silicates leaving modified mg si ratios of residual materials it is suggested that the chemical stratification of mg si in the earth may have been formed as a result of aqueous fluid migration

stability limits of hydrous minerals in sediment and mid ocean ridge basalt compositions implications for water transport in subduction zones phase relationships in hydrated natural sediment and mid ocean ridge basalt morb compositions were investigated experimentally from 6 to 15 gpa and 700 c to 1400 c in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle phengite and hydrous aluminum silicates topaz oh and phase egg are stable in the sediment composition at depths greater than 150 km and constitute h2o reservoirs similarly lawsonite is an h2o reservoir in morb composition at depths greater than 150 km according to mass balance calculations based on experimental run products the weight proportions of phengite and h2o at 6 gpa in the sediment are about 50 and 2 respectively at pressures from 8 to 12 gpa topaz oh al2sio4 oh 2 appears as the hydrous mineral and the sediment retains about 0 5 0 7 wt h2o at higher pressures phase egg alsio3 oh appears as the hydrous phase with a large stability field the h2o content in the sediment is about 0 5 wt at pressures greater than 12 gpa in natural morb compositions lawsonite is the hydrous phase at pressures lower than 10 gpa about 1 wt h2o in the morb composition is retained by lawsonite at 6 gpa and 800 c above 6 gpa a lawsonite out reaction is controlled by compositional changes of garnet and clinopyroxene if the oceanic crust descends with an overlying sediment layer water can be carried down into the transition zone by the sediment the amount of transported water is controlled by the chemical compositions of the subducting sediments water in the morb layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle causing the generation of a peridotite region including hydrous phases chondrodite clinohumite phase a and phase e if this hydrous peridotite is dragged down by the descending oceanic crust water can be transported into the transition zone by the subduction process the amount of transported water is controlled by the thermal state of the slab and the chemical composition of morb

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

a statistical analysis of the distribution of cordierite and biotite in hornfels from the bugaboo contact aureole implications for the kinetics of porphyroblast crystallization

limestone and the problem of radiocarbon dating of land snail shell carbonate in order to test the role of limestone in producing anomalously old radiocarbon ages in land snail shells 14 c analyses were performed on shell carbonate of modern land snails from limestone and nonlimestone areas of jamaica no anomaly was found in snails from the nonlimestone area implying that such material is suitable for radiocarbon dating snails from limestone areas produced variable anomalies of as much as 3 120 yr due to incorporation of 14 c free limestone into shell carbonate all rock scraping snails and most leaf litter feeding snails from limestone areas showed anomalous 14 c contents because of the variability in 14 c content even within species no standard correction factor for limestone anomaly can be applied however dating error can be minimized by selecting ecologically appropriate species or by comparison of analyses of several fossil species within a stratum to their modern counterparts

mg si ratios of aqueous fluids coexisting with forsterite and enstatite based on the phase relations in the mg2sio4 sio2 h2o system direct observation of aqueous fluids coexisting with mgsio 3 enstatite and or mg2sio4 forsterite was performed at 0 5 5 8 gpa and 800 1000 c with an externally heated diamond anvil cell and synchrotron x rays at 1000 c in the mgsio3 h2o system forsterite crystallizes below 3 gpa but not above that pressure at 1000 c in the mg2sio4 h2o system forsterite congruently dissolves into the aqueous fluids up to 5 gpa these observations suggest that the aqueous fluids coexisting with enstatite and forsterite have mg si 1 below 3 gpa and 1 mg si 2 above that pressure comparison with the previous studies reporting mg si ratios of the aqueous fluid coexisting with enstatite and forsterite indicates that the mg si ratios change rapidly from sio2 rich to mgo rich at around 3 gpa and 1000 c this change can be related to possible structural changes of liquid water under these conditions the aqueous fluids coexisting with enstatite and forsterite do have mg si ratios similar to those found in the partial melts of h2o saturated peridotite somewhere within the upper mantle these two fluids unite to form a single regime and cannot be distinguished from each other

aqueous fluid connectivity in pyrope aggregates water transport into the deep mantle by a subducted oceanic crust without any hydrous minerals abstract the effect of pressure and temperature on the dihedral angles of aqueous fluid in a pyrope matrix was investigated experiments were performed on an h 2 o pyrope system in a multianvil apparatus over the pressure and temperature ranges of 4 13 gpa and 900 1200 c respectively the dihedral angle of the fluid in contact with the pyrope exhibited a significant change at pressures around 8 9 gpa the dihedral angles increased with increasing pressure up to 9 gpa at pressures above 9 gpa the dihedral angles were greater than 60 at temperatures below 1000 c therefore the efficient percolation of aqueous fluid in a pyrope matrix is not feasible in the upper mantle and the transition zone the fluid released from the breakdown reactions of the hydrous minerals lawsonite and phengite exists in the oceanic crust which mainly consists of garnet in the upper mantle and transition zone we conclude that a part of the aqueous fluid released from the hydrous minerals may be retained in the subducted oceanic crust and transferred into the deep mantle by the subduction process

a potential method to confirm the previous existence of lawsonite in eclogite the mass imbalance of sr and lrees in multistage epidote ganghe dabie uhp terrane lawsonite is an important hydrous mineral that is stable at low temperature lt and high to ultrahigh pressure hp uhp conditions in subducted slabs the occurrence absence of lawsonite in eclogite is a significant constraint for the construction of the metamorphic tectonic and fluid melt evolution histories of an hp uhp terrane however lawsonite is very rarely preserved in natural eclogites and accurate judgment of its former existence is a significant challenge for petrologists at present whether lawsonite has ever existed in lawsonite absent eclogite is mainly judged by i pseudomorphs after lawsonite and ii phase equilibria modelling in this study major element and trace element distributions in multistage minerals were examined in the ganghe lawsonite absent uhp eclogite in the dabie uhp terrane eastern china this work demonstrates that the whole rock sr and light rare earth elements lrees are mainly dominated by epidote other minerals garnet omphacite quartz kyanite barroisite phengite and accessory minerals play a very limited role in the sr and lrees budgets two stages of epidote which have noticeably different sr and lrees contents were recognized in the eclogite i epidote porphyroblasts ep p core which are suspected to be the pseudomorphic mineral after lawsonite contain significantly high sr 7200 10 300 ppm and lrees 160 1300 ppm for la ii an earlier stage epidote ep in core occurs as inclusions in matrix omphacite or in omphacite inclusions in the suspected pseudomorphic minerals after lawsonite spmal this early epidote has significantly lower sr 990 1890 ppm average 1495 ppm n 17 and lrees contents 60 110 ppm for la average 91 ppm n 17 all of the existing early stage minerals predating the spmal have very low contents of sr and lrees and the total amounts of these elements in the early stage minerals do not balance those in the spmal this indicates that a missing ca al sr and lree rich mineral which was previously in equilibrium with the early stage minerals likely existed in the ganghe eclogite on the basis of the mineral geochemistry and phase equilibria modelling we confirm that the missing mineral cannot be anything but lawsonite this study indicates that examining the mass im balance of sr and lrees between multistage hp uhp epidote can be used as a potential method to confirm the previous existence of lawsonite in lawsonite absent eclogite

hydrous phases and water transport in the subducting slab arc volcanoes are typically located 90 180 km above the surface of downgoing slabs as shown by wadati benioff deep seismic foci gill 1981 tatsumi 1989 the intimate relationship between the dip angles of the subducting slab and the locations of volcanic arcs indicates that subduction zone magmatism is triggered by material input from the subducting slab tatsumi and eggins 1995 the slab derived components are thought to be aqueous fluids or h2o rich partial melts of subducted oceanic crust therefore knowledge of the stability of hydrous phases and the chemical and physical properties of aqueous fluids in downgoing slabs is essential to understand the material transport in subduction zones in this section i will review the stability of hydrous phases in downgoing peridotite basalt and sediment systems and the chemical and the wetting properties of aqueous fluids recent experimental studies indicate that 3 4 gpa equivalent to 90 120 km depth is a key pressure where 1 the chemical compositions of silicate components dissolved in aqueous fluids equilibrated with mantle minerals approach the composition of mantle peridotite itself stalder et al 2001 mibe et al 2002 kawamoto et al 2004 2 the dihedral angle between olivine and aqueous fluids starts becoming smaller than 60 watson et al 1990 mibe et al 1998 1999 and 3 the immisciblity gap between peridotitic melts and aqueous fluids disappears and consequently hydrous minerals liberate supercritical aqueous fluids mibe et al 2004a 2006 the similarity between these pressures and the depths of downgoing slab underneath volcanic fronts where the maximum numbers of volcanoes are formed 124 38 km gill 1981 or 112 19 km tatsumi 1986 suggests that subduction zone magmatism can be triggered by the input of supercritical fluids from the downgoing peridotite and basalt many hydrous crystalline phases

the 10a phase a high pressure expandable sheet silicate stable during subduction of hydrated lithosphere h2o storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases as a result of its relatively large thermal stability and intermediate composition the 10a o phase mg3si4o10 oh 2wnh2o has been regarded as a relevant h2o reservoir in a wide range of rock compositions and mineral assemblages high pressure syntheses of the 10a o phase were carried out at 6 7 gpa and 650 c under fluidsaturated conditions in a walker type multi anvil apparatus from 5 min to 430 h x ray powder diffraction of large platy hexagonal crystals of the 10a o phase up to 100 wm were indexed on the basis of a trioctahedral type structure long term run products s 110 h reveal sensitivity of the 10a o phase to treatment with acetone leading to the appearance of diffractions at greater d spacings 10 2 11 6 a o with respect to the basal peak of the 10a o phase 9 64 10 07 a o this swelling behavior is strongly related to synthesis run duration the raman spectrum of the 10a o phase at frequencies less than 800 cm 31 shows a strong similarity to talc in the si o stretching region 800 1100 cm 31 the 10a o phase exhibits three modes 909 992 and 1058 cm 31 as compared to two in talc the bending mode of water x2 i s found at 1593 cm 31 in the oh stretching region peaks at 3593 3622 and 3668 cm 31 were observed the acetone treated sample shows a c h stretching mode at 2923 cm 31 while the double bond cno signal is absent the swelling behavior of the 10a o phase is interpreted as due to intercalation of acetone with pre existing interlayer water the efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration the relation between the response to acetone treatment and run duration is therefore interpreted as a time dependent hydration of the 10a o phase the fractions transformed from non expandable to expandable fractions was fitted to the avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions the ability to accommodate variable amounts of h2o makes the 10a o phase a major h2o sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere under h2o saturated conditions a fully hydrated 10a o phase occurs when h2o undersaturated conditions prevail a h2o deficient 10a o phase incorporates the volatile component available the exchange capacity of interlayer molecules in the 10a o phase structure opens new scenarios on the control of fluid compositions escaping from subducted

the use of land snail shells in paleoenvironmental reconstruction fossil land snail shells constitute a valuable source of paleoenvironmental information for the quaternary they can be dated by a variety of methods including radiocarbon amino acid racemization and epimerization and perhaps also thu and esr the vast majority of paleoenvironmental studies based on land snail shells have examined the faunal composition of fossil assemblages from which a variety of paleoenvironmental characteristics such as biome temperature and moisture conditions have been reconstructed still there are a number of problems involved in using this approach and these are discussed shell morphology has occasionally been used to reconstruct such factors as rainfall and temperature stable isotope studies on quaternary land snails include analysis of 18o values of organic matter in the shells to reconstruct c4 plant distributions from which rainfall amounts can be inferred and analysis of 18o values of shell carbonate from which trends in the oxygen isotope composition of rainfall can be reconstructed stable carbon isotopes of shell carbonate have also been studied but their interpretation is not clear amino acid epimerization analysis dalloisoleucinel isoleucine ratios of land snail shells has been used for estimation of paleotemperatures some potential uses of land snail shells for paleoenvironmental reconstruction include the study of stable isotopes of h and n periodic growth lines and deposits of pedogenic carbonates on the shells

lithic raw material procurement of the late epigravettian hunter gatherers from kopacina cave island of brac dalmatia croatia this paper considers lithic raw material procurement of epigravettian hunter gatherers from kopacina cave on the island of brac dalmatia croatia the most significant group among the determined petrographic categories are different cherts and a significantly smaller group of radiolarites cherts are both locally and regionally available while radiolarites originate from more distant areas use of raw material that could have been procured within 20 km radius local and in the range of 20 50 km regional is predominant and very similar in all phases raw material that could have been procured from distances ranging from 20 to 50 km shows a gradual trend of increase in frequency from the earliest to the latest phase while the raw material procured from distances greater than 50 km extra regional has an obvious drop in frequency from the eldest to the youngest phase temporal trends in lithic raw material use suggest certain continuity during the whole epigravettian sequence as well as change which shows itself in larger exploitation areas or more intensive long distance contacts in earlier epigravettian phases in kopacina than in the later ones and possibly a higher degree of hunter gatherers mobility presence of radiolarites in kopacina s late upper paleolithic layers as well as their potential outcrops suggest movements of hunter gatherers deeper in east adriatic continental hinterland where for now only a few traces of human settlements from late glacial have been found raw material of several found artifacts indicates possible contacts with the west adriatic coast

pressure and temperature dependence of water solubility in fe free wadsleyite the effects of temperature and pressure on water solubility in wadsleyite in the system mgo sio 2 h 2 o were investigated experiments were carried out using a 1000 ton multi anvil press one series of experiments was performed at a fixed pressure of 15 gpa and at various temperatures and in a second series the temperature was fixed at 1200 c and pressure was varied from 13 to 18 gpa the starting material was a mixture of oxides and hydroxides equivalent to the composition mg 2 sio 4 5 wt h 2 o run products consisted of wadsleyite quenched hydrous melt and minor amounts of clinoenstatite the water content of wadsleyite was quantified by ion probe results show that at 15 gpa the water solubility in wadsleyite decreases significantly with increasing temperature from 2 2 wt h 2 o at 900 c down to 0 9 wt h 2 o at 1400 c the corresponding mg si ratios increase from 1 80 to 1 91 over this temperature range this effect appears to be largely due to changes in the water activity in the coexisting melt the partition coefficient of water between wadsleyite and coexisting melt is nearly independent of temperature with d w a t e r w a d s l e y i t e m e l t 0 08 no significant effect of pressure on water solubility was observed at 1200 c our data suggest that the water storage capacity of wadsleyite in the transition zone is much lower than previously suggested together with previous results on ringwoodite our data imply a strong decrease of the water partition coefficient between wadsleyite and ringwoodite with temperature this decrease could have two important consequences 1 the width of the 520 km discontinuity may vary strongly as a function of temperature 2 during cooling of the earth s mantle since the hadean water may have increasingly partitioned from the lower into the upper part of the transition zone

phase relations and formation of k bearing al 10 a phase in the morb h2o system implications for h2o and k cycles in subduction zones the potassium k and water h 2 o cycles in subduction zones are predominately controlled by the stability of k and h 2 o bearing minerals such as k mica lawsonite and dense hydrous magnesium silicates dhms k micas muscovite or phlogopite are the principal h 2 o and k hosts in subduction zones and earth s upper mantle and play a significant role in the deep h 2 o and k cycles the mg 10 a phase normally appearing in hydrated peridotite in high pressure experiments has been considered as an important water carrier in subducted hydrated peridotite in this study we found a k bearing al 10 a phase in the morb h 2 o system hydrated basalt at high pressures according to x ray diffraction and stoichiometry we experimentally constrained its stability field at high pressure by considering newly and previously documented compositions of the 10 a phase and micas we confirmed a continuous solid solution or mixed layering between the 10 a phase and k mica at the interlayer site suggesting that the k cycle and the h 2 o cycle in subduction zones are coupled from the discussion of the effect of f h 2 o on stability of the al 10 a phase we conclude that a cold subduction zone can host and carry more bulk h 2 o and k into earth s deep mantle than a hot one this work expands the stability regions of the 10 a phase from the ultramafic system mg 10 a phase to the mafic system al 10 a phase and emphasizes the significance of the 10 a phase for the deep h 2 o and k cycle in subduction zone

decomposition of k amphibole at high pressures and implications for subduction zone volcanism abstract the stability of k amphibole has been studied at pressures 12 6 16 5 gpa and at temperatures 940 1450 c k amphibole decomposes into an assemblage of clinoenstatite diopside an unknown phase x stishovite fluid at pressures above 16 gpa and temperatures below 1200 c the phase boundary has a negative clapeyron slope and the high temperature assemblage is clinoenstatite diopside x wadeite type k 2 si 4 o 9 fluid at pressures 14 16 gpa the x phase has a cation ratio of approximately k mg si 1 2 2 and contains 1 7 0 1 wt h 2 o as determined by sims measurements leading to a formula of k 4 mg 8 si 8 o 25 oh 2 the present results suggest that the decomposition of k amphibole in the dragged hydrous peridotite layer at the base of the mantle wedge may produce certain amounts of h 2 o rich fluid at 14 16 gpa 450 km depth while some of h 2 o is trapped in the new hydrous phase x and is further carried into deeper regions of the mantle the aqueous fluid released by the decomposition of k amphibole should react with phase to form hydrous phase in the mantle transition region thus the dehydration of k amphibole would not cause any volcanic activities in the back arc regions in contrast to the dehydrations of amphibole chlorite and phlogopite which are presumably responsible for the first and the second volcanic chains however some of the volcanic activities such as in muriah indonesia may be related to the dehydration of k amphibole in unusually hot regions above the subducting slab

the stability of antigorite in the systems mgo sio2 h2o msh and mgo al2o3 sio2 h2o mash the effects of al3 substitution on high pressure stability the high pressure stability of antigorite in the systems msh and mash was investigated using two structurally and chemically well constrained natural samples careful sample selection and characterization ensured that the only significant difference between the samples was al content one sample being essentially al free and the other containing 3 06 2 wt al 2 o 3 in the system msh the reaction antigorite forsterite clinoenstatite water was bracketed under water saturated conditions between 630 and 650 c at 1 6 gpa between 620 and 660 c at 2 5 gpa between 620 and 660 c at 3 9 gpa and between 4 5 and 5 0 gpa at 520 c in the system mash the reaction antigorite forsterite clinoenstatite chlorite water was bracketed under water saturated conditions between 660 and 700 c at 2 0 gpa between 660 and 680 c at 2 9 gpa and between 5 0 and 5 5 gpa at 600 c at pressures above 5 8 gpa intersection of this reaction with the reaction chlorite clinoenstatite pyrope forsterite water leads to an additional reaction whereby the al component of the antigorite is transferred to pyrope upon antigorite breakdown the addition of a few weight percent al 2 o 3 into antigorite is shown to stabilize the antigorite structure to significantly higher temperatures and pressures possibly by minimizing structural misfit among the component octahedral and tetrahedral sheets this effect partially explains the considerable discrepancies noted between previous studies on the stability of antigorite at high pressure in addition antigorite breakdown in the system mash transfers a significant volume of water to chlorite bearing assemblages thereby greatly increasing the range of temperatures over which water is tied up in hydrous phases relative to the system msh

stability and dynamics of serpentinite layer in subduction zone abstract the hydrous phyllosilicate serpentines have a strong influence on subduction zone dynamics because of their high water content and low strength at shallow and intermediate depths in the absence of data newtonian rheology of serpentinites has been assumed in numerical models yet experimental data show that serpentine rheology is best described by a power law rheology recently determined in subduction zone conditions hilairet n et al 2007 high pressure creep of serpentine interseismic deformation and initiation of subduction science 318 5858 1910 1913 using a simple 1d model of a serpentinized channel and as opposed to previous models in this power law rheology we examine the influence of channel thickness temperature and subduction angle on serpentine flow driven by density contrast serpentinization degree with the surroundings at temperatures of 200 500 c relevant to intermediate depths a fully serpentinized channel is unlikely to be thicker than 2 3 km for channel thicknesses of 2 km upward velocities are comparable to those using a constant viscosity of 10 18 pa s the velocity profile using power law rheology shows shear zones at the edges of the channel and a low strain rate region at its centre consistent with the frequent observation of weakly deformed hp rocks upward velocities estimated for channels 1 to 3 km thick are comparable to the serpentinization rates for maximum estimates of fluid velocities within shear zones in the literature competition between the upward flow and serpentinization may lead to intermittent behavior with alternating growth periods and thinning by exhumation at shallower levels the thickness allowed for a channel may be up to 8 10 km if the rheology has a higher dependence on stress we therefore propose that the exhumation of hp oceanic units in serpentinite channels is organized in two levels the deepest and fastest motion being driven by density contrast with the surrounding mantle and the shallowest circulation being driven by forced return flow the thicknesses estimated here for serpentinized layers at intermediated depths are similar to the precision of seismic studies the deepest serpentinite channel may thus be difficult to detect by seismic methods but it will have a strong influence on the mechanical coupling between the slab and mantle wedge

stability of phase a in antigorite serpentine composition determined by in situ x ray pressure observations abstract here we precisely determined the low pressure stability limit of phase a in the mg end member antigorite bulk composition defined as the reaction forsterite water phase a enstatite water line or water storage line in a multi anvil apparatus pressures were determined by in situ synchrotron x ray diffraction measurements using nacl as an internal pressure standard results demonstrate that the water line is located at 800 c and 8 5 gpa and at 550 c and 5 1 gpa with a clapeyron slope of 13 6 mpa c we also examined the conditions for the formation of phase a by the decomposition of antigorite on the basis of the phase relations in the systems mgo sio 2 h 2 o msh and mgo al 2 o 3 sio 2 h 2 o mash a descending slab peridotite retains water in phase a beyond the stability of antigorite only when temperature in the slab is lower than 550 or 660 c at 5 1 gpa corresponding to 160 km depth in the system msh or mash respectively the double seismic planes observed within the slabs may be caused by the dehydration reaction of antigorite or chlorite their merging depths which could represent the high pressure stability limit of antigorite or chlorite are 160 km or deeper beneath northeast japan aleutian kamchatka and kuril water included in hydrous slab peridotite is transported into the transition zone and deeper mantle in these areas but recycled to the surface in other subduction zones

phase relations of peridotites under h2o saturated conditions and ability of subducting plates for transportation of h2o phase relations of peridotites under h2o saturated conditions up to 28 gpa and 2600 c have been clarified based on the high pressure experimental results in the mgo feo sio2 h2o mgo sio2 al2o3 h2o cao mgo al2o3 sio2 h2o and natural systems based on the phase assemblages deduced and the chemical compositions of the phases the maximum h2o contents for two bulk rock compositions lherzolite and harzburgite have been calculated using mass balance then the potential ability of subducting plates for transportation of h2o is discussed by simple models combining the phase relations and the thermal structures of subduction zones as a function of the age of subducting plate a 15 130 ma the subduction velocity u0 2 25 18 cm year and angle 30 and 60 and the mantle potential temperature tp 1300 and 1350 c the results confirm the importance of choke point a cusp around 6 2 gpa and 550 c at which the stability field of major hydrous phases is minimized in terms of temperature although the choke point is exceeded by the mg sursassite bearing assemblage that has the maximum h2o content of 0 4 0 7 wt around 5 7 gpa and 550 700 c if the geotherm of the coldest part across the subducting plate passes below the choke point maximum 4 6 wt h2o can subduct whereas h2o less than 0 4 0 7 wt can subduct above the choke point the critical conditions are clarified as follows if 30 and tp 1300 c a 15 ma with u0 9 cm year or a 30 ma with u0 2 25 cm year is the critical condition if 60 and tp 1300 c or 30 and tp 1350 c the critical conditions slightly shift to an older age or a greater velocity e g a 30 ma with u0 3 5 cm year the slab thermal parameter au0sin is useful to predict roughly the critical condition however within the younger plate the spatial extent of a cold region with major hydrous phases is thinner hence less h2o is subducted even with the same slab thermal parameter this indicates that the age of the plate and the subduction velocity act differently on the h2o transportation ability of subducting plates

stability of dense hydrous magnesium silicate phases and water storage capacity in the transition zone and lower mantle abstract we evaluated the water storage capacity in the mantle based on the phase relations of silicate systems containing water we conducted high pressure and temperature synthesis experiments on some silicate systems such as the superhydrous phase b phase c and phase g phase d and f compositions in the mgo sio2 h2o system and the cmas pyrolite 2 wt h2o system up to the uppermost part of the lower mantle superhydrous phase b is stable at temperatures below 1200 c at 18 gpa and below 1300 c at 20 gpa the following reaction sequences were observed with increasing temperature from 1200 to 1600 c in the pressure range from 18 to 20 gpa superhydrous phase b phase b liquid wadsleyite periclase liquid anhydrous phase b periclase liquid phase g is stable at temperatures below 1000 c at 18 gpa below 1100 c at 20 22 gpa and below 1200 c at 25 gpa wadsleyite stishovite liquid are stable above 1000 c at 18 gpa ringwoodite or phase e stishovite liquid above 1100 c at 22 gpa ilmenite stishovite liquid above 1100 c at 23 5 gpa and perovskite stishovite liquid above 1200 c at 25 gpa superhydrous phase b is stable at 1200 c at 18 5 and 25 gpa in cmas pyrolite 2 wt h2o composition whereas it decomposes at around 1400 c at 25 gpa in the composition there may be a layered structure in the mantle in its water storage potential solubility of water per unit mass i e the upper and lower mantles have relatively small water storage potentials whereas the transition zone has a larger potential the water content in the transition zone might be large because the primordial water trapped in the lower mantle and the recycled water transported by the slab subduction might have been released upwards and stored in the transition zone during geological time

the high pressure stability of mg sursassite in a model hydrous peridotite a possible mechanism for the deep subduction of significant volumes of h2o the stability of the high pressure phase mg sursassite previously mgmgal pumpellyite in ultramafic compositions has been determined in experiments in the system mgo al2o3 sio2 h2o mash the breakdown of mg sursassite forsterite enstatite to pyrope vapour with increasing temperature was bracketed at 6 0 and 7 0 gpa below 6 0 gpa mg sursassite forsterite vapour reacts to chlorite enstatite this reaction provides a mechanism for transfer of water from chlorite to mg sursassite bearing assemblages at pressures of 7 0 gpa and above the assemblage mg sursassite phase a enstatite was found phase relations involving mg sursassite and phase a are considered for bulk compositions with a low water content the vapour absent reaction mg sursassite forsterite pyrope phase a enstatite determines the upper pressure stability of mg sursassite and provides a mechanism for the complete transfer of water from mg sursassite to phase a bearing assemblages mg sursassite plays an important role in peridotite compositions in the subducting slab because at temperatures below 700 c it can transfer water from hydrous phases such as antigorite and chlorite to high pressure stable phases such as phase a

sound velocities and elasticity of dhms phase a to high pressure and implications for seismic velocities and anisotropy in subducted slabs abstract dense hydrous magnesium silicate dhms phase a forms in cold subducted slabs after the breakdown of antigorite serpentine and may play an important role in the transport of water within the upper mantle in this paper we present acoustic velocities and the single crystal elastic properties of fe bearing phase a mg0 981fe0 019 7si2o8 oh 6 measured by brillouin spectroscopy on a sample compressed to 12 4 2 gpa in a diamond anvil cell a fit to the acoustic data using a 3rd order finite strain eos yields the following adiabatic bulk ks and shear moduli and their pressure derivatives ks 106 1 gpa ks p t0 5 8 3 61 1 gpa p t0 1 8 1 within the experimental resolution the pure longitudinal elastic constants c11 and c33 and the off diagonal c12 constants exhibit positive linear pressure dependence whereas c44 c66 and c13 increase with a quadratic dependence on pressure the axial compressibility of phase a remains highly anisotropic in the investigated pressure range with the a axis being 15 more compressible than the c axis at 12 4 2 gpa compared to forsterite the aggregate compressional vp and shear vs acoustic velocities of phase a are 7 slower at room pressure although the velocity contrast diminishes to 3 5 for vp it is maintained for vs over the investigated pressure range phase a has high shear wave anisotropy as and shear wave polarization anisotropy a s p 0 of as 20 and a s p 0 18 and a more moderate compressional wave anisotropy ap 12 at room pressure the ap of phase a decreases to 8 at 12 4 2 gpa remaining significantly lower than that of forsterite whereas the shear anisotropy is nearly constant at 20 over the same pressure range and exceeds that of forsterite by 12 at 12 4 2 gpa at upper mantle pressures the shear wave splitting in phase a a s p 0 is 20 higher than in forsterite the results of this study were used with thermoelastic data for other relevant minerals to compute the density seismic velocities and vp vs ratios of subducted garnet harzbugite and moderately depleted harzburgite assemblages with various degrees of hydration as a function of pressure along a slab isotherm at 1073 k the results suggest that the seismic velocities of dry and water saturated harzburgites 44 5 vol phase a may be indistinguishable at upper mantle p t conditions because of the increasing concentration of high pressure orthopyroxene upon hydration this phase displays high seismic velocities that offset the decrease in velocities due to phase a rendering hydration difficult to detect anelastic attenuation is not considered combined observations from the analysis of seismic parameters indicate that significant shear wave anisotropy accompanied by high vp vs and poisson s ratios and pronounced shear wave splitting could be major diagnostic features for identifying phase a bearing assemblages at depth 180 350 km in cold subducted slabs

effects of h2o on the phase behaviour of the forsterite enstatite system at high pressures and temperatures and implications for the earth abstract in addition to many of the naturally occurring hydrous minerals known in the system mgo sio2 h2o various dense hydrous magnesium silicates 10 a phase phases a b c and d have been synthesized by high pressure and high temperature experiments in the past two decades in conjunction with these high pressure hydrous phases the effects of h2o on the observed phase transformations in the forsterite mg2sio4 enstatite mgsio3 system at high pressures and high temperatures are investigated on the basis of both experimental data and theory for forsterite the modified spinel phase may exist up to 7 1 wt h2o along with enstatite phase a up to 9 9 wt h2o along with stishovite phase a and up to only 3 2 wt h2o along with stishovite phase b the spinel phase of mg2sio4 is no longer stable when more than 3 2 wt h2o is present although both stishovite and the ilmenite type mgsio3 do not occur in dry mg2sio4 at high pressures a small amount of h2o would yield assemblages containing stishovite and the ilmenite phase in mg2sio4 at high pressures the post spinel phase assemblage perovskite periclase may be stable along with phase d up to 11 wt h2o in the case of enstatite except for the assemblage modified spinel stishovite which exists along with phase a up to 7 1 wt h2o and for the perovskite type mgsio3 along with periclase and phase d which exists up to 15 wt h 2 o all the high pressure phase transformations observed in dry mgsio3 may not occur at high pressures if 2 3 wt h2o is present based on experimental observations the various high pressure hydrous phases are stable from as low as 40 kbar to more than 280 kbar at various temperatures thus there is no lack of storage phases for h2o in the deeper interior of the earth s mantle according to the impact induced dehydration model advanced recently by shock wave experiments it is estimated that the earth s interior contains probably at least more than five times its present water contents in the near surface geochemical reservoirs hydrosphere crust so long as both forsterite olivine and h2o coexist in the upper mantle even in trace amounts there must exist a water line which is defined by the reaction 5mg 2 sio 4 3h 2 o 3mgsio 3 mg 7 si 2 o 14 h 6 forsterite water enstatite phase a in the upper mantle the equilibrium boundary of the water line has been in fact determined in two different phase assemblages by experiments but was not noticed if the earth s surface was covered by large scale melts during and shortly after its accretion h2o would be stored in phases a b below the water line and would liberate as water above the water line during freezing today s oceans were most likely derived from water released in the mantle when the palaeogeotherm dropped below the solidus of mantle materials stripping the free water above the water line from depths near 350 km thus the oceans were formed rapidly and early in the earth s history and slow degassing is still going on today the low velocity zone observed in today s mantle is interpreted as a region containing free water between the water line and the stability field of amphiboles in the suboceanic mantle and between the water line and the stability field of phlogopite in the subcontinental mantle in other words the low velocity zone is the free water zone and the water line is the base of the low velocity zone in the earth s mantle

phase relations of a serpentine composition between 5 and 14 gpa significance of clinohumite and phase e as water carriers into the transition zone run products from high pressure experiments at 800 1 200 c and 5 14 gpa corresponding to depths of 150 to 420 km on a serpentine bulk composition close to mg3si2o5 oh 4 were analysed by optical microscopy micro raman spectroscopy and electron microprobe all charges exhibit strong chemical zoning fluid melt and hydrous solids were mostly concentrated at the top bottom and along the wall of the capsules the central part of the charge was devoid of h2o both fluid and hydrous magnesian phases exhibit a mg si ratio higher than forsterite in contrast the centre of the capsule was enriched in sio2 the observed zoning can neither be explained by gravitational settling nor by a thermal gradient alone most likely the fluid was separated from the solids by surface forces and thereby established the chemical gradient by preferentially dissolving mgo if strong chemical zoning is taken into account the occurrence of more phases than allowed by the phase rule can be explained by separating the bulk into several domains of different bulk compositions results indicate that small amounts of f increase the stability field of clinohumite mg9si4o16 oh f 2 compared to oh clinohumite in pure msh previously reported clinohumite coexists with enstatite up to 975 c at 5 gpa and up to 1 100 c at 12 gpa at 14 gpa close to the mg2sio4 transition phase e becomes the most important water carrier the new results indicate that clinohumite could be an important mantle mineral for transporting water into the earth s transition zone due to its high thermal stability compared to other important water carriers such as serpentine and phase a

chlorite stability in mantle peridotite the reaction clinochlore enstatite forsterite pyrope h2o chlorite crystallising in hydrated mantle peridotite is a potential source of water for subduction zone volcanism the reaction clinochlore enstatite forsterite pyrope h2o is the important reaction for defining the stability of chlorite in these rocks it has been investigated in phase equilibrium experiments in piston cylinder and multi anvil apparatus between 2 and 5 gpa the reaction has a steep negative p t slope at low pressures with brackets at 860 880 c at 2 5 gpa 840 860 c at 3 0 gpa and 820 840 c at 3 5 gpa the reaction flattens out with increasing pressure to 5 0 gpa at 650 c chlorite compositions close to the reaction were determined from unit cell parameters measured using high resolution synchrotron powder diffraction these showed a decrease in al content with increasing pressure along the reaction consistent with calculations using a previously published thermodynamic data set the experimental results are consistent with the results of earlier studies on the reaction defining the maximum thermal stability of clinochlore clinochlore forsterite pyrope spinel h2o they have been used in the derivation of revised thermodynamic data for clinochlore in the latest version of the previously published data set the new results show that chlorite in subducting slab or overlying mantle wedge could dehydrate below subduction zone volcanoes and be a major source of the water required for melting however the dehydration reaction is not sufficiently pressure dependent to be responsible for the narrow range in depth from the volcanoes to the slab and chlorite will not be the only hydrous mineral involved

long term stability of plate like behavior caused by hydrous mantle convection and water absorption in the deep mantle we investigate the cycling of water regassing dehydration and degassing in mantle convection simulations as a function of the strength of the oceanic lithosphere and its influence on the evolution of mantle water content we also consider pseudo plastic yielding with a friction coefficient for simulating brittle behavior of the plates and the water weakening effect of mantle materials this model can generate long term plate like behavior as a consequence of the water weakening effect of mantle minerals this finding indicates that water cycling plays an essential role in generating tectonic plates in vigorous plate motion the mantle water content rapidly increases by up to approximately 4 5 ocean masses which we define as the burst effect a burst is related to the mantle temperature and water solubility in the mantle transition zone when the mantle is efficiently cooled down the mantle transition zone can store water transported by the subducted slabs that can pass through the choke point of water solubility the onset of the burst effect is strongly dependent on the friction coefficient the burst effect of the mantle water content could have significantly influenced the evolution of the surface water if the burst started early in which case the earth s surface cannot preserve the surface water over the age of the earth

stability of hydrous magnesium silicates in the mantle transition zone phase relations in the mg oh 2 sio2 system have been studied up to 18 gpa a new hydrous phase phase e mg2 3si1 3h2 4o6 was encountered at 13 17 gpa and 1000 c phase f mg1 2si1 8h2 4o6 another new hydrous phase was observed at 15 5 gpa and 800 c the stability field of phase a was also briefly studied these new hydrous phases might be stabilized in the mantle transition zone especially in subducting slabs at those depths

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

petrogenetic grid in the system mgo sio2 h2o up to 30 gpa 1600 c applications to hydrous peridotite subducting into the earth s deep interior 1 a semiquantitative petrogenetic grid in the system mgo sio2 h2o up to 30 gpa and 1600 c was constructed using schreinemakers analysis on previous experimental data the grid includes stability relations of hydrous wadsleyite hydrous ringwoodite and dense hydrous magnesium silicates dhmss phase a phase d phase e and superhydrous phase b a sequence of chemical reactions among these hydrous phases was clarified in the mantle transition zone 410 660 km depth hydrous wadsleyite and hydrous ringwoodite are stable even at a standard mantle temperature of 1600 c whereas in the other depths nominal hydrous phases including dhmss are stable below 1400 c newly found phase transitions in hydrous wadsleyite and hydrous ringwoodite are multiphase and multireaction because their compositions are on neither mgo sio2 nor mg2sio4 h2o tie lines different from single transition in the dry mg2sio4 system the grid indicates that water in the subducting slab peridotite would be transported to the bottom of the upper mantle by several dhmss and hydrous polymorphs of olivine via solid solid reactions after antigorite decomposition finally dhmss would dehydrate to liberate free water at the upper mantle lower mantle boundary layer or deeper level we also examined a relation between the predicted depth distributions of dehydration reactions in slab peridotite along several possible pressure temperature paths and the mode of seismic frequency in each subduction zone the result suggests a possible origin of intermediate to deep seismicity by the dehydration of hydrous phases in the subducting slab

transportation of h2o beneath the japan arcs and its implications for global water circulation abstract in order to understand the transportation and circulation of h2o in subduction zones geological and seismological observations and the corresponding quantitative modeling are reviewed and consolidated with a special emphasis on the japan arcs the japan arcs are ideal for such a study in that 1 they are very active magmatically and seismically with abundant high quality observations and 2 subduction parameters change significantly along the japan arcs allowing one to investigate how the variations of input parameters affect the resultant structures and processes first numerical models for the transportation of h2o and melting are introduced the generation and migration of aqueous fluid its interaction with the convecting solid and melting are considered based on the realistic phase relationships of the peridotitic and basaltic systems application of the models to the japan arcs and comparison between the model results and the observations distribution of volcanoes and seismic structures along the japan arcs are then discussed the three cases from the ne central and sw japan arcs clarify the following points first an aqueous fluid released from the subducting oceanic crust forms a serpentinite layer in the mantle wedge just above the subducting slab maintaining a condition close to equilibrium in terms of hydration dehydration reactions during aqueous fluid migration second most of the h2o is subducted to a depth where serpentine and chlorite in the serpentinite layer break down this depth depends on the thermal structure of the slab and is greater for the older plate beneath the japan arcs as a consequence aqueous fluid and melt are in general not supplied straight upward to the volcanic front this result is rather different from the conventional view finally based on the understanding obtained from the japan arcs the global budget and circulation of h2o are discussed even after the completion of dehydration of major hydrous mineral phases in the subducting materials especially at the base of the mantle wedge just above the subducting slab nominally anhydrous phases of the upper mantle assemblage i e olivine pyroxenes garnet can carry a significant amount of h2o 1 1 1011 to 7 8 1011 kg yr 1 into the deep mantle the minimum estimate is comparable to the outflow of h2o from the mantle at mid ocean ridges and hotspots while the maximum estimate greatly exceeds it the large estimated range is mainly due to the uncertainty of the maximum h2o contents in the nominally anhydrous minerals of the upper mantle which controls the net influx of h2o into the deep mantle through subduction zones water subduction magmatism global circulation

geochemistry of the quaternary volcanic rocks of the northeast japan arc major and trace element data are presented for a series of lavas from 17 volcanic centres in the ne japan arc these represent a transect of the quaternary arc from its volcanic front type a volcanoes in the east across its central zone type b to its western margin type c rocks range from basalt to dacite in composition and the variation is attributed to fractionation of plagioclase mafic silicates and titanomagnetite for comparative purposes trace element data for each volcano are presented on a 55 wt sio2 normalised basis the volcanoes display the characteristic features of arc volcanism viz enrichment in large ion lithophile elements rb sr ba k and depletion in high field strength elements zr nb hf ta using these data and derived primary magma compositions tatsumi et al 1983 mixing calculations give the percentages of fractionating mineral phases necessary to derive the 55 wt sio2 liquids these data together with published distribution coefficients allow the calculation of trace element abundances in the primary magmas reciprocal trace element plots indicate that irrespective of position with respect to the volcanic front the elements la ba k sr nb y and zr of the primary magmas all lie on a common line strongly suggesting that the melts are the products of varying degrees of melting of a common homogeneous source however those melts close to the volcanic front type a show anomalous rb th and pb values suggesting that their mantle source was enriched in these elements further calculations suggest that the hfs depletion is not due to retention by residual mineral phases but is a feature of the source thus the data suggest a that the source of the arc volcanism was homogeneous and b that the characteristic chemical features were an inherent feature of the source since zr nb ratios in the source melts are close to those found in n type morb it is suggested that a major component of arc volcanic source rocks was a depleted mantle of the type which has given melts of n type morb composition lil elements added to such a depleted source are derived by the loss of silica rich aqueous fluids from the descending subducting slab such losses occur within the first 100 km and are fixed in the overlying mantle wedge this contaminated material is transported down by drag induced convection producing a homogeneous but metasomatised mantle within the melting zone diapiric uprise produced by small scale melting produces further melting with the major control of melt chemistry being the degree of partial melting superimposed on this chemistry is a localised addition of pb rb and th in the source of those volcanoes nearest the volcanic front

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

fluid inclusions in jadeitite and jadeite rich rock from serpentinite melanges in northern hispaniola trapped ambient fluids in a cold subduction channel abstract freezing point depression was measured in aqueous fluid inclusions to determine salinities in six samples of jadeitite and jadeite rich rock from the jagua clara serpentinite melange of the rio san juan complex dominican republic the melange represents a fossil subduction zone channel from a cold mature subduction zone with a geothermal gradient of 6 c km one hundred and twenty five determinations of salinity in primary inclusions hosted in jadeite quartz apatite and lawsonite range between extremes of 1 2 and 8 7 but yield a well defined mean of 4 5 1 1 wt 1 s d nacl equiv slightly higher than mean seawater 3 5 wt in one sample eight additional fluid inclusions in quartz aligned along grain boundaries yield slightly lower values of 2 7 1 3 wt nacl equiv homogenization temperatures were also measured for 47 fluid inclusions in two samples but primary entrapment densities are not preserved it is significant that the suite includes two types of samples those precipitated directly from an aqueous fluid as well as examples of metasomatic replacement of a pre existing magmatic rock nevertheless the results indicate identical salinity for both types and suggest a much stronger genetic link between the two types of jadeitite and jadeite rich rock than has previously been assumed based on the results of conductivity measurements in modern subduction zones we envision a pervasive fluid in the subduction channel that evolved from salinity levels lower than those in sea water up to the measured values due to on going but largely completed serpentinization in the subduction channel the present data represent a reference marker for the subduction channel of the rio san juan intra oceanic subduction zone at 30 50 km depth and after 50 60 myr of operation

effect of water on melting phase relations and melt composition in the system mg2sio4mgsio3h2o up to 15 gpa melting experiments in the system mg2sio4 mgsio3 h2o up to pressures of 15 gpa show that the presence of h2o drastically changes melting temperature liquidus phases and the coexisting melt composition the system mg2sio4 h2o shows incongruent melting characterized by a high mgsi ratio of the eutectic composition exceeding 2 0 above 12 gpa whereas it exhibits congruent melting up to at least 7 7 gpa in the mgsio3 h2o system incongruent melting is observed up to 3 gpa which switches to congruent melting above 5 5 gpa and persists to at least 12 gpa the variation of the eutectic composition in the system mg2sio4 mgsio3 h2o is consistent with the variations of the melting temperatures in the systems mg2sio4 h2o and mgsio3 h2o the melting temperature solidus in the system mg2sio4 h2o decreases from 1800 c at atmospheric pressure to less than 1200 c at 10 gpa while in the system mgsio3 h2o it is reduced to a minimum value of 1250 c at approximately 5 5 gpa and then increases with pressure the composition of the initial melt in this system drastically changes at around 5 5 gpa the mgsi ratio being about 3 at 15 gpa variations of the ternary phase diagrams mgo sio2 h2o with pressure are compiled and compared with previous work on the relevant systems a modified spinel like hydrous phase is synthesized at 15 5 gpa the maximum amount of h2o accommodated in this phase is estimated to be 3 3 wt from observed chemical composition and crystal chemical considerations implying that a significant amount of h2o can be accommodated in the mantle transition zone the overall effect of h2o on ultrabasic magma genesis in the mantle is discussed a possibility is presented for the formation of komatiite magma involving the effect of h2o in the mantle of the early earth mg rich komatiite magma was possibly generated at a depth of only 200 km at temperatures much lower than that having been assumed previously

predicted elastic properties of the hydrous d phase at mantle pressures implications for the anisotropy of subducted slabs near 670 km discontinuity and in the lower mantle the dense hydrous magnesium silicate called the d phase is the most likely candidate to recycle hydrogen into the lower mantle in subduction zones as seismology represents the preferred method to detect this mineral in subduction zones the single crystal elastic tensor has been calculated using first principle methods a triple cell p3 m1 was used to account for an ordered ideal composition of the d phase to lower mantle pressures the elastic tensor of the mineral at ambient conditions is predicted to be c11 387 7 c33 287 7 c44 100 4 c12 108 0 c13 51 1 c14 14 6 in gpa with bulk modulus at zero pressure ko 163 gpa and a statistical error of about 4 0 gpa for all constants the pressure derivatives of the elastic constants have been determined to pressure of 80 gpa our tensor correctly predicts the published experimental measurements of linear compressibility along the a and c axes the logarithmic pressure derivative of the c a axial ratio and bulk modulus at zero pressure it contradicts the experimentally measured elastic tensor reported by liu l g okamoto k yang y j chen c c lin c c 2004 elasticity of single crystal phase d a dense hydrous magnesium silicate by brillouin spectroscopy solid state commun 132 517 520 which fails to reproduce the experimental compressibility data using the elastic constants and density predicted to 85 gpa the seismic anisotropy of p and s waves has been calculated as a function of pressure the single crystal has high p wave velocities and shear wave splitting in the basal plane below 20 gpa at high pressures the maximum p wave velocities have threefold symmetry normal to a axes in the basal plane whereas the maximum shear wave splitting is parallel to a axes with twofold symmetry the d phase has high vp and vs anisotropy at ambient pressure 17 6 for vp and 19 9 for vs the vp anisotropy decreases with increasing pressure to about 30 gpa while it remains constant at 8 from 30 to 85 gpa the vs anisotropy decreases to 17 at 20 gpa and then increases to 22 at 85 gpa the isotropic velocity ratios of s to p rs p bulk sound to s r s and density to s r s are calculated as a function of pressure the rs p shows very little variation with pressure with low value of 0 6 whereas both r s and r s increase with pressure to peak values of 2 8 and 2 0 respectively at about 60 gpa the presence of the d phase in subducted horizontal stagnant plates in the circum pacific could contribute to a number of seismic observations such as the observed velocity heterogeneity at depths between 500 and 1000 km and the fast s waves travelling horizontally with horizontal polarization in this depth range further the lower density of the d phase compared with

crystal structure of monoclinic hydrous wadsleyite β mg fe 2sio4 wadsleyite b mg fe 2sio4 is known to accept variable amounts of h and may be an important reservoir of h in the transition zone of the mantle the crystal structure of fo 94 6 hydrous wadsleyite mg1 730fe0 098 al0 008si0 991h0 355o4 synthesized at 1400 8c and 17 gpa was refined in space groupi2 m from 1784 measured intensities of which 830 were unique and of these 650 were of intensity greater than 4s unit cell parameters are a 5 5 6715 7 b 5 11 582 2 c 5 8 258 1 a and b5 90 397 9 8 the finalr f for all reflections was 0 026 rw 5 0 024 goodness of fit was 1 68 an h position was located on the nonsilicate o atom o1 partial occupancy of a normally vacant tetrahedral site adjacent to m3 was observed this is postulated to be the result of si moving from m3 to the adjacent tetrahedral void on decompression deviation from orthorhombic symmetry appears to result from ordering of h mg fe and possibly si within the two nonequivalent m3 sites

deformation and mantle flow beneath the sangihe subduction zone from seismic anisotropy abstract subduction of oceanic lithosphere is the most direct feedback between the earth s surface and deep interior however the detail of its interaction with the broader convecting mantle is still unclear mantle flow around subduction zones can be constrained using seismic anisotropy but despite many such studies a simple global picture is lacking the sangihe subduction zone where the molucca sea microplate is subducting westward beneath the eurasian plate is part of the tectonically complex sulawesi philippine region and an ideal natural laboratory to study complex subduction processes we investigate the anisotropic structure of the sangihe subduction zone with shear wave splitting measurements of local s and sks phases at two stations mni in sulawesi dav in the philippines as well as downgoing s phases at five stations at teleseismic distances combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase the broad depth distribution of local events 60 630 km allows us to observe a change in splitting behaviour at 380 km depth above fast directions are trench parallel and delay times t are 0 34 0 53 s with no increase with depth we suggest this anisotropy is caused by aligned cracks possibly melt filled beneath the volcanic arc and fossil anisotropy in the overriding plate below 380 km is predominantly trench normal and t are slightly higher 0 53 0 65 s as no correlation is observed with inferred distance travelled inside the slab we attribute this anisotropy to shear layers atop the slab which are coherent from 200 to 400 km depth and perhaps extend into the transition zone sks and source side measurements show larger t 1 53 and 1 33 s respectively and trench parallel since these phases predominantly sample sub slab mantle we consider along strike lateral flow associated with the double sided subduction of the molucca sea microplate to be the most likely explanation we thus infer three dominant regions of anisotropy at the sangihe subduction zone one within the overriding lithosphere one along the slab wedge interface and one below the subducting molucca sea slab the mantle wedge above 200 km depth and the slab itself do not seem to contribute notably to the measured anisotropy this study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems

the british final magdalenian society settlement and raw material movements revealed through la icp ms trace element analysis of diagnostic artefacts abstract raw material movements and group mobility in the british final magdalenian is poorly understood following a review of the british final magdalenian creswellian this paper presents preliminary results of a major la icp ms trace elemental characterization of british bedrock flint regions and compare to these characterizations of artefacts from three assemblages from the first half of the late glacial interstadial the meiendorf classed as final magdalenian the results successfully distinguish between major bedrock flint regions and artefacts can be linked to these with confidence the results reveal the importance of east anglia the lincolnshire wolds and the salisbury plain as lithic sources for artefacts from farndon fields nottinghamshire bradgate park leicestershire and wey manor farm surrey to which some flints were transported over distances ranging from the local to 200 km the results are reviewed in the context of continental raw material movements

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

the stability and composition of phengitic muscovite and associated phases from 5 5 to 11 gpa implications for deeply subducted sediments abstract the stability and composition of phengitic muscovite was investigated from 5 5 11 gpa 700 1150 c in synthesis experiments performed in a multianvil apparatus starting materials consisted of natural minerals with a bulk composition similar to that of a k deficient intermediate dioctahedral trioctahedral mica phengitic muscovite was found to be stable from 5 5 11 gpa at 900 c phengite melting occurs between 1075 1150 c at 7 8 gpa and 1000 1050 c at 10 gpa at 10 11 gpa 800 c octahedral cation deficient ocd muscovite and k hollandite are observed rather than phengite the average phengite content of muscovite is positively correlated with pressure ranging from 3 65 si pfu at 5 5 gpa to 3 81 si pfu at 11 gpa the maximum phengite content of stable muscovite appears to be 3 80 3 85 si pfu most phengite examined exhibits at least minor solid solution towards phlogopite averaging 2 04 0 06 2 octahedral cations pfu the hydrous phases phengite lawsonite topaz oh and mg pumpellyite occur between 6 8 gpa 700 900 c with increasing pressure lawsonite and mg pumpellyite dehydrate to form garnet between 8 9 gpa with increasing temperature mg pumpellyite lawsonite and topaz oh devolatilize to form garnet and kyanite between 900 1000 c at 7 8 gpa phengitic muscovite and topaz oh would be stable in hydrous sediments in cool mature subduction zones to depths exceeding 360 km while lawsonite and mg pumpellyite would be stable to 240 300 km allowing the transport of h2o contained in these phases deep into the upper mantle in warmer subduction zones the dehydration of mg pumpellyite lawsonite and topaz oh as well as the melting of phengite would result in fluid release at depths of 180 240 km the presence of phengite at far greater depths than the zone of melt generation beneath arcs 100 150 km requires a mechanism such as the partitioning of k be b ba and rb into migrating fluids rather than the simple dehydration of phengite beneath arcs in order to provide for the transfer of these slab signature elements from phengite into arc magmas

phase transformations in serpentine and transportation of water into the lower mantle phase transformations in serpentine the major hydrous mineral in the upper part of the subducting slabs have been studied at pressures to 26 gpa and at temperatures between 500 and 1400 c while serpentine completely dehydrates at temperatures higher than 600 800 c and at pressures to 12 gpa phase assemblages including some dense hydrous magnesium silicates dhms such as phases a e superhydrous b and d were formed at higher pressures phase d was found to possess a stability field of particularly wide pressure and temperature regime suggesting that this phase may play an important role in transportation of water into the lower mantle via subduction of cold slabs

compositions of aqueous fluids in equilibrium with pyroxenes and olivines at mantle pressures and temperatures solubility experiments were performed at 30 kbars in the system mg2sio4 sio2 h2o and at 20 and 30 kbars on omphacitic pyroxene water mixtures they confirm that the solubility of the forsterite component in aqueous fluids remains rather low up to 5 wt whereas the solubility of the sio2 component from solids of appropriate sio2 rich compositions in the system mg2sio4 sio2 h2o increases with temperature up to some 75 at 1 100 c at this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35 mgo sio2 hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind most interestingly the amount of sodium leached at constant temperature increases with decreasing pressure comparison of the results with previous solubility studies in the system k2o mgo al2o3 sio2 h2o indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels where amphiboles become stable this k na fractionation in the upper mantle may explain the generation of k rich or of na rich magmas through partial melting at different depths

phase relations and melt compositions in cmas pyrolite h2o system up to 25 gpa abstract phase relations and melt compositions in the cao mgo al 2 o 3 sio 2 pyrolite under hydrous 2 of h 2 o and anhydrous conditions have been determined at 10 25 gpa and temperatures from 1400 to 2400 c at the pressure of 16 and 25 gpa the apparent solidus temperatures for the hydrous system are about 200 and 100 c lower than the solidus temperatures for the dry system respectively the apparent solidus temperature changes drastically from 1600 c at 13 5 gpa to about 1850 c at 16 gpa majorite is a liquidus phase in the hydrous pyrolite from 10 to 25 gpa the liquidus phases in the dry pyrolite are olivine at 10 14 gpa majorite at 14 20 gpa and periclase at pressures above 20 gpa we observed expansion of the stability field of anhydrous phase b and its complex relations with the other phases in the cmas pyrolite system we suppose it is formed not only by incongruent melting of wadsleyite but also by the subsolidus reaction of wadsleyite and majorite in the dry condition compositions of partial melts formed by low degree of melting 10 20 have al 2 o 3 depleted and cao rich compositions up to 22 gpa and enriched in al 2 o 3 5 7 wt and cao at 25 gpa at 10 gpa dry and hydrous melts formed by low degree of melting have high sio 2 48 51 wt and relatively high al 2 o 3 3 5 5 2 wt their compositions are generally consistent with those of aluminum depleted komatiite at 13 22 gpa dry and hydrous melts have low sio 2 44 49 wt and al 2 o 3 1 7 3 0 wt and high cao contents 9 15 wt an essential depression of the apparent solidus temperature in the hydrous deep upper mantle compared to that in the transition zone provides the dehydration melting of the hydrous plume the present results support a model of komatiite genesis by dehydration melting of rising wet plumes at the base of the upper mantle

feh 1 xtixo2 a new water carrier to the mantle transition zone it is now widely accepted that earth s transition zone located at depth between 410 and 670 km is most likely hydrated however a definite conclusion has yet to be reached regarding the nature of the hydrous phase or phases that have the capacity to efficiently transport water down to such depths in the april issue of american mineralogist nishihara and matsukage 2016 show that feh 1 xtixo2 can be stable in wet basalts and sediments at high pressures and temperatures these phases allow subducting lithosphere to transport far more water to the mantle transition zone than previously thought possible

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains

flat slab subduction trench suction and craton destruction comparison of the north china wyoming and brazilian cratons abstract the mechanisms of growth and destruction of continental lithosphere have been long debated we define and test a unifying plate tectonic driving mechanism that explains the numerous petrological geophysical and geological features that characterize the destruction of cratonic lithospheric roots data from three archean cratons demonstrate that loss of their roots is related to rollback of subducted flat slabs some along the mantle transition zone beneath the cratons during flat slab subduction dehydration reactions add water to the overlying mantle wedge as the subducting slabs roll back they suck in mantle material to infill the void space created by the slab roll back and this fertile mantle becomes hydrated the roll back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting generating new magmas that gradually destroy the roots of the overlying craton through melt peridotite reactions calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2 7 trillion to 70 million cubic kilometers which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons cratonic lithosphere may be destroyed in massive quantities through this mechanism warranting a re evaluation of continental growth rates with time

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

high pressure fluids in the system mgo sio 2 h 2 o under upper mantle conditions fluids and melts have been trapped and analysed in high pressure experiments in the model mantle system mgo sio2 h2o at 6 to 10 5 gpa and 900 to 1 200 c the fluid melt traps consisted of a diamond layer that was added to the experimental charge and was separate from the silicate phases the recovered diamond traps were analysed by laser ablation icp ms starting materials were synthetic mixtures of brucite talc and silica with variable mg si containing 11 31 wt h2o experiments on a serpentine starting composition mg3si2o5 oh 4 result in mgo sio2 weight ratios in the subsolidus fluids close to 1 at 6 gpa and close to 2 at 9 gpa melt compositions at 6 and 9 gpa have mgo sio2 ratios close to that of forsterite at a single pressure the amount of dissolved silicate in the fluid increases steadily with increasing temperature up to 1 150 c where a sudden increase of both sio2 and mgo is observed this discrete step marks the solidus which is more clearly developed at 6 than at 9 gpa thus hydrous melts within the model mantle subsystem mg2sio4 mg2si2o6 h2o are chemically distinct from aqueous fluids up to at least 9 gpa corresponding to 300 km depth extrapolation of the current data set implies that total convergence between fluid and melt along the solidus probably occurs at 12 13 gpa 400 km i e close to the earth s mantle transition zone beneath cratons interactions of hydrous fluids with upper mantle lithologies cause relative silica depletion olivine enrichment at depths greater than 200 km and silica orthopyroxene enrichment at shallower depths

control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge the water released from descending oceanic lithosphere is thought to have an important role in subduction zone magmatism as this water might trigger partial melting of the mantle wedge above the subducting plate1 2 3 4 5 6 7 8 if however there is incomplete wetting of mineral grain boundaries in the mantle that is the dihedral angle9 at the triple junctions between grains is more than 60 then the water would not form an interconnected network and might instead be trapped as interstitial fluid in the mantle peridotite the water would then be transported to deeper parts of the mantle rather than triggering partial melting here we use dihedral angle data to estimate the connectivity of an aqueous fluid phase in a model upper mantle mineral assemblage forsterite at pressures from 3 to 5 gpa corresponding to depths of 80 150 km by combining these data with previous results10 11 we find that the dihedral angle is greater than 60 at low pressure and temperature 1 000 c at 2 gpa and 800 c at 4 gpa and lower than 60 at higher pressures and temperatures suggesting that wetting is incomplete below these conditions this indicates that the connectivity of water in hydrous upper mantle peridotite at convergent plate boundaries might control the position of the volcanic front in island arcs