oxygen diffusion in amphiboles the kinetics of oxygen isotope self diffusion in natural samples of hornblende tremolite and richterite have been measured samples were run under hydrothermal conditions using 18o enriched water profiles of 18o 16o 18o vs depth into the crystal were obtained using an ion microprobe the depths of sputtered holes were measured using an optical interferometer at 1000 bars 100 mpa water pressure the activation energies q and pre exponential factors d0 for diffusion parallel to c are d0 cm2 sec q kcal gm atom t c hornblende 1 20 1 10 741 6 650 800 tremolite 2 30 2 10 8 39 5 650 800 richterite 3 5 2 10 4 57 2 650 800 the diffusion coefficient d for hornblende at 800 c and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions an increase in water pressure from 200 to 2000 bars increases d by a factor of 2 7 for diffusion parallel to c at 800 c the d value for hornblende at 800 c is about 0 01 that for quartz and 0 001 that for anorthite as a result closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars it is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions

water solubility in pyrope to 100 kbar the solubility and incorporation mechanism of water in natural almost pure pyrope from dora maira western alps was investigated the infrared spectrum of the natural untreated sample 58 ppm water shows several exceptionally sharp bands in the oh stretching region including a single band at 3601 9 cm 1 and a band system with main components at 3640 5 3650 8 and 3660 6 cm 1 high temperature and high pressure infrared spectra suggest that the two absorption features arise from almost free oh groups in sites with different compressibility and thermal expansivity with the site causing the 3601 9 cm 1 band being much stiffer pyrope samples were annealed in a piston cylinder or multi anvil apparatus for several days in the presence of excess water excess sio2 and excess al2sio5 to determine the equilibrium solubility of water in pyrope to 100 kbar total solubility increases with pressure however this is exclusively due to the high frequency band system while the intensity of the low frequency band decreases with pressure at 1000 c and the oxygen fugacity of the ni nio buffer the bulk solubility can be described by the equation c oh af h2o 0 5exp p v rt with a 0 679 ppm bar0 5 and v 5 71 cm3 mol this equation implies the incorporation of water in the crystal as isolated oh groups with increasing temperature solubility appears to decrease with h 14 kj mol at fe feo buffer conditions solubility is 30 to 50 lower than with the ni nio buffer suggesting that the incorporation of oh is not coupled to the reduction of fe3 possibly the 3601 9 cm 1 band is associated with the tetrahedral oh b defect and the high frequency system with the dodecahedral oh li defect based on the experimentally established solubility model it is estimated that garnet in a hot subducted slab will transport 170 ppm of water into the mantle beyond the breakdown limit of amphibole in a cold slab 470 ppm of water can be incorporated into garnet at the breakdown limit of phengite these numbers imply that a significant fraction of the total water in the hydrosphere has been recycled into the mantle since the proterozoic

fluid rock interactions during uhp metamorphism a review of the dabie sulu orogen east central china comprehensive review on the characteristics of petrology oxygen isotope fluid inclusion and nominally anhydrous minerals nams for many dabie sulu ultrahigh pressure uhp metamorphic rocks including drill hole core samples reveals that fluid has played important and multiple roles during complicated fluid rock interactions attending the subduction and exhumation of supracrustal rocks we have identified several distinct stages of fluid rock interactions as follows 1 the neoproterozoic supercrustal protoliths of uhp rocks experienced variable degrees of hydration through interactions with cold meteoric water with extremely low oxygen isotope compositions during neoproterozoic snow ball earth time 2 a series of dehydration reactions took place during triassic subduction of the yangtze plate beneath the sino korean plate the released fluid entered mainly into volatile bearing high pressure hp and uhp minerals such as phengite zoisite epidote talc lawsonite and magnesite as well as into uhp nams such as garnet omphacite and rutile 3 silicate rich supercritical fluid hydrous melt existed during the uhp metamorphism at mantle depths 100 km which mobilized many normally fluid immobile elements and caused unusual element fractionation 4 the fluid exsolved from the nams during the early exhumation of the dabie sulu terrane was the main source for hp hydrate retrogression and generation of hp veins 5 local amphibolite facies retrogression at crustal depths took place by infiltration of aqueous fluid of various salinities possibly derived from an external source 6 the greenschist facies overprinting and low pressure lp quartz veins were generated by fluid flow along ductile shear zones and brittle faults during late stage uplift of the uhp terrane

uhp metamorphism and exhumation of the dabie orogen china evidence from shrimp dating of zircon and monazite from a uhp granitic gneiss cobble from the hefei basin a shrimp u pb study of zircons separated from an ultrahigh pressure uhp granitic gneiss cobble sample ds12 from the upper jurassic fenghuangtai formation of the hefei basin north of the dabie orogen has identified three different domains 1 cores some of which show straight boundaries and strong oscillatory zoning with th u ratios of 0 12 0 70 and an imprecise upper intercept age of 777 220 ma mswd 2 4 2 mantles variable in shape and cathodoluminescence cl intensity and containing many uhp mineral inclusions including coesite and omphacite and with th u ratios of 0 02 0 26 and a weighted mean 206pb 238u age of 244 5 ma mswd 4 7 and 3 rims which are more homogeneous and luminescent in cl than the mantles and contain fewer uhp mineral inclusions with th u ratios of 0 01 0 05 and a weighted mean 206pb 238u age of 226 2 ma mswd 0 65 monazites from the same sample do not contain coesite and omphacite inclusions but they do show sector zones and are composed of two contrasting domains the light domains in backscattered electron bse images have a th u ratio of 7 3 18 9 and a weighted mean 206pb 238u age of 221 2 ma mswd 0 94 the dark domains in bse have a th u ratio of 2 8 7 2 and have a weighted mean 206pb 238u age of 218 2 ma mswd 0 96 based on these data and a review of the literature it can be concluded that 1 the protolith of gneiss sample ds12 is a granitoid of neoproterozoic age 2 the ages of 244 and 226 ma for the mantle and rim domains of the zircons are time records of two discrete stages in the uhp evolution of dabieshan 3 the ages of 221 and 218 ma for the light and dark gray domains respectively of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the uhp granitic gneiss to a higher crustal level 4 the exhumation rate of the gneiss is estimated to be 6 km ma and 5 the uhp metamorphic blocks of the dabie orogen provided significant detritus to the hefei basin in the jurassic

fluid evolution during hp and uhp metamorphism in dabie shan china constraints from mineral chemistry fluid inclusions and stable isotopes the dabie shan ultrahigh pressure uhp metamorphic terrane is introduction located in the eastern part of the east west striking qinling dabie the discovery of coesite and micro diamond as phases orogenic belt in china a major mylonitized contact zone of produced by deep metamorphism of continental crust 200 300 m width divides dabie shan into the south dabie revolutionized our understanding of continental collision terrane sdt and the north dabie complex ndc combined zones and mantle dynamics attending subduction of investigation of major and trace element geochemistry fluid inclusions continental lithosphere in general we define metaand oxygen and hydrogen isotopes constrains the fluid history during morphic rocks with coesite and micro diamond as ultrathe metamorphic evolution of the two metamorphic belts which high pressure uhp rocks which have been increasingly differ in their fluid and metamorphic evolution fluid inclusions in recognized so far more than a dozen uhp terranes rocks from the sdt are mainly aqueous with varying salinities have been documented within the major continental whereas those from the ndc are dominated by co2 low o collision belts in the eurasian plate values in the sdt rocks 2 8 to 8 6 indicate meteoric the dabie sulu belt in east central china is the water rock interactions before uhp metamorphism whereas rocks largest among the uhp metamorphic terranes found from the ndc show normal o values 6 7 9 0 with no worldwide research activities over the past decade have obvious meteoric water rock signature whole rock rare earth element documented a number of characteristic features of this ree contents correlate with oxygen isotope compositions samples area including rapid subduction followed by rapid initial from the sdt have higher ree contents and lower o values uplift e g li et al 1993 xiao li 1993 eide et al whereas samples from the ndc have lower ree contents and 1994 the abundance of hydroxyl bearing uhp mineral higher o values during retrograde metamorphism fluids with phases okay 1994 zhang et al 1995 very low o different hydrogen isotope compositions interacted with the rocks from values ranging from 15 to 10 yui et al 1995 1997 the sdt zheng et al 1996 1998 1999 xiao et al 1997 rumble yui 1998 and the world record highest nd 0 values ever measured for eclogite jahn et al 1996 the nature and mobility of fluids in high pressure hp

ultrahigh pressure metamorphic rocks from the chinese continental scientific drilling project i petrology and geochemistry of the main hole 0 2 050 m the main hole mh of the chinese continental scientific drilling project ccsd in southern sulu has penetrated into an ultrahigh pressure uhp metamorphic rock slice which consists of orthogneiss paragneiss eclogite ultramafic rock and minor schist recovered eclogites have a uhp metamorphic mineral assemblage of garnet omphacite rutile phengite kyanite coesite epidote ultramafic rocks contain garnet olivine clinopyroxene orthopyroxene ti clinohumite phlogopite gneisses and schists contain an amphibolite facies paragenesis but their zircons have coesite garnet omphacite or jadeite and phengite inclusions indicating that eclogites and gneisses have been subjected to in situ uhp metamorphism using available geothermobarometers p t estimates of 3 1 4 4 gpa and 678 816 c for eclogites were obtained if surface outcrops and neighboring shallow drill holes are considered together we suggest that a huge supracrustal rock slab 50 km long 100 km wide 5 km deep was subducted to a depth 100 km and then exhumed to the surface the depth interval 0 2 050 m of the ccsd mh can be divided into six lithological units unit 1 consists of alternating layers of quartz rich and rutile rich eclogites with thin interlayers of gneiss and schist eclogites of unit 1 are characterized by nb ta sr and ti depletions low mg number and general lree enrichment unit 2 comprises rutile and ilmenite rich eclogite and minor normal eclogite and is characterized by high tio2 total fe v co and sr and very low sio2 alkali zr ba nb ta and total ree contents and lree depleted ree patterns with slightly positive eu anomalies unit 3 contains ultramafic rock and minor mgo rich eclogite protoliths of uhp rocks from units 1 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth units 4 and 6 consist of interlayered eclogite and paragneiss the eclogites are characterized by th u nb ta and ti depletion and k enrichment and lree enriched ree patterns paragneisses show nb ta sr and ti depletions and lree enriched ree patterns occasionally with slightly negative eu anomalies indicating that their protoliths represent metamorphic supracrustal series unit 5 consists mainly of orthogneisses showing distinct nb ta sr and ti depletions and lree enriched ree patterns with pronounced negative eu anomalies suggesting granitic protoliths in conclusion it is proposed that the southern sulu uhp belt consists of a series of meta supracrustal rocks a layered mafic ultramafic complex and granites

infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks infrared microspectroscopy has been applied to thin sections of various deformed and metamorphosed rocks in order to investigate water content of quartz in these rocks the broad ir band absorbance around 3400 cm 1 probably due to fluid inclusion molecular water h2o was used to calculate water contents deformed granitic rocks from the yanazawa kamimura area near the median tectonic line mtl showed an increase of water content in quartz from about 300 ppm to 2500 ppm toward the mtl with increasing degree of deformation metacherts from sambagawa metamorphic rocks asemigawa route showed a systematic decrease of water content in quartz from about 1000 ppm to 200 ppm with increasing metamorphic degree from chlorite garnet albite biotite to oligoclase biotite zones an archaean metachert from the napier complex granulite facies has only 40 ppm water comparing this with an inuyama unmetamorphosed chert sample having water contents of 3500 to 7000 ppm a starting point of metamorphism the systematic decrease of water in quartz with increasing metamorphic grade may extend from unmetamorphosed cherts to the highest grade metacherts water contents in high pressure metamorphic rocks in japan are mostly of the order of 500 ppm 300 700 ppm except for those from kurosegawa with a water content as high as 1700 ppm these results represent an exploratory analysis of intragranular water contents in quartz in various deformed and metamorphosed rocks further micro ft ir studies of the distribution of water in crustal rocks will provide a quantitative basis for examining the geochemical cycle of water in the earth s crust

hydroxide in pyroxene variations in the natural environment a suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy small amounts of oh occur in nearly all the samples suggesting that oh groups are a minor component of most pyroxenes the oh concentrations vary 0 002 0 12 wt oh as a function of geological setting with the greatest amounts occurring in mantle derived samples the intensities of certain oh absorption bands are somewhat correlated with pyroxene composition in particular with the presence of trivalent cations this suggests that crystal chemical or compositional factors also control the oh incorporation the oh incorporated in the pyroxene structure is easily distinguishable from oh owing to amphibole lamellae which usually occur in diopsides and often in orthopyroxenes oh in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment

mineral inclusions in zircons of para and orthogneiss from pre pilot drillhole ccsd pp1 chinese continental scientific drilling project the pre pilot drillhole ccsd pp1 chinese continental scientific drilling project ccsd with depth of 432 m is located in the donghai area in the southwestern sulu terrane the core samples are mainly comprised of paragneiss orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite bearing kyanite quartzite all analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism coesite and coesite bearing ultrahigh pressure uhp mineral assemblages were identified by raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss eclogite and phengite bearing kyanite quartzite samples in the paragneiss samples uhp mineral inclusion assemblages mainly consist of coe omp grt phe coe jd phe ap preserved in the mantles m and rims r of zircons these uhp mineral inclusion assemblages yield temperatures of 814 852 c and pressures of 28 kbar presenting the p t condition of uhp peak metamorphism of these country rocks according to the mineral inclusions and cathodoluminescence images of zircons the orthogneisses can be divided into two types uhp og1 and non uhp og2 in og1 orthogneisses low pressure mineral inclusion assemblage mainly consisting of qtz phe ab ksp ap were identified in zircon cores c while coesite or coesite bearing uhp mineral inclusions were identified in the mantles m and rims r of the same zircons these features suggest that the og1 orthogneisses together with the paragneisses phengite bearing kyanite quartzite and eclogite experienced widespread uhp metamorphism in the sulu terrane however in the zircons of og2 orthogneiss samples no uhp mineral inclusions were found inclusions mainly comprised qtz phe ap and were identified in cores c mantles m and rims r of og2 zircons the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims these features indicate that the og2 orthogneisses in pre pilot drillhole ccsd pp1 did not experience uhp metamorphism therefore we should not rule out the possibility that some orthogneisses in sulu terrane might represent relatively low pressure granitic intrusives emplaced after the uhp event

eclogite facies quartz veins within metabasites of the dabie shan eastern china pressure temperature time deformation path composition of the fluid phase and fluid flow during exhumation of high pressure rocks

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

water content in natural eclogite and implication for water transport into the deep upper mantle abstract infrared spectroscopy and ion micro probe measurements showed that the major constituent minerals of eclogites from the kokchetav massif which have been subducted to 180 km depths contain significant amounts of water up to 870 ppm h 2 o by weight in omphacite 130 ppm h 2 o in garnet and 740 ppm h 2 o in rutile omphacite shows three hydroxyl absorption bands at 3440 3460 3500 3530 and 3600 3625 cm 1 garnet has a single band at 3580 3630 cm 1 and rutile has a single sharp band at 3280 cm 1 the hydroxyl absorbance at these wavenumbers changes with the crystal orientation in polarized infrared radiation indicating that the water is structurally incorporated in these minerals the water contents in omphacite and garnet increase systematically with the metamorphic pressure of the host eclogites the partitioning coefficient of the water content between coexisting garnet and omphacite is similar in different eclogites d grt omp 0 1 0 2 but decreases slightly at high pressure based on the mineral proportions of the eclogites we estimate bulk rock water content in the eclogites ranging from 3070 to 300 ppm h 2 o by weight although hydrous minerals are absent in the diamond grade eclogite 60 kbar and 1000 c trace amounts of water are incorporated in the nominally anhydrous minerals such as omphacite and garnet the presence of significant water in these minerals implies that the subducting oceanic crust can transport considerable amounts of water into the deep upper mantle beyond the stability of hydrous minerals such water may be stored in the deep upper mantle and have an important influence on dynamics in the earth s interior

different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks abstract garnet in high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction zone conditions it provides an important target to retrieve physicochemical changes in subduction zone processes including those not only in pressure and temperature but also in the durations of metamorphism and anatexis garnet from different compositions of hp to uhp metamorphic rocks may show different types of major and trace element zonation as well as mineral inclusions discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the hp to uhp metamorphic rocks magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction zone conditions with spessartine increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation with spessartine decreasing from core to rim if the intracrystalline diffusion is not too fast the compositional profiles of metamorphic garnet in the abundances of grossular almandine and pyrope are variable depending on the composition of host rocks and co existing minerals peritectic garnet grows through peritectic reactions during partial melting of hp to uhp rocks with the composition of major elements to be controlled by anatectic p t conditions and the compositions of parental rocks and anatectic melts trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages the garnet forming reactions and the property of metamorphic fluids or anatectic melts mineral inclusions not only present key clues to identify the different origins of garnet but also serve as sound candidates for the temporal constraint on garnet growth

ultrahigh pressure metamorphic rocks from the chinese continental scientific drilling project ii oxygen isotope and fluid inclusion distributions through vertical sections in order to reconstruct the formation and exhumation mechanisms of uhp metamorphic terrains the chinese continental scientific drilling program ccsd has been carried out in donghai of the dabie sulu ultrahigh pressure uhp metamorphic belt east china eclogite gneiss amphibolite retrograded from eclogite ultramafic rocks and minor schist and quartzite have been drilled aiming to reveal the fluid behaviour in a vertical sequence of an uhp slab we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot holes pp2 and zk 703 of the ccsd more than 540 laser ablation oxygen isotope analyses on garnet omphacite quartz kyanite amphibole phengite rutile epidote amphibole plagioclase and biotite from various rocks in the depth range of 0 3 000 m mainly eclogite and gneiss show that the investigated rocks can be divided into two groups 18o depleted rocks as low as 18o 7 4 for garnet indicate interaction with cold climate meteoric waters whereas 18o normal rocks with bulk 18o 5 6 have preserved the o isotopic compositions of their protoliths meteoric water rock interaction has reached depths of at least 2 700 m oxygen isotope equilibrium has generally been achieved isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology but heterogeneous on the scale of a few metres oxygen isotope distributions in the vertical sections favour an in situ origin of the uhp metamorphic rocks the very negative 18o eclogites usually have higher hydroxyl mineral contents than the normal 18o rocks indicating higher water content during uhp metamorphism fluid inclusion data suggest that rocks with depleted 18o compositions have had different fluid histories compared to those with normal 18o values rocks with depleted 18o mainly have primary medium to high salinity inclusions in omphacite kyanite and quartz and abundant secondary low salinity or pure water inclusions in quartz indicating a high salinity brine dominated fluid system during peak uhp metamorphism no carbonic inclusions have been identified in these rocks by contrast primary very high density co2 inclusions are commonly found in the rocks with normal 18o values these observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the dabie sulu uhp metamorphic rocks

low δ18o zircons u pb dating and the age of the qinglongshan oxygen and hydrogen isotope anomaly near donghai in jiangsu province china zircons from metamorphosed granites exposed near qinglongshan have 18ovsmow values of 7 to 0 in both grain rims and cores the concordant 238u 206pb ages of zircon cores are 684 to 754 ma with rims at 221 ma discordant 238u 206pb ages range from 242 to 632 ma results demonstrate a neoproterozoic age for the origin of the qinglongshan oxygen and hydrogen isotope anomaly the low 18o values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate groundwater circulation was driven by heat from cooling granitic magma the geologic age of the hydrothermal system correlates with that of the nantuo tillite in the sinian strata of the south china block suggesting that qinglongshan s cold climate may be a manifestation of neoproterozoic snowball earth

geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the raobazhai complex from north dabie shan china a combined study of major and trace elements fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the raobazhai complex in the north dabie terrane ndt well preserved compositional zoning with na decreasing and ca and mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift a p t path with substantial heating from c 750 to 900 c after the maximum pressure reveals a different uplift history compared with most other eclogites in the south dabie terrane sdt fluid inclusion data can be correlated with the metamorphic grade the fluid regime during the peak metamorphism eclogite facies was dominated by n2 bearing nacl rich solutions whereas it changed into co2 dominated fluids during the granulite facies retrograde metamorphism at a late retrograde metamorphic stage probably after amphibolite facies metamorphism some external low salinity fluids were involved in situ uv laser oxygen isotope analysis was undertaken on a 7 mm garnet and impure pyroxene amphibole and plagioclase the nearly homogeneous oxygen isotopic composition 18ovsmow c 6 7 in the garnet porphyroblast indicates closed fluid system conditions during garnet growth however isotopic fractionations between retrograde phases amphibole and plagioclase and garnet show an oxygen isotopic disequilibrium indicating retrograde fluid rock interactions unusual morb like rare earth element ree patterns for whole rock of the garnet pyroxenite contrast with most ultra high pressure uhp eclogites in the dabie sulu area however the age corrected initial end t is 2 9 which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a morb source combined with the present data of oxygen isotopic compositions and the characteristic n2 content in the fluid inclusions we suggest that the protolith of the garnet pyroxenite from raobazhai formed in an enriched mantle fragment which has been exposed to the surface prior to the triassic metamorphism

petrology of ultrahigh pressure eclogites from the zk703 drillhole in the donghai eastern china abstract the 558 m deep zk703 drillhole located near donghai in the southern part of the sulu ultrahigh pressure metamorphic belt eastern china penetrates alternating layers of eclogites gneisses jadeite quartzites garnet peridotites phengite quartz schists and kyanite quartzites the preservation of ultrahigh pressure metamorphic minerals and their relics together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic ultramafic rock assemblages that have experienced in situ ultrahigh pressure metamorphism the eclogites can be divided into five types based on accessory minerals rutile eclogite phengite eclogite kyanite phengite eclogite quartz eclogite and common eclogite with rare minor minerals rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining two retrograde assemblages are observed in the eclogites the first stage is characterized by the formation of sodic plagioclase amphibole symplectite or symplectitic coronas after omphacite and garnet plagioclase biotite after garnet or phengite and plagioclase coronas after kyanite the second stage involved total replacement of omphacite and garnet by amphibole albite epidote quartz peak metamorphic p t conditions of the eclogites were around 32 to 40 kbar and 720 c to 880 c the retrograde p t path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature micro textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh pressure minerals is a domainal equilibrium reaction or disequilibrium reaction the composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions

stable isotope characteristics of eclogites from the ultra high pressure metamorphic terrain east central china abstract a stable isotope study was made for the ultra high pressure uhp metamorphic rocks in the dabie and sulu terrains east central china 18 o values of minerals in eclogites cover a wide range from 4 to 6 the o isotope fractionations between minerals i e omphacite phengite and or amphibole and garnet in eclogites are mostly not in isotopic equilibrium the large range of o isotope composition of garnet may approximate the 18 o variations of eclogites during uhp metamorphism and must have been largely inherited from their pre subduction basaltic precursors which might have interacted with meteoric waters under variable temperature conditions and water rock ratios before uhp metamorphism the o isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration reactions the fluids required for these hydration reactions must have low or even negative o isotope compositions it is postulated that such fluids were either originated from meteoric deep infiltration or were derived from low 18 o para gneisses

multiphase solid inclusions in zoisite bearing eclogite evidence for partial melting of ultrahigh pressure metamorphic rocks during continental collision abstract multiphase solid ms inclusions in both garnet and omphacite were investigated for zoisite bearing eclogite from the ultrahigh pressure uhp metamorphic zone in the sulu orogen the results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust there are three types of ms inclusions 1 plagioclase quartz 2 plagioclase quartz k feldspar and 3 barite plagioclase k feldspar zoisite epidote the host minerals mostly exhibit radial fractures surrounding the ms inclusions the first and second types of ms inclusions were analyzed for their bulk compositions yielding high sio 2 and na 2 o but very low feo mgo tio 2 with variable k 2 o for the second type trace element analyses of representative ms inclusions yield generally very low concentrations except such large ion lithophile elements as sr ba and rb these features suggest different origins for the three types of ms inclusions the first type of ms inclusions would be primarily derived from dehydration melting of paragonite whereas the second type of ms inclusions would be derived from dehydration melting of both paragonite and phengite the third type of ms inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz providing components for growth of garnet during the exhumation therefore the anatexis of zoisite bearing uhp eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite the major and trace element compositions of ms inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of dehydration melting in subduction channel

an experimental investigation of hydroxyl solubility in jadeite and na rich clinopyroxenes the solubility and incorporation mechanisms of water in synthetic water saturated jadeite and na rich clinopyroxenes have been experimentally investigated infrared spectra for water saturated jadeite synthesised from 2 0 to 10 gpa show two prominent sharp peaks at 3 373 and 3 613 cm 1 together with several weaker features in the oh stretching region indicating that there are at least 5 distinct modes of hydrogen incorporation in the structure water solubility in pure jadeite reaches a maximum of about 450 ppm by weight at 2 gpa and slowly decreases with increasing pressure to about 100 ppm at 10 gpa solubility can be described by the function coh a fh2o0 5 exp p vsolid rt where coh is water solubility in ppm h2o by weight a is 7 144 ppm bar0 5 fh2o is water fugacity and vsolid 8 019 cm3 mol is the volume change of the clinopyroxene upon incorporation of oh jadeite provides a good model for understanding hydrogen incorporation mechanisms in more complex omphacite compositions assignment of absorption bands in ir spectra verifies the importance of cation vacancies on the m2 site in providing mechanisms for hydrogen incorporation however results also suggest that substitution of lower valency cations onto the m1 site may also be important solid solution of jadeite with diopside and in particular with ca eskola component leads to a drastic increase of water solubility and the bulk composition has a more important effect on the capacity of omphacite to store water than pressure and temperature omphacite is expected to be the major carrier of water in a subducted eclogite after the breakdown of hydrous minerals

fluid inclusions in granulites granulitized eclogites and garnet clinopyroxenites from the dabie sulu terranes eastern china minor granulites believed to be pre triassic surrounded by abundant amphibolite facies orthogneiss occur in the same region as the well documented triassic high and ultrahigh pressure hp and uhp eclogites in the dabie sulu terranes eastern china moreover some eclogites and garnet clinopyroxenites have been metamorphosed at granulite to amphibolite facies conditions during exhumation granulitized hp eclogites garnet clinopyroxenites at huangweihe and baizhangyan record estimated eclogite facies metamorphic conditions of 775 805 c and 15 kbar followed by granulite to amphibolite facies overprint of ca 750 800 c and 6 11 kbar the presence of na ca ba sr feldspars in garnet and omphacite corresponds to amphibolite facies conditions metamorphic mineral assemblages and p t estimates for felsic granulite at huangtuling and mafic granulite at huilanshan indicate peak conditions of 850 c and 12 kbar for the granulite facies metamorphism and 700 c and 6 kbar for amphibolite facies retrograde metamorphism cordierite orthopyroxene and ferropargasite plagioclase coronas and symplectites around garnet record a strong rapid decompression possibly contemporaneous with the uplift of neighbouring hp uhp eclogites carbonic fluid co2 rich inclusions are predominant in both hp granulites and granulitized hp uhp eclogites garnet clinopyroxenites they have low densities having been reset during decompression minor amounts of ch4 and or n2 as well as carbonate are present in the granulitized hp uhp eclogites garnet clinopyroxenites early fluids are high salinity brines with minor n2 whereas low salinity fluids formed during retrogression syn granulite facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene granulitized hp garnet clinopyxeronite or in quartz blebs in garnet and quartz matrices uhp eclogite for hp granulites a limited number of primary co2 and mixed h2o co2 liquid inclusions have also been observed in undeformed quartz inclusions within garnet orthopyroxene and plagioclase which contain abundant low density co2 carbonate inclusions it is suggested that the primary fluid in the hp granulites was high density co2 mixed with a significant quantity of water the water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions giant myrmekites occurring at quartz feldspar boundaries compared with the uhp eclogites in this region the granulites were exhumed in the presence of massive externally derived carbonic fluids and subsequently limited low salinity aqueous fluids probably derived from the surrounding gneisses

archean mantle heterogeneity and the origin of diamondiferous eclogites siberia evidence from stable isotopes and hydroxyl in garnet data are presented for the o isotopic composition of clinopyroxene and garnet the c isotopic composition of diamond and the oh content of garnet from eclogite xenoliths brought to the surface by the udachnaya kimberlite pipe yakutia siberia radiogenic isotopic data suggest that the eclogites could have been derived from an ultradepleted mantle at approximately 2 9 ga pearson et al 1995 snyder et al in preparation o isotopic compositions generally show equilibration between the eclogitic minerals cpx grt 0 11 0 41 and have 18 o smow for both garnet and clinopyroxene that lie near the range of accepted mantle values of 5 7 0 7 however several eclogites indicate significant deviations from this range at higher values of 6 8 7 0 also two eclogites lie at the lower end of the mantle range at values of 4 8 and 5 0 all in clinopyroxene c isotopic compositions of diamonds all have 13 c pdb in the range of 1 to 7 and are centered at approximately 5 also within the range of accepted mantle values the oh contents of the garnet are generally between 0 and 22 ppm as h 2 0 although two samples exceed 70 ppm this range of oh is similar to eclogitic garnet from the kaapvaal craton of southern africa the mantle c isotopic values of associated diamonds the majority of o isotopic data and the low oh content of the minerals although suggesting a general lack of crustal participation in the formation of the udachnaya eclogites do not rule out the participation of some ancient crustal material that these eclogites include both 18 o enriched and 18 o depleted types suggests that the protoliths may have been overprinted by both low and high temperature hydrothermal events cf jacob et ai 1994 a positive correlation between 18 o and 87 sr 86 sr allows the interpretation of these eclogites as representing a cross section of an archean ophiolite however the lack of a single coherent grouping on a plot of 18 o vs 87 sr 86 sr suggests that if the udachnaya eclogites were derived from oceanic crust they cannot be cogenetic and must represent at least two separate ophiolite sequences conversely if the eclogites are found to be cogenetic then a totally different process affected their formation and a probable metasomatic mechanism was operative because of the total lack of correlation of 18 o with other geochemical parameters we find no compelling evidence that all eclogites are derived ultimately from oceanic crust

solubility of hydrogen and ferric iron in rutile and tio2 ii implications for phase assemblages during ultrahigh pressure metamorphism and for the stability of silica polymorphs in the lower mantle 1 the solubility of hydrogen in fe2o3 doped rutile and tio2 ii at 1100 c has been experimentally determined h incorporation in rutile is coupled to substitution of fe3 onto the octahedral ti4 site in contrast tio2 ii contains no structurally incorporated hydrogen the dominant fe3 incorporation mechanism in both phases is unrelated to h content and involves substitution of fe3 onto octahedral ti4 sites charge balanced by oxygen vacancies substitution of fe2o3 into tio2 ii stabilizes the structure to much lower pressures than in the pure tio2 system results indicate that al bearing stishovite could act as an important carrier of water in subducting oceanic crust but that formation of the post stishovite phase with the pbo2 structure would represent a significant dehydration event at the base of the lower mantle

density of morb eclogite in the upper mantle in order to study the role of eclogite subducted oceanic crust in geodynamics we carried out high pressure experiments on natural mid oceanic ridge basalt morb using a multi anvil apparatus we determined phase relations of morb as a function of pressure and temperature in the temperature range of 1200 1700 c and pressures from 5 to 14 gpa the present results differ from a previous high pressure study of morb earth planet sci lett 77 1986 245 conducted at 1200 c as follows 1 coesite coe or stishovite st appears throughout the studied pressure range and 2 the clinopyroxene garnet cpx gar ratio is smaller particularly in the low pressure range 12 gpa our results show that the calculated bulk density of subducted oceanic crust increases discontinuously at around 9 gpa due to the coesite stishovite phase transition the density is considerably higher than estimates based on the previous experiments at pressures less than 12 gpa thus the role of eclogite in driving subduction should be more important than previously thought on the other hand the density difference between peridotite and eclogite in ascending mantle plumes becomes smaller as temperature increases a result of the increase in clinopyroxene garnet sio2 ratio in eclogite with increasing temperature as eclogite is still denser than peridotite even under subsolidus conditions recycled oceanic crust entrained in ascending plumes must be sustained by the upward flow of ambient peridotite in mantle plumes

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

fluid rock interactions during ultra high pressure metamorphism dabie shan china oxygen isotope analyses of garnet separates and whole rock samples of gneiss and marble hosted eclogites from the ultra high pressure uhp dabie shan terrain fall in the range 6 76 to 10 76 comparison of the isotopic compositions of the eclogites with those of their host rocks leads to the following conclusions 1 the basaltic protolith of the eclogites had isotopic compositions in the range 2 15 to 4 99 it is proposed that this is the result of contamination of the parent basaltic magma with hydrothermally altered18o depleted source materials 2 18o enriched eclogite compositions resulted from local scale 3 m interaction between this basaltic eclogite protolith and fluids produced by decarbonation of impure marble lithologies during uhp metamorphism 3 18o depleted eclogite and gneiss compositions resulted from regional scale meteoric hydrothermal alteration of the basalts and their host rocks prior to uhp metamorphism epidote separates and whole rocks from the cores and rims of mineralogically zoned eclogite boudins within amphibolite grade biotite gneisses show no distinct isotopic differences this suggests that the preferential recrystallisation of the gneisses and the associated amphibolite grade overprint of the rims of eclogite bodies could not have resulted from regional scale infiltration of significant volumes of fluid during the exhumation of the dabie shan terrain

large areal extent of ultrahigh pressure metamorphism in the sulu ultrahigh pressure terrane of east china new implications from coesite and omphacite inclusions in zircon of granitic gneiss abstract coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite facies granitic orthogneiss in the sulu ultrahigh pressure uhp metamorphic terrane of eastern china by raman spectroscopy and microprobe analyses the occurrences of these minerals in the voluminous granitic gneiss of sulu support a regional and pervasive uhp metamorphic event that predated regional amphibolitic retrogression taking into account the widespread discoveries of coesite in other lithologies we thus conclude that a substantial crustal component in the sulu uhp metamorphic terrane appears to have shared a common history of triassic subduction to mantle depths and later exhumation

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

diffusion in silicate minerals and glasses a data digest and guide to the literature the rates of diffusion of cations oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems in order that the earth scientist may readily apply the available experimentally determined diffusion rates to such problems a compilation of data for diffusion in silicate minerals and glasses is presented along with a working guide for its use published data and experimental conditions are listed in tables and these are preceded by an outline of diffusion mechanisms terminology equations and units diffusion regimes factors controlling diffusion rates measurement techniques and diffusional behaviour

the isotopic composition of hydrogen in nominally anhydrous mantle minerals the common nominally anhydrous minerals of the earth s mantle are an important reservoir for hydrogen in the earth s interior due to their ability to incorporate hydroxyl in trace quantities we present measurements of the isotopic composition d h ratio of hydrogen from upper mantle derived garnet enstatite and augite employing a two step extraction procedure with infrared spectroscopic monitoring of oh removal from the samples raw d values of all samples range from 80 to 130 and blank corrected d of high purity separates with 50 blank contribution to the measured gas lie in the range 92 to 113 these d values together with predictions of fractionation behavior from ir spectroscopic systematics suggest that the d h ratio of the nominally anhydrous mineral reservoir may differ from that of other mantle hydrogen in being relatively depleted in deuterium to the extent that nominally anhydrous minerals control the distribution of water between the earth s hydrosphere and mantle isotopic fractionations between oh in these minerals and hydrous melts or fluids may contribute to the large d h fractionation between the mantle and the hydrosphere

diffusion of a multi species component and its role in oxygen and water transport in silicates an important but poorly understood factor that affects diffusion rates is the role of speciation during diffusion of a multi species component the diffusion of such a component is complicated by the different diffusion coefficient of each species and the interconversion reactions among the species these complexities can be treated by a diffusion equation that incorporates the diffusive fluxes of all species contributing to the concentration of the component the effects of speciation on the diffusion of the component can be investigated experimentally in some simple cases by measuring concentration profiles of all species developed during diffusion experiments or by studying some of their other consequences experimental data on water diffusion in rhyolitic glasses indicate that although dissolved water is present as two species h 2o molecules and oh groups molecular h 2o is the dominant diffusing species at very low to high water concentrations this explains the apparently complex behavior of water diffusion experimental data on oxygen diffusion in some silicates using 18 o tracers in the form of h 2 18 o are consistent with the idea that 18 o transport is dominated by diffusion of h 2o molecules even at lower water contents ppm or less this explains why oxygen transport depends on the presence of water and generally depends on water fugacity linearly for this mode of oxygen transport there is a simple theoretical relationship between the effective total oxygen diffusion coefficient and the total water diffusion coefficient that is a function of only the water concentration of the silicate at low water content this relationship appears to describe quantitatively the existing data over a wide range in water contents and diffusion coefficients in several phases

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

quantitative analysis of trace oh in garnet and pyroxenes to calibrate infrared ir spectroscopy for quantitative analysis of trace structural oh in specific minerals we have determined concentrations of h in pure separates of mantle derived pyrope garnet 56 6 ppm h 2o by weight augite 268 8 ppm h 2o and enstatite 217 11 ppm h 2o by manometry after heating the samples and extracting h 2 gas under vacuum ir spectroscopy confirmed the presence of intrinsic oh in these samples prior to extraction and indicated between 86 and 100 removal of h during the extraction procedure the integral specific absorption coefficients of 1 39 0 14 l 7 09 0 32 and 15 6 0 94 ppm h 2o cm 2 for pyrope augite and enstatite respectively allow precise spectroscopic determination of the oh content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million uncertainties in accuracy depend on mineral composition and characteristics of the oh absorption spectrum and are estimated to range between 10 and 50

hydroxyl defects in garnets from mantle xenoliths in kimberlites of the siberian platform a suite of more than 200 garnet single crystals extracted from 150 xenoliths covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites were studied by means of electron microprobe analysis and single crystal ir absorption spectroscopy in the voh vibrational range in search of the occurrence energy and intensity of the voh bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the siberian platform the voh single crystal spectra show either one or a combination of two or more of the following major voh bands i 3645 3662 cm 1 ii 3561 3583 cm 1 iii 3515 3527 cm 1 and minor bands ia 3623 3631 cm 1 iia 3593 3607 cm 1 the type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith mg mg ca ca mg fe or alkremite on the xenolith type as well as on the chemical composition of the respective garnet nearly all garnets contain band systems i and ii band system iii occurs in ti rich garnets with wt tio2 ca 0 4 from xenoliths of the mg ca and mg fe series only the voh spectra do not correspond to those of oh defects in synthetic pyropes or natural ultra high pressure garnets from diamondiferous metamorphics there were no indications of voh from inclusions of other minerals within the selected 60 60 m measuring areas in the garnets the voh spectra of pyrope knorringite and pyrope knorringite uvarovite rich garnets included in diamonds do not show band systems i to iii instead they exhibit one weak broad band voh 200 460 cm 1 near 3570 cm 1 a result that was also obtained on pyrope knorringite rich garnets extracted from two olivine megacrysts the quantitative evaluation on the basis of relevant existing calibrational data bell et al 1995 of the sum of integral intensities of all voh bonds of the garnets studied yielded a wide range of water concentrations within the set of the different garnets between values below the detection limit of our single crystal ir method near 2 10 4 wt up to 163 10 4 wt the water contents vary in a complex manner in garnets from different xenolith types obviously depending on a large number of constraints inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks secondary alteration effects during uplift of the kimberlite play if any only a minor role despite the very complex pattern of the water contents of the garnets preventing an evaluation of a straightforward correlation between water contents of the garnets and the composition of the mantle s fluid phase during garnet formation at least two general conclusions could be drawn 1 the wide variation of water contents in garnets is not indicative of regional or local differences in the composition of the mantle s fluid phase 2 garnets formed in the high pressure high temperature diamond pyrope facies invariably contain significantly lower amounts of water than garnets formed under the conditions of the graphite pyrope facies this latter result 2 may point to significantly lower fh2o and fo2 in the former as compared to the latter facies

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

oxygen and hydrogen isotope geochemistry of gneisses associated with ultrahigh pressure eclogites at shuanghe in the dabie mountains the oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss biotite gneiss and granitic gneiss associated with ultrahigh pressure uhp eclogites in the shuanghe district of the eastern dabie mountains there are significant differences in 18o between the two gneisses the uhp biotite gneiss varying from 4 3 to 10 6 similar to the associated eclogites whereas the non uhp granitic gneiss ranges only from 3 8 to 1 2 the d values are similar in the two gneisses with 37 to 64 for epidote zoisite 92 to 83 for amphibole and 63 to 109 for biotite phengite hydrogen isotope disequilibrium among the coexisting hydroxyl bearing minerals is ascribed to retrograde exchange subsequent to amphibolite facies metamorphism oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock 18o oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals the metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss the isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite facies metamorphism however high temperatures of 550 to 650 c are obtained from those minerals resistant to retrograde oxygen isotope exchange implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers the 18o depletion of both gneisses is interpreted to result from meteoric hydrothermal exchange before during plate subduction therefore the measured 18o values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the uhp metamorphism it is inferred that the uhp unit is in foreign contact with the non uhp unit like a tectonic melange but both of them experienced the two common stages of geodynamic evolution 1 18o depletion prior to the uhp metamorphism 2 uplifting since the amphibolite facies metamorphism

fluid inclusions in coesite bearing eclogites and jadeite quartzite at shuanghe dabie shan china fluid inclusions in coesite bearing eclogites and jadeite quartzite at shuanghe in dabie shan east central china have preserved remnants of early prograde and or peak metamorphic fluids reset during post uhp ultrahigh pressure metamorphic uplift inclusions occur in several minerals e g omphacite epidote notably as isolated primary inclusions in quartz included in various host minerals two major fluid types have been identified non polar fluid species n2 or co2 and aqueous the latter is by far the most predominant aqueous fluids cover a wide range of salinity from halite bearing brines to low salinity fluids for non polar fluids few n2 inclusions occur in undeformed eclogite whereas a number of co2 rich inclusions have been found in microshear zones of eclogite or jadeite quartzite in close proximity to marble occurrences the primary character of n2 and high salinity aqueous inclusions indicates that they are remnants from uhp metamorphic fluids and for some there is the distinct possibility that they are ultimately derived from pre metamorphic fluids this conclusion is supported by the preservation in some samples of microdomains containing synchronous inclusions of variable salinities which tend to relate to the chemical composition of the host crystal carbonic fluids may be derived from neighbouring rocks notably marble and carbonate bearing metasediments during post metamorphic uplift during post uhp exhumation a limited decrease of the fluid density has occurred with formation of new sets of fluid inclusions fluid movements however remained exceedingly limited at the scale of the enclosing crystal

the hydrous component in andradite garnet twenty two andradite samples from a variety of geological environments and two synthetic hydroandradite samples were studied by fourier transform ir spectroscopy their spectra show that h enters andradite in the form of oh amounts up to 6 wt h 2o occur in these samples those from low temperature formations contain the most oh some features in the absorption spectra indicate the hydrogarnet substitution sio 4 4 o 4h 4 4 whereas others indicate additional types of oh incorporation the complexity of the spectra due to multi site distribution of oh increases with increasing complexity of the garnet composition

occurrences of intergranular coesite in ultrahigh p rocks from the sulu region eastern china implications for lack of fluid during exhumation several coesite grains along garnet omphacite grain boundaries were discovered in eclogites from yangkou beach of the sulu region eastern china these grains exhibit variable extents of conversion of coesite to palisade through mosaic to granoblastic quartz aggregates the rare occurrence of intergranular and matrix coesite in ultrahigh pressure uhp crustal rocks is related to the fast rate of exhumation and the low amount of fluid infiltration required during retrograde recrystallization sluggish reactions due to the lack of fluid in both uhp and retrograde metamorphism resulted in the occurrence of intergranular coesite in these eclogites the preservation of gabbroic texture and relict igneous minerals and the metastable persistence of low p assemblages in the coesite stability field

dehydration melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet several types of multiphase solid ms inclusions are identified in garnet from ultrahigh pressure uhp eclogite in the dabie orogen the mineralogy of ms inclusions ranges from pure k feldspar to pure quartz with predominance of intermediate types consisting of k feldspar quartz silicate plagioclase or epidote barite the typical ms inclusions are usually surrounded with radial cracks in the host garnet similar to where garnet contains relict coesite barite aggregates display significant heterogeneity in major element composition with total contents of only 57 73 and highly variable sio2 contents of 0 32 25 85 that are positively correlated with bao and so3 contents the occurrence of ms inclusions provides petrographic evidence for partial melting in the uhp metamorphic rock the occurrence of barite aggregates with variably high sio2 contents suggests the coexistence of aqueous fluid with hydrous melt under hp eclogite facies conditions thus local dehydration melting is inferred to take place inside the uhp metamorphic slice during continental collision this is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust as a consequence the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction zone metamorphism

transition of uhp eclogites to gneissic rocks of low amphibolite facies during exhumation evidence from the dabie terrane central china the shuanghe ultrahigh pressure uhp slab in the dabie mountains consists of layered coesite bearing eclogite jadeite quartzite marble and biotite gneiss and is fault bounded against hosting orthogneiss representative assemblages of eclogite are grt omp coe rt ky phn mgs it formed at p 27 kbar and 680 720 50 c during exhumation these uhp rocks experienced multistage retrograde metamorphism coesite was overprinted by quartz aggregates phengite by biotite muscovite and rutile by titanite garnet was successively replaced by a thin rim of amp amp pl and amp ep bt pl minor omphacite and kyanite were replaced by amp pl cpx or bt and by zo pl ms mrg bt respectively secondary calcite occurs as irregular pockets in some layers an outcrop near the uhp slab border is composed of 20 thin concordant layers of foliated eclogites amphibolite and gneissic rocks of variable bulk composition these layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low amphibolite facies through hydration metasomatism and recrystallization retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals but the fluid for retrograde reactions was internally buffered in the stable isotope compositions retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of co2 rich and low salinity aqueous intensity of deformation and mineral resistance to alteration the fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites and or as isolated pockets in extensive or less altered eclogite layers

timing and nature of the partial melting processes during the exhumation of the garnet bearing biotite gneiss in the southern altyn tagh hp uhp belt western china abstract partial melting of the garnet grt bearing biotite gneiss from the jianggalesayi area in the southern altyn tagh high pressure hp ultra high pressure uhp metamorphic belt has been identified and reported here felsic veins were distributed along the foliation plane and deformed together with the grt bearing biotite gneiss which dominated the main outcrop and preserved diagnostic features of anataxis highly cuspate and irregular quartz qz melt and plagioclase pl melt surrounded or stretched into biotite and or quartz grains with obvious eroded boundaries further provided the microstructural evidences for partial melting of the gneiss cl images showed that zircons from the grt bearing biotite gneiss were characterized by multizone structures of core rim i rim ii rim iii the different zircon zones from the gneiss had integrated recordings of the protolith ages 836 1613 ma the peak metamorphic age of 503 5 ma the retrograded age of 456 4 ma and the anatectic age of 418 3 ma zircons from felsic veins also exhibited core rim structures on cl images the felsic veins yielded similar protolith ages 1099 1072 ma and anatectic age 417 2 ma as the host gneiss the zircons from the host gneiss and felsic veins had plotted identical crustal evolution curves in the ehf t vs u pb age diagram with identical tdm ages the u pb ages and lu hf isotope both suggested that the felsic veins were internally derived by the partial melting of the host gneiss moreover the high content of al2o3 and rb in felsic veins indicated that the decomposition of biotite accounted for the partial melting of the gneiss the temperatures calculated by ti in zircon thermometer for the anatectic zircons from the felsic veins 582 694 c were identical to the conditions of the amphibolite facies stage of the grt bearing biotite gneiss t 619 646 c which indicated that the crystallization of the melt had taken place during the amphibolite facies stage of the exhumation the anatectic ages 418 3 ma and 417 2 ma of the gneiss and felsic veins were determined to be within the range of the fourth episode of the early paleozoic magmatism 420 385 ma which had formed in a post collisional extensional setting therefore the combination of the magmatic heat events and stress release may have played an important role in the partial melting of the gneiss

grain scale sr isotope heterogeneity in amphibolite retrograded uhp eclogite dabie terrane implications for the origin and flow behavior of retrograde fluids during slab exhumation to constrain the origin and flow behavior of amphibolite facies retrograde fluids during slab exhumation we investigate the textures trace element contents and in situ strontium sr isotopic compositions using la mc icp ms of multiple types of epidote and apatite in the uhp eclogite and amphibolites from the hualiangting area dabie terrane china the uhp epidote porphyroblasts in the eclogite ep e which formed at 28 30 kbar and 660 720 c contain high amounts of sr pb th ba and light rare earth elements lrees and have a narrow range of initial 87sr 86sr ratios 0 70431 0 00012 to 0 70454 0 00010 two types of amphibolite facies epidote were recognized in the amphibolites the first type of epidote ep ai developed in all the amphibolites and has slightly lower trace element contents than ep e the ep ai has a same initial 87sr 86sr ratio range as the ep e and represents the primary amphibolite facies retrograde product that is associated with an internally buffered fluid at 8 0 10 3 kbar and 646 674 c the other type of epidote ep aii occurs as irregular fragments veins veinlets or reaction rims on the ep ai in certain amphibolites elemental x ray maps reveal the presence of ep ai relics in the ep aii domains appearing as a patchy texture which indicates that ep aii most likely formed by the partial replacement of the ep ai in the presence of an infiltrating fluid the distinctly lower trace element contents of ep aii are ascribed to element scavenging by a mechanism of dissolution transport precipitation during replacement the ep aii in an individual amphibolite exhibits large intra and inter grain variations in the initial 87sr 86sr ratios 0 70493 0 00030 to 0 70907 0 00022 which are between those of the ep ai and granitic gneisses wall rock of the amphibolites 0 7097 0 7108 these results verify that the infiltrating fluid was externally derived from granitic gneisses the matrix apatite in the amphibolites has the same initial 87sr 86sr ratio range as the ep ai indicating that the amphibolite facies fluid involved in the apatite crystallization was also internally derived we propose that at least two separate stages of fluids were accounted for the amphibolite facies retrogression of the hualiangting eclogite the fluid responsible for the growth of most of the amphibolite minerals was locally derived and behaved in a pervasive manner whereas the influx of gneiss derived fluid was transient episodic and highly channelized with a longer transport distance 60 m the disparate origins and flow behavior of these fluids significantly influence the water budget and element transfer in exhumed hp uhp slabs this study also indicates that examining grain scale sr isotopic variations provides key information regarding the isotopic dis equilibrium fluid origins and fluid flow regimes in metamorphic or metasomatic rocks that form in subduction zone environments

comment on geometry of the deep calabrian subduction central mediterranean sea from wide angle seismic data and 3 d gravity modeling by dellong et al

h2o contents and d h ratios of nominally anhydrous minerals from ultrahigh pressure eclogites of the dabie orogen eastern china abstract garnet and omphacite from ultrahigh pressure eclogites from the dabie orogen eastern china were investigated by micro ftir the results show that all garnet and omphacite grains contain structural water occurring as hydroxyl oh with h2o contents varying from 14 to 1915 ppm h2o wt and from 105 to 695 ppm respectively within the same sample the water contents are either homogeneous at the grain scale or vary systematically from higher in the core to lower in the rim low water contents at crystal rims possibly result from hydroxyl exsolution after pressure decrease upon rock exhumation the d values of omphacites are between 108 4 and 114 2 and independent of water contents heterogeneous water contents of garnet occur at the centimeter scale and fluid mobility during uhp metamorphism was very limited the estimated whole rock water content based on mineral h2o contents is between 260 and 750 ppm thereby implying that eclogitic rocks formed during continental subduction have the potential to recycle at least several hundreds ppm water to mantle depths the preserved chemical differences likely indicate that the eclogitic rocks resided at mantle conditions for a limited time span and imply that they were exhumed shortly after subduction the water released during decompression might represent the early stage retrograde fluid

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

faulting induced by precipitation of water at grain boundaries in hot subducting oceanic crust dehydration embrittlement has been proposed to explain both intermediate and deep focus earthquakes in subduction zones1 2 3 4 5 because such earthquakes primarily occur at shallow depths or within the core of the subducting plate dehydration at relatively low temperatures has been emphasized6 7 8 however recent careful relocation of subduction zone earthquakes9 10 shows that at depths of 100 250 km earthquakes continue in the uppermost part of the slab probably the former oceanic crust that has been converted to eclogite where temperatures are higher here we show that at such pressures and temperatures eclogite lacking hydrous phases but with significant hydroxyl incorporated as defects in pyroxene and garnet develops a faulting instability associated with precipitation of water at grain boundaries and the production of very small amounts of melt this new faulting mechanism satisfactorily explains high temperature earthquakes in subducting oceanic crust and could potentially be involved in much deeper earthquakes in connection with similar precipitation of water in the mantle transition zone 400 700 km depth of potential importance for all proposed high pressure earthquake mechanisms is the very small amount of fluid required to trigger this instability

fluids in metamorphic rocks abstract basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated a major problem relates to the number of inclusions possibly formed on a wide range of p t conditions having also suffered in most cases extensive changes after initial trapping the interpretation of fluid inclusion data can only be done by comparison with independent p t estimates derived from coexisting minerals but this requires a precise knowledge of the chronology of inclusion formation in respect to their mineral host the three essential steps in any fluid inclusion investigation are described observation measurements and interpretation observation with a conventional petrographic microscope leads to the identification and relative chronology of a limited number of fluid types same overall composition eventually changes in fluid density for the chronology the notion of gis group of synchronous inclusions is introduced it should serve as a systematic basis for the rest of the study microthermometry measurements completed by nondestructive analyses mostly micro raman specify the composition and density of the different fluid types the major problem of density variability can be significantly reduced by simple considerations of the shape of density histograms allowing elimination of a great number of inclusions having suffered late perturbations finally the interpretation is based on the comparison between few isochores representative of the whole inclusion population and p t mineral data essential is a clear perception of the relative chronology between the different isochores when this is possible as illustrated by the complicated case of the granulites from central kola peninsula a good interpretation of the fluid inclusion data can be done if not fluid inclusions will not tell much about the metamorphic evolution of the rocks in which they occur

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

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

a survey of hydrous species and concentrations in igneous feldspars the hydrous components in 85 feldspars from various igneous environments spanning the range of naturally occurring compositions were examined with infrared spectroscopy the feldspars contain structural oh 0 512 ppm h 2o h 2o 0 1350 ppm h 2o and nh 4 0 1500 ppm nh 4 groups as well as fluid inclusions and alteration products although composition and structure do influence the type of hydrous species that can be incorporated into a particular feldspar mineral the concentration of these species is not controlled by major element composition coarse perthitic microclines have a heterogeneous distribution of hydrous species and contain h 2o or nh 4 in k rich lamellae and fluid inclusions in na rich areas the structural oh in plagioclase feldspars is not associated with twin boundaries or exsolution lamellae all of the possible structural hydrous species are found in pegmatite feldspars whereas volcanic feldspars contain only structural oh the variation in oh concentration within a given feldspar composition suggests that fluids in the geologic environment play a role in determining the hydrogen concentration of each sample the vast majority of plutonic feldspars have undergone partial or total equilibration with meteoric fluids during low temperature 400 150 c hydrothermal exchange obliterating any structural hydrogen and creating substantial concentrations of fluid inclusions up to 4000 ppm h 2o in the exchanged regions the amount of water stored as fluid inclusions within feldspars in the upper crust 1 10 19 kg is small compared to the 1 35 10 21 kg of water in the oceans but is roughly equivalent to the reservoir of water stored in hydrous minerals in the upper crust

mineral oxygen isotope and hydroxyl content changes in ultrahigh pressure eclogite gneiss contacts from chinese continental scientific drilling project cores a combined study of mineral o isotopes and hydroxyl contents was carried out for the contacts between ultrahigh pressure eclogite and gneiss from main hole of the chinese continental scientific drilling project in the sulu orogen while there is a large d 18 o variation from 8 3 to 7 3 for all minerals different styles of mineral pair fractionation occur at the boundaries of different lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some samples of eclogite this suggests that both eclogite and gneiss acquired their negative d 18 o values by meteoric hydrothermal alteration of their protoliths at high temperatures before subduction and that fluid assisted o isotope exchange did take place across the boundary of different lithologies at local scales during amphibolite facies retrogression fourier transform infrared spectroscopy analysis yielded h2o concentrations of 50 to 1144 p p m by weight for garnet and 139 to 751 p p m for omphacite the state of equilibrium or disequilibrium o isotope fractionations between omphacite and garnet are correlated with variations in their water content at local scales indicating that the internally derived fluid plays a critical role in retrograde metamorphism during exhumation the retrograde metamorphism results in mineral reactions and o isotope disequilibria between some of the minerals but the fluid for retrogression was derived from the decompression exsolution of structural hydroxyl and thus internally buffered in the o isotope composition a quantitative estimate suggests that a hand specimen 3 6 9 cm of eclogite composed of 70 garnet and 30 omphacite can release 0 316 g water by the decompression exsolution of structural hydroxyl which can form 14 4 g amphibole during exhumation this provides sufficient amounts of water for the amphibolite facies retrogression

volume self diffusion of oxygen in biotite muscovite and phlogopite micas the kinetics of oxygen self diffusion have been measured in natural samples of biotite muscovite and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses all experiments were run at 1000 bars 100 mpa water pressure except those designed to measure water pressure dependence which ranged from 200 to 2000 bars the arrhenius relations obtained for layer parallel diffusive transport in the three micas by bulk exchange experiments fit to an infinite cylinder model are mineral biotite muscovite phlogopite 34 2 39 5 42 3 9 1 10 6 7 7 10 5 1 4 10 4 t c 500 800 512 700 600 900 the results of ion microprobe sims depth profiling perpendicular to the layers parallel to c axis yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers a single layer parallel step profiling experiment yields a diffusion coefficient identical within uncertainty to those obtained from bulk exchange experiments a slight dependence of diffusion rate on water pressure may obtain in phlogopite however the difference is approximately within the uncertainty of the measurement diffusion rates are similar within about a half an order of magnitude for a range of biotite composition from annite4 to annite63 as well as for muscovite comparison of oxygen diffusion rates in biotite from this study with ar k and rb diffusion from published data supports the idea that water is the oxygen bearing species in hydrothermal diffusion experiments the implication of these results for natural systems is that the micas especially biotite will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300 c the rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions full size table table options view in workspace download as csv

remnants of premetamorphic fluid and oxygen isotopic signatures in eclogites and garnet clinopyroxenite from the dabie sulu terranes eastern china a combined oxygen isotope and fluid inclusion study has been carried out on high and ultrahigh pressure metamorphic hp uhpm eclogites and garnet clinopyroxenite from the dabie sulu terranes in eastern china coesite bearing eclogites garnet clinopyroxenite and quartz eclogites have a wide range in whole rock 18ovsmow from 0 to 11 the high t oxygen isotope fractionations preserved between quartz and garnet preclude significant retrograde isotope exchange during exhumation and the wide range in whole rock oxygen isotope composition is thought to be a presubduction signature of the precursors aqueous fluids with variable salinities and gas species n2 co2 or ch4 rich are trapped as primary inclusions in garnet omphacite and epidote and in quartz blebs enclosed within eclogitic minerals in high 18o hp uhpm rocks from hujialin and shima high salinity brine and or n2 inclusions occur in garnet porphyroblasts which also contain inclusions of coesite cl rich blue amphibole and dolomite in contrast in low 18o eclogites from qinglongshan and huangzhen the cl concentrations in amphibole are very low 0 2 wt and low salinity aqueous inclusions occur in quartz inclusions in epidote porphyroblasts and in epidote cores these low salinity fluid inclusions are believed to be remnants of meteoric water although the fluid composition was modified during pre and syn peak hp uhpm eclogites at houshuichegou and hetang contain ch4 rich fluid inclusions coexisting with high salinity brine inclusions methane was probably formed under the influence of co2 rich aqueous fluids during serpentinisation of mantle derived peridotites prior to or during plate subduction remnants of premetamorphic low to high salinity aqueous fluid with minor n2 and or other gas species preserved in the dabie sulu hp uhpm eclogites and garnet clinopyroxenite indicate a great diversity of initial fluid composition in the precursors implying very limited fluid rock interaction during syn and post peak hp uhpm

hydrous species in eclogitic omphacite implication for metamorphic dehydration during exhumation abstract water contents were analyzed by infrared spectroscopy for structural hydroxyl in randomly oriented cuboids and doubly polished wafers of single crystal omphacite from an ultrahigh pressure eclogite in the dabie orogen the results are quantitatively compared with bulk water contents measured by the tc ea ms continuous flow technique this enables recognition of various hydrous species in omphacite and their bearings on metamorphic dehydration during continental subduction the two cuboids yield constant maximal structural water contents of 378 ppm h2o wt using the omphacite specific infrared absorption coefficient which are much lower than the bulk water contents of 1496 ppm the excessive water is contributed by the presence of other hydrous species in the form of hydrous mineral inclusions hydrous symplectite and submicron fluid inclusions since the hydrous mineral and or fluid inclusions may be captured at different metamorphic stages during subduction and exhumation of continental crust and the hydrous symplectite is the secondary rather than the primary it is suggested that the bulk water contents represent a mixing of various hydrous species with distinct origins in this regard it is critical to examine whether water in these secondary hydrous species is of internal origin if not it has nothing to do with the transport capacity of water in deeply subducted crustal rocks upon stepwise heating up to 700 c omphacite exhibits a gradual decrease in bulk water contents but an increase in d values indicating the preferential loss of d depleted hydrous species as a whole due to the decompressional dehydration of uhp metamorphic rocks during exhumation therefore the d depleted hydrous species may be dominated in the hydrous symplectite and inclusion water

the eclogite garnetite transformation in the morb h2o system abstract to decipher phase relations of oceanic crust in the coldest slab at the mantle transition zone multi anvil experiments were conducted in the morb h 2 o system at pressures of 10 19 gpa and temperatures of 700 1500 c garnet and stishovite were recognized in all run charges above 15 gpa garnet drastically increases nasi na 2 msi 5 o 12 component m ca mg fe 2 jadeite occurs instead of omphacite na k hollandite containing 7 mol naalsi 3 o 8 and ca perovskite with 60 mol catio 3 were observed at p 17 gpa with decomposition of omphacite and increase of modal ratio of garnet there is a sharp increase of density at 440 km the density increase due to appearance of ca perovskite at 570 km is estimated approximately 100 km shallower than that of previous estimation

hydroxyl in clinopyroxene from the deep subducted crust evidence for h2o transport into the mantle clinopyroxenes in eclogites from the deep subducted crust in the kokchetav kazakhstan ultrahigh pressure metamorphic terrane were found by infrared spectroscopy to contain up to 3020 ppm oh these rocks were recrystallized at pressures over 60 kbar and temperatures over 1000 c where no hydrous minerals are stable infrared spectra of the clinopyroxenes show three hydroxyl absorption bands in the regions 3440 3460 cm 1 3500 3530 cm 1 and 3600 3625 cm 1 the hydroxyl absorbance increases with recrystallization pressure of the eclogites and with vacancy concentration in the pyroxene structure ca eskola component clinopyroxenes represent about 40 50 vol of eclogites such that the bulk eclogites contain approximately 1300 ppm hydroxyl at depths greater than 150 km thus subducted oceanic crust can transport trace amounts of h2o into the upper mantle even in the absence of nominally hydrous minerals

a mechanism for na incorporation in garnet an example from garnet in orthogneiss from the su lu terrane eastern china sodian and yttrian na y garnet occurs in orthogneisses of the su lu terrane of eastern china with major quartz microcline and albite and subordinate biotite titanite hematite and zircon acmite rich pyroxene muscovite epidote allanite and hastingsitic amphibole occur in a few of the samples the na y garnet can be divided into almandine spessartinegrossular ai 1 6 per formula unit pfu for 0 12 hereafter denoted as ai garnet and andradite spessartine feh 1 3 pfu hereafter denoted as feh garnet groups average composition of the ai garnet in each sample ranges from alm3ssps13prp grs49adr2 to aim2lsps67prpogrssadr7 the feh garnet contains a calderite component cdr and its composition is alm4 7spslo 2 prp 2adrs9 8 cdr3 12 na y and yb contents of the ai rich garnet reach 0 37 1 46 and 0 42 oxide wt respectively and na y yb 0 99 t 0 16 the feh garnet contains up to 0 11 2 1 and 0 48 wt of na20 y203 and yb203 respectively and its na y yb value 0 17 t 0 1 is lower than that of the ai garnet we conclude that na y and yb are incorporated in the ai rich garnet by the 8jna 8j y yb ca mn 2substitution whereas in the feh garnet y and yb are dominantly incorporated by the 8j y yb l4j ai feh ca mn sl substitution with up to 3 mol y yb for ca mn comparison ofna y yb of garnets in the orthogneisses and in some pegmatites implies that increasing pressure favors incorporation of na with y and yb and that under the albite epidote amphibolite facies or higher pressure conditions y and yb probably substitute into the garnet structure by 8jna 8j y yb ca mn 2 instead of the 8j y yb 4j al feh ca mn sl type substitution effective under lower pressure conditions

heterogeneity of water in garnets from uhp eclogites eastern dabieshan china abstract garnets in uhp eclogites from bixiling in dabieshan were investigated by fourier transform infrared spectroscopy ftir the results indicate that all garnets contain structural water that occurs as hydroxyl oh and non structural molecular water h 2 o possibly in the form of sub microscopic fluid inclusions the structural hydroxyl contents range from 92 to 1735 ppm h 2 o wt and most are between 200 and 1000 ppm therefore garnet in eclogite can recycle surface water into the mantle various water contents were observed among different samples of the same outcrop 150 m and in different domains of the same sample 1 cm this variability in structural h 2 o contents suggests that the mobility of fluids during uhp metamorphism was very limited and that both subduction and exhumation processes of uhp rocks occurred in a short time interval

water content of mantle garnets garnet megacrysts from colorado plateau diatremes green knobs garnet ridge and the wesselton kimberlite south africa commonly contain a structural hydrous component the colorado plateau samples range from 0 0 to 0 26 wt h 2 o and the wesselton samples contain from 0 01 to 0 07 concentrations were measured using p 2 o 5 cell coulometry h 2 gas manometry and thermogravimetry these were used to calibrate infrared integrated absorbance in the 3 m region which is a more sensitive measure of total o h content than the other analytical methods infrared absorbance patterns were also used to differentiate structural hydrous component from water contained in alteration and included phases the structure of the hydrous component in these garnets appears to be the classic h 4 o 4 4 sio 4 4 profiles at 100 m intervals across these samples show flat concentration profiles or slightly increasing concentration toward the center a large range of water content among samples appears to represent real differences in water fugacity at the point where the garnets equilibrated garnets in eclogite nodules from south africa and the solomon islands were also studied but were either anhydrous or too badly altered to determine the content of structurally bound water the high concentration of hydrous component in the colorado plateau samples is consistent with other indicators of high volatile content in that region of the mantle the water content of mantle garnets may prove to be an accurate indicator of mantle water fugacities

metamorphism of magnesium altered felsic volcanic rocks from bergslagen central sweden a transition from mg chlorite to cordierite rich rocks abstract the ca 1 9 ga old felsic volcanic rocks of the bergslagen province have been affected by synvolcanic hydrothermal mg metasomatism a spatial relationship exists between mg alteration zones and calc silicate carbonate banded iron mineralizations and a genetic link is suggested this is supported by the occurrence of the mg rich tourmaline dravite which exists in the mg alteration zones as well as in the wall rocks to iron oxide mineralizations following later regional metamorphism and deformation these rocks occur as large zones of mg rich schists parallel to sub parallel with the regional foliation s1 the dravite crystals have been affected by this regional deformation a second deformational episode folded s1 along steeply dipping fold axes locally this resulted in a crenulation cleavage and the development of a new generation of micas s2 crosscutting s1 in the westernmost areas the mg schists contain a fine grained network of mg chlorite and muscovite with little or no preferred orientation indicating low pressures and low grade metamorphic conditions the grade of recrystallization and deformation increase towards the east and a boundary between low and medium grade metamorphic terrains is crossed the metamorphic boundary is marked by the disappearance of chlorite and the appearance of cordierite in the mg schists porphyroblasts of mg chlorite or cordierite grew across s1 and cordierite porphyroblasts are transected by s2 micas this indicates that the peak of regional metamorphism occurred between the two deformational episodes retrogression of cordierite to chlorite and muscovite is confined to the vicinity of crosscutting s2 micas

the distribution of hydroxyl in garnets from the subcontinental mantle of southern africa 166 garnets of dominantly mantle origin were analyzed for oh content by infrared ir spectroscopy ir spectra in the 3400 3700 cm 1 region display consistent absorption patterns attributable to oh structurally bound within the garnet crystal occasionally contaminated by low intensity oh absorptions from microscopic inclusions the principal structural oh absorption occurs near 3570 cm 1 with the appearance of additional absorptions near 3512 cm 1 and 3650 cm 1 dependent on garnet composition or paragenesis samples derive from a wide variety of rock types occurring as xenoliths in kimberlites of southern africa oh abundances using the best currently available calibration range from less than 1 up to 135 ppm h 2o and increase in the general order as follows on craton eclogites group 1 basaltic kimberlite alnoite alkali basalt the oh contents of common lithospheric granets from coarse peridotites including several phlogopite bearing samples are typically less than 20 ppm h 2o for tectonic settings of kimberlites both on and off the archaean kaapvaal craton ti rich garnets from deformed peridotites are richer in oh supporting previous suggestions of association of these xenoliths with putative megacryst magmas subcalcic cr rich xenocrysts diamond inclusion garnets and garnets from diamondiferous eclogites have very low oh contents similar to eclogites and depleted peridotites without macroscopic diamonds the oh content of southern african peridotite and eclogite garnets are significantly lower on average than those previously examined from the colorado plateau diatremes while details of emplacement related h mobility in garnets remain to be established our results suggest that garnets record useful information on the role of water or other hydrous volatile species in petrological processes at their source regions in the mantle although garnets do not appear to constitute a large reservoir of mantle hydrogen the large stability range of oh bearing garnet in the crust and mantle implies wide applicability as a qualitative hydrobarometer

cluster of water molecules in garnet from ultrahigh pressure eclogite through the use of transmission electron microscopy and infrared spectroscopy hydrous components have been identified as hydroxyl and free water h 2 o in elongate garnet of ultrahigh pressure eclogite from the dabie mountains eastern china the h 2 o occurs as clusters of water molecules and as hydroxyl in garnet the clusters of water molecules are interpreted to have promoted elongation of garnet through hydrolytic weakening the presence of hydrous species in ultrahigh pressure garnet from continental crust protoliths indicates that such protoliths are able to carry water into the mantle during the subduction of continental lithosphere

hydrogen and oxygen isotope evidence for fluid rock interactions in the stages of pre and post uhp metamorphism in the dabie mountains abstract hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern dabie mountains china the 18 o values of eclogites cover a wide range of 4 2 to 8 8 but the d values of micas from the eclogites fall within a narrow range of 87 to 71 both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some eclogite samples the 18 o values of 4 to 1 for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water rock interaction before the high to ultrahigh pressure metamorphism heterogeneous 18 o values for the eclogite protoliths implies not only the varying degrees of the water rock interaction before the metamorphism at different localities but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks

distribution of hydrous components in jadeite of the dabie mountains two kinds of jadeite occur in the jadeite quartzite from the dabie mountains eastern china associated either with weak or strong deformation the former shows a uniform composition while the latter shows both uniform and zoning composition these jadeites were examined with infrared ir spectroscopy all jadeite displays hydroxyl oh stretching bands implying that hydrous components commonly exist in jadeite the concentration of the hydrous components in the jadeite with weak deformation is homogenous whereas the concentration of the hydrous components in strongly deformed samples is variable the correlation between the intensity of oh band absorbance and compositional zoning of jadeite with strong deformation indicates that na rich jadeite can accommodate more oh than na poor jadeite its interpretation is that there is more na associated with greater oh availability in the initial crystallization environment our data of the comparatively high oh concentrations recorded in the core of the jadeite which contains relative high vacancies in m2 site imply that both oh content and the vacancies in m2 site could decrease during exhumation of the jadeite quartzite the decrease of oh solubility in jadeite would result in the formation of h2o fluid during the early exhumation of uhp metamorphic rocks the h2o fluid transformed from oh during exhumation could trigger and enhance the early retrograde metamorphism of the host rocks and facilitate plastic deformation of jadeite grain by dislocation creep and diffusion creep

the temperature dependence of water solubility in enstatite the solubility of water in pure enstatite was measured on samples synthesized under water saturated conditions at 15 kbar and temperatures ranging from 700 to 1 100 c polarized ftir measurements on millimetre sized clear crystals showed that water solubility increases strongly with temperature from 101 ppm by weight at 700 c to 269 ppm by weight at 1 100 c the position and shape of the infrared bands hardly changes with temperature with one notable exception a band close to 3 380 cm 1 is present in samples synthesized between 700 and 1 000 c while this band is absent from samples synthesized at 1 100 c this effect appears to be very reproducible and points towards a slight change in the crystal structure of enstatite between 1 000 and 1 100 c at 15 kbar the water solubility data of this study as well as those of rauch and keppler contrib mineral petrol 143 525 536 2002 can be reproduced by the equation k a f text h 2 text o exp delta h 1 text bar rt exp delta v text solid p rt where k is water solubility f text h 2 text o is water fugacity a is 0 01354 ppm bar vsolid 12 1 cm3 mol is the volume change of enstatite during incorporation of water and h1 bar 4 563 j mol is the reaction enthalpy at 1 bar this equation predicts the following behaviour of water solubility in enstatite as a function of pressure and temperature 1 water solubility increases with pressure up to a maximum around 80 kbar 2 water solubility decreases with temperature at 1 bar and 3 water solubility increases with temperature between 10 and 100 kbar if the observed temperature dependence for enstatite were representative for other upper mantle minerals as well it would have the following implications 1 lateral temperature gradients in the upper mantle could cause major variations in water contents at the same depth in particular hot mantle plumes may scavenge water from the surrounding shallow upper mantle 2 the scavenging of water by hot plumes could be a major factor in increasing the mobility of plumes 3 the predicted temperature dependence of water solubility at the base of the upper mantle may allow plumes to bypass the transition zone water filter postulated by bercovici and karato nature 425 39 44 2003

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes

hydroxyl rich topaz in high pressure and ultrahigh pressure kyanite quartzites with retrograde woodhouseite from the sulu terrane eastern china hydroxyl rich topaz was recognized in thick kyanite quartzites from both high pressure hp and ultrahigh pressure uhp belts of the sulu terrane china these quartzites contain variable proportions of quartz kyanite and topaz with minor phengite pyrite and rutile some topaz grains from the uhp belt contain abundant inclusions of oriented kyanite whereas those from the hp belt are partially replaced by woodhouseite caal 3 po 4 so 4 oh 6 most topaz crystals contain 9 5 to 13 5 wt f 0 92 1 30 atoms per formula unit apfu indicating 35 55 substitution of f by oh such naturally occurring hydroxyl rich topaz has not been previously reported some topaz grains from the hp belt show distinct zoning 1 decreasing f content from narrow cores 13 3 16 5 wt to thick rims 9 5 wt or 2 oscillatory zoning 9 44 12 77 wt unit cell parameters of topaz show a positive linear correlation between the oh content and a and b as well as volume based on our petrologic data the experimentally determined curve of ky h 2 o topaz oh at very high pressures and calculated oh oh f isopleths of topaz at low pressures the topaz with x oh 0 35 from the uhp belt may have formed at p t conditions within the coesite stability field the p t condition of hp topaz is less well constrained its high x oh 0 40 0 55 may have been caused by lower metamorphic temperatures and higher initial x oh in comparison with uhp topaz hydroxyl rich topaz together with other hydrous minerals in uhp rocks may be important carriers of h 2 o to mantle depths of 100 200 km during continental subduction

petrology of ultrahigh pressure rocks from the southern su lu region eastern china abstract in the su lu ultrahigh p terrane eastern china many coesite bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet quartz jadeite rock and kyanite quartzite in addition to garnet omphacite rutile coesite other peak stage minerals in some eclogites include kyanite phengite epidote zoisite talc nyboite and high al titanite the garnet quartz jadeite rock and kyanite quartzite contain jadeite quartz garnet rutile zoisite apatite and quartz kyanite garnet epidote phengite rutile omphacite assemblages respectively coesite and quartz pseudomorphs after coesite occur as inclusions in garnet omphacite jadeite kyanite and epidote from both eclogites and metasediments study of major elements indicates that the protolith of the garnet quartz jadeite rock and the kyanite quartzite was supracrustal sediments most eclogites have basaltic composition some have experienced variable crustal contamination or metasomatism and others may have had a basaltic tuff or pyroclastic rock protolith the su lu ultrahigh p rocks have been subjected to multi stage recrystallization and exhibit a clockwise p t path inclusion assemblages within garnet record a pre eclogite epidote amphibolite facies metamorphic event ultrahigh p peak metamorphism took place at 700 890 c and p 28 kbar at c 210 230 ma the symplectitic assemblage plagioclase hornblende epidote biotite titanite implies amphibolite facies retrogressive metamorphism during exhumation at c 180 200 ma metasedimentary and metamafic lithologies have similar p t paths several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in situ ultrahigh p metamorphism during the triassic collision between the sino korean and yangtze cratons

the oh content of pyrope at high pressure the oh contents of pyrope garnets synthesised at 1000 c and pressures between 2 and 13 gpa have been measured by infrared spectroscopy we find that under the same conditions of pressure temperature ah2o and asio2 the oh content of pyrope is similar to that of grossular this shows that observed differences in nature between water contents of pyrope and grossular are related to paragenesis rather than reflecting higher intrinsic solubility in grossular in the presence of excess sio2 the h2o content increases with pressure to 5 gpa 1000 ppm then decreases to below the detection limit at pressures above 7 gpa thus garnet becomes more hydrous with pressure to some critical value beyond which dehydration occurs even under h2o saturated conditions this observation is consistent with the measured partial molar volume of water in hydrogarnet which becomes greater than that in fluid water within the pressure range of this study when corrected to upper mantle conditions we find that pyrope should dehydrate with increasing depth below about 250 km pyrope is unlikely therefore to be a major site of water storage in the transition zone and upper mantle