magmatic mn rich garnets in volcanic settings age and longevity of the magmatic plumbing system of the miocene ramadas volcanism nw argentina abstract the miocene corte blanco tuff rhyolite deposit is the product of a large volume and high intensity plinian eruption from the solitary and monogenetic ramadas volcanic centre central andes province of salta nw argentina the corte blanco tuff consists of vitreous tube pumices with rare euhedral sub millimetric mn garnet phenocrysts typically hosting inclusions of u phases as zircon and monazite here we present new textural major and trace elemental analyses of garnet zircon and glass that combined with in situ u th pb zircon and monazite dating are used to reconstruct the thermobaric environment of formation age and longevity of the magmatic plumbing system of the ramadas magma the results indicate to a crystallization path of a peraluminous rhyolitic melt at shallow crustal levels 6 km as sequentially tracked by the initial nucleation of zircon 780 c at 9 16 ma and garnet above or at ca 700 c to the final monazite growth 660 670 c at 8 70 ma in a water saturated h2o 3 5 wt environment shortly before the eruption started these data 1 define for the first time the primary magmatic origin of mn garnet in a rhyolitic volcanic setting 2 provide new partition coefficients of rare earth elements ree between natural garnet zircon and rhyolitic melts and 3 permit reconstruction of the magmatic processes that resulted in the ramadas eruption on a wider scale our results document the spatio temporal p t conditions timing and longevity time scales involved in the petrogenesis of a shallow peraluminous water saturated rhyolitic magmatic plumbing system that is able to generate the conditions for extremely explosive plinian eruptions

high pressure eclogites in northern jiangsu southern shandong province eastern china abstract eclogites are distributed for more than 500 km along a major tectonic boundary between the sino korean and yangtze cratons in central and eastern china these eclogites usually have high p assemblages including omphacite kyanite and or coesite or its pseudomorph and form a high p eclogite terrane they occur as isolated lenses or blocks 10 cm to 300 m long in gneisses type i serpentinized garnet peridotites type ii and marbles type iii type i eclogites were formed by prograde metamorphism and their primary metamorphic mineral assemblage consists mainly of garnet pyrope prp 15 40 mol omphacite jadeite jd 34 64 mol pargasitic amphibole kyanite phengitic muscovite zoisite an sio2 phase apatite rutile and zircon type ii eclogites characteristically contain no sio2 phase and are divided into prograde eclogites and mantle derived eclogites the prograde eclogites of type ii are petrographically similar to type i eclogites the mantle derived eclogites have high mgo feo fe2o3 and cr2o3 compositions in bulk rock and minerals and consist mainly of pyrope rich garnet prp 48 60 mol sodic augite jd 10 27 mol and rutile type iii eclogites have an unusual mineral assemblage of grossular rich grs 57 mol garnet omphacite jd 30 34 mol pargasite rutile pargasitic and taramitic amphiboles calcic plagioclase an68 epidote zoisite k feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites this suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and or amphibolite facies rocks with near isothermal compression reflecting crustal thickening during continent continent collision of late proterozoic age equilibrium conditions of the prograde eclogites range from p 26 kbar and t 500 750 c in the western part to p 28 kbar and t 810 880 c in the eastern part of the high p eclogite terrane the prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part subsequent reactions manifested by 1 narrow rims of sodic plagioclase or paragonite on kyanite and 2 symplectites between omphacite and quartz are interpreted as an effect of near isothermal decompression during the retrograde stage the conditions at which symplectites re equilibrated tend to increase from west p 9 kbar and t 680 c equilibrium temperatures of type ii mantle derived eclogites and type iii eclogite are 730 750 c and 680 c respectively

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

decoding the oxygen fugacity of ore forming fluids from garnet chemistry the longgen skarn pb zn deposit tibet abstract the relationship between the compositional zonation of garnet in skarn deposits and the evolution of physicochemical conditions of ore forming fluids is not clear and needs further investigation this study presents the characteristics of trace elements of garnet from the longgen skarn pb zn deposit tibet with an aim to reveal the evolution trend of the physical and chemical conditions fo2 and ph of ore forming fluids and to explore the fingerprint of garnet trace elements in different types of skarn deposits based on robust alteration halo mineral assemblages and petrographic observation the garnets of the longgen skarn pb zn deposit are divided into two types 1 distal exoskarn garnet near limestone with ore bearing and fine oscillatory zoning adr39 9 75 0 grt1 adr represents andradite and 2 proximal exoskarn garnet core adr32 9 70 7 rim adr83 3 99 5 with pb zn rich mineral and coarse oscillatory zoning grt2 according to the total rare earth element ree content in the garnet compositional zone grt1 is further divided into three component zones grt1 1 heavy rare earth element hree enriched chondrite normalized ree distribution patterns and positive eu anomaly eu 1 eu 2 eusample euchondrite smsample smchondrite gdsample gdchondrite grt1 2 hree enriched ree patterns with negative eu anomaly eu 1 similarly five component zones were identified in grt2 grt2 1 ree rich ree patterns with eu 1 and grt2 5 lree enriched ree patterns with eu the fractionation of lree with hree and the behavior of eu indicate that the physical chemical conditions during the formation of grt1 1 grt1 2 and grt2 1 grt2 3 were near neutral ph 6 7 and changed to mildly acidic in grt1 3 and grt2 4 grt2 5 ph

fluid evolution of triassic and jurassic w mineralization in the xitian ore field south china constraints from scheelite geochemistry and microthermometry abstract rare earth element ree enriched scheelite is widely distributed in many ore deposits and has been utilized in a broad range of geochronological and geochemical studies in an effort to decipher skarn and quartz vein hosted w mineralization process scheelite from triassic and jurassic granites skarn ores and quartz vein hosted ores in the xitian ore field south china were analyzed for major and minor element content and fluid inclusion microthermometry all scheelite grains show homogeneous backscatter electron and cathodoluminescence response and uniform w and ca distribution the chondrite normalized ree patterns of scheelite from triassic granites to skarn ores show similar pr sm enrichments and negative eu anomalies but different hree contents in contrast the chondrite normalized ree patterns of scheelite from jurassic granites to quartz vein hosted ores changes from flat with slight lree enrichment to concave with remarkable lree depletion the homogenization temperatures and salinities of fluid inclusions from the scheelite in triassic granites and skarn ores are similar both reduce significantly from the jurassic granites to quartz vein hosted ores combined with pressure ph and mineral assemblage analysis we propose that the ree signatures in the ore forming fluids of skarn type w deposits are mainly controlled by crystallization of ree enriched skarn minerals e g garnet diopside epidote the similarity in lree patterns and y ho ratios between the triassic granite and skarn ore scheelite indicates that the skarn ore forming fluid was derived from the triassic granite while the distinct ree patterns and variable y ho ratios in scheelite from jurassic granites and quartz vein hosted ores imply possible fluid mixing for jurassic w mineralization in light of these results ree rich scheelite could serve to discriminate fluid source and document mineralization processes in a complex ore forming system

chemical composition genesis and exploration implication of garnet from the hongshan cu mo skarn deposit sw china abstract in this study we present new mineral chemical data on the garnet from the hongshan cu mo deposit one of the largest skarn deposits in the sanjiang tethyan tectonic domain in sw china the aims are to unravel the formation mechanism of the oscillatory zoned garnet at hongshan and to explore possible applications using garnet trace element to distinguish different types of skarn deposits two generations of garnets i e poorly zoned garnet i grt i and the well zoned garnet ii grt ii are identified from the endoskarn and exoskarn at hongshan based on field and petrographic observations epma and la icp ms analyses show that grt i contains a narrow compositional range adr40 07 61 25 grs36 65 57 08 and chondrite normalized hree enriched and lree depleted patterns with minor positive eu anomalies in contrast grt ii shows a wide compositional range adr27 78 67 53 grs30 67 70 92 to almost pure andradite adr94 41 99 99 grs0 00 4 21 with interlayered al rich and fe rich zones in grt ii its al rich zone is chemically similar to grt i whereas its fe rich zone exhibits chondrite normalized lree enriched and hree depleted patterns with distinct positive eu anomalies the positive ree3 vs mg correlations and the poor ree3 vs y and ca correlations altogether indicate that the ree3 incorporated into garnet was co influenced by the menzerite type substitution mechanism c a 2 1 v i i i r e e 3 1 v i i i a l 3 1 v i m g 2 1 v i fluid chemistry and the physicochemical conditions of precipitation the higher u contents and hree enriched patterns of grt i suggest that it was formed under a relatively reducing and near neutral ph condition varying u contents and ree patterns across different oscillatory zones indicate that grt ii was probably formed under alternating physicochemical fluctuations led by repeating crack seal cycles in the hydrothermal system besides chemical data compilation suggests that the garnet from different types of skarn deposits has different compositions and can be used as mineral prospecting tools we suggested that discrimination plots such as eu vs v and ce vs u can distinguish cu skarn deposit from other skarn deposit types whilst the u vs w plot can discriminate between cu w w sn and w mo skarn deposits

the roza member columbia river basalt group a gigantic pahoehoe lava flow field formed by endogenous processes we present studies on the physical volcanology of the 15 ma roza member of the wanapum formation in the columbia river basalt group the roza member represents a compound pahoehoe flood basalt lava flow field with an area of 40 300 km 2 and a volume of 1300 km 3 it consists of 4 major lava flows each composed of numerous decimeter to kilometer long pahoehoe lobes roza lavas feature a wide range of pahoehoe surface structures such as lava rise plateaus tumuli and surface breakouts and we illustrate that the lava morphology is inconsistent with previous proposals of rapid emplacement for these lavas an integral component of the roza flow field is the sheet lobe with internal structures identical to those of inflated pahoehoe sheet lobes from hawaii and iceland both at the same scale and at much larger scales we identify a three part division of the sheet lobes into basal crust lava core and lava crust which are interpreted as the equivalent to the bottom crust the liquid lava core and the surface crust of an active inflating pahoehoe lobe the upper lava crust grows continuously during lava emplacement and its growth rate can be approximated by conductive cooling this relationship is used to calculate the emplacement time for individual roza sheet lobes and to derive a first order estimate on the duration of the roza eruption the results indicate that the emplacement of individual roza lobes lasted for months to years and that the lava flow field was constructed over a period of at least 14 years we propose that the roza flows achieved great areal dimensions and thicknesses by inflation and endogenous growth as the lava flowed from vent to flow front it traveled under an insulating crust which maintained cooling rates of 0 1 c km and allowed for efficient transport of lava over distances up to 300 km

geochemistry of core sediments along the active channel northeastern indian ocean over the past 50 000 years sources and climatic implications abstract major and trace elements of sediments from two gravity cores 10ydy09 and 12i712 in the northeastern indian ocean have been analyzed for sediment provenance and the relationship of the sediments with climate change since 50 ka the rare earth element ree compositions were analyzed and the sediment contributions were estimated for the major potential sources including the ganges brahmaputra g b godavari krishna g k and mahanadi rivers our results showed that the sediments in the study area were a mixture of sediments discharged from the above three rivers the sediment contribution of the g k rivers increased notably during the holocene when the rainfall increased which may be caused by the strengthening of the east indian coastal current eicc and the resulting large amounts of sediments supplied by the g k rivers the higher sediment contribution from the g b rivers demonstrated the role of the active channel in transporting sediments to the distal bengal fan during the last glacial maximum lgm especially in the late period the indian summer monsoon weakened but the winter monsoon strengthened resulting in a decrease in chemical weathering and elemental enrichment in the northeastern indian ocean conversely when the rainfall was low the chemical weathering strengthened and k2o tio2 ratios were lowered due to the longer weathering time and resulting sediment potassium loss however when the sea level reached a certain height during the holocene the fluvial sediments could not easily enter deep water environments through submarine channels which were well developed in the bengal fan

rates of li diffusion in garnet coupled transport of li and y rees numerical simulation of stranded diffusion profiles in partially resorbed garnet crystals from the aureole of the makhavinekh lake pluton labrador canada yields quantitative rates of intracrystalline diffusion for li in garnet diffusion coefficients for li at 700 900 c and 0 53 gpa are 0 5 1 5 log10 units lower than those for divalent cations in the same samples and match those for y and yb this correspondence likely stems from coupled substitution of li and y or ree ions in the garnet structure the requirement of local electroneutrality limits li mobility to be no faster than that of y rees because of this coupling li zoning in garnet is retained to comparatively high temperatures making garnet a valuable monitor of the behavior of li in deep crustal systems

global color variations on io abstract visible and near infrared images of io from the galileo spacecraft reveal a surface more colorful than previously thought red yellow green white and black hues decorate the satellite presumably caused by a varied composition of sulfur compounds and silicates almost a third of io is covered by red and orange materials particularly at polar latitudes above 30 these red regions were scarcely distinguishable in the shorter wavelength voyager observations bright red pyroclastic deposits mark the locations of many hot spots plumes and visible surface changes providing a prominent flag of recent volcanic activity io s equatorial regions are dominated by yellow materials which occupy about 40 of the satellite s surface white and gray materials cover about 27 of io primarily in equatorial areas and in localized deposits at high latitudes these are identified with moderate to coarse grained so2 as mapped by the nims instrument on galileo r w carlson et al 1997 geophys res lett 24 2479 2482 greenish yellow materials in small isolated spots on io s anti jupiter hemisphere were recently discovered in 3 km pixel color imaging from orbit 14 unlike other ionian terrains these regions have a negative near infrared spectral slope suggesting contamination by a nonsulfur component only about 1 4 of io s surface is occupied by dark materials which display a variety of visible colors ranging from black to red and green most dark spots have a shallow spectral absorption feature at 0 9 m suggesting magnesium rich silicates rather than black sulfur little large scale alteration in the global color and albedo pattern has occurred between the voyager and galileo eras 90 of the surface appears unchanged despite the vigorous volcanic activity which must have taken place in the intervening 17 years this suggests that over a time scale of decades the bulk of io s resurfacing is restricted to a few small but persistently active areas

global color and albedo variations on io abstract three multispectral mosaics of io have been produced from voyager imaging data a global mosaic from each of the voyager 1 and voyager 2 data sets and a high resolution mosaic of the region surrounding the volcano ra patera the mosaics are maps of normal albedo and color in accurate geometric map formats io s photometric behavior mapped with a two image technique is spatially variable especially in the bright white areas the disk integrated color and albedo of the satellite have been remarkably constant over recent decades despite the volcanic activity and the many differences between voyager 1 and 2 images acquired just 4 months apart this constancy is most likely due to the consistent occurrence of large pele type plumes with relatively dark red deposits in the region from long 240 to 360 a transient brightening southeast of pele during the voyager 1 encounter was probably due to real changes in surface and or atmospheric materials rather than to photometric behavior the intrinsic spectral variability of io as seen in a series of two dimensional histograms of the multispectral mosaics consists of continuous variation among three major spectral end members the data were mapped into five spectral units to compare them with laboratory measurements of candidate surface materials and to show the planimetric distributions unit 1 is best fit by the spectral reflectance of ordinary elemental sulfur and it is closely associated with the peletype plume deposits unit 2 is strongly confined to the polar caps above about latitude 50 but its composition is unknown unit 5 is probably so 2 with relatively minor contamination it is concentrated in the equatorial region and near the long lived prometheus type plumes units 3 and 4 are gradational between units 1 and 5 in addition to so 2 and elemental sulfur other plausible components of the surface are polysulfur oxides fecl 2 na 2 s and nahs

a refined garnet biotite fe mg exchange geothermometer and its application in amphibolites and granulites a new formulation of garnet biotite fe mg exchange thermometer has been developed through statistical regression of the reversed experimental data of ferry and spear input parameters include available thermo chemical data for quaternary fe mg ca mn garnet solid solution and for excess free energy terms associated with mixing of al and ti in octahedral sites in biotite solid solution the regression indicates that fe mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withw femg bi 1073 490 cal mol h r and s r for the garnet biotite exchange equilibrium were derived to be 4301 cal and 1 85 cal respectively the resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations

sr bearing zoisite and epidote in ultra high pressure uhp metamorphic rocks from the su lu province eastern china an important sr reservoir under uhp conditions sr bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the su lu ultra high pressure uhp province eastern china the sro content of prograde zoisite and epidote reaches 3 2 wt in crystal cores and generally decreases toward crystal margins retrograde epidote is poorer in sro 0 1 wt preliminary rare earth element ree analyses of epidote give la2o3 up to 2 9 wt ce2o3 5 9 wt and nd2o3 3 0 wt ree contents of zoisite are distinctly lower la 2o3 up to 0 16 wt ce2o3 up to 0 26 wt and nd2o3 up to 0 16 wt than coexisting epidote apatite is always more depleted in sro 0 10 0 59 wt on average than coexisting zoisite and epidote and sr ca partition coefficients for zoisite and epidote and apatite sr ca zo ep ap range from 5 to 20 sro content of k white mica 0 012 0 044 wt is an order of magnitude lower than that of apatite an evaluation of the sro content in zoisite and epidote and their modal abundances in seven samples indicates that 70 of the whole rock sro is contained in these minerals apatite and k white mica are only minor reservoirs for sro in these rocks zoisite and epidote are thus regarded as the most important sr reservoirs at uhp conditions where calcic plagioclase and titanite are unstable

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

a smnd and pb isotope study of archaean greenstone belts in the southern kaapvaal craton south africa abstract a sm nd and pb study on a wide variety of lithologies in archaean greenstone belt fragments in the southern kaapvaal craton reveals a complex petrogenetic history the fragments are important because they represent a 350 km transect through the craton south of barberton to its southern margin the commondale greenstone belt yields a precise sm nd age of 3334 18ma on an exceptionally well preserved peridotite suite of komatiitic affinity the wide range of sm nd from 0 6 to 1 0 is attributed to the unusual occurrence of orthopyroxene in the spinifex bearing rocks a considerably younger age of about 3 2 ga is suggested for the nondweni greenstone belt close to the southern margin of the craton on the basis of separate sm nd isochrons on individual lithologies ranging from komatiite through komatiitic basalt and basalt to felsic volcanic rocks the most precise sm nd age for the greenstone belt of 3208 87ma is based on a combined mineral whole rock isochron for komatiitic basalts but the possibility cannot be excluded that this represents a later metamorphic age on the basis of the present study the greenstone belts appear to have been emplaced at progressively younger ages towards the southern margin of the craton of greater significance is that irrespective of age the individual lithologies of the nondweni sequence have isotopically distinct initial ratios combining isotopic data from these lithologies invariably yields apparent older ages the isotopic characteristics are consistent with a model of progressive contamination by older sialic crust thought to be the basement to the nondweni sequence

ultra high pressure uhp marble and eclogite in the su lu uhp terrane eastern china a large mass of dolomitic marble including many eclogite blocks occurs in orthogneisses of the rongcheng area of the su lu province eastern china the marble consists mainly of dolomite calcite formerly aragonite graphite forsterite diopside talc tremolite and phlogopite aggregates of talc and calcite occur at the boundary between dolomite and diopside tremolite is a reaction product between talc and calcite eclogite blocks are rimmed by dark green amphibolite the primary mineral assemblage in the core of eclogite is na bearing garnet up to 0 2 wt na2o omphacitic pyroxene clintonite and rutile secondary minerals are pargasitic edenitic amphibole plagioclase sodic diopside chlorite zoisite and titanite the peak metamorphic conditions based on stability of the dolomite forsterite aragonite now calcite graphite assemblage under conditions where tremolite is unstable are estimated at t 610 660 c and p 2 5 3 5 gpa for xco 0 001 a reaction between dolomite and diopside to form talc under tremolite unstable conditions indicates a temperature decrease under ultra high pressure conditions p 2 4 gpa xco 0 0013 the formation of secondary tremolite is consistent with a nearly adiabatic pressure decrease post dating the ultra high pressure metamorphism the temperature decrease under ultra high pressure conditions preceding decompression may reflect the underplating of a cold slab and the rapid decompression probably corresponds to the upwelling stage promoted by the delamination of a downwelling lithospheric root the p t conditions of the amphibolitization stage are estimated at 0 9 gpa and 460 c and are similar to conditions recorded by the surrounding orthogneisses

metamorphic evolution mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra high pressure eclogites in the dabieshan of central china as is typical of ultra high pressure uhp terrains the regional extent of the uhp terrain in the dabieshan of central china is highly speculative since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and or diamond stability is very small by contrast the common garnet xmn 0 18 0 45 phengite si 3 2 3 35 zoned epidote ps38 97 biotite titanite two feldspars quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low p t conditions of ca 400 50 c at 4 kbar however certain orthogneiss samples preserve garnets with xca up to 0 50 rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with si contents p f u of up to 3 49 overlapping with the highest values 3 49 3 62 recorded for phengites in samples of undoubted uhp schists these and other mineral composition features such as a site deficiencies in the highest si phengites na in garnets linked to y yb substitution and al f ti 1 o 1 substitution in titanites are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated uhp schists and eclogites re evaluated garnet phengite and garnet biotite fe mg exchange thermometry and calculated 5 rutile 3 grossular 2sio2 h2o 5 titanite 2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction related clockwise p t path with the uhp paragneisses and eclogites through conditions of pmax at ca 690 c 715 c and 36 kbar to tmax at ca 710 c 755 c and 18 kbar prior to extensive re crystallisation and re equilibration of these ductile orthogneisses at ca 400 c 450 c and 6 kbar the consequential conclusion that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these dabieshan orthogneisses with undoubted uhp lithologies has far reaching implications for the interpretation of controversial gneiss eclogite relationships in other uhp metamorphic terrains

a komatiite analog to potential ultramafic materials on io ultramafic volcanism is one model which fits currently available data for some eruptions on jupiter s moon io assuming that such activity does occur it is possible to apply komatiitic analogs and terrestrial ultramafic flow models to io we have used such analogs and models along with galileo data to investigate the nature of ultramafic materials that are assumed to occur at high temperature hotspots on io the unusual komatiites of the commondale greenstone belt south africa are consistent with available galileo data on the temperatures and composition of potential ionian ultramafic materials these komatiites had high sio2 50 wt and mgo 31 wt contents which are inferred to result in high liquidus temperatures 1610 c low dynamic viscosities 0 2 pa s and low densities 2680 kg m and in the crystallization of orthopyroxene phenocrysts not found in other komatiites we used the commondale composition and the model of williams et al 1998 to investigate potential ultramafic flow behavior on io compared to their terrestrial counterparts ionian ultramafic lava flows would have less potential for turbulent flow lower maximum emplacement distances and lower thermal erosion rates because of the lower ionian gravity and their more silicic composition shallow 10 m thermal erosion channels seem less likely on io unless 1 lavas are superheated 2 lavas are more ultramafic 31 mgo 3 eruption rates eruption durations or flow volumes are high or 4 substrates are unconsolidated partly consolidated or volatile rich and thus more erodable if volatiles are present in the lava as vesicles then increasing lava vesicularity results in decreasing maximum emplacement distances decreasing thermal erosion rates and decreasing erosion channel depths for given eruption durations

the summer 1997 eruption at pillan patera on io implications for ultrabasic lava flow emplacement galileo data and numerical modeling were used to investigate the summer 1997 eruption at pillan patera on io this event now defined as pillanian eruption style keszthelyi et al this issue included a high temperature 1600 c possibly ultrabasic 140 km high plume eruption that deposited dark orthopyroxene rich pyroclastic material over 125 000 km2 followed by emplacement of dark flow like material over 3100 km2 to the north of the caldera we estimate that the high temperature energetic episode of this eruption had a duration of 52 167 days between may and september 1997 with peak eruption temperatures around june 28 1997 galileo 20 m pixel 1 images of part of the pillan flow field show a widespread rough pitted surface that is unlike any flow surface we have seen before we suggest that this surface may have resulted from 1 a fractured lava crust formed during rapid low viscosity lava surging perhaps including turbulent flow emplacement 2 disruption of the lava flow by explosive interaction with a volatile rich substrate or 3 a combination of 1 and 2 with or without accumulation of pyroclastic materials on the surface well developed flow lobes are observed suggesting that this is a relatively distal part of the flow field shadow measurements at flow margins indicate a thickness of 8 10 m we have modeled the emplacement of putative ultrabasic flows from the summer 1997 pillan eruption using constraints from new galileo data results suggest that either laminar sheet flows or turbulent channelized flows could have traveled 50 150 km on a flat unobstructed surface which is consistent with the estimated length of the pillan flow field 60 km our modeling suggests low thermal erosion rates 20 m erosion channels was unlikely especially distal to the source we calculate a volumetric flow rate of 2 7 103 m3 s 1 which is greater than those for typical mauna loa kilauea flows but comparable to those for the 1783 laki eruption and the inferred flow rates of the roza flows in the columbia river flood basalts the differences in ultrabasic eruption styles on earth and io appear to be controlled by the different eruption environments plumes at sites of ultrabasic eruptions on io suggest strong magma volatile interactions on a low gravity body lacking an atmosphere whereas the geology at sites of komatiite eruptions on earth suggest mostly submarine emplacement of thick flows with a pronounced lack of subaerial explosive activity

high precision lu hf geochronology of eocene eclogite facies rocks from syros cyclades greece abstract garnet bearing high pressure low temperature hp lt metamorphic rocks of the cycladic blueschist unit have been investigated by lu hf geochronology eclogites from syros island yield precise ages of 52 2 0 3 ma 51 4 0 4 ma and 50 2 ma preserved major and trace element growth zoning in garnet suggests that the closure temperature of the lu hf system in garnet was higher than the estimated peak metamorphic temperature of 500 c hence lu hf ages most likely reflect garnet growth in the studied samples our new lu hf grt ages are in excellent agreement with previously published u pb shrimp ages of metamorphic zircon and the maximum reported white mica rb sr and ar ar ages from syros garnet growth in all samples apparently took place over a narrow time interval because the lu hf ages cluster tightly even though the distribution of lu between cores and rims varies among samples we find no evidence for garnet generations that are significantly older or younger than the given isochron ages this in conjunction with the agreement among the ages of metamorphic zircon the maximum white mica ages and the lu hf ages strongly suggests that the 52 ma age dates a single hp event that affected these rocks

in situ la icp ms study of garnets in the makeng fe skarn deposit eastern china fluctuating fluid flow ore forming conditions and implication for mineral exploration abstract most garnets in fe cu skarn deposits have simple petrographic textures lree enriched geochemical patterns are enriched in fe and are distinguished from garnets of typical w or w mo skarn deposits by their extremely low w sn and mo contents however little is known about the garnets in fe skarn systems that have more complex textures and high concentrations of w sn and mo here we describe diverse textures and la icp ms data for garnets in the largest fe skarn deposit makeng in eastern china and we discuss the dynamics of the mineralizing fluids and the significance of garnet for w mo exploration garnets from the makeng deposit fall into four main types corresponding to four stage garnet growth based on their textural characteristics and trace element contents stage 1 al rich relatively homogeneous garnets adr51 55 with a portion adr17 48 and hree enriched patterns and weak negative eu anomalies stage 2 fe rich garnet with lree enriched or flat patterns and weak negative or positive eu anomalies represented by oscillatory zoned rims adr50 90 with dodecahedral faces and oscillatory zones cores adr54 68 rims adr65 80 with composite dodecahedral trapezohedral faces stage 3 nearly pure andradite garnets with significantly lree enriched patterns and strong positive eu anomalies as represented by irregular zones adr70 99 in oscillatory zoned rims and relatively homogeneous garnets adr95 100 with extremely weak zoning and stage 4 al rich garnet with hree enriched patterns and weak negative or weak positive eu anomalies as represented by garnet veinlets adr45 77 with weak zoning these four stages of garnet were formed by periodic fluctuations in the fluid flux which can be explained by a kinetic dispersion model a low flux responsible for the al rich garnet a high flux responsible for the fe rich garnets the highest flux responsible for the nearly pure andradite garnet and a low flux responsible for the veins of al rich garnet relative to garnet in fe cu skarn deposits all garnet types in the makeng fe skarn deposit are rich in trace elements such as w sn and mo especially the pure andradite garnet in stage 3 which has the highest contents of w sn and mo these elemental enrichments are closely correlated with fe lree hree and fo2 signatures the w and mo signatures are identical to those of garnets in typical w or w mo skarn deposits and they are useful geochemical indicators during exploration for w mo ore deposits

tio2 exsolution from garnet by open system precipitation evidence from crystallographic and shape preferred orientation of rutile inclusions we investigated rutile needles with a clear shape preferred orientation in garnet from ultra high pressure metapelites from the kimi complex of the greek rhodope by electron microprobe electron backscatter diffraction and tem techniques a definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that rt 001 is either parallel to grt or describes cones with opening angle 27 6 around grt each rt 001 small circle representing a cone on the pole figure displays six maxima in the density plots this evidence together with microchemical observations in tem when compared to various possible mechanisms of formation corroborates a precipitate origin a review of exchange vectors for ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system it requires that components are exchanged between the garnet interior and the rock matrix by solid state diffusion a process we refer to as open system precipitation osp the kinetically most feasible reaction of this type will dominate the overall process the perhaps most efficient reaction involves internal oxidation of fe2 to fe3 and transfer from the dodecahedral to the octahedral site just vacated by text ti 4 6 text m 2 3 text tial left text alsi 2 right text o 1 2 6 text m 2 2 5 text tialsi 3 text o 1 2 10 text m 2 3 0 text al 1 8 text fe 0 2 text si 3 text o 1 2 text m 2 2 text e 1 2 text tio 2 osp is likely to occur at conditions where the transition of natural systems to open system behaviour becomes apparent as in the granulite and high temperature eclogite facies

transformation of enstatite diopside jadeite pyroxenes to garnet the high pressure stability of enstatite en diopside di jadeite jd pyroxenes has been investigated experimentally with a split sphere anvil apparatus ussa 2000 on the enstatite pyrope join the compositions of garnet coexisting with enstatite were determined at 100 165 kbar and 1450 1850 c the results indicate complete solubility between enstatite and pyrope in the system cao mgo al2o3 sio2 cmas the compositions of coexisting pyroxenes and garnet were determined at 100 165 kbar and 1250 1750 c at 157 kbar 1650 c garnet with the composition en79di21 mol forms on the en di join in the system na2o mgo al2o3 sio2 nmas the compositions of coexisting pyroxenes and garnet were determined at 60 160 kbar and 1200 1850 c on the en jd join the first garnet has the composition en48jd52 at 135 kbar 1650 c and en53jd47 at 140 kbar 1500 c on the di jd join the first garnet with the composition di63jd37 forms around 170 kbar 1650 c in the en di jd system the first appearance of garnet with the composition en42di9jd49 is estimated at 133 kbar 1650 c the new pyroxene with the composition namg0 5si2 5o6 napx transforms to garnet at 154 kbar 1650 c the experimental results indicate that the transformation of a twopyroxene assemblage to garnet and residual pyroxene in the en di jd system could occur at pressures consistent with the 400 km seismic discontinuity and in a pressure interval of 0 3 kbar

lava fountains from the 1999 tvashtar catena fissure eruption on io implications for dike emplacement mechanisms eruption rates and crustal structure the first direct evidence that patterns of basaltic eruptive activity may be very similar on io and earth was seen on november 26 1999 when the solid state imaging system on board the galileo spacecraft obtained high resolution images of a fissure eruption taking place in one of the calderas of the tvashtar catena complex we analyzed the dynamics of the fissure eruption from the morphology of the lava fountains and pyroclastic deposits and derived an estimate of the eruption rate our best estimate of the volume eruption rate per unit length of fissure lies in the range 0 7 to 7 m3 s 1 m 1 and multiplying this rate by the fissure length of 25 km yields a total volume eruption rate of magma of 2 104 to 2 105 m3 s 1 the upper end of the inferred eruption rate is very close to the upper end of the range of eruption rates observed or inferred for fissure eruptions on earth theoretical models of the possible dike systems feeding the fissure show that for eruptions of positively buoyant magma the elastic properties of rocks are such that a magma flux of 6 to 7 m3 s 1 m 1 is the largest value that can ever be realized in such eruptions this limiting value being independent of planetary gravity and thus the same on all silicate bodies the eruption rate inferred for tvashtar can also be achieved by a negatively buoyant magma but in that case the source depth cannot be much greater than a few tens of kilometers we find that some eruptive events are consistent with the effusion of magmas which are buoyant relative to the crust whereas others appear to involve magmas which are not this implies that the distribution of crustal volatiles is laterally very inhomogeneous our analyses suggest that the depths from which magmas make their final ascent to the surface of io are generally of order tens rather than hundreds of kilometers

evidence for intracontinental thrust related exhumation of the ultra high pressure rocks in china coesite and diamond bearing ultra high pressure metamorphic rocks produced during the triassic continental collision between the sino korean and yangtze plates are offset by about 530 km along the sinistral tanlu fault in eastern china the tanlu fault terminates abruptly south of the ultra high pressure metamorphic terrain in dabie shan and does not extend into the foreland fold belt along the yangtze river this suggests that in this region the mesozoic major sinistral displacement along the tanlu fault was transferred to southward directed thrusting which led to the exhumation of the ultra high pressure metamorphic rocks formed earlier postcollisional himalayan type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in china

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

development of the 1990 kalapana flow field kilauea volcano hawaii the 1990 kalapana flow field is a complex patchwork of tube fed pahoehoe flows erupted from the kupaianaha vent at a low effusion rate approximately 3 5 m3 s these flows accumulated over an 11 month period on the coastal plain of kilauea volcano where the pre eruption slope angle was less than 2 the composite field thickened by the addition of new flows to its surface as well as by inflation of these flows and flows emplaced earlier two major flow types were identified during the development of the flow field large primary flows and smaller breakouts that extruded from inflated primary flows primary flows advanced more quickly and covered new land at a much higher rate than breakouts the cumulative area covered by breakouts exceeded that of primary flows although breakouts frequently covered areas already buried by recent flows lava tubes established within primary flows were longer lived than those formed within breakouts and were often reoccupied by lava after a brief hiatus in supply tubes within breakouts were never reoccupied once the supply was interrupted during intervals of steady supply from the vent the daily areal coverage by lava in kalapana was constant whereas the forward advance of the flows was sporadic this implies that planimetric area rather than flow length provides the best indicator of effusion rate for pahoehoe flow fields that form on lowangle slopes

constraining the potential temperature of the archaean mantle a review of the evidence from komatiites the maximum potential temperature of the archaean mantle is poorly known and is best constrained by the mgo contents of komatiitic liquids which are directly related to eruptive temperatures however most archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid relatively fresh lavas from the saskmar suite belingwe greenstone belt zimbabwe 2 7 ga include chills of 25 6 wt mgo and olivines ranging to fo93 6 implying eruption at around 1520 c a chill sample from alexo township ontario also 2 7 ga is 28 wt mgo and associated olivines range to fo94 1 implying eruption at 1560 c however inferences of erupted liquids containing 32 33 wt mgo from lavas in the barberton greenstone belt south africa 3 45 ga and from the perseverance complex western australia 2 7 ga may be challenged on the grounds that they contain excess cumulate olivine or were enriched in mg during alteration or metamorphism re interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt mgo corresponding to an eruption temperature of 1580 c this composition is the highest liquid mgo content of an erupted lava that can be supported with any confidence the hottest modern magma on gorgona island 0 155 ga contained 18 20 mgo and erupted at circa 1400 c if 1580 c is taken as the temperature of the most magnesian known eruption then the source mantle from which the liquids rose would have been at up to 2200 c at pressures of 18 gpa corresponding to a mantle potential temperature of 1900 c these temperatures are in excess of the mantle temperatures predicted by secular cooling models and thus komatiites can only be formed in hot rising convective jets in the mantle this result requires that archaean mantle jets may have been 300 c hotter than the archaean ambient mantle temperature this temperature difference is similar to the 200 300 c temperature difference between present day jets and ambient mantle temperatures an important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions

emplacement and erosion by archean komatiite lava flows at kambalda revisited we have developed a mathematical model to evaluate the flow and erosional potential of submarine channelized komatiite lavas at kambalda western australia field data from kambalda were used to constrain the choice of important input parameters and model results were compared with data from field studies and geochemical analyses our results suggest that thermal erosion is strongly dependent upon the nature and behavior of the substrate if the substrate is treated as an unconsolidated hydrous sediment that can be fluidized by vaporized seawater then our model predicts that an initially 10 m thick basal kambalda komatiite lava could have produced very high thermo mechanical erosion rates 23 10 m day crustal thicknesses of 5 20 cm at distances of 5 35 km from the source and a high degree of lava contamination 3 12 in contrast if the substrate is treated as a more consolidated anhydrous sediment that could not be fluidized then our model predicts that a kambalda komatiite flow would have had much lower thermal erosion rates 1 2 0 4 m day and degrees of contamination 1 3 and would have had crustal thicknesses of 5 20 cm at longer flow distances of 30 165 km field constraints are generally consistent with our predictions for a non fluidized substrate the reentrant embayments at kambalda are thought to form from either erosion of deep channels in a flat basaltic seafioor huppert et al 1984 or erosion of a thin 5 m sediment with minor undercutting of basalt in pre existing topography lesher et al 1984 our modeling indicates that the former was possible for long eruption durations months whereas the latter was possible for short eruption durations 2 weeks as the latter hypothesis is more consistent with the existing field evidence for thermal erosion at kambalda we believe it is the preferred interpretation

prometheus io s wandering plume unlike any volcanic behavior ever observed on earth the plume from prometheus on io has wandered 75 to 95 kilometers west over the last 20 years since it was first discovered by voyager and more recently observed by galileo despite the source motion the geometric and optical properties of the plume have remained constant we propose that this can be explained by vaporization of a sulfur dioxide and or sulfur snowfield over which a lava flow is moving eruption of a boundary layer slurry through a rootless conduit with sonic conditions at the intake of the melted snow can account for the constancy of plume properties

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

io in the near infrared near infrared mapping spectrometer nims results from the galileo flybys in 1999 and 2000 galileo s near infrared mapping spectrometer nims observed io during the spacecraft s three flybys in october 1999 november 1999 and february 2000 the observations which are summarized here were used to map the detailed thermal structure of active volcanic regions and the surface distribution of so2 and to investigate the origin of a yet unidentified compound showing an absorption feature at 1 m we present a summary of the observations and results focusing on the distribution of thermal emission and of so2 deposits we find high eruption temperatures consistent with ultramafic volcanism at pele such temperatures may be present at other hot spots but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations loki is the site of frequent eruptions and the low thermal emission may represent lavas cooling on the caldera s surface or the cooling crust of a lava lake high resolution spectral observations of emakong caldera show thermal emission and so2 within the same pixels implying that patches of so2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another thermal maps of prometheus and amirani show that these two hot spots are characterized by long lava flows the thermal profiles of flows at both locations are consistent with insulated flows with the amirani flow field having more breakouts of fresh lava along its length prometheus and amirani each show a white ring at visible wavelengths while so2 distribution maps show that the highest concentration of so2 in both ring deposits lies outside the white portion visible measurements at high phase angles show that the white deposit around prometheus extends into the so2 ring this suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction so2 mapping of the chaac region shows that the interior of a caldera adjacent to chaac has almost pure so2 the deposit appears to be topographically controlled suggesting a possible origin by liquid flow

a reassessment of the emplacement and erosional potential of turbulent low viscosity lavas on the moon we have reevaluated the role of thermal erosion by low viscosity lunar lavas as a mechanism for the formation of the lunar sinuous rules we have adapted the model of williams et al 1998 and used the compositions of an apollo 12 basalt and a terrestrial komatiitic basalt to investigate the compositional and environmental effects on the flow behavior of low viscosity lavas on the moon and the earth our model predicts that lunar lavas could have erupted as turbulent flows that were capable of flowing hundreds of kilometers on a sufficiently flat unobstructed substrate these results are consistent with previous studies modeling of lavas over a substrate of the same composition shows that thermal erosion rates would have been low 10 cm d 1 as a result long duration eruptions approximately months to years would have been required to incise deep tens to hundreds of meters channels partial melting and mechanical removal of the substrate a mechanism suggested by hulme 1973 to enhance erosion only slightly increases thermal erosion rates other factors such as higher flow rates or lava superheating could have produced deep rules by thermal erosion during shorter duration eruptions a superheated lunar lava not only would have had a higher erosion rate 40 cm d 1 but also would have remained uncrusted for tens of kilometers which is consistent with the open channel morphology of most sinuous rules for lunar lavas with large volatile i e vesicle contents the presence of vesicles would have tended to increase viscosity at low strain rates resulting in shorter turbulent flow distances lower thermal erosion rates and thus shallower erosion channel depths for given eruption durations