chemical geodynamics in the back arc region of japan based on the trace element and srnd isotopic compositions 143nd144nd 87sr86sr trace and major element results are reported for cenozoic alkaline basalts and associated tholeiitic basalts from southwestern japan and some from korea and china most of the samples plot within the mantle array with nd values ranging from 5 0 to 3 2 and 87sr86sr ratios ranging from 0 7037 to 0 7057 the isotopic variations observed along traverse sections of alkaline basalts from the trench to the continental sides of southwestern japan do not suggest a direct relationship between the subduction of the philippine sea and pacific plates and the generation of alkaline basalt magmas however in southwestern japan the alkaline basalts do show evidence of a subducted component and are more enriched in the mobile incompatible elements such as rb ba and k relative to chinese and korean basalts negative correlations between nd and sm nd and between nd and la th as well as a positive correlation between 87sr86sr and la th are recognized in some of the basalts these relationships indicate at least two component magma mixing between 1 magma resulting from a small melting from a depleted mantle and 2 magma resulting from a relatively large amount of melting of an enriched mantle source it is suggested that the cenozoic alkaline basalts from southwestern japan korea and china are product of interaction between a morb type mantle and a deep mantle plume if the mantle plume has ascended from the upper mantle lower mantle or core mantle boundary the subducted pacific and philippine sea plates could not have extended to southwestern japan had they done so they would have arrested before they reached the melting zone the ascent of the plumes that produced the alkaline volcanism in southwestern japan

trends of sr and nd isotopes through time near the japan sea in northeastern japan nd and sr isotopic analyses were made on selected volcanic rocks of known stratigraphic age from a limited area in the green tuff region northeastern japan and in adjacent areas two ultramafic inclusions from ichinomegata were also analyzed the volcanic rocks range in age from 27 to 6 ma and vary in e nd from 0 9 to 5 7 and in e sr from 14 1 to 16 3 the isotopic data plot generally along the region of the mantle array when plotted against time the isotopic ratios are found to be a regular function of geologic age the results indicate that 1 the volcanic rocks in the area are part of a related sequence or evolutionary pattern over a time period of 27 ma 2 the evolutionary pattern goes from an initially enriched source to that of a very depleted source the shift in isotopic composition appears to reflect the ongoing lining of magma conduits or the decreasing extraction of lower crustal contributions with increasing infiltration of oceanic magma types in an environment where the crust is thinning based on the geotectonic development of this area due to the back arc spreading of the japan sea we identify two different stages the pre opening stage 15 ma and the post opening stage 15 ma the volcanic rocks of the pre opening stage are characterized by an enriched isotopic signature and those of the post opening era by a depleted signature the host rocks of the kuroko ore deposit lie within this evolutionary trend and contain a substantial amount of continentally derived materials processed through an oceanic environment in general we infer that the degree of continental crust involvement is high during the pre opening stages of a back arc basin when it is associated with a rifted continental mass

a kinematic subduction model for the genesis of back arc low k volcanoes at a two overlapping subduction zone central japan another volcanic front originated from the philippine sea plate subduction abstract systematic geochemical and volumetric variations have been documented across many island arcs volcanism in the shinetsu highland the northeastern part of central japan shows various anomalies in such variations though located in a subduction zone in this highland the distribution density of volcanoes and total volume of erupted materials decrease from the volcanic front toward the back arc side however they increase again in the more distant back arc area lavas from this back arc area characterized by a low k nature are equivalent to those from the trench side of northeastern japan this volcanism occurred since 1 7 ma taking the place of the usual back arc type high k volcanism furthermore in the fore arc region of this highland the volcanic front had retreated about 20 km toward the back arc side during 4 3 ma presently the depth to the wadati benioff zone beneath the volcanic front is 20 30 km deeper than that beneath the main part of northeastern japan to explain these features a kinematic subduction model is proposed here which assumes the northward subduction of the philippine sea phs plate over the previously subducting pacific pac plate in this model the retreat of the pliocene volcanic front in the fore arc region is considered to have resulted from the migration of a high temperature region of the wedge mantle 1400 c toward the back arc side by the sweeping effect of the subducting phs plate at about 4 3 ma generation of the low k volcanism in the back arc side of this highland is interpreted to have been caused by the increase of the supply rate of h 2 o related to the dehydration of amphibole and chlorite dragged by subduction of the phs plate which occurred slightly before 1 7 ma

mantle contamination under akagi volcano japan as inferred from combined sr o isotope relationships the northeast japan arc is associated with westward subduction of the pacific plate beneath the eurasian plate at the southernmost region of the arc the philippine sea plate subducts northward into the mantle wedge between the eurasian plate and the pacific slab akagi volcano located at this region yields volcanic rocks with combined sr o isotope relationships which are explained in terms of the mixing of the mantle material from the mantle wedge and deep ocean and or continent derived sediments conveyed with the subducting philippine sea plate this implys that the philippine sea plate continues to the location just under akagi volcano although the seismic philippine sea plate does not reach just under the volcano

nd and sr isotopic variations in acidic rocks from japan significance of upper mantle heterogeneity initial nd and sr isotopic ratios have been measured for cretaceous acidic and related intermediate rocks 24 volcanic and two plutonic rocks from the inner zone of southwest japan izswj to investigate the genesis of acidic magmas the initial nd and sr isotopic ratios for these rocks show three interesting features 1 end values for acidic rocks 2 to 9 are negatively correlated with esr values 10 to 90 together with those for intermediate rocks end 3 to 8 esr 0 to 65 2 the end values for silica rich rocks 60 sio2 correlate with the longitude of the sample locality decreasing from west to east in a stepwise fashion four areas characterized by uniform end values are discriminated 3 low silica rocks 60 sio2 in a certain area have distinctly different end values from those of the high silica rocks in the same area

strontium isotope variations in lower tertiary quaternary volcanic rocks from the kurile island arc average 87sr 86sr ratios for lavas from quaternary and pleistocene volcanoes of the kurile island arc nw pacific decrease from 0 7035 in the south to 0 7032 in the north the northern kuriles are characterised by k2oricher volcanics and by an older crust varying ratios show no simple relation to crustal thickness or geochemical indicators of crustal contamination this is thought to reflect the immature character of the crust its simatic composition low rb sr ratios and youthfulness older lavas from the kuriles lower tertiary miocene have similar or slightly higher 87sr 86sr ratios some have suffered slight alteration and possibly crustal contamination quaternary volcanics from the kurile and aleutian arcs have the lowest 87sr 86sr ratios of all circum pacific arcs and this may be ascribed to a the isotopic individuality of the landward north american plate and or b the high degree of mechanical coupling between the pacific and north american plates reducing the amount of subducted 87sr rich sediments and seawater an isotopic boundary between island arcs is located in central hokkaido the primary basaltic magmas of the kuriles were derived from mantle recently contaminated by radiogenic sr subsequent fractionation to andesites and dacites occurred by closed system fractional crystallization

tholeiitic and calc alkaline magma series at adatara volcano northeast japan 1 geochemical constraints on their origin abstract low alkali island arc by jakes and gill 1970 or low k by gill 1978 tholeiitic rocks are closely associated with calc alkaline rocks in the outer volcanic zone called the nasu volcanic zone of northeast japan major element trace element ni cr co sc v zr nb zn rb cs ba sr ce and y and sr isotope compositions are presented for 9 low alkali tholeiitic and 16 calc alkaline samples of adatara volcano belonging to the nasu volcanic zone in the trace elements vs sio 2 diagram the tholeiitic series and calc alkaline series show different trends to each other for co sc v zr rb sc ba and sr as expected from k 2 o contents the calc alkaline samples are enriched in incompatible elements such as rb cs ba ce and zr compared with the tholeiitic samples on the other hand contents of nb in the calc alkaline samples are somewhat lower than those in the tholeiitic samples plots of ni and cr contents vs sio 2 for the tholeiitic samples come near smooth trend lines whereas those for the calc alkaline samples are scattered sr isotopic ratios in the tholeiitic series lie within the range between 0 7056 and 0 7058 and between 0 7048 and 0 7052 in the calc alkaline series smooth major and trace element compositional trends in the tholeiitic series are successfully modelled by simple fractional crystallization of plagioclase hypersthene augite titanomagnetite and minor olivine similarly crystal fractionation models can closely reproduce major and most of the trace element variations in the calc alkaline series but failed to account for irregular variations of ni and cr the scattering of ni and cr probably reflects simultaneous crystal fractionation and subordinate magma mixing in the subvolcanic chamber the inferred fractionated phases for the calc alkaline series are richer in augite and titanomagnetite and slightly poorer in plagioclase than those for the tholeiitic series the tholeiitic magma and calc alkaline magma are not derived from a common parental tholeiitic magma and the calc alkaline parental magma is richer in sio 2 and incompatible elements in particular rb and zr poorer in radiogenic sr and possibly more oxidized than the tholeiitic parental magma

acid mine drainage at cerro rico de potosi i unabated high strength discharges reflect a five century legacy of mining intensive mining and processing of ag sn pb and zn ores have occurred in various locations within and around the city of potosi bolivia since 1545 surface and subsurface waters stream sediments and soils are contaminated with various ecotoxic metals in the headwaters of the economically vital yet highly impacted upper rio pilcomayo watershed previous studies have documented downstream trace metal contamination however not addressed specific sources the amd discharges identified in this study help link downstream pollution to primary origins the majority of amd would be considered high strength due to metal concentrations and acidity orders of magnitude greater than typical amd discharges from both operating and abandoned portals as well as tailings related deposits displayed a high degree of heterogeneity with total metal concentrations ranging from 0 11 to 7480 0 022 to 889 0 0006 to 65 3 0 001 to 310 0 12 to 72 100 0 3 to 402 0 012 to 34 8 and 0 24 to 19 600 mg l of al as cd cu fe mn pb and zn respectively net acidity and ph ranged from 10 to 246 000 mg l as caco3 equivalent and 0 90 6 94 standard units respectively data were gathered during two sampling events centered around the most extreme periods of the dry and wet seasons of one water year loadings to local streams were marginally greater for most metals in the wet season if observed loadings are historically representative cerro rico amd has contributed thousands of tonnes of ecotoxic metals to the upper rio pilcomayo over the last five centuries metal and hydrogen ion concentrations in the majority of amd sampled were several orders of magnitude above discharge limits set by the bolivian government yet no action has historically or contemporarily been taken

effect of the volcanic front migration on helium nitrogen argon and carbon geochemistry of hydrothermal magmatic fluids from hokkaido volcanoes japan abstract active volcanoes from hokkaido are distributed along the sw ne trending volcanic front where the subducting pacific plate changes direction from n s to wsw ese one exception is the tokachi volcano which is located approximately 50 km to the north west of the current volcanic front the tokachi oki slab cracking zone represents the transitional zone between the honshu and kurile trenches this change is responsible for the rapid lateral variation of the subduction angle and slab shape we investigate the consequence of this change on the chemical and isotopic compositions from hydrothermal magmatic fluids and mineral separates collected from three volcanoes of hokkaido northern japan namely tarumae tokachi and meakan our data show that these volcanoes have a distinct range of he n and c isotopic compositions meakan fumaroles have a 3he 4he ratio that ranges from 4 46ra to 6 88ra 15n from 2 2 to 3 5 and 13c co2 from 8 47 to 9 81 consistent with subducted sediment contribution tarumae fumaroles have a particularly low 3he 4he ratio 3 08ra in comparison with their hot springs up to 5 24ra and are associated with high co2 3he ratios 2 2 5 3 1011 hydrothermal crustal interactions from the dome or from a deeper hydrothermal system are the main mechanisms that control the 3he 4he and co2 3he ratios in tarumae fumaroles the 15n and 13c co2 values for tarumae volcano range from 0 6 to 3 1 and from 6 34 to 6 43 respectively tokachi fumaroles are characterized by high 3he 4he up to 7 38ra low 15n 2 4 0 1 and low 13c co2 up to 5 32 consistent with mantle contribution and minor subducted sediment contribution the limestone sediment mantle ratio shows a decreasing contribution from subducted sediments starting from tokachi 1 9 to meakan 3 4 and finally tarumae 5 3 this change of isotopic compositions from the sw ne of hokkaido is the result of the tokachi oki slab cracking zone slab dehydration under tokachi occurs at a deeper level 150 200 km than in other hokkaido volcanoes 100 km which produces the migration of the volcanic front therefore we argue that the isotopic signature of tokachi is the result of i a lower sediment contribution from the slab ii a deeper dehydration of the slab and iii a lower degree of partial melting

geographical distribution of 3 he 4 he ratios in the chugoku district southwestern japan we have collected 34 hot spring and mineral spring gases and waters in the chugoku and kansai districts southwestern japan and measured the 3he 4he and 4he 20ne ratios by using a noble gas mass spectrometer observed 3he 4he and 4he 20ne ratios range from 0 054 ratm to 5 04 ratm where ratm is the atmospheric 3he 4he ratio of 1 39 10 6 and from 0 25 to 36 8 respectively they are well explained by a mixing of three components mantle derived radiogenic and atmospheric helium dissolved in water the 3he 4he ratios corrected for air contamination are low in the frontal arc and high in the volcanic arc regions which are consistent with data of subduction zones in the literature the geographical contrast may provide a constraint on the position of the volcanic front in the chugoku district where it was not well defined by previous works taking into account the magma aging effect we cannot explain the high 3he 4he ratios of the volcanic arc region by the slab melting of the subducting philippine sea plate the other source with pristine mantle material may be required more precisely the highest and average 3he 4he ratios of 5 88 ratm and 3 8 1 6 ratm respectively in the narrow regions near the volcanic front of the chugoku district are lower than those in kyushu and kinki spot in southwestern japan but close to those in ne japan this suggests that the magma source of the former may be related to the subduction of the pacific plate in addition to a slight component of melting of the philippine sea slab

lithium boron and lead isotope and trace element systematics of quaternary basaltic volcanic rocks in northeastern japan mineralogical controls on slab derived fluid composition abstract across arc li b pb isotope systematics together with trace element geochemistry and phase relationships in subducting slabs are used to understand the petrogenesis of lavas from the northeastern japan arc pb nb ratios and pb isotope compositions of the arc lavas decrease with depth to slab whereas li y u nb th nb and la sm show an upward profile across the arc with peaks at 180 km depth to the wadati benioff zone wbz the peaks in these profiles may reflect the influence of lawsonite breakdown in the slab at 180 km depth b nb ratios of the arc lavas do not show clear across arc variation and the 11b values show a concave upward across arc profile with minimum values at around 180 km in depth of the wbz these results possibly reflect the decomposition of tourmaline at 180 km our results suggest that accessory minerals and minor phases in the subducting slab play important roles in determining the chemical compositions of fluid released by dehydration and the resulting trace element and isotope composition of arc lavas li isotope compositions of the lavas are morb like and do not show systematic across arc variation contrasting with izu arc lavas which show maximum 7li values at the volcanic front and a systematic decrease towards the back arc the strikingly different results for the northeastern japan arc and the izu arc are likely to be caused by differing extents of li isotope fractionation due to differing subduction zone physical characteristics such as thermal structure of the subducting slab and subduction angle thus not only chemical characteristics of the subducting slab but also the physical as well as chemical characteristics of the subduction zone could affect the li isotope compositions of subducting slabs producing heterogeneity in the li isotope composition of the mantle

geochemical features of collision related volcanic rocks in central and eastern anatolia turkey in central and eastern anatolia volcanism related to continental collision occurred in neogene to quaternary times major and trace element compositions 87sr86 sr and k ar ages were determined for volcanic rocks from selected volcanoes in this region 87sr86 sr for basalts from erciyes volcano in the eastern part of central anatolia as well as ararat nemrut and karacadag volcanoes in eastern anatolia are in the range 0 7035 0 7040 these ratios are higher than those of n type morb but overlap arc volcanic rock values this suggests that the mantle source which was possibly contaminated by fluids and melts with higher87sr86 sr during the pre collisional events has similar sr isotopic compositions in wide areas of the eastern part of central anatolia and eastern anatolia even across the collision plate boundary in addition chemical compositions of the mantle source are not homogeneous with respect to hfs element contents indicating that the slab derived components resulting from past subductions are not homogeneously distributed the mantle source beneath the western part of central anatolia which corresponds to the transitional region from collision tectonics in the east to subduction tectonics in the west has different chemical and sr isotopic signatures from other parts of anatolia in the western part of central anatolia the87sr86 sr of basaltic rocks from three regions characterized by monogenetic volcanoes are in the range 0 70427 to 0 70581 and differ from region to region in spite of indistinguishable chemical compositions this indicates either source heterogeneity on a small scale or different contributions of local crustal contaminants calc alkaline lavas from ararat and kars in eastern anatolia have chemical compositions with subduction signatures inherited from pre collision subduction events in contrast the chemical and sr isotopic features of volcanic rocks from erciyes volcano in central anatolia indicate that the combined process of fractional crystallization of basaltic magma and crustal assimilation without injection of slab derived components is the dominant process generating calc alkaline andesite under the collision tectonics regime operating in this region

geochemistry of quaternary basaltic lavas in the norikura area central japan influence of the subcontinental upper mantle on the trace elements and sr isotope compositions abstract the trend of across arc major and trace element compositions and 87sr 86sr ratios for quaternary lavas from the western part of central japan are different from those from ne japan basalts from the norikura area located at the backarc side of the western part of central japan are characterized by lower k zr and higher 87sr 86sr 0 705 0 706 than those of the backarc side of ne japan beneath the norikura area the pacific plate is subducting at the depth where dehydration of phlogopite in the down dragged peridotite occurs through dehydration fluid is added to the mantle wedge which consists of a n morb like source triggering partial melting and producing uprising mantle diapirs these diapirs have high k zr and low 87sr 86sr as indicated by basalts in the backarc side of ne japan subcontinental mantle materials which have low lil hfs and high 87sr 86sr exist in the uppermost portions of the mantle wedge beneath the norikura area and interact geochemically with the uprising diapirs the geochemical characteristics of the norikura area may be attributed to the involvement of the subcontinental upper mantle

acid mine drainage at cerro rico de potosi ii severe degradation of the upper rio pilcomayo watershed ag pb sn and zn ores have been intensively mined and processed at cerro rico de potosi bolivia since 1545 acid mine drainage amd and mineral processing plant effluent are prime sources of water contamination in the headwaters of the upper rio pilcomayo watershed streams receiving amd drainage from the slopes of cerro rico and surrounding landscapes were sampled during the dry july august 2006 and wet march 2007 seasons of one water year in stream waters contained total metal concentrations of up to 16 mg l as 4 9 mg l cd 0 97 mg l co 1 100 mg l fe 110 mg l mn 4 1 mg l pb and 1 500 mg l zn with ph ranging from 2 8 to 9 5 amd impacted streams contained elevated concentrations of the same major ecotoxic constituents present in amd discharges at concentrations orders of magnitude greater than in those streams unimpacted by amd many of the amd impacted water bodies are more degraded than class d of the bolivian receiving water body criteria rendering them unfit for domestic or agricultural use natural attenuation is insufficient to render waters safe for use however some of these waters are currently being utilized for irrigation and livestock watering the data indicate that historic and current mining activities have transformed these key natural resources into potential human and environmental health hazards

unabated acid mine drainage from cerro rico de potosi bolivia uncommon constituents of concern impact the rio pilcomayo headwaters intensive mining and processing of the polymetallic sulfide ore body of cerro rico de potosi bolivia has occurred since 1545 leading to severe degradation of surface and subsurface waters stream sediments and soils at the headwaters of the economically vital yet highly impacted rio pilcomayo previous studies have documented extremely elevated concentrations of a limited suite of metals in local waterways from acid mine drainage amd terrestrial and in stream tailings and ore processing plant discharges however contamination from a wider variety of ecotoxic metals metalloids was considered likely due to the highly mineralized polymetallic nature of the ore body to screen for this broader range of ecotoxic elements in amd and receiving streams data were gathered during two sampling events timed for the most extreme periods of the dry and wet seasons of one water year concentrations of ag b ba mo sb se sn and v in amd and receiving streams were greater than bolivian discharge limits and receiving water body guidelines as well as international agricultural use standards locally results indicate that contamination from mining cerro rico has a larger scope than previously thought and underscore the importance of remediation globally the results raise the possibility that other mining regions could have unquantified hazards from overlooked ecotoxic elements and that screening for a broader range of contaminants may be warranted

constraints on the evolution of the japan sea based on 40ar 39ar ages and sr isotopic ratios for volcanic rocks of the yamato seamount chain in the japan sea 40 ar 39 ar and sr isotope analyses were performed on basalts and andesites dredged from the yamato seamount chain in the japan sea the 40 ar 39 ar plateau ages range from about 11 to 17 ma though most samples show ages between 10 and 14 ma the seamounts seem to have formed within a period of a few million years although some of them might have formed earlier based on the present results together with previously reported radiometric age data it is thought that the yamato basin formed during some period prior to 17 ma and probably later than around 25 ma taking into account the radiometric age data on rocks from the japan basin it is conjectured that the opening of the japan sea might have started almost at this time or a little earlier the observed 87 sr 86 sr ratios range from 0 70357 to 0 70388 suggesting incorporation of some time integrated components enriched in incompatible elements such as continental crustal materials this may indicate that in the japan sea area at least the yamato basin had not developed enough to show the characteristics of typical n type morb source materials without being affected by pre existing continental crustal materials

probable mineralogic control on the mantle metasomatic fluid composition beneath the northeast japan arc geochemical data are compiled for basalts sio2 30 from the nasu zone 17 tholeiites at the trench side and the chokai zone 2 high alumina and calc alkalic basalts at the back arc side northeast japan arc the source peridotites of these basalts above the subducted pacific plate have large lil large ion lithophile hfs high field strength element ratios but are differentially enriched in individual lil elements the chokai basalt sources are distinctly more enriched in k and rb relative to sr ce and zr and lower in k rb than the nasu tholeiite sources a phlogopite component appears to have been important in metasomatic fluids related to the chokai basalts which were generated at relatively great depths 140 km deep on the other hand fluids high in sr and k rb and low in k sr and rb sr which are probably generated by dehydration of amphibole at a relatively shallower level 80 km deep enrich the nasu tholeiite sources these metasomatic fluids derived ultimately from the subducted pacific crust are before added to the magma sources probably trapped at the base of the mantle wedge descending by induced convection

geographical distribution of 3he 4he ratios in japan implications for arc tectonics and incipient magmatism the 3he 4he and 4he 20ne ratios of 115 natural gas samples including various types of gases from volcanoes hot springs mineral springs water wells petroleum fields and natural gas fields were measured using a mass spectrometer the observed 3he 4he and 4he 20ne ratios range from 7 47 10 8 to 9 65 10 6 and from 0 26 to 1100 respectively the 3he 4he ratios reflect well the geotectonic structure of the japanese islands in northeastern ne japan there is a clear geographical difference in the 3he 4he ratios between the frontal arc forearc and volcanic arc back arc regions lower ratios were found in the trench side region and higher ratios in the back arc side this result suggests that the mantle derived helium in the volcanic arc region is associated with the diapiric uprise of a magma lower 3he 4he ratios in the frontal arc region may be due to radiogenic he produced by radioactive decay of u and th in the crustal and sedimentary rocks in southwestern sw japan there is no clear geographical contrast in the 3he 4he ratios some samples in the frontal arc region show quite high 3he 4he ratios the tendency of the 3he 4he ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from ne japan the high 3he 4he ratios observed in the frontal arc region in sw japan may be indicative of renewed or incipient magmatism due to a descending young and warm slab

beryllium isotopic systematics in island arc volcanic rocks from northeast japan implications for the incorporation of oceanic sediments into island arc magmas abstract we present beryllium be isotopic ratios 10 be 9 be for 55 volcanic rocks from 21 quaternary volcanoes in the northeast japan arc including the hokkaido and central japan areas the nejh although be isotopic ratios in these rocks are much lower than in other arcs reported in the literature they are much higher than in control samples ocean island basalt oib and tertiary basalts that have no recent addition of 10 be hence our results indicate the incorporation of a component derived from subducted sediments in the japan arc magmas two kinds of be isotopic variations were observed in the nejh 1 variations among lava flows of different ages in the same volcano and 2 variations among volcanoes in an along arc or cross arc direction in the first case there are no apparent differences in be isotopic ratios for lavas of the same age but relatively large variations among lavas of different ages and the degree of variation depends on the regional characteristics indicating that regional differences in near surface conditions may affect these be isotopic variations in the second case the progressive decrease in be isotopic ratios in a cross arc direction suggests the continuous incorporation of subducted sediment even in the back arc region the regional along arc be isotopic variations reflect not only the incorporation of sediments into the arc magma but also regional tectonic and or geological and geochemical processes this is the first systematic study of be isotopic systems in the ne japan arc and we provide direct evidence for the incorporation of sedimentary components from the subducting slab into the arc magma including in the back arc region at 300 km from the volcanic front where the wadati benioff zone is at a depth of 290 km

lithospheric contributions to arc magmatism isotope variations along strike in volcanoes of honshu japan major chemical exchange between the crust and mantle occurs in subduction zone environments profoundly affecting the chemical evolution of earth the relative contributions of the subducting slab mantle wedge and arc lithosphere to the generation of island arc magmas and ultimately new continental crust are controversial isotopic data for lavas from a transect of volcanoes in a single arc segment of northern honshu japan have distinct variations coincident with changes in crustal lithology these data imply that the relatively thin crustal lithosphere is an active geochemical filter for all traversing magmas and is responsible for significant modification of primary mantle melts

long term eruption rates and dimensions of magma reservoirs beneath quaternary polygenetic volcanoes in japan abstract long term eruption rates of 18 quaternary volcanoes in japanese islands are estimated to range from 5 to slightly greater than 0 05 km 3 10 3 y the long term rate correlates well with the variation of phenocryst assemblage which reflects the crystallization temperature the long term eruption rate and temperature of magma from the volcanoes in northeastern japan are higher than those in southwestern japan in northeastern japan the long term rate is higher at volcanoes located near the colcanic front than those on the back arc side applying the estimated long term eruption rates to the model calculation of continuous magma supply the appropriate radius of the magma chamber is estimated to be less than 3 5 km

how long can a hydrothermal system be sustained by a single intrusive event published numerical calculations show that the convective systems which produce near surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years even if the intrusion is large generally hydrothermal circulation near surface geothermal activity and magmatic intrusion should be closely tied in time and space long lived approximately 1 m y hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation however if the permeability of the intruded environment is just above that which allows convection and the volume of the intrusion is very large calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near surface geothermal activity for approximately 800 000 yr for a deep sill heat source these unusually long lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size

brief duration of hydrothermal activity at round mountain nevada determined from ar 40 ar 39 geochronology the round mountain gold silver deposit is one of the world s largest volcanic hosted precious metal deposits reserves plus production total at least 16 million oz 500 000 kg au the deposit occurs in ash flow tuff that is ponded within its source caldera along and overlying the caldera ring fracture host rocks have undergone propylitic potassic and high level silicic and intermediate argillic alteration these are interpreted either as a temporal sequence from early propylitic to late silicic and argillic or as a spatial progression from a potassic core outward to a propylitic halo 40 ar 39 ar ages determined by single crystal fusion of sanidine from host tuff and incremental heating of adularia indicate a most likely duration of hydrothermal activity of approximately 0 1 m y possibly no more than 0 05 m y and a maximum of 0 5 m y eruption of host ash flow tuff and caldera collapse occurred at 26 5 ma a late or postalteration tuff that rests upon mineralized rock is 26 0 ma hydrothermal circulation is unlikely to have been established immediately after caldera collapse so duration was probably considerably less than 0 5 m y eight adularia samples give ages between 25 94 or 0 09 and 26 09 or 0 05 ma and one is 26 20 or 0 05 ma the mean of all nine is 26 03 or 0 08 ma the maximum and minimum values and their uncertainties allows a duration of as much as 0 4 m y but for the following reasons the longevity is probably much less the absolute ages of seven samples that can be placed in the possible temporal alteration sequence do not correlate with that sequence this does not disprove the temporal sequence but does indicate that alteration occurred in a time span which is not resolvable even with precise 40 ar 39 ar dating furthermore all samples show flat age spectra which themselves argue against a long continuously active system kinetic studies of two adularia reveal diffusion domains with ar closure temperatures between approximately 230 degrees and approximately 390 degrees c adularia that crystallized and remained at temperatures of 275 degrees c the maximum temperature inferred from fluid inclusion data would yield age spectra with age gradients spanning the lifetime of the system no spectra are consistent with a thermal history that incorporates sustained 0 1 m y temperatures at or above 275 degrees c episodic activity to account for the one older adularia date cannot be precluded but is inconsistent with most data therefore the hydrothermal system probably was active for less than the uncertainty in the ages of individual samples possibly as little as 50 000 years we conclude that hydrothermal activity at round mountain was a brief event at approximately 26 0 ma and was closely linked to caldera structure and magmatism this activity followed initial ash flow eruption by 0 5 m y and may have been driven by a ring fracture intrusion

duration of magmatic hydrothermal and supergene activity at cerro rico de potosi bolivia new high precision 40ar 39ar dating of sanidine and biotite from two rhyolitic domes and an ignimbrite combined with existing fission track data and a hydrothermal sericite age suggests that the world class ag deposit at cerro rico was emplaced during a protracted period of magma related hydrothermal activity beginning at 13 77 0 03 ma and continuing for at least 0 2 m y this may have been sustained by a large single injection or repeated injections of fractionated ag enriched magma into a high level magma chamber k ar dating of alunite indicates that supergene oxidation had begun by about 13 5 ma soon after dome emplacement and progressed semicontinuously for at least 7 5 m y this oxidation while not leading to significant enrichment has significantly enhanced the economic viability of the disseminated part of the orebody

nd and sr isotopic study of volcanic rocks from japan two older granitic rocks and some selected quaternary volcanic rocks from the japanese islands were analyzed in a reconnaissance study for the purpose of examining the relationships between nd and sr isotopic abundances and the megatectonic structure around the japanese islands model ages of 0 9 ae were determined by the nd and sr methods on a paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the japanese islands the nd and sr isotopic data show that the cretaceous granodiorite is the result of partial melting of older crust the nd of tholeiitic rocks from the izu arc gives e nd ranging from 8 3 to 9 3 and with the corresponding e sr from 14 5 to 18 5 these results are identical to those found for the mariana arc these values are distinctly lower than typical morb by around 1 2 eu this difference in e nd between arcs and morb is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust the hakone volcano is clearly confirmed as belonging to an oceanic source by nd isotopic results e sr e nd values of the volcanics from a section along the fossa magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material the crustal component clearly increases in going from south to north volcanics across the northeast japan arc also show a distinct correlation of e sr e nd related to the position relative to the active subduction zone but with the opposite trend these relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the japan sea in general there appear to be regular isotopic relationships between the izu mariana oceanic island arc and the continental island arc of japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas

origin of low k intermediate lavas at nekoma volcano ne honshu arc japan geochemical constraints for lower crustal melts at lower crustal depths beneath the frontal arc is suggested by extremely low k basaltic andesite to andesite lavas at nekoma geophysical observations these observations further support lowervolcano situated in the frontal volcanic zone of the ne honshu crustal melting beneath nekoma as the origin of the intermediate arc were produced from melts that originated in the lower crust low k lavas multiple incompatible trace element model calculations suggest that extremely low k basalt found in the same arc is a natural analog for the source composition however fractional crystallization magma mixing and crustal contamination models of primary lowk basalt cannot reproduce the nekoma chemical composition

fluid inclusion studies of the polymetallic hydrothermal ore deposits in bolivia homogenization temperature and salinity were determined for fluid inclusions in mostly quartz and partly sphalerite cassiterite and barite from the 28 tin polymetallic ore deposits in bolivia generally the homogenization temperatures and salinities of these fluid inclusions are comparatively high for ore deposits formed by cassiterite mineralization such as morococala and avicaya in the oruro district frequently indicating a temperature higher than 300 c and salinity higher than 20 equiv wt nacl particularly it is quite possible that tin deposits associated with the w bi and tourmaline mineralizations such as viloco and caracoles have been produced by such high temperature hypersaline fluid ranging up to 500 c and 56 equiv wt nacl similar to the porphyry copper type this feature reveals that the hydrothermal fluid related to the sn w bi mineralization may be of magmatic origin homogenization temperatures for the pb zn deposits with no tin minerals are low mostly ranging 170 300 c at the avicaya bolivar mining area in the oruro district as well as at the tasna and chocaya animas mining areas in the quechisla district temperature gradients consistent with the zonal distributions of ore minerals were confirmed

strontium isotopic composition of hot spring and mineral spring waters japan abstract in japan hot springs and mineral springs are distributed in quaternary and neogene volcanic regions as well as in granitic sedimentary and metamorphic regions lacking in recent volcanic activity the87sr 86sr ratio was determined in hot spring and mineral spring waters obtained from 47 sites the ratios of waters from quaternary and neogene volcanic regions were in the range 0 703 0 712 which is lower than that from granitic sedimentary and metamorphic regions 0 706 0 712 the geographical distribution of the ratios coincides with the bedrock geology and particularly the ratios of the waters in quaternary volcanic regions correlate with those of surrounding volcanic rocks these features suggest that subsurface materials control the87sr 86sr ratios of soluble components in the hot spring and mineral spring waters

strontium isotopic and chemical variations of the granitic rocks in the tsukuba district japan initial 87sr 86sr ratios major and trace element compositions have been determined for the paleogene granitic rocks in the tsukuba district japan isotopic ages strongly suggest that the granitic rocks seven units were continuously emplaced and solidified during a short time interval initial 87sr 86sr ratios for seven granitic units vary from 0 7082 to 0 7132 while sedimentary and metasedimentary country rocks have ratios at the time of granitic magma emplacement ranging from 0 7149 to 0 7298 continuous linear arrays for the granitic rocks in the diagrams of initial 87sr 86sr ratios versus some chemical parameters can be explained by either of following two processes one is the assimilation fractional crystallization afc process between the parental magma sio2 of 68 and initial ratio of 0 7078 and sedimentary country rocks and the other is magma mixing process between above parental magma and sediment derived acidic magma melt sio2 of 75 the high initial ratios 0 7078 0 7098 for basic rocks such as gabbro or diorite in the tsukuba district and the similar characteristics observed in the rocks of ryoke belt sw japan suggest that the parental magma had the same source region as the basic rocks probably the lower crustal source

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

two overlapping plates subducting beneath central japan as revealed by strontium isotope data abstract in central japan the pacific plate subducts westward beneath the eurasian plate and the philippine sea plate subducts northwestward into the mantle wedge between the eurasian plate and the subducted pacific slab there the northeast japan arc is joined to the izu ogasawara arc we determined 87 sr 86 sr ratios and rb and sr contents for 47 volcanic rock samples from 15 quaternary volcanoes in central japan and summarized the geographical distribution of the ratios the general trend of slowly increasing 87 sr 86 sr ratio from the back arc side toward the volcanic front in the northeast japan arc is broken by a marked high ratio above 0 7060 centered around akagi volcano located at the southernmost region of the arc elsewhere the ratio along the volcanic front in this arc varies within the range 0 7038 to 0 7045 the marked high 87 sr 86 sr ratio is considered to be due to the addition of slab derived components transported by the philippine sea plate to the magma generating region in the mantle wedge beneath central japan therefore the geographical distribution of the high ratio may correspond to that of the philippine sea slab derived components in the mantle wedge and we may draw the underground outline of the philippine sea plate this outline implies that an aseismic portion of the philippine sea plate continues a few tens km ahead of the seismic one a belt of low 87 sr 86 sr ratios from the izu peninsula northwestward along the northern end of the izu ogasawara arc coincides with the zone where the subducting philippine sea plate is not observed seismologically while it is detected seismologically on both sides of the belt

formation of epithermal sn ag zn vein type mineralization at the pirquitas deposit nw argentina fluid inclusion and noble gas isotopic constraints abstract the pirquitas sn ag zn deposit in northwestern argentina is thought to be an analogue to the miocene polymetallic epithermal sn ag deposits of the southern bolivian tin belt but little is known in detail about the origin and evolution of ore forming fluids at pirquitas this paper reports on a microthermometric study of fluid inclusions in quartz sphalerite ag sn sulfides and ag rich sulfosalts using transmitted near infrared and visible light combined with noble gas isotope analyses of fluids released from mineral separates the study focused on the vein hosted mineralization which formed during two major mineralization events whereby the first event i comprises two stages i 1 and i 2 all studied minerals exclusively contain aqueous two phase inclusions indicating that the ore forming fluids did not undergo two phase phase separation boiling salinity of fluid inclusions in i 1 quartz that precipitated along with pyrite and pyrrhotite ranges between 0 and 7 5 wt nacl equiv and homogenization temperatures th are between 233 and 370 c stage i 2 is characterized by abundant sn ag pb zn sulfides and a variety of ag rich sulfosalts fluid inclusions in stage i 2 ag sn sulfides have salinities up to 10 6 wt nacl equiv and th between 213 and 274 c the deposition of stage i 2 ore is likely related to a new pulse of saline magmatic fluids to the hydrothermal system the mineralization event ii deposited the richest ag ores at pirquitas colloform sphalerite and pyrargyrite deposited during event ii contain two phase aqueous fluid inclusions with homogenization temperatures between 190 and 252 c and salinities between 0 9 and 4 3 wt nacl equiv noble gas concentrations and isotopic compositions of ore hosted fluid inclusions were determined from crushing hand picked ore minerals from both mineralization events with one exception all samples yielded 3he 4he ratios between 1 9 and 4 1 ra which is within the range of published data from the volcanic arc and somewhat higher than typical values of meteoric water derived hot springs in the region this demonstrates a significant contribution of magmatic fluids to the pirquitas mineralization although no intrusive rocks are exposed in the mine region taking the noble gas evidence for a magmatic fluid source we interpret the trends of decreasing th and salinity values in fluid inclusions from events i and ii to represent waning of the magmatic hydrothermal system and or increased admixing of meteoric water to the magmatic fluids

primary magmas at the volcanic front of the ne japan arc coeval eruption of crustal low k tholeiitic and mantle derived medium k calc alkaline basalts at azuma volcano it has long been thought that the low k tholeiitic th and medium k calc alkaline ca lavas in the nejapan arc were produced by fractional crystallization from mantle derived basalt magmas and that the latter formed from mixing of mafic and felsic magmas both derived from a common primaryth basalt through fractionation an alternative view was recently proposed on the basis of sr isotope microanalysis of plagioclase phenocrysts from the zao volcano suggesting that 1 the low k th basaltic andesites formed by melting of lower crustal amphibolite and that 2 the medium k ca basalts to andesites formed by mixing of mantle derived basalt and crustal th melts to investigate further the origin of the primary low k th and medium k ca basalts we investigated basalts and andesites from azuma volcano azuma is a quaternary eruption center at the volcanic front in the ne japan arc that has erupted two types of basalt 1 radiogenic sr sr sr1 4 0 7058 0 7062 low k th basalt lavas without evidence of magma mixing and assimilation 2 unradiogenic sr sr sr1 4 0 7039 0 7041 medium k ca basalt lavas with subtle evidence for magma mixing associated intermediate lavas are voluminous and are all 3 mildly radiogenic sr sr sr1 4 0 7044 0 7055 medium k andesites all of which have ca affinities with evidence for rigorous magma mixing but no crustal assimilation the low k th basalt has an isotopic composition similar to that of crustal granitoids beneath azuma and has a composition indicating that it potentially formed from a high degree lower crustal amphibolite melt the medium kca basalt has a basaltic groundmass with mg rich olivine fo89 and calcic plagioclase phenocrysts an90 and the most unradiogenic sr sr sr1 4 0 7037 0 7038 suggesting that it originated from a primary mantle melt major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high k we conclude that the mantle derived basalt at azuma is the result of a highto medium k magma that was later mixed with a low k th basalt melt from the amphibolitic lower crust to form medium k ca basalts and andesites this supports the view of a lower crustal origin of the low kth basalts and simultaneously requires a reappraisal of the origin of the across arc variation in k contents of the mantle derived primary arc basalts as the highto medium k ca basalt is geochemically fairly similar to the high k rear arc basalt in the ne japan arc

detailed morphology and bubble plumes of daiichi amami knoll in the central ryukyu arc abstract high resolution geophysical mapping operations were conducted from a survey vessel and autonomous underwater vehicle auv in the daiichi amami knoll ryukyu island arc for the first time both the detailed morphology and the extent of the bubble plume in this region are revealed the knoll is a conical edifice rising 500 700 m from the surrounding seafloor with a peak depth at 247 m the summit region of the knoll has a complex structure and is divided into three parts the northeastern rim central caldera and southern cones among the three parts the southern cones has characteristic morphological features such as cones and a depression two cones cone a and cone b rise 30 m above the surrounding basement and have fissure like linear features the depression depression d1 faces the steep slope of cone a and is bounded by cone a and b the horizontal dimensions are 500 m 200 m and it is 10 40 m lower than the surrounding basement water column anomalies detected on side scan sonar images which are thought to be bubble plumes were intensively distributed in and around depression d1 and cone b with horizontal dimensions of 300 m 800 m the volcanic morphology and bubble plumes indicators of hydrothermal activity suggest that the daiichi amami knoll is an active volcanic knoll this result proposes a new insight that dual volcanic chains exist in the north to central ryukyu arc the daiichi amami knoll is located behind the volcanic front not on the front the knoll surrounding volcanoes and volcanic edifice behind the front are aligned with one straight line 20 km behind and parallel to the front making a rear arc volcanic chain the presence of dual volcanic chains in the north to central ryukyu arc revealed by our detailed geophysical survey provides valuable information to consider toward the subduction processes and magma generation in the ryukyu arc

strontium isotopic composition of oil field and gas field waters japan abstract the87sr 86sr ratios of 10 formation waters in petroleum and natural gas reservoirs along the western coast of northeastern honshu japan ranged from 0 7052 to 0 7084 and are distinctly lower than the ratio in seawater this forms a marked contrast to oil field brines from the u s a which have higher87sr 86sr ratios taking into consideration that petroleum and natural gas in japan accumulate in volcanic rocks derived from intense submarine volcanism in the middle miocene the87sr 86sr ratios of the water samples are explained in terms of the isotope exchange between waters containing seawater sr and reservoir rocks with lower ratios

strontium isotopic characteristics of arc volcanic rocks at the initial stage of subduction in western japan abstract in western japan the philippine sea plate subducts northwestward from the ryukyu trench and the nankai trough to form two volcanic arcs the ryukyu and southwest japan arcs respectively these arcs are thought to be in the initial stage of subduction because microearthquake data show that the subducting slab extends only a short distance from the trenches in the southwest japan arc volcanoes are aligned parallel to the nankai trough although the leading edge of the subducted slab does not reach under the volcanic arc to the south the subducted slab reaches just under the ryukyu volcanic arc we determined 87sr 86sr ratios and rb and sr contents for 39 volcanic rock samples from 14 quaternary volcanoes of both arcs and summarized the spatial distribution of the ratios the ranges of 87sr 86sr ratios of volcanic rocks from the ryukyu and southwest japan arcs are 0 7040 0 7069 and 0 7035 0 7066 respectively in the ryukyu arc the regional variation in 87sr 86sr ratios of volcanic rocks is hidden under the wide scatter of ratios of different volcanic rocks from each one volcano in the southwest japan arc the ratios of volcanic rocks from volcanoes on the volcanic front vary in the along arc direction 0 7040 0 7048 for the western part 0 7035 0 7039 for the central part and 0 7048 0 7066 for the eastern part the across arc variations in the ratios are different from place to place these isotopic features suggest the absence of slab derived components in the source mantle and reflect the heterogeneous nature of 87sr 86sr ratios in the upper mantle modified by crustal processes this coincides with the seismological observation that the subducting philippine sea slab does not reach under the volcanic arc at all

age of supergene oxidation and enrichment in the chilean porphyry copper province twenty five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 degrees and 27 degrees s in northern chile yield k ar ages ranging from about 34 to 14 ma therefore supergene oxidation and enrichment processes were active from the early oligocene to the middle miocene a minimum of 20 m y supergene activity at individual deposits lasted for at least 0 4 to 6 2 m y the early oligocene supergene activity affected deposits in the paleocene porphyry copper belt whereas early and middle miocene supergene processes are documented in the early cretaceous paleocene and late eocene to early oligocene porphyry copper belts middle miocene oxidation also affected the oldest epithermal gold silver deposits in the maricunga belt farther east supergene activity commenced no less than 11 m y after generation of each porphyry copper deposit because of the time required to unroof the copper bearing parts of the system supergene activity throughout northern chile ceased at approximately 14 ma the geologic features of deposits and prospects and their morphotectonic positions present latitudes and present elevations display no obvious correlations with the supergene chronology exploration for major cumulative enrichment blankets should not be carried out either beneath thick sequences of piedmont gravels or ignimbrites of oligocene through middle miocene age unless their accumulation is demonstrably late in the documented history of supergene activity or in porphyry copper provinces such as those of central chile and northwestern argentina which formed after approximately 14 ma the uplift responsible for efficient cumulative copper enrichment is difficult to correlate convincingly with the brief pulses of compressive tectonism postulated for northern chile and contiguous areas unless their effects were much more prolonged intensifying aridity is confirmed as the likely reason for the cessation of supergene activity in northern chile and tectonic uplift was its most probable cause however more fundamental global controls producing a period of chemical weathering followed by worldwide dessication also may have played a role

variation of al2o3 content in late cenozoic japanese basalts a re examination of kuno s high alumina basalt al2o3 variation in late cenozoic japanese basalts is re examined using the mg value as the differentiation index there exist no apparent differences in al2o3 among three postulated parental basalt magma types low alkali tholeiite high alumina basalt and alkali olivine basalt when the undifferentiated basalts with high mg values 55 are compared the al2o3 content of each type of basalt increases with decreasing mg value at mg values 55 and most basalts lack phenocryst plagioclase this feature is indicative of al2o3 increase by predominant mafic mineral fractionation and the absence of significant plagioclase fractionation among the differentiated basalts mg value 55 systematic differences of al2o3 content are observed the more alkalic basalt has the higher al2o3 content the origin of this variation is considered to result from a systematic difference in timing of the first appearance of plagioclase during crystallization from tholeiite through high alumina basalt to alkali basalt the progressive delay of plagioclase crystallization relative to mafic silicates may be explained by the gradual increase in h2o content in basalt magmas from tholeiite to alkali basalt

strontium isotope composition in volcanic rocks from the northeast japan arc abstract in the northeast japan arc a number of quaternary volcanoes form a long narrow belt parallel to the japan trench 87 sr 86 sr ratios were determined in 52 specimens of volcanic rocks from 27 volcanoes in the northeast japan arc area the results reveal that the ratios change systematically in space decreasing 87 sr 86 sr ratios across the arc were confirmed over a wide area of northeast japan in the same direction increases in both rb and sr contents were also found the regular trends are considered to be a strong constraint for elucidation of subduction originated magma genesis at the eurasia plate vs pacific plate boundary in the northern region of the northeast japan arc 87 sr 86 sr ratios in volcanic rocks along the volcanic front were almost constant 0 7038 0 7045 and slightly higher than those from the izu ogasawara arc 0 7032 0 7038 this suggests that interactions between the eurasia plate and the pacific plate and those between the philippine sea plate and the pacific plate are slightly different the southern region of the northeast japan arc where the direction of the volcanic front bends from southward to westward showed anomalously high 87 sr 86 sr ratios reaching to 0 7077 this region coincides with the triple junction of the eurasia pacific and philippine sea plates suggesting anomalous interaction at the triple junction

on the na2o content of convergent zone high alumina basalts abstract high alumina basalts in the convergent zone are classified into high medium and low na type the high na high alumina basalts occur in arcs without low alkali tholeiites where younger and warmer oceanic lithospheres are subducting the low na high alumina basalts occur in arcs where on the contrary older and cooler plates are descending and occur also in arcs with extensional tectonics even though younger lithospheres are underthrusting a good correlation is observed between the na2o content of high alumina basalts and the age of the subducting lithosphere this suggests that generation of high alumina basalt magma is controlled by the nature of the descending plate

uthpb zircon dating of the 13 8 ma dacite volcanic dome at cerro rico de potosi bolivia abstract the temporal relationship between the extrusion of the miocene dacite volcanic dome at cerro rico de potosi bolivia and the associated ag sn mineralization has an important bearing on the heat and metal sources for this world class mineral deposit the highly altered nature of the volcanic rock however has prevented previous efforts from definitively determining the emplacement age of the dome rather than subsequent episodes of alteration the present study uses u th pb dating of sparse zircon contained in the dacite to demonstrate that at most only several hundred thousand years separate dome emplacement from main stage mineralization the dacite contains a morphologically heterogeneous population of zircon with considerable variation in size shape and color preliminary analyses revealed the presence of a significant component of inherited zircon most likely as older cores that were overgrown by rims during the crystallization of the dacite magma morphological distinctions shape size color within the zircon population provided the basis for separating the zircon into fractions that reduced or enhanced the inherited component in composite grains by analyzing the tips of zircon with very large length to width ratios it was possible to constrain the crystallization age of the dacite to 13 8 0 2 ma the dispersion of data on a concordia diagram indicates at least two ages of inherited zircon one mesozoic associated with yellow grains and another proterozoic associated with colorless grains such xenocrystic components have useful petrogenetic implications for the source and travel path of the magma

analytical model of surge flow in nonprismatic permeable channels and its application in arid regions a surge running down a dry wadi bed as a consequence of a controlled water release from a reservoir e g for artificial groundwater recharge represents a free boundary problem after some time when aiming for groundwater recharge the infiltration equals inflow and thus forms a kind of standing wave the numerical solution of such phenomena generally involves considerable problems for avoiding the numerical inconvenience resulting from the complex interacting surface subsurface flow we present an analytical solution of the slightly modified zero inertia zi equations the development introduces a momentum representative cross section for portraying the transient development of momentum and refers to a channel with constant slope irregular geometry and a permeable channel bed with significant infiltration due to the structure of the solution any arbitrary infiltration model can be used for quantifying the infiltration losses for both synthetic prismatic and nonprismatic test channels the robust and easy to use analytical zi model shows an excellent match with the results of a comparative numerical simulation finally the zi model is employed for simulating a surge flow downstream of the wadi ahin groundwater recharge dam oman in order to perform a scenario for artificial groundwater recharge in a natural wadi channel reach this realistic application illustrates the potential of the new approach by even computing an almost standing wave and shows its applicability for an accurate and robust evaluation of release strategies

peculiar features of suiyo seamount hydrothermal fluids izu bonin arc differences from subaerial volcanism abstract this is the first report of the dissolved gas composition of a deep sea venting hydrothermal system originating from arc volcanism the origin of dissolved gases in hydrothermal fluids from the suiyo seamount in the southern izu bonin ogasawara arc is discussed on the basis of chemical and isotopic compositions twelve high temperature up to 310 c fluid samples were collected from three vents of the hydrothermal system using the submersible shinkai 2000 the average 3 he 4 he ratio of the dissolved he is 8 1 r r atm which is within the range of values reported for n morb the co 2 3 he ratio is 12 109 which is within the range of arc volcanic gas values the 13c values of co2 and ch4 are 1 and 8 5 respectively which is quite high compared with other subaerial arc volcanic gases and those from mid ocean ridges these results which do not accord with those from subaerial volcanism are unlikely to result from fractionation or addition of carbon sources during the fluid circulation but are regarded as characteristic of the magma source of suiyo seamount a simple mixing of 97 morb mantle with 3 subducting slab explains well the composition of the helium and carbon in the parent magma

geochemical implications from sr isotopes and k ar age determinations for the cook austral islands chain abstract sr isotopes and k ar ages were determined for volcanic rocks from three islands samoa rarotonga and rurutu in the austral gilbert marshall chain we have established that rurutu originated from the same hot spot as tubuai and raivavae in the cook austral chain as indicated by its 11 m y k ar age and 87 sr 86 sr ratios the 87 sr 86 sr ratios for rurutu tubuai and raivavae lie in the restricted range 0 7026 0 7035 k ar ages of all volcanic rocks from samoa and rarotonga are all less than 4 m y and their 87 sr 86 sr ratios are much higher than those in rurutu tubuai and raivavae these findings suggest that samoa and rarotonga did not originate in a single hot spot for the cook austral islands

geochemistry of miocene basaltic rocks temporally straddling the rifting of lithosphere at the akita yamagata area northeast japan abstract voluminous basaltic rocks with arc geochemical signatures were produced in the akita yamagata area the back arc side of ne japan in the middle miocene temporally straddling the opening of the japan sea eleven of these rocks were studied for trace elements among these seven whole rocks and five clinopyroxene separates were analyzed for 87 sr 86 sr and four clinopyroxene separates were analyzed for 143 nd 144 nd the results suggest that the lithosphere has been partly broken up and eliminated or thinned beneath the back arc side of ne japan since 16 ma the new analyses and existing data collectively show that the basaltic magmas have decreased in 87 sr 86 sr and increased in 143 nd 144 nd since 16 ma coincidently the magmas have changed their geochemical signature from a continental margin type to an oceanic arc type on average the basaltic rocks younger than 16 ma are likely to be products of varying degrees of mixing between subduction related basaltic magmas with low 87 sr 86 sr 0 703 0 7035 and partial melts of subarc lithospheric mantle with high 87 sr 86 sr 0 7055 at least in part and arc tholeiitic geochemical characteristics the basaltic rocks older than 16 ma seem to have been more intensely contaminated by crustal material

the age and thermal history of cerro rico de potosi bolivia cerro rico de potosi bolivia is the world s largest silver deposit and has been mined since the sixteenth century for silver and for tin and zinc during the twentieth century together with by product copper and lead the deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia the deposit is compositionally and thermally zoned having a core of cassiterite wolframite bismuthinite and arsenopyrite surrounded by a peripheral lower temperature mineral assemblage consisting principally of sphalerite galena lead sulfosalt and silver minerals the low temperature assemblage also was superimposed on the high temperature assemblage in response to cooling of the main hydrothermal system both the dacite dome and the ore fluids were derived from a larger magmatic hydrothermal source at depth the dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement the dome was extruded at 13 8 0 2 ma 2 based on u th pb dating of zircon mineralization and alteration occurred within about 0 3 my of dome emplacement as indicated by a 40ar 39ar date of 13 76 0 10ma 1 for sericite from the pervasive quartz sericite pyrite alteration associated with the main stage high temperature mineralization the last thermal event able to reset zircon fission tracks occurred no later than 12 5 1 1 ma 1 as indicated by fission tract dating minor sericite and magmatic steam alunite veins were episodically formed around 11 ma and between 8 3 and 5 7 ma the younger episodes occurring at the time of extensional fracturing at cerro rico and wide spread volcanism in the adjacent los frailes volcanic field none of these younger events appear to be significant thermal mineralizing events the exceptionally flat thermal release pattern of 39ar from sericite and the results of the fission tract dating of zircon show that none of the younger events was hot enough and lasted long enough to cause significant loss of ar or annealing of zircon fission tracks u th pb dating of zircon cores indicates a precambrian progenitor for some zircons and ree analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive eu anomaly which constrains interpretations of the origin and evolution of the magmatic hydrothermal system

secular variation of magma source compositions beneath the northeast japan arc abstract sr and nd isotopic compositions were determined for tertiary volcanic rocks with ages of 22 16 and 8 ma along the volcanic front in the ne japan arc in order to examine the secular variation of magma source compositions in the mantle wedge these rocks show no through time variation in both sr and nd isotope ratios 0 70412 0 704759 and 2 09 4 12 units respectively it follows that the subduction components do not increasingly pollute the magma source region during the continuous subduction of oceanic lithosphere the degree of pollution by the subduction components is controlled by the amount of clinopyroxene and aluminous phases in mantle wedge peridotites because h 2 o which causes partial melting of the mantle wedge beneath a volcanic front is supplied by the breakdown of both amphibole and chlorite in the dragged hydrated peridotite formed by the addition of slab derived fluid beneath the fore arc region

thermal and geochemical evolution of the mantle wedge in the northeast japan arc 1 contribution from experimental petrology melting phase relations at high pressures have been examined for four mg rich basalts from both the trench and the backarc sides of the miocene ne japan arc all these basalts crystallize olivine and orthopyroxene on the liquidus at pressures lower and higher than the point of multiple saturation with those two phases respectively the pressure of multiple saturation is rather constant for all samples 1 0 1 4 gpa suggesting that 1 there was no across arc variation in the depth of magma segregation in the miocene ne japan arc 2 the depth of magma segregation remains constant since 20 ma in the trench side of the volcanic arc whereas it has changed from 30 40 km to 60 km beneath the backarc side of the arc this temporal variation may be caused by increasing thickness of the lithosphere beneath the backarc region associated with cooling of the mantle wedge after the opening of the japan sea backarc basin in the early miocene

the kari kari caldera bolivia and the cerro rico stock abstract the rocks comprising the kari kari massif southeast of the city of potosi bolivia consist entirely of welded ignimbrites it is argued that the massif constitutes the resurgent centre of a 20 m y old resurgent caldera plutonic rocks are exposed in the south but volcanic rocks of the caldera rim are exposed in the north and indicate a shallower erosion level there the volcanic rocks consist of a coarse moat deposit consisting of angular fragments of basement material and juvenile clasts overlain by an extensive garnet bearing ignimbrite a plant fossil bearing lacustrine deposit was laid down in a lake within the caldera the cerro rico stock noted for its silver tin mineralisation may be a late intrusion along the caldera ring fractures