isotopic evolution of mauna loa volcano abstract in an effort to understand the temporal helium isotopic variations in mauna loa volcano we have measured helium strontium and lead isotopes in a suite of mauna loa lavas that span most of the subaerial eruptive history of the volcano the lavas range in age from historical flows to ninole basalt which are thought to be several hundred thousand years old most of the samples younger than 30 ka in age kau basalt are radiocarbon dated flows while the samples older than 30 ka are stratigraphically controlled kahuku and ninole basalt the data reveal a striking change in the geochemistry of the lavas approximately 10 ka before present the lavas older than 10 ka are characterized by high 3 he 4 he 16 20 times atmospheric higher 206 pb 204 pb 18 2 and lower 87 sr 86 sr 0 70365 ratios than the younger kau samples having he pb and sr ratios of approximately 8 5 atmospheric 18 1 and 0 70390 respectively the historical lavas are distinct in having intermediate sr and pb isotopic compositions with 3 he 4 he ratios similar to the other young kau basalt 8 5 atmospheric the isotopic variations are on a shorter time scale 100 to 10 000 years than has previously been observed for hawaiian volcanoes and demonstrate the importance of geochronology and stratigraphy to geochemical studies the data show consistency between all three isotope systems which suggests that the variations are not related to magma chamber degassing processes and that helium is not decoupled from the other isotopes however the complex temporal evolution suggests that three distinct mantle sources are required to explain the isotopic data most of the mauna loa isotopic variations could be explained by mixing between a plume type source similar to loihi and an asthenospheric source with helium isotopic composition close to morb and elevated sr isotopic values an asthenospheric source or variation within the plume source is considered more likely than lithospheric sources due to the elevated 87 sr 86 sr ratios in the recent kau basalts however the distinct isotopic characteristics of the historical lavas are inferred to be related to lithospheric involvement in the latest stages of shield building volcanism

he and sr isotopic constraints on subduction contributions to woodlark basin volcanism solomon islands in order to assess the nature and spatial extent of subduction contributions to arc volcanism sr and he isotopic compositions are measured for dredged volcanic rocks from the woodlark basin in the western pacific sup 87 sr sup 86 sr ratios increase geographically from ocean ridge values 7025 7029 at the woodlark spreading center to island arc ratios 7035 7039 in the solomon islands forearc with intermediate values near the triple junction where the woodlark spreading center subducts beneath the solomon islands sup 3 he sup 4 he ratios are also more radiogenic in the forearc 6 9 plus minus 2 r sub a at active kavachi volcano than along the spreading center where values typical of major ocean ridges were found 8 2 9 3 r sub a very low sup 3 he sup 4 he ratios occur in many triple junction rocks 1 to 5 r sub a but consideration of he isotopic differences between crushing and melting analyses suggests that the low ratios were caused by atmospheric 1 r sub a and radiogenic approx 0 2 r sub a helium addition after eruption variations in unaltered magnetic sup 3 he sup 4 he and sup 87 sr sup 86 sr ratios are best explained by subduction related fluid or silicate melt contributions to the magma source region perhaps from ancient pacific lithosphere however more mantle volatiles dominate the generation of woodlark basin rocks despite extensive subduction in the region less

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

evolution of atauro island temporal constraints on subduction processes beneath the wetar zone banda arc abstract atauro is a key to understanding the late stage volcanic and subduction history of the banda arc to the north of timor a volcanic history of bi modal subaqueous volcanism has been established and new whole rock and trace element geochemical data show two compositional groups basaltic andesite and dacite rhyolite 40ar 39ar geochronology of hornblende from rhyo dacitic lavas confirms that volcanism continued until 3 3 ma following the cessation of volcanism coral reef marine terraces have been uplifted to elevations of 700 m above sea level continuity of the terraces at constant elevations around the island reflects regional scale uplift most likely linked to sublithospheric processes such as slab detachment local scale landscape features of the eastern parts of atauro are strongly controlled by normal faults the continuation of arc related volcanism on atauro until at least 3 3 ma suggests that subduction of australian lithosphere continued until near this time this data is consistent with findings from the earthquake record where the extent of the wetar seismic gap to a depth of 350 km suggests slab breakoff as a result of collision commenced at 4 ma leading to subsequent regional uplift recorded in elevated terraces on atauro and neighbouring islands

temporal association of arc continent collision progressive magma contamination in arc volcanism and formation of gold rich massive sulphide deposits on wetar island banda arc whole rock 87sr 86sr and 18o analyses of volcanic rocks and 3he 4he analyses of sulphides and sulphates from mineralized rocks on wetar indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted australian craton to the south these new data support the idea of progressive source contamination with precisely dated events showing that wetar island hosts the most extreme examples of crustal assimilation in the region the increased continental contamination occurs during the pliocene zanclian to piacenzian during distinct magmatic events between 5 and 4 ma and at 2 4 ma when 87sr 86sr ratios in unaltered lavas with whole rock 18o values between 5 7 and 9 6 increase from 0 707484 to extreme radiogenic values of 0 711656 the earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on wetar samples from this yield radiogenic 3he 4he ratios between 0 5 and 1 4 r ra similar to the data from volcanic rocks on nearby romang the later magmatic event coincides with the arrival of the australian continental margin at the subduction zone along the banda arc progressive incorporation of continental sourced components into the source region below the wetar island edifice coincides with the formation of gold rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile

seismotectonics of the central segment of the indonesian arc abstract in this paper a revision of seismicity affecting the central segment of the indonesian island arc ranging between 110 and 126 e is presented using the areal and in depth distribution of seismic activity strain release maps and focal mechanisms lateral changes in the wadati benioff zone have been analyzed to determine possible boundaries between portions of lithosphere with different subduction geometries the seismicity pattern indicates that the sumbawa flores wetar sector shows different forms of behaviour with respect to the adjacent sectors these include driving mechanism inclination and continuity of the subducting slab and subduction features this area therefore seems to be isolated from the sunda and banda arcs by two principal boundaries these having a nearly n s trend in the bali region and with a nearly e w trend in the region ranging between wetar northern timor and tanibar the first boundary characterized by an absolute minimum of seismic activity at all ranges of depth has been interpreted in terms of subduction of the roo rise aseismic bathymetric ridge for the second boundary dividing a northwardly steeply dipping slab from an e w subducting slab dipping with an angle of 30 40 a tear in the upper part depth less than 300 km and a hinge fault system in the deepest part of the lithosphere have been proposed from the analysis of focal mechanisms of shallow earthquakes it was inferred that the central part of the indonesian arc is subject to a vortex shaped stress field centred on the savu basin in this model the compressive axes appear to rotate counterclockwise from sw to nne in the sumba sumbawa western flores region and clockwise from w to nnw in the timor eastern flores zone to interpret these features on the basis of seismological evidence a lateral discontinuity in the arc trench system close to sumba a collision between sumba and sumbawa and a rotation towards the north northeast of sumba have been suggested the proposed structural discontinuity trending nw se may represent a major transcurrent fault zone through which the australian continental lithosphere comes into contact with the indian oceanic lithosphere

diffusion of cosmogenic3he in olivine and quartz implications for surface exposure dating abstract thein situ production of3he in surface rocks by cosmic ray induced nuclear reactions offers an important geochronological tool to evaluate helium loss problems in this technique cosmogenic3he diffusivities were measured in quartz and olivine by incremental heating at 150 600 c arrhenius temperature dependences were observed in both minerals with similar activation energies ea 25 4 kcal mole logd0 3 7 0 9 in olivine andea 25 2 0 9 logd0 0 2 0 4 in quartz and imply very low diffusivities when extrapolated to environmental temperatures less than 10 18 cm2 s in quartz and 10 22 cm2 s in olivine at 20 c these low diffusivities suggest helium loss will not significantly affect cosmogenic helium exposure dating for time scales on the order of 106 years in quartz and 109 years in olivine provided large 2 mm sample grains are used exposure ages obtained with smaller grains need to be corrected for diffusive helium loss and equations are provided for this purpose cosmogenic3he diffuses more than 100 times faster than trapped magmatic3he in olivine or radiogenic4he in quartz the mechanisms responsible for these differences have yet to be determined but may involve both trapping processes at mineral defects and enhanced mobility associated with spallation produced crystal damage therefore it is possible that helium diffusivities may depend on a sample s cosmic ray exposure or radiogenic production history whatever the precise mechanism the higher mobility of3he suggests that incremental heating will be useful in separating cosmogenic helium from magmatic or radiogenic helium in future studies comparison of the diffusivity results with total cosmogenic3he contents of different quartz size fractions for an antarctic quartzite suggests that he loss may have occurred more rapidly than predicted by the laboratory measurements nonetheless retention of cosmogenic3he is quite high equivalent to more than a million years of exposure and cosmogenic3he geochronology is feasible in this and other quartz containing rocks

cosmogenic 3he in terrestrial rocks the summit lavas of maui we have identified terrestrial cosmic rayproduced 3he in three lava flows on the crest of haleakala volcano on maui 3 km above sea level and 0 5 million years old although these lavas like all oceanic basalts contain primordial 3he from the mantle the cosmogenic component 3hec can be identified unambiguously because it is extractable only by high temperature vacuum fusion in contrast a large fraction of the mantle helium resides in fluid inclusions and can be extracted by vacuum crushing leaving a residual component with 3he 4he ratios as high as 75 those in the atmosphere which can be liberated by melting the crushed grains cosmogenic 3he is present in both olivines and clinopyroxenes at 0 8 1 2 10 12 ml stp g and constitutes 75 5 of the total 3he present the observed 3hec levels require a cosmic ray exposure age of only some 64 000 years much less than the actual age of the lavas if there is no erosion using a model that includes effects of uplift or submergence as well as erosion we calculate an apparent erosion rate of the order of 8 5 m 106 years for the western rim of the summit crater as an example of the application of measurements of cosmogenic rare gases to terrestrial geological problems

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

chemical and isotopic compositions of volcanic gases from the east sunda and banda arcs indonesia the easternmost sunda arc and the banda arc represent a continent arc collision zone where magma genesis is influenced by subducted continent derived material chemical and isotopic studies of volcanic gas samples from this environment provide information on the sources of volatiles in arc magmas these volcanic gases some of which last equilibrated at magmatic temperatures are characterized by anomalous low sup 3 he sup 4 he values but by common arc values of c s approx 2 4 delta sup 13 c approx minus 3 per thousand and delta sup 34 s sub tot approx 5 per thousand abundant helium and high he ar ratios are consistent with the subduction of terrigenous components in local sediments or slivers of continental crust although individual concentration and isotope ratios of volatile components may be explicable by complex fractionation in the recycling process the combined data are in agreement with an important role of subducted sedimentary source components comparison of the authors results with volcanic gas data from other arcs indicates that the carbon and sulfur signals in arc gases are relatively insensitive to the amount and nature of sediment on the subsiding plate hence a contribution to arc volcanic gases from subducted altered oceanic crust cannot be excluded

the geochemistry and petrogenesis of k rich alkaline volcanics from the batu tara volcano eastern sunda arc mineralogical major and trace element and isotopic data are presented for leucite basanite and leucite tephrite eruptives and dykes from the batu tara volcano eastern sunda arc in general the eruptives are markedly porphyritic with phenocrysts of clinopyroxene olivine leucite plagioclase biotite set in similar groundmass assemblages these k rich alkaline volcanics have high concentrations of large ion lithophile lil light rare earth lre and most incompatible trace elements and are characterized by high 87sr 86sr 0 70571 0 70706 and low 143nd 144nd 0 512609 0 512450 compared with less alkaline volcanics from the sunda arc they also display the relative depletion of ti and nb in chondrite normalized plots which is a feature of subalkaline volcanics from the eastern sunda arc and arc volcanics in general chemical and mineralogical data for the batu tara k rich rocks indicate that they were formed by the accumulation of variable amounts of phenocrysts in several melts with different major and trace element compositions the compositions of one of these melts estimated from glass inclusions in phenocrysts is relatively fe rich 100 mg mg fe2 48 51 and is inferred to have been derived from a more primitive magma by low pressure crystal fractionation involving olivine clinopyroxene and spinel mg rich mg 90 and cr rich up to 1 7 wt cr2o3 zones in complex oscillatory zoned clinopyroxene phenocrysts probably also crystallized from such a magma the marked oscillatory zoning in the clinopyroxene phenocrysts is considered to be the result of limited mixing of relatively evolved with more primitive magmas together with their phenocrysts along interfaces between discrete convecting magma bodies

a dual doubly vergent orogen in the banda arc continent arc collision zone as observed on deep seismic reflection profiles new deep seismic reflection profiles across the banda arc of indonesia reveal reflectors in the uppermost 50 km of the lithosphere combined with existing earthquake hypocenter locations and focal mechanisms the new structural geometries inferred from the reflectors yield a more complete analysis of deformation during the past 10 m y and provide insights into how strain is partitioned across an orogenic belt in which volcanic arcs and continents converge a clearly defined wadati benioff zone and recent deformation of shelf sediments observed on older shallow seismic profiles indicated to previous workers that substantial convergence occurred at the timor trough a few focal mechanisms seafloor escarpments and recent geodetic surveying indicate that convergence at 7 cm yr 1 currently last 200 kyr occurs at the northern margin of the now inactive volcanic arc the wetar thrust zone reflectors on the new seismic profiles are interpreted as thrust faults and folds that occur throughout the crust and within the uppermost mantle between the timor trough and wetar thrust specifically basement reflectors beneath the toe of the accretionary complex have reverse sense offsets that imply blind thrusts the whole crust is horizontally shortened not only the sedimentary cover rocks that previously deformed into duplexes above a decollement reflectors dipping away from both margins of the forearc basin and at the northern margin of the volcanic arc are interpreted as evidence of thrusting thus each arc represents a doubly vergent fold and thrust belt but only the northern one is currently active crustal thicknesses inferred from seismic velocities reflectors and gravity anomalies are consistent with the merging of a thinned continental shelf margin with oceanic lithosphere to form an orogenic belt with at present 3 4 km of topographic relief in the region of eastern timor

seismic moment release during slab rupture beneath the banda sea summary the highest intermediate depth moment release rates in indonesia occur in the slab beneath the largely submerged segment of the banda arc in the banda sea to the east of roma termed the damar zone the most active western part of this zone is characterized by downdip extension with moment release rates 10 18 nm yr 1 per 50 km strike length implying the slab is stretching at 10 14 s 1 consistent with near complete slab decoupling across the 100 200 km depth range differential vertical stretching along the length of the damar zone is consistent with a slab rupture front at 100 200 km depth beneath roma propagating eastwards at 100 km myr 1 complexities in the slab deformation field are revealed by a narrow zone of anomalous in plane p axis trends beneath damar where subhorizontal constriction suggests extreme stress concentrations 100 km ahead of the slab rupture front such stress concentrations may explain the anomalously deep ocean gateways in this region in which case ongoing slab rupture may have played a key role in modulating the indonesian throughflow in the banda sea over the last few million years

4he diffusion in 40ar retentive minerals abstract 4he vacuum diffusion experiments were carried out by stepwise degassing minerals normally used in k ar chronometry such as hornblende nepheline and others the study aims at 4he diffusion parameters for uppermost crustal temperatures and thus at he retentivities of the minerals the results indicate long time near quantitative 4he retention in hornblende pyroxene langbeinite and perhaps nepheline for 0 5 mm grains at least at temperatures below about 70 c sanidine and muscovite lose 4he over geologic periods at lower temperatures only the minerals of the first group are possible u th he chronometers or geothermometers provided that other dating problems such as alpha loss and u migration can be settled closure temperatures for rapidly cooling 2 mm hornblende crystals are found to be in the range of 190 to 170 c tentative u he indices indicate that diffusion parameters of the same order of magnitude as determined for the he retentive minerals may be used to select minerals to be further examined for their u th he datability e g ore minerals

origin of geochemical variability by arc continent collision in the biru area southern sulawesi indonesia analyses of igneous rocks from the eocene calc alkaline and miocene geochemistry can be modelled by varying contributions potassic volcanic arc in southwest sulawesi indicate that magmas from the mantle wedge fluid from the subducted slab became more heterogeneous in their trace element and pb sr nd and the presence or absence of a subducted sedimentary isotopic signature following the collision of the buton microcontinent component white patchett 1984 gill williams with the arc at 15 ma isotopic ratios become more continental 1990 woodhead johnson 1993 kepezhinskas et al 4 my after the collisional event sr sr 0 7085 nd 1997 peate et al 1997 alves et al 1999 hoogewerff nd 0 5125 pb pb 19 2 pb pb 15 73 1999 pearce et al 1999 eiler et al 2000 despite pb pb 39 4 as the overriding plate consists of young agreement on the importance of these components there sundaland crust whereas the subducted sediment is likely to is still some dispute about the importance of an ocean have been shed from a compositionally distinct microcontinent of island basalt oib component in the wedge stolz et al australian derivation we can be certain that the continental isotopic 1990 edwards et al 1991 and the implied role of signature reflects subduction of continental material rather than residual rutile ryerson watson 1987 foley et al crustal contamination the isotopic compositions of the magmas 2000 if an arc is built upon continental crust or can be explained by the melting of a mixed mantle wedge consisting overlain by a thick layer of sediment there is a potential of fluid fluxed and sediment modified morb mantle in this model contribution from the upper plate via assimilation the maximum amount of sediment added to the mantle source is fractional crystallization afc processes which can 10 sometimes be difficult to distinguish from the signature of subducted sediment davidson 1987 thirlwall et al 1996 gasparon varne 1998 the only unambiguous way to distinguish between subducted sediment and

postcollision extension in arc continent collision zones eastern indonesia postcollisional extension superimposed on arc continent collision complexes is a common feature in the orogenic belts of eastern indonesia in the southern banda arc the arc continent collision event is essentially a pliocene and younger feature but already an element of extension is being superimposed on the compressional structures of the collision complex it is thus likely that the extension commences within a very short period 5 m y after the initiation of collision related compressional deformation a similar history of fold belt development immediately followed by extension can be inferred for the lengguru fold belt and the adjacent wandamen wondiwoi terranes of irian jaya and for the gulf of bone in sulawesi it is suggested that the extension results from decoupling of the subducting oceanic lithosphere from the unsubductable continental lithosphere and that the superimposition of extension is a virtually unavoidable consequence of the arc continent collision

strain accumulation across the carrizo segment of the san andreas fault california impact of laterally varying crustal properties 1 major strike slip faults juxtapose geologically dissimilar terrain which may vary in mechanical properties leading to an asymmetric pattern of strain accumulation we present new gps data on the carrizo segment of the san andreas fault separating the salinian block southwest of the fault from franciscan terrane northeast of the fault to better quantify asymmetric strain accumulation we also present a series of finite element models to investigate the possible role of variable elastic layer thickness and material properties of the upper crust the geodetic data are well fit with a simple model comprising a weak upper crustal zone 10 25 km wide northeast of the fault this model is also consistent with geologic data on the distribution of major rock types and corresponding laboratory data on their material properties as well as paleoseismic seismic and magnetotelluric data using this model we estimate a long term average over several seismic cycles slip rate for the san andreas fault of 36 1 5 2 mm yr in agreement with the known holocene rate within uncertainties and a viscosity for the combined lower crust upper mantle of 2 5 1019 pa s

diatoms of the fortuna basin southeast spain evidence for the intra messinian inundation the fortuna basin in southeast spain part of the betic strait was one of the connecting pathways between the atlantic ocean and the mediterranean sea during the late miocene the basin contains a remarkable record of messinian marine sediments two well laminated marly diatomaceous sections were examined the 1 m thick el fenazar section f section and the 36 m thick rio chicamo diatomite member rcd section both sections are intercalated with gypsum horizons the diatoms in the rio chicamo diatomite suggest an atlantic origin and strong coastal upwelling associated with high productivity on the basis of event stratigraphy the diatomites correlate to the intra messinian inundation of the mediterranean sea during the messinian salinity crisis four biostratigraphically useful species nitzchia jouseae thalassiosira oestrupii t praeoestrupii denticulopsis hustedtii and asterolampra acutiloba occur in the west pacific between 5 35 and 4 5 ma lower reversed gilbert chron this interpretation places the subsequent upper evaporite deposition into the lower reversed gilbert chron our results indicate 1 the existence of a marine connection between the atlantic ocean and the mediterranean sea through the betic strait southeast spain during the messinian salinity crisis 2 a possible earlier first appearance of nitzchia jouseae in the betic strait compared to the atlantic and pacific oceans according to the stratigraphy of the fortuna basin and 3 the probability that many of the tripolilike and younger diatomites in the mediterranean are local deposits and diachronous with each other

helium and argon isotope systematics of the central lau basin and valu fa ridge evidence of crust mantle interactions in a back arc basin abstract we report helium and argon isotope analyses of fresh lau basin volcanic glasses from six sites within the central basin 18 s six localities along the valu fa ridge 21 22 4 s and a single site from the northeastern basin 15 s central basin basalts have 3 he 4 he ratios r between 8 2 8 5 r a r a air 3 he 4 he 40 ar 36 ar ratios significantly greater than atmosphere up to 4900 and 4he contents from 3 5 9 4 10 6cm3 stp g similar to n morbs worldwide these results are consistent with trace element and other radiogenic isotope data on these samples which indicate derivation from a depleted mantle source region in contrast evolved lavas from the valu fa ridge all have r r a and show evidence of mixing with a component rich in radiogenic helium there is a clear relationship between the 3 he 4 he ratios of these samples and their chemistry basaltic andesites have 6 02 r c r a andesites have lower ratios 2 37 2 65 ra and a dacite has the lowest value of the entire sample suite 1 19 ra all lavas have 40 ar 36 ar ratios similar to the atmospheric value and low helium concentrations from 3 11 10 8cm3 stp g or between 30 and 300 times less than the central basin basalts although the helium isotope results of the valu fa lavas mirror the shift to more radiogenic values of other isotope systems e g 206 pb 204 pb 87 sr 86 sr which indicate addition of subducted sediment to these magma sources we find no evidence that the radiogenic helium has a mantle or slab derivation or is in any way coupled to these other tracers instead the most plausible mechanism to explain its incorporation into the valu fa lavas is by assimilation of old lau crust in the near surface environment by previously degassed magma we argue that this mechanism has general applicability and can explain a number of hitherto apparently paradoxical geochemical features of some back arc and ocean ridge lavas such as their high volatile and lil element contents with low rare gas concentrations and their mantle 3 he 4 he ratios with hydrated oceanic crustal d h values the realisation that the helium and argon systematics of the valu fa lavas are controlled by crust mantle interactions has important implications for distinguishing between a number of models proposed for the formation of such evolved lavas and we show that fractional crystallisation processes can most readily account for the low concentrations of and systematic trends in the mantle derived helium and argon component of these lavas in addition because pre existing crust in the lau basin must be old and or altered enough to supply the radiogenic helium and atmospheric like 40 ar 36 ar component to the valu fa lavas the occurrence of crust mantle interactions implies that old forearc crust may have been trapped within the lau basin such a scenario has a clear bearing on ideas of the tectonic development of the basin finally because of the potential of crust mantle interactions to modify 3 he 4 he and 40 ar 36 ar ratios of mantle derived melts we assess the implications for using he and ar tracers to characterise mantle sources in arcs back arcs and spreading ridges and consider the consequences for the combined use of rare gases with other radiogenic isotopic tracers of magma provenance at such settings the basaltic andesite from the northeastern basin may also be influenced by the same kind of crustal interaction as the valu fa lavas as it falls within the 3 he 4 he range 6 9ra of the other basaltic andesites interestingly other helium isotope studies indicate that this part of the basin is characterised by a wide range in 3 he 4 he ratios from morb values up to 22ra the low 3 he 4 he ratio of the basaltic andesite therefore serves to illustrate the possible effects of magma chamber processes on the rare gas and other volatile characteristics of hotspot lavas an observation which is important not only for this part of the lau basin but for other localities worldwide

neodymium isotopic composition of quaternary island arc lavas from indonesia abstract 143 nd 144 nd ratios measured in quaternary lavas from java and the banda arc of indonesia range from 0 51242 to 0 51280 and exhibit an inverse correlation with 87 sr 86 sr isotopically the indonesian samples resemble andean rather than island arc lavas the samples from java plot either within or adjacent to the mantle array towards higher 87 sr 86 sr ratios samples from the banda arc and the anomalous calc alkaline volcano papandajan are characterized by relatively low 143 nd 144 nd and high 87 sr 86 sr ratios these characteristics are consistent with the interpretation that subducted terrigenous material was involved in the genesis of these lavas furthermore the banda arc samples appear to lie on a mixing line between isotopic compositions characteristic of the mantle and upper continental crust a high k trachyte from the alkaline volcano muriah java has isotopic characteristics of the mantle 143 nd 144 nd 0 51270 87 sr 86 sr 0 70424 which implies that the extreme enrichment in large ion lithophile elements in its source must have occurred only shortly before its formation the inferred 143 nd 144 nd ratio of the unmodified mantle beneath java and the banda arc is lower than that observed in mid ocean ridge basalt which may have important implications for a better understanding of the geochemical structure of the mantle

helium strontium isotope constraints on mantle evolution beneath the roman comagmatic province italy european social fund scottish universities carnegie trust for the universities of scotland

mapping magma sources in the east sunda banda arcs indonesia constraints from helium isotopes abstract we report new helium isotope analyses of olivine and clinopyroxene separates from recent lavas for eleven volcanoes from flores in the east sunda arc through the inactive segment between the arcs to banda island at the extreme of the contiguous banda arc in the east sunda arc 3 he 4 he ratios r vary between 4 5 ra r a air 3 he 4 he for the leucitic batu tara volcano to a remarkable low of pure radiogenic helium 0 0075 ra for werung at the southern tip of lomblen island lavas from the inactive zone which represents the locus of collision of the australian continent with the arc have a narrower range in r r a from 3 9 for kisu in the straits of pantar to 1 0 for romang island our one locality banda island in the banda arc gives the highest r r a ratio 3 1 observed along this arc to date the results are consistent with the involvement of crustal material in magma genesis throughout the east sunda banda arcs as far west as iya in central flores we combine these helium isotope results with published and on going strontium isotope studies and show that the source of the helium in the crustal component is unlikely to be terrigenous sediments derived from the australian continent rather degassing of australian continental crust appears to be the dominant process controlling the helium budget the he sr isotope systematics also provide a framework to account for the areal pattern of 3 he 4 he in this part of the arcs the radiogenic crustal component is diluted with mantle helium both in a down dip direction and with increasing lateral distance from the collision zone these factors result in an excellent first order relationship between the 3 he 4 he ratio degree of he sr enrichment relative to the postulated mantle endmember and alkalinity of the erupted lavas such a relationship has a direct bearing on models of the tectonic evolution of the collision zone and on the observation that helium isotopes are decoupled from strontium and other geochemical signatures along the banda arc

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

geochronology and thermochronology by the 40ar39ar method ian mcdougall and t mark harrison oxford university press 1988 212p u s 55 00 isbn 0 19 504302 2

microearthquake seismicity and fault plane solutions related to arc continent collision in the eastern sunda arc indonesia the collision of australia with the eastern sunda and banda arcs of indonesia represents a modern example of the early stages of arc continent collision in order to obtain a more detailed view of the tectonics of the collision zone a microearthquake survey was conducted around the savu sea the region that encompasses the transition from subduction of indian ocean lithosphere on the west to collision or subduction of australian continental litho sphere on the east intermediate depth earth quakes were concentrated near pantar island the easternmost active volcano of the sunda arc and outline a northwest dipping zone that strikes n65oe from 70 to 150 km depth the seismic zone probably marks the western edge of the collision zone and its more northerly strike and the orientations of nodal planes from fault plane solutions indicate a 25 o bend in the subducted slab to a depth of at least 150 km because the convergence rate and the timing of collision from geologic observations on timor suggest that the oart of the slab now at 150 km depth was at the trench when australia first collided with timor we infer that continental crust or at least rifted continental margin crust caused the bend in the convergent margin and has been subducted to 150 km depth fault plane solutions of several events show nearly vertical nodal planes that trend parallel to the strike of the seismic zone along which a northwest side down sense of dis placement is indicated these events are concentrated at the southern top side of the seismic zone and suggest that the subducted lithosphere is presently detaching in the 50 100 km depth range beneath the eastern savu sea fault plane solutions for earthquakes within the upper part of the detaching slab 70 100 km depth have p and t axes whose trends lie parallel to the strike of the seismic zone and horizontal southeast trending b axes these earthquakes are interpreted as due to stresses caused by the bend in the subducting plate

carbon sources in arc volcanism with implications for the carbon cycle new co2 3 he data from the east sunda arc indonesia confirm earlier observations that arc volcanic gases have higher co2 3 he ratios than mor environments on average 80 of arc volcanic co2 is recycled exogene carbon addition of a few percent of carbonate bearing sediments to the mantle wedge explains much of the carbon abundance andcarbon isotopic data of arc gases but can not explain the he isotope observations the co2 3he in arc volcanoes is not strongly dependent on the composition of modem trough sediments e g deep sea clays vs carbonate rich sequences and calcite veins in the hydrothermally altered subducted slab may provide a contribution to the recycled carbon flux of arcs the sum of globally deep subducted sediment and slab carbon exceeds the estimated arc co2 flux and approximately 3 5 teramole of carbon may return annually to the mantle in convergent zones the modem combined processes of mor volcanism slab alteration and subduction volcanism do not produce a substantial carbon flux into the exosphere and rate changes in ocean floor spreading are unlikely to cause major changes in atmospheric co2 as a result of changes in the volcanic co2 fluxes intense pulses of flood basalt volcanism however may alter the co2 contents of the atmosphere over the course of a millenium or so and influence global climate

the origin of the potassic rock suite from batu tara volcano east sunda arc indonesia abstract atu tara is an active potassic volcano in the eastern sunda arc its leucite bearing rock suite can be subdivided into two groups one less evolved with th 20 ppm 87sr 86sr 18o and trace element systematics in the less evolved group suggests that existence of parental magmas with different mantle origins the mantle below batu tara is most likely heterogeneous and several source components are involved in magma genesis trace element and isotopic compositions of batu tara and adjacent volcanoes are consistent with the involvement of a subducted sedimentary crustal component as well as morb and oib mantle the latter with geochemical characteristics comparable to the mantle underlying muriah java melt extraction from this complex mixture is envisioned as a two stage process partial melts of the crust contaminated morb mantle mix in the mantle wedge with partial melts of oib domains different mixtures of these two melts provide the parental magmas that enter the volcanic plumbing system where crystallization hybridization and refilling processes occur the calcalkaline volcanoes in the arc segment show stronger signatures for a subducted crustal component than batu tara which displays a greater influence from the oib mantle source the potassium enrichment can therefore be attributed to contributions both from the enriched mantle and from subducted crustal material mantle type 18o values of the batu tara magmas indicate that the mantle wedge below potassic orogenic volcanoes is not necessarily strongly enriched in 18o

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift