isotope and trace element evidence for late stage intra crustal melting in the high andes abstract purico chascon is an acid igneous complex less than 1 5 ma old rising to 5800 m in the north chilean andes and consisting of andesite dacite cones and dacite domes on an ignimbrite shield the rocks are subdivided into two groups those from chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths whereas among those at purico no xenoliths have been found 87sr 86sr 0 7095 0 7081 at purico 0 7079 0 7069 at chascon and 0 7061 0 7057 in the xenoliths from the chascon lavas 143nd 144nd 0 51222 0 51236 overall the purico lavas are characterised by higher sio2 rb sr 87sr 86sr and ree abundances and lower sr nd sr ba and143nd 144nd than most andean igneous suites there is no indication ofselective crustal contamination of sr or any systematic change in isotope ratios during differentiation nonetheless the trend of for example high sr nd and sr contents in rocks with low87sr 86sr 0 704 ecuador to low sr nd and sr and high sio2 in rocks with87sr 86sr 0 7081 0 7095 at purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity the formation of the least evolved purico lavas 60 sio2 is discussed in terms of bulk assimilation of crustal material mixing between crustal and mantle derived magmas and partial melting of pre existing crust although such models are still extremely primitive the simplest explanation of the observed chemical variations is that the purico rocks evolved from parental magmas derived by crustal anatexies thermal considerations suggest that such late stage crustal anatexis is a predictable response to crustal thickening which in the andes is thought to have taken place during the cenozoic

petrology and geochemistry of potassic and ultrapotassic volcanism in central italy petrogenesis and inferences on the evolution of the mantle sources major trace element sr isotopic and mineral chemical data are reported for mafic volcanic rocks mg value 65 from the northern central sector of the potassic volcanic belt of central italy the rocks investigated range from potassic series ks and high k series hks to lamproitic lmp and kamafugitic kam through a transitional series trans thus covering the entire compositional spectrum of potassic and ultrapotassic magmas kam rocks are strongly silica undersaturated and compared with the other rock series have low sio2 al2o3 na2o sc and v and high cao k na na k al ks and hks have high al2o3 cao and variable enrichment in k2o and incompatible elements lmp rocks are saturated in silica and have high sio2 k2o k2o na2 mgo ni and cr and low al2o3 cao na2o sc and v trans rocks display intermediate compositional characteristics between lmp and ks all the rocks under study have fractionated hygromagmaphile element patterns with high lil hfs element values and negative anomalies of ti ta nb and ba negative sr anomalies are observed in the lmp and trans rocks lil elements show overall positive correlations with k2o whereas different trends of sr and hfse vs k2o are defined by lmp trans and ks hks kam 87sr 86sr range from about 0 710 to 0 716 ks hks and kam rocks have similar 87sr 86sr values clustering around 0 710 lmp and trans rocks have the highest 87sr 86sr values geochemical and isotopic data reported for the most primitive italian potassic and ultrapotassic rocks support the hypothesis that the interaction between crustal and mantle reservoirs was a main process in the genesis of italian potassic magmatism simple mass balance calculations exclude however an important role of crustal assimilation during ascent of subcrustal magmas to the surface and indicate that the sources of central italy volcanics underwent contamination with fluids and or melts released by upper crustal material previously brought into the mantle by subduction processes different trends of incompatible elements vs k2o observed in the studied rocks suggest distinct metasomatic processes for the sources of the investigated magmas liquids derived by bulk melting of pelitic sediments are believed to be the most likely contaminants of the source of lmp rocks fluids or melts rich in ca sr and with high lile hfse value and sr isotopic composition around 0 710 are the most likely contaminant of the source region of ks hks and kam volcanics variations in cao na2o and ferromagnesian element abundances and ratios suggest that in some zones the mantle source of potassic magmas experienced partial melting with extraction of basaltic liquids prior to metasomatism

roman comagmatic province central italy evidence for subduction related magma genesis geochemical data on mafic volcanics show that important affinities exist between the roman and the calc alkaline rocks from the aeolian arc south tyrrhenian sea these affinities together with the close association of calc alkaline and k rich volcanics in the aeolian arc and in the naples area the continuity in the variation of abundances of incompatible elements from calc alkaline to potassic suites and the similarity in terms of major element geochemistry support a genetic relationship of the roman magmatism and the subduction processes that affected the apennines in tertiary time and are still active under the aeolian arc in the genetic model presented here both calc alkaline and k rich magmas were generated within a mantle heterogenously enriched in lil elements composition of the mantle was modified by addition of material probably sediments dragged down by the undergoing slab the geochemical and petrological differences displayed by the calc alkaline and k rich volcanics are accounted for by the different conditions of melting as well as by chemical and isotopic heterogeneities of the source 26 references 3 figures 1 table

helium 3 anomalies and crust mantle interaction in italy abstract the distribution of primordial 3he in the crust of italy has been investigated through a survey of groundwaters hot springs exploration and production geothermal wells the mantle derived component varies from 0 to 60 and reaches a maximum at vulcano in the eolian islands 3he anomalies occur where the conductive heat flow is relatively high but the relationships between heat flow and anomaly are uncertain the distribution of 3he in the main reservoir and cap rock to the larderello geothermal system shows that the mantle derived 3he varies from 5 to 40 of total he and reaches a maximum in fluids extracted from a depth of ca 3 km within the basement bomb produced tritiogenic 3he appears to make a near negligible contribution well 107 at larderello has been monitored over a period of 17 months and the 3 he 4 he ratio r varies from 1 4 to 1 6 times the atmospheric ratio ra whilst he ne varies from 9 to 160 a model is proposed for the larderello geothermal system whereby mantle derived heat and volatiles are advected to shallow 6 8 km depths in association with melts and released into the main geothermal reservoir at larderello in a non uniform manner possibly associated with fracturing isotopic and elemental equilibrium of gaseous species c o h appears to have taken place in the basement to the main reservoir from where fluid with r ra 3 2 has been extracted

the geochemistry of potassic lavas from vulsini central italy and implications for mantle enrichment processes beneath the roman region major and trace element and 143nd 144nd 0 51209 0 51216 and 87sr 86sr 0 70879 0 71105 isotope analyses are presented on a representative group of lavas from the vulsini district of the roman magmatic province three distinct series are identified the high k and low k series are similar to those described from other italian volcanoes while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near primary mantle melts major and trace element variations within the high k series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites crustal contamination resulted in an increase in 87sr 86sr with increasing fractionation but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub continental mantle these are reviewed using the available results from vulsini roccamonfina and ernici transition element abundances and ta yb ratios indicate that the pre enrichment mantle was similar to that of e type morb and that these elements were not mobilised by the enrichment process mixing calculations suggest that three components were involved in the enrichment process mantle comparable with the source of morb and two other components rich in trace elements one the low k component had high sr nd th ta and ba nb and no europium anomaly while the second had lower sr nd a negative europium anomaly and very high th ta it was also characterised by low nb ba and high rb ba ratios similar to those reported from phlogopite rich peridotite xenoliths the trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high k end member reflecting the subduction of sediments and the stabilisation of mantle phlogopite

step filling and development of a three layer magma chamber the neapolitan yellow tuff case history abstract the neapolitan yellow tuff the product of the largest known trachytic phreatoplinian eruption gives a good opportunity to investigate the filling mechanisms and internal dynamics of a trachytic magma chamber a detailed study of the geochemical mineralogical and isotopical features of the deposit was carried out to investigate the behaviour of the magma chamber before the eruption the collected data show three distinct compositional groups separated by gaps single depositional units contain glass shards formed contemporaneously although each of these shards is homogeneous they display the same compositional variations and gaps detected in pumice clasts this feature is taken as an evidence for interpreting the detected compositional gaps as real gaps in the chamber therefore the chamber was filled by three distinct magma bodies separated by compositional gaps the uppermost magma was alkali trachyte and highly homogeneous likely a consequence of vigorous convection the intermediate magma was trachyte with a slight and continuous compositional variation likely resulting from less intense convection the lowermost magma was compositionally zoned from alkali trachyte to latite downward this compositional zonation was most likely acquired during uprise from a deeper reservoir the three magmas entered the chamber sequentially from the uppermost to the lowermost the latter entered the chamber short before the beginning of the eruption its input was interpreted as a possible triggering factor for the eruption the results of this study strongly support a step filling mechanism for the neapolitan yellow tuff magma chamber and allow definition of the temporal succession of input of magma batches furthermore they also suggest that the magma bodies did not mix although at least the uppermost two coexisted inside the chamber for a time long enough to allow internal homogeneization by convection

multiple mantle metasomatism in central southern italy geochemical effects timing and geodynamic implications the regional petrological and geochemical variations observed in the recent magmatism along the italian peninsula are interpreted to testify to the coexistence of distinct sectors of upper mantle these are suggested to result from at least three compositionally and temporally distinct metasomatic events that affected petrologically different premetasomatic mantle sources geological and geochemical evidence suggests that metasomatism in the northern sector occurred during the alpine subduction by introduction of a composition similar to the deeply subducted dora maira metagranites into a residual lithospheric mantle of upper crustal material metasomatic events beneath the roman and neapolitan areas are younger these are related to the addition of melts and fluids into a fertile asthenospheric mantle during the latest stage of the west directed apennine subduction of the adria plate and during the current northwest subduction of the ionian sea floor

a combined o sr nd and pb isotopic and trace element study of crustal contamination in central andean lavas i local geochemical variations systematic variations in o sr nd and pb isotopic ratios and in sr nd pb and rb trace element abundances in central andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization the results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the arequipa and barroso volcanic groups of southern peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of charcani gneiss or an equivalent rock accompanied by dominantly plagioclase fractional crystallization a consequence of this result is that the rbsr pseudoisochrons reported for the arequipa and barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation as are all of the pb isotopic variations measured in the same lavas already interpreted by tilton and barreiro 14 to be the result of crustal contamination isotopic and trace element compositions of the primary magmas for the arequipa and barroso volcanic series are obtained by correcting for assimilation and fractional crystallization the starting magmas are characterized over the entire range of permissible primary compositions by high87sr 86sr ratios low143nd 144nd ratios and high sr concentrations relative to normal island arc andesites these results appear to require prior contamination of the magmas but not involving plagioclase fractionation the uncertainty in pb and o isotope ratios computed for possible primary magmatic compositions is sufficiently large however that it is not possible to preclude a subcrustal origin for at least some regional variation in sr and nd isotopic ratios

atmospheric noble gases in volcanic glasses from the southern lau basin origin from the subducting slab abstract noble gas concentrations and isotopic compositions have been determined for four submarine volcanic glasses from the valu fa ridge vfr in the southern lau basin the samples are the least differentiated ones from this area and they display enrichments in fluid mobile elements similar to the nearby island arc 3he 4he ratios are slightly below average morb 6 8 7 8 times atmospheric whereas ne ar kr and xe have isotopic compositions very similar to air together with previously published data from the valu fa ridge and other spreading segments in the lau basin our data show a systematic latitudinal variation of increasing ne ar kr and xe abundances from north to south as well as ne and ar isotopic compositions changing from morb like to atmosphere like in the same direction moreover isotopic compositions and noble gas abundances of the lavas correlate strongly with ba nb ratios and h2o concentrations based on these observations and mass balance arguments we propose that the atmospheric noble gases come from the subducting oceanic crust and are not due to shallow contamination with air dissolved in seawater or assimilation of old crust our data suggest that the noble gases released from the subducting slab are atmospheric and thus contain little or no solar he and ne in addition to the fact that ratios of he to heavy noble gases are small in aged ocean crust he has possibly fractionated from the other noble gases due to its higher diffusivity and thus he transport from the subducting slab into the mantle wedge is probably insignificant we propose that the 3he 4he ratios lower than morb observed in the vfr lavas result from radiogenic ingrowth of he in a highly depleted and hence degassed mantle wedge after the enrichment of u and th released from the downgoing slab

isotope geochemistry of pantelleria volcanic fluids sicily channel rift a mantle volatile end member for volcanism in southern europe chemical and isotopic ratio he c h and o analysis of hydrothermal manifestations on pantelleria island the southernmost active volcano in italy provides us with the first data upon mantle degassing through the sicily channel rift zone south of the african european collision plate boundary we find that pantelleria fluids contain a co2 he rich gas component of mantle magmatic derivation which at shallow depth variably interacts with a main thermal 100 c aquifer of mixed marine meteoric water the measured 3he 4he ratios and 13c of both the free gases 4 5 7 3 ra and 5 8 to 4 2 respectively and dissolved helium and carbon in waters 1 0 6 3 ra and 7 1 to 0 9 together with their covariation with the he co2 ratio constrain a 3he 4he ratio of 7 3 0 1 ra and a 13c of ca 4 for the magmatic end member these latter are best preserved in fluids emanating inside the active caldera of pantelleria in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir outside the caldera the magmatic component is more affected by air dilution and at a few sites by mixing with either organic carbon and or radiogenic 4he leached from the u th rich trachytic host rocks of the aquifer pantelleria magmatic end member is richer in 3he and has a lower closer to morb 13c than all fluids yet analyzed in volcanic regions of italy and southern europe including mt etna in sicily 6 9 0 2 ra 13c 3 1 this observation is consistent with a south to north increasing imprint of subducted crustal material in the products of italian volcanoes whose he and c but also o and sr isotopic ratios gradually evolve towards crustal values northward of the african eurasian plate collision boundary our results for pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or less contaminated 3he richer mantle source beneath the sicily channel rift zone the lower than morb 3he 4he ratio but higher than morb co2 3he ratio of pantelleria volatile end member are compatible with petro geochemical evidence that this mantle source includes an upwelling himu em1 type asthenospheric plume component whose origin according to recent seismic data may be in the lower mantle

noble gases in mafic phenocrysts and xenoliths from new zealand we have determined the elemental and isotopic compositions of noble gases in young subduction related phenocrystic olivine and clinopyroxene samples from the taupo volcanic zone central north island new zealand and in behind arc intraplate phenocrystic and xenolithic olivine samples from the northland and auckland volcanic provinces northern north island new zealand helium isotopic ratios range from morb like 3he4he values of about 11 10 6 to lower values of about 6 10 6 consistent with previous measurements of helium isotopic ratios in subduction related samples 40ar36ar and 21ne22ne ratios range from atmosphere like compositions to maximum values of about 700 and 0 033 respectively in contrast most of the 21ne22neratios are generally indistinguishable from atmospheric values the variations in helium neon and argon isotopic ratios are interpreted as resulting from mixing of 1 a a primordial 3he rich component derived from the upper mantle and characterised by morb like 3he4he ratios of about 12 10 6 2 b a radiogenic 4he rich component derived from crustal materials and characterised by a 3he4he ratio of less than 2 10 7 and 3 c a helium poor atmosphere derived component which dominates the heavier noble gases by combining the helium and argon isotopic results it is possible to estimate the relative contribution of each of these three components to the total 40ar observed in each sample based on the present understanding of the origin and evolution of arc magmas a simple qualitative model of the mechanisms which introduce noble gases to the parent magmas of the samples is developed however it remains uncertain whether the atmospheric and crustal derived noble gas components are introduced to the mantle source regions of the magmas by subduction or alternatively are introduced by interactions between the ascending parent magmas and the overlying crust

he ar o sr and nd isotope constraints on the origin and evolution of mount etna magmatism abstract the 0 5 ma history of the origin and evolution of mount etna sicily has been investigated by analysing the isotopic composition of he ar o sr and nd in 21 selected lava samples the near constancy of the isotopic compositions of oxygen 18 o 5 4 0 3 and of helium trapped in olivine phenocrysts 6 7 0 4 ra is interpreted as evidence of a single mantle source despite drastic petrological changes during the volcano s history he analyses performed by crushing cogenetic pyroxene and olivine phenocrysts show a tendency to lower 3 he 4 he ratios in pyroxenes this is best explained by crystallization of pyroxenes at a depth shallower than that of olivines and or by exchange of helium trapped in pyroxenes with atmospheric or radiogenic he before eruption 87 sr 86 sr ratios of recent lavas tend to increase with time and to correlate with rb th ratios and for historical lavas these variations are tentatively attributed to shallow selective contamination from underlying sediments based on the similarity of the 3 he 4 he ratios at etna to those of european mantle xenoliths 1 we propose that the baseline geochemical signature of isotopic tracers at etna reflects the composition of the subcontinental mantle comparison to other southern italian active volcanoes etna vulcano ischia campi flegrei vesuvius shows gradual dilution of the predominantly mantle etnean end member by more radiogenic sr and he and material with higher 18o and c 3 he which is reasonably explained by the progressively important influence of subducted continental crust

uranium and 10 be enrichments by fluids in andean arc magmas volcanic rocks from subduction settings frequently contain 10be enrichments that have been attributed to sediment subduction1 6 and are often characterized by 238u 230th ratios greater than unity in contrast to mid ocean ridge and ocean island basalts7 8 here we report such an excess of 238u over 230th in historic lavas from the southern volcanic zone of chile 33 42 s the greatest uranium excess is found south of 39 s where 10be 9 be ratios are highest and the trench is filled with sediment these features suggest that both u and 10be enrichments result from preferential partitioning of u and be into a fluid phase during dehydration of the recently subducted sediments the presence of strong 238u 230th disequilibria in basaltic magmas of the southern volcanic zone suggests that the timescale for dehydration melting and eruption in this arc is probably less than 20 000 yr

experimental determination of au solubility in rhyolite melt and magnetite constraints on magmatic au budgets the solubility of au metal in rhyolite melt and coexisting magnetite has been determined at 800 c 140 mpa and f 0 2 nno in a au metal saturated sulfur free vapor brine silicate melt system whole crystals of magnetite suspended within quenched rhyolite glass were analyzed by la icp ms these data yield a solubility of au in magnetite on the order of 2 g g the solubility of au metal in rhyolite melt hosting the magnetite crystals is on the order of 500 ng g these data indicate a partition coefficient for au between magnetite and melt d m t m e l l au 4 using reasonable estimates of the mass fraction of magnetite that crystallizes in crustal magmatic systems we modeled the proportion of au sequestered by magnetite during fractional crystallization we considered fractionation in two steps the idealized derivation of a rhyolite by fractionation of basalt at depth and the closed system crystallization of the rhyolite in a magma chamber below the site of ore deposition magnetite sequesters 14 54 au from basaltic parent melt that produces a rhyolite melt via crystal fractionation at modal abundances of 1 5 less then 4 au is sequestered from a crystallizing rhyolite melt because of low modal abundances of magnetite 2 our experimental and model results suggest that early crystallization of magnetite can play a mitigating role in the ability of a calc alkaline magmatic system to yield a au rich ore fluid

subsidence at southern andes volcanoes induced by the 2010 maule chile earthquake large earthquakes in subduction zones can affect nearby volcanoes analysis of ground deformation following the 2010 earthquake in maule chile shows that some volcanoes subsided by up to 15 cm probably because of quake triggered release of hydrothermal fluids

petrology geochemistry and sr isotope characteristics of lavas from the area of commenda mts vulsini italy major trace element and sr isotope compositions are reported for a suite of lavas coming from the area of commenda in the se vulsinian district the analyzed samples have all low silica contents and variable but generally high cao mgo and feot based on k2o and k2o na2o ratio the rocks from commenda can be classified as belonging to the potassic series ks and the high potassium series hks the hks rocks appear to have derived by cristal liquid fractionation from the most mafic types with separation of olivine and clinopyroxene and then of clinopyroxene leucite the most primitive hks rocks have aphyric texture and high mg values cr and ni contents which are close or within the range of values of magmas formed by partial melting of periodititic mantle sources the ks rocks have lower incompatible element contents as the hks rocks with similar degree of evolution

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

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

the geochemistry and petrogenesis of the lavas of the vulsinian district roman province central italy the vulsinian lavas are dominated by a suite of undersaturated leucite bearing basic to intermediate compositions the remaining lavas are mainly oversaturated and have shoshonitic affinities one hundred and thirty five samples have been analysed for major elements and most for 20 trace elements twenty seven lavas have been analysed for ree they are all perpotassic for the undersaturated lavas k2o na2o 2 8 and have very high lil element concentrations e g rb 400 800 ppm th 25 150 ppm ree reecho c 200 lree hree cho c 20 even in the most basic rocks

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

petrogenesis and tectonic setting of the roman volcanic province italy abstract the volcanism in the roman province of italy can be modelled by the partial melting of heterogeneously enriched mantle sources the heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central apennines new petrographical and chemical data are presented for the high k calc alkaline and the shoshonitic volcanics of the campania region primary magmas are present only locally the existence of spatial zonation in the volcanism of campania is documented for the first time the shoshonitic leucite basanitic and leucititic volcanics of the phlegraean fields procida ischia and the somma vesuvius areas are at similar degrees of evolution about two times richer in nb and ba than those of northwestern campania and the latium part of the roman province accordingly distinct north western and south eastern subprovinces can be defined the evaluation of enrichment factors that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low k and the high k suites shows that the mantle sources prior to k metasomatism were different in the two sub provinces of the roman province in the north western one they resembled the sources of ocean island tholeiites and moderately enriched mor basalts in the south east they were similar to those of ocean island alkaline lavas and enriched morb s modelling based on k p ce sr rb ba th sm eu gd y nb and 87 sr 86 sr was carried out it indicates that the range of mantle sources of the volcanics in northwestern campania and latium can be modelled by the addition of 3 to 20 of materials derived by partial melting of carbonaceous pelites to a sr enriched mantle wedge comparable to the honolulu mantle source least enriched in nb the production of sr enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust or a small amount of metasomatism caused by the presence of carbonatite melts the near absence of ti ta nb yb and the highly fractionated ree in the metasomatizing component requires the presence of residual garnet and accessory ti rich minerals during the partial melting of the subducted sediments the writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust probably sediments this process played a dominant role in the generation of the mantle sources from which the high k calc alkaline shoshonitic leucite basanitic and leucititic magmas of the roman province were derived