the tolbachik volcanic massif a review of the petrology volcanology and eruption history prior to the 2012 2013 eruption abstract the primary goal of this paper is to summarize all of the published data on the tolbachik volcanic massif in order to provide a clear framework for the geochronologic petrologic geochemical and to a lesser extent the geophysical and tectonic characteristics of the tolbachik system established prior to the 2012 2013 eruption the tolbachik massif forms the southwestern part of the voluminous klyuchevskoy volcanic group in kamchatka the massif includes two large stratovolcanoes ostry sharp tolbachik and plosky flat tolbachik and a 70 km long zone of the basaltic monogenetic cones that form an arcuate rift like structure running across the plosky tolbachik summit the tolbachik massif gained international attention after the 1975 1976 great tolbachik fissure eruption gtfe which was one of the largest eruptions of the 20th century and one of the six largest basaltic fissure eruptions in historical time by the end of the gtfe 2 2 km 3 of volcanic products of variable basaltic compositions with morb like isotopic characteristics covered an area of 1000 km 2 during the following three decades more than 700 papers on various aspects of this eruption have been published both in national and international journals although the recent 2012 2013 eruption which is the main topic of this volume was not as long as the gtfe in duration or as large in area and volume of the erupted deposits it brought to the surface a unique volcanic material never found before in order to understand the data from new eruptions and make significant progress towards a better understanding of the tolbachik magmatic system it is important to be able to put the new results into the historic context of previous research

discriminating assimilants and decoupling deep vs shallow level crystal records at mount adams using 238u 230th disequilibria and os isotopes article a suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the mount adams volcanic field has been analyzed for u th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis all of the lavas have 230 th 238 u n1 and span a large range in 230 th 232 th ratios

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

stress diffusion along rupturing plate boundaries an analysis is made of viscoelastic lithosphere asthenosphere coupling in the time dependent redistribution of stress along plate boundaries or other seismic lineaments following great earthquakes the study is based on a generalization by rice of elsasser s model of stress diffusion in which general elastic plane stress deformations are allowed in lithospheric plates which are coupled in an elementary way to a maxwellian viscoelastic asthenosphere solutions are developed which describe the large scale quasistatic distribution of thickness averaged in the lithosphere at or near stationary or travelling rupture zones modeled here by either crack like zones of fixed stress drop or dislocation type slip zones sudden ruptures shed load onto the asthenosphere which is gradually transferred back to the lithosphere by a slow relaxation process the spatial and temporal characteristics of the predicted stress alterations suggest a significant role of lithosphere asthenosphere coupling effects in triggering interactions of great earthquakes patterns of prolonged aftershock activity and the breaking of barriers or gaps by time dependent stressing

the evolution of weathering profiles through time new insights from uranium series isotopes the sustainability of soil is a major issue for society in principle the evolution of soil resources can be constrained by comparing the rates of soil erosion and production cosmogenic isotopes provide one measurement of soil erosion rates they can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production this implies that the evolution of soil resources thinning thickening or constant since soil thickness is assumed to be constant with time here we utilise an independent method to estimate soil production rates using uranium series u series isotopes the study of a site in temperate australia undisturbed by human activity shows that soil production rates inferred from u series isotopes are similar to erosion rates derived from beryllium 10 10be measurements implying that at this site there is no net accumulation or loss of soil saprolite production rates the migration rate of the weathering front into the bedrock are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady state this study demonstrates that the combination of u series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles preliminary observations suggest that the rate of bedrock weathering i e saprolite production in temperate australia is of the same order of magnitude as that inferred for laterites in tropical climates this may suggest that for thick weathering profiles although the extent of weathering strongly differs between temperate and tropical climates the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions

klyuchevskoy volcano kamchatka russia the role of high flux recharged tapped and fractionated magma chamber s in the genesis of high al2o3 from high mgo basalt

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

u series evidence for crustal involvement and magma residence times in the petrogenesis of parinacota volcano chile in this study we present th u disequilibria as well as radiogenic and trace element data for recent volcanic rocks from the nevados de payachata volcano which erupted through 70 km of continental crust in the central volcanic zone of the andes 18 s 69 w both lavas and mineral separates were analyzed by mass spectrometry for 238u 230th disequilibria the lavas are characterized either by 230th enrichment or depletion relative to its parent nuclide 238u mineral separates are used to derive u th isochron ages and these ages compare favorably with inferred stratigraphic ages or k ar ages although in one case the u th age is significantly older than the stratigraphic age despite relatively constant sr nd and pb isotope ratios the lavas display inverse trends in 230th 238u versus ce yb or ba hf diagrams these trends cannot be interpreted by simple two component mixing rather there must be three and perhaps four components involved in the genesis of the parinacota lavas a mantle wedge a slab fluid and a lower crustal component can be identified a sediment component is more difficult to detect as it is difficult to decipher its signature because of the strong crustal influence the existence of binary arrays can be explained by variable amounts of crustal material the process of crust mantle interaction must have been short enough to preserve u th disequilibrium 300 ka

rapid magma ascent and generation of 230 th excesses in the lower crust at puyehue cordon caulle southern volcanic zone chile basaltic to rhyolitic lavas and tephras erupted over the last 70 kyr at the puyehue cordon caulle volcanic complex in the andean southern volcanic zone svz were analyzed for major and trace element sr isotope and u th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis internal u th mineral isochrons from five lavas and three tephra fall deposits are indistinguishable from their eruption ages indicating a short period b1000 yr of crystal residence in the magma prior to eruption the 230 th 232 th ratios define a narrow range 0 80 0 83 compared to that of all svz lavas 0 72 0 97 suggesting that puyehue basalt was derived from a relatively uniform mantle source dacites and rhyolites have the largest u excesses and likely evolved via fractional crystallization of a plagioclase dominated mineral assemblage in contrast basalts have 1 to 6 230 th excesses a characteristic not previously observed in frontal arc stratovolcanoes of the andean svz the 230 th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust lower crustal processes therefore have dampened the 238 u excesses that were generated during fluid addition to the mantle wedge although prior 238 u 230 th 226 ra studies of lavas from other southern svz stratovolcanoes 36 to 41 s have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc our results suggest that mash processes envisioned by hildreth and moorbath w hildreth s moorbath crustal contributions to arc magmatism in the andes of central chile contrib mineral petrol 98 1988 455 489 in the northern svz also occur in the southern svz where the crust is relatively thin

seismicity and structure of the kamchatka subduction zone the configuration of the pacific plate subducted beneath the kamchatka peninsula and the stress distribution in the kamchatka subduction zone ksz were studied using the catalog of the kamchatka regional seismic network focal mechanism solutions estimated from p wave first motions the formal inversion of long period waveforms and centroid moment tensor solutions to the south of 55 n the slab shows an approximately constant dip angle of 55 to the north of 55 n the dip of the slab becomes shallower reaching 35 the maximum depth of seismicity dm varies from 500 km depth near 50 n to 300 km depth at 55 n the volcanic front is almost linear along the main part of the ksz whereas it is sharply shifted landward to the north of 55 n the variation of dm is apparently consistent with the standard empirical relation dm where is the thermal parameter of the subducted slab to the north of 55 n the slab is offset toward the northwest and it is sharply deformed in a narrow contorted zone which is 30 km wide 56 n 161 e to the north of this contortion dm decreases to 100 km the landward shift of the northern part of the slab is reflected by a sharp deviation of the volcanic front to the northwest which follows the 90 160 km isodepth range of the subducted slab the observed value of dm in the northern segment significantly diverges from the global relation dm we interpret this as an effective decrease of the thermal thickness of the subducted lithosphere

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

earth science a wet mantle conductor arising from x huang y xu s karato nature 434 746 749 2005 huang et al reply the suggestion that the transition zone of earth s mantle 410 670 km in depth is enriched in water is of great possible significance to the geodynamics and geochemistry of earth s interior as well as for the role of the mantle in the global water cycle1 huang et al 2 compare the effect of water on electrical conductivities of transition zone phases to electromagnetic and magnetotelluric soundings of the mantle beneath the north pacific3 and conclude that the transition zone contains between 1 000 and 2 000 p p m of water which is considerably more than the 50 200 p p m present in the upper mantle4 5 6 this conclusion is predicated on the assumption that the transition zone is relatively oxidized but in fact fairly reduced conditions are more likely7 8 here i show that if the transition zone is reduced high conductivities can be explained without the requirement for large enrichments of water

cs rb ba systematics in phengite and amphibole an assessment of fluid mobility at 2 0 gpa in eclogites from trescolmen central alps eclogites from trescolmen that contain abundant hydrous minerals phengite amphibole paragonite zoisite talc apatite show petrographic evidence for fluid infiltration under conditions of 2 0 to 1 8 gpa 650 c large ion lithophile elements lile e g cs rb ba and sr were analysed by in situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid mobile elements at high pressure in situ analysis of carefully chosen metamorphic assemblages circumvents the problem of partial late stage alteration which can severely influence the calculated element budgets of whole rock samples phengite is the dominant host for cs rb and ba in both eclogite and adjacent garnet mica schist samples and incorporates 90 of the budgets of these elements in whole rocks lile contents of phengites in phengite rich rocks are likely to record the cs rb and ba rb ratios of their host rock protoliths the lile patterns of eclogite are consistent with protoliths derived from basalt that underwent seafloor alteration whereas those of mica schist are almost identical to average upper continental crust in contrast lile patterns of eclogite samples that lack phengite but do contain amphibole are unlike any plausible protolith but are identical to those of amphibole in phengite bearing samples this observation points to homogenization of the lile in different lithologies which we correlate with petrographic evidence for fluid infiltration because phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to cs rb and ba homogenization of amphiboles is best explained by fluid infiltration from the surrounding metapelites into eclogite bodies implying at least metre scale fluid mobility the amphibole homogenization can be most easily modelled by a pervasive open system fluid flux through the eclogites possibly facilitated by ductile deformation during the early stages of uplift simple calculations give minimum fluid rock ratios of 0 001 to 0 004 demonstration of the mobility of very small volumes of fluid through eclogite is an important prerequisite of many subduction zone models that try to explain across arc variations in trace element geochemistry the low fluid rock ratios from this study are not in contrast with oxygen isotope heterogeneities reported from other eclogite localities fluid mobile elements such as cs rb and ba are more sensitive indicators of small volume fluid rock interaction and are therefore potentially valuable for understanding fluid infiltration processes in systems where oxygen isotope shifts are not large enough to be detectable

comment on steady state 226ra 230th disequilibrium in mantle minerals implications for melt transport rates in island arcs by m d feineman and d j depaolo earth planet sci lett 215 2003 339 355

origin of 226ra 230th disequilibria in arc lavas from southern chile and implications for magma transfer time abstract improved understanding of mantle melting processes and melt transport requires knowledge of how fast magma is generated and transferred from source region to surface the rate of magma transfer can in favorable cases be estimated from radioactive disequilibria between nuclides of the 238u series young lavas from southern chile in which 238u 230th disequilibria have been measured sigmarsson et al nature 346 1990 163 165 sigmarsson et al nature 394 1998 566 569 were analyzed for 226ra abundances the disequilibrium between 226ra and 230th in these lavas is found to correlate with 238u 230th disequilibria and 10be be morris et al nature 344 1990 31 36 these correlations strongly suggest that the excess of 226ra over 230th is due to the addition of a slab derived fluid to the magma source since ra and u are fluid mobile elements and the cosmogenic 10be is most likely derived from the subducting nazca plate beneath the andes the largest slab signature is observed in the lavas of villarrica volcano which is the most active volcano in south america a model for subduction fluxing is discussed in which the u series disequilibria in arc lavas will reflect the integrated dehydration process during metamorphism of the subducting plate and the metasomatized mantle but be principally controlled by the latest hydrous mineral breakdown in the mantle wedge repeated precipitation and dehydration mineral reactions of the hydrated mantle could be the homogenization process of the slab input needed to explain the 10be be b be correlation for different arcs morris et al nature 344 1990 31 36 the fact that excesses of 226ra and 238u over 230th are correlated indicates that linear arrays on the 230th 232th 238u 232th diagram are not isochrons reflecting time elapsed since a fluid addition but rather mixing lines between a fluid phase and melts the 226ra 230th disequilibrium in arc lavas suggests significantly shorter timescales for magma transfer or less than 8000 years this disequilibrium is consistent with minimum magma transfer rate through the mantle wedge on the order of 10 m year finally the correlations of 226ra 230th with 238u 232th and 10be be in andean magmas imply that magma chamber residence time is of the same order of magnitude beneath the stratovolcanoes studied

melting of a subducting oceanic crust from u th disequilibria in austral andean lavas understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the earth s mantle and for deciphering the long term growth of the continental crust most arc magmas are thought to be generated from fluid induced melting of the mantle wedge above slabs of subducting oceanic crust1 such magmas frequently display 238u enrichments or radioactive equilibrium2 3 between 238u and its radiogenic product 230th but where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc alkaline andesites and dacites termed adakites4 here we report a uniform excess of 230th over 238u but variable th isotope ratios in young adakites from the andean austral volcanic zone south of the triple junction where the chile ridge subducts beneath south america we show that these results are compatible with the adakites having been formed by approximately 20 equilibrium melting due to amphibole decomposition in a heterogeneous5 oceanic crust moreover both the degree of melting of the oceanic crust and its thermal structure appear to be uniform under most of the andean austral volcanic zone such partial melting of subducted oceanic slabs may have occurred throughout the earth s history where young oceanic plates were subducted

rapid magmatic processes accompany arc continent collision the western bismarck arc papua new guinea new u th ra major and trace element and sr nd pb isotope data are presented for young lavas from the new britain and western bismarck arcs in papua new guinea new britain is an oceanic arc whereas the latter is the site of an arc continent collision building on a recent study of the manus basin contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc continent collision and possible slab detachment all three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle however the western bismarck arc lavas diverge from the pb isotope mixing array formed by the new britain and the manus basin lavas toward elevated 208pb 204pb we interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision 238u and 226ra excesses are preserved in all of the lavas and are greatest in the western bismarck arc high mg andesites with high sr y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge data for two historical rhyolites are also presented although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions both formed from mafic parents within millennia

a 210pb 226ra 230th 238u study of klyuchevskoy and bezymianny volcanoes kamchatka abstract lavas from klyuchevskoy and bezymianny volcanoes kamchatka appear to show a link between the extent of partial melting in their mantle source region and the subsequent degree of fractionation suffered by the magmas during passage through the crust this fractionation may have occurred on timescales significantly less than 1000 years if observed 226 ra excesses largely reflect variable residual porosity in the source melting region unlike most arc lavas those with the highest mgo contents and ba th ratios have the lowest 226 ra excess forward models suggest that those portions of the source which had undergone the greatest addition of u by fluids from the subducting plate also underwent the greatest extents of partial melting at the highest residual porosity at kluchevskoy a change from eruption of high mgo to high al 2 o 3 basaltic andesites around 1945 is reflected in an increase in size of 226 ra excess which seems to require a simultaneous decrease in residual porosity and suggests a rapid changes in the melting regime the eruption of andesites at bezyminanny simultaneous with the eruption of basaltic andesites at klyuchevskoy further suggests that different degree melts produced at differing residual porosity can be formed and extracted from the melt region at the same time thus the melting processes beneath klyuchevskoy and bezyminanny are demonstrably complex they have clearly been influenced by both fluid addition from the subducting plate and extension and decompression beneath the central kamchatka depression finally the 210 pb data are with one or two exceptions in equilibrium with 226 ra suggesting that there was restricted relative magma gas movement in this highly productive magmatic system

probability of earthquake recurrence with nonuniform stress rates and time dependent failure current methods for calculation of long term probabilities for the recurrence of large earthquakes on specific fault segments are based upon models of the faulting process that implicitly assume constant stress rates during the interval separating earthquakes and instantaneous failure at a critical stress threshold however observations indicate that the process of stress recovery following an earthquake involves rate variations at all time scales in addition to stress steps caused by nearby earthquakes additionally the existence of foreshocks aftershocks and possible precursory processes suggest that there may be significant time dependence of the earthquake nucleation process a method for determining the conditional probabilities for earthquake occurrence under conditions of irregular stressing is developed that could be useful at all time scales including those pertinent to short and intermediate term prediction used with models for earthquake occurrence at a stress threshold the addition of variable stressing introduces a simple scaling of the conditional probabilities by stress level and stress rate a model for the time dependent nucleation of earthquake slip has been proposed recently that is based upon laboratory observations of fault strength this failure criterion results in large but relatively short duration changes in the probability of earthquake recurrence particularly following stress steps applied to populations of earthquakes the models predicts a 1 t decay of seismicity following stress steps as observed for aftershocks and for frequency of foreshock mainshock pairs the model suggests that variations of seismicity rates of small earthquakes in the nucleation zone of the expected earthquake directly indicate variations in probability of recurrence of the large earthquake

u th pa ra study of the kamchatka arc new constraints on the genesis of arc lavas abstract the 238u 230th 226ra and 235u 231pa disequilibria have been measured by mass spectrometry in historic lavas from the kamchatka arc the samples come from three closely located volcanoes in the central kamchatka depression ckd the most active region of subducted related volcanism in the world the large excesses of 226ra over 230th found in the ckd lavas are believed to be linked to slab dehydration moreover the samples show the uncommon feature of 230th 238u activity ratios both lower and higher than 1 the u series disequilibria are characterized by binary trends between activity ratios with 231pa 235u ratios all 1 it is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting we suggest that they are more likely to reflect mixing between two end members a high magnesia basalt hmb end member with a clear slab fluid signature and a high alumina andesite haa end member reflecting the contribution of a slab derived melt the u th ra characteristics of the hmb end member can be explained either by a two step fluid addition with a time lag of 150 ka between each event or by continuous dehydration the inferred composition for the dehydrating slab is a phengite bearing eclogite equilibrium transport or dynamic melting can both account for 231pa excess over 235u in hmb end member nevertheless dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid derived 226ra 230th a continuous flux melting model is also tested in this model 231pa 235u is quickly dominated by fluid addition and for realistic extents of melting this process cannot account for 231pa 235u ratios as high as 1 6 as observed in the hmb end member the involvement of a melt derived from the subducted oceanic crust is more likely for explaining the haa end member compositions than crustal assimilation melting of the oceanic crust is believed to occur in presence of residual phengite and rutile resulting in no 226ra 230th disequilibrium and low 231pa excess over 235u in the high alumina andesites consequently it appears that high alumina andesites and high magnesia basalts have distinct origins the former being derived from melting of the subducted oceanic crust and the latter from hydrated mantle it seems that there is no genetic link between these two magma types in contrast with what was previously believed

a semi global reference model for electrical conductivity in the mid mantle beneath the north pacific region 1 one dimensional electrical conductivity structure in the mid mantle of the one fourth of the earth beneath the north pacific ocean was obtained by a semi global electromagnetic induction study electromagnetic response functions estimated from electric field variations measured by submarine cables and geomagnetic field variations obtained by magnetic observatories and long term observations sites were inverted into radially symmetric conductivity distribution by taking the distribution of land and ocean at the surface into account as a most preferred model a smooth conductivity depth profile was obtained with two abrupt increases that possibly correspond to the seismic discontinuities at 410 and 660 km

petrological and geochemical evolution of the tolbachik volcanic massif kamchatka russia abstract data on the geology petrography and geochemistry of middle late pleistocene rocks from the tolbachik volcanic massif kamchatka klyuchevskaya group of volcanoes are presented and compared with rocks from the neighboring mount povorotnaya klyuchevskaya group basement and holocene historical tolbachik monogenetic cones two volcanic series of lavas middle k and high k are found in the tolbachik massif the results of our data analysis and computer modeling of crystallization at different p t h 2 o fo 2 conditions allow us to reconstruct the geochemical history of the massif the tolbachik volcanic massif started to form earlier than 86 ka based on k ar dating during the formation of the pedestal and the lower parts of the stratovolcanoes the middle k melts depleted relative to nmorb fractionated in water rich conditions about 3 of h 2 o at the late pleistocene holocene boundary a large fissure zone was initiated and the geodynamical regime changed upwelling associated with intra arc rifting generated melting from the same mantle source that produced magmas more enriched in incompatible trace elements and subduction components these magmas are high k not depleted relative to n morb melts with island arc signatures and rift like characteristics the fissure opening caused degassing during magma ascent and the high k melts fractionated at anhydrous conditions these high k rocks contributed to the formation of the upper parts of stratovolcanoes at the beginning of holocene the high k rocks became prevalent and formed cinder cones and associated lava fields along the fissure zone however some features including 1975 1976 northern breakthrough are represented by middle k high mg rocks suggesting that both middle k and high k melts still exist in the tolbachik system our results show that fractional crystallization at different water conditions and a variably depleted upper mantle source are responsible for all observed variations in rocks within the tolbachik volcanic massif sr nd isotopes are consistent with 2 4 crustal assimilation during formation of the pedestal and stratovolcanoes while the young lava fields do not show evidence of crustal assimilation major and trace element data coupled with k ar dating provide strong evidence that mount povorotnaya located in 8 km northeast of plosky tolbachik is an old block of the tolbachik massif pedestal and for the moment it is the oldest 306 ka known object in klyuchevskaya group of volcanoes

u th systematics of dispersed young volcanoes in ne china asthenosphere upwelling caused by piling up and upward thickening of stagnant pacific slab abstract young 300 to 5500years old dispersed intraplate volcanism characterized by small volume of melt and large compositional variability occurred over ne china at jingbohu long gang tianchi and wudalianchi these young volcanic rocks provide ideal opportunities for investigating the mechanism of dispersed continental volcanism using short lived uranium series isotopes we present systematic u series nd sr pb isotope and trace element analyses of the young lavas from ne china jingbohu lavas display variable extents of 230th excesses 10 to 28 and moderately depleted nd isotopic compositions end 1 5 to 3 3 long gang lavas have pronounced 33 to 35 230th excesses and slightly depleted nd isotopic compositions end 0 5 to 0 7 the tianchi lavas display moderate 12 230th excesses and slightly enriched nd isotopic compositions end 1 0 to 1 1 230th enrichments in all these volcanic rocks are uncharacteristic of melts generated by subduction the strongest evidence against the involvement of subduction related fluids is derived from the long gang lavas having more than 30 230th excesses low u th sr th and ba th ratios and positive nb and ta anomalies the fluids released from the subduction of the pacific plate under the eurasian plate might have already been completely lost before the slab reached the mantle beneath ne china we postulate that in addition to the convective circulation process the continuous piling up and upward thickening of stagnant pacific slab in the mantle transition zone may help drive a broad asthenosphere upwelling far from the trench leading to mantle partial melting and the formation of the young volcanoes in ne china in addition our u th disequilibrium data provide constraints on magma evolution time the magma evolution time from low sio2 47 5 basalt to higher sio2 50 4 basalt at long gang is within 10 thousand years 10ka the magma evolution time from trachyte to rhyolite at tianchi volcano is also less than 10ka if the tianchi silicic rocks were evolved from long gang basaltic magmas with minor crustal assimilation then the magma evolution time from basaltic magma to silicic magmas is less than 110 thousand years

quaternary and geodetically measured crustal movements in the tokai district central honshu japan geodetic measurements made in this century demonstrate that modern movements correlate well with deformation that has occurred during the past 2 m y both show effects of subduction and of the quaternary collision of izu peninsula with central honshu westward from suruga bay deformation is greatest near the coast decreases markedly towards the uplifting akaishi mountains and then increases again in the conjugate block faulted terranes of chubu district quaternary faults historic earthquakes and the distribution of horizontal shear straining since 1880 all reflect this deformation pattern since 1900 the west coast of suruga bay has undergone secular subsidence horizontal contraction and tilting towards suruga trough all typical of the strain buildup process that occurs landward of subduction zones however futher westward active mountain building is occuring and the akaishi ranges are rising 3 mm a unlike the episodic aseismic uplift of the transverse ranges of southern california movements here have been relatively uniform it seems likely that an increase in the tilt rate of the suruga coast recognized since 1973 is part of a long term readjustment to the great 1944 tonankai earthquake data show that a monotonic increase has occurred since at least 1949 and similar effects are seen adjacent to the 1944 rupture zone over 200 km southwest along the nankai trough

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

u series isotopes and destructive plate margin magma genesis in the lesser antilles high precision tims230th232th isotope ratios are presented from a suite of lavas which span the length of the lesser antilles island arc compositions vary from low k tholeiitic lavas with low87sr86sr in the north to high k calc alkaline to alkalic lavas with higher87sr86sr in the south the northern lavas are characterised by 230th238u 1 a three component model is required for magma genesis in the lesser antilles sediments from the subducting slab melt and variably enrich the mantle wedge while the subducting oceanic crust dehydrates releasing fluids enriched in u ba sr and k but with negligible th and low87sr86sr which promote partial melting in the wedge ta zr increases with87sr86sr southwards along the arc and this is inferred to reflect increasing enrichment of the mantle wedge through the addition of partial melts of subducted sediment mass balance calculations are consistent with a relatively constant fluid flux except for u and little variation in the degree of partial melting 10 15 along the arc and they suggest that the sediment contribution to the arc lavas increases from 2 in the north to 15 in the south in the northern parts of the arc the addition of u by these fluids overprints any increases inth u that might have been caused by partial melting and results in lavas with 230th238u 1 m yr 1 which imply channelled flow through the mantle wedge

geochemistry of the neoarchaean high mg basalts boninites and adakites from the kushtagi hungund greenstone belt of the eastern dharwar craton edc implications for the tectonic setting abstract volcanic rocks of the kushtagi hungund greenstone belt from the eastern dharwar craton edc show extreme diversity due to the presence of high mg high fe high mg fe basalts high mg dacites and andesites boninitic affinities and sodic plagioclase quartz rich felsic rocks adakites with na2o 4 6 wt the adakites drummond and defant 1990 martin 1999 and high mg dacites andesites are being reported for the first time from greenstone belts of the dharwar craton trace elements in these different types of volcanic rocks have distinct characteristics for example the basalts have 1 al2o3 tio2 16 pm value of 17 2 la yb n 1 3 3 low order negative anomalies of nb nb zr zr hf hf and ti ti 4 zr nb 4 10 and 5 ce nb 3 4 two samples of andesites having mg enrichment and al2o3 tio2 ratios of 31 35 are interpreted to have boninitic affinities other dacites and andesites are characterized by 1 high mg 8 10 wt 2 high al2o3 tio2 22 27 3 zr nb 8 14 4 la yb n 2 7 5 negative nb nb zr zr hf hf and ti ti anomalies and 6 sc lu 67 172 the adakites are characterized by 1 a narrow range of sio2 67 70 wt 2 enriched na2o 4 6 wt with very high na2o k2o 2 5 7 7 3 low mg 0 38 0 48 4 high la yb n 13 31 with fractionated patterns no eu anomaly and gd yb n 2 4 5 very high al2o3 tio2 50 80 and 6 enriched ni cr and co total 50 ppm these adakites resemble 3 0 ga tonalite trondhjemite gneisses of the dharwar craton which are interpreted as products of partial melting of the basaltic slab the available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

the effect of assimilation fractional crystallization and ageing on u series disequilibria in subduction zone lavas abstract although most arc lavas have experienced significant magma differentiation the effect of the differentiation process on u series disequilibria is still poorly understood here we present a numerical model for simulating the effect of time dependent magma differentiation processes on u series disequilibria in lavas from convergent margins our model shows that in a closed system with fractional crystallization the ageing effect can decrease u series disequilibria via radioactive decay while in an open system both ageing and bulk assimilation of old crustal material serve to reduce the primary u series disequilibria in contrast with recharge of refresh magma significant 226ra excess in erupted lavas can be maintained even if the average residence time is longer than 8000 years the positive correlations of 226ra 230th between sr th or ba th in young lavas from convergent margins have been widely used as evidence of fluid addition generating the observed 226ra excess in subduction zones we assess to what extent the positive correlations of 226ra 230th with sr th and ba th observed in the tonga arc could reflect afc process results of our model show that these positive correlations can be produced during time dependent magma differentiation at shallow crustal levels specifically fractional crystallization of plagioclase and amphibole coupled with contemporaneous decay of 226ra can produce positive correlations between 226ra 230th and sr th or ba th to a lesser extent therefore the correlations of 226ra 230th with sr th and ba th cannot be used to unambiguously support the fluid addition model and the strength of previous conclusions regarding recent fluid addition and ultra fast ascent rates of arc magmas is significantly lessened

relationship between kamen volcano and the klyuchevskaya group of volcanoes kamchatka data on the geology petrography mineralogy and geochemistry of rocks from kamen volcano central kamchatka depression are presented and compared with rocks from the neighbouring active volcanoes the rocks from kamen and ploskie sopky volcanoes differ systematically in major elemental and mineral compositions and could not have been produced from the same primary melts the compositional trends of kamen stratovolcano lavas and dikes are clearly distinct from those of klyuchevskoy lavas in all major and trace element diagrams as well as in mineral composition however lavas of the monogenetic cones on the southwestern slope of kamen volcano are similar to the moderately high mg basalts from klyuchevskoy and may have been derived from the same primary melts this means that the monogenetic cones of kamen volcano represent the feeding magma for klyuchevskoy volcano rocks from kamen stratovolcano and bezymianny form a common trend on all major element diagrams indicating their genetic proximity this suggests that bezymianny volcano inherited the feeding magma system of extinct kamen volcano the observed geochemical diversity of rocks from the klyuchevskaya group of volcanoes can be explained as the result of both gradual depletion over time of the mantle n morb type source due to the intense previous magmatic events in this area and the addition of distinct fluids to this mantle source

fluxes of high versus low temperature water rock interactions in aerial volcanic areas example from the kamchatka peninsula russia abstract volcanic areas play a key role in the input of elements into the ocean and in the regulation of the geological carbon cycle the aim of this study is to investigate the budget of silicate weathering in an active volcanic area we compared the fluxes of the two major weathering regimes occurring at low temperature in soils and at high temperature in the active volcanic arc of kamchatka respectively the volcanic activity by inducing geothermal circulation and releasing gases to the surface produces extreme conditions in which intense water rock interactions occur and may have a strong impact on the weathering budgets our results show that the chemical composition of the kamchatka river water is controlled by surface low temperature weathering atmospheric input and in some limited cases strongly imprinted by high temperature water rock reactions we have determined the contribution of each source and calculated the rates of co 2 consumption and chemical weathering resulting from low and high temperature water rock interactions the weathering rates between 7 and 13 7 t km 2 yr for cations only and atmospheric co 2 consumption rates 0 33 0 46 10 6 mol km 2 yr for kamchatka river due to rock weathering in soils low temperature are entirely consistent with the previously published global weathering laws relating weathering rates of basalts with runoff and temperature in the kamchatka river co 2 consumption derived from hydrothermal activity represents about 11 of the total hco 3 flux exported by the river the high temperature weathering process explains 25 of the total cationic weathering rate in the kamchatka river although in the rivers non affected by hydrothermal activity the main weathering agent is carbonic acid reflected in the abundance of hco 3 in rivers in the region most impacted by hydrothermalism the protons responsible for minerals dissolution are provided not only by carbonic acid but also by sulphuric and hydrochloric acid a clear increase of weathering rates in rivers impacted by sulphuric acid can be observed in the kamchatka river 19 of cations are released by hydrothermal acids or the oxidative weathering of sulphur minerals our results emphasise the important impact of both low and high temperature weathering of volcanic rocks on global weathering fluxes to the ocean our results also show that besides carbonic acid derived from atmospheric co 2 hydrochloric acid and especially sulphuric acid are important weathering agents clearly sulphuric acid with hydrothermal activity are key parameters that cause first order increases of the chemical weathering rates in volcanic areas in these areas accurate determination of weathering budgets in volcanic area will require to better quantify sulphuric acid impact

source composition and distribution of the fluid in the kurile mantle wedge constraints from across arc variations of b nb and b isotopes b and nb concentrations and b isotope composition were determined for lavas covering the entire length of the kurile arc b nb and 11b of these lavas show clear across arc variations in which both are highest at the trench side and continuously decrease as the slab depth increases b nb in the volcanic front lavas 8 25 is significantly higher than those of morb and oib indicating that the mantle wedge is extensively metasomatized by b rich fluid derived from the subducting slab the frontal 11b values are also out of the range of morb and oib and systematic difference exists between northern kurile nk 5 4 0 5 and central and southern kurile csk 4 3 0 3 however these two parameters approach or coincide with mantle values at the most back arc side the 11b and 87sr 86sr linearly correlate with nb b suggesting control by simple mixing of two isotopically homogeneous components that is slab derived fluid and mantle wedge the estimated 11b and 87sr 86sr of the fluid are 7 0 0 6 and 0 70326 0 00005 for nk and 6 3 0 3 and 0 7033 for csk respectively these fluid compositions are consistent with a source constituted by 95 altered oceanic crust and 5 oceanic sediment with the slight difference between nk and csk resulting from the different lithologies of subducting sediments the mantle wedge beneath the kurile arc has b sr and nd isotope compositions identical with morb but shows significantly higher b nb the above trends bear a close resemblance to those observed in the izu arc strongly suggesting their universality in the arc system the across arc decline of b nb observed both in kurile and izu implies that the fluid flux from the slab continuously decreases as the slab depth increases however this relates only to a final stage of slab dehydration since most of the volatile components are lost at the fore arc region in contrast to b k is uniformly enriched across the arc this requires control by a k bearing mineral phase either in the slab phengite or the mantle wedge phlogopite although such a mineral is responsible for only a small part of the whole fluid transport

reappraisal of uranium series isotope data in kamchatka lavas implications for continental arc magma genesis uranium series isotopes can be used to determine constraints on the timescale of slab dehydration and melt production at subduction zones however interpretations of u th ra data suggest very different timescales of slab dehydration here we present new u th ra data from kamchatka along with a number of alternative models for production of radioactive disequilibrium variations in 226ra 230th and 231pa 235u activity ratios are best explained by crystal fractionation with host rock assimilation for a duration of less than c 6000 years the association of the largest 226ra excesses with high sr th in the most primitive lavas suggests that ra th fractionation is controlled by slab dehydration less than 10 ka ago we show that u th data can be explained by dynamic melting of a recently ka metasomatized mantle wedge dynamic melting of an oxidized source metasomatized several hundreds of thousands of years ago cannot produce significant 231pa excess because 238u 230th disequilibrium is inferred to be controlled by partial melting there is no requirement for multi stage slab dehydration commencing 150 ka we suggest that ra th disequilibria constrain the timing of slab dehydration whilst u th fractionation is dominated by partial melting at least at the kamchatka arc

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

gold recycling and enrichment beneath volcanoes a case study of tolbachik kamchatka magmas supply metals to hydrothermal ore deposits although typical arc basalts may be unable to produce a gold rich ore forming fluid as such basalts rarely exceed 5 ppb au consistent with this the occurrence of native gold of magmatic origin is extremely rare and only a few finds of micron sized gold particles in unaltered basalts have been documented surprisingly some lava flows and scoria cones of the historic basaltic eruptions of tolbachik volcano kamchatka are unusually gold rich tolbachik basalts contain up to 11 6 ppb au based on whole rock analyses nuggets of gold electrum up to 900 m in size and native gold droplets up to 200 m plus numerous vapor deposited gold crystals within fumarolic incrustations and directly on surfaces of basaltic lapilli our results demonstrate that the gold nuggets in tolbachik basalt are of hydrothermal origin and were physically scavenged from epithermal veins hosted by country rocks during intrusion of mafic magmas depending on the melt temperature and or time span of the melt rock interaction gold was ejected by the erupting volcano either in the form of abraded nuggets or liquid droplets or was fully assimilated dissolved into the shallow long lived magma chamber to provide a 4 fold increase in gold content over background concentration of 2 7 ppb au characteristic of mafic volcanic rocks in kamchatka upon the end of the eruption the continued discharge of volcanic vapors enriched in gold deposited abundant crystals of gold on cooling lava and scoria similar to tolbachik recycling of metals from prior accumulations ore deposits in the shallow crust may take place in other long lived magma reservoirs thus upgrading the gold and other metal contents and contributing to the ore forming potential of a magma

usercalc a web based uranium series calculator for magma migration problems 1 measured departures from secular equilibrium in the uranium series decay chains provide important constraints on the rates of recent mantle and crustal processes nevertheless the inferences drawn from these observations depend upon the models used to interpret them while several useful models for u series exist they are not all equally easy to use and it is often the ease of use rather than the utility of the model that determines which models are applied the purpose of this paper is to level the playing field by making some of the more general u series transport models accessible to a wider community through the usercalc web site available from lamont doherty earth observatory at www ldeo columbia edu mspieg usercalc these models calculate the effects of both melting and melt transport on two u series chains 238u 230th 226ra and 235u 231pa and allow the user to input their own petrological insight and data to explore the behavior of this model the purpose of the site is to provide a tool that can spur additional collaborations between geophysicists and geochemists as well as drive more rapid exploration evolution of u series models

melting dynamics beneath the tonga kermadec island arc inferred from 231pa 235u systematics measurement of 231pa 235u disequilibrium in tonga kermadec island arc lavas north of new zealand permits discrimination of the time scales and mechanisms of fluid addition and partial melting for tonga lavas the 231pa 235u ratios are less than 1 and can be explained by fluid addition from the subducting plate around 60 000 years ago in contrast the effects of partial melting overprinted those of fluid addition in the kermadec lavas resulting in 231pa 235u ratios greater than 1

interseismic strain accumulation in the marmara sea region the marmara sea region is deforming along the branches of the north anatolian fault naf which is the boundary between the anatolian and the eurasian plates we evaluate global positioning system gps measurements performed from 1992 to 1999 up to the 17 august 1999 izmit earthquake to quantify the interseismic strain accumulation pattern in the marmara sea region we compute interseismic velocities in a frame fixed to the eurasian plate at 136 gps points to compute the frame independent strain rates we first smooth the north south and the east west velocity components separately to obtain the values at grid nodes and then we compute directional derivatives and strain rates shear strain rates reveal that the marmara sea region is within the plate boundary deformation zone having a width of about 110 km the largest shear strain rate accumulation is along the northern branch of the naf with maximum shear strain rate reaching 220 nstrain yr in the marmara sea we calculate clockwise rigid body rotation rates with a maximum of 10 deg m y along the northern branch while the anatolian plate rotates anticlockwise dilatation rates display adjacent local tensional and compressional areas in the east west direction within the marmara sea assuming the uniform simple shear we compute the naf slip rates vary between 11 and 26 mm yr with a minimum around izmit and increase east and west of izmit manuscript received 30 august 2000

earth science a wet mantle conductor reply inference of the spatial distribution of water content in the mantle is critical to our understanding of the dynamics of earth s interior a model1 has been described that indicates there may be a jump in water content at the 410 km discontinuity in the earth s mantle from the electrical conductivity we have inferred2 the water content in the transition zone and concluded that it is significantly larger than that of the upper mantle hirschmann 3 questions our conclusion on the grounds of the trade off between water content and oxygen fugacity

emissions of trace elements during the 2012 2013 effusive eruption of tolbachik volcano kamchatka enrichment factors partition coefficients and aerosol contribution abstract gases and aerosols from the 2012 13 effusive eruption of tolbachik basaltic volcano kamchatka were sampled in february and may 2013 from a lava tube window located 300 m from the eruptive crater temperature at the sampling point was 1060 1070 c the chemical and isotopic compositions of the sampled gases 92 4 h 2 o 3 5 co 2 2 3 so 2 on average d from 25 0 to 38 6 correspond to a typical volcanic arc gas without dilution by meteoric or hydrothermal water halogen contents in the gases 1 37 hcl 0 5 hf were higher than average arc values the total amount of analyzed metallic and metalloid trace elements in the gas exceeded 665 ppm six most abundant trace elements k 250 ppm na 220 ppm si 74 ppm br 48 ppm cu 21 ppm and fe 12 ppm accounted for 95 of the total content of trace elements in the gas the gases contained 24 ppb re 12 ppb ag 4 9 ppb au and 0 45 ppb pt refractory rock forming elements mg al ca and some other elements such as ba and th were transported mainly in the form of silicate microspheres and altered rock particles the concentrations of metals in the eruptive tolbachik gases are higher than the corresponding concentrations in high temperature fumaroles worldwide although the mutual ratios of the elements are approximately the same the gas magma partition coefficients of eleven elements exceed unity including the non metals f s cl br as se and te and the rare metals cd re tl and bi despite the relatively low concentrations of trace elements in the volcanic gases at the highest temperatures superficial magma degassing provides information on the sources and sinks of metals

thermodynamics of water solubility and partitioning when it was realized that nominally anhydrous minerals could be a major storage site of water in the mantle smyth 1987 bell and rossman 1992 it also became rather obvious that many mantle samples probably have lost most of their water during ascent e g ingrin and skogby 2000 while analyses of natural samples therefore in many circumstances only provide a lower limit of the actual water content in the mantle measurements of water solubility give an upper limit of the amount of water that might be stored in a mineral experimental measurements of water solubility have therefore naturally evolved in the last decades as a major tool for understanding the water storage capacity of the mantle closely related to studies of water solubility are studies of water partitioning among minerals if the water solubility in two minerals is known under given conditions the partition coefficient is just the ratio of the water solubilities under the same conditions provided that the composition of the fluid coexisting with the two minerals is the same since the pioneering work of bai and kohlstedt 1992 and of kohlstedt et al 1996 the water solubility in all major upper mantle minerals has been studied as well as the partitioning of water between the minerals of the lower mantle and the transition zone solubility studies by themselves do not directly give the water content in the mantle they constrain the storage capacity of the mantle i e the maximum amount of water that may be stored if the mantle were water saturated however as most of the mantle is very likely water undersaturated the actual water content is probably far below the storage capacity estimates of the actual abundance and distribution of water in the mantle may be obtained by combining solubility and partitioning data with direct

u series disequilibria of trachyandesites from minor volcanic centers in the central andes abstract young trachyandesite lavas from minor volcanic centers in the central andes record the magma differentiation processes at the base of the lower continental crust here we report u series disequilibrium data for the historical lavas from the andagua valley in southern peru to define the time scale and processes of magmatism from melting in the mantle wedge to differentiation in the crust the andagua lavas show 230 th 238 u 231 pa 235 u and 226 ra 230 th above unity except for one more evolved lava with 230 th depletion likely owing to fractional crystallization of accessory minerals the 226 ra excess indicates that the time elapsed since magma emplacement and differentiation in the deep crust is within 8000 years based on the correlations of u series disequilibria with sio 2 content and ratios of incompatible elements we argue that the andagua lavas were produced by mixing of fresh mantle derived magma with felsic melt of earlier emplaced basalts in the deep crust because of the lack of sediment in the chile peru trench there is no direct link of recycled slabs with 230 th and 231 pa excesses in the andagua lavas instead 230 th and 231 pa excesses are better explained by in growth melting in the upper mantle followed by magma differentiation in the crust such processes also produced the 226 ra excess and the positive correlations among 226 ra 230 th sr th and ba th in the andagua lavas the time scale of mantle wedge melting should be close to the half life of 231 pa ca 33 ka while it takes less than a few thousand years for magma differentiation to form intermediate volcanic rocks at a convergent margin

fault interactions and subduction tectonics a re examination of the weber new zealand earthquake sequence of 1990 summary two moderate magnitude mw 6 2 and 6 4 earthquakes in the hikurangi subduction margin north island new zealand occurred 3 months apart in 1990 the epicentres are nearly coincident but the first weber 1 primarily normal faulting occurred within the subducting pacific plate depth about 28 km and the second weber 2 a mix of thrusting and right lateral motion occurred within the overlying australian plate depth about 13 km the plate interface in between the plate interface is interpreted to be locked trenchward se from about the position of these events with a transition to aseismic slip further down dip to the nw several stress interaction questions are examined first to see whether weber 1 triggered weber 2 a range of possible mainshock parameters are used to calculate induced changes in the static coulomb failure stress cfs in most cases the results are consistent with triggering secondly previous work showed that the rate of aftershock occurrence for weber 1 decreased markedly about 35 days before weber 2 recovering afterwards to see whether aseismic pre slip on the weber 2 fault as predicted by rate and state friction could be the cause of the decrease the same fault parameters have been used in reverse the results are ambiguous some fault parameters giving results consistent with the hypothesis and others not the amount of pre slip required for significant inhibition however is about equal to that in the mainshock and distributed over the entire fault plane thirdly observations of episodic aseismic slip down dip from locked sections of other plate interfaces are becoming more common could such slip have triggered both weber events the induced changes in cfs for such slip are uniformly positive on the weber 1 fault plane and mostly positive on the weber 2 fault plane so the answer is yes although there is no independent evidence for aseismic slip prior to the weber sequence this case shows that such slip may trigger events on other nearby faults besides loading the locked section of the plate interface static stress triggering considerations are thus likely to be important in subduction environments

a new method for u th pa ra separation and accurate measurement of 234u 230th 231pa 226ra disequilibria in volcanic rocks by mc icpms abstract a new method for the chemical separation and mc icpms measurements of u th pa ra disequilibria in volcanic samples from a single sample aliquot is presented the accuracy and precision of our techniques is assessed by replicate analyses of the synthetic u and th isotope standards irmm 184 and irmm 35 and the secular equilibrium rock standards bcr 2 w 2a and tml a measure of true sample reproducibility including the errors related to sample processing and the chemical separation is obtained from analysis of multiple digestions of the rock standards 234u 238u 230th 238u 231pa 235u and 226ra 230th activity ratios on ten separate dissolutions of usgs rock standard bcr 2 for example reproduce to 0 4 1 4 1 1 and 2 4 2 sd respectively all elements u th pa ra are separated from a single spiked sample aliquot and the elemental concentrations are measured on the same solutions as the isotopic compositions analytical artefacts from determination of elemental and isotopic concentrations on different sample aliquots due to sample heterogeneity are therefore avoided in addition our new chemical separation protocol is time efficient and allows for rapid processing of volcanic samples with less than 25 fg of pa and ra

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses