oxygen isotope evidence for hydrothermal alteration within a quaternary stratovolcano lassen volcanic national park california brokeoff volcano a quaternary stratocone located in the lassen volcanic center in northern california has been deeply eroded exposing a 10 km2 meteoric hydrothermal alteration zone at the core of the volcano portions of the former volcanic edifice are sufficiently well preserved that an unusual opportunity exists wherein the alteration pattern can be correlated with the position of the volcanic cone the 18o analyses of more than 100 whole rock samples consisting primarily of andesitic lavas vary from 9 8 to 0 6 per mil the highest 18o values occur in bleached solfatarically altered rocks that have interacted with low ph fumarolic hot springs associated with the present day hydrothermal system low 18o values are found in propylitically altered rocks that underwent isotopic exchange with meteoric hydrothermal fluids at elevated temperatures mostly during the stratovolcanic stage 650 400ka of the hydrothermal system but probably continuing today at depth electron microprobe analyses of secondary layer silicate minerals in strongly propylitized samples 18o 5 0 revealed the presence of discrete chlorite suggesting that temperatures up to 200 to 250 c were attained in the shallow levels of the system two zones of pervasive meteoric hydrothermal alteration defined by concentric 18o contours that are probably interconnected at depth are located within the original topographic edifice of the volcano the most intensely altered rocks within these equant zones of alteration define nnw trends that coincide with stream valleys and with regional structural patterns a comparison of the characteristics of the 18o depleted zone at brokeoff with those of more deeply eroded volcanic centers such as the comstock lode mining district criss and champion 1991 permits the construction of composite 18o cross sections through a hypothetical intact stratovolcano at both brokeoff and comstock hydrothermal fluids were strongly focused into plumelike zones of intense 18o depletion at comstock these low 18o plumes are associated with faults although major fault displacements are not observed at brokeoff the topographic and alteration patterns are consistent with the presence of a linear array of faults that acted as conduits for fluid flow up into the shallow levels of the volcano

role of nonlinear ocean dynamic response to wind on the asymmetrical transition of el nino and la nina to understand why el nino is frequently followed by la nina while the opposite occurs rarely we analyze the inherent asymmetry in a delayed negative feedback loop in the framework of delayed oscillator theory using observational data the asymmetrical response of the ocean wave to wind is much larger than that of the wind intensity to sst and that of the subsurface temperature to thermocline strong oceanic response during el nino compared to la nina is presumably due to the relatively shallow mean thermocline over the western pacific which efficiently traps the atmospheric momentum in the shallow upper ocean and the asymmetrical wind pattern response to sst a modified delayed oscillator model experiment verifies that the asymmetrical ocean wave response to wind is more important in the asymmetrical transition of el nino and la nina than asymmetrical wind intensity response to sst and asymmetrical subsurface temperature response to thermocline

crustal modification during subduction zone magmatism evidence from the southern salar de uyuni region 20 22 s central andes late cenozoic volcanic rocks erupted in the southern salar de uyuni region of the central andes provide evidence for recent modification of both the lower and upper continental crust during subduction zone magmatism at a given sio 2 content k 2 o and related incompatible trace element concentrations increase eastward in basaltic andesite samples and are associated with increasing sr isotopic ratios and decreasing sr concentrations these data favor a model in which basaltic andesite magmas are contaminated by lower continental crust that becomes increasingly more mafic toward the active volcanic front because of relatively recent intrusion and hybridization by primary basaltic magmas compositions of andesitic lavas continue the geochemical trends established by the basaltic andesite samples except that sr isotopic ratios of lavas erupted at the arc front are constant to slightly decreasing with increasing sio 2 contents assimilation of upper crustal rocks that also vary in composition across the strike of the arc is the most plausible mechanism to produce the distinct sr isotopic trends of the andesitic lavas at the volcanic front the magmas producing the andesitic lavas assimilated rocks within the quaternary volcanic and plutonic complex of the andean arc itself toward the east the magmas assimilated basement rocks that become progressively older with distance from the volcanic front because of diminishing modification by late cenozoic arc magmatism thus the across arc geochemical trends in the southern salar de uyuni region are consequences of spatial variations in the composition of the lower and upper continental crust and these variations appear to be linked to the subduction process itself

asymmetry of atmospheric circulation anomalies over the western north pacific between el nino and la nina the asymmetry of the western north pacific wnp low level atmospheric circulation anomalies between the el nino and la nina mature winter is examined an anomalous wnp cyclone wnpc center during la nina tends to shift westward relative to an anomalous wnp anticyclone wnpac center during el nino two factors may contribute to this asymmetric response thefirst factor is the longitudinal shifting of el nino and la nina anomalous heating the composite negative precipitation anomaly center during la ninai s located farther to the west of the composite positive precipitation anomaly center during el nino the westward shift of the heating may further push the wnpc westward relative to the position of the wnpac the second factor is the amplitude asymmetry of sea surface temperature anomalies sstas in the wnp namely the amplitude of local cold ssta during el nino is greater than that of warm ssta during la nina the asymmetry of ssta is originated from the asymmetric ssta tendencies during the enso developing summer although both precipitation and surface wind anomalies are approximately symmetric the surface latent heat flux anomalies are highly asymmetric over the key wnp region where the climate mean zonal wind speed is small both the anomalous westerly during el nino and the anomalous easterly during la nina in the region lead to an enhanced surface evaporation strengthening weakening the enhancement of the cold warm ssta in situ during el nino la nina the asymmetry of the ssta in the wnp is further amplified due to anomalous wind differences between el nino and la nina in their mature winter atmo spheric general circulation model experiments demonstrate that both factors contribute to the asymmetry between the wnpac and wnpc the asymmetric circulation in the wnp contributes to the asymmetry of temporal evolutions between el nino and la nina

changing techniques and paradigms for the evaluation of magmatic processes the study of magmatic processes is at a juncture new continuum models for the complex physics of crystallization and melting with convection and crystal melt segregation are now available and the models are amenable to exploration by numerical methods these models which were largely developed in the fields of material science and mechanical engineering will allow petrologists to consider a wide range of transport phenomena crystal settling with reaction as fully multiphase flow combined thermal and compositional buoyancy with realistic material property variations open system and eruptive behavior and the appropriate system geometry and boundary conditions the traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal melt mush subject to reintrusion or mobilization percolative flow and reaction in this mush including contaminants may be as important as near liquidus processes in driving petrological diversity thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes melting of the crust following intrusion by basalt can be considered in light of two end members the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes although more commonly cartooned the large sill type may not be nearly as efficient as repeated diking in driving crustal melting magma mixing and mingling the greatest challenge for those studying magma dynamics will be to key the models to well constrained geological examples and to raise the sophistication of the content of these models in light of well defined tests that have geological significance

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

oxygen isotope constraints on the petrogenesis of volcanic arc magmas from martinique lesser antilles abstract volcanic rocks from the island of martinique in the lesser antilles island arc are highly diverse isotopically values of 18o smow in lavas erupted over the last 5 ma from five stratigraphically discrete and geochemically distinct volcanic centers are morne jacob 7 5to 7 9 pitons du carbet 8 0to 8 9 diamant 6 3to 7 4 piton mt conil 7 2to 7 3 andmt pelee 6 5to 7 4 13 of the 27 samples have been corrected for variable amounts of secondary alteration and low temperature hydration the least18o rich andesites on martinique have 18o values similar to those of the northern lesser antilles 18o 6 0 to 7 3 and other intra oceanic arcs like the marianas 18o 5 5 to 6 8 o isotope ratios are compositionally dependent correlations between 18o and radiogenic isotope ratios 87sr 86sr 143nd 144nd 206pb 204pb and indices of fractionation show that modification of isotopic compositions occurred during magmatic differentiation rather than at source a two stage model is proposed for the origin and differentiation of magmas on martinique primary basaltic melts which are broadly similar in composition to primitive island arc magmas elsewhere e g marianas aleutians south sandwich islands are generated from the subduction modified asthenospheric mantle wedge these magmas ascend into the arc crust and differentiate on martinique crystal fractionation is accompanied by contamination with isotopically distinct crust including a component of terrigenous sediments thus subduction zone processes at the lesser antilles and other island arcs are similar involving only a small albeit significant isotopically volumetric contribution from subducted sediment

compositional and dynamic controls on mafic silicic magma interactions at continental arc volcanoes evidence from cordon el guadal tatara san pedro complex chile

interannual variability in a tropical atmosphere ocean model influence of the basic state ocean geometry and nonlinearity abstract the behavior of a tropical coupled atmosphere ocean model is analyzed for a range of different background states and ocean geometries the model is essentially that of cane and zebiak for the tropical pacific except only temporally constant background states are considered here for realistic background states and ocean geometry the model solutions feature oscillations of period of 3 5 yr by comparing the full model solution with a linearized version of the model it is shown that the basic mechanism of the oscillation is contained within linear theory a simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere ocean system the analog model highlights the properties that produce coupled atmosphere ocean instability in the eastern ocean basin and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed negative feedback into this instability growth the growth rate of the local instabil

geochemistry of the active azufre planchon peteroa volcanic complex chile 35 15 s evidence for multiple sources and processes in a cordilleran arc magmatic system

low δ 18 o tephra from a compositionally zoned magma body fisher caldera unimak island aleutians abstract we present the results of an oxygen isotope study of phenocrysts in pumice clasts and ash layers produced by the 9100 yr bp composite dacite basaltic andesite climactic eruption that formed fisher caldera in the eastern aleutians products of the eruption represent a low 18o magma with 18o plagioclase 4 79 0 24 and clinopyroxene 3 81 0 23 corresponding to equilibrium at magmatic temperatures dacitic and overlying basaltic andesitic tephra of the climactic eruption subsequent intracaldera basaltic to andesitic lavas and a cumulate inclusion are similarly low in 18o other analyzed lavas and pyroclastics of unimak island and the lower alaska peninsula as well as precaldera fisher basalt have normal 18o magmatic values 5 5 we propose a model in which prior to 9100 yr bp normal mantle derived basaltic magma coalesced in a large shallow precaldera magma chamber during late wisconsin glaciation lowering of magmatic 18o resulted then from long term assimilation of 5 10 of syn glacial hydrothermally altered country rocks differentiation of basaltic magma was concurrent with this assimilation and produced low 18o fisher dacites cumulates and post caldera crystal richer lavas we propose the use of 18o values of phenocrysts especially alteration resistant pyroxene in tephra as a tool for tephrochronological and tephrostratigraphic correlation distinctly low 18o values are useful in identification of the fisher ash in the eastern aleutians and in the lower alaska peninsula

asymmetry in the duration of el nino and la nina abstract el nino and la nina are not a simple mirror image but exhibit significant differences in their spatial structure and seasonal evolution in particular sea surface temperature sst anomalies over the equatorial pacific cold tongue are larger in magnitude during el nino compared to la nina resulting in positive skewness of interannual sst variations the associated atmospheric deep convection anomalies are displaced eastward during el nino compared to la nina because of the nonlinear atmospheric response to sst in addition to these well known features an analysis of observational data for the past century shows that there is a robust asymmetry in the duration of el nino and la nina most el ninos and la ninas develop in late boreal spring summer when the climatological cold tongue is intensifying and they peak near the end of the calendar year after the mature phase el ninos tend to decay rapidly by next summer but many la ninas persist through the following year and often reintensify in

tectonic setting of the southern cascade range as interpreted from its magnetic and gravity fields we have compiled and analyzed aeromagnetic data from the southern cascade range and compared them with residual gravity data from the same region in order to investigate regional aspects of these young volcanic rocks and of basement structures beneath them various constant level aeromagnetic surveys were mathematically continued upward to 4 571 m and numerically mosaicked into a single compilation extending from lat 40 10 n to lat 44 20 n these data were reduced to the pole upward continued an additional 10 km and compared with a magnetic topographic model and with residual gravity data upward continued to the same level several intriguing regional features are suggested by these data 1 the trinity ophiolite complex that is exposed west of mount shasta probably dips at a shallow angle to the east and continues in the subsurface at least 10 km east of mount shasta 2 mount shasta lassen peak and medicine lake volcanoes are located in a widespread magnetic low possibly caused by an upwarp of the curie temperature isotherm 3 crater lake caldera is located at the intersection of various linear anomalies interpreted to be related to structure in basement rocks below the cascade range 4 three sisters volcanoes and newberry crater are connected to each other by an arcuate magnetic source 5 the high cascades from lat 40 10 n to at least lat 44 30 n are marked by a residual gravity low which includes the three sisters volcanoes mount shasta medicine lake volcano mount mcloughlin and crater lake we believe this gravity feature represents a major structural depression beneath the high cascades 6 except for newberry crater every major volcano of the study area is located on the perimeter of a local gravitational low we suggest that the gravity lows reflect subsidence of low density volcanic material relative to denser country rock and that the major volcanoes have developed over structures at the perimeters of their respective depressions

osmium isotope constraints on lower crustal recycling and pluton preservation at lassen volcanic center ca osmium isotope compositions of intermediate to silicic composition calc alkaline volcanic rocks from the lassen volcanic region of the cascade arc are significantly more radiogenic qos 23 to 224 than typical mantle these evolved arc rocks in the lassen region have unradiogenic sr nd and pb isotope compositions which overlap with those of contemporaneous mafic lavas crystal fractionation of mafic to intermediate composition magmas produces re os ratios that are high enough to evolve to very radiogenic os isotope compositions in only a few million years providing a potential fingerprint for detecting the involvement of such young relatively mafic crust in magmatic systems however the sr nd and pb isotope compositions will remain constant over such short time intervals due to relatively low parent daughter enrichment during magmatic evolution the radiogenic os isotope compositions in typically evolved lassen rocks are interpreted to reflect significant interaction with lower crustal material that has radiogenic os isotope compositions beneath this section of the cascade arc large amounts of such high qos lower crust may have formed and been isolated from mash zone mixing and homogenization processes during the pliocene or late miocene the results from this study indicate that os isotopes may provide a unique glimpse into lower crustal processes such as recycling in primitive orogenic arcs 9 2002 elsevier science b v all rights reserved

large volume low δ18o rhyolites of the central snake river plain idaho usa the miocene bruneau jarbidge and adjacent volcanic fields of the central snake river plain southwest idaho are dominated by high temperature rhyolitic tuffs and lavas having an aggregate volume estimated as 7000 km3 samples from units representing at least 50 of this volume are strongly depleted in 18o with magmatic feldspar 18ovsmow vienna standard mean ocean water values between 1 4 and 3 8 the magnitude of the 18o depletion and the complete lack of any rhyolites with normal values 7 10 combine to suggest that assimilation or melting of a caldera block altered by near contemporaneous hydrothermal activity is unlikely instead we envisage generation of the high temperature rhyolites by shallow melting of idaho batholith rocks under the influence of the yellowstone hotspot affected by eocene meteoric hydrothermal events the seeming worldwide scarcity of strongly 18o depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths

the origin and evolution of magmas from the san pedro pellado volcanic complex s chile multicomponent sources and open system evolution the san pedro pellado volcanic complex is located at 36 s in the chilean andes the eruptive rocks of the complex record the development and collapse of a caldera followed by voluminous largely basaltic andesite volcanism at each stage of evolution crystal fractionation was accompanied by variable degrees of contamination and mixing large variations in incompatible element ratios cannot be produced by closed system evolution correlations between indices of differentiation and incompatible element ratios together with high 18o values indicate that basaltic andesites have assimilated crust to generate the evolved volcanic rocks at san pedro pellado even in the most mafic rocks however incompatible element characteristics are variable as a result of source heterogeneity and deep level processes the restricted ranges in isotope ratios of sr nd and pb among san pedro pellado rocks are due to the small contrast in isotopic compositions between magma and wallrock three source components are recognized as contributing to parental magmas at san pedro pellado although the relative contributions of each cannot be quantified the volumetrically dominant source component is the sub arc asthenospheric mantle morb source the major source of lile is thought to be slab derived fluids which modified the sub arc mantle other incompatible elements may also have been enriched by interaction with the continental lithosphere mantle and or lower crust during ascent

analysis of the non linearity in the pattern and time evolution of el nino southern oscillation in this study the observed non linearity in the spatial pattern and time evolution of el nino southern oscillation enso events is analyzed it is shown that enso skewness is not only a characteristic of the amplitude of events el ninos being stronger than la ninas but also of the spatial pattern and time evolution it is demonstrated that these non linearities can be related to the non linear response of the zonal winds to sea surface temperature sst anomalies it is shown in observations as well as in coupled model simulations that significant differences in the spatial pattern between positive el nino versus negative la nina and strong versus weak events exist which is mostly describing the difference between central and east pacific events central pacific events tend to be weak el nino or strong la nina events in turn east pacific events tend to be strong el nino or weak la nina events a rotation of the two leading empirical orthogonal function modes illustrates that for both el nino and la nina extreme events are more likely than expected from a normal distribution the bjerknes feedbacks and time evolution of strong enso events in observations as well as in coupled model simulations also show strong asymmetries with strong el ninos being forced more strongly by zonal wind than by thermocline depth anomalies and are followed by la nina events in turn strong la nina events are preceded by el nino events and are more strongly forced by thermocline depth anomalies than by wind anomalies further the zonal wind response to sea surface temperature anomalies during strong el nino events is stronger and shifted to the east relative to strong la nina events supporting the eastward shifted el nino pattern and the asymmetric time evolution based on the simplified hybrid coupled rechoz model of enso it can be shown that the non linear zonal wind response to sst anomalies causes the asymmetric forcings of enso events this also implies that strong el ninos are mostly wind driven and less predictable and strong la ninas are mostly thermocline depth driven and better predictable which is demonstrated by a set of 100 perfect model forecast ensembles

tropical pacific basin wide adjustment and oceanic waves this study examines the basin wide adjustment process associated with el nino southern oscillation enso in terms of oceanic waves using the cane zebiak model the rossby waves induce not only the meridional mass divergence in the ocean interior but also the incoming zonal mass flux at the western boundary which generally overcompensates the interior meridional divergence therefore the rossby waves alone cannot make a negative feedback however the outgoing zonal mass flux associated with the reflected kelvin wave at the western boundary reduces the incoming boundary mass flux associated with the rossby waves resulting in the tendency of equatorial basin wide heat content varying in phase with the meridional divergence also shown is that the nonequilibrium adjustment of the tropical pacific ocean is mainly due to the free kelvin waves reflected at the western boundary

a proposed mechanism for the asymmetric duration of el nino and la nina el nino and la nina exhibit significant asymmetry not only in their spatial structure but also in their duration most el ninos terminate rapidly after maturing near the end of the calendar year whereas many la ninas persist into the following year and often reintensify in boreal winter through atmospheric general circulation model experiments it is shown that the nonlinear response of atmospheric deep convection to the polarity of equatorial pacific sea surface temperature anomalies causes an asymmetric evolution of surface wind anomalies over the far western pacific around the mature phase of el nino and la nina because of the eastward displacement of precipitation anomalies in the equatorial pacific during el nino compared to la nina surface winds in the western pacific are more affected by sst forcing outside the equatorial pacific which acts to terminate the pacific event

the dimensions of magmatic inclusions as a constraint on the physical mechanism of mixing abstract silicic lavas often bear inclusions of more mafic lava several physical processes have been proposed by which these magmas may become mixed measurements of the sizes of mafic inclusions in lavas of two calc alkaline volcanoes are used here to obtain a characteristic length scale for this type of mixing and provide a means to discriminate between different mixing processes we quantitatively assess the proposal that in a stratified reservoir dynamic interactions of rising gas bubbles with the interface between a lower layer of mafic magma and an upper layer of silicic magma lead to the formation of vesiculated blobs of the lower magma in the upper one this is due to gravitational instability of a foam layer growing at the interface between the two magmas our theoretical calculations of this instability predict both the correct order of magnitude for the characteristic size of inclusions and the way in which inclusion size varies with the physical properties of the magmas this fluid dynamic framework is used to map out different possible flow regimes and place constraints on the fraction of gas in these systems

el nino and la nina sea surface temperature anomalies asymmetry characteristics associated with their wind stress anomalies 1 the asymmetric nature of el nino and la nina sea surface temperature sst anomalies is investigated by the use of national centers for environmental prediction reanalysis data and various ocean and atmosphere models it is demonstrated that the relatively weak sst anomalies during la nina compared with those of el nino are related to the westward shift of wind stress anomalies by 10 15 the asymmetric characteristics of atmospheric responses are confirmed by the general circulation model experiments with the two different sst anomalies which have equal amplitude but are of opposite sign from each other the experiments with an intermediate ocean model and a hybrid coupled model clearly show that the sst anomalies over the equatorial pacific become weaker as the zonal wind stress shifts to the west not only the amplitude but also the oscillation timescale of the sst anomaly is shown to be sensitive to the location of wind stress anomalies the duration of la nina which is rather shorter than that of el nino is also related to the longitudinal displacement of the wind stress anomaly

new peculiar cave ceiling forms from carlsbad caverns new mexico usa the zenithal ceiling tube holes abstract during a trip to the hall of the white giant carlsbad caverns nm usa cigar shaped vertically upward developing holes were observed on the ceiling at different heights of the passages they have a circular cross section with diameters of 1 to some centimetres and taper out towards their upper end their walls are smooth and their bottom edges are sharp while their length can reach several decimetres sometimes gypsum can be found inside they often occur randomly distributed in groups and their development is not necessarily controlled by fractures or other bedrock structures we name these peculiar karren like cave microforms zenithal ceiling tube holes because of their origin by h 2 s environment corrosion processes and their vertical zenithal upward growth in ceilings a comparison is made between zenithal ceiling tube holes and other karstic or non karstic similar forms such as bell holes oxidation vents snailholes korrosionskolke mixture solution hollows or pockets rohrenkarren light oriented photokarren borings of often marine organisms and negative stalactites zenithal ceiling tube holes are created by the corrosive effect of sulphuric acid h 2 s g dissolves in water giving rise to widespread sulphuric acid corrosion when h 2 s bubbles are trapped underneath overhanging surfaces or ceilings and water level rises steadily the corrosive effect is concentrated vertically upwards drilling vertical holes that can also completely pass overhanging rock ledges

oxygen isotope constraints on the petrogenesis of aleutian arc magmas the first measurement of 18 o 16 o ratios of plagioclase clinopyroxene orthopyroxene and titanomagnetite phenocrysts from modern aleutian island arc lavas provides new insight and independent constraints on magma sources and intracrustal processes basalts are heterogeneous on the scale of the entire arc 18 oplag 5 5 6 7 and individual volcanic centers ranges of 0 3 to 0 8 combined with sr isotope and trace element data 18 oplag values suggest a variable magma source characterized by differences in the mantle wedge or the subducted sediment component along the volcanic front seven tholeiitic basalt to rhyodacite 50 71 sio 2 lavas from the seguam volcanic center have nearly identical 18 oplag values of 6 0 0 2 reflecting extensive closed system plagioclase dominated crystal fractionation oxygen isotope thermometry and pyroxene and oxide equilibria indicate that differentiation occurred between 1150 100 c basalt and 950 100 c rhyodacite in contrast 18 oplag values of 12 calc alkalic basaltic andesites and andesites 53 62 sio 2 from the smaller kanaga volcanic center span a broader range of 5 9 6 6 and consist of mostly higher values isotopic disequilibrium in the kanaga system is manifest in two ways two types of basaltic inclusions with contrasting 18 o values 6 0 and 6 5 occur in one andesite 18 oplag 6 6 and in two other andesites plagioclase titanomagnetite and clinopyroxene titanomagnetite oxygen isotope temperatures are inconsistent one andesite however yields concordant oxygen isotope temperatures of 850 100 c and a two pyroxene temperature of 920 50 c assimilation of isotopically heavy crustal wall rock and mixing off basaltic and andesitic magmas explains the variable and high 18 oplag values and low eruptive temperature of the kanaga andesites

lithospheric structure of northern california from teleseismic images of the upper mantle teleseismic p wave travel time residuals from 120 earthquakes recorded across the u s geological survey california seismic network were used to determine the lithosphere p wave velocity structure beneath northern california a region characterized by complex interactions between the pacific north american and gorda plates lateral p wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals inversion results for the crust show strong correlations to volcanic features the active volcanic fields shasta medicine lake lassen and clear lake are characterized by crustal low velocity anomalies that average approximately 6 possibly identifying partially molten magma bodies cooled solidified magma bodies beneath the extinct volcanic fields sonoma southern clear lake and sutler buttes are denoted by relative velocity highs averaging 3 the largest upper mantle velocity variations occur in the depth range 30 110 km where velocities vary from 5 5 to 9 5 these velocity variations reflect changes in the thickness and geometry of the pacific north american and gorda plates where they interact at the mendocino triple junction north of the mendocino triple junction the steep 70 east dipping portion of the gorda plate is imaged as a 5 velocity high to depths near 270 km a presumed segment of the gorda plate observed beneath the northern great valley and south of the inferred edge of the plate is characterized by a 9 velocity high in the depth range 30 70 km beneath the northern coast ranges shallow asthenosphere is imaged in the depth range 30 100 km as a pronounced southward tapering 4 low velocity zone which we interpret as the slab window results from this study provide improved constraints on gorda plate subduction evolution of the san andreas fault system and development of the lithosphere beneath western north america

biogenic corrosion caused by bats in drotsky s cave the gcwihaba hills nw botswana abstract located in north west botswana drotsky s cave is a vast cavity hosted within dolomite known for the discovery of archaeological strata dating back to the late stone age it is home to a colony of bats comprising several ten thousands of individuals who are responsible for the accumulation of a thick deposit of guano on which we have conducted research in some of the deeper parts of the cavity a detailed study of the morphologies of the dolomite walls and roofs demonstrates that part of the cave s speleogenesis is probably due to biogenic corrosion this type of weathering and corrosion process is still poorly described in underground environments however it is now accepted that the role played by bats and their droppings on the condition of walls their weathering and the enlargement of galleries and chambers by condensation corrosion processes should be taken into account when considering the way deep networks evolve in this article we discuss the mechanisms of this biogenic corrosion in drotsky s cave as well as their morphogenic impacts the dissolution and recession of walls the formation of lateral notches arches bell holes and ceiling spherical cupolas and the consequences for the wall retreat of conduits finally we propose a new typological classification the consequences of this biogenic corrosion are important for archaeological questions related to the taphonomy of archaeological remains in the buried deposits and the conservation of cave art because of the important impacts of guano on buried biological archaeological remains such a population would have encouraged accelerated weathering corrosion and dissolution of the walls potentially contributing to the total or partial disappearance of any trace of human activity on the surrounding rocks

meridional movement of wind anomalies during enso events and their role in event termination 1 observational analysis has shown that when el nino southern oscillation enso events typically reach their peak amplitude in boreal winter the associated zonal wind anomalies abruptly shift southward so that the maximum anomalous zonal wind is located around 5 7 s here an analysis utilizing multiple wind products identifies a clear enso phase nonlinearity in the extent of this meridional wind movement and its dynamically linked changes in equatorial heat content it is shown that the meridional wind movement and its discharging effect increase with increasing el nino amplitude while both remain relatively small regardless of la nina amplitude this result implies that asymmetries in the extent of the meridional wind shift may contribute to the observed asymmetry in the duration of el nino and la nina events we also evaluate the result sensitivities to wind product selection and discuss eastern pacific ep and central pacific cp el nino event differences

a consistent model for the large scale steady surface atmospheric circulation in the tropics the authors present a new model of the tropical surface circulation forced by changes in sensible heat and evaporative flux anomalies that are associated with prescribed sea surface temperature anomalies the model is similar to the lindzen and nigam ln boundary layer model also driven by the above flux anomalies but here since the boundary layer is assumed well mixed and capped by an inversion the model reduces to a twolayer reduced gravity system furthermore the rate of exchange of mass across the boundary layer free atmosphere interface is dependent on the moisture budget in the boundary layer when moist convection is diagnosed to occur detrainment operates on the timescale associated with the life cycle of deep convection approximately eight hours otherwise the detrainment is assumed to be associated with the mixing out of the stable tropical boundary layer which has a timescale of about one day the model provides a diagnostic estimate of the anomalies in precipitation however it is assumed that the latent heat is released above the boundary layer and it drives a circulation that does not impact the boundary layer the authors discuss the derivations of the gill zebiak gz and lindzen nigam models and highlight some apparent inconsistencies between their derivation and the values of several of the parameters that are required for these models to achieve realistic solutions for the circulations then the new reduced gravity boundary model equations are rewritten in the form of the gz and ln models using realistic values for the parameters in the new model geometry it is shown that the constants combine in the rewritten equations to produce the physically doubtful constants in the gz and ln models hence the reason for the apparent success of these models

stratigraphic lithologic and major element geochemical constraints on magmatic evolution at lassen volcanic center california the lassen volcanic center is the most recent of several long lived volcanic centers in the southernmost cascade range these centers have erupted products ranging from basaltic andesite to rhyolite and are superimposed on a background of regional basaltic to andesitic volcanism the evolution of the lassen volcanic center is described in three stages stages i and ii comprise the brokeoff volcano and 80 km3 andesitic stratocone active from 600 to 400 ka brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the lassen region and is the result of structurally controlled focusing of the diffuse regional mafic magmatism stage iii comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km3 volcanism during stage iii was episodic and is subdivided into four sequences of lithologically and temporally distinct lavas stage iii began at 400 ka with a rhyolitic caldera forming pyroclastic eruption and chemically related lavas additional sequences of dacite erupted between 250 200 ka and 100 0 ka hybrid andesites erupted adjacent to the silicic dome field between 300 and 0 ka porphyritic andesite and dacite with high al2o3 low tio2 medium k2o and feo mgo ratios of 1 5 2 0 are the most abundant rock types in the lassen volcanic center however the single most voluminous unit is sparsely phyric rhyolite pumice in general the lavas of lassen volcanic center form a single coherent trend on major element variation diagrams and in pseudo quaternary phase space consistant with an origin either by fractional crystallization or magma mixing in detail however the lack of systematic temporal change in silica and subtly crossing trends indicate a complex origin a variety of statistically successful fractional crystallization models can be constructed that derive brokeoff andesites from regional magmas an important conclusion of the modeling is that if fractional crystallization is the process responsible for generation of brokeoff andesite then the parent magma must be low to medium k in geochemical affinity in order to explain the variation in k2o however although major element variation can be modeled by fractional crystallization petrographic and stratigraphic evidence indicates that magma mixing is an important but subtle process in brokeoff lavas and suggests that lavas evolved in small independent batches lavas erupted during stage iii while predominantly silicic range from 53 to 75 sio2 disequilibrium mineral assemblages in the stage iii lavas indicate that they are not directly derived from brokeoff andesite by fractional crystallization mixing of silicic magma with regional mafic magma and disaggregation of andesitic quenched magmatic inclusions play dominant roles in the compositional diversity of stage iii lavas

the idea of magma mixing history of a struggle for acceptance abstract in 1851 chemist robert bunsen suggested that the mixing of two magmas one mafic and the other felsic in various proportions might account for the wide range of chemical compositions of igneous rocks based on flaws in several of its secondary provisions the whole hypothesis was rejected by a succession of influential critics and remained in disrepute for a hundred years meanwhile studies of composite dikes and sills indicated that indeed mafic and felsic magmas had coexisted at close quarters and had been emplaced in quick succession this interpretation was also used by some investigators to explain the intimate association of mafic and felsic rock types in the commonly occurring igneous complexes others believed that the mafic components of these complexes were derived from geologically older mafic formations by the early 1900s it had become apparent that mafic magmas crystallized at higher temperatures than felsic magmas this knowledge was not immediately applied to the problem of m

a nonlinear analysis of the enso cycle and its interdecadal changes abstract the nonlinear principal component analysis nlpca via a neural network approach was applied to thermocline anomalies in the tropical pacific while the tropical sea surface temperature sst anomalies had been nonlinearly mapped by the nlpca mode 1 onto an open curve in the data space the thermocline anomalies were mapped to a closed curve suggesting that enso is a cyclic phenomenon the nlpca mode 1 of the thermocline anomalies reveals the nonlinear evolution of the enso cycle with much asymmetry for the different phases the weak heat accumulation in the whole equatorial pacific is followed by the strong el nino and the subsequent strong drain of equatorial heat content toward the off equatorial region precedes a weak la nina this asymmetric enso evolution implies that the nonlinear instability enhances the growth of el nino but dwarfs the growth of la nina the nonlinear enso cycle was found to have changed since the late 1970s for the pre 1980s the enso cycle associated with the ther

geochemistry of the 1984 mauna loa eruption implications for magma storage and supply the 1984 eruption of mauna loa volcano hawaii produced about 220 106 m3 of lava of remarkably homogeneous composition no significant compositional changes occurred during the 22 day eruption and along the 20 km vent system even though crystallinity increased substantially accompanying a decline in the eruption rate such homogeneity in lava composition is typical of many historic mauna loa eruptions the 1984 lava along with most other homogeneous mauna loa lavas is multiply saturated crystallizing olivine clinopyroxene and plagioclase bulk compositions are perched at the intersection of an olivine control trend with a trend for more differentiated lavas there is no evidence to indicate that the 1984 lava is a differentiate of the 1975 magma or that it results from mixing a new batch of magma with differentiated melt remaining in the magmatic plumbing system after the 1975 eruption it is proposed that these lavas are erupted directly from a small continuously replenished magma reservoir located about 3 4 km beneath the summit caldera successive eruptions therefore provide a sampling of magma chamber processes and small but discernible changes in lava composition reflect the competing processes of magma supply differentiation and eruption

mechanisms of locking of the el nino and la nina mature phases to boreal winter the peaks of el nino in the cane zebiak cz model tend to appear most frequently around november when the ocean rossby waves which were amplified during the previous unstable season february may turn back to the eastern pacific and when the local instability in the eastern pacific is very weak the peaks of la nina in the cz model occur most frequently in boreal summer in contrast to the observed counterpart that usually occurs in boreal winter sensitivity experiments indicate that the phase locking of the la nina to boreal summer is primarily caused by seasonal variations of the tropical convergence zone which regulate convective heating through atmospheric convergence feedback the observed thermocline and the wind anomalies in the western pacific exhibit considerable seasonal variations these were missed in the original cz model in a modified cz model that includes the seasonal variations of the western pacific wind anomalies and the basic state thermocline depth the peaks of la nina preferably occur in boreal winter suggesting that the seasonal variation of the western pacific surface wind anomalies and the mean thermocline depth are critical factors for the phase locking of the mature la nina to boreal winter the mechanisms by which these factors affect enso phase locking are also discussed

post speleogenetic biogenic modification of gomantong caves sabah borneo article i nfo the gomantong cave system of eastern sabah malaysia is well known as an important site for harvesting edible bird nests and more recently as a tourist attraction although the biology of the gomantong system has been repeatedly studied very little attention has been given to the geomorphology here we report on the impact of geobiological modification in the development of the modern aspect of the cave an important but little recognized feature of tropical caves basic modeling of the metabolic outputs from bats and birds co2 h2o heat reveals that post speleogenetic biogenic corrosion can erode bedrock by between 3 0 mm ka 1 m 300 ka and 4 6 mm ka 1 m 200 ka modeling at high densities of bats yields rates of corrosion of 34 mm ka or 1 m 30 ka sub aerial corrosion creates a previously undescribed speleological feature the apse flute which is semicircular in cross section and 80 cm wide it is vertical regardless of rock properties developing in parallel but apparently completely independently and often unbroken from roof to floor they end at a blind hemi spherical top with no extraneous water source half dome ceiling conch pockets are remnants of previous apse fluting sub cutaneous corrosion creates the floor level guano notch formed by organic acid dissolution of bedrock in contact with guano speleogenetic assessment suggests that as much as 70 95 of the total volume of the modern cave may have been opened by direct subaerial biogenic dissolution and biogenically induced collapse and by sub cutaneous removal of limestone over a timescale of 1 2 ma 2011 elsevier b v all rights reserved

a complex magma mixing origin for rocks erupted in 1915 lassen peak california the eruption of lassen peak in may 1915 produced four volcanic introduction rock types within 3 days and in the following order 1 hybrid magma mixing is an important process among interblack dacite lava containing 2 undercooled andesitic inclusions mediate magmas in volcanic arcs e g turner 3 compositionally banded pumice with dark andesite and light campbell 1986 philpotts 1990 and references therein dacite bands and 4 unbanded light dacite all types represent laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption they contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms e g cashman bergantz 1991 however use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite the petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid crystal mixtures at magmatic and partially hybridized with it phenocrysts from the dacite magma temperatures and pressures if magmas mix incompletely were reacted cooling crystallization and vesiculation of the hybrid i e mingle features such as compositional banding or andesite magma converted it to a layer of mafic foam the decreased undercooled inclusions are usually apparent but if they density of the andesite magma destabilized and disrupted the foam mix completely mineralogical disequilibrium may be the blobs of foam rose into and were further cooled by the overlying dacite magma forming the andesitic inclusions disaggregation of only direct evidence for a mixing origin andesitic inclusions in the host dacite produced the black dacite and if the thermal and compositional contrasts between light dacite magmas formation of foam was a dynamic process two magmas are great and the ratio of silicic to mafic removal of foam propagated the foam layer downward into the magma is large there is little interaction between them hybrid andesite magma eventually the thermal and compositional and the mafic magma is undercooled e g bacon 1986 contrasts between the hybrid andesite and black dacite magmas were many undercooled inclusions contain reacted phereduced then they mixed directly forming the dark andesite magma nocrysts inherited from their host silicic magma heiken about 40 50 andesitic inclusions were disaggregated into the eichelberger 1980 and bacon 1986 pointed out host dacite to produce the hybrid black dacite thus disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas in general the formation of undercooled an efficient magma mixing process disaggregation of undercooled inclusions retards further mixing sakuyama 1984 inclusions carrying reacted host magma phenocrysts produces cothompson dungan 1985 sparks marshall 1986 existing reacted and unreacted phenocryst populations koyaguchi blake 1991 however fragmentation and or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems thompson dungan 1985 coulon et al 1986 clynne christiansen 1987 clynne 1989 linneman myers

subduction controls on the compositions of lavas from the ecuadorian andes three volcanoes of the ecuadorian andes atacazo antisana and sumaco lie in a transect perpendicular to the trench and the main trend of the andean arc each of the volcanoes lies on crust of substantially different age composition and thickness few compositional or isotopic features correspond in a straightforward way to the type of the crust through which the magmas have passed isotopic data limit assimilation to 15 at each of the volcanoes instead a systematic relationship exists between the compositions of the lavas and the depth to the benioff zone suggesting that subduction imparts the principal control on the compositions of the magmas atacazo s lavas have low concentrations of the incompatible trace elements and very large lil hfs ratios sumaco s lavas are strongly enriched in the incompatible trace elements and have small lil hfs ratios antisana s lavas are intermediate in almost every respect these features are consistent with devolatilization of the subducted slab controlling the extent of partial melting of a depleted mantle source a mixing and melting model suggests the volcanic front magmas are made by large extent of partial melting 15 and include a large slab input 1 1 added to the depleted mantle the magmas of the middle belt of volcanoes are made by smaller extent of partial melting 3 induced by moderate amounts of slab derived fluid 0 06 the back arc magmas result from small degrees of melting 2 and small slab input

homogenization and lowering of 18o 16o in mid crustal rocks during extension related magmatism in eastern nevada the oxygen isotope systematics of tertiary volcanic rocks of east central nevada and of plutonic and metamorphic rocks of the ruby mountains east humboldt range core complex provide complementary evidence for major18o depletion and 18o 16o homogenization of mid crustal rocks during metamorphism and magmatism the 18o value of crustal source material for silicic volcanic rocks decreased from between 9 and 11 to between 7 and 8 over 5 ma mid crustal metasedimentary and granitic rocks in the east humboldt range have 18o values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels exchange with deep seated mantle derived igneous rocks or fluids derived therefrom is the most plausible18o depletion mechanism intrusion of these mafic magmas promoted crustal melting and fluid migration homogenization of 18o 16o resulted from migration of high temperature fluids and melts at mid crustal levels and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks

the origin of lavas and ignimbrites of the taupo volcanic zone new zealand in the light of oxygen isotope data abstract the taupo volcanic zone consists predominantly of andesites at each end and about a 20times larger volume of rhyolites incl ignimbrites and pumice in the center in a 18o vs sio2 diagram the rhyolites do not continue the increasingly 18o contaminated andesite basalt to dacite trend but have lower 18o values than the most differentiated andesites comparison with 18o of greywackes and conjectural granitoids seems to rule out a partial fusion origin in retrospect the sr isotope data of ewart and stipp 1968 are seen to confirm this conclusion isotopic adjustment of source rocks before partial fusion by exchange with circulating subsurface waters is highly unlikely because of the combined demands for o and sr isotope shifts and the constraint offered by the normal igneous to somewhat enriched 18o values of the rhyolites oxygen isotope results confirm separate origins or at least different modes of ascent of basalt and andesites on one hand and rhyolites on the other both magma types appear to have originated below crustal formations and both have become contaminated by greywacke through which they passed to reach the surface

tectonic implications of isotopic variation among jurassic and early cretaceous plutons klamath mountains jurassic and early cretaceous plutons in the klamath mountains province of california and oregon were intruded both before and after thrusting during the nevadan orogeny ca 155 148 ma plutons intruded before nevadan thrusting range from calc alkalic to calcic and display a range of compositions that changes consistently with time among this group rb k 2 o and initial 87 sr 86 sr sr i range 0 7034 to 0 7047 decrease and nd range 2 0 to 4 8 increases in younger plutons within individual plutons sr i varies considerably whereas nd is nearly constant the sr and nd isotopic data typically plot to the right of the mantle array 18 o values in these plutons are greater than 8 0 tonalitic and trondhjemitic plutons intruded after nevadan thrusting are typically calcic contain low rb and k 2 o contents and have 18 o less than 8 0 these plutons have a narrow range of sr i 0 7033 0 7036 and nd 4 7 to 7 4 values that plot in the mantle array the isotopic compositions of the pre nevadan plutons requires assimilation of metasedimentary rocks or mixing with crustal melts at deep crustal levels the deep crustal component is not exposed in the province and was presumably truncated during nevadan thrusting post nevadan plutons show little evidence of assimilation of metasedimentary crust but display trace element evidence low k 2 o rb and rare earth element concentrations consistent with an origin by partial melting of young ophiolitic rocks underthrust during the nevadan orogeny

eruptive stratigraphy of the tatara san pedro complex 36 s southern volcanic zone chilean andes reconstruction method and implications for magma evolution at long lived arc volcanic centers

bats and bell holes the microclimatic impact of bat roosting using a case study from runaway bay caves jamaica article the microclimatic effect of bats roosting in bell holes blind vertical cylindrical cavities in cave roofs in runaway bay caves jamaica was measured and the potential impact of their metabolism on dissolution modelled rock temperature measurements showed that bell holes with bats get significantly hotter than those without bats during bat roosting periods by an average of 1 1 c the relationship is clearest for bell holes with more than about 300 g aggregate bat body mass and for bell holes that are moderately wide and deep of w d ratio between 0 8 and 1 6 measurement of temperature decay after abandonment showed that rock temperature returns to normal each day during bat foraging periods metabolic activity from a typical population of 400 g bat 10 individuals yields 41 g of co2 417 6 kj of heat and 35 6 g of h2o in each 18 hour roost period and could produce a water film of 0 44 mm that is saturated with co2 at 5 the resultant rock dissolution is estimated at 0 005 cm

oxygen isotope geochemistry of the lassen volcanic center california resolving crustal and mantle contributions to continental arc magmatism this study reports oxygen isotope ratios determined by laser fluorination of mineral separates mainly plagioclase from basaltic andesitic to rhyolitic composition volcanic rocks erupted from the lassen volcanic center lvc northern california plagioclase separates from nearly all rocks have o values 6 1 8 4o higher than expected for production of the magmas by partial melting of little evolved basaltic lavas erupted in the arc front and back arc regions of the southernmost cascades during the late cenozoic most lvc magmas must therefore contain high o crustal material in this regard the o values of the volcanic rocks show strong spatial patterns particularly for young rhyodacitic rocks that best represent unmodified partial melts of the continental crust rhyodacitic magmas erupted from vents located within 3 5 km of the inferred center of the lvc have consistently lower o values average 6 3o 0 1o at given sio2 contents relative to rocks erupted from distal vents 47 0 km average 7 1o 0 1o further magmas erupted from vents situated at transitional distances have intermediate values and span a larger range average 6 8o 0 2o basaltic andesitic to andesitic composition rocks show similar spatial variations although as a group the o values of these rocks are more variable and extend to higher values than the rhyodacitic rocks these features are interpreted to reflect assimilation of heterogeneous lower continental crust by mafic magmas followed by mixing or mingling with silicic magmas formed by partial melting of initially high o continental crust 9 0o increasingly hybridized by lower o 6 0o mantle derived basaltic magmas toward the center of the system mixing calculations using estimated endmember source o values imply that lvc magmas contain on a molar oxygen basis approximately 42 to 4 isotopically heavy continental crust with proportions declining in a broadly regular fashion toward the center of the lvc conversely the o values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the lvc has been substantially modified by intrusion of mantlederived basaltic magmas with the degree of hybridization ranging on a molar oxygen basis from approximately 60 at distances up to 12 km from the center of the system to 97 directly beneath the focus region these results demonstrate on a relatively small scale the strong influence that intrusion of mantle derived mafic magmas can have on modifying the composition of pre existing continental crust in regions of melt production given this result similar but larger scale regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction related basaltic magmas

high resolution 3 d mapping using terrestrial laser scanning as a tool for geomorphological and speleogenetical studies in caves an example from the lessini mountains north italy abstract terrestrial laser scanning tls is increasingly used in geomorphology for the study of medium to small scale landforms a light weight compact and portable tls device has been used in the grotta a cave mt lessini n italy to make a detailed 3d model of the underground environment a total of 16 scans were used to survey the about 150 m long cave in

oxygen isotope disequilibrium between quartz and sanidine from the bandelier tuff new mexico consistent with a short residence time of phenocrysts in rhyolitic magma abstract oxygen isotope analyses are reported from co existing quartz and feldspar from the bandelier tuff and cerro toledo high silica rhyolitic pyroclastic deposits erupted from the valles caldera new mexico quartz shows little variation outside analytical error but 18 o in feldspar varies over 1 in most samples 18 o 16 o fractionation between quartz and feldspar is significantly less than is predicted for equilibrium at temperatures appropriate for rhyolitic magma in the tshirege upper member of the bandelier tuff isotopic fractionation between mineral pairs is close to equilibrium in the later erupted ignimbrite but non equilibrium in the initial plinian deposit these relationships are interpreted in terms of a model where most phenocrysts are derived from a highly porphyritic carapace around the magma chamber that was disrupted by eruption thus scattering crystals throughout the magma carapace quartz and feldspar are initially isotopically lighter than the bulk aphyric magma due to chemical communication with low 18 o country rock in the meteoric hydrothermal system surrounding the chamber we assume that quartz and feldspar were initially in isotopic equilibrium diffusive re equilibration of crystals begins when the carapace disintegrates and the minerals are immersed in the bulk magma just prior to and during eruption feldspar is isotopically lighter than quartz at equilibrium but responds more rapidly than quartz to an external change due to a higher diffusion coefficient for oxygen hence immersion in the isotopically heavier bulk magma causes feldspar and quartz 18 o values to initially converge over 10 2 years and then diverge over 10 3 10 4 years as first feldspar and then quartz re equilibrate with the new magma higher 18 o variability of feldspar than quartz indicates that the shorter timescale applies to the bandelier and cerro toledo rhyolites two important implications of this interpretation are 1 that the bandelier magmas developed in an aphyric condition and their porphyritic character is an artifact of eruption and 2 that a protective mechanically rigid cognate carapace around a silicic magma chamber may limit interaction with low 18 o hydrothermally altered crust thus hindering the development of significant volumes of low 18 o silicic magma

feedback process responsible for intermodel diversity of enso variability the origin of the inter model diversity of the el nino southern oscillation enso variability is investigated by applying a singular value decomposition svd analysis between the inter model tropical pacific sea surface temperature anomalies ssta variance and the inter model enso stability index bj index the first svd mode features an enso like pattern for the inter model ssta variance 74 of total variance and the dominant thermocline feedback th for the bj index 51 inter model th is mainly modified by the inter model sensitivity of the zonal thermocline gradient response to zonal winds over the equatorial pacific h and the inter model h is correlated higher with the inter model off equatorial wind stress curl anomalies than the equatorial zonal wind stress anomalies finally the inter model off equatorial wind stress curl is associated with the meridional shape and intensity of enso related wind patterns which may cause a model to model difference in enso variability by influencing the off equatorial oceanic rossby wave response

origin of silicic magmas and the compositional gap at sumisu submarine caldera izu bonin arc japan abstract sumisu caldera volcano izu bonin arc japan is dominated by the bimodal assemblage of basalt basaltic andesite 66 wt sio2 andesites 56 61 wt sio2 are minor in volume and a compositional gap of 6 wt sio2 exists between the andesites and the dacites most andesites contain sieve textured plagioclase and some dacites have resorbed plagioclase containing many glass inclusions some orthopyroxene phenocrysts in andesites and dacites have overgrowth rims with a high ca content a feature consistent with crystallization from a higher temperature magma than their cores dacites rhyolites and glasses in the matrix and plagioclases possess compositions similar to those of partial melts produced by dehydration melting of calc alkaline andesite tanzawa tonalite at low pressure 3 kbar phenocryst assemblages of these andesites dacites and rhyolites resemble restite phase assemblages of the dehydration melting prolonged fractionation or partial melting of basalts in contrast is untenable based on petrography and fractionation calculations we suggest that the silicic magmas at sumisu caldera volcano have been produced by dehydration melting of solidified calc alkaline andesite protolith in the upper to middle crust which segregated efficiently from restite crystals in rare situations reheating and en mass remobilization permitted the same andesites to erupt as andesites 56 61 wt sio2 thus sumisu has a relatively narrow temperature difference 900 1100 c associated with a large compositional range 56 74 wt sio2

age of the rockland tephra western usa abstract the age of the rockland tephra which includes an ash flow tuff south and west of lassen peak in northern california and a widespread ash fall deposit that produced a distinct stratigraphic marker in western north america is constrained to 565 000 to 610 000 yr by 40ar 39ar and u pb dating 40ar 39ar ages on plagioclase from pumice in the rockland have a weighted mean age of 609 000 7000 yr isotopic ages of spots on individual zircon crystals analyzed by the shrimp rg ion microprobe range from 500 000 to 800 000 yr a subpopulation representing crystal rims yielded a weighted mean age of 573 000 19 000 yr overall stratigraphic constraints on the age are provided by two volcanic units including the underlying tephra of the lava creek tuff erupted within yellowstone national park that has an age of 639 000 2000 yr the basaltic andesite of hootman ranch stratigraphically overlies the rockland in the lassen peak area and has 40ar 39ar ages of 565 000 29 000 and 565 000 12 000 yr for plagioclase and groundmass respectively identification of rockland tephra in odp core 1018 offshore of central california is an important stratigraphic age that also constrains the eruption age to between 580 000 and 600 000 yr

direct evidence for the origin of low 18o silicic magmas quenched samples of a magma chamber s partially fused granitoid walls crater lake oregon abstract partially fused granitoid blocks were ejected in the climactic eruption of mount mazama which was accompanied by collapse of crater lake caldera quartz plagioclase and glass in the granitoids have much lower 18 o values 3 4 to 4 9 than any fresh lavas of mount mazama and the surrounding region 5 8 to 7 0 oxygen isotope fractionation between phases in granitoids is consistent with equilibrium at t 900 c following subsolidus exchange with hydrothermal fluids of meteoric origin assimilation of 10 20 of material similar to these granitoids can account for the o and sr isotopic compositions of lavas and juvenile pyroclasts derived from the climactic magma chamber many of which have 18 o values 0 5 or more lower than comparable lavas of mount mazama the o isotope data provide the only clear evidence for such assimilation because the mineralogy and chemical and radiogenic isotopic compositions of the granitoids dominantly granodiorite are similar to those of erupted juvenile magmas the granitoid blocks from crater lake serve as direct evidence for the origin of 18 o depletion in large shallow silicic magma bodies

the effect of the south pacific convergence zone on the termination of el nino events and the meridional asymmetry of enso abstractduring large el nino events the westerly wind response to the eastern equatorial pacific sea surface temperature anomalies sstas shifts southward during boreal winter and early spring reaching latitudes of 5 7 s the resulting meridional asymmetry along with a related seasonal weakening of wind anomalies on the equator are key elements in the termination of strong el nino events using an intermediate complexity atmosphere model it is demonstrated that these features result from a weakening of the climatological wind speeds south of the equator toward the end of the calendar year the reduced climatological wind speeds which are associated with the seasonal intensification of the south pacific convergence zone spcz lead to anomalous boundary layer ekman pumping and a reduced surface momentum damping of the combined boundary layer lower troposphere surface wind response to el nino this allows the associated zonal wind anomalies to shift south of the equator furthermore using a linear sh