mobility of rare earth and other elements during crystallization of peralkaline silicic lavas abstract pantellerite lavas of the dormant caldera volcano mayor island new zealand have thick nonhydrated obsidian selvages at the tops and bases and crystalline microspherulitic interiors the flows were chemically homogeneous on emplacement but the crystalline portions have lost significant na f cl cs y and ree percentage losses of y and ree may exceed 60 ce yb ratios decrease by as much as 20 but eu eu ratios are unchanged loss of y and ree is attributed to the expulsion at near solidus temperatures of y ree 3 chloride complexes but other complexing agents may have contributed to this mobility na is also lost in the same manner groundwater has introduced mg and sr and leached or redistributed na cs and halogens analysis of hydrated glassy plinian pumice shows that secondary hydration has not affected y and ree concentrations the elements na mg f cl sr cs y and ree are highly mobile during the crystallization of peralkaline silicic melts and should not be used in petrogenetic evaluations these elements but not y and ree are also redistributed during secondary hydration processes magmas of undersaturated peralkaline composition will be similarly affected

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

liquid immiscibility and its role at the magmatic hydrothermal transition a summary of experimental studies several types of fluid immiscibility may affect the evolution of volatile rich magmatic systems at the magmatic hydrothermal transition the topology of silicate salt h2o systems implies that three fluid immiscibility silicate melt hydrosaline melt vapour should be stable in a broad range of compositions and p t conditions the most important factor controlling the immiscibility appears to be the coulombic properties electric charges z and ionic radii r of the main network modifying cations and the capacity for immiscibility appears to decrease in the following sequence mg ca sr ba li na k liquid immiscibility is enhanced in peralkaline compositions and in the presence of nonsilicate anions such as f cl co32 and bo33 in volatile rich magmatic systems the h2o is likely to react with the chloride fluoride borate and carbonate species and the chemical effects of high temperature hydrolysis may be greatly enhanced by phase separation in systems with multiple immiscible fluid phases natural granitic magmas can thus exsolve a range of chemically and physically diverse hydrosaline liquids and the role of these fluid phases is likely to be especially significant in pegmatites and li f rare metal granites

late collision rhyolitic volcanism in the north eastern part of the armenian highland abstract plio pleistocene acidic volcanism of the northeastern part of the armenian highland is related to a continental collision zone as a result of convergence between eurasia and arabia lithosphere plates the development of volcanism is divided into three stages of 10 17 4 5 7 5 and 0 1 2 8 ma a crustal origin model is proposed to explain the geochemical and petrogenetic aspects of the acidic volcanism formation spatially separated ba rich and rb rich geochemical types of rhyolites were revealed indicating more evolved and primitive rhyolitic magma types respectively the variation interval of the primary 87sr 86sr ratio was found to be from 0 70438 to 0 70636 age and lateral variation of geochemical and isotopic parameters were determined by differences of initial substrates and degrees of their partial melting the mechanisms of rhyolitic magma evolution were the remelting of heterogeneous sialitic materials and low pressure fractional crystallization in isolated magmatic chambers these facts confirm the model of crustal anatexis origin of the eutectic melts the eutectic rhyolitic magma was generated by an open system melting in the presence of deep horizon mobile k rb fluids initiated by mantle resources

neogene and quaternary ignimbrites in the area of arequipa southern peru stratigraphical and petrological correlations abstract this study focuses on the correlation of four newly identified rhyolitic ignimbrites located around arequipa which were previously described as a single ignimbrite sheet 1 the rio chili ignimbrite ca 13 33 ma crops out in the rio chili canyon and consists of a cooling unit of non welded to partially welded massive crystal rich lapilli tuff this ignimbrite represents the base of the neogene ignimbrite succession in the arequipa area and may be stratigraphically equivalent to the 14 13 ma huaylillas ignimbrites in southern peru 2 the second voluminous 20 km3 and widespread la joya ignimbrite sheet ca 4 87 ma filled the arequipa depression its probable source is now buried underneath the chachani volcanic complex the la joya ignimbrite covers both the arequipa batholith toward the rio vitor piedmont and also part of the altiplano to the east and north of the chachani complex 3 the arequipa airport ignimbrite aai sheet ca 1 6 ma 18 km3 the third and latest infill of the arequipa basin consists of a lower white columnar jointed massive lapilli tuff indurated by vapor phase recrystallization and is overlain by an upper pink lithic rich non welded massive lapilli tuff the source of this ignimbrite is also buried underneath the chachani complex as indicated by anisotropy of magnetic susceptibility measurements and component lithology 4 the yura non welded pumice flow deposits ca 1 02 ma 1 5 km3 are restricted to the northern and western flanks of the chachani complex and may correlate with tuffs of the capillune formation on the altiplano all ignimbrites are high k calc alkaline to alkaline rhyolites which contain variable amounts of plagioclase biotite quartz sanidine and fe ti oxides the modal composition of mineral phases and chemistry of amphiboles and biotites are useful for distinguishing the ignimbrites specific geochemical fingerprints such as sr y rb ba cs ree and sr isotopes were effective discriminator elements magmatic differentiation crustal residence time and contamination and source melting processes are discussed in order to explain compositional variations the older la joya ignimbrite and rio chili ignimbrite reflect initial wetter crustal melts closer to granite composition whereas with time the younger arequipa airport ignimbrite and yura tuffs initial melts are slightly less evolved and dryer due to more extensive crustal melting the voluminous 40 km3 neogene ignimbrites of the arequipa area were produced by three major explosive volcanic pulses in the central volcanic zone of southern peru whose genetic relation with tectonic uplift and crustal thickening of the andean cordillera are still a matter of debate

variations in trace element partition coefficients in sanidine in the cerro toledo rhyolite jemez mountains new mexico effects of composition temperature and volatiles trace element partition coefficients have been measured for one plagioclase and five sanidine mineral separates from the cerro toledo rhyolite new mexico sanidine partition coefficients vary substantially and systematically within the cerro toledo rhyolite partition coefficients for ca sr zn la and eu are lowest in the most evolved rhyolites whereas sm and hree partition coefficients are highest rubidium partition coefficients remain constant while those for ba ce and th are variable variations of the sr zn la and eu partition coefficients are correlated with the ca contents and partition coefficients of the sanidines calcium may have controlled the distribution of these elements in the sanidine by modifying the feldspar structure the low ca partition coefficients in sanidines for the most evolved rhyolites may be the consequence of modification of the melt structure possibly due to increased volatile contents at the top of the magma chamber s during evolution of the cerro toledo rhyolite the zn and la partition coefficients between sanidine and melt also may have been controlled by this change in melt structure modelling using major elements and the constant rb partition coefficient for sanidine indicates 70 crystallization of magma during cerro toledo rhyolite time by removal of 68 sanidine and 32 quartz estimates of the volume 1 of initial parental magma and 2 of the magma that crystallized during this period are 11 670 km3 and 8170 km3 respectively the average intrusion rate of silicic magma during cerro toledo rhyolite activity was 35 10 3 km3 a

a new model for the evolution of la reunion volcanic complex from complete marine geophysical surveys results from recent marine geophysical surveys offer a new perspective for characterizing the evolution processes of volcanic islands in 2006 cruises forever and eroder 1 investigated the submarine flanks and the surrounding abyssal plain of la reunion indian ocean to obtain for the first time a complete geophysical survey of the area combined analyses of these data reveal major differences in the evolution of the two emerged volcanoes piton des neiges and piton de la fournaise we show that debris avalanche deposits extend on the abyssal plain only offshore the active piton de la fournaise volcano attesting the occurrence of large flank collapse events the absence of such deposits offshore piton des neiges and the presence of compressive structures within the sedimentary unit below the edifice support a mechanism of slow deformation of this volcano such as sliding or spreading the slow deformation of piton des neiges has led to numerous secondary submarine slope instabilities and favored some unconfined turbidity flows which generated large sediment waves running downward all around the island this study proposes a new model using the most complete marine data set available slow deformation controls the evolution of piton des neiges whereas piton de la fournaise formed on the flanks of a pre existing edifice experienced catastrophic large flank collapse events

magma mixing recharge and eruption histories recorded in plagioclase phenocrysts from el chichon volcano mexico operative throughout the 200 ky history of the el chichon magma consistent core to rim decreases of sr sr ratios and coincident system increases in sr concentrations in plagioclase phenocrysts of varying size 1 cm to 2 mm are reported from samples of the 1982 and pre 1982 200 ka eruptions of el chichon volcano maximum sr sr ratios of 0 7054 significantly higher than the wholerock isotopic ratios 0 7040 0 7045 are found in the cores

vegetation development in an amazonian peatland recent investigations of wetlands in western amazonia have revealed the presence of extensive peat deposits up to 7 5 m thick developing under a variety of vegetation types we report the first attempt to establish the long term centennial to millennial scale vegetation history of one of these peatland sites quistococha a palm swamp close to iquitos in northern peru pollen and sedimentological analyses show that peat formation began at the core site under sedge fen or floating mat vegetation c 2200 calendar years before present cal yr bp a seasonally flooded woodland developed c 1880 cal yr bp the permanently waterlogged palm swamp which persists today began to form c 1000 cal yr bp with the present vegetation community established by c 400 years ago the vegetation at this site has undergone continuous change throughout the period of peat formation with several abrupt transitions and reversals and repetitions in the apparent trajectory of change the pollen data combined with sedimentary evidence suggest that the dominant control on ecosystem functioning and development is the flooding regime there appears to have been a decrease in fluvial influence over time there is no clear evidence of direct climatic or anthropogenic influence although we cannot rule out the possibility of climatically driven hydrological changes our results caution against adopting a simple model of peatland vegetation succession in this region

sr isotope disequilibrium during differentiation of the bandelier tuff constraints on the crystallization of a large rhyolitic magma chamber the 1 61 ma otowi member of the bandelier tuff jemez mountains volcanic field new mexico is a caldera forming high silica rhyolitic ignimbrite with a precursor fallout deposit the otowi member has high rb sr ratios the 87 rb 86 sr ratios from sanidines and glasses range from 14 to 570 most sanidines from glomerocrysts in the ignimbrite have 87 sr 86 sr i 0 7052 0 7056 whereas the ratios from glasses from glomerocrysts range from 0 7052 to 0 7079 quartz phenocrysts containing glass inclusions from both the initial fallout and the ignimbrite are markedly more radiogenic at 87 sr 86 sr i 0 7105 0 7113 despite having much lower rb sr ratios than glomerocryst glasses these relations require that the inclusion glasses are more contaminated with proterozoic country rock than are glomerocryst glasses textural isotopic and trace element data support a model in which crystals grow in a boundary layer with the most inclusion rich quartz grains closest to the magma country rock contact phenocrysts and glomerocrysts represent fragments from different zones of the chamber9s crystalline carapace disseminated throughout the magma prior to eruption an important implication of these results is that glass inclusions do not necessarily represent precursor magma compositions hence extrapolation of measured volatile contents of inclusion glasses to the entire volume of an erupted magma should be approached with caution

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

a model that helps explain sr isotope disequilibrium between feldspar phenocrysts and melt in large volume silicic magma systems feldspar phenocrysts of silicic volcanic rocks are commonly in sr isotopic disequilibrium with groundmass in some cases the feldspar is more radiogenic and in others it is less radiogenic several explanations have been published previously but none of these is able to accommodate both senses of disequilibrium we present a model by which either more or less radiogenic feldspar or even both within a single eruptive unit can originate the model requires a magma body open to interaction with biotite and feldspar bearing wall rock magma is incrementally contaminated as wall rock melts incongruently biotite preferentially melts first followed by feldspar such melting behavior which is supported by both field and experimental studies first contaminates magma with a relatively radiogenic addition followed by a less radiogenic addition feldspar phenocrysts lag behind melt groundmass of volcanic rock in incorporating the influx of contaminant thus resulting in sr isotopic disequilibrium between the crystals and melt the sense of disequilibrium recorded in a volcanic rock depends on when eruption quenches the contamination process this model is testable by isotopic fingerprinting of individual feldspar crystals for a given set of geologic boundary conditions specific core to rim sr isotopic profiles are expectable moreover phenocrysts that nucleate at different times during the contamination process should record different and predictable parts of the history initial results of sr isotopic fingerprinting of sanidine phenocrysts from the taylor creek rhyolite are consistent with the model more tests of the model are desirable

in situ sr isotopic analysis by laser ablation we demonstrate that the sr isotopic compositions of geological materials can be measured in situ through laser ablation using an icp multiple collector double focusing magnetic sector mass spectrometer mc icpms this provides rapid texturally sensitive high precision 0 004 sr isotopic analysis without the need for chemical preparation of samples the system used employs a q switched nd yag infrared laser the laser ablated material is carried by ar flow to an icp torch the resulting ions are extracted from the plasma and focused with d c quadrupole lenses and an electrostatic analyzer prior to entering a magnetic sector analyzer and collection in an array of faraday collectors a feldspar megacryst with independently known 87sr86sr of 0 703117 13 and a modern gastropod shell with independently known 87sr86sr of 0 709170 10 were used evaluate the method both have 2000 ppm sr laser power was adjusted to give an average 88sr of 2 10 11 amps during analysis to replicate the beam size during typical thermal ionization mass spectrometry tims within run precisions in 87sr86sr of 4 10 5 compare favorably with the precision from tims thirteen analyses of the cameroon feldspar megacryst yielded a weighted average 87sr86sr of 0 703106 22 in perfect agreement with the tims measurement similarly 22 analyses of the modern gastropod shell gave a weighted average of 0 709182 22 identical to the known value no memory effect was observed when switching from the cameroon feldspar sample run for several hours to the modern shell material the power of this technique is illustrated with a preliminary study of systematic differences in 87sr86sr of 0 0004 between two feldspar crystals of a long valley basalt one feldspar has identical 87sr86sr to the host basalt confirming the lack of any matrix effect with this method

crustal forensics in arc magmas abstract the geochemical characteristics of continental crust are present in nearly all arc magmas these characteristics may reflect a specific source process such as fluid fluxing common to both arc magmas and the continental crust and or may reflect the incorporation of continental crust into arc magmas either at source via subducted sediment or via contamination during differentiation resolving the relative mass contributions of juvenile mantle derived material versus that derived from pre existing crust of the upper plate and providing these estimates on an element by element basis is important because 1 we want to constrain crustal growth rates 2 we want to quantitatively track element cycling at convergent margins and 3 we want to determine the origin of economically important elements and compounds traditional geochemical approaches for determining the contributions of various components to arc magmas are particularly successful when applied on a comparative basis studies of suites from multiple magmatic systems along arcs for which differentiation effects can be individually constrained can be used to extrapolate to potential source compositions in the lesser antilles arc for example differentiation trends from individual volcanoes are consistent with open system evolution however such trends do not project back to a common primitive magma composition suggesting that differentiation modifies magmas that were derived from distinct mantle sources we propose that such approaches should now be complemented by petrographically constrained mineral scale isotope and trace element analysis to unravel the contributing components to arc magmas this innovative approach can 1 better constrain true end member compositions by returning wider ranges in geochemical compositions among constituent minerals than is found in whole rocks 2 better determine magmatic evolution processes from core rim isotopic or trace element profiles from the phases contained in magmas and 3 constrain rates of differentiation by applying diffusion controlled timescales to element profiles an example from nguaruhoe volcano new zealand underscores the importance of such a microsampling approach showing that mineral isotopic compositions encompass wide ranges that whole rock isotopic compositions are consequently simply element weighted averages of the heterogeneous crystal cargo and that open system evolution is proved by core rim variations in sr isotope ratios nguaruhoe is just one of many systems examined through microanalytical approaches the overwhelming conclusion of these studies is that crystal cargoes are not truly phenocrystic but are inherited from various sources the implication of this realization is that the interpretation of whole rock isotopic data including the currently popular u series needs careful evaluation in the context of petrographic observations

crustal structure of the main ethiopian rift from gravity data 3 dimensional modeling abstract a three dimensional interpretation of the newly compiled bouguer anomaly map of the main ethiopian rift is presented a high resolution 3 d model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben the bouguer anomalies along the axial portion of the rift floor as deduced from the results of the regional and residual separation are mainly caused by the deep seated structures the inferred zone of intrusion which is the main subject of the present study coincides with the maximum gravity anomaly of the rift floor the intrusion is displaced at several sectors along the east west direction and the two major displacements coincide with the locations of the major rift offsets on the surface because of the asthenospheric uplift the crust under the main ethiopian rift is slightly thinned the zone of crustal thinning 31 km coincides with the location of the intrusion beneath the rift floor and the maximum of which is attained in the northern and central sectors of the graben the trend of the crustal thinning zone which is from south to north is the same as the one obtained in the afar depression the southeastern and western plateaus on the other hand show by far the largest crustal thickness in the region 38 51 km in contrast to the afar depression where the crust is partly oceanized the thickness and density of the crust suggest that the main ethiopian rift is underlain by a purely continental crust the deep and relatively large nature of the intrusion leads to the conclusion that a large scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben

heterogeneous initial sr isotope compositions of highly evolved volcanic rocks from the main ethiopian rift ethiopia many individual mineral sr isotope studies have revealed the complex evolution of highly evolved rocks in upper crustal magmatic systems casting doubts on the meaning of whole rock sr isotopes in such samples in this paper whole rock sr isotope measurements were replicated three to 13 times on six highly evolved peralkaline rhyolites from the main ethiopian rift mer to appraise their internal heterogeneity and thus the significance of such data these rocks were all fresh samples of pumice obsidian and lava their maximum sr contents and ages were 15 g g and 1 7 ma respectively significant small scale heterogeneities of both sr isotopes and rb and sr contents were observed in most samples although not necessarily associated with petrological characteristics suggesting possible inheritance processes only two almost crystal free obsidian give fairly homogeneous sr isotope ratios these results outline the ambiguity of a single whole rock sr isotope determination on highly evolved peralkaline rocks especially when no simultaneous accurate determination of the rb sr is performed they also suggest that the limitations of sr whole rock analyses are not restricted to phenocryst rich samples as phenocryst poor obsidian and almost aphyric pumices and lava are also concerned these data further underscore that unreplicated whole rock sr isotope measurements should always be used with great caution in the petrogenetic modeling of highly evolved rocks however multiple determinations of whole rock 87sr 86sr in the same samples combined with other geochemical and isotopic data may provide constraints on the shallow level evolution of these magmas it is suggested that selective upper crustal contamination and or interactions with halogen bearing hydrous fluids typical of evolved peralkaline magmas were probably involved in the late magmatic evolution of these mer rhyolites the more pervasive character of fluid interaction processes would probably better account for the small scale association of uncontaminated and contaminated signatures in a single sample thus even fresh samples may have their rb sr isotopic system significantly modified by fluid interactions not as a secondary process but at the late magmatic stage

magmatic interactions as recorded in plagioclase phenocrysts of chaos crags lassen volcanic center california the silicic lava domes of chaos crags in lassen volcanic national and mingling of this material into the silicic host these processes park contain a suite of variably quenched hybrid basaltic andesite are commonplace in some orogenic magma systems and may be magmatic inclusions the inclusions represent thorough mixing elucidated by isotopic microsampling and analysis of the plagioclases between rhyodacite and basalt recharge liquids accompanied by some crystallizing from them mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement sr sr ratios 0 7037 0 7038 of the inclusions are distinctly lower than those of the host rhyodacite

evolution of silicic magma through assimilation and subsequent recharge evidence from sr isotopes in sanidine phenocrysts taylor creek rhyolite nm

geochemical comparison between himalayan and hercynian leucogranites abstract isotopic sr pb nd o ree and trace element data from three himalayan nepal and six hercynian brittany france leucogranites are compared for the himalayan granites makulu mustang and especially manaslu the 87sr 86sr versus 87rb 86sr scatter diagrams and hence variability of the initial 87sr 86sr ratios reflect heterogeneity of the source materials these ratios are very high 0 730 0 770 for manaslu the very high 207pb 204pb ratios 15 8 at the corresponding 206pb 204pb ratio of 18 8 the very low 143nd 144nd ratios i nd 13 to 17 and the very high 18o 16o ratios 12 confirm the crustal origin with the paragneisses of the tibetan slab formation i as the probable parent source for the six hercynian granites with an individual rb sr isochron established for many of them the initial 87sr 86sr ratios range from 0 706 to 0 717 the lead neodymium and oxygen isotope compositions confirm their crustal source of presumably palaeozoic age but not specifically identified the rare earth patterns and thorium contents are comparable to those for himalayan granites the similarities of the geochemical signatures isotopes and trace elements of the himalayan and hercynian leucogranites suggest that the petrogenetic processes were comparable and that the himalayan continental collision model can be applied to the hercynian the depletion in ree and th implies either interaction of the granitic liquids with fluid or more probably early precipitation of monazite the increase in sm nd in the granites compared to their parent materials implies that the neodymium model ages of primary mantle extraction are maximum rather than minimum ages in contrast to the hercynian leucogranites the extremely radiogenic sr of the himalayan leucogranites reflects the very long crustal residence time before the melting event the apparent uniformity of the initial 87sr 86sr ratios of the individual hercynian granites may be related to an aging effect where the straight line fitting improves with increase in age of the pluton