reconciling heterogeneity in crystal zoning data an application of shared characteristic diagrams at chaos crags lassen volcanic center california isotope trace element and textural crystal zoning patterns record heterogeneity in magmatic systems not resolved by whole rock analyses these zoning data are used to infer crystal residence times magma mixing and other magmatic processes in many magmatic systems we present the shared characteristic diagram scd as an organizational framework for crystal zoning data that compares information from different phases and chemical tracers in a common framework an example from chaos crags in the cascade arc produces three main results 1 anorthite zoning profiles in plagioclase have fewer shared characteristics in mafic inclusions than in the host rhyodacite 2 single crystal 87sr 86sr data from previous studies tepley et al 1999 are consistent with more shared history between crystals than in anorthite profiles this difference reflects a more homogeneous distribution of 87sr 86sr than the intensive parameters controlling plagioclase composition 3 the chaos crags system exhibits a layer of heterogeneity in crystal populations that is not represented in whole rock analyses that indicate only simple binary mixing the inconsistency between 87sr 86sr and anorthite zoning data highlights decoupling between compositionally controlled and temperature water controlled zoning in plagioclase from chaos crags

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

trace element evidence for plagioclase recycling in calc alkaline magmas abstract trace element zoning in plutonic plagioclase feldspars from the val fredda complex southern adamello massif italy has been measured using secondary ion mass spectrometry sims the studied samples range from hornblende gabbro through diorite to granitoid with an overall range in plagioclase composition from an96 to an15 individual plagioclases display extensive zonation in anorthite through a considerable variety of zoning textures there is equally extensive zoning in trace elements notably sr and ba broadly similar trace element zoning patterns and concentrations are displayed by all plagioclases irrespective of host rock composition using a relationship betweenxan and the distribution coefficientsdsr anddba trace element concentrations in plagioclase are converted to those of the melt which precipitated each crystal zone the calculated variations in ba and sr are greater than can be attributed to changes in pressure or temperature alone indicating that compositional variation in the coexisting melt was the principal cause of zoning the ba sr melt trend for each crystal describes a two stage crystallisation history the calcic cores of all crystals show melt trends of increasing sr and ba which are attributed to mixing and fractionation in parental hydrous picrobasalt magmas in contrast the melt trends recorded by the rims vary from sample to sample but essentially show decreasing sr and ba consistent with fractionation of sodic plagioclase alkali feldspar and or biotite the plagioclase cores are thought to have crystallised in a common magma chamber at 6 10 kbar pressure while the rims crystallised at the emplacement level 1 5 kbar from interstitial melt whose composition is determined by that of the host magma the data suggest that early formed calcic plagioclase was recycled by the magma during fractionation vigorous convection is invoked to both retain relatively dense calcic plagioclase within the evolving magma and to periodically re entrain cumulus crystals into more evolved magmas the occurrence of calcic plagioclase cores in felsic derivative magmas indicates that fractionation from picrobasalt to tonalite occurred on a relatively short time scale

lillooet glacier basalts southwestern british columbia canada products of quaternary glaciovolcanism the retreat of lillooet glacier lg has exposed a succession of pillow basalt and subordinate amounts of breccia and hyaloclastite the lithofacies and physiographic setting suggest that the deposits have a glaciovolcanic origin and represent a partially dissected basaltic pillow dominated tindar chemically the lg volcanic rocks are basalt to basaltic andesite and as a group they represent the highest silica quaternary mafic products in the garibaldi volcanic belt gvb like other northern gvb alkaline basalts they lack the nb ta depletion signature typically associated with subduction related products geochemical and petrologic analysis indicates that the lg basalts are comagmatic and that chemical variations within the suite are consistent with sorting of the observed phenocryst assemblage olivine plagioclase thermodynamic modeling establishes shallow crustal pre eruptive storage conditions at 2 kbar 1 kbar 100 mpa or 7 5 km and an h2o content of 0 5 1 wt we estimate that the

positive degree day factors for ablation on the greenland ice sheet studied by energy balance modelling ice ablation is related to air temperature by the positive degree day factor variations of the positive degree day factor in west greenland are studied using an energy balance model to simulate ablation under different conditions degree day factors for simulated and measured ice ablation at nordbogletscher and qaman rssup sermia agree well with values around 8 mm d 1 c 1 degree day factors for snow are less than half those for ice energy balance modelling shows that degree day factors vary with summer mean temperature surface albedo and turbulence but there is only evidence of large positive degree day factors at lower temperatures and with low albedo 0 3 the greatest effect of albedo variations 0 3 0 7 is at lower temperatures while variations in turbulence have greater effect at higher temperatures current models may underestimate runoff from the greenland ice sheet by several tenths because they use a degree day factor for melting ice that is too small for the colder parts of the ice sheet i e the upper ablation area and the northerly margin

five decades of shrinkage of july 1st glacier qilian shan china a survey of july 1st glacier qilian shan china was carried out in 2002 previously the glacier s boundary had been recorded in 1956 and further research had been carried out in the mid 1970s and 1980s our survey reveals that area shrinkage and surface lowering have accelerated in the past 15 years surface elevation changes can result from changes in accumulation surface melting and emergence velocity the contributions of these elements to surface lowering are evaluated at the lower part of the glacier from observations of surface velocity ice thickness and precipitation and from temperature data near the glacier apart from the effect of glacier ice redistribution our analysis reveals quantitatively that the recent accelerated glacier shrinkage has been caused by increasing temperature furthermore it is established that meltwater discharge from the glacier in the past 17 years has increased due to glacier shrinkage by about 50 over that from 1975 to 1985

observations of plagioclase zoning using interference imaging abstract interference imaging techniques laser interference microscopy in transmitted light and nomarski differential interference contrast in reflected light have been used to study different types of zoning in magmatic plagioclase crystals samples studied come from a wide range of rock types and geological environments analysis of zoning profiles for individual unit zones shows that a saw tooth pattern in which an initially high an decreases as growth proceeds is the most common type of zoning configuration we recognize 2 major types of zoning type 1 small zones usually less than 10 microns thick with compositional amplitudes up to 10 an and type 2 zones up to50 microns thick with large compositional amplitudes of up to 35 an detailed analysis of zoning patterns shows that oscillatory zoning is not actually oscillatory in the sense of a sine wave it is discontinuous unit zones are characteristically bounded by discontinuities this common pattern is consistent with alternating periods of growth and periods of dissolution reaction and lack of growth or possibly very slow growth our observations firmly establish that dissolution and magmatic reaction involving both growth and dissolution are common during growth of the magmatic plagioclase which we have studied type 1 zonation appears consistent with perturbations of local conditions in a diffusion controlled boundary layer around the growing crystal type 2 zoning is probably due to large scale conditions faceted planar growth surfaces appear to be the rule in magmatic crystallization although in some instances rounded or irregular convolute zonation can occur the repetitive nature of many of the features we have observed is consistent with and suggestive of some form of self organization

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

u pb zircon geochronology of silicic tuffs from the timber mountain oasis valley caldera complex nevada rapid generation of large volume magmas by shallow level remelting large volumes of silicic magma were produced on a very short timescale in the nested caldera complex of the sw nevada volcanic field swnvf voluminous ash flows erupted in two paired events topopah spring ts 1 200 km3 12 8 ma tiva canyon tc 1 000 km3 12 7 ma and rainier mesa rm 1 200 km3 11 6 ma ammonia tanks at 900 km3 11 45 ma all cited ages are previously published 40ar 39ar sanidine ages within each pair eruptions are separated by only 0 1 0 15 my and produced tuffs with contrasting isotopic values these events represent nearly complete evacuation of sheet like magma chambers formed in the extensional basin and range environment we present ion microprobe ages from zircons in the zoned ash flow sheets of ts tc rm and at in conjunction with 18o values of zircons and other phenocrysts which differ dramatically among subsequently erupted units bulk zircons in the low 18o at cycle were earlier determined to exhibit 1 5 core to rim oxygen isotope zoning and high spatial resolution zircon analyses by ion microprobe reveal the presence of older grains that are zoned by 0 5 2 5 the following u pb isochron ages were calculated after correcting for the initial u pb disequilibria at zircon rims 11 7 0 2 ma cores 12 0 0 1 ma pre at rhyolite lava 12 0 0 3 ma rm 12 4 0 3 tc 13 2 0 15 ma ts 13 5 0 2 average zircon crystallization ages calculated from weighted regression or cumulative averaging are older than the ar ar stratigraphy but preserve the comparably short time gaps within each of two major eruption cycles ts tc rm at notably every sample yields average zircon ages that are 0 70 0 35 ma older than the respective ar ar eruption ages the th u ratio of swnvf zircons are 0 4 4 7 higher than typically found in igneous zircons which correlates with elevated th u of the whole rocks 5 16 high th u could be explained if uranium was preferentially removed by hydrothermal solutions or is retained in the protolith during partial melting for low 18o at cycle magmas rim ages from unpolished zircons overlap within analytical uncertainties with the 40ar 39ar eruption age compared to core ages that are on average 0 2 0 3 my older than even the age of the preceding caldera forming eruption of rm tuff this age difference the core to rim oxygen isotope zoning in at zircons and disequilibrium quartz zircon and melt zircon isotopic fractionations suggest that at magma recycled older zircons derived from the rm and older eruptive cycles these results suggest that the low 18o at magmas were generated by melting a hydrothermally altered protolith from the same nested complex that erupted high 18o magmas of the rm cycle only 0 15 my prior to the eruption of the at the largest volume low 18o magma presently known

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

the ion microprobe as a tool for obtaining strontium isotopes in magmatic plagioclase a case study at okataina volcanic centre new zealand abstract we investigated the potential of multi collector secondary ion mass spectrometry mc sims as a tool for obtaining sr isotopic compositions in plagioclase a ubiquitous mineral in igneous rocks that serves as a recorder of crystallization history mc sims allows for high spatial resolution analysis 12 m in this study of isotopes and therefore improves the temporal scale at which fluctuations in crystallization conditions can be recognized ultimately improving our understanding of rates of magmatic processes plagioclase crystals from two young rhyolitic deposits from two major eruptive complexes tarawera and haroharo of the okataina volcanic centre in new zealand were analysed results were corrected for matrix effects using linear modelling of mc sims data versus an contents as well as 87sr 86sr ratios acquired via laser ablation inductively coupled plasma mass spectrometry la mc icp ms corrected mc sims sr isotopic ratios had an average 2 uncertainty of 0 0008 per spot and were homogeneous in okataina plagioclase at high spatial resolutions average la mc icp ms 87sr 86sr ratios of plagioclase from both intra caldera volcanic complexes tarawera 87sr 86sr 0 7056 and haroharo 87sr 86sr 0 7054 suggest similar magma sources and similar assimilation and fractional crystallization processes for the two complexes overall homogeneity of plagioclase excluding relict cores indicates no significant changes in contributions i e crustal assimilation mafic influx to the system during the majority of plagioclase crystal growth furthermore lack of 87sr 86sr ratio fluctuations in plagioclase rims suggest interaction between the resident silicic magma and the intruding mafic magma that triggered the eruptions was largely limited to volatile and heat transfer using appropriate standards and analysis this mc sims method can be used to detect short lived open system events in magma reservoirs where differences in 87sr 86sr isotopic ratios are significant

the record of magma chamber processes in plagioclase phenocrysts at thera volcano aegean volcanic arc greece lavas and pyroclastic rocks throughout the volcanic stratigraphy of the tertiary quaternary volcanic complex of thera in the aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures interpreted as indicative of magma mixing plagioclase zoning characteristics studied by nomarski and laser interferometry techniques establish three main categories of plagioclase i inherited plagioclase nucleated in endmember prior to initial mixing event ii in situ plagioclase nucleated in mixed or hybrid magma and iii xenocrystic plagioclase nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases depending on the composition of the lava in which they are hosted these differences reflect the contrasting effects of changes in physical chemical parameters in basic vis a vis more acidic melts during magma mixing and or influx of new magma into the subvolcanic magma chamber as well as the influence of magma chamber dynamics on plagioclase equilibration variations in bulk major and trace element abundances of thera volcanic products reflect the dominant overprint of crystal fractionation but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes consistent with compositional and textural zoning in plagioclases

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

the oases along the keriya river in the taklamakan desert china and their evolution since the end of the last glaciation the area of the keriya river is an important laboratory for studying the formation and evolution of artificial and natural oases the plant communities vary in an orderly way from phragmites to populus to tamarix in a landscape profile crossing a riverbed in the center of the taklamakan desert the green oasis belt along the river course and the oases in the former delta areas have been plagued by desertification for about 400 years especially since the beginning of 1950s while irrigation has continuously increased in the middle reaches of the river the recent deterioration of the natural oases along the keriya river is likely caused mainly by human activities rather than a deduced aridification of climate on the basis of stratigraphy of river terraces and paraglacial fans and on microstructures on the quartz grains it is hereby concluded that the keriya river in the tarim basin was able to flow through the taklamakan desert and to have formed a green oasis corridor extending through the desert during deglaciation

pb isotopic zoning of k feldspar megacrysts determined by laser ablation multi collector icp ms insights into granite petrogenesis this study investigates pb isotopic zoning in magmatic k feldspar megacrysts from the monte capanne pluton elba italy using laser ablation multi collector icpms the studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open system processes and better characterize the components involved in the genesis of complex magma systems earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal derived magmas the investigated k feldspar megacrysts exhibit strong zoning in 207pb 206pb and 208pb 206pb correlated with lead elemental variations we interpret these variations as reflecting growth zoning as opposed to secondary diffusive exchange despite a great variety of zoning patterns we were able to correlate different events of megacryst growth reflecting crystallization in a dynamic magma system our two step model includes 1 growth of a granitic magma chamber by addition of low 208pb 206pb magma to a high 208pb 206pb magma contaminated with crustal material i e the megacryst cores and 2 recharge by mantle derived magma i e the megacryst rims we interpret the thorogenic nature of the megacryst rims to reflect the mantle derived component involved in the mixing process taking account of other data from the tuscan magmatic province the mantle source is inferred to have been metasomatized by continental material during subduction tims sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled

in situ strontium and sulfur isotope investigation of the ni cu pge sulfide ore bearing kevitsa intrusion northern finland the 2 06 ga kevitsa mafic ultramafic intrusion in northern finland hosts a large disseminated ni cu pge deposit the deposit occurs in the ultramafic olivine pyroxene cumulates and shows a range in ni tenors varying from 4 7 wt regular ore to 10 wt ni pge ore there are also a metal poor sulfide mineralization false ore and contact mineralization that are uneconomic ni tenor 0 7045 in comparison to the estimated depleted mantle sr isotope ratio of 0 702 at 2 06 ga the sulfur 34s values are generally higher than 2 which together with the sr isotope data imply involvement of crustal material in the genesis of the kevitsa intrusion and its ores the 87sr 86sr i values obtained from the ore bearing domain of the intrusion show stratigraphic variation and exceed 0 7050 with the maximum value reaching up to 0 7109 in contrast in rocks around the ore domain the initial sr isotope compositions remain more or less constant 0 7047 0 7060 throughout the intrusive stratigraphy the isotope data suggest that the ore bearing domain of the intrusion represents a dynamic site with multiple injections of variably contaminated magma whereas the surrounding part of the intrusion experienced a less vigorous emplacement history no correlation is observed between the strontium and sulfur isotope compositions this is explained by bulk assimilation of the silicate magma in a deeper staging magma chamber and variable assimilation of sulfur during magma transport into the kevitsa magma chamber the low level of metals in false ore and the ni depleted nature of its olivine suggest that some sulfides may have precipitated and deposited in the feeder conduit during the initial stage of magma emplacement cannibalization of early formed sulfides by later magma injections may have been important in the formation of the economic ore deposit

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

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

laser interferometry study of oscillatory zoning in plagioclase the record of magma mixing and phenocryst recycling in calc alkaline magma chambers iztaccihuatl volcano mexico

the amphibole effect a possible mechanism for triggering explosive eruptions abstract the possible effect of pressure induced breakdown of amphibole in triggering explosive eruptions is considered since amphibole is a hydrous mineral when it breaks down to an anhydrous assemblage as pressure is reduced to less than 1 5 2 kbar the water liberated might oversaturate the coexisting melt generating the necessary overpressure to trigger an explosive eruption resorbed amphiboles are commonly observed in evolved lavas and pyroclastic ejecta the amount of a volatile component such as water that will dissolve in a melt is a function of pressure temperature and composition and during crystallization it is also a function of the extent of crystallization and the nature of crystallizing minerals the relation can be expressed by the simple equation where x r is the water content of the residual liquid x i is the initial water content x moh is the water content of hydrous minerals f is the total extent of crystallization and f is the extent of crystallization of hydrous minerals such that 0 f f 1 we suggest that storage of water in hydrous minerals such as amphibole and biotite plays an important role in the eruptive behavior of certain types of magmas the breakdown of these minerals liberates water to the melt at a rate governed by the kinetics of the resorption reaction if the release of water causes the liquid fraction to exceed the solubility limit and the overpressure resulting from expansion of the gas exceeds the strength of the overlying magma and rocks in the conduit the result can be an explosive eruption the amphibole effect can occur at different structural levels depending on the nature of the magma and physical conditions leading to instability

petrogenesis of sierra nevada plutons inferred from the sr nd and o isotopic signatures of mafic igneous complexes in yosemite valley california mafic complexes in the central sierra nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust in the intrusive suite of yosemite valley major and trace element compositions of the hornblende bearing gabbroic rocks from the rockslides mafic complex and of the mafic dikes in the north america wall are compositionally similar to high alumina basalt of these rocks two samples have higher ni and cr abundances as well as higher end values than previously recognized for the intrusive suite plagioclase crystals in rocks from the north america wall and the rockslides have prominent calcic cores and sharply defined sodic rims a texture commonly associated with mixing of mafic and felsic magmas in situ analyses of 87sr 86sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains we propose that the high 87sr 86sr 0 7067 and low end 3 4 of bulk rocks the homogeneity of 87sr 86sr in plagioclase and the high 18o values of bulk rocks 6 6 7 3 and zircon lackey et al in j petrol 49 1397 1426 2008 demonstrate that continental crust was assimilated into the sublithospheric mantle derived basaltic precursors of the mafic rocks in yosemite valley contamination 20 40 likely occurred in the lower crust as the magma differentiated to high alumina basalt prior to plagioclase and zircon crystallization as a consequence the isotopic signatures recorded by whole rocks plagioclase and zircon do not represent the composition of the underlying lithospheric mantle we conclude that the mafic and associated felsic members of the intrusive suite of yosemite valley represent 60 80 new additions to the crust and include significant quantities of recycled ancient crust

observed physical and environmental causes of scatter in whitecap coverage values in a fetch limited coastal zone 1 meteorological and oceanographic data along with sea surface images were recorded at a fetch limited coastal site to investigate the effect of physical and environmental conditions on whitecap coverage w an automated image processing technique allowed over 100 000 images to be analyzed for w data analysis showed that many processes influenced w the presence of tidal currents appeared to have augmented values of w under certain specific conditions analysis of wave spectra indicated the ubiquitous presence of swell propagating northward scatter in w was markedly absent in mixed seas when the spectral intensity of the wind waves was of the same order of magnitude as the spectral intensity of the swell waves swell dominated seas introduced much more scatter in w w was approximately one third lower in swell dominated seas than in mixed seas specifically steep swell waves steepness values greater than 0 01 that propagated opposite to wind wave direction appeared to have reduced w at wind speeds below approximately 7 5 m s 1 but this effect needs more investigation the coastal site enabled the possibility of investigating physical and environmental effects on w that would otherwise have been more difficult to observe in the open ocean

degree day glacier mass balance modelling with applications to glaciers in iceland norway and greenland a degree day glacier mass balance model is applied to three glaciers in iceland norway and greenland for which detailed mass balance measurements are available over a period of several years model results are in good agreement with measured variations in the mass balance with elevation over the time periods considered for each glacier in addition the model explains 60 80 of the year to year variance in the elevation averaged summer season mass balance measurements on the glaciers using a single parameter set for each glacier the increase in ablation on the glaciers due to a warming of 2 c is predicted to range from about 1 m w e year 1 at the highest elevations to about 2 5 m w e year 1 at the lowest elevations predicted changes in the winter balance measured between fixed date are relatively small except at the lowest elevations on the icelandic and norwegian glaciers where the winter balance is significantly reduced equilibrium line altitudes are raised by 200 300 m on the icelandic and norwegian glaciers except at the highest elevations the winter balance of the icelandic and norwegian glaciers is predicted to decrease even if the warming is accompanied by a 10 increase in the precipitation no firm evidence of a climate related variation in the degree day factors or in the temperature lapse rate on the same glacier could be found the model furthermore reproduces large variations in the mass balance with elevation and from year to year on each glacier using the same parameter set we assume therefore that these parameters will not change significantly for the climate scenarios considered here

nomarski interference contrast observations of textural details in volcanic rocks both minute textural details and the overall petrographic features of volcanic rocks are markedly clarified through the examination of etched polished thin sections by using the reflected light differential interference contrast dic apparatus of g nomarski the sensible two dimensional nature of the stage object relief 0 5 m permits precise definition of the forms of the crystalline and noncrystalline components of the rock and therefore defines their precise spatial relations the nomarski dic techniques which can readily be combined with traditional polarized transmitted light microscopy should prove particularly effective in the study of rocks containing abundant dark glass

sr isotope zoning in plagioclase from parinacota volcano northern chile quantifying magma mixing and crustal contamination we present analyses of sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from parinacota volcano central volcanic zone northern chile which were analysed for major and minor element zoning in a previous study although the isotopic range of the bulk rock samples is small at this volcano 0 7067 0 7070 except for one flow of mafic andesite at 0 7061 significant variations are seen 0 70649 0 70700 within and between plagioclase crystals a general negative correlation is observed between sr isotope composition and sr concentration in the liquid in equilibrium with each plagioclase zone as calculated from chemical zoning data and partition coefficients additional scatter is superimposed on this general trend indicating a decoupling between isotopic and chemical variations for sr in one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event we apply energy constrained recharge assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal results suggest 20 assimilation of the local wall rock gneiss at high initial temperatures the isotopic data confirm the involvement of two contrasting mafic magmas which are sampled at flank cinder cone vents one lower ajata has a low sr content with high 87sr 86sr the other upper ajata has a high sr content with lower 87sr 86sr in some samples from parinacota the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87sr 86sr mafic magma in others where chemical zoning profiles suggest that recharge was from the low 87sr 86sr magma the 87sr 86sr of the groundmass and crystal rims is higher than expected this indicates either additional parent magmas to the two previously identified or further crustal assimilation either at lower crustal depths before crystallization of plagioclase or just after the last recharge our results illustrate the complexity of magma crust interaction beneath parinacota which is likely to be representative of many other central andean volcanoes formed on thick crust such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase in a textural petrographic context despite a small whole rock isotopic range the distinct contamination patterns of various samples suggest an important role for the geometry location and evolution of the magma plumbing system and in general variations of the thermal and compositional structure of the crust underneath the volcano

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

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico i disequilibrium phenocryst assemblages as indicators of magma chamber processes

climatic warming glacier recession and runoff from alpine basins after the little ice age maximum records of discharge of rivers draining alpine basins with between 0 and 70 ice cover in the upper aare and rhone catchments switzerland for the period 1894 2006 have been examined together with climatic data for 1866 2006 with a view to assessing the effects on runoff from glacierized basins of climatic warming coupled with glacier recession following the little ice age maximum annual runoff from ice free basins reflects precipitation variations rising from minima between 1880 and 1910 to maxima between the late 1960s and early 1980s the more highly glacierized the basin the more runoff mimicked mean may september air temperature during two periods of warming runoff increased gradually from the 1900s rapidly in the 1940s before decreasing to the late 1970s rising runoff levels during the second warming period failed to exceed those attained during the first despite higher summer temperatures although temperatures continued to rise discharge from glacierized basins declined after reaching maxima in the late 1980s to early 1990s in the first warming period rising specific melt rates augmented by increasing precipitation opposed the impact of declining glacier area on runoff although melt continued to increase in the second period enhanced melting even in the exceptionally warm summer of 2003 appears to have been insufficient to offset reducing glacier surface area exposed to melt low or reducing levels of precipitation and increasing evaporation thus runoff from glacierized basins peaked in the late 1940s to early 1950s

rapid determination of initial 87sr 86sr and estimation of the rb sr age of plutonic rocks by la icpms of variably altered feldspars an example from the 1 14 ga great abitibi dyke ontario canada abstract sr isotope analysis of plutonic rocks is an important method for deriving the provenance and age of ancient magmatic episodes and the petrogenetic processes operating during intrusive activity we determined the 87 sr 86 sr isotopic compositions of fresh and variably altered feldspars from the 1 14 ga gabbroic great abitibi dyke ontario canada through the use of laser ablation inductively coupled plasma mass spectrometry la icpms the data show an alteration trend between fresh low rb plagioclase with 87 sr 86 sr ratios of 0 7045 and high rb alkali feldspar with 87 sr 86 sr ratios of 0 7059 which may be used to constrain the 87 sr 86 sr ratio of the mantle source this trend is incompatible with simple mixing and instead has age significance as post intrusive alteration through hydrothermal circulation starts immediately following emplacement la icpms analysis allows rapid estimation of initial 87 sr 86 sr ratios and intrusive age suitable for sample screening and selection for more precise dating for the great abitibi dyke the rb sr age estimate of emplacement of 1 10 0 23 ga 2 is concordant with the precise u pb emplacement age of the dyke 1140 6 2 0 ma the most altered high rb alkali feldspars return younger ages suggesting that post intrusive alteration lasted several hundred million years this study highlights the need for microanalysis to differentiate between alteration and magmatic processes on basis of sr isotopic signatures and outlines a rapid method to determine initial 87 sr 86 sr ratios and to estimate rb sr ages of plutonic rocks in terranes with long histories of multiple intrusive magmatic episodes

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

insights into magmatic evolution and recharge history in capraia volcano italy from chemical and isotopic zoning in plagioclase phenocrysts plagioclase phenocrysts in dacites from the high k calc alkaline capraia volcano were investigated for major trace element and sr isotope variations in order to gain better insight into the proposed open system behaviour of the volcano repeated dissolution zones in plagioclases from the early erupted monte castello mc and later monte rucitello mr dacitic units are associated with a sharp increase in an content and variation in isotopic compositions 87sr 86sr 0 70872 0 71004 which argue for repeated magma mixing in particular petrographic and mineralogical evidence suggest that recharge with hot basaltic magma occurred repeatedly during the lifetime of the capraia volcanic system we attempted to correlate shared crystallization and resorption histories using chemical mol an fe and sr isotopic zoning in plagioclase in this correlation framework the observed mismatch between chemical and isotopic data in some phenocrysts is interpreted to reflect either growth of phenocrysts in a separate magma chamber or compositional zoning within a single magma chamber a minor role of crustal contamination is inferred instead the intra crystal variations in isotopic composition in plagioclase reflect the existence of distinct components that may have thoroughly mixed at the end of the mc plagioclase growth history the dominant radiogenic component is similar to lamproites occurring in tuscany recharge with this component occurred throughout the genesis of the mr dacites but was restricted to the early growth history of the mc dacites the mr plagioclases are inferred to have crystallized from separate more differentiated batches of magma that were likely to have evolved at the top of a zoned magma reservoir decrease in the an content after most dissolution zones in all phenocrysts and low temperature estimates from zircon saturation thermometry 800 c also emphasize the importance of crystal fractionation during the evolution of the volcano in agreement with the occurrence of a significant amount of rhyolitic material thus it is inferred that a rather large magma chamber may have existed during the first cycle of eruptive activity

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

responses of various sized alpine glaciers and runoff to climatic change this paper presents a glacier ice flow model that simulates changes to alpine glaciers of various sizes and their runoff response to climate change in the yili river basin in the tien shall mountains northwestern china it is suggested that the sensitivity of glaciers to climatic change is determined by glacier size the change in glacial runoff does not keep pace with climactic change as climate warms and glaciers retreat the glacier runoff tends to increase and then decrease the runoff peak and its timing depend not only on glacier size but also on the rate of air temperature rise

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

reconstructions of annual discharge and equilibrium line altitude of glaciers at qilian shan northwest china from 1978 to 2002 discharge was calculated from a mountainous area including discharge from glaciers in the qilian shan qilian mountains of northwest china the studied yingluoxia basin is 9983 km2 in area with glaciers making up 0 3 of the basin the calculation method was based on the heat balance requiring only daily temperature and precipitation calculated annual discharge from the basin corresponded well with the observed data calculated annual discharge from glaciers was 3 6 of the total discharge from the basin the temporal trend of the calculated equilibrium line altitude ela at the july 1st glacier western side of the yingluoxia basin was similar to that of the observed ela the calculated annual mass balance of glaciers within the yingluoxia basin has a larger negative value than the other glaciers in china as the ratio of accumulation area to the total glacier area in the yingluoxia basin is much lower than in neighbouring basins to the west copyright 2010 john wiley sons ltd

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

glacier changes since the little ice age maximum in the western qilian shan northwest china and consequences of glacier runoff for water supply based on aerial photographs topographical maps and the landsat 5 imidge data we have analyzed fluctuations of glaciers in the western qilian shan northwest china from the little ice age lia to 1990 the areas and volumes of glaciers in the whole considered region decreased 15 and 18 respectively from the lia maximum to 1956 this trend of glacier shrinkage continued and accelerated between 1956 and 1990 these latest decreases in area and volume were about 10 in 34 years the recent shrinkage may be due either to a combination of higher temperatures and lower precipitation during the period 1956 66 or to continuous warming in the high glacicrized mountains from 1956 to 1990 as a consequence glacier runoff from ice wastage between 1956 and 1990 has increased river runoff by 6 2 km 3 in the four river basins under consideration besides the equilibrium line altitude ela rise estimated from the mean terminus retreat of small glaciers 1 km long is 46 m which corresponds to a 0 3 c increase of mean temperatures in warm seasons from the lia to the 1950s

effects of volatiles on mixing in calc alkaline magma systems the intimate mixing between different magmas of disparate densities characteristic of some calc alkaline magma systems is explained by a new mechanism involving the emplacement of a layer of wet undersaturated mafic magma at the base of a magma chamber containing more differentiated magma turbulent transfer of heat between the mafic magma and the overlying magma leads to crystallization and exsolution of volatiles in the lower layer for an initial water content of a few per cent the bulk density of the mafic magma can become equal to that of the overlying magma causing overturning and intimate mixing of the magmas the mechanism is most probable in the low pressure environment of a high level magma chamber

a model for mixing basaltic and dacitic magmas as deduced from experimental data it has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours the presence of phenocrysts larger than 0 5 mm increased considerably the efficiency of mixing flow patterns in the experimental system were visualized using pt spheres which indicated that convection occurs in basalt melt but not in dacite melt the reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10 3 and 10 6 respectively mixing proceeds initially by mechanical mixing of the two magmas in a large scale but later by coupling interfacial convection and mutual diffusion thus depending on the depth where vesiculation and following disruption of the magma occurs banded pumice homogeneous pumice and homogeneous andesite lava are erupted the observed textures of mixed rocks of plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model

chemical and hydrogen isotope evidence for in situ dehydrogenation of biotite in silicic magma chambers to examine the potential for volatile fluxing of magma chambers by in situ degassing of hydrous minerals we obtained complete chemical analyses for biotite separates from silicic lavas the separates exhibit unusually low h 2 o contents that inversely correlate with host lava temperatures high fe 3 fe 2 ratios that inversely correlate with host lava oxygen fugacities and the highest d values yet reported for biotite from any silicic igneous rock up to 19 these results are direct evidence for selective loss of protium 1 h from biotite during dehydrogenation in magma chambers heated from below by intrusion of mafic magma the maximum p v energy generated from dehydrogenation alone can approach 2 10 3 joules per kilogram of magma this finding provides support for the concept that injection of mafic magma coupled with sudden degassing of hydrous minerals in a volatile rich magma chamber can increase pressure and thus enhance the possibility of initiating a volcanic eruption