the significance of phenocryst diversity in tephra from recent eruptions at popocatepetl volcano central mexico tephra lapilli from six explosive eruptions between april 1996 and february 1998 at popocatepetl volcano popo in central mexico have been studied to investigate the causes of magma diversification in thick crusted volcanic arcs the tephra particles are sparsely porphyritic 5 vol magnesian andesites sio2 58 65 wt mgo 2 6 5 9 wt that contain phenocrysts of nio rich up to 0 67 wt nio magnesian olivine fo89 91 cores with inclusions of cr spinel cr 59 70 orthopyroxene mg 63 76 clinopyroxene mg 68 86 intermediate to sodic plagioclase an33 66 and traces of amphibole major and trace element systematics indicate magma mixing the liquid mg melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic mg melt 72 76 and an evolved component magma mg melt 35 40 these component magmas form a hybrid magnesian andesite with an intermediate range of mg melt 50 72 the mafic end member mg melt 72 75 is saturated with olivine and spinel and crystallizes at temperatures 1170 1085 c with oxygen fugacities close to the fayalite magnetite quartz buffer and elevated water contents of several wt h2o a likely location of crystallization is at lower crustal levels possibly at the moho olivine is followed by high mg clinopyroxene which could start to crystallize during magma ascent at depths of 4 to 13 km the mafic magma mixes with an evolved composition containing low mg clino and orthopyroxene and plagioclase at a temperature of 950 c the repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing the tephra is similar to the popo magnesian andesites suggesting similar generic processes for the common lavas of the volcano the advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma building on the elemental systematics of the tephra and a comparison to the near primary basalts from the surrounding monogenetic fields we infer that the popo mafic end member is a magnesian andesite with variable but high sio2 contents of 55 62 wt and near primary characteristics such as high mg melt of 72 75 feo mgo ratios 1 if extrapolated to an mg melt of 72 75 and high ni contents 200 ppm ni this model implies that the typical elemental signature of the popo andesites such as the low cao al2o3 feo high na2o contents and the depletion in high field strength elements e g p zr ti are mantle source phenomena thus determining the elemental budget of the magnesian andesite as it is prior to the modifications by crustal differentiation is central to quantifying the subcrustal mass fluxes beneath popo

zircon u pb chronology of the jianping complex implications for the precambrian crustal evolution history of the northern margin of north china craton abstract the neoarchean to paleoproterozoic jianping complex in the western liaoning province is an important component of the precambrian basement of the north china craton this region connects the neoarchean fengning chengde complex in the west the neoarchean to paleoproterozoic north chaoyang complex in the northeast and the yixian fuxin archean greenstone belt in the east the precambrian jianping complex is dominantly composed of metamorphosed supracrustal sequence and dioritic to granitoid gneisses tonalite trondhjemite granodiorite ttg here we present results from la icpms zircon u pb isotope dating from the various lithological units in this complex which reveal that the magmatic precursors of the metavolcanic rocks associated with the supracrustal sequence were generated during 2555 2550 ma and up to 2615 ma a major magmatic pulse of dioritic to granitic suite occurred during 2538 to 2495 ma this was followed by ca 2485 ma granulite facies metamorphism and a retrograde event at ca 2450 2401 ma our data also reveal a major charnockite emplacement event in this region at ca 1694 ma integrating our new results with the available geologic and previous geochronological data we identify three major growth stages in the crustal evolution history of the northern margin of the eastern continental block of the north china craton at ca 2550 2495 ma ca 2403 2394 ma and ca 1700 ma the la icp ms zircon data provide new insights on the neoarchean to paleoproterozoic tectonothermal evolution history in the northern margin of the eastern continental block of the north china craton

igneous or metamorphic hornblende phenocrysts as greenschist facies reaction cells in the half dome granodiorite california the half dome granodiorite yosemite national park california is recognized in the field by euhedral fresh looking black hornblende phenocrysts up to 2 cm in length this variety of granodiorite typifies intermediate age hornblende phyric units of cretaceous nested plutonic suites in the sierra nevada batholith although only inclusions of feldspar are evident in hand samples the phenocrysts are riddled with up to 50 inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt al 2 o 3 to magnesiohornblende with over 8 wt elemental zoning within the amphibole is highly irregular on the micrometer scale showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions the chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite suggesting that the non biotite inclusions are the result of metamorphism of the phenocrysts backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite pressure calculations using the al in hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization these hornblende phenocrysts would thus be unsuitable for geobarometry and caution must be used to avoid similarly zoned phenocrysts in the application of the al in hornblende geobarometer

magmatic hydrothermal evolution and devolatilization beneath merapi volcano indonesia abstract at merapi volcano indonesia explosive eruptions lasting several months result from injections of reduced mafic magmas and are interspersed by periods of lava dome growth and collapse quiescent degassing assimilation of wall rock and fractional crystallization lasting a few years silicate melt inclusions and volcanic gases were analyzed to study processes of magmatic volatile exsolution and volcanic degassing during periods of quiescent degassing at merapi volcanic gases were sampled at merapi in 1994 during a phase of quiescent degassing and in 2006 at the end of a dome growth and collapse explosive eruption silicate melt inclusions were collected and analyzed from lavas and scorias for their major element and volatile components solubility relationships between h 2 o and co 2 in melt inclusions demonstrate that melt was being trapped beneath merapi to depths reaching 19 km host amphibole geobarometry indicates crystallization pressures reaching up to 26 km hence there is evidence of deep pooling and crystallization of magma beneath merapi at appreciable depths using a mass balance model of magmatic volatile exsolution we demonstrate that the magma degassing from 19 km to the surface approaches closed system behavior compared to the bulk magmatic volatiles analyzed in melt inclusions volcanic gases had similar amounts of co 2 were enriched in h 2 o and s and depleted in cl and f we thus propose that the magmatic volatile phase was initially exsolved from the magma as a supercritical fluid and that it subsequently exsolved into a h 2 o cl f rich brine and co 2 s rich vapor according to the h 2 o nacl model brine vapor exsolution occurred at 5 km in depth for a 5 wt nacl 900 c to 1000 c fluid although the presence of h 2 s so 2 and co 2 may cause the supercritical fluid to unmix at greater depths magma at shallower depths exsolved a brine and a vapor directly from the melt the volcanic gases at merapi thus represent the vapor fraction of a magmatic volatile phase as brine ponded below the surface we further suggest that carbon dioxide liberated by the magmatic assimilation of limestone as proposed by other studies fluxed the melt thereby promoting co 2 enrichment of the melt and h 2 o degassing finally we propose that merapi volcanic gases are enriched in sulfur compared to the modeled volatile phase composition because deep reduced mafic magma supplies sulfur to the shallow magmatic plumbing system

seasonal patterns of coupled flow in the active layer at three sites in northwest north america vertical arrays of temperature and electric potential probes were installed in the upper soil at sites along the mackenzie river valley and on the north slope of alaska time series were obtained at subdiurnal frequencies throughout the year in the active layer and upper permafrost if the data acquisition system is properly configured the time series can be used to infer soil physiochemical processes the electric potential develops primarily in response to soil water solute concentration gradients in the soil column and is a crude surrogate of the soil water electrolytic conductivity summer precipitation can cause rapid penetration of the thaw front when percolating rainwater warmed at the ground surface carries sensible heat downward to the thawing front rates of warming at depth occur significantly faster than those typical of conductive heat transfer in early winter as the freezing front penetrates downward toward the permafrost table ions are excluded from the ice and concentrated in the intermediate unfrozen zone nearly instantaneous warming of the active layer is triggered by spring snowmelt at happy valley in northern alaska temperatures at the 29 cm depth rise from 7 to 3 c in 1 h for several hours during this event the temperature at 29 cm is warmer than that at regions both above and below producing a strong thermal inversion time series of electric potential or a surrogate derived from electric potential suggest rapid transport of meltwater from the snowpack to depth probably through soil cracks serial events hasten active layer warming by 1 2 weeks

stable chlorine isotope behavior during volcanic degassing of h 2 o and co 2 at mono craters ca trends in co2 and h2o concentrations and d values of obsidian clasts from mono craters volcanic field california demonstrate clear chemical and isotopic evidence for eruptive degassing however neither cl concentrations nor stable isotopes 35cl and 37cl track the degassing process which is likely because of disequilibrium due to slow diffusion of cl in the cooling melt obsidian pyroclasts n 29 were collected from tuff layers representing a single eruptive sequence that occurred circa 1340 a d as well as rhyolitic obsidian samples n 12 were collected from three high silica 74 sio2 flows forming the domes and coulees in the region the cl h2o and co2 concentrations recorded by the eruptive pyroclastic obsidians track the chemical evolution of the magmatic system during eruption whereas the concentrations of the dome samples represent the final degassed product the h2o and co2 concentrations of the pyroclastic samples range from 0 49 to 2 13 wt and 2 to 35 ppm respectively whereas concentrations in the dome glasses range from 0 17 to 0 33 wt and 1 to 3 ppm respectively h2o and co2 concentrations in the pyroclastic fall and dome samples are strongly correlated and reflect the degassing trend of the eruptive sequence chlorine concentrations of the pyroclastic fall samples and the domes range from 609 to 833 ppm and 681 to 872 ppm respectively cl concentrations do not display a strong correlation with either h2o or co2 concentrations d values of the pyroclastic fall obsidians vary between 84 and 55 whereas the d values of the dome obsidians vary between 117 and 91 d h ratios decrease with total water content following a distillation trend controlled by both closed and open system degassing 37cl values of pyroclastic fall obsidians 1 9 to 0 1 overlap with those of dome samples 1 2 to 0 0 the similar cl concentrations between the pyroclastic fall and dome obsidians argue for lack of cl degassing despite h2o and co2 loss these observations can be explained by disequilibrium effects due to the slow diffusion rate of cl compared to h2o and co2 in silicate melt buffering by a separate brine phase or by fluxing of cl from a deeper magma source with the slow diffusion rate of cl being the preferred explanation the wide range in 37cl values may be indicative of isotopic compositional heterogeneities in the magma source due to assimilation of sedimentary material or fluxing of mantle derived cl to a crustal melt

meso cenozoic multiple exhumation in the shandong peninsula eastern north china craton implications for lithospheric destruction abstract the shandong peninsula in the eastern north china craton ncc forms part of the region that witnessed extensive tectonic reactivation with concomitant craton destruction and lithospheric thinning during the meso cenozoic previous studies concentrated mainly on the timing mechanism and tectonic setting of the meso cenozoic magmatism with inadequate evidence from low temperature thermochronology to constrain the thermo tectonic exhumation history of this region in this study we present new apatite u pb aupb and fission track aft data with corresponding thermal history models for igneous rocks from the two flanks of the tan lu fault zone tlfz that bisects the shandong peninsula with a view to gain insights into the meso cenozoic exhumation history of this region and to evaluate its implications on the lithospheric destruction of the ncc the newly obtained aupb ages of 2 5 1 5 ga for the precambrian intrusive rocks and of 162 112 ma for the mesozoic igneous suite are mainly used to constrain their thermal history models in addition the mesozoic aupb ages of 162 112 ma highly overlap with their corresponding zircon u pb ages 161 115 ma suggesting shallow granitoid emplacement and associated rapid post magmatic cooling in response to the westward subduction of the paleo pacific plate the aft dating results yield two groups of aft central ages at 122 113 ma and 98 59 ma and corresponding thermal history models also reveal two rapid cooling stages during the early cretaceous 130 105 ma and late cretaceous to paleogene 85 55 ma by integrating previous low temperature thermochronological studies with this study we interpret that the early cretaceous rapid exhumation corresponds to the peak timing of craton destruction resulting from the paleo pacific slab rollback within a back arc extensional setting the late cretaceous rapid exhumation is interpreted as a response to continuing craton destruction derived by the nnw directed pacific plate subduction the paleogene cooling might represent the termination of craton destruction of shandong peninsula associated with a dextral motion along the tlfz triggered by the change in direction of the pacific plate from nnw to wnw and or far field effect of the india eurasia collision

dynamics of the ca 4965 yr 14c bp ochre pumice plinian eruption of popocatepetl volcano mexico abstract the ochre pumice op plinian fallout was produced by popocatepetl volcano in central mexico during a major plinian eruption that occurred 4965 65 14 c yr bp 3700 bc the op is part of the ochre pumice sequence ops that consists of surge fall and pyroclastic flow deposits the op plinian fallout shows a bimodal grain size distribution with poor to moderate sorting that improves progressively towards the upper beds the juvenile component is mainly pumice 83 wt whereas the accidental components consist of igneous sedimentary and metamorphic clasts from the walls of the magma chamber and or conduit the vesicularity of the pumice decreases from bottom to top of the stratigraphic sequence while the crystal and glass contents increase this suggests increasing magma degassing prior and during the eruption the chemical composition of the pumice varies insignificantly at the base it is less evolved sio 2 61 wt and it becomes slightly more silicic towards the top sio 2 63 wt pumice clasts have a crystallinity index that ranges between 10 and 25 vol and display a seriate texture with phenocrysts of euhedral plagioclase pl clinopyroxene cpx orthopyroxene opx olivine ol oxide ox apatite ap the size of the maximum lithic 2 5 cm at a horizontal distance of 19 km indicates an eruption column height of 37 41 km the isopach map allows the recognition of a dispersal axis that points towards the nne where an area of 300 km 2 was covered by 6 cm of pumice and ash a minimum volume for the op plinian fallout deposit was calculated using a minimum thickness of 6 cm found at distal outcrops and was estimated at 4 9 km 3 which corresponds to 2 km 3 of dense rock equivalent dre stratigraphic relations indicate that the entire ops was emplaced during a short time interval of a few days to weeks and consisted of three main different phases phreato magmatic plinian and vulcanian it started violently with the emplacement of surges and then culminated with the eruption of the op fallout deposit as the eruption advanced discharge rates became more intermittent and the height of the column fluctuated and finally collapsed generating pumice and ash flows that were emplaced around the volcano this short but intense activity was followed during subsequent years by the growth of domes in the crater and the emplacement of block and ash flows generated by vulcanian explosions

applications of a simplified equation of state for the density of silicate hydrous magmas the volcan de colima mexico buoyancy driven dome growth process similarities and differences with the popocatepetl volcano domes abstract colima and popocatepetl volcanoes began erupting during the second half of the twentieth century after periods or relatively long quiescence both eruptive episodes show similarities persistent dome growth and destruction episodes over periods of decades and differences higher rates of magma and gas production at popocatepetl with similar magmas popocatepetl s slightly more acidic we use a simplified equation of state eos for calculating the density of the hydrous andesitic magmas of volcan de colima in a realistic range of pressures temperatures and dissolved water content to model the density contrast with the host rock causing buoyancy of the magmatic column we use this model to explain some aspects of the dome forming effusive activity of colima and compare the results with a similar study made in a previous work on popocatepetl this dome height isostatic model explains well the observed mean heights around 50 m of the colima lava domes with the buoyant force resulting from a 2 5 wt h 2 o content in the melt a value consistent with recently published measurements of rock inclusions the slightly higher h 2 o contents required for the observed dome heights at popocatepetl is explained in terms of the different depths of the isostatic compensation levels the compositional differences between the magmas and the structural differences between the volcanic edifices

tectonothermal history of the basement rocks in the western zone of the north china craton and its tectonic implications the basement of the north china craton can be divided into the eastern central and western zones based on lithological structural metamorphic and geochronological data the western zone comprises two different petrotectonic units archaean tonalitic trondhjemitic granodioritic ttg grey gneisses and metamorphic mafic rocks and palaeoproterozoic khondalite series the former is characterized by isobaric cooling ibc type anticlockwise p t paths in the north northwestern part of the zone and near isothermal decompression itd type clockwise p t paths in the eastern part adjacent to the central zone on the other hand the tectonothermal evolution of palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling defining clockwise p t paths the archaean ttg gneisses and associated mafic rocks with anticlockwise metamorphic p t paths reflects an origin related to underplating and intrusion of mantle derived magmas which may be derived from mantle plumes they represent a late archaean continental block in the western part of the north china craton the palaeoproterozoic khondalite series rocks represent passive continental margin deposits they were metamorphosed and deformed in the late palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the north china craton the itd type clockwise p t t paths of the palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the north china craton at c 1800 ma

excess volatiles supplied by mingling of mafic magma at an andesite arc volcano we present the results of a study of volcanic gases at soufriere hills volcano montserrat which includes the first spectroscopic measurements of the major gas species co2 and h2s at this volcano using a multisensor gas analyzer system multigas sensor the fluxes of co2 and h2s were 640 2750 t d and 84 266 t d respectively during july 2008 during a prolonged eruptive pause the flux of co2 is similar to estimates for the entire arc from previous geochemical studies while the measured h2s flux significantly alters our interpretation of the sulphur budget for this volcano the fluxes of both sulphur and carbon show considerable excesses over that which can be supplied by degassing of erupted magma we demonstrate using thermodynamic models and published constraints on preeruptive volatile concentrations that the gas composition and fluxes are best modeled by mixing between 1 gases derived from isobaric quenching of mafic magma against cooler andesite magma at depth and 2 gases derived from shallower rhyolitic interstitial melt within the porpyritic andesite the escape of deep derived gases requires pervasive permeability or vapor advection extending to several kilometers depth in the conduit and magma storage system these results provide more compelling evidence for both the contribution of unerupted mafic magma to the volatile budget of this andesitic arc volcano and the importance of the intruding mafic magma in sustaining the eruption from a broader perspective this study illustrates the importance and role of underplating mafic magmas in arc settings these magmas play an important role in triggering and sustaining eruptions and contribute in a highly significant way to the volatile budget of arc volcanoes copyright 2010 by the american geophysical union

source and behavior of pyroclastic density currents generated by vulcanian style explosions of popocatepetl volcano mexico on 22 january 2001 abstract since 1994 popocatepetl volcano has extruded 85 lava domes each of which has been destroyed by vulcanian style explosions the paroxysmal vulcanian explosion of 22 january 2001 has been unique by generating multiple pyroclastic density currents pdcs scientists and civil protection authorities have used those pdcs to remap the high risk hazard zone of the volcano although little study has been carried out to understand the generation and evolution of those pdcs here we examine 38 stratigraphic sections and analyze the granulometry componentry vesicularity and petrology glass melt inclusions in plagioclase of collected samples to interpret the origins of the vulcanian explosions and the pdc behavior the 22 january event generated two series of dense pdcs that detached from the base of the eruption column the first set of pdcs traveled northwest across a glacier on the volcano partially scouring and melting the ice that hours later generated a lahar two pulses of these pdcs were confined within a large ravine surpassed the slope break and traveled up to 3 km away from the vent the deposits are matrix supported throughout their extent the second set of pdcs traveled to the northeast mainly as unconfined currents that split into two lobes prior to the slope break the farthest traveled current followed a western ravine and reached the forest 3 5 km from the summit the eastern lobe buried a local hiking hut and reached 2 1 km the deposits from these northeastern currents are massive and block supported all deposits volume of 5 71 105 m3 have finger like morphology with levees and frontal snouts all pdcs flowed as a dense basal granular current with turbulent upper clouds at minimum speeds of 7 10 m s in terms of hazards pdcs that interact with glaciers on the volcano are expected to travel farther and potentially generate extensive lahars at present small intermittent patches of glacier occur on the northern upper slopes of the volcano the components of the deposits are in order of abundance black scoria 53 62 wt sio2 banded scoria pumice 63 wt sio2 dome lithics 60 61 wt sio2 accidental lithics and free crystals juvenile pumice and scoria contain phenocrysts of plagioclase an47 10 clinopyroxene en48 3 orthopyroxene en64 1 ilmenite and titanomagnetite set in a rhyolitic glass pumice clast 74 39 wt sio2 or microlite rich matrix scoria clast the dome lithics also contain these phenocrysts as well as rare olivine phenocrysts fe2 fe2 mg 0 1 dissolved h2o co2 contents in melt inclusions hosted by pumice plagioclase phenocrysts suggest the felsic magma was stored at depths of 2 4 km thus the 2001 event was triggered by the injection of mafic magmas into a small volume of felsic magma probably forming dikes left from the previous eruptive episode we propose that the vulcanian explosions tap a column of magma 2 km depth equivalent to the volume that contained the felsic dikes this mechanism has been acting during the destruction of all summit domes of the ongoing eruption of the volcano

juvenile magmatism and crustal recycling at the end of the neoarchean in western shandong province north china craton evidence from shrimp zircon dating western shangdong province experienced major crustal growth as a result of 2 75 to 2 50 ga tectonothermal events different from other archean areas in the north china craton besides early neoarchean tonalite trondhjemite granodiorite ttg and supracrustal assemblages there are large volumes of late neoarchean gabbroic dioritic and granitic rocks in western shandong shrimp zircon dating of thirty one samples of different rock types from late neoarchean rocks yielded a narrow age range from 2560 to 2490 ma based on these data and previously published results the following conclusions can be drawn 1 the archean basement can be divided into three belts a late neoarchean 2525 2490 ma crustally derived granite belt in the northeast an early neoarchean 2 75 2 60 ga rock belt in the center and a late neoarchean 2550 2500 ma belt of juvenile rocks in the southwest 2 the tectonic regime in western shandong province probably changed from compressional to extensional at around 2525 ma between 2530 and 2520 ma 3 crustal recycling with addition from the mantle occurred at the end of the neoarchean 4 the late archean magmatic rocks were probably formed in an arc environment

zircon ages and geochemistry of late neoarchean syenogranites in the north china craton a review abstract at the end of the neoarchean continental evolution voluminous syenogranites were emplaced in the north china craton together with other magmatic rocks trondhjemite tonalite granodiorite ttg monzogranite diorite gabbro syenogranites are widely distributed in anshan benxi qinhuangdao and western shandong and also occur in southern jilin northern liaoning northwestern hebei and central henan based on geological relationships degree of metamorphism deformation and magmatic zircon ages two phases of syenogranite magmatism are recognized rocks produced during the first phase show a gneissic texture and were formed between 2 53 and 2 52 ga and locally comprise abundant ttg rocks of the second phase cut late neoarchean ttg and supracrustal rocks display a massive structure and mainly formed between 2 52 and 2 50 ga all syenogranites share the same features in major element compositions being high in sio2 and low in cao total feo mgo tio2 and p2o5 however they are different in trace and ree compositions and can be subdivided into three types 1 type 1 shows a large variation in total ree contents low la yb n ratios strong negative eu eu anomalies and ba depletion 2 type 2 is similar to type 1 but has higher la yb n ratios 3 type 3 shows a large variation in total ree and la yb n ratios and significantly do not show strongly negative eu eu anomalies and ba depletion whole rock sm nd isotopic compositions show large variations in nd t values and tdm nd modal ages ranging from 9 49 to 4 72 and 3 70 to 3 25 ga type 1 0 55 1 03 and 2 77 2 71 ga type 2 and 2 35 to 1 23 and 2 93 2 66 ga type 3 respectively hf isotopic compositions of zircons from three samples have hf t values and tdm1 hf ages of 0 7 7 2 and 2 84 2 56 ga type 1 2 6 7 4 and 2 74 2 56 ga type 2 and 2 1 6 3 and 2 76 2 60 ga type 3 it is concluded that syenogranites were generated by melting of continental crust with different mean crustal residence ages and most of them were emplaced during the second phase 2 52 2 50 ga in an extensional tectonic regime the formation of these voluminous syenogranites marks a tectono magmatic event resulting in stabilization of the north china craton at the end of the neoarchean

caught in the act implications for the increasing abundance of mafic enclaves during the recent eruptive episodes of the soufriere hills volcano montserrat 1 an exceptional opportunity to sample several large blocks sourced from the same region of the growing soufriere hills lava dome has documented a significant increase in the presence of mafic enclaves in the host andesite during the course of a long lived eruptive episode with several phases in 1997 phase i mafic inclusions comprised 1 volume percent of erupted material in 2007 phase iii deposits their volumetric abundance increased to 5 7 percent a broader range of geochemically distinctive types occurs amongst the 2007 enclaves crystal poor enclaves generally have the least evolved basaltic compositions porphyritic enclaves represent compositions intermediate between basaltic and andesitic compositions the absence of porphyritic enclaves prior to phase iii magmatism at soufriere hills volcano suggests that a mixing event occurred during the course of the current eruptive episode providing direct evidence consistent with geophysical observations that the system is continuously re invigorated from depth

characterization of open and closed volcanic systems in indonesia and mexico using insar time series 1 we use 2007 2011 advanced land observing satellite alos data to perform an arc wide interferometric synthetic aperture radar insar time series survey of the trans mexican volcanic belt tmvb and to study time dependent ground deformation of four indonesian volcanoes selected following the 2007 2009 study of chaussard and amelung 2012 our objectives are to examine whether arc volcanoes exhibit long term edifice wide cyclic deformation patterns that can be used to characterize open and closed volcanic systems and to better constrain in which cases precursory inflation is expected we reveal deformation cycles at both regularly active and previously inactive indonesian volcanoes but we do not detect deformation in the tmvb reflecting a lower activity level we identify three types of relationships between deformation and activity inflation prior to eruption and associated with or followed by deflation kerinci and sinabung inflation without eruption and followed by slow deflation agung and eruption without precursory deformation merapi colima and popocatepetl at merapi no significant deformation is detected even during eruption the first two cases correspond to closed volcanic systems and suggest that the traditional model of magmatic systems and eruptive cycles do apply to andesitic volcanoes i e inflation and deflation episodes associated with magma accumulation or volatile exsolution in a crustal reservoir followed by eruptions or in situ cooling in contrast the last case corresponds to open volcanic systems where no significant pressurization of the magmatic reservoirs is taking place prior to eruptions and thus no long term edifice wide ground deformation can be detected we discuss these results in terms of insar s potential for forecasting volcanic unrest

late holocene stratigraphy of the tetimpa archaeological sites northeast flank of popocatepetl volcano central mexico late holocene 2500 yr b p tephras bury a sequence of pre hispanic archaeological sites in the tetimpa area on the northeast flank of popocatepetl volcano from measured stratigraphic sections 14c dates and isopach maps this paper reconstructs the eruptive chronology and the regional extent of deposits associated with the tetimpa archaeological sites a regionally extensive paleosol defines the base of the late holocene sequence in the tetimpa area deposits from two periods of explosive volcanism unconformably overlie this paleosol eruptive sequence i was deposited at ca 2100 yr b p and blanketed late tetimpa archaeological sites with 1 1 5 m of yellow andesitic 62 sio2 pumice and locally with 20 40 m of olivine bearing andesitic lavas isopach and isopleth maps of the pumice deposit suggest a plinian style eruption event that covered 240 km2 on the east side of the volcano with 25 cm of tephra lavas from eruptive sequence i dammed drainage in the lowland area near the town of san nicolas and caused local upstream deposition of as much as 30 m of lacustrine silts clays and sands these lacustrine deposits record an eruptive hiatus for the tetimpa area of about 750 14c yr between ca 2100 and ca 1350 yr b p no major tephras were deposited in the tetimpa area in upland areas this time period is represented by an unconformity and by entisols formed in the top of pumice deposits and lavas from eruptive sequence i artifacts agricultural furrows and dwellings record human reoccupation of this surface at the end of this hiatus several lahars were deposited above the lacustrine sequence and locally above the entisol in upland positions adjacent to streams between ca 1350 and ca 1200 yr b p tephras from eruptive sequence ii buried these paleosols occupation sites lacustrine sediments and lahars andesitic 62 sio2 pumice lapilli deposits in the tetimpa area record three pumice fall eruptions directed northeast and east of the crater the first and smallest of these maximum tetimpa area thickness 12 cm 52 km2 covered by 25 cm took place at ca 1350 yr b p and was accompanied by pyroclastic surge events preserved in the tetimpa area by charcoal sand waves and cross stratified sand sized tephra at ca 1200 yr b p the products of two plinian style events and additional pyroclastic surges reached the tetimpa area the largest of these tephra fall events covered the tetimpa area with 0 5 1 m of tephra and blanketed an area of 230 km2 with a thickness of 25 cm the tetimpa record confirms two of the four periods of explosive volcanism recognized by studies conducted around popocatepetl in the past 30 yr eruptive sequence i corresponds to the explosive period between 2100 and 2500 yr b p and eruptive sequence ii corresponds to the period between 900 and 1400 yr b p the archaeology and lacustrine stratigraphy of the tetimpa area help constrain the timing of the plinian phase of eruptive sequence i to ca 2100 yr b p and suggest that the pumice fall eruptions of eruptive sequence ii took place in at least two intervals between ca 1350 and ca 1200 yr b p

petrogenesis and geodynamics of late archean magmatism in eastern hebei eastern north china craton geochronological geochemical and nd hf isotopic evidence abstract the eastern part of the north china craton contains large volumes of late archean hornblenditic tonalitic dioritic granodioritic and granitic rocks la icpms zircon u pb dating of three plutons from eastern hebei eastern north china craton record emplacement ages of 2526 2515 ma and metamorphic ages of 2500 2490 ma for the diorite granodiorite and monzogranite and 2440 ma for the k feldspar granite hornblendites have high mgo 8 6 10 6 wt cr 674 1126 ppm ni 125 159 ppm and v 224 315 ppm at low basic silica contents sio2 45 7 49 0 wt they are enriched in light rare earth elements lrees and large ion lithophile elements liles and depleted in high field strength elements hfses with positive nd t values 1 2 to 2 0 indicating they were derived from an enriched mantle source the monzogranites have high sr and ba contents and sr y and la yb ratios low y and hrees abundances and positive nd t and zircon hf t values with nd and hf model ages less than 3 0 ga reflecting that they were derived from partial melting of a juvenile lower crust at high pressure however the k feldspar granites with low sr and ba concentrations and high hree and y contents are considered to have originated from partial melting of juvenile crustal materials at shallow crustal levels the diorites and granodiorites have variable major and trace element concentrations magmatic zircons from these rocks have heterogeneous hf t values but record uniform homogeneous u pb ages indicating they resulted from mixing of mafic and felsic magmas coupled with fractional crystallization because of the widespread distribution of late archean magmatism and evidence for magma mixing possibly related to mafic magma underplating it appears likely that this event in the eastern north china craton may relfect mantle plume activity that resulted in significant growth of continental crust during the late archean

active layer thermal regime at different vegetation covers at lions rump king george island maritime antarctica abstract climate change impacts the biotic and abiotic components of polar ecosystems affecting the stability of permafrost active layer thickness vegetation and soil this paper describes the active layer thermal regimes of two adjacent shallow boreholes under the same soil but with two different vegetations the study is location in lions rump at king george island maritime antarctic one of the most sensitive regions to climate change located near the climatic limit of antarctic permafrost both sites are a turbic cambic cryosol formed on andesitic basalt one under moss vegetation andreaea gainii at 85 m a s l and another under lichen usnea sp at 86 m a s l located 10 m apart ground temperature at same depths 10 30 and 80 cm water content at 80 cm depth and air temperature were recorded hourly between march 2009 and february 2011 the two sites showed significant differences in mean annual ground temperature for all depths the lichen site showed a higher soil temperature amplitude compared to the moss site with ground surface 10 cm showing the highest daily temperature in january 2011 7 3 c and the lowest daily temperature in august 16 5 c the soil temperature at the lichen site closely followed the air temperature trend the moss site showed a higher water content at the bottommost layer consistent with the water saturated low landscape position the observed thermal buffering effect under mosses is primarily associated with higher moisture onsite but a longer duration of the snowpack not monitored may also have influenced the results active layer thickness was approximately 150 cm at low lying moss site and 120 cm at well drained lichen site this allows to classify these soils as cryosols wrb or gelisols soil taxonomy with evident turbic features

ca 2 5 billion year old coeval ultramafic mafic and syenitic dykes in eastern hebei implications for cratonization of the north china craton abstract a group of extremely rare coeval ultramafic mafic and syenitic dykes was discovered in the eastern hebei region of the north china craton these dykes intrude the 3 8 2 55 ga old caozhuang complex an olivine gabbro dyke and syenite dyke yield respectively shrimp zircon u pb ages of 2516 26 ma and 2504 11 ma interpreted as the magmatic crystallization age their zircons have single stage hf model ages 2677 ma and 2705 ma the olivine gabbros have mg values of 59 63 similar to high magnesian tholeiitic basalt they show relatively lree enriched patterns without eu anomalies la yb n 9 28 9 78 gd yb n 2 8 eu 0 96 1 00 with enrichment in liles and depletion in hfses they also contain high cr and ni and have zr hf and nb ta ratios similar to those of the primitive mantle the syenites are alkaline in composition with 8 67 8 88 wt na 2 o k 2 o and show high total ree contents 543 854 ppm and strong lree enriched patterns with minor eu negative anomalies la yb n 50 101 gd yb n 5 2 5 3 eu 0 80 0 82 trace element diagrams show strong lile enrichment relative to hfse zr hf and nb ta ratios are 47 49 and 26 76 respectively which are much higher than those of primitive mantle and higher than the average nb ta ratio of post archean continental crust the petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source which implies that the ncc probably had continental crust of considerable thickness at this time combined with evidence for 2 5 ga granite intrusion and metamorphism in the eastern hebei region and adjacent areas we propose that the ncc has been a present scale craton at the end of archean

earliest paleoproterozoic supracrustal rocks in the north china craton recognized from the daqingshan area of the khondalite belt constraints on craton evolution abstract the upper wulashan subgroup at daqingshan in the khondalite belt of the western block of the north china craton contains an early paleoproterozoic 2 5 2 45 ga supracrustal sequence incorporating banded iron formations which we refer to as the daqingshan supracrustal rocks they contain rounded to elliptical zircon grains that commonly show core rim or core mantle rim structures and precise shrimp u pb dating of these domains allows the discrimination of a series of tectono thermal events that straddle the archean proterozoic boundary detrital zircon cores with oscillatory zoning have formation ages ranging from 2 55 to 2 50 ga indicating derivation from late neoarchean magmatic rocks as a result of exhumation and erosion these became incorporated in earliest paleoproterozoic sediments that were deposited between 2 50 and 2 45 ga and then underwent high grade metamorphism at 2 45 2 40 ga this event variously recrystallized the cores into two main domains a dark inner domain and a gray outer domain as imaged in cathodoluminescence the gray domains commonly show sector zoning and have th u ratios of 0 1 0 5 a feature commonly noted in zircons recrystallized under high grade metamorphic conditions metamorphic mantles are generally more homogeneous in structure and have lower th u ratios commonly

el ventorrillo a paleostructure of popocatepetl volcano insights from geochronology and geochemistry volcan popocatepetl mexico was constructed over the remains of a volcanic paleostructure based on fieldwork 40ar 39ar dating u pb dating and geochemistry we have determined the age chemistry and location of this paleostructure and named it el ventorrillo most remnants of el ventorrillo are covered by deposits from subsequent activity of popocatepetl except for the el abanico scarp and the barranca de nexpayantla where the stratigraphy of el ventorrillo eruptive products can be investigated inception of volcanism at el ventorrillo occurred at 331 10 ka with emission of the nexpayantla andesitic lavas and continued with extrusion of the yoloxochitl 267 31 ka and microwave 227 6 ka domes intrusion of dikes occurred at 298 94 and 230 3 ka activity at el ventorrillo continued with the emission of lavas that built the el abanico scarp 193 29 to 96 8 ka and continued until the tutti frutti eruption destroyed the cone 14 1 kyr ago el ventorrillo magmas produced rocks divided into two mineralogical groups the first group contains biotite amphibole rich rocks and the second group consists of biotite amphibole free lavas the rocks that contain biotite and amphibole are older than 198 13 ka whereas the rocks with no hydrous phases are younger than 227 6 ka and contain skarn and granodiorite xenoliths we interpret the change to an anhydrous mineral assemblage and the occurrence of skarn and granodiorite xenoliths as evidence for the formation of a new shallower reservoir a granodiorite xenolith was chosen for 40ar 39ar dating and u pb zircon analyses the u pb method yielded an age of 540 110 ka and the 40ar 39ar an age of 109 24 ka these ages are interpreted to indicate granodiorite crystallization 540 110 ka which metamorphosed the calcareous basement beneath popocatepetl into skarn and an influx of magma 109 24 ka that reheated the granodiorite major and trace elements sr nd and pb isotopic compositions and textural analyses of plagioclase show that el ventorrillo magmas were moderately modified by mixing with more mafic magmas and that assimilation of the calcareous basement was negligible

the 23 500 y 14 c bp white pumice plinian eruption and associated debris avalanche and tochimilco lava flow of popocatepetl volcano mexico abstract the white pumice wp is one of the thickest and most voluminous plinian fallouts produced by popocatepetl volcano in central mexico during the late pleistocene holocene its eruption 23 500 14 c y bp 27 800 cal bp was triggered by the catastrophic failure of the sw flank of the volcano the resulting debris avalanche was highly mobile reaching 72 km from the cone with an apparent coefficient of friction l h of 0 06 the deposit covers an area of 1200 km 2 and has a volume of 10 4 km 3 this gigantic landslide characterized by exceptionally large proximal hummocks 400 m provoked the sudden decompression of the hydrothermal and magmatic systems which produced an initial blast followed by the rise of a plinian column that reached an altitude of 33 km the isopach map allows the recognition of a dispersal axis pointing toward the south where an area of 2490 km 2 was covered by 10 cm of pumice and ash the total volume of the pumice fallout was estimated at 1 9 km 3 dre dense rock equivalent pumice clasts are dacitic 62 66 wt sio 2 anhydrous basis highly vesicular 55 88 vol and display a seriate texture with phenocrysts of plagioclase hornblende augite hypersthene oxides ti magnetite and ilmenite apatite as the eruption advanced discharge rates became more intermittent and the height of the column fluctuated and finally collapsed generating pumice and ash flows that were emplaced around the volcano this short but intense activity was followed during subsequent years by rain induced lahars that reached great distances from the volcano at the same time more degassed andesitic dacitic 61 65 wt sio 2 magma was erupted effusively 4 4 km 3 dre in the new horseshoe shaped 5 km wide crater from which the tochimilco lava flow descended toward the sse where it inundated an area of 68 km 2 and reached as far as 22 km from its source since then multiple eruptions have reconstructed the summit cone almost completely obliterating the horseshoe shaped crater during the course of this catastrophic eruption vei 5 a total volume of 6 3 km 3 dre of juvenile magma pumice and lava were emitted and at least an additional 10 km 3 of pre existing rocks debris avalanche mobilized it surpasses in magnitude most other known plinian eruptions from popocatepetl and can be envisaged as an example of a worst case scenario for hazard evaluation purposes it dramatically changed the morphology of the volcano and had profound and far reaching effects beyond its immediate vicinity along rivers draining surrounding plains as well as on the lacustrine basins e g chalco to the ne a repeat of such an eruption in this densely populated area would certainly cause a major calamity of unprecedented dimensions

hydrous basalt limestone interaction at crustal conditions implications for generation of ultracalcic melts and outflux of co 2 at volcanic arcs abstract high degassing rates for some volcanoes typically in continental arcs e g colli albani volcanic district etna vesuvius italy merapi indonesia popocatepetl mexico are thought to be influenced by magma carbonate interaction in the crust in order to constrain the nature of reaction and extent of carbonate breakdown we simulated basalt limestone wall rock interactions at 0 5 1 0 gpa 1100 1200 c using a piston cylinder and equal mass fractions of calcite caco3 and a hydrous 4 wt h2o basalt in a layered geometry contained in aupd capsules all experiments produce melt fluid calcite clinopyroxene plagioclase calcic scapolite spinel with increasing t plagioclase is progressively replaced by scapolite clinopyroxene becomes cats rich and fluid proportion as inferred from vesicle population increases at 1 0 gpa 1200 c our hydrous basalt is superliquidus whereas in the presence of calcite the experiment produces calcite clinopyroxene scapolite melt with the consumption of calcite with increasing t and decreasing p melt on a volatile free basis becomes silica poor 58 1 wt at 1 0 gpa 1100 c to 34 9 wt at 0 5 gpa 1200 c and cao rich 6 7 wt at 1 0 gpa 1100 c to 43 7 wt at 0 5 gpa 1200 c whereas al2o3 drops e g 19 7 at 1100 c to 12 8 wt at 1200 c at 1 0 gpa as clinopyroxene becomes more cats rich high t or low p melt compositions are ultracalcic potentially presenting a new hypothesis for the origin of ultracalcic melt inclusions in arc lava olivines wall rock calcite consumption is observed to increase with increasing t and decreasing p at 0 5 gpa our experiments yield carbonate assimilation from 21 6 to 47 6 between 1100 and 1200 c using measured co2 outflux rates for mts vesuvius merapi etna and popocatepetl over a t variation of 1100 to 1200 c at 0 5 gpa we calculate 6 92 of magmatic input estimates undergo this extent of assimilation suggesting that up to 3 of the current global arc co2 flux may be crustally derived application of the assimilation extent bracketed in this study to the estimated elevated number of carbonate assimilating arc magmatic systems active during the late cretaceous to early paleogene suggests that magma induced upper plate decarbonation alone had the potential to contribute up to 2 7 10 14 5 6 10 15 g y co2 assuming no dilution and complete gaseous release of all assimilated carbon using an estimated assimilation extent averaged from current systems gives a slightly lower though still significant value of 5 5 10 14 g y of excess co2 being released into the atmosphere

climate warming and active layer thaw in the boreal and tundra environments of the mackenzie valley1 the variability of maximum active layer thickness in boreal and tundra environments has important implications for hydrological processes terrestrial and aquatic ecosystems and the integrity of northern infrastructure for most planning and management purposes the long term probability distribution of active layer thickness is of primary interest a robust method is presented to calculate maximum active layer thickness employing the stefan equation to compute phase change of moisture in soils and using air temperature as the sole climatic forcing variable near surface ground temperatures boundary condition for the stefan equation were estimated based on empirical relationships established for several sites in the mackenzie valley simulations were performed for typically saturated mineral soils overlain with varying thickness of peat in boreal and tundra environments the probability distributions of simulated maximum active layer thickness encompass the range of measured thaw depths provided by field data the effects of climate warming under a2 and b2 scenarios for 2050 and 2100 were investigated under the a2 scenario in 2100 the simulated median thaw depth under a thin organic cover may increase by 0 3 m to reach 1 m depth for a tundra site and 1 6 m depth for a boreal site the median thaw depth in 2100 is dampened by about 50 under a 1 m thick organic layer without an insulating organic cover thaw penetration can increase to reach 1 7 m at the tundra site the simulations provide quantitative support that future thaw penetration in permafrost terrain will deepen differentially depending on location and soil

archean crustal evolution in china the taishan complex and evidence for juvenile crustal addition from long term depleted mantle abstract the main framework for crustal evolution of the late archean taishan complex in the shandong province of china is constructed using data from field investigation petrographic examination geochemical analysis and isotopic age determination the history of the complex began with emplacement of basic and tonalitic magmas about 2 7 2 75 ga ago which were metamorphosed designated as metamorphism i to amphibolites and grey gneisses shortly after their emplacement these rocks constitute a metamorphic basement termed the wangfushan gneisses the basement gneisses were later intruded by a plutonic association including 1 alkali and light rare earth element lree enriched dioritic rocks and their differentiates 2 6 ga ago 2 granites and trondhjemites collectively termed the hushan granites also 2 6 ga ago and 3 the aolaishan type granites sensu stricto 2 4 2 5 ga ago the plutonic association in conjunction with the older basement gneisses was later subjected to a second period of deformation and metamorphism metamorphism ii 2 ga ago as inferred from published k ar biotite dates the chronological order of all the above events has been determined by a multichronometric approach and the age results in ma with 2 uncertainties are summarized as follows the relatively low i sr initial 87 sr 86 sr ratio and high positive nd t values for the wangfushan gneisses and the diorite group provides strong evidence for continental growth by addition of juvenile material ultimately separated from long term depleted mantle sources by contrast the sr and nd isotopic data for the aolaishan type granites coherently suggest that they are likely to be derived by anatexis of the basement gneisses at shallow depths crustal evolution of the taishan complex offers an additional example supporting the concept of a continental accretion differentiation superevent that during such a worldwide tectonothermal episode particularly in the archean important differentiation of the upper mantle has contributed to the growth of the continental crust by addition of juvenile material

conduit convection magma mixing and melt inclusion trends at persistently degassing volcanoes abstract magmas progressively exsolve volatiles as they ascend towards the earth s surface such that their volatile content is a function of pressure water and carbon dioxide concentrations measured in melt inclusions from degassing volcanoes rarely coincide with modelled degassing trends i show that observed melt inclusion trends can be reproduced through mixing of magmas either during convection within the volcanic conduit or within a subterranean magma reservoir no fluxing gas phase or post entrapment loss of water need be invoked a permeable network allowing gas transport is still required to avoid fragmentation of magma at shallow depths

passive infrared remote sensing evidence for large intermittent co2 emissions at popocatepetl volcano mexico abstract passive infrared ftir and correlation spectrometer cospec measurements were conducted at popocatepetl volcano during february 10 to 26 1998 from sites 4 to 17 km distant from the summit volcano behavior was relatively quiet and so 2 flux averaged 1670 1420 t day 51 measurements relatively small for popocatepetl concurrent hcl so 2 and hf so 2 ratios were 0 17 0 01 and 0 031 0 003 respectively about the same as ratios measured from 1994 to 1997 the amount of co 2 in the volcanic plume was quantified using fascode in which atmospheric co 2 is numerically subtracted from the total infrared spectrum to obtain the residual magmatic co 2 surprisingly co 2 so 2 mass ratios rose dramatically to values as high as 140 about 30 times higher than typical values of 2 to 8 measured from 1994 to 1996 these excursions in high co 2 so 2 ratios were short lived lasting no longer than about 0 5 to 3 0 h but co 2 flux occasionally exceeded 100 000 t day we estimate that the average co 2 so 2 ratio for the period was about 23 yielding an average co 2 flux of roughly 38 000 t day chemical and petrographic analyses of lava and pumice erupted during explosions on june 30 1997 and january 1 1998 show conclusively that popocatepetl produces mixed products formed by injection of mafic magma into a more silicic chamber at temperatures and pressures of roughly 1040 c and 5 kbar in addition popocatepetl eruptive products include xenoliths of metamorphosed carbonate rocks containing wollastonite and other calc silicate minerals indicating reaction of magma with cretaceous limestone underlying the volcano using a normal co 2 so 2 ratio of 4 for reference we calculate an average excess co 2 production of 32 000 t day for 17 days this would require assimilation of only 5 10 4 km 3 of limestone an amount easily accessible in the 3 km thick cretaceous section beneath the volcano we also examine two scenarios in which excess co 2 is produced by degassing of subjacent basalt magma but these explanations seem less plausible to us because many other volcanoes are underlain by carbonate sequences short duration bursts of co 2 flux and increased co 2 so 2 ratio might be observed at other sites if simultaneous real time measurements of major gas species are made

transfer of volatiles and metals from mafic to felsic magmas in composite magma chambers an experimental study abstract in order to determine the behavior of metals and volatiles during intrusion of mafic magma into the base of silicic upper crustal magma chambers fluid rock partition coefficients dfluid rock of li b na s cl k mn fe rb sr ba ce cu zn ag cd mo as se sb te w tl pb and bi were determined experimentally at 2 kbar and 850 c close to the solidus of mafic magma in a first step volatile bearing mafic glasses were prepared by melting a natural basaltic trachyandesite in the presence of volatile bearing fluids at 1200 c 10 kbar in piston cylinder presses the hydrous glasses were then equilibrated in subsequent experiments at 850 c 2 kbar in cold seal pressure vessels which caused 80 90 of the melt to crystallize after 0 5 2 0 days of equilibration the exsolved fluid was trapped by means of in situ fracturing in the form of synthetic fluid inclusions in quartz both the mafic rock residue and the fluid inclusions were subsequently analyzed by laser ablation icp ms for major and trace elements reverse experiments were conducted by equilibrating metal bearing aqueous solutions with rock powder and then trapping the fluid in two additional experiments information on relative element mobilities were obtained by reacting fluids that exsolved from crystallizing mafic magma with overlying silicic melts the combined results suggest that under the studied conditions s cl cu se br cd and te are most volatile dfluid rock 10 followed by li b zn as ag sb cs w tl pb and bi dfluid rock 1 10 less volatile are na mg k ca mn fe rb sr mo and rb dfluid rock 0 1 1 and the least fluid mobile elements are al si ti zr ba and ce dfluid rock

twenty years 1990 2010 of geodetic monitoring of galeras volcano colombia from continuous tilt measurements abstract galeras an andesitic stratovolcano part of the galeras volcanic complex is one of the most active volcanoes in colombia historic activity is centered on a small volume cone inside the youngest amphitheater which breaches the west flank of the volcano at least 30 confirmed eruption periods have been recorded in the past 480 years with episodes of unrest ranging from weak fumarolic activity and ash emissions to larger explosive events the most recent eruption periods recorded instrumentally since 1988 have been characterized by minor explosive eruptions and the emplacement of three crater domes and small pyroclastic flow deposits in this paper we discuss the evolution of volcanic activity using a 20 year long record of tilt measurements in particular we focus on three episodes of unrest occurred in 1991 2006 and 2008 when the deformation was clearly associated with shallow magma intrusions and the emplacement and destruction of crater domes the depth of the intrusions varied from a few hundred meters august 2005 to two kilometers january 2009 while the volume change ranged from 10 4 m 3 may october 2009 to 10 6 m 3 january 2009 a comparison with seismic data indicates that the deformation sources were located within the cloud of hypocenters of the volcano tectonic events the lack of a clear correlation between the volume change and depth of the sources and the total so 2 flux could indicate that the unrest at galeras was related to a larger intrusive event with only a small part of the magma erupted in the form of tephra and lava domes

constraining the degassing processes of popocatepetl volcano mexico a vesicle size distribution and glass geochemistry study abstract the explosive activity of popocatepetl volcano is a threat to the surrounding densely populated areas and it is therefore important to recognize indicators of change in eruptive style explosive to dome building within a short period of time in this study we present results of vesicle size distributions vsds and compositional analysis of matrix glass from juvenile clasts from five of the main plinian eruptions of popocatepetl ca 23 1 2 ka the 2001 small eruption during partial dome collapse and four eruptions during 1997 may 11th and june 14th 15th and 30th major element analysis of matrix glass wds epma allows the estimation of the depth from which the erupted magma went into disequilibrium between crystals and melt by calculating the equilibrium pressure using the quartz albite orthoclase ternary system of blundy and cashman 2001 quantitative interpretation of texture in juvenile pumice or scoria clasts via vsd analysis using csd software was used to link physical changes experienced by magma during ascent with conditions responsible for eruptions the extent and style of vesiculation in juvenile clasts is also related to eruption style and duration and has specifically allowed the recognition of changes in vesicular texture that represent variations from explosive to dome building activity mangan and sisson 2000 adams et al 2006 this study highlights a more complicated story in terms of magma storage than that previously accepted for the popocatepetl volcanic system and is an important contribution to ongoing research at the volcano

relations between air and surface temperature in discontinuous permafrost terrain near mayo yukon territory the association of site characteristics with the n factor a ratio of air to ground surface temperature was investigated at five sites in the boreal forest near mayo yukon territory permafrost was in equilibrium with surface conditions at three sites was degrading at another and was absent from the fifth air and near surface ground temperatures were recorded by data loggers between september 2000 and april 2002 and mean daily temperatures were accumulated to calculate n factors for the freezing nf and thawing nt seasons air temperature did not vary between the sites so inter site differences in nf and nt were because of variations in surface temperature variations in nf between the sites over the two winters were primarily because of differences in snow depth but at sites with similar snow cover the surface temperatures were relatively high when the site was underlain by unfrozen ground during summer daily mean surface temperatures were initially less than air temperatures however once

microblock amalgamation in the north china craton evidence from neoarchaean magmatic suite in the western margin of the jiaoliao block abstract the archaean earth is considered to have been characterized by microcontinents that formed dominantly through the accretion of oceanic arcs and plateaus the north china craton ncc provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the archaean here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks the jiaoliao block that forms part of the composite eastern block of the ncc we present petrological geochemical and zircon u pb geochronological data from the taipingzhai charnockite suite and associated amphibolites metagabbros and orthogneisses from the qianxi complex geochemically the rocks show a wide range of sio 2 charnockite suite 52 57 75 50 wt metagabbro 43 71 wt amphibolite 50 24 wt garnet bearing biotite 63 73 wt and mgo charnockite suite 0 89 5 01 wt metagabbro 3 99 wt amphibolite 6 23 wt garnet bearing biotite 2 08 wt the composition of the felsic units straddle from diorite through syeno diorite to granite with both alkalic and subalkalic affinity with dominantly magnesian composition and arc related features their immobile trace element relationships suggest calc alkaline affinity they show positive pb ba la nd and gd and negative nb ta sr th and ti anomalies with slightly negative anomalies of ce and y attesting to arc related features in tectonic classification diagrams the rocks plot in the vag syn colg field or the vag area suggesting subduction related origin the dominant population of zircons in all these rocks displays magmatic crystallization features including high th u values with core rims textures indicating subsequent thermal events the zircon u pb data yield upper intercept ages of 2587 10 ma to 2543 17 ma and 207 pb 206 pb mean ages of 2578 7 3 ma to 2536 8 ma for the charnockite suite marking the timing of emplacement of the arc magmas the overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207 pb 206 pb ages between 2533 ma to 2490 ma zircons in the metagabbro preserve upper intercept ages of 2556 20 ma representing the crystallization age of this rock the younger ages of 2449 58 ma upper intercept age and 1845 25 ma 207 pb 206 pb spot age are interpreted to represent subsequent multiple thermal events in this area zircons in the amphibolite preserve the 207 pb 206 pb mean age of 2539 9 ma representing the crystallization age of this rock the garnet bearing biotite gneiss shows an upper intercept age of 2562 10 ma mswd 0 66 n 36 and the 207 pb 206 pb mean age of 2561 9 ma mswd 0 63 n 33 which is taken to represent the crystallization age of this rock some inherited zircons are also identified with 207 pb 206 pb ages of 2664 26 ma and 2628 26 ma zircon lu hf data show dominantly positive ehf t values and combined with crustal residence ages the results suggest mesoarchean to neoarchean juvenile crust formation in the ncc we interpret the data presented here to represent a phase of major late neoarchaean arc magmatism along the western margin of the jiaoliao block related to the birth of microcontinental nuclei within the ncc our data suggest that the western and eastern blocks might not have existed as discrete crustal blocks and that the construction of the ncc is a result of the assembly of several microblocks or terranes at the end of archaean similar archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early earth

the response 1958 1997 of permafrost and near surface ground temperatures to forest fire takhini river valley southern yukon territory forest fires in permafrost areas often modify ground surface conditions causing deepening of the active layer and thawing of near surface permafrost takhini river valley lies in the discontinuous permafrost zone of southern yukon territory the valley floor is covered by glaciolacustrine deposits which are locally ice rich in 1958 extensive forest fires burned most of the vegetation and the soil organic horizon in the valley but 50 km west of whitehorse 1 km2 of spruce forest adjacent to the alaska highway escaped burning permafrost beneath this stand of trees is in equilibrium with surface conditions the active layer is 1 4 m thick the base of permafrost is at 18 5 m the annual mean temperature at the top of permafrost 1 5 m is 0 8 c and the temperature gradient in permafrost is constant with depth at burned sites nearby there has been little regeneration of forest vegetation since the fire and long term permafrost degradation has occurred at one burned site the permafrost table is mor

the thermal regime of a retrogressive thaw slump near mayo yukon territory the development of a retrogressive thaw slump near mayo yukon territory has been traced from initiation by bank erosion 1949 of the stewart river to stabilization in 1993 1994 the stabilized headwall of the slump is 450 m from the river and the slope of the slump floor is 3 a transect of the slump from the river to the stabilized headwall was drilled in july 1995 to determine the extent and rate of permafrost degradation in the slump floor thermistors were placed in access tubes to 12 m depth at five sites four near the transect and one in undisturbed terrain to determine the magnitude of thermal disturbance due to slump development data loggers at the sites recorded the ground temperature at 1 m depth for two years from august 1995 the annual mean ground temperatures measured by the data loggers varied between 1 2 and 1 8 c in the slump compared with 2 4 c in undisturbed ground indicating a disturbance of about 4 c due to slumping the depth of thaw in the slump floor is consistent wit

deformation history of the hengshan complex implications for the tectonic evolution of the trans north china orogen structural analysis indicates that the hengshan complex underwent five distinct episodes of deformation d1 d5 the d1 episode formed small isoclinal folds f1 penetrative axial planar foliations s1 and mineral stretching lineations l1 d1 fabrics were reworked by following d2 deformation associated with d2 was the development of nw verging asymmetric folds and accompanying thrust faults both d1 and d2 resulted from crustal thickening and coherent with prograde and peak metamorphism d2 deformation was followed by transpressional dextral shearing d3 including top to nw oblique slip shearing and nne ssw dextral strike slip shearing ongoing collision led to development of the zhujiafang ductile shear zone d4 a near e w trending high strain belt across the hengshan complex d5 deformation is characterized by f5 open folds and associated normal faults probably related to exhumation of the complex structural patterns of the hengshan complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the eastern and western blocks underwent eastward directed subduction beneath the western margin of the eastern block and closure of this ocean finally led to collision between the two blocks to form the coherent north china craton

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga

neoarchean crust mantle interactions in the yishui terrane south eastern margin of the north china craton constraints from geochemistry and zircon u pb hf isotopes of metavolcanic rocks and high k granitoids abstract the neoarchean yishui terrane yst is situated in the east of western shandong province wsp south eastern margin of the north china craton ncc the metavolcanic rocks of the yst are fine grained hornblende plagioclase gneisses group 1 and fine grained amphibolites group 2 in the yangzhuangzhen area and fine to medium grained amphibolites group 3 in the leigushan area the high k granitoids associated with groups 1 and 2 are dominated by fine to medium grained monzogranitic gneisses zircon la icp ms u pb dating reveals that the magmatic precursors of groups 1 and 2 were formed at 2641 ma and the magmatic precursors of concomitant monzogranitic gneisses were emplaced from 2615 to 2575 ma whereas group 3 represents a later 2500 ma volcanic eruption and all these metamorphic volcanic rocks and monzogranitic gneisses were subjected to subsequent 2470 2460 ma metamorphism the metamorphic volcanic rock samples in group 1 exhibit the chemical compositions of calc alkaline andesites showing fractionated chondrite normalized ree patterns la yb n 10 48 19 30 and negative nb ta and ti anomalies nb la pm 0 13 0 22 which are akin to those of typical high mg andesites hmas in the subduction related settings the magmatic precursors of the group 1 samples were derived from partial melting of a fluid or melt metasomatized depleted mantle wedge at deep levels in the upper mantle samples in group 2 show calc alkaline chemical compositions with less fractionated chondrite normalized ree patterns la yb n 2 24 3 34 and negative nb ta and ti anomalies nb la pm 0 47 0 76 which are consistent with those of the volcanic rocks in the aleutian island arc the magmatic precursors of group 2 were generated by partial melting of a fluid metasomatized depleted mantle wedge at shallow levels in the upper mantle the monzogranitic gneisses exhibit high sio2 and k2o contents with high k calc alkaline affinities and peraluminous characteristics based on their distinct hree contents and chondrite normalized ree patterns these granitoid samples are subdivided into low yb monzogranitic gneisses lymgs and high yb monzogranitic gneisses hymgs the lymg magma was derived from partial melting of a mixed source of juvenile two mica pelites and minor basic intermediate igneous rocks at lower crustal levels with pyroxene amphibole garnet as the main residual phases and the hymg magma was derived from partial melting of multi sourced juvenile two mica pelites at middle to lower crustal levels with pyroxene amphibole and subordinate plagioclase and garnet as the main residual phases in addition group 3 resembles tholeiitic back arc basalts in the okinawa trough and displays flat chondrite normalized ree patterns la yb n 1 22 2 08 and slightly negative nb and ta anomalies nb la pm 0 35 0 59 this group was most likely derived from partial melting of a depleted mantle source that had been modified by the addition of subducted slab derived fluids at shallow levels in the upper mantle these metavolcanic rocks and concomitant high k granitoids record important neoarchean crust mantle interactions involving the first modification and partial melting of the lithospheric mantle induced by oceanic crust subduction then upwelling and underplating of mantle derived magmas triggered partial melting of the middle to lower crust and mixing between crust and mantle derived magmas these processes imply that neoarchean crust mantle interaction played a crucial role in the evolution of the southeastern margin of the ncc available whole rock sm nd and zircon lu hf isotopic data from metamorphic volcanic rocks and plutonic granitoids from this study and previous studies reveal that yst experienced three crucial juvenile crustal growth events from 2 78 2 69 ga 2 64 2 56 ga and 2 54 2 50 ga

timing magma ascent at popocatepetl volcano mexico 2000 2001 abstract magnetic anomalies may be used to constrain magma ascent and are useful as precursors to eruptions especially when correlated with other geophysical and geochemical data in this paper we present multiparameter data on the magnetics dome morphology geochemistry and seismicity associated with the december 2000 january 2001 eruptions the largest of the recent eruptions at popocatepetl volcano a 6 month data period was studied in order to evaluate the precursors and post eruption processes several cycles of dome construction and destruction occurred from september 2000 through february 2001 in december large amplitude tremor associated with a higher effusion rate resulted in the formation of a large dome which filled the crater to within about 50 m of the lowest part of the crater rim seismic activity in december was marked by many volcanotectonic earthquakes and both high frequency and harmonic tremor on december 12 and 13 an increase in the tremor amplitude was followed by ash eruptions with 1 7 5 km high columns tremor amplitude increased again on december 15 and oscillated for the next four days activity remained high until the end of the month on january 22 an 18 km high plume produced ash and pumice fall to the east as well as pyroclastic flows and mudflows which reached 6 km from the crater the eruption left three concentric explosion pits partially destroying the december dome mixing of a mafic olivine bearing melt with a more evolved magma triggered the larger eruption on january 22 as can be seen from the higher mgo concentrations in some of the ejecta and the presence of a dark andesitic scoria with lower silica content and a white andesitic pumice with higher silica content precursory negative magnetic anomalies up to 5 nt 3 2 nt 5 nt 2 9 nt were associated with the ascent of the larger batches of magma which preceded the increases in seismicity before the december 2000 january 22 vei 3 4 eruptions no significant increases in seismicity were observed prior to the january 22 eruption except for a magnitude 2 4 earthquake on the day of the eruption there was however a 0 8 nt magnetic anomaly which lasted from january 10 to 12 and possibly to january 17 most of the magma ascended in three large batches and several smaller ones from october 22 through december 10 short lived crater domes with small volumes were formed at popocatepetl in september october and november 2000 and in march 2001 even the smaller negative anomalies from september to february were associated with the ascent of several batches of magma they preceded the increases observed in the seismicity and growth of a dome by several days

magnetic signatures associated with magma ascent and stagnation at popocatepetl volcano mexico during 2006 monitoring of real time magnetic signals at popocatepetl during 2006 has allowed discrimination of magma injection and dome growth magnetic signals correlated with seismic volcanotectonic events and harmonic tremor as well as number of small emissions spring water ph ash components and dome evolution helped define upward magma transport and yield a better understanding of the volcanic plumbing system magma ascent occurs mostly in periods of 7 3 days associated with harmonic tremor and decreasing magnetic signals between 21 1 and 215 nt followed by increasing signals linked to cooling of the domes and increased seismicity over periods of 1 to more than 3 months the dome clogs the vent after the negative magnetic anomaly harmonic tremor period associated with magma ascent and forces an explosive crater reopening explosion larger negative changes in the magnetic signals occurred in april 26 nt august 23 to 26 nt and october to december 25 to 215 nt associated with dome formation and growth negative magnetic anomalies preceded eruptions

magma storage and ascent at lipari island aeolian archipelago southern italy at 223 81 ka the role of crustal processes and tectonic influence fluid inclusion studies together with volcanological and petrochemical data allow reconstruction of the magma feeding system of basaltic andesitic to andesitic activity during the oldest and intermediate stages of development of lipari island 223 81 ka a major magma storage zone is active during the overall investigated time span at depths of 22 km close to the crust mantle moho transition at which mantle derived mafic magmas tend to accumulate due to neutral buoyancy conditions beneath central type volcanoes m mazzacaruso m s angelo m chirica costa d agosto a shallower magma reservoir is located within the upper crust at 5 5 3 5 km associated with a major lithological discontinuity for fissural type volcanoes timpone ospedale monterosa m chirica tectonic structures are suggested to influence further magma ascent and storage at mid crustal depths 14 km with no ponding at shallower levels partial crustal melting processes at the roofs of the deep magma reservoirs 17 km are invoked to explain the origin of cordierite bearing lavas beneath m s angelo and m chirica costa d agosto volcanoes which were active during the intermediate stages of development of lipari 105 81 ka the generation of felsic anatectic melts in the lower crust could have created density and rheologic barriers to impede the passage of mafic melts and promote their ponding with influence on the subsequent evolution of lipari volcano

magmatic record of neoarchean arc polarity reversal from the dengfeng segment of the central orogenic belt north china craton abstract the dengfeng segment of the archean central orogenic belt in the north china craton ncc includes circa 2 66 2 5 ga metavolcanics metasedimentary and plutonic rocks all units are intruded by 2 5 2 4 ga mafic dykes and a suite of coeval circa 2 5 2 45 ga potassic granites and leucogranitic veins mafic dykes are characterized by mixed alkaline and calc alkaline affinities enriched lree and negative nb and zr anomalies indicating a lithospheric mantle source region potassic granitic dykes exhibit highly fractionated lree patterns and negative hfse anomalies and positive zircon ehf t values which are similar to those of ttg gneisses in the area leucogranitic veins cutting across the diorites are characterized by high contents of sio2 and k2o lree enriched patterns positive eu and sr anomalies negative nb and ti anomalies we suggest that the granitic dykes and leucogranitic veins were derived from partial melting of thickened older crusts combined with other reported 2 5 ga mafic dykes and potassic granites from the ncc we propose that they demonstrate an ancient example of arc polarity reversal and provide a record of the formation of a new plate boundary in the archean the underplating of the mafic dyke source beneath the newly accreted eastern block may have generated the widespread 2 5 2 47 ht lp metamorphism with anticlockwise p t paths in the eastern block

intermittent mixing processes occurring before plinian eruptions of popocatepetl volcano mexico insights from textural compositional variations in plagioclase and sr nd pb isotopes volcan popocatepetl has explosively erupted in plinian style at least five times in the last 23 000 years extreme deviations in composition and the occurrence of dissolution features in plagioclase and pyroxene and the occasional presence of xenocrysts of cr rich fe ti oxides and mg rich olivines and pyroxenes indicate that magma mixing has been a major process affecting the magmatic system the nearly invariant composition of the erupted products andesitic dacitic suggests however that mixing is not acting alone and must be balanced by assimilation and or crystallization to investigate the magmatic processes that have modified the plinian magmas textural and compositional variations and growth rates in plagioclase were used to approximate the frequency of mixing events affecting each magma systematic analysis of sr nd and pb isotopes was carried out on plagioclase pyroxene and pumice matrix glass to constrain the extent of assimilation of upper crustal rocks additionally a series of phase equilibrium experiments were carried out to constrain the depth where such mixing and assimilation occurred we find that magma was stored at one of two different depths beneath popocatepetl with magma mixing acting in both reservoirs mixing frequency and the relative impact on mineral compositions and textures has varied with time assimilation of calcareous rocks underneath popocatepetl has not been pervasive and does not contribute significantly to the evolution of the plinian magmas the similar compositions of magmas with diverse mixing histories suggest that fractional crystallization and possibly assimilation of deep crust takes place at depth and that intermediate magmas ascend into the upper crust already differentiated

flat slab thermal structure and evolution beneath central mexico recent seismic and magnetotelluric experiments aimed at better characterizing the shape and state of the subducting slab and continental crust beneath central mexico exposed significant differences with conclusions of previous studies a new slab geometry is revealed in which the subducting cocos slab is perfectly flat between 120 to 290 km from the trench after which it plunges into the asthenosphere at a dip angle of 65 in sharp contrast with the previously proposed 20 dip angle seismic tomography studies show negative p wave velocity anomalies 2 to 4 in the mantle wedge beneath the mexican volcanic belt and positive anomalies 2 to 3 for the subducted cocos slab magnetotelluric experiments exposed a very low resistivity area 1 10 m located within the continental crust just below the mexican volcanic arc finally several spots of non volcanic tremors nvts have been recorded inside the continental crust above the flat slab segment while all these experiments provide a better picture of the subduction system beneath central mexico several key processes need further investigation in this study we take advantage of these new observations to better constrain the thermal structure beneath central mexico two different thermal models are computed for a mantle potential temperature t p of 1 350 and 1 450 c respectively the new thermal structures are then converted into p wave velocity anomalies and compared with the observed v p anomalies we found that a t p of 1 450 c produced larger v p anomalies that do not fit the observations however using a t p of only 1 350 c our predicted v p anomalies are positive 2 to 3 for the cold slab and negative 2 to 4 in the mantle wedge these v p estimates are consistent with the observed seismic tomography from p wave arrivals and therefore we conclude that a t p of 1 350 c is a better estimate for the mantle potential temperature beneath central mexico the new thermal model in conjunction with phase diagrams for sediments hydrated basalt and lithospheric mantle have been used to estimate the amount and location of fluids released from the subducting cocos slab several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat slab we found a good correlation between the pattern of these dehydration pulses and the location of nvts suggesting that slab dehydration is responsible for triggering the tremors we suggest that nvt bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening a process that has been going on since 15 ma ago when the cocos slab entered into a flat slab regime such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15 ma allowing the slab to easily roll back the continuous low resistivity region recorded beneath the volcanic front in central mexico might represent the evidence of slab dehydration and crust weakening over time

ca 2 9 ga granitoid magmatism in eastern shandong north china craton zircon dating hf in zircon isotopic analysis and whole rock geochemistry abstract eastern shandong is one of the important early precambrian areas in the north china craton where extensive 2 5 2 7 and 2 9 ga magmatic activities have been identified followed by very strong late neoarchean and paleoproterozoic tectono thermal events we carried out shrimp zircon dating hf in zircon isotope analysis and a whole rock geochemical study on ca 2 9 ga mesoarchean magmatic rocks these include gneissic diorite gneissic quartz diorite gneissic tonalite and gneissic high si trondhjemite and were identified in five new areas all these rocks contain ca 2 5 ga metamorphic zircons the dioritic and tonalitic rocks are enriched in large ion lithosphile lil elements k rb ba and depleted in nb and p with moderate to highly fractionated ree patterns similar in chemical compositions to archean ttg rocks worldwide the high si trondhjemites are characterized by very high sio2 and very low feot mgo the magmatic zircon grains from all these rocks are similar in their hf isotopic compositions and have hf t values ranging from 1 3 to 9 9 combined with previous work our main conclusions are 1 eastern shandong underwent a major magmatic event at ca 2 9 ga 2 diorites and tonalites formed by melting of thickened lower crust or the root of an oceanic plateau whereas the high si trondhjemites were derived from melting of intermediate to felsic rocks 3 the mesoarchean ca 2 9 ga was a major period of juvenile crustal addition from the depleted mantle earlier than the major crustal growth period at 2 7 2 8 ga in the north china craton 4 the 2 7 2 9 ga rocks underwent strong metamorphism at ca 2 5 ga probably as a result of magmatic underplating

characterization of the recent ash emissions at popocatepetl volcano mexico abstract nine representative ash emissions from 1994 1997 were studied to characterize the recent activity and the eruptive process at popocatepetl a series of tephra eruptions began on december 21 1994 and intermittent activity continues to present the first eruptions were phreatomagmatic but in mid march 1996 they turned magmatic cumulative volumes 529 1810 10 3 m 3 were determined for the first eruptions however when eruptions grew larger more widely spaced and magmatic the volumes were then calculated individually 22 1107 10 3 m 3 both using the simpson rule and based on 244 sampling sites this numerical integration method is more precise than other methods especially since sub mm isopachs are neglected in most cases dominant winds carried ash mainly to the east january through april 1995 and april 1996 except for the summer months when ash fell on mexico city to the northwest october 28 1996 and june 30 1997 in march 1996 changing wind direction produced ash fall to the southwest as well during the first year volume calculations indicated that emission rate was higher at the beginning of the eruptions and then declined and stopped activity resumed the following year with a similar pattern until larger amounts of magma ascended detailed studies of the ashfall provided constraints on the dynamics of the volcanic plumbing system tephra emission was related with clearing december 1994 to march 1995 and clogging of the vent may 1995 to february 1996 until a larger new ascending batch was able to clear its way to the surface march 1996 after april 1996 dome formation and explosive destruction were related to individual small ascending magma batches tephra from december 1994 to early march 1996 was made up mostly of andesitic lithic clasts and plagioclase and pyroxene crystals with minor amounts of accidental and accessory minerals in march 1996 prior to dome formation glass was also detected afterwards ash components were mostly dome lithics and scoria with minor amounts of plagioclase pyroxene and olivine crystals from the incoming magma batches

shrimp u pb zircon ages of the fuping complex implications for late archean to paleoproterozoic accretion and assembly of the north china craton the fuping complex is situated in the central part of the north china craton and consists of four major lithological assemblages fuping tonalitic trondhjemitic granodioritic gneisses longquanguan augen gneisses wanzi supracrustal assemblage and nanying granitic gneisses shrimp u pb geochronology combined with u th and cathodoluminescence cl imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the fuping complex cl images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the fuping complex the metamorphic zircons occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores and are all characterized by nebulous zoning or being structureless with extremely high luminescence and very low th contents these features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning comparatively low luminescence and high th and u contents shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic plutons of the fuping gneisses were emplaced at 2523 14 ma 2499 10 ma and 2486 8 ma respectively whereas the monzogranitic and granitic plutons of the longquanguan augen gneisses were intruded respectively at 2510 22 ma and 2507 11 ma prismatic and oscillatory zoned zircons dominate in the pelitic rocks of the wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks the concordant and discordant u pb ages of 2502 5 ma and 2507 14 ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than 2507 ma ago in addition a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 5 1 ma which may provide a maximum depositional age for the wanzi supracrustal rocks shrimp results also reveal that granitic magmatism assigned to the nanying granitic gneisses occurred over a protracted interval from 2077 13 ma to 2024 21 ma the nebulously zoned zircon grains and overgrowth recrystallization zircon rims from different rocks yielded similar concordant 207pb 206pb ages in the range 1875 to 1802 ma interpreted as approximating the age of regional metamorphism of the fuping complex timing of primary zircon crystallization and regional metamorphism of the fuping complex is in general agreement with recent u pb zircon ion probe results for the wutai and hengshan complexes that bound the fuping complex to the northwest these areas are characterized by the emplacement of major granitoid bodies at around 2 50 ga to 2 48 ga ago deposition of supracrustal rocks in the paleoproterozoic intrusion of paleoproterozoic granitic bodies at 2 1 to 2 0 ga and regional metamorphism at 1 875 to 1 802 ga these data indicate that the fuping and hengshan complexes do not represent an older crystalline basement to the wutai complex as suggested in previous tectonic models but together with the wutai complex represent elements of a single late archean to paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western north china blocks at 1 85 ga which resulted in the final assembly of the north china craton

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

temporal evolution of magmatism in the northern volcanic zone of the andes the geology and petrology of cayambe volcanic complex ecuador in the northern volcanic zone of the andes the cayambe volcanic complex consists of 1 a basal mostly effusive volcano the viejo cayambe whose lavas andesites and subordinate dacites and rhyolites are typically calc alkaline and 2 a younger essentially dacitic composite edifice the nevado cayambe characterized by lavas with adakitic signatures and explosive eruptive styles the construction of viejo cayambe began 1 1 myr ago and ended at 1 0 ma the young and still active nevado cayambe grew after a period of quiescence of about 0 6 myr from 0 4 ma to holocene its complex history is divided into at least three large construction phases angureal cone main summit cone and secondary summit cone and comprises large pyroclastic events debris avalanches as well as periods of dome activity geochemical data indicate that fractional crystallization and crustal assimilation processes have a limited role in the genesis of each suite on the contrary field observations and mineralogical and geochemical data show the increasing importance of magma mixing during the evolution of the volcanic complex the adakitic signature of nevado cayambe magmas is related to partial melting of a basaltic source which could be the lower crust or the subducted slab however reliable geophysical and geochemical evidence indicates that the source of adakitic component is the subducted slab thus the viejo cayambe magmas are inferred to come from a mantle wedge source metasomatized by slab derived melts adakites whereas the nevado cayambe magmas indicate a greater involvement of adakitic melts in their petrogenesis this temporal evolution can be related to the presence of the subducted carnegie ridge modifying the geothermal gradient along the wadati benioff zone and favouring slab partial melting

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

mafic magma recharge supplies high co2 and so2 gas fluxes from popocatepetl volcano mexico since late 1994 open vent eruptive activity and degassing at popocatepetl volcano mexico have released large masses of co2 and so2 tephra and lava produced by these eruptions show evidence for mixing of mafic and silicic magmas shortly before eruption we present the first measurements of dissolved co2 in the mafic magma end member based on analyses of olivine hosted melt inclusions that were trapped at pressures as high as 400 mpa 15 km depth beneath the volcano we combine our data with thermodynamic models to show that degassing of mafic magma at 150 350 mpa pressure can explain the co2 so2 mass ratios 1 8 of volcanic gases released from the volcano during 1995 1997 our results demonstrate that mafic magma recharge was responsible for the high measured fluxes of co2 and so2 from 1995 to 1997 the total so2 emission of 9 mt during this period requires intrusion and degassing of a minimum of 0 8 km3 of mafic magma only 0 3 of this new mafic magma has been erupted in the form of mixed hybrid lava and tephra our results suggest that the ongoing eruption of popocatepetl is essentially an intrusive event more generally we suggest that intrusion and deep degassing may explain the high gas fluxes at some other open vent volcanoes rather than convection of magma in the uppermost parts of subvolcanic conduits

redefinition of depositional ages of neoarchean supracrustal rocks in western shandong province china shrimp u pb zircon dating abstract western shandong province is an important terrane in the eastern part of the north china craton where 2 75 2 5 ga tectono thermal events are well recognized supracrustal rocks named the taishan group experienced greenschist to amphibolite facies metamorphism and occur as lenses and strips in granitoids and are subdivided into the yanlingguan shancaoyu and liuhang formations the yanlingguan formation is mainly composed of amphibolite but also contains fine grained biotite hornblende gneiss komatiitic rocks and schist the liuhang formation is similar in rock association to the yanlingguan formation but contains more metasedimentary rocks the shancaoyu formation is mainly composed of fine grained biotite gneiss leptynite preserving bedding and is interlayered with schist its protoliths are mainly sedimentary rocks with some mafic to felsic volcano sedimentary varieties we undertook shrimp zircon dating on eighteen metasedimentary and volcanic samples mainly from the shancaoyu formation and the upper part of the liuhang formation ages of detrital and volcanic zircons vary from 2 75 to 2 54 ga and 2 55 to 2 525 ga respectively and the age distribution patterns vary in different samples we conclude that the shancaoyu formation and the upper part of the liuhang formation were deposited in the late neoarchean 2 55 2 525 ga and not in the early neoarchean 2 8 2 7 ga as thought before western shandong is the only terrane in the north china craton where both early and late neoarchean supracrustal rocks have been identified a magmatic arc model may be applied to explain the tectono thermal evolution at the end of the neoarchean

2 7ga juvenile crust formation in the north china craton taishan xintai area western shandong province further evidence of an understated event from u pb dating and hf isotopic composition of zircon abstract the evolution of the north china craton ncc is well known for a marked 2 55 2 50 ga tectonothermal event however supracrustal and intrusive rocks of 2 75 2 70 ga are in fact widely distributed in the western shandong province the most important area of archaean basement in the eastern part of the ncc this paper reports shrimp u pb dating and la icpms hf isotopic composition of zircons from 2 75 2 70 ga supracrustal and trondhjemite tonalite granodiorite ttg rocks in that area three fine grained hornblende biotite gneiss samples known locally as leptynite with meta volcanic or volcanosedimentary rock protoliths and five ttg samples have shrimp zircon u pb ages varying from 2 75 to 2 70 ga and 2 74 to 2 71 ga respectively zircons from most of the samples have high positive hf t values 4 7 to 10 0 and t dm hf ages 2 85 2 60 ga similar to their zircon u pb ages this indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before however some granitoids show hf t zircon values of 13 6 to 5 1 and t dm hf of 3 51 2 80 ga therefore the strong 2 75 2 70 ga tectonothermal event in the western shandong province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components combined with craton wide data it is shown that the ncc is similar to many other cratons around the world where tectonothermal events of 2 7 ga are well developed however the main difference is that in the ncc superimposed 2 5 ga tectonothermal events were much stronger than in most other cratons

gas composition of popocatepetl volcano between 2007 and 2008 ftir spectroscopic measurements of an explosive event and during quiescent degassing abstract on december 1 2007 the solar absorption infrared spectra of the popocatepetl volcanic plume was recorded during an eruptive event and complementarily on november 17 2008 the passive quiescent degassing was measured from the same site a portable ftir spectrometer with a scanning mirror for fast tracking of the sun provided the flexibility quality and simplicity needed for field deployment slant columns of the gases so 2 hcl hf and sif 4 were retrieved and strong differences could be observed when comparing gas ratios in both time periods during the explosive eruption the so 2 hcl ratio was three times greater and the hf hcl ratio was slightly smaller than during passive degassing while the ratios among so 2 hcl hf and sif 4 describe the chemical composition of the volcanic gas mixture the sif 4 hf ratio provides information about the equilibrium temperatures of the stored gases which in this study were calculated at 150 and 185 c for the explosive and quiescent degassing episodes respectively we conclude that cooling of lava domes in the crater precedes vulcanian explosions as suggested by schaaf et al 2005 based on so 2 flux grutter et al 2008 and measurements and data from the november 2008 event the average fluxes for hcl hf sif 4 and f through quiescent degassing are estimated to be 204 22 7 9 8 and 31 7 tons day respectively these values are similar to those reported by love et al 1998 more than 10 yrs ago

middle neoarchean magmatism in western shandong north china craton shrimp zircon dating and la icp ms hf isotope analysis abstract western shandong province is a typical area of the north china craton ncc where neoarchean plutonic and supracrustal rocks are widely distributed early studies documented that 2 7 ga and 2 5 ga magmato tectono thermal events are well developed in the area here we report shrimp u pb ages and hf in zircon isotopic data from ten samples of different magmatic rock types including hornblendite gneissic tonalite gneissic trondhjemite and gneissic granite magmatic zircon grains have 207 pb 206 pb ages ranging from 2667 to 2598 ma some rocks contain 2 5 ga metamorphic rims and 2 7 ga zircon cores the magmatic zircon grains have hf t values and hf crustal model ages of 1 1 to 11 3 and 3 02 2 4 ga respectively combined with an earlier study our main conclusions are that the middle neoarchean rocks mainly occur together with early neoarchean rocks in the northeastern portion of the central belt juvenile additions to continental crust and crustal recycling played important roles in the middle neoarchean of western shandong province and the entire neoarchean tectonic evolution can be divided into middle to early neoarchean 2 75 2 6 ga and late neoarchean 2 6 2 5 ga events

influence of volcanic activity on spring water chemistry at popocatepetl volcano mexico abstract the results of the 7 years 1994 2000 of monthly monitoring of spring water before and during eruptions show response to volcanic activity low salinity and temperature characterize most of the springs which are located on the flanks of popocatepetl volcano the ph ranges from 5 8 to 7 8 and temperature from 3 to 36 c oxygen and hydrogen isotopic data show that the water is of meteoric origin but so 4 2 cl f hco 3 b and so 4 2 cl variations precede main eruptive activity which is considered linked to influx of magmatic gases and acid fluids that react with sublimates and host rock and mix with the large water system na ca 2 sio 2 and mg 2 concentrations in the water also increased before eruptive activity the computed partial pressure of co 2 in equilibrium with spring waters shows values higher than air saturated water asw with the highest values up to 0 73 bar of pco 2 boron is detected in the water only preceding the larger eruptions when present boron concentration is normally under health standard limits but in two cases the concentration was slightly above other components are within health standard limits except for f in one spring

spatial and temporal variations in active layer thawing and their implication on runoff generation in peat covered permafrost terrain 1 the distribution of frost table depths on a peat covered permafrost slope was examined in a discontinuous permafrost region in northern canada over 4 consecutive years at a variety of spatial scales to elucidate the role of active layer development on runoff generation frost table depths were highly variable over relatively short distances 0 25 1 m and the spatial variability was strongly correlated to soil moisture distribution which was partly influenced by lateral flow converging to frost table depressions on an interannual basis thaw rates were temporally correlated to air temperature and the amount of precipitation input simple simulations show that lateral subsurface flow is governed by the frost table topography having spatially variable storage that has to be filled before water can spill over to generate flow downslope in a similar manner that bedrock topography controls subsurface flow however unlike the bedrock surface the frost table is variable with time and strongly influenced by the heat transfer involving water therefore it is important to understand the feedback between thawing and subsurface water flow and to properly represent the feedback in hydrological models of permafrost regions

zircon u pb geochronology and hf isotopes of major lithologies from the jiaodong terrane implications for the crustal evolution of the eastern block of the north china craton abstract as an important component of the western shandong complex in the eastern block of the north china craton the yishui terrane consists of neoarchean high grade supracrustal rocks granitoid gneisses and charnockites la icp ms zircon u pb dating and hf isotopic analyses on these lithologies have been carried out and the results provide new insights into the neoarchean crustal evolution of the eastern block of the north china craton new zircon dating results reveal that the magmatic precursors of the supracrustal metapelites and granitoid gneisses were generated at 2 54 2 53 ga and 2 57 2 55 ga respectively and the charnockites were emplaced contemporaneously around 2 56 2 53 ga single metamorphic zircon grains and overgrowth rims from these rocks document consistent metamorphic ages at 2 50 ga suggesting that the yishui terrane experienced a high grade metamorphic event at the end of the neoarchean detrital zircons from pelitic gneisses yield 207pb 206pb ages of 2 89 2 65 ga with a major age peak at 2 53 ga and a subordinate age peak at 2 70 ga implying that the sedimentary protoliths of the pelitic gneisses may have been sourced from 2 53 ga rocks in the yishui terrane and 2 70 ga rocks in the adjacent luxi granite greenstone terrane hf isotopic compositions show that the neoarchean magmatic zircons have positive ehf t values ranging from 1 4 to 7 8 and depleted mantle model ages of 2 92 2 60 ga with a prominent peak at 2 8 2 7 ga suggesting that the neoarchean crust was derived mainly from juvenile sources and partly from the recycling of old continental crust in the yishui area combined with previous data from the adjacent luxi granite greenstone terrane and other complexes in the eastern block it is concluded that the neoarchean was an important period of crust accretion in the eastern block of the north china craton

the building of an archean microcontinent evidence from the north china craton abstract continents on the early earth are considered to have been built through the accretion of microterranes oceanic arcs and plateaus the north china craton ncc envisaged in recent models as a collage of several microblocks which were amalgamated along multiple zones of ocean closure during the late neoarchean provide a typical case to investigate the origin and amalgamation of microcontinents through convergent margin processes here we report a suite of magmatic rocks developed at the periphery of one of these microblocks the jiaoliao block that forms part of the composite eastern block of the ncc we integrate our new data with those from various parts of this microblock to elucidate the mechanism of continent building in the archean we present petrological geochemical and zircon u pb geochronological and lu hf isotopic data from the magmatic suite that belongs to the yishui complex geochemically the felsic units of the suite straddle from monzonite through granodiorite to granite with dominantly metaluminous affinity magnesian composition and arc related features the metagranites ttg gneisses and charnockites are characterized by negative nb ta anomalies and positive k and pb anomalies the diorites and gabbros display negative anomalies of th u nb ta and zr hf and positive anomalies at ba pb and sm with negative eu anomalies and minor positive ce anomalies attesting to arc related features in the tectonic discrimination diagrams the rocks plot in the volcanic arc field indicating arc related origin in subduction setting zircon grains from all the rocks display core rim texture with the cores showing magmatic crystallization and the narrow structureless rims corresponding to metamorphic overgrowth the magmatic zircons from the metagranites show upper intercept ages or 207 pb 206 pb weighted mean ages of 2505 29 ma and 2569 20 ma to 2513 27 ma those from the ttg gneisses show 2535 17 ma to 2546 39 ma from charnockites display 2543 20 ma 2555 15 ma and diorite and gabbro show 2587 15 ma and 2516 13 ma respectively the zircon rim ages of 2472 23 ma 2457 35 ma 2545 30 ma and 2511 35 ma suggest the timing of metamorphism ca 2 55 2 45 ga magmatic zircons with slightly older ages of ca 2 73 ga 2 64 ga also occur suggesting multiple magmatic pulses the lu hf isotopic data show positive ehf t values ranging from 0 2 to 5 7 for metagranites with hf model ages of 2602 2815 ma t dm and 2658 3002 ma t dm c whereas for ttg gneisses the positive ehf t values are up to 6 5 and display dominant mesoarchean hf model ages with limited early neoarchean hf model ages charnockite samples show positive ehf t values 2 3 5 7 and display the hf model ages ranging from 2601 ma to 2772 ma t dm and 2658 ma to 2904 ma t dm c diorite and gabbro also show positive ehf t 2 3 6 9 and yield hf model ages of 2625 2788 ma t dm and 2647 ma to 2903 ma t dm c the hf isotopic data indicate that the magmas were derived from neoarchean mesoarchean juvenile sources integrating our data with those from the entire jiaoliao microblock reveals vestiges of hadean crust involved in building the eoarchean nucleus of this microblock vigorous convergent margin processes ranging from mesoarchean to late neoarchean with multiple pules of arc magmatism associated with subduction tectonics led to further growth of continental crust culminating in paired high temperature and high pressure metamorphism during late neoarchean early paleoproterozoic transition

source mechanism of vulcanian degassing at popocatepetl volcano mexico determined from waveform inversions of very long period signals 1 the source mechanism of very long period vlp signals accompanying volcanic degassing bursts at popocatepetl is analyzed in the 15 70 s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium the waveforms of two eruptions 23 april and 23 may 2000 representative of mild vulcanian activity are well reproduced by our inversion which takes into account volcano topography the source centroid is positioned 1500 m below the western perimeter of the summit crater and the modeled source is composed of a shallow dipping crack sill with easterly dip of 10 intersecting a steeply dipping crack northeast striking dike dipping 83 northwest whose surface extension bisects the vent both cracks undergo a similar sequence of inflation deflation and reinflation reflecting a cycle of pressurization depressurization and repressurization within a time interval of 3 5 min the largest moment release occurs in the sill showing a maximum volume change of 500 1000 m3 pressure drop of 3 5 mpa and amplitude of recovered pressure equal to 1 2 times the amplitude of the pressure drop in contrast the maximum volume change in the dike is less 200 300 m3 with a corresponding pressure drop of 1 2 mpa and pressure recovery equal to the pressure drop accompanying these volumetric sources are single force components with magnitudes of 108 n consistent with melt advection in response to pressure transients the source time histories of the volumetric components of the source indicate that significant mass movement starts within the sill and triggers a mass movement response in the dike within a few seconds such source behavior is consistent with the opening of a pathway for escape of pent up gases from slow pressurization of the sill driven by magma crystallization the opening of this pathway and associated rapid evacuation of volcanic gases induces the pressure drop pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles assuming a penny shaped crack at ambient pressure of 40 mpa the observed pressure and volume variations can be modeled with the following attributes crack radius 100 m crack aperture 5 m bubble number density 1010 1012 m 3 initial bubble radius 10 6 m final bubble radius 10 5 m and net decrease of gas concentration in the melt 0 01 wt

magmatic vapor expansion and the formation of high sulfidation gold deposits chemical controls on alteration and mineralization abstract large bulk tonnage high sulfidation gold deposits such as yanacocha peru are the surface expression of structurally controlled lode gold deposits such as el indio chile both formed in active andesite dacite volcanic terranes fluid inclusion stable isotope and geologic data show that lode deposits formed within 1500 m of the paleo surface as a consequence of the expansion of low salinity low density magmatic vapor with very limited if any groundwater mixing they are characterized by an initial sulfate stage of advanced argillic wallrock alteration alunite commonly with intense silicification followed by a sulfide stage a succession of discrete sulfide sulfosalt veins that may be ore grade in gold and silver fluid inclusions in quartz formed during wallrock alteration have homogenization temperatures between 100 and over 500 c and preserve a record of a vapor rich environment recent data for el indio and similar deposits show that at the commencement of the sulfide stage condensation of cu as s sulfosalt melts with trace concentrations of sb te bi ag and au occurred at 600 c following pyrite deposition euhedral quartz crystals were simultaneously deposited from the vapor phase during crystallization of the vapor saturated melt occurs to fe tennantite with progressive non equilibrium fractionation of heavy metals between melt vapor and solid vugs containing a range of sulfides sulfosalts and gold record the changing composition of the vapor published fluid inclusion and mineralogical data are reviewed in the context of geological relationships to establish boundary conditions through which to trace the expansion of magmatic vapor from source to surface and consequent alteration and mineralization initially heat loss from the vapor is high resulting in the formation of acid condensate permeating through the wallrock this sulfate stage alteration effectively isolates the expansion of magmatic vapor in subsurface fracture arrays from any external contemporary hydrothermal activity subsequent fracturing is localized by the embrittled wallrock to provide high permeability fracture arrays that constrain vapor expansion with minimization of heat loss the sulfide stage vein sequence is then a consequence of destabilization of metal vapor species in response to depressurization and decrease in vapor density the geology mineralogy fluid inclusion and stable isotope data and geothermometry for high sulfidation bulk tonnage and lode deposits are quite different from those for epithermal gold silver deposits such as mclaughlin california that formed near surface in groundwater dominated hydrothermal systems where magmatic fluid has been diluted to less than about 30 high sulfidation gold deposits are better termed solfataric gold deposits to emphasize this distinction the magmatic vapor expansion hypothesis also applies to the phenomenology of acidic geothermal systems in active volcanic systems and equivalent magmatic vapor discharges on the flanks of submarine volcanoes

volatiles degassing and crystallization at a primitive cinder cone implications for melt evolution and the plumbing systems of monogenetic volcanoes volcan jorullo mexico

timing of late neoarchean to late paleoproterozoic events in the north china craton shrimp u pb dating and la icp ms hf isotope analysis of zircons from magmatic and metamorphic rocks in the santunying area eastern hebei abstract santunying is an important area for revealing nature of the late neoarchean tectono magmato thermal events in the eastern hebei part of the north china craton it is mainly composed of meta intrusive rocks supracrustal rocks sporadically occur in the meta intrusive rocks the meta intrusive rocks are subdivided into the santunying tonalitic gneiss qiuhuayu tonalitic trondhjemitic gneiss xiaoguanzhuang dioritic gneiss and qingyangshu meta gabbro respectively shrimp u pb zircon dating on fourteen samples yielded weighted mean 207pb 206pb ages of 2525 2537 2532 2546 2530 2544 and 2531 ma for magmatic zircons from them dioritic gneiss of the xiaoguanzhuang gneiss contain abundant 2544 3487 ma xenocrystic zircons shrimp u pb dating on a garnet biotite gneiss sample yielded a weighted mean 207pb 206pb age of 2537 ma for detrital zircons all rocks underwent strong metamorphism deformation and anatexis resulting in formation of leucosomes and residues with some leucosomes concentrating to form large veins they record a strong late neoarchean event by metamorphic zircon ages of 2489 2519 ma some rocks also record metamorphic zircon ages of 1772 1843 ma magmatic zircons from the magmatic rocks show large variations in ehf t values ranging from 1 7 to 8 7 combined with early studies conclusions are 1 intrusive rocks with the involvement of mantle derived materials have a narrow range of magmatic zircon ages from 2525 to 2546 ma and supracrustal rocks were formed during the same period 2 ancient crustal remnants 2600 ma are present consistent with the late neoarchean arc magmatism involving older continental crust similar to phanerozoic andean margins 3 the archean basement underwent a strong tectonothermal event at the end of the neoarchean with the metamorphic zircon ages being 10 30 million years younger than the timing of magmatism a common feature of the north china craton 4 a late paleoproterozoic tectonothermal event widely occurred in the western part of eastern hebei which is linked with regional ductile deformation

a simplified equation of state for the density of silicate hydrous magmas an application to the popocatepetl buoyancy driven dome growth process abstract we present a simplified equation of state eos for calculating the density of hydrous siliceous magmas in a range of pressures temperatures and dissolved water content of interest to study several magma conduit processes a simpler linear form and a more precise non linear yet simple form of the equation both valid in the range 0 to 0 25 gpa 800 to 1000 k and 0 to 7 wt h2o are proposed for the dacitic and andesitic popocatepetl magma melt compositions we apply these equations to explain some aspects of the dome forming effusive activity that has been observed at popocatepetl volcano since 1996 this model poses a density reduction dependent mainly on the volatile contents of the magma filling the volcano conduit system as the driving buoyancy force causing magma extrusions and dome growth

two ages of porphyry intrusion resolved for the super giant chuquicamata copper deposit of northern chile by ela icp ms and shrimp zircon u pb ages measured in situ by excimer laser ablation inductively coupled plasma mass spectrometry ela icp ms and verified by sensitive high resolution ion microprobe shrimp on ore bearing felsic porphyries from the chuquicamata porphyry copper molybdenum deposit northern chile identify two discrete igneous events the volumetrically dominant east porphyry has an age of 34 6 0 2 ma whereas the bench and west porphyries yield ages of 33 3 0 3 ma and 33 5 0 2 ma respectively the age of the east porphyry is indistinguishable from a re os age for early molybdenite mineralization 35 ma and the oldest reported 40 ar 39 ar ages for hydrothermal alteration confirming a genetic link with mineralization previous geological studies and 40 ar 39 ar and re os geochronology identify two main hydrothermal events high temperature potassic alteration with chalcopyrite at 33 4 0 3 ma followed by lower temperature quartz sericite alteration with pyrite at 31 1 0 3 ma the ages of the west and bench porphyries match the ages for potassic alteration younger quartz sericite alteration may reflect an additional fourth intrusion concealed at depth the anomalously large size of chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic hydrothermal systems

the complexities of zircon crystllazition and overprinting during metamorphism and anatexis an example from the late archean ttg terrane of western shandong province china abstract there are different viewpoints on metamorphic and anatectic zircons recording ages of 2 45 2 48 ga or even younger in some areas of the north china craton where both late neoarchean and late paleoproterozoic tectono thermal events are well developed these are 1 partial resetting of the u pb isotopic system in the late neoarchean zircons 2 metamorphism lasting from the late neoarchean to the earliest paleoproterozoic and 3 earliest paleoproterozoic metamorphism as separate different event western shandong province is an area where the late neoarchean tectono thermal event is widely developed but the late paleoproterozoic event has not been identified this provides an opportunity to understand the geological processes around the archean proterozoic boundary based on a field study we carried out shrimp u pb zircon dating on seventeen samples of 2 5 ga old metamorphic and anatectic rocks including tonalite trondhjemite granodiorite gabbro quartz diorite granite and paragneiss with primary emplacement or depositional ages of 2 52 2 68 ga anatectic zircons show some textural and compositional features a homogenous or blurred oscillatory zoning b high u contents and low in th u ratios commonly

crustal growth and reworking of the eastern north china craton constraints from the age and geochemistry of the neoarchean taishan ttg gneisses abstract the taishan complex is located in the central part of the eastern north china craton ncc and preserves extensive neoarchean ttg gneisses and potassic granitoids their formation age geochemical features and petrogenesis can provide important insights into understanding the crustal growth and reworking events involving in the evolution of the craton previous studies were mainly focused on geochronology of these neoarchean rock assemblages but lacked study on their petrogenesis and related tectonic regime in this study we carried out lithological geochronological and geochemical analyses for the representative ttg gneisses of the taishan complex u pb zircon age dating results reveal that the taishan ttg gneisses were mainly emplaced at 2718 2548 ma lu hf isotopic data show uniform positive zircon ehf t values 2 2 to 6 6 with hf model ages of 2949 2665 ma indicating that the ttg gneisses were mostly derived from juvenile crustal materials extracted from the depleted mantle during the mesoarchean to neoarchean the 18o values of zircons vary from 4 6 to 6 7 indicating that the source region was not significantly affected by the supracrustal materials geochemical data show that these ttg gneisses have high na2o 3 81 4 83 wt and ratio of zr sm 24 80 but show low mg 40 56 content of sr 292 508 ppm and ratios of sr y 35 6 66 6 and nb ta 10 1 15 9 these ttg gneisses were probably derived from partial melting of juvenile crustal materials under a relatively low pressure environment which were equilibrated with a certain amount of amphibole and plagioclase and minor garnet in the residue it indicates that a large scale magmatic underplating could be a most possible tectonic regime operating in this region during the neoarchean our new results combined with the available data from the adjacent areas reveal a significant crustal growth event in the eastern ncc at ca 2 94 2 70 ga which was followed by the pervasive crustal reworking event at ca 2 70 2 55 ga and regional metamorphism at ca 2 55 2 45 ga

mineralogical and geochemical constraints for the origin and evolution of magmas in sierra chichinautzin central mexican volcanic belt abstract the sierra chichinautzin volcanic field is a key area for understanding the origin of the controversial mexican volcanic belt mvb this is due to its recent volcanic activity which is located at the front of the central mvb although a direct genetic relationship between the volcanism of the mvb and the subduction of the cocos plate is accepted by most authors geological geophysical and geochemical key features of the mvb depart from those of typical magmatic arcs this fact has promoted a debate on the nature of the complex mantle beneath the mvb 1 slab induced convection in the mantle wedge that causes advection of asthenospheric mantle and 2 anomalously hot mantle related to the eastward channeling of plume related material this is a petrological study of the sierra chichinautzin volcanism using mineralogical data from selected representative samples together with their geochemistry to refine the existing models of magmatic processes the integrated petrological analyses have been used to define four main groups of rocks in sierra chichinautzin 1 ol mafic rocks 2 opx intermediate rocks 3 opx felsic rocks and 4 qtz intermediate felsic rocks moreover two different types of mantle derived primitive mafic magmas have been described based on their mineralogical and geochemical features 1 oib like mafic magmas presence of olivine plagioclase phenocrysts higher incompatible element contents with no hfse lile negative anomalies and 2 mafic magmas derived from a metasomatized mantle no plagioclase phenocrysts and more forsteritic olivine hfse lile negative anomalies we prove that the diversity of magmas basalts to dacites in sierra chichinautzin is un related to fractional crystallization processes from the mafic magmas rather magma mixing phenomena between mafic magmas and two different crustal derived felsic magma types opx dacite and qtz bt amp pl cpx opx dacite explain the textural mineralogical and geochemical signatures of most of the sierra chichinautzin rocks there is a strong genetic relationship between the mafic and felsic end members of each mixing trend the relatively anhydrous oib like mafic magmas promote partial melting of a basaltic lower crust under low water fugacity conditions generating opx bearing felsic rocks on the contrary hydrous mafic magmas derived from a metasomatized mantle promote basaltic crustal partial melting under high water fugacities yielding magmas with a complex hydrous phenocryst assemblage several important geodynamical implications for the origin of the mvb magmatism can be extracted from this petrological study 1 the existence of two different primitive mafic magmas implies the presence of an heterogeneous mantle source beneath sierra chichinautzin 2 present data do not offer enough evidence to constrain the nature of the metasomatizing fluids beneath sierra chichinautzin and 3 the negative nb anomaly of most of the sierra chichinautzin rocks andesites and dacites is due to magma mixing and therefore is not a signature of the role of fluids from the subducting cocos plate these geochemical features together with the geophysical and tectonic scenario of the sierra chichinautzin indicate that volcanism in this area is mainly related to the upwelling of an anomalously hot oib like mantle and cast further doubts on a significant role of the subducting cocos plate in the origin of this magmatism

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

neoarchean sanukitoids and associated rocks from the tengzhou pingyi intrusive complex north china craton insights into petrogenesis and crust mantle interactions abstract quartz monzodiorites granodiorites and monzogranites constitute the dominant lithological assemblage in the neoarchean tengzhou pingyi intrusive complex tpic in the southwestern part of western shandong province wsp in the eastern part of the north china craton ncc and mafic magmatic enclaves mmes are widely developed only in granodiorites of the tpic la icp ms zircon u pb isotopic dating reveals that the tpic was emplaced at 2 54 2 52 ga the 2 54 ga monzogranites display geochemical affinities with high k granites and they were likely derived from the partial melting of metamorphic greywackes at lower crustal levels the 2 54 2 52 ga quartz monzodiorites and granodiorites show significant geochemical similarities to the archean sanukitoids of the western karelian province of finland with higher mg 43 57 values cr 24 88 ppm sr 547 1154 ppm and ba 662 1961 ppm contents and k2o na2o 0 42 1 00 ratios therefore we define the quartz monzodiorites and granodiorites as neoarchean sanukitoids based on their lithological assemblages and geochemical characteristics these sanukitoids can be subdivided into sanukitoid quartz monzodiorites sqms and porphyritic sanukitoid granodiorites psgs the sqms exhibit higher positive hf t2 values from 1 9 to 3 6 with tdm values of 2 70 to 2 77 ga and are strongly enriched in light rare earth elements lrees and large ion lithophile elements liles implying a mantle source that was enriched by either fluids or melts the psgs have a wide range of hf t2 values from 3 4 to 3 8 suggesting that they were formed by crust mantle magma mixing this interpretation is also supported by the petrological textural and compositional disequilibrium of plagioclase phenocrysts from the psgs moreover the mmes and their host psgs have identical crystallization ages 2 54 ga similar mineral chemistries and zircon lu hf isotopic compositions indicating that the mmes are cumulates from the earlier host psgs magma petrogenetic studies and zircon lu hf isotopic features of these lithological assemblages indicate that the interval of 2 55 2 52 ga in the tpic represents an important period of crustal growth and crust mantle interactions considering the spatial temporal relationships and the petrogenesis of the diverse lithological assemblages we propose that the neoarchean tpic was most likely formed in an active continental margin setting

broadband seismic measurements of degassing activity associated with lava effusion at popocatepetl volcano mexico abstract from november 1999 through july 2000 a broadband seismic experiment was carried out at popocatepetl volcano to record seismic activity over a wide period range 0 04 100 s we present an overview of the seismicity recorded during this experiment and discuss results of analyses of long period lp and very long period vlp seismic signals recorded at stations nearest to the crater over a four month interval december 1999 march 2000 three families of lp signals types i ii and iii are identified based on distinctive waveform features observed periods shorter than 1 s periods longer than 15 s and within the period range 0 5 2 5 s type i lp events have impulsive first arrivals and exhibit a characteristic harmonic wave train with dominant periods in the 1 4 1 9 s range during the first 10 s of signal these events are also associated with a remarkable vlp wavelet with period near 30 s type ii lp events represent pairs of events occurring in rapid succession and whose signatures are superimposed these are typically marked by slowly emergent first arrivals and by a characteristic vlp wave train with dominant period near 30 s made of two successive wavelets whose shapes are quasi identical to those of the vlp wavelets associated with type i events type iii lp events represent the most energetic signals observed during our experiment these have an emergent first arrival and display a harmonic signature with dominant period near 1 1 s they are dominated by periods in the 0 25 0 35 s band and contain no significant energy at periods longer than 15 s hypocentral locations of the three types of lp events obtained from phase picks point to shallow seismic sources clustered at depths shallower than 2 km below the crater floor observed variations in volcanic eruptive activity correlate with defined lp families most of the observed seismicity consists of type i events that occur in association with 1 3 min long degassing bursts exhalations eruptive activity increased in intensity in february coinciding with an increasing occurrence of type ii lp events type iii events were first observed at the end of february and during march in coincidence with the formation of a new lava dome vulcanian eruptions occurred in april and may these events typically exhibit broadband signatures extending over the full period range of the sensors and lasting 30 80 min

the giant popocatepetl stirs in about 800 ad the mexican volcano popocatepetl exploded sending up several cubic kilometres of ash and pumice and burying the surrounding area since then it has been relatively quiet but every few decades it threatens to explode again which could this time be catastrophic as the area is now one of the most densely populated in mexico seismic activity and eruptions are now increasing with ash falls even 70 km away in mexico city

carbon dioxide transport in crustal magmatic systems abstract the volatile chemistry of juvenile volcanic glasses has suggested that shallow stored crustal magmas often suffer the open system addition of a carbon dioxide rich fluid from below probably from a mantle derived basaltic source carbon dioxide fluxing however the actual mechanism of such a fluid transport is poorly understood to constrain the volatile transport mechanism we formulated this phenomenon as a reactive transport process and clarified the fundamental characteristics of chemical exchange in the system the model assumes that a carbon dioxide rich fluid is introduced into a water rich rhyolitic magma column from below and ascends at a constant velocity whilst a volatile exchange takes place between the fluid and melt two types of exchange modes were examined one is the equilibrium mode where the volatile exchange is instantaneously achieved at all column depths the second is the diffusive mode where the volatile exchange is rate limited by diffusion in the equilibrium mode the extent of re equilibration of the entire column is controlled solely by the mass ratio of the integrated fluid to the melt in the diffusive mode the extent of re equilibration is controlled by the damkohler number a dimensionless parameter representing the ratio of the advection time to the diffusion time when the damkohler number for carbon dioxide exceeds 10 the diffusive exchange becomes indistinguishable from the equilibrium exchange both exchange modes produce a negative correlation between the concentrations of carbon dioxide and water in the melt which cannot be explained by conventional degassing models without significant crystallisation the fluid emitted from the column as a volcanic gas changes its composition from carbon dioxide rich to water rich and the emission rate decreases monotonically during fluxing the simulation enables us to constrain the mechanism of fluid transport for the melt inclusion data from the bishop tuff wallace et al 1999 anderson et al 2000 fluid velocity in this magma was estimated to be 10 6 10 7 m s the corresponding mechanism of fluid transport may include permeable flow with a permeability of 10 15 m 2 or a buoyant ascent of individual bubbles with a radius of 4 7 mm

granitoid evolution in the late archean wutai complex north china craton abstract two temporally discrete episodes of granite magmatism have been identified in the wutai complex of the north china craton based on shrimp u pb zircon dating of all major granitoid bodies in the area the older period is latest archean and ranges in age from 2560 to 2515 ma whereas the younger episode is paleoproterozoic with magmatism at 2170 and 2120 ma archean magmatism can be further separated into two pulses based on geochemistry and the association of the younger plutons with extensive coeval volcanic activity in the wutai complex the early phase of archean activity is represented by the lanzhishan ekou and portions of the chechang betai granites with ages ranging from 2560 to 2540 ma the lanzhishan granite is the only one at wutaishan that contains inherited zircon with a number of cores and individual grains giving ages in excess of 2700 ma these are relatively evolved granitoids interpreted to be derived from older basement of the eastern block of the north china craton the shifo guangmingshi the main grey phase of the wangjiahui granite and the major components of the chechang betai granite have ages of 2540 2515 ma and are coeval with felsic volcanism in the wutai complex which is a consequence of subduction and related arc magmatism the dawaliang granite and pink phase of the wangjiahui granite are considerably younger with paleoproterozoic ages of 2170 and 2120 ma respectively these are also evolved granitoids most likely derived by partial melting of the older basement although we have found no evidence of inheritance the petrogenesis of the granitoids reflects the precambrian amalgamation history of the north china craton indicating derivation along the western edge of the eastern block during development and amalgamation of the wutai arc and prior to collision with the western block of the craton 1 8 ga ago

diverse middle neoarchean granitoids and the delamination of thickened crust in the western shandong terrane north china craton abstract middle neoarchean granitoids contain crucial clues for understanding crust mantle interactions and further investigating the mechanism of the compositional transformation of the crust in the neoarchean in the western shandong terrane wst diverse neoarchean magmatism is recognized including 2 62 ga tonalitic gneisses 2 59 ga granodioritic gneisses and 2 61 2 58 ga monzogranitic gneisses which generally intruded into 2 70 ga tonalite trondhjemite granodiorite ttg gneisses the 2 70 ga ttg gneisses show high sio2 contents 70 17 72 89 wt but low mgo contents 0 58 0 69 wt and high la yb n ratios 30 48 and originated from partial melting of low k mafic rocks at lower crustal levels the 2 62 ga tonalitic gneisses exhibit low k2o na2o ratios 0 10 0 39 but high la yb n 7 60 12 7 and sr y 18 8 48 ratios and mg 47 57 with high ehf t2 values ranging from 2 7 to 7 4 these rocks were derived from dehydration melting of subducted oceanic slabs the 2 61 2 58 ga monzogranitic gneisses show high sio2 70 35 75 12 wt and k2o 4 35 5 81 wt contents but low mgo 0 20 0 77 wt contents with positive ehf t2 values 2 0 6 6 suggesting they were derived from partial melting of juvenile greywackes the 2 59 ga granodioritic gneisses exhibit high sio2 66 34 73 04 wt k2o 2 23 4 32 wt and mgo 1 39 3 51 wt contents with dispersed ehf t2 values 0 2 5 3 indicating a mixing process between crust and mantle sourced magmas the 2 59 ga granodioritic gneisses show consistent major and trace element compositions i e high mg lack of eu anomalies and uniform chondrite normalized ree patterns and accordingly their magmatic precursors were derived from partial melting of delaminated lower crustal materials the lithological assemblages in the wst became increasingly diverse between 2 71 2 58 ga changing from ttg gneisses to arc related rock series and then to delamination related granodioritic gneisses crust derived monzogranitic and trondhjemitic gneisses and mantle derived tholeiitic basalts and diorites by integrating these data with a reduction in the continental crustal thickness from 2 70 ga to 2 55 ga a subduction delamination model is proposed in which a long term lateral accretion beginning at 2 7 ga and persisting through 2 68 2 62 ga led to the delamination of the lower crust at 2 61 2 58 ga which may have further triggered the transition of the continental crust composition from basaltic to andesitic

characterization of the volcanic and hypabissal rocks of the paleoproterozoic iricoume group in the pitinga region and balbina lake area amazonian craton brazil petrographic distinguishing features and emplacement conditions abstract the iricoume group 1 897 to 1 875 ma is a widespread volcanic sequence in the amazonian craton south american platform in the pitinga region it consists of acidic to intermediate ignimbrites with cogenetic surge and ash fall deposits associated to hypabissal and effusive acidic rocks in the balbina lake area it consists of acidic to intermediate effusive rocks crystal rich ignimbrites and pumice rich ignimbrites detailed petrographic studies of phenocrysts and crystal fragments provided characterization and distinctive features of pyroclastic effusive and hypabissal rocks the phenocrysts of hypabissal rocks were affected by high temperature resorption and flow related physical fragmentation the effusive rocks can have a considerable content of crystal fragments due to intense dissolution and fragmentation of the phenocrysts and the pyroclastic rocks crystal fragments are generated mainly by decompression mechanic fragmentation dissolution and resorption of quartz and feldspar crystals may have acted at pressure conditions between 500 mpa and 100 mpa in response to rapid decompression in the magma ascent the viscosity values of 7 5 10 log pa s for anhydrous conditions decrease exponentially with estimated water addition the amount of phenocrysts and their intratelluric character in the volcanic units of iricoume group can be related to a magma chamber with high content of crystals we suggest that even with the addition of water in the magma the viscosities had a drastic increase with progressive crystal growth due to the viscosity dependence on the solid fraction and the deposits were generated by the extrusion of viscous lava and pyroclastic flows

downward water movement into frozen ground western arctic coast canada field studies carried out mainly since 1975 in permafrost areas of alaska canada china and the soviet union have been combined with the results of laboratory investigations to show that in summer water can move from the thawing active layer into the subjacent frozen active layer and under certain conditions even into the top of permafrost direct field evidence discussed includes data from drilling and neutron probe logging which show a summer increase in the ice content of already frozen ground summer heave of heavemeters with heave occurring in the frozen active layer and increase in the ice content of the subjacent frozen ground in both permafrost and non permafrost areas caused by snowmelt infiltration indirect field and laboratory evidence is also added to support the direct lines of evidence the conditions that favor the downward migration of water from thawed to frozen ground are examined in terms of thermally induced hydraulic gradients hydraulic conductivity content of unfrozen pore