volcanic ash hazard to aviation during the 2003 2004 eruptive activity of anatahan volcano commonwealth of the northern mariana islands abstract within the commonwealth of the northern mariana islands cnmi anatahan is one of nine active subaerial volcanoes that pose hazards to major air traffic routes from airborne volcanic ash the 2003 2004 eruptive activity of anatahan volcano affected the region s aviation operations for 3 days in may 2003 on the first day of the eruption 10 may 2003 two international flights from saipan to japan were cancelled and several flights implemented ash avoidance procedures on 13 may 2003 a high altitude flight through volcanic gas was reported with no perceptible damage to the aircraft toms and modis analysis of satellite data strongly suggests that no significant ash and only minor amounts of so 2 were involved in the incident consistent with crew observations on 23 may 2003 airport operations were disrupted when tropical cyclone winds dispersed ash to the south dusting saipan with light ashfall and causing flight cancellations there and at guam 320 km south of the volcano operational near real time monitoring of ash clouds produced by anatahan has been conducted since the first day of the eruption on 10 may 2003 by the washington volcanic ash advisory center vaac the vaac was among the first groups outside of the immediate area of the volcano to detect and report on the unexpected eruption of anatahan after being contacted about an unusual cloud by national weather service forecasters in guam at 1235 utc on 10 may 2003 the vaac analyzed goes 9 images confirming anatahan as the likely source of an ash cloud and estimating that the eruption began at about 0730 utc the vaac issued its first volcanic ash advisory for anatahan at 1300 utc on 10 may 2003 more than 5 h after the start of the eruption the delay reflecting the difficulty of detecting and confirming a surprise eruption at a remote volcano with no in situ real time geophysical monitoring the initial eruption plume reached 10 7 13 4 km 35 000 44 000 ft well into jet cruise altitudes thereafter the maximum plume height decreased and during the rest of the eruption usually did not exceed 5 km 17 000 ft which lessened the potential hazard to aircraft at higher cruise altitudes drifting ash clouds commonly extended hundreds of kilometers from the volcano occasionally as far west as the philippines over the course of the eruptive activity in 2003 2004 the vaac issued 323 advisories 168 with graphical depictions of ash clouds for anatahan serving as a reliable source of ash cloud information for aviation related meteorological offices and air carriers with a record of frequent eruptions in the cnmi continued satellite and in situ real time geophysical monitoring is needed at anatahan and other marianas volcanoes so that potential hazards to aviation from any future eruptive activity can be quickly and correctly assessed

mechanisms and timescales of generating eruptible rhyolitic magmas at yellowstone caldera from zircon and sanidine geochronology and geochemistry we constrain the physical nature of the magma reservoir and the mechanisms of rhyolite generation at yellowstone caldera via detailed characterization of zircon and sanidine crystals hosted in three rhyolites erupted during the c 170 70 ka central plateau member eruptive episode the most recent post caldera magmatism at yellowstone we present 238 u 230 th crystallization ages and trace element compositions of the interiors and surfaces i e unpolished rims of single zircon crystals from each rhyolite we compare these zircon data with 238 u 230 th crystallization ages of bulk sanidine separates coupled with chemical and isotopic data from single sanidine crystals zircon age and trace element data demonstrate that the magma reservoir that sourced the central plateau member rhyolites was long lived 150 250 kyr and genetically related to the preceding episode of magmatism which occurred c 256 ka the interiors of most zircons in each rhyolite were inherited from unerupted material related to older stages of central plateau member magmatism or the preceding late upper basin member magmatism i e are antecrysts conversely most zircon surfaces crystallized near the time of eruption from their host liquids i e are autocrystic the repeated recycling of zircon interiors from older stages of magmatism demonstrates that sequentially erupted central plateau member rhyolites are genetically related sanidine separates from each rhyolite yield 238 u 230 th crystallization ages at or near the eruption age of their host magmas coeval with the coexisting zircon surfaces but are younger than the coexisting zircon interiors chemical and isotopic data from single sanidine crystals demonstrate that the sanidines in each rhyolite are in equilibrium with their host melts which considered along with their near eruption crystallization ages suggests that nearly all central plateau member sanidines are autocrystic the paucity of antecrystic sanidine crystals relative to antecrystic zircons requires a model in which eruptible rhyolites are generated by extracting melt and zircons from a long lived mush of immobile crystal rich magma in this process the larger sanidine crystals remain trapped in the locked crystal network the extracted melts plus antecrystic zircon amalgamate into a liquid dominated i e eruptible magma body that is maintained as a physically distinct entity relative to the bulk of the long lived crystal mush zircon surfaces and sanidines in each rhyolite crystallize after melt extraction and amalgamation and their ages constrain the residence time of eruptible magmas at yellowstone residence times of the large volume rhyolites 40 70 km 3 are 1 kyr conservatively 6 kyr which suggests that large volumes of rhyolite can be generated rapidly by extracting melt from a crystal mush because the lifespan of the crystal mush that sourced the central plateau member rhyolites is two orders of magnitude longer than the residence time of eruptible magma

origin of crystal poor differentiated magmas insights from thermal gradient experiments crystal poor differentiated magmas are commonly erupted from shallow thermally zoned magma chambers in order to constrain the origin of these magmas we have experimentally investigated crystallization differentiation and crystal melt separation in presence of a thermal gradient experiments have been designed taking advantage of the innate temperature gradient of the piston cylinder apparatus and carried out on a phonolitic system at 0 3 gpa and temperature ranging from 1 050 to 800 c crystallization degree and melt composition in experimental products vary as a function of the temperature gradient in particular melt composition differentiates from tephri phonolite starting material to phonolite moving from the hotter glassy zone t 1 050 c towards the cooler heterogeneously crystallized zone t 900 c of the charge the heterogeneously crystallized zone is made up of 1 a crystal rich mushy region crystallinity 30 vol 2 a rigid crystal framework crystallinity 80 vol and 3 glassy belts of phonolitic glass at the top thermal gradient experiments picture crystallization differentiation and crystal melt separation processes occurring in a thermally zoned environment and reveal that relatively large volumes of crystal poor melt glassy belts can originate as a consequence of the instability and collapse of the rigid crystal framework analogously in thermally zoned magma chambers the development and collapse of a solidification front may represent the controlling mechanism originating large volumes of crystal poor differentiated magmas

mineralogy and geochemistry of microgranular enclaves in palaeoproterozoic malanjkhand granitoids central india evidence of magma mixing mingling and chemical equilibration palaeoproterozoic ca 2 480 ma felsic magmatism of malanjkhand region of central indian precambrian shield referred to as malanjkhand granitoids mg contain xenoliths of country rocks and mesocratic to melanocratic fine grained porphyritic microgranular enclaves me the shape of me is spheroidal ellipsoidal discoidal elongated and lenticular varying in size from a few centimeters to about 2 m across the contact of me with the host mg is commonly sharp crenulate and occasionally diffuse which we attribute to the undercooling and disaggregation of me globules within the cooler host mg the me as well as mg show hypidiomorphic texture with common mineral hbl bt kfs pl qtz assemblage but differ in modal proportions the variation in minerals composition presence of apatite needles elongated biotites resorbed plagiclase ocellar quartz and other mafic felsic xenocrysts strongly oppose the restite and cognate origins of me compositions of plagioclases an3 an29 amphiboles mg mg fe2 0 55 0 69 and biotites mg mg fe2 0 46 0 60 of me are slightly distinct or similar to those of mg which suggest partial to complete equilibration during mafic felsic magma interactions al in amphibole estimates the mg pluton emplacement at ca 3 4 0 5 kbar and therefore magma mixing and mingling must have occurred at or below this level the hbox fe rightleftharpoons hbox mg substitution in biotites of me and mg largely suggests subduction related calc alkaline metaluminous i type nature of felsic melts most major and trace elements against sio2 produce near linear variation trends for me and mg probably generated by the mixing of mafic and felsic magmas in various proportions trace including rare earth elements patterns of me mg pairs however show partial to complete equilibration most likely governed by different degrees of elemental diffusion the available evidence supports the model of me origin that coeval mafic enclave and felsic mg magmas produced a hybrid me magma layer which injected into cooler partly crystalline mg and dispersed mingled and undercooled as me globules in a convectively dynamic magma chamber

episodic intrusion internal differentiation and hydrothermal alteration of the miocene tatoosh intrusive suite south of mount rainier washington the miocene tatoosh intrusive suite south of mount rainier is composed of three broadly granodioritic plutons that are manifestations of ancestral cascades arc magmatism tatoosh intrusive suite plutons have individually diagnostic characteristics including texture mineralogy and geochemistry and apparently lack internal contacts new ion microprobe u pb zircon ages indicate crystallization of the stevens pluton ca 19 2 ma reflection pyramid pluton ca 18 5 ma and nisqually pluton ca 17 5 ma the stevens pluton includes rare statistically distinct ca 20 1 ma zircon antecrysts wide ranging zircon rare earth element ree hf u and th concentrations suggest late crystallization from variably evolved residual liquids zircon eu eu hf covariation is distinct for each of the reflection pyramid nisqually and stevens plutons although most tatoosh intrusive suite rocks have been affected by weak hydrothermal alteration and sparse mineralized veins cut some of these rocks significant base or precious metal mineralization is absent at the time of shallow emplacement each of these magma bodies was largely homogeneous in bulk composition and petrographic features but prior to final solidification each of the tatoosh intrusive suite plutons developed internal compositional variation geochemical and petrographic trends within each pluton are most consistent with differential loss of residual melt possibly represented by late aplite dikes or erupted as rhyolite from crystal rich magma crystal rich magma that formed each pluton evidently accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth assembled by episodic intrusion the tatoosh intrusive suite may be representative of midsized composite plutonic complexes beneath arc volcanoes

extraction of crystal poor rhyolite from a hornblende bearing intermediate mush a case study of the caldera forming matahina eruption okataina volcanic complex the matahina ignimbrite 160 km3 rhyolite magma 330 ka was deposited during a caldera forming eruption from the okataina volcanic centre taupo volcanic zone tvz new zealand juvenile clasts are divided into three groups group 1 the dominant crystal poor rhyolite type group 2 a minor coarse grained mingled mixed intermediate type and group 3 a rare fine grained basalt the ignimbrite consists of the group 1 type and is divided into three members a lower and middle member which is high silica crystal poor 10 vol rhyolite and the upper member which is low silica and slightly more crystal rich up to 21 vol cognate crystal rich up to 50 vol basalt to intermediate pumice occurs on top of lag breccias and within lithic rich pyroclastic density current deposits along the caldera margin groups 2 and 3 several lines of evidence indicate that the intermediate clasts represent the cumulate complement to the melt rich rhyolite 1 continuity in the compositions of plagioclase orthopyroxene hornblende and oxides and normal zoning of individual phenocrysts 2 the silicic glass from the intermediate magma interstitial melt overlaps compositionally with the bulk rock rhyolite and glass 3 high zr and a slight positive eu anomaly in the intermediate magma relative to quenched enclaves from other intermediate tvz eruptions indicates zircon and plagioclase accumulation respectively 4 an increase in the cl contents in glass from the least evolved to most evolved is consistent with the concentration of volatiles during magma evolution most of the compositional variations in the low to high silica rhyolites can be accounted for by continued rayleigh fractionation up to 15 following melt extraction from the underlying mush under varying fo2 fh2o conditions to form a slightly compositionally zoned rhyolitic cap this link to the varying fo2 fh2o conditions is evidenced by the strong correlation between key geochemical parameters e g dy y that qualitatively reflect fh2o conditions presence or absence of hornblende biotite and fo2 estimated from fe ti oxide equilibrium magma mingling mixing between the basalt andesite and the main slightly compositionally zoned rhyolitic magma occurred during caldera collapse modifying the least evolved rhyolite at the lower portion of the reservoir and effectively destroying any pre eruptive gradients

young cumulate complex beneath veniaminof caldera aleutian arc dated by zircon in erupted plutonic blocks mount veniaminof volcano alaska peninsula provides an opportunity to relate quaternary volcanic rocks to a coeval intrusive complex veniaminof erupted tholeiitic basalt through dacite in the past 260 k y gabbro diorite and miarolitic granodiorite blocks ejected 3700 14 c yr b p in the most recent caldera forming eruption are fragments of a shallow intrusive complex of cumulate mush and segregated vapor saturated residual melts sensitive high resolution ion microprobe shrimp analyses define 238 u 230 th isochron ages of 17 6 2 7 ka 5 11 10 ka and 10 2 4 0 ka 2 for zircon in two granodiorites and a diorite respectively sparse zircons from two gabbros give 238 u 230 th model ages of 36 8 ka and 26 7 ka zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid although historic eruptions have been weakly explosive strombolian fountaining and small lava effusions the young ages of plutonic blocks as well as late holocene dacite pumice are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions

evolution of peralkaline silicic complexes lessons from the extrusive rocks abstract young 4 wt and the rhyolitic varieties evolve to temperatures 4 10 6 pa s result in rapid growth of volcanic edifices highly efficient crystal melt separation and the ubiquitous development of compositionally zoned caps to reservoirs no peralkaline equivalent of the monotonous intermediate ignimbrites found in some calc alkaline systems has yet been found

quantifying magma segregation in dykes abstract the dynamics of magma flow is highly affected by the presence of a crystalline load during magma ascent it has been demonstrated that crystal melt segregation constitutes a viable mechanism for magmatic differentiation moreover crystal melt segregation during magma transport has important implications not only in terms of magma rheology but also in terms of differentiation of the continental crust however the influences of the crystal volume percentage of their geometry their size and their density on crystal melt segregation are still not well constrained to address these issues we performed a parametric study using 2d direct numerical simulations which model the ascension of a crystal bearing magma in a vertical dyke using these models we have characterised the amount of segregation as a function of different physical properties including the density contrast between crystals and the melt phase the size of the crystals ac and their aspect ratio r results show that small values of r do not affect the segregation in this case the amount of segregation depends upon four parameters segregation is highest when and ac are large and lowest for large pressure gradient pd and or large values of dyke width wd these four parameters can be combined into a single one the snumber which can be used to quantify the amount of segregation occurring during magma ascent based on systematic numerical modelling and dimensional analysis we provide a first order scaling law which allows quantification of the segregation for an arbitrary snumber and encompassing a wide range of typical parameters encountered in terrestrial magmatic systems although developed in a simplified system this study has strong implications regarding our understanding of crystal segregation processes during magma transport our first order scaling law allows to immediately determine the amount of crystal melt segregation occurring in any given magmatic dyke system

cannibalization of an amphibole rich andesitic progenitor induced by caldera collapse during the matahina eruption evidence from amphibole compositions the diverse range of calcic amphibole compositions in eruptive products from the ca 330 ka matahina eruption ca 160 km3 rhyolitic magma of the okataina volcanic complex taupo volcanic zone including crystal rich basalt dacite pumice from post collapse deposits reveals several pre and syn eruption magmatic processes 1 amphibole phenocrysts in the basaltic andesite and andesite crystallized at the highest pressures and temperatures p to 0 6 0 06 gpa and t to 950 c equivalent to mid crustal depths 13 22 km inter and intra crystalline compositions range from ti magnesiohastingsite ti tschermakite tschermakite magnesiohornblende and some display gradual decreases in t from core to rim both consistent with magma differentiation by cooling at depth 2 the largest amphibole crystals from the basaltic andesite to andesite display several core to rim increases in t to 70 c indicating that new hotter magma periodically fluxed the crystal mush 3 the dominant population of rhyolite amphibole is small and bladed magnesiohornblende and crystallized at low p t conditions p 0 3 gpa t 765 c equivalent to the eruptive p t conditions dacitic and low silica rhyolitic amphibole tschermakite magnesiohornblende form two distinct populations which nucleated at two different t high 820 c and low 750 c these compositional variations governed primarily by differences in t conditions during crystal growth record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive temperature therefore the diversity in amphibole compositions can be reconciled as an exchange of crystals liquid between the basaltic andesite to dacite from the mid crust and rhyolite from the upper crust which quenched against one another modifying the dacite to low silica rhyolite compositions as the eruption progressed

generation of alkalic flood rhyolite insights from evolution of the paisano volcano davis mountains trans pecos texas u s a abstract extensive silicic lava flood rhyolite constitutes part of the mid tertiary paisano volcano shield this lava borderline peralkaline quartz trachyte rhyolite forms part of a sequence of peralkaline rhyolite quartz trachyte trachyte produced as eruptions tapped deeper portions of underlying plutons flood rhyolite lava extended as far as 20 km from dikes of the eruptive center of the volcano creating flows as thick as 100 m flood rhyolite was nearly aphyric to sparsely porphyritic relatively hot 911 c and erupted over a topographically subdued region forming two lobes that extended north northeast and southeast from a highland of previously erupted rhyolite domes flood rhyolite extrusion followed eruption of a local ignimbrite which filled valleys flattening topography successive silicic flows off lapped each other away from the volcano flood rhyolite eruption was followed by a second eruptive sequence that contained coarsely porphyritic mafic trachyte lava silicic lava was most likely generated by filter pressing from mush zones of mafic plutons glomeroporphyritic trachytes of the volcano are mixed rocks silicic lava incompletely segregated from entrained fragments of mafic plutons continued evolution of quartz trachyte low silica rhyolite to peralkaline rhyolite was dominated by fractionation of alkali feldspar

silicic magma differentiation in ascent conduits experimental constraints abstract crystallization of water bearing silicic magmas in a dynamic thermal boundary layer is reproduced experimentally by using the intrinsic thermal gradient of piston cylinder assemblies the standard agv2 andesite under water undersaturated conditions is set to crystallize in a dynamic thermal gradient of about 35 c mm in 10 mm length capsules in the hotter area of the capsule the temperature is initially set at 1200 c and decreases by programmed cooling at two distinct rates of 0 6 and 9 6 c h experiments are conducted in horizontally arranged assemblies in a piston cylinder apparatus to avoid any effect of gravity settling and compaction of crystals in long duration runs the results are conclusive about the effect of water rich fluids that are expelled out the crystal rich zone mush where water saturation is reached by second boiling in the interstitial liquid expelled fluids migrate to the magma ahead of the solidification front contributing to a progressive enrichment in the fluxed components sio 2 k 2 o and h 2 o the composition of water rich fluids is modelled by mass balance using the chemical composition of glasses quenched melt the results are the basis for a model of granite magma differentiation in thermally zoned conduits with application of in situ crystallization equations the intriguing textural and compositional features of the typical autoliths accompanying granodiorite tonalite batholiths can be explained following the results of this study by critical phenomena leading to splitting of an initially homogeneous magma into two magma systems with sharp boundaries magma splitting in thermal boundary layers formed at the margins of ascent conduits may operate for several km distances during magma transport from deep sources at the lower crust or upper mantle accordingly conduits may work as chromatographic columns contributing to increase the silica content of ascending magmas and at the same time leave behind residual mushes that eventually are dragged as enclaves or autoliths

entablature fracture types and mechanisms entablature is the term used to describe zones or tiers of irregular jointing in basaltic lava flows it is thought to form when water from rivers dammed by the lava inundates the lava flow surface and during lava meltwater interaction in subglacial settings a number of different fracture types are described in entablature outcrops from the burfell lava and older lava flows in jorsardalur southwest iceland these are striae bearing column bounding fractures and pseudopillow fracture systems that themselves consist of two different fracture types master fractures with dimpled surface textures and subsidiary fractures with curved striae the interaction of pseudopillow fracture systems and columnar jointing in the entablature produces the chevron fracture patterns that are commonly observed in entablature cube jointing is a more densely fractured version of entablature which likely forms when more coolant enters the hot lava the entablature tiers display closely spaced striae and dendritic crystal shapes which indicate rapid cooling master fracture surfaces show a thin band with an evolved composition at the fracture surface mineral textures in this band also show evidence of quenching of this material this is interpreted as gas driven filter pressing of late stage residual melt that is drawn into an area of low pressure immediately preceding or during master fracture formation by ductile extensional fracture of hot partially crystallised lava this melt is then quenched by an influx of water and or steam when the master fracture fully opens our findings suggest that master fractures are the main conduit for coolant entering the lava flow during entablature formation

distribution and compositions of magmatic inclusions in the mount helen dome lassen volcanic center california insights into magma chamber processes abstract variations in spatial abundances compositions and textures of undercooled magmatic inclusions were determined in a glaciated pleistocene lava dome mt helen 0 6 km3 at the lassen volcanic center lvc southernmost cascades spatial variations were determined by point counting at 86 locations separated by 100 m on the dome major and trace element compositions of host rocks and inclusions at 12 locations along the flow length of the dome were obtained important results include the following 1 inclusion abundances range from 3 19 vol with the highest values generally located along the little eroded northwestern margin and flow front of the dome 2 host rock compositions are markedly uniform across the dome 65 4 0 4 wt sio2 indicating that the degree of inclusion disaggregation was uniform despite large spatial variations in inclusion abundances 3 inclusion sizes range from a maximum of 1 m across to mm sized crystal clots of phenocrysts plus adhering ca rich plagioclase microphenocrysts 4 inclusions have variable macroscopic textures indicating that partial undercooling both prior to and following entrapment in cooler dacitic host magma were important processes 5 inclusions are variably fractionated magmas with large variations in ni 79 11 ppm and cr 87 7 ppm contents that are lower than presumed mantle derived melts furthermore large ranges in incompatible trace elements indicate that inclusion compositions also reflect deep processes involving either melting of variable mantle source rocks or assimilation fractional crystallization 6 inclusions are variably mixed magmas 56 61 wt sio2 that contain up to 50 host dacitic magma 7 correlations between ni and cr contents in hosts and inclusions from individual outcrops indicate that the effect of inclusion disaggregation and magma mingling on host dacitic magma was local e g these features are interpreted to reflect protracted recharge of diverse composition mafic magmas into the base of a shallow magma reservoir containing a lower layer of compositionally evolving hybrid mafic magma and an upper layer of rhyodacitic magma complex interactions among magmas in the upper and lower layers resulted in formation of mafic inclusions by both pre and post entrapment undercooling mechanisms followed by buoyant rise and accumulation near the roof of the reservoir the main stage of inclusion disaggregation probably occurred in the conduit during eruptive ascent and largely ceased upon surface eruption we infer that endogenous growth concentrated inclusions along the margins and top of the dome as more inclusion poor dacite was progressively tapped from deeper regions of the reservoir

magmatic consequences of the transition from orthogonal to oblique subduction in panama the closure of the central american seaway is linked with tectonic and magmatic processes that have controlled the evolution of the isthmus of panama we focus on the terminal stages of arc activity in the central panama region and present new geochemical data from 9 ma explosive silicic volcanism preserved in three syngenetic tuff beds from the gatun formation the magmatic evolution of the gatun formation is controlled by a series of magma mushes where pyroxene is the dominant early forming mafic mineral with amphibole appearing only relatively late in the fractionation sequence our data shows gatun lavas exhibit a strong subduction signature consistent with plate reconstruction models showing arc normal subduction from costa rica to panama pre 8 5 ma however large ion lithophile elements are depleted in the gatun formation in comparison to other regional suites indicative of a lower flux of subduction fluid to the gatun formation mantle source which is explained by a shift towards magma generation by decompression following the collision of the arc with south america oblique subduction commencing 8 5 ma resulted in the shutdown of normal arc activity throughout panama we interpret subsequent regional quaternary adakitic volcanism as a response to this oblique subduction the now more refractory mantle wedge required greater fluid flux in order to melt the resultant volatile rich melts were more prone to deep fractionation of amphibole and garnet cumulates forming adakites deep fractionation was potentially enhanced by changing stress regimes on the upper plate caused by oblique subduction this article is protected by copyright all rights reserved

stratigraphic evolution of the nakdong river valley in response to late quaternary sea level changes abstract sequence analysis using borehole samples and high resolution seismic data in the nakdong river valley yielded a good example of a river valley sequence influenced by the complex interplay between the bottom morphology of the steep paleo valley varying sediment supply and holocene sea level change associated with coastal currents the late quaternary stratigraphic sequence in the nakdong river estuary approximately 60 70 m thick forms a high frequency sequence consisting of a set of lowstand transgressive and highstand systems tracts that corresponds to a fifth order 20 ka sea level cycle four main depositional systems including ten sedimentary facies constitute these systems tracts fluvial estuarine coastal shoreface and deltaic the lowstand systems tract lst consisting of gravelly sand forms a fluvial depositional system unit i which fills the thalweg of river valley mainly developed before 12 ka the transgressive systems tract tst showing a succession of retrograding or backstepping depositional arrangements can be divided into two depositional systems unit ii and iii the riverine sediments were trapped within the paleo estuary forming an estuarine depositional system unit ii developed between 12 and 7 ka as the transgression continued the coarse sediments were deposited and redistributed by coastal processes resulting in coastal shoreface depositional system unit iii it is characterized by an isolated sand body and thin sand veneer the hst is composed of deltaic depositional system including delta plain delta front and prodelta unit iv during the delta progradation most coarse grained sands derived from the river were deposited in the lower delta plain and delta front forming sand bars and shoals less than 15 m deep the remaining fine grained sediments were transported further offshore in a suspension mode and deposited in the inner shelf off the present river mouth forming a subaqueous prodelta radiocarbon and optically stimulated luminescence osl dating suggest that the holocene deltaic system was initiated by aggradational and progradational stacking patterns at approximately 8 ka during the last stage of decelerated sea level rise and was then followed by a prograding clinoform after the highest sea level at approximately 7 ka

water transfer during magma mixing events insights into crystal mush rejuvenation and melt extraction processes many plutons preserve evidence of magma mixing between hydrous mafic magmas and resident felsic crystal rich mushes to investigate water transfer processes in such systems following thermal equilibration we conducted 24 h experiments to establish the petrological evolution of a water undersaturated 4 wt h 2 o in the interstitial melt quartz bearing dacite crystal mush 0 5 0 8 in crystal fraction intruded by a water saturated 6 wt h 2 o initially crystal free andesite magma at 950 c and 4 kbar 12 km depth our results show isothermal undercooling resulting from a change in liquidus temperatures of the interacting magmas due to their changing water content specifically mafic samples dramatically crystallize during water escape into the felsic end members and consequent increase in liquidus temperature conversely the addition of water to the felsic mush reduces the liquidus temperature leading to an increase in melt fraction the experiments provide insights into how volatiles contribute to crystal mush rejuvenation i e increase of melt fraction however h 2 o diffusion alone is not sufficient to promote melt extraction from short and long lived mushes in the earth s crust

incipient melt segregation as preserved in subaqueous pyroclasts melt segregation is the extraction of residual melt from the rigid but permeable network of growing crystals with which it co evolved it is recognized as an effective mechanism of igneous differentiation that acts over many geologic time and length scales here we present evidence for rapid melt segregation in subaqueous basaltic pyroclasts segregation produced intravesicular extrusions i e partly hollow balloons of glass penetrating the walls of earlier formed vesicles set in a microcrystalline groundmass the segregation process can be described as a form of gas filter pressing where microlite crystallization produced local gradients in volatile supersaturation and vapor pressure that drove melt extrusion into adjacent vesicles unlike in previously described segregation features the structures presented here are shown to have formed after the extruded melt became enriched in fast diffusing h2o but before it became measurably enriched or depleted in slower diffusing major elements we show that melt segregation of this embryonic type must occur in seconds rather than in the days to centuries required for similar processes to occur in lava flows or magma chambers and that it occurs within the short time scales that characterize explosive fragmentation of basaltic melt

segregation vesicles cylinders and sheets in vapor differentiated pillow lavas examples from tore madeira rise and chile triple junction abstract we conducted a detailed field and laboratory study of internal segregation structures of two hand size pillow lavas samples they were dredged respectively on the josephine seamount tore madeira rise tmr and on a small quaternary volcanic edifice located on the continental edge of the trench close to the chile triple junction ctj both pillows display a combination of four types of segregation structures spherical vesicles pipe vesicles vesicle cylinders and vesicle sheets observed so far only within subaerial basalt flows typically 2 10 m thick in particular the samples offer a remarkable exposure of the transition between pipe vesicles and cylinders we show that the vesicle sheets are not generated by the same mechanism in both occurrences they do not seem to be connected to cylinders in the ctj pillow as they are in the tmr pillow the two pillows are geochemically distinct the tmr being alkaline and the ctj calc alkaline two types of internal differentiation are proposed the first one implies the extraction of the residual liquid from the host lava and transport towards the segregation structures whereas the other one results from in situ crystallization within one given structure in the latter case glass composition is highly dependant on the nature of the neighbouring crystallizing minerals the degree of crystallization required to produce a crystal framework strong enough for generating the segregation structures seems to be lower in pillows ca 25 crystallization than in vapor differentiated basaltic lava flows 35 crystallization

zircon textures and composition refractory recorders of magmatic volatile evolution zircon textures and composition have been used to infer magmatic processes including closed system fractional crystallization magma mixing or replenishment and country rock assimilation here we propose that zircon textures and composition may also be refractory recorders of magmatic volatile evolution we present field whole rock chemical textural mineral chemical and u pb age data from evolved fine to coarse grained granite intrusions on melville peninsula nunavut canada zircon forms two main populations in these granites type 1 and type 2 zircon type 1 zircon is present in all samples but predominant in fine grained granite crystals are euhedral and inclusion rich and show periodic fine scale oscillatory zoning comparatively low concentrations of u 2 200 ppm and hf 1 6 wt high zr hf 40 62 and pervasive alteration type 2 zircon is predominant in coarse grained granite crystals form overgrowths on type 1 zircon and individual crystals they are subhedral and inclusion poor and show weak irregular large scale oscillatory zoning high u up to 7 250 ppm and hf 1 5 2 0 wt low zr hf 37 44 and only local alteration compatible trace element concentrations and zr hf change sharply across the boundary of type 1 to type 2 zircon 207pb 206pb ages preclude a significant hiatus between crystallization of the two types we argue against magmatic versus hydrothermal crystallization country rock assimilation or magma mixing as causes for the crystallization of type 1 and type 2 zircon we propose instead that type 1 zircon formed from volatile undersaturated magmas and that type 2 zircon formed from volatile saturated magmas magmas fractionated by volatile driven filter pressing into crystal rich mush and crystal poor magma crystal rich mush with abundant type 1 zircon crystallized to fine grained granite volatile rich magma crystallized to type 2 zircon and coarse grained granite while type 1 zircon was pervasively altered by exsolving magmatic volatiles type 2 zircon was only locally affected by subsolidus hydrothermal alteration

pore pressure embrittlement in a volcanic edifice the failure mode of porous rock in compression dilatant or compactant is largely governed by the overlying lithostatic pressure and the pressure of pore fluids within the rock wong solid earth 102 3009 3025 1997 both of which are subject to change in space and time within a volcanic edifice while lithostatic pressure will tend to increase monotonously with depth due to the progressive accumulation of erupted products pore pressures are prone to fluctuations during periods of volcanic unrest for example an increase in pore fluid pressure can result in rock fracture even at depths where the lithostatic pressure would otherwise preclude such dilatant behaviour a process termed pore fluid induced embrittlement we explore this phenomenon through a series of targeted triaxial experiments on typical edifice forming andesites from volcan de colima mexico we first show that increasing pore pressure over a range of timescales on the order of 1 min to 1 day can culminate in brittle failure of otherwise intact rock irrespective of the pore pressure increase rate we record comparable accelerations in acoustic emission and strain prior to macroscopic failure we further show that oscillating pore fluid pressures can cause iterative and cumulative damage ultimately resulting in brittle failure under relatively low effective mean stress conditions we find that macroscopic failure occurs once a critical threshold of damage is surpassed suggesting that only small increases in pore pressure may be necessary to trigger failure in previously damaged rocks finally we observe that inelastic compaction of volcanic rock as we may expect in much of the deep edifice can be overprinted by shear fractures due to this mechanism of embrittlement pore fluid induced embrittlement of edifice rock during volcanic unrest is anticipated to be highest closer to the conduit and as a result may assist in the development of a fractured halo zone surrounding the conduit potentially explaining commonly observed near conduit outgassing at many active volcanoes further rock embrittlement at depth may create transient outgassing pathways by linking fracture networks near the edifice to larger scale regional fault systems our experimental results affirm that pore pressure fluctuations associated with volcanic unrest may play a crucial role in dictating the evolution of a volcanic system

pre eruptive conditions of the huerto andesite fish canyon system san juan volcanic field colorado influence of volatiles c o h s on phase equilibria and mineral composition crystallization experiments at 400 mpa oxidized condition logfo2 nno 1 where nno is nickel nickel oxide buffer and over a range of temperatures 850 950 c and fluid composition xh2oin 0 3 1 have been carried out to constrain the storage conditions of the sulphur rich magma of the huerto andesite an anhydrite pyrrhotite and s rich apatite bearing post fish canyon tuff mafic lava the results are used to evaluate the role of fluids released from the crystallization of magmas such as the huerto andesite on the remobilization of the largely crystallized dacitic fish canyon magma body experiments were performed using the natural andesitic bulk composition with and without added sulphur the presence of sulphur slightly affects the phase equilibria by changing the phase proportions stability fields of plagioclase pyroxenes and ilmenite and also affects the plagioclase composition phase equilibria and mineral composition data indicate that the magma may have contained 4 5 wt water in the melt and that the pre eruptive temperature was 875 25 c assuming that the magma was in equilibrium with a fluid phase the co2 concentration of the melt is estimated to be in the range 2000 4000 ppm at 400 mpa before eruption the andesite had an oxidation state very close to or slightly within the co stability field of anhydrite pyrrhotite at nno 1 1 at these conditions the sulphur content in the melt is 500 ppm assuming open system degassing resulting from continuing crystallization at depth most of the co2 dissolved in the andesitic melt should be released after the crystallization of 10 vol of the magma corresponding to a cooling from 875 to 825 850 c thus the fluids released owing to crystallization processes should be mainly composed of water at temperatures below 825 c

on the mechanical effects of poroelastic crystal mush in classical magma chamber models author posting american geophysical union 2018 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research solid earth 123 11 2018 9376 9406 doi 10 1029 2018jb015985

pc based acquisition and processing of high resolution marine seismic data we have improved the quality of high resolution marine seismic data using a simple pc based acquisition and processing system the system consists of a pc an a d converter and a magneto optical disk drive the system has been designed to acquire single channel data at up to 60 000 samples per second and to perform data processing of seismic data by a simple procedure test surveys have been carried out off pohang southern east sea of korea the seismic systems used for the test were an air gun and a 3 5 khz sub bottom profiling system spectral characteristics of the sources were analyzed simple digital signal processes which include gain recovery deconvolution band pass filter and swell filter were performed the quality of seismic sections produced by the system is greatly enhanced in comparison to analog sections the pc based system for acquisition and processing of high resolution marine seismic data is economical and versatile

variable regimes in magma systems documented in plagioclase zoning patterns el misti stratovolcano and andahua monogenetic cones abstract magmas may experience closed or open system evolution during their ascent and evolution in the upper crust chemically closed system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent on the other hand newly injected magmas can mix with the residing magma and hybridize plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km3 el misti stratovolcano 5822 m and a field of relatively small monogenetic andahua scoria cones both in southern peru in particular fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed vs open system processes large and correlated anorthite fe contrasts at resorption zones identify compositional mixing related to recharge subhorizontal an fe variations record thermal or water induced effects el misti volcano a large but still relatively youthful stratovolcano is dominated during most of its approx 120 ka history by closed system effects during its evolution recharge and mixing mostly occurred cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts a small volume erupted at el misti however is affected by strong open system behaviour as reflected in strongly correlated variations in an and fe contents of plagioclase compositions its crystal population is texturally and chemically very similar to lavas of the monogenetic cinder cones of the andahua volcanic field for the el misti magma reservoir resorption events caused by thermal effects or magma ascent are more frequent in comparison to the actual eruption frequencies many cycles of closed system evolution heat transfer and decompression therefore occurred without resulting in an eruption plagioclase zonations from lavas of the monogenetic andahua field indicate that any mafic batch that intruded the much smaller reservoirs immediately triggered an eruption by contrast a large magma system that feeds el misti volcano can easily accommodate a new batch without causing eruption

residence resorption and recycling of zircons in devils kitchen rhyolite coso volcanic field california zircons from the devils kitchen rhyolite in the pleistocene coso volcanic field california have been analyzed by in situ pb u ion microprobe shrimp rg and by detailed cathodoluminescence imaging the zircons yield common pb corrected and disequilibrium corrected pb u ages that predate a previously reported k ar sanidine age by up to 200 kyr and the range of ages exhibited by the zircons is also approximately 200 kyr cathodoluminescence imaging indicates that zircons formed in contrasting environments most zircons are euhedral and a majority of the zircons are weakly zoned but many also have anhedral embayed cores with euhedral overgrowths and multiple internal surfaces that are truncated by later crystal zones concentrations of u and th vary by two orders of magnitude within the zircon population and by 10 20 times between zones within some zircon crystals indicating that zircons were transferred between contrasting chemical environments a zircon saturation temperature of 750 c overlaps within error a previously reported phenocryst equilibration temperature of 740 25 c textures in zircons indicative of repeated dissolution and subsequent regrowth are probably caused by punctuated heating by mafic magma input into rhyolite the overall span of ages and large variation in u and th concentrations combined with calculated zircon saturation temperatures and resorption times are most compatible with crystallization in magma bodies that were emplaced piecemeal in the crust at coso over 200 kyr prior to eruption and that were periodically rejuvenated or melted by subsequent basaltic injections

second boiling effects on the al content of hornblende rims from an exhumed cretaceous arc pluton stewart island new zealand high resolution transects across amphiboles from the layered mafic felsic halfmoon pluton suggest that amphibole compositions in the outermost 40 m of grains analyzed fluctuate in response to p t p h2o conditions during the final stages of pluton emplacement and crystallization detailed transects 10 15 m point spacing across 15 crystals from six representative rock types within the compositionally stratified halfmoon pluton revealed that in almost all crystals altot increased over at least the outer 40 m coupled with a decrease in temperature and ti content p t estimates for halfmoon pluton amphiboles using the al in hornblende geobarometer and the hornblende plagioclase geothermometer are consistent with upper crustal emplacement depths amphibole altot within the core and midsections is controlled by the temperature dependent edenite and ti tschermak exchanges and is interpreted to reflect the influence of repeated injections of hot mafic magma into the magma reservoir during crystallization increases in altot at all amphibole rims arise from a notable increase in vial and small increases in ival indicating that unlike the core and midsections that exhibit a strong temperature dependence rim altot is a function of the pressure dependent al tschermak exchange pressure estimates calculated for amphibole rims record a rim ward increase in p equivalent to between 0 2 and 1 9 kbar which corresponds to increases in depth of crystallization of 0 7 and 6 7 km respectively this observed increase in pressure at the rims of all amphibole grains analyzed is not consistent with increasing lithostatic pressure as a result of pluton burial but to an increase in p h2o in response to second boiling a common process during late stage crystallization of h2o rich intermediate silicic magmas the presence of water escape structures throughout the pluton and the predominance of biotite rich mafic enclaves that commonly contain amygdules toward the inferred top of the magmatic stratigraphy provide further evidence to support the process of second boiling these results indicate that caution is required in choosing suitable analytical sites when applying the al in hornblende geobarometer particularly within plutonic bodies where there is evidence for vesiculation and water escape structures as the outer rim compositions of amphiboles may not be a function of lithostatic pressure alone

cordieritite and leucogranite formation during emplacement of highly peraluminous magma the el pilon granite complex sierras pampeanas argentina cordieritites and highly peraluminous granites within the el pilon granite complex sierras pampeanas argentina were emplaced during a medium p high t metamorphic event during the initial decompression of a cambrian orogen along the southwestern margin of gondwana very fresh orbicular and massive cordieritite bodies with up to 90 cordieritite are genetically associated with a cordierite monzogranite pluton and a larger body of porphyritic granodiorite the petrogenesis of this association has been studied using petrographical mineralogical thermobarometric geochemical geochronological and isotope methods the granitic magmas were formed by anatexis of mid crustal metamorphic rocks formed earlier in the pampean orogeny the cordieritites appear at the top of feeder conduits that connected the source region located at 6 kbar with the pluton emplacement level at 3 7 0 3 kbar a fall in the liquidus temperature of the melt during emplacement was produced by a marked increase in fluid activity owing to rapid decompression and assimilation of surrounding hydrous metapelitic schists followed by isobaric crystallization high mg cordierite crystallized early on biotite sillimanite restitic mineral assemblages of the assimilated schists or at the wall of the feeder conduits strong convection in the small magma chamber caused flow segregation of cumulate cordierite and restite developing leucogranites and highly evolved pegmatoids that are in isotopic equilibrium rapid ascent of highly peraluminous magmas might explain why emplacement of these granites was simultaneous with the metamorphic peak registered in neighbouring high grade migmatite and granulite terranes

segregation processes in vesiculating crystallizing magmas vesiculation of crystallising magma can produce either a mobile vesicular magma or a rigid network of crystals containing vesicular liquid where partially crystallized rigid mush underlies less crystallized magma such as near the base of a lava flow or in the cumulus pile of a magma chamber evolved interstitial melt and or gas may escape into the main body of magma the consequences of this may include contamination of the overlying liquid with gas and interstitial melt or intrusion of diapirs of vesicular evolved liquids to form vertical vesicle cylinders and other segregation features found in many basaltic lava flows and sills analog experiments were used to investigate some of the phenomena that can arise during vesiculation within a crystal mush which was simulated by pumping air through a porous plate that formed the floor of a container filled with a viscous liquid floored with a layer of glass beads experiments used either a single liquid or two stably stratified liquids with a liquid interface either coincident with the top of the porous layer of beads or slightly above the porous layer for a range of liquid viscosities and air flow rates vesiculation rates individual bubbles emerged from the top of the porous layer of beads and carried a thin trail of interstitial liquid into the overlying liquid the number of bubble trains leaving the surface of the porous bed increased with decreasing liquid viscosity and flow rate and with increasing bead size and hence with increasing permeability analog vesicle cylinders composed of diapirs of bubbly interstitial liquid were produced only when a layer of buoyant bubbly liquid lay above the surface of the porous layer the relative size of the bubbles and constrictions within the porous layer are argued to control whether individual bubbles leading to bubble trains or vesicular liquid leading to vesicle cylinders leaves the porous layer and hence whether vesicle cylinders can form

gas driven filter pressing in magmas most silicic and some mafic magmas expand via second boiling if they crystallize at depths of about 10 km or less the buildup of gas pressure due to second boiling can be relieved by expulsion of melt out of the region of crystallization and this process of gas driven filter pressing assists the crystallization differentiation of magmas for gas driven filter pressing to be effective the region of crystallization must inflate slowly relative to buildup of pressure and expulsion of melt these conditions are satisfied in undercooled magmatic inclusions and in thin sheets of primitive magma underplating cooler magma reservoirs gas driven filter pressing thereby adds fractionated melt to magma bodies gas driven filter pressing is probably the dominant process by which highly evolved melts segregate from crystal mush to form aplitic dikes in granitic plutons this process could also account for the production of voluminous crystal poor rhyolites

structure and dynamics of a silicic magmatic system associated with caldera forming eruptions at batur volcanic field bali indonesia the batur volcanic field bvf in bali indonesia underwent two successive caldera forming eruptions that resulted in the deposition of silicic ignimbrites the magmas erupted during and between these eruptions show a broad range of compositions from low sio2 andesite to high sio2 dacite on the basis of their geochemistry and mineralogy these magmas may be assigned to six groups 1 homogeneous andesites with phenocryst compositions essentially in equilibrium with the whole rock composition 2 remobilized crystal rich low sio2 andesites with resorbed phenocrysts in equilibrium with the whole rock composition 3 mixed low sio2 dacite with a relatively large range of phenocryst compositions with most phenocrysts slightly too evolved to be in equilibrium with the wholerock 4 extensively mixed low sio2 dacites with a very large and discontinuous range of phenocryst compositions with most phenocrysts either more mg rich or more evolved than the equilibrium compositions 5 remobilized crystal rich low sio2 dacites with resorbed and euhedral phenocrysts 6 homogeneous high sio2 dacites lacking evidence for magma mixing and showing narrow ranges of phenocryst compositions in equilibrium with the whole rock composition this range of silicic magmas is interpreted to reflect a combination of closedand open system fractional crystallization magma mixing and remobilization of cumulate piles by heating the variety of magmas erupted simultaneously during the caldera forming eruptions suggests that the magmatic system consisted of several independent reservoirs of variable composition and degree of crystallization the magmatic evolution of individual reservoirs varied from closed system fractional crystallization to fully open system evolution thereby resulting in simultaneous production of magmas with contrasted compositions and mineralogy extensive emptying of the magmatic system during the caldera forming eruptions led to successive or simultaneous eruption of several reservoirs

insights into continental rift related magma chambers cognate nodules from the kula volcanic province western turkey abstract the quaternary volcanism of kula western turkey occurred in response to miocene lower lithospheric n s extension which resulted in the formation of 80 monogenetic cinder cones each with an associated lava flow basanites tephrites and phono tephrites comprising a total magma volume of 2 3 km 3 many cones form closely spaced genetically related groups of up to 5 cones one such group of cones in the immediate vicinity of kula contains large crystal rich nodules dominated by amphibole and clinopyroxene the nodule suite is varied both compositionally and texturally and is likely to comprise solid fractionates of the kula lavas formed at the floor or roof zones of a suite of synchronously active mid to lower crustal magma lenses or sill like chambers and in narrow tectonically active conduits the local occurrence of nodules is probably because the plumbing system in this region was used by a series of eruptions which occurred over a short time interval during which solidification of the conduit system was incomplete the amphibole nodules generally contain more vesicular glass than clinopyroxene dominated nodules comparison of the textures with those of other studies suggests that the highly porous amphibole dominated nodules were derived from a relatively porous thick mush the highly crystalline clinopyroxene nodules were most likely excavated from the deeper levels in the mush at least part of the crystal mush was layered compositionally with irregular variations in porosity and grain size attesting to the dynamic nature of crystallisation in such a small short lived system the presence of adcumulates together with poikilitic heteradcumulates points to a highly variable degree of liquid exchange with the magma adjacent to the mush zone

magma origin and evolution of white island whakaari volcano bay of plenty new zealand white island is an active composite stratovolcano in the bay of of the north island the pacific plate is being subducted plenty new zealand that comprises many small volume westwards under the australian plate at a rate of 50 mm yr to form the taupo hikurangi arc trench with phenocryst contents of 15 44 minor high mg basaltic system volcanic activity is now concentrated in the andesite explosive eruptions such as those of 1976 1992 may taupo volcanic zone tvz the youngest expression have occurred at intervals throughout the history of white island of 22 ma cenozoic continental margin arc volcanism in but are rarely preserved these alternate with major episodes of the north island of new zealand cole 1990 tvz andesite dacite lava extrusion the high mg magmas form by fig 1 inset is at the southern end of the 2800 km hydrous melting of mantle metasomatized by fluids from the tonga kermadec taupo volcanic arc system and the dehydrating slab at the slab mantle wedge interface that rise lau basin havre trough ngatoro basin back arc basin rapidly to shallow magma chambers 2 7 km where limited system southern havre trough and tvz are offset mixing and contamination occurs before eruption some of this sinistrally by 40 50 km and wright 1992 believes the magma remains in the magma chamber where it interacts with the offset to be accommodated by a series of dextrally oblique crystal mush from which it inherits phenocrysts to form so called and en echelon bookshelf faults the southwestern limit dirty lavas total phenocryst content of these lavas is correspondingly of this fault system is placed 20 km north of white higher as more magma is intruded into the chamber the heat flux island and has nw se orientation fig 1 inset will increase and melt fraction will eventually rise to the surface to tvz extends offshore into the bay of plenty as a form high silica andesite dacite magma clean lavas with fewer graben structure of 45 km width fig 1 bounded by the inherited phenocrysts similar multi magma chamber plumbing tauranga fault zone to the west and the white island systems with complex evolution involving fractionation and confault zone to the east wright 1992 the eastern side tamination probably occur in most andesite dacite arc volcanoes of tvz coincides with the current active volcanic front and a broadly linear alignment of andesitic volcanoes extends from ruapehu in the southwest to whakatane

mafic felsic magma mixing limited by reactive processes a case study of biotite rich rinds on mafic enclaves article history mafic enclaves in felsic plutons are often used to argue that intermediate magmas are formed by mafic felsic magma mixing but the extent and nature of mixing remains unclear here we examine biotite rich rinds on mafic enclaves from the cretaceous bernasconi hills pluton in the peninsular ranges batholith of southern california to gain insight into magma mixing processes rinds differ from the enclave interior and the host monzogranite in being more fine grained and more mafic and potassic rinds are also 2 5 times more enriched in rare earth elements than the host monzogranite and up to 3 times more enriched than enclave interiors these observations indicate that the rinds were not generated by isochemical quenching binary mixing between enclave and host monzogranite or in situ magmatic differentiation instead rinds appear to have been formed by chemical reaction between the solidified enclave and a hydrous k rich residual melt or fluid formed after progressive crystallization and cooling of the host magma body transforming amphibole in the enclave into biotite rich rinds field observations show snapshots of biotite rich rinds being eroded away and new rinds simultaneously forming on freshly eroded surfaces of enclaves consistent with rinds being formed by chemical reaction instead of as quenching products deformation of enclaves is accommodated primarily by ductile attenuation of the thin rind while the enclave as a whole tends to rotate as a rigid body with minimal internal deformation other than localized brittle failure a comparison of the aspect ratios and cross sectional areas of mafic bodies in the pluton shows that those with high aspect ratios indicating greater accumulated strain are systematically more biotite rich and have smaller cross sectional areas than those with lower aspect ratios which are amphibole rich these relationships not only confirm that biotite rich lithologies are more deformable but also indicate that the high aspect ratio biotite rich bodies also known as schlieren derive from small parent bodies consistent with a derivation from eroding enclave rinds rather than from the enclave itself finally geochemical and thermodynamic modeling indicates that the biotite rich rinds formed when the host felsic magma had cooled to a low melt fraction state f 0 15 0 3 700 760 c suggesting that such reactions occur late in the lifespan of a magma body thus mafic felsic mixing may not be an efficient process for making intermediate magmas unless the magma body can reside at this low temperature range long enough to permit rind formation and subsequent deformation

gas driven filter pressing in magmas insights into in situ melt segregation from crystal mushes gas driven filter pressing is the process of melt expulsion from a volatile saturated crystal mush induced by the buildup and subsequent release of gas pressure filter pressing is inferred to play a major role in magma fractionation at shallow depths 10 km by moving melt and gas relative to the solid crystalline framework however the magmatic conditions at which this process operates remain poorly constrained we present novel experimental data that illustrate how the crystal content of the mush affects the ability of gas driven filter pressing to segregate melt hydrous haplogranite 2 1 wt water in the melt and dacite 4 2 wt water in the melt crystal mushes with a wide range of crystallinities 34 80 vol crystals were investigated using in situ high temperature 500 800 c synchrotron x ray tomographic microscopy with high spatial 3 m pixel and temporal resolution 8 s per three dimensional data set our experimental results show that gas driven filter pressing operates only below the maximum packing of bubbles and crystals 74 vol above this threshold the mush tends to fracture and gas escapes via fractures therefore the efficiency of gas driven filter pressing is promoted close to the percolation threshold and in situations where a mush inflates slowly relative to build up of pressure and expulsion of melt such observations offer a likely explanation for the production of eruptible crystal poor magmas within earth s crust

petrology and geochemistry of the orbicular granitoid of sierra de velasco nw argentina and implications for the origin of orbicular rocks the velasco orbicular granitoid is a small 65 15 m irregularly shaped body that crops out within the huaco granite central sierra de velasco nw argentina it consists of ellipsoid shaped orbicules of 3 to 15 cm length immersed in an aplitic to pegmatitic matrix the orbicules are formed by a core made up of a k feldspar megacryst partially to totally replaced by plagioclase an inner shell of radial and equant plagioclase crystals a layer of tangentially oriented biotite laths and an outer shell of plumose plagioclase crystals containing diffuse rings of tangentially oriented biotite the orbicular granitoid formed in situ in a pocket of evolved and volatile rich melt segregated from the surrounding partially crystallized huaco granite possibly via a filter pressing mechanism the segregated melt entrained relatively few k feldspar megacrysts into the pocket leaving behind a concentration of megacrysts around the pocket high water concentration caused effective superheating of the melt and destruction of nuclei with only the large megacrysts surviving as solids sudden water pressure loss and exsolution of the volatile phase perhaps related to a volcanic eruption or fracturing of the surrounding granite caused rapid undercooling of the melt the orbicules grew in the undercooled melt by heterogeneous nucleation on the megacrysts which acted as nucleation seeds and crystallization of reversely zoned radial plagioclase and sporadic crystallization of tangential biotite rings according to fluctuations in its saturation orbicular growth gave way to crystallization of the equiaxial inter orbicular matrix in two stages when sufficient polymerization of the melt was attained the time scale of formation of the orbicular granitoid was fast possibly a matter of a few weeks or months

non stationary entrainment and tunneling eruptions a dynamic link between eruption processes and magma mixing the products of many intermediate arc volcanoes and batholiths manifest compositional complexity with mafic and mixed material occurring early and through out a period of eruption and pluton assembly we present a vesiculation model suggesting that the progressive and repeated vesiculation of volatile rich magmas yields a super linear buoyancy acceleration this style of non stationary entrainment provides a mechanism for compositional reversal the intimate co occurrence of mixed and un mixed magmas and rapid chaotic crystal and melt dispersal under some circumstances entrainment can cease entirely producing eruptions with compositional tunneling where nearly unmixed mafic or newly intruded silicic magma exits first

magmatic perturbations in the okataina volcanic complex new zealand at thousand year timescales recorded in single zircon crystals abstract using single crystals to trace the chemical evolution of a magmatic system has long been a goal of igneous petrology crystals utilized for in situ dating hold great potential to link compositional and temporal information to better understand the evolution of a magmatic system if micron scale zoning of trace elements within a single zircon can be directly associated with volcanic events in magmatic systems then new insights into long term maintenance and storage of eruptible magma can be unlocked this study presents new data that directly links the geochemical history recorded in individual zircon crystals with over 50 000 yrs of changing physical and chemical conditions within a magma reservoir trace elements hf and y in zircon have not diffusively equilibrated with their host melt so that they store information about the melt when that zone crystallized thus different stages of growth can be associated with discrete time periods in the magmatic system zircon from the two most recent rhyolite eruptions kaharoa and whakatane within the okataina volcanic complex ovc new zealand have both isotopic age and trace element signatures that correspond with distinct changes in the temperature and phase assemblage of pulses of erupted magma within the caldera system this discrete change is associated with caldera collapse and reflects a change from cold wet source rhyolite to a relatively hotter and drier source rhyolite and back the kaharoa and whakatane eruptions are separated by 4000 yrs and 15 km distance yet the zircon populations record the same distinct thermal and chemical pulse that occurred following caldera collapse suggesting an interconnected magmatic system that houses at least small volumes of rhyolite melt for timescales of 10 3 to 10 5 yrs that is periodically extracted during eruption

intermittent generation of mafic enclaves in the 1991 1995 dacite of unzen volcano recorded in mineral chemistry mafic enclaves in the 1991 1995 dacite of unzen volcano show chemical and textural variability such as bulk sio2 contents ranging from 52 to 62 wt and fine to coarse grained microlite textures in this paper we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types type i mafic enclaves contain high mg plagioclase and low cl hornblende as microlites whereas type iii enclaves include low mg plagioclase and high cl hornblende type ii enclaves have an intermediate mineral chemistry type i mafic enclaves tend to show a finer grained matrix have slightly higher bulk rock sio2 contents 56 60 wt when compared with the type iii mafic enclaves sio2 53 59 wt but the overall bulk enclave compositions are within the trend of the basalt dacite eruptive products of quaternary monogenetic volcanoes around unzen volcano the origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion controlled re equilibration of minerals in a low temperature mushy dacitic magma reservoir mafic enclaves with a long residence time in the dacitic magma reservoir whose constituent minerals were annealed at low temperature to be in equililbrium with the rhyolitic melt represent type iii enclaves in contrast type i mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high temperature mineral chemistry taking temperature ca ca na ratio of plagioclase and water activity of the hydrous unzen magma into account the mg contents of plagioclase indicate that plagioclase microlites in type iii enclaves initially crystallized at high temperature and were subsequently re equilibrated at low temperature conditions compositional profiles of mg in plagioclase suggest that older mafic enclaves type iii had a residence time of 100 years at 800 c in a stagnant magma reservoir before their incorporation into the mixed dacite of the 1991 1995 unzen eruption presence of different types of mafic enclaves suggests that the 1991 1995 dacite of unzen volcano tapped mushy magma reservoir intermittently replenished by high temperature mafic magmas

cooling and crystallization recorded in trachytic enclaves hosted in pantelleritic magmas pantelleria italy implications for pantellerite petrogenesis abstract this study focuses on the comenditic trachyte enclaves hosted in pantelleritic lava and pumices emplaced during one of the most recent eruptive events at pantelleria 6 ka enclaves range from mm sized fragments to dm sized blocks with spheroidal to amoeboid shapes and characteristic globular surfaces they are crystal rich with 30 vol large anorthoclase less abundant fe rich olivine clinopyroxene and fe ti oxides vesicles ranging from a few mm to 1 2 cm in size are distributed throughout the enclave and are commonly filled with microlite free vesicular glass the groundmass presents spectacular textures including fine to coarse spherulites and hopper and skeletal microlites mostly anorthoclase with 10 35 vol residual glass and 10 15 vol small vesicles 2 accordingly the microlite composition is close to that of the host pantellerite these textures were acquired during a magma mixing event prior to the eruption i a vapor saturated comenditic trachyte magma intruded the shallower cooler pantellerite magma body triggering a first degassing event first boiling and rapid crystallization with a high degree of undercooling t 100 150 c ii vapor exsolution induced by rapid crystallization second boiling produced the microvesicular groundmass and iii vapor pressure forced the residual pantelleritic liquid to migrate into the large vesicles and or outside the enclaves based on this interpretation these enclaves can be used as a natural laboratory for identifying the chemical and physical processes driving the evolution of silicic magmas at pantelleria in particular they can be used to assess whether pantellerite melts can be obtained from comenditic trachytes after extensive crystallization followed by gas driven filter pressing and consequent pantellerite melt segregation

generation of porphyritic and equigranular mafic enclaves during magma recharge events at unzen volcano japan mafic to intermediate enclaves are evenly distributed throughout the dacitic 1991 1995 lava sequence of unzen volcano japan representing hundreds of mafic recharge events over the life of the volcano this study documents the morphological textural chemical and petrological characteristics of the enclaves and coexisting silicic host lavas the eruptive products described in this study appear to be general products of magma mingling as the same textural types are seen at many other volcanoes two types of magmatic enclaves referred to as porphyritic and equigranular are easily distinguished texturally porphyritic enclaves display a wide range in composition from basalt to andesite are glass rich spherical and porphyritic and contain large resorbed plagioclase phenocrysts in a matrix of acicular crystals and glass equigranular enclaves are andesitic non porphyritic and consist of tabular medium grained microphenocrysts in a matrix glass that is in equilibrium with the host dacite magma porphyritic enclaves are produced when intruding basaltic magma engulfs melt and phenocrysts of resident silicic magma at their mutual interface equigranular enclaves are a product of a more prolonged mixing and gradual crystallization at a slower cooling rate within the interior of the mafic intrusion

a theoretical model for the formation of ring moat dome structures products of second boiling in lunar basaltic lava flows abstract newly documented ring moat dome structures rmdss low mounds typically several hundred meters across with a median height of 3 5 m and surrounded by moats occur in the lunar maria they appear to have formed synchronously with the surrounding mare basalt deposits it has been hypothesized that they formed on the surfaces of lava flows by the extrusion of magmatic foams generated in the flow interiors as the last stage of the eruption and flow emplacement process we develop a theoretical model for the emplacement and cooling of mare basalts in which the molten cores of cooling flows are inflated during the late stages of eruptions by injection of additional hot lava containing dissolved volatiles crystallization of this lava causes second boiling an increase in vapor pressure to the point of supersaturation due to crystallization of the melt generating copious quantities of vesicles magmatic foam layers at the top and bottom of the central core of the flow flow inflation of many meters is predicted to accompany the formation of the foam layers flexing the cooled upper crustal layer and forming fractures that permit extrusions of the magmatic foams onto the surface to form domes with subsidence of the subjacent and surrounding surface forming the moats by modeling the evolution of the internal flow structure we predict the properties of rmdss and the conditions in which they are most likely to form we outline several tests of this hypothesis

phase equilibria constraints on the chemical and physical evolution of the campanian ignimbrite the campanian ignimbrite is a 4200 km trachyte phonolite pyroclastic deposit that erupted at 39 3 0 1ka within the campi flegrei west of naples italy here we test the hypothesis that campanian ignimbrite magma was derived by isobaric crystal fractionation of a parental basaltic trachyandesitic melt that reacted and came into local equilibrium with small amounts 5 10 wt of crustal rock skarns and foid syenites during crystallization comparison of observed crystal and magma compositions with results of phase equilibria assimilation fractionation simulations melts is generally very good oxygen fugacity was approximately buffered along qfm 1 where qfm is the quartz fayalite magnetite buffer during isobaric fractionation at 0 15 gpa 6 km depth the parental melt reconstructed from melt inclusion and host clinopyroxene compositions is found to be basaltic trachyandesite liquid 51 1 wt sio2 9 3 wt mgo 3 wt h2o a significant feature of phase equilibria simulations is the existence of a pseudo invariant temperature 883 8c at which the fraction of melt remaining in the system decreases abruptly from 0 5 to50 1 crystallization at the pseudo invariant point leads to abrupt changes in the composition properties density dissolved water content and physical state viscosity volume fraction fluid of melt and magma a dramatic decrease in melt viscosity from 1700 pa s to 200 pa s coupled with a change in the volume fraction of water in magma from 0 1 to 0 8 and a dramatic decrease in melt and magma density acted as a destabilizing eruption trigger thermal models suggest a timescale of 200 kyr from the beginning of fractionation until eruption leading to an apparent rate of evolved magma generation of about 10 3 km year in situ crystallization and crystal settling in density stratified regions as well as in convectively mixed less evolved subjacent magma operate rapidly enough to match this apparent volumetric rate of evolved magma production

formation of segregation structures in hafnarhraun pāhoehoe lobe sw iceland a window into crystal melt separation in basaltic magma to gain insights into crystal melt separation processes during basalt differentiation we have studied an 8 m thick p hoehoe lava lobe from the hafnarhraun lava flow field in sw iceland the lobe has abundant melt segregations porous cylindrical and sheet like structures generally interpreted as separated residual melts of a lava lobe we divide these melt segregations into three types based on morphology and composition vesicle cylinders vc type 1 horizontal vesicle sheets hvs1 and type 2 horizontal vesicle sheets hvs2 remarkably the studied vc are not simple residual melts generated by fractional crystallization but their composition points to removal of plagioclase from the parental lava hvs1 resemble vc but have fractionated more olivine ol plagioclase plg augite and have lost most if not all of their olivine phenocrysts hvs2 are fe rich and evolved corresponding to residual melts after 50 60 fractional crystallization of the lobe we suggest that the hafnarhraun vc formed in a two stage process firstly vc forming residual melt and vapor detached as rising diapirs from ol plg melt vapor mush near the lava base and later these vc diapirs accumulated ol phenocrysts and minor plg microphenocrysts in the lava core hvs1 represent accumulations of vc to the viscous base of the solidifying upper crust of the lobe and hvs2 formed as evolved vapor saturated residual melts seeped into voids within the upper crust such vapor aided differentiation here documented for the hafnarhraun lava may also apply to shallow crustal magma storage zones contributing to the formation of evolved basalts

silicic volcanism triggered by increased denudation rates in the quaternary andean arc of central chile between 33 50 34 30 s abstract in central chile 33 50 34 30 s the quaternary arc records a distinctive episode of silicic volcanism sio2 70 wt that occurred during latest pleistocene 0 1 1 ma this episode is recorded in several eroded and inactive arc centers distributed mostly along the highest summits of the andes in the same region covered by the modern arc centers we report new data including field observations petrography whole rock chemical and sr nd isotopic analyses mineral chemistry and geochronology for the quaternary andean arc in the region with emphasis in the latest pleistocene units 0 1 1 ma the silicic episode that the latter units represent constitutes a singularity in the common compositions that characterize in the long term this arc segment as it is preceded and followed by basaltic andesite andesite dacite suite compositions in addition it occurs coevally with a period of increased denudation resulting from the major uplift event of the andean orogen in the area geologic field markers supported by thermobarometric estimations indicate conservative estimates of denudation of 1 5 km during this period in the area which in turn accounts for a significant pressure decrease at upper crustal levels that must have affected the shallow reservoirs that fed the arc volcanoes for evaluating this thermodynamic numerical simulations have been run at different pressure conditions in order to describe for such magmatic systems the evolution in terms of major element composition degree of crystallinity and volatile content with the temperature decrease overall results show that the compositional spectra defined by this arc segment can be reproduced by low pressure

the timing and origin of pre and post caldera volcanism associated with the mesa falls tuff yellowstone plateau volcanic field abstract we present new sanidine 40ar 39ar ages and paleomagnetic data for pre and post caldera rhyolites from the second volcanic cycle of the yellowstone plateau volcanic field which culminated in the caldera forming eruption of the mesa falls tuff at ca 1 3 ma these data allow for a detailed reconstruction of the eruptive history of the second volcanic cycle and provide new insights into the petrogenesis of rhyolite domes and flows erupted during this time period 40ar 39ar age data for the biotite bearing bishop mountain flow demonstrate that it erupted approximately 150 kyr prior to the mesa falls tuff integrating 40ar 39ar ages and paleomagnetic data for the post caldera island park rhyolite domes suggests that these five crystal rich rhyolites erupted over a centuries long time interval at 1 2905 0 0020 ma 2 the biotite bearing moonshine mountain rhyolite dome was originally thought to be the downfaulted vent dome for the pre caldera bishop mountain flow due to their similar petrographic and oxygen isotope characteristics but new 40ar 39ar dating suggest that it erupted near contemporaneously with the island park rhyolite domes at 1 2931 0 0018 ma 2 and is a post caldera eruption despite their similar eruption ages the island park rhyolite domes and the moonshine mountain dome are chemically and petrographically distinct and are not derived from the same source integrating these new data with field relations and existing geochemical data we present a petrogenetic model for the formation of the post mesa falls tuff rhyolites renewed influx of basaltic and or silicic recharge magma into the crust at 1 2905 0 0020 ma led to 1 the formation of the island park rhyolite domes from the source region that earlier produced the mesa falls tuff and 2 the formation of moonshine mountain dome from the source region that earlier produced the biotite bearing bishop mountain flow these magmas were stored in the crust for less than a few thousand years before being erupted contemporaneously along a 30 km long structurally controlled vent zone related to extracaldera basin and range faults these data highlight the rapidity with which magma can be generated and erupted over large distances at yellowstone

a possible link between gabbros bearing high temperature iddingsite alteration and huge pegmatoid intrusions the society islands french polynesia this paper reports new field observations and new petrological textural and geochemical data on two gabbroic intrusions and one pegmatoid dyke from the french polynesian islands maupiti and bora bora respectively olivine crystals from the faataufi and barque de hiro gabbros maupiti include high temperature iddingsite hti exhibiting three distinct morphological facies chemical and crystallographical data performed by electron microprobe scanning microscope x ray diffractometry and near infrared spectroscopy show that hti results from modifications of olivine structure through intense hydroxylation and fe oxidation the hti free 120 m wide cloche de hiro dyke bora bora exhibits a textural zonation from heterogranular intergranular gabbro peripheral part to typical pegmatoid inner area we propose a textural tetrahedral classificatory diagram which can be used to clearly distinguish the textural characteristics of the pegmatoids from those of the other coarse grained samples through comparison with a lava flow containing vesicle rich segregation sheets we suggest that the cloche de hiro dyke may correspond to a huge segregation structure derived from a maupiti type hti bearing gabbro through a vapor differentiation process both intrusions might be considered as representative of two superposed levels in a vertically arranged magmatic complex in this view the maupiti gabbroic bodies would represent the lower part of the postulated system under high oxygen fugacity olivine crystals would have undergone important modifications of their structure leading to hti the huge bora bora pegmatoid dyke would be the upper part of the complex after expulsion from the hti bearing gabbroic solidification zone by build up of gas pressure vesicle rich residual melt would have gathered into a shallow level and crystallized there as a pegmatoid body

compositional zonation of the shallow la gloria pluton central chile by late stage extraction redistribution of residual melts by channelization numerical modeling abstract the origin of highly evolved magmas e g rhyolites has been a long standing controversy in earth sciences they are commonly thought to be generated in the upper crust by melt extraction from mush zones but due to the rapid cooling of magma reservoirs in such shallow and typically cold environments high magma emplacement rates of intermediate magmas are thought to be necessary to maintain large silicic mushes above the solidus long enough for the high sio2 melts extraction to occur late stage redistribution of interstitial melts i e heat and mass by channels dikes within those mushes has been invoked as a mechanism to preserve silicic mushes above their solidi for longer periods i e delaying their final crystallization but the nature of this process and its implications on plutons zonation are still poorly understood here using time dependent numerical modeling we study the feasibility of late stage interstitial melt extraction redistribution by channels dikes from a crystalline mush our model accounts for magma fluid dynamics extraction of residual melts and thermal evolution of the crystallizing magma system and its hosting rocks considering the thermal effect of the redistributing material the model was applied to explain the anatomy of the well documented la gloria pluton lgp central chile which exhibits increasing contents of sio2 and abundant leucocratic dikes toward the margins interpreted as trapped residual melts generated elsewhere in the magma chamber our results suggest that favorable conditions for extracting late stage residual melts are reached at temperatures of 750 c 60 vol crystallinity at least for compositions similar to lgp dikes simulations correspond to 30 kyr of reservoir cooling when the concentric compositional zonation of lgp is reproduced after a short period of extraction

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

the role of superheating in the formation of glass mountain obsidians long valley ca inferred through crystallization of sanidine the glass mountain obsidians long valley ca are crystal poor 8 vol and highly evolved high sio2 low sr and therefore their formation required extremely efficient separation of melts from a crystal rich source a petrologic and experimental investigation of the mineral phases in glass mountain lavas identifies conditions under which phenocrysts grew and the driving mechanism for crystallization which places constraints on the possible processes that generated the obsidians the obsidian in this study gm 11 is saturated in nine phases sanidine quartz plagioclase titanomagnetite ilmenite zircon apatite allanite biotite and results of high resolution sem compositional mapping and electron microprobe analysis reveal that individual sanidine crystals are normally zoned and span a range of compositions or40 78 sanidines have a granophyric texture characterized by intergrowths of quartz and sanidine mineral phases in the natural sample are compared to h2o saturated phase equilibrium experiments conducted in cold seal pressure vessels over a range of conditions 700 850 c 75 225 mpa and all are found to be plausible phenocrysts comparison of sanidine compositions from the natural sample with those grown in phase equilibrium experiments demonstrates that sanidine in the natural sample occurs in a reduced abundance further comparison with phase equilibrium experiments suggests that sanidine compositions track progressive loss of dissolved melt water cooling suggesting that crystallization in the natural obsidian was driven predominantly by degassing resulting from decompression it is paradoxical that an effusively slowly erupted lava should contain multiple phenocryst phases including sanidine crystals that span a range of compositions with granophyric textures and yet remain so crystal poor to resolve this paradox it is necessary that the solidification mechanism degassing or cooling that produced the sanidine crystals and other mineral phases must have an associated kinetic effect s that efficiently hinders crystal nucleation and growth decompression experiments conducted in this study and from the literature collectively demonstrate that the simplest way to inhibit nucleation during degassing induced crystallization is to initiate degassing cooling from superliquidus conditions and therefore the glass mountain obsidians were superheated prior to crystallization

juvenile magma recognition and eruptive dynamics inferred from the analysis of ash time series the 2015 reawakening of cotopaxi volcano forecasting future activity and performing hazard assessments during the reactivation of volcanoes remain great challenges for the volcanological community on august 14 2015 cotopaxi volcano erupted for the first time in 73 years after approximately four months of precursory activity which included an increase in seismicity gas emissions and minor ground deformation here we discuss the use of near real time petrological monitoring of ash samples as a complementary aid to geophysical monitoring in order to infer eruption dynamics and evaluate possible future eruptive activity at cotopaxi twenty ash samples were collected between august 14 and november 23 2015 from a monitoring site on the west flank of the volcano these samples contain a range of grain types that we classified as hydrothermal altered lithic juvenile and free crystals the relative proportions of theses grains evolved as the eruption progressed with increasing amounts of juvenile material and a decrease in hydrothermally altered material in samples from the initial explosion juvenile grains are glassy microlite poor and contain hydrothermal minerals opal and alunite the rising magma came in contact with the hydro thermal system under confinement causing hydro magmatic explosions that cleared the upper part of the plumbing system subsequently the magmatic column produced a thermal aureole in the conduit and dried out the hydrothermal system allowing for dry eruptions magma ascent rates were low enough to allow for efficient outgassing and microlite growth constant supply of magma from below caused quasi continuous disruption of the uppermost magma volume through a combination of shear deformation and gas expansion the combination of increasing crystallinity of juvenile grains and high measured so2 flux indicate decreasing integrated magma ascent rates and clearing of the hydrothermal system along transport pathways in a system open to gas loss the near real time monitoring of ash samples combined with traditional geophysical monitoring techniques during the reawakening of cotopaxi allowed us to gain a much clearer understanding of events than when using traditional geophysical monitoring alone c 2016 elsevier b v all rights reserved

late quaternary inner shelf deposits in response to late pleistocene holocene sea level changes nakdong river se korea abstract the high resolution seismic profiles from the inner shelf off the nakdong river in se korea can be directly correlated with the lithologic characteristics in the borehole data these correlations offer an opportunity to document the evolution of the inner shelf deposits with sediment supplies in response to the late quaternary sea level changes the inner shelf sequence in this area consists of four sedimentary units which comprise a set of lowstand transgressive and highstand systems tracts deposited since the last glacial maximum incised channel fill su1 estuary su2 sand veneer su3 and subaqueous delta su4 the lowermost unit su1 which overlies the sequence boundary is interpreted as fluvial deposits formed during the last glacial period and the early stage of transgression and belongs to the lowstand to transgressive systems tract the lower middle unit su2 lying below the ravinement surface represents a paralic component that consists of estuarine sandy mud or muddy sand developed between approximately 13 and 8 cal ka bp whereas the upper middle unit su3 above the ravinement surface corresponds to a marine component that consists of sand veneer produced by the shelf erosion during the ensuing sea level rise these two units su2 and su3 belong to the transgressive deposits the uppermost unit su4 regarded as the highstand systems tract formed the nakdong subaqueous delta including the proximal and distal systems developed after the highstand sea level at approximately 6 cal ka bp the lateral transition from the proximal to distal facies suggests a prograding delta system in the nakdong river

crystallization dynamics of granite magma chambers in the absence of regional stress multiphysics modeling with natural examples numerical models built linking an internally consistent rheological dataset for a cooling granite magma with equations of heat transfer and fluid motion for geometrically different magma chambers cooling at various crustal depths reveal that granite magmas first undergo a short period of chaotic convection during which wall rock contamination and magma mixing are possible followed by a long period of no convective cooling during which melt segregation occurs convection is driven by the negative density gradient generated in the upper cooling zone by melt to solid phase transformation convection breaks the upper mushy zone and drags the fragments downwards with descending rayleigh taylor fingers such fragments can be preserved as microgranular enclaves the descending rayleigh taylor fingers split low aspect ratio sill like magma chambers into nearly isolated convection cells if the magma is initially heterogeneous this effect divides the chamber into contiguous homogeneous zones with distinct trace element and isotope ratios and finally results in a pluton with marked lateral compositional variations easily misinterpreted as different intrusive batches convective heat loss quickly leads most of the magma chamber to critical crystallinity independently of the vertical coordinate so that a chamber wide three dimensional skeleton of crystals with uniform initial porosity c 0 4 0 5 is formed this configuration is gravitationally unstable therefore it spontaneously compacts towards an equilibrium vertical variation of porosity approaching atty s law in the absence of regional stress the upwards migration of the inter crystalline melts as a result of compaction is the most effective way of melt solid segregation and causes vertically zoned plutons with an upper layer of felsic segregates granite magma chambers fractionated by these mechanisms will produce short range differentiation series from a composition slightly less silicic than the initial magma to high silica segregates in the presence of regional stress tectonic squeezing and shearing during the post convective stage can expel residual fluid more efficiently and lead to wide range granite differentiation series from rocks notably less silicic than the initial magma to high silica leucogranites

temporal evolution of granitic magmas in the luanchuan metallogenic belt east qinling orogen central china implications for mo metallogenesis abstract the luanchuan metallogenic belt located within the eastern part of the qinling orogen central china hosts a number of world class mo deposits that are closely related to small late mesozoic granitic plutons zircon u pb dating of distinct plutons in the luanchuan metallogenic belt has yielded ages of 153 1 154 2 152 2 and 148 1 ma molybdenite re os isotopic compositions of yuku ore district in the southern part of luanchuan metallogenic belt has yielded an isochron age of 146 1 ma which is consistent with the large scale mineralization ages in the northern part of the luanchuan metallogenic belt a combination of previous studies and new geochronological and isotopic data show a concordant temporal and genetic link between granitic magmatism and mo mineralization in the luanchuan metallogenic belt suggesting that this mineralization episode formed the most extensive mo mineralization belt in the east qinling orogen zircon grains from mo related granitic plutons show similar trace element distributions high precision multi collector inductively coupled plasma mass spectrometry mc icp ms pb isotope analysis of k feldspar megacrysts from mineralization related granites suggest that they were derived from the lower crust similarly the pb isotopic compositions of pyrite coprecipitated with molybdenite also suggest that the metals were derived form the lower crust with probably minor mantle contribution a continuum mineralization model that describes the sourcing of mo from an evolving granitic magma over successive differentiation events possibly in separate but connected magma chambers could explain the remarkable mo enrichment in the luanchuan metallogenic belt the volatile and mo bearing granitic magmas ascended as diapirs from the deep crust and were emplaced as dikes in the upper crust lithological differences between these mo bearing granites may relate to different stages in the evolution of individual magmas finally ore forming fluids were exsolved from the granitic melts in shallow crustal magma chambers resulting in the formation of the mo deposits of the luanchuan metallogenic belt this model may account for the widespread late mesozoic mo deposits of eastern china

rejuvenation of the fish canyon magma body a window into the evolution of large volume silicic magma systems voluminous unzoned phenocryst rich pyroclastic deposits considered as erupted batholiths provide a unique opportunity to investigate magmatic processes in silicic magmas the fish canyon tuff a well documented example of these monotonous ignimbrites displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near isobaric reheating from 720 to 760 8c these observations along with a high crystallinity 45 and near solidus mineral assemblage suggest that the fish canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption rejuvenation was triggered by intrusion of water rich mafic magmas at the base of the fish canyon mush but the mechanisms of heat transfer remain poorly understood the growth of amphibole during reheating requires addition of mafic components but the absence of any measurable gradients and the paucity of mafic enclaves in the fish canyon magma rule out a reheating event dominated by convective mixing with a mafic magma closed system processes such as heat conduction and convective self mixing could not account for the transport of externally derived mafic components we performed numerical simulations of upward percolation of a hot low density h2o co2 fluid phase gas sparging through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems sparging by 20 40 km 3 of gas extracted from 3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by 40 8c in 150 200 k y moreover the vertical thermal gradient after 150 k y in most of the mush is small 25 8c in the upper 65 gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal poor rhyolites by expelling interstitial melt however our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites

concentric zoning in the tunk lake pluton coastal maine the tunk lake pluton of coastal maine usa is a concentrically zoned granitic body that grades from an outer hypersolvus granite into subsolvus rapakivi granite and then into subsolvus non rapakivi granite with gradational contacts between these zones the pluton is partially surrounded by a zone of basaltic and gabbroic enclaves interpreted as quenched magmatic droplets and mushes respectively as well as gabbroic xenoliths all hosted by high silica granite the granite is zoned in terms of mineral assemblage mineral composition zircon crystallization temperature and major and trace element concentration from the present day rim interpreted as being closer to the base of the chamber to the core interpreted as being closer to the upper portions of the chamber the ferromagnesian mineral assemblage systematically changes from augite and hornblende with augite cores in the outermost hypersolvus granite to hornblende to hornblende and biotite and finally to biotite only in the subsolvus granite core of the pluton sparse fine grained basaltic enclaves that are most common in the outermost zone of the pluton suggest that basaltic magma was present in the lower portions of the magma chamber at the same time that the upper portions of the magma chamber were occupied by a granitic crystal mush however the slight variations in initial nd isotopic ratio in granites from different zones of the pluton suggest that contamination of the granitic melt by basaltic melt played little role in generating the compositional gradation of the pluton the zone of basaltic and gabbroic chilled magmatic enclaves and gabbroic xenoliths hosted by high silica granite that partially surround the pluton is interpreted as mafic layers at the base of the pluton that were disrupted by invading late stage high silica magma these mafic layers are likely to have consisted of basaltic lava layers and basalt that chilled against granitic magma to produce coarse grained gabbroic mush basaltic and gabbroic magmatic enclaves and gabbroic xenoliths are hornblende bearing suggesting that their parent melts were relatively hydrous the water rich nature of the underplating mafic magmas may have prevented extensive invasion of the granitic magma by these magmas owing to the much greater viscosity of the granitic magma than the mafic magmas in the temperature range over which magma interaction could have occurred

rhyodacite magma storage conditions prior to the 3430 ybp caldera forming eruption of aniakchak volcano alaska this study presents a pre eruptive magma storage model for the rhyodacite and andesite magmas erupted during the 3430 ybp caldera forming eruption of aniakchak volcano alaska derived from phase equilibria experiments and petrological data the compositions of fe ti oxide pairs from the early erupted plinian rhyodacite pumice yield core temperatures of 871 900 c with rims up to 942 c and fo2 from 10 6 to 11 8 log units melt inclusions entrapped in plagioclase phenocrysts have h2o contents between 3 and 5 wt estimated by ftir and electron microprobe volatiles by difference methods with no detectable co2 assuming water saturation this corresponds to entrapment pressures between 65 and 150 mpa phase equilibria results reproduce the natural phase assemblages at p text h 2 text o of 95 150 mpa at 870 880 c assuming water saturation a mismatch in experimental versus natural glass sio2 and al2o3 and melts models for h2o undersaturated conditions indicate that the rhyodacite may not have been h2o saturated melts models with x text h 2 text o 0 8 and ptotal of 125 150 mpa at 870 880 c reproduce the natural groundmass glass al2o3 composition best indicating the magma may have been slightly h2o undersaturated those pressures correspond to storage at 4 5 5 4 km depth in the crust melts models and vbd estimates from melt inclusions in titanomagnetite grains from the andesite indicate pre eruptive conditions of 1 000 c and 110 mpa corresponding to a minimum residence depth of 4 1 km assuming water saturation or greater if the magma was h2o undersaturated previous geochemical studies indicate separate histories of the two magmas though they retain some evidence that they are ultimately related through fractional crystallization processes analogous to the 1912 novarupta magmas the rhyodacite and andesite presumably originated within the same crystal mush zone beneath the edifice yet were separated laterally and underwent different degrees of crustal assimilation the andesite must have resided in close proximity with ascent occurring in response to movement of the rhyodacite and resulting in extensive syn eruptive mingling

eruptive history and geochronology of mount mazama and the crater lake region oregon geologic mapping k ar and 40 ar 39 ar age determinations supplemented by paleomagnetic measurements and geochemical data are used to quantify the quaternary volcanic history of the crater lake region in order to define processes and conditions that led to voluminous explosive eruptions the cascade arc volcano known as mount mazama collapsed during its climactic eruption of 50 km 3 of mainly rhyodacitic magma 7700 yr ago to form crater lake caldera the mazama edifice was constructed on a pleistocene silicic lava field amidst monogenetic and shield volcanoes ranging from basalt to andesite similar to parental magmas for mount mazama between 420 ka and 35 ka mazama produced medium k andesite and dacite in 2 1 proportion the edifice was built in many episodes some of the more voluminous occurred approximately coeval with volcanic pulses in the surrounding region and some were possibly related to deglaciation following marine oxygen isotope stages mis 12 10 8 6 5 2 and 2 magmas as evolved as dacite erupted many times commonly associated with or following voluminous andesite effusion establishment of the climactic magma chamber was under way when the first preclimactic rhyodacites vented ca 27 ka the silicic melt volume then grew incrementally at an average rate of 2 5 km 3 k y 1 for nearly 20 k y the climactic eruption exhausted the rhyodacitic magma and brought up crystal rich andesitic magma mafic cumulate mush and wall rock granodiorite postcaldera volcanism produced 4 km 3 of andesite during the first 200 500 yr after collapse followed at ca 4800 yr b p by 0 07 km 3 of rhyodacite the average eruption rate for all mazama products was 0 4 km 3 k y 1 but major edifice construction episodes had rates of 0 8 km 3 k y 1 the long term eruption rate for regional monogenetic and shield volcanoes was d 0 07 km 3 k y 1 but only 0 02 km 3 k y 1 when the two major shields are excluded plutonic xenoliths and evidence for crystallization differentiation imply that the amount of magma intruded beneath mount mazama is several times that which has been erupted the eruptive and intrusive history reflects competition between 1 crystallization driven by degassing and hydrothermal cooling and 2 thermal input from a regional magma flux focused at mazama before ca 30 ka relatively small volumes of nonerupted derivative magma crystallized to form a composite pluton because the upper crust had not been heated sufficiently to sustain voluminous convecting crystal poor melt subsequently and perhaps not coincidentally during mis 2 a large volume of eruptible silicic magma accumulated in the climactic chamber probably because of heating associated with mantle input to the roots of the system as suggested by eruption of unusually primitive magnesian basaltic andesite and tholeiite west of mazama