the petrology of a large intra oceanic silicic eruption the sandy bay tephra kermadec arc southwest pacific abstract the pumiceous pyroclastic deposits known as the sandy bay tuff on macauley island in the kermadec group represent a medium scale silicic eruption in an oceanic subduction setting the sandy bay eruption occurred about 6310 yr bp forming a large submarine caldera in the summit of the mainly submarine and essentially basaltic macauley volcano the sandy bay magma contained

a new 122 mm electromechanical drill for deep ice sheet coring disc 4 drill cable the deep ice sheet coring disc drill developed by ice coring and drilling services icds under contract with the us national science foundation requires a drill cable capable of transmitting high amounts of electrical power as well as high rates of data the disc cable was designed and manufactured by the rochester corporation to core ice to depths of 4000 m in addition to the steel strength members to support the drill the cable has copper wires for conducting electrical power for the drill sonde cutter pump motors and the sonde electronics and an optical fiber core for data transmission the outer steel layers are filled with a special compound to minimize fluid penetration of the cable

geochemistry of silicic magmas in the macolod corridor sw luzon philippines evidence of distinct mantle derived crustal sources for silicic magmas silicic volcanic deposits 65 wt sio2 which occur as domes lavas and pyroclastic deposits are relatively abundant in the macolod corridor sw luzon philippines at makiling stratovolcano silicic domes occur along the margins of the volcano and are chemically similar to the silicic lavas that comprise part of the volcano pyroclastic flows are associated with the laguna de bay caldera and these are chemically distinct from the domes and lavas at makiling stratovolcano as a whole samples from the laguna de bay caldera contain lower concentrations of mgo and higher concentrations of fe2o3 t than the samples from domes and lavas the laguna de bay samples are more enriched in incompatible trace elements the silicic rocks from the domes makiling volcano and laguna de bay caldera all contain high alkalis and high k2o na2o ratios melting experiments of primitive basalts and andesites demonstrate that it is difficult to produce high k2o na2o silicic magmas by fractional crystallization or partial melting of a low k2o na2o source however recent melting experiments sisson et al contrib mineral petrol 148 635 661 2005 demonstrate that extreme fractional crystallization or partial melting of k rich basalts can produce these silicic magmas our model for the generation of the silicic magmas in the macolod corridor requires partial melting of mantle derived evolved moderate to k rich crystallized calc alkaline magmas that ponded and crystallized in the mid crust major and trace element variations along with oxygen isotopes and ages of the deposits are consistent with this model

phase equilibrium constraints on the viscosity of silicic magmas 1 volcanic plutonic comparison by using recently determined experimental phase equilibria we show that the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at 104 5 pa s irrespective of their temperature and level of emplacement magmas crystallizing as granitic plutons are not water poor and thus not more viscous than their extrusive equivalents instead comparison between pre eruption magma viscosities of extrusive silicic intermediate and intrusive granitic magmas shows that the former are on average slightly more viscous given the typical strain rates in silicic magma chambers magma rheological behavior is expected to be dominantly newtonian bubbles having a minor rheological influence at depth although exceptions can exist thus whether a silicic intermediate magma is erupted or frozen at depth depends primarily on the rheological properties of surrounding terranes or on external tectonic factors but not on the rheology of the magma itself however preemptive viscosities of extrusive magmas rarely exceed 106 pa s which suggests that crystal melt mushes with higher viscosities cannot leave the magma storage regions beneath volcanoes the narrow range of viscosities displayed by silicic intermediate magmas results from both the strong control that pressure exerts on volatile solubilities in silicate melts and thermal limitations required to produce acid magmas considerations of the relationships between magma crystallinities bulk sio2 and preemptive melt h2o contents show that the higher the melt h2o content is the higher the maximum crystallinity that a given magma will be while still being potentially erupted an empirical correlation is proposed that enables us to estimate preemptive melt h2o contents of erupted magmas by knowing their crystallinity and bulk sio2

melt extraction from crystal mushes numerical model of texture evolution and calibration of crystallinity ordering relationships abstract mechanical crystal melt interactions in magmatic systems by separation or accumulation of crystals or by extraction of interstitial melt are expected to modify the spatial distribution of crystals observed as phenocrysts in igneous rocks textural analysis of porphyritic products can thus provide a quantitative means of interpreting the magnitude of crystal accumulation or melt loss and reconstructing the initial crystal percentage at which the process occurred we present a new three dimensional numerical model that evaluates the effects of crystal accumulation or interstitial melt removal on the spatial distribution of crystals both processes lead to increasing apparent crystallinity but also to increasing spatial ordering expressed by the clustering index r the trend of progressive crystal packing deviates from a random texture trend produced by static crystal nucleation and growth and it is universal for any texture with straight log linear crystal size distribution for sparse crystal suspensions 5 vol crystals r 1 03 up to 97 melt can be extracted corresponding to a new crystallinity of 65 vol and r 1 32 when the rheological threshold of crystal interlocking is reached for initially crystal rich suspensions the compaction path is shorter this is because the initial crystal population is more aggregated and it reaches the limit of interlocking sooner crystal suspensions with 35 vol crystals cannot be compacted without mechanical failure these results illustrate that the onset of the rheological threshold of magma immobility strongly depends on the spatial configuration of crystals in the mush the primary rigid percolation threshold 35 vol crystals corresponds to touching or interlocking crystal framework produced by in situ closed system crystallization whereas the secondary rigid percolation threshold 35 to 75 vol crystals can be reached by compaction which is particularly spatially efficient when acting on sparse crystal suspensions illustrative quantitative evaluation of the crystallinity clustering relationships to representative porphyritic granites from a single intrusive unit of the krkonose jizera pluton central europe reveals a single crystal accumulation path starting at low initial crystallinity 5 7 vol k feldspar phenocrysts with 24 84 melt extracted leading to the observed crystallinity of 9 26 vol by contrast a camptonite dyke from the ceske st edoho i volcanic province has experienced the onset of crystal accumulation later 18 vol amphibole crystals and lost 23 interstitial melt only the combination of modal and clustering analysis offers a sensitive tool for identifying differentiation processes in natural magma chambers and here it illustrates examples of mechanically dominated open system vs in situ nearly closed system crystallization from two contrasting magmatic settings

depositional features and stratigraphic sections in granitic plutons implications for the emplacement and crystallization of granitic magma abstract many granitic plutons contain sheet like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature more mafic magma during crystallization of the granitic plutons small scale structures associated with these bodies e g load cast and compaction features silicic pipes extending from granitic layers into adjacent gabbroic sheets indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber on granitic crystal mush and beneath crystal poor magma while the mafic magma was incompletely crystallized these structures indicate way up typically toward the interior of the intrusions and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition hence these plutons preserve a stratigraphic history of events involved in the construction filling replenishment and crystallization of the magma chamber the distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions which usually preserve mutual intrusive contacts and dike sill relations of different magma types the considerable thickness of material that can be interpreted as depositional and the evidence for replenishment suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton thus magma chambers may be constructed much more slowly than presently envisaged the present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber along with the underlying solidified granite and country rock sank during continuing episodes of magma chamber replenishment these internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber

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

calcic amphibole growth and compositions in calc alkaline magmas evidence from the motru dike swarm southern carpathians romania a late pan african calc alkaline dike swarm basalt andesite dacite rhyolite has been investigated in a region of over 2000 km 2 in the alpine danubian window south carpathians romania amphibole phenocrysts and microphenocrysts have been investigated by wavelength dispersive microprobe analysis and bse imaging the ca amphibole population represented in all the lithologies displays a large compositional range interpreted as the result of two processes 1 magmatic evolution kaersutite ti pargasite pargasite ti magnesiohastingsite magnesiohastingsite edenite tschermakite magnesiohornblende linked to magmatic differentiation from andesitic basalt to rhyolite and 2 deuteritic alteration of the primary amphibole related to late emplacement hydrothermal activity yielding numerous varieties comprising those cited above in all rock types amphibole phenocrysts equilibrated at a nearly constant pressure of about 0 6 0 1 gpa but their temperatures of crystallization ranged from 1000 900 c for basaltic andesites to 700 600 c for dacites in rhyolites edenite to magnesiohornblende crystals res ect a continuous range of p t conditions from 700 c 0 6 gpa to 600 c 0 1 gpa in agreement with their change of habit from euhedral to subhedral complex zonations in pargasite magnesiohastingsite including resorption are interpreted in term of self organization of

disc drill and replicate coring system a new era in deep ice drilling engineering the deep ice sheet coring disc drill and replicate coring system rcs were designed and manufactured by ice coring and drilling services ice drilling design and operations group of the university of wisconsin madison the disc rcs is an electromechanical system designed to take 122 mm diameter ice cores from the main borehole and 108 mm diameter replicate ice cores to depths of 4000 m detailed design of the disc drill began in 2003 and the completed drill was tested in greenland in 2006 during five consecutive field seasons at west antarctic ice sheet wais divide 3405 m of ice core were drilled setting the us deep ice drilling record the rcs based on the disc drill was developed and built in 2010 11 tested in antarctica during the 2011 12 wais divide field season and tested further in madison wi during summer fall 2012 during the 2012 13 antarctic field season the system produced five azimuth and depth controlled deviations at four target depth levels a total of 285 m of replicate ice core was recovered in the first coring of its kind the entire main replicate ice core including ductile brittle and warm ice had excellent quality and satisfied the needs of the ice science community

crystal poor multiply saturated rhyolites obsidians from the cascade and mexican arcs evidence of degassing induced crystallization of phenocrysts a detailed petrological study is presented for six phenocryst poor obsidian samples 73 75 wt sio2 erupted as small volume monogenetic domes in the mexican and cascade arcs despite low phenocryst microphenocryst abundances 2 6 these rhyolites are each multiply saturated with five to eight mineral phases plagioclase orthopyroxene titanomagnetite ilmenite apatite zircon hornblende clinopyroxene sanidine pyrrhotite plagioclase and orthopyroxene phenocrysts identified using phase equilibrium constraints span 30 mol an and 15 mg respectively eruptive temperatures 1 on the basis of fe ti two oxide thermometry range from 779 25 to 940 18 c oxygen fugacities 1 range from 0 4 to 1 4 0 1 log units relative to those along the ni nio buffer with temperature known the plagioclase liquid hygrometer was applied maximum water concentrations calculated for the most calcic plagioclase phenocryst in each sample range from 2 6 to 6 5 wt this requires that the rhyolites were fluid saturated at depths 2 7 km it is proposed that the wide compositional range in plagioclase and orthopyroxene phenocrysts despite their low abundance can be attributed to changing melt water concentrations owing to degassing during magma ascent phase equilibrium experiments from the literature show that higher dissolved water concentrations lead to more fe rich orthopyroxene as well as more calcic plagioclase loss of dissolved water leads to a progressive increase in melt viscosity and phenocrysts often display diffusion limited growth textures e g dendritic and vermiform consistent with large undercoolings caused by degassing a kinetic barrier to microlite crystallization occurred at viscosities from 4 5 to 5 0 log10 pa s for these rhyolites presumably because the rate at which melt viscosity changed was high owing to rapid loss of dissolved water during magma ascent

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

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

origin and evolution of a peraluminous silicic ignimbrite suite the violet town volcanics the violet town volcanics are a 373 ma old comagmatic s type volcanic sequence mainly comprising crystal rich intracaldera ignimbrites rock types vary from rhyolites to rhyodacites all containing magmatic cordierite and garnet phenocrysts variation in the suite is primarily due to fractionation of early crystallized quartz plagioclase and biotite plus minor accessory phases in a high level magma chamber prior to eruption early magmatic crystallization occurred at around 4 kb and 850 c with melt water contents between 2 8 and 4 wt this high temperature markedly water undersaturated restite poor granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain leaving a granulite facies residue

hornblende cummingtonite and biotite hydrogen isotopes direct evidence of slab derived fluid flux in silicic magmas of the taupo volcanic zone new zealand abstract hornblende cummingtonite and biotite d values from rhyolitic and mingled mixed mafic rocks from the central taupo volcanic zone were determined in an effort to characterize the isotopic composition of the dissolved fluid of magmas in this predominantly rhyolitic volcanic zone hand samples were obtained from pyroclastic flow airfall and lava flow deposits as well as co eruptive cognate plutonic blocks mineral d values range from 88 to 35 smow standard mean ocean water cummingtonite phenocryst d overlaps with hornblende from the same rocks while biotite values are consistently 7 more negative than hornblende phenocrysts from the same hand sample calculated volcanic steam d values ca 46 to 25 are consistent with an island arc crustal source ca 40 to 15 with a negligible to absent mantle ca 100 to 60 component earlier researchers suggested that the taupo volcanic zone fluid flux included dilute concentrations of slab derived phases i e co 2 cl similar to back arc volcanic systems at other convergent margins however the island arc type fluid composition estimated from the minerals we studied is similar to hornblende and biotite compositions from pyroclastic deposits and volcanic fumaroles from the kamchatka and japanese arcs the silicic rocks from the 2 0 1 0 ma mangakino caldera the most inland caldera system we studied have the lowest d hornblende and biotite suggesting that silicic volcanism distal to the subduction trench was derived from a back arc like i e mixture of mantle and slab fluid however we found that the currently active more proximal locus of silicic volcanism reflects typical arc like fluid compositions our results further suggest that the composition of volcanic steam migrating through the upper crust was similar in composition to regional meteoric waters ca 45 to 23

instruments and methods hot water borehole drilling at a high elevation debris covered glacier while hot water drilling is a well established technique used to access the subsurface of ice masses drilling into high elevation 4000 m a s l debris covered glaciers faces specific challenges first restricted transport capacity limits individual equipment items to a volume and mass that can be slung by small helicopters second low atmospheric oxygen and pressure reduces the effectiveness of combustion limiting a system s ability to pump and heat water third thick supraglacial debris which is both highly uneven and unstable inhibits direct access to the ice surface hinders the manoeuvring of equipment and limits secure sites for equipment placement fourth englacial debris can slow the drilling rate such that continued drilling becomes impracticable and or boreholes deviate substantially from vertical because of these challenges field based englacial and subglacial data required to calibrate numerical models of high elevation debris covered glaciers are scarce or absent here we summarise our experiences of hot water drilling over two field seasons 2017 2018 at the debris covered khumbu glacier nepal where we melted 27 boreholes up to 192 m length at elevations between 4900 and 5200 m a s l we describe the drilling equipment and operation evaluate the effectiveness of our approach and suggest equipment and methodological adaptations for future use

cataclastic flow and semi brittle deformation of anorthosite abstract a series of experiments on dry bushveldt anorthosite an 75 350 m grain size 5 impurities is carried out at 25 700 c 500 1500 mpa pressures and 10 5 s 1 strain rate the strength of the bushveldt anorthosite remains pressure sensitive throughout the applied p t range while weakening occurs with increased temperature at a given pressure a transition from stick slip to stable sliding occurs at 200 300 c and pressures of 1500 mpa reduction in strength at 200 300 c localized cataclastic flow and lack of microscopic evidence of crystal plastic deformation raises the possibility that the transition may be coincident with the propagation of a critical number of randomly distributed thermal cracks uniform ductile flow occurs only at temperatures of 600 and 700 c and 1000 1500 mpa pressures coincident with the development of mechanical twinning patchy and banded undulatory extinction and is likely to be due to semi brittle deformation under these conditions the deformation remains distributed to strains as high as 44

multiple rhyolite magmas and basalt injection in the 17 7 ka rerewhakaaitu eruption episode from tarawera volcanic complex new zealand abstract the 17 7 ka rerewhakaaitu eruption episode volume 5 km3 dre rhyolite magma was the second of five major episodes that have built the tarawera volcanic complex in the okataina volcanic centre over the past 22 kyr the rerewhakaaitu episode produced a widespread tephra fall deposit associated proximal pyroclastic flow deposits and voluminous rhyolite lava extrusions two different rhyolite magmas t1 and t2 were simultaneously erupted from the main vent area throughout much of the eruption episode t1 magma was a crystal poor orthopyroxene hornblende rhyolite that is highly evolved whole rock sio2 77 wt with a moderate temperature 760 c based on fe ti oxides t2 is a crystal rich biotite hornblende rhyolite that is less evolved sio2 75 wt with a fe ti oxide temperature of 700 c ejecta from the simultaneous and sequential eruption of these two magmas include some pumice clasts with mixed hybrid and mingled glass compositions and crystal populations a third rhyolite magma t3 was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome t3 was the least evolved sio2 74 wt and hottest 820 c of the three magmas saturation pressures calculated using dissolved h2o and co2 contents of melt inclusions in quartz crystals indicate that t2 magma stagnated and crystallised at about 12 km depth while small quartz crystals in t1 magma grew during ascent through 8 km depths some t1 and t2 rhyolite clasts contain vesicular brown blebs with widely variable andesite to rhyolite glass compositions accompanied by olivine clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt temperatures over 1000 c estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic magma which is now identified as the priming and triggering mechanism for three of the four post 22 ka tarawera rhyolite eruption episodes however the rhyolite magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry our preferred model for the rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated rhyolite magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event the ascending hotter and less viscous t1 rhyolite magma intersected and further invigorated a stagnant pond of cooler denser and more viscous t2 magma and lubricated its transport to the surface

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

universal stage measurements and transmission electron microscope observations of fractured plagioclase abstract at low to moderate temperatures of deformation fracturing of plagioclase is common the mechanism of fracturing is generally thought to be either a dislocation assisted process with fractures typically exhibiting some crystallographic regularity or a process of breaking along cleavage planes without the involvement of dislocations in this study naturally fractured plagioclase from granodiorites and a gabbro deformed at high strain rates are examined with the transmission electron microscope tem to identify structures at that scale in addition fracture orientations are determined with the universal stage some fractures observed in thin section occur parallel to 001 but many are not so simple but are confined to the 112 1 1 2 101 1 01 zones at the tem scale dislocation walls or arrays are common in plagioclase they also occupy the 101 1 01 112 1 1 2 zones microcracks form when dislocations are pinned in these arrays or when a free dislocation interacts with dislocations within a dislocation wall in this way large scale fractures which develop inherit their crystallographic orientation from the dislocation wall

the miocene pliocene macusani volcanics se peru i mineralogy and magmatic evolution of a two mica aluminosilicate bearing ignimbrite suite

origin of heavy fe isotope compositions in high silica igneous rocks a rhyolite perspective abstract the origin of heavy fe isotope compositions in high silica 70 wt sio 2 igneous rocks remains a highly controversial topic considering that fluid exsolution in eruptive rocks is more straight forward to constrain than in plutonic rocks this study addresses the problem of fe isotope fractionation in high silica igneous rocks by measuring fe isotope compositions of representative rhyolitic samples from the neoproterozoic volcanic sedimentary basins in southern china and the triassic tu le basin in northern vietnam the samples show remarkably varied 56 fe irmm014 values ranging from 0 05 0 05 to 0 55 0 05 which is among the highest values reported from felsic rocks the extensional tectonic setting and short melt residence time in magma chambers for the studied rhyolites rule out soret diffusion and thermal migration processes as causes of the high 56 fe values effects of volcanic degassing and fluid exsolution on bulk rock 56 fe values for the rhyolites are also assessed using bulk rock geochemical indicators and rayleigh fractionation models and these processes are found to be insufficient to produce resolvable changes in fe isotope compositions of the residual melt the most probable mechanism accounting for heavy fe isotope compositions in the high silica rhyolites is narrowed down to fractional crystallization processes in the magma before rhyolite eruption removal of isotopically light fe bearing minerals i e ulvospinel rich titanomagnetite ilmenite and biotite is proposed as the main cause of fe isotope variation in silicic melts during magmatic evolution this study implies that crystal fractionation is the dominant mechanism that controls fe isotope fractionation in eruptive rocks and fe isotopes could be used to study magmatic differentiation of high silica magmas

fracture controlled feldspar shape fabrics in deformed quartzo feldspathic rocks abstract the behaviour of feldspar in the brittle ductile transition region has often been discussed in models dominated by mechanisms of extension fracturing in this example a small caledonian granitic pluton from nw ireland feldspar shape changes are primarily accomplished by small displacements upon numerous shear fractures these fractures developed as riedel and anti riedel shears as the granite was synkinematically deformed in a regional shear zone the deformation took place under greenschist facies conditions at strain rates estimated between 10 13 and 10 14 s 1

a new 122 mm electromechanical drill for deep ice sheet coring disc 3 control electrical and electronics design the deep ice sheet coring disc drill developed by ice coring and drilling services under contract to the us national science foundation is an electromechanical drill designed to take 122 mm ice cores to depths of 4000 m electronic electrical and control systems are major aspects of the disc drill the drill sonde the down hole portion of the drill system requires approximately 5 kw of d c power for the cutter and drill motors and instrumentation power is transmitted via a drill cable from a modified commercially available surface d c power supply operating at 1000v to power modules in the sonde instrumentation section these modules regulate the power to the motors to 300 v d c and to lower voltages for the instrumentation and control electronics cutter and pump motors are controlled by electronics that include motor controllers there are 20 distinct sensors in the drill sonde which measure conditions such as hole fluid temperature motor fluid temperature drill orientation etc on board electronics facilitate communication of control commands and data between the surface and the drill sonde electronics also play an integral part in the operation of surface equipment such as the winch in raising and lowering the sonde in the borehole overall control of the disc drill system is provided by a pc based supervisory control system that allows the drill operators to monitor and control all aspects of the drilling operation

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

melt distribution beneath a young continental rift the taupo volcanic zone new zealand 1 taupo volcanic zone tvz is a zone of intense volcanism and rifting associated with the subduction of the pacific plate beneath the continental crust of new zealand s north island an image of the conductivity structure beneath the central part of the tvz has been constructed using 2 d inverse modeling of magnetotelluric data a rapid increase in conductivity at a depth of 10 km beneath the tvz 3 km beneath the base of the seismogenic zone but well above the base of the quartzo feldspathic crust 16 km is interpreted to mark the presence of an interconnected melt fraction 4 within the lower crust beneath the quartzo feldspathic crust the model shows a zone of increased conductivity on the eastern side of the tvz consistent with an increased concentration of melt at deeper levels the pacific plate is resistive compared with the overlying mantle

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

replicate ice coring system testing drilling the wdc06a borehole at the west antarctic ice sheet wais divide with the deep ice sheet coring disc drill began in december 2007 and was successfully completed in december 2011 to a depth of 3405 m the design and construction of a replicate coring system for use with the existing disc drill began in 2010 in january 2012 the new replicate coring system was tested in the parent borehole at wais divide while a full deviation was not completed during the test period much was learned about the mechanical electrical and operational aspects of the system extensive testing and modifications were done over the northern boreal summer to prepare the system for the upcoming and final season of the project further tuning of the system continued during the 2012 13 field season at wais divide this paper identifies the issues found with the system during the initial test season and discusses solutions methods and testing done to arrive at an operational system

subsurface geology ancient hydrothermal systems and crater excavation processes beneath lake rotomahana evidence from lithic clasts of the 1886 ad rotomahana pyroclastics abstract the craters associated with the 1886 ad phreatomagmatic rotomahana eruption okataina volcanic centre new zealand and the near vent geology are now hidden beneath lake rotomahana and its post eruptive sediment fill lithic clasts from the near vent lithic lapilli ash deposits of the rotomahana pyroclastics are used in this study to trace geological and geothermal conditions before the eruption as well as vent excavation dynamics near vent deposit characteristics were described in the field representative lithic clasts were documented petrographically and unaltered clasts were analysed for major and trace element compositions the majority of the lithic clasts were rhyolites with subordinate ignimbrites and hydrothermally altered clasts and trace siltstone and silicified clasts the rhyolites were classified into four petrographic groups according to phenocryst content and assemblage and were more diverse with respect to geochemical compositions most of the rhyolite lithics in the rotomahana pyroclastics did not match the rhyolite domes exposed subaerially around the lake but did have affinities with the pre matahina caldera wairua rhyolite and potentially other older non exposed domes ignimbrites most likely correlated either to the matahina ignimbrite or older non exposed units hydrothermally altered rhyolite and ignimbrite lithic clasts are common and suggest that there has been a long lived hydrothermal system in this sector possibly dating back to early activity of the okataina volcanic centre the diversity in lithic types indicate a spatial variation in country rock lithology and strength which probably contributed to the vent position and morphology along the rotomahana fissure

a new 122 mm electromechanical drill for deep ice sheet coring disc 1 design concepts the deep ice sheet coring disc drill developed by ice coring and drilling services icds under contract with the us national science foundation is an electromechanical drill designed to take 122 mm diameter ice cores to depths of 4000 m the conceptual design of the disc drill was developed in 2002 03 based on science requirements written by k taylor and the united states ice coring community and on engineering performance objectives detailed design of the drill began in june 2003 special attention was paid to building safety into the design and operation of the drill system the drill was designed and manufactured by a team of engineers and technicians from the university of wisconsin madison and various subcontractors with assistance from the science community the european ice drilling community and polar logistical support organizations icds successfully tested the drill in greenland in 2006 and will continue its development to meet the science objectives of the west antarctic ice sheet divide ice core project

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

a new 122 mm electromechanical drill for deep ice sheet coring disc 2 mechanical design the deep ice sheet coring disc drill consists of four major mechanical drilling subsystems and four subsystems supporting on surface activities the mechanical drilling subsystems are a drill sonde a drill cable a tower and a winch the drill sonde is the down hole portion of the drill system and consists of six distinct sections 1 the cutter head 2 the core barrel 3 the screen section 4 the motor pump section 5 the instrument section and 6 the upper sonde which includes anti torques and drill cable terminations the drill cable not only provides the means of supporting the drill sonde in the borehole but also provides conduits for electrical power and data transmission the tower tilts to allow the drill sonde to be serviced in the horizontal position without removing it from the tower the winch provides a means of quickly raising the sonde from the borehole and providing the fine control necessary for coring operations

basalt triggering of the c ad 1305 kaharoa rhyolite eruption tarawera volcanic complex new zealand abstract the ad 1305 kaharoa eruption episode of tarawera volcanic complex was the largest 4 km3 magma eruption in new zealand in the last 1000 years high silica 76 sio2 rhyolite was erupted from seven vents along an 8 km linear zone to form pumiceous pyroclastic deposits and lava domes initial vent clearing explosions were followed by a series of plinian pumice eruptions that spread tephra downwind and pyroclastic density current deposits over the volcano slopes the early pyroclastic eruptions were followed by extrusion of a dome in the summit vent and the migration of further plinian activity to adjacent vents the episode finished with the extrusion of three summit lava domes accompanied by block and ash flows two main types of rhyolite were erupted 1 a low zr low sr high rb type comprising the early plinian pyroclastics and 2 a high zr high sr low rb type comprising the late eruptives including the summit domes basaltic material basalt and basaltic andesite is commonly present as free clasts within the kaharoa pyroclastic deposits along with rare clasts of granodiorite diorite gabbro and olivine clinopyroxenite the basaltic clasts are often veneered by kaharoa rhyolite basaltic material is also common as inclusions within rhyolite pumices along with olivine and augite xenocrysts derived from the basalt the basaltic clasts and inclusions contain complexly zoned plagioclase and corroded olivine commonly with a reaction rim consistent with disequilibrium caused by magma mixing some basaltic clasts and inclusions are hornblende free others contain groundmass hornblende plumes of brown glass often extend from hornblende bearing basaltic inclusions into the surrounding rhyolite these plumes demonstrate magma mixing as does the presence of rhyolitic glass and quartz xenocrysts in some basaltic inclusions most basaltic inclusions have crenulate boundaries suggesting they were fluidal when coming into contact with the rhyolite magma diorite gabbro and olivine clinopyroxenite clasts have mineralogy similar to the basalts commonly contain patches of fine grained basaltic material and are considered co magmatic with the basalts intrusion of basalt magma into the base of the rhyolite magma chamber appears to have occurred for some time before the kaharoa eruptions began allowing considerable mixing with the rhyolite magma transfer of water to some basalt inclusions and formation of hornblende in their groundmass conversely the hornblende free basalt inclusions did not have time to react with the rhyolite magma and must have been intruded shortly before the start of the eruption intrusion of this hornblende free basalt appears to have triggered the kaharoa eruption after the magma chamber had been primed by the earlier intrusions

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

trace element indicators of crystal accumulation in silicic igneous rocks abstract if fractional crystallization coupled with some crustal assimilation afc is the dominant process for generating evolved magmas in the earth s crust large quantities of igneous cumulates must be present in the rock record however despite some reported occurrences intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive the signature is expected to be subtle in viscous silicic magmas and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern we propose models that use specific trace element concentrations and ratios zr ba eu eu and zr hf within a well constrained framework for the physical extraction of liquid from crystals mostly occurring between 50 and 70 vol crystals to identify silicic cumulates the results of these models indicate that crystal accumulation or melt extraction has occurred in multiple natural examples around the world both in plutonic and volcanic realms however these examples were selected because they represent conditions that are most favorable high amount of melt extraction well constrained initial liquid compositions for unambiguously identifying cumulate residue in many cases we show that the trace element signature for crystal accumulation will remain largely concealed in addition the different liquid lines of descent related to the various tectonic settings on earth dry reduced in hot spots divergent margins and wetter more oxidizing in arc environments will lead to different trace element evolution and each criterion must be applied on a case by case basis despite these limitations the undeniable but subtle presence of silicic cumulates in the mid to upper crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs

initiation of cataclastic flow and development of cataclastic foliation in nonporous quartzites from a natural fault zone abstract microstructural study of foliated quartz cataclasites of a segment of the towaliga fault zone georgia usa indicates that rock strain by cataclastic deformation occurred through microscopic displacements along a distributed system of shear bands the localized cataclastic flow in the bands was in turn the result of frictional sliding along a network of mesoscopic shear fractures that accommodated initial strains and were in part created by high fluid pressures in the fault zone the shear fractures at both the microscopic and mesoscopic scale were formed in the riedel r1 p and y orientations simultaneous displacements along these riedel shears produced the cataclastic foliation rhomb shaped and tabular quartz grain fabrics and straight stepped grain boundaries crystallization by fluids accompanied fracturing and possibly aided the cataclastic flow by forming muscovite and quartz crystals along fractures most fluid inclusion trails and quartz veins cut across the cataclastic features indicating that fluids which assisted the deformation later healed the fractures a right lateral sense of displacement is suggested for the cataclastic flow consistent with the kinematics of the mylonitization along the fault zone

volcanic and structural evolution of the okataina volcanic centre dominantly silicic volcanism associated with the taupo rift new zealand article the okataina volcanic centre ovc contains the northeasternmost caldera complex in onshore taupo volcanic zone tvz new zealand sited largely within the taupo rift pyroclastic fall deposits and ignimbrites deposited in the bay of plenty coast area between 420 and 625 ka were probably erupted from ovc and these provide a maximum age for the centre the earliest ignimbrite for which there is good evidence for eruption from ovc is the c 550 ka quartz biotite ignimbrite exposed to the west of lake okataina and to se of ovc this ignimbrite probably correlates with one of the early ignimbrites found in the kawerau geothermal wells and is large enough to have been accompanied by caldera collapse it was followed by extensive rhyolitic explosive eruptions of the murupara subgroup culminating with eruption of the matahina ignimbrite at c 325 ka this c 160 km 3 magma volume eruption was accompanied by caldera collapse to form the southern part of the present day okataina caldera complex a long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of ovc before eruption of the 100 km

igneous emplacement in a transpressive shear zone ox mountains igneous complex the central and southwestern ox mountains inlier in western ireland comprises amphibolite facies metasediments and metavolcanic rocks which have been deformed by a major sinistral shear zone granitoid emplacement occurred in two distinct styles in the ox mountains shear zone the older pluton the ox mountains granodiorite was emplaced syn kinematically with respect to the main sinistral transpressional events in its wall rocks it was emplaced as a series of compositionally distinct sheets in the central part of the shear zone system thus suggesting the formation of transient dilational sites occurred in response to the transpressive deformation the younger lough talt and easky lough adamellites were emplaced as individual intrusions in dilational cavities produced by reactivated movements on a discrete zone within the ox mountains inlier the different intrusive styles are considered to be a product of the tectonic regimes operating during emplacement intrusion into transient dilational cavities may be one method by which granitoid magmas are emplaced into fault zones undergoing active contractional deformation

late stage mafic injection and thermal rejuvenation of the vinalhaven granite coastal maine the vinalhaven intrusive complex consists mainly of coarse grained granite inward dipping gabbro diorite sheets and a fine grained granite core small bodies of porphyry occur throughout the coarsegrained granite the largest porphyry body roughly 0 5 km by 2 5 km occurs with coeval gabbro hybrid rocks and minor finegrained granite in the vinal cove complex which formed during the waning stages of solidification of the coarse grained vinalhaven granite porphyry contacts with surrounding coarse grained granite are irregular and gradational compositions of whole rocks and minerals in the porphyry and the coarse grained granite are nearly identical neighboring phenocrysts in the porphyry vary greatly in degree of corrosion and reaction indicating that the porphyry was well stirred thermal rejuvenation of a silicic crystal mush by a basaltic influx can explain the composition and texture of the porphyry comparable rejuvenation events have been recognized in recent studies of erupted rocks weakly corroded biotite phenocrysts in the porphyry require that hydrous interstitial melt existed in the granite during remelting field relations along with thermal calculations suggest that cooling and crystallization of coeval mafic magma could have generated the porphyry by thermal rejuvenation of granite crystal mush containing about 20 melt field relations also suggest that some of the porphyrymatrix may represent new felsic magma that was emplaced during remelting

microstructure and geochemistry of plagioclase and microcline in naturally deformed granite abstract naturally deformed feldspars from foliated granites in a shear zone in newfoundland exhibit transitional brittle ductile behaviour brittle failure is subordinate to dynamic recrystallization microcracking strain enhanced diffusion and reaction enhanced ductility during the deformation both plagioclase an 28 and k feldspar are transformed to albite with increasing strain interaction of metamorphic and structural processes at the grain scale is emphasised this is illustrated with examples of quartz filled veins segregation bands in plagioclase and recrystallized polycrystalline aggregates in plagioclase and k feldspar the role of microcracking in plagioclase and of pre existing internal growth structures in the formation of initially coarse grained recrystallized aggregates from large single crystals is suggested

remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs an example from the izu bonin arc

formation of rhyolite at the okataina volcanic complex new zealand new insights from analysis of quartz clusters in plutonic lithics granitoid lithic clasts from the 0 7 ka kaharoa eruption at the tarawera volcano okataina volcanic complex taupo volcanic zone new zealand provide insight into the processes of rhyolite formation the plutonic lithic clasts of the kaharoa eruption consist of 1 quartz phenocrysts which are often grouped into clusters of two to eight quartz grains 2 plagioclase phenocrysts mostly an 40 with up to an 60 cores and 3 interstitial alkali feldspar quartz orientations obtained through electron backscatter diffraction ebsd methods show that 78 of the 82 analyzed clusters have at least one pair of quartz grains with the dominant dipyramidal faces matched variations in cathodoluminescence cl zoning patterns of the quartz suggest that quartz clusters came together after initial crystal growth and that many quartz crystals were subject to one or more resorption events the process of quartz crystals with different magmatic histories coming together into common relative orientations to form clusters is indicative of oriented quartz synneusis and suggests a history of crystal accumulation the quartz clusters are interpreted to have formed as part of a crystal cumulate mush within a shallow magma chamber where quartz crystals rotated into contact along their dominant dipyramidal faces during hindered settling and or compaction the preservation of oriented quartz clusters from the kaharoa plutonic lithics thus provides evidence for synchronous shallow pluton formation from a cumulate mush during active silicic volcanism this result is consistent with models whereby melt rich high silica rhyolite formation occurs via interstitial melt extraction from a low silica rhyolite mush in the shallow crust

middle late pleistocene tephrostratigraphy of the eastern bay of plenty new zealand abstract up to fifty tephras ranging in age from ca 500 to 50 ka are recorded in a terrestrial sequence of loess and paleosols in the eastern bay of plenty north island new zealand tephra correlations are based on the distinctive physical characteristics of the airfall beds and confirmed by microprobe analysis of glass shards the deposits are divided into six subgroups with their boundaries marked by either major tephras or significant changes in the paleo climate indicator deposits such as loess and paleosols age control on these tephras has been provided by fitting the paleoclimatic data seen in the field to the specmap oxygen isotope curve with correlations to a few well dated tephras and ignimbrites providing key time planes within this record

insights into volcanic conduit flow from an open source numerical model 1 numerical models that calculate the fluid dynamics of explosive volcanic eruptions have been used with increasing frequency to understand volcanic processes and evaluate volcanic hazards yet those who develop such models rarely make them publicly available so that they can be verified used and possibly improved by other scientists in this paper i present a visual interactive open source numerical model that calculates steady state flow of magma and gas in vertical eruptive conduits and contains user friendly utilities for quickly determining physical thermodynamic and transport properties of silicate melts h2o gases and melt gas crystal mixtures the model represents an advance over previously published conduit models by incorporating a non arrhenian viscosity relation for hydrous silicate melts a relation between viscosity and volume fraction of gas that depends on capillary number and adiabatic temperature changes in the mixture using established thermodynamic relations for melts and h2o gas respectively volcanologists who have not had access to conduit models have frequently approximated conduit flow using an analytical equation for incompressible laminar newtonian pipe flow which predicts that the mass flux is proportional to the fourth power of conduit radius and inversely proportional to mixture viscosity the model presented here which is not much more difficult to use than a back of the envelope calculation shows that the pipe flow approximation significantly overestimates the sensitivity of mass flux to both conduit radius and mixture viscosity results from the model also show that viscous heating in the lower conduit which is not considered in most other models may increase the mass flux of large silicic eruptions by several percent and decrease the viscosity of the mixture at the fragmentation depth by a few tens of percent

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

the relationship between tectonic strain and the three dimensional shape fabrics of pebbles in deformed conglomerates the bias associated with the use of the shape of deformed inclusions to estimate bulk strain from conglomerate systems is examined mechanical considerations based on eshelby9s theory for the deformation of ellipsoidal inclusions supported by data from naturally deformed conglomerates with more than one type of inclusion reveal a discrepancy between the degree of prolateness oblateness shown by the inclusion and the bulk rock strain ellipsoid this bias is most marked in situations where the difference of competence between the inclusion and matrix is high and the volume fraction of inclusions is low in these situations an inclusion which is more competent than the matrix will deform to a shape which is more prolate than the bulk strain ellipsoid this has important implications for strain analysis of conglomeratic rocks and the interpretation of pebble shape fabrics in the context of regional tectonics