an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

mobilization of gold as a polymetallic melt during pelite anatexis at the challenger deposit south australia a metamorphosed archean gold deposit the challenger gold deposit in south australia is hosted by pelitic migmatites that underwent peak metamorphism at 7 5 1 5 kbars and at least 800 c nd model ages suggest a protolith age of 2900 ma zircon u pb and garnet sm nd dating indicates that peak metamorphism occurred at 2440 ma and that garnet was present during this event at this time in situ vapor absent melting affected the pelitic host rock indicating that peak metamorphism occurred without introduction of a foreign fluid or melt thus evidence indicating the presence of ore minerals during peak metamorphism implies a prepeak mineralization event this evidence includes the occurrence of invisible gold in lollingite but not in adjacent arsenopyrite and the presence of spherical gold sulfide inclusions in peak metamorphic garnet and other silicates textures developed between pyrrhotite lollingite arsenopyrite and gold the lack of silicate alteration minerals supports this conclusion experimental evidence is presented to support our interpretation that a gold rich polymetallic melt was mobilized into leucosomes synchronously with silicate melt during peak metamorphism visible gold is restricted to migmatitic leucosomes and to a lesser extent melanosomes large inclusions of gold with coexisting arsenopyrite pyrrhotite and bismuth are hosted within peak metamorphic silicate minerals and at grain boundaries trails of micrometric spherical inclusions of predominantly gold and bismuth propagate along annealed fractures from these larger inclusions we show that these gold sulfide inclusions represent the crystallized products of an immiscible polymetallic melt metamorphism of the challenger deposit resulted in partial melting of the pelitic host rock and formation of this gold rich melt these two melts were synchronously redistributed to accumulate as leucosomes during development of a stromatic migmatite formation of a polymetallic melt thereby enabled extensive mobilization of gold producing leucosomes enriched in gold this occurred because the immiscible metallic melt was physically entrained rather than chemically dissolved within the silicate melt gold rich shoots at challenger parallel the plunge of ptygmatically folded leucosomes that are shown to be parasitic to a larger scale fold geometry which appears to be structurally related to the ore shoots it is interpreted that concurrent migration of polymetallic and silicate melt allowed concentration of gold into a series of dilational structures which developed within the larger scale fold geometry challenger represents a new deposit type other deposits around the world such as renco and griffins find are hosted in granulite facies rocks but challenger is the first reported example of leucosome hosted gold in migmatites

towards the reconstruction of the shallow plumbing system of the barombi mbo maar cameroon implications for diatreme growth processes of a polygenetic maar volcano abstract understanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention while there is no direct evidence of the presence of diatremes beneath most of the young maars their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars as an alternative this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar forming eruptions based on field investigations pyroclast distribution componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the barombi mbo maar bmm a polygenetic maar volcano in cameroon the bmm consists of three diatremes that formed during distinct eruptive events and coalesced to produce an amalgamated maar diatreme two end member types of eruption styles from the dry magmatic to the wet phreatomagmatic explosions governed the formation of the maar in total a minimum of 0 0658 km 3 of magma dense rock equivalent corrected was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from srtm30m data corrected by rock textural data collected from the field the distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process which originated probably within the uppermost part of the diatreme from the distribution and morphology of juvenile clasts in the deposits it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions sometimes with lateral and vertical variations in the position of the explosion loci a prospective juvenile based conceptual model is proposed for the formation of the bmm the model suggests that during maar forming eruptions explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic dominated upper crater infill to deposit juvenile poor 90 vol

evidence for an earliest oligocene ice sheet on the antarctic peninsula there is growing consensus that development of a semipermanent ice sheet on antarctica began at or near the eocene oligocene e o boundary beyond ice rafted debris in oceanic settings however direct evidence for a substantial ice sheet at this time has been limited and thus far restricted to east antarctica it is unclear where glacier ice first accumulated and how extensive it was on the antarctic continent in the earliest oligocene sediments at the top of the eocene marine shelf section on seymour island antarctic peninsula include glacial marine deposits and a lodgment till with clasts derived from a variety of rock units on the peninsula dinoflagellate biostratigraphy and strontium isotope stratigraphy indicate an age at or very close to the e o boundary glacier ice extending to sea level in the northern peninsula at this time suggests the presence of a regionally extensive west antarctica ice sheet and thus an even more dramatic response to the forcing factors that facilitated high latitude ice expansion in the earliest oligocene

coupled textural and compositional characterization of basaltic scoria insights into the transition from strombolian to fire fountain activity at mount etna italy strombolian and fire fountain activities represent a common expression of explosive basaltic eruptions however the transition between these two eruptive styles and their source mechanisms are still debated we use textural and compositional studies to characterize pyroclastic material from both the strombolian and hawaiian style fire fountain phases of the january june 2000 etna activity we find that basaltic scoria presents distinctive textural and compositional features that reflect different modes of magma vesiculation and crystallization in the two eruptive regimes overall magma that forms strombolian scoria is far more crystallized less vesicular and more evolved indicating strong volatile depletion and longer residence time before being erupted fire fountain scoria indicates a fast rising magma with evidence of moderate syneruptive volatile exsolution the new textural and compositional data set is integrated with previous volcanological and geophysical investigations to provide further insights into the dynamics of fire fountains and to frame the transition from strombolian explosions to fire fountain activity into a model that may apply to future eruptions at mount etna as well as other active basaltic volcanoes

the possibility of a zns bearing sulfide melt at 600 c evidence from the rajpura dariba deposit india supported by laboratory melting experiment abstract inclusions of euhedral pyrite crystals are seen within sphalerite occupying veins and fractures in the rajpura dariba zn pb cu sulfide deposit in rajasthan india sphalerite in the deposit shows wide variation in composition and contains high sulfur with fe s zn varying from 0 49 to 1 06 indicating that a portion of the fe in sphalerite could be expressed as a pyrite fes 2 component and that the pyrite inclusions within sphalerite in the vein ore could have formed due to exsolution we have conducted experiments by the evacuated silica tube method at 600 c pertinent to the peak metamorphic conditions of the rajpura dariba deposit our experiments were aimed at exploring the possibility of a zns bearing sulfide partial melt that could precipitate sufficiently s rich sphalerite which in turn might exsolve pyrite the starting mixture comprised synthetic fes zns pbs cu 2 s and s in the molar proportions of 40 33 9 9 and 9 respectively which approximately corresponded to molar proportions of 27 fes 40 zns 11 pbs and 22 cufes 2 electron probe micro analysis epma of the resulting run product showed the presence of a cu fe zn pb s melt containing 4 61 mol zns in the coexisting sphalerite the estimated fes 2 component was 9 07 mol the eutectic temperature was determined to be 595 c with a melt containing 22 44 mol fes 31 24 mol cu 2 s 5 37 mol zns 35 81 mol pbs and 5 14 mol fes 2 the sphalerite that was in equilibrium with the eutectic melt contained 6 72 mol fes 2 our experimental results suggest that a zns bearing sulfide melt containing as high as 5 mol zns was produced during metamorphism at rajpura dariba by partial melting of the preexisting ore further experiments confirmed that a minimum sulfur fugacity of 2 46 log units is required for initiating melting pyrite and sphalerite containing fe and stoichiometrically excess s are the solidus phases present in all melt bearing experimental run products except one with much higher zns suggesting that these two minerals at rajpura dariba could have simultaneously crystallized from a sulfide partial melt giving rise to the observed poikilitic textural relation between the two

footwall type cu ni pge mineralization in the broken hammer area wisner township north range sudbury structure footwall type cu ni pge platinum group element deposits are a major focus of recent exploration in the sudbury mining camp owing to their higher cu and precious metal contents compared to orebodies situated along the lower contact of the sudbury igneous complex the most important footwall type orebodies occur in the onaping levack area of the north range but economic deposits are found along the east range of the structure as well recently promising occurrences were also discovered in wisner township of the north range in this paper we report results from two main footwall type cu ni pge occurrences the broken hammer zone and the south zone of wisner township both discoveries are situated in intensely brecciated zones of the footwall sudbury breccia 1 300 and 500 m respectively north of the present sudbury igneous complex footwall contact the mineralization consists of 1 massive sulfide veins 2 disseminated replacement sulfides and 3 silicate quartz dominated veins although massive sulfide veins account for the major part of the resource sulfide poor disseminations may have similarly high precious metal contents ore assemblages are dominated by chalcopyrite millerite magnetite and or pyrite quartz and hydrous silicates these are accompanied by platinum group minerals pgm and other trace minerals which commonly occur as composite grains within sulfides and hydrous silicates these include minerals typical of footwall type deposits e g merenskyite michenerite hessite but also minerals found only in a few sudbury deposits e g clausthalite sopcheite naumannite bohdanowiczite an unnamed cupdbis 3 mineral is one of the most abundant pgm at broken hammer and might be a pd equivalent of muckeite cunibis 3 statistical investigation of metal distribution patterns using grab sample and drilling data sets from both localities indicate important differences between the two occurrences and the various mineralization styles e g increasing cu cu ni and decreasing pt pt pd ratio with increasing sulfide content metal concentrations recalculated to 100 sulfides decrease by up to two orders of magnitude for all metals from the disseminated to the massive sulfides fluid inclusion studies revealed three fluid generations which are very similar to those described from well known footwall type deposits of the onaping levack area primary polyphase fluid inclusions in cu ni pge bearing assemblages represent a highly saline 30 40 wt nacl equiv magmatic hydrothermal system with a high temperature min 450 500 c and a lower temperature min 300 350 c stage a later low temperature ca rich fluid min 100 200 c is most probably part of a post sudbury igneous complex regional fluid flow in which temperature was controlled by the geothermal gradient and not related to ore forming processes the fluid inclusion data indicate that mobilization of highly saline and relatively high temperature 400 500 c sudbury igneous complex related magmatic hydrothermal fluids similar to those described from the onaping levack embayment also occurred in the wisner footwall although our studies do not rule out the possibility of an initial magmatic sulfide melt emplacement they prove that magmatic hydrothermal fluids played a significant role in the transport and deposition of metals

origin of high magnesian andesites in the setouchi volcanic belt southwest japan ii melting phase relations at high pressures melting phase relations of an augite olivine high magnesian andesite and an augite olivine basalt from the miocene setouchi volcanic belt in southwest japan have been studied under water saturated water undersaturated and under anhydrous conditions both the andesite and the basalt are characterized by low feo mgo ratios 0 86 and 0 76 in weight respectively and qualify as primary magmas derived from the upper mantle the andesite melt coexists with olivine orthopyroxene and clinopyroxene at 15 kbar and 1030 c under water saturated conditions and at 10 kbar and 1070 c under water undersaturated conditions 7 wt h2o in the melt the basalt melt also coexists with the above three phases at 11 kbar and 1305 c under anhydrous conditions and at 15 kbar and 1205 c in the presence of 4 wt water present studies indicate that high magnesian andesite magmas may be produced even under water undersaturated conditions by partial melting of mantle peridotite it is suggested that two types of high magnesian andesites in the setouchi volcanic belt augite olivine and bronzite olivine andesites were produced by different degrees of partial melting augite olivine andesite magmas whose mantle residual is lherzolite were formed by lower degrees of partial melting than bronzite olivine andesite magmas which coexist with harzburgite the basalt magmas which were often extruded in close proximity to the high magnesian andesite magmas are not partial melting products of a mantle peridotite which had previously melted to yield high magnesian andesite magmas

mechanisms for transition in eruptive style at a monogenetic scoria cone revealed by microtextural analyses lathrop wells volcano nevada u s a explosive activity at lathrop wells volcano nevada u s a originated with weak strombolian ws eruptions along a short fissure and transitioned to violent strombolian vs activity from a central vent with lava effusion during both stages the cause for this transition is unknown it does not reflect a compositional change as evidenced by the consistent bulk geochemistry of all the eruptive products however comparison of agglutinate samples from the early ws events with samples of scoria from the later vs events reveal differences in the abundance and morphology of groundmass phases and variable textures in the rims of olivine phenocrysts scanning electron microscope sem examination of thin sections from the ws samples show euhedral magnetite microlites in the groundmass glass and olivine phenocrysts show symplectite lamellae in their rims secondary ion mass spectrometry sims depth profiles of these symplectites indicate they are diffusion controlled the calculated dfe mg allows an estimation of the oxygen fugacity fo2 and indicates an increased fo2 during eruption of the ws products conversely the vs samples show virtually no magnetite microlites in the groundmass glass a lack of symplectites in the olivines and a lower calculated fo2 these microtextural features suggest that the lathrop wells trachybasalt experienced increased oxidation during ws activity as magma ascended through the original fissure exsolved bubbles were concentrated in the wider part s the protoconduit and this bubble flux drove convective circulation that oxidized the magma through exposure to atmosphere and recirculation this oxidation resulted in groundmass crystallization of magnetite within the melt and formation of symplectites within the olivine phenocrysts bubble driven convection mixed magma vertically within the protoconduit keeping it fluid and driving strombolian bursts while microlite crystallization in narrower parts of the fissure helped to focus flow development of a central conduit increased the magma ascent velocity due to a greater product volume in the later eruptive stages and caused the shift in eruption intensity consequently variations in microtextures of the lathrop wells products reveal how a combination of fluid dynamic and crystallization processes in the ascending magma resulted in different styles of activity while the products maintained a consistent bulk composition

radiocarbon ages of holocene pelado guespalapa and chichinautzin scoria cones south of mexico city implications for archaeology and future hazards pelado guespalapa and chichinautzin monogenetic scoria cones located within the sierra del chichinautzin volcanic field scvf at the southern margin of mexico city were dated by the radiocarbon method at 10 000 2 800 4 700 and 1 835 years b p respectively most previous research in this area was concentrated on xitle scoria cone whose lavas destroyed and buried the pre hispanic town of cuicuilco around 1 665 35 years b p the new dates indicate that the recurrence interval for monogenetic eruptions in the central part of the scvf and close to the vicinity of mexico city is 2 500 years if the entire scvf is considered the recurrence interval is 1 700 years based on fieldwork and landsat imagery interpretation a geologic map was produced morphometric parameters characterizing the cones and lava flows determined and the areal extent and volumes of erupted products estimated the longest lava flow was produced by guespalapa and reached 24 km from its source total areas covered by lava flows from each eruption range between 54 chichinautzin and 80 km2 pelado and total erupted volumes range between 1 and 2 km3 cone an average eruption rate for the entire scvf was estimated at 0 6 km3 1 000 years these findings are of importance for archaeological as well as volcanic hazards studies in this heavily populated region

from feeder dykes to scoria cones the tectonically controlled plumbing system of the rauðholar volcanic chain northern volcanic zone iceland the rau holar volcanic chain located in the northern volcanic zone of iceland has been variably eroded such that in the northern part the original scoria cones are preserved while the central and southern parts expose their shallow feeders the chain thus offers insight into the inner workings of the near surface feeder system of scoria cones the volcanic chain was mapped in 3d using gps the en echelon arranged volcanic chain can be divided into three parts the southernmost part contains only plugs and necks with a thin pyroclastic cover as well as multi tiered lava flows the central part combines partially eroded scoria cones feeder dyke intersections and welded scoria interbedded within rootless and clastogenic lava flows the welded scoria is composed of different kinds of lithics and bombs the northern part preserves almost intact overlapping scoria cones with voluminous lapilli sized scoriaceous deposits the overall dyke trend is orthogonal but shows radial patterns in individual cone complexes feeder dykes observed to depths of about 200 m below the volcanic chain are up to 8 m thick and flare in to conduits in the uppermost 20 50 m the exposed shallow plumbing system shows that magma pathways through the volcanic edifice are very complex with incremental repeated intrusions we interpret the arcuate shape to be the result of a local change in the orientation of the stress field because the rau holar volcanic chain is located within a major relay structure between volcanoes on the eastern fremrinamur rift arm and a rift extension with grabens on the western periphery

eruptive style of the young high mg basaltic andesite pelagatos scoria cone southeast of mexico city the eruption of the pelagatos scoria cone in the sierra chichinautzin monogenetic field near the southern suburbs of mexico city occurred less than 14 000 years ago the eruption initiated at a fissure with an effusive phase that formed a 7 km long lava flow and continued with a phase of alternating and or simultaneous explosive and effusive activity that built a 50 m high scoria cone on the western end of the fissure and formed a compound lava flow field near the vent the eruption ended with the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater products consist of a microlite rich high mg basaltic andesite samples were analyzed to determine the magma s initial properties as well as the effects of degassing induced crystallization on eruptive style although distal ash fallout deposits from this eruption are not preserved a recent quarry exposes a large section of the scoria cone detailed study of exposed layers allows us to elucidate the mode of cone building activity petrological and textural data combined with models calibrated by experimental work and melt inclusion analyses of similar magmas elsewhere indicate that the magma was initially hot 1 200 c gas rich up to 5 wt h2o crystal poor 10 vol fo90 olivine phenocrysts and thus poorly viscous 40 80 pa s during the early phase low magma ascent velocity at the fissure vent allowed low viscosity magma to degas and crystallize during ascent producing lava flows with elevated crystal contents at t 1 100 c and blocky surfaces later the closure of the fissure by cooling dikes focused the magma flow at a narrow section of the fissure this led to an increased magma ascent velocity rapid and shallow degassing 3 km deep triggered 40 vol microlite crystallization limited times for gas escape and higher magma viscosity 6 105 4 106 pa s drove strong explosions of highly 60 80 vol and finely vesicular magma coarse clasts broke on landing which implies brittle behavior due to complete solidification this requires sufficient time to cool and in turn implies ejection heights of over 1 km which is much higher than normal strombolian activity hence magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics of shallow degassing the long lasting eruptions of jorullo and paricutin which produced similar magmas in western mexico were more explosive this can be related to higher magma fluxes and total erupted volumes implications of this study are important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the trans mexican volcanic belt

the engineer mine british columbia an example of epithermal au ag mineralization with mixed alkaline and subalkaline characteristics abstract the engineer mine epithermal precious metal deposit in british columbia canada is related to eocene sloko skukum group ssg volcanism and according to previous studies contains roscoelite in intimate association with electrum roscoelite a vanadian mica is considered characteristic of low sulfidation epithermal deposits related to alkaline magmatism this contradicts the fact that the ssg volcanics are subalkaline in order to address this ambiguity and to accurately classify the style of epithermal mineralization at the engineer mine we conducted detailed petrographic mineralogical geochronological fluid inclusion and stable isotope studies the principal ore assemblage of the engineer mine epithermal veins precipitated in response to boiling during a hydrothermal event at 49 90 ma 0 25 ma during this event electrum arsenopyrite pyrite chalcopyrite sphalerite lollingite tetrahedrite group phases allargentum acanthite hessite dyscrasite stibarsen galena and an unidentified ag rich phase were deposited in conjunction with amorphous silica platy and rhombic calcite k feldspar and vanadian illite fluid inclusion and stable isotope data suggest that the ore forming fluid was boiling at 220 c during vein mineralization and had an isotopic composition derived from local meteoric water based on these results the engineer mine is classified as an epithermal low sulfidation deposit which shares similarities with alkaline and subalkaline epithermal low sulfidation deposits this is attributed to the fact that the ssg volcanic rocks are borderline subalkaline to alkaline in character and that the sedimentary host rocks are vanadium bearing these sedimentary rocks contributed the bulk of the vanadium to the engineer mine epithermal system the presence of roscoelite at the engineer mine could not be confirmed during this study the mica referred to as roscoelite in previous publications instead is vanadian illite to our knowledge the only alkaline low sulfidation epithermal precious metal deposit that contains v mica which exclusively qualifies as true roscoelite is the porgera deposit papua new guinea

partial melting of the broken hill galena sphalerite ore experimental studies in the system pbs fes zns ag2s for many years the genesis of the broken hill pb zn ag deposit has been and continues to be hotly debated mostly because many of the original characteristics of the deposit have been obscured by high grade metamorphism and deformation metamorphism not only overprinted the deposit but fundamentally modified the sulfides through metamorphic remobilization based on experimental studies it had also been speculated that partial melting of the sulfides may have occurred the lack of experimental data at elevated pressures in the pbs fes zns system has limited this suggestion to mere speculation we present the results of an experimental study of the melting behavior of sulfides which show that broken hill ores must have been partially molten under peak metamorphic conditions

textural evidence for extensive melting of the broken hill orebody galena pyrrhotite and chalcopyrite in samples of zn rich ore from the broken hill orebody show textures similar to those in partially melted metamorphic rocks including strings of low melting phases separating lithons of minerals likely to be restite low dihedral angles between low melting temperature minerals and restitic minerals and enlargement of these dihedral angles due to partial annealing analysis of existing experimental work from the system fe pb zn s indicates that at 1 bar galena and sphalerite coexisting with pyrrhotite of the compositions found at broken hill would have been partially melted at temperatures of about 750 c at 5 kbars this melting would occur at 780 c this temperature is well within the range of metamorphic temperatures reported from the area from these data we conclude that the broken hill orebody was partially molten at peak metamorphism with much of the pb cu ag sb and as in the melt and fe and zn residing mostly in the restite differentiation during cooling enriched the residual melt in sb as ag and other low melting temperature chalcophile elements the main sulfide melt froze after the last penetrative deformation to affect the orebody at temperatures below 720 c and possibly as low as 650 c sulfosalts crystallized out of the remaining melt at temperatures of 600 to 450 c and the final melt crystallized native bismuth at ca 250 c

monogenetic vulcanism in sierra chichinautzin mexico the variation in the activity patterns of the chichinautzin volcanic rocks is discussed this sequence of lavas and pyroclastic deposits is located in the central part of the mexican volcanic belt directly south of mexico city and is typical of its quaternary monogenetic vulcanism

textural morphological and compositional varieties of modern arc sulfides a case study of the tolbachik volcano kamchatka abstract magma unmixing into separate sulfide and silicate melts is a key process in the formation of magmatic sulfide ore deposits sulfide inclusions in olivine preserve the original chemical composition of the segregated sulfide melt whereas the phase compositions of the solidified sulfide droplets and their textures record the final stages of sulfide evolution sulfide globules hosted in olivine phenocrysts of magnesian basalts from the tolbachik volcano kamchatka display a variety of textures ranging from homogeneous to fine grained to coarse grained and lamellar porous textures and textures associated with the oxidation and dissolution of sulfides were identified as separate categories the outer surfaces of sulfides vary significantly in shape which includes normal spherical or elliptical globules gravitationally layered inclusions thin sulfide foils and concave deformed sulfides in contact with fluid bubbles the estimated cooling rate of olivine phenocrysts responsible for observed crystallization textures varies by five orders of magnitude the fe ni cu ratio also exerts an influence on the resulting texture whereas the size of the globules does not have any significant effect the morphology and textural patterns of individual grains indicates that fine grained textures resulted from the rapid breakdown of a homogeneous solid phase which the sulfide melt solidified into during extremely rapid quenching the presence of large or abundant small pores in sulfides along with channels at the globule periphery indicates the separation of an appreciable amount of dissolved fluid from the sulfide melt during crystallization sulfides in the form of thin foils and planar swarms in healed cracks indicate cyclic pressure changes in the olivine environment which resulted in phenocrysts rupturing and healing the sulfide mineralogy is represented by high temperature sulfides mss monosulfide solid solution and iss intermediate solid solution as well as pyrrhotite pentlandite chalcopyrite cubanite bornite magnetite assemblages which formed at lower temperatures

noble metals potential of sulfide saturated melts from the subcontinental lithosphere the origin of vast accumulations of nickel and platinum in some continental magmatic rocks is still enigmatic but ultimately linked to silicate sulfide liquid immiscibility the exact composition of pristine sulfide melts has proved extremely difficult to document and understand largely because of the ephemeral reactive qualities and small quantities of such melts here we report the discovery of fe ni sulfide melt globules highly enriched in noble metals pt pd au 120 ppm total platinum group elements pge within an unusual high mg andesitic glass 8 2 wt mgo 57 3 wt sio2 dredged from the southern mid atlantic ridge near the bouvet triple junction the composition of this glass indicates derivation of its parental silicate melt from a garnet pyroxenite mantle source with pronounced continental isotopic pb sr nd hf os o signatures we infer that the chemical properties of this high temperature 1250 c melt notably high sio2 and ni 310 ppm contents promoted sulfide saturation at low pressures in a purely oceanic setting and propose that this unique example with its likely origin in the continental lithospheric mantle may be a useful analogue for incipient ni pge sulfide melt generation and magmatic pge enrichment

record of complex scoria cone eruptive activity at red mountain arizona usa and implications for monogenetic mafic volcanoes abstract scoria cone eruptions are generally modeled as a simple succession from explosive eruption to form the cone to passive effusion of lava generally from the base of the cone sector collapse of scoria cones wherein parts of the cone are rafted on a lava flow is increasingly recognized as common but the reasons that a cone may not be rebuilt are poorly understood red mountain volcano is a pleistocene scoria cone in the san francisco volcanic field of northern arizona usa the cone lies along the trace of a major steeply dipping normal fault that originated during proterozoic tectonism and was reactivated in tertiary time the earliest phase of eruption at red mountain was typical strombolian forming a cone that was followed by or possibly synchronous with lava effusion toward the west from the base of the cone rafting then ensued as the west side of the cone collapsed approximately 15 of the cone is preserved in mounds as much as 30 m high rafting was extensive enough to remove most of the cone over the vent area which effectively reduced the pressure cap on the magma conduit resultant low fountaining fed clastogenic lava flows and minor scoria fallback clastogenic flows traveled as far as 4 km and now form a cliff 30 40 m high at the edge of the lava platform although several possibilities explain the change in vent dynamics and eruptive style we favor the interpretation that an increase in magma rise rate caused collapse of the cone the abrupt removal of 300 m of material over the vent removed a conduit cork and low fountaining began magma that had erupted effusively suddenly became explosive this aspect of scoria cone rafting at red mountain is broadly similar to sector collapse followed by explosive eruption in larger systems a steep walled 150 m high amphitheatre on the northeast side of red mountain exposes weakly to strongly altered scoria cemented by calcite iron and zeolites we suggest that vapor phase alteration was responsible for sealing fine grained ash beds in the cone and a pressurized system developed residual heat from a dike that was emplaced as part of the magmatic activity provided heat that drove groundwater along the regional fault up into the cone eventually the overpressurized system exploded in a phreatic eruption that created the amphitheatre which has subsequently been enlarged by water and wind erosion the combined sequence of events at red mountain illustrates some of the complexities in monogenetic scoria cone eruptions that have received little attention to date

metamorphism graphite crystallinity and sulfide anatexis of the rampura agucha massive sulfide deposit northwestern india located adjacent to the banded gneissic complex rampura agucha is the only sulfide ore deposit discovered to date within the precambrian basement gneisses of rajasthan the massive zn pb sulfide orebody occurs within graphite biotite sillimanite schist along with garnet biotite sillimanite gneiss calc silicate gneisses amphibolites and garnet bearing leucosomes plagioclase hornblende thermometry in amphibolites yielded a peak metamorphic temperature of 720 780 c whereas temperatures obtained from fe mg exchange between garnet and biotite 580 610 c in the pelites correspond to postpeak resetting thermodynamic considerations of pertinent silicate equilibria coupled with sphalerite geobarometry furnished part of a clockwise p t t path with peak p t of 6 2 kbar and 780 c attained during granulite grade metamorphism of the major zn rich stratiform sedimentary exhalative deposits orebody and its host rocks arsenopyrite composition in the metamorphosed ore yielded a temperature and log f s2 range of 352 c 8 2 to 490 c 4 64 thus indicating its retrograde nature contrary to earlier research on the retrogressed nature of graphite raman spectroscopic studies on graphite in the metamorphosed ore reveal variable degree of preservation of prograde graphite crystals 490 43 c with a maximum at 593 c the main orebody is mineralogically simple sphalerite pyrite pyrrhotite arsenopyrite galena deformed and metamorphosed while the pb ag rich sulfosalt bearing veins and pods that are irregularly distributed within the hanging wall calc silicate gneisses show no evidence of deformation and metamorphism the sulfosalt minerals identified include freibergite boulangerite pyrargyrite stephanite diaphorite mn jamesonite cu free meneghinite and semseyite the last three are reported from agucha for the first time stability relations of cu free meneghinite and semseyite in the pb ag rich ores constrain temperatures at 550 c and 300 c respectively features such as 1 low galena sphalerite interfacial angles 2 presence of multiphase sulfide sulfosalt inclusions 3 microcracks filled with galena pyrargyrite without any hydrothermal alteration and 4 high contents of zn ag and sb in galena indicate partial melting in the pbs fe0 96s zns 1 ag2s cufes2 system which was critical for metamorphic remobilization of the rampura agucha deposit

revised stratigraphy and eruption rates of ceboruco stratovolcano and surrounding monogenetic vents nayarit mexico from historical documents and new radiocarbon dates abstract more than a dozen new radiocarbon dates reconstruct the eruptive history of ceboruco volcano six of these further constrain previous results for the important plinian jala eruption which occurred near 1060 55 yr bp a calibrated radiocarbon age of ad 990 1020 was obtained as best overlap range for all samples pottery fragments found directly underneath the pumice deposit indicate that this area was inhabited by human populations that witnessed the eruption this age therefore represents an important time marker in the prehistory of this region because an area of 560 km2 was devastated and covered by a thickness of 50 cm of pumice and ash fallout after the prominent jala eruption vei 6 at least seven major lava flows and several smaller domes were issued from ceboruco s crater analysis of historical documents allows us to conclude that most of these eruptions took place well before the arrival of the spanish conquerors to this area in 1528 ceboruco s last historic eruption 1870 1872 produced a ca 7 km long viscous dacite lava flow its emplacement was accompanied by block and ash flow activity and deposition of ash fallout because a repeat of such an eruption in the future would seriously endanger the nearby population and disrupt important life lines a detailed discussion of eyewitness accounts and other documents including drawings and paintings is provided some of these materials are made available to the broader public for the first time in the present article several surrounding monogenetic vents were previously dated by others using the k ar and the ar ar methods because these dating methods are often not suitable for very young rocks we also dated several holocene monogenetic vents by the radiocarbon method these dates together with geologic mapping allowed calculating eruption rates and recurrence intervals of different types of eruptions accordingly an andesite dacite lava flow accompanied by block and ash flow activity was erupted from ceboruco s crater on average every 143 years during the last 1000 years in contrast a monogenetic eruption forming either a scoria cone or a dome has occurred in the area surrounding ceboruco on average every 1000 years during the holocene during the period between ad 1000 and ad 1500 ceboruco displayed an elevated activity experiencing a rhyodacite plinian eruption 3 4 km 3 dre followed by the emplacement of six andesitic dacitic lava flows also with a total volume of 3 4 km 3 all these figures indicate that the ceboruco volcanic region is one of the most active areas in the entire trans mexican volcanic belt consequently production of a detailed volcanic hazard map and implementation of other preventive measures aimed at reducing potential losses in case of renewed volcanic activity should be mandatory

hydrous sulfide melting experimental evidence for the solubility of h2oin sulfide melts the effect of h 2 o on sulfide melting temperatures has been investigated in the fes pbs zns system at 1 5 gpa revealing that the addition of h 2 o results in a 35 c drop in melting temperature from 900 to 865 c in addition to the melting point depression the solubility of h 2 o is confirmed by the presence of vesicles in the quenched melt no oxide phases were present in any of the run products ruling out oxidation as a cause of the melting point depression confirmation of the solubility of h 2 o in sulfide melts is consistent with the recent suggestion by mungall and brenan 2003 of a magmatic origin for halogen rich alteration associated with magmatic sulfide ore deposits as the hydrous component of the alteration may similarly originate in the fractionating sulfide melt anatectic sulfide melts could be expected to contain more h 2 o than magmatic sulfide melts owing to the lack of a parental silicate melt that buffers the h 2 o content of magmatic sulfide melts fluids expelled from the cooling anatectic melts such as that present during granulite facies metamorphism of sulfide deposits at broken hill australia may have been responsible for associated retrograde hydrothermal alteration

volcanic stratigraphy of a high altitude mammuthus columbi tlacotenco sierra chichinautzin central mexico the discovery of a near complete skeleton of mammuthus columbi in a cornfield located on the northern slopes of the sierra chichinautzin volcanic field south of mexico city sparked the interest of the scientific and public community although remains of this species of mammoth are frequently discovered in central mexico this new find is at the southernmost and highest ca 2770 m asl location yet within the mexico basin in addition the bones were found embedded in dark volcanic ash raising the possibility of a relationship between the death of the animal and explosive activity at a neighboring scoria cone as the site is located 10 km from several young volcanoes stratigraphic sedimentological geochemical and geochronological studies were conducted at the discovery site and within a 5 km radius to determine the tephra stratigraphy in the area and constrain the source of the mammoth ash and the age and taphonomy of the fossil remains results show that the mammoth was buried after death by stream flows dilute lahars that were triggered by torrential rain that remobilized loose scoriaceous ash ejected by the san miguel cone some time after its eruption ca 17 000 bp

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

sedimentology eruptive mechanism and facies architecture of basaltic scoria cones from the auckland volcanic field new zealand abstract scoria cones are a common type of basaltic to andesitic small volume volcanoes e g 10 1 10 5 km 3 that results from gas bubble driven explosive eruptive styles although they are small in volume they can produce complex eruptions involving multiple eruptive styles eight scoria cones from the quaternary auckland volcanic field in new zealand were selected to define the eruptive style variability from their volcanic facies architecture the reconstruction of their eruptive and pyroclastic transport mechanisms was established on the basis of study of their volcanic sedimentology stratigraphy and measurement of their pyroclast density porosity scanning electron microscopy 2d particle morphology analysis and visible and near visible infrared spectroscopy collection of these data allowed defining three end member types of scoria cones inferred to be constructed from lava fountaining transitional fountaining and strombolian type and explosive strombolian type using the physical and field based characteristics of scoriaceous samples a simple generalised facies model of basaltic scoria cones for the avf is developed that can be extended to other scoria cones elsewhere the typical avf scoria cone has an initial phreatomagmatic phases that might reduce the volume of magma available for subsequent scoria cone forming eruptions this inferred to have the main reason to have decreased cone volumes recognised from auckland in comparison to other volcanic fields evolved dominantly in dry eruptive condition e g no external water influence it suggests that such subtle eruptive style variations through a scoria cone evolution need to be integrated into the hazard assessment of a potentially active volcanic field such as that in auckland

on the initiation of metamorphic sulfide anatexis mineral assemblages in common sulfide ore deposits are examined together with phase relations to 1 investigate the pressure temperature conditions required for the onset of metamorphically induced partial melting involving economic minerals and 2 place constraints on the amount of melt produced deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies deposits lacking sulfosalt and or telluride minerals may begin to melt once p t conditions reach the upper amphibolite facies if galena is present or well into the granulite facies if galena is absent the result is two broad melting domains a lowto medium temperature low melt volume domain involving melting of volumetrically minor sulfosalt and or telluride minerals and a high temperature potentially higher melt volume domain involving partial melting of the major sulfide minerals epithermal gold deposits which are especially rich in sulfosalt minerals are predicted to commence melting at the lowest temperatures of all sulfide deposit types massive pb zn cu deposits may start to melt in the lower to middle amphibolite facies if pyrite and arsenopyrite coexist at these conditions and in the upper amphibolite facies if they do not excepting sulfosalt bearing occurrences massive ni cu pge platinum group element deposits will show little to no melting under common crustal metamorphic conditions whereas disseminated cu deposits are typically incapable of generating melt until the granulite facies is reached when partial melting commences in bornite bearing rocks the volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals even at granulite facies conditions this volume is usually less than 0 5 the exception is massive pb zn deposits where melt volumes significantly exceeding 0 5 vol may be segregated into sulfide magma dykes allowing mobilization over large distances

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

analysis of geomagnetic secular variation for the last 1 5 ma recorded by volcanic rocks of the trans mexican volcanic belt new data from sierra de chichinautzin mexico

evidence of sulfide melting and melt fractionation during amphibolite facies metamorphism of the rajpura dariba polymetallic sulfide ores abstract partial melting of sulfide ores during prograde metamorphism could have been more prevalent than generally accepted however identification of such melting is difficult as sulfide melts do not form glasses and the textures generated on quenching are obliterated due to the tendency of sulfides for ready recrystallization the polymetallic base metal sulfide deposit at rajpura dariba rajasthan india is a typical stratiform ore metamorphosed to the middle amphibolite facies the peak metamorphic temperature of 600 c should have been sufficient to initiate sulfide melting as evident from experimental studies in the zns pbs cu 2 s fes 2 s system further syn metamorphic melting of the original sedex ore was abetted by the high f s2 condition that prevailed as a consequence of barite dissolution a zn fe s melt containing minor pb sb and cu but no ag fractionated from an initial melt in the above system resulting in a residual immiscible sulfosalt bearing pbs melt the final metallic melts represented by formation of dyscrasite ag 3 sb from the sulfosalt bearing melt and breithauptite nisb or ullmannite nisbs from the sulfosalt absent melt were a product of independent fractional crystallization of the immiscible sulfide and pbs sulfosalt melts

morphometry of scoria cones and their relation to geodynamic setting a dem based analysis abstract the morphometry of a great number of scoria cones belonging to volcanic fields of various geodynamic settings has been measured and analyzed addressing the question whether there is a relation between the prevalent cone shape in a given field and the geodynamic setting of the field itself morphometric analysis was carried out on freely downloadable digital elevation models dems the accuracy of the used dems and the associated error in scoria cone morphometry were determined by cross comparing high resolution lidar derived dems usgs ned tinitaly dem and aster gdem the 10 m tinitaly 01 and usgs ned dems are proven to be suitable for scoria cone morphometry whereas aster gdem can be used reliably for cones with volume greater than 30 10 6 m 3 according to a detailed morphometry of all scoria cones we propose that the cones related to subductional setting show relatively higher values of h c o w co and lower values of w cr w co than the cones related to extensional setting the detected differences can be imputable to peculiar eruption dynamics resulting in slight but systematic changes in shape and differences in lithological and sedimentological characteristics that govern post eruptive erosion to constrain the pathway of scoria cone erosion the detected morphometric changes were also interpreted using a simple linear degradation model utilizing the obtained simulation results the inferred initial cone base and the age of scoria cones we calculated a diffusion coefficient k for several dated cones which are related to the prevalent climate our results despite the high error associated allow to assess the median k for all volcanic fields due to the complexity of the factors behind it is not easy to understand if the prevalent shape characterizing a certain volcanic field is due mainly to sin eruptive or post eruptive mechanisms however our distinction between the two main geodynamic settings may be the first step to decipher these factors

generation of metal rich felsic magmas during crustal anatexis we show that partial melting of a preexisting mineral deposit during regional anatexis is likely to lead to incorporation of sulfide melt into silicate melt simple dissolution of metals in silicate melt during anatexis leads to minor metal addition to escaping magma but physical entrainment of sulfide melt can lead to extreme enrichment and higher total metal incorporation metal enrichment is enhanced if melt segregation is structurally driven only dissolved metal is incorporated if initial segregation is driven by buoyancy alone however once encapsulated within a larger volume of silicate melt immiscible globules of sulfide melt are physically carried by buoyantly rising magma gradual dissolution of entrained sulfide melt globules is expected as the silicate magma migrates from the lower crust this process may lead to significant variations in metal contents of silicate magmas above regions that underwent anatexis

paricutin re examined a classic example of crustal assimilation in calc alkaline magma following its birth on the 20th of february 1943 the mexican volcano paricutin discharged a total of 1 38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952 until 1947 when 75 of the volume had been erupted the lavas varied little in chemical or isotopic composition all were basaltic andesites with 55 to 56 sio2 18o of 6 9 to 7 0 and 87sr 86sr ratios close to 0 7038 subsequent lavas were hypersthene andesites with silica contents reaching 60 18o values up to 7 6 and 87sr 86sr of 0 7040 to 0 7043 the later lavas were enriched in ba rb li and k2o and depleted in mgo cu zn cr ni sr and co the isotopic and other chemical changes which appeared abruptly over a few months in 1947 are interpreted as the result of tapping a sharply zoned and density stratified magma chamber xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70 18o of 5 6 to 9 9 and 87sr 86sr between 0 7043 and 0 7101 in many respects they resemble samples of basement rocks collected from nearby outcrops three analysed samples of the latter have silica contents of 65 to 67 18o of 7 7 to 8 6 and 87sr 86sr between 0 7047 and 0 7056 these new data provide strong support for the original interpretations of wilcox 1954 who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight continental crust except for a few trace elements particularly ba sr and zr the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late stage lavas some of the discrepancies may be explained by postulating a contaminant that was older and richer in ba sr and zr than those represented by the analysed xenoliths others can be attributed to chemical changes accompanying disequilibrium partial melting contact metamorphism and meteoric hydrothermal alteration of the country rock many of the xenoliths show evidence of having been affected by such processes the lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption the order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities

age and archaeological implications of xitle volcano southwestern basin of mexico city the pedregal lavas are fresh well exposed basaltic flows erupted from the xitle scoria and cinder cone in the southwestern part of the basin of mexico these lavas cover an area of 70 km 2 and were emplaced over pyramids and other buildings e g cuicuilco and copilco archaeological sites today a part of mexico city including the national university is built on the flows initial strombolian activity produced an ash fallout layer which was immediately followed by effusive emplacement of lava flows the xitle cone grew on the north facing slope of ajusco volcano and lava flowed down to the n ne until it reached the basin floor more than 30 radiocarbon dates have been obtained by several workers on charcoal samples from beneath the lava and several ages for the eruption have been proposed from these dates most dated samples were not directly produced by xitle s eruption but instead are artifacts of human activity that predates the eruption thus these ages mostly about 2000 bp are older than the eruption a new age of 1670 35 years bp ad 245 315 obtained on charcoal samples collected just beneath the lavas is favored for the xitle eruption these samples originated by ignition of vegetation during the emplacement of hot scoriaceous tephra the new age is within the classic period of mesoamerican archaeology whereas the earlier reported ages are at the end of the preclassic the new age carries important implications for the timing of population shifts within the basin of mexico q 2000 elsevier science b v all rights reserved

space time evolution of monogenetic volcanism in the mafic garrotxa volcanic field ne iberian peninsula we reconstructed the evolution of the volcanic activity within the central garrotxa monogenetic volcanic field the youngest volcanic area of the iberian peninsula by investigating the stratigraphy of the volcanic successions and the morphology of the monogenetic eruptive centres analysis of this volcanic succession has been conducted following the unconformity bounded stratigraphic units criteria the detailed stratigraphy of the volcanic successions shows that the central garrotxa volcanic field gvf evolved through four main periods of volcanic activity synthems represented by the eruptive products of the mafic monogenetic volcanoes and associated syn eruptive reworked deposits eruptive units and by the inter eruptive deposits epiclastic units the distribution and the morphologies of the monogenetic volcanoes suggest that feeder dykes were emplaced under influence of the present stress filed and along pre existing fractures of the basement our facies analysis of the deposits and their distribution shows that migration of volcanism toward the centre of the basin was accompanied by a trend of increasing explosivity episodic hydromagmatism in central garrotxa occurred without a specific geographic locus or obvious temporal correlation finally by integrating field data with the stratigraphy extracted from water wells we determined the volume of the volcanic deposits the small average volume of products emitted during each eruptive period and the long quiescence separating them allow us to classify the gvf as a low output rate volcanic field

rothenberg scoria cone east eifel a complex strombolian and phreatomagmatic volcano rothenberg scoria cone eifel formed by an alternation of three strombolian and three phreatomagmatic eruptive phases eruptions took place from up to six vents on a 600 m long fissure building an early tuff ring and then two coalescing scoria cones strombolian volcanism dominated volumetrically as the supply of external water was severely limited magma water interaction only occurred during the opening stages of eruption at any vent when discharge rates were low and the fragmentation surface was below the water table the phreatomagmatic deposits consist of relatively well sorted fall beds and only minor surge deposits they contain juvenile clasts with a wide range of vesicularity and grain size implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions the transition from phreatomagmatic to strombolian eruption at any vent was rapid and irreversible and strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as by then the fragmentation surfaces were high in the growing cones and water was denied access to the magma the strombolian deposits are relatively homogenous consisting of alternating coarser and finer grained well sorted fall beds erupted during periods of open vent eruption and partial blockage of the vent respectively the intervals of strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent collapse of the growing cones and repeated recycling of clasts through the vent clear evidence of the instability of the rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones the close of strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters

platinum palladium gold copper rich stringers at the strathcona mine sudbury their enrichment by fractionation of a sulfide liquid recent study of the platinum group elements pge in the strathcona ni cu sulfide deposit has shown that their distribution is zoned pt pd au and cu increase progressively from the hanging wall ore zone through the main zone into the deep ore zone rh ru ir and os show a reverse trend naldrett et al 1982 the copper and deep copper zones at strathcona are the farthest away from contact with the sudbury igneous complex and consist of veins of massive sulfides up to 6 m in width which narrow down to thin veins

did the giant broken hill australia zn pb ag deposit melt abstract recent published research has proposed that some metamorphosed massive sulfide deposits including the broken hill deposit australia have partially melted here we discuss the evidence for and against this process the broken hill deposit is located in the curnamona craton within a rift in an apparent upward coarsening sequence of clastic metasediments into which paleoproterozoic mafic and felsic melts intruded the deposit was metamorphosed to granulite facies conditions 750 800 c and 5 6 kbar and was subjected to at least five periods of deformation at peak p t conditions a melt phase was produced in silicate rocks especially granitoids and psammopelites and in ore characterized by the systems cu as s ag pb s ag sb as s cu sb pb s sb as s cu sb s and fe as s however the proportion of minerals in these s bearing systems is insignificant 2 feo mno cao al 2 o 3 p 2 o 5 co 2 zns pbs fes feass feas 2 which is the most relevant system to broken hill ores p t conditions were never high enough to produce a melt phase in this system authors arguing for sulfide rock melting suggest that mn rich lithologies garnetite and quartz garnetite which are intimately associated with the pb rich ores at the broken hill deposit were produced by a reaction between mn rich sphalerite and aluminous wall rocks however to produce such rocks would require mn contents of sphalerite compositions that are unrealistically high and yet to be found in nature moreover the implication of the melt model is that wherever mn rich quartz garnetite and garnetite are found sphalerite should be located next to them this is clearly not the case the presence of polyphase sulfide inclusions within garnet in garnetite has also been considered by some in the literature to be sulfide melt inclusions with the implication that the enclosing garnet was a product of melting however we consider these sulfides to be products of hydrothermal processes during retrograde metamorphism it is impossible to form garnetite and quartz garnetite in rocks within the curnamona craton that formed at upper greenschist lower amphibolite facies conditions by partial melting p t conditions were too low we consider that the fe and mn component of garnetite and quartz garnetite are products of exhalation and inhalation at or near the sea floor and that these rocks are meta exhalites or meta inhalites some garnet rich rocks also formed by metasomatic processes throughout the protracted metamorphic history that affected the curnamona craton

post eruptive sediment transport and surface processes on unvegetated volcanic hillslopes a case study of black tank scoria cone cima volcanic field california abstract conical volcanic edifices that are made up from lapilli to block bomb pyroclastic successions such as scoria cones are widespread in terrestrial and extraterrestrial settings eruptive processes responsible for establishing the final facies architecture of a scoria cone are not well linked to numerical simulations of their post eruptive sediment transport using sedimentological geomorphic and 2d fragment morphology data from a 15 ky old scoria cone from the cima volcanic field california this study provides field evidence of the various post eruptive sediment transport and degradation processes of scoria cones located in arid to semi arid environments this study has revealed that pyroclast morphologies vary downslope due to syn eruptive granular flows along with post eruptive modification by rolling bouncing and sliding of individual particles down a slope and overland flow processes the variability of sediment transport rates on hillslopes are not directly controlled by local slope angle variability and the flank length but rather by grain size and morphological characteristics of particles such as shape irregularity of pyroclast fragments and block lapilli ratio due to the abundance of hillslopes degrading in unvegetated regions such as those found in the southwestern usa granulometric influences should be accounted for in the formulation of sediment transport laws for geomorphic modification of volcanic terrains over long geologic time

textural development in sulfide matrix ore breccias in the voisey s bay ni cu co deposit labrador canada abstract magmatic ni cu sulfide ores at voisey s bay contain complex assemblages of extremely heterogeneous rocks these range from polymict breccias with rock fragments in sulfide rich and or sulfide poor matrices to heterogeneous vari textured gabbros with rapid short range variations in grain size and content of hydrous phases rock fragment populations in the breccias include endogenous olivine gabbros cumulate and non cumulate and cumulate peridotites along with extensively depleted plagioclase hercynite gneisses interpreted as restites from extensive partial melting of country rock quartzo feldspathic paragneisses using a combination of desk top microbeam xrf mapping at cm scale and 3d x ray tomography we show that both sulfide poor and sulfide rich breccias comprise heterolithic assemblages of clasts within a matrix of olivine gabbro this matrix is characterised by an interconnected 3d framework of plagioclase crystals highly variable in grain size at mm to cm scale with interstitial olivine and poikilitic clinopyroxene and is texturally indistinguishable from clast free olivine gabbro sulfide forms interconnected networks at cm to dm scale and possibly larger much of the plagioclase developed by outgrowth from the margins of paragneiss xenoliths when the porosity was occupied by silicate melt the observed range of textures is explained by a model of percolation of molten sulfide through variably crystalline inter clast matrix displacing the silicate melt to leave the refractory plagioclase olivine or in some cases plagioclase only component now entirely within a sulfide matrix the process is analogous to that believed to have formed interspinifex ore in komatiite hosted deposits biotite rims on plagioclase enclosed in sulfide are interpreted as the result of reaction between plagioclase olivine and a hydrous component derived from the sulfide melt itself with a possible component of migrating residual silicate melt wicking along sulfide silicate contacts this sulfide infiltration model offers an alternative to the current model for upward emplacement of a slurry of silicate melt sulfide melt and breccia fragments as a late stage injection into the dyke sill complex the preserved range of textures is interpreted as being due to gravity driven percolation of sulfide liquid through a pre existing partially molten intrusion breccia in this model the breccia serves as a physical trap site accumulating downward migrating sulfide liquid however the invariable close mutual association of sulfide and rock fragments at voisey s bay implies a common derivation

a mesoscale sediment budget for long island new york abstract a sediment budget encompassing the 140 km shoreline from montauk point to fire island inlet new york derived longshore transport rates for 25 coastal cells using comparative surveys the study area along the south shore of long island a generally straight coast of mainland and barrier island beaches includes the eroding headland at montauk point three jettied inlets two groin fields and three barrier beaches bridging between small scale quantitative surveys and large scale regional geology the sediment budget provides a mesoscale view and quantitative estimates of key coastal processes in the area results integrate changes between the foredunes and lower foreshore to the estimated depth of closure 9 to 12 m below mean sea level for a 24 5 year period 1955 1979 beach fills and losses to washovers and breach channels are incorporated in the analysis the results confirm net westerly transport and provide the first evidence that shoaling in fire island inlet long estimated to be 400 000 m 3 yr is due primarily to cannibalization of the immediate updrift compartments particularly erosion of the foreshore below mean low water this lowers estimates of longshore transport rates along most of the study area compared with previous estimates the offshore source attributed by others to account for fire island s stability appears to be trailing ebb shoals lagging behind inlet migration jettied inlets in the central study area intercept some longshore transport but since stabilization circa 1955 have been ebb dominant preserving the littoral budget seaward of the strandline littoral losses to the lagoons by inlets or washovers were insignificant over the period however breach channels if not closed soon after storms were found to offset years of ebb dominance in jettied inlets by depositing major flood deltas beach fills had measurable and positive effect on the sediment budget along the western half of the study area through the early 1970s the volume of beach fills since 1975 has diminished to about 10 of the earlier rate a field of 15 groins in the central study area is shown to trap 100 of the longshore transport and produce a local drift reversal rates of accretion within the groin cells and erosion along the downdrift compartment dwarf the average rates of change for the majority of littoral compartments in the study area

experimental constraints on sulphur behaviour in subduction zones implications for ttg and adakite production and the global sulphur cycle since the archean we performed crystallization experiments at 2 3 gpa at 700 9508c on basaltic and pelitic lithologies with added water and sulphur to constrain the factors controlling sulphur behaviour in subduction zones and how it may have varied through geological time the resulting hydrous silicic melts have up to 20 times more dissolved sulphur up to 1wt than at 0 2 0 4 gpa when moderately oxidized conditions prevail such high solubilities appear to result from the combined effects of enhanced solubility of water in high pressure silicate melts 10 20 wt h2o which acts to decrease silica activity and oxidizing conditions the results confirm previous findings that high sulphur contents in silicate melts do not necessarily require iron rich compositions suggesting instead that sulphur water complexes play a fundamental role in sulphur dissolution mechanisms in iron poor silicic melts in agreement with recent spectroscopic data the experimental melts reproduce phanerozoic slab derived magmas in particular their distinct ca and mg rich composition the results also show that sulphur increases the degree of melting of basalt lithologies hence we suggest that subducted slabs will preferentially melt where sulphur is present in abundance and that the variability in arc magma sulphur output reflects in part the vagaries of sulphur distribution in the slab source in contrast comparison with the composition of archean felsic rocks suggests that in the early earth much less sulphur was present in subducted slabs in agreement with a number of independent lines of evidence showing that the archean ocean hence the hydrothermally altered subducted archean oceanic crust was considerably poorer in sulphur than at present volcanic degassing of sulphur was thus probably much weaker during the archean than in proterozoic phanerozoic times

the cerro chopo basaltic cone costa rica an unusual completely reversed graded pyroclastic cone with abundant low vesiculated cannonball juvenile fragments cerro chopo is a partially dissected asymmetric isolated pleistocene pyroclastic cone located in front of the cordillera de guanacaste in northern costa rica the cone consists of similar to 0 09 km 3 of basaltic tephra as well as similar to 0 14 km 3 of lateral lava flows tephras are tholeiitic high alumina olivine basalts and represent minor degrees 5 of crystal fractionation major and trace element compositions are consistent with minor fractionation from a mixture of e morb and oib magmas the cone walls consist of alternating coarser and finer grained well sorted beds containing continuous spectra from breadcrust to smooth surface cannonball bombs but also less frequent cylindric fragments and broken clasts cerro chopo is unique compared to other typical scoria cones because it contains ubiquitous reversely graded layers scarcity of scorias and instead a wide range of dense 1 54 2 49 g cm 3 poorly vesicular 5 40 vol juvenile clast morphologies including abundant cannonball juvenile bombs and lapilli these are bombs with concentric layers surrounding vesiculated dense and lapillistone cores and are interpreted to have repeatedly recycled through the vent the cannonball bombs and lapilli have been described in a few scoria cones but are much less abundant than in chopo the reverse graded sequences are interpreted to have resulted from decreasing explosivity at the vent in addition to local failure of tephra on slopes as the consequence of grain flows elsewhere on the earth most of the poorly vesiculated spherical bombs particularly cannonball accretionary composite and core bombs and its equivalent lapilli size pelletal spinning droplets and ellipsoidal lapilli are all related to mafic to ultramafic low viscosity magmas c 2010 elsevier b v all rights reserved

high chlorine alteration minerals and calcium rich brines in fluid inclusions from the strathcona deep copper zone sudbury ontario the strathcona deep copper zone consists of a complex suite of massive sulfide veins that occupy a series of fractures in the footwall gneisses and sudbury breccia it is characterized by the occurrence of branching quartz veinlets and hydrothermal alteration selvages along the margins of massive sulfide veins hornblende or actinolite epidote calcite and ferropyrosmalite fe mn 8 si 6 o 15 oh c1 10 occur in the inner zones of the alteration selvages adjacent to the massive sulfide veins annite chlorite and albite are developed in the outer zones fe rich silicates of the alteration assemblages contain significant amounts of chlorine ferropyrosmalite contains 5 79 to 6 54 wt percent cl hornblende contains up to 2 99 wt percent cl annite contains up to 2 13 wt percent cl primary fluid inclusions in the quartz from the branching veinlets are mutiphase consisting of vapor liquid and more than four solids the solid crystals of opened inclusions identified by scanning electron microscope are na ca k k pb k fe and fe mn chlorides the sequence of melting events and the nature of the solid phases suggest that the trapped fluids are nacl cacl 2 h 2 o solutions with minor k fe mn and pb these fluid inclusions are highly saline avg 60 wt nacl cacl 2 calcium rich ca na 1 by weight and homogenize to liquid at temperatures 260 degrees to 420 degrees c 342 degrees or 35 degrees c mean or std dev two options are proposed for the origin of the fluids melt fluid phase separation during sulfide liquid crystallization and convection of ambient fluids across the contact of the sulfide veins the cl rich fluids may have participated in the precipitation of pd and probably other platinum group elements pge at a later stage but they did not transport the pge very far from the magmatic sulfide veins

water solubility and d h fractionation in the system basaltic andesite h2o at 1250 c and between 0 5 and 3 kbars abstract water solubilities in melt of basaltic andesite composition have been measured in the range 500 to 3000 bars 1200 1250 c the saturation curve obey an empirical relationship very close to the square root of pressure law of hamilton et al hamilton d l burnham c w osborn e f 1964 the solubility of water and effects of oxygen fugacity and water content on crystallisation in mafic magmas j petrol 5 21 39 the best fit is obtained by a power law h 2 o wt ap n p in b where a 0 05 0 01 and n 0 63 the departure from n 0 5 is mainly due to the increasing importance of dissolved molecular water at high pressures hydrogen isotopic fractionation between water vapor and total water dissolved in the melt d v m has been measured at 0 5 2 and 3 kb d v m decreases with pressure from 32 to 20 2 this variation is related to the increase in molecular water proportions relative to hydroxyl group when water solubility increases the variation of the d h ratios in water dissolved in such melts thus appears as a possible indirect measurement of the original oh h 2 o ratio of total dissolved water not subject to variation during quenching as the later is this correlation shows that the d v m isotopic fractionation at very high water vapor pressure tend toward a limit of 16 3 for basic magmas

localization of volcanic activity 2 effects of pre existing structure abstract the capture of vertically rising dikes by faults is examined with a combination of an analytic solution numerical simulations and field observations one of the numerical solutions and the field observations also provide insight into the intrusion of sills into the hanging wall of faults that are reactivated when lubricated by magma the approximate analytic solution indicates that dike capture by a fault will be limited primarily to steeply dipping faults capture will become easier meaning that it will be possible for faults with shallower dips as the intersection of the dike with the fault occurs at smaller depths capture will also be easier as it is harder for crack to grow in the hanging wall this may be interpreted in terms of a higher tensile strength or in terms of shorter pre existing cracks or higher fracture toughness numerical analysis using a discrete element code confirms this conclusion but also shows that the analytic solutions overestimate the difficulty of intruding a pre existing fault the maximum deviation occurring for faults dipping at 45 this finding that dike capture by faults should be restricted to high angle faults and shallow depths is supported by field observations of exhumed miocene basaltic volcanic terrain in central nevada usa differences in style seen in other regions are attributed to both the rock properties and the regional stress field the numerical solutions also indicate that synintrusive fault slip is likely to lead to formation of sills in the hanging wall at depths of a few hundred meters or less a finding that is also consistent with the field observations in nevada

origin of high magnesian andesites in the setouchi volcanic belt southwest japan i petrographical and chemical characteristics high magnesian andesites of middle miocene age occur in southwest japan forming an obvious volcanic belt these andesites have low feo mgo ratios 0 546 0 931 and are rich in ni 101 312 ppm co 30 0 45 1 ppm and cr 208 756 ppm they are relatively aphyric phenocrysts 10 vol and the phenocrysts of magnesian olivine fo88 are in equilibrium with the host high magnesian andesite magmas on the basis of the fe mg exchange partitioning these features suggest that the high magnesian andesites are not differentiated or accumulative they appear to represent primary andesites generated in the upper mantle these southwest japanese high magnesian andesites are rich in incompatible elements and show light rare earth enrichment relative to boninites suggesting that the former is derived from a less depleted mantle source than the latter

multiple hydrothermal processes in footwall units of the north range sudbury igneous complex canada and implications for the genesis of vein type cu ni pge deposits systematic regional sampling along the contact zone of the 1 85 ga age sudbury igneous complex with the archean levack gneiss footwall in the north range of the sudbury structure revealed textural mineralogical and fluid inclusion systematics of post sudbury igneous complex hydrothermal processes that affected the various lithologies at late stages of emplacement of the sudbury igneous complex and formation of magmatic fe ni cu sulfide deposits a partial melt from the levack gneiss invaded the contact zone this partial melt is revealed by small microdikes and irregular bodies of granophyric quartz plagioclase with megacrysts of hornblende clinopyroxene and titanite plagioclase hornblende equilibrium was established at 750 to 800 c and 1 10 to 1 55 kbars pressure corresponding to about a 4 to 6 km depth under lithostatic conditions f cl wt percent ratios around 15 for accessory apatite and hornblende indicate that a cl rich fluid phase separated during the crystallization of the footwall granophyre this is expressed as miarolites the cl rich fluid 50 wt nacl equiv salinity was trapped as primary fluid inclusions in quartz at around 480 c minimum temperature and around 1 1 kbars minimum pressure during supercooling these inclusions are rich in ca fe mn and k as well as na chloride this fluid may have interacted with earlier primary magmatic sulfide causing remobilization and reprecipitation of cu ni platinum group elements pge in veins and disseminations in the footwall these veins are parallel to the sudbury igneous complex footwall contact and are characterized by chalcopyrite pentlandite millerite magnetite stilpnomelane ferropyrosmalite epidote and chlorite michenerite moncheite merenskyite froodite insizwaite sobolevskite gold and bi ni sulfides are associated with sulfides epidote quartz actinolite and chlorite are common in the alteration selvages of the veins compositions and assemblages of platinum group minerals pgm indicate their precipitation below 575 to 485 c primary highly saline fluid inclusions about 40 wt nacl equiv in quartz indicate crystallization at 400 to 480 c and around 1 6 kbars minimum t and p respectively late carbonate epidote actinolite chlorite veins and alteration overprinted the earlier assemblages at about 300 to 400 c with some bornite millerite native silver and other bi sulfides the second stage of hydrothermal activity is characterized by regional carbonic aqueous nacl co 2 ch 4 h 2 o type immiscible fluids that were trapped as secondary inclusions in quartz from various lithologies no mineralization is related to this stage along the north range boiling of these fluids took place during a pressure drop from lithostatic to hydrostatic conditions during uplift from about a 5 to 6 to a 3 to 4 km depth at 300 to 350 c high 20 26 wt nacl equiv and low salinity 6 12 wt nacl equiv fluids coexisted with different carbonic species contents the predominant north south and northwest southeast orientations of fluid inclusion planes reveal that mobilization of carbonic aqueous solutions may be related to the northwesterly oriented thrusting in the sudbury structure during the late stages 1 8 1 7 ga of the penokean orogeny and to predominantly north south faulting in the north range the third stage of fluid mobilization also took place mostly along northerly and northwesterly oriented fractures aqueous fluids were ca rich brines 20 40 wt salinity with temperatures between 150 and 250 c some of these fluids also circulated in fractures parallel to the sudbury igneous complex footwall contact ne sw thus they locally may have interacted with the earlier sulfide assemblages and are also responsible for formation of late veinlets with chalcopyrite epidote quartz chlorite assemblages mobilization of these fluids took place under litho and hydrostatic conditions at about a 4 to 6 km depth this hydrothermal activity is related to a regional tectonothermal event during emplacement of northwesterly oriented sudbury dikes 1 24 ga regional scale fluid inclusion studies along the north range highlight the role of multiple hydrothermal processes in the metallogenic evolution of the sudbury igneous complex and especially in the distribution of copper and precious metals in the footwall units

a model for calculating eruptive volumes for monogenetic volcanoes implication for the quaternary auckland volcanic field new zealand abstract monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios especially transitions between explosive and effusive behaviour and the length of eruptions erosion superposition and lack of exposure limit the accuracy of volume determination even for very young volcanoes in this study a systematic volume estimation model is developed and applied to the auckland volcanic field in new zealand in this model a basaltic monogenetic volcano is categorised in six parts subsurface portions of volcanoes such as diatremes beneath phreatomagmatic volcanoes or crater infills are approximated by geometrical considerations based on exposed analogue volcanoes positive volcanic landforms such as scoria spatter cones tephras rings and lava flow were defined by using a light detection and ranging lidar survey based digital surface model dsm finally the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes considering only those parts with high reliability the overall magma output converted to dense rock equivalent for the post 250 ka active auckland volcanic field in new zealand is a minimum of 1 704 km3 this is made up of 1 329 km3 in lava flows 0 067 km3 in phreatomagmatic crater lava infills 0 090 km3 within tephra tuff rings 0 112 km3 inside crater lava infills and 0 104 km3 within scoria cones using the minimum eruptive volumes the spatial and temporal magma fluxes are estimated at 0 005 km3 km2 and 0 007 km3 ka the temporal volumetric evolution of auckland is characterised by an increasing magma flux in the last 40 ky which is inferred to be triggered by plate tectonics processes e g increased asthenospheric shearing and backarc spreading of underneath the auckland region

evaluation of morphometry based dating of monogenetic volcanoes a case study from bandas del sur tenerife canary islands morphometry based dating provides a first order estimate of the temporal evolution of monogenetic volcanic edifices located within an intraplate monogenetic volcanic field or on the flanks of a polygenetic volcano two widely used morphometric parameters namely cone height width ratio hmax wco and slope angle were applied to extract chronological information and evaluate their accuracy for morphometry based ordering based on these quantitative parameters extracted from contour based digital elevation models dems two event orders for the bandas del sur in tenerife canary islands were constructed and compared with the existing k ar paleomagnetic and stratigraphic data the results obtained suggest that the commonly used hmax wco ratio is not reliable leading to inappropriate temporal order estimates while the slope angle gives slightly better results the overall performance of such descriptive parameters was however generally poor i e there is no strong correlation between morphometry and age the geomorphic morphometric mismatches could be the result of 1 the diversity of syn eruptive processes i e diverse initial morphologies causing geomorphic morphometric variability 2 contrasting edifice specific degradation that depends partly upon the inner facies architecture of the volcanic edifices 3 various external environmental controls e g tephra mantling from pyroclastic density currents unrelated to the edifice evaluated and 4 differences in the scale resolution of input data the observed degradation trend and changes in morphometric parameters over time do not support a simple degradation model for monogenetic scoria cones volcanoes

calculating shoreline erosion potential using nearshore stratigraphy and sediment volume outer banks north carolina 1 despite the acknowledged influence of coastal geological framework on the behavior of beaches and barrier islands and a wealth of geological and bathymetric observations from the inner shelf quantitatively connecting those observations to shoreline behavior has been difficult nearshore geologic and morphologic variability described by recent research is not well represented by conventional geologic parameters such as mean grain size and shoreface slope used in most shoreline change models we propose that total nearshore sediment volume as calculated to a continuous seismic reflection surface provides a flexible and robust metric for use in the prediction of shoreline change this method of determining the volume of sediment in the nearshore accounts for three dimensional sandbar morphologies and heterogeneous seafloor sediments the decadal scale shoreline change rate for northeastern north carolina is significantly correlated to the volume of sediment in the nearshore when a geologically defined base is used in volume determinations suggesting that the shallow stratigraphic framework of transgressive coasts is an important influence on decadal shoreline behavior nearshore sediment volume was overestimated when an arbitrary depth constant baseline was used and was not correlated to decadal shoreline change this implies that a volume metric which accounts for both framework geology and variable seafloor morphology better represents the geologic character of the shoreface and may help to improve existing models of shoreline change an empirical model of regional shoreline erosion potential demonstrates the importance of incorporating nearshore sediment volume shallow framework geology and surface morphology when predicting seasonal to decadal shoreline evolution

late pleistocene to precolumbian behind the arc mafic volcanism in the eastern mexican volcanic belt implications for future hazards an area of widespread alkaline to subalkaline volcanism lies at the northern end of the cofre de perote citlaltepetl pico de orizaba volcanic chain in the eastern mexican volcanic belt mvb two principal areas were active about a dozen latest pleistocene to precolumbian vents form the 11 km wide e w trending cofre de perote vent cluster cpvc at 2300 2800 m elevation on the flank of the largely pleistocene cofre de perote shield volcano and produced an extensive lava field that covers s 100 km 2 more widely dispersed vents form the naolinco volcanic field nvf in the sierra de chiconquiaco north of the city of jalapa xalapa three generations of flows are delineated by cone and lava flow morphology degree of vegetation and cultivation and radiocarbon dating the flows lie in the behind the arc portion of the northeastern part of the mvb and show major and trace element chemical patterns transitional between intraplate and subduction zone environments flows of the oldest group originated from la joya cinder cone radiocarbon ages v42 000 yr bp at the eastern end of the cpvc this cone fed an olivine basaltic flow field of v20 km 2 that extends about 14 km southeast to underlie the heavily populated northern outskirts of jalapa the capital city of the state of veracruz the central cone group ccg of intermediate age consists of four morphologically youthful cinder cones and associated vents that were the source of a lava fields 27 km 2 of late pleistocene or holocene age the youngest group includes the westernmost flow from cerro colorado and a lava flow v2980 bp from the rincon de chapultepec scoria cone of the nvf the latest eruption from the compound el volcancillo scoria cone occurred about 870 radiocarbon years ago and produced two chemically and rheologically diverse lava flows that are among the youngest precolumbian flows in mexico and resemble paired aa pahoehoe flows from mauna loa volcano the el volcancillo eruption initially produced the high effusion rate short duration toxtlacuaya alkaline aa lava flow from the southeastern crater this 12 km long hawaiite average 50 5 sio2 flow was followed by extrusion of the calc alkaline r o naolinco lava flow from the northwestern crater this large volume v1 3 km 3 tube fed basaltic pahoehoe flow average sio2 49 traveled 50 km inferred effusion rates suggest emplacement over a decade long period flows of all three age groups are transected by highway 140 and the railway that form major transportation arteries between jalapa and puebla this area has not previously been considered to be at volcanic risk but volcanism here has continued into precolumbian time future eruptions of similar magnitude and location to those documented here could pose significant hazards to transportation corridors and to densely populated areas in and to the north of jalapa slight

carmacks copper cu au ag deposit mineralization and postore migmatization of a stikine arc porphyry copper system in yukon canada

a blind folded test of equilibrium beach profile concepts with new zealand data abstract methodology for calculating equilibrium beach profiles for uniform sand size characteristics is extended to the case of an arbitrary distribution of sediment characteristics across the profile the application of this method and comparison with actual profiles is posed as a means of interpreting whether the profile contains a deficit or excess of sediment and thus whether long term shoreline recession or advancement can be anticipated various types of profile disequilibrium are reviewed and the significance discussed the methodology is applied using measured profiles sediment sizes and beach face slopes for ten sites on the northern island of new zealand one profile was documented in this study whereas the data for the other nine were obtained from published sources the number of sediment samples available for each profile varied from three to twelve the agreement between the actual and calculated profiles differs considerably for the ten sites the degree of disequilibrium is quantified by calculating the shoreline adjustment y required for the actual profile to equilibrate for depths less than 7 m which represents the near maximum depth available on all profiles these shoreline adjustments ranged from 105 m recession to 159 m advancement with four of the ten sites having positive values three of the sites with negative shoreline adjustment have been or are presently sites of substantial sand extraction from the beach or in the nearshore waters however the differences between the actual and equilibrium profiles are not consistent with anticipated profile forms and or volumes and it is thus concluded that sand mining is not responsible for most of the observed deficits at this stage it is not possible to state with confidence whether differences between actual and calculated equilibrium profiles are due to true disequilibriums or to limitations in the equilibrium beach profile methodology studies of the type reported here when applied to many different areas will advance the methodology and contribute to the confidence in the resulting interpretations

linking variable explosion style and magma textures during 2002 at stromboli volcano italy microtextural characteristics of fresh ejecta from stromboli volcano were examined from three periods of differing eruption style and intensity in 2002 activity shifted from relatively weak and infrequent ash charged explosions during january through may into two broad cycles of waxing activity in june through late september and late september through december followed by the onset on 28 december of the 2002 2003 effusive eruption analyzed sets of lapilli from may september october and 28 december show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity three distinct textures are observed among the analyzed pyroclasts low density ld with an abundance of subspherical bubbles the presence of large irregularly shaped bubbles and a light to transparent glass matrix transitional texture tt with an intermediate number of subspherical bubbles a high frequency of large irregularly shaped bubbles and a honey colored glass matrix and high density hd with sparse relatively small bubbles conspicuous large irregular bubbles and a dark glass matrix observational and quantitative data density vesicle size indicate that these textures are linked through variable residence time in stromboli s shallow conduit with an ongoing evolution from ld to hd magma calculations suggest that residual ld magma will evolve to hd texture in a period of hours to days contrasting amounts of the ld tt and hd magmas are present in each sample with the most tt in may the most ld in september october and the most hd in december this implies that the shallow magma had a different rheology at each collection period the viscosity of ld and hd magmas are calculated to be in the range of 2 000 to 2 600 and 3 000 to 5 000 pa s respectively which with their changing proportions must have implications for rates of bubble slug ascent and processes of fragmentation this study suggests that an increasing maturity of magma in stromboli s shallow conduit with resultant increase in viscosity feeds back to reduce the intensity of explosions whereas a steady flux of ld magma favors more powerful explosions

revisiting the age of the jumento volcano chichinautzin volcanic field central mexico using in situ produced cosmogenic 10be abstract previous studies on jumento volcano based on radiocarbon dating and comparative morphometric analysis suggest that it could be the youngest volcano of the sierra chichinautzin here we establish a new chronology of the emplacement of the late holocene lava flows of jumento volcano using in situ produced 10be cosmic ray exposure cre dating of quartz xenocrysts to test this hypothesis depending on the choice of the cosmogenic nuclide production parameters in the exposure age calculators used the mean 10be cre ages ranges between 1 86 0 68 ka and 2 41 0 97 ka for the inner lava flow and between 1 90 0 29 ka and 2 49 0 41 ka for the breakout lava flow these preliminary mean 10be cre ages confirm that the jumento volcano is one of the youngest volcanoes of the sierra chichinautzin although slightly older than the xitle volcano this first 10be eruption chronology of the sierra chichinautzin shows that 10be cre dating represents an alternative approach to date volcanic rocks not only for the sierra chichinautzin but also for other volcanic fields especially when no charcoal or paleosoils are available for radiocarbon dating furthermore it suggests that in situ produced cosmogenic nuclide dating has considerable potential to throw light on the volcanic history and eruption recurrence of this volcanic field in order to provide enough data for the mitigation of the hazards and related risks in mexico city the city of cuernavaca and the small towns around the volcanic field

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

geomorphology of the post caldera monogenetic volcanoes at deception island antarctica implications for landform recognition and volcanic hazard assessment abstract deception island is one of the most active volcanoes in antarctica with more than 20 monogenetic eruptions during the holocene the latest episodes of 1967 1969 and 1970 have shown that volcanic activity on deception island can become a concern for tourists scientists and military personnel working on or near the island the objective of this work is to identify eruptive processes and the geomorphic evolution of post caldera monogenetic volcanic edifices at deception island by morphometric analyses supported by field observations morphometric analysis has been used since the 1970s to analyse scoria cones but it has rarely been applied to other monogenetic volcanoes such as tuff cones and tuff rings tuff cones and tuff rings represent the most common landforms during deception island s recent geological past with over 70 scattered eruptive vents inside and along the caldera rim these volcanic landforms have been studied based on field observations and later digital elevation model analyses their geometry ranges from 10 to 100 m in height and diameters vary between 300 and 2500 m the morphometric data suggest that tuff cones have a more circular crater than tuff rings with a significant separation between the two landforms using the height hco and basal width wco parameters tuff cones have hco wco ratios with values between 0 04 and 0 16 and outer slopes between 7 and 34 tuff rings have lower hco wco ratios that are between 0 01 and 0 04 and lower outer slopes between 3 and 21 this study shows that basic shape parameters in combination with slope angle analysis can be used to discriminate among different types of monogenetic volcanoes the initial eruptive related morphometric diversity however prevents correlation of eruption ages with morphometric parameters or constructing a relative chronology of the monogenetic eruptions this work provides a better comprehension of the potential evolution of a future eruption and a broader understanding of volcanic hazards on deception island

noril sk talnakh cu ni pge deposits a revised tectonic model the noril sk mining district is located at the northwest margin of the tunguska basin in the centre of the 3 000 4 000 km siberian continental flood basalt cfb province this cfb province was formed at the permo triassic boundary from a superplume that ascended into the geometric centre of the laurasian continent which was surrounded by subducting slabs of oceanic crust we suggest that these slabs could have reached the core mantle boundary and they may have controlled the geometric focus of the superplume the resulting voluminous magma intruded and erupted in continental rifts and related extensive flood basalt events over a 2 4 ma period cu ni pge sulfide mineralization is found in olivine bearing differentiated mafic intrusions beneath the flood basalts at the northwestern margin of the siberian craton and also in the taimyr peninsula some 300 km east of a triple junction of continental rifts now buried beneath the mesozoic cenozoic sedimentary basin of western siberia the noril sk i and talnakh oktyabr sky deposits occur in the noril sk kharaelakh trough of the tunguska cfb basin the cu ni pge bearing mineralized intrusions are 2 3 km wide and 20 km long differentiated chonoliths previous studies suggested that parts of the magma remained in intermediate level crustal chambers where sulfide saturation and accumulation took place before emplacement the 5 7 km thick neoproterozoic to palaeozoic country rocks containing sedimentary cu mineralization and evaporites may have contributed additional metal and sulfur to this magma classic tectonomagmatic models for these deposits proposed that subvertical crustal faults such as the northeast trending noril sk kharaelakh fault were major trough parallel conduits providing access for magmas to the final chambers however geological maps of the noril sk region show that the noril sk kharaelakh fault offsets the mineralization which was deformed into folds and offset by related reverse faults indicating compressional deformation after mineralization in the late triassic to early jurassic in addition most of the intrusions are sills not dykes as should be expected if the vertical faults were major conduits a revised tectonic model for the noril sk region takes into account the fold structure and sill morphology of the dominant intrusions indicating a lateral rather than vertical emplacement direction for the magma into final chambers taking into account the fold structure of the country rocks the present distribution of the differentiated intrusions hosting the noril sk i and talnakh oktyabr sky deposits may represent the remnants of a single 60 km long deformed and eroded palm shaped cluster of mineralized intrusions which are perceived as separate intrusions at the present erosional level the original direction of sill emplacement may have been controlled by a northeast trending paleo rise which we suggest is present at the southeastern border of the noril sk kharaelakh trough based on analysis of the unconformity at the base of the cfb the mineralized intrusions extend along this rise which we interpret as a structure that formed above the extensionally tilted block in the metamorphic basement geophysical data indicate the presence of an intermediate magma chamber that could be linked with the talnakh intrusion in turn this t shaped flat chamber may link with the yenisei khatanga rift along the northwest trending pyasina transform fault which may have served as the principal magma conduit to the intermediate chamber it then produced the differentiated mineralized intrusions that melted through the evaporites with in situ precipitation of massive disseminated and copper sulfide ore the noril sk kharaelakh crustal fault may not relate to mineralization and possibly formed in response to late mesozoic spreading in the arctic ocean

quaternary volcanism near the valley of mexico implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions the valley of mexico and surrounding regions of mexico and morelos states in central mexico contain more than 250 quaternary eruptive vents in addition to the large composite volcanoes of popocatepetl iztaccihuatl and nevado de toluca the eruptive vents include cinder and lava cones shield volcanoes and isolated andesitic and dacitic lava flows and are most numerous in the sierra chichinautzin that forms the southern terminus of the valley of mexico the chichinautzin volcanic field cvf is part of the e w trending mexican volcanic belt mvb a subduction related volcanic arc that extends across mexico the crustal thickness beneath the cvf 50 km is the greatest of any region in the mvb and one of the greatest found in any arc worldwide lavas and scoriae erupted from vents in the cvf include alkaline basalts and calc alkaline basaltic andesites andesites and dacites both alkaline and calc alkaline groups contain primitive varieties that have whole rock mg mgo and ni contents and liquidus olivine compositions fo90 that are close to those expected of partial melts from mantle peridotite primitive varieties also show a wide range of incompatible trace element abundances e g ba 210 1080 ppm ce 25 100 ppm zr 130 280 ppm data for primitive calc alkaline rocks from both the cvf and other regions of the mvb to the west are consistent with magma generation in an underlying mantle wedge that is depleted in ti zr and nb and enriched in large ion lithophile k ba rb and light rare earth la ce elements extents of partial melting estimated from ti and zr data are lower for primitive calc alkaline magmas in the cvf than for those from the regions of the mvb to the west where the crust is thinner the distinctive major element compositions low cao and al2o3 high sio2 of the primitive calc alkaline magmas in the cvf indicate a more refractory mantle source beneath this region of thick crust in contrast primitive alkaline magmas from the cvf and other regions of the mvb show compositional similarities to intraplate type alkali basalts erupted behind the arc in the mexican basin and range province these similarities are consistent with the hypothesis that slab induced convection in the mantle wedge beneath the mvb causes advection of asthenospheric mantle from behind the arc to the region of magma generation trace element systematics of primitive magmas in the mvb reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub alkaline basaltic melts

thermodynamic analysis of fe and mg partitioning between plagioclase and silicate liquid thermodynamic analysis of fe and mg bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions solution properties of ternary feldspar standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid derived mixing properties of fe and mg bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems caal2si2o8 cafesi3o8 and caal2si2o8 camgsi3o8 based on the determined solution properties of the plagioclase a computer program to calculate the element partition relationships between fe and mg bearing plagioclase and multicomponent silicate liquid was developed the feo mgo and mgo mgo feo in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0 2 wt 0 1 wt and 0 1 mol ratio respectively the fe3 content in plagioclase crystallized at high oxygen fugacity can be estimated with this program the fe3 total fe ratio in plagioclase crystallized near the quartz fayalite magnetite buffer ranges from 0 to 0 5 which is consistent with previous study on natural plagioclase in submarine basalt derived solution properties of the fe and mg bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase change of xfo in olivine coexisting with plagioclase affects mgo and feo contents in plagioclase greatly the present model predicts xfo of coexisting olivine from the chemical composition of plagioclase to 0 1 accuracy at given pressure and temperature

reconstruction of the volcanic history of the tacambaro puruaran area michoacan mexico reveals high frequency of holocene monogenetic eruptions the 690 km2 tacambaro puruaran area located at the arc front part of the michoacan guanajuato volcanic field in the trans mexican volcanic belt tmvb records a protracted history of volcanism that culminated with intense monogenetic activity in the holocene geologic mapping 40ar 39ar and 14c radiometric dating and whole rock chemical analyses of volcanic products provide insights to that history eocene volcanics 55 40 ma exposed at uplifted blocks are related to a magmatic arc that preceded the tmvb early tmvb products are represented by poorly exposed pliocene silicic domes 5 2 ma quaternary 30 km may be related to high rates of magma production at depth and a favorable tectonic setting

mammoth bones embedded in a late pleistocene lahar from popocatepetl volcano near tocuila central mexico the accidental discovery in july 1996 of the tocuila mammoth site in the eastern valley of mexico was followed by intensive scientific excavation which yielded remains of at least seven individuals of mammuthus columbi in an area measuring only 28 m 2 the high density of mostly disarticulated mammoth bones identified 3 m below the surface in a massive 130 cm thick lahar deposit of sandy ash and rounded pumice fragments is unusual and suggests a high energy depositional environment samples of charcoal found within this deposit yielded radiocarbon ages ranging between 10 220 75 and 12 615 95 yr b p the lahar was studied in greater detail in order to establish its provenance determination of grain size distribution and magnetic properties of the deposit as well as petrographical mineralogical and geochemical analyses of pumice clasts all indicate that the lahar was derived from volcanic fallout deposits that originated at popocatepetl volcano located 50 km south southeast of the site popocatepetl underwent a major phreato plinian eruption dated ca 14 000 yr b p at the end of the last major glaciation dispersal of the fallout was mostly toward the northwest and north deposits were found on the slopes of tlaloc mountain 4150 m 20 km southeast of tocuila at the beginning of deglaciation and the onset of more humid conditions 3000 yr later lahars derived from the fallout deposits picked up the bones of already dead animals and then deposited them on the shores of lake texcoco and or simply covered bones without major displacement

chlorine and alkali geochemical halos in the footwall breccia and sublayer norite at the margin of the strathcona embayment sudbury structure ontario the ni cu platinum group element ores in the sublayer and footwall breccia of the sudbury structure have distinctive geochemical halos in the fraser mine f cl ratios of the sublayer rocks appear to decrease na2o k2o ratios of the footwall breccia samples decrease and cl in both units increases with proximity to mineralization these halos are interpreted to result from interaction of residual magmatic fluids with host rocks which altered the host rock mineralogy and precipitated cl bearing minerals the results of this study together with the results of previous studies suggest that most ores at sudbury including sublayer and footwall breccia hosted contact ores and sudbury breccia hosted footwall ores are surrounded by cl and possibly alkali halos and that these geochemical signatures may be used to identify rocks that are proximal to mineralization

sand budgets at geological historical and contemporary time scales for a developed beach system saco bay maine usa abstract saco bay hosts one of the largest sand beach and salt marsh systems in northern new england usa unsuccessful remedies for chronic beach erosion have generated controversy and differing theories regarding 1 the source s of sand to the bay 2 the sand transport paths and 3 the role of jetties in beach erosion we constructed a sand budget for the bay by evaluating the past and present sand transport pathways fluxes and reservoir volumes side scan sonar and bottom samples were employed to map the offshore areas seismic reflection profiles and vibracores were used to measure the volume of offshore sand on land ground penetrating radar and cores constrained the sand volume and stratigraphy time series maps and aerial photographs provided data on historical sand movement and current meters and recent harbor dredge data allowed contemporary sand movement to be estimated in the saco river estuary at the present time 56 million m3 of sand reside in the shoreface and 22 million m3 of sand are beneath the barrier beach and dunes the saco river has been the major source of sand to the bay providing an estimated 10 000 16 000 m3 of sand per year the net direction of sand movement is to the north where river derived sand has historically fed growth of barrier spits construction of the north jetty at the saco river mouth profoundly altered the prehistoric patterns of sand movement and storage in the bay causing the northward migration of almost 6 million m3 of sand between 1867 and 1955 contemporary dredging of tidal inlets involves sand volumes approximately equal to the influx of sand delivered by the saco river

sulfide melt inclusions as evidence for the existence of a sulfide partial melt at broken hill australia polyphase sulfide melt inclusions are hosted within garnetite rocks and quartz veins in garnetite surrounding droppers and large masses of the orebody at broken hill australia and record the presence of a former sulfide melt sulfide melt inclusions are either primary or occur along healed fractures in both garnets and quartz veins common daughter minerals in the inclusions are galena sphalerite arsenopyrite chalcopyrite tetrahedrite tennantite and minor amounts of argentite bornite dyscrasite ag3sb and gudmundite fesbs the inclusions exhibit a strong enrichment in low melting point chalcophile elements compared to the main orebody experimental reequilibration of sulfide melt inclusions shows homogenous melt at temperatures as low as 720 10 c and 5 kbars well below that of peak metamorphism at broken hill 800 10 c and 5 kbars thus these inclusions are interpreted to represent a trapped sulfide melt formed during peak metamorphism at broken hill australia

geology and radiocarbon ages of tlaloc tlacotenco cuauhtzin hijo del cuauhtzin teuhtli and ocusacayo monogenetic volcanoes in the central part of the sierra chichinautzin mexico tlaloc tlacotenco cuauhtzin hijo del cuauhtzin teuhtli and ocusacayo monogenetic volcanoes located within the sierra del chichinautzin volcanic field scvf at the southern margin of mexico city were studied to further refine attendant volcanic hazards in this heavily populated region based on fieldwork and landsat imagery interpretation a geologic map was produced morphometric parameters characterizing the cones and lava flows were determined and the areal extent and volumes of erupted products were estimated the longest lava flow was produced by tlacotenco and reached 9 5 km from its source total areas covered by lava flows from each eruption range between 12 8 km2 tlacotenco and 54 4 km2 tlaloc and total erupted volumes range between 0 26 and 1 36 km3 per volcano radiocarbon measurements of a paleosol underneath an ash layer from the tlaloc scoria cone yielded an age of 6200 years bp while charcoal found within block and ash flow and lahar deposits from cuauhtzin dome yielded ages of 7360 and 8225 years bp respectively the tlacotenco dacite lava flow overlies popocatepetl s tutti frutti plinian pumice fall deposit dated at 14 000 years bp and is therefore younger than this prominent stratigraphic marker on the other hand teuhtli and hijo del cuauhtzin scoria cones and the ocusacayo andesite lava flows are overlain by the tutti frutti and therefore older than 14 000 years bp these new dates together with other published dates for scoria cones in the scvf imply that the previously determined recurrence interval during the holocene for monogenetic eruptions in the scvf of 1700 years siebe c rodriguez lara v schaaf p abrams m 2004a radiocarbon ages of holocene pelado guespalapa and chichinautzin scoria cones south of mexico city implications for archaeology and future hazards bull volcanol 66 203 225 needs to be corrected to 1250 years this means that the time of quiescence since the last eruption of the scvf 1670 years bp exceeds that of the estimated recurrence interval during the holocene

wetting facilitates late stage segregation of precious metal enriched sulfosalt melt in magmatic sulfide systems the wetting behavior of as rich sulfosalt melts against monosulfide solid solution mss is investigated using experiments to elucidate late stage fractionation processes in magmatic sulfide systems which may control the distribution of platinum group elements a range of as rich melt compositions is found to wet mss including those that contain significant proportions 1 45 of precious metals pt pd and au however extremely au rich or pt rich sulfosalt melts 40 au 50 pt 5 au do not wet mss these results imply that if magma contamination and or fractionation processes were able to produce a late stage as rich melt sulfosalt melts crystallize cooler than mss that exceeds 0 2 of the rock volume then an interconnected melt drainage network would be able to form along mss crystal triple junctions the dense sulfosalt melt could thereby drain downward progressively sequestering incompatible bi sb te pt pd and au to form sulfosalt melt accumulations that continue to fractionate to form the platinum group minerals this late stage fractionation model is consistent with the observed mineral distribution in numerous magmatic sulfide deposits

nearshore shore oblique bars gravel outcrops and their correlation to shoreline change abstract this study demonstrates the physical concurrence of shore oblique bars and gravel outcrops in the surf zone along the northern outer banks of north carolina these subaqueous features are spatially correlated with shoreline change at a range of temporal and spatial scales previous studies have noted the existence of beach surf zone interactions but in general relationships between nearshore geological features and coastal change are poorly understood these new findings should be considered when exploring coastal zone dynamics and developing predictive engineering models the surf zone and nearshore region of the outer banks is predominantly planar and sandy but there are several discrete regions with shore oblique bars and interspersed gravel outcrops these bar fields have relief up to 3 m are several kilometers wide and were relatively stationary over a 1 5 year survey period however the shoreward component of the bar field does exhibit change during this time frame all gravel outcrops observed in the study region a 40 km longshore length were located adjacent to a shore oblique bar in a trough that had width and length similar to that of the associated bar seismic surveys show that the outcrops are part of a gravel stratum underlying the active surface sand layer cross correlation analyses demonstrate high correlation of monthly and multi decadal shoreline change rates with the adjacent surf zone bathymetry and sediment distribution regionally areas with shore oblique bars and gravel outcrops are correlated with on shore areas of high short term shoreline variability and high long term shoreline change rates the major peaks in long term shoreline erosion are onshore of shore oblique bars but not all areas with high rates of long term shoreline change are associated with shore oblique bars and troughs

the hemlo gold deposit ontario an example of melting and mobilization of a precious metal sulfosalt assemblage during amphibolite facies metamorphism and deformation the timing of mineralization at the hemlo gold deposit has long been a controversial issue we present arguments that use partial melting of the ore assemblage to explain the mechanism of ore mobilization and resolve problems with the model of premetamorphic mineralization the results have implications for mine exploitation and exploration strategies we show that the ore mineral assemblage underwent partial melting during middle amphibolite facies metamorphism 600 650 c 6 7 kbars primarily through breakdown of stibnite and arsenopyrite in a high s2 environment concurrent deformation led to segregation of the resulting sb and as rich sulfosalt melt interaction between this melt and a range of unmelted sulfides led to further melting and incorporation of other elements into the melt the gold bearing melt was mobilized from compressional high strain sites into dilational domains such as boudin necks and extensional fractures developed in competent lithologies ore minerals that did not melt significantly pyrite molybdenite pyrrhotite and sphalerite were not extensively mobilized and largely remained within high strain compressional domains this segregation of melt from residue thus resulted in the observed heterogeneous distribution of ore minerals within the deposit crystallization of the sulfosalt melt produced a diverse suite of ore minerals dominated by stibnite and realgar and containing an array of rare sulfosalts native elements intermetallic compounds and tellurides some sulfosalt melt persisted to low temperature 300 c allowing continued small scale localized mobilization during late deformation although gold occurs at moderate concentrations within the compressional high strain domains it is particularly concentrated in the dilational domains a consequence of its mobilization within a sulfosalt melt our model of partial melting of the ore mineral assemblage with consequent mobilization explains how ore minerals that are unstable even at greenschist facies conditions came to be hosted in structures that formed at or near the peak of amphibolite facies metamorphism

eruptive styles and inferences about plumbing systems at hidden cone and little black peak scoria cone volcanoes nevada u s a we describe two small scoria cone volcanoes hidden cone and little black peak ages between 320 390 ka in the southwestern nevada volcanic field and discuss their eruption mechanisms and inferences about their plumbing systems cone forming pyroclastic deposits are consistent with eruptive styles ranging from strombolian to violent strombolian and lavas emanated from near the bases of the cones the volcanoes are monogenetic rather than polycyclic as allowed by previous geomorphic interpretations vents at each volcano appear to coincide with pre existing normal faults consistent with observations at older deeply eroded volcanoes in the region the existence of these two volcanoes on a topographically high area particularly hidden cone provides evidence for short feeder dike lengths 500 m at the surface we infer that this short length reflects the small length scale of the mantle source region that was tapped to feed each volcano

shallow plumbing systems for small volume basaltic volcanoes eruptive dynamics in basaltic volcanoes are controlled in part by the conduit geometry however uncertainties in conduit shape and dike to conduit transition geometry have limited our predictive capability for hazards assessments we characterize the subvolcanic geometry of small volume basaltic volcanoes magmatic volatile driven eruptions 0 1 to 0 5 km3 based on a synthesis of field studies of five basaltic volcanoes exposed to varying degrees by erosion and exhibiting feeder dikes conduits and vent areas 250 m depth study areas include east grants ridge new mexico usa basalt ridge east basalt ridge paiute ridge and southeast crater flat nevada usa basaltic feeder dikes 250 to 100 m deep have typical widths of 4 12 m with smooth host rock contacts rhyolite tuff at depths less than 100 m heterogeneities in the host rock form preferential pathways for small dike splays and sills resulting in a 30 m effective width at 50 m depth the development of a complex conduit at depths less than 70 m is reflected in bifurcating dikes and brecciation and incorporation of the country rock the overall zone of effect at depths less than 50 m is 110 m wide 220 m elongated along the feeder dike based on comparisons with theoretical conduit flow models the width of the feeder dike at depths from 250 to 500 m is expected to range from 1 to 10 m and is expected to decrease to about 1 2 m at depths greater than 500 m the flaring shape of the observed feeder systems is similar to results of theoretical modeling using lithostatic pressure balanced flow conditions sizes of observed conduits differ from modeled dimensions by up to a factor of 10 in the shallow subsurface 50 m depth but at depths greater than 100 m the difference is a factor of 2 to 4 this difference is primarily due to the fact that observed eroded conduits record the superimposed effects of multiple eruptive events while theoretical model results define dimensions necessary for a single steady eruption phase the complex details of magma host rock interactions observed at the study areas contact welding brecciation bifurcating dikes and sills and stoping represent the mechanisms by which the lithostatic pressure balanced geometry is attained the similarity in the normalized shapes of theoretical and observed conduits demonstrates the appropriateness of the pressure balanced modeling approach consistent with the conclusions of wilson and head j geophys res 86 2971 3001 1981 for this type of volcano

highly explosive eruption of the monogenetic 8 6 ka bp la vache et lassolas scoria cone complex chaine des puys france abstract the eruption of the trachy basaltic la vache and lassolas cone complex was the youngest eruption ca 8 6 ka bp and one of the most violent in the chaine des puys france here we present field data and results of grain size componentry and clast density measurements of different layers of the widespread tephra deposit that is associated with this cone forming eruption our data indicates five main eruption phases comprising a vent opening phase a second sustained highly explosive phase a third and fourth violent strombolian phase and a fifth dominantly effusive phase the layer formed by the opening phase is rich in lithic material which was previously considered to be the result of phreatomagmatic activity the data presented here on the componentry and textures of the pyroclastic material contradict this hypothesis we propose instead that the material of the basal layer results from fragmentation caused by the explosion of a first arriving gas dominated phase the variations in eruption intensity during the main eruption phases are interpreted here to be the result of gas segregation within the plumbing system and fluxes in the magma ascent rate during the eruption significant amount of gas segregation is indicated by the deposition of both gas poor and gas rich material and by the presence of plate tephra this is also supported by the simultaneous ejection of tephra and lava from both cones during most of the explosive activity we suggest that gas segregation occurred within shallow intrusions and that fresh ascending material in the main conduit mixed with degassed material that flow back into the conduit from the intrusion before fragmentation the interaction of the ascending magma and the opening of intrusions may have controlled the evolution and explosivity of the eruption the high explosivity at the beginning of the eruption and the wide dispersal area demonstrate that scoria cone eruptions in monogenetic fields can impose a major threat to the population and infrastructures nearby as these events may occur with little warning and therefore research on this kind of eruptions is of a major importance to better understand the processes driving these events

geochemistry and genesis of the lengenbach pb zn as tl ba mineralisation binn valley switzerland the lengenbach pb zn as tl ba mineralisation is located in triassic dolostones of the penninic zone in the swiss alps where alpine metamorphism reached upper greenschist to lower amphibolite grade geochemical data are used to constrain the origin of this unique occurrence two metamorphic redox environments are present the as iii rich zone is controlled by barite pyrite while the reduced zone contains graphite or pyrrhotitepyrite and formally zerovalent as the as iii rich zone is characterised by a mineral assemblage consistent withfo2 in the stability field of barite pyrite an as pb tl rich sulphide melt coexisted with a hydrothermal fluid at 300 c in this zone mineralised dolostones are anomalous in as pb ag tl hg zn ba cd fe cu mo u v b ga cr and possibly sri and au in order of decreasing enrichment as pb and zn are present in the 0 1 to 1 range tl and ag reach several hundred ppm uraninite is concentrated in silicate rich bands and yields a late alpine u pb age of 18 5 0 5 ma pb and s isotopic variations are interpreted by metamorphic overprinting and re equilibration within an isochemically metamorphosed mineralisation hydrothermal sulphides are more strongly affected by uranogenic pb than massive pb as sulphides representing a former sulphide melt the least overprinted mineralisation is characterised by206pb 204pb 18 44 18 56 207pb 204pb 15 60 15 75 208pb 204pb 38 44 38 84 and 34s sulphide 25 2 s isotopic variations are largely a result of sulphide sulphate re equilibration yielding temperatures of 450 30 c 87sr 86sr ratios of mineralised samples are lower than or equal to host dolostones precluding major infiltration of basement derived fluids during alpine metamorphism the sr source 87sr 86sr close to 0 708 probably was seawater with a radiogenic detrital mineral component the genesis of the unique lengenbach mineralisation is interpreted as the result of isochemical metamorphic overprinting of a carbonate hosted stratiform sulphide mineralisation well crystallised sulphide minerals in fissures and druses formed during retrograde cooling of a sulphide melt in equilibrium with a hydrothermal fluid the primary mineralisation was probably formed at or close below the sea floor and fed by sulphide poor hydrothermal fluids sulphide was largely derived from seawater by open system bacterial sulphate reduction u v and mo may be seawater derived

the al du aythah volcanic cones al madinah city implications for volcanic hazards in northern harrat rahat kingdom of saudi arabia the basaltic al du aythah volcanic cones lie in the northern part of the extensive lava field of harrat rahat and only 13 km from the centre of al madinah city in the kingdom of saudi arabia historical records indicate they may have erupted in ad 641 the four cones are formed by deposits that record a transition from phreatomagmatic to magmatic explosions followed by minor lava effusion three cones display elongated tuff rings at the base and two produced late stage lava flows the cones themselves are symmetrical and constructed mostly by the accumulation of ballistically ejected pyroclasts spherical bombs and lapilli cannonball bombs lapilli occasionally with country rock fragments inside both cored and loaded bombs lapilli are common within the tuff ring deposits lidar data show a total volume of 1 664 10 6 km3 for the four cones 418 10 6 km3 dre whole rock chemical analyses indicate alkali basalt compositions sio2 44 7 45 9 wt with little compositional variation and no relationship between chemistry and eruptive styles small differences in composition may reflect variations in fractional crystallisation of clinopyroxene and olivine a magnetotelluric 2d cross section shows that the cones are located adjacent to a buried sediment filled alluvial channel along a nnw sse fault dipping to the east the al du aythah eruption was related to the ascent of magma through this structure with the first phase of the eruption triggered by the interaction of the magma with water from the northern harrat rahat aquifer that exists in the al madinah basin this initial water source was rapidly exhausted while the eruption progressed roughly from north to south and from west to east the latter motion probably along the fault controlled feeding dyke our work draws attention to the existence of recent explosive phreatomagmatic eruptions in the al madinah basin which despite the hyperarid climate of the area must be considered a potential future eruption hazard

tectonics and volcanism of sierra chichinautzin extension at the front of the central trans mexican volcanic belt abstract because of its recent activity and position at the southern magmatic front of the trans mexican volcanic belt tmvb the sierra chichinautzin volcanic field scn is a key area for the understanding of this controversial volcanic province volcanic activity has built more than 220 monogenetic volcanoes shields scoria cones thick lava flows and hydromagmatic structures during the last 40 000 years for a total volume of about 470 km 3 the scn basalts are geochemically similar to oibs while the intermediate and felsic volcanic rocks show a calc alkaline trend and abundant evidence for magma mixing the structural analysis of this volcanic field and surrounding areas has been based on field data satellite images and a method for detecting volcanic center alignments the tectonic data together with geophysical evidence confirm active general n s extensional conditions with a strike slip component for the scn area the same structural setting that prevails in the rest of the central tmvb extensional tectonics a negative regional bouger gravity anomaly a low velocity mantle high heat flow and shallow seismicity suggest a rift type setting involving the upwelling of anomalous mantle beneath the central tmvb the combined petrological structural and geophysical arguments support that the scn volcanism is rift related and rule out processes involving the subduction of the cocos plate which casts further doubts on the standard subduction model for the tmvb volcanism