segmentation and differential post rift uplift at the angola margin as recorded by the transform rifted benguela and oblique to orthogonal rifted kwanza basins abstract we analyse tectonic and sedimentary field and subsurface data for the angola onshore margin together with free air gravity anomaly data for the offshore margin this enables us to characterize the mode of syn rift tectonism inherited from the precambrian and its impact on the segmentation of the angola margin we illustrate that segmentation by the progressive transition from the benguela transform rifted margin segment to the oblique rifted south kwanza and orthogonal rifted north kwanza margin segments the spatial variation in the intensity of post rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post rift succession of the coastal benguela and kwanza basins upper cretaceous to cenozoic uplifted palaeodeltas erosional unconformities palaeovalleys quaternary marine terraces and perched gilbert deltas the onshore benguela transform margin has a distinctive mainly progradational stratigraphic architecture with long term sedimentary gaps and high elevation marine terraces resulting from moderate upper cretaceous cenozoic to major quaternary uplifting i e 775 1775 mm ky or m ma by contrast repeated synchronous episodes of minor cenozoic to quaternary uplift occurred along the orthogonal rifted north kwanza segment with its cenozoic aggradational architecture short term sedimentary gaps and low elevation pleistocene terraces margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep sea fans along the low lying north kwanza margin sedimentation of the broad cenozoic to pleistocene kwanza submarine fan was probably governed by the width of the kwanza interior palaeodrainage basin combined with the wet tropical neogene climate along the high rising benguela margin the small size of the benguela deep sea fan is related to the interplay between moderate continental sediment dispersal from long lived small catchments and a warm very arid neogene climate however the driving forces behind the epeirogenic post rift uplift of the angola coastal bulge remain a matter of speculation

deposition diagenetic and hydrothermal processes in the aptian pre salt lacustrine carbonate reservoirs of the northern campos basin offshore brazil abstract the discovery of large oil accumulations in the rift and sag pre salt sections of the campos and santos basins has revived interest in the exploration of the lacustrine carbonate reservoirs in the brazilian and african marginal basins more than half of brazilian oil production originates from the pre salt reservoirs of these offshore basins a study integrating systematic petrography cathodoluminescence scanning electron microscopy microprobe and x ray diffraction was performed on seven wells in the northern campos basin this study highlights the major primary diagenetic and hydrothermal features of the pre salt succession with the aim to improve our understanding of the factors that influence the porosity and permeability distribution in these important lacustrine carbonate reservoirs the pre salt deposits correspond to bioclastic grainstones and rudstones syngenetic crusts of fascicular calcite and intraclastic grainstones and rudstones of reworked crust fragments and calcite spherulites magnesian silicates are frequently associated with carbonate deposits in the sag phase stevensitic laminations constitute the substrate for the precipitation of calcite crusts and spherulites which displace and replace the syngenetic magnesian clay deposits in the rift section stevensitic ooids are mixed with bioclasts or form ooidal arenites pre salt carbonate reservoirs have undergone a complex and heterogeneous diagenetic evolution eodiagenetic processes include the dissolution neomorphism and cementation of bivalve bioclasts in the rift as well as the dissolution of magnesian silicates and their replacement by calcite spherulites silica and dolomite in the sag section burial alterations are commonly associated with hydrothermal fluids carried through faults and fracture systems these fluids promote dolomitization silicification and dissolution at varying degrees and intensities eodiagenetic precipitation and dissolution owing to variations in the lake water chemistry and the episodic flow of hydrothermal fluids under burial conditions control the creation redistribution and obliteration of porosity in the pre salt reservoirs

volcano geodesy and magma dynamics in iceland abstract here we review the achievements of volcano geodesy in iceland during the last 15 years extensive measurements of crustal deformation have been conducted using a variety of geodetic techniques including leveling electronic distance measurements campaign and continuous global positioning system gps geodesy and interferometric analysis of synthetic aperture radar images insar results from these measurements provide a comprehensive view of the behavior of icelandic volcanoes between inflation intrusion and eruption episodes volcanoes are likely to deflate or show no sign of seismic activity subsidence rates are often in the range of a few millimeters to a few centimeters a year reducing progressively with time since the last eruption or intrusion at the volcano subsidence can be caused by cooling and contraction of magma outflow of magma it can be related to plate spreading volcano subsidence or lack of deformation is often interrupted by episodic magma flow towards near surface locations such magma recharge has been observed geodetically at hengill hekla eyjafjallajokull katla grimsvotn and krafla volcanoes with inflow inferred to last from a few months up to two decades in the last 15 years five volcanic eruptions three intrusive events and two m6 earthquakes have occurred in recent years the grimsvotn and katla volcanoes have exhibited continuous inflation of a few centimeters per year which at grimsvotn culminated in an eruption on 1 november 2004 hekla and torfajokull volcanoes have inflated at rates an order of magnitude less subsidence is occurring presently at the askja and krafla volcanoes within the period of geodetic measurement signals consistent with no deformation are typical for most of the 35 active volcanoes in iceland

relating petroleum system and play development to basin evolution west african south atlantic basins abstract sedimentary basins can be classified according to their structural genesis and evolutionary history and the latter can be linked to petroleum system and play development we propose an approach in which we use the established concepts in a new way breaking basins down into their natural basin cycle division then defining the characteristics of each basin cycle including the type of petroleum systems and plays they may contain and comparing them with similar basin cycles in other basins thereby providing a means to learn through a greater population of perhaps not immediately obvious analogues furthermore we introduce the use of the trajectory plot as a new tool in such an analysis this methodology has been applied to the west african south atlantic marginal basins between cameroon and angola and we demonstrate that the similar tectonostratigraphic evolution of the individual basins along this margin has led to the development of similar types of petroleum systems and play level s consequently we can make analogue comparisons among these basins in order to evaluate and predict the presence of potential yet undiscovered hydrocarbon accumulations in less well explored parts of the margin

substrate geology controlling different morphology sedimentology diagenesis and geochemistry of adjacent travertine bodies a case study from the sanandaj sirjan zone western iran abstract a travertine fissure ridge and travertine mound are situated 800 m from each other at the western margin of the active tectono volcanic sanandaj sirjan zone western iran despite their close proximity the two geobodies show a difference in morphology lithofacies lithotypes diagenesis and geochemistry petrographic analysis of the fissure ridge carbonates revealed homogeneous sparitic fabrics with dendritic structures reflecting precipitation under fast flowing conditions from calcite supersaturated spring water the mound carbonates are much more heterogeneous and display dominantly micritic fabrics with mainly volcanic clasts reflecting a lower energy flow regime and lower calcite saturation in contrast to the limited diagenetic overprint of the fissure ridge carbonates the more porous mound carbonates are affected by dissolution cementation and formation of mn fe oxide hydroxides attesting of a strong control on porosity by early diagenetic processes the widespread bright luminescent calcite phases especially in sparitic fabrics within both geobodies display high mn concentrations in calcite indicating suboxic precipitation conditions the overlapping 87sr 86sr signatures and 13c signatures point to a mixture of co2 that dominantly originated from dissolution of the marine carbonate qom formation oligo miocene the different 18o signatures indicate different upwelling systems caused by different substrate settings the fissure ridge formed on a hard and brittle limestone substrate with precipitation from dominantly subsurface sourced fluids the mound travertine formed on top of fairly unconsolidated and fractured volcanic strata the depleted 18o signatures and impure micrite dominated fabrics of the mound travertine reflect precipitation from thermal water that mixed with surrounding groundwater this difference is also reflected in a different calculated precipitation temperature that varies between 4 and 21 c for the mound versus 25 50 c for the fissure ridge our findings show that subsurface geology exerts a major control on precipitation processes resulting in differences in travertine morphology sedimentology diagenesis and geochemistry despite that the travertine bodies occur adjacent to each other

dating recent volcanic rocks through 230th 238u disequilibrium in accessory minerals example of the puy de dome french massif central precise dating of very young 10 000 yr silicic volcanic rocks can be achieved by measurements of 230 th 238 u disequilibria in accessory minerals such as apatite and zircon it is illustrated here by the isochron age of 12 000 1000 yr obtained on a trachyte from the puy de dome french massif central the crystallization age is virtually the same as the eruption age determined by thermoluminescence and 14 c dating this indicates a short time interval a few centuries at most between crystallization and eruption in such small volume silicic magmas

source mechanism of the 1987 vatnafjoll earthquake in south iceland the may 25 1987 vatnafjoll earthquake occurred on the mid atlantic plate boundary at a junction between a transform the south iceland seismic zone and a spreading segment the eastern volcanic zone the south iceland seismic zone has had the most destructive earthquakes in the history of iceland with average intervals of 80 100 years and estimated maximum magnitudes around 7 the 1987 vatnafjoll earthquake was the largest earthquake mb 5 8 ms 5 8 mw 5 9 in south iceland since the 1912 magnitude 7 earthquake and is the first event sufficiently large to allow a meaningful study of source process in this geologically interesting region foreshocks and aftershocks of the vatnafjoll earthquake were located with relatively good accuracy they define an elongated n s epicentral area 12 km long and 4 km wide with hypocentral depths mostly between 6 and 13 km we determined centroid source parameters of the vatnafjoll earthquake by inverting long period teleseismic p and sh waves the centroid source mechanism and foreshock and aftershock distribution indicate n s right lateral strike slip faulting on a near vertical plane the rupture initiated near the base of the crust and propagated upward along an approximately 10 km long fault but did not reach the surface the centroid depth is well constrained and is between 5 and 8 km depth at the 95 confidence level the seismic moment is 9 1 1024dyn cm the source has relatively short duration of 3 s which may indicate higher stress drop than global average of earthquakes of this size the south iceland seismic zone is a 70 km long e w striking transform zone between two overlapping rift zones the eastern and western volcanic zones it is characterized by bookshelf type of tectonics that is there is no surface faulting in the direction of the transform instead n s right lateral strike slip faults are distributed along and perpendicular to the transform zone the vatnafjoll earthquake has characteristics of earthquakes in the south iceland seismic zone and is probably not related directly with magmatic activity in the eastern volcanic zone the transform zone thus extends farther into the rift zone than was previously recognized

improved 1d velocity model and deep long period earthquakes in kanlaon volcano philippines implications for its magmatic system abstract we determined an improved 1d p wave velocity model for kanlaon volcano one of the most active volcanoes in the philippines for use in precise earthquake relocation the inversion for velocity model was conducted using the program velest which solved the coupled hypocenter velocity problem from dataset of 251 local earthquakes recorded by the kanlaon volcano seismic network from 2014 to 2017 using the improved velocity model we relocated the volcano tectonic vt earthquakes in kanlaon with hypodd resulting in better constrained hypocenter locations based on the decrease in rms and corresponding error assessment tests subsequently we relocated swarms of long period volcanic earthquakes that occurred from september 1 2017 to august 31 2018 and prompted the philippine institute of volcanology and seismology to raise the volcano s alert level these long period earthquakes had emergent arrivals hence we applied a cross correlation procedure using redpy to obtain an improved arrival time picks these improved hypocenters revealed the existence of deep long period dlp earthquakes that occurred at depths of 23 to 36 km beneath the volcano these are the first dlp events documented in kanlaon since the establishment of the kanlaon volcano observatory in 1961 they were found to belong into families based on similarity of waveforms suggesting they were produced by repetitive non destructive sources the occurrences of dlps in kanlaon reflected the pressurization of its deep seated magmatic system their identification is critical in assessing the current state of kanlaon volcano especially since its last magmatic eruption was in 1902

hydrothermal carbonate chimneys from a continental rift afar rift mineralogy geochemistry and mode of formation carbonate chimney like deposits up to 60 m high are scattered or arranged in rows at the shores of a desiccating hypersaline and alkaline lake from a continental rift setting lake abhe afar rift djibouti the chimneys formed sub aqueously in the lake water body at a higher water level than observed today alternating calcite and low mg calcite silica concentric layers compose the chimney structures mineralogical and geochemical investigations of the chimneys lake water and hot spring hydrothermal fluids suggest that the chimneys are a result of rapid carbonate precipitation during the mixing of hydrothermal fluids with lake water in contrast to the hot spring fluid lake water is enriched in hree and possesses a pronounced positive ce anomaly features that are preserved in the carbonate chimney layers mixing calculations based on sr isotope and concentration data indicate a hydrothermal fluid contribution of 45 in the chimney interior which decreases to 4 in the external chimney layer sr in the hydrothermal fluids is predominantly leached from the underlying volcanic rocks whereas the lake s sr budget is dominated by riverine input considering the fluid mixing ratios calculated by sr data the measured c and o isotope compositions indicate that chimney carbonates precipitated at temperatures between 14 c internal part and 22 c external part with 13c carbonate mainly controlled by isotope equilibrium exchange of lake water with atmospheric co2 the low mg calcite layers including the outermost layer have enhanced signals of lake water inheritance based on elevated concentrations of immobile elements ree and sr and ca isotope compositions ca isotope data reveal that internal chimney layers formed by non equilibrium calcite precipitation with a predominantly hydrothermal ca source the external low mg calcite layer received ca contributions from both hydrothermal fluid and lake water with the latter being the dominant ca source highly positive 44 40ca of lake water likely reflects non equilibrium ca carbonate precipitation during lake water evaporation with resulting 44ca enrichment of residual lake water the strong degree of 44ca enrichment may point towards multiple lake drying and ca reservoir depletion events coupled c o ca isotope data of the sampled carbonate chimney suggest late stage low temperature hydrothermal carbonate chimney formation during strongly evaporative lake conditions at the time of low mg calcite precipitation u th age dating suggests that the chimneys formed no earlier than 0 82 kyr bp 0 28 0 54

ash layers from iceland in the greenland grip ice core correlated with oceanic and land sediments four previously known ash layers ash zones i and ii saksunarvatn and the settlement layer all originating in iceland have been identified in the central greenland ice core grip this correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash this establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately a precise connection with dates obtained by14c beyond the range of dendrochronology is established which provides an excellent confirmation of230th 234u dates from corals four additional icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits

lower crustal earthquakes caused by magma movement beneath askja volcano on the north iceland rift the lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity the icelandic mid atlantic rift is characterized by high heat flow abundant magmatism generating up to 25 30 km thick crust and seismicity within the upper 8 km of the crust in a 20 seismometer survey in july august 2006 within the northern rift zone around the askja volcano we recorded 1700 upper crustal earthquakes cutting off at 7 8 km depth marking the brittle ductile boundary unexpectedly we discovered 100 small magnitude ml 1 5 earthquakes occurring in swarms mostly at 14 26 km depth within the otherwise aseismic lower crust and beneath the completely aseismic middle crust a repeat survey during july august 2007 yielded more than twice as many lower crustal events geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath askja we interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding askja they represent bursts of magma motion opening dykes over distances of a few meters facilitated by the extensional setting of the active rift zone

causes and implications of the middle rare earth element depletion in the eolian component of north pacific sediment abstract previous studies clearly demonstrate that the detrital fraction of central north pacific sediment is derived almost exclusively from wind born particles from the arid and semi arid regions in asia these conclusions are based in part on trajectories observed for westerly wind systems and on a grainsize distribution for central north pacific sediment which can only be explained by eolian transport these observations have been verified by comparing the mineralogy neodymium isotopic composition and trace element geochemistry of the sediment with its asian source region however recent geochemical investigations consistently highlight compositional differences exemplified by a mree middle rare earth element depletion and a lower th sc ratio in eolian material extracted from north pacific sediment when compared to bulk china loess the continental analog of the eolian material these geochemical differences persist even when the bulk loess is subjected to the same extraction procedure here we present experiments demonstrating that these compositional differences are caused by a combination of grainsize fractionation during transport and partial dissolution of ree and th bearing phosphatic phases during the extraction procedure two bulk loess samples from china were separated into several different grain size fractions and a split of each fraction was subjected to the extraction procedure commonly used to isolate terrigenous material from marine sediment all extracted and unextracted sample pairs were analyzed for p th sc fe la ce sm eu yb and lu the amount of p removed by the extraction procedure correlates well with both the observed mree depletion and the decrease in th sc and is most pronounced in the finest grained samples these correlations strongly suggest that the extraction procedure dissolves inorganic phosphate phases which are rich in mrees and th a comparison of the compositions of the 2 3 mm loess size fractions with central north pacific sediment shows that both of these materials are identical

hydrothermal activity in the northern tanganyika rift east africa abstract hydrothermal fluids from sub lacustrine and sub aerial sites were sampled in the north tanganyika rift during the 1991 tanganydro expedition two types of hydrothermal end member fluids a na hco 3 type and a na cl type have been identified within the sample set the na hco 3 hydrothermal fluids are emitted at the level of the sub lacustrine pemba and sub aerial kabezi and mashuzi sites which are located in the northernmost bujumbura sub basin the na cl hydrothermal fluid vents at the level of the sub lacustrine cape banza field on the southeastern edge of the ubwari ridge the two types of hydrothermal fluids display isotopic and chemical evidence of interaction with magmatic volatiles the magmatic effects are strongest in the fluid discharging on the pemba hydrothermal field where abundant co 2 bubbling accompanies fluid emission the 13 c values of the co 2 gas sampled at pemba and at cape banza indicate a magmatic origin the pemba fluid shows the highest alkalinity and boron content and the lowest ph of the hydrothermal end member fluids and it has equilibrated at depth at the highest temperature 214 7 c in addition the sulfide mineralizations associated with the pemba hydrothermal vents are enriched in as and tl which similarly to b can be transported as volatiles in magmatic fluids the northern tanganyika na hco 3 fluids are similar to the na hco 3 fluid of the geothermal system of lake bogoria in the eastern branch of the east african rift and they can be compared with the na hco 3 hydrothermal water that is inferred to flow into lake kivu the cape banza na cl fluids may derive from the mixing between meteoric water and a deep seated basement brine similar to the ones found in old continental shields such as the canadian shield the high equilibrium temperatures and the geochemistry of the north tanganyika rift hydrothermal fluids suggest that degassing magmatic bodies are present at depth in the area these magmatic bodies may be either the southern expression of the south kivu magmatism or a more recent activity not yet expressed as surface volcanism the hydrothermal sites are located along major active rift bounding faults which can be associated with deep and strong seismicity as in the case of the pemba field such deep fractures may have favoured the uprise of the magmatic bodies that are the source of heat and volatiles to the hydrothermal systems the circulation of hydrothermal fluids within the tanganyika lacustrine sedimentary pile could play an important role in the maturation of the organic matter enclosed in the lake sediments and the subsequent oil generation and migration

persistent shallow background microseismicity on hekla volcano iceland a potential monitoring tool abstract hekla is one of iceland s most active volcanoes since 1970 it has erupted four times with a period of quiescence of 14 years since the last eruption we detected persistent levels of background microseismicity with a temporary seismic network in autumn 2012 an amplitude based as well as an arrival time based location method was applied to two populations of events and located them at shallow depths on the northern flank close to the summit this seismicity has not been identified previously by the permanent seismic network in iceland as it is below its detection threshold the detected events were either short higher frequency events with distinct arrivals located beneath the summit on the northern flank of hekla or longer emergent lower frequency events about 4 km northeast of the summit at 200 400 m depth below the surface estimated moment magnitudes were m w 1 1 to 0 1 and m w 0 9 to 0 0 and local magnitudes m l 0 5 to 0 3 and m l 0 3 to 0 3 respectively this seismicity does not show any correlation with gas output but is located at the steepest slopes of the edifice hence we suggest that the current shallow microseismicity at hekla is structurally controlled this offers a possible opportunity of using near summit microseismicity as a tool for monitoring emerging unrest at hekla microseismicity rates will be very sensitive to small stress perturbations due to magma migration at depth currently in the absence of microseismicity monitoring hekla switches from apparently quiescent to fully eruptive on the order of only 1 h

earthquake activity related to the 1991 eruption of the hekla volcano iceland the 1991 eruption of the hekla volcano started unexpectedly on 17 january no long term precursory seismicity was observed the first related activity was a swarm of small earthquakes that began approximately half an hour before the eruption intensive seismicity both earthquakes and volcanic tremor accompanied the violent onset of the eruption almost 400 events up to ml magnitude 2 5 were recorded during the first few hours during the later phases of the eruption the earthquake activity was modest and the main volcano related seismic signal was the persistent volcanic tremor the tremor died away together with the eruption on 11 march and hekla was seismically quiet until the beginning of june 1991 when a sudden swarm of numerous small shallow earthquakes occurred this activity is atypical for hekla and is interpreted to be a failed attempt to resume the eruption

are spherulitic lacustrine carbonates an expression of large scale mineral carbonation a case study from the east kirkton limestone scotland lacustrine carbonate deposits with spherulitic facies are poorly understood but are key to understanding the economically important pre salt mesozoic strata of the south atlantic a major barrier to research into these unique and spectacular facies is the lack of good lacustrine spherulite dominated deposits which are known in outcrop stratigraphy and petrography suggest one of the best analogue systems is found in the carboniferous of scotland the east kirkton limestone here we propose a hydrogeochemical model that explains why the caco3 sio2 mg si al mineral suite associated with spherular radial calcite facies forms in alkaline lakes above basaltic bedrock demonstrating links between igneous bedrock chemistry lake and spring water chemistry and mineral precipitation this model has implications for studies of lacustrine sediments in rift basins of all ages using empirical and theoretical approaches we analyze the relationship between metal mobilization from sub surface volcaniclastic rocks and the potential for precipitation of carbonate minerals various mg bearing minerals and chalcedony in a lacustrine spherulitic carbonate setting this suite of minerals is most likely formed by in gassing of co2 to a carbon limited alkaline spring water consistent with the reaction of alkali igneous rocks in the subsurface with meteoric groundwater we suggest that an analogous system to that at east kirkton caused development of the pre salt spherulitic carbonate deposits

strontium isotopes and rare earth element geochemistry of hydrothermal carbonate deposits from lake tanganyika east africa at cape banza north tanganyika lake fluids and aragonite chimneys have been collected many times since the discovery of this sublacustrine field in 1987 this sampling has been investigated here for the sr isotopic compositions and the rare earth element features of the carbonates and a few fluid samples the 87sr 86sr ratios of the chimneys indicate that they have precipitated from a mixture of lake water more than 95 and hydrothermal fluids no zoning in the chimneys was detected with our sr data for the rare earth elements the situation is more complex the external walls of the chimneys are rare earth element poor la 500 ppb yb 200 ppb la yb 2 to 3 4 their shale normalised rare earth element patterns suggest that they are in equilibrium with the inferred carbonate depositing fluids the rare earth element concentrations of the internal walls of the chimneys are significantly light rare earth elements lree enriched with la contents sometimes up to 5 ppm we suggest that they contain more vent fluid rare earth elements than the external wall samples possibly adsorbed on the surface of growing crystals or simply hosted by impurities it was not possible to constrain the nature of these phases but the variations of the compositions of the internal wall materials of the active chimneys with time as well as data obtained on an inactive chimney indicate that this rare earth element excess is mobile partition coefficients were calculated between the external wall aragonite and carbonate depositing fluid the results are strikingly similar to the values obtained by sholkovitz and shen 1995 on coral aragonite and suggest that there is no significant biologic effect on the incorporation of rare earth elements into coral aragonite and that the various carbonate complexes involved me co3 complexes are the main lree carriers in seawater cantrell and byrne 1987 instead of me co3 2 in banza fluids have the same behaviour during aragonite precipitation

multiphase solid inclusions in uhp rocks su lu china remnants of supercritical silicate rich aqueous fluids released during continental subduction abstract primary multiphase solid ms inclusions without preserved fluid are found within peak minerals in kyanite quartzite topaz and kyanite phengite epidote eclogite from donghai area su lu terrane typical mineral association in inclusions is paragonite muscovite anhydrite corundum alunite type sulphate zircon calcite chlorite sio 2 barite pyrite apatite in quartzites and paragonite rutile apatite amphibole zn staurolite magnetite plagioclase zircon pyrite alunite type sulphate zn mg fe al ti spinel in eclogites on the basis of the fluid inclusion textures and of the daughter phase assemblage calculated fluid composition is as follows in quartzites 24 wt sio 2 30 wt al 2 o 3 9 wt cao 5 wt k 2 o 3 wt na 2 o 11 wt so 3 18 wt h 2 o with traces of tio 2 fe 2 o 3 feo mgo bao p 2 o 5 cl f and co 3 2 and in eclogites 26 wt sio 2 21 wt tio 2 20 wt al 2 o 3 2 wt mgo 4 wt feo 6 wt fe 2 o 3 7 wt cao 3 wt na 2 o 3 wt p 2 o 5 7 wt h 2 o 1 wt cl and traces of zno mno k 2 o so 3 and f originally trapped chloride poor aqueous fluids contained very high amounts in the order of tens of wt of si 4 al 3 and ti 4 solute species reflect the chemical composition of the host rocks mg 2 fe 2 ti 4 p 5 and na are abundant in the fluids present in eclogites while fluids are enriched in al 3 k na so 4 2 co 3 2 in quartzites we propose that multiphase solid inclusions represent remnants of high density supercritical silicate rich aqueous fluids that were in equilibrium with peak minerals at uhp conditions these fluids show characters which are transitional between aqueous fluids and silicate melts and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction

rare earth element composition of precipitation precipitation particles and aerosols we report the rare earth element ree composition of aerosols and precipitation dissolved and particle phases from bermuda lewes delaware and woods hole massachusetts there is large scale fractionation of rees between the continents atmosphere and the oceans the ree compositions of atmospheric samples are highly fractionated relative to 1 each other e g aerosol precipitation pairs 2 the upper crust of the earth and 3 seawater precipitation samples exhibit markedly convex up shale normalized patterns large negative eu anomalies and large and systematic depletions in the heavy ree lu yb er dy aerosols are strongly depleted in the heavy ree which dissolutions experiments show are attributable to the presence of hree enriched refractory minerals we argue that in situ selective dissolution of mineral aerosol particles is responsible for the unusual ree composition of precipitation this has major implications for the interpretation of the biogeochemical cycles of rees in the oceans particularly the partitioning between solution and particle phases residence times and the composition of seawater

physical volcanology of a subglacial to emergent rhyolitic tuya at rauðufossafjoll torfajokull iceland we present the first modern volcanological study of a subglacial to emergent rhyolite tuya at se rau ufossafjoll torfajokull iceland a flat topped edifice with a volume of 1 km3 was emplaced in upper pleistocene time beneath a glacier 350 m thick although it shares morphological characteristics with basaltic tuyas the lithofacies indicate a very different eruption mechanism field observations suggest that the eruption began with vigorous phreatomagmatic explosions within a well drained ice vault building a pile of unbedded ash up to 300 m thick this was followed by a subaerial effusive phase in which compound lava flows were emplaced within ice cauldrons small volume effusive eruptions on the volcano flanks created several lava bodies with a variety of features columnar jointed sides subaerial tops peperitic bases that are used to reconstruct spatially heterogeneous patterns of volcano ice interaction volcaniclastic sediments exposed in a stream section provide evidence for channelised meltwater drainage and fluctuating depositional processes during the eruption we develop models for the evolution of se rau ufossafjoll and discuss the differences between subglacial rhyolitic and basaltic eruption mechanisms which are principally caused by contrasting hydrological patterns

mobility and fluxes of major minor and trace metals during basalt weathering and groundwater transport at mt etna volcano sicily abstract the concentrations and fluxes of major minor and trace metals were determined in 53 samples of groundwaters from around mt etna in order to evaluate the conditions and extent of alkali basalt weathering by waters enriched in magma derived co 2 and the contribution of aqueous transport to the overall metal discharge of the volcano we show that gaseous input of magmatic volatile metals into the etnean aquifer is small or negligible being limited by cooling of the rising fluids basalt leaching by weakly acidic co 2 charged water is the overwhelming source of metals and appears to be more extensive in two sectors of the s sw paterno and e zafferana volcano flanks where out flowing groundwaters are the richest in metals and bicarbonate of magmatic origin thermodynamic modeling of the results allows to evaluate the relative mobility and chemical speciation of various elements during their partitioning between solid and liquid phases through the weathering process the facts that rock forming minerals and groundmass dissolve at different rates and secondary minerals are formed are taken into account at mt etna poorly mobile elements al th fe are preferentially retained in the solid residue of weathering while alkalis alkaline earth and oxo anion forming elements as se sb mo are more mobile and released to the aqueous system transition metals display an intermediate behavior and are strongly dependent on either the redox conditions mn cr v or solid surface related processes v zn cu the fluxes of metals discharged by the volcanic aquifer of etna range from 7 0 10 3 t a th to 7 3 10 4 t a na they are comparable in magnitude to the summit crater plume emissions for a series of elements na k ca mg u v li with lithophile affinity but are minor for volatile elements basalt weathering at mt etna also consumes about 2 1 10 5 t a of magma derived carbon dioxide equivalent to ca 7 of contemporaneous crater plume emissions the considerable transport of some metals in etna s aquifer reflects a particularly high chemical erosion rate evaluated at 2 3 10 5 t a enhanced by the initial acidity of magmatic co 2 rich groundwater

deep magma storage at hekla volcano iceland revealed by insar time series analysis 1 hekla volcano is one of the most active volcanoes in iceland the most recent eruption occurred from 26 february to 8 march 2000 when about 0 19 km3 of magma was erupted we present deformation data from multitemporal analyses of synthetic aperture radar sar images acquired between 1993 and 2008 focusing on pixels with low phase variance using persistent scatterer and small baseline approaches prior to and after the 2000 eruption we find a broad area of inflation around the volcano radius about 20 km with satellite line of sight los shortening of up to 5 mm yr we interpret this signal as the result of pressure increase in a deep seated magma chamber which we model as a spherical source at 14 20 km depth increasing in volume by 0 003 0 02 km3 yr within a 6 km radius of the summit of the volcano a los lengthening is superimposed on the broad inflation signal which correlates partly with recent lava flows we interpret this signal as the result of thermally contracting lava flows combined with viscoelastic yielding due to the load of the volcano and its lavas coeruptive deflation during the 2000 eruption was similar to the cumulative inflation from 1993 to 2000 and is consistent with a spherical magma chamber at 14 18 km depth that decreases in volume by 0 04 0 08 km3 interferograms spanning the 2000 eruption show a local coeruptive deformation signal near the eruptive fissure this is consistent with a dike opening from the surface to depths up to 5 8 km with a volume of 0 005 0 006 km3

geophysical evidence for breakup volcanism in the angola and gabon passive margins abstract serial two dimensional reflection seismic profiles spanning the continent ocean transition in the offshore kwanza and southern gabon basins have been interpreted and integrated with 2 5d gravity and magnetic anomaly modelling these continental margins are characterized by the presence of a distal domain in which continental crust has been highly stretched and faulted overlain by thick sequences of early cretaceous syn rift sediments that are in turn overlain by a thick but highly deformed aptian evaporite unit the evaporites are overlain by post breakup sediments cretaceous to recent in age oceanwards syn rift sediments are overlain by a volcanic unit that thickens basinward this volcanic unit forms the base and the lateral equivalent of the distal evaporite basin developing into positive relief volcanic edifices that acted as the oceanward margin of the evaporite basin these volcanic units extend from the distal domain of the passive margin to the oceanic crust and are interpreted to represent volcanism that immediately predates lithospheric breakup and formation of oceanic crust as indicated by the progressive transition into a constant thickness oceanic crust volcanism synchronous with the late stages of passive margin development and related hydrothermalism can account for the high alkalinity that dominated the pre salt lacustrine environments and could also have contributed to the modification of marine waters that led to the deposition of the thick aptian evaporites in the south atlantic ocean breakup volcanism and its associated thermal impact can also help explain the apparently anomalous shallow water depth of these margins preserved during the latest rift and earliest post rift stages

the mechanics of the south iceland seismic zone the south iceland seismic zone appears to act as a transform fault between offset sections of the mid atlantic ridge however there is no morphological expression of a throughgoing transform between the ridges the anticipated throughgoing east west transform is represented by a series of north south strike slip faults we study the mechanics of the south iceland seismic zone using boundary element methods first we examine the implications of the differences between the geometry of an orthogonal ridge transform ridge system and the geometry of the structures observed in iceland then we consider the role of the north south faults in accommodating transform motion in an east west direction we conclude that the north south faults can accommodate deformation if they are longer or more spatially frequent than mapped the south iceland seismic zone is subject to sequences of large earthquakes every 45 112 years in comparing the seismic moment release derived from earthquake magnitudes with that predicted by our models we confirm that the system of north south faults can act as a transform fault the seismic moment release derived from the earthquake magnitudes is 1 4 times greater than that predicted by our models if our application of the moment magnitude relation is correct the above could imply that the brittle layer extends to 14 km rather than the 10 km assumed in our models other possibilities are that the magnitudes of the earthquakes have been slightly overestimated or that the historical seismic record documents a period of seismicity greater than that expected from rates of plate motion inferred from geological and very long baseline interferometry vlbi

seismicity crisis at the katla volcano iceland signs of a cryptodome abstract katla one of the most active and hazardous icelandic volcanoes has shown signs of unrest since 1999 in july that year a small glacial flood lasting a few hours was observed accompanied by abnormal seismicity it was probably caused by a short lived shallow magma intrusion or possibly a small subglacial eruption in 2002 seismic activity increased dramatically particularly in the go abunga area west of the caldera and earthquakes have occurred continuously ever since a network of four portable seismometers was run at katla in the spring summer 2003 with one station directly at the locus of the go abunga activity these observations are combined with the data of the permanent icelandic digital seismograph network run by the icelandic meteorological office the earthquakes at go abunga are concentrated in a small area with a diameter of 3 to 4 km and the events are shallow mainly within the uppermost 2 km we propose that the unusual pattern of seismicity indicates an intruding hot and acidic cryptodome apparently this dome has been slowly rising for decades but its propagation towards the surface accelerated some time around 1999 possibly triggered by a basaltic intrusion this may lead to a silicic dome building eruption in the go abunga area which should be taken into consideration when estimating the eruption hazard at katla

seismic activity related to the 2000 eruption of the hekla volcano iceland the 2000 hekla eruption took place from february 26 to march 8 its seismic expressions were a swarm of numerous small earthquakes related to its onset and low frequency volcanic tremor that continued throughout the eruption a swarm of small earthquakes was observed some 80 min before the onset of the eruption and the size of the events increased with time low frequency volcanic tremor with a characteristic frequency band of 0 5 1 5 hz and dominant spectral peak s at 0 7 0 9 hz became visible at 18 19 gmt on february 26 marking the onset of the eruption the tremor amplitude rose quickly and was very high in the beginning of the eruption however it soon began to decrease after about an hour in general the seismic activity related to the 2000 hekla eruption was very similar to what was observed in the previous eruption in 1991 based on knowledge gained from seismicity and strain observations from 1991 this was the first time that a hekla eruption was predicted

geodetic investigation of plate spreading along a propagating ridge the eastern volcanic zone iceland summary hotspot ridge interactions lead to the dynamic evolution of divergent plate boundaries including propagating and overlapping ridge segments in southern iceland the eastern volcanic zone evz formed approximately 2 3 ma ago during the last eastward ridge jump from the western volcanic zone wvz and is propagating to the southwest into the tertiary lithosphere of the eastern volcanic flank zone north america eurasia relative plate motion is partitionedbetweentheeasternandwvzs weutilizenewterrestrial dry tilt andspace gps and insar geodetic data to investigate the nature of plate spreading and magma tectonic interaction at the southern terminus of this propagating ridge system we present a new gps derived horizontal velocity field covering the period 1994 2006 new insar analyses for the periods 1993 2000 and 2003 2007 and models of plate spreading across this region the velocity field indicates horizontal surface deformation consistent with plate spreading across and the propagation of the evz the dry tilt and insar data show transient deformation signals associated with magmatic processes the velocity field is corrected for these transient deformation sources in order to investigate the nature of secular plate motion our model results indicate a decrease in spreading rate from northeast 15 mm yr 1 to southwest 9 mm

discovery of sublacustrine hydrothermal activity and associated massive sulfides and hydrocarbons in the north tanganyika trough east african rift massive sulfides and carbonate mineral deposits associated with sublacustrine thermal springs were recently discovered along the zaire side of the north tanganyika trough western branch of the east african rift this hydrothermal activity investigated by scuba diving at a maximum depth of 20 m is located at the intersection of major north south normal faults and northwest southeast faults belonging to the tanganyika rukwa malawi trm strike slip fault zone the preliminary results presented here come from analyses of sulfide deposits hydrothermal fluids and associated hydrocarbons that result from geothermal activity in this part of the east african rift filled by a thick pile of sediment the north tanganyika trough

interpretation of rayleigh wave ellipticity observed with multicomponent passive seismic interferometry at hekla volcano iceland the 2010 eruption of eyjafjallajokull has drawn increased attention to iceland s eastern volcanic zone evz due to the threat it poses to the heavily used air traffic corridors of the northern atlantic ocean within the evz hekla is historically one of the most active volcanoes and has exhibited a decadal eruption pattern for the past 40 years hekla most recently erupted in 2000 and is thus ripe for another decadal eruption because hekla is generally aseismic except for a brief time period hours leading up to an eruption monitoring has previously depended on precursory deformation signals linde et al 1993 as a result seismic tomography of the internal structure of the volcano using phase arrivals of local earthquakes is not possible motivated by hekla s practically aseismic behavior in inter eruptive periods we installed a temporary network of four broadband seismometers around the volcanic edifice in late august 2010 with the intention of investigating the applicability of passive seismic

seismic constraints on magma chambers at hekla and torfajokull volcanoes iceland hekla and torfajokull are active volcanoes at a rift transform junction in south iceland despite their location next to each other they are physically and geologically very different hekla is an elongate stratovolcano built mainly of basaltic andesite torfajokull is a prominent rhyolitic centre with a 12 km diameter caldera and extensive geothermal activity the scope of this study is to examine the propagation of body waves of local earthquakes across the hekla torfajokull area and look for volumes of anomalous s wave attenuation which can be evidence of magma chambers so far the magma chamber under hekla has been modelled with various geophysical means and its depth has been estimated to be 5 9 km a data set of 118 local earthquakes providing 663 seismic rays scanning hekla and torfajokull was used in this study the major part 650 seismograms did not show evidence for s wave attenuation under these volcanoes only six seismograms had clear signs of s wave attenuation and seven seismograms were uncertain cases the data set samples hekla well at depths of 8 14 km and south part of it also at 4 8 km and 14 16 km western torfajokull is sampled well at depths of 4 14 km eastern and southern torfajokull at 6 12 km conclusions cannot be drawn regarding the existence of magma beyond these depth ranges also magma volumes of smaller dimensions than about 800 m cannot be detected with this method if a considerable molten volume exists under hekla it must be located either above 4 km or below 14 km the former possibility seems unlikely because hekla lacks geothermal activity and persistent seismicity usually taken as expressions of a shallow magma chamber an aseismic volume with a diameter of 4 km at the depth of 8 km in the west part of torfajokull has been inferred in earlier studies and interpreted as evidence for a cooling magma chamber our results indicate that this volume cannot be molten to a great extent because s waves travelling through it are not attenuated intense geothermal activity and low frequency earthquakes are possibly signs of magma in the south part of torfajokull but a magma chamber was not detected there in the areas sampled by this study

the hekla eruption 1980 1981 the sixteenth eruption of hekla since 1104 began on august 17th 1980 after the shortest repose period on record only ten years the eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the hekla volcanic ridge the production rate declined rapidly after the first day and the eruption stopped on august 20th a total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity in the following seven months steam emissions were observed on the volcano activity was renewed on april 9th 1981 and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope

the deep structure of the south atlantic kwanza basin insights from 3d structural and gravimetric modelling abstract we present results from 3d gravimetric modelling of the kwanza basin offshore angola accomplished to investigate the deep crustal structure beneath the basin and discuss our findings with respect to their implications for the opening of the central south atlantic the kwanza basin is located in the southern part of the central south atlantic although the post rift evolution of the kwanza basin is well studied little is known about the basin s early history this is mainly due to the missing knowledge of its crustal structure owing to the masking effect of an up to 4 km thick salt layer which seismically obscures the underlying basement to get an insight into the deeper structure of this part of the angolan margin we combined 3d structural isostatic and gravimetric modelling 2d seismic reflection data was used to determine the structural setting and the configuration of the stratigraphic units in the sedimentary part of the basin whereas its crustal structure was constrained by isostatic and gravity modelling our modelling results indicate that high density lower crustal bodies similar to ones found in the northern part of the central segment are present in the southern part and thus seem to be a general feature of the crust of the central south atlantic thinning of the crust occurs gradually rather than in an abrupt manner and block faulting seems to have played a significant role during rifting as is indicated by a strongly structured upper crust in contrast we find little evidence for ductile thinning or exhumation of the deeper lithosphere as previously suggested for the northern segment of the central south atlantic in its southern segment it appears more likely that magmatic intrusions prevented mantle exhumation by sealing crustal detachment faults and giving the margin a distinct magmatic signature

effects of sedimentary sorting on neodymium isotopes in deep sea turbidites the study of neodymium model ages of sedimentary rocks has added considerably to our understanding of crustal growth and evolution so far however the possible effects of various sedimentary processes on nd isotope distributions have not been fully evaluated here we present nd isotope data for a series of sand mud pairs in modern deep sea turbidites from different tectonic settings which show that sands and muds commonly have significantly different nd isotope compositions that in only some cases are associated with differences in sm nd ratio differences are as great as 7 ndunits corresponding to nd model ages that differ by up to 0 44 gyr and may be explained by separation or unmixing of components from different sources during sedimentary transport and sorting for one sample heavy mineral fraction ation may be indicated these results have implications for studies of nd model ages in sedimentary rocks and for the interpretation of provenance from sedimentary petrography and geochemistry

hydrothermal vents in lake tanganyika east african rift system sublacustrine hydrothermal vents with associated massive sulfides were discovered during april 1987 at pemba and cape banza on the zaire side of the northern basin of lake tanganyika east african rift system new investigations by a team of ten scuba divers during the multinational france zaire germany and burundi tanganydro expedition august october 1991 found hydrothermal vents down to a depth of 46 m along north trending active faults bounding the tanganyika rift on the western side temperatures from 53 to 103 degrees c were measured in hydrothermal fluids and sediments veins of massive sulfides 1 10 cm thick pyrite and marcasite banding were found associated with vents at the pemba site at cape banza active vents are characterized by 1 70 cm high aragonite chimneys and there are microcrystalline pyrite coatings on the walls of hydrothermal pipes hydrothermal fluid end members show distinctive compositions at the two sites the pemba end member is a nahco sub 3 enriched fluid similar to the nahco sub 3 thermal fluids form lakes magadi and bogoria in the eastern branch of the rift the cape banza end member is a solution enriched in nacl such brines may have a deep seated basement origin as do the uvinza nacl brines on the eastern flank ofmore the tanganyika basin geothermometric calculations have yielded temperatures of fluid rock interaction of 219 and 179 degrees c in the pemba and cape banza systems respectively abundant white or reddish brown microbial colonies resembling beggiatoa mats were found surrounding the active vents thermal fluid circulation is permitted by opening of cracks related to 130 degrees n normal dextral faults that intersect the north south major rift trend the sources of heat for such hydrothermal systems may relate to the existence of magmatic bodies under the rift which is suggested by the isotopic composition of carbon dioxide released at pemba and cape banza 21 refs 2 figs less

volcanic rock alterations of the kwanza basin offshore angola insights from an integrated petrological geochemical and numerical approach the lower cretaceous presalt section of the kwanza basin angola is in the spotlight following the discoveries of petroleum systems in this basin and more generally in the south atlantic these systems are mostly composed of continental carbonates in close association with volcanic rocks this work is focused on the study of an offshore kwanza presalt volcanic sequence characterized as valanginian trachytic subaerial lava flows a detailed petrological analysis of the altered trachyte in association with fluid inclusion microthermometry was conducted in order to depict the initial mineralogy albite sanidine titanomagnetite and obtain a paragenetic sequence quartz siderite kaolinite calcite thermodynamic equilibrium modelling of the trachytes alteration by meteoric fluids over a range of temperatures 25 c 200 c and co2 partial pressure pco2 0 01 mbar to 100 bar were performed with phreeqc and compared to the observed paragenetic sequence some numerical simulations reflect the observed paragenesis as a result the pco2 is constrained by the occurrence of siderite from 0 1 bar at 50 c to 30 bar at 125 c and kaolinite from 0 2 bar at 50 c to 1 2 bar at 125 c the simulations emphasize the need for a high pco2 in the hydrothermal system to achieve the observed trachyte transformation after reaching equilibrium with the trachytes the simulated fluids highlight a mid alkaline to near neutral ph with high fe hco3 co3 and alkali concentrations the palaeofluids could have evolved from ca and mg rich to ca and mg poor with increasing temperature inversely si concentrations are positively correlated with increasing temperatures this methodology integrating a petrological approach and numerical simulations proves to be a powerful tool leading to better understanding of the proxies pco2 temperature redox conditions controlling paragenesis to push further these simulations are also a step toward improved understanding of palaeofluid evolutions in presalt systems and better prediction of reservoir quality

the 1912 iceland earthquake rupture growth and development of a nascent transform system we have mapped in detail surface ruptures of the 1912 magnitude 7 0 strike slip earthquake in south iceland this earthquake ruptured fresh basalt flows that had covered the pre existing fault the observed style of surface fracturing closely matches both theoretical predictions of the first stages of shear fracture development and microscopic scale observations from laboratory experiments the shear offset distributed across the zone of surface fractures produced by this earthquake is right lateral and is in the range of 1 to 3 m total mapped rupture length is 9 km but total rupture length is probably at least 20 km this interplate earthquake had an exceptionally high ratio of slip to fault length and by inference stress drop the north south trending rupture of the 1912 earthquake is part of the bookshelf faulting in the east west trending south iceland seismic zone we ascribe the bookshelf faulting in the south iceland seismic zone to a combination of the early development stage of the transform and regional strength anisotropy of the crust

tectonic control of the oil rich large igneous carbonate salt province of the south atlantic rift abstract the early cretaceous south atlantic magmatic province samp which includes the parana etendeka lip produced about 8 million km 3 of tholeiitic basalt and diabase over an area of 4 million km 2 huge pre salt oil reserves discovered in 2007 by petrobras in non marine carbonates are estimated at more than 45 billion barrels here we show the close causal relationship of the southward increasing width of the wedge shaped south atlantic rift with the similarly southward increase in igneous activity in the thicknesses of non marine carbonate and salt and in the size of oil reserves all controlled mainly by south america s early clockwise rotation away from africa about a pole in its northeast large diabase dike swarms transversal to the rift witness to south america s rotation that opened in its wake the southward widening south atlantic rift westward increasing pressure on the equatorial margin by south america s clockwise rotation forced open the benue trough and created pre late aptian folds in the demerara plateau and in brazil s solimoes upper amazonas basin prerift and synrift volcanic activity increases southward culminating in the parana etendeka lip and in the offshore volcanic sdrss that continue southward to the cape basin berriasian valanginian rift sediments deposited from about 145 ma 10 ma before the flood basalts of the parana etendeka lip the largest transversal dike swarm continued in the proto walvis ridge that separated the central south atlantic endorheic rift basin from the sea in the south erosion and leaching of basalts supplied ca mg and sio 2 to the endorheic basin for the deposition of non marine carbonates and authigenic clays basalt flows intercalated with carbonates nearly until salt deposition about 113 ma hypogenic leaching of carbonates by mantle derived co 2 created optimal reservoirs supergiant oil deposits occur where the widest endorheic basin and the volcanic province overlap

deep burial hydrothermal alteration of the pre salt carbonate reservoirs from northern campos basin offshore brazil evidence from petrography fluid inclusions sr c and o isotopes abstract petrographic mineralogical elemental isotopic and fluid inclusion analyses were integrated to unravel the diagenetic evolution of brazilian pre salt lacustrine carbonate reservoirs of northern campos basin southeast brazilian margin detailed thin section and cathodoluminescence petrography scanning electron microscopy and electron microprobe analyses established a paragenetic evolution of diagenetic processes and products comprising extensive dolomitization silicification and dissolution a paragenesis including saddle dolomite macrocrystalline calcite mega quartz sr barite celestine fluorite dickite sphalerite galena and other metallic sulfides filling fractures and dissolution porosity and aqueous fluid inclusions with homogenization temperatures of 92 152 c and salinities between 13 and 26 wt eq nacl characterized a hydrothermal system with some analogy to carbonate hosted pb zn mississippi valley mvt and irish type deposits petroleum inclusions and solid bitumen testify atypical oil generation and migration associated with the hydrothermal flow the host pre salt spherulitic and fascicular carbonates present highly radiogenic 87sr 86sr ratios indicating strong interaction with continental crust materials hydrothermal phases show 18o values more negative than syngenetic and diagenetic carbonates the 13c values are interpreted as result of interaction between the hydrothermal fluids and the host rocks the combined data set provides clear evidence of intense hydrothermal alteration of northern campos basin pre salt reservoirs at deep burial conditions 2 km possibly related to late cretaceous or more probably paleogene magmatic activity mixed sourced fluids bearing a basinal signature fed the hydrothermal system and promoted dissolution of the host rocks the hydrothermal alterations had strong impact on the porosity permeability and heterogeneity contributing together with the associated fracturing to the excellent production performance of the pre salt reservoirs

origin and chemical nature of the thermal fluids at caldeiras da ribeira grande fogo volcano s miguel island azores the caldeiras da ribeira grande crg site is characterised by low temperature fumaroles boiling mud pools and gas ground emissions some of them artificially confined to a tank cooling and oxidation of deep hydrothermal fluids under boiling control the formation of acid condensates and strong hydrothermal alteration of the hosting volcanic rocks the latter feature is locally reinforced at shallow levels through flow of mixtures of acid condensates with surface and shallow groundwater triggering the development of thick clay deposits used in mud therapy purposes at an old thermal bath facility the emerging aqueous fluids are acidic ph 3 and 3 rain runoff and subsurface water contributions must be balanced to confine the ph of thermal fluids between 3 and 5 any change in conditions of geothermal fluid boiling at depth or in environmental and hydrological characteristics of the site can cause significant changes in leaching processes and sediment formation

diversity and origin of quartz cements in continental carbonates example from the lower cretaceous rift deposits of the south atlantic margin abstract silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis the lower cretaceous rift deposits of the south atlantic equatorial margin which are intensively explored by petroleum companies provide good examples of such silicifications in carbonates exhibiting a wide diversity of petrographic habit of early to late quartz cements in order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity we integrated detailed petrography of diagenetic sequences and quartz habit with 18oquartz measurements by sims of individual cements observed in samples from the offshore and onshore basins of the west african margin the petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length fast or length slow habit in addition to laminated microquartz and micro or mega quartz forms amongst the isotopic analysis the 18oquartz data show that length slow cements are generally strongly enriched in 18o 18oquartz ranging from 31 to 37 smow whereas length fast forms show less elevated values 25 c and

provenance of dust in the pacific ocean eolian dust preserved in deep sea sediment cores provides a valuable indicator of past atmospheric circulation and continental paleoclimate in order to identify the provenance of eolian dust nd and sr isotopic compositions and rb sr and rare earth element ree concentrations have been determined for the silicate fractions of deep sea sediments from the north and central pacific ocean different regions of the pacific ocean are characterized by distinct air borne inputs producing a large range in end 10 to 1 stsr 86sr 0 705 0 721 la yb 5 15 eun eu 0 6 1 0 and sr nd 4 33 the average nd isotopic composition of pacific deep sea sediments eno 6 is more radiogenic than the average from the atlantic ena 8 in contrast the average 1475m 14and ratio for pacific sediments 0 114 is identical to that of atlantic sediments and to that of global average riverine suspended material the values of ena and 147sm 144nd are positively correlated for the pacific samples but negatively correlated for atlantic samples reflecting a fundamental difference between the dominant components in the end members with radiogenic nd island arc components in the pacific and lree enriched intraplate ocean island components in the atlantic samples from the north central pacific have distinctive unradiogenic end values of 10 87sr 86sr 0 715 high la yb 12 and low eun eu 0 6 and sr nd 3 6 these data are virtually identical to the values for loess from asia and endorse the use of these sediments as indicators of asian paleoclimate and paleowind directions island arc contributions appear to dominate in the northwest pacific resulting in higher end 1 to 1 and lower 87sr 86sr 0 705 and la yb 5 sediments from the eastern pacific tend to have intermediate sr and nd isotopic compositions but regionally variable sr nd and ree patterns they appear to be derived from the west margin of the north and south american continents rather than from asia our results confirm that dust provenance can be constrained by isotopic and geochemical analyses which will facilitate reconstructions of past atmospheric circulation and continental paleoclimate

neodymium isotopic variations in north pacific modern silicate sediment and the insignificance of detrital ree contributions to seawater the neodymium isotopic composition of the silicate fraction of holocene pelagic sediments from the north pacific define two provinces a central north pacific province characterized by unradiogenic and remarkably homogeneous end 10 2 0 5 and a narrow circum pacific marginal province characterized by more radiogenic and variable end 4 2 3 8 the silicate fraction in the central north pacific is exclusively eolian based on prevailing wind patterns meteorological data and neodymium isotopic data the only significant sediment source is chinese loess leaching experiments on chinese loess confirm that leachable nd is isotopically indistinguishable from bulk and residual silicate nd silicates in the circum north pacific marginal province comprise eolian loess volcanic ash and hemipelagic sediments derived from volcanic arcs a compilation of pacific seawater and mn nodule end data shows no clear spatial variation except for a general decrease from surface to deep waters from 3 to 4 and slightly lower end in bottom waters along the western north pacific due to the incursion of antarctic bottom water the relative homogeneity of bottom water end which contrasts sharply with the distinctive variation in sediment end plus the large difference between the average end of bottom waters and the central north pacific eolian silicates 4 vs 10 suggests that any contribution of ree to seawater from eolian materials is insignificant furthermore leaching of ree from eolian particles as they sink though the water column must be insignificant because nd in shallow waters is more radiogenic than nd in deeper waters that there is no contrast in the nd isotopic composition of bottom waters that overlie the central and marginal sediment provinces suggests that the ash and hemipelagic sediments derived from pacific rim volcanic arcs also contribute minimal ree to seawater the elimination of eolian ash and hemipelagic sediments leaves only near shore riverine particulates as a possibly significant particulate source of ree to seawater

experimental studies of leaching and albitization in intermediate basic rocks abstract experimental studies combined with geological evidence indicate that gaseous thermal fluids enriched in nacl or other chlorides leach and metasomatize intermediate basic and intermediate acidic rocks in this process part or all of the rock and ore forming materials are mobilized and enriched to form skarns and fe deposits when gaseous thermal fluids enriched in nacl or other chlorides act on intermediate basic rocks the mobilities of the main components in the rocks are in decreasing order k or na fe ca si mg and al the mobility of al is extremely small and the mobility of mg is not large either the leaching capacities of high temperature fluids for rock and ore forming elements depend mainly on the kinds and numbers of negative and positive ions ph and eh temperature and pressure etc acidic solution and high temperature favor the leaching of fe whereas alkaline solution and high pressure favor the leaching of sio2 at 550 c and 750 1000 bar weak alkaline solution enriched in nacl favors the formation of albite acidic solution favors the formation of minerals such as amphiboles and chlorites while wollastonite can form in wider ranges of ph the mobility of fe and albitization represent the change in ph values of the solutions at different mineralized stages

the 1991 eruption of hekla iceland the eruption that started in the hekla volcano in south iceland on 17 january 1991 and came to an end on 11 march produced mainly andesitic lava this lava covers 23 km2 and has an estimated volume of 0 15 km3 this is the third eruption in only 20 years whereas the average repose period since 1104 is 55 years earthquakes as well as a strain pulse recorded by borehole strainmeters occurred less than half an hour before the start of the eruption the initial plinian phase was very short lived producing a total of only 0 02 km3 of tephra the eruption cloud attained 11 5 km in height in only 10 min but it became detached from the volcano a few hours later several fissures were active during the first day of the eruption including a part of the summit fissure by the second day however the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed the average effusion rate during the first two days of the eruption was about 800 m3 s 1 after this peak the effusion rate declined rapidly to 10 20 m3 s 1 then more slowly to 1 m3 s 1 and remained at 1 12 m3 s 1 until the end of the eruption site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption a notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption but it levelled off on the 6th day and then remained essentially constant the volume and initial silica content of the lava and tephra the explosivity and effusion rate during the earliest stage of the eruption as well as the magnitude attained by the associated earthquakes support earlier suggestions that these parameters are positively related to the length of the preceeding repose period the chemical difference between the eruptive material of hekla itself and the lavas erupted in its vicinity can be explained in terms of a density stratified magma reservoir located at the bottom of the crust we propose that the shape of this reservoir its location at the west margin of a propagating rift and its association with a crustal weakness all contribute to the high eruption frequency of hekla

recent volcanic history of the veidivotn fissure swarm southern iceland an approach to volcanic risk assessment abstract the recent volcanic history of the southwestern part of the veidivotn fissure swarm southern iceland provides a basis for assessment of volcanic risk in an area of large hydropower potential local tephrastratigraphy and regional tephrochronology provide relative and absolute dating of individual eruptions as well as information on the volume and distribution of the products formed in each eruption three large eruptions took place in this area in 1480 a d 900 a d and 150 a d respectively each eruption produced approx 1 km3 dre of basaltic and minor amounts of silicic lava and tephra on fissures up to 42 km long no evidence is found of smaller eruptions during this period the estimated eruption frequency one eruption every 600 800 years implies that this part of the veidivotn fissure swarm is inactive for long periods between relatively large volcanic events a change in the mode of eruption from effusive to explosive took place during this period the hazards posed by this area include far reaching lava flows widespread heavy tephra fall with thicknesses in excess of 2 m at distances of 10 km and damming of a large glacial river with the consequent formation of unstable lakes a volcano tectonic model which explains the observed eruption frequency and provides a basis for a long term monitoring program is proposed eruptions on the veidivotn fissure swarm are interpreted as corollaries of rifting episodes initiated in the bardarbunga central volcano volcano tectonic episodes affect the fissure swarm at an average interval of 100 years minor episodes are limited to the central volcano and adjacents parts of the fissure swarm during the less frequent major episodes rifting and volcanic activity extends to the extreme southwestern part of the fissure swarm seismic monitoring of the bardarbunga central volcano could provide an early warning of renewed activity on the veidivotn fissure swarm a major rifting episode resulting in eruption on its southwestern part can be expected during the next 100 to 300 years

location of a magma reservoir beneath hekla volcano iceland hekla situated on the western boundary of the eastern volcanic zone in south iceland is one of the most active volcanoes in iceland having erupted 16 times since 1104 repeated electronic distance measurements since its eruption in 1980 81 show significant changes in crustal movement in the aftermath of the eruption such movement on and around volcanoes can be used to monitor movement of magma within and below volcanoes1 3 we show here that these measurements can be interpreted in terms of inflating magma reservoirs with a centre at 8 km depth below the volcano over a period of 10 months the volume of the inflation amounted to 55 106 m3

observed ground deformation at hekla iceland prior to and during the eruptions of 1970 1980 1981 and 1991 abstract ground tilt measurements in the vicinity of hekla have demonstrated a slow rate of deformation during periods of repose and rapid deformation during eruptions an eruption started at hekla on january 17 1991 after a repose period of almost 11 years this eruption was preceded by a slow inflation of the volcano centered 4 to 6 km north or northwest of the summit an uplift rate of 3 to 4 cm yr is indicated with a probable depth to the source of deformation of 5 to 6 km a deflation during the eruption is centered at the same location and the maximum subsidence was 30 40 cm the volume of the subsidence bowl is estimated as 0 05 0 1 km 3 if this subsidence is the result of removal of magma from a shallow reservoir then the volume of escaped magma is approximately twice the volume of the subsidence bowl hekla has erupted four times since the initiation of deformation measurements in 1968 first in may july 1970 then in august 1980 and again in april 1981 and finally in january february 1991 if the vertical component of ground deformation is used to estimate the relative magnitude of these eruptive events then the 1991 event is similar to or smaller than the 1970 event while the 1980 and 1981 events are significantly smaller

the great west african tertiary coastal uplift fact or fiction a perspective from the angolan divergent margin 1 we explore exhumation in the coastal kwanza basin by combining analyses of tertiary hiatuses and apatite fission tracks planktonic biozones show five major hiatuses in the oligo miocene and plio pleistocene between gaps oligo miocene strata accumulated under marine conditions a marine setting refutes the idea of a massively raised coastal plateau in the mid tertiary marine conditions continued until 5 ma fission track data suggest three thermal events 150 ma during rifting and volcanism 100 70 ma during shortening and volcanism and 20 10 ma during exhumation tertiary uplift was spatially highly variable for the kwanza basin we infer that tertiary uplift on the west african margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas massive uplift 2000 4000 m of the precambrian craton had little structural effect in the outer basin instead minor uplifts on the shelf drove late tertiary deformation on the slope

low frequency earthquakes at the torfajokull volcano south iceland abstract torfajokull is a large rhyolitic volcanic edifice with a 12 km diameter caldera and abundant high temperature geothermal activity it is located in the neovolcanic zone in south iceland at the junction of the eastern rift zone and a transform zone with the intraplate volcanic flank zone of south iceland the latest eruption at torfajokull occurred about 500 years ago torfajokull is a source of persistent small scale seismicity where two types of earthquakes occur high frequency events are concentrated in the western part of the caldera and low frequency events cluster in the south small low frequency earthquakes have been observed at torfajokull since the installation of a local analogue seismograph station in 1985 they typically occur in swarms up to 300 earthquakes per day have been observed the low frequency events have a frequency content of about 1 3 hz and are difficult to locate because of the emergent nature of their phases the 160 events located during the years 1994 2000 using the permanent icelandic seismic network cluster in the southern part of the torfajokull caldera a closer study of low frequency events was carried out between may and october 2002 with a dense network of twenty guralp 6td broadband seismometers in the torfajokull area no distinct swarm activity was observed during this period but small low frequency events occurred almost on a daily basis about 330 low frequency events were detected during the study period they are located in the southern part of the caldera between two small glaciers areas of intensive geothermal activity surround the cluster of low frequency events it is argued that these earthquakes are associated with active magma in the south part of the torfajokull caldera possibly a rising cryptodome

seismicity around the hekla and torfajokull volcanoes iceland during a volcanically quiet period 1991 1995 the volcano hekla in south iceland had its latest eruption in january march 1991 the eruption was accompanied and followed by considerable seismic activity this study examines the seismicity in the hekla region 63 42 64 18 n 18 30 20 12 w during a period when the high activity related to the eruption had ceased from july 1991 to october 1995 the aim is to define the level of the normal background seismicity of the area that can be compared to the eruption related activity the hekla volcano proper was generally aseismic during the study period the most prominent earthquake cluster is in the neighbouring torfajokull volcano the epicentres are concentrated in the western part of the caldera and west of it the hypocentres are located at all depths from the surface down to 14 km with highest activity at 5 12 km inside this cluster in the northwest part of the caldera is a spherical volume void of earthquakes approximately 4 km in diameter and centred at 8 km depth this is interpreted as a cooling magma body small low frequency events of volcanic origin were occasionally recorded at torfajokull this activity has mainly occurred in swarms and was most abundant during the first year of the study period presumably reflecting some kind of connection to the 1991 hekla eruption our study area also includes the easternmost section of the south iceland seismic zone a transform zone characterized by bookshelf faulting on transverse faults two lineaments of epicentres were identified roughly corresponding to mapped faults of the south iceland seismic zone the hypocentres are relatively deep mainly at 6 12 km matching the general trend of hypocentral depth increasing toward the east the seismicity is highest in the area of the mapped faults however the epicentres extend beyond them and indicate greater width of the south iceland seismic zone or 20 30 km rather than approximately 10 km as indicated by the length of the surface faults the seismicity in the volcanic systems of hekla and vatnafjoll shows some characteristics of the south iceland seismic zone epicentres are concentrated into two n s lineaments one of which coincides with the location of the 1987 vatnafjoll earthquake mw 5 9 a strike slip event on a n to s trending fault the hypocentres of the hekla vatnafjoll events are mainly at 8 13 km depth which indicates a continuation of the depth trend of the earthquakes of the south iceland seismic zone the events located at hekla proper and immediately north of it are all of low frequency character which can be held as an indication of volcanic origin on the other hand they show clear s arrivals at observing stations like normal high frequency tectonic earthquakes

structural segmentation inversion and salt tectonics on a passive margin evolution of the inner kwanza basin angola the kwanza basin angola is divided into the inner and outer kwanza salt basins separated by a chain of synrift basement highs on which aptian 112 122 ma salt is thin or absent north to northwest trending basement structures in the inner kwanza basin have repeatedly been reactivated since neocomian 144 127 ma rifting reactivation formed three northwest striking fold and thrust belts near basement uplifts the thrust belts are bounded by northeast striking rift related transfer fault zones that were apparently reactivated during subsequent shortening three episodes of postrift basement involved shortening are documented in the inner kwanza basin 1 albian early cenomanian 112 96 ma 2 senonian 89 65 ma and 3 oligocene holocene 34 0 ma we relate the albian early cenomanian event to ridge push the senonian event to global plate reorganization and oligocene holocene shortening to uplift of the african superswell structural segmentation of the inner kwanza basin controlled the evolution of salt structures adjacent to basement uplifts diapirs were initiated as buckle folds some anticlines were unroofed by erosion and evolved into passive salt walls elsewhere broad salt walls were triggered by either detached extension or basement block uplift these walls grew until they exhausted their supply of salt thereafter dissolution rates exceeded rates of salt inflow so the walls began to subside withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous trough fill ranges in age from cenomanian to pliocene and this age varies greatly from trough to trough and along strike within troughs

spatiotemporal distribution of low frequency earthquakes in southwest japan evidence for fluid migration and magmatic activity abstract low frequency earthquakes lfes in the lower crust and uppermost mantle are widely observed in southwest japan and they occur not only along the subducting philippine sea phs slab interface but also beneath active arc volcanoes the volcanic lfes are still not well understood because of their limited quantities and less reliable hypocenter locations in this work seismic tomography is used to determine detailed three dimensional 3 d p and s wave velocity vp and vs models of the crust and upper mantle beneath southwest japan and then the obtained 3 d vp and vs models are used to relocate the volcanic lfes precisely the results show that the volcanic lfes can be classified into two types pipe like and swarm like lfes and both of them are located in or around zones of low velocity and high poisson s ratio anomalies in the crust and uppermost mantle beneath the active volcanoes the pipe like lfes may be related to the fluid migration from the lower crust or the uppermost mantle whereas the swarm like lfes may be related to local magmatic activities or small magma chambers the number of lfes sometimes increases sharply before or after a nearby large crustal earthquake which may cause cracks and fluid migration the spatiotemporal distribution of the lfes may indicate the track of migrating fluids as compared with the tectonic lfes along the phs slab interface the volcanic lfes are more sensitive to fluid migration and local magmatic activities high pore pressures play an important role in triggering both types of lfes in southwest japan

presalt stratigraphy and depositional systems in the kwanza basin offshore angola the lower cretaceous presalt section in the kwanza basin contains an excellent petroleum system that includes synrift strata barremian overlain by a sag interval aptian and capped by the loeme salt the upper synrift is generally limestone with widespread mollusk packstones and grainstones coquinas deposited in a fresh to moderately saline alkaline lake the sag interval is characterized by carbonate platforms and silica rich isolated buildups formed in highly evaporated highly alkaline lakes shrubby dendritic microbially influenced boundstones and intraclast spherulite grainstones accumulated in shallow water on platform tops microbial cherts were deposited as organic buildups on large isolated structural highs basinward west of platforms and they apparently formed at low temperatures in very alkaline lake water shrubby boundstones and microbial cherts have vuggy pores that are primary and result in high permeability wackestones and packstones with calcitic grains mainly spherulites in dolomite or argillaceous dolomite were deposited in slightly deeper low energy sag environments in addition clays especially stevensite precipitated out of the silica rich highly alkaline lake waters during sag deposition calcite precipitated on the shallow lake floor with morphologies that ranged from spherulites to shrubs and included a continuum of intermediate forms spherulites probably precipitated just below the sediment water interface spherulites and shrubby calcites are commonly recrystallized spherulites floating in stevensite probably formed in deeper lacustrine environments organic rich mudstones were deposited in even deeper lacustrine environments in synrift and sag intervals and they are likely the source of most hydrocarbons in this system these interpretations are supported by seismic core petrographic and stable isotope data

grain size distribution and depositional processes of the mineral component of abyssal sediments lessons from the north pacific grain size analyses of the mineral component of abyssal pacific surface sediments show two distinctly different size distributions that can be associated with sediments that are dominantly eolian and dominantly hemipelagic respectively in the range of 1 to 30 m eolian grains show a distinct size mode at about 2 m 9 and little material coarser than about 16 m 6 hemipelagic grains are characterized by a much flatter size distribution that has a broad size mode near 2 m 9 and important amounts of material coarser than 16 m 6 this ability to distinguish between hemipelagic and eolian grains will obviate some long standing concerns in the interpretation of deep sea sediments examples of the utility of this technique are given for the north pacific and south atlantic

precise hypocentre relocation of microearthquakes in a high temperature geothermal field the torfajokull central volcano iceland the torfaj okull volcanic system is one of approximately 30 active volcanoes comprising the neovolcanic zones of iceland the central volcano of the system is the largest silicic centre in iceland with a caldera of approximately 12 km diameter its high temperature geothermal system is one of the most powerful in iceland torfaj okull is a source of persistent seismicity where both high and low frequency earthquakes occur to study this microseismicity in detail a temporary array of 20 broad band seismic stations was deployed between 2002 june and november these temporary stations were embedded in the permanent south iceland lowland sil network and data from nine adjacent sil stationswere included in this study aminimum one dimensional 1 d velocity model with station corrections was computed for earthquake relocation by inverting manually picked p and s wave arrival times from events occurring in the torfaj okull volcanic centre and its surroundings high frequency earthquakes from the torfaj okull volcanic centre were then relocated calculating a non linear probabilistic solution to the earthquake location problem subsequently we correlated the waveforms of these 121 events 2000 observations to define linked events calculated the relative traveltime difference between event pairs and solved for the hypocentral separation between these events the resulting high resolution pattern shows a tighter clustering in epicentre and focal depth when compared with original locations earthquakes are mainly located beneath the caldera with hypocentres between 1 and 6 km depth and lie almost exclusively within the geothermal system a sharp cut off in seismicity at 3 km suggests either that there is a marked temperature increase or that this is a structural boundary no seismic activity was observed in the fissure swarms to the northeast ne and southwest sw of the volcanic centre

the stress pattern of iceland moritz ziegler mojtaba rajabi oliver heidbach gylfi pall hersir kristjan agustsson sigurveig arnadottir arno zang

three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland