degassing processes at stromboli volcano inferred from short lived disequilibria 210 pb 210 bi 210 po in volcanic gases abstract volcanic aerosols and gases released by three active craters at stromboli volcano have been regularly collected since 1985 in this paper we present new evidence of the high volatility of some nuclides among radon daughters 210pb 210bi and 210po which are strongly fractionated leading to significant radioactive disequilibria in volcanic exhalations the very low volcanic activity in october 1996 allowed a separate sampling of each crater plume for the first time remote sampling of the bulk plume were also performed these data show that the chemical composition of volcanic aerosols remains constant within the first few hundred meters from their source vents ensuring the validity of remote sampling when the activity does not allow one to approach the active craters moreover it appears that there is no differentiation of gases from one crater to another suggesting that the geometry of the upper plumbing system of the volcano is rather simple gases being directly emitted from a shallow magma chamber without significant cooling inside the edifice on the basis of the assumption of a continuously replenished shallow magma reservoir in steady state we propose a dynamic model of degassing accounting for the variations of radionuclide contents and ratios observed in the gas phase since 1985 this model allows us to relate these variations to changes in the magma chamber dynamics namely the magma residence time inside the chamber and the escape time of gases from it both parameters being closely linked to the volcanic activity while gases are always emitted within a few hours after bubble nucleation suggesting that the chamber is no deeper than a few hundred meters magma residence time varies from less than 20 days during eruptive periods highly explosive or effusive periods to more than 200 days before the 1985 eruption the latter figure is explained by the storage at shallow depth of a poorly renewed magma batch that fed the eruption the variations of the residence time observed over the studied period suggest that the steady state dynamics of the stromboli magma chamber is sustained for rather short periods of a few months at most on basis of the knowledge of the deep undegassed magma supply and the magma residence time we estimate the volume of the shallow magma chamber at about 7 2 10 5 m 3 the good agreement of our results with previous estimates based on both geochemical and geophysical studies suggests that the measurement of radioactive disequilibria in the gas phase provides a robust tool for a deeper understanding of the volcanic activity and the magma dynamics beneath active volcanoes

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

sedimentology stratigraphic occurrence and origin of linked debrites in the west crocker formation oligo miocene sabah nw borneo abstract the west crocker formation oligocene early miocene nw borneo consists of a large 20 000 km2 submarine fan deposited as part of an accretionary complex a range of gravity flow deposits are observed the most significant of which are mud poor massive sandstones interpreted as turbidites and clast rich muddy sandstones and sandy mudstones interpreted as debrites an upward transition from turbidite to debrite is commonly observed with the contact being either gradational and planar or sharp and highly erosive based on their repeated vertical relationship and the nature of the contact between them these intervals are interpreted as being deposited from one flow event which consisted of two distinct flow phases fully turbulent turbidity current and weakly turbulent to laminar debris flow the associated bed is called a co genetic turbidite debrite with the upper debrite interval termed a linked debrite linked debrites are best developed in the non channellised parts of the fan system and are absent to poorly developed in the proximal channel levee and distal basin floor environments due to outcrop limitations the genesis of linked debrites within the west crocker formation is unclear based on clast size and type it seems likely that a weakly turbulent to laminar debris flow flow phase was present when the flow event entered the basin a change in flow behaviour may have led to deposition of a sand rich unit with turbidite characteristics which was subsequently overlain by a mud rich unit with debrite characteristics flow transformation may have been enhanced by the disintegration and incorporation into the flow of muddy clasts derived from the upstream channel floor channel mouth or from channel levee collapse lack of preservation of this debrite in proximal areas may indicate either bypass of this flow phase or that the available outcrops fail to capture the debris flow entry point establishing robust sedimentological criteria from a variety of datasets may lead to the increasing recognition of co genetic turbidite debrite beds and an increased appreciation of the importance of bipartite flows in the transport and deposition of sediments in deepwater environments

trace element degassing and enrichment in the eruptive plume of the 2000 eruption of hekla volcano iceland during its last eruption in february 2000 hekla volcano iceland emitted a sub plinian plume that was condensed and scavenged down to the ground by heavy snowstorms offering the unique opportunity to study the chemistry of the gaseous plume released during highly explosive eruptions in this paper we present results on trace element and minor volatile species sulfates chlorides and fluorides concentrations in snow samples collected shortly after the beginning of the eruption the goal of this study is to better constrain the degassing and mobility of trace elements in gaseous emissions trace element volatility at hekla is quantified by means of enrichment factors ef relative to be well known volatile trace elements e g transition metals heavy metals and metalloids are considerably enriched in the volcanic plume of hekla their abundances are governed by the primary magmatic degassing of sulfate and or halide compounds which are gaseous at magmatic temperature their volatility is however slightly lower than in basaltic systems most likely because of the lower magma temperature and higher magma viscosity at hekla more surprisingly refractory elements e g ree th ba and y are also significantly enriched in the eruptive plume of hekla where their apparent volatility is two orders of magnitude higher than in mafic systems in addition ree patterns normalized to the hekla 2000 lava composition show a significant enrichment of hree over lree suggesting the presence of ree fluorides in the volcanic plume such enrichments in the most refractory elements and ree fractionation are difficult to reconcile with primary degassing processes since ree fluorides are not gaseous at magma temperature ree enrichments at hekla could be attributed to incongruent dissolution of tephra grains at low temperature by f rich volcanic gases and aerosols within the eruptive plume this view is supported by both leaching experiments performed on hekla tephra and thermodynamic considerations on ree mobility in hydrothermal fluids and modeling of glass dissolution in f rich aqueous solutions tephra dissolution may also explain the observed enrichments in other refractory elements e g th y and ba and could contribute to the degassing mass balance of some volatile trace elements provided they are mobile in f rich fluids it thus appears that both primary magmatic degassing and secondary tephra dissolution processes govern the chemistry of eruptive plumes released during explosive eruptions

bubble coalescence in basaltic lava its impact on the evolution of bubble populations morphological properties bubble number density porosity mean radius specific surface area of vesicular rocks provide quantitative information on the rates of bubble nucleation growth and coalescence in magmas when measured as a function of time such data are useful in constraining the timing and style of gas release during volcanic eruptions volcanic rocks commonly show strong zonation with respect to bubble size and porosity indicating a variation in the amount of bubble growth and coalescence preserved within a single sample morphological properties and bubble size distributions bsd s were measured in a suite of zoned alkali basalts using image analysis and the data were compared to theoretical predictions our data indicate that at porosities greater than 35 extensive coalescence occurred during the growth of bubbles with restricted nucleation at lower porosities vesiculation is dominated by nucleation and diffusion with no coalescence the interiors of many of our samples have undergone 4 7 binary coalescence events after eruption in a time of around 15 min the ostwald ripening effect has not significantly modified the bsd s

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

relationship between gases and volcanic activity of mount etna in 1986 the long lived 222rn decay products 210pb 210bi and 210po have been monitored in the plumes of several vents at mount etna sicily from may to october 1986 the results show that the four main craters of this volcano emit gases whose compositions are different from each other the 210bi 210pb ratios for the plumes have similar mean values close to 25 which correspond to a degassing time of 1 5 to 2 7 days according to the model of lambert et al 1985 86 in contrast 210po 210pb ratios have very different mean values in each plume 35 at the voragine crater 20 at the bocca nuova crater and 14 at the south east crater these figures enable us to calculate proportions of deep magma of 50 29 and 19 in the degassing cells of these craters respectively moreover the se crater appears to be a secondary degassing vent not directly related to the main magma reservoir the evolution of these ratios has been related to variations in volcanic activity

revised tectonic implications for the magnetic anomalies of the western weddell sea abstract ten years after the usac u s a rgentina c hile project which was the most comprehensive aeromagnetic effort in the antarctic peninsula and surrounding ocean basins questions remain regarding the kinematics of the early opening history of the weddell sea key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the weddell sea and merging the usac data set with the other magnetic data sets in the region for this purpose we reprocessed the usac data set using a continuation between arbitrary surfaces and equivalent magnetic sources the equivalent sources are located at a smooth crustal surface derived from the existing bathymetry topography and depths estimated by magnetic inversions the most critical area processed was the transition between the high altitude survey over the antarctic peninsula and the low altitude survey over the weddell sea that required downward continuation to equalize the distance to the magnetic source this procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion the enhancement of the mesozoic sequence permits refining the interpretation of the seafloor spreading anomalies in particular the change in shape and wavelength of an elongated positive in the central weddell sea suggests that it was formed during the cretaceous normal polarity interval the older lineations in the southwestern weddell sea are tentatively attributed to susceptibility contrasts modeled as fracture zones numerical experimentation to adjust synthetic isochrons to seafloor spreading lineations and flow lines to fracture zones yields stage poles for the opening of the weddell sea since 160 ma to anomaly 34 time the corresponding reconstructions look reasonable within the known constraints for the motions of the antarctic and south america plates however closure is not attained between 160 and 118 ma if independent published east antarctica africa south america rotations are considered the lack of closure may be overcome by considering relative motion between the antarctic peninsula and east antarctica until 118 ma time an important component of convergence

the 1989 1990 activity of etna magma mingling and ascent of h2oclsrich basaltic magma evidence from melt inclusions abstract the melt inclusions in olivine phenocrysts from basaltic tephra erupted by etna volcano during the 1989 1990 eruptive period were investigated for major elements cl and s by electron microprobe fluorine by pigme particule induced gamma ray emission and h 2 o by sims the results indicate the ascent of basaltic magmas with different k 2 o concentrations possibly related to different feeding dykes lk basaltic melt inclusions rich in cl s and h 2 o are trapped in olivine fo 84 from the 1990 tephra and represent the early crystallization stage their composition in major elements is quite similar to those of most of the historic and even pre etnean alkali basalts but significantly lower in k 2 o compared with the 1989 1990 lavas only scarce hk basaltic melt inclusions were observed in the outer rim fo 79 6 of reversely zoned olivine phenocrysts and correspond to late crystallization stage at low pressure high sulfur concentration s 0 3 wt is a general characteristic of the etnean basaltic magmas with h 2 o s ratio as high as 7 the volatile abundances of the etna primitive basalts were computed at 1 45 wt for h 2 o 0 17 wt for cl and 0 047 wt for f with a high cl f ratio close to 3 6 the 1989 lava samples with cl at 660 90 ppm and f at 770 30 ppm have suffered a significant cl loss the possible behaviour of the volatile phase as co 2 h 2 o even cl is discussed with h 2 o and chlorine late degassing and an active role of h 2 o during the lava fountaining activity

explosive volcanic eruptions ix the transition between hawaiian style lava fountaining and strombolian explosive activity the commonest eruption styles of basaltic volcanoes involve hawaiian lava fountaining or intermittent strombolian explosions we investigate the ways in which magma rise speed at depth magma volatile content and magma viscosity control which of these eruption styles takes place we develop a model of the degree of coalescence between gas bubbles in the magma which allows us to simulate the transition between the two extreme styles of activity we find that magma rise speed is the most important factor causing the transition with gas content and viscosity also influencing the rise speed at which the transition occurs counter to intuitive expectations a decrease in gas content does not cause a transition from hawaiian to strombolian activity but instead causes a transition to passive effusion of vesicular lava rather a change from hawaiian to strombolian style requires a significant reduction in magma rise speed

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

radioactive isotopes and trace elements in gaseous emissions from white island new zealand abstract measurements of the radioactivity of gases released by white island volcano show variations in 210 po concentrations with temperature in fumarolic gases and this is interpreted to be due to dilution of magmatic gases with vapours derived from an envelope of acid brines surrounding the vent system the degassing model derived for mount etna emissions is tested on white island magmatic gases the results are a short degassing time 2 days small degassing magma volume 5 10 6 m 3 and the following estimations of emanation coefficients pb 1 bi 2 7 cd 0 05 and cu 0 07 the initial content of s in the magma is estimated to be within the range 300 600 mg kg fluxes of trace metals kg day emitted by the volcano are estimated to be pb 125 bi 6 cu 300 and cd 9

h2o and halogen f cl br behaviour during shallow magma degassing processes abstract degassing paths of silicic h2o rich magmas during explosive plinian and dome forming eruptions may be represented in simple evolution diagrams relating the residual volatile content of the melt to the volume ratio of gas over melt vg vl these parameters are measurable in erupted magma fragments they are used for interpreting the h2o and halogen contents determined in a series of variously degassed volcanic clasts from the 650 y b p eruption at mt pelee martinique fwi which is characterized by the succession of a dome extrusion and a plinian activity h2o degassing of plinian pumice clasts are modeled by an equilibrium closed system evolution which allows to calculate the partition coefficients of halogen between the aqueous fluid and the rhyolitic melt d v l i 1 for f 10 1 5 for cl and 3 7 1 for br at around 900 c the strong increase of the dv li values of cl and br during dome extrusion is explained by an open system degassing model with a simultaneous crystallization of the melt some significant br enrichments relative to cl in dome fragments are attributed to interactions with an hydrothermal fluid close to seawater in composition the models allow calculation of cl and br contents of aqueous fluids generated by the different degassing paths cl contents are 0 5 mol l for closed system evolution and vary between 0 2 and 1 mol l during open system evolutions these fluids are able to extract significant amounts of chloride complexed metals and silica from the magma during dome forming eruptions they also may generate mineralization of the wall rocks or the rising dome itself e g silica precipitation which may seal the volcanic system and be responsible for the dome explosivity and the transition from dome forming to plinian eruptive style the cl br ratio of initial melts of p1 eruption 250 is much lower than the basaltic and exospheric ratios 400 suggesting that strong cl br fractionation may occur during the production or the differentiation of subduction related magmas

magma budgets and steady state activity of vulcano and stromboli we present three models for magma budget during steady state volcanic activity by which non erupted magma is emplaced as dykes or cumulates or crystallises in place using gas and thermal data we apply our models at vulcano to calculate degassing cooling and crystallisation of magma at a rate of 40 375 kg s 1 within a magma body with an upper surface at a 2 km depth at stromboli we calculate a steady magma supply of 300 1300 kg s 1 to shallow 1 km depths

gas transport model for the magmatic system at mount pinatubo philippines insights from 210pb 226ra abstract measurements of 226ra 210pb disequilibria in eruptive products have the potential to track the accumulation or loss of volatiles in magmatic systems on timescales of decades because the intermediate nuclide 222rn follows the gas phase we present measurements of 210pb 226ra disequilibria for whole rock samples representing a time sequence of tephra through the june 15 1991 cataclysmic eruption of mount pinatubo volcano philippines mount pinatubo volcano is a dacitic system that did not significantly vent gases at the surface prior to eruption and we can therefore isolate the 210pb 226ra disequilibria due to gas accumulation without complications due to 222rn loss during degassing pyroclastic samples have 210pb 226ra 0 ranging from 1 01 to 1 10 averaging 1 06 a sample of the post climactic dome has 210pb 226ra 0 1 12 previous uranium series degassing studies have suggested that 210pb excesses are created by rapid volatile transport carrying the intermediate daughter 222rn and subsequent volatile accumulation and decay of 222rn to 210pb however bubbles in viscous dacite magma cannot rise at speeds needed to provide a flux of 222rn large enough to cause measurable disequilibria in the 210pb 226ra system in addition there is little evidence for magmatic sources large enough to supply a rapid flux of 222rn therefore we present a model in which 210pb 226ra disequilibria is established during basaltic recharge of the pinatubo reservoir the relatively low viscosity of basaltic magma allows for differential gas motion and the production of 210pb excess in localized basaltic melt transport of volatiles and 210pb rich basalt through a crystal matrix and the formation of bubble plumes in the dacitic reservoir produces a mixed magma with 210pb excess through this mechanism the timescale of gas transport and accumulation is constrained not by the half life of 222rn 3 8 days but rather by the half life of 210pb 22 6 years bubble plume motion preserves disequilibria and creates a zone of eruptable dacite with 210pb excess alleviating the need for gas transport on very short time scales using the rate of decay of 210pb coupled with published trace element data we present a quantitative investigation of this new conceptual model and propose that changes in 210pb values with time may suggest changing conditions of magma supply at volcanoes

temporal and spatial variability of seafloor spreading processes in the northern south atlantic the paper reports the results of an aeromagnetic study of the mid atlantic ridge in the northern south atlantic the study consists of 38 000 km of low altitude magnetics tracks centered on the ridge covering nearly 700 000 km2 south of ascension island most tracks extend out to crust of anomaly 5b age large areal coverage coupled with high resolution allows comparison of crustal generation processes over space along ridge strike and time across strike temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion the total spreading rate increased sharply at 10 ma and then decreased between 6 ma and the present the time variation of spreading rate is similar to that seen in other areas such as the north atlantic and the southern and eastern pacific the plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones in several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones the periods of plate boundary modification correlate with the increase and decrease in spreading rate spatial heterogeneities are caused by the influence of the ascension hotspot on the mid atlantic ridge in the northern portion of the study area and include two apparent seamount chains formed by the hotspot ascension island and several of the seamounts appear to have been produced some distance away from the hotspot location presently about 225 km southeast of ascension island by channeled subaxial magma flow beneath the spreading center the seamount chains therefore do not mark hotspot tracks large local variations in spreading rate and degree of asymmetry along strike are also found

merapi volcano central java indonesia a case study of radionuclide behavior in volcanic gases and its implications for magma dynamics at andesitic volcanoes for more than 20 years volcanic gases have been regularly collected at merapi volcano indonesia and have been subsequently analyzed for their 210pb 210bi and 210po activities and so2 contents these new results show the high volatility of the three radionuclides in andesitic gases although their emanation coefficients 0 94 3 5 and 53 for 210pb 210bi and 210po respectively are significantly lower than those observed at basaltic volcanoes this emphasizes the major role of magma temperature on the degassing of these metals which are mainly transported in volcanic gases as pb chloride compounds and as bi and po metallic species radioactive disequilibria between 210pb 210bi and 210po in the gas phase are characteristic of degassing processes and gas paths within the edifice gases released at both gendol and woro fumarolic fields are of magmatic origin but their radionuclide content is strongly altered by secondary processes chemical reactions with surrounding brines and deposition of sublimates in the ground high temperature gases collected in the summit gas plume are of pure primary magmatic origin and are likely directly tapped in the degassing reservoir gases arising from the growing domes are strongly depleted in the most volatile isotopes and gas species this suggests that magma mostly degasses at depth in open system explaining why the volcano usually undergoes no explosive eruptions the gas flux at merapi is sustained by a deep magma supply which is twice the extrusion rate of lava unerupted degassed magma thus progressively accumulates beneath the summit yielding an overpressure which can trigger dome collapse without precursor

high resolution inversion for south atlantic plate kinematics using joint altimeter and magnetic anomaly data we present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone fz and magnetic anomaly location data jointly we define misfit functions that incorporate properties unique to each data type in particular the fz misfit function does not depend upon alignment of conjugate fz traces in the same way as magnetic lineations under the finite rotations this property is useful for fz locations in which the signals on conjugate sides of the ridge may include systematic differences or where data from one side of the ridge are sparse or missing formal error bounds estimated for the pole parameters show that the magnetic and fz data are complementary in their information content error bounds computed for the joint inversion are substantially smaller than for either the fz or magnetics data used separately indicating that simultaneous use of the data in an inversion is crucial we apply this method to seasat altimeter data and magnetic anomaly picks in the south atlantic we solve for finite poles corresponding to magnetic anomalies 5 6 8 13 21 22 25 30 33 33r and 34 these define a smooth path over the past 84 m y indicating that the plate motions have not been as erratic as found previously

a simple model of 222rn accumulation leading to 210pb excesses in volcanic rocks while most rocks from active volcanoes display 210pb 226ra activity ratios lower than or close to the equilibrium value of 1 several other have ratios much higher than 1 transfer of 222rn by magmatic volatiles has often been advocated to explain both 210pb deficits and excesses we develop here a model to account for 210pb excesses through 222rn accumulation we show that large 210pb excesses can be readily obtained even for moderate ratios of degassing magma over accumulating magma in a closed system model where the 222rn atoms produced by decay of 226ra in the degassing magma are continuously extracted by a gas phase for a given duration of volatile transfer relative 210pb excesses are expected to be much larger than 210pb deficits this model is applied to samples from santorini aegean arc and surtsey iceland both of which show high 210pb 226ra ratios of 6 7 and 2 3 respectively at the time of eruption the agreement with field data suggests that 222rn accumulation leading to 210pb excesses might be a rather common process in zoned magma chambers as well as in individual lava flows and that it can be described by a closed system model

atmospheric 210 po anomaly as a precursor of volcano eruptions 1 due to its high volatility 210po can be effectively released from active volcanoes prior to their explosions a continuous record of the fallout rate of 210po versus that of 210pb in northern taiwan reveals that most of the 210po was derived from pre eruption gas emission of philippines mayon volcano in the past two years from this observation we suggest that if a global network can be properly designed it may be possible to employ 210po anomaly as a precursor of volcano eruptions worldwide

strombolian explosions 2 eruption dynamics determined from acoustic measurements strombolian activity consists of a series of explosions due to the breaking of a large overpressurized bubble at the surface of the magma column acoustic pressure due to sound waves has been measured and analyzed at stromboli for more than 50 explosions three parts can be distinguished in the acoustic pressure waveform which are related to the behavior of the bubble before during and after its bursting before the sharp rise in acoustic pressure the signal is dominated by waves with a frequency of 2 hz which develop on the nose of the bubble they produce sound in air by imposing a rapid motion to the interface and one could detect a bubble travelling in the uppermost 30 m of the magma column when the bubble reaches the air magma interface its strong vibration driven by a large overpressure inside the gas generates the main event with a frequency around 9 hz after the bubble has burst kinematic waves of frequency around 4 5 hz are the main source of sound they develop at the surface of the magma left on the conduit side the three types of motion although determined independently give consistent results furthermore combining the results obtained for the two types of kinematic waves the magma viscosity is estimated to be of 300 65 pa s which is in good agreement with petrological constraints and corroborates the validity of our analysis this suggests that acoustic measurements constitute a powerful tool in the understanding of eruption dynamics

thermomechanical evolution of oceanic fracture zones a fracture zone fz model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere as the lithosphere cools by conduction thermal and mechanical boundary layers develop and increase in thickness as age 1 2 surface expressions of this development such as topography and deflection of the vertical i e gravity field are most apparent along major fz s because of the sharp age contrast a simple model including the effects of lateral heat transport but with no elastic layer predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the fz ages observations however show that both the fz topography and the deflection of the vertical remain sharp as the fz evolves these two observations as well as the observed asymmetry in deflection of the vertical profiles across the udintsev romanche and mendocino fz s are explained by including a continuous elastic layer in the model the asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the fz modeling also shows that the evolution of the fz topography is extremely sensitive to the initial thermal structure near the ridge transform intersection model geoid steps and their development with age are used to access techniques for measuring geoid offsets across fz s reasonable step estimation techniques will underestimate the overall step amplitude by up to 50 this implies that abnormally thin thermal boundary layers derived from studies of geoid height versus age are not required by the data

elevated gas flux and trace metal degassing from the 2014 2015 fissure eruption at the barðarbunga volcanic system iceland the 2014 bar arbunga rifting event in iceland resulted in a 6 month long eruption at holuhraun this eruption was characterized by high lava discharge rate and significant gas emission the so2 flux for the first 3 months was measured with satellite sensors and the petrologic method high resolution time series of the satellite data give 1200 kg s that concurs with 1050 kg s obtained from melt inclusion minus degassed lava sulfur contents scaled to the mass of magma produced a high purity gas sample with elevated s cl due to limited chlorine degassing reveals a similar degassing pattern of trace metals as observed at k lauea hawai i and erta ale ethiopia this suggests a common degassing mechanism at mantle plume related volcanoes the trace metal fluxes calculated from trace element to sulfur ratios in the gas sample and scaled to the sulfur dioxide flux are 1 2 orders of magnitude stronger at holuhraun than k lauea and erta ale in contrast volcanoes at convergent margins etna and stromboli italy have 1 2 orders of magnitude higher trace element fluxes most likely caused by abundant chlorine degassing this emphasizes the importance of metal degassing as chlorine species short lived disequilibria between radon daughters 210pb 210bi 210po measured in the gas suggest degassing of a continuously replenished magma batch beneath the eruption site earlier and deep degassing phase of carbon dioxide and polonium is inferred from low 210po 210pb in the gas consistent with magma transfer rate of 0 75 m s

an investigation of small offset fracture zone geoid waveforms approximately 1 500 geoid cross sections across 15 small offset south atlantic fracture zones fz s are compiled from seasat altimeter data and organized according to crustal age these profiles provide a basis for the comparison of the different fz s and the evolution of each over geological time i use an empirical orthogonal function decompostion to investigate the dependence of profile shape and amplitude upon crustal age the geoid cross sections are found to be coherent in form down the length of each fz and possess amplitudes that are inversely related to the relative spreading rate at the time of formation this observation is consistent with a simple model in which the active portion of a fracture zone the transform fault remains a fixed spatial length yielding a variable age offset across the fz

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

seasat altimetry and the south atlantic geoid 2 short wavelength undulations this paper is a discussion of the short wavelength 25 100 km undulations of the geoid recovered from seasat altimeter data in the south atlantic these undulations are known to be highly correlated to seafloor topography and they considerably improve our knowledge about global bathymetry of the south atlantic which remains poorly surveyed the orientation of fracture zones fz with respect to the seasat profiles ensures an efficient recovery of the fz pattern in the younger 65 ma parts of the south atlantic between 20 n and 50 s we used 22 well marked fzs regularly distributed between 12 n and 47 s to compute eulerian poles describing the finite relative movement between africa and south america from the present back to 35 ma fz trends south of the equator are well accounted for with a single rotation and standard statistical tests do not indicate a need for distinct and successive rotations however these tests indicate an improvement of the fit when distinct poles are adjusted to separate subsets of our 22 fzs either fzs north and south of the equator or fzs north and south of tristan da cunha this points out the possibility of the existence of either corridors in which slight relative movements occur or diffuse elastic deformation over the entire south atlantic although this is really a second order effect we also present and discuss a map of the roughness of the geoid for the 25 to 110 km wave band strong disagreements between the roughness map and the gebco maps exist in regions of sparse ship track coverage for example the rio grande rise has an associated roughness feature which appears elongated and more symmetric with respect to the walvis ridge than what is displayed on the gebco maps the roughness map also clearly documents a long wavelength 400 km segmentation of the mid atlantic ridge between the equator and 35 s

the 2007 eruption of stromboli volcano insights from real time measurement of the volcanic gas plume co2 so2 ratio abstract the recent eruption of stromboli in february april 2007 offered a unique chance to test our current understanding of processes driving the transition from ordinary persistent strombolian to effusive activity and the ability of instrumental geophysical and geochemical networks to interpret and predict these events here we report on the results of two years of in situ sensing of the co 2 so 2 ratio in stromboli s volcanic gas plume in the attempt to put constraints on the trigger mechanisms and dynamics of the eruption we show that large variations of the plume co 2 so 2 ratio range 0 9 26 preceded the onset of the eruption since december 2007 interrupting a period of relatively steady and low ratios time averaged ratio 4 3 lasting from at least may to november 2006 by contrasting our observations with numerical simulations of volcanic degassing at stromboli derived by use of an equilibrium saturation model we suggest that the pre eruptive increase of the ratio reflected an enhanced supply of deeply derived co 2 rich gas bubbles to the shallow plumbing system this larger than normal ascent of gas bubbles was likely sourced by a 1 3 km deep gas melt separation region probably a magma storage zone and caused faster convective overturning of magmas in the shallow conduit an increase in the explosive rate and in seismic tremor and finally the collapse of the la sciara del fuoco sector triggering the effusive phase the high co 2 so 2 ratios up to 21 observed during the effusive phase and particularly in the days and hours before a paroxysmal explosion on march 15 2007 indicate the persistence of the same gas source and suggest that de pressurization of the same 1 3 km deep magma storage zone could have been the trigger mechanism for the paroxysm itself

investigations of relative plate motions in the south atlantic using seasat altimeter data fracture zones in the world s oceans appear clearly in the seasat altimeter geoid data at wavelengths of about 100 km and less in the north and south atlantic these signatures are stable in form and amplitude from the ridge crest to crust tens of millions of years old and can be mapped across nearly the width of the basin the signatures diminish in amplitude markedly on crust older than 70 to 80 m y b p apparently when thickly covered by sediments because fracture zone trajectories closely approximate plate motion flow lines they may be used to infer the plate kinematic history of the ocean basin in this paper i describe a new method for estimating the relative poles of opening describing the history of motion between two plates by using fracture zone trajectories the method is an iterative inversion whereby the trajectories of synthetic flow lines are linearized with respect to the pole parameters when applied to the seasat altimeter track data a waveform inversion based upon this method is possible in which the altimeter data are modeled using a synthetic geoid fracture zone signal this waveform inversion is applied to 17 prominent fracture zones in the central and south atlantic to estimate the poles of opening between the south american and african plates altogether 1022 seasat fracture zone crossings are used for each of these a waveform comparison is made the new method is found to be robust and converges to a stable set of pole solutions even if the initial flow lines are small circles about a single pole that poorly approximates the observed trajectories the solution poles document a cenozoic change in plate motion near anomaly 13 time 36 m y b p similar to that in the north atlantic at anomaly 1 3 21 time 36 50 m y b p

gravity changes and passive so2 degassing at the masaya caldera complex nicaragua an understanding of the mechanisms responsible for persistent volcanism can be acquired through the integration of geophysical and geochemical data sets by interpreting data on micro gravity ground deformation and so2 flux collected at masaya volcano since 1993 it is now clear that the characteristically cyclical nature of the activity is not driven by intrusion of additional magma into the system rather it may be due in large part to the blocking and accumulation of gas by restrictions in the volcano substructure the history of crater collapse and formation of caverns beneath the crater floor would greatly facilitate the trapping and storage of gas in a zone immediately beneath san pedro and the other craters another mechanism that may explain the observed gravity and gas flux variations is the convective overturn of shallow pre existing degassed cooled dense magma that is replaced periodically by lower density hot gas rich magma from depth buoyant gas rich magma rises from depth and is emplaced near the surface resulting in the formation and fluctuation of a low density gas rich layer centred beneath nindiri and santiago craters as this magma vigorously degasses it must cool increase in density and eventually sink five stages of activity have been identified at masaya since 1853 and the most recent data suggest that the system may have been entering another period of reduced degassing in 2000 this type of analysis has important implications for hazard mitigation because periods of intense degassing are associated with poor agricultural yields and reduced quality of life a better understanding of persistent cyclically active volcanoes will allow for more effective planning of urban development and agricultural land use

the fifteen twenty fracture zone and the north american south american plate boundary the fifteen twenty fracture zone offsets the mid altantic ridge over 195 km in a sinistral sense it was surveyed in 1982 between 30 w and 60 i e from anomaly 34 on the african plate to anomaly 34 on the american plate where it disappears as the barracuda ridge into the caribbean subduction zone the fossil limbs of the fifteen twenty fracture zone show a remarkable symmetry to both sides of the mid atlantic ridge barracuda ridge appears to have undergone a later deformation the sections of the fifteen twenty fz between 49 w and 54 w to the west of the mid atlantic ridge and between 37 5 w and 42 5 w to the east do not exhibit the normal characteristics of a fracture zone but have elongate and extremely narrow transverse ridges dykes along the fracture zone axis the present transform domain shows signs of leaking this is interpreted as a result of a fairly recent about 7 ma bp change in spreading direction of the area this change in spreading direction seems to have affected only the area south of 17 50 n it may have resulted from a northward migration of the plate boundary between the north american and south american plates and the coming into existence of a triple point near 16 n from this reconstruction the location of the differential pole between the north american and south american plates is 16 n 53 5 w this location leads to increasing compression from 53 5 to 60 w affecting the barracuda ridge and trough with a n s shortening of about 16 km in 7 ma from 53 5 to 45 w n s extension occurred about 19 km near the mid atlantic ridge which led to the origin of the volcano tectonic complex of researcher ridge researcher trough and royal trough to the west of the mid atlantic ridge

understanding magmatic processes at telica volcano nicaragua crystal size distribution and textural analysis telica volcano in nicaragua currently exhibits persistent activity with continuous seismicity and degassing yet it has not produced lava flows since 1529 to provide insight into magma chamber processes including replenishment and crystallization crystal size distribution csd profiles of plagioclase feldspar phenocrysts were determined for quaternary telica basalts and basaltic andesites textural analysis of 14 highly crystalline lavas 20 vol phenocrysts indicates that the samples are dominated by sieve textured plagioclase feldspar phenocrysts whose origin requires thermochemical disequilibrium within the magmatic system the csd curves display an inverse relationship between phenocryst length and population density concave up patterns observed for the telica lava samples can be represented by linear segments that define two crystal populations a steeply sloping segment for small crystals 1 5 mm in length the two crystal populations may be explained by magma replenishment and a mixing model in which a mafic magma is introduced to a stable chamber that is petrologically and geochemically evolving residence times calculated using the defined linear segments of the csd curves suggest these magmatic processes occur over timescales on the order of decades to centuries the crystal size distribution and textural analysis advocate for the current persistent activity as being consistent throughout telica s historic and prehistoric eruptive periods and driven by replenishment of mafic magma

observations of seismic volcanic signals at stromboli volcano italy abstract digital recordings of ground motion during a seismic operation organized at stromboli volcano italy from may 31 1986 to june 26 1986 have been studied the amplitude of background seismic noise is greater at the summit than at the base of the volcano and the spectra calculated on different days are dominated by at least two frequencies 2 0 and 6 0 hz with subdominant frequencies at 4 1 4 8 and 8 hz according to their waveform and their spectral content three types of discrete volcanic signals are distinguishable the first type is characterized by a dominant frequency at 2 hz and a subdominant frequency at 4 1 hz the second type also has a dominant peak frequency at 2 hz with two important subdominant frequencies at 5 and 8 hz the type 3 has a sharp dominant frequency at 6 hz and subdominant frequency at 3 2 hz the frequencies observed for discrete signals and ground noise suggest that they are produced by the same repeated mechanism which probably has a volcanic origin the discrete signals are linked to explosions at the active vents within the craters

fractionation of metals in volcanic emissions abstract the enrichment of some metals al mg na k cu zn cd in volcanic gases is measured by an emanation coefficient ex relating the amount of the element x in lavas and in aerosols ex is determined in the volcanic emissions of mount etna sicily with regard to that of210pb the accuracy of the results is limited by the geochemical behavior of common lead compared to that of210pb concerning the most volatile species it appears that practically all the volcanic aerosols are produced by evaporation followed by gas to particle conversion and the spattered fraction appears to be negligible

escape of unradiogenic osmium during sub aerial lava degassing evidence from fumarolic deposits piton de la fournaise reunion island abstract this study presents new re os isotope and elemental data in gas condensates and corresponding lavas in order to examine the geochemical behavior of these two elements during magma degassing at piton de la fournaise reunion island gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca 100 c include na k sulfate aphthitalite na sulfate thenardite ca cu sulfate e g gypsum ca mg al fe fluoride e g ralstonite and native sulfur the high temperature deposits show trace element typical of volcanic gas with high enrichment in re 24 to 79 ppb almost two order of magnitude higher than the corresponding lavas but with os abundances similar to those of the lavas 14 132 ppt in contrast the os contents of the low temperature fluoride deposits 13 77 ppb are higher than any of the other condensates the fluorides are also enriched in re albeit to lesser extent than os 2 9 15 3 ppb based on high temperature samples the fluid melt partition coefficients estimated for re and os are 100 80 and 1 2 respectively considering 1 of fluid loss these partition coefficients translate into emanation coefficients of 0 50 0 17 0 65 for re and 0 01 0 0 03 for os these results indicate that re unlike os is highly volatile at piton de la fournaise osmium isotopic compositions of samples collected at medium and low temperature 187 os 188 os between 0 130 0 135 and plot within the range of the april 2007 lava flow and the historical lavas of piton de la fournaise i e 0 130 0 137 however the highest temperature condensates na k sulfates with t of 384 400 c yield lower 187 os 188 os ratios i e 0 124 0 129 within the field of mantle signal such unradiogenic compositions are best explained if old mantle sulfides occur in lavas and contribute to volcanic gases within the general frame of osmium mantle geochemistry loss of unradiogenic os during magmas degassing could help to explain osmium isotope disequilibrium between lavas and melting residues

eruption of basalt and andesite lava degasses 222rn and 210po activities of 222rn and 210po were measured in a september 1983 basic andesite lava from arenal and a november 1983 basalt from kilauea starting 3 and 1 days after eruption respectively in both cases in growth patterns show that all rn volatalized during eruption po degassing also was complete at kilauea but only 84 10 at arenal

volcanic emission of radionuclides and magma dynamics 210pb 210bi and210po the last decay products of the238u series are highly enriched in volcanic plumes relative to the magma composition moreover this enrichment varies over time and from volcano to volcano a model is proposed to describe 8 years of measurements of mt etna gaseous emissions the lead and bismuth coefficients of partition between gaseous and condensed phases in the magma are determined by comparing their concentrations in lava flows and condensated volatiles in the case of volatile radionuclides an escaping time is calculated which appears to be related to the volcanic activity finally it is shown that that magma which is degassing can already be partly degassed it should be considered as a mixture of a few to 50 of deep non degassed magma with a well degassed superficial magma cell

relative positions of africa and antarctica in the upper cretaceous evidence for non stationary behaviour of fracture zones abstract new bathymetric and magnetic data have been collected in the enderby basin southern indian ocean during cruise md34 of the n o marion dufresne january february 1983 one can identify in this basin oceanic crust created along the same portion of the southwest indian ridge as the crust of the mozambique basin two sets of magnetic lineations with different amplitude split by a large offset fracture zone are localized in each basin and enable us to obtain accurate reconstructions of the africa antarctica plate boundary at the time of anomalies 34 33 and 32 the data west of prince edward fracture zone are taken into account to adjust the parameters of rotation these reconstructions of isochrons allow us to discuss the evolution of the geometry of accretionary plate boundaries and to emphasize the unstable behaviour of fractures zones in particular we give evidence for an important change in the spreading direction along prince edward fracture zone which demonstrates that stability of a pole of rotation is not guarantueed by major offset fracture zones

some geometrical aspects of uncertainties in combined plate reconstructions the uncertainties in both the position of a pole of rotation and the rotation angle describing a plate reconstruction depend not only on the number and quality of data used but also on two geometrical factors the length of plate boundary represented by the data and the distance from the best fit pole to the center of the data region we describe a simple geometrical method that can be used to calculate minimum uncertainties in reconstructions based on positions of magnetic anomalies and fracture zones with finite uncertainties uncertainties in pole positions and angles corresponding to 10 km uncertainty in individual data points can when combined yield uncertainties in reconstructed positions greatly in excess of 10 km per rotation for example an uncertainty of 10 km in anomaly 6 reconstructions in the south pacific southeast indian northwest indian and north atlantic oceans would result in uncertainty of up to 190 km in the reconstructed position of the pacific plate with respect to north america at 19 8 m y b p existing data show that the realistic uncertainties for this case are twice as large as the calculated minimum uncertainties

variability of alkali and heavy metal fluxes released by mt etna volcano sicily between 1991 and 1995 abstract we regularly collected volcanic gases and aerosols at mt etna volcano over a long period 1991 1995 and measured their chemical composition by icp ms based on these data we present new evidence on the high volatility of both alkali and some heavy metals in contrast both lanthanides and actinides are not produced as significant gaseous compounds at magmatic temperatures and can be used as tracers of spattered materials all metals of each chemical family maintained systematic trends during the study period at both summit craters and effusive vents located in the valle del bove during the 1991 1993 eruption during the eruption we observed high metal enrichments at effusive vents these enrichments seem to be characteristic of the secondary degassing of a magma previously degassed under the summit area lower metal enrichments are recorded in aerosols sampled in summit crater plumes according to the primary degassing type occurring at these vents while a primary degassing type still occurs at both bocca nuova and voragine during the post eruptive period sud est crater exhibits a transition from primary to secondary type according to the variations of the volcanic activity level we confirm that the alkali metals fractionate systematically within their family as they cross the melt vapour interface the heavier members being always more enriched than the lighter ones in aerosols we also show that this fractionation is not related to the volcanic activity and the degassing type but seems to be a significant feature of the alkali family itself estimation of new emanation coefficients for rubidium caesium tin and thallium 0 07 0 15 0 50 and 21 respectively yields annual outputs around 1000 t yr 60 t yr 240 t yr and 1600 t yr respectively mount etna is a very important source of metals injected into the atmosphere contributing to the global volcanic budget about 16 for heavy metals and 19 for alkali metals in eruptive periods and only 2 and 4 respectively in quiet periods

degassing of metals and metalloids from erupting seamount and mid ocean ridge volcanoes observations and predictions recently it has been reported that the element polonium degasses from mid ocean ridge and seamount volcanoes during eruptions published and new observations on other volatile metal and metalloid elements can also be interpreted as indicating significant degassing of magmatic vapors during submarine eruptions this process potentially plays an important role in the net transfer of chemical elements from erupting volcanoes to seawater in addition to that arising from sea floor hydrothermal systems in this paper a framework is constructed for predicting and assessing semiquantitatively the potential magnitude and chemical fingerprints in the water column of metal and metalloid degassing using 1 predictions from a summary of element volatilities during mafic subaerial volcanism worldwide and 2 limited data from submarine volcanic effusives the latter include analyses of polonium and trace metals in near volcano water masses sampled following a submarine eruption at loihi seamount hawaii 1000 m bsl in 1996 the element volatility predictions and observations show good agreement considering the limited dataset some of the highest volatility main group and transition element enrichments in seawater over loihi are predicted by the degassing mass transfer model i present when expanded to cover all submarine volcanic activity it is predicted that exit fluxes of these elements are up to 102 103 greater by degassing than by normal mor hydrothermalism in contrast mor exit fluxes of low volatility alkali and alkaline earth elements are likely 102 106 greater from hydrothermal inputs degassing inputs to the ocean are probably highly episodic occurring almost entirely during eruptions these are times of enhanced and abnormal hydrothermalism as well although major hydrothermal and degassing events may not be chemically recognizable in real water masses as wholly distinct entities it is nevertheless possible to predict to what extent each process flavors the effluents of the other degassing at mid ocean ridges may explain a variety of observations previously ascribed to complexities occurring during hydrothermal venting and or fluid ascent in the buoyant hydrothermal plumes above ridges

the uncertainties of finite rotations in plate tectonics incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates a reconstruction may be represented by the finite rotation that describes one configuration relative to another in this paper a method of reconstruction is provided that reflects the uncertainties in the data the method minimizes a weighted least squares measure of fit as a function of the rotation parameters for a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points representing corresponding former plate margin segments from a common plate margin the common margin is estimated from the locations of both fixed and rotated data points the estimated margin consists of a set of great circle arcs it is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation these techniques are then used to study the uncertainties of reconstructions in the south pacific for the times of anomalies 13 and 18 the best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work however the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigators

evolution transfer and release of magmas and volcanic gases an introduction abstract in this introduction recent progress in the study of the evolution transfer and release of magmas and volcanic gases is briefly reviewed based on discussions at the mag2007 international conference held in taipei taiwan in april 2007 the meeting pooled the diverse expertise of igneous and volcanic gas geochemists this special issue which presents six case studies covering a range of topics from variations in mantle source compositions to the nature of volcanic degassing reflects that diversity

image processing of explosive activity at stromboli abstract a new way to study the dynamics of explosive behaviour at different vents of stromboli volcano is proposed the method is based on computer processing of images taken by an infrared film camera at a rate of 4 pictures per second each picture has been digitized by use of an image scanner with a 75 d p i dots per inch resolution and 256 levels of gray then processed and the total mass of the ejected material has been calculated for each photograph with this procedure the flux of ejected matter throughout every single explosive event has been reconstructed the analysis of trajectories made on adequate samples of ejecta has allowed us to evaluate the ejection velocity the mode of the mass flux distribution and the mean value of the velocities calculated for individual particles quantify the kinetic energy of an explosion vents nos 1 and 3 which remained active after the last effusive eruption of 1985 1986 show several explosive patterns the energy of which is measured the observed flux distributions have been checked both numerically and experimentally by modelling impulsive events the computer simulations of the mass distributions have allowed us to speculate on the possible explosive dynamisms two different models have been used a single impulse model confirms the behaviour actually shown by one of the two vents while the behaviour of the other is better depicted by a cumulative trend of successive impulses in rhythmic sequence moreover through the experimental approach we have had the opportunity to test the overall resolution of the method the observed matter flux distribution has then been associated to the seismic signals in order to correlate the dynamics involved in the explosions with the seismic source mechanisms

pole of rotation analysis of present day juan de fuca plate motion convergence rates between the juan de fuca and north america plates are calculated by means of their relative present day pole of rotation subduction rates as measured normal to the strike of the washington oregon trench are predicted to range from about 40 mm yr under washington to 20 mm yr under oregon an integral part of this analysis is the formulation of a numerical inversion scheme which calculates both the pole of rotation between two plates and the associated errors this method is used to determine the euler pole for pacific juan de fuca this pole can then be vectorially added to previously published poles for north america pacific and hot spot pacific to obtain north america juan de fuca and hot spot juan de fuca respectively the errors associated with these resultant poles are also determined by propagating the errors of the two summed angular velocity vectors with the assumption that hot spots are fixed with respect to a mantle reference frame if is computed that the average absolute velocity of the juan de fuca plate is approximately 15 mm yr thereby making it the slowest moving of the oceanic plates