the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

effusive history of the grande decouverte volcanic complex southern basse terre guadeloupe french west indies from new k ar cassignol gillot ages abstract the grande decouverte volcanic complex gdvc active since at least 0 2 ma is the most recent volcanic complex of the basse terre island guadeloupe lesser antilles arc a detailed geochronological study using the k ar cassignol gillot technique has been undertaken in order to reconstruct the history of effusive activity of this long lived volcanic system twenty new ages permit to suggest that the gdvc experienced at least six main effusive stages from 200 ka to present time to the north of the gdvc the gds grande decouverte soufriere volcano has been active since at least 200 ka and to the south the trmf trois rivieres madeleine field started to be emplaced 100 ka morphological investigations suggest that the whole trmf volcanism was emitted from vents distinct from the gds most probably a large e w fissure network linked to the marie galante rift the mean age of 62 5 ka obtained for the e w madeleine le palmiste alignment suggests that a fissure opening event occurred at that time however whole rock major and trace element signatures are similar for both systems suggesting that a common complex magma plumbing system has fed the overall gdvc we report very young ages for lava flows from the trmf which implies that

geochronology of the tumpangpitu porphyry au cu mo and high sulfidation epithermal au ag cu deposit evidence for pre and postmineralization diatremes in the tujuh bukit district southeast java indonesia the tumpangpitu porphyry and high intermediate sulfidation epithermal deposit is the largest deposit in the tujuh bukit district southeast java indonesia the porphyry resource contains 1 9 billion tonnes 0 45 cu and 0 45 g t au for 28 1 moz au and 19 billion lbs of cu there are an additional 2 1 moz au and 72 9 moz of ag in oxidized high sulfidation epithermal mineralization tumpangpitu is located along a nw striking structural corridor covering an area of 12 5 km that hosts several cu au mo mineralized tonalitic porphyries each with varying degrees of metal enrichment at least eight discrete intrusions spanning the alteration mineralization sequence have been identified at tumpangpitu what is unusual however is the presence of both a premineralization relatively dry volcanic breccia pipe tanjung jahe and a late mineralization diatreme complex associated with a significant large magmatic hydrothermal system tumpangpitu in the same district magmatism mineralization and alteration at tumpangpitu occurred in response to north directed subduction of the indo australian plate beneath the asian continental plate margin the tujuh bukit district is floored by early to late miocene sedimentary and andesitic volcanic rocks volcanic hydrothermal activity at tujuh bukit began with the formation of the weakly altered tanjung jahe diatreme complex u pbzircon ages of 8 78 0 22 8 52 0 21 ma mineralization at tumpangpitu was preceded by the intrusion of a large equigranular dioritic batholith 5 81 0 20 5 18 0 27 ma hydrothermal activity associated with mineralization has been constrained by u pb age determinations from syn to late mineralization porphyries that were emplaced in the early pliocene from 5 40 0 46 to 3 94 0 69 ma high and intermediate sulfidation au ag cu mineralization and associated advanced argillic alteration part of a district scale lithocap has overprinted and significantly upgraded the top of the porphyry orebody ar ar dating of alunite 4 385 0 049 ma and re os dating of molybdenite 4 303 0 018 ma have defined a short time period between the high sulfidation epithermal and porphyry mineralization events this suggests extreme rates of uplift exhumation and erosion in the vicinity of the sunda banda magmatic arc volcanic hydrothermal activity associated with the tumpangpitu diatreme occurred during epithermal mineralization breccia matrix zircon age of 2 7 1 0 ma with systematic errors clasts of high sulfidation state mineralized rocks are a minor but significant component of the diatreme and late stage epithermal veins cutting the diatreme demonstrate an intermineralization timing with respect to epithermal activity in the district implying that epithermal mineralization continued intermittently for 1 to 1 5 m y after porphyry mineralization ceased at tumpangpitu

volcanic evolution of central basse terre island revisited on the basis of new geochronology and geomorphology data twenty six new and seven previous k ar ages obtained on groundmass separates for samples from the axial chain massif guadeloupe f w i associated with geomorphological investigations allow us to propose a new model of the volcanic evolution of the central part of basse terre island the axial chain is composed of four edifices moustique mateliane capesterre and icaque mounts showing coeval activity from 681 12 to 509 10 ka which contradicts a previous hypothesis that flank collapse affected them successively our geomorphological reconstruction shows that the axial chain can be considered as a single large volcano named the southern axial chain volcano sca rather than a succession of several smaller volcanoes it raises questions regarding the formation of a large depression within the sca volcano prior to the construction of the sans toucher volcano between 451 13 and 412 8 ka given presently available evidence a slump affecting the western part of the sca volcano is the most probable scenario to reconcile the complete age dataset and the present day morphology of central basse terre finally our study shows that the sca volcano had a post activity volume of 90 km3 implying a construction rate of 0 5 km3 kyr this value strongly constrains interpretations of magma generation processes throughout the lesser antilles arc

crystals stirred up 2 numerical insights into the formation of the earliest crust on the moon 1 this is the second paper in a two part series examining the fluid dynamics of crystal settling and flotation in the lunar magma ocean in the first paper we develop a direct numerical method for resolving the hydrodynamic interactions between crystals and their feedback on the flow field in magmatic liquid in this paper we use this computational technique to test the leading model for the formation of the earliest crust on the moon the anorthositic lithology of the lunar crust is thought to have been formed by the flotation of buoyant plagioclase crystals at a time when the lunar mantle was still wholly or largely molten this model is appealing from an observational point of view but its fluid dynamical validity is not obvious because 1 plagioclase probably started crystallizing very late i e when the magma ocean was already 80 solidified and 2 a significant portion of the shallow lunar crust consists of almost pure plagioclase 90 vol requiring very efficient plagioclase segregation the goal of this study is to better understand the fluid dynamical conditions that hinder or facilitate crystal settling or flotation our approach complements earlier studies by explicitly linking the petrological and fluid dynamical evolution and by focusing on the effect of increasing crystal fraction we find that crystal settling was probably possible throughout the entire solidification history of the lunar magma ocean as long as crystal sizes were sufficiently large r 1 mm and crystal fraction sufficiently low 13

pyroclastic density currents pdc of the 16 17 august 2006 eruptions of tungurahua volcano ecuador geophysical registry and characteristics abstract tungurahua located in the eastern cordillera of the ecuadorian andes is a 5023 m high active volcano notable for its extreme relief 3200 m steep sides and frequent eruptive cycles from 1999 until 2006 tungurahua experienced short periods of low to moderate strombolian activity characterized by fire fountaining explosions frequent ash falls and debris flows and no pdc events without warning tungurahua initiated pdc activity on 15 16 july 2006 which became more intense on the night of 16 17 august 2006 which is the focus of this study continuous monitoring of tungurahua has employed seismic both short period and broadband bb instruments so 2 gas emission cospec and doas and geodetic methods edm tilt meters and gps in addition to thermal imagery airborne and ground based acoustic flow monitors afm installed to monitor lahar activity were important for detecting pdc events acoustic signals were monitored at riobamba 40 km to the sw as well as by infrasound sensors at tungurahua s bb seismic stations based on geophysical parameters visual observations and pdc deposit characteristics four phases of distinct eruptive activity are recognized during the 16 17 august episode phase i 08h37 to 21h13 of 16 aug local time experienced low to moderate strombolian activity with occasional high energy impulsive bursts and small pdc phase ii 21h13 16 aug to 00h12 17 aug was characterized by a number of discrete events with high amplitude seismo acoustic signals followed by the generation of larger pdc that overran monitoring stations and had velocities of 30 33 m s after midnight phase iii 00h12 to 01h14 saw an intense period of unrelenting eruptive activity corresponding to the episode s greatest energy release it was characterized by subplinian activity accompanied by a series of high energy outbursts and constant low frequency jetting that together formed a continuous plume it was during this phase that the largest pdc were produced reaching the surrounding river valleys phase iv after 01h14 followed the cessation of the paroxysmal eruption but witnessed many granular pdc generated by degassed lava spill outs from the crater that developed lobe and channel morphology on the cone s lower flanks hours later a blocky lava flow issued from the crater during these episodes more than 30 pdc events were detected the majority being small flows that remained high on the cone the two largest pdc occurred after midnight probably generated by fountain collapse their descent down the cone s upper steep flanks 28 and 2 4 km in length favored air entrainment resulting in pdc with greater fluidity these flows had volumes of 9 to 17 10 6 m 3 and produced widespread but relatively thin 1 2 m thick normally graded deposits at their distal ends the character and evolution of the pdc activity apparently reflect decreasing volatile contents of the magma and a diminishing magma supply

the ad 1300 1700 eruptive periods at tungurahua volcano ecuador revealed by historical narratives stratigraphy and radiocarbon dating tungurahua is a frequently active and hazardous volcano of the ecuadorian andes that has experienced pyroclastic flow forming eruption in 1773 1886 1916 18 and 2006 08 earlier eruptions in late pre hispanic and early colonial times have remained poorly known and are debated in the literature to reconstruct the eruptive chronology in that time interval we examine relevant historical narratives recently found in sevilla spain and rome italy and we combine stratigraphic field constraints with 22 new radiocarbon age determinations results show that pyroclastic flow forming eruptions and tephra falls took place repeatedly since similar to 700 c 14 yr bp when the tungurahua region was already populated radiocarbon ages averaging around 625 yr bp reveal a period of notable eruptive activity in the 14th century late integration cultural period the associated andesitic eruptions produced ash and scoria falls of regional extent and left scoria flow deposits on the western flanks of the edifice the fact that tungurahua was known by the puruhas indians as a volcano at the time of the spanish conquest in 1533 perhaps refers to these eruptions a group of ages ranging from 380 to 270 yr bp is attributed to younger periods of activity that also predates the 1773 event and calibration results yield eruption dates from late 15th to late 17th centuries i e inca and early colonial periods the historical narratives mention an early colonial eruption between the spanish conquest and the end of the 16th century followed by a distinct eruptive period in the 1640s the descriptions are vague but point to destructive eruptions likely accompanied by pyroclastic flows the dated tephras consist of andesitic scoria flow deposits and the contemporaneous fallout layers occur to the west these findings reveal that the eruption recurrence rate at tungurahua is at least one pyroclastic flow forming event per century since the 13th century and the occurrence of such eruptions in 2006 08 is thus fully consistent with the rate inferred for the past seven centuries in addition historical chronicles suggest that a notch opened in the crater margin during the 1640 decade as has occurred repeatedly in the past millennium at tungurahua such small volume collapse events represent a previously unrecognized source of hazards which deserve special attention for risk assessment purposes in the context of the currently ongoing eruption c 2008 elsevier b v all rights reserved

ecuadorian andes volcanism a review of late pliocene to present activity abstract this article provides background information about ecuador s general volcanic setting as an introduction to the numerous articles in this special issue of the journal dealing with the many facets of ecuadorian volcanism it also provides a brief description of ecuador s physiography as well as its geodynamic setting a comprehensive bibliography is included as a guide to further reading the ecuadorian andes contain at least 20 volcanoes active during the holocene several reach altitudes of 5000 to 6000 m and are glacier clad eleven have erupted in pre columbian and historic times and have greatly affected local populations and caused fatalities

eruptive chronology of tungurahua volcano ecuador revisited based on new k ar ages and geomorphological reconstructions abstract this study focuses on the evolution through time of tungurahua volcano ecuador and provides new information regarding its history eighteen new k ar ages constrain its construction and the activity of its three successive edifices we show that the volcano is much younger than expected indeed the older edifice activity only began around 293 10 ka and ended at 79 3 ka after 50 ka of quiescence the second edifice started growing at 29 2 ka after a major sector collapse and itself collapsed at 3 ka since then the third edifice filled the amphitheatre and is still active together with numerical reconstructions of the morphology of the three edifices flanks before erosion these new ages allow us to quantify the magmatic productivity rates during their construction from 0 6 0 3 and 0 9 0 2 km3 ka for the two older edifices to 2 5 1 0 km3 ka for the youngest as well as an erosion rate of 0 2 0 1 km3 ka occurring since the end of tungurahua i construction major and trace element contents of lavas from the three edifices display rather similar trends combined with our new ages the magmatic signature through time does not seem to have been significantly affected either by the sector collapses experienced by the volcano or by changes of the deep magmatic source finally our results show that the k ar dating method by the unspiked cassignol gillot technique performed on groundmass can be successfully applied to lava flows older than the holocene while the uncertainties related to younger units can prevent an accurate age determination particularly this method can be applied to quaternary volcanoes from the ecuadorian arc with many of them remaining without knowledge of the timing of their past activity

crystal settling in a vigorously converting magma chamber there has been much debate concerning the mechanism of fractional crystallization in magma chambers the traditional hypothesis of crystal settling has been widely replaced by the concept of in situ crystallization coupled with compositional convection observations from layered intrusions however are equivocal1 4 doubts have been raised about crystal settling on theoretical grounds because convective velocities in magma chambers are often much greater than the crystal settling velocities predicted by stokes law5 but there has been no experimental study of crystal settling in such vigorous convection here we present physical considerations and laboratory experiments which show that the phenomenon of particle settling in these conditions can be accounted for by a simple theory application of this theory to crystal settling in magma chambers suggests that crystal settling may be an efficient differentiation mechanism at least in basaltic magma chambers despite large convective velocities

solidus and liquidus profiles of chondritic mantle implication for melting of the earth across its history abstract we investigated the melting properties of a synthetic chondritic primitive mantle up to core mantle boundary cmb pressures using laser heated diamond anvil cell melting criteria are essentially based on the use of x rays provided by synchrotron radiation we report a solidus melting curve lower than previously determined using optical methods the liquidus curve is found between 300 and 600 k higher than the solidus over the entire lower mantle at cmb pressures 135 gpa the chondritic mantle solidus and liquidus reach 4150 150 k and 4725 150 k respectively we discuss that the lower mantle is unlikely to melt in the d layer except if the highest estimate of the temperature profile at the base of the mantle which is associated with a very hot core is confirmed therefore recent suggestions of partial melting in the lowermost mantle based on seismic observations of ultra low velocity zones indicate either 1 a outer core exceeding 4150 k at the cmb or 2 the presence of chemical heterogeneities with high concentration of fusible elements our observations of a high liquidus temperature as well as a large gap between solidus and liquidus temperatures have important implications for the properties of the magma ocean during accretion not only complete melting of the lower mantle would require excessively high temperatures but also below liquidus temperatures partial melting should take place over a much larger depth interval than previously thought in addition magma adiabats suggest very high surface temperatures in case of a magma ocean that would extend to more than 40 gpa as suggested by siderophile metal silicate partitioning data such high surface temperature regime where thermal blanketing is inefficient points out to a transient character of the magma ocean with a very fast cooling rate

tectonics and geology of porphyry cu au deposits along the eastern sunda magmatic arc indonesia the discovery of the world class cu au deposit at tumpangpitu east java 1 9 gt 0 45 cu 0 45 g t au has reinforced the eastern sunda arc as a significant metallogenic belt that is highly promising for the discovery of major porphyry deposits the arc hosts three premier porphyry cu au deposits at batu hijau elang and tumpangpitu 300 t au and 5 mt cu these giant porphyry deposits are confined to the eastern segment of the eastern sunda arc eastern java to sumbawa constructed on island arc crust where the roo rise is being subducted by contrast major epithermal low sulfidation dominant deposits associated with poorly endowed porphyry prospects occur along the western segment of the arc western java developed on thick continental crust on the southern margin of sundaland associated with subduction of typically thin indian oceanic crust porphyry cu au deposits of the eastern sunda arc are spatially associated with small nested dioritic to tonalitic intrusive complexes with low k calc alkaline to weakly alkaline signatures hypogene alteration and mineralization developed during three main temporally and spatially overlapping events termed early transitional and late the hypogene cu au mineralized zone of the giant deposits forms a bell shaped body of 0 3 cu shell greater than 1 km in both diameter and vertical extent that is centered on multiphase elongate pencil like tonalite intrusions 200 to 1 200 m in diameter and 2 km vertical extent large 20 km 2 lithocaps associated with high sulfidation epithermal systems and postporphyry mineralization diatreme breccias characterize productive porphyry districts at batu hijau elang and tumpangpitu the known porphyry deposits with metal content exceeding 300 t au and 5 mt cu all formed later than 5 ma suggesting an important change in the metallogeny of the arc at this time these giant porphyry deposits formed relatively rapidly typically over a period of 30 to 860 k y during the pliocene pleistocene between 5 and 2 5 ma porphyry deposits along the eastern sunda arc display characteristic geologic features similar to porphyry deposits elsewhere significant differences include low k systems dominance of hydrothermal biotite over k feldspar early intense actinolite alteration associated with ore widespread shreddy chlorite after secondary biotite narrow quartz sericite pyrite alteration zones lack of development of chalcopyrite veins large overlying lithocaps and abundant chalcocite bornite covellite veinlets associated with late overprinting epithermal systems hydrothermal oligoclase is also more abundant than k feldspar in the central biotite magnetite zones

thermodynamics of silicate liquids in the deep earth article i nfo we discuss the geophysical implications of recent first principles molecular dynamics simulations of mgsio3 liquid in particular the findings that the gruneisen parameter increases by a factor of three on two fold compression and the predicted melting curve the predicted melting temperature of mgsio3 perovskite at 136 gpa is 5400 600 k which if lowered by freezing point depression by 1300 k yields a mantle solidus temperature of 4100 k identical to recent estimates of the temperature at the base of the mantle we argue on this basis that partial melting at the base of the mantle is plausible while mgsio3 perovskite is denser than the isochemical liquid reasonable values of iron partitioning and bulk iron contents leads to the conclusion that a melt at the base of the multi component mantle would be denser than its surroundings and therefore buoyantly stable we predict the bulk sound velocity of this melt to be 10 9 km s 1 less than that of ultra low velocity zones and consistent with the explanation of these regions as being due to partial melting the increase on compression of the gruneisen parameter in the liquid state yields isentropes that are much hotter than previously thought a potential temperature of only 2450 k is sufficient to melt the entire mantle and a magma ocean will begin crystallization at mid mantle depths rather than at the base of the mantle

evolution of the late pleistocene mojanda fuya fuya volcanic complex ecuador by progressive adakitic involvement in mantle magma sources the mojanda fuya fuya volcanic complex consists of two nearby volcanoes mojanda and fuya fuya the older one mojanda volcano 0 6 to 0 2 ma was first constructed by andesites and high silica andesites forming a large stratovolcano lower mojanda this edifice was capped by a basaltic andesite and andesitic cone upper mojanda which collapsed later to form a 3 km wide summit caldera after large phreatomagmatic eruptions the lower fuya fuya edifice was constructed by the extrusion of viscous si rich andesitic lavas and dacitic domes and the emission of a thick sequence of pyroclastic flow and fallout deposits which include two voluminous rhyolitic layers an intermediate construction phase at fuya fuya is represented by a mainly effusive cone andesitic in composition san bartolo edifice the construction of which was interrupted by a major sector collapse in the late pleistocene finally a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre forming the upper fuya fuya volcanic centre this paper shows that the general evolution from an effusive to an explosive eruptive style is related to a progressive adakitic contribution to the magma source although all the rocks of the complex are included in the medium k field of continental arcs the fuya fuya suite 61 75 wt sio2 shows depletion in y and hree and high sr y and la yb values compared to the less silicic mojanda suite 55 66 5 wt sio2 the mojanda calc alkaline suite was generated by partial melting of an adakite metasomatised mantle source that left a residue with 2 garnet followed by fractional crystallization of dominant plagioclase pyroxene olivine at shallow intra crustal depths for fuya fuya geochemical and mineralogical data suggest either 1 partial melting of a similar metasomatised mantle with more garnet in the residue 4 followed by fractional crystallization involving plagioclase amphibole and pyroxene or 2 mixing of mafic mantle derived magma from the mojanda suite and slab melts followed by the same fractional crystallization process

tungurahua volcano ecuador structure eruptive history and hazards abstract tungurahua one of ecuador s most active volcanoes is made up of three volcanic edifices tungurahua i was a 14 km wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series tungurahua ii mainly composed of acid andesite lava flows younger than 14 000 years bp was partly destroyed by the last collapse event 2955 90 years ago which left a large amphitheater and produced a 8 km3 debris deposit the avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for 15 km a large lahar formed during this event which was followed in turn by dacite extrusion southwestward the damming of the chambo valley by the avalanche deposit resulted in a 10 km long lake which was subsequently breached generating another catastrophic debris flow the eruptive activity of the present volcano tungurahua iii has rebuilt the cone to about 50 of its pre collapse size by the emission of 3 km3 of volcanic products two periods of construction are recognized in tungurahua s iii history from 2300 to 1400 years bp high rates of lava extrusion and pyroclastic flows occurred during this period the magma composition did not evolve significantly remaining essentially basic andesite during the last 1300 years eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition andesite dacite and end with more basic andesite lava flows or crater plugs this pattern is observed in the three historic eruptions of 1773 1886 and 1916 1918 given good age control and volumetric considerations tungurahua iii growth s rate is estimated at 1 5 106 m3 year over the last 2300 years although an infrequent event a sector collapse and associated lahars constitute a strong hazard of this volcano given the 3000 m relief and steep slopes of the present cone a future collapse even of small volume could cover an area similar to that affected by the 3000 year old avalanche the more frequent eruptive episodes of each century characterized by pyroclastic flows lavas lahars as well as tephra falls directly threaten 25 000 people and the agoyan hydroelectric dam located at the foot of the volcano

physical state of the very early earth abstract the earliest surface environment of the earth is reconstructed in accordance with the planetary formation theory formation of an atmosphere is an inevitable consequence of earth s formation the atmosphere near the close of accretion is composed of 200 300 bars of h2 and h2o and several tens of bars of co and co2 either by the blanketing effect of the proto atmosphere or heating by large planetesimal impacts a magma ocean is formed during accretion we can distinguish three stages for the thermal evolution of the magma ocean and proto crust stage 0 is characterized by a super liquidus or completely molten regime near the surface at this stage the surface of the earth is covered by a super liquidus magma ocean no chemical differentiation is expected during this stage once the energy flux released by planet formation decreases to the 200 w m2 level the super liquidus magma ocean then disappears within a time interval of 1 m y this is the transition from stage 0 to 1 stage 1 is characterized by a partially molten magma ocean in the magma ocean consisting of 20 30 partial melt heat transport is controlled by melt solid separation a type of compositional convection rather than thermal convection chemical differentiation of the mantle mainly occurs in this stage once the energy flux drops to the 160 w m2 level more than 90 of water vapor in the proto atmosphere condense to form the proto oceans several tens of bars of co and co2 remain in the atmosphere just after formation of the oceans water oceans are occasionally evaporated by large impacts after each such event recondensation of the ocean takes several hundred years although the surface is covered by a chilled proto crust it is short lived because of extensive volcanic resurfacing activity as well as meteorite impacts resurfacing this stage ends when the energy flux drops to 0 1 1 w m2 level the duration time of stage 1 is estimated to be several hundred million years the best estimate is about 400 m y stage 2 is characterized by solid state convection this stage continues to the present day one of the most important change on the proto earth is the transition from stage 1 to 2 which occurs several hundred million years after the earth formation long lived crust is formed only after this transition

low sulfidation epithermal gold deposits in java indonesia characteristics and linkage to the volcano tectonic setting abstract java island a part of the eastern sunda magmatic arc is becoming an emerging part of the region for gold and copper endowment a total of 15 4 of 1044 tons of java gold endowment is originated from low sulfidation ls epithermal deposit type as a matter of fact the mineralization type that has given a significant credit to the gold endowment of java is intriguing to be geologically evaluated this paper is aimed to document some data information and summarizing the characteristics of selected 10 ls epithermal gold deposits in java including cibaliung kerta cisoka cikidang pongkor gunung peti cihar cijaringao petungkriyono and dalang turu as well as discussing their relationship with the volcano tectonic setting the formation of 10 ls epithermal deposits took place in the time span between early miocene and pleistocene which is coincident with the mio pliocene magmatic arc activities and even probably continue on to pleistocene all deposits show typical ls epithermal characteristics i e quartz veins with crustiform colloform textures relatively high au ag au ratio the cibaliung veins seem to have genetic linkage with the dioritic intrusion and diatreme phreatomagmatic breccias as well as with rhyolitic domes in kerta the veins exhibit close relationship with the rhyolitic rhyodacitic plugs the pongkor and cisoka veins are genetically related to rhyodacite dome plugs within caldera setting which is controlled by extensional structures cikidang and gunung peti may be related with the andesitic sub volcanic plugs cihar and cijaringao might be related to andesite dykes petungkriyono is linked with andesite and dacite plugs as well as diatreme breccia formed in an extensional basin with few calderas developed the dalang turu has been considered having a close linkage with andesite plug at extensional structures in relation to the volcano tectonic setting six deposits cibaliung kerta pongkor cisoka cikidang and gunung peti are obviously emplaced in bogor trough a back arc rift basin related to the mio pliocene arc the presence of rhyolitic to dacitic magmatism that could be related with caldera systems e g in kerta pongkor cisoka and the common association with subvolcanic plugs e g kerta are evidences of their linkage to the back arc rift basin setting the basaltic rocks in close proximity with the rhyolite rhyodacite as identified in cibaliung and kerta suggest the bimodal volcanism which is commonly occurred in the back arc rifting environment exploration for ls epithermal prospects therefore should be targeted along the back arc rifting basins e g bogor trough preferably over the areas with bimodal magmatism the bogor trough is reported extending to east java as kendeng trough basin which is also reasonably targeted for exploration

pyroclastic flow erosion and bulking processes comparing field based vs modeling results at tungurahua volcano ecuador pyroclastic density currents pdcs are high temperature and high velocity mixtures that threaten populations in the vicinity of many active volcanoes deciphering the cause of their remarkable mobility is essential for volcanic hazard analysis but remains difficult because of the complex processes occurring within the flows here we investigate the effect of bulking on dense pdc mobility by means of a double approach first we estimate the amount of material incorporated into scoria flows emplaced during the august 2006 eruption of tungurahua volcano ecuador for this we carry out a detailed analysis of 3d corrected digital images of well exposed scoria flow deposits componentry analysis indicates that pdc bulking occurs principally on the steep 25 upper slope of the volcano and the deposits typically comprise 40 50 wt of non juvenile i e accessory and accidental material secondly we develop a simple stress related grain by grain equation of erosion combined with two simple depth averaged geophysical mass flow models that compare the bulking mechanism to a non fluidized and a fluidized flow two behaviors based on coulomb and plastic rheologies are used to reproduce on a first order basis the 2006 tungurahua pdcs cross check comparisons between these modeled cases and the erosion pattern inferred from field based data allow us to evaluate the accuracy of our modeling assumptions regardless of the rheological regime the pdc induced erosion pattern of the 2006 tungurahua eruption can only be reproduced by fluctuations of the flow s basal shear stress during emplacement such variations are controlled by flow thinning thickening processes notably through the formation of a thick non erosive flow body that pushes a thin frictional erosive front during pdc emplacement the input volume of juvenile material as well as the thickness of the erodible layer available prior to the eruption are additional key parameters our work highlights complexities in pdc erosion and bulking processes that deserve further study in terms of hazard assessment our findings reveal that incorporation and bulking translate into increased flow mobility i e the augmented flow mass enhances both flow velocity and runout distance up to 20 these outcomes should be considered closely for hazard analysis at many other andesitic volcanoes worldwide where similar pdc events are common

construction and destruction rates of volcanoes within tropical environment examples from the basse terre island guadeloupe lesser antilles abstract in order to better constrain the construction and the erosion rate affecting the volcanic island of basse terre island guadeloupe f w i an enlarged k ar age dataset has been combined with reconstruction of the paleo topography two different methods of interpolation of the present topography have been cross checked to better support the erosion rates obtained and their associated uncertainties the present study focusses on the monts caraibes volcanoes and on the main geomorphic feature of the piton de bouillante volcano the beaugendre valley the monts caraibes volcanoes were constructed in 83 kyr at a rate of 0 12 0 04 km 3 kyr during the last 450 kyr they have experienced an erosion rate of 610 550 t km 2 yr in the piton de bouillante volcano eleven new k ar ages have been obtained constraining the duration of its volcanic activity between 880 14 and 712 12 ka and involving a construction rate of 0 70 0 20 km 3 kyr for this volcano an erosion rate of 1220 700 t km 2 yr has been obtained for the last 700 kyr our study also shows based on the contemporaneity of the ages in the entire beaugendre valley added to the mean erosion rate of 1350 550 t km 2 yr that the flank collapse hypothesis cannot explain the formation of this valley finally the similarity of the erosion rates computed for different locations of the basse terre island shows that the time integrated erosion appears independent to the trade wind effect and suggests that the barrier effect due to the relief is not present here

dem based reconstruction of southern basse terre volcanoes guadeloupe archipelago fwi contribution to the lesser antilles arc construction rates and magma production abstract we combine radiometric ages and geomorphologic investigations to quantify the relief creation of well preserved volcanic surfaces applied to the volcanoes of southern basse terre guadeloupe lesser antilles arc the last 650 ka volcanic evolution of this island has been modeled using ten main stages constrained by k ar ages previously obtained on lava flows and domes from the volcanic massifs of the axial chain 1000 435 ka and from the grande decouverte volcanic complex 250 ka present based on the construction of a 250 000 point database inferred from the analysis of the guadeloupe digital elevation model 3d reconstructions of the successive volcanic stage landforms were calculated and the correlated geochronological maps drawn using arcgis software volumes and rates of construction were computed for each time span separating these ten stages the average construction rates calculated here are 2 4 0 3 10 4 km 3 yr for the last million years 0 9 0 3 10 4 km 3 yr for the last 100 ka and 0 9 0 2 10 4 km 3 yr for the last 15 ka although basse terre volcanism is characterized by a marked dominance of effusive products our estimates should be considered as minimum values because the material that went into the sea during explosive events was not taken into account however we note that a relatively high construction rate of 4 5 1 3 10 4 km 3 yr has been obtained for the icaques volcano which was emplaced during the 630 600 ka time interval within the depression formed by the first large scale flank collapse having affected southern basse terre the sudden release of the lithostatic load induced by this mass wasting event could explain this value which is significantly higher than the average value of 0 8 0 1 10 4 km 3 yr obtained for the last 650 ka finally the comparison with other analogous volcanic massifs from islands and continental arcs points to a relatively low magmatic production for southern basse terre which could be tentatively related to the relatively slow subduction rate of the atlantic plate

energetics of a two phase model of lithospheric damage shear localization and plate boundary formation summary the two phase theory for compaction and damage proposed by bercovici et al 2001a j geophys res 106 8887 8906 employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a model for damage void generation and microcracking and a continuum description of weakening failure and shear localization here we examine further variations of the model which consider 1 how interfacial surface energy when averaged over the mixture appears to be partitioned between phases 2 how variability in deformational work partitioning greatly facilitates localization and 3 how damage and localization are manifested in heat output and bulk energy exchange microphysical considerations of molecular bonding and activation energy suggest that the apparent partitioning of surface energy between phases goes as the viscosity of the phases when such partitioning is used in the two phase theory it captures the melt compaction theory of mckenzie 1984 j petrol 25 713 765 exactly as well as the void damage theory proposed in a companion paper ricard bercovici submitted calculations of 1 d shear localization with this variation of the theory still show at least three possible regimes of damage and localization at low stress is weak localization with diffuse slowly evolving shear bands at higher stress strong localization with narrow rapidly growing bands exists and at yet higher shear stress it is possible for the system to undergo broadly distributed damage and no localization however the intensity of localization is strongly controlled by the variability of the deformational work partitioning with dilation rate represented by the parameter f or 1 extreme localization is allowed with sharp profiles in porosity weak zones nearly discontinuous separation velocities and effectively singular dilation rates finally the bulk heat output is examined for the 1 d system to discern how much deformational work is effectively stored as surface energy in the high stress distributed damage cases heat output is reduced as more interfacial surface energy is created yet in either the weak or strong localizing cases the system always releases surface energy regardless of the presence of damage or not and thus slightly more heat is in fact released than energy is input through external work moreover increased levels of damage represented by the maximum work partitioning f make the localizing system release surface energy faster as damage enhances phase separation and focusing of the porosity field thus yielding more rapid loss of net interfacial surface area however when cases with different levels of damage are compared at similar stages of development say the peak porosity of the localization it is apparent that increased damage causes smaller relative heat release and retards loss of net interfacial surface energy the energetics and energy partitioning of this damage and shear localization model are applied to estimating the energy costs of forming plate boundaries and generating plates from mantle convection

geomorphic evolution of the piton des neiges volcano reunion island indian ocean competition between volcanic construction and erosion since 1 4 ma abstract reunion island indian ocean is a volcanic complex whose eruptive history was dominated by the activity of two main edifices piton des neiges pn and piton de la fournaise pf volcanoes the tropical climate induces erosion processes that permanently compete with volcanic constructional processes exposed to the trade winds and associated heavy rainfalls the northeastern part of the island exhibits the most complex morphological evolution geomorphological analysis performed on a 50 m dem and associated to new k ar ages has clarified the overall history of pn volcano each massif is assigned to one of the main building stages of the edifice in addition the arrangement of these different massifs reveals that the eruptive phases have led to successive relief inversions and successive excavations of large central depressions in the proximal area as a result the younger massifs are always located in more proximal parts of the volcano the youngest being close to the edifice center in distal areas early lava flows were channeled into valleys incised along the massif boundaries leading to a more complex geochronological organization quantitative study of the dissection of pn volcano allows us to propose a minimum eroded volume of 101 44 and 105 41 km3 for the mafate and cilaos cirques depressions respectively during the last 180 kyr and a minimum average long term erosion rate of 1 2 0 4 km3 ka this leads us to estimate the removed volume during the whole history of pn volcano 1000 km3 as equivalent to the volume of the deposits identified on the submarine flanks of piton des neiges volcano therefore as regressive erosion appears to be the prevailing geomorphic process during the whole pn history it questions the presence of major flank collapses younger than 1 4 ma on this volcano erosion processes have largely been neglected in recent models but our study emphasizes them as a key component of landscape development and a major process in the morphological evolution of reunion island that has to be fully integrated in future studies

new radiometric and petrological constraints on the evolution of the pichincha volcanic complex ecuador fieldwork radiometric 40ar 39ar and 14c ages and whole rock geochemistry allow a reconstruction of eruptive stages at the active mainly dacitic pichincha volcanic complex pvc whose eruptions have repeatedly threatened quito most recently from 1999 to 2001 after the emplacement of basal lavas dated at 1100 to 900 ka the eruptive activity of the old rucu pichincha volcano lasted from 850 ka to 150 ka before present bp and resulted in a 15 20 km wide edifice which comprises three main building stages 1 a lower stratocone lower rucu 160 km3 in volume developed from 850 to 600 ka 2 this edifice was capped by a steeper sided and less voluminous cone the upper rucu 40 50 km3 the history of which started 450 430 ka ago and ended around 250 ka with a sector collapse 3 a smaller 8 10 km3 but more explosive edifice grew in the avalanche amphitheatre and ended rucu pichincha s history about 150 ka ago the guagua pichincha volcano ggp was developed from 60 ka on the western flank of rucu with four growth stages separated by major catastrophic events 1 from 60 to 47 ka a basal effusive stratocone developed terminating with a large ash and pumice flow event 2 this basal volcano was followed by a long lasting dome building stage and related explosive episodes the latter occurring between 28 30 and 22 23 ka these first two stages formed the main ggp 30 km3 a large part of which was removed by a major collapse 11 ka bp 3 sustained explosive activity and viscous lava extrusions gave rise to a new edifice toaza 4 5 km3 in volume which in turn collapsed around 4 ka bp 4 the ensuing amphitheatre was partly filled by the 1 km3 cristal dome which is the historically active centre of the pichincha complex the average output rate for the whole pvc is 0 29 km3 ka nevertheless the chronostratigraphic resolution we obtained for lower rucu pichincha and for the two main edifices of guagua pichincha main ggp and toaza leads to eruptive rates of 0 60 0 65 km3 ka during these construction stages these output rates are compared to those of other mainly dacitic volcanoes from continental arcs our study also supports an overall sio2 and large ion lithophile elements enrichment as the pvc develops in particular distinctive geochemical signatures indicate the involvement of a new magma batch at the transition between rucu and guagua at the ggp the same phenomenon occurs at each major collapse event marking the onset of the ensuing magmatic stage since the 11 ka bp collapse event this magmatic behaviour has led to increasingly explosive activity four explosive cycles of between 100 and 200 years long have taken place at the cristal dome in the past 3 7 ka and repose intervals between these cycles have tended to decrease with time as a consequence we suggest that the 1999 2001 eruptive period may have initiated a new eruptive cycle that might pose a future hazard to quito 2 million inhabitants

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

potassium rich volcanic rocks of the muriah complex java indonesia products of multiple magma sources abstract the extinct pleistocene volcano muriah situated behind the main pleistocene recent sunda magmatic arc in north central java has erupted at least two contrasted groups of lavas one group forms a well defined compositional series anhydrous series from leucite basanite to tephritic phonolite with olivine and tschermakitic clinopyroxene the main phenocrysts the other group the hydrous series includes compositionally variable tephrites and high k andesites with common plagioclase biotite and amphibole lavas of the anhydrous series are much richer in lil trace elements than the most potassic lavas of neighbouring active volcanoes but relative hfs element enrichment is less pronounced ree patterns have almost constant slopes from la 250 600 times chondrites to yb 5 10 times chondrites while those of lavas of active centres are less light enriched and show flattening in the heavy ree anhydrous series initial 87sr 86sr ratios 0 7043 0 7046 are lower than those of active centres 0 7047 0 7053 hydrous series lavas are intermediate in all these geochemical characteristics the most mafic a series leucite basanite with mg mg fe 2 0 69 140 ppm ni and 620 ppm cr was probably derived from the primary magma for the series by fractionation of only 5 wt olivine its ree pattern suggests derivation from a garnet bearing source experiments on this basanite with up to 10 olivine and 20 orthopyroxene added and in the presence of h2o and h2o co2 mixtures have shown that for all but very high magma water contents the olivine and garnet liquidus fields are widely separated by fields of phlogopite and clinopyroxene there is no liquidus field of orthopyroxene hence if magma production involved an equilibrium melting process alone the most probable sources are of garnet bearing phlogopite clinopyroxenite type alternatively this magma may represent the end product of interaction between a low k basanite magma from a garnet lherzolite source in the asthenosphere and a phlogopite bearing lherzolite zone in the lower lithosphere its production was probably related to crustal doming and extension superimposed on the dominant subduction regime hydrous series magmas may have resulted from mixing between anhydrous series magmas and high k calc alkaline basaltic to andesitic magmas more directly related to subduction processes

radiometric dating of three large volume flank collapses in the lesser antilles arc abstract it is now recognised that flank collapses are a recurrent process in the evolution of the lesser antilles arc volcanoes large magnitude debris avalanche deposits have been identified off the coast of dominica martinique and st lucia with associated volumes up to 20 km 3 deplus c le friant a boudon g komorowski j c villemant b harford c segoufin j cheminee j l 2001 submarine evidence for large scale debris avalanches in the lesser antilles arc earth planet sci lett 192 145 157 we present new radiometric dating of three major events using the k ar cassignol gillot technique in the qualibou depression of st lucia a collapse has been constrained by dome emplacement prior to 95 2 ka in dominica where repetitive flank collapse events have occurred le friant a boudon g komorowski j c deplus c 2002 l ile de la dominique a l origine des avalanches de debris les plus volumineuses de l arc des petites antilles c r geoscience 334 235 243 the plat pays event probably occurred after 96 2 ka inside the depression caused by this event scotts head which is interpreted as a proximal megabloc from the subsequent soufriere avalanche event has been dated at 14 1 ka providing an older bound for this event on martinique three different domes within the carbets structure dated at 337 5 ka constrain the age of this high magnitude event finally these results obtained from three of the most voluminous flank collapses provide constraints to estimate the recurrence of these events which represent one of the major hazards associated with volcanoes of the lesser antilles arc

sangay volcano ecuador structural development present activity and petrology abstract sangay 5230 m the southernmost active volcano of the andean northern volcanic zone nvz sits 130 km above a 32 ma old slab close to a major tear that separates two distinct subducting oceanic crusts southwards quaternary volcanism is absent along a 1600 km long segment of the andes three successive edifices of decreasing volume have formed the sangay volcanic complex during the last 500 ka two former cones sangay i and ii have been largely destroyed by sector collapses that resulted in large debris avalanches that flowed out upon the amazon plain sangay iii being constructed within the last avalanche amphitheater has been active at least since 14 ka bp only the largest eruptions with unusually high plinian columns are likely to represent a major hazard for the inhabited areas located 30 to 100 km west of the volcano however given the volcano s relief and unbuttressed eastern side a future collapse must be considered that would seriously affect an area of present day colonization in the amazon plain 30 km east of the summit andesites greatly predominate at sangay there being few dacites and basalts in order to explain the unusual characteristics of the sangay suite highest content of incompatible elements except y and hree of any nvz suite low y and hree values in the andesites and dacites and high nb la of the only basalt found a preliminary five step model is proposed 1 an enriched mantle in comparison with an morb source or maybe a variably enriched mantle at the site of the sangay prior to quaternary volcanism 2 metasomatism of this mantle by important volumes of slab derived fluids enriched in soluble incompatible elements due to the subduction of major oceanic fracture zones 3 partial melting of this metasomatized mantle and generation of primitive basaltic melts with nb la values typical of the nvz which are parental to the entire sangay suite but apparently never reach the surface and subordinate production of high nb la basaltic melts maybe by lower degrees of melting at the periphery of the main site of magma formation that only infrequently reach the surface 4 afc processes at the base of a 50 km thick crust where parental melts pond and fractionate while assimilating remelts of similar basaltic material previously underplated producing andesites with low y and hree contents due to garnet stability at this depth 5 low pressure fractionation and mixing processes higher in the crust both an enriched mantle under sangay prior to volcanism and an important slab derived input of fluids enriched in soluble incompatible elements two parameters certainly related to the unique setting of the volcano at the southern termination of the nvz apparently account for the exceptionally high contents of incompatible elements of the sangay suite in addition the low cr ni values of the entire suite another unique characteristic of the nvz also requires unusual fractionation processes involving cr spinel and or clinopyroxene either in the upper mantle or at the base of the crust

numerical models of mantle lithosphere weakening erosion and delamination induced by melt extraction and emplacement continental rifting caused by extension and heating from below affects the lithosphere or cratons in various ways volcanism and melt intrusions often occur along with thinning weakening and even breaking lithosphere although mechanical necking models of the lithosphere are often applied the aspects of melting and the implications due to melt transport and emplacement at shallower depths are not well understood a two phase flow approach employing melt extraction and shallow emplacement associated with thermal weakening is developed and compared with observations the results of this comparison indicate the importance of partial melts and an asthenospheric magma source for increasing the rising rate of the lithosphere asthenosphere boundary during extension thermo mechanical physics of visco plastic flow is approximated using the finite difference method with eulerian formulation in 2d the conservation of mass momentum and energy equations are solved for a multi component crust mantle and two phase melt matrix system rheology is temperature and stress dependent in consideration of depletion and enrichment melting and solidification are controlled by a simplified linear binary solid solution model melt is extracted and emplaced in predefined depth regions emplacement zones in the lithospheric mantle and crust the compaction boussinesq approximation was applied its validity was tested against the full compaction formulation and found fully satisfactory for the case of sublithospheric melting models a simple model guided by the geodynamic situation of the rwenzori region typically results in updoming asthenosphere with melt assisted erosion of the lithosphere s base even with a conservative approach for a temperature anomaly melting alone doubles the lithospheric erosion rate in comparison with a model without melting with melt extraction and intrusion lithospheric erosion and upwelling of the lithosphere asthenosphere boundary speeds up by a factor 3 4 in an extreme case delamination may occur if weakening fully decouples a hanging mantle block models with an emplacement zone of up to approximately 70 km agree well with observations especially for a concept based on seismological and petrological data

simultaneous melting and compaction in deformable two phase media summary melt generation and extraction are typically modelled using the two phase equations developed by mckenzie or scott and stevenson various approximations are often made to simplify the problem which may lead to some unphysical results e g thermodynamically inconsistent conditions of melting and unrealistic porosity profiles we discuss a generalized version of the set of equations introduced by bercovici et al that allows for mass transfer between the two phases in a single component system and consider a self consistent set of equations in our description the two phases solid and melt are submitted to individual pressure fields whose difference is related to the surface tension at the interfaces changes in porosity and the melting rate a kinetic relation for the melting rate arises from the second law of thermodynamics the condition of chemical equilibrium corresponds to the usual univariant equality of the chemical potentials of each phase when the matrix and melt are motionless in the most general form phase equilibrium is influenced by both the gibbs thomson effect that arises naturally from thermodynamic considerations on surface tension and by the viscous deformation of the phases we apply these new equations to a steady state problem of pressure release melting in a univariant system we treat melting and compaction simultaneously and observe several new effects including multiple domains near the onset of melting that correspond to various force balances a consequence of matrix compaction and melt expulsion or matrix dilation and melt accumulation is a pressure difference between melt and solid that facilitates inhibits melting for parameters corresponding to mid oceanic ridge magmatism compaction permits melting to start as much as 2k m below the standard solidus numerical solutions are necessary to determine the magnitude of the melt zone shift numerical results support the boundary layer solutions obtained analytically and suggest that in most of the melting zone the movement of melt and matrix should be close to the darcy equilibrium where the buoyancy of melt is balanced by the viscous drag between the phases the darcy equilibrium follows an initial stage where the matrix viscous stresses balance darcy drag in all situations the steady state porosity profile remains a monotonic function of depth the existence of a compaction layer following a melting zone where the porosity is maximum as described in various earlier publications has never been found

magma eruption rates constrained by 40ar 39ar chronology and gis for the ceboruco san pedro volcanic field western mexico 40ar 39ar geochronology is coupled with quantitative volume determinations utilizing field mapping digital elevation models orthophotos and geographic information system gis software to constrain magma eruption rates at the ceboruco san pedro volcanic field 1600 km2 in the western trans mexican arc ages are reported for 40 volcanic units including volcan ceboruco an active andesitic stratovolcano peripheral domes shields cinder cones and fissure fed flows after a hiatus of 3 m y volcanic activity recommenced to produce 80 5 3 5 km3 of magma at a rate of 63 m3 km2 per year over the past 0 8 m y however 75 of this volume erupted in the past 100 k y including the 51 2 5 km3 of volcan ceboruco equivalent to an eruption rate of 377 m3 km2 per year there have been at least two stages of cone building activity at volcan ceboruco the main edifice is composed of 38 km3 of precaldera andesites the youngest dated at 45 8 ka their eruption was followed by a hiatus interrupted by a plinian eruption at 1 ka the plinian eruption and subsequent lava flows are andesite to dacite in composition and constitute 13 km3 of the total volume of volcan ceboruco overall the relative proportions of lava types erupted in the past 0 8 m y are 18 19 basaltic andesite 56 60 andesite 18 22 dacite and 3 rhyolite the peripheral lavas are each of small volume geochemically diverse and show little evidence of prior storage in an upper crustal chamber the eruptive sequence proportions of lava types phenocryst assemblages textures and geochemistry imply that the lavas do not reflect the differentiation of a single parental liquid in a long lived magma chamber the distinct geochemical signatures were present prior to magma emplacement in the upper crust whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages

multi technique equation of state for fe2sio4 melt and the density of fe bearing silicate melts from 0 to 161 gpa we have conducted new equation of state measurements on liquid fe 2 sio 4 in a collaborative multi technique study the liquid density the bulk modulus k and its pressure derivative k were measured from 1 atm to 161 gpa using 1 atm double bob archimedean multi anvil sink float and shock wave techniques shock compression results on initially molten fe 2 sio 4 1573 k fitted with previous work and the ultrasonically measured bulk sound speed c o in shock velocity u s particle velocity u p space yields the hugoniot u s 1 58 0 03 u p 2 438 0 005 km s sink float results are in agreement with shock wave and ultrasonic data consistent with an isothermal k t 19 4 gpa and k 5 33 at 1500 c shock melting of initially solid fe 2 sio 4 300 k confirms that the gruneisen parameter of this liquid increases upon compression where o o q yields a q value of 1 45 constraints on the liquid fayalite eos permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system cao mgo al 2 o 3 sio 2 feo at elevated temperatures and pressures in our model a whole mantle magma ocean would first crystallize in the mid lower mantle or at the base of the mantle were it composed of either peridotite or simplified chondrite liquid respectively in regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra low velocity zones neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales but our model defines a compositional range of liquids that would be gravitationally stable

identifying potentially active volcanoes in the andes radiometric evidence for late pleistocene early holocene eruptions at volcan imbabura ecuador abstract recent eruptions from volcanoes with no previously known historical activity in chile and indonesia have raised the importance of the early identification of potentially active centers for the purpose of hazard assessment here we bring radiometric evidence 14 c 39ar 40ar of previously unrecognized but significant magmatic activity at partly eroded imbabura volcano ecuador in late pleistocene to early holocene times on whose perimeter live more than 300 000 persons following an effusive stage from 50 to 30 ka with the emplacement of andesitic lava flows on different flanks of its edifice the activity became explosive with the generation of andesitic block and ash flows on its eastern side beginning at 35 ka cal bp subsequently a flank collapse associated with a volcanic blast occurred on the volcano s sw flank at 30 ka cal bp the resulting debris avalanche and blast breccias cover an area now heavily populated around san pablo lake and its source was later concealed by successive dome building episodes at huarmi which produced 2 8 km3 of silicic andesite renewed dome activity at the edifice s taita summit occurred at 17 ka cal bp and continued intermittently into early holocene times as indicated by pyroclastic flow deposits overlying a palaeosoil dated at 9 ka cal bp in summary this study reveals an eruptive behavior characterized by a low recurrence rate but with quite large eruptions a pattern which is also observed at other silicic volcanoes of ecuador s western cordillera it is now imperative to reconsider the origin and source of the many tephra layers catalogued in holocene lacustrine sediments in the imbabura area tephra and lava volume estimates for imbabura volcano converted to dense rock equivalent values yield a minimum magmatic output rate of 0 13 km3 ka in the past 35 000 years which argues for sustained magma production for this volcano in recent geological times the imbabura example thus raises the question of how to improve population preparedness for volcanoes with infrequent eruptions and how to guide authorities decisions concerning the development of urban areas and infrastructures near presently inactive but potentially highly dangerous volcanoes

numerical modelling of magma transport in dykes the rheology and dynamics of an ascending pure melt in a dyke have been extensively studied in the past from field observations it is apparent that most dykes actually contain a crystalline load the presence of a crystalline load modifies the effective rheology of such a system and thus the flow behaviour indeed the higher density and viscosity of each crystal compared to the melt cause a decrease of the ascent velocity and modify the shape of the velocity profile from a typical poiseuille flow to a bingham type flow a common feature observed in the field is the arrangement of crystals parallel or at a very low angle to the edge of the dyke such a structural arrangement is often interpreted as the result of magma flow which caused the crystals to rotate and align within the flow direction but this process remains unclear another issue related to the introduction of a crystalline load concerns the possibility for crystals to be segregated from a viscous granitic melt phase during magma ascent the implications of such a process on magmatic differentiation have not previously been considered nor has such a process been previously investigated via numerical models in this study we examine the flow dynamics of a crystal bearing granitic melt ascending in a dyke via numerical models in our models both the crystal and melt phases are represented as highly viscous fluids in a stokes regime our results reveal that the presence of crystals in the melt modifies the magma velocity profile across the dyke furthermore we observe that whilst crystals continually rotate in the shear flow over one period of revolution their major axis has a high probability to be aligned parallel to the flow direction moreover some experiments showed that the melt phase can effectively be squeezed out from a crystal rich magma when subjected to a given pressure gradient range this demonstrates that crystal melt segregation in dykes during granitic magma ascent constitutes a viable mechanism for magmatic differentiation

lifting the cover of the cauldron convection in hot planets convection models of planetary mantles do not usually include a specific treatment of near surface dynamics in all situations where surface dynamics is faster than internal dynamics the lateral transport of material at the surface forbids the construction of a topography that could balance the internal convective stresses this is the case if intense erosion erases the topography highs and fills in the depressions or if magma is transported through the lithosphere and spreads at the surface at large distances in these cases the usual boundary condition of numerical simulations that the vertical velocity cancels at the surface should be replaced by a condition where the vertical flux on top of the convective mantle equilibrates that allowed by the surface dynamics we show that this new boundary condition leads to the direct transport of heat to the surface and changes the internal convection that evolves toward a heat pipe pattern we discuss the transition between this extreme situation where heat is transported to the surface to the usual situation where heat diffuses through the lithosphere this mechanism is much more efficient to cool a planet and might be the major cooling mechanism of young planets even the modest effect of material transport by erosion on earth is not without effect on mantle convection and should affect the heat flow budget of our planet

morphometry and evolution of arc volcanoes volcanoes change shape as they grow through eruption intrusion erosion and deforma tion to study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs central america and the southern central andes using shuttle radar topography mission srtm digital elevation models we compute and defi ne parameters for plan ellipticity irregularity and profi le height width summit basal width slope shape as well as size height width volume we classify volcanoes as cones sub cones and massifs and recognize several evolutionary trends many cones grow to a critical height 1200 m and volume 10 km 3 after which most widen into sub cones or massifs but some grow into large cones large cones undergo sector collapse and or gravitational spreading without sig nifi cant morphometry change other smaller cones evolve by vent migration to elliptical sub cones and massifs before reaching the critical height the evolutionary trends can be related to magma fl ux edifi ce strength structure and tectonics in particular trends may be controlled by two balancing factors magma pressure versus lithostatic pressure and conduit resistance versus edifi ce resistance morphometric analysis allows for the long term state of individual or volcano groups to be assessed morphological trends can be integrated with geological geophysical and geochemical data to better defi ne volcano evolution models

potassium argon geochronology of a basalt andesite dacite arc system the mount adams volcanic field cascade range of southern washington high precision k ar dating and detailed mapping have established an eruptive chronology for a cascade stratovolcano and its surrounding array of coeval basaltic centers mount adams is a 200 km 3 andesitic cone that stands at the center of a coeval 1250 km 2 quaternary volcanic field that contains 60 discrete vents k ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping within analytical error there are no discrepancies between k ar ages and stratigraphic sequence major activity began in the area ca 940 ka and inception of the central stratovolcano took place at ca 520 ka a plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was 3 k y and 80 of the products were basaltic andesites are volumetrically dominant and were emplaced in three main cone building episodes centered at 500 450 and 30 ka at eruptive rates of 1 6 5 km 3 k y at a lower rate of 0 05 0 1 km 3 k y the magmatic system remained almost continuously active between the main pulses although breaks in activity as long as 30 k y are permitted by the k ar data andesitic dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k y and their products are interstratified the last main episode of cone construction occurred ca 40 10 ka the oldest andesite identifiably derived from within the present day edifice having an age of 33 14 ka andesites forming the south summit rim and the true summit have ages of 13 8 ka and 15 8 ka respectively the time volume composition data bear upon several fundamental questions concerning the long term behavior of arc volcanoes stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity large stratocone systems may remain active for half a million years subdivision of complex stratovolcanoes into eruptive or constructional stages without detailed mapping geochronology and compositional data should be treated skeptically discussion of volumetric eruptive rates can be misleading without an adequate time scale stratovolcanoes need never develop large upper crustal magma chambers basalt erupts peripherally but its ascent is suppressed beneath stratovolcanoes by deep crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting

nonfractional crystallization of a terrestrial magma ocean it has been suggested that evolution of a terrestrial magma ocean does not unavoidably follow a fractional crystallization scenario convection is able to preclude differentiation until a sharp viscosity increase occurs near some critical crystal fraction however this kind of crystallization and its physical and chemical consequences have not been previously studied we consider an end member called here nonfractional crystallization we begin with a simple equilibrium thermodynamical model of partial melts which is based on an ideal three component phase diagram it allows a self consistent calculation of physical and chemical parameters in the melting range at all interesting pressures in particular adiabats of the convecting magma ocean are calculated the sharp increase in the viscosity is supposed to occur near the maximum packing crystal fraction however almost independently of this value convection occurs even in the highly viscous quasi solid part of the magma ocean and it is strong enough to prevent differentiation in deep regions a kind of compositional convection occurs due to the layered differentiation although it is weaker than the thermal convection only a surface region undergoes an essential differentiation via melt expulsion by compaction the thickness of this layer depends on the rheology of partial melts critical crystal fraction and crystal sizes but in any case the basal pressure hardly can exceed 5 10 gpa because of lower pressures in the moon the thickness of the differentiating layer is large and thus the entire lunar magma ocean could undergo a strong differentiation remelting due to the energy released by differentiation is crucial only for much deeper layers possibly deeper than about 1000 km for the earth for the remaining shallow layer p 5 10 gpa the predicted increase of the melt fraction is less than 40 at the surface and decreases to zero at the bottom of the differentiating layer thus the nonfractional crystallization is suggested to be a likely alternative to the fractional crystallization the crucial and still poorly understood factors are suspension in convective layers rheology of partial melts crystal size and surface conditions the most pronounced chemical consequence of the nonfractional crystallization is an almost completely preserved undifferentiated lower mantle and possibly a significant undifferentiated part of the upper mantle at all depths in the beginning of differentiation not only the first liquidus solid phase but also subsequent phases have been partially crystallized so when the differentiation begins it involves mixtures of phases it is important for the remaining layer where differentiation is unavoidable this layer does not have as strong differentiation of minor elements as in the case of fractional crystallization but it will still involve differentiation of major elements future geochemical calculations of this multiphase differentiation considering both major and minor elements could help to constrain the differentiation further

eruptive history geochronology and magmatic evolution of the puyehue cordon caulle volcanic complex chile forty three 40 ar 39 ar age determinations of lava flows domes ignimbrites and dikes plus 14 c dates from seven distal tephra layers combined with stratigraphy geochemistry and sr and th isotope data establish an eruptive chronology for the puyehue cordon caulle volcanic complex at 40 5 s in the andean southern volcanic zone svz the complex preserves 131 km 3 of lava and tephra that erupted from numerous vents widely separated in time and space approximately 80 of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka the modern puyehue stratovolcano was built on the southerly shield during the past 69 k y following a hiatus of 25 k y puyehue has erupted 15 km 3 of basaltic to rhyolitic magma that spans the entire compositional range found in the southern svz and evolved via at least six phases including 1 basaltic andesitic to dacitic lavas between 69 and 32 ka 2 a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of mgo rich basaltic andesite at 34 ka 3 dacitic to rhyolitic flows and domes from 19 to 12 ka 4 basaltic to basaltic andesitic flows between 15 and 12 ka 5 subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka followed by 6 a powerful series of phreatomagmatic and sub plinian eruptions at ca 1 1 ka that obliterated the preceding rhyolite domes and formed the present 2 5 km diameter 280 m deep summit crater along the cordon caulle fissure zone 5 km 3 of rhyodacitic to rhyolitic lavas domes and cones have formed during the past 16 5 k y including explosive and effusive eruptions in 1921 1922 and 1960 eruptive rates were nonuniform over time with background growth at 0 04 km 3 k y or less punctuated by spurts at up to 0 90 km 3 k y the time averaged rate 0 42 km 3 k y is nearly double that at the tatara san pedro complex 500 km to the north during the past 300 k y these findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable the last three stratocone building events on puyehue began during periods of deglaciation suggesting a relationship between unloading of ice and ease of magma ascent puyehue basalt exhibits subtle changes in 238 u 230 th 87 sr 86 sr and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge or the extent to which basalt was modified by assimilation of heterogeneous crustal melts the complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt despite the high flux of basalt during the past 300 k y no large silicic magma reservoir formed in the upper crust instead 238 u 230 th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust promoted perhaps by conduits that reflect strike slip faulting beneath the complex

degassing patterns of tungurahua volcano ecuador during the 1999 2006 eruptive period inferred from remote spectroscopic measurements of so2 emissions this paper presents the results of 7 years aug 1999 oct 2006 of so2 gas measurements during the ongoing eruption of tungurahua volcano ecuador from 2004 onwards the operation of scanning spectrometers has furnished high temporal resolution measurements of so2 flux enabling this dataset to be correlated with other datasets including seismicity the emission rate of so2 during this period ranges from less than 100 to 35000 tonnes day t d with a mean daily emission rate of 1458 t d and a standard deviation of 2026 t d average daily emissions during inferred explosive phases are about 1 75 times greater than during passive degassing intervals the total amount of sulfur emitted since 1999 is estimated as at least 1 91 mt mostly injected into the troposphere and carried westwards from the volcano our observations suggest that the rate of passive degassing at tungurahua requires so2 exsolution of an andesitic magma volume that is two orders of magnitude larger than expected for the amount of erupted magma two possible and not mutually exclusive mechanisms are considered here to explain this excess degassing gas flow through a permeable stagnant magma filled conduit and gas escape from convective magma overturning in the conduit we have found that real time gas monitoring contributes significantly to better eruption forecasting at tungurahua because it has provided improved understanding of underlying physical mechanisms of magma ascent and eruption

k ar dating extended into the last millennium application to the youngest effusive episode of the teide volcano spain less than one thousand years dating of the youngest volcanic episode of the teide volcano canary islands spain has been performed using the k ar cassignol technique analyses of about 20 g of pure alkali feldspar yielded a weighted mean age of 800 300 years stratigraphic historic and archeomagnetic dating validate this age and rule out the hypothesis of the presence of significant excess argon contamination for this flow our result shows that the last effusive activity of the teide volcano took place shortly before european settlement in tenerife island this study confirms that the cassignol technique can be applied with success for historic dating and demonstrates that it can now even be extended to the last millennium

plio quaternary volcanism in ecuador extensive sampling major element chemistry on over 300 samples and k ar radiometric dating have been carried out on the ecuadorian upper tertiary quaternary volcanoes the results show important space time variations of the volcanic activity between late miocene time and the present in late miocene time a continuous volcanic belt located approximately along the present volcanic front vf affected the whole country from the cuenca basin to the south up to colombia to the north major changes occurred at about 5 ma volcanic activity stopped south of the guayaquil fault belt and never resumed to the north the active volcanic axis shifted eastward to the cordillera real cr area with a simultaneous relative decrease in intensity since early quaternary time the volcanic belt widened westward to the western cordillera where the volcanism resumed at about 1 5 1 0 ma giving rise to the very wide active volcanic zone of ecuador the plio quaternary products show significant longitudinal and latitudinal chemical and mineralogical changes volcanics of the vf and interandean depression contain amphibole and define a calc alkaline trend with a k 2 o content lower than that of the cr products which are characterized by a mostly anhydrous phenocryst assemblage in both areas andesites dominate but extreme compositions basaltic andesites and rhyolites are more diffuse in the cr than the vf no significant transverse zoning has been detected in the northern region north of the chota mira transverse tectonic line the observed temporal and spatial variations are interpreted as a result of the subduction of the carnegie ridge anomalous oceanic crust underthrusting of which began approximately 6 ma ago

temporal evolution of magmatism in the northern volcanic zone of the andes the geology and petrology of cayambe volcanic complex ecuador in the northern volcanic zone of the andes the cayambe volcanic complex consists of 1 a basal mostly effusive volcano the viejo cayambe whose lavas andesites and subordinate dacites and rhyolites are typically calc alkaline and 2 a younger essentially dacitic composite edifice the nevado cayambe characterized by lavas with adakitic signatures and explosive eruptive styles the construction of viejo cayambe began 1 1 myr ago and ended at 1 0 ma the young and still active nevado cayambe grew after a period of quiescence of about 0 6 myr from 0 4 ma to holocene its complex history is divided into at least three large construction phases angureal cone main summit cone and secondary summit cone and comprises large pyroclastic events debris avalanches as well as periods of dome activity geochemical data indicate that fractional crystallization and crustal assimilation processes have a limited role in the genesis of each suite on the contrary field observations and mineralogical and geochemical data show the increasing importance of magma mixing during the evolution of the volcanic complex the adakitic signature of nevado cayambe magmas is related to partial melting of a basaltic source which could be the lower crust or the subducted slab however reliable geophysical and geochemical evidence indicates that the source of adakitic component is the subducted slab thus the viejo cayambe magmas are inferred to come from a mantle wedge source metasomatized by slab derived melts adakites whereas the nevado cayambe magmas indicate a greater involvement of adakitic melts in their petrogenesis this temporal evolution can be related to the presence of the subducted carnegie ridge modifying the geothermal gradient along the wadati benioff zone and favouring slab partial melting

ascending seismic source during an explosive eruption at tungurahua volcano ecuador 1 application of seismic techniques to an explosion event at tungurahua volcano ecuador provided clear images to elucidate its source process a source location method using high frequency seismic amplitudes with an s wave velocity of 2000 m s indicates that the event was triggered at a depth of 6 km below the summit and the source ascended toward the summit at a speed of about 1600 m s waveform inversion of low frequency signals at the event onset points to an isotropic mechanism with initial deflation at a similar depth of 6 km the ascending source suggests that a pressure wave propagated along the magma conduit triggering fragmentation of magma at shallow depths rapid decompression of magma in a shock tube has been considered to be an important mechanism for explosive eruptions triggered by ruptures at the magma surface however our study suggests that explosive eruptions are triggered by pressure disturbances in magma at depth

lunar magma transport phenomena abstract an understanding of the most significant events during the first ga or so of lunar history global differentiation associated with a lunar magma ocean lmo and the later generation of about 107 km3 of mare basalts depends profoundly on magma transport dynamics lmo evolution is dominated by the characteristics of hard turbulent convection of a fluid undergoing phase change quench protocrust formed at the ocean surface in early times is repeatedly disrupted by bolide impact ocean body tides and associated crustal foundering during this epoch the ocean temperature is set by the balance between convective heat brought to the surface and brown body radiation from the surface at later times the convective flow is organized into a number of regimes from top to bottom these include conductive crustal lid a viscous sublayer an inertial core a lower viscous sublayer and a mushy cumulate region crystal settling and flotation is restricted to the viscous sublayer regions dilatancy pumping may prevent the efficient expulsion of intercumulus melt during cumulate formation typical velocities within the inertial region are of order 10 m s because late stage ti rich cumulates are dense relative to the earlier mg rich cumulates hyper to sub solidus convection tends to mix lmo crystallization products numerical simulations that account simultaneously for compositional and thermal buoyancy effects indicate a characteristic mixing time 200 ma statistical measures of mixing variance and spatial correlation length quantify the details of mixing the darcy percolative and vein drain models of melt extraction are briefly discussed because compaction lengths are small 102 m the rate of melt extraction is governed by the balance between melt buoyancy and darcy friction differential melt matrix velocities are of order 10 km ma for nominal values appropriate for mare basalt melt extraction from a heterogeneous mixed lmo source transport of magma through the lunar lithosphere via a connected crack network seems highly probable for the range of inferred discharge rates applicable for lunar mare volcanism lmv 103 to 107 m3 s typical fracture widths and ascent speeds are 10 1 to 50 m and 10 1 to 10 m s respectively a simple model is proposed to evaluate the extent of fractionation as magma traverses cold lunar lithosphere if apollo green glasses are primitive and have not undergone significant fractionation en route to the surface then mean ascent rates of 10 m s and cracks of widths 40 m are indicated lunar tephra and vesiculated basalts suggest that a volatile component plays a role in eruption dynamics the predominant vapor species appear to be co co2 and cos near the lunar surface the vapor fraction expands enormously and vapor internal energy is converted to mixture kinetic energy with the concomitant high speed ejection of vapor and pyroclasts to form lunar fire fountain deposits such as the apollo 17 orange and black glasses and apollo 15 green glass

modeling phase separation and phase change for magma ocean solidification dynamics just after accretion the earth s mantle was significantly molten by the heat dissipation due to large impacts and to the segregation of the core the mineralogical observations and thermodynamics models of solid liquid equilibrium of silicates show that several types of crystallization may have happened at different depths in the mantle solids were probably formed first at the bottom of the lower mantle or at mid mantle leaving two possible magma oceans a shallow one and an abyssal one near the bottom of the mantle the liquid phase might become denser than solids due to iron enrichment in the shallow magma ocean the crystallizing solid phase was denser and sank through the magma to settle and compact at depth to understand these complex dynamics we develop a two phase numerical code that can handle simultaneously convection in each phase and in the slurry and the compaction or decompaction of the two phases although our code can only run in a parameter range rayleigh number viscosity contrast between phases prandlt number far from what would be realistic we think it already provides a rich dynamics that illustrates what could have happened we show situations in which the crystallization front is gravitationally stable and situations were the newly formed solids are gravitationally unstable and can snow across the magma our study suggests that the location of a density contrast between solid and magma must be considered of equal importance with that of the intersection between liquidus and isentrope for what concerns mantle solidification

effective shear and bulk viscosity of partially molten rock based on elastic moduli theory of a fluid filled poroelastic medium summary the effective bulk and shear viscosity of the matrix of a partially molten rock are important properties for the process of melt migration and matrix compaction a set of porosity dependent effective bulk and shear viscosity models is presented based on a self consistent poroelastic formulation for partially molten rock where the melt occurs in spherical ellipsoidal film like or tubular inclusions for these melt geometries a graphical user interface operated matlab program is provided which allows the calculation of effective bulk and shear viscosities as a function of porosity as well as effective elastic moduli and seismic velocities for all melt geometries an inverse porosity dependence of the effective bulk viscosity is found depending on the assumed melt geometry this formulation predicts both the effective bulk and shear viscosity to drop to zero at and above finite melt fractions of 20 50 per cent fitting equations are derived allowing direct implementation into two phase flow melt matrix formulations

the geology and mesozoic collisional history of the cordillera real ecuador abstract the geology of the metamorphic rocks of the cordillera real of ecuador is described in terms of five informal lithotectonic divisions we deduce that during the mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the northern andes the oldest event recognised of probable late triassic age may be related to the break up of western gondwana and generated a regional belt of s type plutons during the jurassic major calc alkaline batholiths were intruded following this in latest jurassic to early cretaceous time a volcano sedimentary terrane of possible oceanic or marginal basin origin the alao division and the most westerly gneissic chaucha arenillas terrane were accreted to continental south america the accretion of the oceanic western cordillera took place in latest cretaceous to earliest tertiary time this latter event coincided with widespread thermal disturbance as evidenced by the large number of young k ar mineral ages recorded from the cordillera real

holocene recurrent explosive activity at chimborazo volcano ecuador abstract ice capped chimborazo is one of the large composite ecuadorian volcanoes whose recent eruptive activity is poorly known this paper presents the characteristics and the ages of a newly discovered holocene sequence of pyroclastic deposits on the east and north sides of the cone lying upon a moraine of the late glacial period the most complete section of 4 5 m in thickness is located 5 km from the present summit crater it consists of seven massive or diffusely stratified ash flow layers and four fallout layers interbedded with seven paleosoils based on field study most flow deposits were assessed as surge layers and six radiocarbon analyses obtained from charcoal fragments and paleosoils indicate that eruptions occurred at quite regular intervals between about 8000 and 1000 years ago the first two and most potent events generated thick lahars over the north and west flanks of the cone surface textures of volcanic clasts were analysed by scanning electron microscopy blocky and blocky fluidal vitric clasts indicate fragmentation during vulcanian explosions of a quite solidified shallow magma body in addition aggregates either cemented at a cooling stage with surface fluidal textures or consisting of fine particles moss looking aggregates form a large part of the surge deposits these characteristics indicate powerful explosions and intense fragmentation due to phreatic water reaching the conduit probably from the ice cap since the last eruption occurred between the early part of the 5th century ad 420 and the end of the 7th century these results highlight that chimborazo is a potentially active volcano given its dominating presence over the densely populated ambato and riobamba basins and owing its large ice cap chimborazo should be considered a dangerous volcano

timing of mantle overturn during magma ocean solidification abstract solidification of magma oceans mos formed early in the evolution of planetary bodies sets the initial condition for their evolution on much longer time scales ideal fractional crystallization would generate an unstable chemical stratification that subsequently overturns to form a stably stratified mantle the simplest model of overturn assumes that cumulates remain immobile until the end of mo solidification however overturning of cumulates and thermal convection during solidification may act to reduce this stratification and introduce chemical heterogeneity on scales smaller than the mo thickness we explore overturning of cumulates before the end of mo crystallization and the possible consequences for mantle structure and composition in this model increasingly dense iron rich layers crystallized from the overlying residual liquid mo are deposited on a thickening cumulate layer overturn during solidification occurs if the dimensionless parameter r c measuring the ratio of the mo time of crystallization m o to the timescale associated with compositional overturn o v g h exceeds a threshold value if overturn did not occur until after solidification this implies that the viscosity of the solidified mantle must have been sufficiently high possibly requiring efficient melt extraction from the cumulate for a given rate of solidification for the lunar mo possible implications for the generation of the mg suites and mare basalt are suggested

volcanic evolution of volcan aucanquilcha a long lived dacite volcano in the central andes of northern chile volcan aucanquilcha northern chile has produced 37 km3 of dacite 63 66 wt silica mainly as lavas with ubiquitous magmatic inclusions 59 62 wt silica over the last 1 million years a pyroclastic flow deposit related to dome collapse occurs on the western side of the edifice and a debris avalanche deposit occurs on the eastern side the 6 000 m high edifice defines a 9 km e w ridge and lies at the center of a cluster of more than 15 volcanoes the aucanquilcha volcanic cluster that has been active for at least the past 11 million years the e w alignment of vents is nearly orthogonal to the arc axis a majority of volcan aucanquilcha was constructed during the first 200 000 years of eruption whereas the last 800 000 years have added little additional volume the peak eruptive rate during the edifice building phases was 0 16 km3 ka and the later eruptive rate was 0 02 km3 ka comparable dacite volcanoes elsewhere show a similar pattern of high volcanic productivity during the early stages and punctuated rather than continuous activity volcan aucanquilcha lavas are dominated by phenocrysts of plagioclase accompanied by two populations of amphibole biotite clinopyroxene fe ti oxides and or orthopyroxene accessory phases include zircon apatite and rare quartz and sanidine one amphibole population is pargasite and the other is hornblende the homogeneity of dacite lava from volcan aucanquilcha contrasts with the heterogeneity 52 66 wt silica at nearby volcan ollague which has been active over roughly the same period of time we attribute this homogeneity at aucanquilcha to the thermal development of the crust underneath the volcano resulting from protracted magmatism there whereas volcan ollague lacks this magmatic legacy

tropical south america during the last glacial maximum evidence from glacial periglacial and fluvial records abstract the records of the lgm climates of tropical south america are still sparse compared to the vast region under consideration nevertheless a number of records hold up to critical evaluation early attempts to reconstruct the lgm temperature and precipitation conditions for the lgm around 21 14 c ka bp have suggested that relative to present lowland temperatures were on average only a few c cooler whereas in the andes the temperature depression was in the range of 6 8 c these reconstructions further suggested that tropical precipitation was more limited in both the lowlands and in the andes although recent pollen evidence supports significantly cooler tropical lowland temperatures during the lgm additional alkenone evidence from sea surface temperatures and model analysis are contradictory the data from tropical south america suggest that the late quaternary climatic changes of the lowlands as well as of the andean mountains reflect a response 1 to environmental changes in the source area of the moisture and 2 to global temperature fluctuations based on glacial periglacial and pollen evidence a lgm temperature depression of 5 6 c can be assumed for both tropical lowland south america and the andes these lgm values for the cooling correspond with recent coral records from barbados and the southwestern pacific ice core records from peru noble gas measurements in brazil and ocean core records from the western equatorial atlantic no difference in the value of lgm cooling can be recognised between the tropical lowlands and the high mountains of south america furthermore in all regions the lgm climates were more arid than the present day climates terrestrial records have not yet yielded records of younger dryas age apart from the tropical andes of ecuador where the younger dryas time is characterised by a cooler and drier climate pollen evidence for cooler younger dryas conditions comes from the bolivian andes

construction and destruction of mont pelee volcano volumes and rates constrained from a geomorphological model of evolution this study presents long term volumes and construction rates for the mont conil mont pelee volcano and rate estimates at which volcanic activity creates relief an algorithm shapevolc is used to numerically model topographic surfaces volcano morphology is analyzed using current digital elevation model in combination with mapped geology to produce 10 paleotopographies at the end of four constructional stages and three destructional events volumes of each constructional stage were estimated at about 35 2 km3 26 2 km3 8 3 km3 and 2 5 km3 for a total cumulative erupted volume of 72 2 km3 we estimate that mont pelee accounted for about 10 of the lesser antilles arc production in the last 100 kyr the volcano has been built at an average rate of 0 13 km3 kyr during the last 550 kyr during that time construction rates varied by a factor of 15 from 0 04 km3 kyr in early stages up to 0 52 km3 kyr after the second flank collapse volumes displaced by each flank collapse were estimated at 14 7 km3 8 8 km3 and 3 5 km3 thus about 37 of the total constructed volume integrated over the volcano s lifetime the rate at which flank collapses removed material off the island is 0 15 km3 kyr in contrast long term erosion rates outside collapsed areas are estimated at about 0 05 0 7 km3 kyr or 11 km3 of material removed this latter rate is not negligible which strengthens the importance of taking into account recurrent small erosional events on the geomorphological evolution of a volcanic island in a tropical context

solid liquid iron partitioning in earth s deep mantle iron in partially molten rocks under deep mantle conditions partitions into the melt phase less than previously reported suggesting that melt generated near the core mantle boundary should segregate upwards denis andrault et al investigate phase relationships in partially molten rocks under deep mantle conditions and find that iron in the rocks partitions into the melt phase to a lesser extent than has previously been reported the calculated densities of the solid and melt phases suggest that melt generated near the core mantle boundary should be buoyant and hence should segregate upwards melting processes in the deep mantle have important implications for the origin of the deep derived plumes believed to feed hotspot volcanoes such as those in hawaii1 they also provide insight into how the mantle has evolved geochemically and dynamically since the formation of earth2 melt production in the shallow mantle is quite well understood but deeper melting near the core mantle boundary remains controversial modelling the dynamic behaviour of deep partially molten mantle requires knowledge of the density contrast between solid and melt fractions although both positive and negative melt buoyancies can produce major chemical segregation between different geochemical reservoirs each type of buoyancy yields drastically different geodynamical models ascent or descent of liquids in a partially molten deep mantle should contribute to surface volcanism or production of a deep magma ocean respectively we investigated phase relations in a partially molten chondritic type material under deep mantle conditions here we show that the iron partition coefficient between aluminium bearing mg fe sio3 perovskite and liquid is between 0 45 and 0 6 so iron is not as incompatible with deep mantle minerals as has been reported previously3 calculated solid and melt density contrasts suggest that melt generated at the core mantle boundary should be buoyant and hence should segregate upwards in the framework of the magma oceans induced by large meteoritic impacts on early earth our results imply that the magma crystallization should push the liquids towards the surface and form a deep solid residue depleted in incompatible elements

three dimensional p wave velocity structure of tungurahua volcano ecuador abstract tungurahua volcano in the ecuadorian andes is a large andesitic stratovolcano 5023 m that has been erupting mildly since 1999 we studied the three dimensional 3 d p wave velocity vp structure beneath the volcano down to 5 km below the summit we inverted 1708 p wave first arrival times from 263 volcano tectonic vt earthquakes recorded by 5 to 10 short period vertical seismic stations on the volcano from august 1999 to may 2003 a tomographic inversion method was used to image the velocity structure in which first arrival times were calculated with a finite difference method the root mean square of the arrival time residuals rms was reduced by 43 after running 10 iterations from the initial rms of 0 15 s the relocated hypocenters in our model are tightly clustered along a vertical structure at depths between sea level and the summit crater a high velocity zone exists above the central base of the volcano under the vertically aligned hypocenters and may be interpreted as the source zone for recharge of the shallow magmatic system high velocity zones are also identified under the lower northeastern and southern flanks of the edifice the southern high velocity anomaly lies close to the surface and is connected to the high velocity zone in the central base of the volcano a feature suggesting an old lateral dike system except for these high velocity zones in the central northern and southern flanks the volcanic edifice is composed of low velocity materials down to a depth of 2 km above sea level these low velocity zones correlate with young unconsolidated deposits and older highly fractured and or altered volcanic materials

the eruptive history of the tequila volcanic field western mexico ages volumes and relative proportions of lava types the eruptive history of the tequila volcanic field 1600 km 2 in the western trans mexican volcanic belt is based on 40 ar 39 ar chronology and volume esti mates for eruptive units younger than 1 ma ages are reported for 49 volcanic units including volcn tequila an andesitic stratovolcano and peripheral domes flows and scoria cones volumes of volcanic units 1 ma were obtained with the aid of field mapping ortho aerial pho tographs digital elevation models dems and arcgis software between 1120 and 200 kyrs ago a bimodal distribution of rhyolite 35 km 3 and high ti basalt 39 km 3 dominated the volcanic field between 685 and 225 kyrs ago less than 3 km 3 of andesite and dacite erupted from more than 15 isolated vents these lavas are crystal poor and show little evidence of storage in an upper crustal chamber approximately 200 kyr ago 31 km 3 of andesite erupted to form the stratocone of volcn tequila the phenocryst assemblage of these la vas suggests storage within a chamber at 2 3 km depth after a hiatus of 110 kyrs 15 km 3 of andesite erupted along the w and se flanks of volcn tequila at 90 ka most likely from a second discrete magma chamber lo cated at 5 6 km depth the youngest volcanic feature 60 ka is the small andesitic volcano cerro tomasillo 2 km 3 over the last 1 myr a total of 128 22 km 3 of

combining historical and 14 c data to assess pyroclastic density current hazards in banos city near tungurahua volcano ecuador pyroclastic density currents pdcs from tungurahua volcano ecuador recurrently rush down the northern vazcun valley and threaten the small tourist city of banos located on its mouth estimating pdc hazards i e pdc return rate and extent is difficult in banos and vazcun because data from the literature are rare and debated while geological exposures are few and vegetated to improve our awareness of pdc hazards in the banos area we examined historical documents conducted new geological research in the vazcun valley and obtained additional 14c data we highlight complex radiocarbon results that may lead to difficulties when interpreting the pdc chronology and offer recommendations to improve sampling strategies for 14c age determinations of pdc deposits overall our results show that the 1640 1773 and 1886 pdc forming eruptions are recorded in the valley while products of the 1918 and 2006 events were likely removed through considering recorded unrecorded bias in the reconstruction of past pdc emplacement in vazcun we calculate a minimal average return rate of 18 pdc forming events for pre columbian and historical eruptions since the devastating 3 ka bp eruption which translates into a maximum average return rate of 150 200 years in the valley however the archives suggest that most pdcs did not affected the now urbanized banos area e g 1918 2006 and we estimate that the city is impacted on average every 350 500 years a duration that is likely perceived as fairly long for people living in the area the banos case study thus raises questions on how to communicate on uncertain threats and impacts and how to improve alert messages in a town where economic incomes are essentially based on tourist activities to better address these issues we plan in future works to focus on cognitive perception of risk and volcanic hazard judgment by populations of banos and nearby rural communities

magma oceans in the inner solar system theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early accreting planetesimals the apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet the processes of magma ocean formation and solidification therefore strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets and they form the starting point for cooling to clement habitable conditions and for the onset of thermally driven mantle convection and plate tectonics this review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean distribution and degassing of volatiles and cooling

large scale inflation of tungurahua volcano ecuador revealed by persistent scatterers sar interferometry the tungurahua volcano in ecuador has been experiencing a substantial activity period since 1999 with several eruptions including those of 2006 and 2008 we use a persistent scatterers approach to analyze a time series of envisat synthetic aperture radar sar data over the period 2003 2009 to investigate surface deformation in the region of the volcano we measure a continuous large scale uplift with a maximum line of sight displacement rate of about 8 mm yr which is the first evidence of a sustained inflation in the andes for an active volcano encompassing several eruptions we model this signal as magma emplacement in a permanent storage zone at 11 5 km below sea level with a net inflow rate of 7 million m3 yr the paroxysmal eruptions in 2006 and 2008 did not seem to disrupt this long term signal however we observe significant deformation during the 2006 eruption consistent with an additional intrusion of 4 5 million m3 of magma

on the cooling of a deep terrestrial magma ocean several episodes of complete melting have probably occurred during the first stages of the earth s evolution we have developed a numerical model to monitor the thermal and melt fraction evolutions of a cooling and crystallizing magma ocean from an initially fully molten mantle for this purpose we numerically solve the heat equation in 1d spherical geometry accounting for turbulent heat transfer and integrating recent and strong experimental constraints from mineral physics we have explored different initial magma ocean viscosities compositions thermal boundary layer thicknesses and initial core temperatures we show that the cooling of a thick terrestrial magma ocean is a fast process with the entire mantle becoming significantly more viscous within 20 kyr due to the slope difference between the adiabats and the melting curves the solidification of the molten mantle occurs from the bottom up in the meantime a crust forms due to the high surface radiative heat flow the last drop of fully molten silicate is restricted to the upper mantle among the studied parameters the magma ocean lifetime is primarily governed by its viscosity depending on the thermal boundary layer thickness at the core mantle boundary the thermal coupling between the core and magma ocean can either insulate the core during the magma ocean solidification and favor a hot core or drain the heat out of the core simultaneously with the cooling of the magma ocean reasonable thickness for the thermal boundary layer however suggests rapid core cooling until the core mantle boundary temperature results in a sluggish lowermost mantle once the crystallization of the lowermost mantle becomes significant the efficiency of the core heat loss decreases since a hotter liquidus favors crystallization at hotter temperatures a hotter deep mantle liquidus favors heat retention within the core in the context of an initially fully molten mantle it is difficult to envision the formation of a basal magma ocean or to prevent a major heat depletion of the core as a consequence an earth s geodynamo sustained only by core cooling during 4 gyr seems unlikely and other sources of motion need to be invoked

eruptive history of chimborazo volcano ecuador a large ice capped and hazardous compound volcano in the northern andes abstract new fieldwork radiometric and whole rock chemical data permit the reconstruction of the main eruptive stages of the chimborazo compound volcano the highest summit of the northern andes chimborazo is composed of three successive edifices the basal edifice ch i was active from 120 to 60 ka and resulted in a large mostly effusive edifice which was built up during two stages of cone building terminating with the formation of a dome complex this edifice was affected by a huge sector collapse around 65 60 ka which produced a major debris avalanche that spread out into the riobamba basin covering about 280 km2 with an average thickness of 40 m and a total volume of 10 12 km3 after the emplacement of the riobamba debris avalanche eruptive activity resumed at the eastern outlet of the avalanche scar and was responsible for the construction of a less voluminous intermediary edifice ch ii whose current remnants are the politecnica and martinez peaks this edifice developed from 60 to 35 ka lastly eruptive activity shifted to the west leading to the construction of the morphologically well preserved young cone ch iii which currently forms the highest summit whymper the average eruptive rate of chimborazo volcano is 0 5 0 7 km3 ka however looking at the three successive edifices individually we estimate that there has been a progressive decrease in magma output rate from the basal edifice 0 7 1 0 km3 ka through the intermediary edifice 0 4 0 7 km3 ka to the young cone 0 1 km3 ka however during the main cone building stages the peak eruption rates are markedly higher indicating significant variations in the magma output rate during the lifespan of this arc volcano during the holocene the chimborazo eruptive activity consisted of small volume explosive events that occurred at quite regular intervals between about 8000 and 1000 yr ago since the last eruption occurred between the early part of the 5th century and the end of the 7th century and the average time interval between the events is about 1000 yr chimborazo must be considered as a potentially active volcano the presence of a thick ice cap covering the summit its steep flanks and its position above the populated lowland area of riobamba and ambato are factors that result in a high potential risk

pongkor west java indonesia a pliocene supergene enriched epithermal au ag mn deposit the pongkor gold silver epithermal deposit with reserves of at least 98 tonnes of gold and 1026 tonnes of silver average grades 16 4 g t au and 171 2 g t ag is one of the most recent and largest gold and silver discoveries in indonesia proven within a short period 1988 1991 40ar 39ar dating on adularia samples give an age of 2 05 0 05 ma the deposit is of the low sulfidation epithermal type and consists of four main mineralized quartz veins located close to the internal rim of a volcano tectonic depression caldera this resulted from an explosive ignimbritic eruption that produced pyroclastic flows and accretionary lapilli with rare intercalations of epiclastic rocks this volcanic unit unconformably overlies miocene subaqueous volcanic andesitic rocks with interbedded epiclastic rocks the mineralized bodies are thick average 4 2 m steeply dipping quartz carbonate adularia veins with a very low sulfide content 0 5 wt their genesis is related to an extensional episode within a tectonic corridor showing nw se and nne ssw conjugate strike slip faults the major vein being located on the inner rim of the caldera the vein fill reveals four successive stages of deposition marked by a specific facies 1 carbonate quartz breccia with dominant quartz and calcite and minor kutnahorite rhodochrosite and rhodonite cq facies 2 a network of banded quartz and former carbonate transformed into manganese oxides through supergene alteration moq facies 3 banded opaline milky quartz boq facies and 4 grey locally banded sulfide rich quartz breccia cutting all the other types gsq facies adularia was deposited at the same time as the quartz the mineralogy and internal structures of the veins crustiform banding vugs collapse breccia clearly indicate a dilational context which is common in low sulfidation epithermal systems gold and silver grades as well as sulfide mineral abundances increase steadily through stages 1 to 4 locally reaching 1 kg t in the gsq facies the sulfides are dominated by pyrite accompanied by common acanthite aguilarite polybasite pearceite and electrum in which the gold content ranges from 48 to 74 wt sphalerite galena chalcopyrite and hessite are fairly rare although present within the cq facies the fluid inclusions of the four facies show homogenization temperatures ranging from 150 to 382 c indicating boiling of a hydrothermal fluid with an initial temperature of around 205 c no marked difference is seen in the gsq facies which has the highest gold content salinities are low generally below 1 wt eq nacl lead isotope compositions of the associated volcanic rocks and the mineralization are very similar 206pb 204pb between 18 706 and 18 814 and between 18 744 and 18 801 respectively demonstrating a genetic link between the pliocene volcanism and the auriferous hydrothermal activity the isotopic signature suggests that the source of the mineralization and associated volcanic rocks is an underlying ancient continental crust that melted and remobilized during the pliocene volcanic and hydrothermal events these conclusions seem applicable to the entire bayah dome the existence of both a tectonic corridor and a caldera favoured channelling of the hydrothermal fluids and the deposition of primary ore in the veins late intense weathering of the ore deposit to depths of 250 m below the surface has given rise to manganese oxide layers limonite zones and silver micronuggets within the veins as well as to gold enrichment

melting temperature distribution and fractionation in the lower mantle abstract the melting curve of perovskite mgsio3 and the liquidus and solidus curves of the lower mantle were estimated from thermodynamic data and the results of experiments on phase changes and melting in silicates the initial slope of the melting curve of perovskite mgsio3 was obtained as d t m d p 77 k gpa 1 at 23 gpa the melting curve of perovskite was expressed by the kraut kennedy equation as t m k 917 1 29 6 v v 0 where tm 2900 k and p 23 gpa and by the simon equation p gpa 23 21 2 t m k 2900 1 75 1 the liquidus curve of the lower mantle was estimated as t liq 0 9 t m perovskite and this gives the liquidus temperature tliq 7000 500 k at the mantle core boundary the solidus curve of the lower mantle was also estimated by extrapolating the solidus curve of dry peridotite using the slope of the solidus curve of magnesiowustite at high pressures the solidus temperature is 5000 k at the base of the lower mantle if the temperature distribution of the mantle was 1 5 times higher than that given by the present geotherm in the early stage of the earth s history partial melting would have proceeded into the deep interior of the lower mantle estimation of the density of melts in the mgo feo sio2 system for lower mantle conditions indicates that the initial melt formed by partial fusion of the lower mantle would be denser than the residual solid because of high concentration of iron into the melt thus the melt generated in the lower mantle would tend to move downward toward the mantle core boundary this downward transportation of the melt in the lower mantle might have affected the chemistry of the lower mantle such as in the d layer and the distribution of the radioactive elements between mantle and core

thermodynamics of the mgo feo sio2 system up to 140 gpa application to the crystallization of earth s magma ocean at the end of earth s accretion and after the core mantle segregation the existence of a basal magma ocean at the top of the core mantle boundary cmb depends on the physical properties of mantle materials at relevant pressure and temperature present day deep mantle structures such as ultralow velocity zones and low shear velocity provinces might be directly linked to the still ongoing crystallization of a primordial magma ocean we provide the first steps toward a self consistent thermodynamic model of magma ocean crystallization at high pressure we build a solid liquid thermodynamic database for silicates in the mgo feo sio2 system from 20 gpa to 140 gpa we use already published chemical potentials for solids liquid mgo and sio2 we derive standard state chemical potential for liquid feo and mixing relations from various indirect observations using this database we compute the ternary phase diagram in the mgo feo sio2 system as a function of temperature and pressure we confirm that the melt is lighter than the solid of same composition for all mantle conditions but at thermodynamic equilibrium the iron rich liquid is denser than the solid in the deep mantle we compute a whole fractional crystallization sequence of the mantle and show that an iron rich and fusible layer should be left above the cmb at the end of the crystallization

suspension in convective layers and style of differentiation of a terrestrial magma ocean recent physical theories for the formation of the earth suggest that about 4 5 b y ago the mantle of the earth was partially or completely molten fractional crystallization of this hypothetical magma ocean would result in a strong chemical stratification of the earth s mantle such a scenario is controversial from the geochemical point of view however it has been noted that the simple scenario of fractional crystallization could be avoidable in a convective magma ocean if crystals remain suspended in this paper the problem of suspension is developed with the help of an energetic approach convection must do some work against gravitational settling we distinguish three regimes of convective suspensions absolute or complete sedimentation occurs when the energy dissipation due to the settling exceeds the heat loss from the convective layer this is possible only in large scale systems like magma oceans and implies that cooling can proceed only together with sedimentation crystallization and a decrease in the liquidus temperature at a constant pressure a regime of partial differentiation occurs when the energy dissipation due to the settling is less than the total heat loss but larger than the power which can be spent by convection on the crystal reentrainment process the differentiation is not complete and a competition between the rate of cooling the rate of sedimentation and the rate of turbulent diffusion determines the degree of differentiation the third regime is an absolute suspension which could be sustained for an indefinitely long time in this case sedimentation starts only when the crystal fraction reaches the maximum packing value when the viscosity of the magma rapidly increases the power which can be spent by convection on reentrainment is equal to e gd cp of the total energy supply to the convective layer where e 15 gpa or 10 3 10 1 cm during crystallization of shallow layers the first regime fractional crystallization is unavoidable the estimates depend on various poorly constrained parameters and processes such as heat flux viscosity thermodynamical disequilibrium and highly variable viscosity convection for absolute suspension the crystal size must be at least e times less or 10 3 10 1 cm and 10 4 10 2 cm respectively if e 0 01 the partial differentiation occurs in a narrow one decade range between these two regimes the radius of about 1 cm must be considered as an absolute upper bound above which fractional differentiation is guaranteed these estimates for the critical crystal size are orders of magnitude lower than suggested previously and thus the problem of crystal sizes becomes a central one for magma oceans a necessary condition for reentrainment is the existence of local mechanisms the absence of such mechanisms to reentrain the particles from the bottom would mean that an absolute suspension is impossible even if the energetics allows it turbulence is considered as a possible important factor a simple model of convection predicts a strong turbulence provided the viscosity is less than 10 9 10 10 p rotation reduces this critical viscosity to 10 5 10 8 p but this is still sufficiently large and is reached only near the maximum packing crystal fraction power law or bingham rheology of partial melts can exclude any turbulence already at 20 30 of crystal fraction we also show that the energetic criterion for the absolute suspension with e 1 coincides with the condition that the particle concentration gradient suppresses the turbulence

evolving metasomatic agent in the northern andean subduction zone deduced from magma composition of the long lived pichincha volcanic complex ecuador geochemical studies of long lived volcanic complexes are crucial for the understanding of the nature and composition of the subduction component of arc magmatism the pichincha volcanic complex northern andean volcanic zone consists of 1 an old highly eroded edifice the rucu pichincha whose lavas are mostly andesites erupted from 1 100 to 150 ka and 2 a younger essentially dacitic guagua pichincha composite edifice with three main construction phases basal guagua pichincha toaza and cristal which developed over the last 60 ka this structural evolution was accompanied by a progressive increase of most incompatible trace element abundances and ratios as well as by a sharp decrease of fluid mobile to fluid immobile element ratios geochemical data indicate that fractional crystallization of an amphibole rich cumulate may account for the evolution from the guagua pichincha andesites to dacites however in order to explain the transition between the rucu pichincha andesites and guagua pichincha dacites the mineralogical and geochemical data indicate the predominance of magma mixing processes between a mafic trace element depleted mantle derived end member and a siliceous trace element enriched adakitic end member the systematic variation of trace element abundances and ratios in primitive samples leads us to propose that the rucu pichincha magmas came from a hydrous fluid metasomatized mantle wedge whereas guagua pichincha magmas are related to partial melting of a siliceous melt metasomatized mantle this temporal evolution implies a change from dehydration to partial melting of the slab which may be associated with an increase in the geothermal gradient along the slab due to the presence of the subducted carnegie ridge at the subduction system this work emphasizes the importance of studying arc magma systems over long periods of time of at least 1 million of years in order to evaluate the potential variations of the slab contribution into the mantle source of the arc magmatism

transition from calc alkalic to adakitic magmatism at cayambe volcano ecuador insights into slab melts and mantle wedge interactions in volcanic arcs two main types of magmatism are recognized 1 widespread calc alkalic magmatism generated by hydrous partial melting of a metasomatized mantle wedge and 2 less common adakitic volcanism produced by subducted slab melting the cayambe volcanic complex in ecuador shows a progressive temporal change from an older calc alkalic volcano viejo cayambe to a younger adakitic edifice nevado cayambe this evolution may be related to the unusual geodynamic setting of the ecuadorian andes controlled mainly by the subduction of the nazca plate including the carnegie ridge thickened oceanic crust the viejo cayambe magmas appear to be generated from a mantle wedge source slightly metasomatized by slab derived melts conversely the nevado cayambe magmas imply either stronger and more advanced interactions between slab melts and mantle peridotite or that adakitic magma could reach the surface due to higher degrees of slab melting fractional crystallization assimilation and mixing processes also contributed to lava diversity we propose that the magmatic evolution of the cayambe volcanic complex is due to increasing efficiency of interaction between slab melts and the mantle wedge because of higher degrees of slab melting in response to the subduction of the carnegie ridge

volcano tectonic and geochemical evolution of an oceanic intra plate volcano tahiti nui french polynesia abstract geological mapping accurate k ar dating and geochemical analyses of lavas allow a detailed reconstruction of the geological history of tahiti nui island french polynesia the exposed volcanic activity is first characterized by the construction of a main shield from 1 4 ma to 870 ka with a maximum aerial eruptive rate around 2 km3 kyr lavas from this early building stage are alkaline slightly silica undersaturated with 87sr 86sr and 143nd 144nd compositions rather constant and close to the enriched mantle ii type vent locations were first concentrated along a main e w rift zone which was responsible for the lateral collapse of the northern and southern flanks of the main shield around 0 87 ma ago the subsequent activity was first restricted to the northern depression corresponding to an eruptive rate of about 5 km3 kyr in the period 850 760 ka significant variations in la sm 87sr 86sr and 143nd 144nd occur in lavas erupted immediately after the main northern landslide indicating a sudden increase in the extent of partial melting likely caused by the decompression subsequent to collapse however the later activity declined and the lavas exhibit a gradual change toward strongly silica undersaturated basanites likely indicating a decreasing extent of partial melting of the upper mantle the evolution of radiogenic isotope ratios over time indicates a change in the source toward more depleted compositions until around 500 ka post erosion volcanic activity following an apparent hiatus of 240 kyr exhibits similar major and trace element and isotope compositions the volcano structural and geochemical evolution of tahiti nui and the overall alkaline character of the lavas from other eruptive complexes of the society alignment suggest a relative weakness low temperature and low eruptive rate of the society plume compared to the hawaiian hot spot

erosion rates and erosion patterns of neogene to quaternary stratovolcanoes in the western cordillera of the central andes an srtm dem based analysis abstract erosion patterns and rates of 33 stratovolcanoes in the arid to hyperarid central andean volcanic zone 14 s to 27 s have been constrained by morphometric modelling all selected volcanoes belong to the short lived symmetrical circular andesitic stratocone type with ages spanning 14 ma to recent starting from the initial youthful volcano morphology of this type represented in our study by parinacota volcano and comparing reconstructed volumes of progressively eroded volcanoes such a time span allows us to infer long term erosion rates typical erosion rates of