evaluation of recent tectonomagmatic discrimination diagrams and their application to the origin of basic magmas in southern mexico and central america discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years although old diagrams have been extensively used the recent ones based on discriminant functions of ratio variables with or without log transformation proposed during 2004 2010 for the discrimination of four tectonic settings of island arc continental rift ocean island and mid ocean ridge were newly evaluated to show their high success rates of 57 3 100 and 58 5 100 for major element and immobile element based diagrams respectively for the continental arc of the andes evaluated for its similarity to island arc these four sets of diagrams showed success rates of 62 1 83 8 these four sets of five diagrams per set were therefore used to infer tectonic setting of the mexican volcanic belt mvb los tuxtlas volcanic field ltvf and central american volcanic arc cava using this approach the mvb especially its western central and eastern parts and the ltvf of southern mexico show a dominantly continental rift setting and the cava shows an arc setting the west central part of the mvb is consistent with dual tectonics of arc and rift these results confirm the application of an unusual mantle upwelling rift model for the mexican on land volcanism whereas the conventional plate tectonic subduction model seems to be applicable for the cava from guatemala to north western costa rica

mechanism of arrested charnockite formation at nemmara palghat region southern india abstract the formation of arrested charnockite is an excellent example of structurally controlled channellised fluid flow along specific sites accompanied by selective elemental mobility and mineralogical changes the present paper recognises and focuses study on three types of arrested charnockite formation from palghat region namely shear controlled foliation parallel and boudin neck types and address their spatio temporal relations to regional scale charnoenderbite the shear controlled and foliation parallel types post date deformation and migmatisation the boudin neck type on the contrary is coeval with partial melting and followed the path of cooling and decreasing water activity in the gneiss k feldspar veining around plagioclase and quartz symplectitic intergrowth of biotite quartz after orthopyroxene and k feldspar and fluid inclusion data suggests the presence of alkalic supercritical brine and low density co2 rich fluid during charnockite formation charnockite domains developed following the breakdown of hornblende biotite and quartz are characterised by a more or less pronounced depletion of fe ca mg and ti and trace elements y and zr compared to their counterpart gneiss ree spectra indicate a subtle depletion in the hree near the centre of the charnockite domain although close pair samples of gneiss charnockite are isochemical on a scale of a few millimetres bi directional element movement related to the formation of new mineral was noted it is postulated that arrested charnockite formation developed in situ on local scale within the granitic domains of the hornblende biotite gneiss in the presence of co2 rich fluids and alkalic supercritical saline brine this process post dated the time of regional granulite charnoenderbite and large regional scale retrogression and migmatisation

relations entre dynamismes eruptifs et realimentations magmatiques d origine profonde au popocatepetl the modern volcano popocatepetl is 30 000 50 000 years old until 5000 years bp its volcanic activity led to the construction of a 2000 m high cone the el fraile volcano this edifice was later topped by the popocatepetl summit the volcanic activity was characterized by long term construction by lava flows alternating with periods of 1000 2000 years of mixed explosive and effusive activity the el fraile volcano experienced three periods of this type marked by back falling pyroclastic flows with heterogeneous magma products and thick air fall deposits ash and scoria the first one occurred more than 10 000 years bp the second between 10 000 and 8000 years bp the third from 5000 to 3800 years bp each of these periods showed violent explosive episodes alternating with lava flows in cycles of 100 to several hundreds of years in duration whenever the explosive activity occurred it destroyed the upper part of the volcano opening large craters after a 2500 year period of lava flow constructi

catastrophic debris avalanche deposit of socompa volcano northern chile between 10 000 and 500 yr ago the socompa volcano in northern chile experienced a catastrophic collapse of a 70 deg sector of the original cone causing a debris avalanche that descended nearly 3000 m vertically and traveled more than 35 km from the volcano the deposits cover some 490 sq km and have a minimum volume of 15 cu km parts of the original cone slumped in a nearly coherent form and are now preserved as large blocks more than 400 m high the primary avalanche traveled northwestward over sloping ground before coming to rest transiently forming a prominent marginal ridge and then slid away northeastward to form a secondary flow overriding much of the primary avalanche deposit abundant prismatic jointed dacite blocks within the debris avalanche deposit and a thin fine grained pumiceous deposit beneath it suggest that the collapse was triggered by magmatic activity and may have been accompanied by a violent lateral blast collapse was followed by eruption of pumiceous pyroclastic flows and extrusion of voluminous dacite domes

the colima volcanic complex mexico part ii late quaternary cinder cones since the late pleistocene eleven cinder and lava cones have erupted on the floor of the southern colima graben ne and nw of the large active andesitic volcano colima scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites 9 leucite basanites 3 and minettes 15 the commonest variety of mica lamprophyre these samples represent primitive high temperature magmas with 47 6 50 3 sio2 7 4 15 3 mgo 2 5 4 4 k2o and 2 2 9 9 normative nepheline all members of this basic alkalic suite contain mg olivine fo75 94 chromite augite and late plagioclase and titanomagnetite the petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile bearing phases phlogopite apatite and analcime during late stages of crystallization as these phases increase in abundance presumably reflecting a rise in magmatic volatile content there are corresponding increases in the whole rock concentrations of 16 incompatible elements although these incompatible elements are relatively abundant even in the basanites many are highly concentrated in the minettes ba 4 200 ppm sr 3 100 ppm zr 550 ppm ce 190 ppm hf 18 ppm among the incompatible elements the degrees of enrichment in the minettes relative to the basanites decrease in the order h th ce la nd zr hf u ba sm eu pb p nb sr ti these enrichments may reflect the increasing importance of minor incompatible element rich mantle phases during partial melting the concentrations of alkali metals k na rb and cs do not correlate with these other elemental enrichments the leucite basanties have similar incompatible element contents to many minettes differing from them only in the presence of leucite rather than analcime and ti f rich groundmass phlogopite rather than hydrous phlogopite phenocrysts thus the leucite basanites represent relatively dry equivalents of minettes two of the eleven cinder cones are calc alkaline in nature and do not belong to the basanite minette group the easternmost cone is constructed of high al basalt and the northernmost of basaltic andesite the high al basalt 49 5 sio2 9 3 mgo 221 ppm ni closely approximates a parental magma to the post caldera andesitic suite of v colima 56 5 61 6 sio2 the basaltic andesite is relatively enriched in incompatible elements compared to the high al basalt v colima trend the ne normative basanite minette samples are highly enriched in incompatible elements while the contemporaneous hy qz normative calc alkaline suite encompassing the high al basalt and v colima s andesites is characterized by relatively low abundances of these elements no likely mineral assemblage can relate the alkaline and calc alkaline suites through crystal fractionation they probably represent fundamentally different melting events during the quaternary the main focus of andesitic volcanism in the southern colima graben has migrated southward with time volcan colima marks its present position 5 km south of the pleistocene volcano nevado de colima and another 15 km from the still older volcan cantaro the eruptions of basic alkalic magma probably occurred during the late stages of nevado s life and through the life of v colima they generally migrated from west to east with time towards v cantaro the most recent cone v apaxtepec is the only one east of the andesitic colima cantaro axis the oldest and the two youngest cones produced basanites while minettes dominated at cones of intermediate ages the cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of v colima the southern end of the colima graben can be viewed then as the volcanic analog of many classical post plutonic hypabyssal lamprophyre localities

oxide and sulphide minerals in highly oxidized rb depleted archaean granulites of the shevaroy hills massif south india oxidation states and the role of metamorphic fluids the shevaroy hills of northern tamil nadu southern india expose the highest grade granulites of a prograde amphibolite facies to granulite facies deep crustal section of late archaean age these highly oxidized quartzofeldspathic garnet charnockites generally show minor high tio2 biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements rb and th peak metamorphic temperatures garnet orthopyroxene and pressures garnet orthopyroxene plagioclase quartz are near 750 degrees c and 8 kbar respectively pervasive veinlets of k feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene they are suggested to have been produced by a low h2o activity migrating fluid phase under granulite facies conditions most likely a concentrated chloride carbonate brine with high alkali mobility accompanied by an immiscible co2 rich fluid silicate oxide and sulphide mineral assemblages record high oxygen fugacity pyroxenes in the felsic rocks have high mg mg fe 0 5 0 7 the major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite utilizing three independent oxygen barometers ferrosilite magnetite quartz ferrosilite hematite quartz and magnetite hematite in conjunction with garnet orthopyroxene exchange temperatures samples with x hm ilm 0 1 yield a consistent oxygen fugacity about two log units above fayalite stability less oxidized samples x hm ilm 0 4 pyrite is the dominant sulphide and pyrrhotite is absent pyrite grains in these samples have marginal alteration to magnetite along the rims signifying a high temperature oxidation event moderately oxidized samples 0 1 x hm ilm 0 4 have abundant co existing pyrrhotite pyrite and magnetite the most reduced granulite samples have pyrrhotite as the dominant sulphide with little or no pyrite and no coexisting magnetite chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples textures in some samples suggest that it formed as an exsolution product from pyrrhotite extensive vein networks of magnetite and pyrite associated principally with the pyroxene and amphibole give evidence for a pervasive highly oxidizing fluid phase thermodynamic analysis of the assemblage pyrrhotite pyrite and magnetite yields consistent high oxidation states at 700 800 degrees c and 8 kbar the oxygen fugacity in our most oxidized pyrrhotite bearing sample is 10 12 65 bar at 770 degrees c there are strong indications that the shevaroy hills granulites recrystallized in the presence of an alkali rich low h2o activity fluid probably a concentrated brine it cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids the high correspondence of maximally rb depleted samples with the highest recorded oxidation states suggests that the rb depletion event coincided with the oxidation event probably during breakdown of biotite to orthopyroxene k feldspar we speculate that these alterations were effected by exhalations from deep seated alkali basalts which provided both heat and high oxygen fugacity low a h2o fluids it will be of interest to determine whether greatly rb depleted granulites in other precambrian terranes show similar highly oxidizing signatures

analysis of the 2002 may earthquake sequence in the central pyrenees consequences for the evaluation of the seismic risk at lourdes france summary three earthquakes of magnitudes 4 6 4 3 and 3 7 occurred in 2002 may at two locations 20 km from the pilgrimage city of lourdes in the french pyrenees they were well recorded by the permanent pyrenean seismic networks by a temporary local network as well as by accelerometric stations in order to understand their tectonic contexts and to come to a better evaluation of the seismic risk at lourdes a detailed analysis of these events is performed the first two events are located south of lourdes in an area where only a few earthquakes have occurred up to now their focal solutions derived from first motion polarities indicate reverse faulting with a n110 e strike consistent with the geological structures 10 aftershocks were recorded and relocated with respect to the main events benefiting from the waveform similarity of the various events this analysis reveals that the two main events concern probably the same fault the second rupture being in the prolongation of the first one whereas the other small aftershocks are located on fault segments in the vicinity of the hypocentre of the second event the third large event located to the se of lourdes involves a normal mechanism with a n120 e plane parallel to the main geological structures it occurred in a region of intense activity including in particular an event of maximum macroseismic intensity ix in 1660 the first two events are at the boundary of a large quiet zone in order to understand the related structural context a new crustal tomographic model has been computed it reveals that this quiet zone coincides with a block of high p velocity in contrast the seismicity appears to be stronger at the northern and eastern boundaries of this block the accelerometric data of the three main events recorded at lourdes have been used to estimate the maximum peak ground accelerations in this city if a large event occurred similar to those which damaged the city in the seventeenth and eighteenth centuries horizontal accelerations of 0 25 0 07 g are predicted in the frequency domain 1 5 hz at the location of the sanctuary for a magnitude 6 event occurring 10 km away from the city taking into account the error bars these values could in some cases exceed those specified by the building codes in this region

petrologic evidence for k feldspar metasomatism in granulite facies rocks abstract we present evidence for k feldspar metasomatism in charnockitic granulites from two well known terranes the shevaroy hills massif s india 750 c 8 kbars and the bamble sector s e norway 790 c 7 5 kbars in the form of k feldspar veins principally along plagioclase and quartz grain boundaries and in the form of highly variable antiperthitic patches of k feldspar in an uneven scattering of plagioclase grains with one exception orthopyroxene or amphibole grains in contact with these k feldspar veins show no alteration to secondary biotite indicating that the h 2 o activity of the fluids responsible for these veins must have been relatively low a high ba concentration in these veins also suggests a metasomatic origin point counted back scattered electron photomicrographs along with microprobe analyses provide reintegrated k feldspar and plagioclase compositions for 8 to 12 predetermined random areas per thin section for three samples from bamble and three samples from the shevaroy hills these reintegrated feldspar compositions plot over a range of temperatures on the feldspar ternary for each sample and indicate saturation temperatures above the mean temperature for either region with a few reintegrated compositions above the 1000 c isotherm and with the lower cut off temperatures at 700 c and 600 c for the shevaroy and bamble samples respectively these patterns suggest that exsolution alone could not have been responsible for the formation of the k feldspar veins and patches in these rocks we suggest that these veins are due to the influx of complex supercritical low h 2 o activity brines shortly after peak metamorphic conditions that this influx continued during the initial phases of post peak metamorphic uplift and that these fluids represent the first stage in a series of fluid influxes in which the h 2 o activity increased as uplift continued reaction of these brines with potassium undersaturated plagioclase grains formed k feldspar veins along grain boundaries and fractures as well as diffusing into the plagioclase which became supersaturated and exsolved k feldspar as antiperthite patches during uplift and cooling formation of secondary biotite halos around orthopyroxene in one sample associated with these k feldspar veins can be explained by heightened h 2 o activity in the brines due to enrichment in h 2 o emplacement of the veins at lower pressure during uplift or both compared to the other shevaroy and bamble samples

may 18 1980 mount st helens deposits in south coldwater creek washington beginning at 0832 pacific daylight time pdt mount st helens washington produced a rockslide debris avalanche and a lateral explosion blast that generated a laterally directed pyroclastic surge called here a blast surge the resulting deposits are thickest in the east west trending valley of north fork toutle river immediately north of the volcano but are almost as thick in south coldwater creek a small tributary of the toutle river in the lee of an east west trending ridge johnston ridge just north of the north fork toutle river the avalanche surmounted johnston ridge 300 m relief and flowed down the valley of south coldwater creek closely following the avalanche and in part contemporaneously with it the blast surge left thin up to 0 5 m deposits on johnston ridge and thick up to 15 m deposits in south coldwater creek these deposits have a lower layer layer a1 which is friable massive poorer in fines particles 1 16 mm and more poorly sorted than the upper layer layer a2 layer a2 is less friable because it contains more silt and clay sized material beneath layer a1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface this zone designated as layer a0 is a poorly mixed layer containing schlieren of soil shattered volcanic fragments and considerable quantities of organic material layer a1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge layer a2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head layer a0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge it is concluded that a strongly fines depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high velocity particulate density current

experimental partitioning of rb and k between phlogopite and a concentrated k rb cl brine implications for the role of concentrated kcl brines in the depletion of rb in phlogopite and the stability of phlogopite during charnockite genesis in this study phlogopite synthesis experiments were carried out at 800 c and at 1000 and 2000 mpa for low concentrations of rb xrb rb rb k 0 1 in a concentrated k rb cl brine rb k cl salt xh2obulk 0 3 for each experiment solid run products consisted of approximately 70 80 phlogopite xrb 0 03 0 12 with both a minor celadonite component 10 as well as a cl component in the order of 5 12 m relative to oh the remaining 20 30 consisted of approximately equal amounts of newly grown sanidine and quartz there is no physical evidence that any of these phases are metastable rb fractionated strongly into the phlogopite whereas for the k feldspar rb was almost equally distributed between the feldspar and fluid salt at 800 c the derived distribution coefficients kdphlg fluid rb k and kdsnd fluid rb k for rb henry s law are 1 88 0 12 and 0 86 0 14 respectively at 2000 mpa and 1 52 0 05 and 0 6 0 4 respectively at 1000 mpa several conclusions may be drawn from these experimental results in conjunction with observations in the field the first is that phlogopite with a minor celadonite component and 5 12 m cl relative to oh can form in a relatively dry system consisting of a concentrated rb k cl brine k rb cl salt xh2obulk 0 3 at high temperature and pressures i e 800 c and 1000 2000 mpa the presence of celadonite and cl apparently stabilises the phlogopite in the presence of quartz at low h2o activities as compared to end member oh phlogopite which breaks down to enstatite k feldspar in the presence of quartz a second conclusion is that in concentrated kcl brines rb is preferentially partitioned into the phlogopite under these p t and h2o conditions this agrees with similar studies involving 1 and 2 m k rb cl salt solutions as well as a simple k rb exchange experiments between end member oh phlogopite and a concentrated k rb cl brine xh2ofluid 0 6 xrb 0 05 at 800 c and 1000 mpa

the sounds of small earthquakes quantitative results from a study of regional macroseismic bulletins an original database of regional bulletins is analyzed to derive first order relationships that link the sounds generated by small earthquakes to the amplitude of ground motions and to local magnitude scales the database is a collection of about 1000 macroseismic observations collected during the 1984 1999 period in the french pyrenees from a staff of regular observers for each observation the main parameters are the qualitative perception of the sound faint or loud the hypocentral distance and the earthquake local magnitude expressed on the scale used by the french laboratoire de detection et de geophysique ldg network a local relationship between the magnitude the hypocentral distance and the amplitude of the p wave vertical ground displacement recorded in local seismological stations is established audibility thresholds of small earthquakes are then expressed as a function of richter m l magnitude and hypocentral distance the values obtained provide lower bounds for human perception of earthquakes

ground coupled air waves and diffracted infrasound from the arequipa earthquake of june 23 2001 1 on june 23 2001 a strong earthquake measuring mw 8 4 occurred along the coast of south central peru coherent infrasonic waves were detected over a period of one hour by the is08 infrasound station in bolivia analysis of the ground coupled air waves shows that the rupture propagated from the northwestern to the southeastern part of the fault with a rupture velocity of 3 3 km s the azimuth variation of the infrasonic waves is attributed to a distribution of secondary sources along the highest mountain ranges which excite infrasonic waves that are diffracted to the ground the predominant source of infrasound is likely distributed along the andean cordillera using the azimuth and arrival time determination the horizontal scale size of the distant source regions of infrasonic waves is reconstructed over distances greater than 400 km

shock melting of k feldspar and interlacing with cataclastically deformed plagioclase in granitic rocks at toqqusap nunaa southern west greenland implications for the genesis of the maniitsoq structure abstract folded sheets of mesoarchaean leucocratic plagioclase k feldspar mesoperthite bearing granitic rocks in the toqqusap nunaa area of the maniitsoq structure west greenland are characterised by their very fine grain sizes and microstructures without normal igneous or planar linear tectonic fabrics quartz forms equidimensional and branching ductilely deformed aggregates and bifurcating panels with protrusions constrictions and chains of ball shaped grains with healed radiating intergranular fractures plagioclase an10 20 was cataclastically deformed and comminuted whereas k feldspar and mesoperthite are devoid of cataclastic microstructures k feldspar forms dispersed highly irregular grains with numerous cusps and saddles indicating almost ubiquitous direct shock melting of this mineral it is commonly located along former fractures in plagioclase resulting in an interlaced feldspar microstructure with contact shapes indicating subsequent melting of plagioclase directly adjacent to k feldspar mesoperthite forms separate rounded and irregular grains with protrusions and cusped margins indicating crystallisation from melts some mesoperthite grains are texturally and compositionally heterogeneous and contain internal lenses of k feldspar and or plagioclase other mesoperthite grains comprise coarsened unzipped areas presumably due to localised fluid controlled dissolution reprecipitation processes the ternary feldspar precursor of the mesoperthite is interpreted as having crystallised from variably effectively mixed k feldspar shock melts and plagioclase contact melts direct melting of k feldspar but no whole rock melting requires shock metamorphism with a short lived temperature excursion to above the melting temperature of k feldspar 1300 c the presence of three different feldspar species and absence of chemical zonation magmatic mantling or metamorphic coronas furthermore hinders interpretations solely by means of endogenic magmatic or ultrametamorphic processes the feldspar microstructures with direct k feldspar melts in the leucocratic orthogneisses of the toqqusap nunaa area thus confirm that the recently discovered maniitsoq structure represents the deep crustal remnants of a giant mesoarchaean meteorite impact

a gigantic bezymianny type event at the beginning of modern volcan popocatepetl the history of volcan popocatepetl can be divided into two main periods the formation of a large primitive volcano approximatively 30 km wide on which is superimposed a modern cone 6 8 km in diameter and 1700m high a major event of bezymianny type marks the transition between these two dissimilar periods the activity of the primitive volcano was essentially effusive and lasted several hundred thousands of years the total volume of products ejected by the volcano is of the order of 500 600 km3 its last differentiated magmas are dacitic a gigantic debris flow d f spread on the southern side is related to the bezymianny type event which destroyed the summit area of the ancient edifice an elliptical caldera 6 5 11 km wide was formed by the landslide its deposits with a typical hummocky surface cover 300 km2 for a volume of 28 30 km3 numerous outcrops belonging to this debris flow show slabs of more or less fractured and dislocated rocks that come from the primitive volcano these deposits are compared to two studied debris flows of similar extent and volume the mount shasta and colima s d f this eruption takes a major place in the volcanologic and magmatic history of popocatepetl pyroclastic products of surge type with laminites and crude layers ashflows and pumiceous airfall layers are directly related to this event and begin the history of the modern volcano probably less than 50 000 years ago in addition a second andesitic and dacitic phase rose both from the central vent forming the basis of modern popo and from lateral vents the terminal cone is characterized by long periods of construction by lava flows alternating with phases of destruction the duration of these episodes being 1000 to 2000 years the cone is composed of two edifices the first volcan el fraile began with effusive activity and was partly destroyed by three periods of intense explosive activity the first period occurred prior to 10 000 years b p the second from 10 000 to 8000 years b p and the third from 5000 to 3800 years b p each period of destruction shows cycles producing collapsing pyroclastic flows or nuees of the st vincent type related to the opening of large craters plinian air fall deposits and minor lava flows the second edifice the summit popo produced lava flows until 1200 years b p and since that time entered into an explosive period two cataclysmic episodes each including major pyroclastic eruptions occurred 1200 and 900 1000 years ago during the pre hispanic and historic times effusive activity was restricted entirely to the summit area alternating with plinian eruptions nevertheless despite the quiet appearance of the volcano the last period of pyroclastic activity which started 1200 years ago may not have ended and can be very dangerous for the nearby populations

catastrophic debris avalanche from ancestral mount shasta volcano california a debris avalanche deposit extends 43 km northwestward from the base of mount shasta across the floor of shasta valley california where it covers an area of at least 450 km 2 the surface of the deposit is dotted with hundreds of mounds hills and ridges all formed of blocks of pyroxene andesite and unconsolidated volcaniclastic deposits derived from an ancestral mount shasta individual hills are separated by flat topped laharlike deposits that also form the matrix of the debris avalanche and slope northwestward about 5 m km radiometric ages of rocks in the deposit and of a postavalanche basalt flow indicate that the avalanche occurred between about 300 000 and 360 000 yr ago an inferred average thickness of the deposit plus a computed volume of about 4 km 3 for the hills and ridges indicate an estimated volume of about 26 km 3 making it the largest known quaternary landslide on earth

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

spatial patterns of earthquake sounds and seismic source geometry earthquake sounds are heard during or immediately before an event up to several kilometres around the epicentral area sound data as heard by people have been collected by way of macroseismic questionnaires showing that sound audibility does not follow isotropic patterns the explanation of this effect is found in the behaviour of the particular radiation pattern of the p waves for each earthquake in connection with its focal mechanism this result opens the possibility to get more information about the earthquake source parameters in the future and past events making use of non instrumental observations

caldera formation at volcan colima mexico by a large large holocene volcanic debris avalanche abstract about 4 300 years ago 10 km 3 of the upper cone of ancestral volcan colima collapsed to the southwest leaving a horseshoe shaped caldera 4 km in diameter the collapse produced a massive volcanic debris avalanche deposit covering over 1550 km 2 on the southern flanks of the volcano and extending at least 70 km from the former summit the avalanche followed a steep topographic gradient unobstructed by barriers resulting in an unusually high area volume ratio for the colima deposit the apparent coefficient of friction fall height distance traveled for the colima avalanche is 0 06 a low value similar to those of other large volume deposits the debris avalanche deposit contains 40 75 angular volcanic clasts from the ancestral cone a small proportion of vesicular blocks that may be juvenile and in distal exposures rare carbonate clasts plucked from the underlying surface by the moving avalanche clasts range in size to over 20 m in diameter and are brecciated to different degrees pulverized and surrounded by a rock flour matrix the upper surface of the deposit shows prominent hummocky topography with closed depressions and surface boulders a thick coarse grained compositionally zoned scoria fall layer on the upper northeastern slope of the volcano may have erupted at the time of collapse a fine grained surge layer is present beneath the avalanche deposit at one locality apparently representing an initial blast event most of the missing volume of the ancestral volcano has since been restored at an average rate of 0 002 km 3 yr through repeated eruptions from the post caldera cone as a result the southern slope of volcan colima may again be susceptible to collapse over 200 000 people are now living on primary or secondary deposits of the debris avalanche and a repetition of this event would constitute a volcanic disaster of great magnitude ancestral volcan colima grew on the southern trenchward flank of the earlier and larger volcano nevado de colima trenchward collapse was favored by the buttressing effect of nevado the rapid elevation drop to the south and the intrusion of magma into the southern flank of the ancestral volcano other such trenchward younging paired volcanoes are known from mexico guatemala el salvador chile and japan the trenchward slopes of the younger cones are common sites for cone collapse to form avalanche deposits as occurred at colima and popocatepetl in mexico and at san pedro volcano in chile

the evolution of volcanic systems following sector collapse abstract sector collapses affect volcanic edifices across all tectonic settings and involve a rapid redistribution of mass comparable in scale to the largest magmatic eruptions the eruptive behaviour of a volcano following sector collapse provides a test of theoretical relationships between surface loading and magma storage which imply that collapse driven unloading may lead to changes in eruption rate and erupted magma compositions large sector collapses are infrequent events globally with all historical examples being relatively small in comparison to many of the events documented in the geological record as a result exploration of the impacts of sector collapse on eruptive behaviour requires detailed investigation of prehistoric collapses but this is often hindered by poorly resolved stratigraphic relationships and dating uncertainties nevertheless observations from a number of volcanoes indicate sharp changes in activity following sector collapse here a global synthesis of studies from individual volcanoes in both arc and intraplate settings is used to demonstrate a number of common processes in post collapse volcanism multiple examples from large 5 km3 sector collapses in arc settings show that collapse may be followed by compositionally anomalous large volume and often effusive eruptions interpreted to originate via disruption of a previously stable upper crustal reservoir these anomalous eruptions highlight that magma compositions erupted during periods of typical i e unperturbed by sector collapse volcanism may not be representative of the range of compositions stored within a vertically extensive crustal reservoir if eruptible magma is not present upper crustal reservoirs may rapidly solidify following collapse without further eruption allowing more mafic compositions to ascend to the surface with only limited upper crustal modification resulting in edifice regrowth at temporarily elevated eruption rates subsequent re establishment of an upper crustal reservoir further supports a relationship between surface loading and crustal storage but long term chemical and mineralogical differences between pre and post collapse evolved magmas imply that a newly developed reservoir can overprint the influence of a preceding reservoir forming a spatially and compositionally distinct plumbing system these broad patterns are replicated in intraplate settings despite differences in scale and melting processes current evidence suggests that post collapse evolution of intraplate volcanoes can be explained by unloading induced destabilisation of the magma plumbing system rather than increased melt production what emerges from an apparently diverse set of observations is a systematic behaviour that strongly supports a coupling between edifice growth and magma ascent storage and pressurisation eruption rates erupted compositions and the style of volcanism at any particular system may thus be modulated from the surface and long term shifts in surface behaviour may occur without any changes in the deep parts of magmatic systems observations of sharp post collapse changes in erupted compositions including the ascent of primitive mafic magmas also require a crystal dominated mid to upper crustal reservoir consistent with recent models of crustal magmatic systems

the source of infrasound associated with long period events at mount st helens 1 during the early stages of the 2004 2008 mount st helens eruption the source process that produced a sustained sequence of repetitive long period lp seismic events also produced impulsive broadband infrasonic signals in the atmosphere to assess whether the signals could be generated simply by seismic acoustic coupling from the shallow lp events we perform finite difference simulation of the seismo acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere the effects of topography velocity structure wind and source configuration are considered the simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of p sv and rayleigh wave energy converted locally along the propagation path and acoustic energy originating from the source epicenter although the horizontal acoustic velocity of the latter is consistent with our data the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source the observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic lp events the observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated highly permeable material heating by magmatic activity causes pressure to rise periodically reaching the pressure threshold for rupture of the valve sealing the crack sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack initiating resonance of the remaining fluid subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13 4 km to the nw of the volcano are attributed primarily to atmospheric boundary layer propagation effects superimposed upon amplitude changes at the source

storage conditions and magma processes triggering the 1818 ce plinian eruption of volcan de colima abstract volcan de colima has an eruptive history punctuated with explosive plinian eruptions followed by long periods of effusive events both of the best documented plinian eruptions which occurred in 1818 ce and 1913 ce emitted andesitic magma 58 wt sio 2 that contains pl opx cpx amph fe ti oxides ap resorbed ol whereas effusive magmas from the 1819 1868 and 1962 2015 periods are particularly scarce in amphibole and are slightly more silicic 59 61 wt sio 2 pumice from the 1818 and 1913 deposits contain three groups of amphibole with different abundance texture and composition composition is the most distinctive between them the first group consists of crystals with high al iv 1 84 2 32 the second group of amphiboles is less abundant in al iv 1 45 1 76 and the third has low al iv 1 29 1 39 in contrast amphibole from two colima prehistoric lavas can only be grouped as medium al iv 1 45 1 76 a set of hydrothermal experiments were carried out using a 1818 natural sample to investigate amphibole compositional variability with changes in pressure and temperature two groups of amphibole found in the plinian deposits can be reproduced experimentally between 875 and 900 c and 150 200 mpa another type of amphibole could not be equilibrated in the 1818 explosive colima magma and their origin can be attributed to mixing with more mafic magmas we found that amphibole abundance is greater in explosive events and that mafic inherited amphiboles can only be found in colima plinian deposits when the influx of mafic magma is large or frequent enough and they mix with more silicic colima magma then explosive events are more likely if mafic melts cannot mix with the resident magmas or just pond and heat the reservoir regardless of location in the plumbing system then magmas can decompress more slowly and amphibole will have time to completely react out

volcano stratigraphy and physical characterization of the san mateo pomez unit cerritos member temascalcingo volcano central mexico abstract the temascalcingo volcano is a dacitic stratovolcano located at the eastern part of the acambay graben a tectonic basin in the central sector of the trans mexican volcanic belt tmvb which is an active mostly calc alkaline volcanic arc that crosses mexico from the pacific ocean to the gulf of mexico this study characterises the explosive activity of the san mateo pumice eruption that is the most recent and largest paroxysmal episode of this volcano and the last unit of the cerritos member indeed this eruption is associated with an undated debris avalanche event consisting of a major sector collapse developed on the western flank of the volcano stratigraphic data suggest a highly explosive eruption that generated a relatively thick up to 10 m and widespread pumice fallout deposit trending to the ne and an aerial distribution of at least 100 km2 this subplinian type eruption of vei 4 lasted about 9 h and produced a plume height up to 12 km above the vent i e 14 km above sea level this type of explosive eruption is relatively common in the tmvb so a comprehensive geological study with the corresponding numerical simulations like the one that was carried out contributes to the understanding of the explosive volcanism of modest size stratovolcanoes that are common in the tmvb

titaniferous magnetite ilmenite thermometry and titaniferous magnetite ilmenite orthopyroxene quartz oxygen barometry in granulite facies gneisses bamble sector se norway implications for the role of high grade co 2 rich fluids during granulite genesis oxygen fugacities have been estimated for a wide distribution of samples from the granulite facies terrane region c of the bamble sector se norway using both the titaniferous magnetite ilmenite and orthopyroxene titaniferous magnetite quartz oxygen barometers these oxygen fugacities are estimated using temperatures calculated from the titaniferous magnetite ilmenite thermometer of ghiorso and sack 1991 and are both internally consistent with each other as well with the thermometer in samples for which the estimated temperature is high the two oxygen barometers show good agreement whereas agreement is poor for low temperature samples in these low temperature samples oxygen fugacities estimated from titaniferous magnetite ilmenite are considerably less than those estimated from orthopyroxene titaniferous magnetite quartz an increase in this discrepancy with decrease in temperature appears to reflect preferential resetting of the hematite component in the ilmenite grains without significant alteration of the more numerous titaniferous magnetite grains this is due in part to greater re equilibration of the ilmenite grains during retrograde interoxide resetting between the ilmenite grains and the titaniferous magnetite grains the mean temperature for the non reset samples 791 17 c 1 is in good agreement with temperatures obtained from garnet orthopyroxene kd exchange thermometry in the same region 785 795 c 1 harlov 1992 2000a most non reset oxygen fugacities range from log10 f o2 14 to 11 8 or approximately 0 5 1 5 log units above quartz fayalite magnetite at 7 5 kbar both these temperatures and the range of oxygen fugacities are in good agreement with those estimated using the titaniferous magnetite ilmenite thermometer oxygen barometer of andersen et al 1991 the quilp equilibrium quartz ulvospinel ilmenite pyroxene is used to project self consistent equilibrium temperatures and oxygen fugacities for samples reset due to hematite loss from the ilmenite grains these projected temperatures and oxygen fugacities agree reasonably well with the non reset samples the mean projected quilp temperature is 823 6 c 1 this result supports the conclusion that low titaniferous magnetite ilmenite temperatures down to 489 c and accompanying low oxygen fugacities are the result of hematite loss from the ilmenite grains non reset oxygen fugacities lie approximately 1 5 log10 units above the upper graphite stability curve indicating that the stable c o h fluid phase interacting with these gneisses whether regionally or locally was co2 this is borne out by the presence of numerous co2 rich fluid inclusions in these rocks

infrasound from earthquakes 1 infrasonic signals have been observed from 31 earthquakes by arrays of microphones operated by the los alamos national laboratory between 1983 and 2003 the properties of the signals are presented signal amplitudes corrected for propagation and distance show a relation with seismic magnitude the variance in the relation is understood primarily in terms of the uncertainties or errors in the ground motion deduced from an independent data set and the stratospheric winds which strongly influence signal propagation signal durations can extend over many minutes a relation is found between signal duration and magnitude to understand this we propose a model in which regions distant from the epicenter are excited by seismic surface waves the surface motion of these regions in turn produces signals which precede or follow the signals from the epicenter analysis failed to detect signals from 56 earthquakes during the observation period predicted signal to noise ratios for these earthquakes indicated that the signals would have been too weak for detection

the nevados de payachata volcanic region 18 s 69 w n chile subduction related volcanism in the nevados de payachata region of the central andes at 18 s comprises two temporally and geochemically distinct phases an older period of magmatism is represented by glaciated stratocones and ignimbrite sheets of late miocene age the pleistocene to recent phase 0 3 ma includes the twin stratovolcanoes volcan pomerape and volcan parinacota the nevados de payachata volcanic group and two small centers to the west i e caquena and vilacollo both stratovolcanoes consist of an older dome and flow series capped by an andesitic cone the younger cone i e v parinacota suffered a postglacial cone collapse producing a widespread debris avalanche deposit subsequently the cone reformed during a brief second volcanic episode a number of small relatively mafic satellitic cinder cones and associated flows were produced during the most recent activity at v parinacota at the older cone i e v pomerape an early dome sequence with an overlying isolated mafic spatter cone and the cone forming andesitic dacitic phase mostly flows have been recognized the two nevados de payachata stratovolcanoes display continuous major and trace element trends from high k2o basaltic andesites through rhyolites 53 76 sio2 that are well defined and distinct from those of the older volcanic centers petrography chemical composition and eruptive styles at v parinacota differ between pre and post debris avalanche lavas precollapse flows have abundant amphibole at sio2 59 wt and lower mg numbers than postcollapse lavas which are generally less silicic and more restricted in composition compositional variations indicate that the magmas of the nevados de payachata volcanic group evolved through a combination of fractional crystallization crustal assimilation and intratrend magma mixing isotope compositions exhibit only minor variations pb isotope ratios are relatively low 206pb 204pb 17 95 18 20 and208pb 204pb 38 2 38 5 87sr 86sr ratios range 0 70612 0 70707 143nd 144nd ratios range 0 51238 0 51230 and 18osmow values range from 6 8 o to 7 6 o smow a comparison with other central volcanic zone centers shows that the nevados de payachata magmas are unusually rich in ba up to 1800 ppm and sr up to 1700 ppm and thus represent an unusual chemical signature in the andean arc these chemical and isotope variations suggest a complex petrogenetic evolution involving at least three distinct components primary mantle derived melts which are similar to those generated by subduction processes throughout the andean arc are modified by deep crustal interactions to produce magmas that are parental to those erupted at the surface these magmas subsequently evolve at shallower levels through assimilation crystallization processes involving upper crust and intratrend magma mixing which in both cases were restricted to end members of low isotopic contrast

the cohesive naranjo debris flow deposit 10 km3 a dam breakout flow derived from the pleistocene debris avalanche deposit of nevado de colima volcano mexico abstract mass movement processes on volcanic terrains such as landslides and debris avalanches can cause the obstruction of main drainages producing the formation of temporary dams a good example of this occurred 18 5 ka ago when the eastern flank of the nevado de colima volcano collapsed producing a debris avalanche deposit that was previously described as one of the largest in the world the deposit extended from the volcanic summit as far as the pacific coast 120 km away new stratigraphic sedimentological and componentry data suggest that the volcanic collapse of nevado de colima resulted in a debris avalanche that traveled 20 km southeast to the naranjo river there it crashed against a topographic barrier consisting of cretaceous limestones cerro la carbonera and the flow direction was diverted to the south down the naranjo river channel for another 25 km before the avalanche came to a halt the obstruction of the drainage produced a temporary dam that stored ca 1 km3 of water and deposited fluvial and slack water sediments some time after the damming the accumulated water sediment load was able to overtop the obstructing material and to release a breakout flow with a calculated initial flow discharge of 3 5 million m3 s the resulting flood cohesive debris flow followed the channel of the naranjo river and due to the high erodibility of the channel and introduction of substrate material the debris flow progressively increased its volume up to 10 km3 six times its initial volume this study highlights the relevance of evaluating the potential remobilization of debris avalanche deposits to initiate large magnitude cohesive debris flows therefore the hazard and risk analysis of future potential events of this nature must consider the pre eruption conditions and the topography surrounding a volcano

eyewitness stratigraphy chemistry and eruptive dynamics of the 1913 plinian eruption of volcan de colima mexico abstract based on the stratigraphic record of the deposits analysis of previous works historic archives and eyewitness accounts the 1913 eruption of volcan de colima was reconstructed the eruption started in 17 january and peaked in 20 january 1913 it occurred in three main phases 1 an opening phase with the generation of merapi type pyroclastic flows units f1 f2 f3 and a pyroclastic surge s1 2 a vent clearing phase with strong explosions that produced vulcanian soufriere type pyroclastic flows f4 and a pyroclastic surge s2 which destroyed the summit dome decompressing the magma system and 3 a plinian phase with the establishment of a 23 km high column dispersing a fallout c1 to the ne followed by the collapse of the column that generated a pyroclastic surge s3 and 15 km long pumice rich pyroclastic flows f5 after the eruption remobilization of the pyroclastic material generated lahars in main gullies around the crater fallout c1 blanketed an area of 191 318 km2 covering the cities of guzman guadalajara and saltillo coahuila located at 725 km from the source it had a volume of 1 4 km3 0 57 km3 dre dense rock equivalent the total volume of pyroclastic flow and surge deposits was 0 26 km3 0 07 km3 dre giving a total volume of the eruption of 1 66 km3 0 64 km3 dre the plinian column lasted 4 6 h with a total mass of 1 5 1012 kg and a mass eruption rate of 9 02 107 kg s the column height and the ejected magma volume indicate that the 1913 eruption had a vei 5 being the largest event in the historical record of colima volcano juvenile scoria and pumice consisted of pl opx cpx hbl accessory titanomagnetite ap and ol with reaction rims chemistry of juvenile scoria and pumice samples is andesitic very homogeneous 58 3 0 5 wt sio2 and similar to the 1818 juvenile products 58 9 0 2 wt sio2 the presence of banded scoria olivine phenocrysts with reaction rims and trace element variations strongly suggest that the 1913 eruption was caused by a magma mixing event

petrology and geothermobarometry of granulite facies metapelites from the hisoy torungen area south norway new data on the sveconorvegian p t t path of the bamble sector the high grade rocks metapelite quartzite metagabbro of the hisoy torungen area represent the south westernmost exposures of granulites in the proterozoic bamble sector south norway the area is isoclinally folded and a metamorphic p t t path through four successive stages m1 m4 is recognized petrological evidence for a prograde metamorphic event m1 is obtained from relict staurolite chlorite albite staurolite hercynite ilmenite cordierite sillimanite fine grained felsic material quartz and hercynite biotite sillimanite within metapelitic garnet the phase relations are consistent with a pressure of 3 6 0 5 kbar and temperatures up to 750 850 c m1 is connected to the thermal effect of the gabbroic intrusions prior to the main m2 sveconorwegian granulite facies metamorphism the main m2 granulite facies mineral assemblages quartz plagioclase k feldspar garnet biotite sillimanite are best preserved in the several metre wide al rich metapelites which represent conditions of 5 9 9 1 kbar and 790 884 c these p t conditions are consistent with a temperature increase of 80 100 c relative to the adjacent amphibolite facies terranes no accompanying pressure variations are recorded up to 1 mm wide fine grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction bt sil qtzgrt lq this dehydration reaction together with the absence of large scale migmatites in the area suggests a very reduced water activity in the rocks and xh2o 0 25 in the c o h fluid system was calculated for a metapelitic unit a low but variable water activity can best explain the presence or absence of fine grained felsic material representing a former melt in the different granulitic metapelites the strongly peraluminous composition of the felsic veinlets is due to the reaction grt former melt silcrd bt qtz h2o which has given poorly crystalline cordierite aggregates intergrown with well crystalline biotite the cordierite and biotite producing reaction constrains a steep first stage retrograde relative to m2 uplift path decimetre to metre wide strongly banded metapelites quartz plagioclase biotite garnet sillimanite inter layered with quartzites are retrograded to m3 amphibolite facies assemblages a p t estimate of 1 7 5 6 kbar 516 581 c is obtained from geothermobarometry based on rim rim analyses of garnet biotite plagioclase sillimanite quartz assemblages and can be related to the isoclinal folding of the rocks m4 greenschist facies conditions are most extensively developed in millimetre wide chlorite rich calcite bearing veins cutting the foliation

high grade k feldspar veining in granulites from the ivrea verbano zone northern italy fluid flow in the lower crust and implications for granulite facies genesis we present observations of extensive networks of k feldspar microveins associated with quartz and plagioclase in granulite facies rocks from the ivrea verbano zone northern italy back scattered electron images of the granulite facies samples show a continuous system of k feldspar veins along quartz plagioclase and plagioclase plagioclase grain boundaries that can be traced across the entire length of a thin section quartz grain rims in contact with these veins show evidence of extensive corrosion in contrast contact between the veins and the orthopyroxene and garnet grains is very clean with no signs of secondary alteration k feldspar quartz plagioclase of the same compositions as the veins are commonly found as near rim inclusions within garnet microprobe traverses from the interiors of the k feldspar veins to their rims show increasing ba indicating a metasomatic origin some veins display a boudinaged texture possibly related to the d2 deformation event in the ivrea verbano zone and the

repetitive mixing events and holocene pyroclastic activity at pico de orizaba and popocatepetl mexico the holocene volcanic activity which built up the present terminal cones of pico de orizaba and popocatepetl in eastern mexico was characterized by repeated pyroclastic saint vincent type eruptions radiocarbon data show that these paroxysmal events occurred at more or less regular intervals and were followed by moderate activity producing ash and pumice falls and andesitic lava flows from the summit craters

local infrasound signals from the tokachi oki earthquake 1 a great earthquake mw 8 3 and aftershock mw 7 4 occurred near hokkaido japan on september 25 2003 this earthquake and aftershock were sources of local and epicentral infrasound signals observed at two seismo acoustic arrays in the republic of korea local infrasound results when seismic signals near the receiver couple to the atmosphere the local cross coupling from seismic ground motion to infrasound signal at two seismo acoustic arrays enabled us to compute a seismic to infrasound transfer function at each array the observed transfer function quantifies the microphone response when the seismometer response is known when seismometers and infrasound gauges are collocated this procedure can provide field calibration

infrasonic imaging of the kunlun mountains for the great 2001 china earthquake 1 on november 14 2001 a strong earthquake occurred on the high plateau of western china at a distance of 1800 km from the epicenter coherent infrasonic waves were detected for more than one hour using both an inverse location procedure and a complete simulation of the radiated pressure field distant source regions are accurately located the seismo acoustic coupling along a mountain belt is clearly demonstrated such an event offers an opportunity to validate velocity models in the atmosphere as well as to improve the understanding of the amplification of ground displacement caused by the topography

mixed magma pyroclastic surge deposits associated with debris avalanche deposits at colima volcanoes mexico the transition between the terminal cones and the ancestral edifices of nevado de colima and fuego de colima volcanoes is marked by the deposits of gigantic volcanic debris avalanches of the mount st helens msh or bezymianny type unusual mafic juvenile fragments and cauliflower bombs as well as juvenile fragments of mixed and more evolved composition are abundant in dune bedded pyroclastic surge deposits directly associated with these catastrophic events at both volcanoes at nevado these mafic juvenile fragments represent the most primitive magma ever erupted by the volcano sio2 52 50 the lavas directly preceding and following the debris avalanche event are silicic andesites sio2 59 at fuego these juvenile fregments have 56 sio2 the lavas from the upper parts of the caldera wall are dacites 65 sio2 whereas the terminal cone is composed of andesites 57 to 62 sio2 at nevado petrologic evidence for interaction of mafic magma with andesitic or dacitic magma in a high level magma chamber just before the eruption of pyroclastic surge deposits consists of 1 banded juvenile bombs of intermediate composition 2 the range of composition of these bombs from sio2 52 to 58 3 the presence of highly magnesian olivine with reaction rims 4 inverse zoning in clinopyroxene with strong mg enrichment towards the rim 5 resorption of plagioclase and 6 significant compositional heterogeneity in the vitric phase volcanic debris avalanche events at nevado and fuego de colima may thus correspond with major breaks in the petrological evolution of the volcanoes and the start of a new magmatic cycle injection of mafic magma into the presently perched viscous surface dome of the active fuego cone as occurred in 1818 and 1913 could enhance the likelihood of southward collapse of the flank of an already unstable edifice and it must be considered in future hazard assessment of this active volcano risk to life and property for the entire colima region associated with such catastrophic phenomena would be immeasurably greater in comparison with hazards related to the last explosive outburst in 1913 which resulted in emplacement of pyroclastic flows over uninhabited areas of the upper flanks of the volcano

nb and zr behavior in rutile during high grade metamorphism and retrogression an example from the ivrea verbano zone abstract detailed textural observations and in situ analyses emp sims and la icp ms are used to characterize trace element behavior during prograde and retrograde metamorphic reactions involving rutile the ivrea verbano zone is a classic granulite area and rocks from the strona and d ossola valleys are an example of the amphibolite to granulite facies transition although different rock types occur in the area detailed sampling and petrographic work show that rutile only occurs in granulite facies paragneisses these rocks show a rich inventory of textures that allow not only for the investigation of trace element behavior in response to prograde rutile growth but also for the effect of post peak processes on rutile chemistry nb concentrations in rutile from lower grade samples show a larger spread from 500 to 5000 ppm within one sample when compared to those from higher grades this pattern can be modeled using prograde rutile growth formed from biotite breakdown zr concentrations in rutile are characterized by an anomalously large spread and a bimodal distribution maximum zr concentrations increase according to the general metamorphic gradient known for this area temperatures from zr in rutile thermometry although feasible are considerably higher than previous calculations increasing from ca 850 to 930 c a second cluster of zr concentrations in rutile occurs at rather constant concentrations ca 1000 ppm for all localities and is interpreted to be related to intense fluid influx at high temperature and or to post peak diffusional resetting favored by slow cooling rates alteration textures characterized by a complex network of micro veins are evidence for the late fluid influx the fluid strongly affected the rutiles which is evidenced by corrosion of older rutile grains and formation of rutile veinlets

solubility of anhydrite caso4 in nacl h2o solutions at high pressures and temperatures applications to fluid rock interaction anhydrite solubility in h2o nacl solutions was measured at 6 14 kbar 600 800 c and nacl mole fractions xnacl of 0 0 3 in piston cylinder apparatus solubilities were determined by weight changes of natural anhydrite in perforated pt envelopes confined with fluid in larger pt capsules in initially pure h2o at 10 kbar and 800 c caso4 concentration is low 0 03molal though much larger than at the same temperature and 1 kbar hematite buffered experiments showed slightly lower solubilities than unbuffered runs caso4 solubility increases enormously with nacl activity at 800 c and 10 kbar and xnacl of 0 3 caso4 molality is 200 times higher than with pure h2o whereas caso4 solubility in pure h2o decreases with rising t at low t and p the high p results show that anhydrite solubility increases with t at constant p at all xnacl investigated the effects of salinity and temperature are so great at 10 kbar that critical mixing between sulfate rich hydrosaline melts and aqueous salt solutions is probable at 900 c at xnacl 0 3 recent experimental evidence that volatile laden magmas crystallizing in the deep crust may evolve concentrated salt solutions could in light of the present work have important implications regarding such diverse processes as mount pinatubo type s rich volcanism high fo2 regional metamorphism and emplacement of porphyry cu mo ore bodies where anhydrite hematite alteration and fluid inclusions reveal the action of very oxidized saline solutions rich in sulfur

the colima volcanic complex mexico iii a 34 meter section of ash and scoria fall units has been studied on the upper ne flank of the active volcan colima charcoal and soil horizons are restricted to the topmost 12 m nine 14c dates show a smooth progression from 235 years near the surface to 8 300 years at 10 meters depth indicating a post pleistocene accumulation rate of 1 3 m 1 000 years at 6 km from the vent this figure allows an estimate of the magmatic eruption rate for fallmaterial of 0 31 km3 1 000 years less than 15 of colima s lava eruption rate an unusually thick and coarse grained scoria fall horizon at about 4 m depth in the section appears to have been produced by the caldera forming eruption of colima some 4 300 years ago

chronology and pyroclastic stratigraphy of the may 18 1980 eruption of mount st helens washington many timed observations make it possible to subdivide the 9 hour plinian eruption of mount st helens on may 18 1980 into six phases defined by eruption style the phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits the suite of pyroclastic deposits indicates that the eruption became more pumice rich and compositionally diverse with time perhaps owing to concurrent eruption of less evolved gas poor parts of the magma body with the more evolved gas rich parts the paroxysmal phase i 0832 0900 consisted of landslides lithic pyroclastic flows of a lateral blast and other explosions and a weak pre plinian column phase i pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer the early plinian phase ii 0900 1215 consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks 1010 1035 a brief period of lithic ash ejection 1035 1100 and a pumice rich pulse that accompanied growth in height of the eruption column 1100 1215 deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer the lower lithic ash layer and the middle pumice lapilli layer all of which consist of evolved white dacitic pumice 63 64 sio2 during the early ash flow phase iii 1215 1500 the height of the eruption column decreased vertical ejection of tephra ceased and pyroclastic flows were fed from intermittent fountains phase iii deposits consist of a poorly exposed sequence 12 m of ash flow tuff that consist of many thin flow units 2 m each containing pumiceous white dacite 63 64 sio2 and denser gray silicic andesite 61 62 sio2 and fine grained ash cloud deposits interbedded with a nongraded middle pumice ash layer the climactic phase iv 1500 1715 developed in two stages fountain fed pyroclastic flows followed by a short pulse 1625 1715 of vigorous vertical ejection of tephra these stages were accompanied by the peak seismic energy release and peak eruption column height respectively climactic deposits consist of a thick 35 m sequence of thick lapilli rich ash flow sheets 4 12 m each with white and gray pumice and streaky scoria bands 60 sio2 in pumice breccia clasts and the reversely graded upper pumice lapilli layer that is interbedded with fine grained ash cloud deposits during the late ash flow phase v 1715 1815 eruption intensity waned but included a brief episode of small pyroclastic flows 1745 1815 phase v deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice and minor fine ash deposits phase vi activity 1815 to may 19 1980 consisted of a low energy ash plume with transient increases in intensity while seismicity continued at depth sustained vertical discharge of phase ii prodeced evolved dacitewith high s cl ratios ash flow activity of phase iii is attributed to decreases in gas content indicated by reduced s cl ratios and increased clast density of the less evolved gray pumice climactic events are attributed to vent clearing and exhaustion of the evolved dacite

comparative oxygen barometry in granulites bamble sector se norway oxygen fugacities have been estimated for the high grade portion of the bamble granulite facies terrane se norway using both titaniferous magnetite ilmenite and orthopyroxene titaniferous magnetite quartz oxygen barometers the two oxygen barometers show good agreement for samples indicating high titaniferous magnetite ilmenite temperatures whereas agreement is poor for low temperature samples oxygen fugacities estimated from titaniferous magnetite ilmenite are considerably lower than those estimated from orthopyroxene titaniferous magnetite quartz this discrepancy increases with a decrease in temperature which appears to reflect preferential resetting of the hematite content in the ilmenite grains without much alteration of the more numerous titaniferous magnetite or orthopyroxene grains the mean temperature for the non reset samples 795 60 c 1 agrees well with temperatures obtained from garnet orthopyroxene k d exchange thermometry in the same region 785 60 c 1 most non reset o

whole rock phosphate and silicate compositional trends across an amphibolite to granulite facies transition tamil nadu india chemical trends from north amphibolite facies to south granulite facies along a 95 km traverse in tamil nadu southern india include whole rock depletion of rb cs th and u enrichment in ti and f and depletion in fe and mn in biotite and amphibole increases in al and decreases in mn in orthopyroxene enrichment of fluorapatite in f k feldspar blebs are widespread along quartz plagioclase grain boundaries and could indicate either partial melting or metasomatism in the northernmost portion of the traverse the principal rare earth element ree bearing minerals are allanite and titanite south of a clinopyroxene isograd monazite grains independent of fluorapatite are the major ree and th bearing phase further south independent monazite is rare but th free monazite inclusions are common in fluorapatite during prograde metamorphism independent monazite was replaced by ree rich fluorapatite in which the monazite inclusions later formed the loss of independent monazite was accompanied by a loss of whole rock th and possibly a small depletion in light ree most mineralogical features along the traverse can be accounted for by progressive dehydration and oxidation reactions trace element depletion is best explained by the action of an externally derived low h2o activity brine migrating from a source at greater depth possibly preceded or accompanied by partial melting

infrasound radiated during the montana earthquake of 1959 august 18 summary seismic waves caused by earthquakes radiate infrasound into the atmosphere as they proceed over the earth s surface several instances of such sound waves radiated locally by seismic waves passing through the washington d c area have been observed at the infrasonic station there a notable instance was the great montana earthquake of 1959 august 18 measurements of the radiated infrasound gave data on the seismic waves including their travel times local speeds directions of travel amplitudes and waveforms

interaction of biotite amphibole gneiss with h2o co2 k na cl fluids at 550 mpa and 750 and 800 с experimental study and applications to dehydration and partial melting in the middle crust to constrain effects of chloride bearing h2o co2 fluids on complex natural assemblages during high grade metamorphism and anatexis we report the results of experiments on the interaction of biotite hornblende tonalitic gneiss from the sand river formation limpopo complex south africa with h2o co2 h2o co2 kcl h2o co2 nacl and h2o co2 k na cl fluids at 550mpa 750 and 8008c and varying chloride h2o co2 ratios with molar co2 co2 h2o 1 4 0 5 heating of solid cylinders of gneiss at both temperatures in the absence of a free fluid phase produced no changes in the gneiss phase assemblage the equimolar h2o co2 fluid at 7508q also did not significantly influence the phase assemblage addition of kcl to the fluid at 7508q resulted in formation of the clinopyroxene k feldspar ilmenite titanite assemblage after biotite hornblende and plagioclase orthopyroxene accompanied by amphibole appeared only at 8008c as a result of biotite breakdown in the presence of h2o co2 and low salinity h2o co2 kcl fluids increase in the kcl content in the fluid at 8008q resulted in the production of a clinopyroxenebearing assemblage increase of the nacl content stabilized amphibole in an assemblage with either orthopyroxene at low nacl concentrations or clinopyroxene nevertheless clinopyroxene albite is stable only at high salt concentrations comparison of the experimental results with the results of thermodynamic modeling using the gibbs free energy minimization method perple x software showed that mineral reactions and assemblages in the run products were governed by the activities of alkali components imposed by kcl and nacl in the h2o co2 fluids and decrease of the water activity served as an additional factor stabilizing anhydrous assemblages no melts formed at 7508c in the presence of the h2o co2 kcl fluids these fluids provoked melting only at 8008c with formation of rhyolitic melts with increasing kcl content of the fluid the melt composition changed to potassic rhyolitic with al2o3513 5 wt cao52 wt k2o na2o47 wt feo feo mgo 40 8 k2o na2o41 and moderate enrichment in cl 0 2 0 6 wt increasing nacl content caused melting at 7508q and shifted the melt composition towards trachytic and trachyandesitic compositions at both 750 and 8008q the experiments support a model for the formation of ferroan a type granite syenite complexes via crustal melting in the presence of h2o co2 salt fluids in extensional tectonic settings they demonstrate a possible link between a type granitoids and mid crustal dehydration zones in amphiboliteto granulite facies terrains and allow a new interpretation of mineral assemblages within these zones in terms of variations in fluid salinity

eruptive and magmatic cycles at fuego de colima volcano mexico the fuego de colima volcano displays a pattern of eruptive cyclicity characterized by lava flows and or slow effusions in an open crater alternating with short explosive events historical accounts field investigations and petrological data allow us to refine our understanding of the relationships between recurrent acidic acidic andesite and mafic andesitic products and this eruptive behaviour the second to last 1818 1913 and present 1913 present day cycles are instructive in this respect the short explosive events in 1818 and 1913 correspond to the initial mixing stage of a differentiated magmatic body with a new mafic input mathematical modeling shows that the pyroclastic products of 1913 result from the mixing between an acidic andesite 61 5 sio2 and an olivine andesite 56 sio2 in various ratios the final andesite mixture is made of 38 basaltic andesite and 62 acidic andesite the long effusive extrusive phases correspond to the ensuing differentiation stage of the new magmatic body for example the 1961 lava flows were derived from the final mixed and homogenized magma after the eruption of 1913 by fractionation of 7 plagioclase 9 pyroxene clinopyroxene orthopyroxene and 0 7 titanomagnetite the fractionation of 11 plagioclase 9 pyroxene and 1 4 titanomagnetite is required to produce the composition of the 1986 summit lava dome from the same parent in conclusion at fuego de colima an eruptive cycle begins with a short violent explosive event related to a mixing process and continues with a long effusive phase characterized by magmatic differentiation this scheme differs from others that have been proposed which consider that the eruptive cycles end with an explosive event

earthquake sound perception 1 sound is an effect produced by almost all earthquakes using a web based questionnaire on earthquake effects that included questions relating to seismic sound we collected 77 000 responses for recent shallow italian earthquakes an analysis of audibility attenuation indicated that the decrease of the percentage of respondents hearing the sound was proportional to the logarithm of the epicentral distance and linearly dependent on earthquake magnitude in accordance with the behavior of ground displacement even if this result was based on italian data qualitative agreement with the results of theoretical displacement and of a similar study based on french seismicity suggests wider validity we also found that given earthquake magnitude audibility increased together with the observed macroseismic intensity leading to the possibility of accounting for sound audibility in intensity assessment magnitude influenced this behavior making small events easier to recognize as suggested by their frequency content

fluid immiscibility in metamorphic rocks evidence of fluid immiscibility in metamorphic rocks comes in the best of all worlds from fluid inclusion observations in fact fluid immiscibility should be anticipated over the full range of metamorphic conditions provided that appropriate bulk fluid compositions are present in a review paper entitled metamorphic fluids the evidence from fluid inclusions crawford and hollister 1986 summarized the role of fluid immiscibility in metamorphism was only recognized after serious study of fluid inclusions in metamorphic rocks was underway in the late 1970s and has yet to attract the attention of more than a handful of petrologists studying metamorphic rocks since then tremendous progress has been made a huge amount of fluid inclusion studies have convincingly shown that immiscible fluids can be present in rocks of all bulk compositions and at all p t conditions thermodynamic databases of minerals solid solutions and fluid mixtures along with the development of gibbs free energy minimization programs now allow for detailed prediction of the evolution of phase assemblages and mineral compositions in p t space significant progress has also been made in understanding reaction kinetics diffusional and convective mass transport and the interpretation of fluid rock interaction during metamorphism given the vast fluid inclusion evidence of immiscibility it would appear however that phase petrology and mass transport in any specific rock where fluids are involved are not correctly interpreted if possible or proven immiscibility is not considered relatively few studies directly link mineral reactions with immiscible fluids therefore this review aims at working out the effects of fluid immiscibility on the progress of metamorphic reactions and fluid transport over a wide range of conditions questions arise directly from the fluid inclusion record in metamorphic rocks this chapter however is organized the other way round at first it summarizes phase relations of fluid systems pertinent to

seismoacoustic recordings of small earthquakes in the pyrenees experimental results an experiment was conducted in the lesponne valley central french pyrenees to record the sounds that frequently accompany earthquakes in this region seismic and acoustic signals of a short period seismometer and a condenser microphone were continuously monitored for ten months during this period four small m l p transmission but also sv to p conversion into the atmosphere are identified thus confirming former hypotheses formulated by hill et al 1976 unambiguous quantitative constraints on the seismic to acoustic conversion are provided by the computation of transmission coefficients the dominant audio frequencies are recorded in the 5 to 60 hz band that is at the boundary between sounds and infrasounds and at the lower bound of human audibility high frequency infrasounds about 8 hz are interpreted as the transmission to the atmosphere of rayleigh waves induced locally by a thin soil layer

eruptive history of the colima volcanic complex mexico abstract the evolution of the colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes emission of andesitic to dacitic lava flows acid ash and pumice flow deposits fallback nuees ardentes leading to pyroclastic flows with heterogeneous magma plinian air fall deposits scoriae cones of alkaline and calc alkaline nature four caldera forming events resulting either from major ignimbrite outbursts or mount st helens type eruptions separate the main stages of development of the complex from the building of an ancient shield volcano 25 30 km wide up to two summit cones nevado and fuego the oldest caldera c1 7 8 km wide related to the pouring out of dacitic ash flows marks the transition between two periods of activity in the primitive edifice called nevado i the first one which is at least 0 6 m y old was mainly andesitic and effusive whereas the second one was characterized by extrusion of domes and related pyroclastic products a small summit caldera c2 3 3 5 km wide ended the evolution of nevado i two modern volcanoes then began to grow the building of the nevado ii started about 200 000 y ago it settled into the c2 caldera and partially overflowed it the other volcano here called paleofuego was progressively built on the southern side of the former nevado i some of its flows are 50 000 y old but the age of its first outbursts is not known however it is younger than nevado ii these two modern volcanoes had similar evolutions each of them was affected by a huge mount st helens type or bezymianny type event 10 000 y ago for the paleofuego and hardly older for the nevado ii the landslides were responsible for two horseshoe shaped avalanche calderas c3 nevado and c4 paleofuego each 4 5 km wide opening towards the east and the south in both cases the activity following these events was highly explosive and produced thick air fall deposits around the summit craters the nevado iii formed by thick andesitic flows is located close to the southwestern rim of the c3 caldera it was a small and short lived cone volcan de fuego located at the center of the c4 caldera is nearly 1500 m high its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls the evolution of the primitive volcano followed a similar pattern leading to formation of c1 and then c2 the analogy between the evolutions of the two modern volcanoes nevado ii iii paleofuego fuego is described their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir or two independent magmatic chambers after the evolution of a common structure represented by the primitive volcano

characteristics of dune bedded pyroclastic surge bedsets abstract the grainsize characteristics of dune bedded pyroclastic surge bedsets are surveyed the variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets and it increases as the grainsize of the coarsest bed increases reflecting an increasing velocity of emplacement deposits of wet surges identified as those which contain accretionary and ash coated lapilli tend to be finer and show less variance this partly because wet ash is cohesive but mainly because wet surges tend to be weaker dry surge bedsets are strongly fines depleted wet ones less so the lack of erosion of underlying ash layers shows that the environment is a strongly depositional one individual bedsets are demarcated by thin intervening fine ash fall layers which are the complementary ash cloud deposits settled or flushed out after the passage and decay of each turbulent surge surge deposits are generally less coarse than the coarsest associated airfall deposits which shows that they are formed by generally weaker events this study helps interpret the dune bedded parts of the landscape mantling may 18th 1980 blast deposit of mount st helens the blast was a very violent event but the variance and the grainsize of the coarsest bed are those of a relatively weak surge this suggests that the dune bedding was produced by a weak effect such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs

magma mingling in an andesite pyroclastic flow of the pourahu member ruapehu volcano new zealand abstract we describe a magma mingling episode from ruapehu volcano between two andesite magmas one very much minor in volume relative to the other the event acted to trigger eruption of the andesitic pourahu pyroclastic flow which is preserved in a thick sequence of tephras and laharic deposits in the southeastern ring plain of the volcano the predominant andesite is pale brown coloured and porphyritic containing phenocrysts of plagioclase clinopyroxene orthopyroxene fe ti oxides rare clasts of a darker andesite are different texturally less vesicular and contain distinctive microphenocrysts of plagioclase and quench olivine equally rare clasts of streaky pumice consisting of interbanded dark and light andesite attest to mingling between these two andesite components chemical analyses of discrete clasts demonstrate that the pourahu pyroclastic flow andesites span much of the compositional spectrum of ruapehu andesites this observation demonstrates heterogeneity in the products of a relatively small eruption the darker clast analyses and those from associated distal fall deposits lie within the fields defined by the dominant light coloured clasts phenocryst and microphenocryst geothermometry suggest slightly higher temperatures in the dark component however glasses from groundmass and phenocryst inclusions in the same specimen may differ considerably leading us to conclude that many phenocrysts are in fact xenocrystic and were incorporated in the melts as they migrated towards the surface we prefer a model in which a small volume of hot andesite magma injects a vent feeding magma chamber triggering vesiculation and eruption we infer that the process of magma withdrawal extended downward into the magma body causing the dark component to intermingle with the lighter dominant component sucking more dark magma into the chamber our observations are entirely consistent with the existence of a plexus of small possibly interlinked magma chambers beneath ruapehu

the tancitaro debris avalanche characterization propagation and modeling abstract the tancitaro volcano 3860 m is an andesitic dacitic stratovolcano located in the western portion of the trans mexican volcanic belt within the state of michoacan mexico the tectonic activity of this area has likely contributed to a large sector collapse of the volcano the first findings of a multidisciplinary investigation into this debris avalanche are presented here geomorphological analyses based on the interpretation of orthophotos satellite imagery and on gis elaborations had the objective of determining the main morphometric features of the landslide the collapse structure is an east facing horseshoe shaped crater 4 km wide and 5 3 km long while the deposit forms a large fan that is 66 km long covers an area of approximately 1155 km2 and has an estimated volume of 18 km3 event volume was established by reconstructing the paleo edifice in a gis and taking into account volumetric expansion cross sections measured in the field were also used for this purpose field investigations also highlighted the presence of two texturally distinct units which are referred to as the block facies and the matrix facies respectively the first is responsible for the typical hummock morphologies found in the proximal area a transitional zone contains a mixed block and matrix facies while in the distal portion blocks and megablocks some of which have a jigsaw puzzle texture gradually decrease in size until they disappear entirely a number of matrix samples were collected to conduct direct shear tests granulometric analyses and classification of the materials the data and analyses described above were used to discuss the mechanism controlling the long runout of the avalanche based on the comparison between the tancitaro debris avalanche and similar events we propose that mechanical fluidization was the mechanism responsible for the remarkable mobility of the landslide the predisposing factors leading to the collapse were also considered field observations suggest that these are mainly related to weakening processes operating both in volcanoes and in non volcanic areas the runout of the tancitaro debris avalanche was numerically modeled using dan w the geotechnical parameters determined in the field and from the laboratory analyses were used as input the dan w code models longitudinal spreading and the thickness and velocity of the failed mass by opportunely selecting a specific rheological model voellmy frictional bingham thus it was determined that the two parameter voellmy model provided the best simulation of the tancitaro debris avalanche movement and the best fitting rheological parameters have been found through back analysis

fluid induced nucleation of y ree phosphate minerals within apatite nature and experiment part i chlorapatite this study investigates chlorapatite ideally ca10 po4 6cl2 from two spatially separated locations in the odegardens verk apatite mine bamble sector se norway which was partially metasomatized under amphibolite facies conditions relative to the original chlorapatite metasomatized areas in sample dl136 are enriched in oh and f depleted in na si and y ree and contain numerous inclusions 1 15 m of monazite and xenotime in contrast metasomatized areas in sample tn174 are enriched in f and si depleted in na show y ree abundances similar to the original chlorapatite and host only a few monazite and xenotime grains in order to define the role of fluids in the formation of y ree phosphate minerals associated with apatite a series of experiments covering wide ranges of temperature 900 300 c and pressure 1000 500 mpa for a series of fluid compositions pure h2o a 50 50 molar mix of h2o and caf2 and co2 with 1 2 wt h2o were conducted on unaltered chlorapatite from the odegardens verk in the h2o experiments features due to metasomatic alteration depletion in na si and y ree as well as numerous monazite and xenotime inclusions as observed in the natural sample dl136 were fully reproduced monazite and xenotime grains were also observed growing on the surface of the apatite in dissolution embayments in metasomatized areas in the 50 50 caf2 h2o experiments metasomatized regions are strongly enriched in f and si depleted in na show unchanged y ree abundances and contain only a very few small monazite and xenotime grains similar to what was observed in sample tn174 natural and experimental observation imply the presence of two coupled substitutions na y ree 3 2 ca2 and si4 y ree 3 p5 ca2 in the case of dl136 and h2o fluid experiments na and si4 went into solution whereas the released y ree contributed to the growth of monazite and xenotime within the metasomatized apatite structure for sample tn174 and the 50 50 caf2 h2o fluid experiments na in apatite became depleted but si was enriched thereby stabilizing the y ree in the now recrystallized fluor hydroxylapatite structure which retarded the growth of monazite or xenotime inclusions the basic conclusion of this study is that nucleation of monazite and xenotime in the metasomatized chlorapatite is principally a function of the composition of the infiltrating fluid and to a much lesser extent temperature and pressure

volcanic hazards from bezymianny and bandai type eruptions major slope failures are a significant degradational process at volcanoes slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years the historic record provides evidence for at least six of these events in the past century several historic debris avalanches exceed 1 km3 in volume holocene avalanches an order of magnitude larger have traveled 50 100 km from the source volcano and affected areas of 500 1500 km2 historic eruptions associated with major slope failures include those with a magmatic component bezymianny type and those solely phreatic bandai type the associated gravitational failures remove major segments of the volcanoes creating massive horseshoe shaped depressions commonly of caldera size the paroxysmal phase of a bezymianny type eruption may include powerful lateral explosions and pumiceous pyroclastic flows it is often followed by construction of lava dome or pyroclastic cone in the new crater bandai type eruptions begin and end with the paroxysmal phase during which slope failure removes a portion of the edifice massive volcanic landslides can also occur without related explosive eruptions as at the unzen volcano in 1792 the main potential hazards from these events derive from lateral blasts the debris avalanche itself and avalanche induced tsunamis lateral blasts produced by sudden decompression of hydrothermal and or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s 1 the ratio of area covered to distance traveled for the mount st helens and bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume the potential for large scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice as at mount st helens and bezymianny debris avalanches can move faster than 100 ms 1 and travel tens of kilometers when not confined by valley walls avalanches can affect wide areas beyond the volcano s flanks tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions the probable travel distance l of avalanches can be estimated by considering the potential vertical drop h data from a catalog of around 200 debris avalanches indicates that the h l rations for avalanches with volumes of 0 1 1 km3 average 0 13 and range 0 09 0 18 for avalanches exceeding 1 km3 h l ratios average 0 09 and range 0 5 0 13 large scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts the nature and duration of precursory activity vary widely and the timing of slope faliure greatly affects the type of associated eruption bandai type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity