the significance of phenocryst diversity in tephra from recent eruptions at popocatepetl volcano central mexico tephra lapilli from six explosive eruptions between april 1996 and february 1998 at popocatepetl volcano popo in central mexico have been studied to investigate the causes of magma diversification in thick crusted volcanic arcs the tephra particles are sparsely porphyritic 5 vol magnesian andesites sio2 58 65 wt mgo 2 6 5 9 wt that contain phenocrysts of nio rich up to 0 67 wt nio magnesian olivine fo89 91 cores with inclusions of cr spinel cr 59 70 orthopyroxene mg 63 76 clinopyroxene mg 68 86 intermediate to sodic plagioclase an33 66 and traces of amphibole major and trace element systematics indicate magma mixing the liquid mg melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic mg melt 72 76 and an evolved component magma mg melt 35 40 these component magmas form a hybrid magnesian andesite with an intermediate range of mg melt 50 72 the mafic end member mg melt 72 75 is saturated with olivine and spinel and crystallizes at temperatures 1170 1085 c with oxygen fugacities close to the fayalite magnetite quartz buffer and elevated water contents of several wt h2o a likely location of crystallization is at lower crustal levels possibly at the moho olivine is followed by high mg clinopyroxene which could start to crystallize during magma ascent at depths of 4 to 13 km the mafic magma mixes with an evolved composition containing low mg clino and orthopyroxene and plagioclase at a temperature of 950 c the repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing the tephra is similar to the popo magnesian andesites suggesting similar generic processes for the common lavas of the volcano the advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma building on the elemental systematics of the tephra and a comparison to the near primary basalts from the surrounding monogenetic fields we infer that the popo mafic end member is a magnesian andesite with variable but high sio2 contents of 55 62 wt and near primary characteristics such as high mg melt of 72 75 feo mgo ratios 1 if extrapolated to an mg melt of 72 75 and high ni contents 200 ppm ni this model implies that the typical elemental signature of the popo andesites such as the low cao al2o3 feo high na2o contents and the depletion in high field strength elements e g p zr ti are mantle source phenomena thus determining the elemental budget of the magnesian andesite as it is prior to the modifications by crustal differentiation is central to quantifying the subcrustal mass fluxes beneath popo

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

intermittent mixing processes occurring before plinian eruptions of popocatepetl volcano mexico insights from textural compositional variations in plagioclase and sr nd pb isotopes volcan popocatepetl has explosively erupted in plinian style at least five times in the last 23 000 years extreme deviations in composition and the occurrence of dissolution features in plagioclase and pyroxene and the occasional presence of xenocrysts of cr rich fe ti oxides and mg rich olivines and pyroxenes indicate that magma mixing has been a major process affecting the magmatic system the nearly invariant composition of the erupted products andesitic dacitic suggests however that mixing is not acting alone and must be balanced by assimilation and or crystallization to investigate the magmatic processes that have modified the plinian magmas textural and compositional variations and growth rates in plagioclase were used to approximate the frequency of mixing events affecting each magma systematic analysis of sr nd and pb isotopes was carried out on plagioclase pyroxene and pumice matrix glass to constrain the extent of assimilation of upper crustal rocks additionally a series of phase equilibrium experiments were carried out to constrain the depth where such mixing and assimilation occurred we find that magma was stored at one of two different depths beneath popocatepetl with magma mixing acting in both reservoirs mixing frequency and the relative impact on mineral compositions and textures has varied with time assimilation of calcareous rocks underneath popocatepetl has not been pervasive and does not contribute significantly to the evolution of the plinian magmas the similar compositions of magmas with diverse mixing histories suggest that fractional crystallization and possibly assimilation of deep crust takes place at depth and that intermediate magmas ascend into the upper crust already differentiated

el ventorrillo a paleostructure of popocatepetl volcano insights from geochronology and geochemistry volcan popocatepetl mexico was constructed over the remains of a volcanic paleostructure based on fieldwork 40ar 39ar dating u pb dating and geochemistry we have determined the age chemistry and location of this paleostructure and named it el ventorrillo most remnants of el ventorrillo are covered by deposits from subsequent activity of popocatepetl except for the el abanico scarp and the barranca de nexpayantla where the stratigraphy of el ventorrillo eruptive products can be investigated inception of volcanism at el ventorrillo occurred at 331 10 ka with emission of the nexpayantla andesitic lavas and continued with extrusion of the yoloxochitl 267 31 ka and microwave 227 6 ka domes intrusion of dikes occurred at 298 94 and 230 3 ka activity at el ventorrillo continued with the emission of lavas that built the el abanico scarp 193 29 to 96 8 ka and continued until the tutti frutti eruption destroyed the cone 14 1 kyr ago el ventorrillo magmas produced rocks divided into two mineralogical groups the first group contains biotite amphibole rich rocks and the second group consists of biotite amphibole free lavas the rocks that contain biotite and amphibole are older than 198 13 ka whereas the rocks with no hydrous phases are younger than 227 6 ka and contain skarn and granodiorite xenoliths we interpret the change to an anhydrous mineral assemblage and the occurrence of skarn and granodiorite xenoliths as evidence for the formation of a new shallower reservoir a granodiorite xenolith was chosen for 40ar 39ar dating and u pb zircon analyses the u pb method yielded an age of 540 110 ka and the 40ar 39ar an age of 109 24 ka these ages are interpreted to indicate granodiorite crystallization 540 110 ka which metamorphosed the calcareous basement beneath popocatepetl into skarn and an influx of magma 109 24 ka that reheated the granodiorite major and trace elements sr nd and pb isotopic compositions and textural analyses of plagioclase show that el ventorrillo magmas were moderately modified by mixing with more mafic magmas and that assimilation of the calcareous basement was negligible

the caldera forming eruption of volcan ceboruco mexico of magma erupted 95 of which was deposited as fall layers during most of the deposition of p1 eruptive intensity mass flux was almost constant at 4 8 107 kg s 1 producing a plinian column 25 30 km in height size grading at the top of p1 indicates however that mass flux waned dramatically and possibly that there was a brief pause in the eruption during the post p1 phase of the eruption a much smaller volume of magma erupted although mass flux varied by more than an order of magnitude we suggest that caldera collapse began at the end of the p1 phase of the eruption because along with the large differences in mass flux behavior between p1 and post p1 layers there were also dramatic changes in lithic content p1 contains 8 lithics post p1 layers contain 30 60 and magma composition p1 is 98 rhyodacite post p1 layers are 60 90 rhyodacite however the total volume of magma erupted during the jala pumice event is close to that estimated for the caldera these observations appear to conflict with models which envision that after an eruption is initiated by overpressure in the magma chamber caldera collapse begins when the reservoir becomes underpressurized as a result of the removal of magma the conflict arises because firstly the p1 layer makes up too large a proportion 75 of the total volume erupted to correspond to an overpressurized phase and secondly the caldera volume exceeds the post p1 volume of magma by at least a factor of three the mismatches between model and observations could be reconciled if collapse began near the beginning of the eruption but no record of such early collapse is evident in the tephra sequence the apparent inability to place the jala pumice eruptive sequence into existing models of caldera collapse which were constructed to explain the formation of calderas much greater in volume than that at ceboruco may indicate that differences in caldera mechanics exist that depend on size or that a more general model for caldera formation is needed

the 23 500 y 14 c bp white pumice plinian eruption and associated debris avalanche and tochimilco lava flow of popocatepetl volcano mexico abstract the white pumice wp is one of the thickest and most voluminous plinian fallouts produced by popocatepetl volcano in central mexico during the late pleistocene holocene its eruption 23 500 14 c y bp 27 800 cal bp was triggered by the catastrophic failure of the sw flank of the volcano the resulting debris avalanche was highly mobile reaching 72 km from the cone with an apparent coefficient of friction l h of 0 06 the deposit covers an area of 1200 km 2 and has a volume of 10 4 km 3 this gigantic landslide characterized by exceptionally large proximal hummocks 400 m provoked the sudden decompression of the hydrothermal and magmatic systems which produced an initial blast followed by the rise of a plinian column that reached an altitude of 33 km the isopach map allows the recognition of a dispersal axis pointing toward the south where an area of 2490 km 2 was covered by 10 cm of pumice and ash the total volume of the pumice fallout was estimated at 1 9 km 3 dre dense rock equivalent pumice clasts are dacitic 62 66 wt sio 2 anhydrous basis highly vesicular 55 88 vol and display a seriate texture with phenocrysts of plagioclase hornblende augite hypersthene oxides ti magnetite and ilmenite apatite as the eruption advanced discharge rates became more intermittent and the height of the column fluctuated and finally collapsed generating pumice and ash flows that were emplaced around the volcano this short but intense activity was followed during subsequent years by rain induced lahars that reached great distances from the volcano at the same time more degassed andesitic dacitic 61 65 wt sio 2 magma was erupted effusively 4 4 km 3 dre in the new horseshoe shaped 5 km wide crater from which the tochimilco lava flow descended toward the sse where it inundated an area of 68 km 2 and reached as far as 22 km from its source since then multiple eruptions have reconstructed the summit cone almost completely obliterating the horseshoe shaped crater during the course of this catastrophic eruption vei 5 a total volume of 6 3 km 3 dre of juvenile magma pumice and lava were emitted and at least an additional 10 km 3 of pre existing rocks debris avalanche mobilized it surpasses in magnitude most other known plinian eruptions from popocatepetl and can be envisaged as an example of a worst case scenario for hazard evaluation purposes it dramatically changed the morphology of the volcano and had profound and far reaching effects beyond its immediate vicinity along rivers draining surrounding plains as well as on the lacustrine basins e g chalco to the ne a repeat of such an eruption in this densely populated area would certainly cause a major calamity of unprecedented dimensions

the chimborazo sector collapse and debris avalanche deposit characteristics as evidence of emplacement mechanisms abstract chimborazo is a late pleistocene to holocene stratovolcano located at the southwest end of the main ecuadorian volcanic arc it experienced a large sector collapse and debris avalanche da of the initial edifice ch i this left a 4 km wide scar removing 8 0 0 5 km 3 of the edifice the debris avalanche deposit dad is abundantly exposed throughout the riobamba basin to the rio chambo more than 35 km southeast of the volcano the dad averages a thickness of 40 m covers about 280 km 2 and has a volume of 11 km 3 two main dad facies are recognized block and mixed facies the block facies is derived predominantly from edifice lava and forms 80 vol of the dad with a probable volume increase of 15 25 vol the mixed facies was essentially created by mixing brecciated edifice rock with substratum and is found mainly in distal and marginal areas the dad has clear surface ridges and hummocks and internal structures such as jigsaw cracks injections and shear zone features are widespread structures such as stretched blocks along the base contact indicate high basal shear substratum incorporation is directly observed at the base and is inferred from the presence of substratum derived material in the dad body based on the facies and structural interpretation we propose an emplacement model of a lava rich avalanche strongly cataclased before and or during failure initiation the flow mobilises and incorporates significant substrata 10 14 vol while developing a fine lubricating basal layer the substrata dominated mixed facies is transported to the dad interior and top in dykes invading previously formed fractures

a 550 year old plinian eruption at el chichon volcano chiapas mexico explosive volcanism linked to reheating of the magma reservoir 1 some 550 years ago 1320 1433 a d a powerful plinian eruption at el chichon volcano in southern mexico produced a widespread pumice fall deposit we subdivided the deposit into three parts on the basis of structural and textural characteristics pumice lithology and density granulometry and petrologic geochemical attributes the deposit covers an area of 1500 km2 within the 1 cm isopach and has a minimum estimated bulk volume of 2 8 km3 1 1 km3 dense rock equivalent dre its eruptive column reached an altitude of 31 km consideration of field evidence the presence and nature of mafic enclaves and chemical data strongly suggest that the 550 year b p eruption is linked with the intrusion of a high temperature basaltic magma into preexisting but stagnated trachyandesitic magma beneath el chichon thorough mixing of the two magmas produced a compositionally uniform hybrid trachyandesite magma average sio2 55 3 wt which subsequently underwent crystal growth and gas exsolution ultimately overpressurizing the zoned magmatic system to erupt explosively on the basis of el chichon s known eruptive history the intrusion mixing event occurred sometime after the 900 year b p eruption the hybrid magma had a preeruption temperature of 820 830 c and was water undersaturated 5 6 wt h2o at pressures of 2 2 5 kbar

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

popocatepetl s crater filled to the brim significance for hazard evaluation abstract after 67 years of quiescence popocatepetl volcano reawoke on december 24 1994 emitting vigorous ash plumes juvenile magma appeared for the first time on march 29 1996 with the emplacement of a dacitic lava dome inside the crater since then more than 25 dacitic domes have been extruded and destroyed by subsequent explosions on january 22 2001 the activity peaked with a strong explosion that produced pyroclastic flows that reached the timberline and ignited fires at a distance of 4 5 km from the crater these pyroclastic flows were contained within the red zone of the volcano hazards map after 10 years of discontinuous activity popocatepetl s crater is almost completely filled with lava dome rock this scenario poses a new threat to the populations settled in the orange zone intermediate hazard level because future explosions will not be contained by the crater walls and pyroclastic flows will probably reach greater distances from the crater

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

a preliminary view of the coexistence of mammoth and early peoples in mexico abstract a progress report about human mammoth interactions in mexico is provided based primarily on a literature search more than 270 mammoth localities are known in mexico but only 17 of them have shown some evidence of an association between early peoples and mammoth however that number is even less when each locality is assessed in detail due to the lack of a supportable association or the loss of the actual specimens that precludes their analysis using current techniques only six localities have modified mammoth bone among them the greatest potential for demonstrating such a relationship is at santa isabel iztapa valsequillo villa de guadalupe and tocuila establishing an analysis methodology for the materials from those as well as new localities is warranted to provide the basis for interpreting the human mammoth relationship in mexico future research calls for detailed stratigraphic and radiometric control and an analytical protocol for bone analysis focused on taphonomy and biotechnology

lake levels since about 40 000 years ago at lake chalco near mexico city abstract diatoms magnetic susceptibility organic content and14c ages of sediments from a 26 m core suggest that lake chalco in the southern part of the basin of mexico went through a series of major fluctuations during the late pleistocene and the holocene before ca 39 00014c yr b p the lake was very deep about 8 10 m alkaline and saline it then became shallow

concentration and mode of occurrence of trace elements in marine oil shale from the bilong co area northern tibet china abstract the bilong co oil shale zone is located in the south qiangtang depression this zone together with the shengli river changshe mountain oil shale zone in the north qiangtang depression northern tibet plateau represents a potentially large marine oil shale resource in china with the aim of better understanding geochemistry of marine oil shale 18 samples from the bilong co area are studied and 56 elements in them are determined the contents of mo se cd cs as bi u rb pb th li sr and zn are enriched from 1 3 to 35 0 times as compared with the average concentration in the crust clarke values whereas the other elements are slightly higher lower than the respective clarke values compared to common australian oil shales the contents of ce co cs f ga hf nb nd pb pr rb sc sn th and zr in the bilong co oil shale samples exhibit relatively high levels whereas as br cd cr cu dy er eu ho lu mo ni se tb tm u v y yb and zn are relatively depleted however the distribution patterns of trace elements are quite similar indicating that trace element speciation in oil shale is governed by general processes rather than by individual geochemical mechanisms within one particular sample the elements in the bilong co oil shale may be classified into three groups of association according to their modes of occurrence i e groups a b and c group a si al k na ti zr sc th sn nb ga ta be hf cs li rb ash rees is strongly correlated with ash yield and mainly has an inorganic affinity group b cr w mg mn br f sr ba has weakly or negatively positive correlation coefficients with an exception of f with ash yield group c zn bi cd ca fe pb hg co cu ni p mo se u as v shows negative correlation coefficients with ash yield with an exception of bi and possibly has an organic affinity

fragmentation of magma during plinian volcanic eruptions the ratio of the volume of vesicles gas to that of glass liquid in pumice clasts v g v l reflects the degassing and dynamic history experienced by a magma during an explosive eruption v g v l in pumices from a large number of plinian eruption deposits is shown here to vary by two orders of magnitude even between pumices at a given level in a deposit these variations in v g v l do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities despite large ranges in these variables gas volume ratios of pumices do however vary systematically with magma viscosity estimated at the point of fragmentation and we infer that pumices do not quench at the level of fragmentation but undergo some post fragmentary evolution on the timescale of plinian eruptions pumices with viscosities 109 pa s can expand after fragmentation as long as their bubbles retain gas at a rate inversely proportional to their viscosity once the bubbles connect to form a permeable network and lose their gas expansion halts and pumices with viscosities 105 pa s can collapse under the action of surface tension textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place the magnitudes of expansion and collapse therefore depend critically on the timing of bubble connectivity relative to the final moment of quenching we propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching after accounting for these effects we derive new information on the fragmentation process from two characteristics of pumices the most important is a relatively constant minimum value of v g v l of 1 78 64 vol vesicularity in all samples with viscosities 105 pa s this value is independent of magma composition and thus reflects a property of the eruptive mechanism the other characteristic is that highly expanded pumices 85 vol vesicularities are common which argues against overpressure in bubbles as a mechanism for fragmenting magma we suggest that magma fragments when it reaches a vesicularity of 64 vol but only if sheared sufficiently strongly the intensity of shear varies as a function of velocity in the conduit which is related to overpressure in the chamber so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes

trace elements in the lower pliocene fossiliferous kangal lignites sivas turkey abstract the kangal lignite basin which is one of the most productive lignite basins in eastern anatolia turkey includes two lignite seams containing abundant gastropoda shells in the lower pliocene kalburcayiri formation the seams are currently exploited in an open cast mine in the kalburcayiri field and supply feed coal to a power plant with 300 mw capacity the average thickness of each of the two seams in this mine is about 10 m and they are separated by about 20 m of tuffaceous sedimentary rocks a 15 20 cm thick tuff bed occurs within the upper seam the purpose of the study was to understand mineralogy and trace element concentrations of the two lignite seams in the kalburcayiri formation which were affected by volcanic ashes fifty nine core samples of the lignites from seven boreholes across the basin and eight channel samples from the working lignite seams were collected and analysed by proximate analysis x ray powder diffraction xrd inductively coupled plasma mass spectrometry icp ms and scanning electron microscopy sem with energy dispersive x ray microanalysis techniques the samples show that the worked lignite seams on an as received basis contain high moisture contents 43 4 57 8 all the samples on an air dried basis have high ash yields from 19 5 to 81 7 and a broad range of total sulfur contents from 1 16 to 7 04 and calorific values 684 to 4621 kcal kg except for two samples with no temperature rise due to high ash yield the mineral matter of the studied samples is made up of calcite quartz feldspar pyrite opal ct clay minerals gypsum and traces of aragonite the trace element analyses on a whole coal dry basis indicate that most of the borehole samples showed enrichment in specific elements including zn 15 1394 g g mo 13 368 g g and u 5 5 131 g g when compared to the range of these elements for most coals in the world the contents of zn mo and u for the samples from the worked lignite seams are respectively 38 187 54 161 and 28 55 g g the sem studies show that zn is hosted mainly in micron sized zn bearing mineral sphalerite but no mo and u bearing minerals have been detected during these studies except for monazites containing th and u traces the mo and u concentrations are negatively correlated with the ash yields and these elements are contained mainly within organic matter

dynamics of the ca 4965 yr 14c bp ochre pumice plinian eruption of popocatepetl volcano mexico abstract the ochre pumice op plinian fallout was produced by popocatepetl volcano in central mexico during a major plinian eruption that occurred 4965 65 14 c yr bp 3700 bc the op is part of the ochre pumice sequence ops that consists of surge fall and pyroclastic flow deposits the op plinian fallout shows a bimodal grain size distribution with poor to moderate sorting that improves progressively towards the upper beds the juvenile component is mainly pumice 83 wt whereas the accidental components consist of igneous sedimentary and metamorphic clasts from the walls of the magma chamber and or conduit the vesicularity of the pumice decreases from bottom to top of the stratigraphic sequence while the crystal and glass contents increase this suggests increasing magma degassing prior and during the eruption the chemical composition of the pumice varies insignificantly at the base it is less evolved sio 2 61 wt and it becomes slightly more silicic towards the top sio 2 63 wt pumice clasts have a crystallinity index that ranges between 10 and 25 vol and display a seriate texture with phenocrysts of euhedral plagioclase pl clinopyroxene cpx orthopyroxene opx olivine ol oxide ox apatite ap the size of the maximum lithic 2 5 cm at a horizontal distance of 19 km indicates an eruption column height of 37 41 km the isopach map allows the recognition of a dispersal axis that points towards the nne where an area of 300 km 2 was covered by 6 cm of pumice and ash a minimum volume for the op plinian fallout deposit was calculated using a minimum thickness of 6 cm found at distal outcrops and was estimated at 4 9 km 3 which corresponds to 2 km 3 of dense rock equivalent dre stratigraphic relations indicate that the entire ops was emplaced during a short time interval of a few days to weeks and consisted of three main different phases phreato magmatic plinian and vulcanian it started violently with the emplacement of surges and then culminated with the eruption of the op fallout deposit as the eruption advanced discharge rates became more intermittent and the height of the column fluctuated and finally collapsed generating pumice and ash flows that were emplaced around the volcano this short but intense activity was followed during subsequent years by the growth of domes in the crater and the emplacement of block and ash flows generated by vulcanian explosions

petrologic diversity in mount st helens dacites during the last 4 000 years implications for magma mixing mount st helens has explosively erupted dacitic magma discontinuously over the last 40 000 years and detailed stratigraphic data are available for the past 4 000 years during this last time period the major element composition of the dacites has ranged from mafic 62 64 wt sio2 to felsic 65 67 wt sio2 temperature has varied by about 150 c 770 920 c and crystallinity has ranged between 20 and 55 water content of these dacites has also fluctuated greatly although the source for the dacitic magmas is probably partial melting of lower crustal rocks there is strong physical evidence such as banded pumices thermal heterogeneities in single pumices phenocryst disequilibrium contrasts between compositions of glass inclusions and host matrix glass and amphibole reaction rims that suggests that magma mixing has been prominent in the dacitic reservoir indeed we suggest that the variations in major and trace element abundances in mount st helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas magma mixing has also controlled the composition of andesites erupted at mount st helens and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite or possibly rhyodacite and basalt the flux of the felsic endmember to the reservior appears to have been relatively constant whereas the flux of basalt may have increased in the past 4 000 years as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period

palaeolandforms and morphotectonic evolution around the baie des chaleurs eastern canada abstract a morphological study of the baie des chaleurs area between northern new brunswick and the gaspe peninsula eastern canada leads to the identification of several types of palaeolandforms in the landscapes of the northeastern appalachians one of them the exhumed sub carboniferous palaeosurface was recognized along the shores of the baie des chaleurs around the western part of the carboniferous basin the maritimes basin which underlies the gulf of saint lawrence by analyzing the conditions of its exhumation and its relationships with escarpments limiting the higher planation surfaces of inner gaspesie and western new brunswick it was possible to identify recent deformations these are en bloc tilting and uplift flexuring and faulting they partly reflect the reactivation of carboniferous or older structures the study of palaeolandforms has already proved to be an appropriate method for reconstructing the morphological evolution of basement areas in this paper it is applied to the study of the evolution of an emerged part of the eastern canada rifted margin after the opening of the north atlantic ocean

last glacial droughts and fire regimes in the central mexican highlands over the last glacial cycle climate variations in the neotropics were related to diverse forcing mechanisms whose understanding has been limited because of the scarcity of continuous long records here we use high resolution charcoal particle accumulation inorganic carbon precipitation and magnetic susceptibility data from lake chalco located in high altitude tropical central mexico to assess the fire regimes and drought periods between 85 and 10 8k cal a bp overall drought intervals were characterized by high carbonate deposition and were contemporary with high spring insolation most of the high fire activity periods were coupled with droughts whereas some of them seem to have responded to volcanic activity in the basin periods of more fire activity during the latest part of marine isotope stage 3 were associated with a long interval of higher spring insolation increase in drought frequency during 47 27k cal a bp were possibly linked to a more active laurentide ice sheet influencing the oceanic and atmospheric controls of the tropical climatic system in fact chalco and cariaco records show similar millennial scale trends suggesting a role of the intertropical convergence zone in modulating moisture availability in the region

the origin of a primitive trondhjemite from the trans mexican volcanic belt and its implications for the construction of a modern continental arc a remarkable suite of miocene high silica trondhjemites discovered in the central trans mexican volcanic belt indicates that slab melts can ascend through the mantle and crust while suffering only minor compositional modifications despite carrying an assortment of deep crustal xenoliths the trondhjemites preserve the most depleted isotopic compositions ever measured in the mexican arc with values that are nearly identical to those of the pacific mid oceanic ridge basalts these rocks also have high sr y ratios and extremely fractionated heavy rare earth element patterns at relatively high mg number mg features that are all consistent with melts from the subducted oceanic crust that had only limited interaction with mantle peridotite during ascent nonetheless modeling results indicate that these unusual geochemical features can be modified by more extensive mantle assimilation resulting in compositions that could match those of more typical intermediate rocks from mexico the data thus indicate that the slab melt component uniquely recorded by the miocene trondhjemites represents a likely constituent for most volcanic sequences of the mexican arc and suggest that a modern andesitic continental crust can be constructed directly from mantle modified slab melts without a basaltic precursor

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

geochemistry of the blue gem coal bed knox county kentucky abstract the blue gem coal bed middle pennsylvanian westphalian b breathitt formation has low ash with some sample sites having less than 1 ash and low sulfur contents through parts of knox county kentucky most collection sites exhibit similar vertical elemental trends in ash geochemistry the relatively high ash 1 ash lower part of the bed displays enrichment in tio2 zr cr v and ni co zn rb and mn show enrichment at some sites a low ash 1 ash and a relative enrichment in sio2 k2o and rb and a relative decrease in cu ni co ba and mn when compared to the middle part of the bed principal components analysis indicates that the samples with an ash content 0 63 1 0 show associations for ni cu cr and co samples with between 1 01 2 0 ash display strong element associations that suggest increased clastic sediment contributions tio2 zr k2o rb sio2 the low ash content and the upwards decrease in trace concentrations both suggest that the blue gem peat swamp was ombrotrophic through at least a part of its geologic history

elemental concentrations and their distribution in two bituminous coals of different paleoenvironments abstract the vertical distributions of major and minor elements in two high volatile bituminous coals of eastern tennessee are compared the coals studied are the pewee deposited in a fresh water environment and the upper grassy spring previously thought to be influenced by a late incursion of marine water x ray fluorescence analyses were performed on a continuous series of specimens obtained from full face channel samples representing the entire thickness of each seam correlation coefficients r for all element pairs show high positive correlations for element pairs normally associated with clay minerals equally good correlations were observed for si ti and al ti suggesting that titanium is associated with clay minerals in both coals strontium and phosphorous showed a high positive r factor for both the upper grassy spring and the pewee major differences were found to exist in the concentrations and distributions of sulfur iron and arsenic in the upper grassy spring and pewee seams the upper grassy spring contains an order of magnitude more sulfur than the pewee as well as several times as much iron and arsenic the element concentrations and especially their distributions in the upper grassy spring suggest an earlier time for the marine incursion than previously reported in fact there is evidence that this coal deposit may have been influenced by marine conditions from its inception

geologic evidence for age of deposits at hueyatlaco archeological site vasequillo mexico direct tracing of beds during excavation in may 1973 confirmed that the artifact bearing layers at hueyatlaco underlie 10 m of fine grained water laid deposits that constitute part of the wide spread valsequillo gravels dissection of these deposits by the adjacent rio atoyac has reached a depth of 50 m the stratigraphic section at hueyatlaco includes four distinctive tephra units the oldest one occupies a small channel in a series of cut and fill stream deposits that have yielded bifacial tools it lies more than a meter above flat lying fine grained beds from which edge retouched tools have been recovered the three other tephra units occur higher in the section fission track ages on zircon phenocrysts from two of the younger tephra layers 370 000 200 000 and 600 000 340 000 yr 2 agree with concordant uranium series dates for a camel pelvis that was found associated with bifacial tools at hueyatlaco 245 000 40 000 yr by 230th and 180 000 yr by 231pa these dates are compatible with the depth of burial and subsequent dissection of the hueyatlaco deposits as well as with the degree of hydration of volcanic glass shards and with the extent of etching of heavy mineral phenocrysts from within the tephra layers these findings suggest to us that further search for archaeological remains in deposits as old as those at hueyatlaco would be warranted

holocene plinian eruption of la virgen volcano baja california mexico a plinian eruption occurred approximately 6500 yr ago at la virgen volcano the youngest volcano of the tres virgenes volcanic complex tvvc located in baja california mexico deposits of the eruption suggest a sequence of events that started with the opening of the volcanic conduit and development of a plinian eruption column up to 18 km in height this eruption column produced a fallout deposit with a dispersal axis toward the southwest an areal extent of about 500 km 2 and a minimum volume of 1 14 km 3 vulcanian activity hydromagmatic followed the plinian phase producing pyroclastic surge and fallout deposits the eruptive activity ceased after a basaltic andesite lava flow was emplaced closing the eruptive activity petrological and geochemical evidence indicates that the eruption was triggered by magma mixing processes our studies confirm that la virgen is a dormant volcano with the potential for future violent eruptions the present study provides important information for the construction of a volcanic hazards map significant hazards are presented to the population living within a distance of 30 km from the volcano together with the interstate road connecting the entire peninsula of baja california which runs at a distance of only 3 km from the volcano

estimations of clarkes for carbonaceous biolithes world averages for trace element contents in black shales and coals abstract black shale and coal clarke values are the average trace element contents in the world black shales and coals these calculations are made in russian geochemistry but up to now are poorly known in the west modern tables of black shale and coal clarkes are presented based on comprehensive calculations using very large amount of information thousands analyses of black shales coals and coal ashes for trace elements in black shale geochemistry three figures were calculated for each main lithologies terrigenous tuff chert and carbonate two clarke estimations are presented named lithological k1 and lithostratigraphical k2 in coal geochemistry seven figures were calculated for each trace element average content in hard coals and their ashes average content in brown coals and their ashes average content in all coals and their ashes and coal affinity index or coalphile index average content in all ashes clarke values of sedimentary rocks the black shale and coal clarkes presented here provide an important scientific base for many geochemical comparisons and issues

on the geochemistry of strontium in bulgarian coals sr geochemistry in coal has been studied on the basis of its distribution in 457 samples of coal coaly shales and mineral interlayers from several bulgarian coal deposits model experiments were also carried out the analysed sr contents are lower than the average values for coals only the burgas deposit is enriched in sr due to marine influence the coal lithotypes liptain vitrain xylain and fusain are usually enriched in sr as compared to the whole samples from which they were separated sr occurs in both organic srorg and inorganic srinorg forms the relation to organic matter is demonstrated by 1 sr enrichment in low ash coals and in lighter specific gravity fractions 2 leaching of sr bound to humic acids by treatment with 2 naoh 3 sorption experiments on xylain at ph 4 5 5 up to 92 of sr in solution was taken up by the sorbents a mechanism of cation exchange as well as formation of metallo organic complexes is proposed host minerals of srinorg are aragonite gypsum calcite and feldspar the highest srinorg content is related to aragonite

the giant popocatepetl stirs in about 800 ad the mexican volcano popocatepetl exploded sending up several cubic kilometres of ash and pumice and burying the surrounding area since then it has been relatively quiet but every few decades it threatens to explode again which could this time be catastrophic as the area is now one of the most densely populated in mexico seismic activity and eruptions are now increasing with ash falls even 70 km away in mexico city

geochemistry of boron in bulgarian coals a set of 466 individual full channel samples coals coaly shales and partings of thirteen bulgarian coal deposits as well as vitrain lenses gagatite and coalified woods embedded in sedimentary rocks have been analyzed by atomic emission spectroscopy the deposits consist of lignites subbituminous and bituminous coals the average boron content ranges from 15 to 164 ppm in the coals the boron content of the individual coal samples ranges from 2 to 164 ppm about 86 of the samples have a b content from 2 to 110 ppm which corresponds to the most probable interval of concentration for coals worldwide the boron content is not related to rank the lignites have the lowest boron content some of the vitrain and xylain samples studied are enriched in boron in comparison with the whole coal samples boron occurs in both organic borg and inorganic binorg forms excluding one deposit the boron content in the ash correlates negatively with the ash content whereas in coals it usually slightly increases with increasing ash content organically associated b dominates in the low ash coals and in the deposits enriched in boron the coals of the burgas deposit have the highest b content which is in agreement with the geological evidence for marine influence during the early stages of coalification the distribution of b is also independent of other geological factors

geochemical surveillance of magmatic volatiles at popocatepetl volcano mexico surveillance of popocatepetl volcanic plume geochemistry and so 2 flux began in early 1994 after fumarolic and seismic activity increased significantly during 1993 volatile traps placed around the summit were collected at near monthly intervals until the volcano erupted on december 21 1994 additional trap samples were obtained in early 1996 before the volcano erupted again emplacing a small dacite dome in the summit crater abundances of volatile constituents ppm day of cl s total f co 2 hg and as varied but most constituents were relatively high in early and late 1994 however ratios of these constituents to cl were highest in mid 1994 34 s s total in trap solutions ranged from 1 5 to 6 4 lowest values generally occurred during late 1994 13 c co 2 of trap solutions were greatly contaminated with atmospheric co 2 and affected by absorption kinetics when trap data are combined with so 2 flux measurements made through november 1996 popocatepetl released about 3 9 mt so 2 16 mt co 2 0 75 mt hcl 0 075 mt hf 260 t as 2 6 t hg and roughly 200 mt h 2 o near vent gas concentrations in the volcanic plume measured by correlation spectrometer cospec and fourier transform infrared ftir commonly exceed human recommended exposure limits and may constitute a potential health hazard volatile geochemistry combined with petrologic observations and melt inclusion studies show that mafic magma injection into a preexisting silicic chamber has accompanied renewed volcanism at popocatepetl minor assimilation of cretaceous wall rocks probably occurred in mid 1994