the petrogenesis of the lac guyer komatiites and basalts and the nature of the komatiite komatiitic basalt compositional gap a sequence of ultramafic rocks in the lac guyer archean greenstone belt exhibit brecciated flow tops pillow structures and spinifex textures testifying to their volcanic origin massive spinifex textured and differentiated flows in the sequence have the chemical characteristics of peridotitic komatiite with mgo ranging from 19 25 wt associated pillowed flows have compositions that straddle the conventional boundary between komatiite and komatiitic basalt with mgo contents ranging from 16 to 19 wt mgo and are best termed pyroxenitic komatiites unlike other komatiitic occurrences the peridotitic and pyroxenitic komatiites at lac guyer constitute a continuous chemical spectrum with no evidence of population minimum near 18 wt mgo the contrasting behaviour of highly compatible elements such as ni and cr versus incompatible elements such as zr indicate that this compositional spectrum was produced by a variation in the extent of partial melting 10 40 of a garnet lherzolite source in the archean mantle the pyroxenitic komatiites represent liquids produced during lower 10 20 degrees of melting during which garnet remained in the mantle residue however a change in slope in the distribution of zr vs y between the pyroxenitic and the peridotitic komatiites indicates that garnet was completely consumed at the more extensive degrees of melting which produced the peridotitic komatiites the lac guyer volcanic rocks display a population minimum at 15 wt mgo separating komatiitic magmas whose compositions are controlled by partial melting from basalts whose composition is controlled by crystal fractionation the population minimum near 18 wt mgo which is taken as the boundary between komatiite and komatiitic basalt may have a similar origin

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

is there a link between faulting and magmatism in the south central aegean sea a distinct spatial relationship between surface faulting magmatic intrusions and volcanic activity exists in the aegean continental crust in this paper we provide detailed structural observations from key onshore areas as well as compilations of lineament maps and earthquake locations with focal plane solutions from offshore areas to support such a relationship although pluton emplacement was associated with low angle extensional detachments the nne to ne trending strike slip faults also played an important role in localizing the middle miocene plutonism providing ready pathways to deeper magma batches and controlling the late stage emplacement and deformation of granites in the upper crust additionally the linear arrangements of volcanic centres from the quaternary volcanoes along the active south aegean volcanic arc are controlled primarily by ne trending faults and secondarily by nw trending faults these volcanic features are located at several extensional settings which are associated with the main ne trending faults such as i in the extensional steps or relay zones between strike slip and oblique normal fault segments ii at the overlap zones between oblique normal faults associated with an extensional strike slip duplex and iii at the tip zone of a ne trending divergent dextral strike slip zone the ne trend of volcano tectonic features such as volcanic cone alignments concentration of eruptive centres hydrothermal activity and fractures indicates the significant role of tectonics in controlling fluid and magma pathways in the aegean upper crust furthermore microseismicity and focal mechanisms of earthquakes in the area confirm the activity and present kinematics of these ne trending faults

deposits and soils of the past 130 000 years at the desert loess transition in northern china the desert loess transitional zone in north central china has long been thought sensitive to quaternary climatic change however reconstruction of quaternary climates in this area has been hindered by incompleteness of geological sections here we report the analytical results of two recently found sand loess soil sections both sections have thick eolian deposits from the last interglacial glacial cycle and can be correlated with one another field observations thermoluminescence dating and other laboratory analyses show that the last interglacial period produced three paleosols and two intercalated loess layers loess from the last glacial period is interbedded with three sand horizons that represent desert extension the expansion and contraction of desert in northern china may have been forced by the east asia monsoon

stacked 2 6 ma grain size record from the chinese loess based on five sections and correlation with the deep sea δ18o record 1 this study aims to establish a stacked climate record of the quaternary period from the chinese loess sequence and to address the forcing mechanisms for the regional climate history of the loess plateau by correlating the stacked record with a composite 18o record in deep sea sediments a total of 18 352 samples were obtained from five loess sections located at baoji lingtai jingchuan puxian and pingliang in the southern and middle loess plateau these yielded high resolution grain size records between section correlation of these shows that although small depositional hiatuses are present in places within a single section most parts of the sections display near continuous dust deposition throughout the quaternary the grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the earth s orbit the grain size records plotted on their orbital timescales were then averaged to form a stacked loess grain size time series termed the chiloparts record this resolves most of the orbital timescale paleoclimate events buried in the loess soil sequences of the southern and middle loess plateau and can be used as a regional archive of the pleistocene climate history in the loess plateau comparison of the chiloparts record with a composite marine 18o record shows that for the past 1 8 ma the loess paleosol record can be correlated almost cycle by cycle with the marine record several discrepancies in the climatic events between the two records have also been identified implying that regional forcing mechanisms may have played a part in the climatic evolution of the chinese loess plateau

water rock interaction in the magmatic hydrothermal system of nisyros island greece abstract in this work we investigated the water rock interaction processes taking place in the hydrothermal reservoir of nisyros through both 1 a review of the hydrothermal mineralogy encountered in the deep geothermal borehole nisyros 2 and 2 a comparison of the analytically derived redox potentials and acidities of fumarolic related liquids with those controlled by redox buffers and ph buffers involving hydrothermal mineral phases the propylitic zone met in the deep geothermal borehole nisyros 2 from 950 to 1547 m total depth is characterised by abundant well crystallised epidote adularia albite quartz pyrite chlorite and sericite muscovite accompanied by less abundant anhydrite stilpnomelane wairakite garnet tremolite and pyroxene these hydrothermal minerals were produced in a comparatively wide temperature range from 230 to 300 c approximately hydrothermal assemblages are well developed from 950 to 1360 m whereas they are less developed below this depth probably due to low permeability based on the r h values calculated for fumarolic gases and for the deep geothermal fluids of nisyros 1 and nisyros 2 wells redox equilibrium with the feo feo 1 5 rock buffer appears to be closely attained throughout the hydrothermal reservoir of nisyros this conclusion may be easily reconciled with the nearly ubiquitous occurrence of anhydrite and pyrite since r h values controlled by coexistence of anhydrite and pyrite can be achieved by gas separation the ph of the liquids feeding the fumarolic vents of stephanos and polybote micros craters was computed by means of the eq3 code based on the cl d relationship which is constrained by the seawater magmatic water mixing occurring at depth in the hydrothermal magmatic system of nisyros the temperature dependence of analytically derived ph values for the reservoir liquids feeding the fumarolic vents of stephanos and polybote micros craters suggests that some unspecified ph buffer fixes the acidity of these reservoir liquids at values of 4 72 4 85 and 4 88 5 23 respectively many of these ph values are lower than those expected for the full equilibrium condition although they are close to those of the reservoir liquids of nisyros 1 5 16 and nisyros 2 4 87 it is likely that this excess of acidity producing species chiefly co 2 promotes release of fe ii and fe iii to the reservoir liquids through rock dissolution permitting the attainment of redox equilibrium with the feo feo 1 5 rock buffer as already suggested by the late werner giggenbach

pre eruption history of phyric basalts from dsdp legs 45 and 46 evidence from morphology and zoning patterns in plagioclase phyric basalts recovered from dsdp legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains megacrysts or aggregates glomerocrysts and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition almost all the plagioclase crystals are complexly zoned in most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts core mantle and rim the cores exhibit two major types of morphology tabular with a euhedral to slightly rounded outline or a skeletal inner core wrapped by a slightly rounded homogeneous outer core the mantle region is characterized by a zoning pattern composed of one to several spikes plateaus superimposed on a gently zoned base line with one large plateau always at the outside of the mantle and by in most cases a rounded internal morphology the inner rim is typically oscillatory zoned the width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow the presence of a skeletal inner core and the concentration of glass inclusions in low an zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount the resorption features at the outer margins of low an zones indicate superheating of the liquid with respect to the crystal

magma mixing at nisyros volcano as inferred from incompatible trace element systematics abstract rocks of the calc alkaline volcanic complex of nisyros at the eastern end of the quaternary south aegean volcanic arc range in composition from basaltic andesite to rhyodacite major and compatible trace element systematics are dominated by crystal fractionation involving olivine clinopyroxene plagioclase and magnetite in the mafic to intermediate compositional range and plagioclase clinopyroxene hypersthene fe ti oxides and apatite in the intermediate to silicic range although there is a marked paucity of lavas with 60 66 wt sio 2 the occurrence of andesitic inclusions in pre and post caldera dacites as well as reversely zoned and or partly resorbed phenocrysts exotic mineralogy e g forsteritic olivine compositionally bimodal plagioclase phenocryst populations and the observation of two distinct glass phases in melt inclusions and pumices all indicate that magma mixing processes have played a role in the petrogenesis of nisyros incompatible trace element systematics are consistent with mixing between mafic and silicic end members major element binary mixing calculations between end members successfully reproduce the dacite composition the eruptive sequence at nisyros suggests the existence of a compositionally zoned magma chamber in which mixing between underlying more mafic magma and overlying rhyodacitic melt result in the formation of dacitic magmas inherent mineral assemblages in basaltic andesites indicate that mixing between mafic liquids and between mafic and more silicic liquids played also a role in their petrogenesis

seismic properties in the eastern part of the south aegean volcanic arc the south aegean active volcanic arc lies along the 150 km seismic isodepth of a benioffzone and consists in andesitic dacitic and rhyolitic volcanoes of an orogenic calc alkaline type in the eastern part of the arc there are two main volcanic sites in the nisyros and kos islands high shallow and intermediate depth seismic activity occurs in this volcanic area seismological data concerning the 1911 1980 period have been used to investigate seismic properties in that area two distinct regions with different seismotectonic features have been defined the internal region nisyros active volcano and its proximity is characterized by a shallow thin seismogenetic layer with abnormally highb value locally concentrated stresses low seismicity and highly heterogeneous structure these features are probably due to a magmatic body intruded in shallow depths within the crust the possibility of magma formation in the upper surface of the descending slab or within the upper mantle wedge overlying this slab in the nisyros kos area is herein discussed on the contrary the remaining external region including the non active volcanic island of kos is characterized by normal for tectonic shocksb value thick seismogenetic layer and high seismicity it seems that in this region there is not a magmatic intrusion at least in shallow depths within the crust

trace element geochemistry of high alumina basalt andesite dacite rhyodacite lavas of the main volcanic series of santorini volcano greece trace element systematics throughout the cal calkaline high alumina basalt basaltic andesite andesite dacite rhyodacite lavas and dyke rocks of the main volcanic series of santorini volcano greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the main series into two distinct crystallisation intervals an early basalt to andesite stage characterised by calcic plagioclase augite olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite hypersthene magnetite apatite crystallisation percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition subtraction diagrams and indicate that basaltic andesites represent 47 69 andesites 70 76 dacites ca 80 and rhyodacite ca 84 crystallisation of the initial basalt magma least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative main series lavas though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition subtraction techniques main series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine pyroxene cr spinel crystallisation on ascent through the sub aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the aegean crust residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of santorini the eruption of large volumes of basic lavas and silicic pyroclastics from santorini may have a volcanological rather than petrological explanation

paleomagnetic stratigraphy and radiometric dating of the pliocene volcanic rocks of aegina greece the island of aegma includes some of the oldest volcanic rocks in the south hellenic arc previous radiometric dates range from 3 87 to 4 4 ma the volcanic sequence is divided into nine units on the basis of field relations petrography and geochemistry and the characteristic paleomagnetic polarity of each unit has been determined two new radiometric dates 2 1 and 3 9 ma show that the volcanism spans the pliocene a detailed chronology is proposed using the paleomagnetic data which places the most voluminous volcanism in the mid pliocene

physical properties and state of the crust and upper mantle of the eastern mediterranean sea deduced from geophysical data abstract by evaluating the geophysical information available for the eastern mediterranean sea ems a crustal thickness map and some conclusions as to the nature of the crustal type were drawn it was found that the ionian abyssal plain is floored by an oceanic type of crust which is 14 km thick and covered by sediments of the order of 8 km thick the herodotus abyssal plain in the levantine sea is floored by a crust 22 km thick and covered by sediments ranging between 10 and 15 km thick the eastern mediterranean rise extending more or less continuously from the apulian plateau to rhodes and the south of the antalya basin is of a thickness ranging between 28 and 34 km the thickness of its sedimentary cover is of the order of 10 km the borders of the eastern mediterranean sea are built up of thick continental crust for example the hellenides of western greece range between 40 and 44 km thick and western turkey has a similar crustal thickness the crust of sicily also exceeds 40 km whereas the southern and eastern borders of the ems are approximately 30 32 km thick tectonically the northern border of the ems is built up of active continental margins defined by the calabrian hellenic and cyprus antalya arcs whereas the southern border represents a passive continental margin controlled mainly by subsidence and stretching to the east the dead sea megashear limits the area the deep basins of the eastern mediterranean sea are tectonically controlled by subsidence and partial stretching the mediterranean ridge or rise is still an area of dispute regarded by some authors as a subsiding continental fragment and separated from the hellenic arc by the hellenic trench whereas others consider it to be a zone of crustal shortening thickened mainly by thrusting both the western and southwestern borders of the mediterranean rise and the hellenides are controlled by compressional processes similar to those deforming the calabrian arc the stravo and pliny trenches to the east southeast border of the hellenic arc are partly affected by strike slip movements various plate tectonic models of the area have been proposed by several authors though the complexity of the deformation hardly permits reliable reconstructions the lack of reliable paleomagnetic information and the absence of linear magnetic anomalies do not favour the application of plate tectonic concepts vertical processes due to the uprising of lithothermal systems below the hellenic and calabrian arcs may also be regarded as the driving force controlling the present day deformation

petrology and evolution of transitional alkaline sub alkaline lavas from patmos dodecanesos greece evidence for fractional crystallization magma mixing and assimilation petrographic mineral chemical and whole rock major oxide data are presented for the lavas of the main volcanic series of patmos dodecanesos greece these lavas were erupted about 7 m y ago and range in composition from ne trachybasalts through hy trachybasalts and trachyandesites to q trachytes to some extent the ne trachybasalts are intermediate in composition to the alkaline lavas found on oceanic islands and the calc alkaline lavas of destructive plate margins major oxide variation is largely explicable in terms of fractional crystallization involving removal of the observed phenocryst and microphenocryst phases viz olivine plagioclase clinopyroxene and ti magnetite in the mafic lavas plagioclase clinopyroxene mica and ti magnetite in the evolved lavas apatite which occurs as an inclusion in other phenocrysts or as microphenocrysts must also have been removed however mass balance calculations indicate that the chemistry of the hy trachybasalts is inconsistent with an origin via fractional crystallization alone and the complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas show that they are hybrids formed by the mixing of 80 77 ne trachybasalt with 20 23 trachyandesite it is estimated that the mixing event preceded eruption by a period of 12 h 2 weeks suggesting that mixing triggered eruption combined fractionation and mixing cannot explain the relatively low mgo contents of the hy trachybasalts and it is concluded that assimilation also occurred assimilation and especially addition of volatiles to the magmas may be responsible for the evolutionary trend from ne normative to hy normative magmas and was probably facilitated by intensified convection resulting from mixing a model is presented whereby primitive magma undergoes fractionation in an intracrustal magma chamber to yield more evolved liquids influx of hot primitive magma into the base of the chamber facilitates assimilation but eventually mixing yields the hy trachybasalts and finally the ne trachybasalts are erupted

the loch scridain xenolithic sill complex isle of mull scotland fractional crystallization assimilation magma mixing and crustal anatexis in subvolcanic conduits

inverse differentiation pathway by multiple mafic magma refilling in the last magmatic activity of nisyros volcano greece based on detailed field petrographic chemical and isotopic data this paper shows that the youngest magmas of the active nisyros volcano south aegean arc greece are an example of transition from rhyolitic to less evolved magmas by multiple refilling with mafic melts triggering complex magma interaction processes the final magmatic activity of nisyros was characterized by sub plinian caldera forming eruption 40 ka emplacing the upper pumice up rhyolitic deposits followed by the extrusion of rhyodacitic post caldera domes about 31 10 ka the latter are rich in magmatic enclaves with textural and compositional basaltic andesite to andesite characteristics that reveal they are quenched portions of mafic magmas included in a cooler more evolved melt dome lavas have different chemical isotopic and mineralogical characteristics from the enclaves the latter have lower 87sr 86sr and higher 143nd 144nd values than dome lavas silica contents and 87sr 86sr values decrease with time among dome lavas and enclaves micro scale mingling processes caused by enclave crumbling and by widespread mineral exchanges increase from the oldest to the youngest domes together with enclave content we demonstrate that the dome lavas are multi component magmas formed by progressive mingling mixing processes between a rhyolitic component post up and the enclave forming mafic magmas refilling the felsic reservoir from 15 wt to 40 wt of mafic component with time we recognize that only the more evolved enclave magmas contribute to this process in which recycling of cumulate plagioclase crystals is also involved the post up end member derives by fractional crystallization from the magmas leftover after the previous up eruptions the enclave magma differentiation develops mainly by fractional crystallization associated with multiple mixing with mafic melts changing their composition with time a time related picture of the relationships between dome lavas and relative enclaves is proposed suggesting a delay between a mafic magma input and the relative dome outpouring we also infer that the magma viscosity reduction by re heating allows dome extrusion without explosive activity

the record of magma chamber processes in plagioclase phenocrysts at thera volcano aegean volcanic arc greece lavas and pyroclastic rocks throughout the volcanic stratigraphy of the tertiary quaternary volcanic complex of thera in the aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures interpreted as indicative of magma mixing plagioclase zoning characteristics studied by nomarski and laser interferometry techniques establish three main categories of plagioclase i inherited plagioclase nucleated in endmember prior to initial mixing event ii in situ plagioclase nucleated in mixed or hybrid magma and iii xenocrystic plagioclase nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases depending on the composition of the lava in which they are hosted these differences reflect the contrasting effects of changes in physical chemical parameters in basic vis a vis more acidic melts during magma mixing and or influx of new magma into the subvolcanic magma chamber as well as the influence of magma chamber dynamics on plagioclase equilibration variations in bulk major and trace element abundances of thera volcanic products reflect the dominant overprint of crystal fractionation but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes consistent with compositional and textural zoning in plagioclases

interpretation of nisyros volcanic terrain using land surface parameters generated from the aster global digital elevation model abstract to model the morphotectonic evolution of nisyros stratovolcano in the aegean volcanic arc 36 35 n 27 10 e a 30 m resolution aster gdem was used nisyros is characterized by a relative pristine volcanic terrain elevation slope and aspect images the corresponding frequency distributions and rose diagrams enabled the geomorphometric analysis of nisyros revealing the major geomorphological structures that are associated to both endogenetic and exogenetic processes acting on the island either new or previously reported in the literature new elements include the number loci of issue relative age ogive structures of the voluminous precalderan nikia flows and their contact relationships with the avlaki flows the tectonic control fine feature morphology and flow paths of lavas and smaller domes associated with the main postcalderan domes become visually apparent particularities of the hydrographic network accentuate and bring forward non mapped radial faults intense landslide scarring and the volcanic stratigraphy of the intact units were revealed in the northeastern quadrant of nisyros major new volcano tectonic features include the division of the island into three northwesterly trending sectors and the dipping of nisyros towards the southeast as a result of segmentation by two major ring faults the kos ring fault krf and perigussa ring trapdoor fault prf which represent ring faults of the kos sagging caldera the aster gdem has provided suitable thematic information content in the geomorphometric analysis of nisyros and therefore it offers a reconnaissance tool in the geomorphological analysis of a volcanic landscape

dependence of near surface magnetic susceptibility on dust accumulation rate and precipitation on the chinese loess plateau magnetic susceptibility ms of surface sediment varies systematically across the loess plateau in central china decreasing exponentially from 200 10 8 m3 kg at the northern margin of the qinling shan to 30 10 8 m3 kg near the southern margin of the mu us desert ms correlates highly with loess median grain size r2 0 79 which decreases south southeastward across the plateau it also correlates with mean annual temperature mat and mean annual precipitation map r2 0 58 and 0 60 respectively and with their product mat map r2 0 83 which is considered a measure of potential pedogenic activity because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel it is difficult to determine their relative influence on ms a simple ms model based on the observed spatial variation in loess thickness permits quantitative assessment of the effect of the dust accumulation rate on the ms signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals the model suggests that 84 of the loess ms variance is dictated by the diluting effect of dust and 10 11 is associated with meteorological factors primarily precipitation the observed and modeled relationships support hypotheses that attribute variations in ms in the loess paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils both of which are controlled by monsoon climate

laser interferometry study of oscillatory zoning in plagioclase the record of magma mixing and phenocryst recycling in calc alkaline magma chambers iztaccihuatl volcano mexico

late holocene pelean style eruption at tacana volcano mexico and guatemala past present and future hazards tacana volcano located on the border between mexico and guatemala marks the northern extent of the central american volcanic chain composed of three volcanic structures it is a volcanic complex that has had periodic explosive eruptions for at least the past 40 k y the most recent major eruption occurred at the san antonio volcano the youngest volcanic edifice forming the complex about 1950 yr ago the pelean style eruption issued from the southwest part of the dome and swept a 30 sector with a hot block and ash flow that traveled about 14 km along the cahoacan ravine deposits from this event are well exposed around the town of mixcun and were therefore given the name of that town the mixcun flow deposit the mixcun flow deposit is in the channel facies a light gray massive thick 10 m matrix supported unit with dispersed lithic clasts of gravel to boulder size divisible in some sections into a variable number of flow units the overbank facies is represented by a thin 1 m massive matrix supported unit in both of these facies the deposit has disseminated charcoal fumarolic pipes and juvenile lithics with cooling joints the mixcun flow deposit contains clasts of 1 light gray dense andesite 2 dark gray glassy and banded andesite and 3 minor altered red andesite from the edifice set in a matrix of sand and silt the mixcun flow deposit covers an area of at least 25 km2 and has a minimum estimated volume of 0 12 km3 basaltic andesite inclusions 54 sio2 and various signs of disequilibrium in the mineral assemblage of the two pyroxene andesitic products 60 63 sio2 suggest that magma mixing may have triggered the eruption following deposition of the mixcun flow deposit andesitic to dacitic 62 64 sio2 lava flows were extruded and a dacitic dome 64 4 sio2 at the san antonio summit formed syn eruptive and posteruptive lahars flooded the main drainages of the cahoacan and izapa mixcun valleys in the area of the present city of tapachula population 250000 and the pre hispanic center of izapa three radiocarbon ages date this event between a d 25 and 72 range 1 38 b c a d 216 which correlates with a halt in construction at izapa hato phase of ca 50 b c a d 100 probably due to temporary abandonment of the city caused by lahars another similar event would produce extensive damage to the towns population of about 68 000 people now built upon the mixcun flow deposit the main summit of tacana volcano continues to show signs of fumarolic activity the most recent period of activity in 1985 1986 culminated in a minor phreatic explosion

laser interference and nomarski interference imaging of zoning profiles in plagioclase phenocrysts from the may 18 1980 eruption of mount st helens washington

magmatic interaction between mantle and crust during the evolution of the archean lac guyer greenstone belt new quebec the lac guyer greenstone belt was one of a series of volcanic troughs active during the archean in the james bay territory of the superior province of quebec the belt consists of a succession of i

is there a compositional gap at nisyros volcano a comment on magma generation at the easternmost section of the hellenic arc hf nd pb and sr isotope geochemistry of nisyros and yali volcanoes greece lithos 83 2005 29 46

the potential for future explosive volcanism associated with dome growth at nisyros aegean volcanic arc greece monitoring of the explosivity controlling parameters in magmas associated with dome growth in the quaternary volcano at nisyros aegean sea involves determining the chemical composition including dissolved volatiles h2o co2 f cl and s of melt inclusions in phenocrysts and interstitial matrices in pre and post caldera volcanic products our results indicate that the volatile content of the pre caldera magma was a dominant factor responsible for the catastrophic explosivity of the caldera forming episode the relatively high estimated volatile contents and apparent interstitial viscosities of the post caldera magmas as well as the geophysical evidence for the existence of an active magma chamber beneath nisyros all suggest that this volcano is presently dormant but has a high potential for future explosive volcanism

chinese loess record of monsoon climate during the last glacial interglacial cycle the thick loess paleosol succession of central china contains a detailed record of changes in east asian monsoon climate since the late miocene epoch the alternating loess paleosol stratigraphy closely resembles the marine oxygen isotope record implying that episodic dust deposition and pedogenesis are in phase with global ice volume fluctuations and controlled by variations in solar radiation reaching the land surface deposits equivalent to marine isotope stages mis 5 through 1 have been dated by 14c 10be luminescence and magnetic susceptibility and grain size models malan loess mis 4 2 decreases in thickness southeastward from 30 m in the western loess plateau to 5 m east of xi an downwind asian dust is found in japan the north pacific ocean and the greenland ice sheet where it has a chemical signature that points to a primary source in western china during glacial times the mean dust flux in the central loess plateau reached 0 35 mm year whereas in interglacial times it fell to 0 1 mm year times of high dust influx and reduced soil development coincided with strengthened winter monsoon conditions which are inferred from variations in loess grain size times of decreased dust accumulation and strong pedogenesis marked by high values of magnetic susceptibility represent times of strengthened summer monsoon conditions high frequency fluctuations in dust influx during deposition of the malan loess include peaks that likely correlate with north atlantic heinrich events the younger dryas oscillation is recorded in chinese eolian and organic deposits by a shift from cool humid late glacial conditions to cold and dry conditions followed by a return to milder humid climate at the beginning of the holocene warm moist climate associated with a strong summer monsoon characterized the first half of the holocene epoch when a widespread paleosol complex developed on the loess plateau increased dust deposition during the middle to late holocene marked a shift to cooler drier conditions

the upper and lower nisyros pumices revisions to the mediterranean tephrostratigraphic record based on micron beam glass geochemistry abstract the greek island of nisyros is the source of tephra found in distal settings throughout the aegean in this contribution we define geochemical fingerprints for the lower and upper pumice eruptions of nisyros based on micron beam major empa and trace la icp ms element data these two eruptions are compositionally distinct from each other the high phenocryst content of proximal lower and upper pumice tephra means that the differences between the two magmas are masked in whole rock analyses we demonstrate that all previous reports of distal marine and continental tephras correspond to the younger upper pumice not the older lower pumice this has important implications for the marine tephrochronology of the mediterranean and for the age of the upper and lower pumice eruptions we suggest an age of at least 47 ka for the upper pumice and an older unconstrained age for the lower pumice

monitoring oxygen fugacity conditions in pre syn and postcaldera magma chamber of nisyros volcano aegean island arc greece abstract the magmatic oxygen fugacities of the pre syn and postcaldera products of the quaternary volcano of nisyros in the aegean volcanic arc eastern mediterranean sea have been measured from analyzed compositions of titaniferous magnetite ilmenite equilibrium pairs quench temperatures in the precaldera rhyodacitic flows range from 877 to 836 c and oxygen fugacities from 11 4 to 11 8 log units one sample of the climactic pumice displays average quench temperatures of 853 c with f o 2 of 11 7 log units crystallization temperatures of the dacitic domes that intercept the nisyros caldera range from 843 to 804 c while oxygen fugacities range from 11 4 to 12 1 log units the rocks of the nisyros volcano record no significant change in oxygen fugacity relative to buffer curves immediately prior during and after the caldera forming event this is surprising given the considerable potential for interaction between the magmas and atmospheric oxygen during caldera collapse and the concomitant massive exsolution of volatiles however the nisyros fugacity data may not be atypical of stratovolcanoes with products of similar composition and histories of caldera forming events we speculate that only in the volcanoes where caldera collapse coincides with magma meteoric water interaction do the climactic or early postcaldera units show anomalously high oxygen fugacities

an analytical and experimental study of zoning in plagioclase a detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars discontinuous sector and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends the relative accuracy of 3 element na ca k microprobe traverses was found statistically to be 2 mole percent comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

east asian monsoon forcing of suborbital variability in the sulu sea during marine isotope stage 3 link to northern hemisphere climate 1 we have generated a new high resolution record of variations in planktonic foraminiferal oxygen isotopes 18o and mg ca from a sediment core images 97 2141 in the sulu sea located in the philippine archipelago of western tropical pacific this record reveals distinct suborbital scale 18o changes most notably during marine isotope stage 3 mis3 30 000 to 60 000 years b p the amplitudes of these 18o fluctuations 0 4 to 0 7 exceed that which can be attributed to sea level changes and must be due to changes in sea surface conditions in the same interval variations in planktonic foraminifera mg ca suggest that suborbital surface ocean temperature variations of 1 to 1 5 c in the sulu sea were not in phase with 18o combined this evidence indicates that the mis3 millennial 18o events in the sulu sea were primarily the result of changes in surface water salinity which today is directly related to the east asian monsoon eam and its influence on the balance between surface water contributions from the south china sea and western pacific warm pool wpwp within dating uncertainties the mis3 sulu sea 18o suborbital variability indicates that times of fresher surface conditions in the sulu sea coincide with similar conditions in the wpwp stott et al 2002 and also with intensifications of the summer eam as recorded in the u th dated chinese hulu cave speleothem 18o record wang et al 2001 and thus by inference with interstadials in the greenland ice core records combined these results indicate that pronounced suborbital variability in the summer eam and intertropical convergence zone itcz during mis3 was tightly coupled with climate conditions in the northern high latitudes