differentiation and crystallization conditions of basalts from the kerguelen large igneous province an experimental study phase relations of basalts from the kerguelen large igneous province have been investigated experimentally to understand the effect of temperature fo2 and fugacity of volatiles e g h2o and co2 on the differentiation path of lip basalts the starting rock samples were a tholeiitic basalt from the northern kerguelen plateau odp leg 183 site 1140 and mildly alkalic basalt evolved from the kerguelen archipelago mt crozier on the courbet peninsula representing different differentiation stages of basalts related to the kerguelen mantle plume the influence of temperature water and oxygen fugacity on phase stability and composition was investigated at 500 mpa and all experiments were fluid saturated crystallization experiments were performed at temperatures between 900 and 1 160 c under oxidizing log fo2 qfm 4 and reducing conditions log fo2 qfm in an internally heated gas pressure vessel equipped with a rapid quench device and a pt membrane for monitoring the fh2 in all experiments a significant influence of the fo2 on the composition and stability of the mg fe bearing mineral phases could be observed under reducing conditions the residual melts follow a tholeiitic differentiation trend in contrast melts have high mg mg2 mg2 fe2 and follow a calk alkalic differentiation trend at oxidizing conditions the comparison of the natural phenocryst assemblages with the experimental products allows us to constrain the differentiation and pre eruptive conditions of these magmas the pre eruptive temperature of the alkalic basalt was about 950 1 050 c the water content of the melt was below 2 5 wt h2o and strongly oxidizing conditions log fo2 qfm 2 were prevailing in the magma chamber prior to eruption the temperature of the tholeiitic melt was above 1 060 c with a water content below 2 wt h2o and a log fo2 qfm 1 early fractionation of clinopyroxene is a crucial step resulting in the generation of silica poor and alkali rich residual melts e g alkali basalt the enrichment of alkalis in residual melts is enhanced at high fo2 and low ah2o

mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

experimental melting of biotite plagioclase quartz muscovite assemblages and implications for crustal melting abstract in order to understand the role of mica rich rocks as a source of granite magmas a series of melting experiments was performed on two different starting materials the first composition is a model biotite gneiss consisting of 30 wt biotite 30 wt plagioclase and 40 wt quartz the second composition is a model two mica pelites consisting of 15 wt biotite 15 wt muscovite 30 wt plagioclase and 40 wt quartz experiments were performed under vapor absent conditions at 1 0 gpa and between 750 o and 950oc with only biotite in the starting material the volume of melt is always less than 15 vol below 900oc and reaches 25 vol at 950oc in experiments that involve both biotite and muscovite in the starting material the melt proportion increases up to 28 vol at 825oc and reaches 60 vol at 950oc for the biotite plagioclase quartz bpq assemblage the solidus is located at 800oc at 1 0 gpa the melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet biotite orthopyroxene plagioclase quartz in addition the experiments show that at 1 0 gpa biotite can be stable above 950oc with both micas in the starting material bpqm the solidus at 1 0 gpa is located at 750oc the melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet sillimanite biotite quartz plagioclase kfeldspar in experiments below 900oc at 950oc the residuum consists of garnet orthopyroxene biotite plagioclase the melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz mineral modes of the source rocks particularly the amount of quartz are at least as important as the amount of available h20 in controlling the melt fraction generated during crustal anatexis

paleomagnetism of the silurian volcanism at almaden southern spain the silurian volcanism of the almaden mining district southern spain has been the object of a very detailed paleomagnetic study the 175 cores 25 sites sampled all along the eastern termination of the almaden syncline were subjected to magnetic susceptibility measurements thermomagnetic cycles progressive alternating field and thermal demagnetizations as well as microscopic observations the magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms including small cubic distributed grains and large elongated crystals the observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases showing a complete inversion of the susceptibility tensors this anisotropy has been related to the emplacement of the flows on a gentle slope dipping 10 toward the southwest implying that their volcanic source was located to the northeast the paleomagnetic analysis has revealed two characteristic components of magnetization the first direction d 109 i 11 95 9 pole position at 11 s 69 e dp 5 dm 9 yields a negative fold test and is interpreted as a late paleozoic partial overprint its declination is 46 away of the expected late paleozoic field direction which confirms that rotations are still occurring after the main folding phases of the variscan orogeny the second direction d 62 i 36 95 14 pole position at 8 n 118 e dp 9 dm 13 is prefolding as evident from the significant clustering when applying appropriate tectonic corrections and we interprete it as the original magnetization of silurian age the compilation of all paleozoic paleomagnetic results from spain reveals that the iberian meseta has been moving along with gondwana at high drift rates throughout ordovician and silurian times

the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

constraining the components of the kerguelen mantle plume a hf pb sr nd isotopic study of picrites and high mgo basalts from the kerguelen archipelago 1 we report geochemical and hf pb sr nd isotopic analyses of a suite of picrites and associated high mgo volcanic rocks 6 17 wt mgo from the kerguelen archipelago which are rare compared to other oceanic islands to better constrain the nature and the origin of components present in the kerguelen mantle plume source the sr and nd isotopic compositions of the transitional to mildly alkalic picrites and high mgo basalts closely match those of the 24 ma mildly alkalic basalts from the courbet peninsula whose compositions are considered to reflect the present geochemical expression of the enriched component of the kerguelen mantle plume however linear trends in pb isotopic compositions in the studied samples reflect involvement of a component with lower 206pb 204pb and 208pb 204pb than that inferred for the enriched kerguelen plume contamination of the mgo rich magmas by the kerguelen plateau cannot account for the observed hf pb sr nd isotopic variations isotopic systematics in the picrites and the high mgo basalts are inconsistent with simple binary mixing between two distinct end members and indicate the presence of small scale heterogeneities within the kerguelen plume itself as has been observed in other hot spot environments such as hawaii and iceland the 34 to 26 ma kerguelen plume related basalts that formed when the archipelago was close to the ridge axis 50 to 250 km show geochemical evidence for significant involvement of a southeast indian ridge seir like source in contrast the 24 25 ma mildly alkalic basalts from the eastern and southeastern parts of the archipelago which erupted about 400 km away from the seir and the picrites and high mgo basalts from this study show little or no contribution from a seir like component thus chemical interaction between the seir axis and the kerguelen plume effectively ceased prior to 25 ma

flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

tephrological implications of beam size sample size effects in electron microprobe analysis of glass shards this paper concerns the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe application of electron probe microanalysis to tephrostratigraphical methods has increasingly facilitated the resolution and refinement of quaternary chronology associated with records of proxy environmental or proxy climatic change the geographical range over which tephras are recovered has expanded significantly with the identification and analysis of crypto or hidden tephras in areas far removed from tephra sources these tephras are dominated by glass shards which in many distal environments may be either small in size m or may be highly pumiceous with low glass void ratios and thin 10 m shard walls we demonstrate that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls this approach distorts the geochemical data creating analytical differences that may generate inappropriate tephrogeochemical fingerprints additional distortion of the geochemical fingerprint in the form of hybrid analyses may be encountered in glass fragments containing micron sized crystalline phases such as feldspar copyright 2001 john wiley sons ltd

an experimental investigation of the influence of water and oxygen fugacity on differentiation of morb at 200 mpa crystallization experiments were performed at 200mpa in the temperature range 1150 950 c at oxygen fugacities corresponding to the quartz fayalite magnetite qfm and mno mn3o4 buffers to assess the role of water and fo2 on phase relations and differentiation trends in mid ocean ridge basalt morb systems starting from a primitive mgo 9 8 wt and an evolved morb mgo 6 49 wt crystallization paths with four different water contents 0 35 4 7 wt h2o have been investigated in primitive morb olivine is the liquidus phase followed by plagioclase clinopyroxene amphibole is present only at water saturated conditions below 1000 c but not all fluid saturated runs contain amphibole magnetite and orthopyroxene are not stable at low fo2 qfm buffer residual liquids obtained at low fo2 show a tholeiitic differentiation trend the crystallization of magnetite at high fo2 mno mn3o4 buffer results in a decrease of melt feo mgo ratio causing a calc alkaline differentiation trend because the magnetite crystallization temperature is nearly independent of the h2o content in contrast to silicate minerals the calc alkaline differentiation trend is more pronounced at high water contents residual melts at 950 c in a primitive morb system have compositions approaching those of oceanic plagiogranites in terms of sio2 and k2o but have ca na ratios and feo contents that are too high compared with the natural rocks implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites

interpretation of magnetic fabrics in the early proterozoic diabase dykes of keuruu central finland abstract magnetic fabrics petrophysics petrography and chemical compositions are described for 14 diabase dykes located in the keuruu area central finland these early proterozoic 1880 ma dykes sharply cut both low density granodiorites and high density diorites and gabbros the diabase dykes with a nw se strike and an almost vertical dip have a maximum length of a few kilometers and widths of about 1 m strongly developed magnetic fabrics of deformation type anisotropy degree up to 1 4 were observed in six of the dykes and weakly developed fabrics of flow type anisotropy degree below 1 1 in eight dykes no systematic differences in the dyke widths mean densities palaeomagnetic poles mineral compositions degrees of alteration or chemical characteristics were observed between the two types of dykes suggesting that the dykes had a common magma source all the fabrics of deformation type are dominantly ferrimagnetic with tightly aligned horizontal foliations and well grouped lineations which in most cases are parallel to the dyke trends it is suggested that the deformation fabrics were generated as tholeiitic magma was forced to intrude laterally along its level of neutral buoyancy lnb which may have been close to the present erosion level pel in the low density granodiorite environment ferri and paramagnetic fabrics of flow type characterize the dykes located in the denser mafic parts of the crust in these parts the lnb was possibly somewhat higher in the crust so that magma flow passed the pel and formed weakly developed flow fabrics with gently plunging lineations and vertical or variable magnetic foliations

differentiation conditions of a basaltic magma from santorini and its bearing on the production of andesite in arc settings santorini volcano in the aegean region greece is characterized by andesitic to silicic dominated explosive activity and caldera forming eruptions sourced from magmatic reservoirs located at various structural levels beneath the volcano there is a good understanding of the silica rich magmatism of the island whereas the andesite dominated volcanism and the petrogenesis of the parental mafic magmas are still poorly understood to fill this gap we have performed crystallization experiments on a representative basalt from santorini with the aim of determining the conditions of differentiation pressure temperature volatile fugacities and the parental magma relationship with the andesitic eruptive rocks experiments were carried out between 975 and 1040 c in the pressure range 100 400 mpa fo2 from qfm to nno 3 5 where qfm is quartz fayalite magnetite and nno is nickel nickel oxide with h2omelt contents varying from saturation to nominally dry conditions the results show that basalt phenocrysts within the basalt crystallized at around 1040 c in a magma storage reservoir located at a depth equivalent to 200 400 mpa pressure with 3 5 wt dissolved h2o and fo2 around qfm comparison with the xenocryst and phenocryst assemblages of the upper scoria 1 andesite shows that andesitic liquids are produced by fractionation of a similar basalt at 1000 c and 400 mpa following 60 80 wt crystallization of an ol cpx plag ti mag opx 6 pig ilm assemblage with melt water contents around 4 6 wt at santorini the andesitic low viscosity and water rich residual liquids produced at these depths segregate from the parent basaltic mush and feed the shallow magma reservoirs eventually erupting upon mixing with resident magma changes in prevailing oxygen fugacity may control the tholeiitic calc alkaline character of santorini magmas explaining the compositional and mineralogical differences observed between the recent thyra and old eruptive products from akrotiri

generation of alkalic basalt by crystal fractionation of tholeiitic magma contemporaneous alkalic and tholeiitic basalts from cerro azul volcano in the galapagos archipelago are related by fractional crystallization in the lower crust and upper mantle the higher pressures enhance augite crystallization which depletes the residual liquids in silica while enriching them in k and na just as predicted by phase equilibria experiments such occurrences have also been reported from other ocean islands the common aspect of these alkalic suites is that they occur at volcanoes that have relatively low magma supply rates which leads to the development of deep magma bodies and the production of alkali olivine basalts by fractional crystallization of a tholeiitic parent during more robust phases of magmatism tholeiitic magmas ascend into the upper crust and produce a low pressure tholeiitic differentiation trend

from alkali basalt to phonolite in hand size samples vapor differentiation effects in the bouzentes lava flow cantal france abstract the bouzentes lava flow is a 20 m thick alkali basalt flow emplaced during the last stage of formation of the cantal stratovolcano at 4 2 ma its upper part has 1 to 20 cm thick vesicle rich segregation sheets which recur every 0 1 2 m these horizontal veins are hawaiitic in composition they are characterized by hypertrophic development of their minerals pegmatoids and by glassy phonolitic segregation vesicles internal differentiation within the bouzentes lava flow was triggered by an unusually high water content as suggested by pre emptive iddingsite alteration of olivine phenocrysts the proposed model of formation of the segregation sheets includes the upward motion of diapirs of residual melt plus addition of vapor from the bottom of the central liquid lens to the base of the upper solidified crust of the cooling lava flow olivine settling appears to have been inhibited or at least retarded by upward migration of melt plus vesicles most of the features observed in bouzentes recall the internal differentiation processes usually described within thick hawaiian lava lakes the segregation vesicles are believed to result from an increase of gas solubility in residual melt during the crystallization process

submarine basalts of the northern kerguelen plateau interaction between the kerguelen plume and the southeast indian ridge revealed at odp site 1140 during ocean drilling program leg 183 basaltic cores were retrieved from the northern kerguelen plateau nkp at site 1140 on the extreme north of the plateau 270 km north of the kerguelen archipelago amongst the six basement units recovered five were pillow basalts with fresh glassy rims together with flow lobes this is the first evidence for submarine eruption anywhere on the kerguelen plateau each flow unit has distinct geochemical and isotopic characteristics that span the range from southeast indian ridge seir mid ocean ridge basalt to tholeiitic transitional basalts derived from the kerguelen mantle plume relationships between element abundance ratios involving incompatible elements and isotopic ratios reflect mixing of near primary melts from an seir source and the kerguelen plume with an increasing role for an incompatible element rich end member in the order unit 1

geothermometry using minor and trace elements in igneous and hydrothermal magnetite abstract the temperature t and oxygen fugacity fo2 dependence of minor and trace elements mg al mn cr ni in magnetite mag in equilibrium with water bearing silicate liquids varying in composition from basalt to rhyolite 49 77 wt sio2 is examined in a compilation of experimental data n 299 from 700 to 1050 c 50 900 mpa and five order of magnitude range in fo2 the xmg mg mg fetot of mag is strongly dependent on t and can be formulated into an empirical geothermometer tmg mag c 8344 320 lnxmg 4 1 0 28 273 r2 0 83 that reproduces 72 of the experimentas to 50 c and conventional fe ti oxide thermometry in tuffs and vitrophyres to better than 60 c for 85 of the samples the tmg mag thermometer may have potential application in igneous or hydrothermally altered rocks where conventional thermometry or oxybarometry is not possible application to natural mag n 476 compiled from several settings shows igneous rocks mostly record 700 c but with some lower temperatures in plutonic rocks due to slow cooling hydrothermal samples skarns porphyry deposits show consistently subsolidus conditions tmg mag of 700 350 c consistent with other mineral and o isotopic equilibria the element correlations in natural mag from various settings are explored using multivariate statistical analysis to identify which minor or trace elements most critically depend on t fo2 or fluid composition

magma flow directions of shallow dykes from the east greenland volcanic margin inferred from magnetic fabric studies abstract the role played by plume generated crustal magmatic complexes in the segmentation of volcanic margins is highlighted by a preliminary study of magma flow directions in shallow intrusives from the east greenland volcanic margin we investigate the magmatic texture using anisotropy of magnetic susceptibility for eight dykes of tholeitic affinity belonging to a dyke swarm associated with the tertiary opening of the north atlantic ocean the thickness of the sampled dykes ranges from 3 to 37 m the dykes are of doleritic texture and contain up to 12 of opaque minerals and 35 of plagioclase laths dykes showing a magnetic foliation plane within the dyke plane i e a magnetic fabric usually attributed to magmatic processes represent 40 of the samples from the studied swarm these dykes have a low degree of anisotropy and their ellipsoid of magnetic susceptibility is strongly oblate inverse magnetic fabrics where the maximum principal susceptibility axis lies near the pole to the dyke account for 60 of the data we interpret these inverse fabrics either as alteration minerals with highly prolate ellipsoids of magnetic susceptibility or primary titanomagnetites with oblate ellipsoid of low anisotropy the flow direction is inferred for normal fabric dykes using the mirror imbrication of magnetic lineation analysis of thin sections shows a good agreement between magnetic fabric directions and phenocryst preferred orientations the inferred flow directions are predominantly horizontal throwing a new light on volcanic margin development

dike surface lineations as magma flow indicators within the sheeted dike complex of the troodos ophiolite cyprus mesoscopic flow lineations and anisotropy of magnetic susceptibility ams have been measured for dikes within the cretaceous age troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge and groove structures termed hot slickenlines found throughout the complex a unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back flow of magma during the waning stages of intrusion surface lineations generally lie subparallel to the direction of flow inferred from ams determinations on cores within 5 cm of dike margins surface lineations also lie subparallel to the long axis e1 of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from ams cores correlation of surface lineations with interior indicators of flow ams plagioclase trachytic texture indicate that the surface features are good proxies for grain scale magma flow directions during dike propagation in troodos dikes orientations of surface flow features in the dikes of the troodos ophiolite indicate an approximately equal mix of subhorizontal to near vertical magma flow contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest

storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands the case of tenerife canary islands spain abstract we report the results of phase equilibrium experiments on a phonolite produced during one of the most voluminous flank eruptions ca 1 km3 of the teide pico viejo complex tenerife island combined with previous experimental and volcanological data we address the factors that control the structure of the phonolitic plumbing system of teide pico viejo stratovolcanoes the roques blancos phonolite erupted ca 1800 bp and contains 14 wt phenocrysts mainly anorthoclase biotite magnetite diopside and lesser amounts of ilmenite crystallization experiments were performed at temperatures of 900 c 850 c and 800 c in the pressure range 200 mpa to 50 mpa the oxygen fugacity fo2 was varied between nno 0 3 0 3 log units above to the ni nio solid buffer to nno 2 whilst dissolved water contents varied from 7 wt to 1 5 wt the comparison between natural and experimental phase proportions and compositions including glass indicates that the phonolite magma was stored prior to eruption at 900 15 c 50 15 mpa with about 2 2 wt h2o dissolved in the melt at an oxygen fugacity of nno 0 5 0 5 the difference in composition between the rim and the cores of the natural anorthoclase phenocrysts suggests that the phonolite was heated by about 50 c before the eruption upon intrusion of a hotter tephriphonolitic magma the comparison between the storage conditions of roques blancos and those inferred for other phonolites of the teide pico viejo volcanic complex shows that flank eruptions are fed by reservoirs located at relatively shallow depths 1 2 km compared to those feeding teide central eruptions 5 km

shallow intrusive directions of sheeted dikes in the troodos ophiolite anisotropy of magnetic susceptibility and structural data sheeted dikes play a central role in the formation of oceanic crust it is commonly assumed that sheeted dikes intrude vertically upward from elongated mid ocean ridge mor magma chambers but there are no direct observational data bearing on this hypothesis this assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes the authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust the troodos ophiolite structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow 20 degree to nearly vertical upward magma transport directions these data suggest that horizontal emplacement has to be considered for sheeted dikes at mors implying more centralized mor plumbing systems than previously thought such plumbing systems provide ample opportunity for complex mixing fractionation and contamination of mor lavas in magma chambers and tabular magma storage volumes whether the mor magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems

petrological and experimental constraints on the pre eruption conditions of holocene dacite from volcan san pedro 36 s chilean andes and the importance of sulphur in silicic subduction related magmas we present an experimental and petrological study aimed at estimating the pre eruptive conditions of a holocene dacitic lava from volcan san pedro 36 s chilean andes phase equilibrium experiments were performed at temperatures t from 800 to 950 c and mainly at 200 mpa but also at 55 150 and 406 mpa oxygen fugacity fo2 ranged from the ni nio buffer nno to 3 5 log units above nno 3 5 and water contents from 3 to 6 wt we also report several experiments where we added sulphur 0 1 1 wt s to the dacite the main mineral assemblage of the dacite hornblende orthopyroxene plagioclase is stable at 200 50 mpa 850 10 c with 4 5 5 5 wt h2o in the melt and at fo2 of nno 1 2 0 2 in accord with the crystallinity mineral proportions and t fo2 determination from fe ti oxides of the lava however biotite which is also present in the dacite is stable at these same t fo2 conditions only in experiments with 0 1 wt s added this result is in accord with the occurrence of pyrrhotite in the lava and with the presence of s in glass inclusions and biotite 300 ppm and up 170 ppm respectively moreover the zoning patterns and compositions of plagioclase phenocrysts together with the presence of high temperature minerals e g clinopyroxene in the lava suggest that the petrological history of the dacite is more complex than a single near equilibrium crystallization stage and could be explained by short lived 100 years temperature fluctuations 50 c in the magma reservoir

variations in magnetic anisotropy and opaque mineralogy along a kilometer deep profile within a vertical dyke of the syenogranite porphyry at cinovec czech republic abstract vertical variations in magnetic fabric and paragenesis of the ferrimagnetic minerals for a virtually upright dyke of the altenberg syenogranite porphyry were investigated using profile data of borehole e 16 to a depth of 922 7 m eastern krusne hory mts nw bohemia it was revealed that this dyke likely consists of two magma pulses indicated both by magnetic fabric and by opaque mineralogy in most of the profile the magnetite grains are oriented parallel to the sub vertical dyke only at the base of the upper magma pulse in a depth interval of 200 400 m are these grains oriented sub horizontally being thus perpendicular to the dyke walls this pattern can be interpreted as a consequence of a static vertical compaction of the magma of the basal portions of the upper pulse due to the pressure of the ascending lower pulse the large planes of the ferrimagnetic minerals are oriented perpendicular to the shortening direction

ionospheric scintillation over antarctica during the storm of 5 6 april 2010 on 5 april 2010 a coronal mass ejection produced a traveling solar wind shock front that impacted the earth s magnetosphere producing the largest geomagnetic storm of 2010 the storm resulted in a prolonged period of phase scintillation on global positioning system signals in antarctica the scintillation began in the deep polar cap at south pole just over 40 min after the shock front impact was recorded by a satellite at the first lagrangian orbit position scintillation activity continued there for many hours on the second day significant phase scintillation was observed from an auroral site 81 degrees s during the postmidnight sector in association with a substorm particle data from polar orbiting satellites provide indication of electron and ion precipitation into the antarctic region during the geomagnetic disturbance total electron content maps show enhanced electron density being drawn into the polar cap in response to southward turning of the interplanetary magnetic field the plasma enhancement structure then separates from the dayside plasma and drifts southward scintillation on the first day is coincident spatially and temporally with a plasma depletion region both in the dayside noon sector and in the dayside cusp on the second day scintillation is observed in the nightside auroral region and appears to be strongly associated with ionospheric irregularities caused by e region particle precipitation

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

anisotropy of magnetic susceptibility in the ponta grossa dyke swarm brazil and its relationship with magma flow direction abstract measurements of anisotropy of magnetic susceptibility ams in 95 mafic dykes mainly tholeiites 10 200 m in width from the mesozoic ponta grossa swarm southern brazil revealed two main types of magnetic fabric type i fabric plane k 1 k 2 parallel to the dyke plane represents magma flow within the dykes whereas type ii plane k 1 k 3 parallel to the dyke plane is compatible with a fabric pattern reflecting vertical compaction of the magma column fabric type i dominates 51 of the dykes within the swarm whereas type ii 38 of the dykes concentrates mainly in the western region where the dykes intrude sediments considering the dykes with type i fabrics it is concluded that 58 of the dykes were fed by horizontal or sub horizontal k 1 inclinations less than 30 magmatic flow and 42 were fed by inclined to vertical k 1 inclinations more than 30 magma flow the latter are more frequent in the southeastern part of the swarm suggesting a magma source close to this area although there may have been other sources in other regions where dykes with inclined flow and distinct chemical characteristics are also found

ultrahigh temperature mafic granulite in the huai an complex north china craton evidence from phase equilibria modelling and amphibole thermometers abstract the huai an complex in the north western trans north china orogen north china craton is recognized to have undergone high pressure metamorphism with controversial ages two mafic granulite samples 17ht13 and 16ht22 from huangtuyao of the huai an complex are documented for metamorphism based on petrography mineral chemistry phase equilibria modelling and zircon dating the rocks comprise garnet clinopyroxene orthopyroxene amphibole plagioclase and ilmenite they show clockwise p t paths involving the pre tmax decompressional heating the tmax and the post tmax cooling stages the pre tmax decompressional heating is revealed by the core rim mantle anorthite ascending zoning in coarse grained plagioclase the tmax stage is constrained to 8 5 10 5 kbar 980 1010 c 17ht13 and 7 5 9 5 kbar 1000 1030 c 16ht22 by the high ti content of amphibole and the anorthite content of coarse grained plagioclase rims mantles and fine grained plagioclase cores these temperatures are slightly higher than the results of amphibole thermometers 940 980 c the post tmax cooling is defined by the later growth of amphibole and some conventional geothermobarometers because it is dominated by temperature with ilmenite present the ti content of amphibole is proposed as a reliable thermometer and the ti rich amphibole with ti 0 28 p f u or tio2 2 4 2 6 wt is indicative of ultrahigh temperature 900 c uht conditions the anorthite content of plagioclase is also significant in confining peak temperatures for uht granulite but requires defining proper water content zircon dating for 17ht13 yields an upper intercept age of 1910 35 ma representing the cooling of uht granulite we conclude that the huai an complex may have at least locally undergone uht metamorphism at 1 92 1 91 ga after high pressure granulite metamorphism at 1 95 ga similar to the jining complex therefore the huai an and jining complexes share the same tectonic evolution involving 1 95 ga crustal thickening 1 92 1 91 ga extension with regional or local uht metamorphism and subsequent cooling and uplifting

the system albite h2o co2 a model for melting and activities of water at high pressures the melting of albite in the presence of h2o co2 v por has been determined in pistoncylinder apparatus from 5 to 25 kbar for mole fractions of hzo in the vapor xfir6 from 1 0 to 0 0 albite in the presence of co2 rich vapor melts as much as 150 300 c lower than indicated by previous theoretical and experimental studies the albite co2 melting curve is indistinguishable from the vapor absent melting curve up to l5 kbar but it is significantly lower above 15 kbar as a result of a substantial increase in the solubility of coz in the liquid calculated activity coefficients y for water in h2o co2 vapor in the range 7001000 c 5 15 kbar are everywhere greater than unity and increase isobarically and isothermally toward coz rich compositions for xfir6 0 3 calculated lffro are significantly greater than those predicted by the modified redlich kwong equation

an experimental study of the influence of oxygen fugacity on fe ti oxide stability phase relations and mineral melt equilibria in ferro basaltic systems

magnetic and plagioclase linear fabric discrepancy in dykes a new way to define the flow vector using magnetic foliation in basaltic dykes the magnetic lineation k1 maximum magnetic susceptibility axis is generally taken to indicate the flow direction during solidification of the magma this assumption was tested in tertiary basaltic dykes from greenland displaying independent evidence of subhorizontal flow the digital processing of microphotographs from thin sections cut in k1 k2 planes yields the preferred linear orientation of plagioclase which apparently marks the magma flow lineation in up to 60 of cases the angular separation between k1 and the assumed flow direction is greater than 45 this suggests that the uncorroborated use of magnetic lineations in dykes is risky a simple geometrical method is proposed to infer the flow vector from ams in dykes based solely on magnetic foliations

shape fabrics of particles in low concentration suspensions 2d analogue experiments and application to tiling in magma abstract jeffery s equations ascribe a theoretical cyclic nature to the shape fabric of non interacting rigid particles immersed in a viscous fluid undergoing simple shear flow this theoretical behaviour is confirmed at low shear strains 5 first experiment 134 identical particles or 1 second experiment 178 particles with two size classes then become stable at 17 first experiment and at more than 50 second experiment of the population of particles due to the contribution of the tiled particles the shape fabric becomes asymmetrical in its orientation distribution with a maximum lying above the shear plane the latter result provides a new shear sense indicator in addition to the statistical determination of the tiled features the study also suggests that crystalline fabrics in magmas could be acquired at high melt fractions i e early in the crystallization history of the magma

volatiles in submarine basaltic glasses from the northern kerguelen plateau odp site 1140 implications for source region compositions magmatic processes and plateau subsidence submarine pillow basalts 34 ma recovered from the northern introduction kerguelen plateau at odp site 1140 contain abundant unaltered the kerguelen plateau and broken ridge formed mostly glass providing the first opportunity to measure the volatile contents during the cretaceous as a single giant oceanic plateau of tholeiitic basaltic magmas related to the kerguelen mantle plume that is interpreted to represent voluminous magmatism the glasses have la sm and nb zr ratios that vary from values associated with emplacement of a mantle plume head similar to southeast indian ridge seir morb unit 1 to below young indian ocean lithosphere duncan slightly more enriched unit 6 to values transitional between storey 1992 weis et al 1992 during ocean drilling seir morb and basaltic magmas formed by melting of the program odp leg 183 igneous basement rock and kerguelen plume units 2 and 3 volatile contents for glasses in sediment cores were obtained from five sites on the units 1 and 6 are similar to depleted mid ocean ridge basalt kerguelen plateau and two on broken ridge fig 1 the morb values 0 25 0 27 wt h2o 1240 1450 ppm s results combined with subsequent radiometric dating 42 54 ppm cl in contrast h2o contents are higher for the demonstrate that from south to north the age of the enriched glasses unit 2 0 44 wt h2o unit 3 0 69 wt uppermost crust decreases from 120 110 ma in the as are s 1500 ppm and cl 146 206 ppm cl k ratios southern kerguelen plateau to 110 to 95 ma in the for all glasses are relatively low 0 03 0 04 indicating that central kerguelen plateau broken ridge and elan bank assimilation of hydrothermally altered material did not occur during to 35 ma in the northern kerguelen plateau coffin shallow level crystallization h2o ce for the enriched glasses et al 2000 pringle duncan 2000 duncan 2002 at units 2 and 3 is significantly lower than pacific and south six of seven basement sites as well as at four sites drilled atlantic morb values suggesting that low h2o ce may be an on earlier odp cruises subaerially erupted basalt flows inherent characteristic of the kerguelen plume source vapor saturation were recovered indicating that the growth rate of the pressures calculated using the h2o and co2 contents of the glasses kerguelen large igneous province lip was sufficient to indicate that 1700 m of subsidence has occurred on this part of form a subaerial landmass however a sequence of the plateau since eruption of the basalts at 34 ma subsidence submarine erupted pillow basalt flows of 90 m thickness estimates for site 1140 and other odp drill sites indicate that the was drilled at site 1140 on the northern kerguelen various parts of the kerguelen plateau subsided at a rate comparable plateau coffin et al 2000 with that for normal indian ocean lithosphere the site 1140 submarine glasses provide the first

paleomagnetic study of french guyana early jurassic dolerites hypothesis of a multistage magmatic event a detailed paleomagnetic and anisotropy of magnetic susceptibility ams study was carried out on 34 sites of early jurassic dolerite dykes from french guyana which formed during the initial opening of the central atlantic ocean four types of ams fabrics are recognized i normal fabric 21 dykes defined by clustering of k1 k2 axes on the dyke plane whereas the k3 axis is nearly perpendicular to it this fabric is interpreted as due to magma flow the subhorizontal inclination of the k1 axis permitted to suggest that the french guyana dykes could be fed by horizontal magma fluxes from a distant magma source ii reversal fabric 8 dykes is characterized by the k2 k3 plane close to the dyke plane and the k1 perpendicular to dyke orientation such fabric was attributed to the local shearing stress iii intermediate fabric 1 dyke is defined by k1 k3 axes close to the dyke plane and k2 axis is perpendicular to this plane it was interpreted as due to vertical compaction of a static magma column iiii other fabric 4 dykes does not show any preferential orientation scanning electronic microscope and susceptibility versus temperature experiments show that minerals of the titanomagnetite family are main magnetic remanence carriers two magnetic components were isolated ages of magnetic remanences are estimated at 198 3 2 0 ma to 192 3 1 5 ma their virtual geomagnetic poles are calculated pole a v 73 2 n p 15 3 e k 288 8 a95 3 4 n 8 and pole b v 81 6 n p 89 1 e k 69 8 a95 4 2 n 18 these two groups probably correspond to two distinct magmatic events which occurred in a short period this hypothesis is consistent with published 40 ar 39 ar radiometric ages though with mini plateau spectra these paleomagnetic results suggest the presence of magmatic pulses led to the construction of the central atlantic magmatic province in french guyana during the early jurassic fl 2000 elsevier science b v all rights reserved

40ar 39ar geochronology of flood basalts from the kerguelen archipelago southern indian ocean implications for cenozoic eruption rates of the kerguelen plume the 6500 km2 kerguelen archipelago formed on the northern kerguelen plateau nkp 4 105 km2 which is a shallow submarine plateau belonging to the kerguelen large igneous province in the southern indian ocean flood basalts make up 85 of the archipelago and are interpreted as the most recent volcanism 40 ma from the kerguelen hotspot which has erupted basalt for the last 115 million years based on 40ar 39ar incremental heating of acid leached groundmass separates we report isochron ages ranging from 29 26 0 87 ma to 24 53 0 29 ma for 15 basalts from five stratigraphic sections from diverse regions of the archipelago the oldest dated basalt from the archipelago 29 ma is much younger than the 40 ma age of conjunction between the hotspot and the southeast indian ridge basalt eruption seems to have ceased shortly after 24 ma although small volume highly evolved lavas and plutons continued to form in the archipelago the basalt age data suggest an average lava accumulation rate of 1 6 0 9 km my during the oligocene the archipelago s volumetric eruption rate 0 009 km3 yr is lower than estimates made for the cretaceous kerguelen plateau 1 7 km3 yr and the ninetyeast ridge hotspot track 0 18 km3 yr suggesting that the late cenozoic extrusive activity of the kerguelen plume is waning cenozoic volcanism attributed to the kerguelen plume occurs over a diffuse area with quaternary eruptions at heard and mcdonald islands and within the kerguelen archipelago the decreasing eruption rate and areally diffuse volcanism may be explained by the thick lithosphere of the cretaceous kerguelen plateau overriding and insulating the plume however if the undated nkp which underlies the archipelago formed during the cenozoic then the crustal production rate of the plume from 40 ma to the present 0 25 km3 yr would be similar to the crustal production rate 0 23 km3 my previously estimated for the formation of the ninetyeast ridge 82 38 ma

magma flow inferred from anisotropy of magnetic susceptibility in the coastal parana etendeka igneous province evidence for rifting before flood volcanism the parana etendeka igneous province is one of the largest flood volcanic provinces in the world peak magmatic activity at 132 ma is believed to have occurred about 5 m y before the birth of south atlantic sea floor and development of rift basins along the brazilian coastal margin anisotropy of magnetic susceptibility ams measurements on 283 samples 28 flows and 3 sills from the etendeka igneous province of namibia and 180 samples 21 flows from the parana province in brazil reveal remarkably consistent fabric orientations with maximum susceptibility k1 axes subhorizontal and parallel to the rifted margin the ams results are most likely due to shape anisotropy reflecting magma flow directions suggesting that lava flows and intrusive conduits near the eventual rifted margin were controlled by structures having topographic expression in existence at the time of peak flood volcanism these results imply that rifting preceded flood volcanism at least in the portion of the magmatic province within 100 km of the nascent mid atlantic ridge

magnetic fabric and palaeomagnetic analyses of the plio quaternary calc alkaline series of aegina island south aegean volcanic arc greece abstract the south aegean volcanic arc represents the magmatic expression of active subduction of the african plate beneath eurasia in the region of greece plio quaternary calc alkaline dacite and andesite lava flows exposed on the island of aegina have been sampled for palaeomagnetic analyses to determine whether the western arc area has experienced vertical axis rotations in addition these units allow an assessment of the use of magnetic fabric anisotropy of magnetic susceptibility ams data as indicators of lava flow directions in silicic extrusive rocks providing a complement to the existing literature on ams of basaltic flows the magnetic mineralogy of both dacites and andesites is dominated by fine grained magnetite ti poor titanomagnetite sampled flows are characterised by oblate magnetic fabrics at site level which are co axial to flow surfaces although most sites also contain specimens with prolate ams ellipsoids where magnetic lineations are developed they are systematically aligned either parallel or orthogonal to average exposure scale lava flow directions magnetic lineations in these rocks therefore may be interpreted in terms of flow directions only when the latter are known a priori observed variability in the directions of remanent magnetisation recorded by the aegina lava flows may be explained wholly in terms of geomagnetic variations several sites record spot readings of the geomagnetic field during polarity transitions scatter of directions at the remaining sites is consistent with that expected from secular variation post eruption tilting of magnetisation vectors is shown to be limited the in situ mean direction of magnetisation of the non transitional sites is indistinguishable from the geocentric dipolar field used as a reference direction for these young

temporal geochemical trends in kerguelen archipelago basalts evidence for decreasing magma supply from the kerguelen plume abstract the kerguelen archipelago a 39 ma to recent volcanic plutonic complex is interpreted to be a manifestation of the kerguelen plume most 85 of the surface area is covered by flood basalts ranging in age from 29 to 25 ma the youngest 25 ma studied flood basalts are in the southeast se province of the archipelago a composite 460 m section of this southeast flood basalt dominantly consists of evolved 3 to 6 mgo alkalic basalt and trachybasalt with a few interbedded highly evolved lavas trachyandesites a 40 70 m conglomerate which contains lignite beds and a trachytic breccia tuff unit all of the lavas in this composite section have sr and nd isotopic ratios that are typical of the kerguelen plume e g 80 of the 115 analyzed archipelago lavas with 2 3 mgo have 87 sr 86 sr i 0 70515 12 and 143 nd 144 nd i 0 51259 5 these ranges include the southeast flood basalts pb isotopes however are more variable these 25 ma lavas have high 206 pb 204 pb at 18 4 to 18 6 relative to other archipelago lavas the temporal trend of the archipelago flood basalt from older 29 ma transitional basalts to younger 25 ma alkalic basalt with an increasing proportion of highly evolved lavas and intra bedded sediments in the relatively young southeast section indicates a a temporal decrease in extent of melting and b a decreasing supply of magma from the plume to the crust these temporal trends are attributed to increasing lithosphere thickness as the plume evolved from a spreading ridge centered plume at 43 ma to its intraplate setting supporting evidence for this interpretation is a the absence of a morb geochemical signature in these 25 ma lavas and b the relatively low abundances of heavy rare earth elements in these southeast lavas which reflect partial melting within the garnet stability field

fabric of the rio ceara mirim mafic dike swarm northeastern brazil determined by anisotropy of magnetic susceptibility and image analysis 1 anisotropy of magnetic susceptibility ams and petrofabric studies of the early cretaceous rio ceara mirim dike swarm reveal that shape preferred orientation of opaque grains controls the magnetic fabric the 350 km long swarm is characterized by relatively thick dikes of the order of 20 m up to 150 m in width the strong magnetic susceptibility of these rocks around 5 10 2 si is attributed to magnetite with low ti content the magnetite usually forms euhedral to subhedral equant to skeletal grains the resulting ams has a large proportion of abnormal fabric types as observed in 58 of the 50 dikes studied the remaining 42 of the dikes show a normal fabric regionally characterized by steep magnetic foliations and subhorizontal lineations in the central eastern part of the swarm however the lineation plunges downdip suggesting a magmatic feeder zone the shape alignment of the plagioclase supports the inferred regional flow pattern indicating a primary origin for the normal ams inverse to highly imbricate magnetic fabrics have lineations perpendicular to or at a high angle to the dike wall a shape lineation of opaque grains lies generally close to the magnetic lineation while the foliation is provided by the oblate shaped plagioclase fabric petrofabric studies indicate that the abnormal magnetic fabric is unrelated to flow competition of interstitial magnetite grains crystallizing either along as well as perpendicular to the magma stretching direction seems to account for the weak magnetic anisotropy and the irregular distribution of the ams types on the swarm

inverse magnetic fabric in carbonate bearing rocks abstract the observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric pole of bedding or schistosity is referred to as an inverse magnetic fabric the investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates that c axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones a mineralogical model based on the property of single domain grains to have a zero susceptibility parallel to their long axis is proposed however more complex cases are also encountered

correlation of deposits and vent locations of the proximal campanian ignimbrite deposits campi flegrei italy based on natural remanent magnetization and anisotropy of magnetic susceptibility characteristics abstract correlation of the distal deposits of the campanian ignimbrite with their proximal equivalents in the campi flegrei caldera is complicated by a lack of medial exposures complex and limited proximal stratigraphic sections and large lateral facies changes paleomagnetic data from 10 sites in and near the campi flegrei yield natural remanent magnetizations nrm that are statistically indistinguishable from the distal campanian ignimbrite in addition their virtual geomagnetic pole vgp yields a possible correlation with lac du bouchet france secular variation data that indicate an age of approximately 32 850 14 c years the secular variation curve at this age was only briefly at this vgp and did not return to it for 10 000 years so the paleomagnetic correlation of proximal and distal deposits is unique and robust the date is consistent with 14 c dates from the campanian ignimbrite but younger than 39 ar 40 ar dates for the same rocks this suggests that a better correction factor for cosmic flux for this time period is needed to calibrate older 14 c dates anisotropy of magnetic susceptibility ams data show that the proximal deposits have an oblate disk shaped poorly lineated magnetic fabric the distal deposits are much better lineated the difference may be due to chaotic depositional currents in the proximal areas in which particles were not well aligned with greater distance of travel and loss of energy particles within the flow became aligned and developed stronger ams lineations early eruptions of the piperno tuff were from a central vent north of pozzuoli whereas later tuffs that underlie the breccia museo may have been emplaced by flows associated with ring vents located on the northern and southern caldera margins

flow direction and magnetic mineralogy of lava flows from the central parts of the peninsula of baja california mexico magnetic properties of two apparently unrelated lava suites one tholeiitic and the other alkalic coexisting in the central parts of the peninsula of baja california mexico were measured in this study macroscopic indicators and measurements of anisotropy of magnetic susceptibility ams were combined to infer the flow direction on those lavas these measurements were used to investigate the likelihood of the existence of an abnormally long tholeiitic lava flow in this part of the peninsula the obtained results indicate that the tholeiitic flows in the region constitute regular length flows and are likely to have been issued through vents located within the area of study additionally comparison of the magnetic signature of tholeiitic and alkalic lavas reveals a systematic difference in some of the magnetic parameters bulk susceptibility and curie temperatures in particular it is proposed that the narrow range of curie temperatures characteristic of each lava type can be used as a reliable proxy for the identification of each lava type in the region all of these findings show that the alkalic and tholeiitic lavas of the region are more closely related to each other than previously suspected

magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy

structure of the plumbing system at tungurahua volcano ecuador insights from phase equilibrium experiments on july august 2006 eruption products understanding the plumbing system structure below volcanoes and the storage conditions temperature pressure volatile content and oxygen fugacity of erupted magmas is of paramount importance for eruption forecasting and understanding of the factors controlling eruptive dynamics phase equilibria experiments have been performed on a tungurahua andesite ecuador to shed light on the magmatic conditions that lead to the july august 2006 eruptions and the parameters that controlled the eruptive dynamics crystallization experiments were performed on a representative august 2006 mafic andesite product between 950 1025oc at 100 200 and 400 mpa and nno 1 and 2 and water mole fractions in the fluid xh2o from 0 3 to 1 water saturation comparison of the natural phenocryst assemblage proportions and phenocryst compositions with our experimental data indicates that the natural andesite experienced two levels of ponding prior to the eruption during the first step the magma was stored at 400 mpa 15 16 km 1000oc and contained ca 6 wt dissolved h2o in the second step the magma rose to a confining pressure of 200 mpa 8 10 km where subsequent cooling down to 975oc and water degassing of the magma led to the crystallization of reversely zoned rims on pre existing phenocrysts the combination of these processes induced oxidation of the system and overpressure of the reservoir triggering the july 2006 eruption the injection of a new hot volatile rich andesitic magma from 15 16 km into the 200 mpa reservoir shortly before the eruption was responsible for the august 2006 explosive event our results highlight the complexity of the tungurahua plumbing system in which different magmatic reservoirs can co exist and interact in time and are the main controlling factors of the eruptive dynamics

mineral chemistry of mildly alkalic basalts from the 25 ma mont crozier section kerguelen archipelago constraints on phenocryst crystallization environments phenocryst compositions and mineral melt equilibria in the mildly alkalic basalts from the 25 ma mont crozier section on the kerguelen archipelago are used to estimate the depths at which magmas stalled and crystallized and to constrain the rule of crustal structure in the evolution of magmas produced by the kerguelen mantle plume the crozier section of nearly 1000m height consists of variably porphyritic flows up to 21 vol phenocrysts dominated by plagioclase clinopyroxene olivine fe ti oxides feldspars show an extreme range of compositions from high ca plagioclase an to sanidine and variable textures that are related to extensive fractionation degassing and mixing in relatively low pressure sub volcanic magma chambers although clinopyroxene is a minor phenocryst type 0 3 vol its non quadrilateral components principally al 1 9 8 6 wt al o vary widely the results of clinopyroxene liquid thermobarometry and clinopyroxene structural barometry indicate that the crozier magmas crystallized at pressures ranging from 1 kbar to 11 12 kbar with high al clinopyroxene recording the highest pressures of crystallization high al clinopyroxene rich cumulates may represent an important component of the seismic crust to mantle transition zone at 14 16 km depth high pressure high al clinopyroxene crystallization became important in the mildly alkalic basaltic magmas from the kerguelen archipelago as ascending magmas stalled and fractionated at or near the crust mantle interface which became deeper as a result of progressive crustal thickening as the archipelago moved from a ridge centered setting at 40 ma to an intraplate position by 25 ma

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

differentiation of ferro basaltic magmas under conditions open and closed to oxygen implications for the skaergaard intrusion and other natural systems because processes such as fractional crystallization and crystallization under conditions closed to oxygen are difficult to simulate in the laboratory there is a need for quantitative models of magma crystallization behaviour comparison of experimental data on an iron rich basaltic composition with predictions of the melts free energy minimization algorithm shows that although liquidus temperatures and silicate mineral equilibria are predicted relatively well the saturation of fe ti oxides is not we have used the same experimental data to construct an alternative crystallization model based on known equilibrium phase relations mineral melt partitioning of major elements and mass balance constraints the model is used to explore the consequences of equilibrium and fractional crystallization in systems open and closed to oxygen liquid lines of descent for perfect equilibrium and perfect fractional crystallization are predicted to be very similar in a system open to oxygen the model predicts that magnetite saturation leads to strongly decreasing iron and increasing silica contents of residual liquids whereas systems closed to oxygen crystallize less abundant magnetite leading to a less pronounced iron depletion in the liquid predicted bulk solid compositions and variations offo with falling temperature agree well with those observed or inferred from the cumulates of the skaergaard intrusion but none of the predicted liquid lines of descent are consistent with the extreme iron enrichment proposed for this intrusion based on mass balance calculations compositional factors such as water and phosphorus are not thought to be the source of the discrepancy as the cumulates of the basistoppen sill which closely resemble those of skaergaard may be used to calculate a liquid line of descent in agreement with that predicted by the model for fractional crystallization closed to oxygen a comparison of the predicted t io paths and liquid lines of descent with those inferred from natural systems suggests that volcanic centres such as iceland and hawaii evolve under conditions open to oxygen whereas evidence from plutonic environments e g skaergaard and kiglapait layered intrusions suggests that they evolved under conditions more closed to oxygen the compositional evolution of the melt phase in volcanic and plutonic systems may therefore be different although the results of this study suggest that magnetite saturation will limit fe enrichment in all environments to 20wt feo consistent with enrichments reported for volcanic glasses

rock magnetic evidence of inflation of a flood basalt lava flow the anisotropy of magnetic susceptibility ams of lava flows is an innovative method which has been proved to be directly related to the shear history of lava one of the advantages of this method is that it can be used in the absence of other morphological features commonly employed to study the mechanism of emplacement of lava flows this feature of the ams method makes it very attractive to gain insight into the mechanism of emplacement of massive relatively featureless long lava flows such as those forming flood basalt provinces in this work we report the results of the measurement of ams as a function of vertical position within the birkett lava flow one of the columbia river basalt group flows the observed variation of ams allows us to identify at least 16 discrete events of lava injection and to estimate the thickness of individual injection events the ams estimated thickness of each injection event in the range of 0 5 4 0 m coincides with the range inferred for injected lava pulses in modern hawaiian lava flows thus the evidence provided by the ams method supports the notion that at least some flood basalt lava flows were emplaced by the same mechanism as many present day inflated pahoehoe flows regarding the orientation of the principal susceptibilities in the central part of the flow they define a preferred orientation along an e w trend whereas in the outer parts of the flow they have a nne ssw trend this difference in the orientation of the principal susceptibilities is interpreted as the result of a change of flow direction of the lava as emplacement progressed electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00445 002 0203 8

phase relations and liquid lines of descent of an iron rich peralkaline phonolitic melt an experimental study we experimentally investigated the phase relations of a peralkaline phonolitic dyke rock associated with the ilimaussaq plutonic complex south greenland the extremely evolved and iron rich composition magnesium number 2 alkalinity index 1 44 feo 12 wt may represent the parental magma of the ilimaussaq complex this dyke rock is therefore perfectly suited for performing phase equilibrium experiments since in contrast to the plutonic rocks of the complex no major cumulate formation processes complicate defining a reasonable starting composition experiments were carried out in hydrothermal rapid quench cold seal pressure vessels at p 100 mpa and t 950 750 c h2o contents ranging from anhydrous to h2o saturated 5 wt h2o and varying fo2 logfmq 3 to 1 where fmq represents the fayalite magnetite quartz oxygen buffer were applied reduced and dry conditions lead to substantial crystallization of alkali feldspar nepheline hedenbergite rich clinopyroxene fayalite rich olivine and minor amounts of ulvospinel rich magnetite which represent the phenocryst assemblage of the natural dyke rock oxidized and h2o rich conditions however suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation accordingly combined low fo2 and ah2o force the evolution of the residual melt toward decreasing sio2 increasing feo and alkalinity index up to 3 55 on the contrary high fo2 and ah2o produce residual melts with relatively low feo high sio2 and a relatively constant alkalinity index we show that variations of ah2o and fo2 lead to contrasting trends regarding the liquid lines of descent of iron rich silica undersaturated peralkaline compositions moreover the increase in feo and alkalinity index reduced and dry conditions in the residual melt is an important prerequisite to stabilize late magmatic minerals of the dyke rock for example aenigmatite na2fe5tisi6o20 coexisting with the most evolved melts at 750 c contrary to what might be expected experiments with high ah2o and interlinked high fo2 exhibit higher liquidus t s compared with experiments performed at low ah2o and fo2 for experiments where magnetite is liquidus phase this is because ulvospinel poor magnetite crystallizes at higher fo2 and has a higher melting point than ulvospinel rich magnetite which is favored at lower fo2

experimental constraints on the storage conditions of phonolites from the kerguelen archipelago phase relations of a phonolite k1 and a tephri phonolite k2 from the upper miocene lavas in the southeast province of the kerguelen archipelago have been investigated in the p t range 100 500 mpa and 700 900 c at two fo2 conditions nno and nno 2 3 to clarify the differentiation and pre eruptive conditions of these magmas crystallization experiments were performed in cold seal pressure vessels cspv and internally heated pressure vessels ihpv at various xh2o under reducing log fo2 nno and oxidizing conditions log fo2 nno 2 3 under reducing conditions the resulting phase assemblage for k1 was titanomagnetite nepheline alkali feldspar clinopyroxene and biotite under oxidizing conditions the assemblage was magnetite plagioclase alkali feldspar nepheline titanite minerals given in the order of appearance with decreasing t at 200 mpa for 4 wt water in the melt it is emphasized that an effect of fo2 on the phase stability of feldspars and feldspathoides was observed comparison of the natural and experimental phase assemblages shows that the pre eruptive conditions for k1 must have been in the log fo2 range nno 1 nno 2 at pressures above 200 250 mpa assuming a temperature of 800 c the water content of the melt is constrained to be between 4 and 6 wt h2o the pre eruptive fo2 conditions for the less evolved sample k2 are more oxidizing with log fo2 close to nno 2 3 the experimental results show that the enrichment of alkalis in residual melts during differentiation of tephri phonolites is enhanced at high fo2

the internal structure of lava flows insights from ams measurements ii hawaiian pahoehoe toothpaste lava and a ā abstract we studied the anisotropy of magnetic susceptibility ams of 22 basaltic flow units including s type pahoehoe p type pahoehoe toothpaste lava and a emplaced over different slopes in two hawaiian islands systematic differences occur in several aspects of ams mean susceptibility degree of anisotropy magnetic fabric and orientation of the principal susceptibilities among the morphological types that can be related to different modes of lava emplacement ams also detects systematic changes in the rate of shear with position in a unit allowing us to infer local flow direction and some other aspects of the velocity field of each unit a flows are subject to stronger deformation than pahoehoe and also their internal parts behave more like a unit according to ams the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes probably resulting from endogenous growth

experimental constraints on parameters controlling the difference in the eruptive dynamics of phonolitic magmas the case of tenerife canary islands we have performed phase equilibrium experiments to determine the pre eruptive conditions of the explosive eruption of montana blanca 2020 bp that occurred from a satellite vent located in the east flank of teide volcano tenerife island crystallization experiments were performed using a phonolitic obsidian from the fall out deposit that contains 5 wt anorthoclase diopside and magnetite with minor amounts of biotite and ilmenite set in a glassy matrix that contains microlites of ca rich alkali feldspars temperature was varied between 850oc and 800oc and pressure between 200mpa to 50 mpa the oxygen fugacity fo2 was varied between nno 0 2 0 2 log units above to the ni nio solid buffer to nno 2 whilst dissolved water contents varied from 7 to 1 5 wt comparison between natural and experimental phase proportions and compositions indicates that the main body of phonolite magma was stored at 850 15oc 50 20mpa 2 5 0 5wt h2o at an fo2 around nno 0 5 prior to eruption equivalent to depths between 1 to 2 km below the surface some clinopyroxene hosting h2o rich melt inclusions possibly originates from intermittent supply of phonolitic magma stored at somewhat deeper levels 100 mpa the ca and fe rich composition of alkali feldspar phenocrysts rims and microlites attests to the intrusion of a mafic magma in the reservoir just prior to eruption as borne out by banded pumices appearing in the later products of the eruptive sequence the comparison with other phonolitic magmas from tenerife and elsewhere e g vesuvius laacher see shows that differences in the eruptive dynamic of phonolitic magmas can be correlated to differences in storage depths along with variation in pre eruptive volatile contents

factors affecting the relative importance of shape and distribution anisotropy in rocks theory and experiments abstract the effect of magnetic interactions has been commonly neglected in the interpretation of the anisotropy of magnetic susceptibility ams of igneous rocks despite the likely occurrence of magnetic interactions due to their relatively high content of ferromagnetic minerals current models of magnetic interactions apply to highly simplified systems and are not directly applicable to natural rocks on this work a model that includes variables previously neglected is used to assess the relative importance of magnetically interactive and non interactive fractions in a rock results of the model show that a variable proportion of magnetically interactive grains the relative values of the bulk susceptibility degree of anisotropy and orientation of the anisotropies characteristic for each fraction all play an important role in controlling the rock ams experimental evidence illustrating the importance of the distribution of the ferromagnetic grains within a specimen is also reported and supports the model predictions both theory and experiments indicate that if a rock has unequant magnetic grains and clusters of magnetic grains the contribution to the ams of the rock will depend on the relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed

paleomagnetism and magnetic fabric of middle jurassic dykes from western patagonia argentina abstract jurassic volcanism in patagonia is widespread although associated dyke swarms are conspicuous they have been almost neglected in previous geologic studies of this region radiometric paleomagnetic and magnetic anisotropy studies are reported from a middle jurassic basic to intermediate dyke swarm exposed in the sierra de mamil choique western patagonia in an area of over 350 km2 two whole rock k ar determinations indicate that these dykes were intruded at around 170 ma an anisotropy of magnetic susceptibility ams study on 15 dykes 74 block samples shows that they carry different kinds of magnetic fabric both normal and inverse apparently governed by compositional differences a paleomagnetic study of these samples suggests that ti poor titanomagnetite is the probable carrier of the characteristic remanence mean site characteristic directions pass a reversal test grade c a paleomagnetic pole computed by averaging vgps from each individual dyke is situated at 70 2 s 190 4 e n 13 95 9 7 not substantially apart from other middle jurassic poles from south america further refinement of the jurassic south american apparent polar wander path is needed to establish whether or not the mamil choique dykes were affected by a small tectonic rotation

effect of oxygen fugacity and water on phase equilibria of a hydrous tholeiitic basalt the influence of oxygen fugacity and water on phase equilibria and the link between redox conditions and water activity were investigated experimentally using a primitive tholeiitic basalt composition relevant to the ocean crust the crystallization experiments were performed in internally heated pressure vessels at 200 mpa in the temperature range 940 1 220 c the oxygen fugacity was measured using the h2 membrane technique to study the effect of oxygen fugacity three sets of experiments with different hydrogen fugacities were performed showing systematic effects on the phase relations and compositions in each experimental series the water content of the system was varied from nominally dry to water saturated conditions causing a range of oxygen fugacities varying by 3 log units per series the range in oxygen fugacity investigated spans 7 log units systematic effects of oxygen fugacity on the stability and composition of the mafic silicate phases cr spinel and fe ti oxides under varying water contents were recorded the mg of the melt and therefore also the mg of olivine and clinopyroxene changed systematically as a function of oxygen fugacity an example of the link between oxygen fugacity and water activity under hydrogen buffered conditions is the change in the crystallization sequence olivine and cr spinel due to a change in the oxygen fugacity caused by an increase in the water activity the stability of magnetite is restricted to highly oxidizing conditions the absence of magnetite in most of the experiments allows the determination of differentiation trends as a function of oxygen fugacity and water content demonstrating that in an oxide free crystallization sequence water systematically affects the differentiation trend while oxygen fugacity seems to have a negligible effect

fractionation of metaluminous a type granites an experimental study of the wangrah suite lachlan fold belt australia abstract crystallisation experiments were performed on two metaluminous a type granite compositions ab412 and ab422 from the wangrah suite a representative a type granite suite from the lachlan fold belt south eastern australia ab412 is more mafic than ab422 and zircon saturation temperatures are 897 and 843 c respectively experiments were performed at 200 mpa f o 2 between nno and nno 1 05 in a temperature range between 700 and 900 c for various melt h 2 o contents experimental results were compared to the natural phase compositions to constrain the water content of the melt and the phases involved in fractionation processes in the wangrah suite with decreasing temperature fe ti oxide orthopyroxene and plagioclase are the first phases to crystallise followed by biotite the orthopyroxene stability is mainly restricted to temperatures above 750 c hornblende is only stable in the less evolved composition if the water content of the melt is at least 4 5 wt h 2 o the initial water content in the more mafic melt ab412 was between 2 and 3 wt h 2 o although orthopyroxene is not observed in the natural assemblages of the wangrah suite it has influenced the early fractionation of the wangrah suite granites mass balance calculations show that composition ab422 can be obtained after the crystallisation of 4 7 wt iron rich orthopyroxene plus 8 5 wt plagioclase from ab412

petrogenesis of the flood basalts forming the northern kerguelen archipelago implications for the kerguelen plume the thick 20 km crust of the kerguelen archipelago formed as the tectonic setting of the kerguelen plume changed from an oceanic ridge centered location at 43 ma to its present location beneath the antarctic plate the uppermost crust is dominantly flood basalt with a thickness of up to 10 km inverse isochron 40ar 39ar ages for upper and lower lavas in a 630 m section of basalt flows from mont bureau are 30 4 and 29 0 ma re os isotopic systematics are consistent with this age most of the lavas in two stratigraphic sections mont bureau and mont rabouillere from the northern part of the archipelago have sr nd and pb isotopic characteristics similar to the youngest upper miocene to pleistocene lavas erupted in the southeast part of the archipelago i e initial 87sr 86sr 0 7050 143nd 144nd 10 mgo alkalic lavas in the southeast province of the archipelago in these northern sections it is confined to transitional lavas with

ropy flow structures a neglected indicator of magma flow direction in sills and dikes ropy flow structures that form in dikes and sills are very similar to ropy lava structures pahoehoe but are of intrusive origin they are unusual among magma flow indicators in that they provide a frozen record of the magma flow our observations suggest that they occur in large vesicles which form as a result of local repeated pressure drops during propagation and emplacement of the magma adiabatic gas expansion causes a ductile rim to form around a vesicle this rim is then sheared by the underlying flow anisotropy of magnetic susceptibility data from the contact zones confirm the flow data from the ropy flow structures but the results from deeper within the body both locally and regionally yield different yet consistent flow orientations this discrepancy suggests that a distinction can be made between the magma flow close to the contact and the deeper regional flow patterns in sills and dikes

geochemical characteristics of the youngest volcano mount ross in the kerguelen archipelago inferences for magma flux lithosphere assimilation and composition of the kerguelen plume mount ross is the youngest 1 0 1 ma volcanic edifice recognized in the kerguelen archipelago which is located on the northern part of the submarine kerguelen plateau lava types range from basaltic trachyandesite to trachyte and extensive glaciation has exposed an intrusive core ranging from gabbro to syenite mount ross rocks are not as silica undersaturated as the basanite to phonolite upper miocene lavas erupted in the southeast province of the archipelago however both lava suites are characterized by relatively high 87sr 86sr 0 7051 0 7054 and 0 7054 0 7058 respectively and low 206pb 204pb 18 02 18 14 and 18 06 18 27 respectively the abundant trachytes and phonolites in the age range from lower miocene to pleistocene on the kerguelen archipelago indicate that the supply of basaltic magma has been low for the last 20 m y despite the prolonged time period required for extensive mineral fractionation there is no evidence for assimilation of geochemically distinctive lithosphere i e most of the trachytes and syenites have initial isotopic ratios within the narrow range of the more mafic rocks volcanic and plutonic rocks of diverse composition and age 0 1 30 ma in the kerguelen archipelago have similar initial isotopic ratios consequently we infer that the kerguelen plume is characterized by 87sr 86sr 0 7051 0 7058 143nd 144nd 0 51263 0 51249 and 206pb 204pb 18 02 18 27

trace of the kerguelen mantle plume evidence from seamounts between the kerguelen archipelago and heard island indian ocean 1 the gravity and bathymetric highs on the kerguelen plateau in the southern indian ocean between the kerguelen archipelago and heard island are seamounts formed of miocene alkalic basalts similar to those found on the islands dredging during the kerimis survey cruise recovered 1 ton of mostly basaltic rocks one of the dredges d6 yielded a large volume of in situ alkalic picritic pillow basalts the first picritic lavas recovered that are related to the kerguelen plume k ar and 40ar 39ar ages are between 18 and 21 ma for all but one sample and these ages are only slightly younger than the main phase of volcanism on the archipelago the dredged lavas form three distinct groups based on chemical and isotopic compositions incompatible element abundance ratios overlap with compositional groups defined by lavas from both the kerguelen archipelago and heard island indicating that alkalic volcanism in this region of the kerguelen plateau has been spatially diverse olivine and picritic basalts have sr and nd isotopic characteristics similar to most of the lavas exposed on the archipelago and those proposed for the kerguelen plume however compared to kerguelen archipelago lavas the picritic basalts have relatively low 206pb 204pb which is a characteristic of cretaceous basalt forming some parts of the kerguelen plateau we propose that the apparent age trend of the lavas from 34 ma in the northern kerguelen plateau odp leg 183 site 1140 to 24 30 ma on the kerguelen archipelago to 18 21 ma on the dredged submarine volcanoes and even possibly to recent volcanism on heard and mcdonald islands may correspond to the tertiary hot spot track of the kerguelen plume

ams measurements on obsidian from the inyo domes ca a comparison of magnetic and mineral preferred orientation fabrics abstract measurements of the anisotropy of magnetic susceptibility ams on lavas can yield information concerning flow directions deformation regimes and relative shear rates here we report results of measurements made on samples of rhyolitic obsidian collected from obsidian dome ca comparisons of ams measurements with previously determined 3d microlite orientation distributions in these samples serve as a calibration of the ams technique despite the low values of bulk susceptibility 1 06 7 04 10 3 si found in the dome relative to typical values in basalts 10 2 si the degree of anisotropy is extremely high in all specimens 3 4 10 7 furthermore susceptibility tensors exhibit a wide range of shapes varying from prolate to triaxial and flattened ellipsoids b parameter from 8 6 to 6 4 corresponding to various degrees of microlite dispersion our results also indicate that despite the tight clustering of principal susceptibility axes obtained on each site bulk flow directions inferred from the orientations of k max axes may be misleading because the orientation of the local extension changes from place to place within the dome however it is shown that the ams of each specimen provides a good measure of the direction of local extension consequently it is possible to interpret spatial variations in flow regime from systematic changes in the orientation of principal susceptibility axes as the proposed association between the local deformation regime and ams is strongly dependent on the aspect ratio of the crystals embedded in the flowing lava some caution must be exerted when interpreting ams results from other flows and a direct comparison between different flows may not be granted in many occasions

ams detection of inverse fabrics without aarm in ophiolite dikes inverse fabric contributions from single domain sd magnetite complicate interpretation of magmatic flow from anisotropy of magnetic susceptibility ams in the s troodos ophiolite dike swarm flow directions and magma chamber dimensions may only be inferred correctly if magnetic fabrics correspond to the orientation distribution od of minerals magnetite shape fabrics were isolated by anisotropy of anhysteretic remanence aarm but ams ambiguously merges magnetite and silicate fabrics however with the retrospective benefit of aarm in this area we recommend avoiding sd complications by excluding magnetite rich samples for which the bulk low field susceptibility 27 500 si magma flow was steeply upwards to the sw proximal to an axial magma chamber

anisotropy of magnetic susceptibility in welded tuffs application to a welded tuff dyke in the tertiary trans pecos texas volcanic province usa consideration of published anisotropy of magnetic susceptibility ams studies on welded ignimbrites suggests that ams fabrics are controlled by groundmass microlites distributed within the existing tuff fabric the sum result of directional fabrics imposed by primary flow lineation welding and if relevant rheomorphism ams is a more sensitive indicator of fabric elements within welded tuffs than conventional methods and usually yields primary flow azimuth estimates detailed study of a single densely welded tuff sample demonstrates that the overall ams fabric is insensitive to the relative abundances of fiamme matrix and lithics within individual drilled cores ams determinations on a welded tuff dyke occurring in a choked vent in the trans pecos texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width the dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit followed by residual squeezing between the failed block and in situ wallrock irrespective of the precise mechanism widespread occurrence of both welded tuff dykes and point welded aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process

rotation of uniaxial ellipsoidal particles during simple shear revisited the influence of elongation ratio initial distribution of a multiparticle system and amount of shear in the acquisition of a stable orientation abstract the rotation of uniaxial ellipsoidal particles has provided the base for many studies of shape preferred orientations of mineral grains in a large variety of rock types in this paper we present reports of a study of the evolution of a multiparticle system as a function of the elongation ratio of the particles and of the initial particle distribution our model reveals that both factors are important in controlling the acquisition of a stable orientation upon deformation the more elongated particles r short axis long axis 0 5 will define a stable fabric in almost any situation of geologic interest shear deformation 0 5 can also be considered to have achieved a stable orientation depending on the total amount of shear experienced by the rock however in systems with the less elongated particles r 0 8 it is possible to find mean orientations that are perpendicular to the direction of shear although the occurrence of such behavior constitutes only a minor fraction of the studied examples in any case it is suggested that a systematic study of mineral fabrics in samples from the same rock can yield enough information to evaluate whether the mean shape preferred orientation of the minerals is parallel to flow direction

experimental constraints on ultrapotassic magmatism from the bohemian massif durbachite series czech republic the equilibrium phase relations of a mafic durbachite 53 wt sio2 from the t ebic pluton representative of the variscan ultrapotassic magmatism of the bohemian massif 338 335 ma have been determined as a function of temperature 900 1 100 c pressure 100 200 mpa and h2o activity 1 1 6 1 wt h2o in the melt two oxygen fugacity ranges were investigated close to the ni nio nno buffer and 2 6 log unit above nno buffer nno 2 6 at 1 100 c olivine is the liquidus phase and co crystallized with phlogopite and augite at 1 000 c for the whole range of investigated pressure and water content in the melt at 900 c the mineral assemblage consists of augite and phlogopite whereas olivine is not stable the stability field of both alkali feldspar and plagioclase is restricted to low pressure 100 mpa at nearly water saturated conditions 3 4 wt h2o and t 900 c a comparison between experimental products and natural minerals indicates that mafic durbachites have a near liquidus assemblage of olivine augite ti rich phlogopite apatite and zircon followed by alkali feldspar and plagioclase similar to the mineral assemblage of minette magma natural amphibole diopside and orthopyroxene were not reproduced experimentally and probably result from sub solidus reactions whereas biotite re equilibrated at low temperature the crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica lamprophyre rocks the similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that k and mg rich magmas in the bohemian massif may have been generated from partial melting of a phlogopite clinopyroxene bearing metasomatized peridotite experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle derived components at mid crustal pressures

magma storage conditions and differentiation of the mafic lower pollara volcanics salina island aeolian islands italy implications for the formation conditions of shoshonites and potassic rocks crystallization experiments of basaltic andesite mafic endmember from the 24 ka lower pollara eruption salina aeolian islands italy were investigated at 200 mpa 950 1100 c in the h2o activity ah2o range 0 3 to 1 and at two ranges of oxygen fugacity fo2 between fmq to fmq 1 and fmq 2 to fmq 3 3 log bars fmq is fayalite magnetite quartz comparison of the produced phase assemblages and phase compositions with the natural sample reveals that the storage conditions were 1050 c 2 8 wt h2o in the melt ah2o 0 5 and relatively oxidizing fmq 2 5 the composition of plagioclase in the groundmass indicates a period of cooling to 950 c the overall differentiation trends of the salina volcanics can be explained by fractional crystallization close to h2o saturated conditions 5 wt h2o in the melt at 200 mpa and most likely by accumulation of plagioclase i e in basaltic andesites and by various degree of mixing mingling between the corresponding differentiates the slightly elevated k2o contents of the most mafic basaltic andesites that can be found in the lowermost unit of the lower pollara pyroclastics reveal earlier processes of moderately hydrous fractional crystallization at higher temperature 1050 c fractional crystallization with decreasing influence of h2o causes a moderate decrease of mgo and a significant increase of k2o relative to sio2 in the residual liquids it is exemplarily shown that the crystallization of sio2 rich phases at high temperature and low ah2o of only moderately k2o rich calc alkaline basalts can produce shoshonitic and high potassic rocks similar to those of stromboli and volcano this suggests that the observed transition from calc alkaline to shoshonitic and high potassic volcanism at the aeolian arc over time can be initiated by a general increase of magmatic temperatures and a decrease of ah2o in response to the extensional tectonics and related increase of heat flow and declining influence of slab derived fluids

significance of anisotropy of magnetic susceptibility fabrics in proterozoic mafic dykes hopedale block labrador abstract anisotropy of magnetic susceptibility fabrics have been studied in a suite of mafic early proterozoic kikkertavak dykes and two mid proterozoic kokkorvik sills in the hopedale block of labrador two sets of en echelon dyke arrays from the kikkertavak swarm are considered to represent two linear dyke intrusions at depth three main fabrics were revealed in the kikkertavak dykes in the first the maximum susceptibility axis is sub horizontal or gently inclined and parallel to the dyke walls and is interpreted as the primary magma flow direction a second fabric affects only dykes along strike from a much younger dyke and is interpreted as the result of local stress anisotropy the third fabric with significantly lower susceptibility is only locally developed in the southeast near the mid proterozoic kanairiktok shear zone and is also interpreted as the result of local superimposed tectonic stress the kokkorvik sheets preserve a single flow induced fabric sub parallel to the measured crack propagation lineation indicating that the magma flow was parallel to the crack propagation direction the gently inclined magma flow direction inferred for the kikkertavak dykes is believed to reflect later stage magmatic emplacement when consolidation had already commenced at this stage the buoyancy contrast between magma and host rock had been reduced to the point when the initially vertical magma flow was no longer sustainable and was replaced by lateral flow

egan range volcanic complex nevada geochronology paleomagnetism and magnetic fabrics abstract in a relatively brief interval near the eocene oligocene boundary trachydacite lavas were erupted over an area of about 300 km2 to form the egan range volcanic complex in the southern butte mountains and adjacent egan range of east central nevada the mean radiometric age for this volcanic episode is 34 1 0 2 ma based on whole rock and biotite k ar ages detailed comparisons were made between anisotropy of magnetic susceptibility ams and the macroscopic petrofabric at 31 sites great variation in flow structures is attributed to complex flow in viscous lava over a surface of considerable relief the magnetic fabric as expressed by ams axes agrees well with the flow petrofabric typically flow lineations are parallel to k1 and flow foliation is perpendicular to k3 as expected extension cracks and joints are normal to k1 late stage deformation of pipe vesicles also impressed a measurable magnetic fabric because of significant initial dip and complexity of flow fabric paleohorizontal is not generally demonstrable paleomagnetic studies indicate an exclusively normal polarity the mean in situ characteristic direction is relatively well grouped d 350 i 52 95 8 for 28 selected sites and provides a mean pole at 124 e 81 n which is not significantly different from the coeval cratonic reference pole these results indicate that large oligocene or younger rotations have not affected this sector of the basin and range province the combination of age and magnetic polarity suggests a correlation with a normal polarity subchron of chron 13 or 15

preferred orientation developed by rigid markers in two dimensional simple shear strain a theoretical and experimental study abstract in a homogeneous two dimensional deformation linear markers embedded in a viscous matrix develop a preferred orientation along the principal axis of the strain ellipse theoretical analysis of marker rotation under two dimensional simple shear is used to derive an orientation distribution function this function shows that the relative density maximum increases with progressive strain its value being equal to the major minor strain ellipse axis ratio moreover the density is equal to the unity along the lines of no finite longitudinal strain experimentally observed preferred orientation is in good agreement with the expected theoretical distribution

an experimental study of a lower proterozoic a type granite from theeastern amazonian craton brazil the jamon granite is representative of the lower proterozoic 1 88 introduction ga oxidized a type granites of the eastern part of the amazonian proterozoic anorogenic granites are found in several craton the dominant facies is a metaluminous to slightly percratonic areas such as in the fennoscandian shield aluminous hornblende biotite monzogranite with k2o na2o be nurmi haapala 1986 haapala ramo 1990 ramo tween 0 8 and 1 5 and feot feot mgo between 0 8 and haapala 1991 in north america bridgwater 0 9 in contrast to many other a type granites the jamon granite windley 1973 barker et al 1975 van schmus et al is characterized by early hornblende crystallization and the presence 1987 anderson bender 1989 gower et al 1990 of magnetite crystallization experiments were performed on glass emslie 1991 emslie stirling 1993 and australia at 300 mpa between 700 and 900 c for various melt h2o wyborn et al 1992 pollard mitchell 1995 they contents and for both oxidizing and reducing fo2 nno nickel are widespread in several tectonic provinces of the amnickel oxide 2 5 and nno 1 5 on average for nno azonian craton which is now recognized as one of 2 5 and under h2o rich conditions ilmenite clinopyroxene the world s major provinces of anorogenic magmatism magnetite and hornblende are near liquidus phases followed by dall agnol et al 1994 1999a bettencourt et al 1995 plagioclase the orthopyroxene stability field is restricted to high anorogenic amazonian granites have ages ranging from temperatures and h2o contents in the melt 5 wt in contrast 1 9 to 1 0 ga machado et al 1991 dall agnol et al for nno 1 5 magnetite and titanite are absent and orthopyroxene 1994 bettencourt et al 1995 and intrude either ar never observed in the granite clinopyroxene and ilmenite are the chaean or early to middle proterozoic sequences they liquidus phases conditions of crystallization of amphibole magare petrologically and geochemically diverse with rock netite and plagioclase constrain the initial melt h2o content to types similar to those found in rapakivi granites from the between 4 5 and 6 5 wt plagioclase cores crystallized from fennoscandian shield haapala ramo 1992 ramo 870 to 720 c clinopyroxene amphibole and biotite fe fe haapala 1995 the amazonian anorogenic granites mg values suggest fo2 around nno 0 5 during crystallization are classified as a type dall agnol et al 1994 1999a of the granite the demonstration of relatively hydrous conditions 1999b many granite complexes are mineralized sn w and oxidizing fo2 for the jamon granite stresses the diversity of ree zr f nb be ta and the tin deposits associated a type magmas in terms of h2o contents and redox states the with these granites are largely responsible for brazil jamon granite was most probably generated from oxidized archaean igneous rocks of mafic intermediate composition becoming a major tin producer in the last 20 years given the importance of the province large time span covered variety of rock types and the significant mineral resources associated there is clearly a need for a better

the anisotropy of magnetic susceptibility of lava flows an experimental approach measurements of the anisotropy of magnetic susceptibility ams of natural lavas have shown that ams varies with depth within a lava flow we have investigated the reasons for such variation by studying the effects of temperature and strain rate on the ams of recent lava in the laboratory samples of lava from kilauea were melted and subjected to a range of strain rate and cooling histories the results show that the degree of anisotropy is a function of both the thermal and shearing history of a sample high degrees of anisotropy were found only in samples that were deformed at temperatures close to those encountered during eruption and then rapidly quenched lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation additionally lava subjected to complex shearing yield a lower degree of anisotropy even when high strain rates were imposed on it these results lead to the conclusion that only the last phase of deformation is detectable using ams and that high strain rates will not result in high degrees of anisotropy if either deformation ends while lava is still fluid or if the orientation of the maximum shear stress varies with time the relation between the orientation of the principal susceptibilities and that of shear is less sensitive to variation on shear with time consequently flow directions can be inferred confidently with this type of measurements

post emplacement tilting of lava flows inferred from magnetic fabric study the example of oligocene lavas in the jeanne d arc peninsula kerguelen islands abstract statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results a difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement for 2 successive series of flows on the jeanne d arc peninsula presently nnw dipping this method shows lava emplacement along a ssw nne direction this indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago additionally the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow