magmatic precursors to the 18 may 1980 eruption of mount st helens usa perhaps the most difficult task facing volcanologists today is that of distinguishing between low level volcanic restlessness and activity that presages a full scale eruption we illustrate these difficulties by reexamining the sequence of events that led to the 18 may 1980 eruption of mount st helens activity that is often presented as a classic example of early phreatic explosions leading to a magmatic eruption this interpretation is problematic however given the known ascent of juvenile magma to within 250 m of the surface prior to the climactic event reexamination of ash samples from several precursory events shows that small amounts of juvenile material were erupted as early as 28 march 1980 just two weeks after the first seismic signals of reawakening and that the juvenile content of the ash had probably increased by early may the magmatic component of these eruptions was not recognized at the time because of the high crystallinity of the juvenile material a signature of extensive degassing induced crystallization during magma ascent identification of juvenile material in the precursory ash from mount st helens has important implications for initiation of eruptive activity

long term atmospheric deposition as the source of nitrate and other salts in the atacama desert chile new evidence from mass independent oxygen isotopic compositions abstract isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the atacama desert in northern chile has shown anomalous 17 o enrichments in both minerals 17 o values of 14 21 in nitrate and 0 4 to 4 in sulfate are the most positive found in terrestrial minerals to date modeling of atmospheric processes indicates that the 17 o signatures are the result of photochemical reactions in the troposphere and stratosphere we conclude that the bulk of the nitrate sulfate and other soluble salts in some parts of the atacama desert must be the result of atmospheric deposition of particles produced by gas to particle conversion with minor but varying amounts from sea spray and local terrestrial sources flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200 000 to 2 0 m years during hyper arid conditions similar to those currently found in the atacama desert correlations between 17 o and 18 o in nitrate salts from the atacama desert and mojave desert california indicate varying fractions of microbial and photochemical end member sources the photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture biologic activity and nitrogen cycling these isotopic signatures have important implications for paleoclimate astrobiology and n cycling studies

tracking magma dynamics by laser ablation la icpms trace element analysis of glass in volcanic ash the 1995 activity of mt etna 1 glass fragments in tephra erupted at mt etna from may to december 1995 have been analyzed by laser ablation icpms the trace element compositional variability of ashes deposited during this interval reveals the presence of discrete magma batches with different crystallization degrees in the shallow plumbing system from may to october a highly crystalline magma is predominant within the conduit with only minor sporadic input of fresh and more primitive magma batches after october new and less evolved magma batches become more prevalent and become progressively homogenized within more evolved resident magma in december ashes closely match the chemistry of the volcanics subsequently erupted till february 1996 this study demonstrates that the trace element characterization of ashes has important implications for volcanic monitoring and is a useful tool for the forecasting of paroxysmal events at mt etna

secondary fabrics revealed by remanence anisotropy methodological study and examples from plutonic rocks summary the fabric of fine grained magnetite is isolated by partial remanence anisotropy paar measurements that combine cycles of anhysteretic remanence acquisition and alternating frequency af tumbling demagnetization of the low coercivity fraction since remanence intensities of the most coercive fine grained magnetite are very weak rarely exceeding 10 per cent of the measured remanence in the studied samples a methodological study is presented that defines optimal measurement schemes and examines the role of gyroremanence and background remanences due to hard remanence carriers this procedure is applied to some reddened granites from tana corsica and wichita oklahoma and to the tellnes ilmenite rich norite norway the paar fabrics for fine grained magnetites are compared to the fabrics of the coarse grained primary magnetite grains as shown by both the anisotropy of magnetic susceptibility ams and the remanence anisotropy of the low coercivity fraction in tellnes fine grained magnetite is believed to be of secondary origin this is strongly suggested by the correlation between the paar fabrics and the orientations of sets of microfractures decorated by oxides in the reddened granites although no direct correlation between paar fabrics and microstructures could be evidenced the comparison between unaltered specimens and hydrothermally altered ones reveals changes in magnetic properties and fabrics that are ascribable to this secondary geological process

petrology of the 1991 1995 eruption at unzen effusion pulsation and groundmass crystallization abstract effusive eruption of dacite magma 2 1 108 m3 during 1991 1995 formed a lava dome at the summit of unzen volcano japan the effusion rate was highest at the beginning 4 0 105 m3 day 4 6 m3 s and decreased roughly with time to almost zero before this pattern was repeated with a second pulse of magma supply the whole rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance the more mafic the more abundant the phenocrysts the pattern of chemical variation with time shows some difference from that of the effusion rate all phenocrysts in dacite plagioclase hornblende biotite quartz and magnetite show evidence of disequilibrium with melt although a glomerophyric aggregation of phenocrysts suggests coexistence with each other phenocrysts are isotopically heterogeneous from species to species the calculated initial melt composition was rhyodacite and was nearly constant throughout the activity in contrast the bulk phenocryst population is andesite a model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments or xenocrysts by parental rhyodacite magma it is suggested that when effusion rate was high less viscous crystal poor magma exited from the reservoir groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate the higher the effusion rate the more evolved the compositions groundmass crystallinity increased with decreasing effusion rate from 33 to 50 textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top the fe ti oxide temperature 880 780 c was low when the crystallinity was high micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit porosity of dome lavas was lower at lower effusion rates collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low frequency earthquakes phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction when effusion waned degassing and the resultant crystallization proceeded more completely so that the magma became too viscous to flow in the conduit top and behaved as a plug resulting in a temporary halt of effusion in turn groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling the plug was scavenged when rising excess pressure overcame its effective strength then the second pulse of magma supply began strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason

isotopic evidence for the source of ca and s in soil gypsum anhydrite and calcite in the atacama desert chile abstract the origin of pedogenic salts in the atacama desert has long been debated possible salt sources include in situ weathering at the soil site local sources such as aerosols from the adjacent pacific ocean or salt encrusted playas salars and extra local atmospheric dust to identify the origin of ca and s in atacama soil salts we determined 34 s and 87 sr 86 sr values of soil gypsum anhydrite and 87 sr 86 sr values of soil calcite along three east west trending transects our results demonstrate the strong influence of marine aerosols on soil gypsum anhydrite development in areas where marine fog penetrates inland results from an east west transect located along a breach in the coastal cordillera show that most soils within 90 km of the coast and below 1300 m in elevation are influenced by marine aerosols and that soils within 50 km and below 800 m in elevation receive 50 of ca and s from marine aerosols 34 s values 14 and 87 sr 86 sr values 0 7083 in areas where the coastal cordillera is 1200 m in elevation however coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced most pedogenic salts from inland soils have 34 s values between 5 0 to 8 0 and 87 sr 86 sr ratios between 0 7070 and 0 7076 these values are similar to average 34 s and 87 sr 86 sr values of salts from local streams lakes and salars 5 4 2 34 s and 0 70749 0 00045 87 sr 86 sr in the andes and atacama suggesting extensive eolian reworking of salar salts onto the surrounding landscape ultimately salar salts are precipitated from evaporated ground water which has acquired its dissolved solutes from water rock interactions both high and low temperature along flowpaths from recharge areas in the andes therefore the main source for ca and s in gypsum anhydrite in non coastal soils is indirect and involves bedrock alteration not surficially on the hyperarid landscape but in the subsurface by ground water followed by eolian redistribution of ground water derived salar salts to soils the spatial distribution of high grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local marine and salar salt supporting arguments for tropospheric nitrogen as the main source for soil nitrate

calcrete palaeosols from the lower carboniferous llanelly formation south wales abstract three calcrete bearing palaeosol units are described from the lower carboniferous llanelly formation in south wales and are named the tyle r bont pedocomplex the cwm dyar pedoderm and the llanelly pedocomplex these units were formed on semi arid to arid floodplains and are associated with sheet flood stream flood and meandering channel deposits three major processes operated during pedogenesis calcification pedoturbation and solution pedoturbation resulted in the formation of brecciated horizons pseudo anticlinal and prismatic structures and a predominance of disorthic nodules the soils developed in smectite rich clays under a seasonally arid climate resulting in strong argillipedoturbation

the henkel petrophysical plot mineralogy and lithology from physical properties the henkel plot logarithm of magnetic susceptibility versus density of rock samples reveals that most rocks fall on either a magnetite trend or a paramagnetic trend i

towards fast and routine analyses of volcanic ash morphometry for eruption surveillance applications the morphometry of volcanic ash produced by explosive eruptions yields ample information on fragmentation processes e g magmatic vs magma water interactions and on transport and sedimentation mechanisms most previous works on volcanic clast morphometry focused on the apparent 2d projected shape of ash grains here termed apash to infer processes and eruptive styles however textural analyses of ash grains has remained a long and tedious task that made such approaches inappropriate for eruption surveillance duties in this work we show that new technological advances on automated dispersion of granular materials imaged with a camera coupled microscope and enhanced computer capabilities enable fast and high resolution image acquisition of thousands of ash grains that resolve this limitation with a morpho grainsizer designed for such fast and routine measurements we perform a series of apash analyses on selected ash fractions of tephra deposits from known eruptive styles we record the size aspect ratio circularity and convexity of apash images and assess resolution reproducibility minimum population size and total analytical duration and offer recommendations for the reporting of apash data for inter laboratory comparisons to avoid fractal geometry concerns our analyses are carried out at constant size range 250 300 m and optical magnification 5 on 3000 grains per samples collected from homogenized samples results from the andesitic 1999 ongoing eruption of tungurahua volcano ecuador show that ash particles from the moderate 2001 phase are relatively equant and convex in shape while the stronger 2006 subplinian phase produced ash grains with more elongated less circular and less convex apash signatures ash grains from a basaltic scoria cone forming eruption show even more ragged apash characteristics overall our protocol allows obtaining accurate and reproducible morphometric measurements that reveal subtle variations of the morphological signature and the short duration 1 8 hours of the whole analytical process renders high resolution analyses of ash shape achievable for volcano surveillance applications this research ultimately aims to set up a morphometric database of apash results for well defined eruptive styles in order to interpret on a short term basis any apash data from active volcanoes for monitoring purposes

landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 ma atacama desert chile the landscape of the hyperarid atacama desert in northern chile records extremely slow change on earth s surface disputed ages for the onset of hyperaridity range from the late paleogene through the pleistocene a long term paleoclimate record is recorded in a nonmarine basin whose fill is primarily alluvial strata for this setting the primary proxies for climate state are the mineralogical and chemical composition of soil which varies across a precipitation gradient and the landforms and deposits of alluvial fans during the most recent 15 million years five climate related landscape stages are resolved for the pampa del tamarugal sedimentary basin with each successively younger stage inset lower in the local topography than its predecessor the earliest landscape stage is expressed as a set of alluvial strata inherited from a time of arid or semi arid climate ca 14 12 ma the younger four landscape stages generated a composite long lasting exposure surface predominantly hyperarid conditions have persisted since ca 12 1 ma during which four intervals of arid to semi arid climate occurred each wet interval was short lived a million years or less whereas some of the hyperarid periods were lengthy 1 5 m y the hyperarid intervals were roughly 11 5 5 ma 4 5 4 ma 3 6 2 6 ma 2 2 1 ma and repeated intervals during the last 1 m y the onset of hyperaridity ca 12 ma likely reflects the growth of the andes mountains above a climate threshold in contrast sea surface temperature variability likely has controlled atacama paleoclimate changes since the late miocene

the magnetic susceptibility of granitic rocks as a proxy for geochemical composition example from the saruhan granitoids ne turkey the magnetic susceptibility of rocks is determined by their bulk chemistry and magnetic mineralogy in this case study of the saruhan pluton in turkey a detailed survey based on 925 field measurements and 37 whole rock analyses reveals a strong correlation between magnetic susceptibility and feot r 2 0 8 petrographic observations and calculations based on whole rock major element analyses indicate the presence of magnetite grains further calculations show that magnetite accounts for 88 of the magnetic susceptibility the spatial correlation between feototal and the magnetic susceptibility is particularly spectacular which suggests that km is a proxy for geochemical differentiation the zoning pattern of magnetic susceptibility across the pluton is concentric and reverse with the highest values of magnetic susceptibility in the center this type of zoning is consistent with a previously proposed petrogenetic model the comparison of the saruhan granites with a classic example of paramagnetic granites reveals two contrasted feot km trends this suggests that these variations can be a useful tool to distinguish between leucocratic paramagnetic and leucocratic ferrimagnetic granites 2007 elsevier b v all rights reserved

a threshold in soil formation at earth s arid hyperarid transition the soils of the atacama desert in northern chile have long been known to contain large quantities of unusual salts yet the processes that form these soils are not yet fully understood we examined the morphology and geochemistry of soils on post miocene fans and stream terraces along a south to north 27 to 24 s rainfall transect that spans the arid to hyperarid transition 21 to 2 mm rain y 1 landform ages are 2 my based on cosmogenic radionuclide concentrations in surface boulders and ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2 1 my a chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall in the arid 21 mm rain y 1 soil total mass loss to weathering of silicate alluvium and dust 1030 kg m 2 is offset by net addition of salts 170 kg m 2 the most hyperarid soil has accumulated 830 kg m 2 of atmospheric salts including 260 kg sulfate m 2 and 90 kg chloride m 2 resulting in unusually high volumetric expansion 120 for a soil of this age the composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in soil chemical transformation and loss long term rates of atmospheric solute addition were derived from the ion inventories in the driest soil divided by the landform age and compared to measured contemporary rates with decreasing rainfall the soil salt inventories increase and the retained salts are both more soluble and present at shallower depths all soils generally exhibit vertical variation in their chemistry suggesting slow and stochastic downward water movement and greater climate variability over the past 2 my than is reflected in recent 100 y rainfall averages the geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold in wetter soils the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion the specific geochemistry of hyperarid soils is a function of atmospheric sources and is expected to vary accordingly at other hyperarid sites this work identifies key processes in hyperarid soil formation that are likely to be independent of location and suggests that analogous processes may occur on mars

magnetic susceptibility anisotropy as a predictive exploration tool of metasomatic iron oxide deposits example from the panj kuh iron ore body ne iran abstract the panj kuh iron deposit hosted by late eocene igneous rocks in north of central iran is associated with a syenitic intrusive body magnetic fabric investigations indicate that the panj kuh pluton was emplaced within a sinistral shear zone corridors of sheared and metasomatized rocks show that the shear zone was active after emplacement of the pluton and in the final steps of crystallization the sheared rocks allowed for circulation of hydrothermal fluids during cooling of the magma and caused the pervasive na ca alteration the ferrimagnetic phase in fresh syenite is mostly euhedral or subhedral magnetite and is observed adjacent to or as inclusion in biotite and pyroxene anisotropy of magnetic susceptibility ams in the fresh syenite is due to preferred shape orientation of magnetite grains in na ca altered syenites magnetite has reduced or has been totally absorbed the alteration results in decreased bulk susceptibilities km and higher anisotropy degrees p accompanied by changes in the shape of magnetic susceptibility anisotropy ellipsoid this study of the panj kuh pluton suggests that ams can be used for mapping lithology and hydrothermal alteration types since various types of hydrothermal alteration create transform or destroy ferrimagnetic minerals in a mineralized system susceptibility measurements aid mapping of igneous rock type alteration zoning and mineralization integrating petrography magnetic mineralogy bulk magnetic susceptibility and preferred orientations of principal axes of the ams in fresh and altered rocks can be used to generate predictive magnetic exploration models for metasomatic iron oxide deposits

mapping of apparent magnetic susceptibility and the identification of fractures a case study from the eye dashwa lakes pluton atikokan ontario in situ magnetic susceptibility measurements are only possible on outcrops which are often limited by overburden and water bodies an alternative approach is to derive an apparent susceptibility map from total magnetic intensity tmi surveys which was done in this study for the eye dashwa lakes pluton near atikokan ontario susceptibility logs of cores directly link alteration to systematic changes in the amount and composition of magnetic minerals the surficial distribution of alteration zones was originally estimated from a limited number of in situ magnetic susceptibility measurements here through forward modeling of the tmi data set susceptibility data are used to validate the apparent susceptibility data set the modeling accounts for the bathymetric surface of all lakes that cover the area a two step process of bulk and local scale modeling was used to estimate apparent susceptibility patterns bulk magnetic susceptibility is used as an indicator of overall alteration content and local scale apparent magnetic susceptibility values are computed using a forward modeling routine the new apparent magnetic data set indicates northwest and northeast linears which are the same as those seen in previous studies

furthering the investigation of eruption styles through quantitative shape analyses of volcanic ash particles abstract volcanic ash morphology has been quantitatively investigated for various aims such as studying the settling velocity of ash for modelling purposes and understanding the fragmentation processes at the origin of explosive eruptions in an attempt to investigate the usefulness of ash morphometry for monitoring purposes we analyzed the shape of volcanic ash particles through a combination of 1 traditional shape descriptors such as solidity convexity axial ratio and form factor and 2 fractal analysis using the euclidean distance transform edt method we compare ash samples from the hydrothermal eruptions of iwodake japan in 2013 tangkuban perahu indonesia in 2013 and marapi sumatra indonesia in 2015 the dome explosions of merapi java indonesia in 2013 the vulcanian eruptions of merapi in 2010 and tavurvur rabaul papaua new guinea in 2014 and the plinian eruption of kelud indonesia in 2014 particle size and shape measurements were acquired from a particle size analyzer with a microscope camera attached to the instrument clear differences between dense blocky particles from hydrothermal or dome explosions and vesicular particles produced by the fragmentation of gas bearing molten magma are well highlighted by conventional shape descriptors and the fractal method in addition subtle differences between dense blocky particles produced by hydrothermal explosions dome explosions or quench granulation during phreatomagmatic eruptions can be evidenced with the fractal method the combination of shape descriptors and fractal analysis is therefore potentially able to distinguish between juvenile and non juvenile magma which is of importance for eruption monitoring

87sr 86sr in recent accumulations of calcium sulfate on landscapes of hyperarid settings a bimodal altitudinal dependence for northern chile 19 5 s 21 5 s an elevation dependent relationship of the 87sr 86sr ratio of holocene surface accumulations of sulfate salts is demonstrated for a continental margin hyperarid setting in the atacama desert of northern chile gypsum and anhydrite of multiple origins exist widely on superficial materials that originated during the last 10 000 years an important source of calcium sulfate is from offshore generated stratocumulus clouds that are advected onto the continent where they generate fog that transfers water droplets to the ground surface which upon evaporation leaves calcium sulfate crystals meteorological measurements of the cloud base and top altitudes average 400 m and 1100 m above sea level masl respectively the seawater ratio of 87sr 86sr 0 70917 is distinctively higher than that reported for weathered mean andean rock less than 0 70750 samples of 28 modern surface salt accumulations for locations between 200 and 2950 masl and between 19 30 and 21 30 s verify that 87sr 86sr varies as a function of site altitude sites below 1075 masl and above 225 masl display calcium sulfate 87sr 86sr of mean value 0 70807 0 00004 while the ratio outside this altitudinal domain is 0 70746 0 00010 thus the 87sr 86sr ratio of holocene salt accumulations differentiates two altitudinal domains

origin of crystalline cold desert salts in the mcmurdo region antarctica abstract the distribution of crystalline salt minerals in deposits in the mcmurdo region of antarctica has been examined to study the origin of these salts sulphate chloride sodium and calcium salts are most frequently encountered salts containing chloride and sodium ions become less common away from the coast sulphate salts are more regularly distributed but tend to be related isotopically to sea water sulphate salts containing magnesium ion tend to exist mainly on substrates composed of basic igneous rocks whereas calcium and carbonate salts are present on all rock types in the region these distributions show that salts of marine origin are regionally and quantitatively most important but that chemical weathering of mafic materials in rocks and soils is also significant however biological volcanic and hydrothermal processes are or have been active contributing to salts in local areas that include penguin rookeries and eastern taylor valley the summit area of erebus volcano and subsurface rocks respectively

geochemistry of bolivian salars lipez southern altiplano origin of solutes and brine evolution abstract the southern bolivian altiplano is a volcanic area which contains numerous undrained basins occupied by playas and saline lakes locally named salars solutes carried by springs and rivers into the salars originate mostly from the alteration of the volcanic rocks and the re solution of ancient buried evaporites both weathering and hydrothermal alteration lead to na hco 3 inflow waters of similar composition high contents of lithium and boron are not specifically related to hydrothermal activity evaporite leaching leads to na cl inflow waters of higher salinity atmospheric precipitation contributes only a small amount of ca and so 4 and very little na or cl the calculated evaporative evolution of the inflow waters shows that about half of them should become highly alkaline brines and the other half should end as near neutral brines however alkaline soda lakes are rare in this region most lake brines are of the na cl and na cl so 4 types to understand this anomaly one characteristic sequence of evaporating waters was studied in detail precipitation of calcite and mg smectites accounts only partially for the reduction of alkalinity a further loss of alkalinity is probably related to the oxidation of wind blown native sulfur in peripheral ponds wind blown sulfur is also deposited around the lakes on the drainage basins in the rainy season ephemeral streams carry along sulfur particles in permanent pools of the preconcentration areas a hydrologic control of brine chemistry is suspected inflow waters that should become alkaline appear to evolve to neutral brines if they remain in contact with atmospheric dust and peripheral colluvium for a long time on the contrary inflow waters to soda lakes are generally close to the shore and have only minimal contact with atmospheric dust and peripheral sediments presently all lakes are at relatively low levels and inflow waters often drain an important area of the drainage basin before reaching the topographic low in the past lakes were at higher levels and less interaction occurred between inflow waters atmospheric dust and peripheral sediments as a result the chemistry of some lakes may have changed during the quaternary

ballooning emplacement and alteration of the chah musa subvolcanic intrusion ne iran inferred from magnetic susceptibility and fabric the porphyritic diorite chah musa subvolcanic intrusion is located in the toroud chah shirin magmatic arc in the northern central iranian structural zone the elliptical chah musa body hosts a copper deposit and intrudes an eocene sequence of volcanic breccia agglomerate and red tuffaceous sediment high magnetic susceptibility values are attributed to the presence of magnetite as a magnetic carrier changes in bulk magnetic susceptibility correlate with zonation of alteration in the intrusion although the degree of anisotropy of magnetic susceptibility decreases due to hydrothermal alteration the field observations confirm that this parameter can be used as a strain marker strongly oblate magnetic ellipsoids are found in the eastern half of the intrusion where isolated outcrops of flat lying tuffaceous host cover dioritic rocks roof zone stations with prolate ellipsoids mostly belong to the centre of the intrusion where the magnetic lineations plunge steeply they are interpreted as indicating the main feeder zone the concentric fabric pattern at the periphery of intrusion the oblate magnetic ellipsoids at the roof the highest anisotropy degree along the small diameter of the intrusion and an intense deformation of the host rocks especially at the western margin all are evidence that the intrusion was ballooning during the late stages of its emplacement ascent and emplacement of the chah musa body is ascribed to the tensional space provided by a dextral shear zone created by the regional left lateral movement on the bounding anjilow and toroud strike slip faults

neogene climate change and uplift in the atacama desert chile the relationship between andean uplift and extreme desiccation of the west coast of south america is important for understanding the interplay between climate and tectonics in the central andes yet it is poorly understood here we use soil morphological char acteristics salt chemistry and mass independent fractionation anomalies 17 o values in dated paleosols to reconstruct a middle miocene climatic transition from semiaridity to extreme hyperaridity in the atacama desert paleosols along the southeastern margin of the calama basin change from calcic vertisols with root traces slickensides and gleyed horizons to an extremely mature salic gypsisol with pedogenic nitrate we interpret this transition which occurred between 19 and 13 ma to represent a change in precipitation from 200 mm yr to 20 mm yr this drastic reduction in precipitation likely resulted from uplift of the central andes to elevations 2 km the uplift blocked moisture from the south american summer monsoon from entering the atacama the mid miocene gyp sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the cal ama basin significantly higher than modern nitrate soils which occur below 2500 m modern and quaternary soils in this elevation zone contain soil carbonate and lack ped ogenic gypsum and nitrate we infer that 900 m of local surface uplift over the past 10 m y displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the calama basin by decreasing local air temperatures and creating an orographic barrier to pacific air masses

stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern chile and southern california u s a natural surficial accumulations of nitrate rich salts in the atacama desert northern chile and in the death valley region of the mojave desert southern california are well known but despite many geologic and geochemical studies the origins of the nitrates have remained controversial n and o isotopes in nitrate and s isotopes in coexisting soluble sulfate were measured to determine if some proposed n sources could be supported or rejected and to determine if the isotopic signature of these natural deposits could be used to distinguish them from various types of anthropogenic nitrate contamination that might be found in desert groundwaters high grade calich a type nitrate deposits from both localities have 15n values that range from 5 to 5 but are mostly near 0 values of 15n near 0 are consistent with either bulk atmospheric n deposition or microbial n fixation as major sources of the n in the deposits 18o values of those desert nitrates with 15n near 0 range from about 31 to 50 v smow significantly higher than that of atmospheric o2 23 5 such high values of 18o are considered unlikely to result entirely from nitrification of reduced n but rather resemble those of modern atmospheric nitrate in precipitation from some other localities assuming that limited modern atmospheric isotope data are applicable to the deposits and allowing for nitrification of co deposited ammonium it is estimated that the fraction of the nitrate in the deposits that could be accounted for isotopically by atmospheric n deposition may be at least 20 and possibly as much as 100 34s values are less diagnostic but could also be consistent with atmospheric components in some of the soluble sulfates associated with the deposits the stable isotope data support the hypothesis that some high grade caliche type nitrate rich salt deposits in some of the earth s hyperarid deserts represent long term accumulations of atmospheric deposition possibly in the order of 104 yr for the death valley region 107 yr for the atacama desert in the relative absence of soil leaching or biologic cycling the combined n and o isotope signature of the nitrate in these deposits is significantly different from those of many other natural and anthropogenic sources of nitrate

degassing crystallization and eruption dynamics at stromboli trace element and lithium isotopic evidence from 2003 ashes during its 1800 year long persistent activity the stromboli volcano has erupted a highly porphyritic hp volatile poor scoriaceous magma and a low porphyritic lp volatile rich pumiceous magma the hp magma is erupted during normal strombolian explosions and lava effusions while the lp one is related to more energetic paroxysms during the march april 2003 explosive activity stromboli ejected two typologies of juvenile glassy ashes namely highly vesicular lp shards and volatile poor hp shards their textural and in situ chemical characteristics are used to unravel mutual relationships between hp and lp magmas as well as magma dynamics within the shallow plumbing system the mantle normalized trace element patterns of both ash types show the typical arc lava pattern however hp glasses possess incompatible element concentrations higher than lp glasses along with sr and eu negative anomalies hp shards are generally characterized by higher li contents to 20 ppm and lower 7li values 1 2 to 3 8 with respect to lp shards li contents of 7 14 ppm and 7li ranging between 4 6 and 0 9 fractional crystallization models based on major and trace element compositions combined with a degassing model based on open system rayleigh distillation and on the assumption that melt fluiddli 1 show that abundant 30 plagioclase precipitation and variable degrees of degassing can lead the more primitive lp magma to evolve toward a differentiated isotopically lighter hp magma ponding in the upper conduit and undergoing slow continuous degassing induced crystallization this study also evidences that in march 2003 stromboli volcano poured out a small early volume of lp magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending lp magma erupted during the april 5 paroxysm the different ascent rates and cooling rates of the two lp magma batches i e pre and post paroxysm resulted in small but detectable differences in their chemical signatures finally this study highlights the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic activity and of li isotopes as tracers of degassing processes within the shallow plumbing system

juvenile volcanic glass erupted before the appearance of the 1991 lava dome unzen volcano kyushu japan fugen dake the main peak of unzen volcano began to erupt on 17 november 1990 after 198 years of dormancy following a period of steam dominated tephra eruptions a new lava dome appeared at the bottom of jigokuato crater in the summit region on 20 may 1991 non hydrated vesiculated glass was detected in the precursory tephra eruptions beginning 12 february 1991 determination of whether glass pyroclasts are hydrated or not appears to be an effective means of ascertaining whether the tephra contains juvenile material from ascending magma the detection of juvenile pyroclasts at fugen dake more than 3 months before the appearance of the lava dome suggests that monitoring of early tephra emissions is an effective way to help forecast impending magmatic eruptions

hydration and refractive indices of holocene tephra glass in hokkaido northern japan abstract refractive indices of incompletely hydrated tephra glasses vary widely thus glass refractive index is not a practical indicator for identifying holocene tephras hydration near the surface of tephra glass shards can be removed by either hydrofluoric acid treatment or annealing for 12 h at 400 c the annealing procedure is a particularly reliable and simple way to dehydrate tephra glasses standard deviations s of refractive indices for glasses after 12 h annealing are small s 0 0014 0 0018 in contrast with untreated glasses s 0 0034 0 0405 the refractive indices of dehydrated tephra glasses are 0 006 0 014 lower than those of the untreated hydrated glasses using the 400 c 12 h annealing procedure values for the refractive indices of eight holocene tephras in hokkaido were obtained these refractive indices are useful indicators for identification and correlation of holocene tephras

a preliminary petrographic study of the chilean nitrates the nitrate deposits of the northern atacama desert occurin a narrow zone between the low lying pampa del tamarugal to the east and the hills of the coastal range to the west high purity nitrate ore occurs as stratiform seams about 20 cm thick present at depths of 3 7 m below thedesert surface the ore is hosted by a variety of lithologies and overlain by salt cemented regolith the ore is largely composed of nitratite sodanitre or chilean salt peter and halite with locally abundant humberstonite polyhalite and mirabilite other salts present include ca iodates and a variety of sulphate borate and chromate minerals textural relationships can be used to deduce paragenetic sequences for individual salt samples and these combined to produce a generalized paragenetic web for the nitrate ore these textural data can be combined with published solubility data to investigate the course of fluid evolution during ore genesis the multiplicity of paragenetic relationships within the ore reflects the derivation of precipitating fluids from a variety of sources westwards flowing andean groundwater coastal fogs occasional rainfall and andean derived surface floodwaters the unusual mineralogy of the nitrate ore reflects the extreme chemical evolution of the precipitating brines through multiple episodes of salt precipitation and remobilization during transport to the nitrate horizons the formation of high purity nitrate ore appears to be the result of multiple phases of dissolution reprecipitation and recrystallization that have separated the highly soluble nitrate salts from less soluble salts in the overlying profile salts have largely accumulated through displacive growth but some of the host silicate and carbonate lithologies have also undergonea small degree of salt replacement

on the lognormal distribution of oxides in igneous rocks using magnetic susceptibility as a proxy for oxide mineral concentration summary trace element concentrations in igneous rocks are frequently lognormally distributed and shaw 1961 suggested that minor minerals in igneous rocks might also be lognormally distributed there is accumulating evidence to show that bulk magnetic susceptibility bms often closely follows a lognormal distribution in fresh igneous rocks but because bms is dependent upon oxide grain size and mineralogy it is not obvious why this should be so we have adopted shaw s simple theoretical model as an argument to account for the lognormal distribution of bms the obvious prevailing conditions must be that the original melt was uniform that pressure temperature and oxygen fugacity were constant and that the minerals were formed at equilibrium in addition because bms is also a function of grain size and mineralogy the distribution of these latter parameters must be insensitive to changes in p t etc at equilibrium it appears that there must be a limit to the lognormal law in trying to apply it to minerals of greater concentration however bms proxies for oxide content and because it does closely follow the lognormal distribution it thus appears that in crystalline igneous rocks the lognormal law is obeyed by minerals of concentrations at least up to a few percent knowledge of the mean and variance of such distributions can be useful in a variety of petrological applications especially in drillcore logging a simple example is presented

hydrothermally induced changes in mineralogy and magnetic properties of oxidized a type granites abstract the changes in magnetic mineralogy due to the hydrothermal alteration of a type granitic rocks have been thoroughly investigated in samples from the granite of tana corsica france and compared with other a type granites meruoca ne brazil bushveld south africa mount scott wichita mountains oklahoma usa and the stratoid hypersolvus granites of madagascar the altered red colored samples and their non altered equivalents were magnetically characterized by means of magnetic susceptibility measurements hysteresis loops remanent coercivity spectra and lowrie test it is shown that hydrothermalization in magnetite bearing granites is related to the formation of fine grained magnetite and hematite and to coeval depletion in the content of primary low coercive coarse grained magnetite these mineralogical changes give typical rock magnetic signatures namely lower susceptibility magnitudes and anisotropy degrees prolate ams anisotropy of magnetic susceptibility fabrics and increased coercivities optical microscopy and sem scanning electronic microscopy images suggest that the orientation of the secondary magnetic minerals is related to fluid pathways and micro fractures formed during the hydrothermal event and therefore may be unrelated to magma emplacement and crystallization fabrics changes in magnetic mineralogy and grain size distribution have also to be considered for any paleomagnetic and iron isotope studies in granites

paleomagnetism of alkaline complexes and remagnetization in the kapuskasing structural zone ontario canada the shenango and nemegosenda alkaline complexes of the southern kapuskasing structural zone ksz an uplifted crustal block in northern ontario preserve primary thermoremanent magnetizations with mean directions d 112 i 46 k 13 95 13 n 9 sites and d 304 i 51 k 31 95 10 n 7 sites for shenango and d 124 i 57 k 13 9520 n 4 sites and d 312 i 54 k 17 95 13 n 7 sites for nemegosenda their average paleomagnetic pole is 45 n 189 e dp 6 dm 8 falling on the keweenawan track of the north american apparent polar wander path apwp between 1098 1107 ma reverse polarity r and 1087 1098 ma normal polarity n poles of lake superior lavas and dikes unlike the keweenawan paleopoles which have a reversal asymmetry of 30 the shenango and nemegosenda r and n poles are almost 180 reversed which was found also by lewchuk and symons 1990b this symmetry of r and n poles casts doubt on the previously accepted predominance of asymmetric reversals of the earth s field in keweenawan time the archean shawmere anorthosite is overprinted by a b magnetization with mean direction d 97 i 68 k 12 95 10 n 18 sites the shawmere b paleopole at 38 n 226 e dp 13 dm l6 falls between the carbonatite poles and 1850 ma cratonic poles and the b magnetization seems to be a composite of middle and late proterozoic overprints sites in the western part of the shawmere body close to the carbonatite line of faults linking the various alkaline complexes are severely altered with an abundance of secondary magnetite and hematite virtual geomagnetic poles for these sites fall close to the keweenawan track and have an average pole of 189 53 n dp 15 dm 20 suggesting that the b nrm at these sites is an 1100 ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion more easterly sites are less altered their vgps resemble 1850 ma cratonic poles and average to 235 e 30 n dp 15 dm 17 these b natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle proterozoic hydrothermal activity associated with intrusion of early alkaline bodies fault movement on the ivanhoe lake fault zone or late stage uplift of the ksz

interpretation of magnetic susceptibility a new approach to geophysical evaluation of the degree of rock alteration magnetic susceptibility measurements on cores from boreholes in the eye dashwa lakes pluton an archean granitic pluton near atikokan ontario give a quantitative evaluation of the alteration leve

the history and nature of wind erosion in deserts recently the importance of wind erosion has been reevaluated many low angle surfaces developed on susceptible materials possess closed depressions pans and these often have a distinctive morphology and lunette dunes on their lee sides it has also become apparent that in drylands there are extensive areas of stone pavements some of which have been molded by deflation however it is now recognized that other horizontal sorting processes have played a role especially movement of fines by overland flow vertical sorting associated with wetting and drying salt heave bioturbation frost action and dust accretion have also played a role in hyperarid areas especially with unidirectional winds bedrock outcrops and old lake beds have been molded to give aerodynamic forms called yardangs the study of dust storms by analysis of climatological data and remote sensing has revealed the importance of deflation especially in hyperarid areas with centripetal drainage deflation hot spots such as the bo

soils at the hyperarid margin the isotopic composition of soil carbonate from the atacama desert northern chile we evaluate the impact of exceptionally sparse plant cover 0 20 and rainfall 2 114 mm yr on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet the atacama desert in northern chile d 13 c and d 18 o values of carbonates from the atacama are the highest of any desert in the world d 13 c vpdb values from soil carbonate range from 8 2 at the wettest sites to 7 9 at the driest we measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of cerling 1984 earth planet sci lett 71 229 240 in which we assumed an exponential form of the soil co2 production function and relatively shallow 20 30 cm average production depths overall we find that respiration rates are the main predictor of the d 13 c value of soil carbonate in the atacama whereas the fraction c3 to c4 biomass at individual sites has a subordinate influence the high average d 13 c value 4 1 of carbonate from the driest study sites indicates it formed perhaps abiotically in the presence of pure atmospheric co2 d 18 o vpdb values from soil carbonate range from 5 9 at the wettest sites to 7 3 at the driest and show much less regular variation with elevation change than d 13 c values d 18 o values for soil carbonate predicted from local temperature and d 18 o values of rainfall values suggest that extreme 80 in some cases soil dewatering by evaporation occurs at most sites prior to carbonate formation the effects of evaporation compromise the use of d 18 o values from ancient soil carbonate to

paleomagnetism of ordovician alkalic intrusives and host rocks from the pedernal hills new mexico positive contact test in remagnetized rocks jackson m van der voo r and geissman j w 1988 paleoma etism of ordovician alkalic intrusives and host rocks from the pedemal hills new mexico positive contact test in remagnetized rocks tectonophysics 147 313 323 a set of thin dikes from central new mexico dated at 469 7 ma rb sr loring and armstrong 1980 have yielded a virtual geomagnetic pole which lies on the late paleozoic segment of the north american apparent polar wander path the remanence of the dikes appears to be a product of late paleozoic hydrothermal alteration paradoxically however the magnetization of the host rocks is most simply explained in terms of a positive contact test samples collected between 0 2 and 0 5 dike widths from the contact contain a component of remanence parallel to the magnetization in the dikes with unblocking temperatures which decrease with distance from the dikes host rocks from a distance of more than 1 dike width show no evidence of the characteristic dike magnetization there are two possible resolutions of this paradox 1 the magnetization of the host rocks is secondary despite the apparent positive contact test and is a product of hydrothe al fluid migration through the dikes or along the contact zones or 2 the magnetization of the dikes is primary but not representative of the ordovician paleofield for north america possible reasons for inaccurate representation include a incomplete averaging of secular variation b tectonic rotation with respect to the stable craton or c erroneous age determination for the rocks we argue that explanation 1 is the most likely

different sulfate sources and their post depositional migration in atacama soils abstract sulfate is one of the major salts accumulated in the hyperarid atacama desert chile its origin has been attributed to local weathering volcanic source sea salt and atmosphere deposition previous studies concluded that atmospheric sulfate is present in the desert but this term was poorly defined in terms of chemical origin and its contribution to the total sulfate budget in soil is debated the contribution of each potential sulfate source is expected to vary geographically due to different source locations efforts to quantify the contribution from each source have been hampered by the lack of well defined isotopic compositions for its end members in this study we discard the vague term of atmospheric sulfate by recognizing two major sulfate sources on the basis of their formation pathways primary and secondary sulfate primary sulfate including those from rock weathering sea salt and volcanic ash leachates do not have an 17o anomaly secondary sulfate derived from the oxidation of reduced sulfur gases biological or volcanic emissions by atmospheric ozone or hydrogen peroxide however has an 17o anomaly we developed a method to extract water soluble anions sequentially or entirely from bulk soil samples and analyzed the sulfate concentrations and 17o anomalies for four geographically representative soil profiles in the central atacama desert 23 s to 25 5 s we found that sequential sulfate extractions on the same soil sample produce sulfate with decreasing 17o values except for a positive bulge at 20 to 50 cm in depth there is a general increase in the 17o value for total sulfate with soil depth for all the four soil profiles that we examined these findings are consistent with the hypothesis that secondary sulfates 17o anomalous sulfates are more soluble and thus preferentially leached in liquid water than primary sulfate the finding also warns that incomplete sulfate extraction from sulfate rich soils yields unpredictable results we infer a 17o value of 4 6 for secondary sulfate based on data from partial sulfate extraction experiments total contribution of secondary sulfate in the central atacama desert is estimated to range from 9 to 24 with the highest percentage in the central depression

87sr 86sr of calcium sulfate in ancient soils of hyperarid settings as a paleoaltitude proxy pliocene to quaternary constraints for northern chile 19 5 21 7 s

juvenile magma recognition and eruptive dynamics inferred from the analysis of ash time series the 2015 reawakening of cotopaxi volcano forecasting future activity and performing hazard assessments during the reactivation of volcanoes remain great challenges for the volcanological community on august 14 2015 cotopaxi volcano erupted for the first time in 73 years after approximately four months of precursory activity which included an increase in seismicity gas emissions and minor ground deformation here we discuss the use of near real time petrological monitoring of ash samples as a complementary aid to geophysical monitoring in order to infer eruption dynamics and evaluate possible future eruptive activity at cotopaxi twenty ash samples were collected between august 14 and november 23 2015 from a monitoring site on the west flank of the volcano these samples contain a range of grain types that we classified as hydrothermal altered lithic juvenile and free crystals the relative proportions of theses grains evolved as the eruption progressed with increasing amounts of juvenile material and a decrease in hydrothermally altered material in samples from the initial explosion juvenile grains are glassy microlite poor and contain hydrothermal minerals opal and alunite the rising magma came in contact with the hydro thermal system under confinement causing hydro magmatic explosions that cleared the upper part of the plumbing system subsequently the magmatic column produced a thermal aureole in the conduit and dried out the hydrothermal system allowing for dry eruptions magma ascent rates were low enough to allow for efficient outgassing and microlite growth constant supply of magma from below caused quasi continuous disruption of the uppermost magma volume through a combination of shear deformation and gas expansion the combination of increasing crystallinity of juvenile grains and high measured so2 flux indicate decreasing integrated magma ascent rates and clearing of the hydrothermal system along transport pathways in a system open to gas loss the near real time monitoring of ash samples combined with traditional geophysical monitoring techniques during the reawakening of cotopaxi allowed us to gain a much clearer understanding of events than when using traditional geophysical monitoring alone c 2016 elsevier b v all rights reserved

magnetic effects of hydrothermal alteration in porphyry copper and iron oxide copper gold systems a review abstract magnetic anomaly patterns can be used as a tool for mapping lithology metamorphic zones and hydrothermal alteration systems as well as identifying structures that may control passage of magmas or hydrothermal fluids associated with mineralisation reliable geological interpretation of mineralised systems requires an understanding of the magmatic metamorphic and hydrothermal processes that create alter and destroy magnetic minerals in rocks predictive magnetic exploration models for porphyry copper and iron oxide copper gold iocg deposits can be derived from standard geological models by integrating magnetic petrological principles with petrophysical data deposit descriptions and modelling of observed magnetic signatures of these deposits even within a particular geological province the magnetic signatures of similar deposits may differ substantially due to differences in the local geological setting searching for look alike signatures of a known deposit is likely to be unrewarding unless pertinent geological factors are taken into account these factors include the tectonic setting and magma type composition and disposition of host rocks depth of emplacement and post emplacement erosion level depth of burial beneath younger cover post emplacement faulting and tilting remanence effects contingent on ages of intrusion and alteration and metamorphism because the effects of these factors on magnetic signatures are reasonably well understood theoretical magnetic signatures appropriate for the local geological environment can qualitatively guide exploration and make semiquantitative predictions of anomaly amplitudes and patterns the predictive models also allow detectability of deposit signatures to be assessed for example when deposits are buried beneath a considerable thickness of nonmagnetic overburden are covered by highly magnetic heterogeneous volcanic rocks or there is a strong regional magnetic gradient this paper reviews the effects of hydrothermal alteration on magnetic properties and magnetic signatures of porphyry copper and iron oxide copper gold systems and presents examples of predictive magnetic exploration models and their predicted signatures in various geological circumstances

characterization of the luminance and shape of ash particles at sakurajima volcano japan using ccd camera images we develop a new method for characterizing the properties of volcanic ash at the sakurajima volcano japan based on automatic processing of ccd camera images volcanic ash is studied in terms of both luminance and particle shape a monochromatic ccd camera coupled with a stereomicroscope is used to acquire digital images through three filters that pass red green or blue light on single ash particles we measure the apparent luminance corresponding to 256 tones for each color red green and blue for each pixel occupied by ash particles in the image and the average and standard deviation of the luminance the outline of each ash particle is captured from a digital image taken under transmitted light through a polarizing plate also we define a new quasi fractal dimension d qf to quantify the complexity of the ash particle outlines we examine two ash samples each including about 1000 particles which were erupted from the showa crater of the sakurajima volcano japan on february 09 2009 and january 13 2010 the apparent luminance of each ash particle shows a lognormal distribution the average luminance of the ash particles erupted in 2009 is higher than that of those erupted in 2010 which is in good agreement with the results obtained from component analysis under a binocular microscope i e the number fraction of dark juvenile particles is lower for the 2009 sample the standard deviations of apparent luminance have two peaks in the histogram and the quasi fractal dimensions show different frequency distributions between the two samples these features are not recognized in the results of conventional qualitative classification criteria or the sphericity of the particle outlines our method can characterize and distinguish ash samples even for ash particles that have gradual property changes and is complementary to component analysis this method also enables the relatively fast and systematic analysis of ash samples that is required for petrologic monitoring of ongoing activity such as at the sakurajima volcano

giant evaporite belts of the neogene central andes large volumes of continental evaporites accumulated within the central andes during neogene uplift of the altiplano puna plateau and development of the andean volcanic arc halite and gypsum are dominant minerals along with local and economically important borates playa conditions have existed since ca 15 ma halite and borate deposition has occurred for the past 7 to 8 m y evaporites formed in salar environments e g playa lakes and are characterized by complex mineral assemblages occurrence zonation and geochemistry evaporite deposition was controlled by volcanism geothermal activity closed drainage and climate these andean deposits and their controls differ from evaporites in other continental and marine environments

late stage oxide growth associated with hydrothermal alteration of the western granite isle of rum nw scotland 1 the paleogene western granite isle of rum nw scotland represents a westward tilted shallow level felsic intrusion associated with the early growth stages of the classic early paleogene rum igneous centre rock magnetic data reveal an east to west variability in magnetic properties across the intrusion that is also reflected in petrographic and geochemical observations it is argued that the subtle intraplutonic variations in oxide grain size and composition observed in the western granite might be a common feature of shallow epizonal igneous intrusions as these mineralogical differences are not easily detectable without detailed rock magnetic study this research underscores the need for comprehensive rock magnetic studies when interpreting the postemplacement history of shallow level plutons paleomagnetic data indicate that the eastern part of the intrusion represents the shallower part of the exposed intrusion as further evidenced by roof pendants of lewisian gneiss and abundant drusy miarolitic cavities indicative of shallow emplacement in contrast the western part represents a deeper 900 m part of the intrusion as evidenced by the lack of roof pendants and existence of only minor drusy cavities rock magnetic data indicate that the dominant magnetic phase is low ti titanomagnetite with some titanomaghemite magnetic grain sizes range from pseudosingle domain in the west and multidomain in the east with a slight increase in ti content of the titanomagnetite phases in the east as revealed by curie point estimates we interpret these data to indicate that late stage subsolidus hydrothermal deuteric fluids associated with emplacement of the stage 2 rum layered suite produced secondary growth of fe ti oxide mineral phases along silicate grain boundaries in the eastern part of the intrusion the secondary oxide growth resulted in a slightly coarser magnetic grain size fraction and an increase in ti content of the titanomagnetite grains the deeper structural levels of the western part of the intrusion and its distance from the contact of the rum layered suite inhibited secondary oxide growth and thus this part of the intrusion preserves a primary magmatic oxide phase of a smaller magnetic grain size and near pure magnetite chemistry

the origin and distribution of salts on alluvial fans in the atacama desert northern chile the atacama desert has been predominantly hyperarid since the middle miocene combined ionic and x ray diffraction analysis shows that calcium sulphate is prevalent on three similar alluvial fans in salar basins across a transect of different environments in antofagasta province northern chile differences between the fans were largely due to the effect of local factors on salt input secondary redistribution and deposit preservation thus carbonate was notable in the high andes fan c and pre cordillera fan b whereas in the central valley fan a greater qualities of the more soluble salts chlorides and nitrates probably reflect the higher level of aridity and an oceanic source the camanchaca calcium sulphate distribution on the alluvial fans and on adjacent hilltops indicates an aeolian origin evidence from the pre cordillera fan b however suggests that salt input may have been episodic related to changes in conditions within the general aridity and possibly to stone pavement and soil development calcium sulphate sources range from an input in andean precipitation and the camanchaca to abundant regional evaporite deposits it is probable however that products of andean volcanism constitute the dominant primary source of calcium sulphate and that the salt has subsequently been distributed widely within the endoreic basins of the region by a combination of groundwater surface flow and the wind 1997 john wiley sons ltd

interpreting regional 3d fault networks from integrated geological and geophysical data sets an example from the guichon creek batholith british columbia abstract the effects of fault processes on rocks commonly include a lowered magnetic susceptibility and an increased predisposition to erosion we present a workflow that uses these characteristics of faults and geological observations to interpret a 3d fault network the location of faults on the topographic surface is first interpreted by a multi layer lineament mapping method that includes geological and geophysical data sets we then demonstrate how magnetic intensity data can be used to estimate the dip of fault related magnetic anomalies by performing 2d inverse modeling along profiles extracted from a magnetic intensity grid the accuracy of the method is assessed by modeling the dip of low magnetic anomalies of known geometry created in a synthetic 3d magnetic susceptibility model modeled dips are consistently accurate to better than 5 for input dips 60 a requirement of the method however is that the magnetic susceptibility contrast between the background field and the magnetic anomaly must be 75 for the anomaly to be accurately modeled comparison between fault orientations modeled from magnetic data and measured in the field in the guichon creek batholith confirms that the methodology can be successfully applied to brittle faults in real albeit relatively simple geological environments