new carbonatite complexes in the archaean in ouzzal nucleus ahaggar algeria mineralogical and geochemical data several massifs of very old carbonatites have been discovered in the archaean granulitic block of in ouzzal western ahaggar algeria these carbonatites are original since they are associated with silica saturated syenitic magmatism and present in the late stages of evolution a very uncommon mineralogy with silicate minerals especially wollastonite allanite and quartz the mineralogy c and o isotopes and r e e distributions indicate that the late stages of crystallization occurred under high sio2 activities and produced the uncommon mineralogy and extremely high r e e concentrations in phosphate minerals apatite and britholite interaction with continental crust is a possible mechanism to explain the original features of these carbonatite complexes

ac magnetic susceptibility studies of chinese red clay sediments between 4 8 and 4 1 ma paleoceanographic and paleoclimatic implications 1 recent studies show that measuring ac magnetic susceptibility with in phase and quadrature components versus temperature provides an effective way to map the grain size distribution of pedogenic maghemite applying this method we mapped the pedogenic maghemite grain size distribution of samples of the red clay sediments deposited on the chinese loess plateau clp during the interval 4 8 4 1 ma during which the validity of magnetic susceptibility as an east asian summer monsoon precipitation intensity proxy has been questioned the results show that the pedogenic maghemite grain size distribution is the same as in the overlying loess paleosol sequence and in the other intervals of the red clay sequence suggesting similar pedogenesis mechanisms and marked precipitation seasonality on the clp between 4 8 and 4 1 ma compared with the other intervals we argue that the seasonal position shift of the polar westerlies produces the required precipitation seasonality and thus produces a uniform pedogenic grain size distribution during this time interval reconstructing paleo rainfall on the central clp on the basis of surface calibration results suggests that the intensification of the east asian summer monsoon might be a key trigger for the subsequent significant ocean and cryosphere adjustments during the pliocene

enhancement mechanisms of magnetic susceptibility in the chinese red clay sequence 1 little is known about the mechanisms of magnetic susceptibility enhancement for the red clay sequence on the chinese loess plateau clp in comparison to the overlying loess paleosol sequence here we present a rock magnetic study of the red clay sediments from the central clp our results show that frequency dependence of fd lf hf where lf and hf are measured at 470 hz and 4700 hz respectively lf and susceptibility of anhysteretic remanent magnetization arm are linearly correlated within the red clay sequence this linear correlation indicates that the pedogenic magnetic minerals of the red clay have a rather uniform grain size distribution as in the loess paleosol sequence and the grain size is independent of the degree of pedogenesis nevertheless red clay sediments are slightly more enriched in superparamagnetic magnetic particles than the overlying loess paleosol sediments as indicated by the higher slope of the regression line between fd and lf

partly pedogenic origin of magnetic variations in chinese loess quaternary sequences of interbedded aeolian silts loess and buried soils palaeosols potentially provide one of the best terrestrial records of past climates1 5 magnetic susceptibility variations in loess and palaeosol sequences from china are strongly correlated with climate induced fluctuations of oxygen isotope ratios in deep sea sediments2 4 6 8 as an explanation of this correlation it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6 8 it has also been suggested that such a model involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7 here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units this suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non magnetic loess is inadequate our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols

massive recrystallization of low mg calcite at high temperatures in hydrocarbon source rocks implications for organic acids as factors in diagenesis we document massive recrystallization of low mg calcite lime mudstone source rocks at moderately high temperatures in the smackover formation of mississippi this process was driven by organic acids produced during kerogen maturation and holds the implications for studies of primary migration of hydrocarbons secondary porosity generation in reservoirs and global environmental change this interpretation is based on contrasting the following petrographic and bulk rock geochemical characteristics of organic poor vs organic rich lime mudstones

quantitative estimates of pedogenic ferromagnetic mineral formation in chinese loess and palaeoclimatic implications abstract the quasi continuously deposited loess sediments of the central chinese loess plateau constitute one of the most complete and long lasting records of quaternary palaeoclimate on land much of this record has been uncovered by measurement and interpretation of the magnetic properties of the loess the polarity of natural remanent magnetization has provided absolute age dates by correlation with the geomagnetic polarity time scale magnetic low field susceptibility has been used to reconstruct the cyclic changes of pleistocene palaeoclimates although the origin of the signal is still poorly understood from comparison with the 10 be concentration throughout the sequence we are able to determine the contributions to susceptibility from inherited and authigenic sources for the last 130 ky since the production of authigenic magnetite is directly related to regional precipitation annual palaeoprecipitation rates can be calculated palaeoprecipitation rates similar to present day rates are derived for oxygen isotope stage 5 including the last interglacial in contrast cold periods corresponding to oxygen isotope stages 2 and 4 experienced negligible precipitation the beginning and end of the time interval represented by oxygen isotope stage 3 however was characterized by short episodes when precipitation exceeded the holocene average

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

natural hydraulic fractures in the wessex basin sw england widespread distribution composition and history bedding parallel veins of fibrous calcite beef are historical in the wessex basin the veins are common in mesozoic mudstones and shales especially of liassic to mid cretaceous ages cone in cone structures which consist of multiple nested cones are also well developed within the beef to investigate the distribution and the context of formation of beef in the basin we have made several field studies and analysed numerous samples the veins are widespread vertically and horizontally in the sedimentary sequence but they are especially common near or within potential source rocks for petroleum or near major tectonic faults the internal structures of some beef veins have revealed that they formed during late cretaceous to tertiary compressional inversion of the basin the typical composition for beef is of calcite with some pyrite and fragments of shale however inclusions or patches of hydrocarbons liquid or solid occur within calcite crystals or between fibres respectively according to some previous studies as well as ours beef veins of the wessex basin represent natural hydraulic fractures which formed as a result of fluid overpressure this may have resulted in part from chemical compaction of petroleum source rocks during late cretaceous to tertiary times indeed source rocks at outcrop in the wessex basin could be more mature than previously thought and the beef veins may be good markers of maturation

paleorainfall reconstructions from pedogenic magnetic susceptibility variations in the chinese loess and paleosols the rock magnetic properties of the chinese loess and paleosols constitute a unique and sensitive record of east asian paleoclimate through the quaternary period systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal which can be easily and rapidly measured at many sites across the loess plateau variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages as well as glaciations and interglaciations past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations the susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the loess plateau our rainfall reconstructions identify enhanced summer monsoonal activity in the chinese loess plateau region in the early holocene and the last interglaciation in the presently semiarid western area of the plateau annual precipitation in interglacial times was up to 80 higher than at present in the more humid southern and eastern areas values were up to 20 higher than today s levels during the last glaciation precipitation decreased across the entire plateau typically by 25 the relationship between pedogenic susceptibility climate and weathering age was examined over the northern hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate rather than soil age is the predominant factor that controls pedogenic susceptibility enhancement in loess soils

formation of bedding parallel fibrous calcite veins in laminated source rocks of the eocene dongying depression a growth model based on petrographic observations abstract bedding parallel fibrous calcite veins commonly referred to as beefs are widely developed within eocene lacustrine laminated organic rich source rocks in the dongying depression of bohai bay basin east china understanding the formation of this type vein is essential to elucidate the diagenetic evolution and fluid migration in muddy source rocks this study investigates the mechanism s of growth of bedding parallel veins in hydrocarbon source rocks of the es4 upper and the es3 lower units in the dongying depression based on systematic observations of macro and micro scale features both in veins and their wall rocks combined with x ray diffraction and analyses of geochemical indicators the source rocks are heterogeneous in both composition and structure the beefs mainly occur in laminated rocks consisting of organic rich laminae which occur together with horizontally continuous banded lamalginites and rare carbonate rich laminae overall corresponding to high toc low carbonate contents and low maturity average ro 0 57 most beefs are embedded in the neighboring lamalginites of organic rich laminae with low clay mineral contents the beefs are products of diagenesis accompanying burial and both the initial fracture opening and subsequent dilation represent early stages of hydrocarbon generation and expulsion the presence of bedding parallel bituminite veins and the median plane or parting is evidence for horizontally oriented hydrofracturing due to fluid overpressure in turn fluid overpressure reflects the generation and primary migration of hydrocarbon bearing fluids hydrocarbon generation organic matter evolution and clay mineral dehydration illitization of smectite were responsible for the fluid overpressure crystal growth within the beefs was a continuous process with an antitaxial growth direction and there was no growth competition between adjacent fibres these observations indicate that the crystal growth and fracture dilation were synchronous and growth space was limited the force exerted by crystallizing vein fill contributed to the high fluid pressure suggesting that fibrous vein minerals actively promoted further dilation as the veins grew vein dilation accompanied the migration of hydrocarbon bearing fluids evinced by the occurrence of primary hydrocarbon inclusions found in the fibrous calcites therefore the hydrocarbon bearing fluids were present at the time of horizontal fluid flow from the start of fracture opening to the end of the dilation

link between benthic oxygen isotopes and magnetic susceptibility in the red clay sequence on the chinese loess plateau 1 recent rock magnetic work on the red clay sequence on the chinese loess plateau clp convincingly demonstrates that the enhancement mechanisms of low frequency magnetic susceptibility i e measured at 470 hz lf in the red clay sequence are similar to those in the loess paleosol sequence therefore lf in the red clay sequence should indicate precipitation intensity received by the clp as is the case in the overlying loess paleosol sequence based on this result we compared lf in the red clay sequence with benthic oxygen isotope records we infer that the primary precipitation source on the clp varies over time in three phases during 8 1 4 5 ma the east asian summer monsoon dominates during 4 5 4 ma the polar westerlies dominate during 4 0 ma the east asian summer monsoon dominates we attribute these precipitation source shifts on the clp to the closure of the panama seaway around 4 5 ma and the tibetan uplift during the interval 4 2 6 ma

petroleum accumulation and leakage in a deeply buried carbonate reservoir nispero field mexico abstract the filling history of the nispero deeply buried lower cretaceous carbonate reservoir below 4000 m from the south part of gulf of mexico was studied using a combination of data from petrography stable isotopes and fluid inclusions and compared with a one dimensional burial model to derive timing a combination of techniques such as microthermometry confocal laser scanning microscopy clsm and raman microspectroscopy have been used to characterize the different fluid inclusion assemblages and discriminate between the reequilibrated and the preserved fluid inclusions the nispero reservoir is composed of a fractured finely crystalline replacement dolostone a first generation of fractures is filled by a non planar dolomite cement and a second generation by a planar e dolomite cement and a blocky sparite the reservoir is lastly stylolitised dolomite has 13c values between 0 9 and 4 0 pdb and 18o values between 4 5 and 10 8 pdb calcite has 13c values between 3 8 and 4 4 pdb and 18o values between 4 9 and 5 5 pdb these values suggest that the calcite represents a change in water composition and that dolomite and calcite cements do not precipitate from petroleum derived fluids however geological fluids are trapped as aqueous and oil inclusions in the calcite cements three petroleum aqueous inclusion assemblages were identified pseudo primary brown oil paired with brine inclusions and pseudo primary yellow oil paired with low salinity water inclusions distributed in local concentrations and secondary colourless oil paired with low salinity aqueous inclusions distributed in healed microfractures the burial model calibrated with bottom hole temperatures has been calculated using petromod 1d to generate the rock pt history under hydrostatic and lithostatic regimes reconstructed temperatures of initial trapping of fluid inclusion and calculated temperatures from 18o measured in dolomite and calcite cements are in good agreement data suggests that the reservoir was being filled at 97 c 315 bar by heavy black oils accompanied by high salinity water during the miocene period 11 6 m y the maximum estimated pressure of 700 bar at 130 c is reconstructed by petroleum inclusions trapping black oil and associated low salinity aqueous inclusions these pt conditions were reached during the pliocene 3 2m y present day reservoir conditions of 145 c and 500 bar are recorded by reequilibration of most of the fluid inclusions of the two assemblages and correspond to hydrostatic pressure the drop of pressure from 700 bar to 500 bar may be an explanation for the late stylolitisation

petroleum seepages and impregnations in great britain abstract this paper documents 173 occurrences of surface petroleum seepages and impregnations in great britain a brief description is given of the seven main areas where they are found the descriptions range from pore fillings of bitumen to steady flows of oil and gas the principal areas in which seepages are found are as follows the orcadian basin of north east scotland the midland valley of scotland the flanks of the pennines the welsh borderlands cornwall the dorset coast of the wessex basin and the south eastern weald stratigraphically the seepages range from caledonide basement to holocene peat the principal sources for the petroleum are devonian oil shales orcadian basin carboniferous shales and coals midland valley pennines welsh borderlands devon and cornwall and the liassic wessex and wealden basins several anomalous seepages are recorded from welsh and scottish basement there are three clearly defined styles in which the seepages occur many are found where permeable carrier beds unconformably overlie impermeable basement around the margins of basins orcadian basin midlands carboniferous basin others are fault related especially in the wessex and wealden basins the seepages in the cornish tin mines are best explained as sourced from the culm series and drawn by the convective flow of connate waters into the thermal chimneys of the granite intrusions

on the arrangement of solid inclusions in fibrous veins and the role of the crack seal mechanism the arrangement of solid inclusions in veins was examined in three different rock types and tectonic settings common to all samples is a new type of sinusoidal inclusions associated with inclusion bands oriented parallel to the vein wall and inclusion trails at high angle to the vein wall crosscutting relationships of inclusion bands and the sinusoidal inclusions imply that inclusion bands do not reflect individual crack seal increments in our veins inclusion bands are not a sufficient criterion to infer the crack seal mechanism further evidence for solid inclusions not formed by the classic crack seal mechanism is given by inclusion bands in gypsum crystals grown in soft clay during incorporation solid inclusions can become folded or boudinaged depending on their orientation with respect to the opening direction the complex displacement field within the vein is recorded by curved crystal fibres which track the opening direction based on the arrangement of solid inclusions within the vein we suggest that our veins grew continuously during syntectonic vein formation solid inclusions formed by a fluctuation of adhesion at the vein wall interface rather than by crack sealing and vein growth was at least partly driven by the force of crystallisation q 2005 elsevier ltd all rights reserved

magnetic enhancement is linked to and precedes hematite formation in aerobic soil 1 soil formation usually increases magnetic susceptibility most often by increasing the concentrations of magnetite and maghemite which are two ferrimagnetic iron oxides here we provide evidence that magnetic enhancement in aerobic soils not affected by detrital magnetic inputs or thermal transformation of other iron oxides is mostly due to the formation of maghemite which is later transformed into hematite the iron oxide that gives red color to soil we show that the maghemite hematite ratio is influenced by the particular environment and the degree of soil development so it constitutes an effective tool for paleoenvironmental and planetary studies

biomarkers in a lower jurassic concretion from dorset uk textural petrographic and stable isotopic evidence suggest that a zoned concretion birchi bed lower lias west dorset uk formed under very shallow burial with carbonate cement passively filling the pore spaces the calcitic core of the concretion formed initially whilst the intermediate and outer edge cements which are dominated by a dolomite and a calcite respectively precipitated successively mixtures of calcite and dolomite occur in the intermediate zone and outer rim suggesting initial incomplete cementation and later back filling the enveloping fibrous calcite vein beef formed later by displacive crystallization the concretionary carbonates preferentially preserve labile organic compounds i e unsaturated fatty acids not found in the surrounding shales fatty acid distributions in the concretion are distinct and informative the presence of 10 methylhexadecanoic acid provides direct evidence for sulphate reducing bacteria in the calcitic core whereas the abundance of unsaturated fatty acids in the concretion as a whole is attributed to localized bacterial production and its early formation the core probably formed in the sulphate reduction zone whereas dolomite in the intermediate zone was derived largely from methanogenesis and iron reduction although direct biomarker evidence for methanogenesis was not found the outer rim and the fibrous calcite vein probably resulted from late bacterial processes probably including renewed sulphate reduction the complex biogeochemistry of the sedimentary environment is reflected by the concentrations and distributions of biomarkers and by the detailed petrography nevertheless carbonate concretions can provide a snapshot of early diagenesis in ancient mudstones

a 48 m y history of fracture opening temperature and fluid pressure cretaceous travis peak formation east texas basin quartz cement bridges across opening mode fractures of the cretaceous travis peak formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low porosity sandstone incremental crack seal fracture opening is inferred based on the banded structure of quartz cement bridges consisting of up to 700 cement bands averaging 5 m in thickness as observed with scanning electron microscope cathodoluminescence crack seal layers contain assemblages of aqueous two phase fluid inclusions based on fluid inclusion microthermometry and raman microprobe analyses we determined that these inclusions contain methane saturated brine trapped over temperatures ranging from 130 c to 154 c using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening in combination with published burial evolution models this reconstruction indicates that fracture opening started at ca 48 ma and above hydrostatic pore fluid pressure conditions and continued under steadily declining pore fluid pressure during partial exhumation until present times individual fractures opened over an 48 m y time span at rates of 16 23 m m y these rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings

hydrocarbon bearing fluid inclusions in calcite filled horizontal fractures from mature posidonia shale hils syncline nw germany abstract hydrocarbon bearing fluid inclusions in calcite filled horizontal fractures from mature posidonia shale 0 88 r o were investigated by microscopic microthermometric and chemical methods the fluid inclusions predominantly contain both a liquid hydrocarbon phase which shows bright uv fluorescence colours yellow brownish light blue and a non fluorescent hydrocarbon gas phase the different uv fluorescence colours probably reflect the different gross chemical compositions of the trapped liquid hydrocarbons the saturated hydrocarbon fraction of the fluid inclusions mostly secondary inclusions predominantly consists of low molecular weight n alkanes c 9 c 12 while c 12 c 22 n alkanes are more abundant in the adjacent rock matrix the molecular composition of the fluid inclusions appears to reflect a mobile hydrocarbon phase which was entrapped within the calcitic fill of the horizontal fractures pressure corrected trapping temperatures were estimated for primary fluid inclusions 123 162 c this temperature range is in agreement with the results from numerical simulation studies which indicate a maximum temperature of 148 c coinciding with the period of fracture formation calcite filling and entrapment of primary fluid inclusions

magnetic minerals in calcic luvisols chromic developed in a warm mediterranean region of spain origin and paleoenvironmental significance abstract soil formation under aerobic conditions usually enhances magnetic signals mainly through neoformation of sub micron magnetite and or maghemite in order to better understand the nature and magnitude of the enhancement in this work we characterized the iron fe oxides and magnetic properties of two calcic luvisols chromic developed on calcareous orthoquartzites in a spanish region with a warm mediterranean climate weathering of the fe bearing minerals in the parent material mostly fe rich smectite had resulted in the neoformation of hematite and maghemite rather than goethite thus causing a substantial increase in magnetic susceptibility saturation magnetization m s and other magnetic signals the absolute frequency dependent magnetic susceptibility fd which is a typical proxy for the concentration of sub micron superparamagnetic pedogenic maghemite was strongly correlated with the concentration of hematite hm the estimated percent frequency dependent magnetic susceptibility fd was 15 7 this and various other magnetic indicators indicate that the grain size distribution gsd of the pedogenic maghemite is narrower around the superparamagnetic single domain threshold than is that of maghemite in soils and paleosols developed under other temperate climates e g the chinese loesss plateau where fd fd ratio fd the hematite goethite ratio and several indicators of the degree of weathering provides a better tool for paleoclimatic reconstructions than does the current use of alone

changing color of chinese loess geochemical constraint and paleoclimatic significance abstract two loess paleosol sequences on the central chinese loess plateau were investigated to understand spatial and temporal variations in the soil color e g lightness and redness and factors that control those variations color difference between the original samples and pretreated samples suggests that loess lightness is influenced by the pedogenic matter e g iron oxides organic matter and calcium carbonate to varying degrees depending on the concentrations iron oxides and organic matter darken the loess lightness whereas carbonate lightens the loess lightness by contrast the redness is dominantly controlled by the types and concentrations of iron oxides variations in magnetic susceptibility and redness are associated with different magnetic minerals that formed mainly during post depositional processes and therefore both proxies can be employed to reconstruct the history and variability of the east asian summer monsoon since the sensitivity of these two proxies to the summer monsoon variation is different we generate a stacked summer monsoon index by combining these two records and interpret the stacked index as most indicative of precipitation variability the fidelity of this new index is supported by an independent ratio estimated from the first derivative values of the color reflectance spectra our loess based proxies provide a new understanding of the east asian summer monsoon variability as a two phase strengthening of summer monsoon intensity during the penultimate deglaciation

hydrothermal alteration and ree th mineralization at the rodeo de los molles deposit las chacras batholith central argentina ree rare earth element and th mineralization at the rodeo de los molles deposit occurs within an elliptical body of hydrothermally altered rocks fenite located in a biotite monzogranite of the las chacras batholith ore assemblages are found as isolated patches of intergrown britholite allanite apatite bastnaesite fluorite sphene quartz and aegirine augite as well as nodules of uranothorite and late stage veins of calcite fluorite and bastnaesite composition volume computations suggest that the fenite was produced by alteration of the biotite monzogranite by addition of k and na and loss of ca and sr petrographic evaluations indicate that microcline and plagioclase have been replaced by perthite and biotite was converted to aggregates of clinochlore anatase kaolinite and hematite relict biotite is characterized by lower fe fe mg and ti values with progressive alteration fluorine rich phlogopite is present in mineralized areas but textural evidence suggests that it was not produced via biotite alteration mass balance constraints also show that ca and mg in ore zones may result from redistribution rather than their being a result of external derivation the 18o values of quartz 8 6 11 1 and feldspar 7 8 10 6 suggest that feldspar continued to exchange oxygen isotopes with a fluid to lower temperatures than did quartz feldspars equilibrated with a fluid of 18o 8 at a fluid rock ratio less than 1 the 18o values of quartz and aegirine augite that crystallized during ree mineralization also suggest equilibration with a fluid of 18o 8 the d values of biotite 83 to 120 are relatively low for igneous rocks and are thought to have resulted from exsolution of a d enriched magmatic vapor the d values of both mineralized and barren fenites are consistent with equilibration with fluid of magmatic origin meteoric water was involved in the production of calcite and clinochlore alteration and late stage calcite fluorite bastnaesite veins the 13c values of calcite and bastnaesite 7 8 to 13 5 suggest that carbon was derived by leaching of carbon from igneous and or enclosing metamorphic rock types and that a majority of carbon ultimately was derived from sedimentary organic meterial

testing the basin centered gas accumulation model using fluid inclusion observations southern piceance basin colorado the upper cretaceous mesaverde group in the piceance basin colorado is considered a continuous basin centered gas accumulation in which gas charge of the low permeability sandstone occurs under high pore fluid pressure in response to gas generation high gas pressure favors formation of pervasive systems of opening mode fractures this view contrasts with thatofothermodelsoflow permeabilitygasreservoirsinwhich gas migrates by buoyant drive and accumulates in conventional traps with fractures an incidental attribute of these reservoirs we tested the aspects of the basin centered gas accumulation model as it applies to the piceance basin by determining the timing of fracturegrowth and associated temperature pressure and fluid composition conditions using microthermometry and raman microspectrometry of fluid inclusions trapped in fracture cement that formed during fracture growth trapping temperatures of methane saturated aqueous fluid inclusions record systematic temperature trends that increase from approximately 140 to 185 c and then decrease to approximately 158 c over time which indicates fracture growth during maximum burial conditions calculated pore fluid pressures for methanerich aqueous inclusions of 55 to 110 mpa 7977 15 954 psi indicate fracture growth under near lithostatic pressure conditions consistent with fracture growth during active gas maturation and charge lack of systematic pore fluid pressure trends

bedding parallel fibrous veins beef and cone in cone worldwide occurrence and possible significance in terms of fluid overpressure hydrocarbon generation and mineralization abstract bedding parallel fibrous veins are common worldwide in sedimentary basins especially within strata of low permeability the term beef refers to bedding parallel veins of fibrous minerals where the fibres are mutually parallel and have formed quasi vertically more complex on a smaller scale are cone in cone structures yet these are also common within bedding parallel veins for both beef and cone in cone we have compiled a worldwide catalogue 157 localities typically the veins consist mainly of white gangue minerals for example calcite gypsum or quartz but may also contain accessory minerals of economic interest for example bitumen sulphides emerald pitchblende or gold fluid inclusions may contain oil or gas calcite beef 110 localities is common in organic rich shale of marine carbonate origin especially of 1 cambrian ordovician 2 devonian carboniferous 3 early jurassic or 4 cretaceous to palaeogene ages gypsum beef 30 localities is common in evaporitic or lacustrine strata of continental origin especially of triassic or neogene ages quartz beef 17 localities is common within meta turbidite sequences especially of ordovician or proterozoic ages because these modal ages seem to reflect climatic controls we infer that the fibre forming mineral species have not travelled far vertically the same conclusion holds for accessory minerals typical temperatures of formation are 1 up to 60 c for gypsum beef 2 70 c to 120 c for calcite beef and 3 200 c to 350 c for quartz beef hydrocarbon bearing calcite beef may be a good indicator of a petroleum system in which oil or gas migrate together with aqueous solutions we argue that beef and cone in cone layers result from tensile fracturing and vertical dilation coeval with fibre growth possible causes are either 1 force of crystallization or 2 seepage forces due to fluid overpressure for layers that form at depths of several km fluid overpressure is the more likely cause

quantifying grain size distribution of pedogenic magnetic particles in chinese loess and its significance for pedogenesis quaternary glacial interglacial cycles have been imprinted on the chinese loess paleosol sequences through pedogenesis in order to accurately decode the paleoclimatic signals carried by these pedogenic particles it is essential to quantify the pedogenically produced magnetic particles in terms of mineralogy as well as grain size distribution gsd to date the gsd has not been accurately determined because of the dearth of available means for analyzing extremely fine grained nanometer scale pedogenic magnetic particles using low temperature techniques we systematically investigated the temperature dependency of fd defined as 1hz 10hz where 1hz and 10hz are ac magnetic susceptibility measured at 1 and 10 hz respectively from two characteristic loess profiles one located at the western chinese loess plateau and the other in the central plateau on the basis of neel theory for a shape anisotropy dominant grain and experimental analysis at low temperatures a quantitative gsd for pedogenic particles in chinese loess paleosols was constructed we found that the dominant magnetic grain size lies just above the superparamagnetic single domain threshold 20 25 nm and that the gsd is almost independent of the degree of pedogenesis this observation agrees well with other constraints from previous studies this new gsd model improves our understanding of the pedogenic processes in chinese loess enabling further explicit linkage of environmental magnetism to paleoclimate changes

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

quantitative estimates of palaeoprecipitation at xifeng in the loess plateau of china abstract using10be recent studies heller et al 1993 beer et al 1993 have tried to reconstruct palaeoclimatic records by separating the secondary pedogenic and primary aeolian detrital contributions to the magnetic susceptibility of chinese loess here we use a technique which monitors the temperature dependence of magnetic remanence to estimate the magnetisation and relative contribution of the superparamagnetic sp single domain sd and multidomain md ferrimagnetic components in the xifeng loess section on the assumption that the sp ferrimagnetic component is dominantly of pedogenic origin the magnetisation of this component has also been determined for samples taken from the topmost palaeosol s0 at six sites over the loess plateau and directly correlated with present day precipitation at these sites this correlation is then used to estimate quantitatively palaeoprecipitation for the topmost 16 m of the xifeng section which covers about the last 140 000 years the data indicate a considerably reduced precipitation during the glacial loess forming periods and a significantly higher level of precipitation than today during the last interglacial period

rainfall dependent transformations of iron oxides in a tropical saprolite transect of hainan island south china spectral and magnetic measurements 1 the iron oxide content of soils and sediments controlled by weathering and pedogenesis is generally considered a reasonable indicator of climate previous studies in temperate zones have established a positive correlation in aerobic soils between ferrimagnets and low to moderate rainfall the correlation seems to be reversed under extreme climates with high rainfall here we present a transect of saprolitic soils from hainan island south china with high rainfall 1440 2020 mm yr little temperature variation 23 24 c and extreme weathering along this transect we observed that both hematite concentration and magnetic susceptibility decrease with increasing rainfall whereas goethite concentration displays a large increase however there is no systematic trend in the total amount of iron oxides related to chemical weathering intensity along the transect goethite is the favored mineral phase of iron oxide with increasing rainfall and accumulates at the expense of hematite and maghemite through the dominance of rainfall driven processes these pedogenic processes coincide with the fundamentals of previous nonmonotonic models of hematite magnetic susceptibility and rainfall control of an inflection point this study also verifies a common genetic relationship between hematite and pedogenic ferrimagnets across a wide climate range a conceptual model considering both rainfall and temperature is proposed to help interpret the mechanism of ferrimagnet formation and changes in the rainfall inflection point

late miocene early pleistocene paleoclimate history of the chinese loess plateau revealed by remanence unmixing previous studies show that the quaternary loess sequence and the late miocene pliocene red clay sequence on the chinese loess plateau clp share similar magnetic properties despite their being deposited under different climate conditions to solve this paradox we apply a novel remanence unmixing technique and demonstrate that loess and red clay on the central clp contain a similar low coercivity pedogenic component but their high coercivity components differ dramatically reflecting different oxidation and temperature conditions we infer that temperatures on the chinese loess plateau cooled from the late miocene to the quaternary in a manner similar to sea surface temperature records for the same time interval this coherency between marine and terrestrial records argues for a co2 forcing on long term paleoclimatic variations key points backfield remanence data of chinese loess and red clay consist of two components loess and red clay have similar pedogenic but different detrital components the loess plateau experienced a cooling trend from the late miocene to the quaternary

fluid inclusion studies of the rodeo de los molles ree and th deposit las chacras batholith central argentina abstract the rodeo de los molles rare earth element ree and thorium deposit is located in granitic rocks of the las chacras piedras coloradas batholith in the southern block of the eastern pampean ranges central argentina mineralization occurs within an elongate 2 km 0 6 km body of alkalifeldspar granite alaskite localized along the northeastern edge of a composite batholith the surrounding lithology is predominantly a biotite monzogranite both the alaskite and localized areas of quartz alkalifeldspar syenite within the alaskite have been produced by hydrothermal alteration of a late crystallizing phase of the monzogranite ree minerals are primarily of the cerium group and include britholite and allanite both partially replaced by bastnaesite or thorbastnaesite these minerals occur as nodules with quartz fluorite aegirine augite sphene and fe ti oxides within aplitic to pegmatoidal quartz alkalifeldspar syenite uranothorite along with a second generation of fluorite and minor amounts of mn ba oxides occurs in the alaskite as nodules or within quartz lined miarolitic cavities but is not found with the ce mineralization studies of fluid inclusions contained in quartz and fluorite indicate a complex history of open system fluid migration and interaction with monzogranite host rocks fluids responsible for ree mineralization and quartz deposition along with initial alteration of the monzogranite to alaskite and quartz alkalifeldspar syenite were of relatively high temperature th of fluid inclusions in quartz 356 535 c and moderate salinity 15 25 eq wt nacl mixed co2 h2o fluids xco2 13 07 found as both primary and secondary inclusions within fluorite are representative of fluids involved in the replacement of britholite allanite by bastnaesite and sphene aegirine augite and plagioclase by calcite minimum pressures of mineral deposition estimated from h2o co2 nacl phase relations range from 1 to 2 kbars secondary aqueous fluid inclusions in quartz define a trend of low salinity high temperature to high salinity low temperature thought to be a result of hydration reactions occurring in alaskite and quartz alkali feldspar syenite the highest salinity fluids 35 37 eq wt nacl detected in the area are associated with the formation of uranothorite and late fluorite multiple periods of hydrothermal fluid introduction are consistent with recent geological data that indicate that the batholith is composed of several stock like bodies the location of the mineralized area near the top of the magma chamber the presence of numerous miarolitic cavities and the bulk composition of inclusion fluids na k ca suggest that the fluids responsible for ree and th mineralization were of magmatic origin

coupled substitutions involving rees and na and si in apatites in alkaline rocks from the ilimaussaq intrusion south greenland and the petrological implications ansrnlcr apatites from the ilimaussaq augite syenite the sodalite foyaite and a qtartz bearing peralkaline pegmatite have been analyzed using an electron microprobe for the major elements plus na si and selected rare earth elements rees in all three rock types the maximum reeto3 content in the apatite exceeds 16 0 wt0 0 apatite from the augite syenite and primary apatite from the sodalite foyaite are characterizedby a low na content that varies independently of the ree content the composition of these apatites approaches 25 molo o of the end member carreq sio4 3 oh f cl lessingite in response to the substitution ree3 sia ca2 ps ree and na enriched apatites from the quartzbearing peralkaline pegmatite are almost si free and rees and na are positively correlated the composition of these apatites approaches 25 molo o of the hypothetical endmember na5ree5 po4 6 oh f ci 2 in response to the substitution na ree3 2ca2 the compositional trend of late crystalllzing apatite from the sodalite foyaite reflects almost equal contributions of the two substitutions the relationship between these substitutions in a given apatite and its associated mineral assemblage indicates that the na component is only present in considerable amounts when apatite crystallizes in a strongly peralkaline environment

climatic thresholds for pedogenic iron oxides under aerobic conditions processes and their significance in paleoclimate reconstruction abstract iron oxides are widely distributed across the surface of the earth as a result of the aerobic weathering of primary fe bearing minerals pedogenic iron oxides which consist mainly of hematite hm goethite gt maghemite mgh are often concentrated synchronously in aerobic soils under low to moderate rainfall regimes magnetic susceptibility and redness which respectively reflect the content of mgh and hm in soils are considered reasonable pedogenic and climatic indicators in soil taxonomy and paleorainfall reconstruction however under high rainfall regimes the grain growth of mgh and transformation to hm combined with the prior formation of gt under conditions of high relative humidity rh can result in magnetic reduction and dramatic yellowing of soils and sediments which explains the existence of rainfall thresholds for mgh and hm at a large scale even before the pedogenic environment turns anaerobic in order to capture the rainfall thresholds for mgh and hm occurring under aerobic conditions we explored a tropical transect across a granitic region where the soil color turned from red to yellow under a wide rainfall range of 900 2200 mm yr and a corresponding mean annual rh range of 77 85 we observed a lower rainfall threshold of 1500 mm yr and a corresponding rh 80 for mgh and hm along this transect as well as a higher rainfall threshold of 1700 mm yr and a corresponding rh of 81 for gt and total pedogenic iron oxides citrate bicarbonate dithionite extractable fe fe d cross referencing with comparable studies in temperate and subtropical regions we noted that the rainfall or rh thresholds for fe d and hm or mgh likewise increase with temperature moreover the different thresholds for total and individual iron oxide phase indicates that a negative correlation between chemical weathering intensity and redness or in sediment sequences can occur under the prevalent climate regime just between their thresholds finally we developed an integrated model to interpret the sequential formation of rainfall thresholds for different pedogenic iron oxides in soils and sediments with conditions from aerobic to anaerobic

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

beef veins and thrust detachments in early cretaceous source rocks foothills of magallanes austral basin southern chile and argentina structural evidence for fluid overpressure during hydrocarbon maturation abstract we describe 1 bedding parallel veins of fibrous calcite beef and 2 thrust detachments which we believe provide good evidence for fluid overpressure in source rocks for petroleum our examples are from the surface or subsurface of the magallanes austral basin which lies at the southern tip of south america there the best source rocks for petroleum are of early cretaceous age in the central parts of the basin these source rocks have become overmature but at the eastern edge onshore and offshore they are today either immature or in the oil window in tierra del fuego the foothills of the andes consist mainly of sedimentary rocks which have undergone thin skinned thrusting in the vicuna area chile early cretaceous source rocks have reached the surface above thrust detachments which are visible on seismic data and well data at the surface we have found calcite beef containing hydrocarbons solid and or fluid in the rio jackson and vicuna formations which have reached the wet gas window in the rio gallegos area argentina the source rocks have not reached the surface but seismic and well data provide good evidence for thin skinned thrusting above flat lying detachments in early cretaceous source rock where it is in the early oil window in contrast there is little or no deformation where the source rock is still immature thus the deformation front coincides with the maturity front next to the central parts of the basin where the source rocks have reached the surface within the andes proper they have undergone low grade metamorphism within these source rocks we have found beef veins but of quartz not calcite to the east within the foreland basin seismic and well data provide evidence for a few compressional structures including thin skinned detachments in the deeply buried source rock finally in the northern part of the basin santa cruz province argentina where it is shallower the source rocks have reached the surface in the foothills above a series of back thrusts at lago san martin the source rocks have reached the oil window and they again contain calcite beef in conclusion where we have examined early cretaceous source rocks at the surface they contain either calcite beef if they have reached the late oil window or wet gas window or quartz beef if they are overmature independent evidence for overpressure in the form of source rock detachments comes from subsurface data especially at the southern end of the basin where the source rocks are not overmature and deformation is relatively intense thus we argue that hydrocarbon generation has led to overpressure as a result of chemical compaction and load transfer or volume changes or both

crystallization pressure versus crack seal as the mechanism for banded veins banded fibrous veins are often pointed to as evidence for episodic crack opening driven by oscillations in fluid pressure or bulk strain advances in understanding the geochemistry of precipitation data on veins and experiments suggest that pressure due to growing crystals may be an alternate explanation for many observations on these types of veins we propose that some veins originate at sites of precipitation and then propagate due to the pressure exerted by the crystal growth as materials precipitate the vein walls are pushed apart the resulting veins have shapes typical for mode i cracks except that mechanically crystallization pressure replaces the role of internal fluid pressure in their propagation a nonzero remote differential stress serves to align veins mechanical and geochemical considerations suggest that this process will be most important in fine grained rocks such as greenschist grade pelites where diffusion from sites of dissolution rather than advection is the dominant mass transport process veins may owe their orientation to tectonism but their initiation and growth are due to processes that supersaturate the pore fluid veins formed by this mechanism involve cracking to the extent that precipitation forces an original flaw to extend during precipitation cyclic quartz mica bands may indicate geochemical self organization at the vein wall driven by pressure solution enhanced supersaturation in the pore fluid and nonlinear precipitation kinetics at the vein wall

petroleum geochemistry of the kimmeridge clay of onshore southern and eastern england abstract the kimmeridge clay is considered a major oil source rock for the north sea hydrocarbon province the formation is also developed onshore in an organic rich mudstone facies this paper examines the possibility of onshore oil generation from the kimmeridge clay geochemically onshore basin margin sediments contain rich potential source horizons with mainly type l type ll oil prone kerogen but are immature some deeper cleveland basin sediments have reached marginal maturity burial history reconstruction suggests significant formation palaeoburial depths in central areas of the cleveland and wessex basins computed vitrinite isoreflectance contours show the wealden and isle of wight kimmeridge clay to be thermally mature basin modelling suggests an early palaeogene onset of oil generation in parts of the cleveland basin while maximum oil generation could have been reached by the formation base in the isle of wight area during the late cretaceous although basin subsidence ceased in the neogene in the weald and isle of wight where the formation is still deeply buried oil generation probably continued for some time during uplift thus significant quantities of oil could have been generated whether or not this oil is present today however would depend on the correct timing of suitable migration and trap structures

six million years of magnetic grain size records reveal that temperature and precipitation were decoupled on the chinese loess plateau during 4 5 2 6 ma abstract magnetic grain size variations have been used as sensitive paleoclimate proxies to investigate the evolution of the east asian summer monsoon but their relationship with temperature and precipitation is not entirely clear here we find that two magnetic grain size proxy records arm lf and arm sirm where arm lf and sirm are anhysteretic remanent magnetization susceptibility magnetic susceptibility measured at 470 hz and saturation isothermal remanent magnetization respectively of chinese loess and red clay sediments co vary during the last 6 ma except between 4 5 and 2 6 ma when these two records had opposite trends we attribute this disparate behavior to the different responses of arm lf and arm sirm to temperature and precipitation during 4 5 2 6 ma when temperature and precipitation on the chinese loess plateau were decoupled a comparison of the loess and red clay arm lf and arm sirm records with the global ice volume proxy records reveals that arm lf is more sensitive to temperature variations than arm sirm the results suggest that temperature on the chinese loess plateau had a cooling trend from 4 5 to 2 6 ma whereas rainfall tended to increase our studies demonstrate that joint analysis of loess arm lf and arm sirm records can reveal paleoclimatic information that cannot be revealed by a single parameter

the superparamagnetism of yucca mountain tuff yucca mountain tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range schlinger et al 1988 magnetic measurements on three samples comprising hysteresis loops at low and ambient temperatures acquisition and demagnetization of isothermal remanent magnetization thermal demagnetization of the saturation remanence and of a low temperature thermoremanence and frequency and temperature dependent susceptibilities allow evaluation of the magnetic properties in terms of neel s 1949 single domain theory precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results in contrast an attempt to derive mean particle volumes by fitting a langevin curve to the room temperature magnetization curves fails probably because the precondition for the langevin function kv kt 1 is not met it is only for the sample with the smallest grains and in weak fields 20 mt that a langevin fit provides a reasonable volume estimate there is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility thus verifying that neel s theory is sufficient for the magnetic description of single domain assemblages spanning the superparamagnetic stable single domain boundary however some deviations between modeled and measured susceptibilities exist and the physical causes may include size dependent anisotropy nonuniform magnetizations and also an uncertain preexponential time constant 0 while 0 10 11 s gives the best fit for the sample with the largest grains 0 10 9 s is more reasonable for the others thus 0 may indeed be size and temperature dependent as predicted by brown 1959 the commonly cited parameter fd frequency dependence of susceptibility reaches 30 at room temperature rt for one sample with a blocking temperatures just below rt while fd 0 at rt for a superparamagnetic sample with smaller grains these results thus exemplify that fd is not limited to 15 as a number of studies suggest and that fd 0 must not be taken to imply the absence of superparamagnetic grains

fibrous gypsum and fibrous anhydrite in veins fibrous calcium sulphate occurs in nature mainly as gypsum satinspar in extensional veins several mechanisms have been put forward to account for these veins and their fillings but a number of features remain unexplained or are not in accord with the interpretation s given this paper describes anhydrite as a fibrous vein precipitate which shows most features typical of satinspar subhorizontal attitude of the vein s vertical fibres a parting separating a lower from an upper seam and incorporated wall rock fragments new observations include the stacked nature of the fibres pressure shadows next to wall rock fragments and parabolic alignment of the latter these features suggest that the veins opened due to vertically oriented tensile stress the vein filling crystals grew centrifugally outwards from the initial plane of rupture which now forms the parting keeping pace with the dilation the mode of incorporation of wall rock fragments in the fibres requires repeated differential opening of the fissure thereby either wall rock fibre interface ruptured at a time and statistically the ruptures alternated so that trace element trends across the vein s are roughly symmetrical to the parting the source of the calcium and sulphate ions may have been adjacent evaporite beds but an external source is indicated where no evaporite beds are present in contrast to previous interpretations hydration of anhydrite to gypsum in the host rocks is not a prerequisite for the formation of satinspar veins the exact causes for the vertical dilation remain somewhat speculative and may have been different in various locations the vertically oriented tensile stress may have resulted from lateral tectonic compression or hydraulic overpressure or a combination of the two

reply to comment by a e williams jones and s a wood on fluid inclusion studies of the rodeo de los molles ree and th deposit las chacras batholith central argentina this paper is in response to a rebuttal made against these authors original research on hydrothermal sytem mineralization of rare earth elements ree the original text was regarding physiochemical characteristics of bastnaesite and fluorite which are the principal ree minerals in hydrothermal environments this paper reviews all the procedures regarding the temperature and pressure of formation calculations reaching in the authors previous research

late holocene climate reconstructions for the russian steppe based on mineralogical and magnetic properties of buried palaeosols abstract insights into past climate changes and corresponding evolution of soils and the environment can be gained by multidisciplinary studies of palaeosols here we focus on palaeosols buried beneath archaeological monuments specifically funerary mounds kurgans in the russian steppe the kurgans were constructed and each of the palaeosols buried over a range of different timesteps from the mid holocene to 600 years before present yr bp integrated magnetic mineralogical and pedological data were used to obtain estimates of past climate especially precipitation changes through both time and space a soil magnetism based climofunction derived previously from modern steppe soils and modern climate was applied to each set of palaeosols to obtain quantitative reconstructions of annual precipitation for the time at which the soils were buried independent soil property data clay mineralogy salt content iron mineralogy from mossbauer analysis and optical and electron microscopy were also obtained in order to test and substantiate the magnetic inferences the data obtained indicate that the climate of the lower volga steppe area has varied from the mid holocene onwards precipitation minima occurred at 5000 3800 and 1600 yr bp with intervals of enhanced precipitation at 1900 yr bp and 600 yr bp these rainfall variations appear to occur synchronously with changes in middle eastern precipitation and lake levels suggesting they are controlled by index changes in the north atlantic oscillation

fibrous calcite veins overpressures and primary oil migration the fibrous calcite veins found parallel to the bedding of certain shales may have been formed during burial as a consequence of overpressuring their presence could even be indicative of former overpressures this suggestion if it can be upheld carries implications for theories of primary oil migration

combined use of confocal laser scanning microscopyand pvt simulation for estimating the composition andphysical properties of petroleum in fluid inclusions we present a method to determine the composition and pvt properties of petroleum inindividual petroleum fluid inclusions confocal laser scanning microscopy is used to generatethree dimensional images of single petroleum inclusions because liquid petroleum fluorescesunder the laser the images readily distinguish the liquid and vapour within the inclusion and canbe used to determine the inclusions volumetric liquid vapour ratio using pvt modellingsoftware the liquid vapour ratio is used along with the homogenisation temperature to determinethe bulk composition phase envelope isochore and a range of physical properties of the includedpetroleum this is done using an iterative series of pvt calculations which match twoparameters 1 the molar volume of the petroleum at room and homogenisation temperatures 2 the liquid vapour ratio of the inclusion at room temperature key uncertainties in the methodare explored including the accuracy with which the liquid vapour ratio can be determined thecomposition of the titrant gas used in the iterative procedure and the composition of thepetroleum chosen to model the physical properties of the included petroleum data from coevalinclusions suggest that the saturation pressure gas oil ratio viscosity molar volume densityand surface tension of included petroleum are determined with a precision of a few percent confirmation of the accuracy of the method awaits tests using inclusions grown in the laboratoryunder carefully controlled pvtx conditions however it is likely that the physical properties ofincluded petroleums are more accurately modelled by fluids which are genetically related tothem if proven to be accurate the method will provide a routine method for determiningpalaeopressure in petroleum systems finally we present data from a central north sea examplewhich are geologically realistic and which for the first time record the evolution of fluid pressureand petroleum composition in a petroleum reservoir

diagenesis of the kimmeridge clay formation onshore uk study of sedimentology carbonate geochemistry and clay mineralogy of four laterally continuous beds from the outcrop of the kimmeridge clay formation indicates strong lateral variations in the depositional and early diagenetic processes reflecting the basin and swell topography of the depositional basin the mudstones from the swell environment with low sedimentation and subsidence rates are shelly and carbonate rich with thin poorly developed oil shales they were deposited under dysaerobic conditions with consequent poor organic matter preservation early diagenetic processes were limited to the sulphate reduction and methanogenesis zones with the formation of discrete horizons of concretions the shallow burial depths attained are reflected by the immaturity of the organic matter in the mudstones although ordered illite smectites are present sediments from other locations comprise black organic rich mudstones the thick beds and good organic matter preservation indicating deposition under high sedimentation and subsidence rates with anoxic bottom waters in a basinal environment these indurated mudstones contain a few shell fragments and much pyrite indicating intense reaction in the sulphate reduction zone carbonates were precipitated in the methanogenesis and decarboxylation zones as ferroan calcites and dolomites the much greater burial depths are indicated by the higher maturity of the organic matter and by the ordered nature of the illite smectites and the loss of kaolinite

new type of kinematic indicator in bed parallel veins late jurassic early cretaceous vaca muerta formation argentina e w shortening during late cretaceous vein opening abstract in the late jurassic early cretaceous vaca muerta formation previously unrecognized yet abundant structures constituting a new category of kinematic indicator occur within bed parallel fibrous calcite veins bpvs in shale domal shapes result from localized shortening and thickening of bpvs and the intercalation of centimeter thick host rock shale inclusions within fibrous calcite beef forming thrust fault bounded pop up structures ellipsoidal and rounded structures show consistent orientations lineaments of interlayered shale and fibrous calcite and local centimeter scale offset thrust faults that at least in some cases cut across the median line of the bpv and indicate e w shortening continuity of crystal fibers shows the domal structures are contemporaneous with bpv formation and help establish timing of fibrous vein growth in the late cretaceous when shortening directions were oriented e w differences in the number of opening stages and the deformational style of the different bpvs indicate they may have opened at different times the new domal kinematic indicators described in this study are small enough to be captured in core when present in the subsurface domal structures can be used to either infer paleostress orientation during the formation of bpvs or to orient core in cases where the paleostress is independently known

individual characterization of petroleum fluid inclusions composition and p t trapping conditions by microthermometry and confocal laser scanning microscopy inferences from applied thermodynamics of oils abstract this paper proposes a new method to characterize individual oil bearing fluid inclusions it uses both the homogenisation temperatures measured by microthermometry and the degree of gas bubble filling measured by confocal laser scanning microscopy in conjunction with thermodynamic modelling for describing liquid gas phase transitions and the volumetric behaviour of hydrocarbon mixtures it is associated with a two parameter compositional model that describes the wide range of compositions of petroleums we show that this method can give 1 useful estimations of the compositions and pressure temperature entrapment conditions of oils in fluid inclusions and 2 insights into the various processes that have affected these fluids either before entrapment liquid gas unmixing gas leaching mixing etc or after leakage etc

fluid evolution in fracturing black shales appalachian basin opening mode veins in cores drilled from the mudrocks overlying and underlying the major silurian salt decollement in the appalachian plateau tioga and lawrence counties pennsylvania have mineralogic and isotopic compositions generally matching those of their host mudrocks suggesting opening and filling amid little cross stratal fluid motion calcite and most trace minerals probably entered the veins via dissolution reprecipitation from nearby host rock consistent with this interpretation are the observations that 1 trace minerals within the veins including quartz pyrite and dolomite are invariably also present within the layers hosting the veins with vein cement minerals generally reflecting the abundance and solubility of minerals in the host rock and 2 carbon and oxygen isotopic compositions of vein filling calcite are similar to those of calcite within the host rock with vein filling 18o slightly depleted and 13c slightly enriched modeling the fluid isotopic evolution assuming vein opening and filling amid immobile connate formation water accounts for these minor but systematic differences which are attributable to increasing temperature and hydrocarbon maturation an exception to the above trend is barite which despite its low solubility is systematically enriched in veins with respect to the host rock it is unclear whether barite precipitation resulted from the influx of external fluids perhaps deriving from silurian salt or from barium mobilized at depth from local clays or organic material

fluid inclusions in sedimentary and diagenetic systems abstract some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals important fluids in the sedimentary realm include atmospheric gases fresh water of meteoric origin lake water seawater mixed water evaporated water formation waters deep in basins oil and natural gas preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global change research is essential for improving understanding of diagenetic systems and provides useful information in petroleum geology applications of fluid inclusions to sedimentary systems are not without their complexities some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration which results in stretching or leakage and refilling of some fluid inclusions similarly overheating in the laboratory can also cause reequilibration of fluid inclusions so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment fluid inclusion assemblages fia consistency in homogenization temperatures within a fluid inclusion assemblage consisting of variably sized and shaped inclusions is the hallmark of a data set that has not been altered through thermal reequilibration in contrast fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration if a fluid inclusion assemblage has not been altered by thermal reequilibration its fluid inclusions may be useful as geothermometers for low and high temperature systems or useful as geobarometers applicable throughout the sedimentary realm if a fluid inclusion assemblage has been altered partially by thermal reequilibration techniques for distinguishing between altered and unaltered fluid inclusions may be applied in studies of global change fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments fluid inclusions also preserve microsamples of ancient seawater and atmosphere the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans in petroleum geology fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons they can delineate the evolution of the chemistry of hydrocarbons and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems in studies of diagenesis fluid inclusions can be the most definitive record most diagenetic systems are closely linked to temperature and salinity of the fluid thus fluid inclusions are sensitive indicators of diagenetic environments

grain growth and transformation of pedogenic magnetic particles in red ferralsols soil magnetism depends on the mineral composition and grain size distribution of magnetic particles the accumulation of magnetic particles with different grain sizes has been found in the laboratory to precede the formation of hematite hm which competes with goethite gt in natural systems to explore changes in the correlation between magnetic particles and hm during pedogenesis we focus on three representative red ferrasol profiles with comparable total concentrations of iron oxides but different hm hm gt ratios from 0 14 to 0 74 superparamagnetic particles and single domain particles commonly become enriched as the concentration of hm increases in the profiles however the accumulation rates of the particles decrease with hm hm gt along the profiles we attributed these changes to the simultaneous grain growth of pedogenic magnetic particles and their partial transformation into hm once a large amount of pedogenic magnetic particles have accumulated in hematitic soils

mineral magnetic properties of a weathering sequence of soils derived from basalt in eastern china abstract the magnetic properties and magnetic mineralogy of a weathering sequence of soils developed on basalt parent material from eastern china were studied by rock magnetism x ray diffraction and soil chemical analyses to establish the connection between mineral magnetic properties and pedogenic development in a subtropical region the magnetic susceptibility of soils formed on basalt varied greatly and did not increase with the degree of pedogenic development the frequency dependent susceptibility fd values of soils ranged from 1 0 to 11 1 and increased with the pedogenic development highly significant linear relationship was found between the frequency dependent susceptibility and the fed content r2 0 683 and fed fet ratio r2 0 780 in soils indicating that pedogenic sp ferrimagnetic grains were associated with enrichment of the secondary iron oxide minerals in the weathering process of soil rock magnetism analysis showed that the major magnetic carriers in the weakly weathered soil profiles are magnetite and or maghemite and the highly developed soil profiles are generally enriched in magnetite maghemite grains of pedogenic origin and the magnetically hard haematite indicating that the magnetic component was transformed from a ferrimagnetic phase magnetite to antiferromagnetic phase hematite during pedogenic development results indicated that some of the magnetic parameters of soils in this case fd can be useful for pedogenic comparisons and age correlations in the weathering sequence of soil it is thus suggested that multiparameter rock magnetic investigations represent a more powerful approach for pedogenesis

early and late lithification of aragonitic bivalve beds in the purbeck formation upper jurassic lower cretaceous of southern england abstract beds of euryhaline bivalves alternating with shales constitute much of the middle purbeck formation they originated on tidal flats at the western margin of an extensive brackish lagoon when these shell beds are thin and enclosed in shale they are often still preserved as aragonite and are associated with beef fibrous calcite formed during compaction in most cases however the shell debris has been converted by diagenesis into calcitic biosparrudite limestones a compacted type has been lithified at a late stage after deep burial in this pyrite is abundant and most of the shell aragonite has been replaced neomorphically by ferroan pseudopleochroic calcite a contrasting uncompacted type of biosparrudite is characterised by bivalve fragments with micrite envelopes shells and former pores are occupied by non ferroan sparry calcite cement and there is little pyrite these limestones frequently contain dinosaur footprints and originated in supratidal environments where they were cemented early mainly in meteoric water once lithified they were unaffected by compaction this uncompacted type indicates phases of mild uplift or halts in subsidence these shell bed lithologies and also intermediate types described here will probably be recognised in other lagoonal formations

climatic interpretation of the luochuan and lingtai loess sections china based on changing iron oxide mineralogy and magnetic susceptibility abstract hematite and goethite estimated from diffuse reflectance spectra and magnetic susceptibility ms have been determined from both the luochuan and lingtai loess sections of the loess plateau of central china changing concentrations of these iron oxides and ms were used to determine paleoclimatic change based on a model that relates pedogenic production of hematite and magnetite to precipitation we propose two precipitation phases on the loess plateau during the last 2 6 ma from the wucheng red clay boundary up to paleosol s5 mis 13 precipitation on the loess plateau was moderate probably not frequently exceeding 450 mm yr and highly seasonal from s5 to the present precipitation during interglacial times has gradually increased to the present levels of 600 650 mm yr for the localities examined throughout the pleistocene spanning both glacial and interglacial conditions hematite to goethite ratios display a consistent decrease this decrease suggests that precipitation on the loess plateau has been getting less seasonal and or has been occurring during times when temperature was lower in addition to these long term changes it also appears that iron oxide records from the loess plateau exhibit 400 500 ka cycles from the wucheng red clay boundary to s5 these 400 500 ka cycles may be analogous to similar length cycles recorded in 13 c marine records in the south china sea and may be linked through weathering and silica production as a result of the asian monsoon

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

geopressuring mechanism of organic matter cracking numerical modeling since the 1950s hydrocarbon generation has been considered as one of the most important abnormal pressuring mechanisms hydrocarbon generation effects on pore pressure evolution 1 it increases the pore fluid volume through the conversion of solid kerogen into liquid or gaseous hydrocarbon and 2 it decreases the sediment permeability because hydrocarbons remain an immiscible phase in pore water in this paper the two products of organic matter decomposition liquid hydrocarbon oil and gaseous hydrocarbon methane are assumed to occur in sequence as a supplementary phase in pores as a basis for evaluating the effect of this pressuring mechanism a simplified three stage first order kinetic reaction model is applied to simulate the generation of oil and gas we u e a numerical basin model with a two phase hydrodynamic formulation to calculate the effect of organic maturation on geopressure evolution oil generation does not play an important role except when the organic matter content of rocks is relatively large 5 however the generation of gas has a large influence the results show that beneath a certain depth oil to gas cracking strongly influences overpressuring the effect of this mechanism becomes more important with increasing organic matter content of rocks several environmental conditions such as kerogen type temperature gradient and lithological characteristics influence to various degrees the effect of organic matter maturation on overpressure development

origin of overpressures in shales constraints from basin modeling techniques for detection evaluation and prediction of pore pressures in low permeability rocks and equations for fluid pressure computations in most integrated basin modeling software are based on relationships between porosity and effective stress in shales however recent data show that overpressured shales in the north sea do not exhibit higher porosities than the normally pressured shales of the same formation at similar depths to further evaluate the existence of porosity vs effective stress relationships in shales fluid flow simulations and porosity modeling in a typical high pressure and high temperature well in the north sea were undertaken the parameters in the permeability and porosity equations were adjusted until a satisfactory fit was achieved between the observed and modeled porosity and fluid pressure at present however the modeled porosity and pore pressure vs depth history of the sediments deviated significantly from known porosity and pore pressure vs depth relationships that have been observed in north sea shales and elsewhere today because the results from basin modeling based on porosity stress relationships were unacceptable irrespective of parameter choices and the well data from the north sea show no signs of elevated porosities in the overpressured shales it is inferred that effective stress driven compaction alone has not generated the hard overpressures observed in deeply buried north sea shales these conclusions are suggested to be generally applicable to shales with low porosities and hard overpressures worldwide both because of the physics involved and because similar results can be extracted from published modeling in the niger delta

diagenesis in mesozoic carbonate rocks in the north pyrenees france from mineralogy and fluid inclusion analysis example of rousse reservoir and caprock abstract fluid rock interaction is of key interest for deciphering the geodynamic history of complex geological environments in this paper we describe the mineralogy and petrography of the rousse reservoir the upper jurassic dolomie de mano formation as well as the reconstruction of its thermal and pressure history we also investigate the base of the reservoir caprock the campanian breccia a satellite of the giant gas field of lacq in the south west of france rousse was a gas producer until it moves to a co2 storage site in 2010 rousse is located within a deep isolated faulted carbonate jurassic horst overlain by a 4500 m thick overburden that is composed of a series of turbiditic flysch deposits of upper cretaceous cenomanian to tertiary eocene age cores were sampled from two wells rousse 1 and 2 and were analyzed by icp ms xrd sem epma tem cathodoluminescence and fluid inclusion techniques pressure temperature p t conditions were derived by modeling data from fluid inclusions trapped in the fracture minerals of the dolomie de mano formation and the campanian breccia in the dolomie de mano diagenesis was dominated by three dolomite episodes followed by a calcite stage in fractures the second and third dolomitizations were associated with fracturing episodes these dolomites precipitated from low saline

britholite monazite ree carbonates and calcite products of hydrothermal alteration of allanite and apatite in a type granite from stupne western carpathians slovakia abstract an uncommon assemblage of primary and secondary accessory ree minerals was identified in a permian a type granite clast in polymict conglomerates intercalated in the cretaceous flysch sequence of the pieniny klippen belt western carpathians northwest slovakia a detailed electron microprobe study of the granite reveals extensive subsolidus alteration of primary magmatic allanite ce to ferriallanite ce and fluorapatite the y ce rich fluorapatite was replaced by the dissolution reprecipitation process to the britholite group mineral members fluorbritholite y britholite y fluorcalciobritholite and its hydroxyl dominant analogue calciobritholite britholite y contains up to 5 2 wt tho2 0 15 apfu th the highest th content yet reported in naturally occurring y dominant britholites moreover the alteration of ferri allanite ce resulted to complex pseudomorphs and overgrowths including mainly ree carbonate phases synchysite ce to its hydroxyl dominant analogue hydroxylsynchysite ce bastnasite ce and calcite rarely monazite ce epidote clinochlore titanite tio2 phase and pseudorutile in some cases secondary carbonate minerals mainly synchysite and calcite replaced a substantial part of former allanite crystals moreover primary magmatic biotite annite was partly transformed to acicular stilpnomelane textural and compositional data indicate extensive replacement and breakdown of the primary magmatic allanite and apatite by aqueous fluids rich in fluorine and carbon liberated during a younger post magmatic low temperature hydrothermal metamorphic overprint of the granite

evidence of crystallization in residual cl f rich agpaitic trachyphonolitic magmas and primitive mg rich basalt trachyphonolite interaction in the lava domes of the phlegrean fields italy the lava domes in the northwestern cuma northern punta marmolite and central accademia parts of the phlegrean fields are the subject of this study the cuma and punta marmolite trachyphonolitic lava domes are among the oldest phlegrean products cropping out the cuma rocks have an agpaitic groundmass with early alkali feldspar fe rich clinopyroxene fe edenite and sodalite and late rosenbuschite fluorite baddeleyite pyrochlore britholite monazite aegirine often zr rich and exceptionally fe mn rich olivine the bulk rock compositions at cuma have some of the highest concentrations of zn mn zr nb th u and lanthanides among the phlegrean fields rocks and some of the lowest mgo p 2 o 5 sr eu and ba the punta marmolite dome is chemically less evolved and lacks characteristic agpaitic minerals but features alkali feldspar sodalite nepheline and relatively na poor fe rich hedenbergite with rare ca rich plagioclase xenocryst cores the accademia dome belonging to the recent activity is latitic to trachytic in composition has highly forsteritic olivine with chromiferous spinel inclusions calcic plagioclase and mg rich diopside phlogopite xenocrysts in an evolved host rock with phenocrysts and microlites of alkali feldspar fe rich clinopyroxene fe rich amphibole magnetite fe rich olivine and accessory baddeleyite zirconolite and fluorite there is clear evidence of open system magma crystallization in the form of interaction between a crystallizing primitive shoshonitic basalt in a reservoir already filled by rather evolved trachytic magma the magmatic evolution towards the evolved compositions is dominated by crystallization of more and more na rich alkali feldspar in a cl f rich and relatively h 2 o poor environment input of mafic magma is evident in many trachytic eruptions of the phlegrean fields and even in the products of the campanian ignimbrite but eruptions having mineral assemblages rich in xenocryst phases as well as eruptions virtually free of mafic magma input are also frequently observed throughout the history this suggests a variable pattern of open and closed system crystallization which may or may not be linked to explosive activity and that can be caused by intermittent supply of basaltic magma from depth

petrogenesis of the postcollisional middle devonian monzonitic to granitic magmatism of the sierra de san luis argentina abstract middle devonian granitoids intruded the eastern sierras pampeanas basement ca 600 km east of the inferred proto pacific margin of gondwana along which a ca 390 ma collisional event developed in the sierra de san luis voluminous middle devonian 393 382 ma batholiths are composed of i to a type hybrid monzonite and granite suites shoshonite and subordinated high k series stocks synplutonic dikes and enclaves make up the monzonite suite rocks are metaluminous alkali calcic magnesian porphyritic or equigranular monzonite quartz monzonite monzodiorite and scarce monzogabbro high k and subordinated shoshonite series metaluminous to mildly peraluminous magnesian alkali calcic to calc alkalic porphyritic or equigranular quartz monzonite granodiorite monzogranite and equigranular leucomonzogranites make up the granite suite plutons and batholiths only a small group of highly evolved granites are ferroan sio 2 46 62 cr ni v sc lile lree th zr and variable sr y la yb n and tb yb n smooth eu eu moderate na 2 o ca 3 5 and troughs at nb and ta for monzonite suite rocks suggest an subduction related enriched lithospheric mantle source sm nd data t dm 0 98 1 08 ga end 380 ma 0 66 1 47 and 87 sr 86 sr i 0 703520 0 704203 are compatible with an enriched mantle source the metaluminous porphyritic quartz monzonite monzogranite and the mildly peraluminous equigranular biotite monzogranites of the granite suite are characterized by relatively moderate al 2 o 3 cao and 87 sr 86 sr i high lile cr variable sr y la yb n and eu eu and low rb sr dm 1 20 1 28 ga end 380ma 3 02 to 3 3 87 sr 86 sr i 0 706578 0 707027 and the biotite monzogranites t dm 1 31 ga end 380ma 3 3 87 sr 86 sr i 0 707782 would share a common source the equigranular alkali calcic leucomonzogranites are characterized by rb sr 1 5 asi 1 05 1 18 and ga al 2 6 3 9 end 380 ma 3 74 to 3 95 and 87 sr 86 sr i 0 710743 0 712955 which would point to metasedimentary or felsic igneous crustal sources nevertheless their t dm 1 36 1 38 ga is considerably younger than the mean 1 8 1 6 ga eastern sierras pampeanas crustal residence age and less radiogenic middle devonian magmatism would record an episode of crustal growth by enriched mantle derived magma input and variable degrees of partial melting of a lower crustal source at the waning stages of the achalian orogeny

what do the hirm and s ratio really measure in environmental magnetism the hard isothermal remanent magnetization hirm and the s ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations respectively of antiferromagnetic minerals hematite and goethite in mineral mixtures we demonstrate that synthetic al substituted hematite and goethite exhibit a wide range of coercivities which significantly influences the hirm and s ratio these parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite goethite to circumvent this problem we propose a new parameter the l ratio which is the ratio of two remanences after alternating field af demagnetization of an irm imparted in a 1 t field with a peak af of 100 mt and 300 mt irmaf 300mt irmaf 100mt these parameters are easily measured using modern vibrating sample or alternating gradient magnetometers changes in hirm only reflect changes in the absolute concentration of hematite and or goethite if the l ratio is relatively constant conversely l ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite goethite corresponding hirm and s ratio variations should be interpreted with caution in such cases the l ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols for example the hirm is equivalent to irmaf 300mt likewise 0 5 sirm irm 100mt where irm 100mt represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back field of 100 mt is equivalent to irmaf 100mt the hirm 0 5 sirm irm 100mt ratio is therefore a suitable substitute for the l ratio when measurements are made with a long core magnetometer the newly proposed l ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism the hirm and s ratio

consistent grain size distribution of pedogenic maghemite of surface soils and miocene loessic soils on the chinese loess plateau abstract there is agreement that ultra ne maghemite grains 100nm are responsible for themagnetic enhancement of chinese loess recent studies show that grain size distribution of ultra nepedogenic maghemite grains within the aeolian sequences on the central chinese loess plateaudeposited in the last 8 ma is consistent however whether the observed grain size distribution ofultra ne pedogenic maghemite grains can be observed in the western chinese loess plateau andmodernsoilsisnotentirelyclear herewe ndthatyoungsurfacesoilsamplesacrossthechineseloessplateau and early neogene loessic soils from the western chinese loess plateau also show consistentgrain size distribution with that of ultra ne pedogenic maghemite grains on the central chinese loessplateau the fact that young surface soil samples show apparent magnetic enhancement suggests thatthe ultra ne pedogenic maghemite grains derive from oxidation of ultra ne magnetite grains copy right 2009 john wiley sons ltd keywords loess plateau magnetic susceptibility arm miocene loess maghemite

the las chacras potrerillos batholith pampean ranges argentina structural evidences emplacement and timing of the intrusion within the southern part of the sierra pampeanas the sierra de san luis argentina a series of extensive intrusive bodies are regarded to post date the famatinian cycle but were emplaced during the achalian a period of heterogeneous deformation along crustal scale fault zones the largest of those is the las chacras potrerillos batholith that is situated at the northern end of the transpressive sinistral guzman shear zone this composite pluton exhibits three sub domains that comprise two granitoid sub units each the southern potrerillos stock muscovite bearing red granite and biotite bearing red granite and the central biotite porphyritic granite and giant porphyritic granite and northern domain equigranular granite and porphyritic granite of the las chacras stock the crystallisation ages of the biotite porphyritic granite is around 381 ma u pb on zircons and pb pb on sphene while the host rock was already cooled below 350 c at 420 ma thermal modelling approaches favour a pulsed intrusion with a duration of 1 5 ma the emplacement was followed by rapid cooling below the muscovite cooling temperature biotite cooling ages in different sub units reflect either a long lasting cooling history of approximately 30 ma which is supported by the modelling or a reheating effect at around 350 ma devonian age determinations on the fault rocks and granitoids point to a syn tectonic emplacement of the batholith the pluton is interpreted to be positioned at the crossover of sinistral shear zones the origin of this nne directed extensional setting in a transpressive regime seems to be related to the transfer of displacement along a secondary set of nnw trending sinistral faults the final emplacement is due to a subsequent ballooning of the batholith following the direction of space creation this model is based on the relative timing of the emplacement sequence and macroscopically visible planar fabrics in the field as well as magnetic fabric data our results indicate that the emplacement is syn kinematic with respect to the achalian deformation event

the magnetic mechanism of paleosol s5 in the baoji section of the southern chinese loess plateau abstract in the central chinese loess plateau clp the magnetic susceptibility has the highest value in the s5 paleosol unit however at the southern of the clp such as in baoji the magnetic susceptibility value in the s5 paleosol unit is on the lower range of the top 6 paleosols s0 s1 s5 the studies of rock magnetism and geochemistry showed that the baoji s5 experienced a stronger pedogenesis and chemical weathering than the s3 the paleosol s3 was formed under an oxidizing environment where the maghemite and hematite are formed simultaneously maghemite is the main contributor to magnetic enhancement however the concentrations of maghemite in the s5 are lower than in the s3 with the development of pedogenesis the proportion of the maghemite component decreased in the s5 while the concentrations of anti ferromagnetic minerals mainly goethite increased in the field measurable amounts of dark brown iron manganese cutans can be observed on the cranny surface of s5 these indicate that the iron manganese elements in the s5 were assembled and pedogenesis occurred intermittently between wet and dry and strong leaching soil conditions this may have led to fine grained magnetite maghemite and hematite which were gradually converted into weak magnetic goethite therefore resulting in the lower magnetic susceptibility of s5

magnetism of a red soil core derived from basalt northern hainan island china volcanic ash versus pedogenesis red soil volcanic ash versus pedogenesis similar to loess paleosol sequences in northwestern china terrestrial sedimentary sequences red soils in southern china also provide sensitive quaternary records of subtropical tropical paleoclimate and paleoenvironment compared with red clay sequences originated from eolian dust red soils derived from bedrock have received little attention in this study a long core of red soil derived from weathered basalt in northern hainan island china was systematically investigated by using detailed magnetic measurements and rare earth element analyses the results show that an extremely strong magnetic zone with a maximum magnetic susceptibility 10 10 5 m3 kg 1 is interbedded in the middle of the core profile this layer contains a significant amount of superparamagnetic magnetite maghemite particles that primarily originated from volcanic ash with secondary contributions from pedogenesis the former has an average grain size of 19 nm with a normal distribution of volume and the latter has a much wider grain size distribution the presence of volcanic ash within the red soil indicates that these quaternary basalts were not formed by continuous volcanic eruptions moreover the magnetic enhancement patterns differ between the upper and lower zones the upper zone is more magnetically enhanced and experienced higher precipitation and temperature than the lower zone discrimination of superparamagnetic particles originating from pedogenic processes and volcanic ash thus provides a sound theoretical base for accurate interpretation of magnetism in red soils in this region

conditions of vein formation in the southern appalachian foreland constraints from vein geometries and fluid inclusions abstract the nolichucky shale in the whiteoak mountain thrust sheet of the southern appalachian foreland contains four coeval bed normal calcite vein sets that trended 015 055 090 and 320 before thrusting the pretectonic veins were displaced by syn tectonic bed parallel slickensided calcite veins during local alleghanian thrusting fluid inclusion thermometry indicates precipitation of both bed normal and bed parallel veins from nacl cacl2 brines 27 wt nacl equiv salinity at 80 110 c for bed normal veins and 110 c for bed parallel veins these temperatures correspond to burial depths of 2 4 3 6 km which were attained after the middle mississippian these pressure corrected homogenization temperatures are comparable to time temperature indicators such as cai and illite crystallinity and indicate that inclusions were trapped near maximum burial conditions and were not deformed after trapping this conclusion is supported by the absence of petrographic evidence for inclusion failure and a worst case mechanical analysis that indicates a lack of brittle deformation to inclusions after trapping the four coeval bed normal vein sets could not be formed by one simple stress field nor by the development of a single structure but required the interaction of at least two stress components or structures it is proposed that the interaction of the alleghanian and ouachita orogenic stress components beyond the limits of thrusting produced the sets as coeval alleghanian orthogonal 055 320 and ouchita sub orthogonal 015 090 mode i fractures

apatite in igneous systems apatite is a minor but ubiquitous mineral in most igneous rocks although the modal proportion of apatite in common rocks is generally low it can reach high concentrations in enclaves cumulates and other rocks of low abundance i e rocks that constitute a small volume of the crust and mantle e g nelsonites the presence of apatite in most rocks is due not only to its low solubility in naturally occurring melts and aqueous solutions but also to the limited ability of common rock forming minerals to accept the amount of phosphorus that occurs in most rocks into their structure in this paper we will discuss some aspects of the occurrence texture composition physical chemistry and petrogenetic significance of apatite in felsic rocks i e andesite to rhyolite and their plutonic analogs in mafic i e basalts and related rocks and plutonic analogs and ultramafic rocks of the earth s crust and mantle fluorapatite is by far the most common member of apatite family found in igneous rocks however most natural fluorapatite contains some chlorine and hydroxyl as well and these constituents can attain high concentrations in some cases the other halogens bromine and iodine also occur in apatite but their concentrations are much lower than chlorine and fluorine many cations commonly substitute for calcium and phosphorus in apatite however they rarely reach concentrations that warrant the definition of a separate mineral species apatite can be described by the general formula a5 xo4 3z following sommerauer and katz lehnert 1985 the a site accommodates large cations e g ca2 sr2 pb2 ba2 mg2 mn2 fe2 ree3 eu2 cd2 na pan and fleet ch 2 in this volume and comprises two sites that exhibit vii fold

displacive widening of calcite veins in shale insights into the force of crystallization abstract the geometry microtextures and c axis fabrics of calcite beef veins in the lower jurassic black shales wessex basin uk were characterized to investigate the mechanism responsible for widening following fracture propagation isolated beef veins exhibit planar tapering tips whereas closely spaced veins are characterized by blunt tips vein surfaces are generally smooth and flat however circular ridges appear on vein surfaces that protrude into the host clays where there are solid inclusions below or above the ridges fossils with well preserved morphologies which are separated by subvertical calcite fibers are observed on both the lower and upper surfaces of single veins the shale laminations around beef veins are folded and parallel to vein margins the beef veins commonly contain blocky zones of small equant calcite crystals pyrite and organic matter the fibers exhibit a preferred subvertical c axis orientation whilst crystals in the median zones and blocky zones have random c axis orientations the different crystal sizes morphologies and c axis orientations of the fibers from the blocky crystals suggest that the fibers grew without competition with each other under a nonhydrostatic stress field the displacive widening of calcite beef veins which is evident from vein interactions and deformation of individual fossil skeletons demonstrates that fibers grew incrementally because the crystallization pressure of calcite exceeded the overburden load the force of crystallization is suggested to be responsible for the c axis orientations of calcite fibers whereby crystals with free surfaces normal to c axis orientations grew preferentially the present study suggests that the fibrous widening of calcite veins in shale postdates their initiation and may result from displacive crystallization rather than fluid overpressure

chemical and textural relations of britholite and apatite group minerals from hydrothermal ree mineralization at the rodeo de los molles deposit central argentina