oxygen diffusion in amphiboles the kinetics of oxygen isotope self diffusion in natural samples of hornblende tremolite and richterite have been measured samples were run under hydrothermal conditions using 18o enriched water profiles of 18o 16o 18o vs depth into the crystal were obtained using an ion microprobe the depths of sputtered holes were measured using an optical interferometer at 1000 bars 100 mpa water pressure the activation energies q and pre exponential factors d0 for diffusion parallel to c are d0 cm2 sec q kcal gm atom t c hornblende 1 20 1 10 741 6 650 800 tremolite 2 30 2 10 8 39 5 650 800 richterite 3 5 2 10 4 57 2 650 800 the diffusion coefficient d for hornblende at 800 c and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions an increase in water pressure from 200 to 2000 bars increases d by a factor of 2 7 for diffusion parallel to c at 800 c the d value for hornblende at 800 c is about 0 01 that for quartz and 0 001 that for anorthite as a result closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars it is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions

paleomagnetism of the permian triassic intrusions from the tunguska syncline and the angara taseeva depression siberian traps large igneous province evidence of contrasting styles of magmatism abstract based on the detailed paleomagnetic investigation we distinguished different styles of intrusive magmatic activity in two regions of the siberian traps large igneous province lip the emplacement of intrusions in the angara taseeva depression the southern periphery of the siberian traps lip occurred as brief but intense bursts of magmatic activity that led to the emplacement of large and extensive sills we argue that this pulsating style of intrusive magmatic activity is common for the margins of the siberian traps lip we also estimated the duration of the main magmatic events as 200 250 km in diameter and dozens of thousands km2 in square on the contrary in the central part of the siberian traps lip the tunguska syncline the intrusive magmatism was more or less continuous without intense peaks of magmatic activity furthermore we obtained the first reliable magnetostratigraphic data from the volcanic section of the tunguska syncline finally we analyzed the available paleomagnetic and geochronological data from the siberian platform and suggested the correlation scheme of the studied intrusive complexes with the volcanic sequences of the siberian traps lip

vegetation changes and hydrological fluctuations in the central ebro basin ne spain since the late glacial period saline lake records abstract although the central ebro basin northeastern iberian peninsula is both the northernmost semi arid area in europe and one of the regions with the largest biodiversity it has been insufficiently studied in terms of past climate variability due to the scarcity of suitable sites for palaeoenvironmental analyses previous studies from ephemeral saline lakes in the area mainly based on palynological data show abrupt and rapid arid humid transitions throughout the last glacial cycle highlighting a complex palaeohydrological evolution new cores from two saline lakes la playa and la salineta in the los monegros area provide multi proxy records including sedimentology geochemistry and pollen indicators this study together with a detailed review of the main saline records from the central ebro basin enables us to reconstruct the main features of the palaeoclimate evolution during the last glacial cycle one of the main results of this study is the alternation of humid and dry phases as a characteristic of the climate evolution during the late glacial additionally the study suggests an important role of the increased flow from the pyrenean rivers during deglaciation in the hydrological balance of the central ebro basin the early holocene is the wettest period of the studied sequences contrasting with the aridity of the middle holocene interval which is frequently absent as a result of intense aeolian erosive processes the lack of archaeological remains associated with the middle holocene neolithic also supports increased aridity that would have impeded human settlements although anthropogenic activity partially masks the climate signal from the palynological data in the uppermost part of the studied sequences there is some sedimentological evidence for a climate change during the last 2000 yr resulting in a recovery of average saline lake levels in the central ebro basin

volume loss from antarctic ice shelves is accelerating the floating ice shelves surrounding the antarctic ice sheet restrain the grounded ice sheet flow thinning of an ice shelf reduces this effect leading to an increase in ice discharge to the ocean using 18 years of continuous satellite radar altimeter observations we have computed decadal scale changes in ice shelf thickness around the antarctic continent overall average ice shelf volume change accelerated from negligible loss at 25 64 cubic kilometers per year for 1994 2003 to rapid loss of 310 74 cubic kilometers per year for 2003 2012 west antarctic losses increased by 70 in the past decade and earlier volume gain by east antarctic ice shelves ceased in the amundsen and bellingshausen regions some ice shelves have lost up to 18 of their thickness in less than two decades

oxygen diffusion in quartz diffusional oxygen exchange between quartz and water was studied by determining 18o profiles in the quartz using an ion microprobe inverse error function plots of the data are linear as required for a diffusion mechanism one natural and two synthetic samples were used and although they had different impurity concentrations oh na and al per 106 si atoms are 7 4000 35 835 and 67 127 respectively the d values were the same within experimental uncertainty diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700 c and diffusion normal to the rhombohedron is intermediate the data indicate a change in slope on an arrhenius plot at the boundary at 100 mpa 1 kbar water pressure the preexponential factors do in square centimeters per second and activation energies q in kilojoules per mole for quartz 500 550 c are as follows do 190 and q 284 parallel to c and do 8 10 2 and q 238 normal to the rhombohedron for quartz 600 800 c do 4 10 7 and q 142 parallel to c do 9 10 7 and q 155 normal to the rhombohedron and do 1 10 4 and q 234 normal to c d increases with ph 2 o and between 25 and 350 mpa log d versus log h 2 o has a slope of approximately 1 1 these d values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water free systems

lead mobilization during tectonic reactivation of the western baltic shield abstract lead isotope data from sulfide deposits of the western part of the baltic shield define mixing lines in the 206 pb 204 pb 207 pb 204 pb diagram lead from two types of sulfide deposits have been investigated 1 1 exhalative and volcanogenic deposits that are syngenetic with their host rocks 2 2 vein deposits the syngenetic deposits locally show a very wide range of lead isotopic compositions that reflect a variable addition of highly radiogenic lead while the vein deposits although they have radiogenic lead isotopic compositions exhibit only limited isotopic variations in different provinces of the shield both types of deposits fall on the same lead mixing array the slope of the lead mixing lines varies as a function of the age of basement rocks and the age of the tectonic event which produced the lead mobilization and therefore relates the source rock age with the age of lead mobilization calculated mixing ages fall into several short time periods that correspond either to orogenic events or to major phases of continental rifting the orogenic events are the ca 360 430 ma caledonian ca 900 1100 ma sveconorwegian and the ca 1800 1900 ma svecofennian orogenic cycles the rifting events correspond to the formation of the ca 280 ma oslo rift and the ordovician ca 450 ma graben system in the area of the present gulf of bothnia each mixing age indicates that lead was mobilized probably as a consequence of mild thermal disturbances and that the crust was permeable to lead migration the data show that the geographic distribution of sulfide deposits with highly radiogenic lead isotopic compositions coincides with old graben systems orogenic belts and orogenic forelands on the baltic shield the ages of vein deposits and their geographic distribution demonstrate multiple tectonic reactivation of the interior of the baltic shield in response to orogenic events at its margin

development of planar and linear fabrics in dauphinois shales and slates french alps studied by magnetic anisotropy and its mineralogical control the progressive development of planar and linear fabrics due to alpine deformation in dogger calcareous shales has been studied in the dauphinois zone of the french alps using measurements of the anisotropy of magnetic susceptibility the axes and shapes of magnetic ellipsoids and petrofabrics show good qualitative correlation through a series of different fabric types from a sedimentary to a highly deformed one together with various petrological techniques magnetization measurements in a variable field up to 5 t and under variable temperature with a cryogenic magnetometer were used to achieve a complete analysis of the magnetic mineralogy with its dia para and ferromagnetic s l contributions the effects on the anisotropy of the complex mineralogy and various geological processes including pressure solution metamorphism and weathering are discussed

geodynamic settings for paleoproterozoic gold mineralization in the svecofennian domain a tectonic model for the faboliden orogenic gold deposit northern sweden abstract northern sweden is currently experiencing active exploration within a new gold ore province the so called gold line situated southwest of the well known skellefte vms district the largest known deposit in the gold line is the hypozonal faboliden orogenic gold deposit mineralization at faboliden is hosted by arsenopyrite rich quartz veins in a reverse mainly dip slip high angle shear zone in amphibolite facies supracrustal host rocks the timing of mineralization is estimated from field relationships at ca 1 8 ga the gold mineralization is hosted by two sets of mineralized quartz veins one steep fault fill vein set and one relatively flat lying extensional vein set ore shoots occur at the intersections between the two vein sets and both sets could have been generated from the same stress field during the late stages of the svecofennian orogen the tectonic evolution during the 1 9 1 8 ga svecofennian orogen is complex as features typical of both internal and external orogens are indicated the similarity in geodynamic setting between the contemporary svecofennian and trans hudson orogens indicates a potential for world class orogenic gold provinces also in the svecofennian domain the swedish deposits discussed in this paper are all structurally associated with roughly n s striking shear zones that were active at around 1 8 ga when gold bearing fluids infiltrated structures related to conditions of e w shortening

spatial and temporal antarctic ice sheet mass trends glacio isostatic adjustment and surface processes from a joint inversion of satellite altimeter gravity and gps data we present spatiotemporal mass balance trends for the antarctic ice sheet from a statistical inversion of satellite altimetry gravimetry and elastic corrected gps data for the period 2003 2013 our method simultaneously determines annual trends in ice dynamics surface mass balance anomalies and a time invariant solution for glacio isostatic adjustment while remaining largely independent of forward models we establish that over the period 2003 2013 antarctica has been losing mass at a rate of 84 22 gt yr 1 with a sustained negative mean trend of dynamic imbalance of 111 13 gt yr 1 west antarctica is the largest contributor with 112 10 gt yr 1 mainly triggered by high thinning rates of glaciers draining into the amundsen sea embayment the antarctic peninsula has experienced a dramatic increase in mass loss in the last decade with a mean rate of 28 7 gt yr 1 and significantly higher values for the most recent years following the destabilization of the southern antarctic peninsula around 2010 the total mass loss is partly compensated by a significant mass gain of 56 18 gt yr 1 in east antarctica due to a positive trend of surface mass balance anomalies

plio pleistocene lacustrine evaporites of the baza basin betic chain se spain abstract the baza basin the eastern part of the guadix baza basin in se spain underwent a significant evaporitic sedimentation in non marine settings during the plio pleistocene the largest of these settings developed in the central part of the basin as a shallow saline lake system where abundant gypsum deposits occur benamaurel and galera gypsum units this paper studies these deposits from the cartographic stratigraphic sedimentologic petrographic and isotopic points of view all the gypsum has been preserved as primary enabling a good characterization of the environmental conditions of the original precipitates three interconnected subenvironments or zones inner intermediate and marginal are differentiated in this saline lake system in the inner zone shallow to relatively deeper central lake a regular alternation of gypsum and carbonate laminae accumulated suggesting a seasonal regime many of the gypsum laminae display size gradation both reverse and symmetrical indicating free precipitation in a stable brine body in the intermediate zone mosaic of shallow lakes carbonate dolomitic beds formed in association with dark lutite levels gypsum beds and gypsum nodules and micronodules moreover anoxic conditions developed in this zone which resulted in the formation of native sulphur occurrences of economic value in the marginal zone the evaporitic deposits are developed only locally the galera gypsum unit in particular the various gypsum lithofacies in these marginal deposits suggest the presence of a number of shallow to exposed settings saline mudflat saline marsh and gypsiferous pond the isotopic composition 34 s 18 o of the gypsum samples is consistent with a triassic origin of the sulphate as a result of chemical recycling these data also suggest that a similar recycling mechanism operated in the saline lake system where the gypsum sediments from the margins were coevally recycled towards the inner part both the evaporitic mineralogy and the gypsum and carbonate lithofacies suggest that semiarid conditions prevailed in this basin during the accumulation of the evaporite units

isotopic exchange in mineral fluid systems ii oxygen and hydrogen isotopic investigation of the experimental basalt seawater system abstract oxygen and hydrogen isotopic exchange reactions between basalt and seawater at t 300 to 500 c were investigated using oceanic tholeiitic basalt 18 o 5 7 d 70 natural seawater 18 o 1 2 d 5 and artificial seawater 18 o 5 3 as starting materials the starting basalts varied in the crystallinity from holocrystalline to glass but were ground to approximately the same grain size 100 mesh the water rock mass ratios ranged from 1 to 3 and the duration of the experiments ranged from 167 to 576 days in general depletion of 18o and enrichment of d in basalts occur at all temperatures with the magnitudes of change being greater as temperature time and to a lesser degree glass content increases the trends in isotopic shifts are directly related to changes in the style and intensity of mineralogic alterations in the basalt e g smectite at 300 c talc actinoiite at 400 500 c the changes in the 18 o values of basalts and seawater in the experimental systems were observed to follow closely with those expected from a first order rate law rate constants for the oxygen isotopic exchange between rock and water range from 10 9 5 to 10 8 0 moles oxygen m2 of solid surface sec for temperatures of 300 to 500 c the activation energy for the isotopic exchange reaction was calculated to be 11 5 kcal mol an application of our experimental rate data to natural systems suggests that the oxygen isotope equilibrium between basalt and seawater in the mid oceanic ridge may take place within approximately 1000 years at 350 c our experimental data also suggest the equilibrium oxygen isotopic fractionation factors between the altered basalt and seawater to be 3 5 0 5 at 300 c 2 0 0 4 at 400 c and 0 5 0 25 at 500 c the observed hydrogen isotopic fractionation factors between the altered basalts and seawater in our experimental systems were about 74 at 300 c about 62 at 400 c and about 48 at 500 c these fractionation factors are probably attributable to the equilibrium fractionation between fe rich secondary phases and seawater

geochemistry and habitat of natural gases in italy abstract systematic chemical and isotopic analyses were carried out on natural gases from 92 fields which represent more than 90 of the original gas reserves of italy a quantitative assessment of the various genetic gas types indicates that most of the gaseous hydrocarbons 80 were generated through bacterial activity and or diagenetic processes while only a minor amount 20 was produced by thermal degradation of organic matter the formation of the gas accumulations was geologically controlled by the neogene tectonic phases and by the sedimentary events associated with the growth of the apennines and southern alps in fact about 80 of italian natural gas reserves have been found in the apennine foredeep while the remaining part is equally distributed in the folded belt and in the foreland furthermore the influence exerted by tectono sedimentary characteristics is emphasized by the fact that thermal degradation of organic matter was the only process responsible for most of the gas generated in the apennine folded belt 97 thermogenic gases on the other hand the high rates of subsidence turbiditic sedimentation and very early compressional tectonics in the foredeep favored processes leading to the accumulation of very large quantities of biogenic gas 82 moreover geochemical and geological data as well as mathematical modelling demonstrate that migration and accumulation of italian natural gases mainly occurred later than 5 my ago and in some cases these phenomena are still active in fact in the multipay zone gas fields of the northern adriatic a kind of steady equilibrium is reached between loss of gas through the thin cap rocks often less than 1 m and continous replenishment from the bacterial fermentation and or low temperature thermochemical reactions

origins and ages of proterozoic granitoids in the bothnian basin central sweden isotopic and geochemical constraints abstract the paleoproterozoic bothnian basin in central sweden and south central finland is dominated by a 10 km thick sequence of metasediments mainly metaturbidites these have been intruded by granitoids which have traditionally been classified as early late and postorogenic relative to the c 1 9 1 8 ga s vecofennian orogeny the early orogenic granitoids are part of a calcalkaline suite ranging in composition from ultramafic to granitic while the late orogenic granitoids mainly are purely granitic and associated with abundant pegmatites the postorogenic group forms large massifs of mainly granitic composition and associated pegmatites are rare an early orogenic tonalite yields a u pb zircon age of 1877 6 ma which is within the age range for s vecofennian granitoids of this tectonic position elsewhere in sweden and finland a late orogenic granite belonging to the harno granites from the southern part of the bothnian basin gives a u pb monazite age of 1822 5 ma this provides a minimum age for the main phases of svecofennian deformation in the region and a correlation with the 1 84 1 83 ga old granite migmatite zone of southern finland is suggested a postorogenic granite belonging to the revsund granitoids gives a u pb zircon age of 1798 8 ma together with previously published age data this indicates that the revsund granitoids were emplaced successively during an age interval of at least between 1 80 and 1 77 ga ree patterns and trace element model calculations indicate that late orogenic granites could have been produced by partial melting of metasediments similar to those exposed at the surface a contribution from the early orogenic granitoids is possible but not discerned by these calculations the postorogenic granites on the other hand have too high concentrations of sr ba y and hree to be derived from such a metasedimentary source the initial e nd values for the early orogenic granitoids range between 2 and 1 corresponding values for both late and postorogenic granites range between 1 and 3 which is on average slightly higher than those for surrounding metasediments the initial sr isotope ratios for the harno granites are intermediate between those for the metasediments and the early orogenic granitoids while the initial sr isotope ratios for the revsund granites are distinctly lower the geochemical and isotopic data support a model in which the harno granites were derived mainly from bothnian basin metasediments with a contribution from early orogenic granitoids the composition of the revsund granitoids is however inconsistent with such a derivation for these rocks a deeper as yet unknown crustal source with a significant pre 1 9 ga crustal component is indicated

paleomagnetism of latest anisian middle triassic sections of the prezzo limestone and the buchenstein formation southern alps italy a paleomagnetic study was carried out at six stratigraphic sections 309 specimens in the latest anisian middle triassic prezzo limestone and the overlying buchenstein formation these units outcrop over a wide area in the western southern alps although most of the sampled sections are in the vicinity of the late eocene early oligocene adamello batholith three sites suffered a complete remagnetization induced by the adamello whereas a characteristic component with a positive fold test has been isolated at the three other sites the mean pole of the characteristic component lat 63 2 n long 229 3 e n 3 a95 8 k 236 is in agreement with the triassic portion of the west gondwana apparent polar wander path apwp supporting the use of paleopoles from well dated rocks in the southern alps as useful proxies for the african apwp the characteristic component where isolated in the sampled sections is of normal polarity only corresponding to the latest anisian on the basis of well defined ammonoid and conodont biostratigraphy but the present results suggest that there are good opportunities for extending middle triassic magnetostratigraphy in these southern alps rock units the mean pole of the adamello induced component lat 74 5 n long 172 1 e n 4 a95 7 6 k 145 lies close to the early tertiary portion of the apwp for stable europe the post folding adamello induced directions confirm that the 30 42 ma adamello batholith intruded after alpine deformation and that no further deformation apparently occurred in post emplacement times

geology petrology and alteration geochemistry of the palaeoproterozoic intrusive hosted algtrask au deposit northern sweden the agtrask intrusive hosted au deposit skellefte district northern sweden is situated in the oldest most heterogeneous part of the c 1 89 1 86 ga jorn granitoid complex which intruded a comp

application of in situ produced cosmogenic 10be and 26al to the study of lateritic soil development in tropical forest theory and examples from cameroon and gabon abstract depth profiles of in situ produced cosmogenic nuclides including 10 be t 1 2 1 5 10 6 years and 26 al t 1 2 0 73 10 6 years in the upper few meters of the earth s crust may be used to study surficial processes quantifying denudation and burial rates and elucidating mechanisms involved in landform evolution and soil formations in this paper we discuss the fundamentals of the method and apply it to two lateritic sequences located in african tropical forests

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

metallogeny and tectonic evolution of the early proterozoic skellefte district northern sweden abstract the various mineralizations in the early proterozoic skellefte district are discussed and placed in a palaeotectonic context the metallogeny together with volcanic stratigraphy geochemistry isotope studies and structural geology favour a former destructive plate margin at 1 89 ga the area resembled a modern island arc environment where volcanogenic massive sulphide and gold rich porphyry type deposits formed mafic to ultramafic sills and dykes hosting ni mineralizations intruded a greywacke sequence immediately south of the skellefte district as a result of large scale rifting at the end of the volcanic period gold was emplaced 20 60 ma later during peak metamorphism at the end of the svecofennian orogeny finally at 1 80 ga after accretion of the island arc to the archaean continent in the northeast large granitoids intruded some of these have associated w u mo and li mineralizations an evidence for a crustal or cratonic influence

oxygen isotope geothermometers for metamorphic rocks the chicago mineral carbonate oxygen isotope fractionation curves have been combined with mineral water fractionation data for jadeite zoisite and rutile and new data for grossular water to provide a set of self consistent mineral pair calibrations the a coefficients in the equation 1000 in a 106t 2 of the new mineral pair fractionations are jadeite zoisite grossular rutile quartz 1 69 2 00 3 03 5 02 jadeite 0 31 1 34 3 33 zoisite 1 03 3 02 grossular 1 99 the isotopic fractionation properties of natural pyralspite garnet ca fe mg mn 3al2si3o12 can be approximated by those of the grossular end member appropriate substitutions also yield coefficients for the solid solution minerals sodic pyroxene and epidote e g aquartz sodic pyroxene 2 75 1 06xjd aquartz epidote 2 00 0 75xps where xjd and xps are the mole fractions of the jadeite and pistacite components respectively the new data set is particularly suitable for the geothermometry of metamorphic rocks 18o data from minerals of the high pressure metamorphic rocks of the sesia zone of italy and cyclades complex of greece yield well constrained mean temperatures of 572 and 478d c respectively type iii blueschist metabasalts of the franciscan formation of california give mean quartz garnet temperatures of 354d c

synvolcanic mixing of ore lead and the development of lead isotopic provinces in the skellefte district sweden ore lead isotope ratios have been analysed in galenas and sulphosalts from nineteen massive sulphide deposits hosted by svecofennian 1 9 ga supracrustals in the skellefte ore district northern sweden the ore lead data can be grouped on the basis of their geographical distribution most probably this feature is reflecting a number of lead isotopic provinces which correspond to crustal blocks the obtained ore lead data define linear trends in conventional pb pb plots the ore lead signatures are due to synvolcanic mixing processes as lead was leached from mafic and acid volcanic sources the relationships inferred for initial ratios in source rocks at 1 89 ga require a pre svecofennian crustal history it is suggested that magmatic processes at c 2 0 ga involved recycling of archean sedimentary material into the mantle and the formation of a crystalline felsic basement the metasomatized mantle and the basement melted at the time of svecofennian magmatism 1 89 ga which created mafic and acid magma sources subsequently basalts and rhyolites were extruded onto the sea floor ore was formed as hot solutions penetrated isotopically different levels of the volcanic pile

pedogenic and early diagenetic processes in palaeogene alluvial fan and lacustrine deposits from the sado basin s portugal abstract the palaeogene deposits of the sado basin were deposited in a continental basin that shows a typical pattern with alluvial fans system in the margins of the basin passing towards distal areas of mudflat facies where in some areas the installation of shallow water bodies favoured the development of palustrine conditions the deposits of this basin vary form coarse conglomerates and sandstones to palustrine carbonates these sediments were affected by pedogenesis and early diagenetic processes that promoted important modifications on their primary features these modifications have been studied by the analyses of four profiles developed on proximal middle and distal fan deposits and the fourth one on lacustrine deposits the overall analyses of the sedimentological pedogenic diagenetic features and their relationships indicate that three main processes took place throughout the basin soil formation palygorskite neoformation and dolomitization soil formation processes led to illuviation of clays and carbonate precipitation mostly around roots pedogenic carbonates increase towards distal areas whereas hydromorphic features are present throughout the basin palygorskite neoformation was partially diagenetic being maximum in proximal areas and palustrine deposits this neoformation is attributed to the percolation of alkaline mg rich soil and groundwaters through smectitic rich sediments promoting important clay transformation dolomitization was an early diagenetic process that occurred mainly in carbonate rich deposits of distal and lacustrine environments as a result of the increasing mg ca ratio of the percolating groundwaters in all these processes there has been a close spatial and temporal interplay between pedogenesis and diagenesis driven by the chemistry of soil particles and groundwaters

magnetic properties of the high himalayan leucogranites structural implications the magnetic properties of the high himalayan leucogranites have been investigated on 527 specimens in three plutons everest makalu 6 sites and manaslu 40 sites in nepal and gangotri 43 sites in india susceptibility varies between 2 and 100 10 6 si with an anisotropy ratio up to 1 16 high field and low temperature magnetic measurements together with comparison with weight percent iron demonstrate that anisotropy of magnetic susceptibility is carried by paramagnetic biotite and tourmaline the latter produces an inverse fabric i e with the minimum axis parallel to mineral lineation the magnetic fabric demonstrates complex patterns of stretching lineations during magmatic emplacement and its usefulness in semi quantitatively estimating petrofabric intensity is demonstrated for the biotite bearing facies natural remanent magnetization was measurable at only two sites in everest makalu where there are well defined reverse directions carried by titanomagnetite and pyrrhotite comparison of these preliminary results with predicted directions for stable india suggests northward tilting of about 10 and a small clockwise rotation of this massif

scheelite geochemistry in porphyry skarn w mo systems a case study from the gaojiabang deposit east china abstract scheelite cawo4 a hydrothermal mineral commonly displaying enrichment in mo up to 16 and or rare earth elements ree is the main economic mineral in the gaojiabang porphyry skarn type w mo deposit east china based on microscopic observations and in situ la icp ms and la mc icp ms analysis three groups of scheelite each with different geochemical characteristics can be recognized this evidence provides a good basis for considering the behavior of some trace elements in scheelite and how they may constrain ore genesis in porphyry skarn systems p group scheelite occurs inside the porphyry rocks in the form of vein hosted or disseminated scheelite these have the lowest 87sr 86sr i values 0 7089 0 7108 lowest mo concentration mean 213 ppm n 51 lree hree ratios of 16 33 84 91 and highest ree concentration 191 405 ppm with downwards sloping ree fractionation trends s group scheelite occurs in skarns coexists with skarn minerals garnet diopside epidote etc and has the highest 87sr 86sr i values 0 7103 0 7113 highest mo concentration mean 1 323 ppm n 29 highest lree hree ratio 62 6 164 and lowest ree concentrations 68 5 112 ppm also with strongly downwards sloping ree fractionation trends h group scheelite occurs within veins hosted by hornfels and displays moderate 87sr 86sr i 0 70104 0 70122 mo mean 806 ppm n 17 lree hree ratio 16 32 42 55 and ree concentration 98 3 167 6 ppm during hydrothermal precipitation of scheelite changing redox state plays a major role in controlling mo behavior both the precipitation of early skarn minerals and changing redox states of ore forming fluids likely result in an increase in mo and corresponding decrease of hree in scheelite particularly in s group scheelite the sr isotope study indicates that crustal materials provided the main source for w mo bearing ore forming fluids furthermore both fluid mixing and fluid rock interaction played an important role in the evolution of ore forming fluids formation water or groundwater was likely involved in ore formation a geochemical model is described combining data for the different types of scheelite and the behavior of mo rees and sr isotopes to constrain the evolution of ore forming fluids and constrain ore genesis in the porphyry skarn system at gaojiabang this study contributes to the fingerprinting of ore deposits using scheelite geochemistry

cooling history of the dauphinoise zone western alps france deduced from the thermopaleomagnetic record geodynamic implications a post tectonic thermoremanent magnetization carried by pyrrhotite has been previously demonstrated in the dauphinoise zone of the french western alps the paleomagnetic signal is interpreted to record successive partial thermoremanent magnetizations acquired in normal and reverse geomagnetic fields during the slow cooling of the epimetamorphic marly limestones a comparison between the derived magnetic polarity sequence versus temperature thermopaleomagnetic record and the magnetostratigraphic time scale taking into account radiochronological constraints allows us to derive 21 age temperature couples between 24 1 ma 302 c and 20 2 ma 182 c for the bourg d oisans area combining these thermopaleomagnetic data with other p t t and structural data a geodynamical model is proposed for the tectono metamorphic evolution of the dauphinoise zone the thickening contemporaneous with the main syncleavage folding occurs around 26 5 ma depending on the hypothesis considered to explain the excess of heat the bourg d oisans area was buried to 8 6 km fluids hypothesis or 12 5 km tectonic hypothesis in both cases these depths imply the existence of overlying units which might come from the internal zones actually cropping out eastward of the penninic front a tectonic denudation is proposed just before 24 1 ma in order to explain our observations after this denudation a faster cooling occurs it is probably related to the dissipation of the thermal anomaly induced by the tectonic denudation and with the increase of the erosion due to the underthrust of a more external crustal slice this interpretation is supported by the similar fluctuations of cooling rate derived here and subsidence of the molasse basin afterwards the cooling slowed down during this time the studied area seems to be passively moved westward in the hanging wall of the external crystalline massifs thrust without significant deformations

empirical calibration of the oxygen isotope fractionation between quartz and fe mg chlorite abstract an empirical calibration has been made of the oxygen isotope fractionation between chlorite and quartz using samples from localities in metamorphic terrains where the formation temperature has been constrained by independent methods e g fluid inclusion homogenization temperatures mineral assemblages and petrology of the host rocks to the veins thermodynamic modeling and raman spectrometry textural equilibrium of chlorite and quartz was a pre requisite and the chemical composition of chlorite was determined the chlorite analyzed had a range in fe mg varying between 0 35 and 0 7 except for pure mg chlorite fe mg substitution within the range measured does not change the oxygen isotope fractionation between quartz and chlorite measurably the results support a linear correlation for chlorite quartz fractionation over the temperature range given by this study 240 550 c 1000 ln 0 01323 0 002 t c 10 97 0 92 the presently established geothermometer can be applied to quartz chlorite fractionations in low grade metamorphic rocks and may be extrapolated to diagenetic systems

facies identification within the playa lakes of the monegros desert spain from field and satellite data abstract the monegros desert and its saline wetlands called saladas literally translated as the salties are a unique european landscape of great scientific and ecological value the saladas i e playa lakes and other small saline depressions are dynamic environments changing their surface morphology on a seasonal diurnal basis in response to both climate and groundwater fluxes to depict changes in these natural systems we have identified five surface facies classes which are detectable both in the field and from remote sensing data these facies are crucial for describing and promoting the protection of these habitats remote sensing has provided worthwhile historical data and additional information that compensate for scarce field records combined field and satellite criteria are used to catalog these facies with a new conceptual integration that manages the asynchronism between the field and satellite data the catalog of facies is intended to be helpful for monitoring these wetlands and for understanding the current hydrological patterns and trends in the playa lakes this work will serve as a baseline for studying the future evolution of the saladas which may soon fall under manmade environmental forces such as increased water input from adjacent newly irrigated lands it is hoped that identification of these facies will be useful with minor adaptations in using more advanced sensors or in studying similar habitats

scheelite elemental and isotopic signatures implications for the genesis of skarn type w mo deposits in the chizhou area anhui province eastern china scheelite is well developed in hydrothermal deposits providing a window into genetic processes and facilitating comparative studies however few studies have focused on characterizing scheelite in skarn type w mo deposits the primary ore mineral in the jitoushan and baizhangyan skarn type w mo deposits anhui province eastern china scheelite was analyzed for major trace and rare earth element ree abundance and for sr nd isotopes the analysis revealed two unique geochemical characteristics that distinguish the scheelite from skarn type w mo deposits to that from vein type au w and porphyry type w mo deposits higher mo content with a negative correlation between moo 3 and wo 3 and a strong hree depletion skarn type scheelite mainly inherited ree signatures from ore forming fluids and the early precipitation of skarn minerals e g garnet diopside and amphibole has most likely resulted in the observed strong hree depletion in scheelite and the decoupling of lrees and hrees of the numerous substitution mechanisms suggested by previous workers 3ca 2 2ree 3 ca where ca is a ca site vacancy is preferred for the substitution of ree 3 for ca 2 and in this study particularly given the low salinity of ore fluids as the scheelite eu anomalies were inherited from ore forming fluids with variable redox conditions and ph the complex deu mo correlation indicates that mo increasingly entered the scheelite under oxidizing conditions and reached a maxim at deu values of 0 8 to 1 in contrast under reducing conditions mo contents in scheelite decrease gradually and mo is precipitated as molybdenite as a result of the change in dominant valence state unlike the sr nd isotope compositions of scheelite from vein type au w and w sb au deposits the scheelite from skarn type w mo deposits has low 143 nd 144 nd t most 87 sr 86 sr t values most between 0 708 and 0 715 the end t values of the scheelite varied from 16 to 12 3 in the baizhangyan deposit and from 9 5 to 9 1 in the jitoushan deposit indicating that the ore forming materials in the two w mo deposits were mainly derived from crustal sources

properties of chemical remanence in synthetic hematite testing theoretical predictions we synthesized hematite under controlled conditions of the applied magnetic field in order to test the behavior of chemical remanent magnetization crm predicted on the basis of theories developed to explain thermoremanent magnetization trm we restrict this investigation to the remanence acquired during mineral crystallization grain growth crm and do not consider chemical remanence acquired during mineral alteration the material produced by our technique is hematite up to 10 m in diameter thermal demagnetization to 200 c removes the viscous component acquired under identical field temperature conditions to those prevalent during synthesis as predicted by analogy to trm the crm acquired parallels the orientation of the magnetic field applied during growth the intensity of chemical remanence is a linear function of the field in which it grew bcrm up to 7 5 mt the maximum growth field investigated furthermore the ratio of intensities of crm to trm acquired in the same field is independent of the strength of that field the intensity of bcrm controls the growth and orientation of hematite crystals a result not predicted from theory derived by analogy to trm the control on crystal growth by the field is reflected in the dependence on bcrm of the orientation of ellipsoids of anisotropy of isothermal remanence and the acquisition of isothermal and anhysteretic remanence

chemical mineralogical and micro morphological study of coarse fragments in mediterranean red soils abstract we studied the gravels derived from mica schists and micaceous quartzites of three mediterranean red soils haploxeralf and cryoboralf from the sierra nevada andalusia spain with emphasis on the following aspects morphology visual evidence of weathering thin section description and scanning electron microscopy composition x ray diffraction extractable fe al and si and electron microprobe analysis and chemistry ph cation exchange capacity specific surface area and organic carbon content the gravels had undergone considerable modification during weathering in soil a loss of the primary rock structure was detected together with a decrease in compactness an increase in roundness color value and chroma neoformed kaolinite was present and relatively high contents of extractable forms of iron aluminium and silica citrate dithionite and oxalate were detected from an analytical point of view the surface properties cec and ssa increased and the equilibrium ph decreased in the clasts which had undergone most weathering the sem study shows that the fabric changes from laminar schistose in the parent rock to other types in the gravels due to the weathering process electron microprobe analysis revealed the presence of phyllosilicates together with iron oxides in the illuvial infilling materials of fissures in the most altered clasts these infillings affect the properties of the coarse fragments the relationship detected between the properties of fine earth and those of the gravel fractions suggest that the coarse fragments have a significant influence on the properties of the soil

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

thermochemically induced transformations in al smectites a spanish natural analogue of the bentonite barrier behaviour in a radwaste disposal abstract the thermal effect induced by the morron de mateo volcanic dome cabo de gata volcanic region spain on the adjacent bentonitised tuffaceous beds has been studied as a natural analogue of the thermal behaviour of the bentonite engineered barrier of a geological radwaste repository these bentonites consist mainly of fe rich smectites and were formed in equilibrium with seawater at temperatures between 75 and 95 c according to the 18o and d values in contrast bentonites from other localities in the region consist mainly of al smectites formed in equilibrium with meteoric water below 25 c this investigation is focussed on the detection of the chemical differences between smectites from proximal and distal zones to the dome as well as to test whether the temperatures calculated based on the o and h isotopic values correspond to their formation or transformation the initial hypothesis was that the chosen smectites could be formed under marine conditions being later transformed and isotopically re equilibrated as a result of the intrusion to check this hypothesis a detailed mineralogical chemical geochemical and isotopic study has been performed on the smectitised tuffaceous materials and the overlaying biocalcarenites outcropping near and far from the dome the results show that distal smectites are dioctahedral al smectites similar to those from other deposits in the region while proximal smectites are fe and mg rich smectites showing two evolutionary trends on a fe mg al ternary diagram similar features are observed when their structural formulae are plotted on the muscovite celadonite pyrophylite diagram thus they plot in the smectite domain with interlayer charge less than 1 which is mainly due to octahedral substitution for distal smectites while for proximal ones it is caused by both octahedral and tetrahedral substitutions in this ternary diagram the domains of both proximal and distal smectites are partially overlapped the coexistence of di and trioctahedral smectites was only detected in one proximal sample further proximal biocalcarenites are enriched in fe rich dolomite in relation to the distal ones the 87sr 86sr and 13c values in carbonates and d in smectites indicate equilibrium with seawater in contrast 18o values of carbonates and smectites indicate that they were transformed and re equilibrated between 40 and 90 c and between 55 and 66 c respectively independently of their location with respect to the dome these data suggest that the transformation of calcite into mg fe carbonates and the occurrence of fe and mg rich smectites near to the dome resulted from a chemically induced process at similar temperatures the compositional differences among samples suggests that fe mg and minor mn were supplied by a contaminant plume originated from the dome migrating through the sediments and becoming more diluted away from the source the absence of a well defined thermal gradient in the system could be due to the small size semi closed and shallow character of the basin as well as to its high underlying volcanic activity finally the results are discussed in terms of analogue processes that can be expected in the bentonite barrier of a radwaste geological repository

a miocene polarity transition r n from the agno batholith luzon a geomagnetic reversal record has been obtained from the agno batholith 16 15 n 120 39 e in luzon island the philippines this body is a biotite hornblende quartz diorite dated at between 14 5 and 15 0 m y by the k ar whole rock method hysteresis properties show that the magnetic remanence is dominated by multidomain low titanium titanomagnetite a stable component of magnetization is isolated upon both thermal and alternating field demagnetization normal and reversed polarity zones within the batholith give tightly grouped results which when compared to other miocene poles from the baguio area allow a tilt correction to be made the resulting reversed to normal reversal sequence has a far sided vgp path and a decrease in intensity of magnetization of approximately a factor of 5 with a longer duration than the directional change the vgp path is confined in a longitudinal band 120 from the site and so differs from the characteristic reversed to normal transition paths in that it is not near sided intermediate field geometry cannot therefore always be the same for individual reversals the agno reversal record is consistent with a quadrupole field and reversal initiated in the northern hemisphere an octupolar intermediate field would require simultaneous flooding from both polar latitude bands

granites and early proterozoic crustal evolution in sweden evidence from smnd upb and o isotope systematics abstract u pb zircon and whole rock rb sr sm nd and o isotope results are reported for granitoids from three proterozoic provinces in sweden that represent different crustal environments the radiometric ages span the period 1 89 1 65 ga and record events ranging from original igneous crystallisation to late metamorphic resetting these events occurred at different times in the crustal segments the o isotope ratios identify different crustal source materials for the granitoids the sm nd results indicate that the major crustal source components common to most of the granitoids formed shortly before 1 9 ga close in time to the formation of the earliest granitoids contributions from recycled archaean basement are minimal the geographically widespread occurrence of such crustal sources indicates that the period around 2 ga was of great importance for the growth of new continental crust in sweden

late archean granitoid hosted lode gold deposits yilgarn craton western australia deposit characteristics crustal architecture and implications for ore genesis granitoid hosted lode gold deposits constitute a sub group of late archean lode gold deposits in the yilgarn craton of western australia they share a number of common characteristics including 1 a strong structural control on a variety of scales 2 in most instances the presence of mineralisation in adjacent supracrustal sequence 3 wallrock alteration and vein assemblages consistent with the addition of sio2 k2o co2 s na2o but dependent on the p t conditions of the host environment and ore fluid 4 a metal association comprising au ag as bi te w mo and low concentrations of cu pb and zn and 5 deposition from a low moderate salinity near neutral h2o co2 ch4 fluid over a temperature and pressure range from 250 to 600 c and 1 to 4 kbar which corresponds to a range of crustal environments from lower greenschist to upper amphibolite facies these characteristics suggest that the granitoid hosted lode gold deposits represent a coherent genetic group that is equivalent to the more abundant greenstone hosted lode gold deposits of the craton in combination lode gold deposits irrespective of their host rocks and host terrane formed over a range of crustal levels and developed late in the tectonic sequence of the craton granitoids which host lode gold mineralisation are older than the inferred age of mineralisation by at least 10 million years and in combination with thermodynamic and isotope studies indicate that the host granitoids are not the dominant source of the ore fluid however the host granitoids do provide a structural and to a lesser extent chemical trap for gold bearing fluids in combination trace element and isotope studies of greenstone and granitoid hosted deposits suggest that the ore fluid originated from a deep mid to lower crustal source and at least interacted with granitic rocks at some stage the ore fluid composition is quite constant over the range of crustal levels from deep to shallow implying that the extent of modification by interaction with fluid conduit rocks is variable but not substantial

late holocene evolution of playa lakes in the central ebro depression based on geophysical surveys and morpho stratigraphic analysis of lacustrine terraces abstract the origin and morpho stratigraphic evolution of the largest playa lake system la playa el pueyo in the bujaraloz sastago endorheic area located in the semiarid central sector of the ebro depression are analysed the enclosed depressions are developed on gypsiferous tertiary bedrock and show a prevalent wnw ese orientation parallel to the direction of the prevalent strong local wind cierzo yardangs have been carved in bedrock and unconsolidated terrace deposits in the leeward sector of the largest lake basins a sequence of three lacustrine terrace levels has been identified by detailed geomorphological mapping the treads of the upper middle and lower terrace levels are situated at 9 m 6 m and 0 5 m above the playa lake floors respectively seismic refraction and electrical resistivity profiles acquired in la playa reveal a thin basin fill 2 m with a planar base these data allow ruling out the genetic hypothesis for the depressions involving the collapse of large bedrock cavities and support a mixed genesis of combined widespread dissolution and subsidence by groundwater discharge and eolian deflation during dry periods the 5 m thick deposit of the middle terrace was investigated in hand dug and backhoe trenches six ams radiocarbon ages from this terrace indicate an aggradation phase between 3 9 ka and ca 2 ka these numerical ages yield a maximum average aggradation rate of 2 6 mm yr and a minimum excavation rate by wind deflation of 3 mm yr subsequent to the accumulation of the middle terrace the latter figure compares well with those calculated in several arid regions of the world using yardangs carved in palaeolake deposits the aggradation phase between 4 and 2 ka is coherent with other iberian and mediterranean records showing relatively more humid conditions after 4 ka including the iron ages and the iberian roman period

river piracy and drainage basin reorganization led by climate driven glacier retreat river piracy the diversion of the headwaters of one stream into another one can dramatically change the routing of water and sediment with a profound effect on landscape evolution stream piracy has been investigated in glacial environments but so far it has mainly been studied over quaternary or longer timescales here we document how retreat of kaskawulsh glacier one of canada s largest glaciers abruptly and radically altered the regional drainage pattern in spring 2016 we use a combination of hydrological measurements and drone generated digital elevation models to show that in late may 2016 meltwater from the glacier was re routed from discharge in a northward direction into the bering sea to southward into the pacific ocean based on satellite image analysis and a signal to noise ratio as a metric of glacier retreat we conclude that this instance of river piracy was due to post industrial climate change rapid regional drainage reorganizations of this type can have profound downstream impacts on ecosystems sediment and carbon budgets and downstream communities that rely on a stable and sustained discharge we suggest that the planforms of slims and kaskawulsh rivers will adjust in response to altered flows and the future kaskawulsh watershed will extend into the now abandoned headwaters of slims river and eventually capture the kluane lake drainage river piracy the diversion of one stream s headwaters into another has occurred on long timescales an analysis of streamflow and digital elevation models documents river re routing in response to glacier retreat in yukon canada in may 2016

lacustrine and palustrine facies in the bembridge limestone late eocene hampshire basin of the isle of wight southern england abstract the carbonate lake system represented by the late eocene bembridge limestone formation isle of wight southern england comprises two main facies lacustrine and palustrine these facies are arranged vertically in transgressive regressive cycles corresponding to a model of carbonate lakes of low gradient and fluctuating margins with extensive palustrine fringes these lakes developed in a coastal plain of limited relief where two paleogeographical domains predominate a lacustrine domain is represented in northern and eastern sections in the isle of wight by cycles representing upward passage from central lacustrine sub facies marl marly limestone through marginal lacustrine sub facies biomicrite to sub facies indicative of exposure brecciated nodular limestone a palustrine domain predominant in west wight is represented by a predominantly pedogenic palustrine facies with clotted peloidal to ooidal textures and some intercalated laminated laminar calcrete horizons within the pedogenic facies textures have evolved from those of unaltered biomicrites through clotted peloidal to dense ooidal to open ooidal textures a third domain developed in close geographical proximity to more saline waters is represented by gypsiferous lake margin facies found in north wight in which microlenticular gypsum developed post depositionally as a result of evaporation following subaerial sediment exposure in the lacustrine domain there appears to be an association between cyclicity and changes in base sea level reflecting a likelihood that the lakes were part of a paralic coastal plain complex with distal marine connections alternatively the cyclicity might have developed as a result of the alternation of wetter and drier climatic periods

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

carbonate deposition in an ephemeral siliciclastic alluvial system jurassic shuttle meadow formation newark supergroup hartford basin usa abstract nonmarine carbonate accumulation from a sedimentologic perspective has multiple possible origins clastic load springs groundwater discharge soil development biochemical seasonal changes cave speleothems and eolian influx many of these carbonates contain diagnostic textures from which origin is easy to ascertain however differentiating between clastic and spring groundwater carbonates especially in depositional systems dominated by siliciclastics is not always simple here criteria for recognition are developed subsurface hydrology and tectonic origin of the depositional basin source rocks surrounding the basin sedimentologic packaging and extent of the carbonates in question as well as their sedimentary structures and fossil content and stable isotopic analyses of the carbonates can all aid in determining carbonate origin and correctly assessing sedimentary and basin evolution the present study focuses on a nonmarine carbonate deposit found within the jurassic shuttle meadow formation of the hartford basin one of many rift basins which formed as a result of the opening of the north atlantic ocean during the late triassic to early jurassic the upper 50 m of the formation at plainville connecticut is interpreted as continental in origin and is composed of five facies red ripple cross laminated muddy siltstones to sandstones fr sr massive red siliciclastic mudstones fm buff fine to medium grained horizontally laminated sandstones sh red to white disrupted calcareous shales fl3 and rare micritic carbonates pm facies fr sr sh and fm are interpreted as an alluvial plain system in an ephemeral setting with bedload sheetflooding as the dominant depositional mechanism similar to the channel country drainage system of central australia in addition preservation of lateral accretion sandstone sheets accretionary benches indicate that the shuttle meadow alluvial system was rapidly accreting the anomalous existence of carbonate beds in facies fl3 and pm in the smf bedload dominated siliciclastic sequence is attributed mainly to the unique nature of the clastic sediment load in the basin no direct evidence for a spring groundwater input is apparent the shallow carbonate lake margin or a pond with vegetation and periodic exposure is envisioned as the depositional environment the intraclastic texture of the micrites with rare primary relict lamination indicates early diagenetic alteration from root penetration desiccation cracks and burrowing

experimental evaluation of thermal recording of successive polarities during uplift of metasediments summary the discovery of a post tectonic natural remanent magnetization nrm carried by pyrrhotite in the dauphinoise zone of the western alps allows us to test the ability of these rocks to record a sequence of magnetic polarity versus temperature acquired during their slow cooling several laboratory experiments composite partial thermal remanent magnetizations ptrms thellier thellier experiments trm thermal demagnetization as well as nrm demagnetization were performed this shows that the pyrrhotite grains are single domain sized and that they are able to record successive independent ptrms the nrm thermal demagnetization analysis demonstrates the occurrence of several antiparallel components a comparison between nrm and trm thermal demagnetizations is attempted in order to highlight small ptrms naturally recorded in a temperature interval of only a few degrees we attempt to retrieve the temperature at which each reversal occurs during the post metamorphic cooling this method called thermopalaeomagnetism is discussed and developed in order to delineate the sequence of polarity chrons recorded versus temperature

geochronological stable isotopes and fluid inclusion constraints for a premetamorphic development of the intrusive hosted bjorkdal au deposit northern sweden the bjorkdal gold deposit bound to a quartz vein system which is mainly hosted by a quartz monzodioritic intrusion is situated at the easternmost part of the 1 9 ga skellefte base metal district in the fennoscandian shield three fluid stages may be distinguished referred to as a barren stage a main gold stage and a remobilization stage respectively from oxygen and hydrogen isotope evidence it is argued that fluids of different origins magmatic and surface waters penetrated the ore zone at the inferred stages but regional metamorphic fluids appear essentially only to have redistributed elements early quartz veining took place during a pre metamorphic stage at ca 1 88 ga as evidenced by unradiogenic galena data and an sm nd scheelite errorchron of 1 915 32 ma mswd 0 25 temporarily the main ore forming stage was closely related to the first barren stage and took place during a major uplift event close to 1 88 ga although other source rocks cannot be totally ruled out available isotope data o s sr and pb are seemingly consistent with the view that these elements and by inference other ore elements were derived from the host intrusion

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

thermodynamics and lattice vibrations of minerals 5 applications to phase equilibria isotopic fractionation and high pressure thermodynamic properties in previous papers in this series a model that uses available elastic structural and spectroscopic data on minerals has been used to predict the thermodynamic functions cv heat capacity s entropy e internal energy and f helmholtz free energy in this paper four applications to problems of current geochemical and geophysical interest are presented 1 interpretation of complex trends of calorimetric data 2 calculation of phase equilibria 3 calculation of oxygen isotopic fractionation factors and 4 estimation of the effect of pressure on thermodynamic functions the model demonstrates that trends in high temperature thermodynamic properties of silicates are determined by the position and relative numbers of high frequency modes generally antisymmetric si al o stretching modes the position of these modes varies systematically with degree of polymerization of tetrahedra and therefore high temperature calorimetric behavior is relatively systematic as a function of crystal structure and mineral composition trends at low frequency are much more complex because the low frequency optic modes that most strongly influence the low temperature thermodynamic functions depend in a complex way on the size coordination and mass of cations and various polyhedra in the minerals the heat capacity curves of kyanite andalusite and sillimanite and of quartz coesite and stishovite show crossovers that cannot be explained by debye theory which accounts only for acoustic mode behavior but can be explained by the model spectra proposed because proper account is taken of the changing low and high frequency optic modes upon polymorphic transformations the proposed model is sufficiently accurate that phase equilibrium problems can be addressed the quartz coesite stishovite equilibrium curves the kyanite andalusite sillimanite triple point and the breakdown of albite to jadeite plus quartz are cited as specific examples for each example predicted slopes of equilibrium curves agree moderately well to excellently with slopes determined experimentally the calculated slopes are sensitive to spectroscopic parameters particularly to the distribution of optic modes in the far infrared this sensitivity is discussed in detail for the albite breakdown reaction the model can be used for prediction of isotopic fractionation factors if spectra of the isotopic forms of the mineral are known or postulated a simple set of rules for generating hypothetical spectra of 18o minerals from measured spectra of the 16o forms is given reduced partition functions are calculated for 13 minerals at 298 k the model values of reduced partition function 1000 ln of these minerals decrease in the order quartz calcite albite muscovite clinoenstatite anorthite diopside pyrope grossular zircon forsterite andradite rutile in good agreement with experimental data at 1000 k the first six minerals show small crossovers so that the order becomes calcite muscovite albite quartz anorthite and clinoenstatite the differences in 1000 ln at high temperature for these minerals are so small that the model probably cannot address the deviations from experimental trends the model clearly defines the region in which the fractionation factors do not follow a 1 t trend and should be useful for extrapolation of experimental data to low temperatures finally a modified gruneisen parameter model is proposed for shift of the lattice vibrational frequencies under compression and thermodynamic properties to 1000 kbar 1000 k are given for nine minerals at 1 mbar the predicted decrease in entropy at 298 k ranges from 54 of the 1 bar value for periclase to 25 for stishovite

experimental evaluation of magnetic interaction in pyrrhotite bearing samples pyrrhotite bearing metamorphic limestones have recently experienced an increasing relevance in paleomagnetic research simple univectorial remanences document the metamorphic uplift whereas more complex multicomponent ptrms may constrain its age for a successful application of the latter it is important to estimate the degree of magnetic interactions to ensure the additivity of individual ptrm segments we therefore have subjected the sized dispersed suite 5 250m of tte pyrrhotite to forc analysis and

geochemistry and mineral formation in the earth surface edited by r rodriguez clemente and yves tardy proceedings of the intermational meeting geochemistry of the earth surface and processes of mineral formation held in granada spain 16 22 march 1986 csic madrid 1987 893p 7000 pesetas isbn 84 00 06683 9

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

volume self diffusion of oxygen in biotite muscovite and phlogopite micas the kinetics of oxygen self diffusion have been measured in natural samples of biotite muscovite and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses all experiments were run at 1000 bars 100 mpa water pressure except those designed to measure water pressure dependence which ranged from 200 to 2000 bars the arrhenius relations obtained for layer parallel diffusive transport in the three micas by bulk exchange experiments fit to an infinite cylinder model are mineral biotite muscovite phlogopite 34 2 39 5 42 3 9 1 10 6 7 7 10 5 1 4 10 4 t c 500 800 512 700 600 900 the results of ion microprobe sims depth profiling perpendicular to the layers parallel to c axis yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers a single layer parallel step profiling experiment yields a diffusion coefficient identical within uncertainty to those obtained from bulk exchange experiments a slight dependence of diffusion rate on water pressure may obtain in phlogopite however the difference is approximately within the uncertainty of the measurement diffusion rates are similar within about a half an order of magnitude for a range of biotite composition from annite4 to annite63 as well as for muscovite comparison of oxygen diffusion rates in biotite from this study with ar k and rb diffusion from published data supports the idea that water is the oxygen bearing species in hydrothermal diffusion experiments the implication of these results for natural systems is that the micas especially biotite will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300 c the rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions full size table table options view in workspace download as csv

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

comments on stable isotope geothermometry the system quartz water we show that the results of quartz water partial oxygen exchange experiments under hydrothermal conditions are determined primarily by exchange between water and dissolved silica species and not by diffusion in the solid hence these experiments do not provide an accurate value of the variation of fractionation with temperature but only a rough indication of it we conclude that the bottinga and javoy 1 equation relying on this type of information and on the precise calculation of the coefficient a 3 7 in the thermometric equation 1000 ln a b 106 t2 is not seriously questioned by these17o 18o partial exchange experiments

detailed analysis of successive ptrms carried by pyrrhotite in himalayan metacarbonates an example from hidden valley central nepal summary low grade metacarbonates from the tethyan himalaya were sampled for palaeomagnetic studies in hidden valley central nepal the remanence is carried by pyrrhotite evidenced by thermomagnetic runs of susceptibility hopkinson peak at y300 uc alternating field demagnetization isothermal remanent magnetization acquisition and subsequent thermal demagnetization the palaeomagnetic directions reflect a tertiary overprint after the main folding event probably synchronous with the metamorphism normal and reverse remanence directions were separated and vary with altitude it is also possible to retrieve several antiparallel components versus temperature during thermal demagnetization of a single sample at higher altitudes 4920 5500 m the first component recorded is reverse r1 at a lower temperature a normal component can be extracted n1 for sites sampled at lower altitudes 4700 4900 m the hightemperature reverse component disappears but a medium temperature reverse component r2 demagnetized in a narrow temperature range can be identified in between two normal components n1 at high temperature and n2 at low temperature at the lowest altitudes 4450 4700 m only a normal component n2 appears the occurrence of successive normal and reverse polarities in one sample is interpreted as the record of successive reversals of the geomagnetic field during the post metamorphic tertiary cooling of the studied area the polarity versus altitude function is a powerful argument for a thermomagnetic origin of the magnetization no obvious rotations around a vertical axis with respect to the stable indian plate are evidenced for the tertiary however the inclination is not consistent with the expected inclination main central thrust ramping can be invoked to explain our observations r1 n1 and n2 inclinations are slightly different and their tendency is consistent with tilting towards the north during magnetization acquisition the minimum total amount of such tilting is around 25u accurate geochronological data from the tethyan himalaya would be of a great help for better resolution

fluid inclusions stable isotopes and gold deposition at bjorkdal northern sweden the bjorkdal gold deposit is located in the eastern part of the early proterozoic skellefte district in northern sweden the ore zone is hosted by a granitoid which intrudes a 1 9 ga old supracrustal sequence and consists of a network of quartz veins between two shear zones the ore mineralogy alteration assemblages ore fluid characteristics and general setting of the bjorkdal deposit reveal many similarities with mesothermal archean systems three types of fluids are represented by fluid inclusions observed in quartz scheelite and calcite the first type consists of a co2 rich fluid which is syngenetic with the formation of the quartz veins these inclusions occur in quartz and scheelite isotopic equilibrium temperatures derived from quartz scheelite pairs reflect depositional temperatures around 375 c molar volumes of the carbonic fluid inclusions ranging down to 55 cm3mole indicate a maximum lithostatic trapping pressure of 1 8 kbar these fluids were generated at depth in conjunction with early orogenic magma forming processes the gold was introduced to the vein system by the carbonic fluid but the gold was deposited after reactions between this fluid and the wall rock producing a slightly alkaline more ch4 rich aqueous type 2 fluid fluid inclusions of this chemically modified fluid indicate that the precipitation of the gold together with pyrrhotite pyrite and chalcopyrite occurred under heterogenous conditions at a temperature of 220 c and a hydrostatic pressure of 0 5 kbar the gold deposition occurred from fluids with a 18o signature of around 8 and d values close to zero per mil any metamorphic influence on the stable isotopic signatures is regarded as minimal the isotope data suggest rather that a surface derived fluid component had access to the vein system during this process at a post vein forming stage metamorphic stage a secondary episode of gold mobilization occurred as suggested by the aqueous type 3 inclusions trapped in cross cutting microfractures in quartz and randomly in calcite and with homogenization temperatures between 145 220 c and a salinity up to 11eq wt nacl

age and provenance of host rocks and ores in the paleoproterozoic skellefte district northern sweden the skellefte district in northern sweden is a ca 1 9 ga extensively mineralized mainly felsic submarine volcanic belt within the district the volcanic rocks skellefte group are overlain by

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

structural evolution of the bjorkdal gold deposit skellefte district northern sweden implications for early proterozoic mesothermal gold in the late stage of the svecokarelian orogen the bjorkdal gold deposit is situated in the eastern part of the paleoproterozoic skellefte district in northern sweden the skellefte district constitutes a 1 89 to 1 88 ga volcanic arc with numer

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

post middle miocene rotations recorded in the bourg d oisans area western alps france by paleomagnetism abstract in order to test the occurrence of post metamorphic rotations or tiltings a paleomagnetic study was undertaken in 44 sites from the inverted half graben of bourg d oisans zone dauphinoise western alps the magnetization carried by pyrrhotite was acquired after metamorphic culmination around the oligo miocene limit no obvious rotations around a vertical axis are recorded instead the dispersion of paleomagnetic data is interpreted as due to a variable tilting around a horizontal n20e axis consistent with structural observations this gentle folding in the sedimentary cover is probably associated with thrusting of the belledonne external crystalline massif over the foreland basement the age of the tiltings reported in this paper is estimated as being younger than 9 13 ma

melting behavior of fluid inclusions in laboratory grown halite crystals in the systems naclh2o naclkclh2o naclmgcl2h2o and naclcacl2h2o abstract the chemical composition of aqueous fluid inclusions in crystals of halite can be accurately determined from observed melting behaviors of ice hydrohalite and sylvite some fluid inclusion melting behaviors observed in laboratory grown halite crystals systems nacl h 2 o nacl kcl h 2 o nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o differ from predicted stable equilibrium relations in the nacl h 2 o and nacl kcl h 2 o systems observed first melt temperatures are up to 15 c below the equilibrium eutectic temperatures of 21 2 and 22 9 c respectively the final melting temperature of ice in the presence of hydrohalite and the final melting temperature of hydrohalite are reproducible and match predicted melting temperatures the limit of detection of sylvite daughter crystals in the nacl kcl h 2 o system is approximately 5 wt 1 molal kcl final melting temperatures of sylvite match published equilibrium data to within 0 3 c in the nacl kcl h 2 o system at halite saturation m kcl can be determined from the final sylvite dissolution temperature or from the final melting temperature of hydrohalite fluid inclusions in the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems that form stable salt hydrates mgcl 2 12h 2 o and cacl 2 6h 2 o during freezing first melt within 3 c of predicted eutectic temperatures 37 and 52 c however fluid inclusions with mgcl 2 or cacl 2 may also start melting at temperatures as low as 80 c such low first melt temperatures indicate the presence of metastable salt hydrates presumably mgcl 2 8h 2 o mgcl 2 6h 2 o or cacl 2 4h 2 o the formation of metastable phases during freezing of fluid inclusions can lead to misinterpretation of the chemical composition of fluid inclusions in natural samples this is especially true for fluid inclusions with first melt temperatures below 37 c which may be erroneously interpreted as being rich in cacl 2 the final melting of ice in the presence of hydrohalite may vary by more than 15 c in fluid inclusions of different size but identical bulk composition and occurs at lower temperatures than predicted in fluid inclusions from the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems however the final melting temperature of ice in inclusions which fail to nucleate hydrohalite and the final melting temperature of hydrohalite are reproducible to within 0 1 c and can be used to determine mgcl 2 and cacl 2 molalities

saying goodbye to glaciers global glacier volume is shrinking this loss of earth s land ice is of international concern rising seas to which melting ice is a key contributor are expected to displace millions of people within the lifetime of many of today s children but the problems of glacier loss do not stop at sea level rise glaciers are also crucial water sources integral parts of earth s air and water circulation systems nutrient and shelter suppliers for flora and fauna and unique landscapes for contemplation or exploration

setting of zn cu au ag massive sulfide deposits in the evolution and facies architecture of a 1 9 ga marine volcanic arc skellefte district sweden the skellefte mining district occurs in an early proterozoic mainly 1 90 1 87 ga svecofennian magmatic province of low to medium metamorphic grade in the baltic shield in northern sweden the district contains over 85 pyritic zn cu au ag massive sulfide deposits and a few vein au deposits and subeconomic porphyry cu au mo deposits the massive sulfide deposits mainly occur within and especially along the top of a regional felsic dominant volcanic unit attributed to a stage of intense extensional continental margin arc volcanism from facies analysis we interpret the paleogeography of this stage to have comprised many scattered islands and shallow water areas surrounded by deeper seas all the major massive sulfide ores occur in below wave base facies associations however some ores occur close to stratigraphic intervals of above wave base facies associations and the summits of some volcanoes that host massive sulfides emerged above sea level intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism uplift resulting in subregional disconformities and then differential uplift and subsidence resulting in a complex horst and graben paleogeography uplift of the arc is attributed to the relaxation of crustal extension and the emplacement of granitoids to shallow crustal levels a few massive sulfide ores formed within the basal strata of this second stage the horst and graben system was filled by prograding fluvial deltaic sediments and mainly mafic lavas and during this stage the skellefte district was a transitional area between renewed arc volcanism of more continental character to the north and subsidence and basinal mudstone turbidite sedimentation to the south this whole volcanotectonic cycle occurred within 10 to 15 m y we define 26 main volcanic sedimentary and intrusive facies in the skellefte district the most abundant facies are 1 normal graded pumiceous breccias which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris 2 porphyritic intrusions and 3 mudstone and sandstone turbidites facies associations define seven main volcano types which range from basaltic shields to andesite cones and rhyolite calderas despite this diversity of volcano types most massive sulfide ores are associated with one volcano type subaqueous rhyolite cryptodome tuff volcanoes these rhyolite volcanoes are 2 to 10 km in diameter 250 to 1 200 m thick at the center and are characterized by a small to moderate volume rhyolitic pyroclastic unit intruded by rhyolite cryptodomes sills and dikes massive sulfide ores occur near the top of the proximal near vent facies association the remarkable coincidence in space and time between the ores and this volcano type indicates an intimate genetic relationship between the ores and the magmatic evolution of the volcanoes many of the massive sulfide ores occur within rapidly emplaced volcaniclastic facies and are interpreted to have formed by infiltration and replacement of these facies some of the ore deposits have characteristics of both marine massive sulfides and subaerial epithermal deposits we suggest that massive sulfides in the skellefte district span a range in ore deposit style from deep water sea floor ores to subsea floor replacements to shallow water and possibly subaerial synvolcanic replacements facies models are provided for the mineralized rhyolite volcanoes and volcanological guides are provided for exploration for blind ores within these volcanoes

sm nd and ree characteristics of tourmaline and scheelite from the bjorkdal gold deposit northern sweden evidence of an intrusion related gold deposit the bjorkdal quartz vein hosted gold deposit is located 25 km northwest of skellefte in northern sweden within a paleoproterozoic volcanosedimentary sequence at the margin of a quartz monzodiorite granitoid northeast trending 030 050 quartz veins from the eastern open pit within the bjorkdal deposit contain quartz scheelite tourmaline calcite and sulfides with visible gold vein quartz shows undulatory extinction and sutured margins or is polycrystalline in form features which suggest postcrystallization deformation coarse scheelite crystals 5 mm within the quartz veins are crosscut by thin veins of quartz calcite sulfides and gold the calcite in these crosscutting fractures is variably replaced by biotite or actinolite tourmaline from the quartz veins has low total ree contents 1 chondrite and lree enriched patterns la sm n 2 8 4 5 la yb n 1 8 5 1 with strong positive eu anomalies eu eu 3 9 17 4 in contrast the scheelite has a bell shaped ree pattern enriched in mree but also with positive eu anomalies eu eu 1 4 2 4 the ree pattern of scheelite results from a strong crystallographic effect largely due to the size of the ca site and charge balance sm nd dating of scheelite from the bjorkdal ore yields an age of 1893 34 ma which coincides with a previously suggested age of the host intrusion the end values of the scheelite 1 8 and 87sr 86sr initial ratios of the tourmaline 0 7013 0 7014 are also consistent with derivation of ree and sr in these minerals from the jorn granitoids overall the petrographic geochemical and isotopic data strongly suggest that bjorkdal is an intrusion related gold deposit and that there is no requirement for involvement of external post magmatic hydrothermal fluids

rates of continental erosion and mountain building the first objective of this work was to obtain values for the rates at which continental erosion can smooth out or remove the topographic expression produced by orogeny the dominant part is played by mechanical erosion which acts most strongly in regions of large topographic expression chemical erosion depends strongly on precipitation or runoff in individual river drainage basins but because most continents have very similar average rainfall chemical erosion is fairly uniform for continental sized areas and will succeed in planing down all continents to a level peneplain if given enough time the exception to this rule is australia which has a very low chemical erosion rate because of its dryness the time constants for mechanical and chemical erosion so obtained vary between about 30 and 300 my depending on the continent and the assumptions made mountain building occurs throughout the geological time scale but at a non uniform rate although there will not be a balance between erosion and mountain building over a short time scale due to the non uniform rate of mountain building the long term situation must be that the two phenomena should balance out it is shown that the freeboard of continents will respond to the long term balance between mountain building and erosion an expression has been derived for the average continental elevation in which the rate of mountain building depends on the rate of radiogenic heat production within the earth it is shown that relatively small changes in average elevation above sea level of a few hundred metres are predicted to have occurred since the beginning of the proterozoic as mountain building is predicted to decrease on average with time because of the reduction in internal heat generation and as erosion is dependent on the average elevation this average elevation will decrease slowly through time the opposite of what some workers have predicted a more complicated model of mountain building is then investigated in which one component of mountain building has a sinusoidal signal the oscillations in average elevation depend on the period of the sinusoid being smaller for shorter periods finally an average continental elevation is derived using a list of real orogenic events although this list of orogenies is incomplete there is some indication that the actual continental elevation as seen in the flooding history of the continents is similar to that derived in this paper

crustal magnetization of mars controlled by lithology or cooling rate in a reversing dynamo 1 to explain the lack of magnetization in large portions of the noachian crust a lithological origin has been put forward corresponding to the lack of formation of magnetic minerals in the crustal rocks an alternative hypothesis is proposed here with constant amount of magnetic minerals but variable reduction of total magnetization over the magnetic thickness due to variable record of successive polarities of the magnetizing field large crustal cooling rates would correspond to strongly magnetized zone and slow cooling rates to poorly magnetized zone based on this model we derive an explanation for the proposed confinement of magnetized zones within the inter tropical band

time temperature relations for the remagnetization of pyrrhotite fe7s8 and their use in estimating paleotemperatures abstract paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains one method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences over subcrustal heat sources such as plumes or at convergent plate margins the overprinted natural remanent magnetization nrm of a rock records both the age and the paleotemperature of remagnetization but a temperature correction from laboratory to geological time scales is required using theoretical time temperature relations time temperature relations are well known for magnetite fe 3 o 4 but are reported here for the first time for pyrrhotite fe 7 s 8 another common nrm carrier data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made paleotemperature can be estimated without geological input if data for both minerals are combined together with the age of remagnetization determined from the paleomagnetic pole of the nrm overprint these paleotemperature estimates can be used to infer the history of heating and uplift following burial as a test case we examine thermally acquired nrm overprints carried by pyrrhotite fe 7 s 8 and magnetite fe 3 o 4 in the milton monzonite of southeastern australia these overprints record a heating event about 100 ma ago probably thermal doming prior to rifting of the tasman sea that upgraded coal rank in the sydney basin extrapolating from laboratory to geological times using the new time temperature contours for pyrrhotite we estimate that the presently exposed sydney basin in the vicinity of the milton monzonite was remagnetized by heating to 165 30 c for 100 ka assuming a paleogeothermal gradient of 70 c km appropriate for young or incipient rifts the depth of burial at the time of remagnetization is estimated to have been 2 3 0 4 km this figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite

geochemistry of agates a trace element and stable isotope study abstract agate samples of acidic intermediate and basic volcanics from 18 localities around the world and of different age precambrian to tertiary were studied by trace element and stable isotope analysis to provide information about the process of agate formation and the origin of mineral forming fluids trace element data are similar for agates from acidic and basic volcanics the general chondrite normalized ree distribution pattern is characterized by a slope from la to lu with enriched lree and a positive eu anomaly in some samples the similarity in the shape of the ree patterns between agates and the parent volcanic rocks suggests that the elements are mobilized by circulating fluids during syn and postvolcanic alteration of the volcanic wall rocks observed positive eu anomalies in agates probably originate from feldspar alteration deuterium and oxygen isotope analyses of agates and associated quartz incrustations d 44 to 130 18o 16 4 to 33 4 reveal variations in isotopic composition between samples of different localities but also within single agate samples up to 10 for 18o in general oxygen isotope compositions become heavier as volcanic host rocks grow more acidic furthermore agates have higher 18o values than associated quartz incrustations variations within single agate samples can be explained either by kinetic effects during isotope fractionation e g the formation of agate from a noncrystalline precursor or by mixing processes of meteoric and magmatic fluids remarkably high 18o values of the parent volcanic rocks up to 19 5 suggest that the circulation of 18o enriched hydrothermal fluids originated from heated meteoric water and or residual magmatic fluids this conclusion is supported by 13c and 18o data of paragenetic calcite the temperature of agate formation was calculated for different fluid compositions and they indicate a temperature range of ca 50 c to 250 c

thermochronometry and cooling rates deduced from single sample records of successive magnetic polarities during uplift of metamorphic rocks in the alps france summary pyrrhotite bearing metamorphic calcschists from the western alps very probably carry a trm acquired during slow cooling of the unit during the late tertiary sequences of successive polarities have been recorded and can be unravelled by very detailed thermal demagnetization polarity versus temperature profiles on single samples are compared to the polarity time scale allowing on a statistical basis the estimation of a cooling rate of soocmyr at the time of trm acquisition 20 25 ma low apparent palaeointensities i0 pt are estimated by comparison with artificial trm