palaeomagnetism of the ferrar dolerite in the northern prince albert mountains victoria land antarctica summary during the sixth italian expedition to victoria land antarctica jurassic ferrar dolerite sills were sampled at 23 sites in the northern prince albert mountains the rock magnetic properties were similar in all sites saturation remanence remanent coercive force and curie temperature values pointed to titanomagnetite as the main carrier of magnetization thermal and af demagnetizations were both effective in removing weak secondary magnetization components below 250 c and 20 mt peak field further demagnetization resulted in single component stable chrm palaeomagnetic directions were derived after af demagnetization at 20 mt and site mean vgps computed for all sites but one thern a where systematic changes of the chrm inclination have resulted in single sample vgps dispersed astride the equator the site mean vgps distribution is clearly bimodal 15 vgps cluster around a palaeopole lat 48 s long 226 e in good agreement with the literature data whereas seven vgps are displaced toward a palaeopole at higher latitude lat 79 s long 181 e all vgps can be regarded as jurassic because no evidence of substantial secondary components was found and experimental data showed that the ptvrm partial thermal viscous remanent magnetization acquisition rate is too low to account for complete remagnetization of the rocks giving the anomalous vgps the thern a and the southward displaced vgps plot along a great circle through the east antarctica jurassic palaeopole as a high reversal rate in the jurassic is well documented by magnetostratigraphy and the reversal vgp path is often confined to a narrow longitudinal band we suggest that the great circle is a palaeomeridian and the sills characterized by anomalous vgps were intruded during transitional phases of the geomagnetic field similar anomalous vgps previously reported from the ferrar supergroup rocks of northern victoria land also plot along the same palaeomeridian which is only 30 away from the palaeomeridian derived from the jurassic intrusives of the dufek massif in the transantarctic mountains burmester sheriff 1980 the jurassic palaeopole of east antarctica has been recalculated from 103 vgps available in the literature the position is 57 s 221 e when all vgps are taken into account and 51 s 223 e when the high latitude vgps from victoria land are discarded as anomalous

felsic magmatic phases and the role of late stage aplitic dykes in the formation of the world class cantung tungsten skarn deposit northwest territories canada abstract a field and petro chemical classification of felsic magmatic phases fmps at the world class cantung w skarn deposit was undertaken to document the evolution of magmatism and the relationships between different fmps metasomatism and mineralization early fmps include moderately differentiated zr hf 18 26 ti zr 14 15 biotite monzogranitic plutons and early biotite rich granitic dykes and compositionally similar quartz feldspar porphyry dykes late highly fractionated zr hf 8 17 ti zr 3 13 fmps sourced from a deeper monzogranitic intrusion include 1 leucocratic biotite or tourmaline bearing dykes derived from localized entrapments of residual magma and 2 sub vertical ne trending aplitic dykes derived from a larger segregation of residual fluid and incompatible element enriched magma the aplitic dykes have textures morphologies spatial associations and a pervasive calcic metasomatic mineral assemblage ca plagioclase quartz or clinozoisite indicative of syn mineralization emplacement very late stage overpressuring and initiation of sub vertical fractures into the overlying plutonic carapace and country rocks by supercritical magmatic fluid led to an interaction with calcareous country rocks that resulted in an increased aca2 in the fluid and the concurrent precipitation of w skarn residual magma also ascended with and quenched in equilibrium with the magmatic fluid to from the aplitic dykes then was metasomatized by the fluid as it interacted with calcareous country rocks overall highly fractionated and moderately to very highly undercooled fmps at cantung provide evidence for a large and evolving felsic magmatic system at depth that segregated and maintained a stable fluid and incompatible element enriched residual magma until the latest stages of crystallization the detailed study of fmps associated with magmatic hydrothermal mineral deposits allow us to refine our understanding of these mineralizing systems and better define metallogenic and exploration models for intrusion related mineralization

crustal outgassing and lile enrichment in major lithosphere structures archean abitibi greenstone belt evidence on the source reservoir from strontium and carbon isotope tracers major structural discontinuities in the abitibi greenstone belt acted as conduits for outgassing of the archean crust as reflected in fixation of a select group of lithophile elements including si c k rb ba li cs b and pb in metasomatized faults for two of the largest structures the destor porcupine dp and kirkland lake cadillac kc fault zones 6 1015 g si 3 1015 g co2 and 1015 g k were introduced into the faults during expulsion of an estimated 6 1018 g aqueous fluids strontium isotope ratios of tourmaline piemontite actinolite and scheelite mineral separates characterized by rb sr 0 02 are concordant with respect to 87sr 86sr initial ratios over local sectors of the faults the sr isotope data record geographic variations which from east to west on the kc fault is 0 7031 0 7041 val d or 0 7008 0 7022 bourlemaque 0 7017 0 7019 bousquet 0 7029 0 7031 noranda and 0 7013 to 0 7015 kirkland lake at timmins on the pd fault 87sr 86sr initial ratios cluster at 0 7010 to 0 7020 metasomatised fault zones are systematically more radiogenic than contiguous host lithologies and imply a source reservoir 0 7010 to 0 7041 generally more radiogenic than the upper mantle at 2690 ma 0 700 0 001 or contemporaneous volcanic rocks of mafic to ultramafic composition 0 700 to 0 7012 whereas certain minerals are concordant and retentive rb sr isochrons based on suites of rocks at progressive intensities of metasomatism have been systematically reset over an elpased time of 200 ma after termination of outgassing due to disturbance accompanying incremental displacements on structures

the drift history of adria and africa from 280 ma to present jurassic true polar wander and zonal climate control on tethyan sedimentary facies article i nfo the orogenic belts surrounding the undeformed adriatic sea represent the margins of an area known as adria the african promontory we have undertaken a critical appraisal of paleomagnetic data from regions of adria considered parautochthonous relative to africa obtained either from biostratigraphically dated sed imentary rocks corrected for inclination shallowing or from igneous rocks that are regarded as free from any inclination shallowing bias paleomagnetic directions were used to calculate paleomagnetic poles for compar ison with coeval and inclination flattening free paleomagnetic poles from stable africa visual coherence of paleopoles for several time slices from the early permian to the eocene supports the construction of a com posite apparent polar wander path apwp valid for parautochthonous adria and stable africa this compos ite apwp is compared to previous apwps finding good agreement with the global apwp of kent and irving 2010 both apwps show a remarkable and rapid polar shift of 40 in the jurassic that other apwps tend to underestimate we interpret this shift to represent a major episode of true polar wander tpw from 183 ma in the early jurassic to 151 ma in the late jurassic using a simple zonal climate model the drift motion of adria attached to africa appears to be consistent with the distribution of permian cretaceous sed imentary facies on adria

a comparison of two sequential geomagnetic polarity transitions upper olduvai and lower jaramillo from the southern hemisphere abstract two normal to reverse n to r geomagnetic polarity transitions were obtained from a southern hemisphere deep sea sediment core 35 91 s 59 97 e the upper jaramillo reversal was continuously sampled by taking 0 5 cm thick samples across 55 cm of section and the upper olduvai reversal was sampled in a similar manner across 60 cm both records are characterized by full normal and reverse polarity directions which are in very good agreement with those predicted by axial dipole fields for the core site latitude each record also exhibits a zone yielding intermediate directions because of a large increase in both the within level and the between level scatter in the upper jaramillo transition zone it is not considered to represent an accurate record of the geomagnetic field the upper olduvai transition however appears to be a more coherent record it is characterized by directions that shallow early in the reversal but then rapidly steepen to nearly vertical upward directions the declinations do not change until the inclinations have passed through the vertical the total directional change occurs within a broad nrm intensity minimum the virtual geomagnetic pole vgp path calculated for this record is not longitudinally constrained but instead exhibits a westward progression through the reversal when considered together with the lower jaramillo transition previously reported from this core these records constitute a set of sequential southern hemisphere transitions a number of striking similarities exist in the two records such as steep upward directions and a shallowing which occurs between the near vertical and the full polarity directions the results of zonal harmonic modeling of these records are consistent with the idea of a standing field which persisted across both the upper olduvai and lower jaramillo reversals

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

rapid changes and near stationarity of the geomagnetic field during a polarity reversal one of the most important suggestions from many of the new reports of sedimentary geomagnetic records is that the time dependence of the changes of the geomagnetic field during a reversal might be more complex than early records indicated indeed alternating periods of rapid and slow field changes are apparent in some sedimentary records1 4 in volcanic records very fast directional changes recorded by individual lava flows during their progressive cooling have been reported5 6 but a major problem when inferring the temporal behaviour of the transitional field is that the extrusion rate of the successive lava flows is much more variable and more difficult to evaluate than the rate of accumulation of the sedimentary sequences here we report the results of a detailed study of a geomagnetic reversal recorded in middle miocene marine clays with temporal resolution almost comparable with that of the volcanic records and in our opinion a more reliable chronological control the record displays large fluctuations in the rate of directional change and different episodes of collapse and recovery of the field intensity a very fast n s directional change may be entirely controlled by a frozen flux dominated reversal mechanism

link between geomagnetic reversal paths and secular variation of the field over the past 5 myr palaeomagnetic records provide information about the behaviour of the geomagnetic field during reversals1 2 existing records are incompatible with transitional field configurations that are either entirely dipolar or entirely zonal dependent only on latitude 3 4 recent compilations5 8 have indicated that the transi tional paths of virtual geomagnetic poles vgps for the past few reversals are located preferentially within two antipodal longitudinal bands suggesting that simple but non zonal field configurations dominate during reversals here i point out that one of the longitudinal bands coincides with that expected from the reversal of a non axial dipole field exactly like that present today the other requires only a sign change in the non axial dipole terms of today s field evidence for persistent non zonal contributions to the field has generally9 13 but not always14 13 been regarded as not statistically significant in the light of poor data distributions i show here that a non zonal bias similar to that observed in reversal data is evident in data on secular variation of the field over the past 5 myr even after normalization according to site locations these results suggest that the time averaged field does indeed contain persistent but not constant non zonal contributions

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

geochemistry and genesis of two unconformity associated gold deposits at the base of the flinton group grenville province southeastern ontario canada the addington and ore chimney mines are proterozoic gold bearing quartz carbonate vein deposits that occur in lower amphibolite facies metamorphic rocks on and near a major regional unconformity the ore chimney deposit has a typical opaque mineral association of galena sphalerite pyrite chalcopyrite pyrrhotite ag rich tetrahedrite ilmenite the vein system is hosted by a 4 m thick biotite schist enclosed by metabasaltic lava flows the metavolcanic rocks are unconformably overlain successively by a 10 m thick metapelitic garnet biotite schist the ore chimney formation and a coarse siliciclastic succession the flinton group the ore chimney deposit occurs 30 m below the unconformity the addington deposit has a typical opaque mineral assemblage of arsenopyrite pyrite chalcopyrite pyrrhotite ilmenite this deposit is hosted by the ore chimney formation and is located 8 km along strike from the ore chimney mine arsenopyrite geothermometry and sphalerite geobarometry yield temperature and pressure estimates ranging from 420 degrees to 530 degrees c and 0 35 to 0 59 gpa respectively consistent with estimated peak metamorphic conditions geochemical textural and structural evidence suggest that vein formation and mineralization occurred during the main episode of metamorphism and deformation and that gold was deposited by metamorphic fluids the mineralizing process was associated with weak carbonatization of the wall rock gold and base metals may have been remobilized from preexisting concentrations resulting from hydrothermal and or weathering processes

chemical weathering in two adirondack watersheds past and present day rates rates of chemical weathering in two forested adirondack watersheds were determined from mineral and elemental depletion trends in soil profiles and from input output budgets based on precipitation and surface water chemistry long term rates of weathering have averaged about 500 to 600 eq ha yr for both watersheds since the glaciers retreated from the region about 14 000 yr ago present day denudation rates average 1679 eq ha yr in the panther lake watershed and only 198 eq ha yr in the woods lake watershed mineral weathering reactions in both watersheds involve primarily the dissolution of plagioclase potassium feldspar and hornblende mass balance calculations however indicate that hornblende weathers in disproportionately large amounts in the panther lake watershed relative to its abundance in soils and till cation exchange in the glacial till mantling the panther lake watershed may also play an important role in controlling surface water chemistry in the basin in the panther lake watershed the current rate of weathering which is about a factor of 3 greater than the long term average may reflect a recent adjustment for higher hydrogen ion fluxes brought about by acid deposition the capacity of the thick till deposits in the panther lake watershed to neutralize acidity is ultimately reflected by the circum neutrality of panther lake in the woods lake watershed the soils and thin till deposits cannot effectively neutralize current acid loadings and therefore woods lake is acidic with a ph below 5

rock magnetic stratigraphy of a mafic layered sill a key to the karoo volcanics plumbing system abstract the insizwa sill is an 1 km thick subhorizontal layered mafic intrusion and part of the karoo large igneous province in south africa this well exposed intrusion consists of several superimposed petrologically and geochemically distinct units magnetic methods were used to study the intrusion in order to constrain the physical processes active in these types of bodies during crystallization rock magnetism studies indicate that within different petrologic units bulk susceptibility is controlled by primary magnetite with minor pyrrhotite and or paramagnetic minerals olivine pyroxene new magnetic data based on 659 specimens obtained from 3 vertical borehole cores each spaced 5 km apart confirm the prominent vertical zonation in low field magnetic susceptibility k lf degree of anisotropy p j and orientation of the anisotropy of magnetic susceptibility ams axes the magnetic susceptibility correlates very well with petrographic units and the lateral continuity of magnetic units between boreholes is very consistent petrologic units with high but variable k lf also show moderate anisotropy and dominantly vertical foliations we interpret these patterns to result from inverse fabrics from single domain magnetite the degree of anisotropy is low in petrologic units with low k lf which also show shallowly dipping magnetic foliations we interpret that the magnetic properties of these units are dominated by the paramagnetic minerals these low k lf petrologic units also show no systematic increase in k lf suggesting that only minor differentiation is occurring in these units the dataset derived from 2 surface sampling traverses are consistent with borehole core ams data showing a pattern of dominantly steep magnetic foliation and variably plunging magnetic lineation with a nw se trend

analytical errors in the determination of high field strength elements and their implications in tectonic interpretation studies abstract the quality of analytical data obtained from cost effective commercial multi element packages is examined with respect to its use in tectonic interpretation studies good accuracy and precision in the determination of high field strength elements zr y ti nb hf ta and the rare earth elements ree is essential in extracting the correct and maximum information for discrimination diagrams misinterpretation can result from the use of noisy and biased analytical data this is shown by the poor quality of results for y and nb where the concentrations measured are close to the method s detection limits a round robin study to determine zr nb hf and ta demonstrates the significant variability to be expected amongst laboratories even for elements with relatively high abundance levels such as zr the round robin results are compared with a sensitive analytical method based upon inductively coupled plasma mass spectrometry icp ms and libo 2 fusion the inefficiency of the common mixed acid attack hf hclo 4 hno 3 hcl to dissolve the heavy ree in some samples is proven by comparison with results employing a fusion decomposition and with instrumental neutron activation analysis such a partial attack prior to analysis can lead to an erroneous chondrite normalised pattern incorrectly suggesting light ree enrichment and masking positive or negative eu anomalies

application of geochemical discrimination diagrams for the tectonic interpretation of igneous rocks hosting gold mineralisation in the canadian shield abstract application of the high field strength elements zr y ti nb and ta to the tectonic interpretation of mafic rocks hosting gold mineralisation is illustrated by example of the joburke mine property near timmins ontario canada discrimination diagrams using these analytical data suggest that the mafic rocks are predominantly high fe tholeiitic basalts distinguishable from average archean basalts the derived geochemical signatures of the host rocks for the joburke mine will be of assistance for gold exploration in the timmins camp or in greenstone belts generally

lithologic and climatologic controls of river chemistry abstract the chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff in all 101 different rivers each draining a primary lithology were examined across the united states puerto rico and iceland to minimize seasonal effects only rivers with at least two years of data were used basaltic catchments were examined in the greatest detail in a survey of hawaiian island watersheds the average river chemistries could be related to the distribution of soil associations within each catchment an analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium ion exchange control on cation ratios among different lithologies unique weathering rate relationships were developed with yields at typical present day runoff rates 1 100 cm y increasing in the order sandstones granites basalts shales and carbonates the bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering thus a warm wet climate or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal

craton scale distribution of archean greenstone gold deposits predictive capacity of the metamorphic model genetic models should not only attempt to explain features of archean gold mineralization on the scale of mines or districts but should also explain its marked spatial and temporal heterogeneity in greenstone belts the application of genetic models developed with such a philosophy can have predictive capacity in gold exploration the metamorphic model one of a number of current genetic models for archean gold deposits involves generation of auriferous h 2 o co 2 fluids by devolatilization of the stratigraphically lowermost greenstones during metamorphism and broadly synchronous synkinematic granitoid intrusion crustal scale fault shear systems are required to induce channeled fluid flow and to focus ore fluids toward suitable host rocks and structural sites declining temperature below the amphibolite greenschist transition combined with sulfidation of fe rich host rocks is a particularly efficient au depositing mechanism the model suggests that the major controlling factors in declining order of importance are 1 high temperature low pressure generally low strain metamorphic regimes 2 crustal scale fault systems 3 unaltered fe rich rocks tholeiites banded iron formation at a suitable stratigraphic metamorphic level and 4 abundant au enriched komatiites and s rich sediments in source regions the low pressure metamorphic conditions combined with the occurrence of elongate high strain zones in most if not all generally low strain greenstone belts may explain the ubiquitous occurrence of gold deposits in archean terranes all four favorable parameters appear to act in concert to produce the largest gold deposits and most abundant gold mineralization in 2 7 or 0 1 b y old rift phase greenstone belts interpreted to have formed under conditions of high crustal extension and extensive crustal thinning i e with predicted high geothermal gradients high fault density limited synvolcanic alteration and abundant komatiites and sulfidic sediment horizons more widespread commonly older platform phase greenstone belts which formed under conditions of lesser crustal extension and thinning are less mineralized as predicted by the metamorphic model the ability of the model to explain the regional distribution of known archean gold deposits lends credence to its predictive capacity for example 1 greenstone belts in any one craton can be broadly ranked in order of likely relative gold potential 2 the likelihood of discovery of large deposits in poorly exposed or poorly explored belts of known type can be estimated 3 areas with the highest potential can be defined in rift phase greenstone belts by the coincidence of critical parameters such as specific host rock sequences and metamorphic facies 4 prospective structural sites for gold mineralization can be postulated in platform phase greenstone belts affected by diapiric tectonics and 5 the possible spacing of large deposits can be estimated further the potential use of the model in exploration emphasizes serious gaps in our knowledge of parameters crucial to understanding greenstone belt metallogeny including alteration patterns metamorphic p t paths and regional scale structural regimes

magnetic anisotropy produced by magma flow theoretical model and experimental data from ferrar dolerite sills antarctica summary volcanic rocks forming sills dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement we model a sill as a steadystate flow of a bingham fluid driven by a pressure gradient in a horizontal conduit the magma velocity as a function of depth is calculated from the motion and constitutive equations vorticity and strain rate are determined for a reference system moving with the fluid the angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations it is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction and that the magnetic foliation plane changes periodically from horizontal to vertical the results are compared with the magnetic fabric of ferrar dolerite sills victoria land east antarctica derived from low field susceptibility measurements

palaeomagnetism and 40ar 39ar dating from lower jurassic rocks in gastre central patagonia further data to explore tectonomagmatic events associated with the break up of gondwana new 40 ar 39 ar data indicate ages of c 185 ma for the lonco trapial volcanic field in gastre north central patagonia implying that this andesitic unit is roughly coeval with the marifil silicic province that crops out in the eastern part of northern patagonia these volcanic fields are therefore roughly coeval with the karoo ferrar large igneous province further contributing to the huge 185 180 ma magmatic outpouring in southern gondwana the comparison of palaeomagnetic results from the unit with early jurassic reference palaeopoles suggests that gastre records a small anticlockwise finite rotation with respect to stable south america this contrasts sharply with the large clockwise rotation recorded by roughly coeval dolerites from the falkland malvinas islands and argues against a direct relationship between the rotation of the islands and the tectonic activity in gastre the results support previous suggestions that the gastre fault system is not the locus of major strike slip displacement activity in the jurassic as some workers have suggested taken as a whole palaeomagnetic data from patagonia suggest a more complex than previously thought pattern of distributed and variable deformation during the early stages of gondwana breakup the crustal block boundaries in the region remain cryptic supplementary material 40 ar 39 ar analytical procedures and expanded data are available at http www geolsoc org uk sup18522

magnetic study of an antarctic weathering profile on basalt implications for recent weathering on mars abstract in order to better interpret the martian surface weathering related mineralogy we focused on a relevant analogue of the martian subsurface in terms of lithology and paleoclimate the jurassic aged ferrar dolerite priestley glacier transantarctic mountains weathered in cold and dry climate together with chemical and mineralogical studies rock magnetic properties were investigated and completed with mossbauer measurements weathering of the decimetric block is evidenced by chemical profiles showing an increase in fe content from 10 5 in the core to 13 wt in the surface and a decrease in si from 57 to 53 wt respectively according to mineralogical thermomagnetic and hysteresis properties the main opaque mineral is inherited titanomaghemite with a concentration about 1 enhancement in low field magnetic susceptibility from 4 10 6 to 10 10 6 m 3 kg respectively and saturation magnetization from 0 44 to 0 96 am 2 kg respectively indicates the neoformation of metastable maghemite neoformed red ferric oxy hydroxides are abundant in the surface high field susceptibility normalized to iron concentration and mossbauer spectra indicate a replacement of fe 2 from primary silicates pyroxene by poorly crystalline antiferromagnetic fe 3 oxy hydroxides thus highly magnetic titanomaghemite and maghemite coexist with nanosized ferric oxy hydroxides and primary silicates in accordance with martian in situ observations therefore this study supports the formation of the martian regolith as resulting from a slow weathering process in near present day conditions

the blake geomagnetic event transition geometry dynamical characteristics and geomagnetic significance abstract two records of the blake geomagnetic event have been obtained from marine cores in the mediterranean the upper bound for duration of the event calculated from accurate oxygen isotope stratigraphy and or tephrochronology is 4000 years one record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved the directional changes do not occur at constant speed rather a stop and go behaviour is observed as in detailed records of reversals the virtual geomagnetic pole vgp path confined to two bands of longitude over the americas and antipodal to them is virtually identical to the only other path of this event obtained from lacustrine sediments in the western usa suggesting that a dipolar component may be present in the transitional field during the blake event the two bands of longitude coincide with those of many recent reversals suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time

jurassic magnetostratigraphy 3 bathonian bajocian of carcabuey sierra harana and campillo de arenas subbetic cordillera southern spain abstract four sections in majocian bathonian middle jurassic pelagic limestone with standard ammonite zonation have yielded magnetic polarity sequences magnetic directions in these red to white limestones were obtained by thermal demagnetization and were stable from about 300 c to in excess of 450 c the polarity patterns indicate that the majority of the bajocian and bathonian is characterized by quite frequent reversals of the magnetic field lengthy periods of constant polarity particularly constant normal polarity were not observed the average frequency of reversals is about 6 per ammonite zone which roughly may be interpreted as a frequency of a reversal every 260 000 years a rate comparable to that of the miocene pliocene paleolatitudes of these sites 25 28 are about 10 south of their present positions variable clockwise block rotations within the subbectic region have rotated these sites relative to stable iberia

oxidation processes and their effects on the magnetic remanence of early cretaceous subaerial basalts from sierra chica de cordoba argentina fil geuna silvana evangelina consejo nacional de investigaciones cientificas y tecnicas oficina de coordinacion administrativa ciudad universitaria instituto de geociencias basicas aplicadas y ambientales de buenos aires universidad de buenos aires facultad de ciencias exactas y naturales instituto de geociencias basicas aplicadas y ambientales de buenos aires argentina

application of the geographical information systems approach to watershed mass balance studies this study was undertaken to test the utility of a geographical information systems gis approach to problems of watershed mass balance this approach proved most useful in exploring the effects that watershed scale lithology and land use have on chemical weathering rates and in assessing whether mass balance calculations could be applied to large multilithological watersheds water quality data from 52 stations were retrieved from storet and a complete gis database consisting of the watershed divide lithology and land use was compiled for each station water quality data were also obtained from 7 experimental watersheds to develop a methodology to estimate annual fluxes from incomplete data sets the methodology consists of preparing a composite of daily flux data calculating a best fit sinusoid and integrating the equation to obtain an annual flux comparison with annual fluxes calculated from high resolution data sets suggests that this method predicts fluxes within about 10 of the true annual flux annual magnesium fluxes moles km 2 yr 1 were calculated for all stations and adjusted for fluxes from atmospheric deposition magnesium flux was found to be a strong function of the amount of carbonate in the watershed and silica fluxes were found to increase with the fraction of sandstone present in the watershed all fluxes were strongly influenced by mining practices with magnesium fluxes from affected watersheds being 6 10 times higher than fluxes from comparable pristine watersheds mining practices enhance chemical weathering by increasing the surface area of unweathered rock to which water has access and by increasing acidity and rate of mineral weathering fluxes were also found to increase with watershed size this scale dependence is most likely caused by the sensitivity of weathering fluxes to even minor quantities of carbonates which are likely to be found in all lithologies at larger scales mass balances were carried out in watersheds where gauged sub watersheds made up more than 95 of the area the calculations show large magnesium flux and water balance discrepancies these errors may be a result of significant groundwater inputs to streams between gauges the results suggest that improvements in how we measure discharge and estimate fluxes may be required before we can apply mass balance techniques to larger scales 1997 john wiley sons ltd

kar dates and paleomagnetic evidence for cretaceous alteration of mesozoic basaltic lava flows mesa range northern victoria land antarctica abstract the mesozoic tholeiite basalts in the transantarctic mountains are probably jurassic in age based on a fragmentary fossil record on their stratigraphic relation to the older sedimentary rocks of the beacon supergroup and on a few 40 ar 39 ar and rb sr age determinations however the whole rock k ar dates of these basalts vary widely between 100 and 200 ma and do not constrain the age of these rocks new whole rock a ar dates of flows on the summit of pain mesa in the mesa range of northern victoria range from 103 4 to 174 7 ma and confirm previously published results however chemical analyses indicate that the k2o concentrations of these flows on pain mesa decrease with increasing volatile content loss on ignition and that most of the low k flows in the mesa range yield low k ar dates this evidence suggests that k was lost as a result of alteration of the flows by aqueous solutions and that the alteration was accompanied by even greater losses of radiogenic 40ar accordingly the new k ar dates of these rocks indicate that the age of alteration is 176 8 ma the discordance of k ar dates of the lava flows of the mesa range as well as those at litell rocks reported by other investigators suggests that northern victoria land experienced a geologic event during the cretaceous period that caused low grade thermal metamorphism and hydrothermal alteration of the flows the occurrence of such an event is supported by paleomagnetic evidence indicating that the basalt flows of the mesa range and the litell rocks as well as sedimentary rocks of early paleozoic age of the bowers terrane in northern victoria land have anomalous virtual geomagnetic pole positions caused by remagnetization during the cretaceous period the tectonothermal and hydrothermal activity in northern victoria land during the cretaceous period indicated by these results may have been caused by the reactivation of deep crustal faults in the area during the separation of australia from antarctica

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

new paleomagnetic pole for the upper jurassic chon aike formation of southern argentina south america testing the tectonic stability of patagonia with respect to south america and implications to middle jurassic early cretaceous true polar wander abstract a paleomagnetic pole and a new ar ar date of 156 08 0 05 ma are reported for the chon aike formation an extensive plateau of ignimbrites outcropping in the deseado massif southern patagonia argentina the geographic coordinates and statistical parameters for the paleomagnetic pole validated by a reversal test are lat 84 3 s long 191 3 e a95 8 6 k 13 3 n 23 this pole shows a complete coherence with late jurassic and early to mid cretaceous poles of south america obtained exclusively from igneous rocks avoiding any potential inclination shallowing it is suggested that a southern patagonia was an integral part of south america in the late jurassic b any reported vertical axis tectonic rotations must pre date the late jurassic in this region c according to updated paleomagnetic data from igneous rocks south america had very limited latitudinal drift between about 160 and 140 ma and therefore d its apparent polar wander apw path when compared with global apparent polar wander paths does not support a large global shift or true polar wander tpw between 160 and 140 ma

stratigraphy and geochemistry of the igneous rocks in the elu link between hope bay and elu greenstone belts northeast slave craton tectonic setting and implications for gold mineralization geological investigation of the rocks in the elu link has provided new information on the geodynamic origin of the neoarchean ca 2716 2663 ma hope bay and elu granite greenstone belts stratigra

the behavior of gold in unaltered island arc tholeiitic rocks from izu oshima fuji and osoreyama volcanic areas japan high au concentrations 4 25 ppb in titanomagnetite concentrates from basalts and basaltic andesites of the izu oshima and fuji volcanic areas in the east japan arc indicate that au is strongly partitioned into titanomagnetite relative to the bulk rock the concentration of an in volcanic rocks increases during the early stages of fractional crystallization dominated by olivine pyroxenes and plagioclase but decreases with subsequent fractional crystallization involving titanomagnetite a delay in titanomagnetite formation during fractional crystallization subsequently results in comparatively high concentrations of au in island arc tholeftes wide variations in the au concentration are observed in rocks derived from primary magmas across a single volcanic area and these can be positively correlated with the concentrations of incompatible elements such as rb and zr these features are inherited from a s depleted mantle source since variable degrees of partial melting do not significantly affect the au concentrations of the primary magmas

mafic to felsic magmatism and crustal recycling in the obonga lake greenstone belt western superior province evidence from geochemistry nd isotopes and u pb geochronology abstract the central wabigoon subprovince of the western superior province contains mesoarchean granitoid and supracrustal rocks 3 01 2 83 ga and 2 78 2 69 ga granitoid plutons and supracrustal sequences it is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement the obonga lake greenstone belt contains two distinct assemblages 1 a ta nb th they have end values of 0 7 to 2 4 a dacitic unit in the northern assemblage has high la yb high sr y low nb y and heavy ree and is interpreted to represent a mantle modified slab melt similar to adakites in cenozoic arcs the associated enriched basalts may represent melts from the mantle wedge modified by slab melt adakitic metasomatism or they may represent an enriched oib like asthenospheric source rocks of the southern assemblage mostly show th and light ree enrichment and negative nb and ta anomalies low end values down to 0 9 can be modelled through contamination of a mafic liquid by 3 3 3 2 ga sialic crust although rocks of that age have not been observed suggested mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years the southern assemblage volcanism may have occurred in a continental arc to continental back arc system where crustal recycling played an important role

cenozoic strike slip faulting from the eastern margin of the wilkes subglacial basin to the western margin of the ross sea rift an aeromagnetic connection

paleomagnetism of the jurassic transantarctic mountains revisited evidence for large dispersion of apparent polar wander within less than 3myr the transantarctic mountains predominantly consist of jurassic continental flood basalts kirkpatrick and sills ferrar emplaced during the earliest phase of the break up of pangea published ages based on a variety of geochronological methods all agree rather well and cluster around 180 ma suggesting emplacement during a rather short time interval of not more than 3 myr paleomagnetic studies mostly carried out between the 60s and 90s of the last century however yield two well defined but significantly different groups of paleomagnetic pole positions plotting either in intermediate latitudes a or at latitudes exceeding 60 degrees s b pole positions belonging to group a are generally interpreted to be of primary origin whereas the significance of group b poles remains unclear here we report new data from the kirkpatrick basalts of northern victoria land antarctica where 157 oriented paleomagnetic samples were taken covering about 800 m of stratigraphy and 23 volcanic flows in gair mesa 73 4666 degrees s 162 8666 degrees e after removal of a steep magnetic overprint with rather low coercivities straight linear segments of exclusively normal polarity trending toward the origin of projection are identified in 151 samples from 22 sites maximum unblocking temperatures do not exceed 580 degrees c and maximum coercivities not 60 mt the resulting mean virtual geomagnetic pole vgp for the gair mesa plots at 66 4 degrees s 227 7 degrees e alpha 95 of 6 2 degrees k 25 7 comparable to group b poles reflecting light microscopy reveals that the magnetic inventory of all samples analyzed is dominated by magnetite showing shrinking cracks and broad ilmenite lamellae the latter being diagnostic for high temperature oxidation analysis of the distribution of the 22 pole positions obtained here shows that secular variation has successfully been averaged s b 15 9 degrees suggesting that the rocks studied here have recorded the time averaged geomagnetic field during early mid jurassic times based on these new results we postulate that both clusters a and b of pole positions reflect primary magnetizations and that the kirkpatrick basalts of gair mesa have been emplaced in a rather short normal polarity time interval of 3 myr if our reasoning is correct the apparent polar wander path for the latest early jurassic might be more complex than previously thought whether this complexity is evidence for massif true polar wander remains speculative c 2015 international association for gondwana research published by elsevier b v all rights reserved

lithophile element systematics of archean greenstone belt au ag vein deposits implications for source processes reply potassic alteration domains of greenstone belt lode gold deposits are characterized by systematic partitioning between lithophile elements k rb and ba are generally co enriched and linearly correlated over almost three orders of magnitude in abundance where k rb 220 400 and k ba 30 85 k cs 104 and k tl 2 104 are also correlated though more weakly lithium abundances and rb sr ratios are erratic in altered rocks these interelement trends collectively are present in deposits variously hosted by ultramafic mafic or felsic volcanic rocks and sediments or granitoids magmatic processes involving crystal fractionation of biotite k feldspar and plagioclase generate trends to systematically diminished k rb 50 k li k cs and k tl but enhanced k ba 8 103 and rb sr in most late stage differentiates such late stage trends are the rule in magmatophile deposits including the archean cadillac molybdenite deposit phanerozoic cu and mo porphyry deposits sn w greisens and most

a possible lacustrine paleomagnetic record of the blake episode from pringle falls oregon u s a abstract we have studied a high resolution paleomagnetic record of a magnetic polarity episode recovered from a 20 m section of diatomaceous lacustrine sediments from pringle falls near la pine oregon 43 7 n 238 6 e a total of 474 samples 982 specimens were collected at about 5 cm intervals from the 20 m section the age of this record is at present constrained between 0 6 ma and 18 000 yr based upon a pollen date from the older part of the lakebed sequence and the age of the younger glacial deposits that overlie the lakebeds the characteristic magnetization of the samples was determined by alternating field af demagnetization to 15 mt the average normal polarity stable declination and inclination of the non magnetic polarity episode sampled section 61 95 2 6 is statistically equivalent to that expected at this site for a geocentric axial dipole 62 the record provides a virtual geomagnetic pole vgp path that identifies a well defined clockwise ne sw loop with a radius of 60 followed by a second younger feature that traces out a counter clockwise loop 15 in radius residing in the ne quadrant and characterized by low latitudes interpretation of the natural remanent magnetization anhysteretic remanent magnetization nrm arm ratios for the samples indicates that the field intensity may have fallen and then increased above the average dipole field intensity the low intensity interval ends before the onset of the polarity episode as defined by directions and may suggest that the episode is a result of an unstable regeneration of the field after a significant lowering of overall field intensity these two characteristic features are similar to the two features present in the blake geomagnetic polarity episode at 114 000 and 105 000 yr bp observed in different parts of the world