a high precision u pb age of metamorphic rutile in coesite bearing eclogite from the dabie mountains in central china a new constraint on the cooling history this paper first reports a high precision u pb age of 218 1 2 ma for rutile in coesite bearing eclogite from jinheqiao in the dabie mounteins east central china this work shows that the u pb mineral rutile omphacite isochron age of 218 2 5 ma and conventional rutile u pb concordia age of 218 1 2 ma obtained by common pb correction based on the pb isotopic composition of omphacite in the same eclogite sample are consistent proving that the omphacite with low u pb ratio 2 8 can be used for common pb correction in u pb dating of rutile oxygen isotope analysis of rutile aliquots gave the consistent 18o values of 6 1 0 1 demonstrating oxygen isotope homogenization in the rutile of different grains as inclusion in garnet and grain in matrix oxygen isotope thermometry yields temperatures of 695 35 and 460 15 c for quartz garnet and quartz rutile pairs respectively these oxygen isotopic observations suggest that the diffusion of oxygen in rutile as inclusion in garnet is not controlled by garnet according to field based thermochronological studies of rutile an estimate of the tc of about 460 c for u pb system in rutile under rapid cooling conditions 20 c ma was advised based on this u pb age as well as the reported chronological data with their corresponding metamorphic and or closure temperature an improved t t path has been constructed the t t path confirms that the uhpm rocks in south dabie experienced a rapid cooling following the peak metamorphism before 220 ma and a long isothermal stage from 213 to 180 ma around 425 c

the deep structure of venusian plateau highlands abstract magellan gravity data confirm that several of the large tectonically deformed plateau like highlands on venus are shallowly compensated most likely by crustal thickness variations apparent depths of isostatic compensation computed in the spatial domain range from 20 to 50 km for alpha tellus ovda and thetis regiones using a two layer model for isostatic compensation alpha tellus and ovda are best represented as nearly completely compensated in crust that is regionally 20 40 km thick around these highlands with little contribution from deeper mantle sources in contrast to these three areas a stronger regional gravity high associated with thetis requires a significant upper mantle component to compensation this is evident in the spectral admittance as a pronounced deep long wavelength anomaly in the two layer isostatic model a broad deeply compensated upland underlies a shallowly compensated central block of thetis if this deep component is interpreted as a thermal anomaly the loci of maximum upwelling agree well with sites of recent extension alternatively dense mantle could be responsible for the broad gravity anomaly although a different style of dynamic coupling is called for the plateau highlands are thus physiographically and isostatically equivalent to terrestrial continents though probably not compositionally they also share the record of a long tectonic history the large regional gravity anomaly of thetis indicates that active mantle processes continue even beneath some areas tessera thought to be relics of a former geological regime the excellent agreement of modeled crustal thicknesses around alpha tellus and ovda regiones suggests that 20 40 km is a representative global value for the plains such a crust is thicker than previously estimated and about twice as thick as the expected thickness of crust produced at venusian spreading centers if crust was produced mostly at spreading centers during global resurfacing the mantle potential temperature must have been 1800 k about 100 k hotter than predicted if hotspot activity dominated lateral variations in crustal production must be smoothed out by creep of the lower crust

structural analysis of a sampling of tesserae implications for venus geodynamics abstract understanding the nature and timing of tessera formation is fundamental to venus tectonic and geodynamic models tesserae are commonly considered to exhibit complex deformation histories to represent the oldest global stratigraphic unit to have formed during a global phase of tesserization and to require weak lithosphere for their formation although these characterizations of tesserae are gaining widespread mention in the literature they are essentially unsupported by an assemblage of data yet they figure prominently in the way we frame and address questions about venus tectonics thus they are quickly becoming accepted paradigms despite a lack of scientific foundation open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of venus we examined tessera terrain in ishtar terra crustal plateaus and as inliers within the plains using high resolution magellan radar imagery we describe several types of tessera terrain that record a wide range of structural histories fold and s c terrains are found only in ishtar terra ribbon lava flow and basin and dome terrains reside within the interior of crustal plateaus whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus inliers are divisible into fracture dominated and graben dominated tesserae although some inliers host early contractional fabrics the range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit tessera deformation records local to regional surface strain patterns and reflects near surface rheology at the time of deformation progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation tesserae likely formed in several tectonic environments including 1 as a result of subsurface flow in ishtar terra 2 as sequences of surface layer extension and contraction in crustal plateaus 3 as flooded crustal plateaus and 4 as densely fractured surface layers fractured as a result of corona and chasma formation

heavy mineral compositions of the changjiang yangtze river sediments and their provenance tracing implication abstract a total of 71 floodplain sediments were collected from the mainstream and major tributaries of the changjiang for heavy mineral examinations the upper changjiang samples contain more heavy minerals in the very fine sand fraction than the middle lower samples which are relatively enriched in polygenetic heavy minerals the heavy mineral assemblages are characterized by fe ti oxide minerals calcic amphibole epidote garnet biotite and zircon which exhibit large variations in percent between different tributaries and the mainstream the widely distributed granitoids and permian emeishan basalt in the drainage basins account for the high contents of zircon and hypersthene in the xiangjiang and daduhe samples respectively most of the detrital magnetite grains are homogeneous in typomorphic feature and almost stoichiometric in chemical composition the discrimination plot of tio2 v2o3 versus mgo mgo al2o3 suggests that most of the detrital magnetite grains are sourced from felsic plutonic volcanic and metamorphic parent rocks and those from different tributaries have distinct chemical compositions our study suggests that the combination of transparent heavy mineral assemblages and varietal study of individual heavy minerals sheds new light on the provenance discrimination of the changjiang sediments and contributes to the mineralogical determination of ancient source rocks within a large drainage basin despite the sediment recycling and complex source rock types

reliability and application of detrital opaque fe ti oxide minerals in provenance determination a survey of detrital opaque fe ti oxide minerals in holocene sands derived exclusively from known source rocks in the rocky mountains usa semi arid climate high relief and the appalachian mountains usa humid climate low relief shows that despite obvious differences in weathering rates the abundance of detrital ilmenite relative to parent rocks is similar in these two regions because ilmenite is very common in many crystalline rocks ilmenite is considered to be a useful fe ti oxide mineral for varietal studies about 30 to 50 of detrital fe ti oxide minerals are polymineralic grains with intergrowths of two or more phases discriminant function analysis indicates that morphological varieties of detrital fe ti oxide minerals in combination with their chemical compositions are useful guides to provenance an igneous or metamorphic source of 96 of the holocene samples is correctly identified by this method the method is used to determine the relative contribution of igneous and metamorphic source rocks to the sandstones of the part of the tertiary renova formation which was deposited within a dissected magmatic arc in southwestern montana usa a separate discriminant function is calculated using the chemical compositions of detrital ilmenite and morphological properties of detrital fe ti oxide minerals in rocky mountain holocene sands classification of detrital fe ti oxide minerals in renova sandstones using this function suggests that igneous and metamorphic rocks contributed 77 and 23 respect ively to the renova sandstones in this area

chemical fingerprinting a precise and efficient method to determine sediment sources abstract an accurate method of determining source locations for detrital sediment is presented using the chemical composition of fe oxide minerals like a fingerprint this method is an improvement in the use of fe oxide minerals for provenance determinations because it requires less time and fewer source samples a rigorous test of the method uses a database of more than 38 000 grains from known locations the average error of matching grains back to 45 source locations designated for this database is less than 2 the method allows for proportional matching of a grain to multiple sources if other grains in the source database meet the compositional match criteria which helps reduce the error of incorrect matches most provenance studies do not involve source basins as large as the entire arctic ocean where sediment can be ice rafted several thousand kilometers for most studies only a few samples 100 grain analyses sample would be required to characterize a source if strategically placed such as near a river mouth deposits more than 40 million years old can be traced to specific sources using this method because fe oxide grains are relatively stable in most deposits

recent developments in dating ancient crustal fluid flow several isotopic techniques have recently been developed or substantially improved allowing the accurate determination of the timing of fluid related processes these include k ar rb sr and sm nd dating of diagenetic minerals in clastic rocks u pb dating of carbonates sm nd dating of fluorite and uraninite re os and rb sr dating of sulfides 40ar 39ar and rb sr dating of fluid inclusions and u pb dating of ilmenite magnetite sphene and rutile while many of these techniques require further evaluation some have already resulted in new constraints on a variety of crustal fluid flow models

resurfacing history of venus the stratigraphy impact cratering record and geoid of venus indicate that a major change in surface geologic activity has occurred before 800 ma venus was dominated by horizontal movement and tectonic deformation of the crust a brief period of global volcanism subsequently filled in all low lying areas and left much of the surface at a nearly uniform elevation current geologic activity is dominated by large volcanoes and limited horizontal movement of the crust this geologic history may have resulted from mantle cooling causing the lithosphere to become positively buoyant globally

numerical investigations of the mantle plume initiation model for flood basalt events recent studies have linked both the eruption of continental flood basalts and the emplacement of large oceanic plateaus to the initial phase of mantle plume activity i e the plume head model here we develop this concept further through numerical models of a large thermal diapir rising through the mantle and impinging upon the base of the lithosphere a finite element analysis in axisymmetric geometry is used to model the dynamics of solid state convective flow and heat transport in the mantle and an anhydrous batch melting model is used to estimate melt volumes and compositions obtained from partial melting in the shallow mantle we explore the effects of a number of model variables including mantle viscosity structure plume temperature and diameter and the mechanical response of the lithosphere our study emphasizes cases in which the lithosphere does not undergo significant extension prior to large scale melting a standard model is found to be consistent with some observed characteristics of flood basalt plume initiation events this model includes an initial diapir of radius 400 km and initial excess temperature 350oc rising through a mantle of viscosity 102 pa s and impinging beneath a model oceanic lithosphere 100 ma in age if lithospheric extension does not occur melting is almost entirely sublithospheric the melt volume produced is 3 5x106 km 3 and its composition has high mgo content 15 20 wt a time sequence of precursory central axial uplift of several kilometers followed by major melting volcanism is a robust feature of all the models regardless of mantle viscosity structure or lithospheric response low viscosity in the upper mantle compresses the timescale for these events and results in a constriction of the plume head initially impinging on the lithosphere the radial extent of the melting region about 500 km at the time of maximum melt production is only slightly affected by the upper mantle viscosity for nonrifting lithosphere no significant melt generation occurs for initial plume excess temperatures less than about 300oc the initial plume radius has only a modest effect upon the volume of melt obtained the most important aspect of lithospheric response is whether or not the uppermost crust and lithosphere above 40 km depth undergo extension otherwise the ductile strength of the lower lithosphere has little effect if the lithosphere is allowed to extend freely the melt volume increases by about an order of magnitude and it is comparable with some of the largest oceanic plateaus such as ontong java if continental or oceanic flood basalt events occur on old nonrifting lithosphere the primary melts are likely to be much more mgo rich than the basalts erupted at the surface this is consistent with abundant evidence for olivine fractionation in many flood basalt provinces along with the occurrence of picrite lavas and we hypothesize that fractionation occurs at a density trap at the crust mantle boundary moho at this stage and following it is likely that significant lower crustal and mantle lithospheric contamination of mantle plume derived magmas may occur both isotopically and in the trace elements the sequence and timing of both precursory and post magmatic events in the vast well exposed oceanic flood basalt terrane known as wrangellia se alaska british columbia are consistent with the predictions of our model the uplift predicted by our models is of order 2 4 km if we require that mantle plumes are hot enough to melt beneath unrifted lithosphere effects not modeled here such as small scale convection and hydrous melting will tend to reduce the model excess temperature and uplift in order to better understand the relation between mantle plumes and large igneous provinces several important advances are required implementation of a fractional melting model some realistic assessment of melt transport through the mantle and crust and a fully three dimensional convection model

geological correlations with the interior density structure of venus a model has been constructed by stipulating that venusian long wavelength topography and geoid elevations are explained by the sum of density anomalies associated with mantle convection and lithospheric compensation mechanisms for convenience the latter are represented by a single airy surface because individual lithospheric components cannot be resolved at the wavelengths considered in this paper mantle convection is represented at a single depth by mass perturbations that drive viscous flow we invert spherical harmonic representations of the topography and the geoid to create global representations of the mantle convection pattern and near surface density anomalies the mantle convection pattern matches well with previous numerical simulations of mantle convection that show a pattern of isolated upwellings amidst an interconnected network of downwellings the near surface density anomalies show no particular pattern the inversion results have been compared with a global synthesis of magellan data impact crater density does not correlate with the mantle convection pattern or the near surface density anomalies large volcanic structures are associated with upwellings planitiae are associated with downwellings coronae are anticorrelated with high amplitude upwellings and downwellings and apparently are passive features associated with rifting that is related to mantle convection in a complex fashion tesserae are associated with negative lithospheric density anomalies interpreted as areas of thickened crust tesserae show no definitive relationship to the present mantle convection pattern

weathering and abrasion of fe ti oxides during rock degradation and fluvial transport implications for sedimentary provenance studies physical and chemical degradation of individual grains of titanomagnetite and ilmenite during rock weathering and fluvial transport processes in the river eden catchment eastern scotland have been traced using backscattered electron images and electron probe microanalysis magnetic characteristics have been used to detect differences in magnetic mineralogy concentration paragenesis and grain size between rock samples showing different stages of weathering and between different sources and sediment samples results have shown higher physical and chemical durability of ilmenite during this segment of the geological cycle than titanomagnetite which is rapidly altered during progressive subaerial weathering of a dolerite the increasingly intensive and extensive oxidation and development of internal fractures in titanomagnetite are observed however under subaqueous conditions titanomagnetite was found to be progressively replaced by titanite and ti impoverished magnetite due to hydrocirculation neither the total physical nor the total compositional degradation of titanomagnetite is achieved under rock weathering and fluvial transport processes thus the importance of titanomagnetite as a provenance indicator remains despite its degradation during the geological processes studied in the river eden catchment

mantle flow tectonics the influence of a ductile lower crust and implications for the formation of topographic uplands on venus convective flow within the interiors of terrestrial planets is a primary source of large scale topographic and tectonic features we treat the crust and mantle as a layered viscous half space and find solutions for flow driven by a buoyancy force distribution within the mantle and by relief at the surface and crust mantle boundary the crust initially responds to mantle upwelling with uplift but continued flow causes significant crustal thinning and subsidence downwelling leads to initial subsidence and subsequent crustal thickening and surface uplift changes in crustal thickness are driven by 1 vertical normal stresses due to mantle flow and 2 shear coupling of horizontal mantle flow into the crust the degree and time scale of crustal deformation are sensitive to crustal thickness and viscosity and the presence of elastic layers in the upper crust and mantle deformation can occur on time scales less than several hundred million years for reasonable values of these parameters strong or elastic upper crustal layers enhance crustal thinning or thickening while large viscosity contrasts between the lower crust and upper mantle or elastic mantle layers tend to diminish deformation for time dependent mantle flow due to a rising or sinking diapiric body crustal deformation depends upon the ratio of diapir radius to crustal thickness and the ratio of crustal to mantle viscosity a sinking diapir can lead to crustal thickening elevated topography and a specific sequence of time varying deformation while regions such as beta regio have long been suggested to result from convective upwelling in the venus mantle we suggest that a mantle downwelling origin is consistent with many of the characteristics of the compressional mountain ranges of western ishtar terra

zr and hf diffusion in rutile abstract chemical diffusion of zr and hf under anhydrous conditions has been measured in synthetic and natural rutile the sources of diffusant used were hafnon or zircon powders or a hafnon rutile mixture experiments were run in crimped pt capsules in air or in sealed silica glass capsules with solid buffers to buffer at nno or qfm rutherford backscattering spectrometry rbs was used to measure zr and hf diffusion profiles from these measurements the following arrhenius relations were obtained for zr diffusion parallel to c over the temperature range 750 1100 c d zr c 9 8 10 15 exp 170 30 kj mol 1 rt m 2 s 1 for hf diffusion parallel to c over the temperature range 800 1000 c d hf c 9 1 10 15 exp 169 36 kj mol 1 rt m 2 s 1 for hf diffusion normal to c over the temperature range 750 1050 c d hf c 2 5 10 12 exp 227 62 kj mol 1 rt m 2 s 1 diffusivities for experiments buffered at qfm and nno are similar to those run in air diffusivities in synthetic and natural rutile are likewise similar indicating that these findings can be applied directly in determining zr diffusivities in rutile in natural systems these data indicate that rutile should be moderately retentive of zr chemical signatures with zr diffusivities within an order of magnitude of those for pb in rutile over most geologic conditions when applied in evaluation of the relative robustness of the recently developed zr in rutile geothermometer t zack r moraes a kronz temperature dependence of zr in rutile empirical calibration of a rutile thermometer contributions to mineralogy and petrology 148 2004 471 488 e b watson d a wark j b thomas crystallization thermometers for zircon and rutile contributions to mineralogy and petrology 151 2006 413 433 these findings suggest that zr concentrations in rutile will be somewhat more likely to be affected by later thermal disturbance than the geothermometer based on zr concentrations in titanite l hayden e b watson d a wark a thermobarometer for sphene abstract 16th v m goldschmidt conference 2006 and much less resistant to diffusional alteration subsequent to crystallization than the ti in zircon geothermometer e b watson d a wark j b thomas crystallization thermometers for zircon and rutile contributions to mineralogy and petrology 151 2006 413 433 e b watson t m harrison zircon thermometer reveals minimum melting conditions on earliest earth science 308 2005 841 844

hot spot evolution and the global tectonics of venus the global tectonics of venus may be dominated by plumes rising from the mantle and impinging on the lithosphere giving rise to hot spots global sea floor spreading does not take place but direct convective coupling of mantle flow fields to the lithosphere leads to regional scale deformation and may allow lithospheric transport on a limited scale a hot spot evolutionary sequence comprises i a broad domal uplift resulting from a rising mantle plume ii massive partial melting in the plume head and generation of a thickened crust or crustal plateau iii collapse of dynamic topography and iv creep spreading of the crustal plateau crust on venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea floor spreading

geology of a rift zone on venus beta regio and devana chasma beta regio is a region of rifting and volcanism on venus the nature of beta a major topographic rise and rift zone is herein characterized using pioneer venus arecibo and venera 15 16 data high resolution 1 2 km arecibo and venera radar images reveal details of faulting and volcanism and pioneer venus altimetry illustrates the density and location of faults in relation to topography faults are distributed throughout beta but are concentrated in devana chasma where they are spaced 5 20 km apart the pattern of faulting and distribution and sequence of volcanic activity in beta can be used to help understand how the rift has evolved including the origin of the high topography of beta and the origin and nature of devana chasma on the basis of geologic mapping relations and map patterns beta appears to have formed as a result of doming in response to a mantle anomaly at the same time rhea mons a major shield volcano was formed formation of devana chasma followed with extensive faulting in the rift trough geometry of the trough and fault patterns suggests that some degree of lithospheric stretching has occurred later volcanism produced a second major shield volcano theia mons which is superimposed on the western bounding fault of the rift zone both uplift and extension have been involved in forming devana chasma and beta regio and may be important in the formation of other equatorial high land regions with systems of chasmata on venus such as aphrodite terra the bifurcation of devana chasma in the vicinity of theia mons and the extension of the rift system south to phoebe regio and west toward aphrodite terra suggest that their origins may be linked

dating volcanism and rifting on venus using impact crater densities although the distribution of impact craters on venus is indistinguishable from a completely spatially random population on the basis of crater data alone the addition of geologic information indicates that the craters are not random with respect to geology areas of low crater density correlate with concentrations of tectonic and volcanic features as well as high proportions of faulted and embayed craters high resolution mapping of large volcanoes flood type lava flow fields rifts and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance because these features also appear young based on stratigraphic data difference in age is the most likely reason for differences in crater density between geologically defined terrains crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters as long as the mapping criteria are independent of impact crater locations this result provides a tool to begin identifying and dating major geologic provinces and to begin developing a geologic history of venus using a simple data model and calculating crater ages relative to an assumed global mean age of 300 ma we show that the ages of large volcanoes 72 45 ma flow fields 128 91 ma rifts 130 145 ma and coronae 120 115 ma are all substantially younger than the mean plains age and probably represent ongoing volcanic and tectonic activity rather than the end of a brief global resurfacing event with a mean age of 300 ma

regional topographic rises on venus geology of western eistla regio and comparison to beta regio and atla regio regional topographic rises in the equatorial region of venus western eistla regio beta regio and atla regio form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes the geologically least complex of these regions is western eistla regio an elongated domical rise that contains the volcanoes sif and gula montes sif mons is a large volcano diameter 100 km whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing widespread outpouring of lava additional fracturing along its northern flank and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion sif contains a 40 km diameter summit caldera within which are nested smaller calderas and pit structures theoretical considerations suggest multiple levels of magma emplacement within the volcano with the primary reservoir residing in the crust below the edifice gula mons is made up of multiple volcanic source regions and is associated with guor linea a zone of lithospheric stretching faulting and rifting the geologically complex highland of beta regio is a tectonic junction with three arms of devana chasma intersecting at theia mons magellan data reveal 1 the details of rifting and volcanism concentrated at theia mons 2 that rhea mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question 3 that tesserae are a major unit arrayed in a pattern around theia from east to west starting on the eastern flank of beta and continuing westward to the northern part of asteria regio 4 that the northern periphery of the highland contains coronae connected by fractures forming chains and 5 the presence of a deformed crater within devana chasma split and extended in an east west direction indicating that part of the rift has undergone as much as 20 30 extension like beta atla regio is a complex area of converging rifts five separate arms that are centered on the volcano ozza mons the magellan images show that volcanism and rifting have occurred contemporaneously at ozza with lava flows both covering and being crosscut by faults a second volcano maat mons contributes to the infilling of the rift dali chasma and does not appear to be deformed by faulting two of the rifts at atla contain chains of coronae suggesting that upwelling along linear zones of extension has occurred beta atla and western eistla are similar in that they are broad regional rises are associated with major rift zones have large apparent depths of isostatic compensation and contain large volcanoes their associations with features mapped as coronae and tesserae are quite variable suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment

u pb and ar ar geochronology of the fujiawu porphyry cu mo deposit dexing district southeast china implications for magmatism hydrothermal alteration and mineralization abstract the fujiawu porphyry cu mo deposit is one of several porphyry cu mo deposits in the dexing district jiangxi province southeast china new zircon shrimp u pb data yield a weighted mean 206 pb 238 u age of 172 0 2 1 and 168 5 1 4 ma from weakly altered granodiorite porphyry and quartz diorite porphyry respectively two hydrothermal biotites from granodiorite porphyry give an ar ar step heating plateau age of 169 9 1 8 and 168 7 1 8 ma hydrothermal apatite exsolved from altered biotite yields an isotope dilution thermal ionization mass spectrometry isochron age of 164 4 0 9 ma the apatite age is similar to the ages obtained from hydrothermal rutile 165 0 1 1 and 164 8 1 6 ma and indicates that the magmatism and hydrothermal activity in the fujiawu deposit occurred in the middle jurassic hydrothermal fluid circulation related to multiple stages of magma emplacement resulted in cu mo mineralization in the fujiawu porphyry deposit the zircon shrimp u pb ages and the published molybdenite re os age 170 9 1 5 ma represent the timing of magma crystallization and mo mineralization whereas the rutile and apatite u pb ages reflect the timing of cu mineralization following quartz diorite emplacement the data suggest slow cooling after emplacement of the quartz diorite porphyry

hydrothermal zircon and baddeleyite in val d or archean mesothermal gold deposits characteristics compositions and fluid inclusion properties with implications fortiming of primary gold mineralization zircon and baddeleyite occur within quartz tourmaline veins at four gold deposits in the val d or district of the archean abitibi southern volcanic zone host rocks have experienced intense metasomatic enrichment of zr hf y and rare earth elements the zircons contain primary inclusions of quartz tourmaline pyrite albite k mica scheelite and gold and gold occurs in primary fluid inclusions in zircons magmatic zircons in host rocks do not have this suite of inclusions consequently a wall rock inheritance model for the vein zircons is implausible compositionally the zircons feature pronounced interzone and intergrain variations of hf y yb th and u and sporadic anomalous ce contents of 1100 ppm distinct from magmatic counterparts two principal types of primary fluid inclusion occur in the vein zircons type 1 h2o co2 inclusions have low salinities variable quantities of co2 and homogenization temperatures of 260 380 c and type 2 co2 rich inclusions contain minor h2o and ch4 the vei

sediment dispersal patterns in a deep marine back arc basin evidence from heavy mineral provenance studies heavy mineral provenance studies can aid reservoir description in deep marine depositional systems by providing a clearer understanding of the lateral extent of discrete sediment bodies in this study the lower gustav group lagrelius point kotick point and whisky bay formations of james ross island antarctica which was deposited in a deep marine slope apron submarine fan complex has been investigated the kotick point and whisky bay formations crop out in two separate areas on the west coast of james ross island the provenance of the group has been studied by both standard sandstone petrography and electron microprobe analysis of the heavy minerals garnet pyroxene and fe ti oxides the sandstones are lithic to arkosic arenites derived from the adjacent antarctic peninsula magmatic arc although the general source area can be recognized individual stratigraphical or depositional units cannot be distinguished on the basis of sandstone petrography however electron microprobe analyses of the detrital heavy minerals allows a more detailed discrimination of sediment provenance in particular analyses of the detrital fe ti oxide minerals show that in the whisky bay formation the southern outcrop area contains ilmenite whilst the northern outcrop area contains titanomagnetite in conjunction with the available biostratigraphical and palaeocurrent evidence this can be interpreted in two ways firstly if the two outcrop areas are direct age equivalents then they were derived from separate point sources and deposited as discrete sediment lobes alternatively if they are not directly time equivalent the depositional system may have switched laterally with time with sediment derived from source rocks of varying composition either interpretation has implications for the understanding of the deep marine depositional system detrital opaque minerals are commonly source specific and should not be ignored during provenance studies

ejecta correlations with spatial crater density and venus resurfacing history we examine the alternative hypotheses that on venus i the spatial randomness of the crater population reflects predominantly the spatially stochastic nature of impact crater formation or ii a significant amount of spatial variation in crater density is due to variations in the geological rate of crater removal three endmember models of resurfacing are formulated case i assumes that a single age production surface accounts for the spatial distribution of craters case ii assumes that variations in spatial crater density reflect local production surface ages while case iii assumes that individual craters are removed regionally so that variations in spatial crater density reflect local retention ages these endmember models are tested by examining correlations of crater properties with crater spatial density we analyze the distribution of embayed and tectonized craters and the relative abundances of extended ejecta deposits termed halos the three endmembers provide specific predictions as to the occurrence of halos as a function of for 1 5 craters per 106 km case iii provides the best explanation for the deficiency of halos as well as the relative abundances of modified craters for the interval 1 5 2 5 case ii is applicable but with only a modest spread in production ages for densities 2 5 case ii is the only endmember model that matches the global observations suggesting that regions with these high densities represent a series of distinct production surfaces with ages increasing from the planetary mean retention age

a precise u pb age on cassiterite from the xianghualing tin polymetallic deposit hunan south china we report the first precise u pb isotope data on cassiterite from the large xianghualing tin polymetallic deposit in the central nanling district south china the results show that four separates from sample xf 51 have a relatively narrow range of 206pb 238u apparent ages varying from 152 to 157 ma and the three 206pb 238u apparent ages yield a weighted average value of 156 4 ma mswd 0 32 separates from two other cassiterite samples do not have sufficient radiogenic pb to generate a reliable 206pb 238u age seven separates from the above three cassiterite samples define a well constrained 238u 206pb isochron corresponding to an age of 157 6 ma mswd 34 a comparison of the u pb cassiterite ages with published ar ar dates on muscovite from this deposit and k ar age data on biotite from the pluton genetically related to the tin mineralization in this area demonstrates that the u pb isotope system of cassiterite is a potential geochronometer combined with the ar ar dates of muscovite from this deposit we can constrain the absolute age of tin polymetallic mineralization in xianghualing at 154 157 ma the dates obtained in this study consistent with the published geochronological results from other important deposits in this region reveal that the large scale tungsten tin mineralization in the central nanling region was predominantly emplaced during 150 161 ma

early thermal profiles and lithospheric strength of ganymede from extensional tectonic features abstract estimates of the brittle lithosphere thickness derived from the width and spacing of extensional tectonic features coupled with lithospheric strength envelopes brittle and ductile yield stress versus depth appropriate for ice allow the quantitative determination of early thermal profiles and lithospheric strength and stability on ganymede furrows and grooves indicate brittle lithospheric thicknesses of 5 10 and 2 5 km respectively assuming that their spacing is controlled by an extensional instability or that their width is controlled by the intersection depth of their bounding faults plots of the brittle and ductile yield stress versus depth for the icy lithosphere of ganymede show a linear increase in brittle strength with depth to a maximum at the brittle ductile transition followed by an exponential decrease in ductile yield stress with depth because the depth to the brittle ductile transition depends primarily on the thermal gradient the thickness of the brittle lithosphere can be used to calculate early thermal profiles of 1 5 6 and 4 5 20 km during the formation of the furrows and grooves respectively lithospheric strength the integral of the yield stress versus depth curve varied from 30 125 gpa m when the furrows formed to 5 30 gpa m when the grooves formed which correspond to maximum yield stresses of 6 11 and 2 5 6 mpa respectively these results indicate that the thermal gradient and lithospheric strength varied laterally by as much as a factor of 5 and that ganymede cooled in a highly inhomogeneous manner with significant lateral thermal anomalies finally this analysis provides a straightforward explanation for the stability of large remnants of cratered terrain such as galileo regio that had a low thermal gradient and strong lithosphere in contrast to small remnants of cratered terrain that were fractured and broken up by grooved terrain as a result of higher thermal gradients and weaker lithospheres

magnetite chemistry as a provenance indicator in archean metamorphosed sedimentary rocks abstract conventional provenance studies of high grade e g amphibolite facies metamorphic sandstone are limited due to intense recrystallization destruction of original minerals and growth of new minerals magnetite is a heavy mineral common in a wide range of sedimentary rocks it can originate from a wide range of rock sources from high temperature to low temperature magmatic metamorphic or hydrothermal environments laser ablation la icp ms and electron microprobe analytical techniques allowed determination of a wide range of trace elements at sub ppm levels in magnetite to identify the provenance of two archean sedimentary formations in eeyou istchee baie james quebec canada results were plotted on multi element diagrams the most compatible elements in magnetite were identified as the most discriminant elements to differentiate four potential different rock sources for the magin formation overlap between the spectra of the magin and keyano formations substantiate the existence of a common source interbedded conglomerate magnetite bearing clasts were examined to determine potential source rocks mafic clasts apparently do not contain magnetite detrital magnetites analyzed have low ti values that would support either felsic to intermediate plutonic or a hydrothermal source the trace element spectra of detrital magnetite from the sandstone beds were compared to magnetite spectra from conglomerate clasts of banded iron formation and felsic to intermediate plutonic rocks only magnetites from felsic plutonic rocks have a signature similar to those from the sandstone to further substantiate a felsic plutonic source a detailed study of magnetite chemistry from ten felsic and intermediate regional plutons from a regional databank allowed us to recognize that each of these plutons has a particular signature on the basis of wide variations in ni and cr from this data base four individual plutons were identified as potential sources for the magin formation recently published magnetite discrimination diagrams proposed for mineral deposits can be applied to sedimentary magnetite provenance however our database shows the importance of identifying low ti magnetite from felsic rocks since these could be misidentified as low ti magnetite from hydrothermal mineral deposits thus we propose improvement to these diagrams by differentiating magnetite from felsic magmatic low ni cr and hydrothermal high ni cr sources before applying the mineral deposit discrimination diagrams

geological evolution of venus rises plains plumes and plateaus crustal plateaus and volcanic rises major physiographic features on venus both formed over mantle plumes crustal plateaus were produced by large degrees of plume melting beneath thin lithosphere the oldest tectonic features in crustal plateaus are ribbon like troughs indicating early uplift and tensile stretching their shallow depths suggest that surface temperature there was higher in the past widespread volcanic plains derived from the broad upwellings of internally heated mantle convection were continually erupted during the time of crustal plateau formation discrete volcanic rises younger than crustal plateaus formed over a thicker lithosphere with less plume partial melting the presence of only one transitional form indicates that the lithosphere thickened rapidly thermal and magmatic models show that the formation of these major features can be tied to an expected change in mantle convective style about 1 billion years ago

localization of gravity and topography constraints on the tectonics and mantle dynamics of venus summary we develop a method for spatio spectral localization of harmonic data on a sphere and use it to interpret recent high resolution global estimates of the gravity and topography of venus in the context of geodynamical models our approach applies equally to the simple spatial windowing of harmonic data and to variable length scale analyses which are analogous to a wavelet transform in the cartesian domain using the variable length scale approach we calculate the localized rms amplitudes of gravity and topography as well as the spectral admittance between the two fields as functions of position and wavelength the observed admittances over 10 per cent of the surface of venus highland plateaus and tessera regions are consistent with isostatic compensation of topography by variations in crustal thickness while admittances over the remaining 90 per cent of the surface rises plains and lowlands indicate that longwavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere the global average crustal thickness is less than 30 km but can reach values as large as 40 km beneath tesserae and highland plateaus we also note that an earth like radial viscosity structure cannot be rejected by the gravity and topography data and that without a mechanical model of the lithosphere admittance values cannot constrain the thickness of the thermal boundary layer of venus modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae the effective elastic plate thickness k was less than 20 km estimates of t at highland rises are consistently less than 30 km our inability to find regions with t 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models

blob tectonics a prediction for western aphrodite terra venus it is proposed that the western aphrodite terra region of venus was formed by four diapirs of heated mantle material or blobs rising to the surface in a region of convective upwelling the four blobs are in different stages of evolution producing different topography gravity anomalies and morphology we show that this model is consistent with available data and make specific predictions that can be tested with magellan radar data

are the alteration halos of massive sulfide deposits syngenetic evidence from u pb dating of hydrothermal rutile at the kidd volcanic center abitibi subprovince canada the kidd volcanic complex is composed of felsic volcanic and subvolcanic rocks of archean age the felsic rocks are intercalated with sedimentary rocks and are in contact with komatiitic ultramafic rocks large diorite plutons were emplaced into the rhyolite and the complex is overlain by younger basalts metasomatic events affecting the lithology of the kidd volcanic complex include silicification extensive co 2 metasomatism carbonate k metasomatism sericite fuchsite and chlorite and minor carbonate alterations petrographic evidence supported by stable isotope and fluid inclusion studies suggests that silicification and early carbonate alteration were synvolcanic and therefore related to ore deposition during subsequent extensive k metasomatism sericite precipitated in the rhyolite and fuchsite precipitated in the ultramafic rocks although chlorite post dates k metasomatism the micas and chlorite are both found in anastomosing microfissures commonly occupying the same set of fractures hydrothermal rutile formed by the breakdown of magnetiteilmenite during k metasomatism and chlorite alteration gives an age of 2624 62 ma 95 confidence level by pb pb isotopic measurements it is therefore approximately 100 m y younger than syngenetic massive sulfide mineralization 2717 2 ma sulfide stringers within sericite and chlorite veins suggest some remobilization of the ores during these later events this alteration assemblage generally thought to be intimately associated with mineralization is identical to that found associated with many lode gold deposits in the superior province recent dating of micas and rutile associated with gold deposits in the abitibi subprovince gives comparable ages to the rutile in the kidd volcanic complex which must therefore record a widespread late hydrothermal event affecting mineralized rocks

venusian highlands geoid to topography ratios and their implications abstract using pioneer venus line of sight gravity data and orbit simulation procedures we have estimated apparent depths of isostatic compensation adcs for twelve venusian highland features asteria atla bell beta ovda phoebe tellus thetis and ulfrun regiones and nokomis gula and sappho montes adcs range from 50 km to 270 km half of the values are less than 100 km using these adcs we estimate geoid to topography ratios gtrs for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots swells and plateaus the geoid is estimated in the wavenumber domain from the isostatic formula using the topography and adc for each region in the space domain the gtr is equal to the least squares slope of the linear fit of the geoid to the topography the resulting gtr range is 7 31 m km which is much higher than terrestrial oceanic values 1 to 5 m km the features fall into two distinct groups one with a gtr range of 7 13 m km and one with a range of 19 25 m km the exception is beta regio which has a gtr of 31 m km a model for thermal thinning of a 100 km thick lithosphere fits all values in the lower gtr group to within one standard deviation airy compensation could also be present but cannot fully compensate these features partial dynamic compensation of the lower gtr group in combination with lithospheric mechanisms is also possible but not required to fit the data the upper gtr range 19 25 m km can be fit with an upper mantle constant viscosity convection model the large gtr values are inconsistent with the presence of a low viscosity zone if more than one compensation mechanism is present in the regions in the higher gtr group the gtrs will be underestimated in terms of a dynamic interpretation we thus fit the convection models to the upper end of the gtr range 25 m km rayleigh numbers in the range 10 4 10 6 will produce a gtr of 25 m km when combined with conductive lid thicknesses of 85 150 km the 6 m km range in both of the gtr groups is probably due to varying degrees of crustal and thermal compensation combined with dynamic compensation in the upper gtr group the difference between terrestrial and venusian gtr ranges can be explained largely by the lack of a low viscosity zone on venus

weathering of ilmenite from granite and chlorite schist in the georgia piedmont ilmenite grains from weathering profiles developed on granite and ultramafic chlorite schist in the georgia piedmont were studied for evidence of morphological and chemical alteration ilmenite rich concentrates from the fine sand 90 150 m component were studied to test the assumption that there is no difference between ilmenite in the parent rock and that in colluvium delivered to primary drainage systems ilmenite grains in the granite profile are rounded to subhedral and commonly contain hematite exsolution blebs dissolution pits are observed along the boundaries of the exsolution blebs with goethite occurring as an alteration product ilmenite grains in the schist profile occur as fractured anhedral grains with uncommon lamellae of rutile grain fractures are filled with goethite and hematite particularly in the b horizon ilmenite from the granite profile is mn rich 7 15 mol mntio3 whereas ilmenite from the schist profile contains only 1 2 mol mntio3 and up to 8 mol mgtio3 two populations of grains develop in both profiles grains with abundant exsolution blebs and fractures alter through a proposed two step reaction mechanism it is proposed that ilmenite first undergoes a solid state transformation to pseudorutile via an anodic oxidation mechanism oxidized fe and mn diffuse from the structure and precipitate as goethite and mno2 pseudorutile is ephemeral and undergoes incongruent dissolution to form anatase hematite and goethite the second population of grains experienced only slight oxidation and dissolution on grain surfaces and they persist through the weathering profile the fe2 content of competent ilmenite grains is somewhat lower in the c horizon compared with grains in the host rock in horizons above the c horizon the fe2 contents of the ilmenite are similar to those in the host rock this study shows that using ilmenite minor element chemistry as a tracer for sediment provenance is a valid technique however textural features of ilmenite in colluvium may be distinct from those in the parent rock also the production of secondary phases such as anatase goethite and hematite in soil profiles results in part from the alteration of ilmenite

cassiterite la mc icp ms u pb and muscovite 40 ar 39 ar dating of tin deposits in the tengchong lianghe tin district nw yunnan china the tengchong lianghe tin district in northwestern yunnan china is an important tin mineralization area in the sanjiang tethyan metallogenic domain there are three n s trending granite belts in the tengchong lianghe area with emplacement ages ranging from early cretaceous to late cretaceous and early cenozoic tin mineralization is spatially associated with these granitic rocks however the petrogenetic link between the tin deposits and the host granites is not clear because of the lack of age data for the tin mineralization we investigate the possibility of direct dating of cassiterite from three typical tin deposits in the tengchong lianghe tin district using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms in situ la mc icp ms dating of seven cassiterite samples from the lailishan lls 1 and lls 2 xiaolonghe xlh wds dsp and hjs and tieyaoshan tys tin deposits yielded well defined 206pb 207pb 238u 207pb isochron ages to assess the accuracy of the in situ u pb dating of cassiterite 40ar 39ar dating of coexisting muscovite in samples lls 1 dsp and tys was also performed the cassiterite in situ u pb ages 47 4 2 0 71 9 2 3 and 119 3 1 7 ma respectively are in excellent agreement with the coexisting muscovite 40ar 39ar ages 48 4 0 3 71 9 1 4 and 122 4 0 7 ma respectively the u pb ages of cassiterite combined with the 40ar 39ar ages of muscovite indicate that there are three tin mineralization events in this district the lailishan tin deposit at 47 4 2 0 to 52 2 7 ma the xiaolonghe tin deposit at 71 6 2 4 to 3 9 2 0 ma and the tieyaoshan tin deposit at 119 3 1 7 to 122 5 0 7 ma these ages are highly consistent with the zircon u pb ages of the host granites it is su ggested that the cretaceous tin mineralization might have taken place in a subduction environment while the early tertiary tin metallogenesis was in a postcollisional geodynamic setting

evidence for active hotspots on venus from analysis of magellan gravity data abstract the 500 myr average crater retention age for venus has raised questions about the present day level of tectonic activity in this study we examine the relationship between the gravity and topography of four large volcanic swells beta atla bell and western eistla regiones for clues about their stage of evolution the magellan line of sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation local compensation a flexural model with local and regional compensation of surface and subsurface loads and a hotspot model of compensation that includes top loading by volcanoes and subsurface loading due to a deep low density mass anomaly the coherence is also calculated in each region but yields an elastic thickness estimate only at bell regio in all models the long wavelengths are compensated locally the long wavelength estimates of the deep compensation depth for bell atla western eistla and beta regiones are 125 175 200 and 225 km respectively with an error of approximately 35 km the flexural model is rejected primarily because it gives values of effective elastic thickness of approximately 100 km which is at least a factor of two larger than estimates of effective elastic thickness obtained from the coherence spectra at bell regio and from other studies that model the topographic expression of flexure at bell regio and in other tectonic settings globally assuming a crustal thickness of 30 km the fit to the hotspot model at atla regio gives an effective elastic thickness of 30 5 km atla regio is interpreted as an active hotspot because of the deep compensation depth and a strong subsurface loading signature at bell regio effective elastic thickness is 30 5 km at short wavelengths and 50 5 km at long wavelengths using a crustal thickness of 10 km the 30 km value is interpreted as the effective elastic thickness at the time when the volcanoes are emplaced the 50 km value is believed to reflect the present day effective elastic thickness bell regio has a relatively shallow compensation depth 125 km and a weak bottom to top loading ratio determined from the coherence spectra these results may indicate a relatively old possibly inactive plume the data at beta and western eistla regiones are of relatively poor quality and do not permit detailed interpretations these areas are interpreted as active hotspots because of their large compensation depths greater than typical of the rest of the planet

mean age of rifting and volcanism on venus deduced from impact crater densities unlike the extensively cratered highlands of the moon and mars the surface of venus does not preserve a record of heavy bombard ment from the early history of the solar system1 3 those craters that are found on venus appear to be statistically indistinguishable from a random spatial population and rarely show modification by folds faults and lava flows1 3 although the volcanic and tec tonic history of venus is still much debated2 5 there is mounting evidence for near global resurfacing 300 500 myr ago1 2 6 moreover it has recently been noted that the density of impact craters on large volcanic structures is less than the average crater density of the planet suggestive of significant activity after the resurfacing event7 it is not clear however whether these features represent late remnants of the global event or continuing volcanism and tectonism of a still active planet to address this question we have used the regional variations in crater density to date volcan oes rifts and coronae which based on stratigraphic evidence clearly post date the main resurfacing event8 11 the calculated mean ages of 70 125 myr exclude the possibility that the majority of these features represent the final stages of the global event

geochemical discrimination of hydrothermal and igneous zircon in the iberian pyrite belt spain intensely altered felsic volcanic subvolcanic rocks located at vhms footwalls in the aznalcollar mining district iberian pyrite belt spain contain both igneous and hydrothermal zircon grains according to textural evidence in order to discriminate their respective features we present a detailed description showing that these contrasting textural characteristics can be correlated with distinctly different chemical compositions in comparison with igneous zircons emp study shows that hydrothermal zircon grains have lower sio2 and zro2 contents whereas they are enriched in p2o5 al2o3 y2o3 and cao on the other hand la icpms study shows that the ree patterns of hydrothermal zircons strongly differ from those of igneous ones in that a hydrothermal zircons are highly enriched in ree b they display patterns with maximum values for intermediate ree often showing tbn ybn ratios 1 and c hydrothermal zircon grains do not display ce or eu anomalies which are widespread in igneous zircons we conclude that these chemical differences are consistent with zircon xenotime solid solution having reached here values close to a 10 mol which represent the maximum possible degree of xenotime substitution according to the available experimental data the absence of ce and eu anomalies is explained according to the particular characteristics of the hydrothermal environment during intense footwall alteration involving temperatures up to 350 c and destruction of plagioclase or other eu 2 hosting minerals we suggest that discrimination of hydrothermal zircons may open new ways in dating hydrothermal zircons in vhms and other mineralized areas related to intense hydrothermal alteration

weathering of ilmenite from chavara deposit and its comparison with manavalakurichi placer ilmenite southwestern india abstract the magnetic fractions of ilmenite from the beach placer deposit of chavara southwest india have been studied for mineralogical and chemical composition to assess the range of their physical and chemical variations with weathering chavara deposit represents a highly weathered and relatively homogenous concentration significant variation in composition has been documented with alteration the most magnetic of the fractions of ilmenite separated at 0 15 a and with a susceptibility of 3 2 10 6 m 3 kg 1 indicates the presence of haematite ilmenite intergrowth an iron poor titanium rich component of the ilmenite ore has been identified from among the magnetic fractions of the chavara ilmenite albeit with an undesirably high nb 2 o 5 0 28 cr 2 o 3 0 23 and th 149 ppm contents the ilmenite from chavara is compared with that from the nearby manavalakurichi deposit of similar geological setting and provenance the lower ferrous iron oxide 2 32 14 22 and higher tio 2 56 31 66 45 contents highlight the advanced state of alteration of chavara this is also evidenced by the relatively higher fe 3 fe 2 ratio compared to manavalakurichi ilmenite in fact the ilmenite has significantly been converted to pseudorutile leucoxene

variation in ilmenite element composition within and among drainage basins implications for provenance abstract the elemental composition ti fe mn mg v zn cr ni and cu of detrital ilmenite grains from three large drainage basins in virginia was found to be relatively unchanged over hundreds of kilometers distance downstream from the blue ridge mountains to the coastal plain the igneous blue ridge complex apparently contributes a sufficient quantity of ilmenite in each river system such that the input of distinctly different ilmenite compositions from tributaries does not change the overall ilmenite composition in the main river this is not the case for nonopaque heavy minerals which change in abundance and type downstream due to tributary input despite a similar primary source each major river has a distinctly different ilmenite composition which can be discriminated at better th n a 0 95 confidence level this enhances the ability to determine the source river for sands in the depocenter initial results on ilmenite from small drainage basin tributaries dominated by one or two source rock types known to contain ilmenite show that compositional differences exist for granitic mafic igneous metamorphic and sedimentary sources

u pb dating of minerals in alteration halos of superior province massive sulfide deposits syngenesis versus metamorphism u pb geochronology of igneous zircon from rhyolitic host rocks to the archean kidd creek geco and winston lake massive sulfide deposits in the superior province of ontario shows that volcanism which accompanied mineralization occupied a narrow time span 2717 2 ma 2720 2 ma and 2723 2 ma respectively precise ages of hydrothermal monazite allanite and rutile from alteration zones surrounding the above deposits indicate that these minerals crystallized 40 70 million years after volcanism monazite from kidd creek mine is 2659 3 ma old in agreement with spatially associated 2664 25 ma old rutile monazite from a biotite schist at geoco mine gives a similar age of 2661 1 ma however monazite from a sericite schist which hosts the ore at geco mine is 2675 2 ma old abraded large monazite grains from three units in the winston lake deposit are coeval with biotite crystallization and record an age of 2677 2 ma approximately the same as monazite in the sericite schist at geco data points from allanite fractions from both the winston lake and geco deposits fall on a pb pb isochron that gives an age of 2672 5 ma rutile from winston lake gives a younger age of 2651 6 2 ma and may date retrograde alteration of biotite to chlorite the ca 2676 ma age of monazite from winston lake and in the sericite schist at geco mine probably dates a regional metamorphic event that affected most of the southern superior province the ca 2660 ma old monazite in the biotite schist at geco mine and in the chlorite sericite alteration at kidd creek may date later k metasomatism caused by metamorphically derived fluids that were focussed along old fault structures such fluids were also responsible for local sulfide remobilization monazite and rutile are spatially associated with chlorite and sericite alterations at kidd creek their young ages indicate that these originally syngenetic mineral assemblages may have been significantly affected by regional metamorphism formation of monazite at all three deposits studied was a result of significant ree remobilization during metamorphism the discrete character of syn metamorphic hydrothermal activity in different units of the same deposit as well as its synchroneity among different widely separated deposits requires a mechanism for episodic injection of heat and fluid into the crust on a regional scale these activities are broadly coeval with and probably related to plutonism within adjacent metasedimentary subprovinces and middle to lower crustal metamorphism in the superior province

the tectonic evolution of western ishtar terra venus the origin and evolution of ishtar terra is modeled using numerical simulations of crust mantle interaction on venus based on our modeling we favor a two phase evolutionary sequence for ishtar the first phase is driven by bulk mantle flow due to thermal convection and results in the formation of a thickened crustal block over a mantle downwelling the second phase a result of the thermal perturbation exerted on the mantle by the region of thickened crust is driven by the sinking of a lithospheric slab this slab driven tectonic pulse leads to low angle subduction and orogenesis at the peripheries of the crustal block throughout its evolution ishtar remains coupled to flow within the deep mantle due to the lack of an asthenosphere and ultimately bulk mantle flow comes to dominate as the effects of the slab driven pulse wane

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

impact crater densities on volcanoes and coronae on venus implications for volcanic resurfacing the density of impact craters on large volcanoes on venus is half the average crater density for the planet the crater density on some classes of coronae is not significantly different from the global average density but coronae with extensive associated volcanic deposits have lower crater densities these results are inconsistent with both single age and steady state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on venus over the last 500 million years

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

magellan a new view of venus geology and geophysics within the past fifteen years the surface of venus has gone from being the least well understood of all the terrestrial planets to the most thoroughly mapped surface of any terrestrial planet including the earth this is primarily due to the magellan mission which has collected a variety of data on the surface morphology physical properties and interior density structure of venus amounting to more than 1 terabit 1012 bits of data synthetic aperture radar images have been obtained for over 95 of the surface their high resolution reveals most surface features larger than 100 200 meters across using its radar altimeter magellan has collected data on surface elevations sub meter scale roughness and radar reflectivity at a resolution of approximately 10 km further information on the physical properties of the surface was gathered by measuring the passive microwave emissivity of the surface pettengill et al 1992 two way doppler tracking of the spacecraft has yielded line of sight los gravity data and a spherical harmonic model of gravity and geoid out to degree 75 collection of high resolution gravity data has been aided by an innovative aerobraking maneuver which used venus atmosphere to brake the spacecraft and lower it from a highly elliptical orbit to a near circular orbit

convection driven tectonics on venus an analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on venus theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a newtonian viscous medium with an exponential depth dependence of viscosity a green s function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth it is shown that during the transient phase of the solution the field quantities stress velocity displacement in both media depend upon the rheology of both media whereas in steady state the solutions become uncoupled due to this uncoupling stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate i show for such a plate that 1 shear stress imparted at the base of the lower boundary can induce large in plane stretching mode forces and 2 these forces can be estimated directly from free air gravity anomalies without detailed knowledge of the density distribution in the underlying flow free air gravity anomalies for venus imply in plane forces of about 2 3 1012 n m in plane forces of this magnitude are capable of extensive disruption of the venusian lithosphere to demonstrate this an elastic plastic analysis is carried out for the deformation of a model venus lithosphere the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow the results predict that dynamic uplift of venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere geoid anomalies and geoid to topography ratios for venusian topographic features are large compared to values for thermal swells in earth s ocean basins this suggests that dynamic support of topography must be an important process on venus further a low viscosity zone is probably not present beneath the venusian lithosphere a comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on earth is unimportant

anatomy of a venusian hot spot geology gravity and mantle dynamics of eistla regio eistla regio is a series of broad swells each up to a few thousand kilometers in diameter in the equatorial highlands of venus it is characterized by strong positive free air gravity anomalies shield volcanoes and rift systems we present a two part study of the western and central portions of this venusian hot spot first magellan radar images were mapped in order to understand the general geologic history of the region radial fracture systems both on the rises and volcanoes indicate uplift and associated faulting accompanied volcanic construction prominent fracture zones strike wnw to nw parallel to the long axis of the highlands the largest of these guor linea displays a progressive deformation history that may include minor clockwise rotation in addition to bulk nne ssw extension other regional structural patterns also suggest this sense of rotation the parts of eistla studied here appear to be structurally distinct from surrounding regions although volcanic radar bright plains and ridge and groove deformation to the west indicate an earlier more diffuse thermal uplift in the second part of our analysis pioneer venus line of sight accelerations were inverted for vertical gravity which when combined with topography were used to solve for mass anomalies on the crust mantle boundary and in the upper levels of the mantle convective system both western and central eistla appear to be sites of vigorous mantle upwelling a linear upwelling appears beneath guor linea indicating that rifting is actively driven by mantle flow minor crustal thinning may be present beneath western eistla and the northern plains whereas some thickening occurs beneath central eistla and the southern plains heng o corona in particular may be the site of a small shallow mass anomaly of either compositional thickened crust or thermal origin mantle return flow is asymmetrically distributed about eistla being much stronger to the south than the north and little flow is inferred at all to the ne in bereghinya planitia these differences introduce a clockwise rotation to finite strain in agreement with observations

provenance discrimination of upper yangtze river basin sediments new insights from heavy mineral signatures and detrital magnetite geochemistry abstract magnetite is typomorphic and its crystal textures and chemical fingerprint form unique signatures that can be used to determine sediment provenance determination of the provenance of sediments in the upper yangtze river is critical for understanding its evolution and the uplift of the tibetan plateau in this study petrographic analysis and electron probe microanalysis epma of 800 detrital magnetite grains were performed to differentiate the provenance of sediments within the mainstream and major tributaries of the upper yangtze river catchment chemical analyses show that the elemental composition of magnetite grains shows pronounced variations among the different sections of the mainstream and tributaries based on the observed chemical fingerprints we infer that the source rocks control the elemental composition of magnetite grains the magnetite contribution is closely related to the tectonics climate and erosion of parent rocks within different drainage basins magnetite from the trunk stream of the jinsha river is found to be a significant contributor to the upper yangtze river magnetite grains of the upper jinsha river are primarily derived from source areas composed of silicic plutonic rocks and ophiolite lithologies that crop out on the se tibetan plateau magnetite grains within the sands of the lower jinshajiang sediments are characterized by high concentrations of ti mg cr al and mn and are derived from the widely distributed emeishan flood basalts and pan xi layered basic ultrabasic igneous rocks this source is also a major contributor of magnetite to the upper yangtze river and the parent rock distributed in this region is of typomorphic significance to the development of the yangtze river the origin of magnetite grains in the tributaries depends on the parent rocks distributed in their catchments in summary the presence of detrital magnetite grains provides valuable information for discriminating the provenance of sediments in the upper yangtze river drainage basins

tessera deformation and the contemporaneous thermal state of the plateau highlands venus abstract geodynamic modeling of tessera deformation at the ancient venusian plateau highlands implies lithospheric thermal gradients exceeding 17 k km 1 regularly spaced contractional ridges in the highlands manifest the unstable growth of perturbations at a dominant wavelength of 20 km controlled by a mechanical layer of relatively uniform thickness we model the deformation as the shortening of a plastic layer over a viscous substrate a layer thickness of less than 6 km is consistent with the observed ridge spacings assuming an anhydrous diabase crustal rheology and a minimum strain rate of 10 16 s 1 the geotherm was at least 17 k km 1 higher strain rates consequently higher thermal gradients are more probable we rule out the possibility that the plateau highlands were locally anomalously hot at the time ridges formed in the tessera that the highlands are dominantly composed of rock more felsic than diabase or that the crust was weak because it was hydrated therefore the average global lithospheric heat flux when the highlands tesserae were deforming was considerably higher than it has been since

a holocene record of changing arctic ocean ice drift analogous to the effects of the arctic oscillation 1 the arctic oscillation ao controls the configuration of the transpolar drift tpd if thicker ice from the beaufort gyre were exported the volume of fresh water sea ice in the nordic seas would significantly increase decreasing the formation of north atlantic deep water this would cool large parts of the northern hemisphere and affect global climate therefore in order to understand how the global climate system functions it is imperative to know how the tpd changed over the last millennium or more the provenance of grains in a sediment core located near the confluence of the tpd and the beaufort gyre provides a direct proxy for changes in seaice drift owing to these circulation systems the core has more than 200 cm of holocene sediment with intervals dominated by grains from the russian shelves alternating with intervals of abundant grains from north american sources grains matched to russian shelves indicate that the tpd was shifted toward north america similar to what occurs during a more positive phase of the ao this condition alternated with intervals where few grains matched to russian sources presumably because the tpd was restricted to the russian half of the arctic far from the core site during the last 1300 years increased influx of russian grains occurred approximately every 50 150 years this fluctuation might represent the long term oscillation of the ao which modulates the same tpd shift today

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

the role of fluids during formation and evolution of the southern superior province lithosphere an overview models for fluid flow and hydrothermal alteration for the abitibi greenstone belt are reviewed in the light of lithoprobe results in the region in the abitibi greenstone belt eruption of volcanic sequences over 2750 2700 ma was accompanied by pervasive low temperature hydrothermal alteration at high water rock ratios giving systematic 18o enrichment archean ambient ocean water bottom temperatures were likely ca 30 c and 18o 0 1 chert iron formations precipitated from low temperature hydrothermal discharge base metal massive sulphide deposits formed at or near the seafloor from focussed discharge of high temperature 300 400 c fluids in convective cells sited above subvolcanic intrusions the ore fluids were evolved seawater that had undergone compositional and isotopic evolution by high temperature low water rock exchange with the volcanic pile to nacl 3 7 wt or cacl2 nacl up to 30 wt brines of 18o 0 8 these volcanic associated hydrothermal deposits are associated with greenst