periglacial geomorphology and landscape evolution of the tempe terra region mars a systematic survey was undertaken and an investigation carried out into the geomorphological characteristics of lobate debris aprons in the tempe terra region of mars based on the most recent high resolution sub 15 m per pixel imagery and on new topography data this study endeavoured to raise and discuss questions regarding their formation emplacement and modification deformation sequence as well as the role of a mantling deposit found at mid latitude locations on mars furthermore a model for the formation of debris aprons in the tempe terra mareotis fossae settings is proposed image survey in combination with basic morphometric observations within a geomorphological context provided additional insights into the source emplacement and modification of hillslope debris material our results imply that lobate debris aprons are not mainly relicts of remnant degradation but are substantially composed of mantling material probably deposited episodically in the course of planetary obliquity changes and over a long timespan as derived erosion rates suggest crater size frequency statistics and the derivation of absolute ages show ages of sub recent modification and document earlier resurfacing events

morphologic and topographic analyses of debris aprons in the eastern hellas region mars this paper presents new detailed analyses of small scale morphologic and topographic characteristics of martian debris aprons that support viking based interpretations of debris aprons as ice rich flow features derived from local uplands fifty four debris apron complexes in the eastern hellas region of mars were examined using mars global surveyor data sets including mars orbiter camera images and mars orbiter laser altimeter topographic profiles consistent patterns in a suite of small scale surface textures and geomorphic features observed throughout the population reflect a history of viscous flow and surface degradation through wind ablation and loss of contained ice a wide variety of shapes seen in topographic profile reveal variations in distribution of contained ice and different stages of apron development and degradation the results of this study provide new evidence consistent with multiple models of apron formation including rock glacier debris covered glacier and ice rich landslide models typical eastern hellas debris aprons formed from a series of large scale events emplacing debris that was enriched initially or later by ground ice complemented by small scale mass wasting of multiple styles and postemplacement flow of apron masses

a u pb age for the sulitjelma gabbro north norway further evidence for the development of a caledonian marginal basin in ashgill llandovery time the sulitjelma gabbro in the central norwegian caledonides has been dated with the u pb zircon sphene method to 437 2 ma geological relationships as well as trace element geochemistry and nd and sr isotopic compositions suggest that the gabbro and associated dyke swarms and pillow lava formed during the initial stages of back arc spreading the age constrains the timing of juxtaposition of an older island arc terrane and continental margin lithologies exposed within the caledonian nappes the age of the gabbro is similar to that of the solund stavfjord ophiolite in west norway and confirms that spreading related magmatism and marginal basin development took place along the margin of iapetus in late ordovician early silurian times

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

mars global surveyor observations of martian fretted terrain the martian fretted terrain between latitudes 30 and 50 n and between 315 and 360 w has been reexamined in light of new mars orbiter camera moc and mars orbiter laser altimeter mola data from mars global surveyor much of the terrain in the 30 50 latitude belt in both hemispheres has a characteristic stippled or pitted texture at moc 1 5 m scale the texture appears to result from partial removal of a formerly smooth thin deposit as a result of sublimation and deflation a complex history of deposition and exhumation is indicated by remnants of a former thicker cover of layered deposits in some hollows and on some slopes particularly those facing the pole are smooth textured deposits outlined by an outward facing escarpment throughout the study area are numerous escarpments with debris flows at their base the escarpments typically have slopes in the 20 30 range at the base of the escarpment is commonly a deposit with striae oriented at right angles to the escarpment outside this deposit is the main debris apron with a surface that typically slopes 2 3 and complex surface textures suggestive of compression sublimation and deflation the presence of undeformed impact craters indicates that the debris flows are no longer forming fretted valleys contain lineated fill and are poorly graded they likely form from fluvial valleys that were initially like those elsewhere on the planet but were subsequently widened and filled by the same mass wasting processes that formed the debris aprons slope reversals indicate that downvalley flow of the lineated fill is minor the ubiquitous presence of breaks in slope formed by mass wasting and the complex surface textures that result from mass wasting deflation and sublimation decreases the recognizability of the shorelines formerly proposed for this area

recent high latitude icy mantle in the northern plains of mars characteristics and ages of emplacement 1 martian high latitude zones are covered with a smooth layered ice rich mantle containing characteristic polygonal patterns the mantle is especially uniform and homogeneous in the northern plains we performed a survey of decameter scale mantle textures and circular features including impact craters in the northern lowlands down to 50 n latitude strongly altered and mantled craters of impact and non impact origin form a population of subtle circular features below the mantle pits of non impact origin are numerous in some regions at lower latitudes impact craters superposed on the mantle are small and very sparse the inferred mean crater retention age of the mantle is 0 1 ma the spatial distribution of young craters suggests an age difference between the highest latitudes younger and some lower latitude regions older latitudinal trends in polygon textures agree with impact crater evidence for a latitudinal age progression of mantle properties

high ice purity of martian lobate debris aprons at the regional scale evidence from an orbital radar sounding survey in deuteronilus and protonilus mensae

context camera investigation on board the mars reconnaissance orbiter 1 the context camera ctx on the mars reconnaissance orbiter mro is a facility instrument i e government furnished equipment operated by a science team not responsible for design and fabrication designed built and operated by malin space science systems and the mro mars color imager team marci ctx will 1 provide context images for data acquired by other mro instruments 2 observe features of interest to nasa s mars exploration program e g candidate landing sites and 3 conduct a scientific investigation led by the marci team of geologic geomorphic and meteorological processes on mars ctx consists of a digital electronics assembly a 350 mm f 3 25 schmidt type telescope of catadioptric optical design with a 5 7 field of view providing a 30 km wide swath from 290 km altitude and a 5000 element ccd with a band pass of 500 700 nm and 7 m pixels giving 6 m pixel spatial resolution from mro s nearly circular nearly polar mapping orbit raw data are transferred to the mro spacecraft flight computer for processing e g data compression before transmission to earth the ground data system and operations are based on 9 years of mars global surveyor mars orbiter camera on orbit experience ctx has been allocated 12 of the total mro data return or about 3 terabits for the nominal mission this data volume would cover 9 of mars at 6 m pixel but overlapping images for stereo mosaics and observation of changes and meteorological events will reduce this area ctx acquired its first instrument checkout images of mars on 24 march 2006

sharad detection and characterization of subsurface water ice deposits in utopia planitia mars morphological analyses of utopia planitia mars have led to the hypothesis that the region contains a substantial amount of near surface ice this paper tests this hypothesis using ground penetrating radar techniques we have identified an expansive radar reflective region spanning approximately 375 000 km2 in shallow radar sharad data over western utopia planitia the sharad reflective regions coincides with high densities of scalloped depressions and polygonal terrain the reflectors are associated with layered mesas 80 170 m thick we find a value of 2 8 0 8 for the dielectric constant of the material overlying the reflectors this work finds that the dielectric constant is consistent with a mixture of ice air and dust containing a water ice volume up to 14 300 km3 in this unit

mineralogy of the sulphide deposits at sulitjelma northern norway abstract the pyritic vms deposits at sulitjelma northern norway have been regionally metamorphosed at amphibolite facies during the caledonide orogen the deposits contain a wide diversity of sulphide sulphosalt and related minerals which are systematically discussed with reference to their occurrence paragenesis and mineral chemistry most of these minerals are however rare to absent within the massive deposits which are dominantly composed of pyrite with subordinate chalcopyrite sphalerite and pyrrhotite galena arsenopyrite cubanite molybdenite stannite and tetrahedrite occur in minor amounts syn metamorphic mobilisation has however resulted in the redistribution of a range of elements including cu pb sb as ni and au these elements have been reconcentrated into mobilised segregations where they have crystallised under retrograde conditions into a range of discrete phases ore mobilisation which took place along tectonically controlled pathways is however of a highly localised nature with a considerable range in mineral textures and parageneses observed in different samples typical associations within these mobilisates include a range of cu pb sb and cu pb as sulphosalts including bournonite meneghinite seligmannite and jordanite breithauptite nisbite costibite arsenopyrite and gudmundite co existing galena is characterised by elevated concentrations of ag bi and se argentian tetrahedrite 9 17 wt ag the chief ag carrier in the deposits as well as other ag bearing phases including pyrargyrite hessite and empressite are strongly enriched in the mobilisates gold as electrum 50 75 wt au and more rarely as aurostibite is also relatively enriched in the mobilisate samples over those from more massive parts of the deposits a further range of minerals including native antimony native arsenic allargentum dyscrasite argentite mckinstryite jalpaite and native silver occur as low temperature breakdown products of earlier formed minerals preserved textures do not contradict a model involving more than a one phase of mobilisation during the metamorphic evolution

lineated valley fill and lobate debris apron stratigraphy in nilosyrtis mensae mars evidence for phases of glacial modification of the dichotomy boundary 1 the nilosyrtis mensae region is important among dichotomy boundary fretted terrain outcrops as it provides evidence of overprinting of ancient landscapes by a suite of glacial features providing a composite view of the variety of midlatitude glacial modification processes that can occur during recent martian ice ages on the basis of a series of criteria developed for the identification of a glacial origin for lineated valley fill and lobate debris aprons we interpret stratigraphic topographic and textural relationships between lineated valley fill and lobate debris apron morphological units as evidence of local and regional glacial overprinting of the landscape during the recent amazonian we document 1 stratigraphic relationships between lineated valley fill subunits including the presence of apparently superposed and small scale lobate features 2 the regional integration and flow of lineated valley fill material 3 lineated valley fill degradation and 4 the nature and stratigraphic position of lobate debris aprons these observations suggest multiple phases or episodes of midlatitude valley glacier activity these observations together with those of surface units elsewhere in northern arabia terra interpreted as glacially modified landforms suggest the possibility of midlatitude glacial deposits extending over broad portions of the martian dichotomy boundary within the past several hundred million years

martian cratering 10 progress in use of crater counts to interpret geological processes examples from two debris aprons abstract recent controversies about systems of crater count dating have been largely resolved and with continuing refinements crater counts will offer a fundamental geological tool to interpret not only ages but also the nature of geological processes altering the surface of mars as an example of the latter technique we present data on two debris aprons east of hellas the aprons show much shorter survival times of small craters than do the nearby contiguous plains the order of magnitude depths of layers involved in the loss process can be judged from the depths of the affected craters we infer that ice rich layers in the top tens of meters of both aprons have lost crater topography within the last few 10 8 yr probably due to flow or sublimation of ice rich materials mantling by ice rich deposits associated with climate change cycles of obliquity change has probably also affected both the aprons and the plains the crater count tool thus adds chronological and vertical dimensional information to purely morphological studies

the role of fluids in syntectonic mass transport and the localization of metamorphic vein type ore deposists abstract in low to medium grade metamorphic environments the mobilization of species is closely associated with the operation of solution precipitation creep processes during foliation development such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields mass transfer occurs via solute diffusion in an essentially static fluid network or by transport of dissolved species in a migrating fluid precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces grain scale microcracks and veins sink sites may be close to or distantly removed from source sites the transport of material during deformation involving solution precipitation creep is largely controlled by the mobility of metamorphic fluids in regions of high fluid mobility significant mass transfer may occur over large distances fluid flow mass transfer paths and the development of sink sites in such cases are constrained by the structurally controlled development of high fluid pressure domains and the consequent mechanical enhancement of permeability by hydraulic fracture processes and the development of grain scale dilatancy these processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions a proportion of syntectonically mobilized material is precipitated to form vein type deposits in hydraulic fracture arrays which develop in high fluid pressure environments the microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization the formation of syntectonic gold quartz vein deposits in central victoria australia illustrates the importance of structural controls on both the large scale migration of metamorphic fluids and the development of sites suitable for the precipitation of metamorphically mobilized species

probing supraglacial debris on mars 2 crater morphology abstract lobate debris aprons lda lineated valley fill lvf and concentric crater fill ccf on mars interpreted to be debris covered glaciers possess craters with a suite of distinct interior landforms called ring mold craters that have been attributed to the presence of glacial ice at depth or surface modification processes we tested current hypotheses for the formation of ring mold craters by conducting a comprehensive analysis of the size and morphology of 16 457 impact craters 125 m in diameter formed within glacial deposits in deuteronilus mensae two major groups bowl shaped craters and ring mold craters are found with at least nine distinct crater types while there is statistical difference in median diameters between these crater types this difference is relatively small and is within the estimated uncertainty in diameter measurements and may be related to more enhanced erosion of the rims of ring mold craters clear degradation sequences are observed supporting a role for post impact modification in producing at least some of the diversity in crater landforms the spatial density of ring mold craters is also directly correlated with the development of lda lvf and ccf surface textures flow lineations cross cut two ring mold crater types but they maintain their circular planforms in some cases suggesting that the craters initially formed completely within mantling layers deposited after glacial flow had ceased we also find analogous craters in non glacial units glacial ice is therefore not required to form the observed morphologic diversity our observations are most consistent with formation of crater landforms by emplacement and modification of at least two depositional episodes of icy dust i e mantle this mantle was initially tens of meters in thickness to support crater formation and has experienced much downwasting and erosion since emplacement derived crater retention ages of 460 ma for lda lvf and ccf features in the region therefore reflect deposition of mantle units and only give a very minimum age for the formation of lda lvf and ccf

flow patterns of lobate debris aprons and lineated valley fill north of ismeniae fossae mars evidence for extensive mid latitude glaciation in the late amazonian a variety of late amazonian landforms on mars have been attributed to the dynamics of ice related processes evidence for large scale mid latitude glacial episodes existing within the last 100 million to 1 billion years on mars has been presented from analyses of lobate debris aprons lda and lineated valley fill lvf in the northern and southern mid latitudes we test the glacial hypothesis for lda and lvf along the dichotomy boundary in the northern mid latitudes by examining the morphological characteristics of lda and lvf surrounding two large plateaus proximal massifs and the dichotomy boundary escarpment north of ismeniae fossae centered at 45 3n and 39 2e lineations and flow directions within lda and lvf were mapped using images from the context ctx camera the thermal emission imaging spectrometer themis and the high resolution stereo camera hrsc flow directions were then compared to topographic contours derived from the mars orbiter laser altimeter mola to determine the down gradient components of lda and lvf flow observations indicate that flow patterns emerge from numerous alcoves within the plateau walls are integrated over distances of up to tens of kilometers and have down gradient flow directions smaller lobes confined within alcoves and superposed on the main lda and lvf represent a later less extensive glacial phase crater size frequency distributions of lda and lvf suggest a minimum youngest age of 100 ma the presence of ring mold crater morphologies is suggestive that lda and lvf are formed of near surface ice rich bodies from these observations we interpret lda and lvf within our study region to result from formerly active debris covered glacial flow consistent with similar observations in the northern mid latitudes of mars glacial flow was likely initiated from the accumulation and compaction of snow and ice on plateaus and in alcoves within the plateau walls as volatiles were mobilized to the mid latitudes during higher obliquity excursions together with similar analyses elsewhere along the dichotomy boundary these observations suggest that multiple glacial episodes occurred in the late amazonian and that lda and lvf represent significant reservoirs of non polar ice sequestered below a surface lag for hundreds of millions of years

effects of greenschist facies metamorphism and related deformation on the mobrun massive sulfide deposit quebec canada the mobrun zn cu ag au deposit in the noranda mining camp is hosted by archean mafic and felsic submarine volcanic rocks the deposit comprises three massive sulfide complexes the main and satellite lenses near surface and the 1100 orebody at depth the rocks have been subjected to lower greenschist facies metamorphism and related deformation which resulted in changes in ore textures development of shear zones and veins systems remobilization of gold and formation of a new mineral electrum within the orebodies both mechanical and chemical processes operated to produce secondary textures and structures resulting from brittle deformation ductile deformation and annealing the specific deformation mechanisms include brittle failure and cataclastic flow dislocation glide dislocation creep and solution precipitation creep the main and satellite lenses are characterized by excellent preservation of primary sulfides deposited from and reworked by synvolcanic hydrothermal fluids these orebodies were affected to a limited degree by mechanical processes of deformation in contrast the 1100 orebody is characterized by a higher degree of development of textures and structures related to metamorphism and deformation especially those formed by chemical processes the differences may be due to the greater depth of the 1100 orebody relative to the other lenses as regional metamorphic isograds are subhorizontal and more extensive interaction between metamorphic fluids and the 1100 lens

geochronology and geochemistry of 2 5 to 2 4 ga granitic plutons from the southern margin of the north china craton implications for a tectonic transition from arc to post collisional setting abstract the xutai and lujiagou plutons exposed in the songshan jishan area in the southern margin of the north china craton ncc are studied to constrain the tectonic regime of the ncc between 2 5 and 2 4 ga the xutai pluton dated at 2509 33 ma mainly consists of tonalite and monzogranite with a calc alkaline affinity rittmann index

the timing of mineral growth across a regional metamorphic sequence if regional metamorphism occurs in response to crustal thickening then low grade rocks are likely to record earlier thermal conditions than will high grade rocks in the same exhumed terrain1 4 at sulitjelma north norway regional metamorphism during the caledonian orogeny created a type sequence of progressive metamorphic zones5 7 garnet and coexisting minerals from the high grade part of this terrain yield precise and concordant sm nd u pb and rb sr ages which relate directly to the pressure temperature conditions of mineral growth8 here we present pressure temperature time p t t information for lower grade rocks occurring at the garnet isograd taken together these data provide a precise estimate of the timing of mineral equilibration across a prograde metamorphic terrain and indicate that peak metamorphic conditions at the garnet isograd at 458 20 c were attained 8 3 3 0myr t sm nd or 8 7 3 2myr t u pb before the peak in the high grade sample at 544 16 c these results clearly illustrate that the exhumed p t array recorded in such terrains cannot be used to deduce any single geotherm that existed during metamorphism

sphalerite stars in chalcopyrite are they always the result of an unmixing process sphalerite skeletal crystals in inclusions in chalcopyrite so called sphalerite stars are widely thought to be the result of high temperature unmixing processes detailed observations in the chalcopyrite ores at ain barbar algeria demonstrate that the unmixing theory cannot work in this case it is here proposed that skeletal crystals are the result of a local supersaturation in sphalerite at the growing fronts of chalcopyrite this implies a high precipitation rate estimated to be ca 10 6 m sec 1 remaining compatible however with a recharge in ore forming components by the flowing mineralizing solutions in addition to ain barbar this explanation is probably of more general significance since the kind of relationships described here is by no means restricted to this particular deposit

geologic and geochemical controls on the mineralogy and grain size of gold bearing phases eastern australian volcanic hosted massive sulfide deposits the mineralogy and occurrence of gold varies in a systematic manner in volcanic hosted massive sulfide deposits of eastern australia in deposits where it is concentrated with zinc at or near the top of the massive sulfide deposit gold occurs in auriferous arsenopyrite and pyrite in the center of the lens and electrum of variable fineness 300 900 in the baritic top of the lens the former occurrence results from precipitation of the gold into the lattice of arsenopyrite and coprecipitation of submicroscopic gold grains in arsenic poor pyrite the latter occurrence results from direct precipitation of electrum from thio complexes owing to the decrease in a sub h 2 s caused by dilution and oxidation as hydrothermal fluids mix with seawater in deposits where gold is concentrated with copper at the base of the sulfide lens and in the stringer zones the gold occurs as electrum of high fineness 750 1 000 or as gold tellurides that are commonly associated with chalcopyrite and or pyrite this occurrence is caused by the direct precipitation of gold from chloro complexes owing to an increase in ph or a decrease in temperature as the hydrothermal fluids move up through the sulfide body gold in auriferous pyrite is readily mobilized during deformation as the pyrite fractures gold migrates via solution precipitation mechanisms to cracks and grain boundaries to form electrum by combination with silver which is released from sulfide minerals in a similar manner with increasing stress the electrum is released from pyrite to recrystallize with other easily remobilized minerals such as galena during this process electrum grain size increases thermodynamic modeling of the variation in electrum fineness shows good agreement with variations measured in massive sulfide ores for gold transported as a thio complex the predicted fineness of 190 to 990 for temperatures of 200 degrees to 300 degrees c and typical volcanogenic fluids matches the observed variation in deposits of the zn au association the very high finenesses observed in deposits of the cu au association can be modeled assuming gold chloro complex transport and high ag au ratios in the hydrothermal fluids

ore mineralogy of the applachian caledonian stratabound sulfide deposits abstract the diverse ore mineralogies of more than 100 stratabound sulfide deposits that occur along the length of the appalachian caledonian orogen have been tabulated the minerals reflect the varying cu zn pb ratios that have been used as a means to separate the ore deposits into six distinct types following the igcp project 60 guidelines the ore deposits each containing at least one million metric tons of ore are typical of massive sulfide deposits throughout the world and provide a basis for contrasting and comparing this orogen with others differing grades of metamorphism have significantly modified the ore mineral grain sizes and textures but do not seem to have effectively influenced the number or variety of mineral species present

formation of lobate debris aprons on mars assessment of regional ice sheet collapse and debris cover armoring abstract lobate debris aprons lda are lobate shaped aprons surrounding scarps and isolated massifs that are concentrated in the vicinity of the northern dichotomy boundary on mars ldas have been interpreted as 1 ice cemented talus aprons undergoing viscous flow 2 local debris covered alpine like glaciers or 3 remnants of the collapse of a regional retreating ice sheet we investigate the plausibility that ldas are remnants of a more extensive regional ice sheet by modeling this process we find that as a regional ice sheet collapses the surface drops below cliff and massif bedrock margins exposing bedrock and regolith and initiating debris deposition on the surface of a cold based glacier reduced sublimation due to debris cover armoring of the proto lda surface produces a surface slope and consequent ice flow that carries the armoring debris away from the rock outcrops as collapse and ice retreat continue the debris train eventually reaches the substrate surface at the front of the glacier leaving the entire lda armored by debris cover using a simplified ice flow model we are able to characterize the temperature and sublimation rate that would be necessary to produce ldas with a wide range of specified lateral extents and thicknesses we then apply this method to a database of documented lda parameters height lateral extent from the dichotomy boundary region and assess the implications for predicted climate conditions during their formation and the range of formation times implied by the model we find that for the population examined here typical temperatures are in the range of 85 to 40 c and typical sublimation rates lie in the range of 6 14 mm a lobate debris apron formation times from the point of bedrock exposure to complete debris cover cluster near 400 500 ka these results show that lda length and thickness characteristics are consistent with climate conditions and a formation scenario typical of the collapse of a regional retreating ice sheet and exposure of bedrock cliffs this scenario helps resolve many of the unusual characteristics of lobate debris aprons lda and lineated valley fill lvf for example the distribution of lvf is very consistent with extensive flow of glacial ice from plateau icefields and the acquisition of a debris cover in the waning stages of retreat of the regional cover as the bedrock scarps are exposed the typical concentric development of lda around massifs is much more consistent with ice sheet retreat than insolation related local accumulation and flow we thus conclude that the retreating ice sheet model is robust and should be investigated and tested in more detail in addition these results clearly show that the lobate debris aprons in the vicinity of the dichotomy boundary could not have attained temperatures near or above the ice melting point and retained their current shape a finding that supports subzero temperatures for the last several hundred million years the age of the lda surfaces a further implication is that the lda ice has been preserved for at least several hundred million years and could potentially contain the record of the climate of mars preserved since that time below a sublimation lag deposit

enhanced erosion rates on mars during amazonian glaciation abstract observations of mars from the surface and from orbit suggest that erosion rates over the last 3 gyr the amazonian have been as slow as 10 5 m myr and have been dominated by aeolian processes while ancient noachian erosion rates may have been orders of magnitude higher due to impact bombardment and fluvial activity amazonian aged glacial deposits are widespread on mars but rates of erosion responsible for contributing debris to these remnant glacial deposits have not been constrained here we calculate erosion rates during amazonian glaciations using a catalog of mid latitude glacial landforms coupled with observational and theoretical constraints on the duration of glaciation these calculations suggest that erosion rates for scarps that contributed debris to glacial landforms are 4 7 orders of magnitude higher than average amazonian rates in non glaciated low slope regions these erosion rates are similar to terrestrial cold based glacier erosion and entrainment rates consistent with cold based glacier modification of parts of mars

an extended period of episodic northern mid latitude glaciation on mars during the middle to late amazonian implications for long term obliquity history mars is the only planet other than earth in the solar system that has a preserved nonpolar geological record of glaciation on its surface nonpolar ice deposits on mars have been linked to variations in spin axis obliquity that cause mobilization of polar ice and redeposition at lower latitudes forming ice rich and glacial deposits remnant nonpolar glacial deposits are found across the northern mid latitudes where surface ice is not currently stable implying that different climatic conditions existed on mars in the past individual glacial deposits are often too small to date reliably using impact crater size frequency data we describe a novel approach that allows us to derive new information about when glaciation occurred in broad areas of the northern mid latitudes in this region we have classified 1 craters that superpose preexisting glacial deposits and were modified by later accumulation and therefore formed during an epoch when glaciation was occurring and 2 craters that are superposed on glacial deposits but are themselves unmodified by ice accumulation and thus post date significant glaciation the sparse population of post glacial craters reveals that the last period of extensive ice deposition of this type in this latitude band was recent late amazonian the substantial number of craters formed during the recurring glacial periods implies that northern mid latitude glaciation was a long lived recurring process occurring over a period of at least 600 m y on the basis of mars atmospheric general circulation models these results are consistent with higher obliquity being common in the past with recurring periods of obliquity exceeding the 25 axial tilt of mars today these observations support the statistical prediction of j laskar and colleagues that the median obliquity during the amazonian was 35 40

the age of the medusae fossae formation evidence of hesperian emplacement from crater morphology stratigraphy and ancient lava contacts abstract the medusae fossae formation mff covering about 2 1 10 6 km 2 with an estimated volume of 1 4 10 6 km 3 and straddling the equatorial region of mars east of tharsis has historically been mapped and dated as amazonian in age analysis of the mff using a range of new observations from recent mission data at multiple resolutions reveals evidence that the formation is older than previously hypothesized with parts of the mff having formed in the hesperian and parts having been reworked and reformed throughout the amazonian up to the present ancient outcroppings of the mff edged with jagged yardangs became a mold for embaying hesperian aged lavas the erosion of the mff left solidified lava casts in the embaying lava unit this lava edge morphology permits the identification of ancient contacts between the mff and hesperian aged lava terrain additionally the flanking fan of the hesperian aged apollinaris patera volcano embays the formation at its foot indicating that parts of the mff were formed in the hesperian erosion has erased and inverted many of the superposed craters in the region showing that very young amazonian ages derived from impact crater size frequency distributions are resurfacing ages and not emplacement ages we find abundant evidence that the formation is extremely mobile and continuously reworked we conclude that a significant part of the mff may have originally been emplaced in the hesperian these observations place new constraints on the mode of origin of the mff

lineated valley fill lvf and lobate debris aprons lda in the deuteronilus mensae northern dichotomy boundary region mars constraints on the extent age and episodicity of amazonian glacial events abstract in order to assess the nature degradational processes and history of the dichotomy boundary on mars we conducted a detailed morphological analysis of a 70 000 km 2 region of its northern portion north central deuteronilus mensae south of lyot in the vicinity of sinton crater this region is characterized by the distinctive sinuous 2 km high plateau scarp boundary outlying massifs to the north and extensive fretted valleys dissecting the plateau to the south these features represent the first order modification and retreat of the dichotomy boundary and are further modified by processes that form lineated valley fill lvf in the fretted valleys and lobate debris aprons lda along the dichotomy scarp and surrounding the outlying massifs we use new high resolution image and topography data to examine the nature and origin of lvf and lda and to investigate the climatic and accompanying degradational history of the escarpment on the basis of our analysis we conclude that 1 lvf and lda deposits within the study region are comprised of the same material show integrated flow patterns and originate as debris covered valley glaciers a significant amount of ice hundreds of meters is likely to remain today beneath a thin cover of sublimation till 2 there is depositional evidence to suggest glacial highstands at least 800 m above the present level implying previous conditions in which the distribution of ice was much more widespread this is supported by similar deposits within many other areas across the dichotomy boundary 3 the timing of the most recent large scale activity of the lda lvf in this area is about 100 500 million years ago similar to ages reported elsewhere along the dichotomy boundary 4 there is evidence for a secondary but significantly limited phase of glaciation the deposits of which are limited to the vicinity of the alcoves similar later phases have also been reported elsewhere along the dichotomy boundary 5 modification of the fretted valleys of the dichotomy boundary has been substantial locally but we find no evidence that the amazonian glacial epochs caused retreat of the dichotomy boundary of the scale of tens to hundreds of kilometers our findings support the results of an analysis just to the east of the study region and of studies carried out elsewhere along the dichotomy boundary that find further evidence for the remnants of debris covered glaciers and extensive valley glacial land systems

the timing of martian valley network activity constraints from buffered crater counting abstract valley networks concentrations of dendritic channels that often suggest widespread pluvial and fluvial activity have been cited as indicators that the climate of mars differed significantly in the past from the present hyperarid cold desert conditions some researchers suggest that the change in climate was abrupt while others favor a much more gradual transition thus the precise timing of valley network formation is critical to understanding the climate history on mars we examine thirty valley network incised regions on mars including both cratered upland valley networks and those outside the uplands and apply a buffered crater counting technique to directly constrain when valley network formation occurred the crater populations that we derive using this approach allow assessment of the timing of the last activity in a valley network independent of the mapping of specific geological units from these measurements we find that valley networks cluster into two subdivisions in terms characteristics and age 1 valley network activity in the cratered highlands has an average cessation age at the noachian hesperian boundary and all valleys that we crater counted are early hesperian or older no evidence is found for valley networks in the cratered uplands of late hesperian or amazonian age the timing of the cessation of cratered upland valley network activity at the noachian hesperian boundary also corresponds to a decline in the intensity of large crater formation and degradation and to the apparent end of phyllosilicate type weathering 2 a few valley network incised regions formed outside of the cratered uplands on volcanic edifices in association with younger impact craters and on the rim of valles marineris we applied our buffered crater counting technique to four such valleys on the volcanoes ceraunius tholus hecates tholus and alba patera and on the rim of echus chasma and find that each has distinctive and different late hesperian or early amazonian ages indicating that valley networks formed from time to time in the post noachian period unlike the cratered upland valley networks these isolated occurrences are very local and have been interpreted to represent local conditions e g snowpack melted during periods of intrusive volcanic activity in contrast to a gradual cessation in the formation of valley networks proposed by some workers our new buffered crater counting results indicate a relatively abrupt cessation in the formation of the widespread cratered upland valley networks at approximately the end of the noachian followed only by episodic and very localized valley network formation in later mars history very likely due to specific conditions e g local magmatic heating these valley network ages and correlations are thus consistent with a major change in the near surface aqueous environment on mars at approximately the noachian hesperian boundary the noachian environment supported surface running water and fluvial erosion across mars in the cratered uplands enhanced crater degradation and a weathering environment favoring the formation of phyllosilicates the hesperian amazonian environment was more similar to the hyperarid cold desert of today with valley networks forming only extremely rarely and confined to localized special conditions sources of water for these latter occurrences are likely to be related to periodic mobilization and equatorward migration of polar volatiles due to variations in spin axis orbital parameters and to periodic catastrophic emergence of groundwater

geochronology and geochemistry of the ttg and potassic granite of the taihua complex mts huashan implications for crustal evolution of the southern north china craton abstract tonalite trondhjemite granodiorite ttg suite and potassic granite volumetrically expose in the neoarchean to paleoproterozoic taihua complex mts huashan region southernmost segment of the trans north china orogen tnco zircon u pb dating on the trondhjemite granitic gneiss k feldspar granite and coarse grained granite samples show three episodes of magmatism 2 55 2 49 ga 2 33 2 25 ga and 1 87 1 80 ga with distinct geochemical features in this area during the paleoproterozoic the trondhjemite samples have high sio2 67 46 71 73 wt na2o 4 83 5 95 wt low mg 31 40 and hree contents with moderate la yb n values 16 8 to 35 3 and sr y ratios 15 8 to 34 the potassic granite samples show high sio2 65 2 75 43 wt k2o 3 9 8 6 low na2o 1 74 4 17 wt mg 2 46 and hree contents with moderate to high la yb n values 10 3 to 226 and sr y ratios 12 8 to 208 all these granitoid samples are characterized by high sio2 and low mg cr ni with moderate to high la yb n values and sr y ratios the absence of evidence of any pre existing high sr y and la yb sources in this region led to that the granitoids probably generated from partial melting of hydrous mafic rocks with garnet and amphibole in the residue therefore partial melting of thickened lower curst is the most likely origin combined with previous studies the first episodic magmatism 2 55 2 49 ga represented by ttg and granitic gneisses with positive whole rock nd t and zircon hf t values is interpreted as melts from partial melting of juvenile thickened lower crust the second episodic magmatism 2 33 2 25 ga represented by ttg granitic gneisses and k feldspar granite with positive to negative whole rock nd t and zircon hf t values is suggested as the product of partial melting of both juvenile and pre existing crustal material the third episodic magmatism 1 87 1 80 ga is represented by these potassic granites which is synchronism with the metamorphism recorded in this region and probably resulted from partial melting of pre existing crustal materials in a syn orogenic or post orogenic setting during the collision between the eastern and western blocks of the north china craton multistage continental accretion at 2 84 2 72 ga 2 57 2 43 ga and 2 36 2 25 ga and reworking at 2 36 2 25 ga 2 19 2 07 ga and 1 87 1 80 ga occurred in the southern segment of the tnco from neoarchean to paleoproterozoic transformation from an andean type continental margin arc setting on the western margin of the eastern block to continent arc continent collision in the southern tnco during late neoarchean to late paleoproterozoic is suggested in this paper

deformation of stratiform znpb barite deposits in the northern canadian cordillera abstract the major stratiform sediment hosted zn pb barite deposits of the northern canadian cordillera occur in the palaeozoic selwyn basin kechika trough they formed in euxinic fine grained clastic dominated extensional basins that developed within the carbonate platform of the passive margin of ancestral north america during the late jurassic to mid cretaceous these basins and the stratiform ore deposits were subjected to low greenschist facies regional metamorphism and fold and thrust deformation as they were incorporated into the western part of the rocky mountains mackenzie mountains foreland fold and thrust belt this paper summarises the results of detailed field and microscopic studies on four of these deposits the tom deposit the howards pass deposit the cirque deposit and the driftpile creek deposit the dominant mineralogy of these deposits is sphalerite galena pyrite and in some cases significant barite is present low greenschist grade metamorphism and deformation is characterised by cataclastic flow and brecciation in coarse grained massive pyritic sulphides and by folding recrystallisation and cleavage development in fine grained sulphides and barite relict primary depositional diagenetic structures are found within the orebodies deformation is characterised by strong pressure solution fabrics in barite in fine grained laminated pyrite layers and in mixed sulphide carbonate beds marked local transposition is found with significant remobilisation of galena barite and pyrite the deformation mechanics of these sulphide barite deposits are illustrated using schematic deformation mechanism maps

late cretaceous neo tethyan slab roll back evidence from zircon u pb o and whole rock geochemical and sr nd fe isotopic data of adakitic plutons in the himalaya tibetan plateau

regional distribution of white k mica polymorphs and their phengite content in the central alps some 150 white k micas from the central alps were analysed for their polymorph and phengite content

late neoarchean to early paleoproterozoic tectonic evolution of the southern north china craton evidence from geochemistry zircon geochronology and hf isotopes of felsic gneisses from the taihua complex abstract the archean paleoproterozoic taihua complex extending from east to west along the southern margin of the north china craton ncc is an ideal window to understand the tectonic evolution of the early earth here we present new data on petrology zircon geochronology whole rock geochemistry and isotopes of ttgs granitic gneisses and felsic leucosomes of migmatites from the taihua complex in the xiaoqinling area zircon la icp ms analyses of rocks yield the formation ages of 2564 2503 ma for ttgs 2481 2476 ma for felsic leucosomes and 2359 2334 ma for granitic gneisses respectively the zircon lu hf isotopes suggest that crustal growth occurred in the late neoarchean and crustal reworking occurred in the end of neoarchean and the early paleoproterozoic the ttg gneisses are characterized by high na2o k2o la yb sr y and low nb ta ratios and they were likely generated by partial melting of subducted juvenile oceanic crust with garnet or amphibole in residuals a mechanism similar to the slab melting model both the felsic leucosomes and granitic gneisses have high sio2 contents and various hf isotopic compositions and they probably were formed in an extensional setting attributing to partial melting as well as mixing of the preexisting ttg suite statistically we report a major transitional phase in the tectonic milieu of the southern ncc from compression to extension during the late neoarchean ca 2 57 2 42 ga the compressional tectonics can be attributed to amalgamation of micro continental blocks during the late neoarchean ca 2 57 2 5 ga as the leading period of crustal growth and associated multistage ttg magmatism in contrast the extensional phase occurred during the end of neoarchean ca 2 5 2 42 ga that may represent the end of cratonic stabilization period and formation of large amounts of k rich granitic rocks widespread high grade metamorphism and migmatites additionally the formation of a large amount of crust derived granites and mafic dykes during the early paleoproterozoic in southern ncc reflect another stage ca 2 36 2 24 ga of crustal extension which represents an episode of initial worldwide rifting events probably linking to the great oxygen event goe therefore the present study proposes a new profile on multi stage crustal growth reworking and craton stabilization during neoarchean paleoproterozoic period along the southern ncc and provides useful insights on complex and heterogeneous evolutionary processes of early earth

gold distribution in the mobrun volcanic associated massive sulfide deposit noranda quebec a preliminary evaluation of the role of metamorphic remobilization the mobrun zn cu au ag orebodies show metal zonation typical of volcanic associated massive sulfide deposits although some metamorphic remobilization of metals has occurred the nature and distribution of primary facies are the result of synvolcanic deposition and reworking of sulfides by hydrothermal fluids secondary facies resulting from metamorphism and deformation locally overprint primary facies and include transgressive veins which contain remobilized au as electrum the distribution of secondary facies was controlled by the mechanical properties of the mineralization as well as of the surrounding rocks the mechanical response of different facies to deformation influenced the development and distribution of zones containing easily recoverable gold preservation of primary textures is best in the main and satellite lenses the 1100 lens also shows significant preservation of primary facies however development of secondary facies is pronounced textural structural and chemical evidence indicates that the 1100 lens was subjected to more extreme metamorphic recrystallization and deformation than the other lenses metamorphic remobilization was important in releasing refractory au from sulfides and locally concentrating it in a recoverable form electrum

zircon u pb ages geochemistry and sr nd pb hf isotopes of the nuri intrusive rocks in the gangdese area southern tibet constraints on timing petrogenesis and tectonic transformation abstract abundant magmatic rocks of various ages are exposed in gangdese southern tibet these rocks play an important role in understanding the tectonic transformation from the subduction of neo tethyan oceanic crust to the collision of the indian and asian continents based on zircon u pb ages geochemistry and sr nd pb hf isotopic data of the late cretaceous to early oligocene 96 30 ma intrusive rocks in the nuri cu w mo deposit we discuss the late cretaceous to early oligocene tectonic transformation of the region and the origin of oligocene cu w mo mineralization in southern gangdese the nuri intrusive rocks represent three magmatic episodes 96 91 56 52 and 33 30 ma the 96 91 and 56 52 ma rocks have relatively low 87sr 86sr i 0 7041 to 0 7060 and high end t 3 1 to 3 5 and ehf t values 3 7 to 15 the 33 30 ma rocks have relatively high 87sr 86sr i 0 7061 to 0 7063 and pb isotopes and low end t 3 8 to 1 8 and ehf t values 0 6 to 10 1 the three stages of intrusive rocks have geochemical characteristics that are similar to those of coeval rocks in gangdese the 96 91 and 33 30 ma rocks are adakitic whereas the 56 52 ma rocks have characteristics of arc calc alkaline magmatic rocks the 96 91 ma rocks were produced by the partial melting of neo tethyan basaltic oceanic crust and minor sediments whereas the 56 52 ma rocks were generated by the partial melting of juvenile crust and the 33 30 ma rocks were formed by the melting of indian plate lower crust contaminated with overlying mantle materials on the basis of the regional tectonic and magmatic characteristics we suggest that neo tethyan oceanic slab subduction and slab roll back occurred from 100 to 65 ma collision between the indian and asian continents occurred at 65 to 40 ma neo tethyan oceanic slab break off took place at 50 ma and the indian continent subducted northwards beneath the asian continent at 30 ma from the late cretaceous 96 91 ma to oligocene 30 ma the geodynamic setting of gangdese was changed from neo tethyan oceanic subduction to indian continental subduction in contrast to the miocene metallogenic rocks the magmatic source of oligocene metallogenic rocks contains old indian continental crust materials which result in the formation of intense unique cu w mo mineralization especially w mineralization in the southern gangdese

expanded secondary craters in the arcadia planitia region mars evidence for tens of myr old shallow subsurface ice abstract a range of observations indicates widespread subsurface ice throughout the mid and high latitudes of mars in the form of both pore filling and excess ice it is generally thought that this ice was recently emplaced and is not older than a hundred thousand to a few millions of years old based on ice stability and orbital induced climate change we analyze the distribution of subsurface ice in arcadia planitia located in the northern mid latitudes by mapping thermokarstically expanded secondary craters providing additional evidence for extensive excess ice down to fairly low latitudes less than 40 n we further infer the minimum age of this subsurface ice based on the ages of the four primary craters that are thought to be the source of a large portion of these secondaries which yields estimates on the order of tens of millions of years old much more ancient than anticipated this estimated ancient age suggests that ice can be preserved in the shallow subsurface for long periods of time at least in some parts of arcadia planitia where expanded secondary craters are especially abundant we estimate the amount of ice lost to sublimation during crater expansion based on measurements of expanded secondary craters in hirise digital terrain models the loss is equivalent to a volume of ice between 140 and 360 km 3 which would correspond to a global layer of 1 2 5 mm thick we further argue that much more ice at least 6000 km 3 is likely preserved beneath the un cratered regions of arcadia planitia since significant loss of this excess ice would have caused extensive terrain dissection and the removal of the expanded secondary craters both the loss of ice due to secondary crater expansion and the presence of this ice today have implications for the martian climate

late amazonian glaciation at the dichotomy boundary on mars evidence for glacial thickness maxima and multiple glacial phases lineated valley fill lvf in fretted valleys at the dichotomy boundary has been interpreted as glacial in origin unknown are 1 the original thickness of the glacier ice 2 the amount of ice surface lowering through sublimation and retreat to its presently observed level and 3 whether there were multiple periods of glaciation we address these questions through analysis of an lvf glacial system the elevation difference between the upper limit of a previous highstand and the current surface of the lvf at the study site is 920 m we interpret this difference to be the minimum amount of ice surface lowering of the glacier system consistent with a general lowering of the ice surface are multiple moraines and or trimlines and changes in lvf flow patterns as the ice retreated and decreased in thickness the superposition of several lobes onto the current surface of the lvf indicates that a phase of alpine glaciation followed the lowering of the valley glacial system these data suggest that the late amazonian glaciation that produced lvf in this region involved significantly larger amounts of ice than previously thought and that subsequent alpine glaciation followed

titanium mobility in metamorphic systems a review abstract titanium is a dispersed lithophile element commonly regarded as chemically immobile at sub magmatic temperatures however a growing number of field studies demonstrate that ti is somewhat mobile in certain environments this review examines the current state of knowledge concerning the mobility of ti during alteration and metamorphism of crustal rocks also considered are relevant ti oxide solubility data from experiments and determinations of ti concentrations in natural waters where ti mobility is well documented its length scale is 10 m however ti may move greater distances in deep crustal shear zones the solubility of ti oxides in geologic fluids is strongly temperature dependent under low to medium grade metamorphic conditions ti mobility is limited by the low solubility of ti oxides in the associated fluids under eclogite conditions ti mobility is probably limited not by ti oxide solubility which reaches 1 ti but by restricted fluid flow in low permeability rocks

cold based debris covered glaciers evaluating their potential as climate archives through studies of ground penetrating radar and surface morphology we describe the morphology and internal structure of mullins and friedman glaciers two cold based debris covered alpine glaciers that occur in neighboring valleys in the mcmurdo dry valleys antarctica both glaciers are overlain by a single dry supraglacial debris layer 8 75 cm thick each mantling debris layer is marked with near identical patterns of arcuate ridges and steps as well as corresponding changes in bulk grain size meter scale surface topography and thermal contraction crack polygons results from 24 km of ground penetrating radar data show that the ice within the uppermost 1 2 km of mullins and friedman glaciers is essentially free of englacial debris 1 by volume but thereafter is interspersed with bands of englacial debris each 3 m thick and dipping up glacier that intersect the ice surface at all major surface ridges and steps the similarity in number and pattern of englacial debris bands and corresponding surface ridges and steps across both glaciers along with model results and observations that call for negligible basal entrainment is best explained by episodic environmental change at valley headwalls our working hypothesis is that layers of englacial debris originate as supraglacial lags that form in ice accumulation areas during times of reduced net ice accumulation following renewed net ice accumulation these lags are subsequently buried by snow and ice flow englacially and intersect the ice surface to impart distinctive changes in the texture of supraglacial debris and topographic relief the implication is that the englacial structure and surface morphology of these cold based debris covered glaciers preserves a consistent record of climate and environmental change

radar evidence for ice in lobate debris aprons in the mid northern latitudes of mars 1 subsurface radar sounding data indicate that lobate debris aprons found in deuteronilus mensae in the mid northern latitudes of mars are composed predominantly of water ice the position in time delay and the relatively low amount of signal loss of the apparent basal reflectors below the debris aprons indicate that aprons contain only a minor component of lithic material the current presence of large ice masses at these latitudes has important implications for the climate evolution of mars and for future targets for in situ exploration

northern mid latitude glaciation in the late amazonian period of mars criteria for the recognition of debris covered glacier and valley glacier landsystem deposits lobate debris aprons lda and lineated valley fill lvf have been known to characterize the mid latitude regions of mars since documented by viking their flow like character suggested that deposition of ice in talus pile pore space caused lubrication and flow during an earlier climatic regime a number of factors have remained uncertain however including the detailed structure and texture of lda lvf the relationships between them their direction of flow the origin and abundance of the lubricating agent and their exact mode of origin e g ice assisted rock creep ice rich landslides rock glaciers debris covered glaciers we use new high resolution stereo camera hrsc image and topography data in conjunction with a range of other post viking data sets and new insights provided by cold based terrestrial glacial analogs to assess the characteristics of lda lvf in the northern mid latitudes of mars we find evidence that the characteristics and flow patterns of the lda and lvf are most consistent with late amazonian debris covered glacial valley landsystems the broad distribution and integrated characteristics of the lda lvf systems suggest that earlier in the amazonian climatic conditions were such that significant snow and ice accumulated on mid latitude plateaus and in valleys producing integrated glacial landsystems the remnants of which are preserved today beneath residual sublimation till derived from adjacent valley walls atmospheric general circulation models suggest that these climatic conditions occurred when mars was at a spin axis obliquity of 35 and the atmosphere was relatively dusty glacial flow modeling under these conditions produces patterns similar to those documented in the lda lvf and sharad radar data suggests that significant amounts of ice remain sequestered below the sublimation lag today

mineral chemistry study of progressive metamorphism in calcareous schists from ankarvattnet swedish caledonides abstract the composition of muscovite chlorite and biotite has been studied in a low grade calcareous phyllite metagreywacke sequence blasjo phyllite in the seve koli nappe complex at ankarvattnet swedish caledonides the b0 lattice constant of muscovite is shown to be a good measure of the mg fe2 content i e the phengite content in muscovite according to the following equation b0 8 992 0 358rm or d 060 3 31 1 499 0 060 rm where rm mole prop 2fe2o3 feo mgo in the chlorite zone the b0 values are mostly controlled by the rock composition thus obliterating the effect of metamorphic conditions in the limiting assemblages of the biotite zone muscovite is continuously de phengitizated and na enriched with increasing grade furthermore the element distribution of mg and fe among coexisting phengite and chlorite is controlled by metamorphic grade temperature the appearance of biotite is found to be due to the following continuous reaction phengitic muscovite chlorite less phengitic more paragonitic muscovite al enriched chlorite biotite quartz water biotite quartz water

chemical evidence for and implications of a primary fes phase in the ankarvattnet zn cu pb massive sulphide deposit central swedish caledonides the origin of the pyrrhotite in the ankarvattnet massive sulphide deposit is discussed based on chemical relations between the different mineralization types a metamorphic genesis is rejected and a premetamorphic origin is alternatively suggested which is in agreement with similar ideas for the origin of the pyrrhotite in other deposits the pyrrhotite is thought to have formed in the stringer zone to a massive pyritic layer with similar formation mechanisms as for volcanogenic massive sulphide deposits the recognition of a stringer zone mineralogy provides better possibilities to understand the physico chemical character of the ore forming environment

concentric crater fill in utopia planitia history and interaction between glacial brain terrain and periglacial mantle processes abstract at martian mid to high latitudes the surfaces of potentially ice rich features including concentric crater fill lobate debris aprons and lineated valley fill typically display a complex texture known as brain terrain due to its resemblance to the complex patterns on brain surfaces in order to determine the structure and developmental history of concentric crater fill and overlying latitude dependent mantle ldm material brain terrain and polygonally patterned ldm surfaces are analyzed using hirise images from four craters in utopia planitia containing concentric crater fill brain terrain and mantle surface textures are classified based on morphological characteristics 1 closed cell brain terrain 2 open cell brain terrain 3 high center mantle polygons and 4 low center mantle polygons a combined glacial and thermal contraction cracking model is proposed for the formation and modification of the brain terrain texture of concentric crater fill a similar model related to thermal contraction cracking and differential sublimation of underlying ice is proposed for the formation and development of polygonally patterned mantle material both models require atmospheric deposition of ice likely during periods of high obliquity but do not require wet active layer processes crater dating of brain terrain and mantled surfaces suggests a transition at martian mid latitudes from peak glacial conditions occurring within the past 10 100 my to a quiescent period followed by a cold desert periglacial period during the past 1 2 my

concentric crater fill in the northern mid latitudes of mars formation processes and relationships to similar landforms of glacial origin abstract hypotheses accounting for the formation of concentric crater fill ccf on mars range from ice free processes e g aeolian fill to ice assisted talus creep to debris covered glaciers based on analysis of new ctx and hirise data we find that concentric crater fill ccf is a significant component of amazonian aged glacial landsystems on mars we present mapping results documenting the nature and extent of ccf along the martian dichotomy boundary over 30 to 90 e latitude and 20 80 n longitude on the basis of morphological analysis we classify ccf landforms into classic ccf and low definition ccf classic ccf is most typical in the middle latitudes of the analysis area 30 50 n while a range of degradation processes results in the presence of low definition ccf landforms at higher and lower latitudes we evaluate formation mechanisms for ccf on the basis of morphological and topographic analyses and interpret the landforms to be relict debris covered glaciers rather than ice mobilized talus or aeolian units we examine filled crater depth diameter ratios and conclude that in many locations hundreds of meters of ice may still be present under desiccated surficial debris this conclusion is consistent with the abundance of ring mold craters on ccf surfaces that suggest the presence of near surface ice analysis of breached craters and distal glacial deposits suggests that in some locations ccf related ice was once several hundred meters higher than its current level and has sublimated significantly during the most recent amazonian crater counts on ejecta blankets of filled and unfilled craters suggests that ccf formed most recently between 60 and 300 ma consistent with the formation ages of other martian debris covered glacial landforms such as lineated valley fill lvf and lobate debris aprons lda morphological analysis of ccf in the vicinity of lvf and lda suggests that ccf is a part of an integrated lvf lda ccf glacial landsystem instances of morphological continuity between ccf lvf and lda are abundant the presence of formerly more abundant ccf ice coupled with the integration of ccf into lvf and lda suggests the possibility that ccf represents one component of the significant amazonian mid latitude glaciation s on mars

late carboniferous adakitic granodiorites in the qiongkusitai area western tianshan nw china implications for partial melting of lower crust in the southern central asian orogenic belt abstract carboniferous granitic rocks are wide spread in the western tianshan but their tectonic contexts remain controversial we present zircon u pb age major element trace element and in situ zircon hf isotopic data for the qiongkusitai granodiorites from the southern part of the yili block in the western tianshan the granodiorites were emplaced in the late carboniferous ca 314 ma and are characterized by high sio 2 62 1 65 7 wt high sr 376 485 ppm low y 13 7 17 7 ppm and low yb 1 34 1 81 ppm with sr y ratios of 23 34 such characteristics imply adakitic affinities their low concentrations of cr 3 85 6 75 ppm co 7 63 10 7 ppm and ni 2 94 6 80 ppm suggest little if any interaction with a mantle source compared to the thickened lower crustal derived adakitic rocks the relatively low nb ta ratios of the qiongkusitai adakitic granodiorites indicate that they were probably generated by partial melting of amphibolite facies rocks but not eclogite facies rocks the qiongkusitai adakitic granodiorites have positive zircon e hf t values of 0 7 to 5 1 and t dm2 of 1 28 1 68 ga and they are interpreted as being derived from remelting of amphibolite facies mesoproterozoic basement with an input of juvenile material late carboniferous magmatism in the western tianshan was possibly triggered by asthenospheric upwelling as a result of the roll back of the subducted southern tianshan oceanic lithosphere

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

tropical to mid latitude snow and ice accumulation flow and glaciation on mars images from the mars express hrsc high resolution stereo camera of debris aprons at the base of massifs in eastern hellas reveal numerous concentrically ridged lobate and pitted features and related evidence of extremely ice rich glacier like viscous flow and sublimation together with new evidence for recent ice rich rock glaciers at the base of the olympus mons scarp superposed on larger late amazonian debris covered piedmont glaciers we interpret these deposits as evidence for geologically recent and recurring glacial activity in tropical and mid latitude regions of mars during periods of increased spin axis obliquity when polar ice was mobilized and redeposited in microenvironments at lower latitudes the data indicate that abundant residual ice probably remains in these deposits and that these records of geologically recent climate changes are accessible to future automated and human surface exploration

evidence for amazonian northern mid latitude regional glacial landsystems on mars glacial flow models using gcm driven climate results and comparisons to geological observations a fretted valley system on mars located at the northern mid latitude dichotomy boundary contains lineated valley fill lvf with extensive flow like features interpreted to be glacial in origin we have modeled this deposit using glacial flow models linked to atmospheric general circulation models gcm for conditions consistent with the deposition of snow and ice in amounts sufficient to explain the interpreted glaciation in the first glacial flow model simulation sources were modeled in the alcoves only and were found to be consistent with the alpine valley glaciation interpretation for various environments of flow in the system these results supported the interpretation of the observed lvf deposits as resulting from initial ice accumulation in the alcoves accompanied by debris cover that led to advancing alpine glacial landsystems to the extent observed today with preservation of their flow texture and the underlying ice during downwasting in the waning stages of glaciation in the second glacial flow model simulation the regional accumulation patterns predicted by a gcm linked to simulation of a glacial period were used this glacial flow model simulation produced a much wider region of thick ice accumulation and significant glaciation on the plateaus and in the regional plains surrounding the dichotomy boundary deglaciation produced decreasing ice thicknesses with flow centered on the fretted valleys as plateaus lost ice scarps and cliffs of the valley and dichotomy boundary walls were exposed providing considerable potential for the production of a rock debris cover that could preserve the underlying ice and the surface flow patterns seen today in this model the lineated valley fill and lobate debris aprons were the product of final retreat and downwasting of a much larger regional glacial landsystem rather than representing the maximum extent of an alpine valley glacial landsystem these results favor the interpretation that periods of mid latitude glaciation were characterized by extensive plateau and plains ice cover rather than being restricted to alcoves and adjacent valleys and that the observed lineated valley fill and lobate debris aprons represent debris covered residual remnants of a once more extensive glaciation 2011 elsevier inc

probing supraglacial debris on mars 1 sources thickness and stratigraphy abstract geomorphic and geophysical evidence supports a debris covered glacier origin for a suite of landforms at the mid latitudes of mars including lobate debris aprons lda lineated valley fill lvf and concentric crater fill ccf these large reservoirs of ice and their near surface structure provide a rich record for understanding the planet s climate and history of global volatile exchange over the past billion years lda lvf and ccf are also potential sites for future robotic and human missions but the accessibility of glacial ice for direct sampling and in situ resource utilization depends largely on the geotechnical properties of the surface debris supraglacial debris including its thickness grain sizes and density structure the physical properties of this supraglacial debris layer have been poorly constrained we use images of morphology digital elevation models thermal inertia data and radar sounding data to probe the near surface of lda lvf and ccf in deuteronilus mensae in order to place constraints on the sources grain sizes thickness and stratigraphy of supraglacial debris we find evidence for at least a two layer stratigraphy layered mantle consisting of atmospherically emplaced dust and ice superposes boulder rich sediment sourced by rockfalls glacially transported downslope high thermal inertia boulder rich termini and debris bands reminiscent of medial moraines are found throughout the study region supporting a rockfall origin for at least a fraction of the debris exposed at the surface this supraglacial debris layer would have thickened with time from sublimation of glacial ice and liberation of englacial sediment and dust at present the entire supraglacial debris package is a minimum of a few meters in thickness and is likely tens of meters in thickness in many locations possibly thinning regionally at lower latitudes and locally thinning toward the headwalls the lack of terracing or interior structures in craters formed within lda lvf and ccf and the absence of near surface reflectors in sharad radar data further suggest that no strong contrasts in permittivity or strength occur at the interface of the layers

martian surface near surface water inventory sources sinks and changes with time today a 34 m global equivalent water layer gel lies in the martian polar layered deposits and shallow ground ice during the amazonian 3 m was outgassed and 31 m was lost to space and to the surface leaving 62 m at the end of hesperian during the hesperian volcanic outgassing added 5 m 7 m was lost and 40 m gel of groundwater was added to form outflow channels leaving 24 m carryover of surface water from the noachian into the hesperian the hesperian budget is incompatible with a northern ocean during this era these figures are for near surface water substantial amounts of water may have existed as deep ground ice and groundwater our estimate of approximately 24 m near surface water in the late noachian is insufficient to support an ocean at that time also and favors episodic melting of an icy highlands to produce the fluvial and lacustrine features

multi stage crustal growth and neoarchean geodynamics in the eastern dharwar craton southern india abstract the dharwar craton is a composite archean cratonic collage that preserves important records of crustal evolution on the early earth here we present results from a multidisciplinary study involving field investigations petrology zircon shrimp u pb geochronology with in situ hf isotope analyses and whole rock geochemistry including nd isotope data on migmatitic ttg tonalite trondhjemite granodiorite gneisses dark grey banded gneisses calc alkaline and anatectic granitoids together with synplutonic mafic dykes along a wide northwest southeast corridor forming a wide time window in the central and eastern blocks of the dharwar craton the dark grey banded gneisses are transitional between ttgs and calc alkaline granitoids and are referred to as transitional ttgs whereas the calc alkaline granitoids show sanukitoid affinity our zircon u pb data together with published results reveal four major periods of crustal growth ca 3360 3200 ma 3000 2960 ma 2700 2600 ma and 2570 2520 ma in this region the first two periods correspond to ttg generation and accretion that is confined to the western part of the corridor whereas widespread 2670 2600 ma transitional ttg together with a major outburst of 2570 2520 ma juvenile calc alkaline magmatism of sanukitoid affinity contributed to peak continental growth the transitional ttgs were preceded by greenstone volcanism between 2746 ma and 2700 ma whereas the calc alkaline magmatism was contemporaneous with 2570 2545 ma felsic volcanism the terminal stage of all four major accretion events was marked by thermal events reflected by amphibolite to granulite facies metamorphism at ca 3200 ma 2960 ma 2620 ma and 2520 ma elemental ratios la yb n sr y nb ta hf sm and hf nd isotope data suggest that the magmatic protoliths of the ttgs emplaced at different time periods formed by melting of thickened oceanic arc crust at different depths with plagioclase amphibole garnet titanite ilmenite in the source residue whereas the elemental ba sr la yb n sr y nb ta hf sm and hf nd isotope data ehf t 0 67 to 5 61 end t 0 52 to 4 23 of the transitional ttgs suggest that their protoliths formed by melting of composite sources involving mantle and overlying arc crust with amphibole garnet clinopyroxene plagioclase ilmenite in the residue the highly incompatible and compatible element contents ree k ba sr mg ni cr together with hf and nd isotope data ehf t 4 5 to 3 2 end t 1 93 to 1 26 of the sanukitoids and synplutonic dykes suggest their derivation from enriched mantle reservoirs with minor crustal contamination field elemental and isotope data ehf t 4 3 to 15 0 end t 0 5 to 7 0 of the anatectic granites suggest their derivation through reworking of ancient as well as newly formed juvenile crust secular increase in incompatible as well as compatible element contents in the transitional ttgs to sanukitoids imply progressive enrichment of neoarchean mantle reservoirs possibly through melting of continent derived detritus in a subduction zone setting resulting in the establishment of a sizable continental mass by 2700 ma which in turn is linked to the evolving earth the neoarchean geodynamic evolution is attributed to westward convergence of hot oceanic lithosphere with continued convergence resulted in the assembly of micro blocks with eventual slab break off leading to asthenosphere upwelling caused extensive mantle melting and hot juvenile magma additions to the crust this led to lateral flow of hot ductile crust and 3d mass distribution and formation of an orogenic plateaux with subdued topography as indicated by strain fabric data and strong seismic reflectivity along an e w crustal profile in the central and eastern blocks of the dharwar craton

flow mechanisms in sulphide minerals abstract the sulphide minerals as a group display a diverse range in strength and mechanical behaviour in crustal environments deforming by a range of different flow mechanisms which can lead to the development of characteristic microfabrics the dependence of creep rate on imposed parameters such as stress difference temperature grainsize effective confining pressure and the activities of various components varies for each type of flow mechanism for particular phases changes in the dominant operative flow mechanism may occur in response to changes in the imposed environmental variables at high applied stress differences low effective confining pressures and low temperatures brittle failure and cataclastic flow are significant deformation mechanisms in some sulphide minerals at elevated confining pressures brittle failure can be suppressed and low temperature plasticity processes involving dislocation glide and or deformation twinning may operate with decreasing applied stress difference and at elevated temperatures even though strain may be accommodated largely by dislocation glide deformation may be rate controlled by recovery processes and dynamic recrystallization may be significant in influencing microfabric development the operation of dislocation glide processes leads to the development of crystallographic preferred orientations at very low applied stress difference various grainsize dependent atomic transfer deformation mechanisms may be important in many fluid present low to medium grade metamorphic environments solution precipitation creep processes are active in the deformation of some sulphide phases in fluid absent situations grain boundary diffusion creep may be significant at higher temperatures where intragranular diffusion is relatively rapid lattice diffusion creep processes are expected to be a major deformation mechanism in fine grained sulphide assemblages at elevated temperatures grain boundary sliding may be an important deformation mechanism

the arditurri pb zn f ba deposit cinco villas massif basque pyrenees a deformed and metamorphosed stratiform deposit stratiform banded and massive pb zn f ba mineralizations in metasediments of the westphalian cinco villas formation basque pyrenees show several features related to hercynian deformation and low grade regional metamorphism however some primary depositional textures and structures are preserved minerals like pyrite reveal preregional metamorphic textures framboidal colloform pyrite with automorphic overgrowths dendritic and atoll textures typical of rapid precipitation from supersaturated solutions during hydrothermal sedimentary activity deformation and subsequent micro and mesoscale remobilization have caused morphological and textural changes in the mineralizations producing folding mineral foliation transposition boudinages piercements etc as well as the development of veinlike orebodies pyrite shows a general tendency toward cataclasis whereas sphalerite galena and chalcopyrite exhibit a variety of microstructures subgrains twining recrystallization etc indicative of ductile deformation microscopy suggests the deformation of more plastic sulfide occurred mainly by a dislocation type mechanism at low temperatures 350 c which are compatible with the regional metamorphism recorded in the host rocks

evidence for recent climate change on mars from the identification of youthful near surface ground ice ground ice in the crust and soil may be one of the largest reservoirs of water on mars1 2 3 near surface ground ice is predicted to be stable at latitudes higher than 40 ref 4 where a number of geomorphologic features indicative of viscous creep and hence ground ice have been observed5 mid latitude soils have also been implicated as a water ice reservoir6 the capacity of which is predicted to vary on a 100 000 year timescale owing to orbitally driven variations in climate7 it is uncertain however whether near surface ground ice currently exists at these latitudes and how it is changing with time here we report observational evidence for a mid latitude reservoir of near surface water ice occupying the pore space of soils the thickness of the ice occupied soil reservoir 1 10 m and its distribution in the 30 to 60 latitude bands indicate a reservoir of 1 5 6 0 104 km3 equivalent to a global layer of water 10 40 cm thick we infer that the reservoir was created during the last phase of high orbital obliquity less than 100 000 years ago and is now being diminished

albitized microcline grains of post depositional and probable detrital origins in brottum formation sandstones upper proterozoic sparagmite region of southern norway untwinned albite ab 99 4 an 0 4 or 0 2 grains forming during diagenesis and very low grade metamorphism are common in sandstones of the brottum formation upper proterozoic in the sparagmite region of southern norway these untwinned albite grains are usually clouded by vacuoles and tiny inclusions which are mostly made of carbonaceous material although some albite grains also contain some combination of carbonates quartz phengite chlorite pyrite halite and anatase the presence of microcline grains replaced partly by albite indicates that the albite grains are pseudomorphs after microcline chessboard albite which occurs in the brottum formation is suggested to be detrital in origin

radar sounding evidence for buried glaciers in the southern mid latitudes of mars lobate features abutting massifs and escarpments in the middle latitudes of mars have been recognized in images for decades but their true nature has been controversial with hypotheses of origin such as ice lubricated debris flows or glaciers covered by a layer of surface debris these models imply an ice content ranging from minor and interstitial to massive and relatively pure soundings of these deposits in the eastern hellas region by the shallow radar on the mars reconnaissance orbiter reveal radar properties entirely consistent with massive water ice supporting the debris covered glacier hypothesis the results imply that these glaciers formed in a previous climate conducive to glaciation at middle latitudes such features may collectively represent the most extensive nonpolar ice yet recognized on mars

ring mold craters in lineated valley fill and lobate debris aprons on mars evidence for subsurface glacial ice 1 ring mold craters rmcs concentric crater forms shaped like a truncated torus and named for their similarity to the cooking implement are abundant in lobate debris aprons lda and lineated valley fill lvf in the northern mid latitudes on mars but are not seen in surrounding terrain lda and lvf have been interpreted to form by flow of debris but uncertainty remains concerning the mechanism of flow with hypotheses ranging from pore ice assisted creep of talus to debris covered glaciers rmcs average less than a few hundred meters in diameter and occur in association with normal bowl shaped impact craters whose average diameters are commonly less than rmcs on the basis of their morphologic similarities to laboratory impact craters formed in ice and the physics of impact cratering into layered material we interpret the unusual morphology of rmcs to be the result of impact into a relatively pure ice substrate below a thin regolith with strength contrast properties spallation viscous flow and sublimation being factors in the development of the ring mold shape associated smaller bowl shaped craters are interpreted to have formed within a layer of regolith like sublimation till overlying the ice substrate estimates of crater depths of excavation between populations of bowl shaped and ring mold craters suggest that the debris layer is relatively thin these results support the hypothesis that lda and lvf formed as debris covered glaciers and predict that many hundreds of meters of ice remain today in lda and lvf deposits beneath a veneer of sublimation till rmcs can be used in other parts of mars to predict and assess the presence of ancient ice related deposits

a geometric model for excavation and modification at terrestrial simple impact craters a geometric model for the excavation and modification of simple crater development is presented modification is modelled by considering the relative dimensions and geometries of the final crater and the transient cavity formed by excavation and displacement the input parameters required are the rim crest diameter of the final crater the depth to autochthonous basement the rim crest height and the outer rim slope the model calculates the diameter of the final crater at the original ground plane the dimensions of the transient cavity and the rim crest volume difference between the final and transient cavities this volume is considered to be the material which slumped off the unstable overheightened transient cavity wall during modification and corresponds to the observed volume of the breccia lens within the final crater excavation is approximated by a steady state z model with the diameter of excavation equivalent to the previously modelled diameter of the transient cavity at the original ground plane the center of flow taken as 1 projectile diameter and z 2 5 3 0 the z model predicts such parameters as the depth and volume of excavation which can be compared with observational data the applicability of the model has been tested with data from meteor and brent craters in the case of meteor the modelled final crater diameter at the original ground plane is within 26 m of the observed value and the modelled breccia lens and the rim crest volumes of the final true crater are within 9 5 and 2 5 respectively of observed values the results of the z model for meteor crater with z 2 8 0 1 give results consistent with observational data on the depth and volume of excavation the correspondence for the more degraded brent crater is less precise as there is greater uncertainty in the input parameters it is possible however to match the model parameters with the breccia lens volume and published estimates of original dimensions the modelled excavated volumes for meteor and brent are 58 and 55 respectively of the transient cavity volume below the original ground plane in the absence of additional observational data for further testing it is concluded that the model may provide first order spatial information on the formation of simple craters

comment on the paper by a d smith j s fox r farquhar and p smith geochemistry of ordovician koli group basalts associated with besshi type cu zn deposits from the southern trondheim and sulitjelma mining districts of norway

formation of patterned ground and sublimation till over miocene glacier ice in beacon valley southern victoria land antarctica a thin glacial diamicton informally termed granite drift occupies the floor of central beacon valley in southern victoria land antarctica this drift is 40 ar 39 ar analyses of presumed in situ ash fall deposits that occur within granite drift at odds with the great age of this ice are high centered polygons that cut granite drift if polygon development has reworked and retransported ash fall deposits then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice our results show that the surface of granite drift is stable at polygon centers and that enclosed ash fall deposits can be used to define the age of underlying glacier ice in our model for patterned ground development active regions lie only above polygon troughs where enhanced sublimation of underlying ice outlines high centered polygons the rate of sublimation is influenced by the development of porous gravel and cobble lag deposits that form above thermal contraction cracks in the underlying ice a negative feedback associated with the development of secondary ice lenses at the base of polygon troughs prevents runaway ice loss secondary ice lenses contrast markedly with glacial ice by lying on a d versus 18 o slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess the latter indicates that secondary ice lenses likely formed by melting downward percolation and subsequent refreezing of snow trapped preferentially in deep polygon troughs the internal stratigraphy of granite drift is related to the formation of surface polygons and surrounding troughs the drift is composed of two facies a nonweathered matrix supported diamicton that contains 25 striated clasts in the 16 mm fraction and a weathered clast supported diamicton with varnished and wind faceted gravels and cobbles the weathered facies is a coarse grained lag of granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies the concentration of cosmogenic 3 he in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice these profile patterns and the 3 he surface exposure ages of 1 18 0 08 ma and 0 18 0 01 ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr although granite drift is stable at polygon centers low frequency slump events occur at the margin of active polygons slumping together with weathering of surface clasts creates the large range of cosmogenic nuclide surface exposure ages observed for granite drift maximum rates of sublimation near active thermal contraction cracks calculated by using the two 3 he depth profiles range from 5 m m y to 90 m m y sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates nevertheless these rates are orders of magnitude lower than those computed on theoretical grounds during eruptions of the nearby mcmurdo group volcanic centers ash fall debris collects at the surface of granite drift either in open thermal contraction cracks or in deep troughs that lie above contraction cracks these deposits subsequently lower passively as the underlying glacier ice sublimes the fact that some regions of granite drift have escaped modification by patterned ground for at least 8 1 ma indicates long term geomorphic stability of individual polygons once established polygon toughs likely persist for as long as 10 5 10 6 yr our model of patterned ground formation which applies to the hyperarid cold desert polar climate of antarctica may also apply to similar sized polygons on mars that occur over buried ice in utopia planitia

ice and debris in the fretted terrain mars viking moderate and high resolution images along the northern highland margin were studied monoscopically and stereoscopically to contribute to an understanding of the development of fretted terrain results suport the hypothesis that the fretting process involved flow facilitated by interstitial ice the process apparently continued for a long period of time and debris apron formation shaped the fretted terrain in the past as well as the present interstitial ice in debris aprons is most likely derived from ground ice obtained by sapping or scarp collapse debris aprons could have been removed by sublimation if they consisted mostly of ice or by deflation if they consisted mostly of debris to remove the debris wind erosion was either very intense early in martian history or was intermittent perhaps owing to climatic cycles the location of debris aprons in two midlatitudinal belts can be attributed to atmospheric conditions that permit ice to remain in the ground at moderate depth in those latitudes and climatic factors that permit temperatures to be warm enough for ice to flow channels have minimum gradients of 5 8 m km which permits flowing ice to deform by dislocation creep at exceedingly low strain rates alternately ice charged debris may have been buoyed by brines by analogy with the fretted channels ice could have flowed through the outflow channels if the atmospheric pressure was somewhat higher in the past and permitted ice to exist in the equatorial area

geomorphic analysis of lobate debris aprons on mars at mars orbiter camera scale evidence for ice sublimation initiated by fractures 1 lobate debris aprons known to be geomorphic landform indicators of the presence of ground ice are of special interest for future missions devoted to the research of water on mars lobate debris aprons in fretted terrains of deuteronilus and protonilus mensae 35 50 n show typical convex shapes interpreted to be the result of viscous deformation at the scale of mars orbiter camera moc high resolution images the surface of these debris aprons shows complex patterns with small pits and buttes these patterns can be explained by the mantling of dust the accumulation of interstitial ice and the subsequent removal of ice by sublimation the sublimation of the ground ice is especially initiated and accelerated by subsurface heterogeneities like fractures theoretical quantification of sublimation rates therefore minimizes sublimation which is not a homogeneous process at least over the landforms studied crater counts show that the sublimation occurred in the last tens of millions of years up to the recent past in the point of view of future searching of subsurface ice only surface layers are submitted to sublimation favoring the conservation of ground ice in deeper layers since the formation of the landform the geophysical survey of lobate debris aprons would give interesting insights into the subsurface distribution of ice and its seasonal variations especially in order to measure current sublimation of ground ice

geochronology and petrogenesis of gray gneisses from the taihua complex at xiong er in the southern segment of the trans north china orogen implications for tectonic transformation in the early paleoproterozoic the taihua complex in the xiong er area along the southern segment of the trans north china orogen tnco consists of gray gneisses with minor amphibolites zircon u pb dating of the gray gneisses reveals at least two episodes of early paleoproterozoic magmatism 2 30 2 32 ga and 2 07 2 19 ga each with distinct geochemical features the tieluping ttg gneisses of the early suite have very low hree ybn 0 58 4 75 and y contents 2 05 13 2 ppm with moderate to high la yb n 14 3 192 6 and sr y ratios 38 5 220 1 pronounced negative ta nb and ti anomalies but positive sr and pb anomalies the protoliths of these gneisses were most likely derived from partial melting of thickened lower crust with residual garnet and amphibole and underwent garnet amphibole fractionation plagioclase accumulation and or fractionation the tieluping ttg gneisses have variable zircon hf model ages tdm hfc 2 48 3 06 ga and whole rock end t values 4 89 4 98 and the youngest whole rock depleted mantle nd model age tdm 2 32 ga is close to the crystallization age of the ttg 2318 8 ma the protolith of the gneisses formed from a combination of pre existing continental crust and juvenile materials implying an andean type continental margin arc environment it is suggested that crustal thickening was the result of accretionary orogenesis accompanied by basaltic underplating the ganshugou potassium rich gneisses of the later suite have low na2o and high k2o and show fractionated lree and nearly flat hree patterns with strong negative ta nb and ti anomalies they are characterized by negative end t values 5 01 to 2 33 and heterogeneous zircon ehf t values 10 65 2 10 with decoupling of zircon hf model ages tdm hfc 2 64 3 23 ga and whole rock nd model ages tdm 2 55 2 81 ga the protolith of the ganshugou gneisses was a potassium rich igneous rock which was likely the result of partial melting of pre existing crustal materials at shallow depth compatible with a post collisional setting there is therefore a tectonic transformation from an accretionary orogenesis to an extensional regime as a consequence of post collisional uplift in the southern segment of the tnco in the early paleoproterozoic

dating very young planetary surfaces from crater statistics a review of issues and challenges determining the ages of young planetary surfaces relies on using populations of small often sub km diameter impact craters due to the higher frequency at which they form smaller craters however can be less reliable for estimating ages as their size frequency distribution is more susceptible to alteration with debate as to whether they should be used at all with the current plethora of meter scale resolution images acquired of the lunar and martian surfaces small craters have been widely used to derive model ages to establish the temporal relation of recent geologic events in this review paper we discuss the many factors that make smaller craters particularly challenging to use and should be taken into consideration when crater counts are confined to small crater diameters establishing confidence in a model age ultimately requires an understanding of the geologic context of the surface being dated as reliability can vary considerably and limitations of the dating technique should be considered in applying ages to any geologic interpretation

the resurfacing history of venus constraints from buffered crater densities because of atmospheric shielding and endogenic resurfacing the population of impact craters on venus is small about a thousand and consists of large craters this population has been used in numerous studies with the goal of deciphering the geologic and geodynamic history of venus but the nearly spatially random nature of the crater population has complicated efforts to understand this history here we utilize the recent 1 15 m scale global geological map of venus ivanov m a head j w 2011 planet space sci 59 1559 1600 to help address this problem the global geological map provides a stratigraphic sequence of units and known areas where each unit is exposed on the planet for each crater on venus we identify the specific geological units predating and postdating the crater we perform a statistical analysis of this set of observations with a buffered crater density approach which rigorously and consistently takes into account the large size of craters and the fact that many craters are known to predate and or postdate more than one unit in this analysis we consider crater emplacement as random and resurfacing history as determined although unknown we obtain formal confidence intervals for the mean ages of geological units and the mean age differences between the pairs of units at the unit boundaries we find that 1 size frequency distributions of craters superposed on each unit are consistent with each other 2 regional plains and stratigraphically older units have similar crater retention ages 3 stratigraphically younger units have a mean crater retention age significantly younger than the regional plains these findings are readily and consistently explained by global resurfacing scenarios and are difficult to reconcile with equilibrium resurfacing scenarios our analysis also shows that the latest recorded part of intensive resurfacing period lasted on the order of 10 of the mean surface age tens of millions of years the termination of intensive resurfacing may or may not be synchronous over the planet our results also indicate that there are extended deposits associated with large craters that are almost indiscernible in the radar images but obscure radar contrasts between predating lava flows we do not see evidence for any significant and prolonged change of atmospheric pressure following the termination of the intensive resurfacing epoch

petrogenesis of the archean tonalite trondhjemite granodiorite ttg and granites in the lushan area southern margin of the north china craton implications for crustal accretion and transformation abstract tonalite trondhjemite granodiorite ttg assemblages and abundant high k calc alkaline magmatism are considered to be closely related to craton stabilization and usually record a tectonic transition three episodes of granitic magmatism are identified in the lushan area at the southern margin of the north china craton ncc on the basis of new la icp ms zircon u pb ages and whole rock geochemical data they are ttg gneisses sodic potassic granite spg suite and potassic granite dated at 2840 ma 2760 ma and 2570 ma respectively the ttg gneisses have high sio 2 variable mg cr and ni contents with high la yb n and sr y ratios suggesting that the parental magmas were derived by partial melting of subducted oceanic crust they show no obvious correlations between sr y and dy yb or la yb n and dy yb and all have negative nb and ta but positive zr and hf anomalies and variable dy yb nb ta zr hf and zr sm ratios suggesting melting of a deep source 45 km with rutile eclogite and garnet amphibolite in the residue the ttg samples have positive hf t values 2 6 to 7 0 with the youngest t dm c 2896 ma close to the crystallization age indicating that they were produced by partial melting of juvenile crustal materials in an arc setting rocks of the spg suite have high sio 2 contents variable na 2 o k 2 o ratios 0 41 10 91 and low y and yb contents with high sr y ratios indicating an affinity to i type granites they display depleted and concave upward ree patterns between middle and heavy rees and higher zr sm than ttg gneisses possibly resulting from amphibole removal by fractional crystallization or as a residual phase during melting meanwhile the rocks have a large variation of dy yb without obvious correlations between sr y and dy yb and la yb n and dy yb in combination with variable but uniformly low y contents therefore we suggest that the spg samples were derived from partial melting with garnet in the residue magmatic zircons have hf t values from 4 3 to 4 2 with t dm c ages from 3200 ma to 2877 ma indicative of older mafic lower crustal sources the rocks formed within a short time interval with the ttg and their emplacement was accompanied by regional granulite and amphibolite metamorphism implying that they may also have formed in an arc setting the potassic granite has high sio 2 74 79 wt and k 2 o na 2 o 9 22 wt contents with a low 10 000ga al 1 89 ratio and t zr 670 c magmatic zircons have a narrow range of hf t values 2 0 to 4 1 with t dm c ages 2942 2692 ma older than the formation age but consistent with the formation age ca 2760 ma of the lushan spg suite the potassic granite has a hf isotope composition identical to that of the spg suite and is enriched in lree and depleted in y with a positive eu anomaly suggesting that the spg suite is the main source one zircon yielded an inherited age of 2837 36 ma consistent with the age of the ttg gneisses indicating that the gneisses could also be the source of the potassic granite the two types of source rocks indicate a shallow crust depth hence the heat required for partial melting of granodioritic rocks to form the potassic granite could have been provided by the upwelling of mantle or mantle derived melts in an extensional setting therefore the ttg gneisses spg suite and potassic granite in the lushan area record a tectonic transition from a compressive to an extensional setting in the southern margin of the ncc between 2 85 and 2 50 ga

geochronological and geochemical constraints on the petrogenesis of the 2 6 2 5 ga amphibolites low and high al ttgs in the wangwushan area southern north china craton implications for the neoarchean crustal evolution abstract the crustal evolutionary processes of the north china craton ncc during the late neoarchean ca 2 6 2 5 ga are still under large debate the systematic investigations of widespread ca 2 6 2 5 ga meta volcanic basalts and ttg assemblages can provide a better understanding of tectonic evolution at this period the wangwushan neoarchean amphibolites high and low al ttgs in southern ncc were formed at 2 57 2 52 ga the amphibolites are tholeiitic in composition they have whole rock end t values of 2 8 4 0 tdm1 3 7 2 6 ga and zircon ehf t values of 1 2 7 2 tdm1 2 81 2 57 ga indicative of a depleted mantle source with involvement of older crustal materials their compositional spectrum accords with those in island arc field or straddles the boundary between morb and island arc fields suggesting significant contributions from subduction zone derived fluids during magma formation the high al ttgs contain high mg cr and ni contents and la yb n and sr y ratios together with positive whole rock end t values 0 6 5 2 tdm2 2 95 2 48 ga indicating that they derived from partial melting of a subducted oceanic slab with older crustal materials their geochemical features are corresponding to slab melt identification criterion in an arc setting and illustrate a deep source 45 km with garnet amphibolite in the residue without rutile the low al ttgs show low mg and la yb n ratios negative eu anomalies relatively flat hree and y patterns their zircon ehf t 2 2 8 8 and 1 3 6 9 and whole rock end t values ca 2 23 are positive all the evidences support that the low al ttgs derived from a low degree partial melting of basaltic crust with the residue lack of garnet consistent with a shallower depth about

a recent ice age on mars evidence for climate oscillations from regional layering in mid latitude mantling deposits 1 two end member hypotheses have been proposed to account for the emplacement and distribution of ice in the near subsurface of mars at mid to high latitudes during recent spin axis orbital variation induced climate change in the first diffusion of atmospheric water vapor into and out of a porous regolith forms ice cemented soils whose latitudinal stability migrates as a function of orbitally controlled climatic conditions in the alternative hypothesis atmospheric deposition of ice snow and dust produces dusty ice rich layers during periods of higher obliquity new image data reveal meters thick layered deposits exposed on mid latitude pole facing slopes supporting the latter hypothesis these observations suggest that the near surface ice detected by the grs instrument suite and the phoenix lander at high latitudes is linked to thick buried ice that was atmospherically deposited during recent ice ages and that significant amounts of subsurface ice may remain today in the 30 50 mid latitude regions where fresh craters imaged by hirise expose abundant ice

extensive middle amazonian mantling of debris aprons and plains in deuteronilus mensae mars implications for the record of mid latitude glaciation abstract the mid latitudes of mars are host to a record of recent episodes of accumulations of ice rich materials the record includes debris aprons interpreted to be debris covered glaciers that may represent the preserved remnants of a much more extensive ice sheet we assessed the possibility of former glacial extents by examining debris aprons and the surrounding plains in deuteronilus mensae geomorphic units and stratigraphic relationships were mapped and documented from mars reconnaissance orbiter mro context ctx and high resolution imaging science experiment hirise camera images and crater retention ages were estimated from crater size frequency distributions three major units are observed within the study area debris aprons lower plains and upper plains debris aprons exhibit characteristics typical for these features documented elsewhere and in previous studies including integrated flow lineations and patterns convex upward profiles and knobby and brain terrain surface textures a lower bound on the age for debris aprons is estimated to be 0 9 ga debris aprons are superposed on a lower plains unit having a lower bound age of 3 3 3 5 ga a 50 100 m thick upper plains unit superposes both debris apron landforms and lower plains units and has a best fit minimum age of 0 6 ga the upper plains unit exhibits characteristics of atmospherically emplaced mantle material including fine grained nature sublimation textures cyclic layering draping character and widespread spatial distribution fracturing and subsequent sublimation erosion of upper plains on debris aprons has contributed to many of the surface textures on debris aprons the upper plains unit has also been eroded from the lower plains and plateaus evidenced by isolated blocks of upper plains in the interiors of craters and on the walls and tops of plateaus while no conclusive evidence diagnostic of former cold based ice sheets are observed in the plains within the study region such landforms and units may have been poorly developed or absent as is often the case on earth and would have been covered and reworked by later mantling episodes these observations suggest that emplacement of thick ice rich mantle deposits extended at least to near the early middle amazonian boundary and overlapped with the waning stages of glaciation in deuteronilus mensae

sphalerite geobarometry and arsenopyrite geothermometry applied to metamorphosed sulfide ores in the swedish caledonides metamorphosed massive sulfide deposits in the swedish caledonides were investigated using sphalerite geobarometry and arsenopyrite geothermometry the sphalerite investigations yield pressure estimates of 3 to 5 kb for six deposits from host rocks of greenschist lower amphibolite facies a bimodal fes distribution is apparent for the chlorite grade deposits whereas the fes distribution in biotite grade deposits is unimodal the arsenopyrite geothermometry is apparently sensitive to high contents 0 2 wt of either of the trace elements co and ni and possibly also of sb temperature estimates for six deposits from the chlorite zone of greenschist facies yield 371 degrees or 45 degrees c whereas one deposit from the biotite zone gives a temperature of 422 degrees or 25 degrees c the pressure and temperature data obtained for the deposits correspond excellently with the metamorphic grade of the host rocks thus sphalerite geobarometry and arsenopyrite geothermometry can be applied successfully to metamorphosed sulfide orebodies if attention is paid to textural relationships and minor element distribution in the minerals investigated

an inventory and population scale analysis of martian glacier like forms abstract martian glacier like forms glfs indicate that water ice has undergone deformation on the planet within its recent geological past but the driving mechanisms behind the origin evolution and dynamics of these landforms remain poorly understood here we present the results of a comprehensive inventory of glfs derived from a database of 8058 context camera ctx images to describe the physical controls on glf concentration and morphometry the inventory identifies 1309 glfs 727 glfs in the northern hemisphere and 582 in the southern hemisphere clustered in the mid latitudes centred on a mean latitude of 39 3 in the north and 40 7 in the south and in areas of rough topography morphometric data show inter hemispheric similarity in glf length mean 4 66 km and width mean 1 27 km suggesting a common evolutionary history and a poleward orientation of glfs in both hemispheres indicating a sensitivity to climate and insolation on a local scale elevation is shown to be the most important control on glf location with most glfs occurring above an altitude threshold of 3000 m aod and in areas of moderate but not high relief we propose therefore that martian glfs may not exhibit accumulation or ablation areas as is the norm for terrestrial glacial systems but reflect ice motion in response to local relief within a widely distributed reservoir of ice deposited within prescribed latitudinal and elevational ranges

scalloped terrains in the peneus and amphitrites paterae region of mars as observed by hirise abstract the peneus and amphitrites paterae region of mars displays large areas of smooth geologically young terrains overlying a rougher and older topography these terrains may be remnants of the mid latitude mantle deposit which is thought to be composed of ice rich material originating from airfall deposition during a high obliquity period less than 5 ma ago within these terrains there are several types of potentially periglacial features in particular there are networks of polygonal cracks and scalloped shaped depressions which are similar to features found in utopia planitia in the northern hemisphere this area also displays knobby terrain similar to the so called basketball terrains of the mid and high martian latitudes we use recent high resolution images from the high resolution imaging science experiment hirise along with data from previous mars missions to study the small scale morphology of the scalloped terrains and associated polygon network and knobby terrains we compare these with the features observed in utopia planitia and attempt to determine their formation process while the two sites share many general features scallops in peneus amphitrites paterae lack the diverse polygon network i e there is little variation in the polygon sizes and shapes and large curvilinear ridges observed in utopia planitia this points to a more homogeneous ice content within the substrate in the peneus amphitrites paterae region and implies that scallop formation is independent of polygon formation this work shows that as in utopia planitia sublimation of interstitial ice is a likely process explaining the formation of the scalloped depressions in the region of peneus amphitrites paterae therefore we provide a simplified scallop formation model based on sublimation of interstitial ice as proposed for utopia planitia we also show that the differences in scallop morphologies between the two regions may be explained by differences in near surface ice content sublimation rates and age of formation of the scalloped terrains

the quantitative relationship between small impact crater morphology and regolith depth abstract small impact craters 10 300 m that encounter a strength transition in the target like a regolith over bedrock have unique morphologies previous studies have used these morphologies as indicators of regolith depth this paper reports on several new analyses that expand our understanding of the quantitative relationship between small crater morphology and target layering i describe three practical situations where the application of the updated method is ambiguous because the specific relationship between the target layering and the crater morphology has never been analyzed in order to resolve the ambiguity i report on new analyses of computer models and lunar data that demonstrate how the dimensions of the crater shape relate to layer depth i also analyze the boundary conditions under which the crater layering relationship will enable determination of layering depth finally in light of the greater understanding of the crater layering relationship i discuss the possible application of this method to mars

early middle jurassic 182 170 ma ruocuo adakitic porphyries southern margin of the lhasa terrane tibet implications for geodynamic setting and porphyry cu au mineralization abstract jurassic adakitic rocks in the southern lhasa subterrane are relatively rare poorly documented and their petrogenesis and geodynamic setting are unclear to explore these issues we present zircon u pb ages along with hf isotopic whole rock geochemical and sr nd pb isotopic data for hornblende quartz diorite porphyry hqdp and quartz diorite porphyry qdp in the ruocuo area southern margin of the lhasa terrane tibet zircon u pb dating for these rocks indicates that they were emplaced in the early middle jurassic 182 170 ma geochemically hqdp and qdp both show characteristics of adakites with intermediate sio2 60 62 65 63 wt and 57 46 60 04 wt respectively relatively high al2o3 14 23 18 23 wt and 18 32 20 14 wt respectively and sr 344 571 ppm and 514 614 ppm respectively and low y 9 12 13 0 ppm and 11 0 14 9 ppm respectively and yb 0 93 1 31 ppm and 1 08 1 52 ppm respectively in addition they show relatively low ratios of 87sr 86sr i 0 7038 0 7045 206pb 204pb i 18 31 18 42 207pb 204pb i 15 56 15 66 and 208pb 204pb i 38 36 38 77 and relatively high values of end t 5 54 to 6 33 and ehf t 11 8 to 17 3 the sr nd pb hf isotopes of hqdp and qdp are similar to those of the yarlung tsangpo ophiolites indicating that they were predominantly generated by the partial melting of the subducted neo tethys oceanic slab the subducted neo tethys oceanic slab derived adakitic rocks that are present in the southern lhasa subterrane provide strong evidence for proving the presence of oceanic subduction during the early middle jurassic by combining our results with the previously reported results we suggest that the ruocuo adakitic rocks and early mesozoic late triassic to middle jurassic magmatic rocks in the southern lhasa subterrane formed in a magmatic arc setting related to the northward subduction of the neo tethys oceanic slab that the initial subduction of the neo tethys oceanic slab occurred prior to the early jurassic and that the upwelling asthenosphere triggered by the roll back of the subducted neo tethys oceanic slab provided the heat that was required for slab melting however contrasting these results with the geochemical characteristics of the no 2 deposit in the xiongcun cu au district leads us to consider that the early middle jurassic subducted oceanic slab derived adakitic porphyries contain a greater potential for porphyry cu au mineralization than the early mesozoic normal magmatic rocks in the southern lhasa subterrane furthermore mineralized jurassic porphyry has been reported in the ruocuo area adjacent to the northern xigaze forearc basin indicating that the northern xigaze forearc basin also has the potential for subduction related porphyry cu au mineralization

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

amazonian northern mid latitude glaciation on mars a proposed climate scenario recent geological observations in the northern mid latitudes of mars show evidence for past glacial activity during the late amazonian similar to the integrated glacial landsystems in the dry valleys of antarctica the large accumulation of ice many hundreds of meters required to create the observed glacial deposits points to significant atmospheric precipitation snow and ice accumulation and glacial flow in order to understand the climate scenario required for these conditions we used the lmd laboratoire de meteorologie dynamique mars gcm general circulation model which is able to reproduce the present day water cycle and to predict past deposition of ice consistent with geological observations in many cases prior to this analysis however significant mid latitude glaciation had not been simulated by the model run under a range of parameters in this analysis we studied the response of the gcm to a wider range of orbital configurations and water ice reservoirs and show that during periods of moderate obliquity 25 35 and high dust opacity sdust 1 5 2 5 broad scale glaciation in the northern mid latitudes occurs if water ice deposited on the flanks of the tharsis volcanoes at higher obliquity is available for sublimation we find that high dust contents of the atmosphere increase its water vapor holding capacity thereby moving the saturation region to the northern mid latitudes precipitation events are then controlled by topographic forcing of stationary planetary waves and transient weather systems producing surface ice distribution and amounts that are consistent with the geological record ice accumulation rates of 10 mm yr

across arc geochemical variations in the paleogene magmatic belt of iran abstract across arc geochemical variations between igneous rocks from the magmatic front mf to rear arc ra are commonly observed for convergent margins but the processes that are responsible for these variations are unclear to address these questions we studied a well exposed cenozoic continental arc the urumieh dokhtar magmatic belt udmb of iran and its rear arc counterpart to the north north iran ra magmatism is bimodal characterized by mafic to felsic igneous rocks of calc alkaline to shoshonitic affinity the new zircon u pb ages show that plutonic rocks are 42 38 ma whereas volcanic rocks are older at 52 ma 87sr 86sr t and end t values of udmb ra igneous rocks range from 0 70483 and 0 70783 and 2 5 and 3 6 respectively zircons from plutonic rocks show ehf t values ranging from 1 8 to 9 8 higher than volcanic rocks with ehf t between 2 6 and 9 2 isotopic data agrees with inferences from incompatible trace element ratios e g th yb zr nb and la yb that mafic plutonic rocks originated from an enriched mantle source whereas felsic volcanic rocks show more interaction with continental crust high magmatic fluxes did not occur at the same time in the mf and ra udmb mf flared up at 54 37 ma and 20 5 ma overlapping ra magmatic flare ups at 45 40 ma and 30 25 ma the available major and trace element data from the udmb show that most magmatic rocks experienced strong fractional crystallization and assimilation fractional crystallization processes making it difficult to identify across arc geochemical variations moreover the bulk rock end t and zircon ehf t values are highest at the mf but the across arc isotopic variability seems to be mainly controlled by crustal influence high flux magmatism in the udmb was related to protracted heating of the crust as a result of subduction of neotethys oceanic lithosphere this weakened the continental lithosphere leading to strong extension in paleogene time accompanied by decompression melting and magmatic flare ups

sphalerite geobarometry in the cu fe zn s system phase relations have been determined among sphalerite pyrite pyrrhotite and intermediate solid solution iss from 439 degrees to 725 degrees c at 1 bar and 353 degrees to 760 degrees c at 5 kb the extents of solid solution in the four phase assemblage are described as functions of temperature in kelvins by the following equations log mole zns in iss 1 bar 2 795 1 632 t super 1 log mole zns in iss 5 kb 2 659 1 608 t super 1 log mole cus in pyrrhotite 1 bar 2 546 1 866 t super 1 log mole cus in sphalerite 1 bar 4 202 3 755 t super 1 and log mole cus in sphalerite 5 kb 4 084 3 791 t super 1 the pressure dependence in kb of the composition of sphalerite in equilibrium with pyrite and pyrrhotite with or without iss can be expressed for the temperature independent portion of the sphalerite geobarometer as p 42 30 32 10 log mole fes because of the low solubility of copper in sphalerite in the four phase assemblage below 650 degrees c the presence of chalcopyrite in natural ores should be of no consequence to sphalerite geobarometry over its temperature independent portion nevertheless sphalerites which are inhomogeneous in fes and which contain abundant blebs of chalcopyrite in excess of the amount that can be reasonably accounted for by an equilibrium process of solubility and subsequent exsolution are a common feature of many sulfide ores such sphalerite grains should be avoided in applying the geobarometer the chalcopyrite blebs may be a product of a premetamorphic sphalerite chalcopyrite intergrowth or represent true exsolution from sphalerite in any one of 1 a former sphalerite pyrrhotite iss equilibrium 2 a stable four phase assemblage at temperatures above the temperature independent portion of the geobarometer or 3 a metastable four phase assemblage in which pyrite was inert use of the sphalerite geobarometer requires a thorough understanding of the textural relationships and discrimination of equilibrium from disequilibrium or low temperature reequilibrated assemblages