anomalous sulfur poor platinum group element mineralization in komatiitic cumulates mount clifford western australia an unusual occurrence of sulfur poor platinum group element pge mineralization has been discovered at mount clifford in the agnew wiluna belt western australia within the lower part of a 1 000 m thick komatiitic dunite body the host body consists primarily of strongly serpentinized olivine adcumulates with accessory chromite high precision whole rock icp ms data show that the pge mineralization extends over tens of meters whereas the lateral extent is unknown palladium and pt abundances are closely associated with base metal sulfides but show offset distribution patterns with maximum concentrations associated with the relatively sulfide poor base of a disseminated sulfide bearing layer whole rock concentrations range up to 4 100 ppb pd and 920 ppb pt iridium rh and ru follow the trends in pd and pt but are strongly depleted ir 0 4 ppb ru 0 5 ppb rh 0 1 ppb platinum group minerals pgm were located in essentially sulfide free serpentine 2 m below the peak in sulfide abundance and can be categorized into three groups pd antimonides pt dominated pgm and pt bearing ni antimonides the pge mineralization zone is underlain by an interval of strong ni depletion within olivine mesocumulates that is characterized by an unusual negative correlation between ni and the mg number of the rock this relationship is interpreted to be the result of an equilibration of olivine of variable forsterite content with a relatively large mass of magmatic sulfide however there is no direct evidence of this sulfide mass anywhere in the entire mount clifford dunite body the ni depletion is accompanied by strong depletion in ipge ir ru and rh the association of the pge zone with a reversal in the mg fe ratio of the host cumulates and a distinctive offset relationship of multiple pt pd and cu s peaks is suggestive of a primary magmatic origin however this is inconsistent with current models for offset reef pge concentrations which involve the onset of sulfide liquid saturation from a previously sulfur undersaturated pge undepleted magma the association of enriched pt and pd with highly depleted ni and ipge in immediately underlying cumulates conflicts with this model we suggest a combination of processes early sulfide segregation at low r values producing ni and ipge depleted olivine partial redissolution and removal of most of this sulfide component during a major replenishment event and subsequent overprint by a migrating pt pd cu rich fluid

conceptual thoughts on continuum damage mechanics for shallow ice shelves we consider a theory for shallow ice shelves that includes an isotropic damage variable a zeroth order shallow shelf approximation allows a simple yet consistent treatment of both ice dynamics and damage evolution we find that the damage variable like temperature has in general to vary with depth a purely two dimensional membrane theory can only be considered a rough approximation for isothermal ice shelves

a new model for the emplacement of columbia river basalts as large inflated pahoehoe lava flow fields extensive flows of the columbia river basalt crb group in washington oregon and idaho are dominantly inflated compound pahoehoe sheet lavas early studies recognized that crb lavas are compound pahoehoe flows with textures suggesting low flow velocities but it was thought that the great thickness and extent of the major flows required very rapid emplacement as turbulent floods of lava over a period of days or weeks however small volume 1 km compound pahoehoe flows on kilauea hawai i demonstrate that such flows can thicken by at least an order of magnitude through gradual inflation and the same mechanism has been proposed for larger 10 20 km3 pahoehoe flows in iceland the vertical distribution of vesicles and other morphologic features within crb lava flows indicate that they grew similarly by inflation small pahoehoe lobes at the base and top of many crb pahoehoe lava flows indicate emplacement in a gradual piecemeal manner rather than as a single flood we propose that each thick crb sheet flow was active for months to years and that each group of flows produced by a single eruption a flow field was emplaced slowly over many years

a physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics we present results from numerical ice flow models that include calving criteria based on penetration of surface and basal crevasses which in turn is a function of longitudinal strain rates near the glacier front the position of the calving front is defined as the point where either 1 surface crevasses reach the waterline model cdw or 2 surface and basal crevasses penetrate the full thickness of the glacier model cd for comparison with previous studies results are also presented for a height above buoyancy calving model qualitatively both models cdw and cd produce similar behaviour unlike previous models for calving the new calving criteria are applicable to both grounded termini and floating ice shelves and tongues the numerical ice flow model is applied to an idealized geometry characteristic of marine outlet glaciers results indicate that grounding line dynamics are less sensitive to basal topography than previously suggested stable grounding line positions can be obtained even on a reverse bed slope with or without floating termini the proposed calving criteria also allow calving losses to be linked to surface melt and therefore climate in contrast to previous studies in which calving rate or position of the terminus is linked to local water depth the new calving criterion is able to produce seasonal cycles of retreat and advance as observed for greenland marine outlet glaciers the contrasting dynamical behaviour and stability found for different calving models suggests that a realistic parameterization for the process of calving is crucial for any predictions of marine outlet glacier change

oceanic forcing of ice sheet retreat west antarctica and more ocean ice interactions have exerted primary control on the antarctic ice sheet and parts of the greenland ice sheet and will continue to do so in the near future especially through melting of ice shelves and calving cliffs retreat in response to increasing marine melting typically exhibits threshold behavior with little change for forcing below the threshold but a rapid possibly delayed shift to a reduced state once the threshold is exceeded for thwaites glacier west antarctica the threshold may already have been exceeded although rapid change may be delayed by centuries and the reduced state will likely involve loss of most of the west antarctic ice sheet causing 3 m of sea level rise because of shortcomings in physical understanding and available data uncertainty persists about this threshold and the subsequent rate of change although sea level histories and physical understanding allow the possibility that ice sheet response could be quite fast no strong constraints are yet available on

were intercalated komatiites and dacites at the black swan nickel sulphide mine yilgarn craton western australia emplaced as extrusive lavas or intrusive bodies the significance of breccia textures and contact relationships intercalated archean komatiites and dacites sit above a thick footwall dacite unit in the host rock succession at the black swan nickel mine north of kalgoorlie in the yilgarn craton western australia both lithofacies occur in units that vary in scale from laterally extensive at the scale of the mine lease to localized thin irregular bodies from 100 m thick to only centimetres thick some dacites are only slightly altered and deformed and are interpreted to post date major deformation and alteration late porphyries however the majority of the dacites display evidence of deformation especially at contacts and metamorphism varying from silicification and chlorite alteration at contacts to pervasive low grade regional metamorphic alteration represented by common assemblages of chlorite sericite and albite texturally the dacites vary from entirely massive and coherent to partially brecciated to totally brecciated strangely some dacites are coherent at the margins and brecciated internally breccia textures vary from cryptically defined to blocky closely packed in situ jig saw fit textures with secondary minerals in fractures between clasts to more apparent matrix rich textures with round clast forms giving apparent conglomerate textures some clast zones have multi coloured clasts giving the impression of varied provenance strangely however all these textural variants have gradational relationships with each other and no bedding or depositional structures are present this indicates that all textures have an in situ origin the komatiites are generally altered and pervasively carbonate veined preservation of original textures is patchy and local but includes coarse adcumulate mesocumulate orthocumulate crescumulate harrisite and occasionally spinifex textures where original contacts between komatiites and dacites are preserved intact i e not sheared or overprinted by alteration the komatiites have chilled margins whereas the dacites do not the margins of the dacites are commonly silicified and inclusions of dacite occur in komatiite even at the top contacts of komatiite units but komatiite clasts do not occur in the dacites the komatiites therefore were emplaced as sills into the dacites and the intercalated relationships are interpreted as intrusive the brecciation and alteration in the dacites are interpreted as being largely due to hydraulic fracturing and alteration induced by contact metamorphic effects and hydrothermal alteration deriving from the intrusion of komatiites into the felsic pile the absence of autobreccia and hyaloclastite textures in the dacites suggest that they were emplaced as an earlier intrusive sill complex at a high level in the crust

effect of orbital scale climate cycling and meltwater drainage on ice sheet grounding line migration 1 ice shelf response to climate cycling introduces hysteretic behavior in outlet glaciers with the evolution of meltwater drainage networks enhancing this effect the 2002 collapse of the larsen b ice shelf one of the floating extensions formed from glaciers flowing off the antarctic peninsula into the weddell sea followed the development of widespread poorly drained surface melt ponds during the atypical warmth of the 2001 2002 summer melt season yet for at least the past half century the floating termini of outlet glaciers in greenland did not collapse despite being exposed to summer mean temperatures and melt rates that far exceed those recorded in 2001 2002 on the antarctic peninsula we suggest that this paradox can be explained by the advection of well developed englacial drainage networks within greenland s extensive ablation zone the efficiency of this plumbing system limits meltwater ponding within crevasses and the subsequent ice shelf failure observed on the antarctic peninsula whether progressive or abrupt ice shelf removal leads to near synchronous glacier retreat conversely glaciation of fjords is delayed during ice shelf formation ultimately the relatively rapid loss and gradual establishment of ice shelf buttressing amid climate cycling leads to an asymmetric response in outlet glacier dynamics this asymmetry may have implications for the future of existing and recently calved ice shelves eustatic sea level and the hysteretic behavior of ice sheets

a novel method for predicting fracture in floating ice basal crevasses may play an important precursory role in determining both the location and propagation of rifts and iceberg dimensions for example icebergs calved recently from thwaites glacier antarctica have the same width as surface undulations strengthening the connection between basal crevasses rifting and calving we explore a novel method for estimating the heights of basal crevasses formed at the grounding lines of ice shelves and ice streams we employ a thin elastic beam teb formulation and tensional yielding criterion to capture the physics of flexed ice at grounding lines observations of basal crevasse heights compare well with model predictions in the siple coast region of the ross ice shelf we find that the teb method is most accurate in areas of low strain rate we also test the method in other areas of antarctica to produce order of magnitude maps of grounding line basal crevasses and find general agreement with reported observations assuming basal crevasses develop in spatio temporal sequence and are advected downstream this method is computationally cheap and could be relatively easy to implement into damage oriented large scale ice models which aim at physically simulating calving and fracture processes

district to camp controls on the genesis of komatiite hosted nickel sulfide deposits agnew wiluna greenstone belt western australia insights from the multiple sulfur isotopes the agnew wiluna greenstone belt of western australia is the largest komatiite hosted nickel sulfide belt in the world and contains two world class ni cu pge deposits and a host of smaller deposits this study focuses on the broader scale geology of this greenstone belt in order to understand the key controls on the genesis of the komatiite hosted ni cu pge deposits with specific focus on camp to district controls we apply multiple sulfur isotopes to this geologic framework and conclude not only that the addition of crustal sulfur is a prerequisite for ore genesis in komatiite systems but above all that the sulfur required to generate world class deposits is most likely derived from barren volcanic massive sulfide lenses which are spatially and genetically associated with felsic volcanic and volcaniclastic sequences that were emplaced coevally with large komatiitic sills and channelized lava flows multiple sulfur isotope data can be utilized in exploration at the deposit to district scales at the deposit scale the spatial pattern of mass independent s isotope values 33 s provides crucial insight into the identification of proximal high grade and high tenor ores in mineralized systems in fact sulfur data reflect the assimilation process that occurred upon komatiite emplacement whereby hot turbulent magma thermomechanically eroded and assimilated exhalative sulfides spatially located close to vent with negative to near zero 33 s values whereas less turbulent flows interacted with distal sulfidic shales having 33 s values above 0 per mil accordingly the spatial variation of multiple sulfur isotope values in magmatic sulfides and associated host rocks may be utilized as a vector towards high grade ores of poorly known systems at the district scale rather than ascertaining what controls the distribution of komatiite hosted ni cu pge deposits the appropriate question to ask is what controls the distribution of country rock sulfides considering that exhalative sulfides may be crucial to ore genesis in komatiite systems we propose that felsic lava domes unambiguously mark their vents and can be directly mapped or inferred from gravity data this work provides the first step in identifying district scale control on komatiite hosted ni cu pge deposits this is the scale that has high impact on exploration for new komatiite hosted nickel sulfide belts globally

analytical numerical modeling of komatiite lava emplacement and thermal erosion at perseverance western australia abstract we have applied a thermal fluid dynamic geochemical model to investigate the emplacement and erosional potential of archean komatiite flows at perseverance western australia perseverance has been proposed as a site of large scale thermal erosion by large volume komatiite eruption s resulting in a 100 150 m deep lava channel containing one of the world s largest komatiite hosted fe ni cu pge sulfide deposits using constraints based on field theoretical and geochemical data we have modeled the emplacement of a range of flow thicknesses over felsic tuffaceous substrates with various degrees of consolidation and water contents thermo mechanical erosion becomes more effective for substrates that are increasingly unconsolidated and water rich for thermo mechanical erosion to be responsible for the formation of the 100 m deep concave perseverance embayment and the highly contaminated 10 20 perseverance komatiites the most likely scenarios require emplacement of thick e g 10 30 m turbulent channelized liquidus or superheated komatiite lavas over a welded or unconsolidated submarine tuff flow distances must have been long tens to hundreds of kilometers and flow volumes must have been very high hundreds to thousands of km 3 lava channels and tubes 110 km long have not been observed on earth but are consistent with those formed by low viscosity lavas on venus the moon mars and io flow volumes are consistent with those in continental flood basalt eruptions and oceanic plateau large igneous provinces and may represent the initial outpourings of komatiite lavas from archean mantle plume activity

thermomechanical erosion of ore hosting embayments beneath komatiite lava channels textural evidence from kambalda western australia abstract archean komatiite hosted nickel deposits at kambalda western australia are located in embayments within channelized lava flows the origin of these embayments is controversial the currently favoured model invokes pre existing topographic features filled in by the komatiite and sulfide melt and subsequently modified by thermomechanical erosion new evidence from sulfide deposits on the eastern limb of the kambalda dome includes well preserved magmatic textural relationships of the sulfides with the surrounding rocks suggesting the entire embayment was formed by thermomechanical erosion these textures are particularly well preserved within the moran deposit of the long victor complex which lacks the younger intense structural deformation that other kambalda deposits have undergone the vestiges of an older broad concave embayment are represented by sediment sulfide textures present on the flanks up to 150 m away from the younger steep sided embayment sediment sulfide contacts are marked here by a globular silicate sulfide intergrowth interpreted as emulsion and cumulate like mono sulfide solid solution mss grains surrounded by interstitial silicate melt textures preserved from the formation of a steep sided embayment are frozen into the basalt sulfide contact and include an undulating contact with sulfide filled microfractures and a ferrichromite layer basalt sulfide breccia and interpreted crystallized silicate sulfide liquid emulsions and basalt plumes rising into the sulfides textures from the last step of embayment formation are found in the pinchouts i e where the massive sulfide ore is bounded top and bottom by basalt they include the same textures that were formed during the excavation of the steep sided embayment but they also feature interpreted basalt sulfide emulsions on the upper contact cm scale silicate sulfide ferrichromite layering on the upper contact and a vesicular basalt scum layer floating on the sulfides beneath the solid older basalt all these textures represent frozen examples of different stages during the excavation and formation of the embayment and reflect physical processes responsible for thermomechanical erosion the initial disposition and orientation of ore hosting channels was controlled by paleo topography probably defined by syn volcanic faults but the textures we describe show that within the channel the geometry of the entire embayment was the product of erosion the sulfide melt itself played a major role in the process owing to its high density high heat content and very low viscosity but the hydrated nature of the underlying altered basalt was also critical fluids were released from the basalt due to the high temperature in the 1 2 cm immediately beneath the sulfide melt with instant flash boiling creating a network of microfractures pre existing fracture networks and breccia in the basalt allowed the sulfide melt to locally infiltrate the substrate to a greater depth increasing the hydraulic head at the tip of the fracture thus accelerating the erosion process once a critical sulfide melt thickness is reached partly aided by a growing pile of matrix net textured sulfides the pressure difference on the embayment floor compared to the basalt at the bottom edges of the embayment becomes high enough to allow the liquid sulfides to melt sideways to form a pinchout

tectono stratigraphy of late archaean greenstone terranes in the southern eastern goldfields western australia abstract late archaean greenstone terranes in the southern part of the eastern goldfields are regionally extensive faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure the greenstones comprise volcano sedimentary successions that were all deposited at the same time c 2720 2675 ma on sialic crust hence the terranes are interpreted to represent contemporaneous probably adjacent basins these basins were affected by the same history of granite plutonism and regional deformation c 2675 2620 ma early subhorizontal deformation is poorly understood but includes at least locally significant thrust stacking d1 with a south to north movement direction this was followed by regional east west extension affecting all supracrustal successions above a province wide detachment now lying at depths between 4 and 7 km this detachment may have evolved from the greenstone basement contact but only after extensive reworking clastic basins developed along or above major extensional faults but no volcanism is associated with this extension subsequent east west d2 shortening resulted in regional scale thrusting folding and associated late stage transcurrent fault movements the d2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement c 2660 ma and associated peak regional metamorphism in the greenstone succession predominantly monzo granite derived from older sialic crust was emplaced throughout the period of basin formation and deformation early synvolcanic granite c 2719 2710 ma is a minor component late synvolcanic granite c 2685 2675 ma is widespread and syn d2 granite c 2660 ma is abundant there are a few small late tectonic intrusions c 2640 ma to 2600 ma the lateral lithostratigraphic persistence of the contemporaneous basins the syn d2 low pressure regional metamorphism and the province wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres the basins closed in response to far field compressional stresses

facies architecture of an archean komatiite hosted ni sulphide ore deposit victor kambalda western australia implications for komatiite lava emplacement nickel sulphide mineralisation is located in structural embayments or troughs at the base of thick basal komatiite lava flows at the victor ore body kambalda western australia primary contact relations and textural and vesicle distribution of the host komatiite lavas particularly the development of coherent or quench fragmented margins is consistent with emplacement of komatiites under laminar flow conditions however the absence of sedimentary units beneath komatiites with coherent flow tops in the ore environment seems contradictory and suggests both turbulent and passive laminar emplacement respectively the presence of erosive basal contacts in the ore environment suggests the komatiite lava was initially turbulent and probably open channel fed it is during this initial stage that both erosion and deposition of the nis deposits occurred as the flow evolved widened and thickened laminar flow conditions prevailed the komatiites are inferred to have flowed in a laminar state through the development of interior magma tubes beneath surface crusts and to have grown endogenously

potential antarctic ice sheet retreat driven by hydrofracturing and ice cliff failure abstract geological data indicate that global mean sea level has fluctuated on 103 to 106 yr time scales during the last 25 million years at times reaching 20 m or more above modern if correct this implies substantial variations in the size of the east antarctic ice sheet eais however most climate and ice sheet models have not been able to simulate significant eais retreat from continental size given that atmospheric co2 levels were relatively low throughout this period here we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model data conflict in response to atmospheric and ocean temperatures typical of past warm periods floating ice shelves may be drastically reduced or removed completely by increased oceanic melting and by hydrofracturing due to surface melt draining into crevasses ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing and may fail structurally if stresses exceed the ice yield strength producing rapid retreat incorporating these mechanisms in our ice sheet model accelerates the expected collapse of the west antarctic ice sheet to decadal time scales and also causes retreat into major east antarctic subglacial basins producing 17 m global sea level rise within a few thousand years the mechanisms are highly parameterized and should be tested by further process studies but if accurate they offer one explanation for past sea level high stands and suggest that antarctica may be more vulnerable to warm climates than in most previous studies

propagation of long fractures in the ronne ice shelf antarctica investigated using a numerical model of fracture propagation long rifts near the front of the ronne ice shelf antarctica are observed to begin as fractures along the lateral boundaries of outlet streams feeding the shelf these flaws eventually become the planes along which tabular icebergs calve the fractures propagate laterally as they advect through the shelf with orientations that can be explained by the glaciological stress field fracture length remains constrained over much of the advective path and locations of crack tip arrest are observed to coincide with structural boundaries such as suture zones between ice from adjacent outlet glaciers geomechanical principles and numerical models demonstrate that in the absence of these suture zones crack tips are unlikely to arrest in these locations we conclude that lateral inhomogeneity in the ice plays an important role in fracture mechanics through most of the ice shelf only near the shelf front are these local structural effects overcome such that the large rifts required for tabular iceberg production develop

continent wide estimates of antarctic strain rates from landsat 8 derived velocity grids strain rates are fundamental measures of ice flow and are used in a wide variety of glaciological applications including investigations of bed properties calculations of basal mass balance on ice shelves and constraints on ice rheological models however despite their extensive application strain rates are calculated using a variety of methods and length scales and the details are often not specified in this study we compare the results of nominal and logarithmic strain rate calculations based on a satellite derived velocity field of the antarctic ice sheet generated from landsat 8 satellite data our comparison highlights the differences between the two common approaches in the glaciological literature we evaluate the errors introduced by each approach and their impacts on the results we also demonstrate the importance of choosing and specifying a length scale over which strain rate calculations are made which can strongly influence other derived quantities such as basal mass balance on ice shelves finally we present strain rate data products calculated using an approximate viscous length scale with satellite observations of ice velocity for the antarctic continent

seasonal and interannual variabilities in terminus position glacier velocity and surface elevation at helheim and kangerlussuaq glaciers from 2008 to 2016 the dynamic response of greenland tidewater glaciers to oceanic and atmospheric change has varied both spatially and temporally while some of this variability is likely related to regional climate signals glacier geometry also appears to be important in this study we investigated the environmental and geometric controls on the seasonal and interannual evolution of helheim and kangerlussuaq glaciers southeast greenland from 2008 to 2016 by combining year round satellite measurements of terminus position glacier velocity and surface elevation while helheim remained relatively stable with a lightly grounded terminus over this time period kangerlussuaq continued to lose mass as its grounding line retreated into deeper water by summer 2011 kangerlussuaq s grounding line had retreated into shallower water and the glacier had a 5 km long floating ice tongue we also observed seasonal variations in surface velocity and elevation at both glaciers at helheim seasonal speedups and dynamic thinning occurred in the late summer when the terminus was most retreated at kangerlussuaq we observed summer speedups due to surface melt induced basal lubrication and winter speedups due to ice shelf retreat we suggest that helheim and kangerlussuaq behaved differently on a seasonal timescale due to differences in the spatial extent of floating ice near their termini which affected iceberg calving behavior given that seasonal speedups and dynamic thinning can alter this spatial extent these variations may be important for understanding the long term evolution of these and other greenland tidewater glaciers

a 3 d geological and structural synthesis of the leinster area of the agnew wiluna belt yilgarn craton western australia with special reference to the volcanological setting of komatiite associated nickel sulfide deposits the leinster area is situated toward the southern end of the agnew wiluna belt in the yilgarn craton of western australia and hosts the giant perseverance komatiite associated nickel sulfide deposit at in excess of 1 2 mt contained ni this is the biggest type 1 basal sulfide rich accumulation deposit in the world and it is associated with a number of smaller deposits that extend over a strike length of 20 km adjacent to the perseverance fault on the eastern margin of the belt in broad terms the greenstone sequence comprises an eastern rift proximal komatiite dacite sequence and a western and more marginal basalt dominated sequence with minor komatiite the komatiites display a wide range of compositions and textures from highly magnesian olivine meso to adcumulate rocks composed of olivine with forsterite contents of up to 94 5 to sequences of thin spinifex textured komatiite flow units this compositional range indicates considerable variation in volcanic facies from rift proximal to distal loci of focused flow termed pathways indicated by thickened cord like bodies of more olivine rich rock occur at all scales and are intimately associated with the ni sulfide mineralization the pathways are linked by thinner sequences of lower mgo rocks either layered olivine orthocumulate rocks or sequences of thin spinifex textured flow units some of the lower mgo komatiite sequences appear to emanate from the tops of pathways in the manner of breakout flows in tube fed basaltic terrains variably sulfidic black shales and cherts and barren exhalative iron sulfide horizons are intimately interlayered with the komatiitic rocks the overprinting effects of regional deformation and metamorphism spanning at least 80 m y are considerable at least eight local deformation events have been recognized by previous workers these have been used to constrain the 3 d model and can be linked to craton wide structural schemes some of the major faults are clearly accretionary and reflect reactivation of underlying basin forming structures early north over south thrusting was followed by e w directed basin closure and the development of nnw trending open regional folds extension accompanied batholithic granitoid intrusion and reactivated rift parallel nnw trending faults producing the major strike slip fault systems that divide the greenstone succession into a series of structural and stratigraphic domains the ensuing orogenic collapse resulted in overturning of the stratigraphy to the east subsequent brittle deformation events have produced fault sets in a variety of orientations that can be related to ongoing shortening with a variable principal stress orientation there is a complex interplay of structural and volcanological controls on the formation and subsequent deformation of the ni sulfide deposits in this ancient terrane three dimensional modeling has furthered a detailed understanding of the geologic and structural evolution of the belt allowed the identification of komatiite pathway positions and their plunges and provided quantitative data on the magnitude and direction of fault offsets of different generations the modeling exercise has shown that it is possible to strip away the effects of overprinting metamorphism and deformation to reveal some of the primary controls on komatiite volcanism and the location of ni sulfide deposits even in those portions of greenstone belts that are highly deformed and have undergone amphibolite facies metamorphism this is illustrated with case studies of the perseverance venus rocky s reward harmony and sir lancelot deposits three dimensional modeling has the capacity to integrate geologic geochemical and geophysical datasets in a way that fully utilizes the results of past exploration programs and produces a premium geologic product that can be used to guide further exploration the recent discovery of the venus deposit is in part a tribute to the power of this approach

sea ice free conditions during the sturtian glaciation early cryogenian south australia in the neoproterozoic snowball earth hypothesis shutdown of the planet9s hydrological system has been attributed to a global ice cover during one or more extreme glaciations in the central flinders ranges south australia the yudnamutana subgroup of sturtian age includes diamictite sandstone and siltstone units of glaciomarine origin as much as 5000 m thick and is overlain by postglacial transgressive siltstone and shale of the tindelpina shale member tapley hill formation in the central flinders ranges the yudnamutana subgroup consists of 1 the pualco tillite gravity resedimented glacial deposits 2 the holowilena ironstone glacioturbidites 3 poorly stratified pebbly diamictite of the warcowie dolomite member lowermost wilyerpa formation gravity resedimented glacial deposits succeeded by 4 siltstones and sandstones with abundant hummocky cross stratification hcs storm deposits and 5 a lonestone bearing succession with cobble sized clasts in the upper wilyerpa formation ice rafted debris interpreted to record a glacial re advance in which hcs is absent because the action of oscillating waves is required to produce hcs on the seafloor its presence indicates an interval of significant meltback prior to glacial re advance given that the hcs occurs 2 km beneath the tindelpina shale member it signifies a major ice free interval during the sturtian glaciation

a unifying framework for iceberg calving models we propose a general framework for iceberg calving models that can be applied to any calving margin the framework is based on mass continuity the assumption that calving rate and terminus velocity are not independent and the simple idea that terminus thickness following a calving event is larger than terminus thickness at the event onset the theoretical near steady state analysis used to support and analyze the framework indicates that calving rate is governed to first order by ice thickness thickness gradient strain rate mass balance rate and backwards melting of the terminus the analysis furthermore provides a physical explanation for a previously derived empirical relationship for ice shelf calving alley and others 2008 in the calving framework the pre and post calving terminus thicknesses are given by two unknown but related functions the functions can vary independently of changes in glacier flow and geometry and can therefore account for variations in calving behavior due to external forcings and or self sustaining calving processes positive feedbacks although the calving framework does not constitute a complete calving model any thickness based calving criterion can easily be incorporated into the framework the framework should be viewed as a guide for future attempts to parameterize calving

the effect of buttressing on grounding line dynamics determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice sheet models here we investigate the role of lateral shear and ice shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets we derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non local ice shelf properties application of these results to a laterally confined version of the mismip 1a experiment shows that the boundary condition at the ice shelf front i e the calving law is a major control on the location and stability of the grounding line in the presence of buttressing allowing for both stable and unstable grounding line positions on downwards sloping beds these results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice shelf properties in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed consequently the marine ice sheet instability hypothesis may not apply to buttressed marine ice sheets

the physical volcanology of archaean komatiite sequences from forrestania southern cross province western australia abstract the archaean supracrustal sequence at forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km lithologies range from olivine cumulates which crystallised at the interface between substrate and flowing lava to spinifex textured rocks which underwent rapid crystallisation within ponded lava lobes characteristic associations of different rock types are interpreted in terms of variations in eruption rate which presumably reflect proximity to the vent or to a major lava river thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment where lava flow was continuous and eruptive velocities were high sheeted bodies of olivine cumulate formed from voluminous turbulent flow of lava over relatively refractory substrates such as basalt and oxide facies banded iron formation ribbon shaped olivine cumulate bodies formed in an equally dynamic environment within major lava channels where the substrate was less refractory and consisted of quartz rich sedimentary rocks which were susceptible to thermal erosion distributary lava channels in more distal settings are represented by thinner ribbon like accumulations of olivine mesocumulate to orthocumulate unusual poikilitic pyroxene bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks pyroxene and plagioclase bearing cumulates are present at the top of other channel sequences and reflect late stage ponding and in situ differentiation of komatiitic lava within the channel the channel facies rocks are flanked by sequences comprising multiple thin spinifex textured flow units which formed from the episodic overflow of lava from the distributary channels similar rocks also occur stratigraphically above many of the thick olivine cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle

an observationally validated theory of viscous flow dynamics at the ice shelf calving front an analytical theory is developed for ice flow velocity in a boundary layer couplet at the calving front the theory has simple quantitative characteristics that relate ice front velocity to thickness strain rate and shelf width matching one set of empirically derived relationships alley and others 2008 and implying that these relationships predict ice velocity rather than calving rate the two boundary layers are where longitudinal and transverse flow fields change from the interior flow to patterns consistent with the calving front stress condition numerical simulations confirm the analytical theory the quantitative predictions of the theory have low sensitivity to unmeasured parameters and to shelf plan aspect ratio while its robustness arises from its dependence on the scale invariance of the governing equations the theory provides insights into calving the stability of ice shelf calving fronts the stability of the grounding line of laterally resisted ice streams and also suggests that the calving front is an instructive dynamical analogue to the grounding line

ice shelf disintegration by plate bending and hydro fracture satellite observations and model results of the 2008 wilkins ice shelf break ups abstract satellite remote sensing observations of three break up events in 2008 for the wilkins ice shelf 28 february to 6 march 27 may to 31 may and 28 june to mid july provide unprecedented detail of ice shelf calving during rapid break up the observations reveal that the wilkins break ups occur through a distinctive type of shelf calving which we term disintegration as well as more typical rifting and calving here we focus on the disintegration process which is characterized by repeated rapid fracturing that creates narrow ice edge parallel blocks with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass we use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free mobile water is available high resolution satellite images and laser altimetry of the first break up event provide details of fracture spacings ice thicknesses and plate bending profiles that agree well with our model predictions we suggest that surface or near surface meltwater is the main pre condition for disintegration and that hydro fracture is the main mechanism brine layers from near waterline brine infiltration can support a similar process but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing a combination of brine enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two wilkins events

impact of tide topography interactions on basal melting of larsen c ice shelf antarctica 1 basal melting of ice shelves around antarctica contributes to formation of antarctic bottom water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers tides influence ice shelf basal melt rate wb by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf we use a three dimensional ocean model thermodynamically coupled to a nonevolving ice shelf to investigate the relationship between topography tides andwb for larsen c ice shelf lcis in the northwestern weddell sea antarctica using our best estimates of ice shelf thickness and seabed topography we find that the largest modeled lcis melt rates occur in the northeast where our model predicts strong diurnal tidal currents 0 4 m s 1 this distribution is significantly different from models with no tidal forcing which predict largest melt rates along the deep grounding lines we compare several model runs to explore melt rate sensitivity to geometry initial ocean potential temperature 0 thermodynamic parameterizations of heat and freshwater ice ocean exchange and tidal forcing the resulting range of lcis averagedwb is 0 11 0 44 m a 1 the spatial distribution of wb is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of wb is influenced by 0 these sensitivities in wbpredictions reinforce a need for high resolution maps of ice draft and sub ice shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

the mechanisms behind jakobshavn isbrae s acceleration and mass loss a 3 d thermomechanical model study the mechanisms causing widespread flow acceleration of jakobshavn isbrae west greenland remain unclear despite an abundance of observations and modeling studies here we simulate the glacier s evolution from 1985 to 2016 using a three dimensional thermomechanical ice flow model the model captures the timing and 90 of the observed changes by forcing the calving front basal drag in the trough is low and lateral drag balances the ice stream s driving stress the calving front position is the dominant control on changes of jakobshavn isbrae since the ice viscosity in the shear margins instantaneously drops in response to the stress perturbation caused by calving front retreat which allows for widespread flow acceleration gradual shear margin warming contributes 5 to 10 to the total acceleration our simulations suggest that the glacier will contribute to eustatic sea level rise at a rate comparable to or higher than at present

ice velocity mapping of ross ice shelf antarctica by matching surface undulations measured by icesat laser altimetry abstract we present a novel method for estimating the surface horizontal velocity on ice shelves using laser altimetry data from the ice cloud and land elevation satellite icesat 2003 2009 the method matches undulations measured at crossover points between successive campaigns elevation measurements are first relocated into a time varying moving coordinate system using an initial velocity e g from velmap and then crossover height differences are minimized with an adjustment vector errors in geolocation of the icesat tracks result in some error in the adjustment vectors but these are small relative to the velocity adjustment for fast moving ice shelves we use the algorithm to estimate changes in the ice velocity of ross ice shelf between an earlier mapping from velmap and the icesat period the new velocity field is compared with velocities from in situ measurements and satellite radar interferometry the slowdown of 98 34 m yr 1 23 is observed in the ice shelf downstream of whillans ice stream and the deceleration rate is 3 1 1 1 m yr 2 during last three decades the method can be expanded to the simultaneous mapping of ice horizontal velocity ice thickness change and surface deformation for antarctic ice shelves as well as a more accurate mapping using future icesat 2 measurements

what caused the low water phase of glacial lake agassiz abstract first order modeling suggests that a low water phase in late glacial lake agassiz can be explained through changes in the balance between evaporation precipitation and runoff rather than drainage the low water moorhead phase is often attributed to drainage through outlets opened by isostatic depression and retreat of the laurentide ice margin however new data indicate that the proposed outlets were ice covered during the moorhead phase instead the lake water levels dropped to the moorhead phase before the start of the younger dryas chronozone and remained there until 11 3 ka thus drainage seems to be an implausible explanation for younger dryas aged low water levels in lake agassiz an alternative explanation is that evaporation equaled or exceeded water inputs from the adjacent ice margin and the deglaciated parts of the drainage basin to evaluate whether this hypothesis is plausible we constructed a simple model that considers the paleo basin geometry hydrology and meltwater production from the adjacent ice margin modest hydrologic changes within the range of present day variability coupled with low meltwater production produce a closed basin shifts in the location of the polar jet driven by increased arctic albedo may explain our inferred hydrologic changes

a sensitivity study of annual area change for greenland ice sheet marine terminating outlet glaciers 1999 2013 the mass flux from marine terminating glacier outlets from the greenland ice sheet is a significant mass budget term yet glacier calving sensitivity to climate parameters is not well understood in this study the front area changes of 42 marine terminating greenland outlet glaciers were measured annually from 1999 to 2013 using optical satellite imagery thirty five glaciers retreated while seven remained stable the average front position change was 240 m a 1 and the total rate of front area change was 114 km 2 a 1 the yearly average area changes of all measured glaciers are compared with interannual variations in sea surface temperature sst sea ice concentration sic surface melt northern hemisphere nh surface air temperatures and the north atlantic oscillation nao index we find 1 northwestern glaciers area change have a significant correlation with surface melt and nh land temperature variations 2 southeastern glaciers have a significant correlation with local sst sic and nh temperatures and 3 southwestern glaciers area change correlate significantly with local sst and nao index we conclude that a climate sensitivity signal emerges considering a population of glaciers further we find a significant correlation between the date of minimum extent and glacier latitude area change data are available from http gac geus dk

komatiites and nickel sulfide ores of the black swan area yilgarn craton western australia 2 geology and genesis of the orebodies the black swan ultramafic succession hosts a number of magmatic fe ni cu pge sulfide ore shoots ranging from high grade massive ore to low grade disseminated sulfides of these the most economically significant is the silver swan massive sulfide orebody associated with the basal contact of the succession the deposit varies in thickness between 5 and 20 m reaches a n s strike length of 75 m extends for at least 1 2 km of vertical plunge and is open at depth overlying matrix net textured ore is rare inclusions of dacite are abundant within the lower 5 m of the massive sulfide they range from angular fragments through smooth sinuous and plumose morphologies to fine lace like intergrowths with the sulfide matrix and comprise variable proportions of cores of porphyritic dacite and carapaces with skeletal plagioclase phenocrysts dynamic crystallisation and kinetic melting textures in the carapaces indicate that the inclusions have been heated to various temperatures some well above their liquidus temperature the composition of the inclusions ranges from a perfect match with the immediate footwall dacites to mixtures of dacite with up to 30 komatiite the consistent thickness of the inclusion bearing basal layer within the massive sulphide is interpreted as the extent of 3 d physical connectivity between the inclusions and a partially molten underlying hybrid layer primary contacts between the silver swan massive sulfide orebody and overlying ultramafic rocks are marked by thin rinds containing coarse grained chevron textured chromites with skeletal textures compositions of these chromites match those from kambalda perseverance and other localities and are inconsistent with a metamorphic origin they are interpreted as markers of primary magmatic contacts the combination of this feature with the general paucity of matrix ore implies that the massive ore accumulated and solidified before the accumulation of the overlying thick sequence of olivine cumulates taken together with observations on the internal fractionation of platinum group elements within the massive ores these observations are consistent with a model where the massive ore were emplaced at the floor of a small partially drained lava tube the floor of the tube had been previously heated by passage of large volumes of lava such that it had reached its melting range the felsic inclusions within the ore are the result of buoyant ascent of partially molten substrate into the ore magma this constitutes strong evidence for the operation of thermo mechanical erosion during ore emplacement the disseminated cygnet and black swan orebodies show a number of distinctive features cygnet contains a assemblage of clasts and inclusions which are interpreted as the result of rip up transport and redeposition of sulfides from a pre existing massive sulfide orebody of which black duck may be a remnant the black swan orebody by contrast does not show xenolithic features but is characterised by an association of sulfide blebs with segregation vesicles and by unusually coarse grained olivine the black swan orebody is interpreted as the result of transport of sulfide droplets within a lava charged with a suspended load of coarse olivine crystals

relationship between microstructures and grain scale trace element distribution in komatiite hosted magmatic sulphide ores komatiite hosted nickel sulphides from the yilgarn craton australia consist of two main sulphide phases pyrrhotite fe7s8 and pentlandite fe ni 9s8 two minor sulphide phases chalcopyrite cufes2 and pyrite fes2 and trace arsenides samples of massive sulphides from three deposits with diverse deformation and metamorphic histories the silver swan perseverance and flying fox deposits have been studied by electron backscatter diffraction and laser ablation inductively coupled plasma mass spectrometry and nano scale secondary ion mass spectrometry these ore bodies were selected to investigate the relationship between microstructures and mineral trace element chemistry in three dominant sulphide species in each deposit in all three samples pyrrhotite preserves a strong evidence of crystal plasticity relative to both pentlandite and pyrite the trace element composition of pyrrhotite shows significant variation in specific elements pb bi and ag this variation correlates spatially with intragrain pyrrhotite microstructures such as low angle and twin boundaries minor signatures of crystal plasticity in pyrite and pentlandite occur in the form of rare low angle boundaries pentlandite and mild lattice misorientation pyrite trace element compositions of pentlandite and pyrite show no correlation with microstructures variations in pyrrhotite are interpreted as a result of intragrain diffusion during the syn and post deformation history of the deposit intragrain diffusion can occur either due to bulk diffusion dislocation impurity pair diffusion or by pipe diffusion i e along fast diffusion pathways at high and low angle and twin boundaries this contribution examines three different diffusion models and suggests that dislocation impurity pair diffusion and pipe diffusion are the most likely processes behind increased trace element concentration along the microstructures in pyrrhotite the same phenomenon is observed in samples from three different deposits that experienced widely different metamorphic conditions implying that the final disposition of these elements reflects a post peak metamorphic stage of the geological history of all three deposits

widespread rifting and retreat of ice shelf margins in the eastern amundsen sea embayment between 1972 and 2011 the major outlet glaciers that drain the eastern sector of the amundsen sea embayment smith haynes thwaites and pine island are among the largest fastest flowing and fastest thinning glaciers in west antarctica their recent ice flow acceleration is linked to ocean induced ice shelf thinning but may also arise from additional losses of ice shelf buttressing that are not well understood here we present a comprehensive history of coastal change in the eastern amundsen sea embayment between 1972 and 2011 derived mostly from landsat imagery the termini of all four major outlet glaciers have retreated but retreat is most rapid along the ice shelf margins where progressive rifting has occurred this pattern of retreat coincides with the recent acceleration of grounded ice and contributed to loss of ice shelf buttressing the observed pattern of margin led gradual ice shelf disintegration appears to be common in accelerating ocean terminating outlet glaciers we hypothesize that this pattern is part of a positive feedback between glacier acceleration and rift growth that could drive further buttressing loss in the eastern amundsen sea embayment

a simple law for ice shelf calving a major problem for ice sheet models is that no physically based law for the calving process has been established comparison across a diverse set of ice shelves demonstrates that iceberg calving increases with the along flow spreading rate of a shelf this relation suggests that frictional buttressing loss which increases spreading also leads to shelf retreat a process known to accelerate ice sheet flow and contribute to sea level rise

links between ocean temperature and iceberg discharge during heinrich events the last glacial period was punctuated by several periods of massive iceberg discharge from the northern hemisphere ice sheets numerical simulations indicate that these discharge events are linked to an interplay between ice sheet accumulation marine ice shelf stability and periodically oscillating surface ocean temperature

evolution of basal crevasses links ice shelf stability to ocean forcing abstract iceberg calving is one of the primary mechanisms responsible for transferring ice from the antarctic ice shelves to the ocean but remains poorly understood previous theories of calving have sought to explain the calving process as a brittle phenomenon that occurs rapidly when surface or basal crevasses penetrate the entire ice thickness here we show that the strain rate weakening nature of ice permits initially narrow basal crevasses to seed an instability that gives rise to locally enhanced ductile deformation and thinning over length scales that are large compared to the ice thickness this ductile failure process called necking amplifies long wavelength features of bottom topography and allows basal crevasses to penetrate an increasing fraction of the ice thickness as they advect downstream application of the model to the four largest antarctic ice shelves shows that necking occurs downstream of pinning points and sharp protrusions in the ice shelf embayment where stress is highly concentrated however model predictions are sensitive to assumptions about basal melting and refreezing within crevasses suggesting that the combination of mechanical instability and ice ocean interaction on the scale of an individual crevasse may play a leading role in controlling ice shelf stability

a three dimensional calving model numerical experiments on johnsons glacier livingston island antarctica calving from tidewater glaciers and ice shelves accounts for around half the mass loss from both polar ice sheets yet the process is not well represented in prognostic models of ice dynamics benn and others proposed a calving criterion appropriate for both grounded and floating glacier tongues or ice shelves based on the penetration depth of transverse crevasses near the calving front computed using nye s formula the criterion is readily incorporated into glacier and ice sheet models but has not been fully validated with observations we apply a three dimensional extension of benn and others criterion incorporated into a full stokes model of glacier dynamics to estimate the current position of the calving front of johnsons glacier antarctica we find that two improvements to the original model are necessary to accurately reproduce the observed calving front 1 computation of the tensile deviatoric stress opening the crevasse using the full stress solution and 2 consideration of such a tensile stress as a function of depth our modelling results also suggest that johnsons glacier has a polythermal structure rather than the temperate structure suggested by earlier studies

the agnew nickel deposit western australia part i structure and stratigraphy the agnew formerly perseverance nickel deposit in the agnew wiluna greenstone belt of the yilgarn block has previously been assigned to the category of intrusive dunite associated deposits on the basis of detailed reinterpretation of the geology of the deposit we conclude that the nickel sulfide mineralization is hosted by komatiite flows which predate the associated dunite body which is itself extrusive nickel sulfide mineralization is hosted by a suite of metamorphosed olivine rich ultramafic rocks containing widely varying proportions of tremolite and chlorite chemical analyses of these rocks show them to be metamorphosed komatiites in which original asymmetric differentiation profiles can be recognized these provide facing directions which form the basis for structural and stratigraphic interpretation much of the massive sulfide has been structurally remobilized into fold noses and planar fractures but some retains primary igneous contacts with underlying flow tops primary contacts are characterized by a zone of skeletal chromite developed across the interface between massive sulfide and flow top the primary massive ore postdates a thick komatiite flow which hosts the matrix ore the dunite body shows a discordant lower contact which truncates stratigraphic marker units in the country rock at the northern limit of the mine area the base of the dunite is separated from the top of the main mineralized flow by more than 150 m of felsic volcanic rocks at the southern limit of mine development the dunite directly overlies the matrix ore zone of the mineralized flow this discordant contact is interpreted as the base of a thermal erosion channel formed by an extensive komatiite lava river the geometry of this contact and the competence contrast between the dunite body and the country rocks exerted a dominant influence on subsequent deformation and sulfide remobilization

geodynamics of the eastern yilgarn craton abstract over the last decade there have been significant advances in our understanding of the stratigraphy magmatism deformation metamorphism and timing of mineralisation in the eastern yilgarn craton eyc of western australia the integration of these disciplines has enabled a holistic review of the tectonic history of the eyc which favours a paraautochthonous tectonic model a significant advance has been the recognition of a 2810 ma rifting event off the eastern margin of the youanmi terrane which set up the north northwest trending architecture of the eyc rifting was followed by a 100 myr long phase of tectonic activity initiated by the establishment of a convergent margin at 2715 ma characterised by a west dipping subduction zone to the east of the eyc subduction was associated with the deposition of 2715 2670 ma volcanic stratigraphy and the emplacement of voluminous tonalite trondhjemite granodiorite magmatism which resulted in magmatic thickening of the crust volcanism was terminated by a 5 myr pulse of east northeast contraction followed by mid orogenic extension possibly linked to lithospheric and lower crustal delamination the lack of ultra high pressure metamorphism and the presence of high geothermal gradients preclude this contractional event from recording continent continent collision mid orogenic extension at 2665 ma resulted in the introduction of metasomatised mantle melts mafic granites and syenites deposition of siliciclastic basins which record metamorphic anticlockwise ptt paths and the start of significant economic gold mineralisation in the eyc high stretching factors and or delamination associated with this extension event resulted in significant heat input into the base of the crust which eventually led to the emplacement of low ca crustal melt granites and cratonisation of the eyc major gold mineralisation postdates mid orogenic extension and was associated with renewed compression transpression 2650 ma and the development of steep predominantly north northwest striking sinistral and later north to north northeast striking dextral strike slip faults during low ca granite emplacement

on the coupled response to ice shelf basal melting ice shelf basal melting is tightly coupled to ice shelf morphology ice shelves in turn are coupled to grounded ice via their influence on compressive stress at the grounding line ice shelf buttressing here we examine this interaction using a local parameterization that relates the basal melt rate to the ice shelf thickness gradient this formulation permits a closed form solution for a steady state ice tongue time dependent numerical simulations reveal the spatial and temporal evolution of ice shelf ice stream systems in response to changes in ocean temperature and the influence of morphology dependent melting on grounding line retreat we find that a rapid 1 year re equilibration in upstream regions of ice shelves establishes a spatial pattern of basal melt rates relative to the grounding line that persists over centuries coupling melting to ice shelf shape generally but not always increases grounding line retreat rates relative to a uniform distribution with the same area average melt rate because upstream ice shelf thickness gradients and retreat rates increase nonlinearly with thermal forcing morphology dependent melting is more important to the response of weakly buttressed strongly forced ice streams grounded on beds that slope upwards towards the ocean e g those in the amundsen sea

bounds on the calving cliff height of marine terminating glaciers increased calving and rapid retreat of glaciers can contribute significantly to sea level rise but the processes controlling glacier retreat remain poorly understood we seek to improve our understanding of calving by investigating the stress field controlling tensile and shear failure using a 2d full stokes finite element model using idealized rectangular geometries we find that when rapidly sliding glaciers thin to near buoyancy full thickness tensile failure occurs similar to observations motivating height above buoyancy calving laws in contrast when glaciers are frozen to their beds basal crevasse penetration is suppressed and calving is minimal we also find shear stresses are largest when glaciers are thickest together the tensile and shear failure criteria map out a stable envelope in an ice thickness water depth diagram the upper and lower bounds on cliff height can be incorporated into numerical ice sheet models as boundary conditions thus bracketing the magnitude of calving rates in marine terminating glaciers

collapse of marine based outlet glaciers from the scandinavian ice sheet abstract we present a reconstruction of the timing and retreat rates of more than 2000 m thick younger dryas yd fjord glaciers in western norway using a detailed chronology of 10 be exposure ages from lateral moraines and 14 c dated end moraines a primary conclusion is that ice margins retreated up the 120 170 km long fjords at mean rates of 240 340 m yr 1 during the early holocene we further show that part of the south western sector of the scandinavian ice sheet collapsed in two distinct steps the first step occurred between 19 5 and 18 5 ka bp as break up of the norwegian channel ice stream which drained the ice sheet during the last glacial maximum lgm the second step was the rapid retreat up the fjords mentioned above dated to 11 6 11 1 ka bp during the intervening 7000 years no net retreat occurred despite oscillations of the ice margin this stepwise ice margin retreat strongly contrasts with the more monotonic decay of the ice sheet as a whole indicating that water depths set the pace for climate triggered ice margin retreat in this part of the ice sheet calving and melting of marine margins has dominated mass loss from modern ice sheets in recent decades however the mechanisms and long term 100 1000 yr rate of ice front retreat is less certain and empirical examples such as those given here may help in developing better numerical models

komatiites and nickel sulphide orebodies of the black swan area yilgarn craton western australia 1 petrology and volcanology of host rocks the black swan succession is a bimodal association of dacitic and komatiitic volcanic rocks located about 50 km nne of kalgoorlie within the 2 7 ga eastern goldfields greenstone province of the yilgarn craton the komatiite stratigraphy comprises a steep dipping east facing package about 700 m in maximum thickness and about 2 5 km in strike length fig 1 which hosts a number of economically exploitable ni sulphide orebodies including the silver swan massive ore shoot approximately half a million tonnes at about 10 5 ni the sequence can be subdivided into a lower felsic unit comprising coherent and autobrecciated facies of multiple dacite lava flows an upper eastern and lower western ultramafic unit each showing marked lateral facies variation and an upper felsic unit coeval with the eastern ultramafic unit the komatiite sequence has been metamorphosed at sub greenschist facies in the presence of high proportions of co2 rich fluid giving rise to pervasive talc carbonate and talc carbonate quartz assemblages with extensive preservation of pseudomorphed igneous textures cores of lizardite serpentinite are present in the thickest parts of the ultramafic succession the degree of penetrative deformation is generally very low and original stratigraphic relationships are largely intact in much of the sequence the eastern ultramafic unit and western ultramafic unit are interpreted as components of a single large komatiite flow field representing overlapping stages in the emplacement of a series of distributory lava pathways and flanking sheet flows the western ultramafic unit which hosts the bulk of the high grade massive and disseminated ores is a sequence dominated by coarse grained olivine cumulates 2 km wide and up to 500 m thick with major magma pathways represented by thick homogenous olivine mesocumulate piles at its northern and southern ends respectively 400 and 200 m thick the sequence between the two major pathways consists of olivine orthocumulates ooc with minor spinifex textured intervals the unit is capped by a persistent spinifex textured crust less than 1 m thick and is locally vesicular the eastern ultramafic unit contains the black swan cumulate zone a 500 m thick sequence of very coarse grained hopper textured locally vesicular ooc containing disseminated sulphides in its lower 200 m the zone is flanked to the north and south by complexly interdigitated sequence of highly irregular spinifex capped olivine cumulate rich flow lobes between 1 and 100 m thick and dacitic lavas and tuffs the complexity of the 3 d spatial relationship of these units suggests a combination of simultaneous eruption of dacite and komatiite combined with thermal or thermomechanical erosion the eastern and western units are interpreted as the result of more or less continuous prolonged eruption of olivine charged komatiite lava which developed localised thermo mechanical erosion channels in the dacitic substrate komatiite and dacite eruption was synchronous giving rise to complex interdigitation and extensive contamination and hybridisation

textural relationship of sulfide ores pge and sr nd os isotope compositions of the triassic piaohechuan ni cu sulfide deposit in ne china the 216 ma piaohechuan no 4 intrusion in northeastern china situated in the easternmost part of the central asian orogenic belt is composed of hornblende olivine gabbro overlain by hornblende gabbro the intrusion is about 630 m long 40 to 250 m wide and 50 to 150 m thick nickel sulfide mineralization is restricted to the lower part of the intrusion and contains 10 000 metric tons t of ni with average pge concentrations of 40 ppb os 30 ppb ir 36 ppb ru 26 ppb rh 255 ppb pt and 161 ppb pd in 100 sulfide

verification of model simulated mass balance flow fields and tabular calving events of the antarctic ice sheet against remotely sensed observations the antarctic ice sheet ais has the greatest potential for global sea level rise this study simulates ais ice creeping sliding tabular calving and estimates the total mass balances using a recently developed advanced ice dynamics model known as segment ice segment ice is written in a spherical earth coordinate system because the ais contains the south pole a projection transfer is performed to displace the pole outside of the simulation domain the ais also has complex ice water granular material bedrock configurations requiring sophisticated lateral and basal boundary conditions because of the prevalence of ice shelves a girder yield type calving scheme is activated the simulations of present surface ice flow velocities compare favorably with insar measurements for various ice water bedrock configurations the estimated ice mass loss rate during 2003 2009 agrees with grace measurements and provides more spatial details not represented by the latter the model estimated calving frequencies of the peripheral ice shelves from 1996 roughly when the 5 km digital elevation and thickness data for the shelves were collected to 2009 compare well with archived scatterometer images segment ice s unique non local systematic calving scheme is found to be relevant for tabular calving however the exact timing of calving and of iceberg sizes cannot be simulated accurately at present a projection of the future mass change of the ais is made with segment ice forced by atmospheric conditions from three different coupled general circulation models the entire ais is estimated to be losing mass steadily at a rate of 120 km3 a at present and this rate possibly may double by year 2100

the eruption of komatiites and picrites in preference to primitive basalts abstract many archaean greenstone and post archaean flood basalt provinces exhibit flows of komatiites or picrites in close spatial and temporal association with fractionated basalts it follows that picrites and komatiites can erupt through the same crustal sections in which basaltic magmas apparently pond and fractionate this is surprising since considerations of melt density alone would suggest that komatiites and picrites should be trapped and fractionated more easily than basalts some of the factors that control the ascent of magmas through the crust are examined from a theoretical viewpoint with particular reference to the eruption of komatiites and archaean basalts ascent in dykes is characterized by a local balance between the viscous pressure drop due to flow and the buoyancy forces due to the local difference in density between the melt and the host rock the level of neutral buoyancy of the magma and solidification in the dyke tip are identified as the key controls for the trapping of mafic melts in solid crust and the formation of crustal magma chambers it is suggested that komatiites and picrites can erupt in preference to basalts because they are less prone to freezing at the dyke tip during dyke propagation

modeling hydraulic fracture of glaciers using continuum damage mechanics the presence of water filled crevasses is known to increase the penetration depth of crevasses and this has been hypothesized to play an important role controlling iceberg calving rate here we develop a continuum damage based poro mechanics formulation that enables the simulation of water filled basal and surface crevasse propagation the formulation incorporates a scalar isotropic damage variable into a maxwell type viscoelastic constitutive model for glacial ice and the effect of the water pressure on fracture propagation using the concept of effective solid stress we illustrate the model by simulating quasi static hydrofracture in idealized rectangular slabs of ice in contact with the ocean our results indicate that water filled basal crevasses only propagate when the water pressure is sufficiently large and that the interaction between simultaneously propagating water filled surface and basal crevasses can have a mutually positive influence leading to deeper crevasse propagation which can critically affect glacial stability therefore this study supports the hypothesis that hydraulic fracture is a plausible mechanism for the accelerated breakdown of glaciers

iceberg calving during transition from grounded to floating ice columbia glacier alaska 1 the terminus of columbia glacier alaska unexpectedly became ungrounded in 2007 during its prolonged retreat visual observations showed that calving changed from a steady release of low volume bergs to episodic flow perpendicular rifting propagation and release of very large icebergs a style reminiscent of calving from ice shelves here we compare passive seismic and photographic observations through this transition to examine changes in calving mechanical changes accompany the visible changes in calving style post flotation generation of seismic energy during calving is substantially reduced we propose this is partly due to changes in source processes

structural event framework for the eastern yilgarn craton western australia and its implications for orogenic gold abstract the nnw trending tectonic grain of the eastern yilgarn craton eyc was established as a result of predominantly ene wsw directed extension d1 and d3 and e ene w wsw d2 d4 to ne sw directed d5 contraction the result has been a succession of nnw striking temporally discrete fabric elements which can be difficult to interpret reliably at any single location despite this many past workers interpreted the nnw striking fabric as the result of only one regional contractional event and used it as a marker for correlating structural events across the region in order to unravel the complexity this paper presents a new sixfold d1 d6 deformation nomenclature based on 10 000 new mesoscale structural observations including their kinematic analysis and cross cutting relationships these mesoscale data were referenced with regional 3d map patterns stratigraphic magmatic metallogenic considerations and deep seismic reflection images this integrated geodynamic architectural approach is applicable to solving structural event histories in other polydeformed terrains gold mineralisation occurred during the first five events but was particularly concentrated from d3 onwards the d3 event marked the most profound change in the tectonic evolution of the eyc with changes in greenstones granites and tectonic mode lithospheric extension and core complexes with the first significant gold deposited within extensional shear zones that dissect the crust later contraction d4 was imposed at a high angle to the previously established anisotropic architecture the outcome was the creation of a new dynamic permeability framework which resulted in gold mineralisation during nnw striking sinistral strike slip faulting and associated thrusting a further stress switch d5 further modified the architecture resulting in n to nne striking dextral strike slip faulting and the final period of gold mineralisation before late stage extension d6

the volcanology of komatiites as deduced from field relationships in the norseman wiluna greenstone belt western australia abstract komatiites in the western part of the norseman wiluna greenstone belt in the yilgarn block of western australia display a wide variety of volcanic facies ranging from very thin differentiated munro township type flow units through to very thick olivine rich cumulate flow units containing high proportions of adcumulate dunite cumulate flow units have been mapped in detail in the agnew wiluna segment of the norseman wiluna greenstone belt in the yakabindie and mt keith areas thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet like sequences of finer grained olivine orthocumulates they show fine scale internal layering broad scale cryptic layering and upper fractionated sequences containing harrisites pyroxene bearing cumulates and in some cases gabbroic derivatives low grade disseminated sulphide mineralisation occurs within the dunite lenses in the stratigraphically equivalent kathleen east area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows a similar relationship is also seen further south where the perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows these field relationships indicate an extrusive origin for the adcumulate dunite bodies the walter williams formation in the west central part of the norseman wiluna belt is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent it consists of a central sheet like body of adcumulate dunite underlain and overlain by olivine orthocumulates the adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite at its northernmost extent the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros dolerites and pyroxene spinifex textured material interpreted as a periodically replenished lava lake sequence both the lenticular dunite bodies of the agnew wiluna belt and the sheet like dunite body of the walter williams formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates the adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates field evidence from perseverance and the general geometry of the dunite lenses suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low melting felsic volcanic substrates the textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion dunite sheets form close to the eruption sites and dunite lenses as a result of channellisation further away from the vent or close to the vent in the situation where the substrate is non refractory and thermal erosion can take place readily kambalda type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic and munro type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions or close to the vent of very small ones waning eruption rates leads to proximal facies being overridden by distal ones a common observation in komatiite sequences the size of thermal erosion channels requires very rapid eruption rates comparable to those in phanerozoic flood basalt terrains

the gravitationally consistent sea level fingerprint of future terrestrial ice loss 1 we solve the sea level equation to investigate the pattern of the gravitationally self consistent sea level variations fingerprints corresponding to modeled scenarios of future terrestrial ice melt these were obtained from separate ice dynamics and surface mass balance models for the greenland and antarctic ice sheets and by a regionalized mass balance model for glaciers and ice caps for our mid range scenario the ice melt component of total sea level change attains its largest amplitude in the equatorial oceans where we predict a cumulative sea level rise of 25 cm and rates of change close to 3 mm yr from ice melt alone by 2100 according to our modeling in low elevation densely populated coastal zones the gravitationally consistent sea level variations due to continental ice loss will range between 50 and 150 of the global mean this includes the effects of glacial isostatic adjustment which mostly contributes across the lateral forebulge regions in north america while the mid range ocean averaged elastic gravitational sea level variations compare with those associated with thermal expansion and ocean circulation their combination shows a complex regional pattern where the former component dominates in the equatorial pacific ocean and the latter in the arctic ocean

a mass flux perspective of the tidewater glacier cycle i explore the tidewater glacier cycle with a 1 d depth and width integrated flow model that includes a mass flux calving parameterization the parameterization is developed from mass continuity arguments and relates the calving rate to the terminus velocity and the terminus balance velocity the model demonstrates variable sensitivity to climate from an advanced stable configuration a small warming of the climate triggers a rapid retreat that causes large scale drawdown and is enhanced by positive glacier dynamic feedbacks eventually the terminus retreats out of deep water and the terminus velocity decreases resulting in reduced drawdown and the potential for restabilization terminus readvance can be initiated by cooling the climate terminus advance into deep water is difficult to sustain however due to negative feedbacks between glacier dynamics and surface mass balance despite uncertainty in the precise form of the parameterization the model provides a simple explanation of the tidewater glacier cycle and can be used to evaluate the response of tidewater glaciers to climate variability it also highlights the importance of improving parameterizations of calving rates and of incorporating sediment dynamics into tidewater glacier models

the statistical physics of iceberg calving and the emergence of universal calving laws determining a calving law valid for all glaciological and environmental regimes has proven to be a difficult problem in glaciology for this reason most models of the calving process are semi empirical with little connection to the underlying fracture processes in this study i introduce methods rooted in statistical physics to show how calving laws valid for any glaciological domain can emerge naturally as a large spatial scale long temporal scale limit of an underlying continuous or discrete fracture process an important element of the method developed here is that iceberg calving is treated as a stochastic process and that the probability an iceberg will detach in a given interval of time can be described by a probability distribution function using limiting assumptions about the underlying probability distribution the theory is shown to be able to simulate a range of calving styles including the sporadic detachment of large tabular icebergs from ice tongues and ice shelves and the more steady detachment of smaller sized bergs from tidewater outlet glaciers the method developed has the potential to provide a physical basis to include iceberg calving into numerical ice sheet models that can be used to produce more realistic estimates of the glaciological contribution to sea level rise

surface elevation changes at the front of the ross ice shelf implications for basal melting 1 rapid melting beneath the ross ice shelf ris occurs near the ice front likely in response to a secondary buoyant plume with tidal mixing and is sensitive to seasonal water temperatures in front of the ice shelf the front of the ris is examined using glas icesat laser altimetry data spatial and temporal changes in surface elevations are attributed to enhanced basal melting of the ice shelf near the ice front melt rates b increase exponentially as the front is approached from approximately zero at 40 km from the front to an average of 2 8 1 0 m a 1 within the front kilometer melt estimates within the front 60 km are best fit by the relationship b 2 0 exp x 11900 m a 1 where x denotes distance from the front frontal melt totals approximately 16 km3 a 1 in the front 40 km which accounts for between 10 and 40 of current published estimates of total melt beneath the ris spatial averaging along the front reveals a different pattern of melt in regions that have recently calved as compared to regions that have not calved frontal melt is modeled as a one dimensional buoyant plume with tidal currents included these model results imply that modest increases in sea surface temperatures will lead to considerable increases in melting beneath the ice shelf front

semiempirical and process based global sea level projections we review the two main approaches to estimating sea level rise over the coming century physically plausible models of reduced complexity that exploit statistical relationships between sea level and climate forcing and more complex physics based models of the separate elements of the sea level budget previously estimates of future sea level rise from semiempirical models were considerably larger than those from process based models however we show that the most recent estimates of sea level rise by 2100 using both methods have converged but largely through increased contributions and uncertainties in process based model estimates of ice sheets mass loss hence we focus in this paper on ice sheet flow as this has the largest potential to contribute to sea level rise progress has been made in ice dynamics ice stream flow grounding line migration and integration of ice sheet models with high resolution climate models calving physics remains an important and difficult modeling issue mountain glaciers numbering hundreds of thousands must be modeled by extensive statistical extrapolation from a much smaller calibration data set rugged topography creates problems in process based mass balance simulations forced by regional climate models with resolutions 10 100 times larger than the glaciers semiempirical models balance increasing numbers of parameters with the choice of noise model for the observations to avoid overfitting the highly autocorrelated sea level data all models face difficulty in separating out non climate driven sea level rise e g groundwater extraction and long term disequilibria in the present day cryosphere sea level system

calving event detection by observation of seiche effects on the greenland fjords with mass loss from the greenland ice sheet accelerating and spreading to higher latitudes the quantification of mass discharge in the form of icebergs has recently received much scientific attention here we make use of very low frequency 0 001 0 01 hz seismic data from three permanent broadband stations installed in the summers of 2009 10 in northwest greenland in order to monitor local calving activity at these frequencies calving seismograms are dominated by a tilt signal produced by local ground flexure in response to fjord seiching generated by major iceberg calving events a simple triggering algorithm is proposed to detect calving events from large calving fronts with potentially no user interaction our calving catalogue identifies spatial and temporal differences in calving activity between jakobshavn isbrae and glaciers in the uummannaq district similar to 200 km to the north the uummannaq glaciers show clear seasonal fluctuations in seiche based calving detections as well as seiche amplitudes in contrast the detections at jakobshavn isbrae show little seasonal variation which may be evidence for an ongoing transition to winter calving activity the results offer further evidence that seismometers can provide efficient and inexpensive monitoring of calving fronts

geochemistry of mineralised and barren komatiites from the perseverance nickel deposit western australia the perseverance formerly known as agnew and the neighbouring rocky s reward nickel deposits are associated with metamorphosed komatiite flows erupted onto a substrate of felsic crystal tuffs the deposits are overlain by the perseverance ultramafic complex which consists of a central dunite lens 700 m thick by 3 km wide flanked by olivine orthocumulates and spinifex textured komatiites a suite of samples of a zones of komatiite flows from the area has been analysed for major and trace elements rare earth elements and platinum group elements the samples fall into two distinct geochemical groups on the basis of major and rare earth elements 1 1 samples from a zones of spinifex textured flows flanking the central dunite show well constrained linear trends of major and trace elements typical of komatiite suites related by simple olivine fractionation 2 2 samples from the perseverance mine mineralised and basal flows from rocky s reward and from unmineralised flows at the base of the perseverance ultramafic complex all show evidence for lree enrichment relative to the first group they are also generally higher in sio2 and lower in feo for a given mgo content geochemical features of the second group are consistent with the effects of crustal assimilation this is in line with current models for nickel sulphide genesis based largely on kambalda which emphasise the role of thermal erosion of sulphidic footwall sediments the chalcophile element ni ir ru pt pd au and cu concentrations in a zone samples from the study area are typical of komatiites komatiites from the perseverance mineralised units and from rocky s reward show evidence of depletion of the more strongly chalcophile pge relative to ni and cu this feature is also evident in massive and matrix sulphide ore samples although less evident in the main perseverance cloud sulphides quantitative geochemical computer modelling indicates that the major and rare earth element chemistry of the samples analysed is consistent with derivation by combined assimilation of felsic material and crystallisation of olivine from a primitive komatiite magma the parental composition is represented by the spinifex textured units flanking the dunite lens compositions of komatiite liquids sulphides and cumulus olivines can be modelled successfully by varying degrees of batch equilibration between komatiite magma olivine and assimilated sulphidic sediment models involving combined assimilation and fractional crystallisation in which olivine and sulphide are fractionally segregated as they form are much less successful at matching the data in physical terms this suggests that the mineralisation is due to wholesale assimilation of floor rocks close to the site of sulphide accumulation comparison with quantitative computer model trends suggests an average ratio of assimilation to olivine fractionation in the order of 0 5 to 1 in order to generate the observed degrees of lree enrichment in flowtops ni depletion in olivine and pge depletion in sulphides and flow tops

fracture field for large scale ice dynamics recent observations and modeling studies emphasize the crucial role of fracture mechanics for the stability of ice shelves and thereby the evolution of ice sheets here we introduce a macroscopic fracture density field into a prognostic continuum ice flow model and compute its evolution incorporating the initiation and growth of fractures as well as their advection with two dimensional ice flow to a first approximation fracture growth is assumed to depend on the spreading rate only while fracture initiation is defined in terms of principal stresses the inferred fracture density fields compare well with observed elongate surface structures since crevasses and other deep reaching fracture structures have been shown to influence the overall ice shelf dynamics we propose the fracture density field introduced here be used as a measure for ice softening and decoupling of the ice flow in fracture weakened zones this may yield more accurate and realistic velocity patterns in prognostic simulations additionally the memory of past fracture events links the calving front to the upstream dynamics thus the fracture density field proposed here may be employed in fracture based calving parameterizations the aim of this study is to introduce the field and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms

future sea level rise from tidewater and ice shelf tributary glaciers of the antarctic peninsula abstract iceberg calving and increased ice discharge from ice shelf tributary glaciers contribute significant amounts to global sea level rise slr from the antarctic peninsula ap owing to ongoing ice dynamical changes collapse of buttressing ice shelves these contributions have accelerated in recent years as the ap is one of the fastest warming regions on earth further ice dynamical adjustment increased ice discharge is expected over the next two centuries in this paper the first regional slr projection of the ap from both iceberg calving and increased ice discharge from ice shelf tributary glaciers in response to ice shelf collapse is presented an ice sheet model forced by temperature output from 13 global climate models gcms in response to the high greenhouse gas emission scenario rcp8 5 projects ap contribution to slr of 28 16 to 32 16 mm by 2300 partitioned approximately equally between contributions from tidewater glaciers and ice shelf tributary glaciers in the rcp4 5 scenario sea level rise projections to 2300 are dominated by tidewater glaciers 8 18 mm in this cooler scenario 2 4 1 mm is added to global sea levels from ice shelf tributary drainage basins as fewer ice shelves are projected to collapse sea level projections from ice shelf tributary glaciers are dominated by drainage basins feeding george vi ice shelf accounting for 70 of simulated slr combined total ice dynamical slr projections to 2300 from the ap vary between 11 2 and 32 16 mm sea level equivalent sle depending on the emission scenario used these simulations suggest that omission of tidewater glaciers could lead to a substantial underestimation of the ice sheet s contribution to regional slr

assessment of ice flow dynamics in the zone close to the calving front of antarctic ice shelves this work was supported by a british antarctic survey studentship funded by the natural environment research council grant no ne k50094x 1

komatiites of the wildara leonora belt yilgarn craton wa the missing link in the kalgoorlie terrane abstract the wildara leonora belt in the kalgoorlie terrane is located between the world class nickel provinces of the agnew wiluna belt to the north and the kambalda domain to the south it extends from the wildara station to the gold mining center of leonora and consists of a deformed package of mafic komatiitic and sedimentary rocks this package of rocks which hosts a number of small nickel sulfide occurrences has been explored extensively for komatiite hosted nickel deposits most recently within the sullivans unit in the south east of the wildara leonora belt lithostratigraphic similarities together with shared physical geochemical and textural properties support a common affiliation between the wildara leonora and the agnew wiluna belts on this evidence the wildara leonora belt represents the southern extension of the largest komatiite hosted nickel camp in the world the apparently lesser prospectivity for nickel sulfide deposits is interpreted as a consequence of contrasting volcanic facies assemblages komatiites in the wildara leonora belt present geochemical and volcanological characteristics consistent with turbulent lava flow emplacement however compared with the agnew wiluna belt the wildara leonora belt komatiites are not spatially associated with felsic volcanic units but rather overlie a thick sequence of pillowed basalts and sulfidic sediments the lower proportion of adcumulate greater degrees of fractionation and lower degrees of contamination are consistent with komatiites in the wildara leonora belt having been emplaced in a more distal position to the rift environment with lower magma flux and more tranquil emplacement than the agnew wiluna belt komatiites and consequently having lower potential for giant nickel sulfide deposits such as those at perseverance and mt keith

instability of the antarctic ross sea embayment as climate warms collapse of the antarctic ice sheet since the last glacial maximum 18 000 years ago is most pronounced in the ross sea embayment which is partly ice free during antarctic summers thereby breaching the o ring of ice shelves and sea ice surrounding antarctica that stabilizes the ice sheet the o ring may have vanished during early holocene 5000 to 3000 b c roman 1 to 400 a d and medieval 900 to 1300 a d warm periods and reappeared during the little ice age 1300 to 1900 a d we postulate further collapse in the embayment during the post 1900 warming may be forestalled because east antarctic outlet glaciers nail the ross ice shelf to the transantarctic mountains so it can resist the push from west antarctic ice streams our hypothesis is examined for byrd glacier and a static ice shelf using three modeling experiments having plastic viscous and viscoplastic solutions as more data and improved modeling became available observed crevasse patterns were not reproduced a new research study is needed to model a dynamic ross ice shelf with all its feeder ice streams outlet glaciers and ice calving dynamics in three dimensions over time to fully test our hypothesis the required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf calving must then successively pull the outlet glacier nails so collapse of the marine west antarctic ice sheet proceeds to completion

avalanching glacier instabilities review on processes and early warning perspectives avalanching glacier instabilities are gravity driven rupture phenomena that might cause major disasters especially when they are at the origin of a chain of processes reliably forecasting such events combined with a timely evacuation of endangered inhabited areas often constitute the most efficient action recently considerable efforts in monitoring analyzing and modeling such phenomena have led to significant advances in destabilization process understanding improving early warning perspectives the purpose of this paper is to review the recent progress in this domain three different types of instabilities can be identified depending on the thermal properties of the ice bed interface if cold 1 the maturation of the rupture is associated with a typical time evolution of surface velocities and passive seismic activity a prediction of the final break off is possible using these precursory signs for the two other types water plays a key role in the development of the instability if the ice bed interface is partly temperate 2 the presence of meltwater may reduce the basal resistance which promotes the instability no clear and easily detectable precursory signs are known in this case and the only way to infer any potential instability is to monitor the temporal evolution of the thermal regime the last type of instability 3 concerns steep temperate glacier tongues switching for several days weeks during the melting season into a so called active phase followed in rare cases by a major break off event although the prediction of such events is still far from being achievable critical conditions promoting the final instability can be identified

spontaneous ice front retreat caused by disintegration of adjacent ice shelf in antarctica abstract antarctic ice discharge constitutes the largest uncertainty in future sea level projections floating ice shelves fringing most of antarctica exert retentive forces onto the ice flow while abrupt ice shelf retreat has been observed it is generally considered a localized phenomenon here we show that the disintegration of an ice shelf may induce the spontaneous retreat of its neighbor as an example we reproduce the spontaneous but gradual retreat of the larsen b ice front as observed after the disintegration of the adjacent larsen a ice shelf we show that the larsen a collapse yields a change in spreading rate in larsen b via their connecting ice channels and thereby causes a retreat of the ice front to its observed position of the year 2000 prior to its collapse this mechanism might be particularly relevant for the role of east antarctica and the antarctic peninsula in future sea level

glaciological advances made with interferometric synthetic aperture radar spaceborne interferometric synthetic aperture radar insar techniques for measuring ice flow velocity and topography have developed rapidly over the last decade and a half revolutionizing the study of ice dynamics spaceborne interferometry has contributed to major progress in many areas of glaciological study by providing the first comprehensive measurements of ice stream flow velocity over the major outlets of greenland and antarctica revealing that ice stream and outlet glacier flow can change rapidly months to years improving understanding of several ice sheet and ice shelf processes providing velocity for flux gate based mass balance assessment mapping flow of mountain glaciers and capturing the geomorphic traces of past ice flow we review the basic technique development the measurement characteristics and the extensive set of results yielded by these measurements