geochemistry and nd isotope systematics of archaean granites of the eastern goldfields yilgarn craton australia implications for crustal growth processes abstract the archaean granites and granitic gneisses of the eastern goldfields in the yilgarn craton western australia can be divided into two major groups high ca and low ca and three minor groups high hfse high field strength elements mafic and syenitic the high ca group 68 77 sio 2 with high al 2 o 3 na 2 o and sr and low y shares many features with typical archaean tonalite trondhjemite suites but has higher k 2 o rb and th contents the low ca group 70 76 sio 2 differs from the high ca group in having lower al 2 o 3 cao and na 2 o but higher k 2 o rb th zr y la and ce contents granites of the high hfse mafic and syenitic groups form a minor component 10 20 of the eastern goldfields granites the siliceous 74 77 sio 2 high hfse granites are restricted to a narrow nnw trending zone and are characterised by high tio 2 total feo mgo y zr and ce but only moderate rb th and pb contents the a type syenites 50 68 sio 2 are distinguished by their high total alkalies and mainly occur along tectonic lineaments the mafic group 55 70 sio 2 is lithologically diverse and exhibits a wide range of k 2 o rb th la and ce contents granite emplacement occurred between 2 69 and 2 60 ga contemporaneous with or post dating greenstone formation nd values for the high ca mafic and syenitic groups are similar mostly 0 0 to 2 5 whereas those for the low ca group show a pronounced polarity from 4 5 in the west to 2 0 in the east nd depleted mantle model ages for the low ca granites range from 3 2 ga in the west to 2 75 ga in the east model ages for the high ca group 2 75 2 9 ga overlap with ages for inherited zircons from these granites the high ca granites appear to have been derived at high pressures by partial melting of a mafic to intermediate source subducted oceanic crust crustal underplate or thickened crust or remelting of an older tonalitic source derived by such a process if the high ca granites were largely crust derived then their extensive source rocks must have formed less than 250 ma prior to granite formation the low ca granites appear to be the products of crustal reworking probably from a tonalitic to granodioritic protolith that was progressively younger towards the east perhaps in the order of several hundred million years younger or involved an additional younger component the localisation of the crustally derived high hfse granites suggests either geochemical zonation of the crust or specific tectonic processes such as rifting the mafic granites and possibly the syenites imply a significant mantle contribution and new crustal growth although the extent is equivocal the crust of the eastern goldfields appears to have been generated by both lateral and vertical accretion over a significant period from or before 3 0 ga until the time of granite emplacement

asymmetric impact of the north and south pacific on the equator in a coupled climate model 1 relative contributions of the north and south pacific to the equatorial ocean are estimated in a coupled climate model idealized experiments show that the south pacific contributes 30 50 more than the north pacific to the equatorial ocean temperature change because the warming in the north pacific can induce an additional cold rossby wave at around 20 n that tends to block the warm effect from the central north pacific while the south pacific influence can be easily convoyed to the equator through the interior pathway

transformation of an archean craton margin during proterozoic basin formation and magmatism the albany fraser orogen western australia abstract the albany fraser orogen is a well preserved example of proterozoic modification of an archean craton margin the formation of two successive basin systems accompanied by magmatism along the southern and southeastern yilgarn craton margin in western australia reflect distinct changes in tectonic regimes resulting in significant transformations of the archean craton crust provenance analysis from detrital zircons indicates that the first basin system the c 1815 1600 ma barren basin was dominantly filled with neoarchean detritus derived from the yilgarn craton and paleoproterozoic detritus derived from coeval voluminous mostly felsic magmatism the abundance of locally derived sediment deposited onto a reworked archean yilgarn craton substrate coupled with the progressively more juvenile isotopic signature of magmas indicates a largely extensional tectonic setting consistent with either a continental rift basin or a long lived backarc basin system this extensional regime led to the formation of a passive margin and oceanic basin along the craton edge the c 1600 1305 ma arid basin and formation of an ocean continent transition in the most heavily modified outboard region of the craton margin the eastern nornalup zone a change in tectonic regime is denoted by the formation of an oceanic arc the c 1410 ma loongana arc preserved in the adjoining madura province oceanic subduction led to accretion of the loongana arc onto the craton edge recorded by the presence of exotic detritus derived from this arc deposited into a foreland basin uppermost arid basin this accretion event triggered crustal thickening and the earliest phase of magmatism of stage i of the albany fraser orogeny at c 1330 ma a change to west dipping subduction beneath the accreted portion of the loongana arc placed the orogen into a backarc setting into which the remainder of the recherche supersuite mafic and felsic magmas were intruded thus the albany fraser orogen is regarded as an archean craton margin that preserves a long history of proterozoic transformation dominated by extensional processes that resulted in the formation of orogen wide basin systems accompanied by magmatism the recognition that the albany fraser orogen has always been part of the west australia craton and the tectonic regimes that have affected it provide important constraints on reconstructions of the nuna and rodinia supercontinents

pacific thermocline bridge revisited abstract the coupling on decadal time scales of the mid latitude and tropical pacific via an oceanic bridge in the thermocline is investigated using ocean general circulation model hindcasts and a coupled ocean atmosphere model results indicate that in the tropics decadal anomalies of isopycnal depth are forced by ekman pumping and are largely independent of the arrival of subducted anomalies in the thermocline that originate in the mid latitudes of either hemisphere in the coupled model temperature anomalies on isopycnals show little coupling from the tropics to the northern hemisphere but are lag correlated between southern hemisphere mid and low latitudes however anomaly magnitudes on the equator are small these results suggest that the oceanic bridge to the northern hemisphere explains only a small part of the observed decadal variance in the equatorial pacific coupling to the southern mid latitudes via temperature anomalies on isopycnals remains an intriguing possibility

ventilation of the subtropical north pacific the shallow salinity minimum abstract the shallow salinity minimum of the subtropical north pacific is shown to be a feature of the ventilated wind driven circulation subduction of low salinity surface water in the northeastern subtropical gyre beneath higher salinity water to the south causes the salinity minimum variation of salinity along surface isopycnals causes variations in density and salinity at the minimum a model of ventilated flow is used to demonstrate how the shallow salinity minimum can arise the model is modified to account for nonzonal realistic winds it is also extended to examine the three dimensional structure of the western shadow zone the boundary between the subtropical and subpolar gyres is given by the zero of the zonal integral of ekman pumping the western shadow zone fills the subtropical gyre at the base of the ventilated layers and decreases in extent with decreasing density for parameters appropriate to the north pacific the eastern shadow zone is of very limited extent observations of salini

oxygen isotope exchange and closure temperatures in cooling rocks retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model which predicts both the zonation profile of 18o within grains and the bulk 18o value of each mineral in the rock apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the 18o values of each mineral pair within the rock in systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values slow cooling the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the dodson closure temperatures tc for the two phases and the mode of the rock adding a third phase into a system which undergoes slow cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two phase system in some systems oxygen isotope reversals can be developed if cooling is not slow then the mineral 18o values resulting from cooling will be partly dependent upon the initial temperature of the system concerned the model successfully simulates the mineral 18o values that are often observed in granitic rocks application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems

on thermocline ventilation of active and passive tracers the ventilation propagation of temperature and passive tracer anomalies are compared in a numerical model although both anomalies tend to have pathways close to the mean ventilation flow the temperature anomaly could have a slower propagation speed and a more complex spatial structure the temperature propagation is better understood in terms of baroclinic planetary waves

translating the mineral systems approach into an effective exploration targeting system abstract holistic mineral systems approaches are being increasingly adopted in the study of ore deposits but application in industry has been suboptimal because of the lack of a conceptual framework to help translate the mineral system understanding to practical application a four step process is proposed for linking the conceptual mineral system with data available to support exploration targeting these steps include translation from 1 critical processes of the mineral system to 2 constituent processes of the mineral system to 3 targeting elements reflected in geology and 4 targeting criteria used to detect the targeting elements directly or by proxy this translation process is illustrated using examples from komatiite hosted ni cu pge and orogenic au mineral systems ranking of targets must be at the critical process level rather than the current practice of mixing processes elements and proxies the significance of targeting elements and their mappable criteria changes as a function of the scale at which the critical processes operate and the availability of relevant geoscience datasets this scale dependence is rarely recognised in current practice sources of uncertainty when translating the mineral system understanding into an effective exploration targeting system include 1 resolution of a mappable criterion in a dataset 2 how well the targeting criterion reflects the targeting element 3 how well the targeting element reflects the critical process and 4 whether the process being mapped is critical for the genesis of a mineral deposit the first three sources of uncertainty collectively are termed the representativeness of the targeting criteria i e how representative is a mappable criterion of the constituent process for which it is a proxy the last source of uncertainty reflects our lack of understanding of the processes that control mineralisation and highlights areas for further research

the ventilation of the world s oceans maps of the potential vorticity field abstract maps of potential vorticity q within four different density layers are presented for the north and south atlantic north and south pacific and indian oceans potential vorticity is evaluated as f 0 z a prescription that is valid for large slow scales and that allows its mapping from hydrographic data alone here f is the coriolis frequency p0 the potential density and z the vertical coordinate it is shown that the character of a layer within the thermocline of a subtropical gyre will vary greatly depending on the extent to which the layer is isolated from surface boundary conditions and forcing fluid particles recirculating within ventilated layers have their potential vorticity and other properties reset with virtually every circuit round the gyre when they pass through the outcrop zone by contrast particles within unventilated or dynamically isolated layers have time to share their properties with neighboring particles and approach a common homogenized state the north

origin of upper ocean warming and el nino change on decadal scales in the tropical pacific ocean the cause of decadal scale variability in the tropical pacific ocean such as that marked by the 1976 77 shift in the el nino southern oscillation1 2 3 4 5 6 7 is poorly understood unravelling the mechanism of the recent decade long warming in the tropical upper ocean is a particularly important challenge given the link to el nino variability but establishing the hypothesized interannual decadal oceanic connections between middle latitudes and tropics has proved elusive8 here we present observational evidence that pacific upper ocean warming and decadal changes in the el nino southern oscillation after 1976 may originate from decadal mid latitude variability in the middle 1970s the north pacific ocean is observed to have undergone a clear phase transition a see saw subsurface temperature anomaly pattern that rotates clockwise around the subtropical gyre at middle latitudes a subsurface warm anomaly formed in the early 1970s from subducted surface waters and penetrated through the subtropics and into the tropics thus perturbing the tropical thermocline and driving the formation of a warm surface water anomaly that may have influenced el nino in the 1980s the identification of this teleconnection of extratropical thermal anomalies to the tropics through a subsurface ocean bridge may enable improved prediction of decadal scale climate variability

crustal structure of a proterozoic craton boundary east albany fraser orogen western australia imaged with passive seismic and gravity anomaly data abstract we use passive seismic and gravity data to characterize the crustal structure and the crust mantle boundary of the east albany fraser orogen in western australia a proterozoic orogen that reworked the southern and southeastern margin of the archean yilgarn craton the crustal thickness pattern retrieved from receiver functions shows a belt of substantially thickened crust that follows the trend of the orogen but narrows to the southwest common conversion point profiles show a clear transition from a wide symmetric moho trough in the northeast to a one sided northwestern moho dip in the southwest where the moho appears to underthrust the craton towards its interior this change appears to coincide with the inferred trace of the ida fault a major terrane boundary within the yilgarn craton forward modelling of gravity anomaly data using the retrieved moho geometry as a geometric constraint shows that a conspicuous elongated gravity low on the northwestern side of the east albany fraser orogen is almost certainly caused by thickened archean crust to obtain a model that resembles the regional gravity pattern the following assumptions are necessary high density rocks occur in the upper crust of the fraser zone at depth inside the moho trough and in parts of the eastern nornalup zone east of the moho trough although our gravity models do not constrain at which crustal level these high density rocks occur active deep seismic surveys suggest that large extents of the east albany fraser orogen s lower crust include a mesoproterozoic magmatic underplate known as the gunnadorrah seismic province the simplest interpretation of the imaged crustal structure is that the gunnadorrah seismic province is underthrust beneath the yilgarn craton likely as a consequence of crustal shortening during accretion further east the imaged geometry overall appears to show a wedge of archean lower crust that was driven between the exhumed fraser zone and the gunnadorrah seismic province effectively splitting the paleo to mesoproterozoic crust of the east albany fraser orogen the vertical splitting of proterozoic crust by a cratonic crustal wedge comparable to what we image in this study may be a process that contributed to forming many craton margins around the world

investigating the role of the tibetan plateau in the formation of atlantic meridional overturning circulation abstractthe tibetan plateau tp over the eurasian continent has significant effects on both regional and global climate it can even affect the remote atlantic meridional overturning circulation

the affinity of archean crust on the yilgarn albany fraser orogen boundary implications for gold mineralisation in the tropicana zone abstract craton margins can be subject to a wide array of gold genesis and redistribution processes although high grade terrains on craton margins are frequently viewed as less prospective than lower grade counterparts in contrast to this the high grade tropicana zone a newly defined archean crustal component on the eastern margin of the yilgarn craton within the albany fraser orogen afo contains a significant proterozoic gold deposit this deposit and zone comprise mid amphibolite to granulite facies gneissic rocks with evidence of partial melting and granite injection the tropicana zone contains significant low si lile enriched granites classed as sanukitoids along with the distinctive compositions the rarity of these rocks within any archean craton suggests that the granitoid protoliths represent a single suite emplaced during one event due to the intense granulite facies overprinting of the tropicana zone rocks determination of the magmatic protolith age for these sanukitoids is challenging nonetheless the best age estimate for magmatism is 2692 16 ma based on the youngest zircons preserving textural evidence of growth within a viscous silicate melt this age is older than compositionally similar magmatism found within the yilgarn craton although a sanukitoid in the northern foreland of the afo has a similar age furthermore the granulite facies metamorphic zircon growth in the tropicana zone at 2718 2554 ma was prolonged compared to that in the yilgarn craton nonetheless the hf isotopic signature of the tropicana zone zircon shares strong similarity to that from the eastern goldfields superterrane of the yilgarn craton this implies that the tropicana zone reflects a deeper crustal level of the yilgarn craton exhumed and thrust nw an unknown distance over the craton edge in addition we observe that the granulite facies zircons have a less radiogenic hf isotope signature than the preserved pre metamorphic zircon cores based on correlations with alpha dose u and th content and 176hf 177hf we suggest this reflects the preferential destruction and release of unradiogenic hf from inherited zircon whereas the protolith sanukitoid zircon with lower u and th content was more resistant to mobilisation during high grade metamorphism we note this situation may be a more general response of the hf isotopic system in which zircon grown in a more mafic melt is less likely to contribute to the metamorphic hf reservoir than its felsic counterpart juvenile granitic veins dated at c 1780 ma intruded into the tropicana zone indicate that the tropicana zone was structurally emplaced at or before c 1780 ma given similar proterozoic magmatic events are well documented from para autochthonous adjacent units re os dating of pyrite coeval with one generation of gold in these rocks indicates model ages of c 2100 ma supportive of a palaeoproterozoic age of mineralisation this mineralisation event is distinct from major proterozoic tectonothermal events elsewhere in the afo sanukitoid magmas are well known for gold fertility and were likely the original source of gold in the tropicana zone which was subsequently concentrated into brittle structures during several episodes gold mineralisation post dated peak metamorphic conditions and is significantly younger than gold mineralisation within other parts of the adjoining yilgarn craton

ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites diffusive exchange as a guide to cooling history ion microprobe analysis of magnetites from the adirondack mountains ny yields oxygen isotope ratios with spatial resolution of 2 8 m and precision in the range of 1 1 sigma these analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of 18o at this scale high precision micro analyses of this sort will permit wide ranging new applications in stable isotope geochemistry the analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite magnetites are near to fe3o4 in composition and optically and chemically homogeneous a combination of ion probe plus conventional brf5 analysis shows that individual grains are homogeneous with 18o 8 9 1 smow from the core to near the rim of 0 1 1 2 mm diameter grains depth profiling into crystal growth faces of magnetites shows that rims are 9 depleted in 18o these low 18o values increase in smooth gradients across the outer 10 m of magnetite rims in contact with calcite these are the sharpest intracrystalline gradients measured to date in geological materials this discovery is confirmed by bulk analysis of 150 350 m diameter magnetites which average 1 2 lower in 18o than coarse magnetites due to low 18o rims conventional analysis of coexisting calcite yields 18o 18 19 suggesting that bulk 18o cc mt 9 3 and yielding an apparent equilibration temperature of 525 c over 200 c below the temperature of regional metamorphism consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories the data for oxygen in calcite under hydrothermal conditions at high p h2o indicates that diffusion is faster in magnetite and modelling of the low 18o rims on magnetite would suggest that the adirondacks experienced slow cooling after grenville metamorphism followed by a brief period of rapid cooling possibly related to uplift conversely the data for calcite at low p h2o show slower oxygen diffusion than in magnetite modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals suggesting that the low 18o rims form by exchange with late low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets in either case the ion microprobe results indicate that two distinct events are recorded in the post metamorphic exchange history of these magnetites recognition of these events is only possible through microanalysis and has important implications for geothermometry

influence of extratropical thermal and wind forcings on equatorial thermocline in an ocean gcm the equatorial thermocline variability in the pacific in response to the extratropical thermal and wind forcings is investigated with an ocean general circulation model modular ocean model version 3 mom3 sensitivity experiments show that the extratropical wind forcing and thermal forcing contribute equally to the equatorial variability the wind induced response is attributed to the off equatorial wind within 30 8 of the equator the thermal induced response can be traced to higher latitudes the thermal forcing affects the equator mainly through the equatorward transport of the perturbation temperature by mean subduction flow the wind forcing affects the equator by changing the strength of meridional overturning circulations it is also found that the southern hemisphere contributes more to the equatorial variability than the northern hemisphere under both external forcings

lateral ramps as an aid to the unroofing of deep crustal rocks seismic evidence from the grenville province we suggest that lateral ramps in the basement to orogenic wedges may be important agents in the rapid unroofing of deeply buried continental crust our suggestion is based on a study of the distribution of high pressure metamorphosed continental rocks in the mesoproterozoic grenvillian orogen in eastern quebec these rocks were buried to depths of more than 50 km through crustal thickening during a continental collision and were then rapidly unroofed they are currently exposed in a 100 km 60 km lobate embayment north of the manicouagan reservoir using a combination of surface structural data and seismic reflection data from lithoprobe line 55 we reconstruct the three dimensional form of the high pressure rocks our reconstruct ion methods are based on demonstrable correlations between the attitudes of gneissic layering at the surface and the attitudes of reflectors on the seismic profile they make extensive use of the fact that much of the seismic reflectivity on line 55 comes from outside the vertical plane of the seismic line because of the sinuous nature of the line and its obliquity to tectonic strike the validity of the reconstructions is demonstrated by the generation of a synthetic seismogram from the interpreted geometry that is very similar to the observed seismogram our reconstruction shows that a steep lateral ramp extends to the base of the crust immediately behind the broad lobe of high pressure rocks at the surface the lateral ramp probably provided a favorable pathway for the extrusion of deep seated rocks from the deep crust and their transport onto the foreland

development of microporosity diffusion channels and deuteric coarsening in perthitic alkali feldspars turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the klokken syenite using sem and tem it is caused by the presence of myriads of tubular micro inclusions either fluid filled micropores or sites of previous fluid inclusions and is associated with coarsening of microperthite and development of sub grains micropores are abundant in coarsened areas in which porosities may reach 4 5 but are almost absent from uncoarsened pristine braind microperthite areas the coarsening is patchy and involves a scale increase of up to 103 without change in the composition of the phases low albite and low microcline or in the bulk composition of the crystal it occurs abruptly along an irregular front within individual crystals which retain their original shapes the coherent braid microperthite gives way across the front to an irregular semi coherent film perthite over a few m and then to a highly coarsened irregular patch perthite containing numerous small sub grains on scales of a few hundred nm in both phases the coarsening and micropore formation occured at a t 400 450 c and it is inferred to have been driven by the release of coherent strain energy low angle grain boundary migration being favoured by a fluid the patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin possibly arising in part through exsolution of water from the feldspar the sub grain texture and microporosity modify profoundly the permeability of the rock and greatly enhance the subsequent reactivity of the feldspars

major crustal boundaries of australia and their significance in mineral systems targeting abstract for over 35 years deep seismic reflection profiles have been acquired routinely across australia to better understand the crustal architecture and geodynamic evolution of key geological provinces and basins major crustal scale breaks have been interpreted in some of the profiles and are often inferred to be relict sutures between different crustal blocks as well as sometimes being important conduits for mineralising fluids to reach the upper crust the widespread coverage of the seismic profiles now allows the construction of a new map of major crustal boundaries across australia which will better define the architecture of the crustal blocks in three dimensions it also enables a better understanding of how the australian continent was constructed from the mesoarchean through to the phanerozoic and how this evolution and these boundaries have controlled metallogenesis starting with the locations in 3d of the crustal breaks identified in the seismic profiles geological e g outcrop mapping drill hole geochronology isotope and geophysical e g gravity aeromagnetic magnetotelluric data are used to map the crustal boundaries in plan view away from the seismic profiles some of the boundaries mapped are subsurface boundaries and in many cases occur several kilometres below the surface hence they will not match directly with structures mapped at the surface for some of these boundaries a high level of confidence can be placed on the location whereas the location of other boundaries can only be considered to have medium or low confidence in other areas especially in regions covered by thick sedimentary successions the locations of some crustal boundaries are essentially unconstrained unless they have been imaged by a seismic profile from the mesoarchean to the phanerozoic the continent formed by the amalgamation of many smaller crustal blocks over a period of nearly 3 billion years the identification of crustal boundaries in australia and the construction of an australia wide gis dataset and map will help to constrain tectonic models and plate reconstructions for the geological evolution of australia and will provide constraints on the three dimensional architecture of australia deep crustal penetrating structures particularly major crustal boundaries are important conduits to transport mineralising fluids from the mantle and lower crust into the upper crust there are several greenfields regions across australia where deep crustal penetrating structures have been imaged in seismic sections and have potential as possible areas for future mineral systems exploration

a geological assessment of airborne electromagnetics for mineral exploration through deeply weathered profiles in the southeast yilgarn cratonic margin western australia abstract mineral exploration in regolith dominated environments is challenging requiring the development of new technical tools and approaches when airborne electromagnetics aem is combined with information on stratigraphy mineralogy geochemistry drilling and landscape observations in a geological context it becomes a powerful approach to describe the architecture of the regolith cover this has significant implications for mineral exploration in any regolith dominated terrain rdt this research presents two case studies of aem data integrated in a geological context for mineral exploration in the yilgarn craton margin albany fraser orogen afo in one of the study sites presented study site 1 neale tenement the availability of aem data allowed for lateral and vertical extrapolation of the information contained in datasets at specific locations thereby creating a 2d architectural model for the regolith cover in addition it was determined 1 the total thickness of the regolith cover and its variability between 2 m and 65 m 2 that low conductivity transported overburden and silcrete units with a total thickness between 5 and 45 m is widely distributed capping the upper saprolite and 3 that the silcrete unit varies laterally from being completely cemented to permeable and that these permeable areas windows coincide vertically with mineralogical textural moisture salt content changes in the underlying saprolite resulting in increased conductivity this has been interpreted as resulting from more intense vertical weathering and consequently a higher vertical geochemical dispersion of the basement signature towards surface aem has been used to assist in identifying and describing the lateral continuity of these windows in areas with no direct field observations surface geochemical sampling above these permeable areas may deliver more reliable geochemical basement signatures in the second study site silver lake tenement the aem data was strongly influenced by the high conductivity of the hypersaline groundwater this had a significant effect on the aem response resulting in reduced depth penetration and reduced resolution of subtle conductivity contrasts between cover units despite this the aem data set combined with geological observations in the area was able to map the presence and extent of a buried palaeochannel network the most significant architectural sedimentary feature in the cover this interpretation allowed for a more efficient drilling campaign to be designed to sample the fresh basement rock suites in the area by avoiding drilling into palaeochannels integrated and constrained by the geological context the application of aem conductivity models by geologists is envisioned as one of the most promising tools within the exploration geologist toolbox to understand the architecture of the cover

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

tropicana translated a foreland thrust system imbricate fan setting for c 2520 ma orogenic gold mineralization at the northern margin of the albany fraser orogen western australia

upper ocean thermal variations in the north pacific during 1970 1991 abstract a newly available extensive compilation of upper ocean temperature profiles was used to study the vertical structure of thermal anomalies between the surface and 400 m depth in the north pacific during 1970 1991 a prominent decade long perturbation in climate occurred during this time period surface waters cooled by 1 c in the central and western north pacific and warmed by about the same amount along the west coast of north america from late 1976 to 1988 comparison with data from coads suggests that the relatively sparse sampling of the subsurface data is adequate for describing the climate anomaly the vertical structure of seasonal thermal anomalies in the central north pacific shows a series of cold pulses beginning in the fall of 1976 and continuing until late 1988 that appear to originate at the surface and descend with time into the main thermocline to at least 400 m depth individual cold events descend rapidly 100 m yr 1 superimposed upon a slower cooling 15 m yr 1 the inter

formation of earth s early archaean continental crust abstract subduction of oceanic crust at an unusually low angle has been proposed as a model for the growth of continental crust older than about 2 5 ga at modern zones of low angle or flat subduction magmatic additions to new crust come from partial melting of both the subducting oceanic crust slab and the thin wedge of mantle above the slab evidence for both a slab and wedge source is preserved in most late archaean 3 0 2 5 ga terrains but we find little evidence that a mantle wedge contributed to crustal growth prior to 3 1 ga this lack of evidence in part reflects a dearth of exposed crust aged between 3 0 and 3 3 ga but also suggests that subduction enriched mantle source regions did not develop before 3 3 ga and possibly not before 3 1 ga in contrast to most modern terrains and some late archaean terrains early archaean 3 3 ga continental crust evolved through direct melting of thick mafic crust we invoke a process of subduction that does not include the development of a mantle wedge and call this process archaean flat subduction to distinguish it from modern low angle subduction

decadal thermocline variability in the north pacific ocean two pathways around the subtropical gyre yearly in situ temperature anomaly data in the north pacific ocean for 1961 90 have been analyzed along constant density surfaces isopycnals in order to better describe and understand decadal thermocline variability in the region various empirical orthogonal function analyses are performed on isopycnals to depict the dominant three dimensional patterns the major finding is of two preferential pathways associated with decadal temperature variability around the subtropical gyre a subduction pathway with a large signal in the upper thermocline originates from the north pacific central eastern outcrop regions about 408n 1508w and then basically follows the mean gyre circulation southwestward along isopycnals toward the western tropics a subtropical pathway extends from the eastern subtropical tropical and boundary regions and appears to continue predominantly westward across the southern part of the gyre between 158 and 308n and then along the kuroshio path toward the midlatitudes along these two pathways thermal anomalies show coherent phase relationships to one another in the surface layer and in the thermocline around the gyre with their source regions variability centers being out of phase on decadal timescales two examples of each type of anomaly pattern can be illustrated for the periods analyzed in the 1960s a negative temperature anomaly signal propagated predominantly westward across the subtropics followed by a subducted warm anomaly from the outcrop region in the early 1970s that subsequently moved southwestward along isopycnals toward the western tropics a similar pattern was observed in the late 1970s and in the 1980s but with the opposite sign a westward propagating positive temperature anomaly signal along the subtropics in the late 1970s through the 1980s and a subducted cold anomaly in the early 1980s that also made its way southwestward with the expected gyre circulation to the western tropics in the late 1980s it is suggested that the southwestward subduction pathway provides a mechanism that connects surface anomalies in the outcrop region to thermocline variations in the western subtropics and in the tropics and that the westward subtropical pathway presents a possible link of tropical subtropical variability to surface temperature anomalies around the kuroshio and its extension regions which may further force variations in the overlying atmospheric circulation in the midlatitudes the results provide an observational basis for verification of theoretical studies and model simulations

subduction of decadal north pacific temperature anomalies observations and dynamics abstract observations of oceanic temperature in the upper 400 m reveal decadal signals that propagate in the thermocline along lines of constant potential vorticity from the ventilation region in the central north pacific to approximately 18 n in the western pacific the propagation path and speed are well described by the geostrophic mean circulation and by a model of the ventilated thermocline the approximate southward speed of the thermal signal of 7 mm s 1 yields a transit time of approximately eight years the thermal anomalies appear to be forced by perturbations of the mixed layer heat budget in the subduction region of the central north pacific east of the date line a warm pulse was generated in the central north pacific by a series of mild winters from 1973 to 1976 and reached 18 n around 1982 after 1978 a succession of colder winters initiated a cold anomaly in the central north pacific that propagated along a similar path and with a similar speed as the warm anomaly then arrived in the west

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

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

thermocline forced by annual and decadal surface temperature variation abstract a two layer thermocline model is modified by adding an essentially passive mixed layer above it the surface temperature variation is simulated by a moving outcrop line it is found that in contrast to a surface wind stress a surface temperature variation causes strong variability in the ventilated zone through subducted water while it affects the shadow zone little two types of buoyancy forced solution are found when the outcrop line moves slowly the solutions are nonentrainment solutions for these solutions the surface beat flux is mainly balanced by the horizontal advection in the permanent thermocline the mixed layer never entrains the time mean thermocline is close to the steady thermocline with the time mean outcrop line when the outcrop line moves southward rapidly during the cooling season the solutions become entrainment solutions now deep vertical convection must occur because the horizontal advection in the permanent thermocline is no longer strong enough to balance the s

oxygen diffusion in magnetite the diffusion rate of oxygen in magnetite fe3o4 has been measured under hydrothermal conditions between 800 and 500 c at 1000 bars pressure and yields the arrhenius relation d 3 5 10 6 exp 45 000rt kcal g atom o the diffusion rate was found to be the same within the estimated uncertainty of a factor of 2 for measurements at 100 1000 and 2000 bars 20 200 mpa water pressure and for oxygen fugacities buffered by magnetite hematite and approximately ninio the activation energy and diffusion mechanism probably change to that given by castle and surman 10 at temperatures below 460 c the oxygen diffusion rate in magnetite is between those of quartz and hornblende closure temperatures fall in the range 380 c for 0 1 mm radius grains cooling at 1 c m y to 675 c for 1 mm radius and cooling at 1000 c m y if the present data are used but will be lower for the fine grain sizes and slow cooling rates if the castle and surman data are used

granulite hosted gold tectonic setting and lithogeochemistry of the tropicana deposit western australia the tropicana gold deposit 7 9 moz is hosted by the neoarchean amphibolite to granulite grade tropicana gneiss these rocks form part of the plumridge terrane a tectonic unit that separates the in situ eastern margin of the yilgarn craton from proterozoic elements of the albany fraser orogen western australia the gneissic host package to the orebody originally constituted a suite of medium to high k calc alkaline and adakitic basaltic to rhyolitic lavas emplaced in a relatively thick crustal continental margin arc amphibolitized mafic intrusions have back arc basin basalt like compositions consistent with their having formed during extension and incipient rifting of the arc gold mineralization occurs mainly within a sheet like syenitic gneiss that ranges compositionally from basaltic andesite to rhyolite and derived through metasomatism of compositionally equivalent lavas within an interpreted shear zone coarse perthite textures in microcline ruffle exsolution in brown biotite and occasional pyrite inclusions away from fractures in microcline porphyroblasts indicate that this shear zone predated peak metamorphism relative to the unmineralized host gneisses the mineralized syenitic gneisses show significant enrichments in s and the ore elements mo te tl ag au w and occasionally bi and sb but not as cu or zn in the k group elements and also in p light rare earth elements lree th u zr and hf depletions in most major elements si ti fe mg and ca as well as v cr ni and co are considered to reflect dissolution of relatively low volume mafic phases hornblende augite fe ti oxides and plagioclase in the protoliths some of the distinctive enrichments p lree zr hf are linked to volume loss in excess of 50 associated with dissolution of silica and concentration of insoluble apatite and zircon in the mineralized unit but the ore elements and strongly enriched similar to x10 th and u were added from the hydrothermal fluids involved exhumation of the tropicana gneiss into the upper crust was accompanied by fluid ingress and retrogression this produced mm to cm scale wavy biotite pyrite shears solution seams and disseminations throughout the syenitic gneiss which were further retrogressed to sericite chlorite assemblages in places both of which host most of the gold in the deposit thus although the present deportment of gold is unambiguously associated with the postpeak metamorphic greenschist grade retrogression at ca 2520 ma there is compelling evidence that the paucity of quartz and carbonate veining atypical element enrichments and sheet like nature of the tropicana deposit in part reflect metasomatism associated with a prepeak metamorphic relatively deep shear zone at present we have no direct evidence for the timing of introduction of gold and a protracted history of gold precipitation redistribution and enrichment is implied

evolution of subduction planetary waves with application to north pacific decadal thermocline variability abstract the evolution of decadal subduction temperature anomalies in the subtropical north pacific is studied using a simple and a complex ocean model it is found that the amplitude of the temperature anomaly decays faster than a passive tracer by about 30 50 the faster decay is caused by the divergence of group velocity of the subduction planetary wave which is contributed to significantly by the divergent sverdrup flow in the subtropical gyre the temperature anomaly also seems to propagate southward slower than the passive tracer or mean ventilation flow this occurs because the mean potential vorticity gradient in the ventilated zone is directed eastward the associated general beta effect produces a northward propagation for the temperature anomaly partially canceling the southward advection by the ventilation flow

thermocline forced by varying ekman pumping part ii annual and decadal ekman pumping abstract thermocline variability forced by zonally uniform ekman pumping with annual to decadal periods is investigated both analytical and numerical solutions are obtained by the method of characteristics as found in part i there is little thermocline variability in the ventilated zone or pool zone in contrast strong variability may exist in the shadow zone for annual forcings nonlinearity is negligible however the linear solution is influenced substantially by the basic state thermocline structure as a result local responses dominate for a shallow interface while remote rossby waves dominate for a deep interface under a strong decadal forcing nonlinearity may become important the time mean thermocline in the shadow zone is shallower than the steady thermocline under the mean ekman pumping particularly in the western part of a shadow zone where the mean deviation may reach the order often meters this shallower mean thermocline is caused by the nonlinear rossby wave

structural controls on gold mineralization on the margin of the yilgarn craton albany fraser orogen the tropicana deposit western australia the tropicana gold deposit is located adjacent to the margin of the yilgarn craton in the albany fraser orogen western australia the deposit is hosted in granulite facies quartzo feldspathic gneisses of the archean tropicana gneiss ore bodies comprise biotite pyrite alteration concentrated in shear zones that formed during ne sw shortening in the late archean and clearly postdate the formation and deformation of high grade gneiss fabrics d1 and d2 the orientation of the ore bodies is controlled by the shear zones that are in turn localised by the gneissic banding mineralization also involved solution and coeval microfracturing and veining of more competent pegmatitic units the mineralizing event d3 was followed by at least two further deformations which reactivated and overprinted the biotite fabrics with sericite and chlorite created new shear zones and affected gold distribution d5 consisted of dextral shear on e w shear zones which subdivide the deposit into five major structural domains the importance of structurally controlled permeability at tropicana is similar in cratonic lode gold deposits as is the protracted deformation fluid flow history like renco mine in zimbabwe tropicana gold deposit was formed by hydrothermal fluid flow peripheral to the craton economic gold mineralization was clearly post peak metamorphism

reequilibration processes in magnetite from iron skarn deposits textural and compositional data for magnetite from nine iron skarn deposits in canada romania and china show that most samples have reequilibrated by dissolution and reprecipitation oxy exsolution and or recrystallization the dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined whereas the oxy exsolution occurs only in high ti magnetite forming exsolution lamellae of fe ti al oxides electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite notably si mg ca al mn and ti whereas oxy exsolution is effective in decreasing the ti content of high ti magnetite many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations bif iron oxide copper gold iocg or porphyry cu fields using the ti v versus ca al mn discrimination diagram this pattern suggests that trace element data for magnetite that has unusual composition and or reequilibrated cannot be reliably used as a petrogenetic indicator mixing of externally derived saline fluids with fe rich magmatic hydrothermal solutions an increase in temperature and local decreasing pressure and f o 2 are considered the most important causes for the dissolution and reprecipitation or recrystallization of the magnetite increasing f o 2 and decreasing temperature may facilitate oxy exsolution of fe ti al oxides in high ti magnetite results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits