structural controls on sulfide mobilization at the high grade flying fox ni cu pge sulfide deposit forrestania greenstone belt western australia the flying fox komatiite hosted ni deposit is located in the forrestania greenstone belt of the yilgarn craton in western australia the host volcanosedimentary succession has been metamorphosed to amphibolite facies and deformation has sheared and dismembered the orebody creating 11 distinct ore shoots the ore shoots are hosted at the base of a komatiite sequence structurally overlying deformed metasedimentary rocks the ni sulfide orebody has undergone six phases of deformation 1 d 1areg e w compression resulted in tilting of the stratigraphy 2 d 1bff continued e w compression resulted in coaxial flattening along the footwall sedimentary rock sulfide komatiite contact and the basalt hanging wall sedimentary rock contact 3 d 2ff ene wsw compression resulted in noncoaxial shearing of the d 1bff structures 4 d 3ff ese wnw compression resulted in shallow angle thrusts 5 d 4ff during e w extension normal ductile brittle faults formed by the pooling and dilation of granitic magma along the f 2 structural contact between the footwall sedimentary and komatiite rocks and 6 d 5ff intrusion of a proterozoic dolerite dike during n s extension accompanied by late stage brittle normal faulting during deformation the ni sulfides reverted to monosulfide solid solution mss and were mechanically relocated along the footwall sedimentary rock komatiite contact offset into the footwall sedimentary rocks and entrained in the granitic magma creating atypical granite hosted sulfides the ni sulfides were mobilized up to 5 m away from the ultramafic rocks during the d 1 and d 2 events however subsequent dismemberment of the original orebody into separate ore shoots along flat lying faults during the d 3 event led to offsets of up to 300 m the detailed structural analysis of the flying fox deposit has shown that restoration of the lithostratigraphy along known faults can be used to define the volcanic stratigraphy including primary ni ore horizons this information can be used as a guide to the location of concealed extensions to known ore shoots or completely unknown orebodies at depth or along strike

the characteristics and origin of the big lake mafic ultramafic hosted volcanogenic massive sulfide occurrence marathon ontario canada the big lake volcanogenic massive sulfide vms occurrence located in the schreiber hemlo belt in the wawa subprovince of the superior province is hosted in an archean mafic ultramafic metavolcanic sequence lacking felsic volcanic or volcaniclastic rocks discovered in 2006 the occurrence is a subeconomic assemblage of pyrrhotite chalcopyrite sphalerite cobaltite and galena currently defined over an area of 0 5 0 5 km in plan view the mineralized zone is less than 10 m thick with the highest grade intersection containing 4 0 m of 7 5 cu 2 2 zn 138 0 gpt ag and 9 2 gpt au mineralization is hosted in metavolcanic rocks of basaltic to komatiitic basalt compositions the sulfides and spatially associated hydrothermal alteration regionally metamorphosed to chlorite actinolite hornblende biotite phlogopite and anthophyllite are located 5 to 50 m structurally below a thick ultramafic cumulate complex the host sequence to the occurrence is overturned with vms sulfides stratigraphically overlying the ultramafic cumulates rocks stratigraphically below the vms mineralization are transitional to alkaline basalts and komatiitic basalts with convex upward lree and fractionated hree when normalized to primitive mantle basalts stratigraphically overlying the vms mineralization have flat normalized ree patterns la ybcn 1 2 typical of tholeiitic plateau basalts found elsewhere in the wawa belt the lithostratigraphic assemblage at big lake is consistent with an oceanic plateau setting and is interpreted to have formed by eruption of plateau basalts and thick ultramafic flows from a mantle plume unlike vms systems that formed above subvolcanic intrusions 0 5 km below sea floor the big lake vms occurrence may have formed as a result of cooling of the stratigraphically underlying ultramafic cumulates and demonstrates that similar styles of ponded flow driven vms systems albeit small ones may be preserved elsewhere in the superior province or in other archean cratons

late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

influence of changing atmospheric circulation on precipitation δ18o temperature relations in canada during the holocene postglacial precipitation 18o history has been reconstructed for two regions of canada long term shifts in the oxygen isotope composition of annual precipitation 18op in southern ontario appear to have occurred with a consistent isotope temperature relation throughout the past 11 50014c yr the modern isotope temperature relation in central canada near present boreal treeline evidently became established between 5000 and 4000 years ago although the relation during the last glacial maximum and deglaciation may also have been similar to present in the early holocene however unusually high 18opapparently persisted in spite of low temperature locally probably associated with high zonal index a rudimentary sensitivity analysis suggests that a small reduction in distillation of moisture in pacific air masses traversing the western cordillera perhaps accompanied by a higher summer winter precipitation ratio could have been responsible for the observed effect equivalent isotope temperature anomalies apparently occurred elsewhere in western north america in response to changing early holocene atmospheric circulation patterns suggesting that a time slice map of 18opfor north america during this period might provide a useful target for testing and validation of atmospheric general circulation model simulations using isotopic water tracers

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

a 350014c yr high resolution record of water level changes in lake titicaca bolivia peru abstract sediment cores collected from the southern basin of lake titicaca bolivia peru on a transect from 4 6 m above overflow level to 15 1 m below overflow level are used to identify a new century scale chronology of holocene lake level variations the results indicate that lithologic and geochemical analyses on a transect of cores can be used to identify and date century scale lake level changes detailed sedimentary analyses of subfacies and radiocarbon dating were conducted on four representative cores a chronology based on 60 accelerator mass spectrometer radiocarbon measurements constrains the timing of water level fluctuations two methods were used to estimate the14c reservoir age both indicate that it has remained nearly constant at 25014c yr during the late holocene core studies based on lithology and geochemistry establish the timing and magnitude of five periods of low lake level implying negative moisture balance for the northern andean altiplano over the last 3500 cal yr between 3500 and 3350 cal yr b p a transition from massive inorganic clay facies to laminated organic matter rich silts in each of the four cores signals a water level rise after a prolonged mid holocene dry phase evidence of other significant low lake levels occurs 2900 2800 2400 2200 2000 1700 and 900 500 cal yr b p several of the low lake levels coincided with cultural changes in the region including the collapse of the tiwanaku civilization

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

mid and late holocene limnogeology of laguna del negro francisco northern chile and its palaeoclimatic implications lacustrine sediments from the high elevation endorheic saline lake negro francisco 27 28 s 69 14 w 4125 m provide a detailed mid and late holocene environmental history of the southern chilean altiplano mineralogy chemical composition and sedimentary facies analyses of the deposits confirm the regional significance of mid holocene aridity between 6000 and 3800 bp precipitation rates were signifi cantly lower than the c 250 mm yr 1 of today fully arid conditions were interrupted by short term moist spells with an average return period of about 200 years effective moisture increased after 3800 bp and peaked between 3000 and 2600 bp and between 2200 and 1800 bp in two phases with conditions more humid than today comparison with regional palaeodata suggest that these two late holocene humid spells were most likely related to pacific moisture sources salinity levels increased again in laguna del negro francisco after 1800 bp the onset of modern climatic conditions was a stepwise nonlinear proce

the nickel sulfide mineralization and metamorphic setting of the forrestania area western australia the forrestania group of nickel sulfide occurrences of the archean southern cross province is localized in intrusive dunite bodies at the base of a lower komatiite unit which is interlayered with tholeiitic mafic rocks para amphibolites quartz biotite schists chemical sedimentary rocks iron formation and sulfide cherts and komatiites concordant lenticular metamor phosed sill like mineralized dunites have komatiitic affinities and appear to be feeder chambers to overlying thick sequences of dominantly extrusive ultramafic komatiites and rare pyroclastic komatiites unmineralized portions of the dunites are extremely rich in magnesium with mgo values ranging between 45 and 51 percent the important nickel sulfide occurrences are pentlandite rich disseminations distributed in the basal portions of metamorphosed intrusive dunites e g cosmic boy and digger rocks south digger rocks similar mineralization also occurs in hanging wall and internal portions of dunites but is generally of less importance than the basal mineralization because of lower nickel tenor massive pyrrhotite rich sulfides occur within reaction zones between dunite and footwall country rocks and as remobilized veins and stringers in footwall metasediments this type of mineralization forms important small tonnage high grade deposits at new morning and flying fox supergene alteration of primary sulfides is extensive and massive violarite pyrite magnetite zones are developed in weathering profiles polyphase deformation and metamorphism with peak conditions estimated at 655 degrees or 30 degrees c and 4 0 or 1 0 kb resulted in widespread recrystallization and tectonic remobilization of ductile sulfides cumulate dunites were metamorphosed to bladed granular and granoblastic aggregates of forsterite talc forsterite talc anthophyllite and forsterite talc enstatite formed under varying f sub co 2 and f sub h 2 o conditions retrograde serpentinization and talc carbonate alteration followed by later supergene alteration has caused textural modification and oxidation of primary sulfides with development of pyrite violarite and magnetite rich ores nickel values and ni cu ratios vary for different types of mineralization the economically important basal disseminations of pentlandite pyrite pyrrhotite chalcopyrite have ni values in the range 2 to 5 percent and constant ni cu ratios in the range 14 1 to 22 1 bulk composition of primary sulfides of this type at cosmic boy is s rich with fe 39 4 percent ni co 18 6 percent and s 42 0 percent massive remobilized veins of pyrrhotite pentlandite pyrite chalcopyrite have high ni 3 to 8 and erratic ni cu ratios generally in the range from 10 1 to 200 1 with a mean of 45 1 this mineralization is fe rich with fe 50 4 percent ni co 7 4 percent and s 42 2 percent broad irregular sparse disseminations less than 2 of pentlandite heazlewood chalcopyrite form in the internal portions of cumulate and metamorphosed dunites they have low ni 0 3 to 0 5 high ni cu ratio 50 1 to 100 1 and bulk composition close to pentlandite composition with fe 31 8 percent ni co 34 2 percent and s 34 0 percent the bulk compositions of sulfides generally plot on the s rich side of the monosulfide solid solution field at 650 degrees c and this appears to reflect nickel and sulfur mobility during retrograde metamorphism

a calculated petrogenetic grid for ultramafic rocks in the system cao feo mgo al2o3 sio2 co2 h2o at low pressures calculated phase equilibria among the minerals amphibole chlorite clinopyroxene orthopyroxene olivine dolomite magnesite serpentine brucite calcite quartz and fluid are presented for the system cao feo mgo al2o3 sio2 co2 h2o caf masch with chlorite and h2o co2 fluid in excess and for a temperature range of 440 c 600 c and low pressures the minerals chosen in cafmasch represent the great majority of phases encountered in metamorphosed ultramafic rocks the changes in mineral compositions in terms of femg 1 and mg fe sial 1al 1 are related to variations in the intensive parameters for example equilibria at high x text co text 2 in the presence of chlorite involve minerals which are relatively aluminous compared with those at low x text co text 2 the calculated invariant univariant and divariant equilibria are compared with naturally occurring greenschist and amphibolite facies ultramafic mineral assemblages the correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good

holocene environments from stable isotope stratigraphy of ostracods and authigenic carbonate in chilean altiplano lakes abstract sediment cores from three shallow 4000 m a s l paleoclimatic profiles show coherency within tropical subtropical circulation patterns climatic signals however are partially masked by local environmental systems results suggest similar arid mid holocene to less arid late holocene trends in the three lakes although each lake shows distinct response characteristics linked to differences in regional moisture balance isotopic signatures for each lake group in discrete 18o vs 13c populations high isotopic variability results from episodic flooding and brine evolution in the shallow lakes laguna seca 18 11 s 69 14 w 4500 m a s l is characterized by a diverse ostracod assemblage of at least 8 species indicating highest moisture levels among the three sites high 13c values are the result of continuous evasion of co2 derived from volcanically charged groundwaters and springs plus the formation of travertine and photosynthetic activity in the lake a 3 decrease in 18o about 4 7 m suggests an abrupt shift to wetter conditions at the transition from mid to late holocene lagunas miscanti 23 44 s 67 46 w 4140 m a s l and del negro francisco 27 28 s 69 14 w 4125 m a s l each contain only one ostracod species limnocythere sappaensis attesting to higher salinities although sediment facies and ostracod continuity and abundances imply a history of higher effective moisture at laguna del negro francisco sediment cores from each lake document more arid conditions in the lower sections with a sharp shift to wetter conditions in the upper sections highly variable isotope values at the transition from mid to late holocene suggest rapid short term climate shifts during this transition this overall trend can be correlated from laguna seca laguna miscanti and laguna del negro francisco to lago titicaca implying climatic coherency along the altiplano isotopic profiles show second order fluctuations and drought events that suggest short term variability along the transect in order to explain differences in the effective moisture among the three sites we postulate interactions of regional air masses in the north convective tropical precipitation dominates laguna seca as for lakes in the titicaca region effective moisture for lagunas del negro francisco and miscanti follows a south to north gradient generated by cold air masses from cut offs of pacific westerlies colliding with moist tropical air masses coherent shifts in the past therefore imply changes in available tropical air mass moisture

geochemistry of ostracode calcite part 2 the effects of water chemistry and seasonal temperature variation on candona rawsoni abstract reconstruction of lake paleochemistry from the isotope and trace element composition of fossil ostracodes requires an understanding of geochemical variations induced by the seasonality of shell formation monthly field collections of live candona rawsoni from two hyposaline lakes in the eastern dakotas reveal large variations in 18 o values 2 5 o and mg ca and sr ca ratios up to 35 among individual adults within the same assemblage because the measured variations in water chemistry were small the amplitude of ostracode 18 o and mg ca variations must result from seasonal temperature variation ostracode sr ca ratios which should be temperature independent shows strong positive covariance with shell mg ca these results imply that sr uptake in ostracode calcite increases with the mg content of the shell the partitioning coefficient for mg on the other hand appears to decreases at high values of mg ca in the host water the geochemical variability in the field collections indicates that c rawsoni populations are composed of multiple overlapping generations that attain maturity at different times many juveniles apparently undergo several molts in mid summer and persist as penultimate instars a 1 into the following spring geochemical analysis of late instar juvenile c rawsoni from lacustrine sedimentary records is likely to provide information on mid summer conditions large numbers of fossil ostracodes should be analyzed from each stratigraphic interval to eliminate seasonal noise from the paleochemistry records of temperate region lakes

ice core pollen record of climatic changes in the central andes during the last 400 yr abstract this paper presents a high resolution ice core pollen record from the sajama ice cap bolivia that spans the last 400 yr the pollen record corroborates the oxygen isotopic and ice accumulation records from the quelccaya ice cap and supports the scenario that the little ice age lia consisted of two distinct phases a wet period from ad 1500 to 1700 and a dry period from ad 1700 to 1880 during the dry period xerophytic shrubs expanded to replace puna grasses on the altiplano as suggested by a dramatic drop in the poaceae asteraceae p a pollen ratio the environment around sajama was probably similar to the desert like shrublands of the southern bolivian highlands and western andean slopes today the striking similarity between the sajama and quelccaya proxy records suggests that climatic changes during the little ice age occurred synchronously across the altiplano

climate variation and the rise and fall of an andean civilization paleolimnological and archaeological records that span 3500 years from lake titicaca and the surrounding bolivian peruvian altiplano demonstrate that the emergence of agriculture ca 1500 b c and the collapse of the tiwanaku civilization ca a d 1100 coincided with periods of abrupt profound climate change the timing and magnitude of climate changes are inferred from stratigraphic evidence of lake level variation recorded in14c dated lake sediment cores paleo lake levels provide estimates of drainage basin water balance archaeological evidence establishes spatial and temporal patterns of agricultural field use and abandonment prior to 1500 b c aridity in the altiplano precluded intensive agriculture during a wetter period from 1500 b c to a d 1100 the tiwanaku civilization and its immediate predecessors developed specialized agricultural methods that stimulated population growth and sustained large human settlements a prolonged drier period ca a d 1100 1400 caused declining agricultural production field abandonment and cultural collapse

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

a 15 000 year isotopic record from lake zurich of deglaciation and climatic change in switzerland abstract ostracod and pelecypod carbonate preserved since the latest pleistocene in the bottom sediments of lake zurich was probably formed under constant temperature conditions temporal shifts in the oxygen isotype ratios for those carbonates therefore correspond to shifts in the isotopic character of the net lake inflow alpine deglaciation which released isotopically light meltwaters commenced prior to ca 15 000 yr b p and was essentially completed in northern switzerland by 12 4000 250 yr b p well before substantially increased atmospheric heat was available from either increased insolation or an increased atmospheric co 2 content the early holocene climate for the region was characterized by minor short term fluctuations in mean annual air temperatures and an overall amelioration of up to 2 c contemporaneously precipitated authigenic and biogenic carbonates have recorded partitioning of carbon isotopes between surface and bottom waters during photosynthetic production and subsequent oxidation of organic matter in the lake the degree of that partitioning provides an index of the relative production rates which were low in the late pleistocene meltwater lake but considerably enhanced during the holocene

palaeoclimates in central and south america since 18 000 bp based on pollen and lake level records abstract the last 18 ka of palaeoclimates are discussed at 3 ka intervals based on palaeoenvironmental changes shown in pollen and lake level records from central and south america a total of 61 pollen records fulfill the requirements of continuity and chronology to be used in this synthesis 8 records from tropical and subtropical lowlands in central and south america 7 records from the central american highlands 26 records from the high andes north and south of the equator 2 records from the southern subtropics and 18 from the southern andes and subantarctic islands palaeoclimatic interpretation of the pollen records is semiquantitative based on bioclimatic parameters such as e g elevational temperature gradients or latitudinal precipitation gradients that can be related to vegetation zonation additional palaeoclimatic information is obtained from lake level records derived either from dated shorelines or from the limnological history of lake sediment sections large scale palaeoclimatic comparison of the proxy data records indicates that full and late glacial climates throughout central and south america were cooler than today by about 4 to 5 c but that moisture patterns showed regional differentiation thus in both the southern subtropics and high latitudes in southern chile conditions were substantially moister than today during full and late glacial times whereas everywhere else conditions were far drier than today in terms of palaeocirculation this scenario can be interpreted as a poleward shift of the subtropical as well as the westerly circulation a scenario that has been suggested by palaeoclimatic simulations from general circulation models gcm another major palaeoclimatic aspect predicted by the gcm simulations is the suppressed early holocene summer monsoon in the southern subtropics although proxy records are few from this zone their palaeoenvironmental history during that time shows a period of aridity that would support the model one of the most controversial aspects of south american palaeoclimates concerns the existence of a younger dryas equivalent a cold period between 11 and 10 ka bp based on the existing data from south america that show a great diversity of palaeoclimatic trends the concept of an interval that is characterized by the same climatic signal as documented for the north atlantic seems too simplistic moreover at this point only very few records have been analysed in sufficient detail to yield unambiguous climatic information for the interval in question the principal outcome of this palaeoenvironmental and palaeoclimatic synthesis is the better understanding of the type and quality of data available at present and the realization of regions and periods where much more and concerted effort is needed to address problems of intra and interhemispheric palaeoclimatic correlations only then can questions concerning the mechanism of climatic change be tackled successfully

the perseverance ultramafic complex western australia the product of a komatiite lava river the perseverance ultramafic complex is a body of olivine rich komatiitic rocks spatially associated with the agnew nickel deposit in the agnew wiluna greenstone belt of the archaean yilgarn block in western australia the complex consists of a central lenticular body up to 700 m thick of olivine adcumulates flanked by laterally extensive sheet like bodies of olivine orthocumulates and spinifextextured komatiite flows rocks progressively further away from the central lens have chemical compositions reflecting higher original proportions of komatiite liquid to cumulus olivine parent liquids had mgo contents between 25 and 32 mgo approximately chondritic al ti ratios and hree patterns and moderate depletion in lree olivines within the adcumulate lens show a progressive increase in forsterite content from fo93 at the bottom to fo94 5 at the top calculated original olivine compositions in ihe flanking rocks are similar to those at the base of the central lens original olivine nickel contents show a symmetrical variation from maximum values of 3500 ppm at the top of the central lens through minimum values of 1000 ppm at the base and margins of the central lens to intermediate values in the distal rocks the complex as a whole shows evidence for nickel depletion relative to other komatiite suites these observations are explained in terms of prolonged eruption and flow of komatiitic lava down a major flow channel or lava river adcumulates crystallized on the floor and sides of the central channel which was formed at an early stage by thermal erosion of floor rocks episodic overflow of the central channel produced distal flood plain rocks consisting of olivine orthocumulates and layered flows lavas became more magnesian and nickel rich with time giving rise to the observed spatial variation in primary olivine composition nickel depletion of the earliest lavas is attributed to pre eruption segregation of large volumes of immiscible fe ni sulfide which were concentrated to form the underlying agnew nickel deposit

a three dimensional numerical investigation of a carolina coastal front and the gulf stream rainband abstract a three dimensional mesoscale planetary boundary layer pbl numerical model is used to investigate mesoscale circulations over the carolina coastal and gulf stream baroclinic zones idealized ambient onshore and offshore flows are investigated which represent the synoptic conditions during the intensive observation period 2 iop 2 of the 1986 genesis of atlantic lows experiment gale for the easterly onshore flow a confluence zone appears west of the gulf stream in response to the effect of the oceanic baroclinicity the confluence zone is nearly parallel to the coastline and the sst isotherms with northeasterly southwesterly flow to the west east a shallow coastal front forms below 2 km as the cyclonic shear of the ageostrophic flow becomes strong quasi stationary rainbands are produced by cumulus convection along the coastal front the northern part of the front and the rainbands later encroach inland as the cold air intensity over ground weakens due to onshore warm air advection

climate in the western cordillera of the central andes over the last 4300 years abstract the distichia peat core obtained in the carhuasanta valley near nevado mismi cordillera chila provides information on climatic and environmental conditions over the last 4300 years the relative changes in the stable carbon isotope composition of plant remains preserved in the core reflect major temperature fluctuations in the western cordillera of the southern peruvian andes these temperature variations can be additionally linked with the changes in precipitation patterns by analysing c and c n ratio in the core relatively warm and moist conditions prevailed from 4280 to 3040 cal yrs bp bc 2330 1090 with a short colder dry episode around 3850 cal yrs bp bc 1900 the most prominent climate changes recorded in the peat occurred between 3040 and 2750 cal yrs bp bc 1090 800 when the initial warming turned to a rapid cooling to temperatures at least 2 c lower than the mean for the late holocene initially drier conditions within this event turned to a short wet phase after 2780 cal yrs bp bc 830 when the temperature increased again this event coincides with significant changes in peat and ice core records in the central andes matching the timing of the global climate event around 2 8 cal ka bp climatic conditions in the study area became relatively dry and stable after the event for about 800 years highly variable temperatures and humidity prevailed during the last 2000 years when an extended warm and relatively humid period occurred between 640 and 155 cal yrs bp ad 1310 1795 followed by predominantly colder and drier conditions the established 13 c peat record represents the first continuous proxy for the temperature in the southern peruvian andes dated by the ams 14 c distichia peat is wide spread in the andes and the proposed approach can be applied elsewhere in high altitudes where no other traditional climate proxies are available

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

experimental deformation of a synthetic dunite at high temperature and pressure i mechanical behavior optical microstructure and deformation mechanism abstract we have performed a series of 27 deformation experiments on a very dry synthetic dunite using the griggs solid medium apparatus strain rates ranged from 1 10 4 to 1 10 7 sec 1 temperatures varied from 1100 to 1300 c and the confining pressure was maintained at 10 or more usually 15 kbar macor im a commercially available anhydrous machinable glass ceramic with a low melting point was used in many of the experiments we find that strength measurements performed using macor as a confining medium are similar to results obtained using sodium chloride as the confining solid tests performed in undried sample assemblies using air dried dunite specimens resulted in creep strengths greater than those found in any earlier studies except those of post 1973 1977 a single test on a sample for which both sample and assembly were dried at a temperature sufficiently high to drive off any adsorbed water resulted in a creep strength comparable to that determined by post 1973 1977 for very dry mt burnett dunite despite our experimental difficulties we are led to believe that our synthetic dunite exhibits mechanical behavior consistent with that determined in other experimental studies in which natural dunites and peridotites were used furthermore we conclude that dry dunite is very probably as strong as the oft disputed results of post earlier indicated finally we show that the optical textures of our highly recrystallized experimental specimens are essentially identical to naturally produced porphyroclastic textures and that the deformation mechanism in both the experimental and natural specimens is probably dislocation creep with recovery by dynamic recrystallization

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

stability and composition relations of calcic amphiboles in ultramafic rocks calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions experiments were performed on the stability of tremolite plus forsterite in the system h2o cao mgo sio2 from 5 to 20 kbar a univariant curve was fitted to the experimental brackets using volume water fugacity and heat capacity data the results indicate that the maximum stability of tremolite in the presence of forsterite is about 825 c at 5 kbar addition of al2o3 to this system increases the stability of tremolitic amphibole by only 20 40 c and induces solubility of 5 7 wt al2o3 in the amphibole as determined from quantitative sem analyses of individual amphibole crystals thus substitution of the tschermakite component ca2 mg3al2 si6al2 o22 oh 2 alone cannot lead to the greatly enhanced al2o3 contents or thermal stability of natural calcic amphiboles

synthetic and natural tremolite in equilibrium with forsterite enstatite diopside and fluid the following equilibrium among tremolite forsterite diopside and orthorhombic enstatite has been investigated using either synthetic tremolite or natural amphibole in the starting materials ca2mg5si8o22 oh 2 mg2sio4 2 camgsi2o6 5mgsio3 h2o a significant increase in the stability of the reactants was observed with natural rather than synthetic tremolite for example in nearly pure h2o with the h2 content of the fluid buffered by nickel bunsenite at one kilobar 108 pascals the breakdown of the assemblage with synthetic amphibole occurs at 708 20 c the breakdown of the assemblage with natural amphibole ca2 16mg4 94fe0 03si7 92 al0 01o22 oh 2f0 03 occurs at 841 47 c the shift in the breakdown curve is attributed to variation in the properties of the amphiboles since all other factors were common in the experiments the reactions have also been investigated with hydrogen fugacity defined by the methane buffer and the nb oh xg coh buffer analysis of the experimental data by linear programming indicates that the enthalpy of reaction is tightly constrained when the calorimetrically determined entropy of 160 92 joules degree is used the resulting enthalpy of reaction is 113 96 1 82 kilojoules with the natural amphibole and 104 83 0 12 kilojoules with synthetic tremolite deviation of the natural amphibole from the ideal tremolite formula as well as a greater number of defects and dislocations in the synthetic amphibole may have contributed to the change in stability

high resolution seismic reflection profiles from lake titicaca peru bolivia evidence for holocene aridity in the tropical andes high resolution seismic reflection profiles of the sediments of lake titicaca peru bolivia suggest that lake levels in the recent past were considerably lower than today incised channels on the major deltas extend to depths of 85 m below modern lake level erosional truncation of onlapping seismic reflectors is found at similar depths this interpretation of the seismic data is supported by analyses of sediment cores from the lake which indicate that there was a significant drop in lake level during the early to mid holocene

lake highstands on the altiplano tropical andes contemporaneous with heinrich 1 and the younger dryas new insights from 14 c u th dating and δ 18 o of carbonates this study provides new geochronological and stable isotope constraints on late pleistocene fluctuations in lake level that occurred in the closed watershed of the central altiplano between 25 and 12 ka u series isochrons and 14 c ages from carbonates are used to confirm and refine the previous chronology published placzek et al 2006b our new data support three successive lake highstands during the late pleistocene i the lake sajsi cycle from 25 to 19 ka that culminated at 3670 m at about 22 ka almost synchronously with the global last glacial maximum ii the lake tauca cycle that lasted from 18 to 14 5 ka and was characterized by the highest water level reached at least 3770 m from 16 5 to 15 ka iii the lake coipasa cycle from 12 5 to 11 9 ka that reached an elevation of 3700 m 42 m above the elevation of the salar de uyuni 3658 m these high amplitude lake level fluctuations are in phase with the cold warm oscillations that occurred in the north atlantic and greenland during the late pleistocene heinrich 1 bolling allerod younger dryas such temporal coincidence supports the hypothesis that wet events recorded in the central altiplano are controlled by the northesouth displacement of the inter tropical convergence zone resulting from changes in the meridional temperature gradient finally the oxygen isotope ratios measured in these lacustrine carbonates allows for calculation of the 18 o value of paleolake waters estimates of water 18 o v smow are 2 8 0 7 for lake tauca and 1 6 0 9 for lake coipasa these data are used to constrain changes in lake hydrology and can be interpreted to indicate that the proportion of precipitation arising from local water recycling was less than 50

the little ice age as recorded in the stratigraphy of the tropical quelccaya ice cap the analyses of two ice cores from a southern tropical ice cap provide a record of climatic conditions over 1000 years for a region where other proxy records are nearly absent annual variations in visible dust layers oxygen isotopes microparticle concentrations conductivity and identification of the historical a d 1600 huaynaputina ash permit accurate dating and time scale verification the fact that the little ice age about a d 1500 to 1900 stands out as a significant climatic event in the oxygen isotope and electrical conductivity records confirms the worldwide character of this event

the holocene paleolimnology of lake issyk kul kyrgyzstan trace element and stable isotope composition of ostracodes the stable isotope and trace element geochemistry of ostracode shells from lake issyk kul kyrgyzstan provide an important new paleoclimate record for central asia the n 18 o n 13 c sr ca mg ca and u ca composition of shells from the species candona neglecta found in two piston cores constrains the paleohydrology of the lake from 8700 to 1000 calendar yr before present these data combined with robust chronologies based on 16 ams 14 c dates suggest that the region went from a relatively moist climate in the early holocene to a more arid climate in the late holocene intense freshwater input presumably glacial runoff flowed into the lake from 8700 to 8300 cal yr bp as evident from the n 18 o and n 13 c values from 8300 to 6900 cal yr bp n 18 o and sr ca values indicate that the lake was relatively fresh and an open basin and u ca values suggest that the lake was well mixed with well oxygenated bottom waters there is a transition zone between 6900 and 4900 cal yr bp when n 18 o and sr ca values rapidly increase and mg ca and u ca values rapidly decrease during this period the lake evolved from an open freshwater well mixed lake to a closed more saline less well mixed lake for the remainder of the record n 18 o sr ca mg ca and u ca are relatively constant implying conditions similar to modern conditions although the slight increasing trend in sr ca may suggest an increase in salinity over this period the n 13 c values were approximately in equilibrium with atmospheric co2 in the early part of the record but became relatively negative in the later part of the record this indicates that the dominant mechanism controlling the isotopic composition of dissolved inorganic carbon shifted from exchange with the atmosphere to variations in biological productivity fl 2001 elsevier science b v all rights reserved

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

partial melting and recrystallization of archeaan komatiites by residual heat from rapidly accumulated flows spinifex textured komatiites at honeymoon well western australia show evidence of partial melting and recrystallization of original igneous textures their textures and mineral compositions differ markedly from those typical of komatiites spinifex olivine plates are bent and broken while interstitial space between spinifex and cumulus olivine is occupied by polygonal aggregates of clinopyroxene orthopyroxene minor olivine and plagioclase similar granular pyroxene plagioclase aggregates occur as diffuse veins cutting spinifex zones and cumulate zones of the flows and in places form the matrix to a breccia containing corroded fragments of spinifex rock thermometry based on the two pyroxene assemblages yields temperatures of 1055 to 1141 c just below the low pressure komatiite solidus mineral compositions are different from those of typical komatiites clinopyroxenes are al poor and cr rich olivines are unusually iron rich and depleted in cr and ca and the low ca pyroxene is bronzite rather than the more typical pigeonite we interpret these observations as the results of thermal metamorphism partial remelting and subsequent slow crystallization of originally normal spinifex textured komatiite flows the rocks in question occupy a 40 70 m interval sandwiched between two olivine rich units an underlying 90 m thick olivine adcumulate layer forming part of the cumulate zone of a basal 160 m thick flow and an overlying 1 km thick extrusive body composed mostly of olivine mesocumulate and adcumulate and capped in turn by spinifex textured flows thermal modelling shows that a sinusoidal temperature profile of cool flow tops and hot flow centres would exist within this sequence shortly after eruption conductive thermal relaxation of this profile could reheat spinifex zones to the extent of inducing partial melting and textural reconstitution such reheating is largely dependent on the time interval between the emplacement of successive flows calculations suggest that at honeymoon well the emplacement interval must have been of the order of 10 years or less textural reconstitution may have contributed to the development of the thick orthocumulate sequences characteristic of komatiites in the agnew wiluna belt

late quaternary vegetational change in the central peruvian andes abstract new fossil pollen records from the central peruvian andes at elevations above 4000 m including pollen concentration data and additional radiocarbon dates provide information on late quaternary vegetation change for the region a pollen assemblage typical of superpuna plus low pollen concentrations and organics in the late glacial section of laguna tuctua indicate a cold dry climate but strong evidence for a late glacial climate reversal i e the younger dryas is not presently confirmed an abrupt increase in urticales pollen as compositae and polylepis acaena type pollen decline signals the beginning of the holocene subsequent increases in urticales and plantago rigida type pollen point to increased moisture as well as higher temperatures from about 11 000 to 7000 yr b p a possible correlation exists between the rapid expansion of puna and closed continuous forest and the rise in importance of camelids in the early holocene in the mid holocene plantago rigida cushion bogs and continuous montane forest elements decline suggesting a drier climate grasses and cheno ams then increase reaching maxima about 4000 yr b p pastoralism and agriculture are thought to have developed at this time according to ethnobotanical studies from this area and pollen evidence seems to support these findings between 3000 and 2000 yr b p pollen percentages of poaceae plantago rigida type and other local taxa decline as glacial activity was renewed in the higher mountains zea mays is present by 2000 yr b p and pollen of weeds and secondary successional plants associated with increasing agricultural disturbance are evident

isovolumetric silicification of early archean komatiites geochemical mass balances and constraints on origin stratiform quartz phyllosilicate carbonate alteration zones are a common feature of archean greenstone terranes and determining their origin is an important aspect of understanding the nature of shallow crustal alteration during the early history of the earth komatiites and komatiitic basalts of the uppermost onverwacht group ca 3 4 3 5 ga barberton greenstone belt south africa were regionally metasomatized prior to major tectonic deformation to the assemblage quartz dioctahedral mica chlorite pm dolomite subsequent metamorphism of these rocks has been restricted to greenschist facies and they thus record processes that may be obscured in analogous sequences from more highly metamorphosed terranes the isovolumetric nature of much of the alteration and the relative immobility of elements such as al zr and y permit a quantitative assessment of elemental gains and losses during metasomatism for each m 3 of komatiite altered approximately 10 3 kg one ton of mgo were removed

isotope hydrology of groundwaters in the pampa del tamarugal chile the water resources in northern chile are extremely scarce and development is limited by this this paper discusses the isotope hydrology of groundwaters in the pampa del tamarugal an area of extreme aridity rainfall 1 mm yr whose groundwater resources are mined for the town of iquique as well as for industrial and agricultural purposes the aim of the project was to obtain information on modern recharge and to delineate if possible recharge environments to obtain the necessary background information a precipitation survey in the high andes as well as spring and surface water studies were carried out the results show that a well defined meteoric water line exists where 2h 7 8 18o 10 3 and altitude effects depend on air mass movements and cannot be defined without a broader regional and detailed sampling programme however it is still possible to assign maximum altitudes of recharge to springs in the andes and at the eastern border of the pampa del tamarugal comparison of these data with groundwater compositions show that these groundwaters originate from infiltrating surface water rather than directly infiltrated precipitation a dependence of individual groundwater systems on specific quebradas river valleys is recognized however low 14c activities indicate that most of the waters pumped today are fossil and at least several hundreds if not thousands of years old some minor subsurface recharge does occur at the foot of the andes especially at pica where high altitude waters discharge but even there groundwater appears to be a diminishing resource

the cold event 8200 years ago documented in oxygen isotope records of precipitation in europe and greenland stable oxygen isotope ratios of ostracod valves in late glacial and holocene sediments of core as 92 5 from deep lake ammersee southern germany reflect variations of mean oxygen isotope ratios in past atmospheric precipitation the record reconfirms the strong similarity of climate evolution in europe and greenland during the last deglaciation for the first time in europe we find a 200 year long negative 18o excursion which is contemporaneous with the strongest negative 18o excursion in the greenland ice around 8 2 ky before present the 8 2 ky isotopic event on both sides of the north atlantic ocean is interpreted as a cold period most probably induced by a perturbation of the north atlantic thermohaline circulation we discuss two possible triggering mechanisms 1 weak forcing as proposed by alley et al and 2 forcing by a strong and sudden freshwater pulse from the collapse of the hudson ice dome

late quaternary spatio temporal limnological variations in the altiplano of bolivia and peru abstract between the western and eastern andean cordilleras in peru and bolivia there are three main lacustrine basins lake titicaca lake poopo and the group of coipasa uyuni for the past few millennia highly variable environmental conditions have been recorded in their sediments today a latitudinal meteorological gradient influences the lakes status leading to specific deposits and ostracod communities lake titicaca in the north is oligohaline whereas lake poopo further south is polyhaline in the south the coipasa uyuni depression is characterized by a 12 000 km2 surficial salt crust during the late pleistocene ca 40 000 to 25 000 yr b p the water depth and salinity in paleolake poopo fluctuated widely and paleolake titicaca was slightly larger than at present sedimentation was mostly biocarbonate in the shallower areas and it was detrital organic in the deepest zones during the holocene a dry period transformed lake poopo into a salar with evaporite precipitation lake titicaca registered a large decline in water level 8100 3600 yr b p initially inducing gypsum precipitation followed by short influxes of water with an ostracod faunal composition similar to that of the modern brines of lake poopo lake titicacas present condition only appeared between 2200 and 1500 yr b p

archaean carbon reservoirs and their relevance to the fluid source for gold deposits it is commonly assumed1 2 that seawater derived carbon in altered volcanic rocks is the only major pre metamorphic carbon in carbonate reservoir in archaean greenstone belts thus carbonates from archaean gold deposits with stable isotope ratios too negative median 13c 3 to have been derived from such carbon 13c usually assumed to be near 0 have been interpreted as being derived from a local felsic magmatic source1 barley and groves3 however recognize two carbonate alteration styles that predate regional metamorphism and gold mineralization in the norseman wiluna greenstone belt of western australia evidence is summarized below these are seafloor alteration and fault controlled regional alteration which as shown below have completely different carbon isotope compositions the latter median 13c 4 8 implies a major juvenile carbon co2 flux from the mantle during greenstone belt evolution and is thus of fundamental importance in itself however here we show why the mere existence of two carbon in carbonate reservoirs and variation in carbon isotope ratios of carbonates between individual gold deposits in one area negate some of the fundamental assumptions used to support magmatic fluid models1

paleohydrology of the laguna lejia north chilean altiplano and climatic implications for late glacial times paleoenvironmental and sedimentological data from laguna lejia 23 30 s 67 42 w 4325 m in the high altiplano of the chilean atacama desert indicate that climatic conditions during late glacial times were significantly wetter than today a water and energy budget model was used to simulate climatic scenarios that would have resulted in the observed changes in the water levels water surfaces water volumes and salinity during the time interval different climatic scenarios for the laguna lejia catchment include precipitation increases of over 100 up to 400 500 mm yr today 200 mm yr to account for the late glacial lake levels with shorelines 15 25 m higher than at present and a lake surface of 9 11 km2 compared to the present 2 km2 during the lake maximum about 13 500 11 300 yr b p finely laminated sediments consisting of mg calcite diatoms and sometimes gypsum were deposited the paleoenvironmental evidence indicates that north easterly wind direction prevailed and the summer rainfall resulted from a seasonal poleward shift of the tropical circulation the lake history of laguna lejia is representative for the altiplano area between 21 and 24 s

holocene paleohydrology of the tropical andes from lake records two century scale time series in northern bolivia constrain the ages of abrupt changes in the physical geochemical and biological characteristics of sediments obtained from lakes that formed during deglaciation from the late pleistocene glacial maximum the watersheds of laguna viscachani 16 12 s 68 07 w 3780 m and lago taypi chaka kkota 16 13 s 68 21 w 4300 m located on the eastern and western slopes of the cordillera real respectively contain small cirque glaciers a high resolution chronology of the lake sediments is provided by 23 ams14c dates of discrete macrofossils late pleistocene glaciers retreated rapidly exposing the lake basins between 10 700 and 970014c yr b p the sedimentary facies suggest that after 890014c yr b p glaciers were absent from the watersheds and remained so during the middle holocene an increase in the precipitation evaporation balance is indicated above unconformities dated to 230014c yr b p in both lago taypi chaka kkota and laguna viscachani an abrupt increase in sediment accumulation rates after 140014c yr b p signals the onset of neoglaciation a possible link exists between the observed millennial scale shifts in the regional precipitation evaporation balance and seasonal shifts in tropical insolation

palaeoenvironments and palaeohydrology of a tropical closed lake lake asal djibouti since 10 000 yr b p in eastern africa republic of djibouti the hypersaline lake asal occupies an exceptional geological topographical and hydrological situation at 155 m below sea level and at 12 km from the sea it lies in the youngest and the deepest basin in a chain of closed depressions of central afar decreasing in elevation from west to east and represents the hydrogeological low of the whole system presently infiltration of marine water represents the major water input to the lake due to high evaporation conditions halite precipitates halocene water level fluctuations 315 m sedimentary conditions from fresh oligosaline waters to heavy brines and palaeohydrology are reconstructed field observations are supported by 85 14c dates on inorganic carbonates mollusc shells stromatolites and hydrothermal travertines stable isotope analyses on carbonates and sulphates mineralogy geochemistry and studies of biological remains discussion of the water balance for the major stages is based on reasonable assumptions about holocene conditions of precipitation and evaporation after an arid terminal pleistocene phase marked by coarse deposits due to exceptional floods the holocene cycle begins with complex strontium rich carbonate phases aragonite high and low mg calcites and dolomite these carbonates could reflect the first underground marine supply and waters concentrated through evaporation a high lake level ca 160 m a s l from ca 8600 to 6000 yr b p corresponds to the deposition of high mg calcite precipitated in a deep fresh water lake lake asal was draining the awash river system ethiopian plateau and escarpment through groundwater circulations the terminal basin of the system was probably the present day marine gulf of ghoubbat al kharab hydrologically separated from the open ocean by a sill at 4 m below sea level possibly due to fracture opening a dramatic regression in the lake level from 160 to 150 m occurred between ca 6000 and 5000 yr b p the inputs of continental water were consequently overwhelmed by marine infiltrations the carbonate phase exhibits clear resemblance with that of the transgressive episode from 5000 yr b p lake asal experienced minor water level fluctuations aragonite rich carbonates precipitated followed by calcium sulphates and halite stable isotopes of gypsum reflect its marine origin and various diagenetic processes the holocene evolution of asal which clearly differs from that of the neighbouring closed basins is due to both its specific geological and hydrological setting and to the regional climatic changes

primary stratigraphic controls on ore mineral assemblages in the wannaway komatiite hosted nickel sulfide deposit kambalda camp western australia abstract the 18 m long uwa 04 02 drillcore from the fe ni cu pge wannaway deposit in the widiemooltha dome district eastern goldfields western australia intersects the whole sequence of a komatiite hosted layer of metal rich sulfide magma in spite of regional deformation and amphibolite facies metamorphism the sequence in the drillcore still preserves some of the original magmatic textures and assemblages and these were examined in a great detail the magmatic orebody typically consists of basal massive sulfides grading to net textured matrix and disseminated sulfide mineralization upward into the komatiite host the ore zone is underlined by sulfide rich black shale passing to basalts country rock xenoliths are locally enclosed in the massive sulfides portions of the drillcore untouched by penetrative deformation and with minimal imprint by late stage serpentinization allow the construction of a fairly complex framework where mineral assemblages and mineral chemistry of sulfides spinels and silicates vary systematically with stratigraphy and may reflect original conditions of ore deposition the general ore assemblage is dominated by fe sulfide and pentlandite with minor sphalerite and chalcopyrite spinels zn rich chromite ti magnetite alabandite mns accessory pge rich sulfarsenides and tellurides and rare molybdenite monoclinic and high s hexagonal pyrrhotite and fresh zn mn rich chromite characterize the basal massive facies whereas the matrix ore facies is marked by magnetite sphalerite and a gradually s depleted and reduced assemblage now represented by troilite exsolving low s hexagonal pyrrhotite and alabandite compositional modifications of the fe sulfides across the whole orebody and occurrence of alabandite testify to progressive sulfur loss concomitant with the establishment of low fo2 conditions over several meters upsequence in the matrix ore facies pge rich sulfarsenides disseminated across the whole mineralized sequence display igneous textures and pge fractionation trends the composition of olivine intergrown with matrix sulfides and in the serpentinized hangingwall komatiite deviates from the typical unmetamorphosed komatiite related highly forsteritic type however the fe mn and zn contents of olivine crystals decrease systematically and gradually with distance from mineralization towards the hangingwall komatiite contamination may be an alternative to metamorphic recrystallization of olivine as the cause of these trends the role of contamination is also shown by the trends of whole rock data from the mineralized sequence across the entire drillcore textures and mineral chemistry of minerals from the different rock facies in the drillcore are evaluated in terms of metamorphic effects although the remarkable relationship observed between stratigraphy and several major and accessory phases over metric distances is suggestive of alternative options including primary processes involving the komatiitic lava flow in its interaction both with the black shale substrate and with the sulfide melt ponding at its base

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

a 200 year mid european air temperature record preserved in lake sediments an extension of the δ18op air temperature relation into the past abstract a detailed history of the water oxygen isotope ratios of lake ammersee southern germany was established for the last two centuries by analyzing oxygen isotope ratios of ostracod shells from a sediment core in a three to five year resolution empirical analysis and modelling of the isotope hydrology of the lake show that the oxygen isotopic composition of the lake water is mainly controlled by the isotopic composition of local precipitation for the last 200 years the lake isotope record is strongly correlated to the air temperature record from hohenpeisenberg situated within the lake s catchment area thus the mid european relation between 18 o in precipitation and air temperature documented for the last 20 years can be extended over a longer time and a larger temperature range the results provide further support for the validity of ostracod 18 o from ammersee sediments as a quantitative proxy for mean annual air temperature probably extending back to the last glaciation

the role of fluids in the metamorphism of komatiites agnew nickel deposit western australia the agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy olivine tremolite chlorite cummingtonite enstatite assemblages occur at the margin of the ultramafic lens giving way to olivine anthophyllite olivine talc and olivine antigorite assemblages successively inwards these rocks are interpreted as having crystallized from komatiitic lavas and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates the relative modal abundances of metamorphic olivine tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith peak metamorphic conditions are estimated at 550 c based on garnet biotite thermometry at a maximum pressure of 3 kb this temperature falls within the narrow range over which metamorphic olivine may co exist with enstatite anthophyllite talc or antigorite depending upon the fugacity of water in the metamorphic fluid the observed mineralogical zonation is therefore attributed to infiltration by co2 rich fluids generated by decarbonation of talc carbonate rocks formed during pre metamorphic marginal alteration of the ultramafic lens metamorphic fluids were essentially binary mixtures of water and co2 with minor h2s having a maximum partial pressure less than 1 percent of total pressure enstatite bearing assemblages formed in the presence of co2 rich fluids at fluid rock volume ratios close to one while anthophyllite talc and antigorite bearing assemblages formed in the presence of progressively more water rich fluids at progressively lower fluid rock ratios

yakabindie revisited volcanological and structural controls on the komatiite hosted six mile well and goliath north deposits and implications for the architecture of the 2 7 ga rift event in the agnew wiluna belt yilgarn craton western australia the yakabindie area hosts significant deposits of disseminated fe ni cu sulfide in the six mile well 2 0 mt contained ni and goliath north 0 6 mt contained ni dunite lenses these dunite lenses form part of the same komatiitic ultramafic unit that hosts the giant mount keith mkd5 deposit 2 6 mt contained ni to the north but the morphology and internal stratigraphy of the host unit differs significantly three dimensional modeling of the yakabindie region reveals that each dunite lens is located at the intersection between steeply dipping nnw and nne trending synvolcanic faults which appear to have acted as conduits from which komatiitic lava vented onto the seafloor the igneous contacts dip more shallowly to the west and describe a w facing wedge of olivine cumulate rock probably less than 1 km in plunge extent and cored by adcumulate rock the mineralization at both six mile well and goliath north consists of multiple stacked lenses of disseminated fe ni cu sulfide associated with the olivine adcumulate core this core is interpreted as the pathway position that focused the flow of komatiite lava away from the vent rather than being linked by sheeted bodies of orthocumulate rocks as is typically the case along strike to the north the dunite lenses at yakabindie appear to be capped by a strike continuous sequence of thin komatiite flow units there is no evidence of the fractionated sequences of pyroxenite and dolerite which occur in pathway positions within the mount keith ultramafic unit to the north and which reflect late stage ponding and in situ fractionation of komatiite lava three sets of synvolcanic structures are recognized in the broader agnew wiluna belt and a model of oblique extension at ca 2 7 ga is proposed a nnw trending set of structures marks the overall rift orientation at ca 2 7 ga but is probably inherited from an earlier ca 2 81 ga phase of rifting a nw trending set is thought to represent ca 2 7 ga transfer faults but may also have much older roots these structures produce marked sinistral offsets in both the greenstone stratigraphy and the gravity response and can be used to divide the agnew wiluna belt into rift segments the third set of early structures trends nne subparallel with the volcanic grain of the greenstones as inferred from the orientation of komatiite lava pathways these are thought to have originated as extension orthogonal normal faults at ca 2 7 ga at the regional scale there is a strong spatial association between the larger fe ni cu sulfide deposits of the agnew wiluna belt and the locus of postulated nw trending transfer zones whereas at the deposit scale intersecting nw and nne trending synvolcanic faults exert a control on the location and best development in terms of thickness and or grade of fe ni cu sulfide mineralization in some deposits these spatial associations have major implications for exploration targeting at both regional and deposit scale

southern westerlies during the holocene deduced from geomorphological and pedological studies in the norte chico northern chile 27 33 s abstract in the norte chico precipitation is controlled by atmospheric perturbations and frontal systems associated with the belt of westerly wind circulation the holocene climatic history of the region can be reconstructed from geomorphic and pedologic investigations in comparison to modern climatic conditions increased influence of the westerlies is deduced for the time periods prior to 7300 yr b p between 5000 3700 yr b p 3000 1800 yr b p and around 270 yr b p it remains uncertain whether the increased influence of the westerlies was the consequence of northward shifts of the westerlies as a whole of a meridional expansion of the westerly belt or only of intensity changes of the atmospheric dynamics almost synchronous with changes in the westerlies the coastal fogs in the region were apparently reduced or the fog belt in the coastal range was elevated probably due to a weakened upwelling and warmer water conditions in the eastern pacific in general the climate of the norte chico has become more variable since 5000 yr b p especially since 3000 yr b p with frequent changes of the geoecological conditions

chromite in komatiites ii modification during greenschist to mid amphibolite facies metamorphism chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes particularly above 500 c metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors as a result of morphism of host rocks this modification is the subject mg fe exchange with silicates and carbonates chromite metaof this paper morphosed to amphibolite facies is enriched in zn and fe and metamorphic modification of chromite has been disdepleted in ni relative to lower metamorphic grades relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions cr al and fe are not ophiolitic or alpine ultramafic complexes onyeagocha greatly modified by metamorphism up to lower amphibolite facies 1974 ulmer 1974 evans frost 1975 hoffman although minor fe depletion occurs during talc carbonate alwalker 1978 loferski lipin 1983 kimball 1990 teration at low temperature significant al is lost from chromite burkhard 1993 and in a few studies of komatiitic rocks cores above 550 c as a result of equilibration with fluids in bliss maclean 1975 donaldson 1983 gole hill equilibrium with chlorite elevated zn content in chromite is 1990 and in a detailed study of the pechenga intrusions restricted to rocks with low metamorphic mg fe ratios and is abzalov 1998 these studies have highlighted two the result of introduction of zn during low temperature alteration important effects first chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or fermetamorphism cobalt and mn also behave similarly except where ritchromit second chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage progressively modified during prograde metamorphism chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite this is the result of incomplete metamorphic reaction silicate minerals evans frost 1975 abzalov 1998 between chromite and chlorite bearing silicate assemblages magthis paper examines the nature and magnitude of these netite compositions at the inner chromite magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade eastern goldfields province of the archaean yilgarn block in western australia fig 1 brief locality descriptions are given in appendix b

mid holocene paleoenvironments in northwestern argentina main patterns and discrepancies abstract this paper summarizes the available paleoenvironmental information for the mid holocene in northwestern argentina in order to systematize a multi scale scenario for human adaptation in the area the main results of the studies carried out by several research teams that have studied the conditions during this period in the tropical andean region are described and compared taking into account their geographical setting the proxies involved and their particular space time resolution the paleoenvironmental situation prior to the onset of mid holocene conditions i e starting at the late glacial maximum is summarized to estimate the nature and extent of the changes occurred during this period paleoenvironmental results their characteristics and interpretation are also considered a general trend towards an aridization process that fostered a hydrologic stress process is traceable along the mid holocene in northwestern argentina and the tropical andes however several localities seem to have retained wetter conditions increasing the contrast between highly productive environments and the generally dry landscape this paper is only a first step towards a full understanding of the diversity and complexity of the mid holocene in different space time scales but a necessary one in order to start to model the resources structure available for human groups in the past in the andes

palaeohydrology of laguna de tagua tagua 34 30 s and moisture fluctuations in central chile for the last 46 000 yr central chile 32 35 s lies at the northern border of the strong influence of the westerly circulation belt and thus exhibits a steep rainfall gradient a new core from laguna de tagua tagua 34 30 0 s provides a sedimentologic geochemical and palynological record of regional hydrologic balance for the last 46 000 cal yr bp according to our age model relatively humid conditions occurred during glacial times before 43 500 cal yr bp and from 40 000 to 21 500 cal yr bp reduced moisture conditions and likely lower temperatures occurred from 42 400 40 100 cal yr bp higher lake levels and pollen assemblages with valdivian rainforest taxa imply much higher precipitation during glacial times 40 100 21 000 cal yr bp compared to today and therefore enhanced westerly activity in northern central chile afterwards the general decrease in moisture was punctuated by two abrupt arid periods at 21 000 19 500 cal yr bp and 17 000 15 000 cal yr bp and two more humid intervals 19 500 17 000 almost coincident with the global last glacial maximum lgm and 13 500 11 500 cal yr bp the early and mid holocene were the most arid periods in central chile for the studied time interval millennial scale palaeohydrological reconstructions from tagua tagua are consistent with regional climatic records copyright 2005 john wiley sons ltd

carbonate and organic matter sedimentation and isotopic signatures in lake chungara chilean altiplano during the last 12 3 kyr abstract sediments in lakes in the andean volcanic setting are often made up of diatomaceous ooze together with volcaniclastics and small amounts of carbonates despite their scarcity carbonates along with organic matter provide significant paleoenvironmental information about lake systems this study focuses on the carbonates in lake chungara their morphologies distribution and origin deduced from the isotopic markers these markers reflected changes in the water and the biomass between the onset of the holocene and around 9 6 cal kyr bp these changes are marked by general increases in toc tn and tn 15nair and by fluctuating values of toc 13cvpdb in its sediments and are probably related to major shifts in the lake surface volume associated with rises in lake level an increase in salinity around 10 cal kyr bp is thought to be linked to a short dry period giving rise to the onset of carbonate production the mid holocene arid period between 7 3 and 3 5 cal ka bp with a maximum of aridity around 6 0 cal kyr bp was deduced from 18ovpdb values in the endogenic carbonates these results match the reconstructions in lake titicaca based on benthic diatoms and paleoshore levels offshore sediments mainly consist of a diatomaceous ooze laminated in the lower half unit 1 and banded massive with tephra layers in the upper half of the sequence unit 2 toc 13cvpdb and the c n ratio confirm that phytoplankton was the main source of organic matter in these sediments shallower sediments units 3 to 5 developed in platform and littoral settings providing evidence of subaqueous macrophytes and to a lesser extent land plants carbonate content ranges between 0 1 and 6 wt in offshore settings 30 to 40 m water depth and reaches the maximum values in the lower part of unit 2 carbonate minerals low magnesium calcite and minor amounts of high magnesium calcite and aragonite are scarce and are arranged in mm thick layers commonly forming cm thick levels or bioclasts carbonate layers are made up of euhedral to subhedral spindle shaped calcite crystals and to a lesser extent aragonite needles all in the m range aragonite spheroids coexist in littoral sediments with other carbonate shapes and charophyte remains where carbonate reaches locally up to 20 wt co2 photosynthetic depletions related to seasonal phytoplankton blooms were responsible for the high frequency deposition of mm thick carbonate layers the average values for 13cvpdb in lake water plankton and sediments of lake chungara as organic matter or as carbonate are around 15 higher than commonly reported values in other lakes this 13c enrichment is attributed to carbon assimilation from a dic affected by methanogenesis in which hco3 is the dominant species the 13cvpdb and 18ovpdb variations and their covariation in endogenic carbonates suggest that lake water volume and lake level increased along the holocene

vegetation invasions into absolute desert a 45 th000 yr rodent midden record from the calama salar de atacama basins northern chile lat 22 24 s plant macrofossils percentage abundance of grass taxa fecal pellet d13 c and plant cuticle contents from 49 fossil rodent middens dated by 14 c record changes in local vegetation and precipitation since 45 ka calibrated or measured to thousands of calendar years before present in the central atacama desert lat 228 248s of northern chile the midden sites are along the hyperarid upper margin 2400 3100 m of the absolute desert in an extreme environment sparsely vegetated by annual herbs and halophytic shrubs conditions between 40 and 22 ka may have been at least intermittently dry and possibly cooler as implied by four middens with low species richness we infer a large increase in summer rainfall between 16 2 and 10 5 ka on the basis of the lowering of steppe grasses by as much as 1000 m prominence of c4 grasses and summer annuals high species richness and displacement of northern species at least 50 km south of their modern ranges the precipitation increase was greatest for a cluster of middens between 11 8 and 10 5 ka abrupt drying evident in a dramatic decrease in grass abundance occurred after 10 5 ka at all four midden localities increased percentages of

low holocene level 7700 to 3650 years ago of lake titicaca bolivia abstract preliminary results of sedimentological and paleontological analysis of two cores taken in lake titicaca bolivia indicate that between 7700 and 3650 yr b p the lake level was up to 50 m lower than at present evaporitic deposits occurred at the maximum low lake level

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

constraining the potential temperature of the archaean mantle a review of the evidence from komatiites the maximum potential temperature of the archaean mantle is poorly known and is best constrained by the mgo contents of komatiitic liquids which are directly related to eruptive temperatures however most archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid relatively fresh lavas from the saskmar suite belingwe greenstone belt zimbabwe 2 7 ga include chills of 25 6 wt mgo and olivines ranging to fo93 6 implying eruption at around 1520 c a chill sample from alexo township ontario also 2 7 ga is 28 wt mgo and associated olivines range to fo94 1 implying eruption at 1560 c however inferences of erupted liquids containing 32 33 wt mgo from lavas in the barberton greenstone belt south africa 3 45 ga and from the perseverance complex western australia 2 7 ga may be challenged on the grounds that they contain excess cumulate olivine or were enriched in mg during alteration or metamorphism re interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt mgo corresponding to an eruption temperature of 1580 c this composition is the highest liquid mgo content of an erupted lava that can be supported with any confidence the hottest modern magma on gorgona island 0 155 ga contained 18 20 mgo and erupted at circa 1400 c if 1580 c is taken as the temperature of the most magnesian known eruption then the source mantle from which the liquids rose would have been at up to 2200 c at pressures of 18 gpa corresponding to a mantle potential temperature of 1900 c these temperatures are in excess of the mantle temperatures predicted by secular cooling models and thus komatiites can only be formed in hot rising convective jets in the mantle this result requires that archaean mantle jets may have been 300 c hotter than the archaean ambient mantle temperature this temperature difference is similar to the 200 300 c temperature difference between present day jets and ambient mantle temperatures an important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions

primary and diagenetic carbonates in the anoxic sediments of lake bosumtwi ghana cores of organic rich muds from the tropical meteorite crater lake bosumtwi ghana contain laminae of authigenic calcite polyhedra and aragonite needles as well as scattered diagenetic calcite mg calcite spherulites and aggregates of dolomite crystals their respective origins are traced by analyses of oxygen and carbon stable isotopes anoxic dolomite and mg calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis carbon dioxide reduction must have been the dominant methanogenic pathway producing diagenetic carbonates with delta sup 13 c values up to 27 pdb this example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to mg calcite to aragonite to dolomite merely in terms of mg sup 2 ca sup 2 in lake waters it provides a model for the interpretation of ancient lacustrine carbonates from organic rich environments

isotopic evolution of river water in the northern chile region two main northern chilean rivers the loa and tarapaca were investigated regarding their isotopic characteristics groundwater associated with various recharge zones and the input of tributaries along their courses mainly control their 180 2h isotopic composition of the rivers in the loa river carbon isotopic exchange between dissolved inorganic carbon dic and atmospheric coplays a minor role in the inorganic carbon cycle of the loa river the carbon isotopic composition 4 c 13 c of this river is probably controlled by carbon source associated with volcanic and geothermal activities and by the deposition of travertines for the tarapaca river the carbon isotope content of the dic reflects the input of recirculated water from irrigated areas along the river course and carbon isotopic exchange these findings imply that the input of volcanic and or geothermal co2 into the dic pool has to be evaluated in order to use carbon isotopes as a dating tool for groundwater in the loa basin and that modern conditions are not analogous to the paleohydrology of the tarapaca river

petrology and geochemistry of metamorphosed ultramafic bodies in a portion of the blue ridge of north carolina and virginia precambrian ashe formation no correlation between mineral assemblages and metamorphic zones modal analyses and quantitative chemical analyses for mg al ca fe ni mn cu co k and zn lead to the recognition of two distinct types of metamorphosed ultramafic bodies based on the degree of metasomatic alteration a retrograde hydration of the ultramafic protolith produced the tremolite chlorite talc anthophyllite assemblage with relict olivine in a second stage serpentine replaced some of the tremolite and olivine a later partial dehydration led to the more strongly foliated tremolite chlorite talc schist or phyllonite modified journal abstract

experimental study of carbonate formation in oceanic peridotite abstract interactions of co2 rich aqueous fluids with mantle peridotite have major implications for geochemical budgets and microbial life in the shallow oceanic lithosphere through the formation of carbonate minerals and reduced carbon species however the underlying mechanisms controlling the transformation of co2 to carbonates in ultramafic hosted hydrothermal systems remain incompletely understood a long term laboratory experiment was conducted at 300 c and 35 mpa to investigate serpentinization and carbonate formation pathways during hydrothermal alteration of peridotite powdered harzburgite was initially reacted with a ca rich aqueous fluid for 14 592 h 608 days and changes in fluid composition were monitored with time once the system reached a steady state a co2 aq rich fluid was injected and allowed to react with the system for 5907 h 246 days fluid speciation and mineral analyses suggest that serpentinization of harzburgite in the co2 poor system led to the precipitation of serpentine brucite magnetite and minor calcite in addition to other minor phases including chlorite and sulfur poor ni sulfides the addition of the co2 aq rich fluid caused dolomite ca rich dolomite and high mg calcite to form at the expense of olivine calcite and brucite while serpentine remained unreactive replacement textures and mineral assemblages mimic those documented in carbonate altered seafloor serpentinites particularly those from the mid atlantic ridge and the iberia margin in contrast to thermodynamic predictions magnesite did not form in the experiment because the dissolution of clinopyroxene in combination with the lack of serpentine reactivity maintained low mg ca ratios in solution clinopyroxene dissolution and unreactive serpentine may similarly maintain low mg ca ratios in submarine serpentinization systems and limit magnesite formation in subseafloor environments results of this study suggest that the formation of ca mg carbonates by mineral carbonation is favorable in subseafloor serpentinization systems and likely represents a significant but poorly quantified carbon sink in hydrothermally altered oceanic lithosphere at slow spreading mid ocean ridges

geochemistry of komatiites from forrestania southern cross province western australia evidence for crustal contamination the 2 9 ga old forrestania greenstone belt is intermediate in character between typical 3 0 ga and 2 7 ga belts in containing both al depleted and al undepleted komatiites the extensive nickel exploration database on the belt provides a unique opportunity to examine geochemical variations in the context of volcanological evolution within each of the six komatiitic units in the greenstone belt there is a transition from komatiites 18 28 wt mgo in the liquid to high and low magnesium komatiitic basalts 5 18 wt mgo with stratigraphic height and much of the chemical variance within the komatiitic suite can be explained in terms of the addition or fractionation of olivine there are however aspects of the incompatible element signatures which must be attributed to other factors the more primitive komatiites from the lower ultramafic units are al depleted al2o3 tio2 10 5 13 0 and depleted in both light and heavy ree lasmn 0 5 gdybn 1 5 komatiites from the uppermost ultramafic units are variably less depleted in al al2o3tio2 16 5 20 5 this is interpreted to reflect decreasing retention of garnet in the source region of the forrestania komatiites over time and possibly a decrease in the depth of melting within the al depleted suite there are also anomalous enrichments in sio2 feo tio2 and zr which correlate with regional variations in the composition of substrate metasedimentary rocks banded iron formation versus felsic tuff coupled with field evidence for trough structures at the base of lava channel sequences these chemical anomalies provide strong evidence for supracrustal thermal erosion by komatiite lavas the forrestania data indicate that 2 9 ga old komatiites were generated over a range of melting conditions spanning those typical of older and younger archaean sequences and that contamination by supracrustal thermal erosion can influence komatiite chemistry on a regional scale

palynological evidence for increased aridity on the central chilean coast during the holocene abstract the late quaternary vegetation of the semiarid coast of central chile is inferred from the palynological analysis of profiles from quereo 31 55 s and quintero 32 47 s prior to 11 400 yr b p wet conditions are suggested by the abundance of pollen indicators of swamp and aquatic taxa such as cyperaceae and myriophyllum and by the presence of traces of arboreal pollen since ca 10 000 yr b p a trend toward increasingly drier conditions is implied by the almost complete absence of arboreal and aquatic taxa and a general decrease in the diversity of the semiarid shrubland indicators from 3000 yr b p onward the pollen records show the reappearance of swamp and aquatic taxa presumably associated with wetter conditions which led to recolonization by forest taxa at 1720 yr b p in quintero the drier climate detected along the semiarid coast of central chile during most of the holocene extended inland to the andean foot hills within the present mediterranean type climate zone of chile and also affected the distribution of the winter deciduous nothofagus forests and the northern boundary of the temperate rain forests

a late glacial holocene pollen record from the eastern andes of northern peru abstract a pollen and sediment record of a core 4 2 m long from laguna baja 7 42 s 77 32 w 3575 m in the cordillera oriental of northern peru suggests several episodes of major vegetational and climatic change over the past 13 000 yr the oldest pollen assemblage consists of a mixture of paramo elements tropical alpine vegetation including high percentages of poaceae 40 that decline upward moist montane forest compositae and polylepis and wet montane forest e g hedyosmum and podocarpaceae organic carbon content range from 60 with high percentages of jamesonia a fern characteristic of paramo and decreasing values of plantago tubulosa and the wet montane forest elements hedyosmum and podocarpaceae charcoal percentages are at a maximum during this period magnetic susceptibility and sand percentages are high and percentages of organic matter are low several explanations for these changes are possible including a reduction in temperature and moisture more frequent periods of aridity with increased fires or natural succession the holocene record begins with pronounced increases in organic carbon and pollen of wet montane forest primarily hedyosmum podacarpaceae and urticales high values of podocarpaceae pollen 35 and a decline in charcoal suggest temperature and moisture levels above modern day values wet montane forest pollen remain high and charcoal values are low from about 10 000 to 6000 yr b p suggesting that warm and moist conditions prevailed for about 4000 yr subsequently podocarpaceae and urticales decline and for a brief time alnus is prominent in the pollen record following the alnus maximum at about 5000 yr b p poaceae ambrosia and chenopodiaceae amaranthaceae become frequent increased paramo and disturbance indicator pollen suggest increased anthropogenic activities in this region from the middle holocene to the present

on the functioning of the southern oscillation in the south american sector part i surface climate abstract the functioning of the southern oscillation so over south america and the surrounding tropical atlantic and eastern pacific oceans is studied through analyses of bimonthly patterns of correlation between an so index and the fields of pressure wind temperature and rainfall hydrometeorological anomalies being further ascertained from river discharge and lake level records and measurements of outgoing longwave radiation most pronounced is the anomalously low pressure over the eastern pacific during the negative so phase defined by anomalously low high pressure at tahiti darwin consistently positive so pressure correlations over the eastern tropical pacific extend eastward into the north atlantic region and southern south america during the respective winter semester during the austral winter the tendency for anomalously abundant rainfall in central chile during the negative so phase appears related to an anomalously weak and northward displaced south pacific subtropical high while relativ