the transition from peraluminous to peralkaline granitic melts evidence from melt inclusions and accessory minerals fractional crystallization of peraluminous f and h2o rich granite magmas progressively enriches the remaining melt with volatiles we show that at saturation the melt may separate into two immiscible conjugate melt fractions one of the fractions shows increasing peraluminosity and the other increasing peralkalinity these melt fractions also fractionate the incompatible elements to significantly different degrees coexisting melt fractions have differing chemical and physical properties and due to their high density and viscosity contrasts they will tend to separate readily from each other once separated each melt fraction evolves independently in response to changing t p x conditions and further immiscibility events may occur each generating its own conjugate pair of melt fractions the strongly peralkaline melt fractions in particular are very reactive and commonly react until equilibrium is attained consequently the peralkaline melt fraction is commonly preserved only in the isolated melt and mineral inclusions we demonstrate that the differences between melt fractions that can be seen most clearly in differing melt inclusion compositions are also visible in the composition of the resulting ore forming and accessory minerals and are visible on scales from a few micrometers to hundreds of meters

rapid stepwise onset of antarctic glaciation and deeper calcite compensation in the pacific ocean the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth at present this depth is 4 500 m with some variation between and within ocean basins the calcite compensation depth is linked to ocean acidity which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate1 geological records of changes in the calcite compensation depth show a prominent deepening of more than 1 km near the eocene oligocene boundary 34 million years ago 2 when significant permanent ice sheets first appeared on antarctica3 4 5 6 but the relationship between these two events is poorly understood here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical pacific ocean that cover the eocene oligocene boundary we find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40 000 years each synchronous with the stepwise onset of antarctic ice sheet growth the glaciation was initiated after climatic preconditioning7 by an interval when the earth s orbit of the sun favoured cool summers the changes in oxygen isotope composition across the eocene oligocene boundary are too large to be explained by antarctic ice sheet growth alone and must therefore also indicate contemporaneous global cooling and or northern hemisphere glaciation

the overdeepening hypothesis how erosional modification of the marine scape during the early pliocene altered glacial dynamics on the antarctic peninsula s pacific margin abstract a new synthesis of diatom assemblage data from ocean drilling program odp leg 178 suggests that the pacific margin of the antarctic peninsula underwent a transition from a shallow shelf to an overdeepened shelf in the early pliocene this modification of the marine scape was due to a relatively brief interval of erosion begun at 5 2 ma the erosion was caused by high frequency advances of a super inflated antarctic peninsula ice sheet apis the frequent advances of the higher elevation ice sheet were a consequence of abundant moisture delivered to the region as the polar front migrated southward by 5 12 ma ice streams incised foredeepened glacial troughs into basement on the inner shelf sediment eroded from the inner shelf was transported through cross shelf troughs and deposited in large trough mouth fan depocenters on the upper slope overdeepened shelf conditions became widespread as troughs widened and intra trough banks beveled by 4 25 ma trough mouth fan construction ceased and subsequent advances of the apis have been infrequent we propose that the reduced frequency of grounding events signaled the transition to a modern foredeepened and overdeepened shelf we hypothesize that a new glacial dynamic emerged in the early pliocene because overdeepening led to accelerated heat exchange between the ocean and apis in two ways firstly the overdeepened shelf required that a larger area of the grounded ice sheet s marine terminus be in contact with the ocean secondly erosional deepening of the outer shelf was equivalent to lowering a shelf edge sill that permitted frequent and voluminous intrusion of warm circumpolar deep waters that upwell in the region the resultant accelerated melting at the apis marine terminus caused the super inflated apis to deflate on the mainland which further decreased the possibility that grounded ice could advance on the overdeepened shelf

influence of fluorine on the solubility of manganotantalite mnta2o6 and manganocolumbite mnnb2o6 in granitic melts an experimental study abstract solubility experiments of manganotantalite mnta 2 o 6 and manganocolumbite mnnb 2 o 6 were conducted under nearly fluid saturated conditions in synthetic haplogranitic hpg and natural granitic ng melts containing different amounts of fluorine at 800 to 1000 c and 200 mpa the experiments were carried out in cold seal pressure vessels at 800 c with water or ar as a pressure medium and in internally heated pressure vessels 1000 c with ar as a pressure medium the f contents vary from 0 to 11 43 wt in experiments using hpg melts and from 0 61 to 3 78 wt in experiments using ng melts additionally the ng melt contains minor amounts of p 2 o 5 0 44 to 0 64 wt in hpg melts alumina saturation index including mn asi mn 0 88 to 0 99 the solubility product values at 800 c yield log k sp ta and log k sp nb ranging from 3 36 0 10 to 2 92 0 10 mol kg and from 3 01 0 11 to 2 87 0 06 mol kg respectively in ng melts the log k sp ta and log k sp nb vary from 3 00 0 04 to 2 35 0 13 mol kg and from 2 83 0 06 to 2 67 0 18 mol kg at 800 c respectively the solubility products of both mnta 2 o 6 k sp ta and mnnb 2 o 6 k sp nb are not significantly affected by the f content for a given melt composition data are virtually identical within error the variation of the solubility products observed in this study is rather attributed to the changes in melt structure e g polymerization and in particular to the alumina saturation index by contrast there is no evidence for the formation of complexes associating f and nb and or ta our results also indicate that the structural changes caused by the incorporation of f in natural water rich silicate melts are not expected to significantly influence the maximum concentrations of nb and ta in granitic or pegmatitic melts coexisting with manganotantalite and manganocolumbite

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

global cooling during the eocene oligocene climate transition about 34 million years ago earth s climate shifted from a relatively ice free world to one with glacial conditions on antarctica characterized by substantial ice sheets how earth s temperature changed during this climate transition remains poorly understood and evidence for northern hemisphere polar ice is controversial here we report proxy records of sea surface temperatures from multiple ocean localities and show that the high latitude temperature decrease was substantial and heterogeneous high latitude 45 degrees to 70 degrees in both hemispheres temperatures before the climate transition were 20 c and cooled an average of 5 c our results combined with ocean and ice sheet model simulations and benthic oxygen isotope records indicate that northern hemisphere glaciation was not required to accommodate the magnitude of continental ice growth during this time

tegula atra lesson 1830 mollusca gastropoda in the marine quaternary of patagonia argentina sw atlantic biostratigraphical tool and palaeoclimate palaeoceanographical signal results of the systematic review and regional palaeobiogeographical context of tegula atra including morphometric multivariate and cladistic analyses show that it is a keystone species in the marine quaternary of argentina that can be used as pleistocene biostratigraphical tool and paleoclimate palaeoceanographical signal while it was absent in warmer than present high sea level episodes during the miocene entrerriense transgression ca 10 ma and pleistocene mis11 it exhibits an excellent and abundant fossil record within dominantly cool coastal settings exclusively during the late pleistocene mis9 7 and 5 between rio negro and southern santa cruz provinces patagonia it first appeared in the sep during the late pliocene cooling trend dispersed during the late pleistocene into the swa presumably by rafting on macroalgae along the cabo de hornos and malvinas falkland currents but became extinct in the mar argentino magellan malacological province during the holocene amelioration trend its absence at present represents a climate change driven range shift and independent evidence of palaeoceanographical changes after the lgm and at the pleistocene holocene transition changes in sst ca 2 c higher wind velocities less light less nutrient availability less extension and intensity of cold less and warm increased shallow water currents altering water masses and biogeographical boundaries altogether these changes and the holocene scenario were disadvantageous causing direct effects on its physiology and survival in turn preventing the occurrence of the associated macroalgae durvillaea antarctica and its successful dispersal in the swa or retraction to the cold humboldt system waters this study reinforces the importance of dispersalist models to explain the origin of key taxa adding for a better understanding of molluscan taxonomic differences along the swa and sep margins of south america with implications for future coastal scenarios the distribution of t atra across time is a new example of the strong linkage between earth history climatic cycles atmospheric and oceanic circulation and the late quaternary biotic responses showing a possible consequence of future climate change on nearshore communities

partial high grade alteration of monazite using alkali bearing fluids experiment and nature recent advances in the dating of monazite using the electron microprobe indicate that thsio 4 and or cath po 4 2 enriched intergrowths in monazite can give ages younger than the original monazite the morphology of the intergrowths suggests that the original monazite grain has been partly altered by a fluid in which th si and ca are mobile this hypothesis has been tested in the piston cylinder apparatus at 1000 mpa and 900 c utilizing a natural unzoned th and pb bearing monazite ce and na 2 si 2 o 5 h 2 o during the experiment a subset of the monazite grains acquired thsi0 4 enriched areas with sharp compositional boundaries devoid of pb and show evidence of being a pseudomorphic partial replacement of the monazite rather than an overgrowth of the monazite these experiments support the hypothesis that similar th enriched or th depleted patches with sharp compositional boundaries observed in natural monazite could be the result of fluid induced alteration via coupled dissolution reprecipitation if so such altered regions would yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of single or multiple metasomatic events assuming that all pre existing radiogenic pb is removed during alteration

melt inclusions in pegmatite quartz complete miscibility between silicate melts and hydrous fluids at low pressure fluorine boron and phosphorus rich pegmatites of the variscan ehrenfriedersdorf complex crystallized over a temperature range from about 700 to 500 c at a pressure of about 1 kbar pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth a silicate rich h2o poor melt and a silicate poor h2o rich melt both melts were simultaneously trapped on the solvus boundaries of the silicate fluorine boron phosphorus water system the partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 c in steps of 50 c by conventional rapid quench hydrothermal experiments glasses of completely rehomogenized inclusions were analyzed for h2o by raman spectroscopy and for major and some trace elements by emp electron microprobe both types of melt inclusions define a solvus boundary in an xh2o t pseudobinary system at 500 c the silicate rich melt contains about 2 5 wt h2o and the conjugate water rich melt about 47 wt h2o the solvus closes rapidly with increasing temperature at 650 c the water contents are about 10 and 32 wt respectively complete miscibility is attained at the critical point 712 c and 21 5 wt h2o many pegmatites show high concentrations of f b and p this is particularly true for those pegmatites associated with highly evolved peraluminous granites the presence of these elements dramatically reduces the critical pressure for fluid melt systems at shallow intrusion levels at t 720 c water is infinitely soluble in a f b and p rich melt simple cooling induces a separation into two coexisting melts accompanied with strong element fractionation on the water rich side of the solvus very volatile rich melts are produced that have vastly different physical properties as compared to normal silicate melts the density viscosity diffusivity and mobility of such hyper aqueous melts under these conditions are more comparable to an aqueous fluid

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

atmospheric carbon dioxide through the eocene oligocene climate transition geological and geochemical evidence1 2 3 indicates that the antarctic ice sheet formed during the eocene oligocene transition4 33 5 34 0 million years ago modelling studies5 6 suggest that such ice sheet formation might have been triggered when atmospheric carbon dioxide levels fell below a critical threshold of 750 p p m v but the timing and magnitude of relative to the evolution of the ice sheet has remained unclear here we use the boron isotope ph proxy7 8 on exceptionally well preserved carbonate microfossils from a recently discovered geological section in tanzania9 10 to estimate before during and after the climate transition our data suggest that a reduction in occurred before the main phase of ice growth followed by a sharp recovery to pre transition values and then a more gradual decline during maximum ice sheet growth was between 450 and 1 500 p p m v with a central estimate of 760 p p m v the ice cap survived the period of recovery although possibly with some reduction in its volume implying as models predict11 a nonlinear response to climate forcing during melting overall our results confirm the central role of declining in the development of the antarctic ice sheet in broad agreement with carbon cycle modelling12 and help to constrain mechanisms and feedbacks associated with the earth s biggest climate switch of the past 65 myr

a refined calcareous nannofossil biostratigraphy for the middle eocene early oligocene southern ocean odp sites abstract this work presents the stratigraphic distribution of several species of calcareous nannofossil in the middle eocene early oligocene from four ocean drilling program odp sites located between 60 and 65 s paleolatitude in the southern atlantic and indian oceans useful nannofossil datums that should facilitate construction of age models and contribute to an integrated chronology for the upper paleogene southern ocean sediments from 42 to 33 ma are summarized the distribution patterns of calcareous nannofossils studied by means of quantitative and semiquantitative methods provide an improvement of the classical southern ocean biozonations introducing new biostratigraphically useful biohorizons and testing their reproducibility within and outside the region

tantalite mn from the borborema pegmatite province northeastern brazil conditions of formation and melt and fluid inclusion constraints on experimental studies carbon dioxide rich fluid and carbonate rich aluminosilicate melt inclusions in tantalite mn from the alto do giz pegmatite in the borborema pegmatite province northeastern brazil were investigated to constrain the formation of the host crystals the results demonstrate that in the alto do giz pegmatite water and alkaline carbonate rich fluids and melts are responsible for the transport and deposition of tantalite mn at temperatures around 600 c and about 4 kbar moreover evidence is presented to show that during crystallization of the tantalite mn three different components coexisted which are now trapped as separate inclusions two immiscible silicate melts types a and b melt inclusions and a co2 rich aqueous fluid we hypothesize that immiscible fluid separation may have been a critical factor in producing the water and alkaline carbonate rich fluids and melts necessary for ta and nb transport since the tantalite mn crystallized during pegmatite formation this mechanism must also have implications for pegmatite genesis in general

transient nature of the earth s climate and the implications for the interpretation of benthic δ18 o records abstract from the marine benthic 18 o records it is known that the earth s climate has experienced significant variability over the past 40 million years in general a number of assumptions are often needed to disentangle the benthic 18 o data into its temperature and ice volume contributions in this study a transient 1 d ice sheet model is used which overcomes these shortcomings by relating temperature to the benthic 18 o data leading to a self consistent and continuous record of 18 o temperature and sea level the contribution of land ice to benthic 18 o is examined with a set of sensitivity experiments varying the mean 18 o of the ice for different ice sheets it is shown that the scaling factor of sea water 18 o w to sea level is not constant over the long time scales however our sensitivity experiments do show that over the long time scale the general assumed 1 0 1 1 per 100 m of sea level is in reasonable agreement with our model results moreover the sea level ice volume response to temperature s t is shown to vary through time with the largest response found when variations in antarctic ice volume dominates the sea level variations during the oligocene to early miocene furthermore this response is quite large compared to the individual response of ice sheets during the plio pleistocene whereas the combined s t in this period is comparable to that during the oligocene accordingly the transient behaviour is very important for the interpretation of data records with respect to climate sensitivity and climate change during the past 40 million years

thresholds for cenozoic bipolar glaciation the long standing view of earth s cenozoic glacial history calls for the first continental scale glaciation of antarctica in the earliest oligocene epoch 33 6 million years ago1 followed by the onset of northern hemispheric glacial cycles in the late pliocene epoch about 31 million years later2 the pivotal early oligocene event is characterized by a rapid shift of 1 5 parts per thousand in deep sea benthic oxygen isotope values3 oi 1 within a few hundred thousand years4 reflecting a combination of terrestrial ice growth and deep sea cooling the apparent absence of contemporaneous cooling in deep sea mg ca records4 5 6 however has been argued to reflect the growth of more ice than can be accommodated on antarctica this combined with new evidence of continental cooling7 and ice rafted debris8 9 in the northern hemisphere during this period raises the possibility that oi 1 represents a precursory bipolar glaciation here we test this hypothesis using an isotope capable global climate ice sheet model that accommodates both the long term decline of cenozoic atmospheric co2 levels10 11 and the effects of orbital forcing12 we show that the co2 threshold below which glaciation occurs in the northern hemisphere 280 p p m v is much lower than that for antarctica 750 p p m v therefore the growth of ice sheets in the northern hemisphere immediately following antarctic glaciation would have required rapid co2 drawdown within the oi 1 timeframe to levels lower than those estimated by geochemical proxies10 11 and carbon cycle models13 14 instead of bipolar glaciation we find that oi 1 is best explained by antarctic glaciation alone combined with deep sea cooling of up to 4 c and antarctic ice that is less isotopically depleted 30 to 35 than previously suggested15 16 proxy co2 estimates remain above our model s northern hemispheric glaciation threshold of 280 p p m v until 25 myr ago but have been near or below that level ever since10 11 this implies that episodic northern hemispheric ice sheets have been possible some 20 million years earlier than currently assumed although still much later than oi 1 and could explain some of the variability in miocene sea level records

eocene circulation of the southern ocean was antarctica kept warm by subtropical waters 1 near the eocene s close 34 million years ago the climate system underwent one of the largest shifts in earth s history antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed previous studies conjectured that poleward penetration of warm subtropical currents the east australian current eac in particular caused eocene antarctic warmth late eocene opening of an ocean gateway between australia and antarctica was conjectured to have disrupted the eac cooled antarctica and allowed ice sheets to develop here we reconstruct eocene paleoceanographic circulation in the tasmanian region using 1 biogeographical distributions of phytoplankton including data from recently drilled ocean drilling program leg 189 sites and 2 fully coupled climate model simulations we find that the eac did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern our results do not support changes in thermal isolation as the primary driver of the eocene oligocene climatic transition

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

the miarolitic pegmatites from the konigshain a contribution to understanding the genesis of pegmatites in this paper we show that the crystallization of miarolitic pegmatites at konigshain started at about 700 c in melts containing up to 30 mass water such high water concentration at low pressures 1 3 kbar is only possible if the melts are peralkaline such peralkaline melts are highly corrosive and reacted with the wall rock here the granite host forming the graphic granite zone in part via a magmatic metasomatic reaction with cooling the water concentration in some melt fractions increased up to 50 mass h2o the melt dominated system ends below 600 c and passes into a fluid dominated system the beginning of which is characterized by strong pressure fluctuations caused by the change of oh and co32 in the melt to molecular water and co2 we note two generations of smoky quartz one crystallized above the transition of quartz 573 c and one below both of which contain melt inclusions this indicates that some melt fraction remains during at least the higher temperature portion of the growth of minerals into the miarolitic cavity contradicting the view that minerals growing into a pegmatite chamber only do so from aqueous fluids we show that the konigshain miarolitic pegmatites are part of the broad spectrum of pegmatite types and the processes active at konigshain are representative of processes found in most granitic pegmatites and are thus instructive in the understanding of pegmatite formation in general and constraining the composition and characteristics of pegmatite forming melts

the role of magmatic and hydrothermal processes in the evolution of be bearing pegmatites evidence from beryl and its breakdown products beryl and euclase crystals from the mina do santino and the jacu pegmatites in the borborema pegmatite province in northeastern brazil contain several generations of melt and fluid inclusions which allow interpretation of p t x conditions responsible for beryl crystallization and for alteration of a primary pegmatitic mineral assemblage to a mixture of hydrothermal minerals euclase bertrandite kaolinite and quartz primary melt and fluid inclusions hosted by beryl were trapped simultaneously however their homogenization temperatures are significantly higher 870 900 c than the values previously reported for pegmatitic systems 712 c and should be treated with caution an isobaric drop of temperature resulted in the exsolution of a fluid a low salinity co2 enriched phase and a saline water rich phase were trapped in pseudosecondary inclusions in beryl at a pressure of 2 1 2 7 kbar and temperature of 390 480 c cooling of the country rocks below 400 c caused a ductile to brittle transition and allowed infiltration of cold groundwater which further decreased the temperature in the system to 190 240 c at the same time the pressure dropped from a lithostatic 2 1 2 7 kbar to a hydrostatic value 0 57 0 73 kbar consequently minerals deposited under magmatic conditions feldspars and beryl became unstable and a newly formed hydrothermal mineral paragenesis euclase bertrandite kaolinite and quartz overprinted the earlier one the hydrothermal fluids responsible for the alteration differ from the earlier exsolved fluids in having a lower salinity lower homogenization temperature the absence of co2 and the presence of ch4

cyclochronology of the eocene oligocene transition from the cape roberts project 3 core victoria land basin antarctica abstract about 34 million years ago at the eocene oligocene e o transition earth s climate underwent a substantial change from relatively ice free green house conditions to a glacial state marked by the establishment of a permanent ice sheet on antarctica our understanding of the antarctic cryospheric evolution across the e o climate transition relies on indirect marine geochemical proxies and hitherto it has not been possible to reconcile the pattern of inferred ice sheet growth from these far field proxy records with direct physical evidence of ice sheet behaviour from the proximal antarctic continental margin here we present a correlation of cyclical changes recorded in the crp 3 drill hole sediment core from the western ross sea that are related to oscillations in the volume of a growing east antarctic ice sheet with well dated lower latitude records of orbital forcing and climate change across the e o transition we evaluate the results in the light of the age model available for the crp 3a succession our cyclostratigraphy developed on the basis of repetitive vertical facies changes and clast peak abundances within sequences matches the floating cyclochronology developed in deep sea successions for major glacial events the astrochronological calibration of the crp 3 succession represents the first high resolution correlation of direct physical evidence of orbitally controlled glaciation from the antarctic margin to geochemical records of paleoclimate changes across the e o climate transition

orbitally paced climate oscillations across the oligocene miocene boundary the late oligocene and early miocene periods some 21 to 27 million years ago have generally been viewed as times of moderate global warmth and ice free conditions yet several lines of evidence suggest that this interval was punctuated by at least one and possibly several episodes of high latitude cooling and continental glaciation1 2 3 here we present stable isotope and per cent coarse fraction data from an equatorial western atlantic deep sea sediment core that provide high resolution records of the climate variability across the oligocene miocene transition 22 5 25 7 million years ago a strong 40 kyr periodicity in the oxygen isotope record is consistent with a high latitude orbital obliquity control on ice volume and temperature orbital influences are also apparent at precession and eccentricity frequencies including a series of 400 kyr oscillations that culminate in distinct maxima at the oligocene miocene boundary about 23 7 million years ago covariance between the carbon and oxygen isotope records suggests that the oceanic carbon cycle may have contributed to global cooling during the 400 kyr cycles particularly at the oligocene miocene boundary

antarctic circumpolar current s role in the antarctic ice system an overview abstract the antarctic circumpolar current acc provides fundamental control on the antarctic ice system the tilt of the isopycnals of the acc in response to strong westerlies serves to thermally isolate the antarctic continent from directly receiving the overwhelming subtropical ocean surface heat this same tilt provides the northern boundary of the polar seas as such it contains the statically stable cold fresh surface polar waters required for sea ice formation in this manner it effectively sets the northern limit for seasonal sea ice formation the isopycnal tilt also allows warm deep water to upwell to the surface near the continental margin in western antarctica where the acc skirts the continental shelf leading to excessive ocean heat flux to the atmosphere in winter and providing heat to melt the underside of the glacial ice

the velocity and vorticity structure of the agulhas current at 32 s the full depth velocity field of the agulhas current the western boundary current of the southwest indian ocean has been directly measured using a lowered acoustic doppler current profiler ladcp fifteen combined conductivity temperature depth ctd and ladcp stations were occupied across the current at 32 s off the east coast of south africa in february march 1995 as part of the world ocean circulation experiment indian ocean program the deep velocity structure of the agulhas current was found to be very different than previously described using geostrophic estimates in particular ladcp results reveal a v shaped pattern for the level of no motion across the agulhas current and an agulhas undercurrent is observed flowing equatorward below 800 m depth directly beneath the surface core of the poleward flowing agulhas current comparisons of direct and geostrophic velocities suggest that the velocity structure of the agulhas current is essentially geostrophic below about 200 m depth where differences are generally less than estimated errors above these depths the ladcp measured shears and geostrophic shears exhibit differences and velocities can diverge shipboard adcp data support ladcp results near the surface indicating that both instruments are measuring real signals the geostrophic and ladcp measured shears are well matched away from the surface and a depth averaged fit below 200 m between them exhibits small standard deviations the geostrophic volume transport of the agulhas current as referenced to ladcp is 73 sv and is just 3 different from the direct ladcp transport estimate the most recent full depth estimate of agulhas current transport in the literature is given by toole and warren 1993 who estimate a geostrophic volume transport of 85 sv the baroclinic velocity structures from their section and from this work are similar indicating that the difference in the transport estimates is due to the choice of geostrophic reference level and ultimately to the presence of the previously unobserved agulhas undercurrent combined ctd ladcp data are very useful for potential vorticity analysis since velocity and density measurements are simultaneous and coincident the vorticity structure of the agulhas current suggests the presence of a mixing boundary at middepths similar to features observed in the gulf stream the feature appears to be associated with a sharp upturn in isopycnals coincident with the foot of the continental slope the boundary inhibits cross stream mixing of intermediate water masses and may help explain the appearance of a discrete filament of red sea water inshore of antarctic intermediate waters in the same density layer despite intense cross stream shears

basement control on past ice sheet dynamics in the amundsen sea embayment west antarctica abstract the development of landscapes and morphologies follows initially the tectonic displacement structures of the basement and sediments such fault zones or lineaments are often exploited by surface erosional processes and play therefore an important role in reconstructing past ice sheet dynamics observations of bathymetric features of the continental shelf of the amundsen sea embayment and identification of tectonic lineaments from geophysical mapping indicate that the erosional processes of paleo ice stream flows across the continental shelf followed primarily such lineaments inherited from the tectonic history since the cretaceous break up between new zealand and west antarctica three major ice flow trends correspond to different tectonic phases in east west northwest southeast and north south directions east west oriented basement trends correlate with coastline trends and overlay tectonic lineaments caused by former rift activities directional trends with northwest southeast orientation are observed for the glacial troughs of the western embayment outer shelf the western pine island bay coastal zones and the inner pine island glacial trough and are associated with a distributed southern plate boundary zone of the former bellingshausen plate the north south trend of the main pine island glacial trough and the north northeast trend of the abbot ice shelf trough on the outer shelf follow the predicted lineation trend of an eastern branch of the west antarctic rift system extending from the thwaites drainage basin northward into pine island bay an understanding of this context helps better constrain the geometries and sea bed substrate conditions for regional paleo ice sheet models

columbite solubility in granitic melts consequences for the enrichment and fractionation of nb and ta in the earth s crust the behaviour of niobium and tantalum in magmatic processes has been investigated by conducting mnnb2o6 and mnta2o6 solubility experiments in nominally dry to water saturated peralkaline aluminium saturation index a s i 0 64 to peraluminous a s i 1 22 granitic melts at 800 to 1035 c and 800 to 5000 bars the attainment of equilibrium is demonstrated by the concurrence of the solubility products from dissolution crystallization mn doped and nb or ta doped experiments at the same pressure and temperature the solubility products of mnnb2o6 ksp nb and mnta2o6 ksp ta at 800 c and 2 kbar both increase dramatically with alkali contents in water saturated peralkaline melts they range from 1 2 10 4 and 2 6 10 4 mol2 kg2 respectively in subaluminous melt a s i 1 02 to 202 10 4 and 255 10 4 mol2 kg2 respectively in peralkaline melt a s i 0 64 this increase from the subaluminous composition can be explained by five non bridging oxygens being required for each excess atom of nb5 or ta5 that is dissolved into the melt the ksp nb and ksp ta also increase weakly with al content in peraluminous melts ranging up to 1 7 10 4 and 4 6 10 4 mol2 kg2 respectively in the a s i 1 22 composition columbite tantalite solubilities in subaluminous and peraluminous melts a s i 1 02 and 1 22 are strongly temperature dependent increasing by a factor of 10 to 20 from 800 to 1035 c by contrast columbite tantalite solubility in the peralkaline composition a s i 0 64 is only weakly temperature dependent increasing by a factor of less than 3 over the same temperature range similarly ksp nb and ksp ta increase by more than two orders of magnitude with the first 3 wt h2o added to the a s i 1 02 and 1 22 compositions whereas there is no detectable change in solubility for the a s i 0 64 composition over the same range of water contents solubilities are only slightly dependent on pressure over the range 800 to 5000 bars the data for water saturated sub and peraluminous granites have been extrapolated to 600 c conditions at which pegmatites and highly evolved granites may crystallize using a melt concentration of 0 05 wt mno 70 to 100 ppm nb or 500 to 1400 ppm ta are required for manganocolumbite and manganotantalite saturation respectively the solubility data are also used to model the fractionation of nb and ta between rutile and silicate melts predicted rutile melt partition coefficients increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions it is demonstrated that the nb2o5 ta2o5 activity coefficient ratio in the melt phase depends on melt composition this ratio is estimated to decrease by a factor of 4 to 5 from andesitic to peraluminous granitic melt compositions accordingly all the relevant accessory phases in subaluminous to peraluminous granites are predicted to incorporate nb preferentially over ta this explains the enrichment of ta over nb observed in highly fractionated granitic rocks and in the continental crust in general

early stall of west antarctic ice sheet advance on the eastern ross sea middle shelf followed by retreat at 27 50014cyrbp abstract marine geological and geophysical data suggest that the west antarctic ice sheet wais deposited three thick seismically resolvable units in the eastern ross sea since the last glacial maximum lgm but the chronology of these grounding events is debated existing ice retreat chronologies for the eastern ross sea are considered suspect because nearly all have been developed using radiocarbon dating of acid insoluble organics aio from bulk sediment we used a new strategy to isolate in situ forams from diamict sediment deposited at the middle shelf grounding zone wedge gzw a subaqueous ice marginal landform previously interpreted to represent deposition during the third grounding event since the lgm advance to the shelf edge here we present a synthesis of our new radiocarbon dates of foram tests and aio with previously published radiocarbon dates we propose that the wais s lgm advance in the eastern ross sea sector reached the middle shelf circa 27 500 14c yr bp and that open marine sedimentation began shortly thereafter there is as of yet no clear explanation for why wais retreat in this sector of antarctica apparently began prior to the peak of the last glacial cold period a precipitation deficit localized warm water intrusion an ice dynamic response in the absence of climate forcing or another as of yet unknown forcing mechanism could be responsible

fluid inclusion microthermometry in be ta li sn bearing pegmatites from the borborema province northeast brazil abstract fluid inclusions in be ta li sn bearing pegmatites have been reported in the literature as hypersaline aqueous brines or low salinity aqueous or aqueous carbonic fluids the evolution paths of volatile fluids in equilibrium with the different crystallizing phases of rare element pegmatites do not seem to be unique the aim of this work is to contribute to this discussion on the basis of fluid inclusion information from the borborema pegmatite province bpp in northeast brazil some well known heterogeneous be ta li sn mineralized pegmatites of this province were systematically sampled along cross sections with the purpose of studying the fluid inclusions in the different pegmatite zones i muscovite rich contact zone ii homogeneous or graphic pegmatite iii blocky feldspar zone and iv quartz core the observed fluid inclusions mainly in quartz were almost always secondary or pseudosecondary grouped along healed fractures with random orientation even some trails paralleling quartz grain limits were not undoubtedly primary almost all these fluid inclusions were filled with low salinity aqueous or aqueous carbonic fluids primary inclusions were recognized only in some well formed crystals from druses with erratic distribution in the pegmatites microthermometric data from these samples confirm that the various crystallizing phases of the pegmatites were in equilibrium with low salinity aqueous or aqueous carbonic fluids with insignificant n 2 or ch 4 contents with frequent accidentally trapped solids the various types of primary inclusions found in these crystals compare well with the types of secondary inclusions of the samples from the systematic cross sections the relative age of the crystals hosting primary fluid inclusions the relative age of primary inclusion types in successive crystal growth zones the relative age of secondary fluid inclusion trails and the corresponding microthermometric data allowed to establish the most probable evolution of the fluid characteristics during pegmatite crystallization an early low salinity aqueous carbonic fluid with co 2 contents of 20 mol was present during the crystallization of early minerals such as garnet and tourmaline of the border zone and graphic quartz beryl manganotantalite and blue tourmaline of zones ii and iii the end of zone iii crystallization occurred in the presence of a low salinity aqueous fluid with very low co 2 contents as indicated by inclusions in the smoky core of quartz crystals at the limit between zones iii and iv later aqueous fluids barren in co 2 and with salinities increasing from 5 to 20 coexisted with the formation of the pegmatite nucleus and replacement bodies as indicated by inclusions in the border of these crystals and in euclase the estimated trapping conditions of 3 5 kbar and 580 c for the earliest inclusion types agree with the regional metamorphic conditions and with the known liquidus conditions of rare element bearing pegmatite crystallization the trapping conditions of inclusions in euclase and coexisting quartz crystal rims in the pegmatitic core at 3 5 kbar and 400 c are slightly below the conditions admitted for the solidus of rare element pegmatites and indicate an isobaric cooling during pegmatite crystallization

do blue ice moraines in the heritage range show the west antarctic ice sheet survived the last interglacial abstract we present a hypothesis that best explains cosmogenic isotope data on blue ice moraines in the heritage range west antarctica the age of the moraines implies that they and the related ice sheet surface with which they are associated have persisted on the flanks of nunataks throughout at least the last interglacial glacial cycle the implication is that although the west antarctic ice sheet wais may have fluctuated in thickness during glacial cycles the central dome has remained intact for at least 200 kyr and possibly even for 400 kyr such a finding if substantiated would contribute to our understanding of the sensitivity of the wais to climate change further it would be a powerful geomorphic constraint on models of the past behaviour of the ice sheet during glacial cycles and thus those predicting the future of the ice sheet in a warming world

sea ice feedback and cenozoic evolution of antarctic climate and ice sheets 1 the extent and thickness of antarctic sea ice have important climatic effects on radiation balance energy transfer between the atmosphere and ocean and moisture availability this paper explores the role of sea ice and related feedbacks in the cenozoic evolution of antarctic climate and ice sheets using a numerical climate model with explicit dynamical representations of sea ice and continental ice sheets in a scenario of decreasing cenozoic greenhouse gas concentrations our model initiates continental glaciation before any significant sea ice forms around the continent once variable ice sheets are established seasonal sea ice distribution is highly sensitive to orbital forcing and ice sheet geometry via the ice sheet s control on regional temperature and low level winds although the expansion of sea ice has significant climatic effects near the coast it has only minimal effects in the continental interior and on the size of the ice sheet therefore the cenozoic appearance of antarctic sea ice was primarily a response to the growth of grounded ice sheets and was not a critical factor in episodes of paleogene and neogene glaciation the influence of the east antarctic ice sheet on sea ice southern ocean surface temperatures and winds has important implications for ocean circulation the marine carbon cycle and the development of the west antarctic ice sheet the sensitivity of sea ice to grounded ice sheets implies reconstructions of sea ice based on marine diatoms are good indicators of glacial conditions in the continental interior and may provide insight into the long term stability of antarctic ice sheets

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

a coupled climate ice sheet modeling approach to the early cenozoic history of the antarctic ice sheet abstract the sudden widespread glaciation of antarctica and the associated shift toward colder temperatures near the eocene oligocene boundary 34 ma represents one of the most fundamental reorganizations of the global climate system recognized in the geologic record this glacial inception and the subsequent evolution of the early east antarctic ice sheet eais are simulated using a new coupled global climate dynamical ice sheet model accounting for the paleogeography greenhouse gas concentrations changing orbital parameters and varying ocean heat transport suites of long 105 yr climate ice sheet simulations are used to investigate the effects of declining atmospheric co2 compared to those of the tectonic opening of southern ocean gateways and the timing of mountain uplift in the antarctic interior in contrast to the established paradigm for the glaciation of antarctica which centers on the opening of the southern ocean gateways and the thermal isolation of the continent our results show that declining cenozoic pco2 may have played the dominant role first small isolated ice caps formed on the highest antarctic plateaus then as a co2 threshold between 3 and 2 pre industrial level pal was crossed height mass balance feedbacks were initiated during orbital periods with cold austral summers triggering much larger highly dynamic terrestrial ice sheets as co2 continued to decline these isolated ice caps eventually merged into a permanent continental scale eais in our model neither the opening of the southern ocean gateways nor mountain uplift significantly affected the timing of the major ice sheet transition given a scenario of gradually declining co2 from 4 to 2 pal over 10 million years around the eocene oligocene boundary

the eocene oligocene transition changes in sea level temperature or both abstract the eocene oligocene transition eot 34 ma reflects the onset of major antarctic glaciation the primary geochemical signature of the eot is two 300 kyr spaced shifts in increasing deep sea oxygen isotope values possibly reflecting both global cooling and or increasing ice volume a way to assess the respective contribution of continental ice is to quantify concomitant glacio eustatic sea level change this is usually expressed in relatively shallow marine depositional settings one potentially suitable region is in the vicentinian alps ne italy where marginal marine deposits document sea level changes during the eot by correlating stable isotope bio and magnetostratigraphic information between three distant regions we are able to relate the shallow marine sections to the pacific oxygen isotope record from ocean drilling program odp site 1218 of coxall et al 2005 microfacies sedimentological and biotic analysis suggests that associated with the first isotope shift eot 1 sea level fell 20 m and with the ultimate shift the oligocene isotope event 1 oi 1 sea level fell some 50 60 m distribution patterns of temperature sensitive dinoflagellates from a coeval central italian section reveal that the early stages of the eot were accompanied by sea surface cooling whereas no sustained cooling is noted in association with the oi 1 this suggests that the initial eot shift s reflect a mixed signal of ice volume and temperature whereas the oi 1 primarily reflects expansion of the antarctic cryosphere

experimental phase equilibria in the system lialsio4 sio2 h2o a petrogenetic grid for lithiumx rich pegmatites phase relations for the bulk composition lia1sis01z system lia1si04 sioz hzo have been investigated experimentally over the range 340 950 c and 0 5 6 0 kbar p hzo stability relations among eucryptite spodumene petalite b spodumene tetragonal and virgilite hexagonal have been determined for quartz saturated subsolidus conditions the result is a quantitative p t phase diagram that is applicable to quartz saturated lithium aluminosilicate assemblages in pegmatites and some volcanic rocks thermochemical data derived from these experimental results are internally consistent and in close agreement with values obtained from calorimetry under the quartz saturated conditions that prevail in pegmatites stability relations among the lithium aluminosilicates are a function of p and t and are largely independent of the nature and proportions of other phases in the chemically complex pegmatite system thus the lithium aluminosilicate phase diagram constitutes a petrogenetic grid from which p t conditions of emplacement and crystallization can be ascertained for many lithium alumino silicate bearing pegmatites

antarctic topography at the eocene oligocene boundary we present a reconstruction of the antarctic topography at the eocene oligocene ca 34 ma climate transition this provides a realistic key boundary condition for modeling the first big antarctic ice sheets at this time instead of using the present day bedrock topography which has changed significantly from millions of years of tectonism and erosion we reconstruct topography using a set of tools including ice sheet erosion models models of thermal subsidence and plate movement erosion estimates are constrained with offshore sediment volumes estimated from seismic stratigraphy maximum and minimum topographic reconstructions are presented as indicators of the range of uncertainty our results point to a significant upland area in the ross sea marie byrd land and weddell sea sectors in addition east antarctic coastal troughs are much shallower than today due to the restoration of material that has been selectively eroded by the evolving ice sheets parts of east antarctica have not changed since the e o boundary because they were protected under non erosive cold based ice the reconstructions provide a better defined boundary condition for modeling that seeks to understand interaction between the antarctic ice sheet and climate along with more robust estimates of past ice volumes under a range of orbital settings and greenhouse gas concentrations

nb ta ti sn oxide mineral chemistry as tracer of rare element granitic pegmatite fractionation in the borborema province northeastern brazil the borborema pegmatitic province bpp northeastern brazil is historically important for tantalum mining and also famous for top quality specimens of exotic nb ta oxides and more recently for the production of gem quality turquoise blue paraiba elbaite with more than 750 registered mineralized rare element granitic pegmatites the bpp extends over an area of about 75 by 150 km in the eastern part of the neoproterozoic serido belt the late cambrian pegmatites are mostly hosted by a sequence of neoproterozoic cordierite sillimanite biotite schists of the serido formation and quartzites and metaconglomerates of the equador formation the trace element ratios in feldspar and micas allow to classify most pegmatites as belonging to the beryl columbite phosphate subtype electron microprobe analyses empa of columbite tapiolite niobian tantalian rutile ixiolite and wodginite group minerals from 28 pegmatites in the bpp are used to evaluate the effectiveness of nb ta oxide chemistry as a possible exploration tool to trace the degree of pegmatite fractionation and to classify the pegmatites the columbite group mineral composition allows to establish a compositional trend from manganoan ferrocolumbite to manganocolumbite and on to manganotantalite this trend is typical of complex spodumene and or lepidolite subtype pegmatites it clearly contrasts with another trend from ferrocolumbite through ferrotantalite to ferrowodginite and ferrotapiolite compositions typical of pegmatites of the beryl columbite phosphate subtype large scatter and anomalous trends in zoned crystals partially overlap and conceal the two main evolution patterns this indicates that a large representative data set of heavy mineral concentrate samples collected systematically along cross sections would be necessary to predict the metallogenetic potential of individual pegmatites other mineral species e g garnets and or tourmaline with a more regular distribution than nb ta oxides would be more appropriate and less expensive for routine exploration purposes the currently available nb ta oxide chemistry data suggest the potential for highly fractionated ta li cs pegmatites in the bpp so far undiscovered and encourages further more detailed research

extreme alkali bicarbonate and carbonate rich fluid inclusions in granite pegmatite from the precambrian ronne granite bornholm island denmark our study of fluid and melt inclusions in quartz and feldspar from granite pegmatite from the precambrian ronne granite bornholm island denmark revealed extremely alkali bicarbonate and carbonate rich inclusions the solid phases daughter crystals are mainly nahcolite nahco3 zabuyelite li2co3 and in rare cases potash k2co3 in addition to the volatile phases co2 and aqueous carbonate bicarbonate solution rare melt inclusions contain nahcolite dawsonite naal co3 oh 2 and muscovite in addition to fluid and melt inclusions there are primary co2 rich vapor inclusions which mostly contain small nahcolite crystals the identification of potash as a naturally occurring mineral would appear to be the first recorded instance from the appearance of high concentrations of these carbonates and bicarbonates we suggest that the mineral forming media were water and alkali carbonate rich silicate melts or highly concentrated fluids the coexistence of silicate melt inclusions with carbonate rich fluid and nahcolite rich vapor inclusions indicates a melt melt vapor equilibrium during the crystallization of the pegmatite these results are supported by the results of hydrothermal diamond anvil cell experiments in the pseudoternary system h2o nahco3 sio2 additionally we show that boundary layer effects were insignificant in the bornholm pegmatites and are not required for the origin of primary textures in compositionally simple pegmatites at least

solubility of manganotantalite zircon and hafnon in highly fluxed peralkaline to peraluminous pegmatitic melts the behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of ta and zr solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous li f p and b bearing pegmatitic melts the pegmatitic melts are synthetic and enriched in flux elements 0 7 1 3 wt li2o 2 5 5 wt f 2 8 4 wt p2o5 and 0 2 8 wt b2o3 and their aluminum saturation index ranges from peralkaline to peraluminous asili al na k li 0 8 to 1 3 with various k na ratios dissolution and crystallization experiments were conducted at temperatures varying between 700 and 1 150 c at 200 mpa and nearly water saturated conditions for dissolution experiments pure synthetic end member manganotantalite and zircon were used in order to avoid problems with slow solid state kinetics but additional experiments using natural manganotantalite and zircon of relatively pure composition i e close to end member composition displayed similar solubility results zircon and manganotantalite solubilities considerably increase from peraluminous to peralkaline compositions and are more sensitive to changes in temperature or asi of the melt than to flux content a model relating the enthalpy of dissolution of manganotantalite to the asili of the melt is proposed hdiss kj mol 304 asili 176 in the peralkaline field and hdiss kj mol 111 asili 245 in the peraluminous field the solubility data reveal a small but detectable competitivity between zr and ta in the melt i e lower amounts of zr are incorporated in a ta bearing melt compared to a ta free melt under the same conditions a similar behavior is observed for hf and ta the competitivity between zr or hf and ta increases from peraluminous to peralkaline compositions and suggests that ta is preferentially bonded to non bridging oxygens nbos with al as first neighbors whereas zr is preferentially bonded to nbos formed by excess alkalies as a consequence zr ta ratios when buffered by zircon and manganotantalite simultaneously are higher in peralkaline melts than in peraluminous melts