fluid absent melting of a layered crustal protolith implications for the generation of anatectic granites we report the result of h2o undersaturated melting experiments on charges consisting of a layer of powdered sillimanite bearing metapelite hq36 and a layer of powdered tonalitic gneiss agc150 experiments were conducted at 10 kbar at 900 925 and 950 c when run alone the pelite yielded 40 vol strongly peraluminous granitic melt at 900 c while the tonalite produced only 5 vol weakly peraluminous granitic melt at 950 c the pelite and the tonalite yielded 50 vol and 7 vol granitic melt respectively when run side by side the abundance of melt in the tonalite was 10 times higher at all temperatures than when it was run alone in the pelite the melt abundance increased by 25 vol when run alone biotite dehydration melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt when run side by side only garnet was produced in addition to granitic melt experiments of relatively short duration however also contained al rich orthopyroxene we suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of al2o3 in the melt which lowers the temperature of the biotite dehydration melting reaction in the pelite the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite layer to the pelite layer this has the effect of changing the bulk composition of this layer in the direction of minimum temperature granitic liquids our results show that rocks which are poor melt producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase s and facilitate dehydration melting because of this effect the continental crust may have an even greater potential for granitoid melt production than previously thought our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks which will be reflected in their isotopic and geochemical compositions

three s type volcanic suites from the lachlan fold belt southeast australia the concept that granitoids of the lachlan fold belt in southeast australia are derived from either igneous or sedimentary source rocks i or s type can be extended to volcanic rocks of the same age three suites of late silurian s type volcanics are described two of which can be matched quite closely with plutonic equivalents a large part of the paleozoic continental margin volcanic activity in southeast australia consisted of the magmatic recycling of old metasedimentary crust probably of late proterozoic age the three volcanic suites are moderately to strongly peraluminous with the corresponding presence of al rich minerals variation within the volcanic suites is ascribed chiefly to progressive removal of restite or source material residual from partial melting the most mafic suite the hawkins suite contains plagioclase cordierite orthopyroxene biotite and quartz as restite components with less abundant almandine these rocks are chemically equivalent to mafic biotite rich and cordierite bearing granitoids from parts of the berridale and murrumbidgee batholiths garnet is absent from the goobarragandra suite volcanics which are a little more felsic and close in compositon to granitoids of the young and maragle batholiths the s type character of the laidlaw suite is less pronounced although a sedimentary source seems to be established such a source would be less mature than those for the other two volcanic suites the laidlaw suite resembles but cannot be closely identified with felsic s type granitoids of the murrumbidgee and berridale batholiths low grade regional metamorphism in most of the volcanic rocks has resulted in much mineralogical alteration and mobility of alkali and alkaline earth elements despite this alteration unaltered rocks are present in all three suites compositional data for the phenocryst phases within the unaltered rocks has allowed estimation of some intensive parameters hawkins suite volcanics were extruded directly from their source region and preserve phenocryst equilibria established at 5 6 kbar at 800 c the laidlaw and goobarragandra suites reequilibrated at lower pressures than their source before extrusion the laidlaw suite reequilibrated at estimated temperatures of 725 730 c o2 of 10 15 5 to 10 14 bars and h2o of 2 to 2 5 kbar differing mineral compositions in the three suites are related to differing source rock compositons and oxygen fugacities relative biotite and orthopyroxene mg is possibly pressure dependent

incremental growth of an upper crustal a type pluton argentina evidence of a re used magma pathway abstract carboniferous igneous activity in the sierra de velasco nw argentina led to the emplacement of several magmas bodies at shallow levels t 5 8 and ehf t in zircon 2 9 to 4 5 the second group comprises the metaluminous to peraluminous granitic rocks of the youngest units san blas and hualco which were formed by a heterogeneous mixture between mantle and crustal sources end t 0 6 to 4 8 and ehf t in zircon 3 to 6 our results provide a comprehensive view of the evolution of an intrusive complex formed from multiple non consanguineous magma intrusions that utilized the same magmatic plumbing system during downward transfer of host materials as the plutonic system matures the ascent of magmas is governed by the visco elastic flow of host rock that for younger batches include older hot magma mush the latter results in ductile downward flow of older during rise of younger magma such complexes may reflect the plutonic portion of volcanic centers where chemically distinct magmas are erupted

inherited and magmatic zircon from neogene hoyazo cordierite dacite se spain anatectic source rock provenance and magmatic evolution

andalusite and na and li rich cordierite in the la costa pluton sierras pampeanas argentina textural and chemical evidence for a magmatic origin the la costa pluton in the sierra de velasco nw argentina consists of s type granitoids that can be grouped into three igneous facies the alkali rich santa cruz facies scf sio2 67 wt distinguished by the presence of andalusite and na and li rich cordierite na2o 1 55 1 77 wt and li2o 0 14 0 66 wt the anillaco facies sio2 74 wt with a significant proportion of mn rich garnet and the anjullon facies sio2 75 wt with abundant albitic plagioclase the petrography mineral chemistry and whole rock geochemistry of the scf are compatible with magmatic crystallization of na and li rich cordierite andalusite and muscovite from the peraluminous magma under moderate p t conditions 1 9 kbar and ca 735 c the high li content of cordierite in the scf is unusual for granitic rocks of intermediate composition

a high resolution14c dated sediment sequence from southwest sweden age comparisons between different components of the sediment a macrofossil rich glaciomarine marine lacustrine sediment from a soft water lake in southwestern sweden has provided an opportunity to 14c date different components of its sediments bulk sediment dates are 100 to 500 yr older than fragile terrestrial macrofossils of corresponding levels with a mean age difference of ca 300 yr this is explained by the presence of old and reworked organic material in the sediment five age comparisons between terrestrial macrofossils and periostraca of marine bivalves probably arctica islandica of allerod age give a mean age difference of 380 yr i e slightly greater than the present reservoir age of 340 30 yr for the swedish west coast this difference is roughly the same as for the marine macroalga desmarestia aculeata a date from shell carbonate fragments of mytilus edulis yields an age that is more than 1000 yr older than corresponding periostraca and terrestrial macrofossils and 500 1000 yr older than the age of the supposed deglaciation of the site altogether this indicates a larger marine reservoir effect during the allerod than at present 1998 john wiley sons ltd

miocene andalusite leucogranite in central east himalaya everest masang kang area low pressure melting during heating abstract the studied miocene andalusite bearing leucogranites intrude the upper part of the high himalayan crystallines hhc and the north himalayan domes and outcrop in an area stretching from mt everest to the kula khangri massif bhutan towards the east the leucogranites constitute both dykes as well as sills and parts of larger andalusite free leucogranite plutons e g makalu they represent mainly of two mica muscovite biotite tourmaline cordierite andalusite sillimanite dumortierite leucogranite and tourmaline muscovite tourmaline biotite andalusite sillimanite garnet kyanite spinel corundum leucogranites microstructures reveal several generations of andalusite from residual peritectic early magmatic to cotectic late magmatic even in the same sample the occurrence of residual and or peritectic andalusite together with inclusions of sillimanite biotite in cordierite indicates that melts formed by dehydration melting of biotite at t 660 700 c during prograde heating at low pressure conditions p according to current models leucogranites are produced by dehydration melting of muscovite and or biotite during exhumation of the hhc in this case micas are consumed in the sillimanite stability field as a consequence these models cannot explain the occurrence of residual and or peritectic magmatic andalusite conditions for anatexis in the andalusite field may have been achieved by heat transfer within the exhuming extruding hhc from structurally lower and hotter rocks towards upper and colder fertile lithologies

gas bubble transport and emissions for shallow peat from a northern peatland the role of pressure changes and peat structure gas bubbles are an important pathway for methane release from peatlands the mechanisms controlling gas bubble transport and emissions in peat remain uncertain the effects of hydrostatic pressure and peat structure on the dynamics of gas bubbles in shallow peat were therefore tested in laboratory experiments a peat monolith was retrieved from a raised bog and maintained in a saturated state three distinct layers were identified from noninvasive permittivity measurements supported by soil physical properties porosity bulk density phase i of the experiment involved monitoring for the accumulation of gas bubbles under steady pressure and temperature conditions the data showed evidence for gas bubbles being impeded by a shallow semiconfining layer at depths between 10 and 15 cm visible gas bubbles observed on the side of the sample box were recorded over time to estimate changes in the vertical distribution of volumetric gas content porosity estimates derived using the complex refraction index model crim suggest that gas bubbles enlarge the pore space when the exerted pressure is high enough phase ii involved triggering release of trapped bubbles by repeatedly increasing and decreasing hydrostatic pressure in an oversaturated condition comparison of changes in pressure head and methane density in the head space confirmed that the increasing buoyancy force during drops in pressure is more important for triggering ebullition than increasing mobility during increases in pressure our findings demonstrate the importance of changes in hydrostatic pressure on bubble size and variations in resistance of the peat fabric in regulating methane releases from peatlands

hydrologic and hydraulic effects of riparian root networks on streambank stability is mechanical root reinforcement the whole story abstract riparian vegetation has a number of effects on the mechanisms by which streambanks fail some positive and some negative previous research has shown that the effect of mechanical root reinforcement on soil stability can be considerable and can be successfully quantified and included in streambank stability models root networks contained within a soil matrix however also have effects on the hydrologic and hydraulic processes acting on a streambank and although these effects are often discussed they have generally been difficult to quantify this paper summarizes the results of field data collection laboratory testing and computer simulations carried out to better quantify the effects of riparian vegetation on hydrologic and hydraulic processes occurring along streambanks first the evapotranspiration potentials of different riparian species were isolated by setting up an experiment to grow young riparian trees and switch grass in separate soil columns each instrumented with tensiometers at 30 cm and 70 cm depths and compared against bare control columns the hydrological reinforcement provided to the soil from increased apparent cohesion as a result of enhanced matric suction was estimated to range from 1 0 to 3 1 kpa in spring when bank stability was most critical and up to a maximum of 5 0 kpa in the summer second a vertical jet test device was used to measure rates and volumes of scour in soils permeated by switch grass roots results showed that the volume of soil scoured during a test declined non linearly with increasing root volume per unit volume of soil and with increasing root length density rld and increasing root biomass calculation of relative soil detachment rates rsd showed that with the highest rooting densities measured in the field jet tests eroded soil volume was 10 of that in the tests with no roots third the effects of enhanced matric suction from evapotranspiration and decreased soil erodibility because of the presence of plant roots were modeled using bstem 5 1 to quantify the effects on streambank factor of safety fs and to compare with the effects of mechanical root reinforcement the sensitivity analysis showed that the change in soil matric suction from evapotranspiration provided the greatest potential benefit to fs but only during the summer months during the winter and spring months root reinforcement remained the most important contributor to fs the sensitivity analysis conducted here also showed that whilst roots are capable of reducing the volume of hydraulic scour the resulting effect on streambank geometry did not increase fs as much as changes in soil matric suction and or mechanical root reinforcement

comprehensive chemical analyses of natural cordierites implications for exchange mechanisms abstract cordierite samples from pegmatites and metamorphic rocks have been analysed for major electron microprobe analysis empa and trace elements inductively coupled plasma mass spectrometry icp ms secondary ion mass spectrometry analyses sims as well as for h 2 o and co 2 coulometric titration and the results evaluated in conjunction with published data in order to determine which exchange mechanisms are significant apart from the homovalent substitutions femg 1 and mnmg 1 on the octahedral site some minor kna 1 on the ch0 channel site and fe 3 al 1 on the t 1 1 tetrahedral site the three most important substitution mechanisms are those for the incorporation of li on the octahedral sites nali 1 mg 1 and of be and other divalent cations on the tetrahedral t 1 1 site nabe 1 al 1 and na mg fe 2 1 al 1 the dominant role of the last vector is clearly demonstrated we propose a new generalized formula for cordierite ch na k 0 1 vi mg fe 2 mn li 2 iv si 5 iv al 3 iv al be mg fe 2 fe 3 o 18 x ch h 2 o co 2 our results show that the population of mg fe 2 on the t 1 1 site is limited to about 0 08 a p f u other exchange mechanisms that were encountered in experiments operate only under p t conditions or in bulk compositions that are rarely realized in nature routine analyses by electron microprobe in which li and be are not determined can be plotted as mg fe mn versus si al to assess whether significant amounts of li and be could be present these amounts can be calculated as li a p f u al na 4 and be a p f u 10 2al m 2 na

dissolution of corundum and andalusite in h2o saturated haplogranitic melts at 800 c and 200 mpa constraints on diffusivities and the generation of peraluminous melts of h2o in the aluminum alkali eigenvector could not be obtained the mechanisms and kinetics of equilibration between peraluminous from our results we propose that some h2o may provide charge minerals and granitic melt were investigated experimentally by balance for excess aluminum multicomponent diffusion models the dissolution of corundum and andalusite into h2o saturated yield eigenvalues for the corresponding aluminum alkali eigenvector metaluminous haplogranitic melt at 800 c and 200 mpa mineral of 0 3 10 to 1 9 10 cm s calculated effective and haplogranitic glass rods were juxtaposed inside platinum binary diffusion coefficients for al2o3 are higher 1 0 10 to capsules and then subjected to experimental conditions for times 3 6 10 cm s these diffusion coefficients are used to ranging from 12 to 2900 h upon melting the mineral melt calculate equilibration times between peraluminous minerals and interface retreats with the square root of time the composition of granitic melts in a model system in which two grains of corundum the melt at the interface changes with time but its asi aluminum or andalusite 1 cm apart are connected via a narrow melt column saturation index molar al2o3 cao na2o k2o at the conditions of the experiments diffusive equilibrium through remains constant at 1 20 after 480 h this value is close to initially metaluminous h2o saturated melt would be attained within the asi of an h2o saturated haplogranitic melt in equilibrium with 10 10 years at h2o activity well below saturation of the corundum or andalusite which may indicate that the composition of melt lower diffusivity of all components may increase the time to the melt at the interface follows the liquidus of these minerals in equilibration to 10 10 years approaching possible time frames the system sodium and potassium diffuse rapidly uphill across the for the generation and extraction of crustal melts entire length of the charge towards the interface resulting in a rapid and uniform increase in asi throughout the entire melt column this uphill diffusion of alkalis is due to coupling with excess aluminum entering the melt at the interface and involves long range

stability of streambanks formed in partially saturated soils and effects of negative pore water pressures the sieve river italy abstract streambanks of alluvial channels are usually composed of loose materials which are unsaturated in ambient conditions unsaturated soils are subject to negative pore water pressures which cause an apparent cohesion the latter is the main factor in allowing the stability of near vertical banks even during moderate in bank flow events the apparent cohesion can be strongly reduced as the material approaches full saturation therefore during the drawdown phase as the confining pressure of the water in the channel disappears a bank failure is likely to occur channel bed level lowering along the sieve river central italy has caused widespread bank instability a geomorphological reconnaissance of forms and processes was followed by in situ tests to determine the shear strength of the banks interpretation of the tests and a streambank stability analysis were based on concepts of soil mechanics for unsaturated soils in order to obtain relations between bank angle and height in limit equilibrium conditions a stability chart was obtained with curves for different apparent cohesion values and a stability analysis was performed taking into account the effects of flow events in order to investigate the pore pressure effects a series of piezo tensiometers were installed in a streambank of the sieve river data from a 1 year monitoring period show variations in pore water pressure and matric suction as a consequence of rainfall evapotranspiration and water stage variations a planar failure with a tension crack occurred in the upper cohesive part of the bank during december 1996 the safety factor has been expressed as a function of the geometry of the bank and of the shear strength of the material safety factor variations through time are therefore shown as a function of seasonal variations in matric suction

soil creep in salt marshes a puzzling aspect of marsh morphodynamics is the occurrence of channel bank slumping despite the absence of channel widening and lateral migration an apparently unrelated conundrum is why the vertical accretion rate on the low marsh adjacent to channels is higher than the rate of relative sea level rise this sedimentation excess should not occur at steady state or in a regime of accelerated sea level rise here we suggest that lateral erosion and vertical sedimentation on channel banks are linked by creep the process by which soil is moved downslope by gravity the model presented herein predicts a dynamic equilibrium configuration for the cross section such that sedimentation excess on the channel banks is transferred by creep toward the channel where tidal flow erosion and suspended sediment transport directed toward the banks close the sediment budget this model predicts that bank slumping is an equivocal indicator of ongoing marsh loss and that high sedimentation rates on channel banks do not necessarily constitute a margin of resilience to accelerated sea level rise

a multiproxy peat record of holocene mangrove palaeoecology from twin cays belize the extent and function of coastal mangrove ecosystems are likely to be influenced by future changes in sea level multiple proxies of past mangrove ecosystems preserved in a 780 cm long peat core tcc2 taken from twin cays belize record palaeoecological changes since 8000 cal yr bp the proxies included pollen and the stable isotope c n and o compositions of mangrove leaf fragments rhizophora mangle red mangrove has been dominant at this site on twin cays for over 8000 years variations in 13 c and 15n suggest past changes in stand structure between dwarf transition and tall r mangle through the holocene marked changes in the 18o up to 4 of mangrove leaf fragments throughout tcc2 most likely record variations in the proportion of seawater versus precipitation taken up by past mangroves reflecting the degree of inundation of the site with seawater resulting from changes in the rate of holocene sea level rise notably a decline in peat accumulation rate at 7200 cal yr bp correlate

assessing bulk assimilation in cordierite bearing granitoids from the central system batholith spain experimental geochemical and geochronological constraints

origin of cordierite bearing granites by assimilation in the central iberian massif cim spain abstract the central iberian massif cim is characterized by the great abundance and variety of granitoids emplaced during hercynian times one of the most conspicuous is cordierite bearing biotite granite cg which in the western part of the cim is associated with other granites lacking cordierite amphibole bearing biotite granite ag and biotite granite bg and with anatectic host rocks cordierite rich nebulites nb major trace and rare earth element data for ag and bg show that these granites have chemical affinities and parallel or overlapping differentiation trends suggesting two different granite magmas with a common or similar deep seated source cg show two extreme facies a marginal cordierite rich facies mcg in direct contact with host rocks schists migmatites and an internal cordierite poor facies icg farther away from the host rocks gradual transitions between mcg and icg are commonly observed as well as between icg and bg an anatectic hypothesis for cg is difficult to accept because a cg have higher cao contents relative to nb or other country rocks a smaller eu eu ratio and similar sr contents and b the similar ree absolute contents and patterns in cg bg and nb and the absence of ree fractionation with respect to country rocks simple mixing equations are computed to test assimilation processes involving bg as a primary magma and nb as the contaminant in producing cg cao and mgo data have been used for this purpose since they are the most discriminating elements between bg and nb the results of the computation indicate that a assimilation is possible for nearly the whole cao range in nb if cao in the bg ranges between 1 91 and 2 86 and mgo is between 0 93 and 1 18 these extreme values are comprised within the observed range for cao and mgo in bg b cao and mgo data for mcg are compatible with assimilation for nearly the whole range of data observed and c the percentage of assimilation is

the probable occurrence of interstitial al in hydrous f bearing and f free aluminosilicate melts experiments carried out on the system sio2 naalsi3o8 kalsi3o8 qz ab or at 1 kbar in the presence of h2o and f show that the quartz alkali feldspar field boundary is progressively displaced towards the feldspar join as f contents increase from 0 to 4 wt f increasing p h 2 o in the absence of f has already been shown to have a similar effect tuttle and bowen 1958 luth jahns and tuttle 1964 the increased size of the quartz field in the f bearing system compared to the hydrous system is believed to be caused by progressive removal of al from the tetrahedral network of the melt by complexing with f the residual network in the melt is thus enriched in si and this stabilizes precipitation of quartz rather than feldspar for certain bulk compositions the common presence of quench cryolite na3alf6 in certain experiments carried out with 4 wt f supports this interpretation and indicates that some al in the melt may be present in six fold coordination with f the effect of h2o in the absence of f may be similar with al being progressively removed from four fold coordination as more h2o is dissolved in the melt although a proportion of al in hydrous melts may occur in six fold coordination dry melts predominantly contain al in four fold coordination this major difference in al complexing may be one of the main causes for differences in the high pressure phase relations of wet and dry albearing silicate systems

geochemical evidence for relict degassing pathways preserved in andesite andesitic arc volcanoes degas large quantities of volatiles evidence for vapour transport in the erupted lavas is rarely preserved and poorly understood but is crucial for understanding eruption style we present geochemical evidence for the transport of metal bearing vapour in shear zones preserved in lavas erupted from soufriere hills volcano montserrat textural evidence suggests that shear induced brittle failure occurred in a narrow zone at the conduit wall or in the lava dome elevated metal concentrations cu au ag pb zn within the zones indicate that the fractures acted as a transient pathway for metal bearing magmatic gases during slip frictional heating to temperatures of 1000 c caused partial melting at the slip surface resorption of volatiles and metals into the partial melt preserved the geochemical signature of magmatic vapour in the shear zone cordierite which is highly unusual in volcanic rocks crystallised from the peraluminous partial melt with metal bearing sulphides and oxides the shear zones provide the first geochemical evidence for vapour segregation and transport through viscous andesite magmas and provide an insight into controls on eruption style

atmospheric14c changes resulting from fossil fuel co2 release and cosmic ray flux variability a high precision tree ring record of the atmospheric14c levels between 1820 and 1954 is presented good agreement is obtained between measured and model calculated 19th and 20th century atmospheric 14c levels when both fossil fuel co2 release and predicted natural variations in14c production are taken into account the best fit is obtained by using a diffusion model with an oceanic eddy diffusion coefficient of 3 cm2 s a co2 atmosphere ocean gas exchange rate of 21 moles m 2 yr 1 and biospheric residence time of 60 years for trees in the state of washington the measured 1949 1951 atmospheric 14c level was20 0 1 2 below the 1855 1864 level model calculations indicate that in 1950 industrial co2 emissions are responsible for at least 85 of the 14c decline whereas natural variability accounts for the remaining 15

estimating carbon accumulation rates of undrained mires in finland application to boreal and subarctic regions equations based on empirical relationships of peat physical properties were used to estimate the average long term apparent rate of carbon accumulation lorca in finnish mire vegetation regions the results were generalized to the boreal and subarctic regions analyses of 1302 dated peat cores were used to infer carbon accumulation for each mire vegetation region of finland the area weighted lorca for finnish undrained mire areas was 18 5 g m 2 yr 1 and the total carbon sink 0 79 tg yr 1 1 tg 1012g the total carbon pool of finnish undrained mires was estimated as 2257 tg the aapa mire region included 80 of the total net accumulation rate of carbon and 85 of the total carbon reservoirs of finnish undrained mires lorca was signi cantly higher in the raised bog region 26 1 g m 2 yr 1 compared with the aapa mire region 17 3 g m 2 yr 1 and bogs generally had a higher lorca 20 8 g m 2 yr 1 than fens 16 9 g m 2 yr 1 the total c sink for boreal and subarctic mires was estimated at 66 tg yr 1 whi

phase relations of peraluminous granitic rocks and their petrogenetic implications peraluminous granitic rocks are magmatic rocks that contain quartz po tassic feldspar and or sodic plagioclase and one or more aluminum rich minerals thus these rocks contain more aluminum than could be accom modated by feldspar in a cipw normative calculation and show normative corundum peraluminous granites are defined by their chemical nature but their recognition is often based on petrography this discrepancy thus may lead to inconsistency although peraluminous granitic rocks are com monly true granite streckeisen 1973 they range from tonalite to granite of special interest are strongly peraluminous as defi ned below granite granodiorite or in rare instances trondhjemite these rocks contain alumi nous minerals almandinic garnet cordierite tourmaline muscovite that are thought to be stable in silicate magmas only if these magmas are themselves peraluminous the origin of peraluminous granite and related intrusive rocks is of much current interest many of these form shallow level intrusions and they can reach enormous sizes examples are the batholiths in the lachlan fold belt in southeastern australia white chappell 1983 the cornub ian batholith in england exley stone 1966 the south mountain batholith in nova scotia mckenzie clarke 1975 the manaslu batho lith in the himalayas lefort 1981 much of the idaho batholith

phase equilibria and compositions of fe mg al minerals and melts in water saturated peraluminous granitic systems phase assemblages and phase compositions were studied experimentally in water saturated biotite bearing peraluminous granitic melts as a function of alumina excess and temperature the runs were performed at 2 and 5 kbars under nno buffer biotite was stable only in composition containing 5 of normative corundum it coexisted with cordierite and hercynite at 2 kbars and with hercynite at 5 kbars in composition containing 10 of normative corundum biotite was not observed abundant cordierite and hercynite were the only fe mg al minerals these relationships show that at constant pressure the amount of cordierite increases with increasing excess of alumina simultaneously the stability of biotite decreases due to preferential partitioning of mg into cordierite and fe into biotite besides the distribution of fe mg and al among the coexisting solid phases solubility of these elements in the melts is given below 900 c melts are poor in iron and magnesium and correspond in terms of these elements to leucogranites it is suggested that the leucogranitic magmas such as parental magmas of european hercynian and himalayan leucogranites must have been formed through highly efficient separation of partial melt from restite in which ferromagnesian components are concentrated peraluminous granites rich in ferromagnesian minerals originate supposedly from restite bearing magmas

origin and evolution of a peraluminous silicic ignimbrite suite the violet town volcanics the violet town volcanics are a 373 ma old comagmatic s type volcanic sequence mainly comprising crystal rich intracaldera ignimbrites rock types vary from rhyolites to rhyodacites all containing magmatic cordierite and garnet phenocrysts variation in the suite is primarily due to fractionation of early crystallized quartz plagioclase and biotite plus minor accessory phases in a high level magma chamber prior to eruption early magmatic crystallization occurred at around 4 kb and 850 c with melt water contents between 2 8 and 4 wt this high temperature markedly water undersaturated restite poor granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain leaving a granulite facies residue

stability of andalusite and sillimanite and the al_ 2 sio_ 5 triple point constraints from the ballachulish aureole scotland andalusite and and sillimanite sil occur in well defined zones in the ballachulish aureole in most of the aureole s sillimanite bearing rocks sillimanite grain diameters range from 2 m to 10 m the boundary between prograde andalusite and prograde sillimanite is constrained to a zone of max 30 m width upgrade of the muscovite quartz andalusite k feldspar h 2 o reaction isograd this boundary is interpreted to correspond to the and sil equilibrium the common occurrence of andalusite well upgrade of the first occurrence of sillimanite is ascribed to sluggish kinetics of the polymorphic inversion the p t position of the and sil equilibrium in the aureole is estimated as 645 20 c and 3 0 0 4 kbar based on a combination of 1 location of the ms qtz and kfs h 2 o reaction corrected for non end member fluid and mineral compositions and 2 independent pressure estimates obtained from equilibria in metapelites marbles calcsilcates and granitoids all in exce

a record of late pleistocene and holocene carbon accumulation and climate change from an equatorial peat bog kalimantan indonesia implications for past present and future carbon dynamics a 9 5 m core from an inland peatland in kalimantan indonesia reveals organic matter accumulation started around 26 000 cal yr bp providing the oldest reported initiation date for lowland ombrotrophic peat formation in se asia the core shows clear evidence for differential rates of peat formation and carbon storage a short period of initial accumulation is followed by a slow rate during the lgm with fastest accumulation during the holocene between 13 000 and 8000 cal yr bp 450 cm of peat were deposited with highest rates of peat 2 mm yr 1 and carbon 90 g c m 2 yr 1 accumulation between 9530 and 8590 cal yr bp these data suggest that kalimantan peatlands acted as a large sink of atmospheric co2 at this time slower rates of peat 0 15 0 38 mm yr 1 and carbon 7 4 24 0 g c m 2 yr 1 accumulation between 8000 and 500 cal yr bp coincide with rapid peat formation in coastal locations elsewhere in se asia the average lorca long term apparent carbon accumulation rate for the 9 5 m core is 56 g c m 2 yr 1 these data suggest that studies of global carbon sources sinks and their dynamics need to include information on the past and present sizeable peat deposits of the tropics copyright 2004 john wiley sons ltd

origin and evolution of mid cretaceous garnet bearing intermediate and silicic volcanics from canterbury new zealand abstract the mt somers volcanics are part of a suite of mid cretaceous 89 2 ma intermediate to silicic volcanics erupted onto an eroded surface of torlesse sediments rock types vary from basaltic andesite to high silica rhyolite andesites are medium to high k with phenocrysts of plagioclase orthopyroxene and pigeonite dacites are peraluminous and commonly contain granulite facies xenoliths and garnet xenocrysts equilibrium mineral assemblages indicate metamorphic pressures of close to 6 kbar at 800 c rhyolites are peraluminous with phenocrysts of quartz sanidine plagioclase biotite garnet and orthopyroxene the ferromagnesian phases show textural evidence of magmatic crystallization and are chemically distinct from xenocryst phases in dacites equilibrium assemblages indicate that early magmatic crystallization occurred at close to 7 kbar 20 km depth at above 850 c with melt water contents of less than 3 5 major element contents trace element contents and an initial 87 sr 86 sr ratio of 0 7085 indicate that the rhyolites formed by partial melting of dominantly quartzo feldspathic torlesse sediments leaving a granulite facies residue the chemical variation displayed by the rhyolites is best explained by fractional crystallization of the observed high pressure phenocryst assemblage most elements show a compositional gap between rhyolite and dacite the major element trace element and sr isotope compositions of the intermediate lavas are best explained by assimilation of lower crustal material combined with fractional crystallization in mantle derived tholeiitic magmas magmatism was the result of heat and magma flux from the mantle during the change from compressive to extensional tectonics after the culmination of the rangitata orogeny

petrology of the coyaguayma ignimbrite northern puna of argentina origin and evolution of a peraluminous high sio2 rhyolite magma abstract the coyaguayma ignimbrite is a strongly peraluminous sp sillimanite garnet bearing silicic rhyolite which erupted in the northern puna segment of the central andean plateau in the upper miocene 11 ma a period that was characterized by the eruption of voluminous 100s to 1000s km3 dacitic ignimbrites of high k calc alkaline affinity in this region the sp magmatic rocks are both rare and small but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central andes the phenocryst assemblage of the coyaguayma ignimbrite comprises plagioclase an26 18 quartz ba rich and ba poor sanidine minor al rich ferromagnesian minerals alt and alvi rich biotite almandine spessartine garnet and sillimanite as well as accessory zircon and monazite textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars mineral equilibria suggest that crystallization of the rhyolite magma began at 5 kbar and 800 c and continued almost isobarically to 720 c causing the residual liquid to increase h2o contents from 4 5 to 7 5 before eruption most major features e g high sio2 a cnk 1 3 low cao mgo tio2 and feo and trace element patterns low ba sr th lree and eu eu high rb u y and nb along with the thermobarometric constraints on magmatic p t and water contents are consistent with an origin by mica dehydration melting of metapelitic sources e g typical biotite muscovite gneisses from the outcropping s puna basement however the relatively low initial 87sr 86sr 0 7125 and high 143nd 144nd 0 512200 ratios invalidate a pure crustal origin instead we propose contamination of calc alkaline dacitic magmas similar to typical puna ignimbrites by metapelite at mid crustal settings 18 km depth geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70 dacite and 30 of metapelite partial melts followed by extensive 70 fractionation of plagioclase k feldspar quartz and biotite with minor magnetite and apatite from the hybrid magma sillimanite in the coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite which was preserved intact in the hybrid melt due to local equilibrium this petrogenetic model explains most characteristics of crystal poor sp rhyolites from the puna plateau e g tocomar ramadas rhyolites and it may be more generally applicable to occurrences of sp magmas in andean type continental arcs dominated by calc alkaline magmatism

s type granites and their probable absence in southwestern north america criteria that have been and still are used to characterize s type granites of the lachlan fold belt lfb of southeastern australia are reviewed and comparisons are made with various peraluminous granites of southwestern north america some of which have been classified as s types on the basis of insufficient data virtually all of the vast volume of s type granites in the lfb are near surface batholithic granites that are commonly associated with s type volcanics and are not associated with regional metamorphic rocks and migmatites they are strongly peraluminous as shown by the presence of cordierite granites with primary muscovite are rare all are low in na ca and sr as a result of chemical weathering during formation of the sedimentary sources peraluminous granites of various ages in southwestern north america are distinctly different they rarely contain cordierite a mineral characteristic of lfb s types but some are highly evolved such that fe mn rich garnet has crystallized they are dominantly two mica granites indicating crystallization at higher water fugacities and greater depths than most peraluminous granites of the lfb cordierite bearing volcanics s types have not been reported sodium is generally high in the peraluminous granites of southwestern north america some of these rocks have trondhjemitic affinities the parent magmas seem more likely to have been produced by partial melting of altered basaltic rocks locally some peraluminous rocks marginal to metaluminous types may owe their compositions to high level contamination of i types these are not s type rocks no compelling evidence has been presented that any of the peraluminous granites of southwestern north america are s types

f rich strongly peraluminous a type magmatism in the pre andean foreland sierras pampeanas argentina geochemical geochronological isotopic constraints and petrogenesis abstract the petrogenetic nature of a type granites is a controversial problem the vinquis batholith in the sierras pampeanas of argentina contains unusual f rich and strongly peraluminous a type monzogranites a new la mc icp ms u pb zircon crystallization age of 355 7 ma indicates emplacement in latest devonian or earliest carboniferous time overlapping with extensive metaluminous a type magmatism in the area the monzogranites have a restricted range of sio 2 content 71 5 74 8 they are poor in ca 0 54 1 4 cao and rich in feo t with relatively high feo t feo t mgo values ranging from 0 77 to 0 86 average 0 80 both feo t feo t mgo vs sio 2 and na 2 o k 2 o cao vs sio 2 plots indicate ferroan and alkali calcic signatures typical of a type granitoids the samples have mgo tio 2 1 2 and are moderately enriched in total alkalis average 8 18 with high k 2 o na 2 o values of 1 40 2 24 the granites are strongly peraluminous with asi molar al 2 o 3 cao na 2 o k 2 o values of 1 2 to 1 3 the high p 2 o 5 content 0 23 0 37 is distinctive and close to values reported for other paleozoic f rich peraluminous a type granites in the sierras pampeanas they have moderate contents of high field strength elements e g zr nb th y etc and moderately fractionated to flat ree patterns la yb n in the range 4 8 19 6 showing significant negative eu anomalies eu eu 0 41 biotite has a distinctive composition with relatively high fe 2 fe 2 mg ratios 0 61 0 74 and high f 0 55 1 42 wt content together with the whole rock chemistry this may be useful in identifying strongly peraluminous a type granites in addition the rb sr vs th zr ce diagram may be an appropriate discriminant between metaluminous and peralkaline a type granites strongly peraluminous a type granites and strongly peraluminous orogenic granites the geochemical evidence indicates that differentiation of the granitic rocks occurred by mineral fractionation from a f rich peraluminous parental magma dominant of plagioclase k feldspar biotite and accessory minerals such as zircon monazite xenotime and oxides the peraluminous composition and isotope data e hf 355 and e nd 355 ranging from 9 5 to 1 5 and 6 1 to 7 8 respectively together with abundant inherited ordovician and cambrian zircon strongly suggest a dominantly metasedimentary source the whole rock and biotite compositions indicate that the vinquis batholith crystallized under mainly oxidizing conditions whereas early carboniferous metaluminous to weakly peraluminous a type granites of the sierras pampeanas crystallized under dominantly reduced conditions

forest dynamics and tip up pools drive pulses of high carbon accumulation rates in a tropical peat dome in borneo southeast asia singapore national research foundation singapore mit alliance for research and technology

development and carbon sequestration of tropical peat domes in south east asia links to post glacial sea level changes and holocene climate variability abstract tropical peatlands of se asia represent a significant terrestrial carbon reservoir of an estimated 65 gt c in this paper we present a comprehensive data synthesis of radiocarbon dated peat profiles and 31 basal dates of ombrogenous peat domes from the lowlands of peninsular malaysia sumatra and borneo and integrate our peatland data with records of past sea level and climate change in the region based on their developmental features three peat dome regions were distinguished inland central kalimantan borneo kutai basin borneo and coastal areas across the entire region with the onset of the holocene the first peat domes developed in central kalimantan as a response to rapid post glacial sea level rise over the sunda shelf and intensification of the asian monsoon peat accumulation rates in central kalimantan strongly declined after 8500 cal bp in close relation to the lowering rate of the sea level rise and possibly influenced by the regional impact of the 8 2 ka event peat growth in central kalimantan apparently ceased during the late holocene in association with amplified el nino activity as exemplified by several truncated peat profiles peat domes from the kutai basin are all younger than 8300 cal bp peat formation and rates of peat accumulation were driven by accretion rates of the mahakam river and seemingly independent of climate most coastal peat domes the largest expanse of se asian peatlands initiated between 7000 and 4000 cal bp as a consequence of a holocene maximum in regional rainfall and the stabilisation and subsequent regression of the sea level these boundary conditions induced the highest rates of peat accumulation of coastal peat domes the late holocene sea level regression led to extensive new land availability that allowed for continued coastal peat dome formation until the present the time weighted mean holocene peat accumulation rate is 0 54 mm yr 1 for central kalimantan 1 89 mm yr 1 for kutai and 1 77 mm yr 1 for coastal domes of sumatra and borneo the mean holocene carbon sequestration rates amount to 31 3 g c m 2 yr 1 for central kalimantan and 77 0 g c m 2 yr 1 for coastal sites which makes coastal peat domes of south east asia the spatially most efficient terrestrial ecosystem in terms of long term carbon sequestration

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

magmatic evolution of the penon rosado granite petrogenesis of garnet bearing granitoids garnet is an uncommon accessory mineral in igneous rocks but is petrologically significant the penon rosado granite 469 4ma at cerro asperecito is an s type granite that contains an unusual amount of magmatic garnet combined petrology chemistry mineralogy and whole rock geochemistry indicates that the magma was produced by partial melting of the surrounding metasedimentary rocks and subsequent differentiation by fractional crystallization during emplacement with garnet occurring throughout the crystallization sequence three facies are recognised prg1 sio2 65 70 represents cumulates prg2 sio2 70 88 represents a differentiated melt and prg3 sio2 74 59 a residual melt the fractionation of mn in garnet and the proportion of garnet crystallizing are roughly controlled by the evolving composition of the different granitic facies geothermobaric calculations reveal an initial crystallization temperature of 764 792 cand a pressure of 5 9 6 0 kb indicating that the parental magma was emplaced atmiddle crustal depths 1920 km in moderate low magmatic temperature conditions major cao na2o k2o and trace element rb sr ba contents in the penon rosado granite strongly suggest anatexis was the outcome of h2o fluxed melting of metagreywacke with heat input from a major metaluminous suite our studies reveal that garnet formed by direct crystallization from peraluminous magma in equilibrium with solid phases such as biotite and white mica we confirm previous suggestions that zoning in garnet is strongly temperature dependent thus magmatic garnets in granitic rocks crystallized above 700 c have spessartine inverse bell shaped profile or are unzoned whereas garnet exhibiting spessartine bell shaped profile must be considered of metamorphic origin i e xenocrystic or formed in very felsic magmas sio2 73 76 crystallizing below 700 c 2006 elsevier b v all rights reserved

mineral leucosome trace element partitioning in a peraluminous migmatite a laser ablation icp ms study abstract low pressure peraluminous migmatites from the pena negra complex central spain stayed partially molten for 5 10 ma until the latest hercynian deformations allowed the segregation of melt as discordant leucosome veins due to long residence within its source the melt had time enough to equilibrate with residual phases other than accessories included within major refractory minerals we estimated crystal melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals which were analyzed for trace elements with a laser probe coupled to an icp mass spectrometer our data reveal that when biotite is stable li rb cs tl sc v cr ni nb and ta become strongly compatible the role of biotite in fractionating y th u and the ree is insignificant cordierite strongly fractionates li and be and also has some effect on the hree and u garnet produces extreme fractionation of sc y and the hree the ree partition coefficients for ferromagnesian silicates increase with the atomic number this effect being progressively more important through biotite cordierite and garnet k feldspar strongly fractionates ba and pb but plays a secondary role to biotite for rb and cs k feldspar fractionation does not change the lree hree ratio but plagioclase fractionation produces a significant decrease in the lree hree ratio both feldspars greatly fractionate sr and eu monazite fractionation produces a dramatic depletion in ree th and u as well as a decrease in the lree hree and th u ratios apatite also fractionates the ree although in contrast with monazite it increases the lree hree ratio and does not affect the th u ratio zircon fractionation like apatite and garnet produces a strong depletion in the hree and a concomitant increase in the lree hree ratio in contrast with monazite zircon fractionation causes the th u ratio to increase

petrology of spinel granulites from araku eastern ghats india and a petrogenetic grid for sapphirine free rocks in the system fmas spinel quartz cordierite and spinel quartz are found as relic prograde assemblages in fe rich granulites from the araku area eastern ghats belt india subsequent reactions produced orthopyroxene sillimanite in the former association and garnet sillimanite in the latter the first reaction is univariant in the fmas system but is trivariant in the present case because of the presence of zn and fe3 in spinel the second reaction also has high variance because of zn and fe3 but also because of the presence of ca in garnet thermobarometry shows that the metamorphic conditions were approximately 950 c and 8 5 kbar and the fo2 was near the nno buffer in fe rich bulk compositions and low p high t conditions of metamorphism two of the univariant reactions around the invariant point sa namely sa hy and sa cd change topology due to reverse partitioning of fe mg between coexisting garnet and spinel an alternative partial petrogenetic grid in the system fmas is constructed for such conditions and is applied satisfactorily to several sapphirine free spinel granulites it is shown that bulk composition xfe and zn exerts greater control on the stability of spinel quartz than fo2 the effect of the presence of zn and fe3 in spinel on the proposed grid is evaluated reaction textures in the araku spinel granulites can be explained from the petrogenetic grid as due to near isobaric cooling

calculated relationships between activity of alumina and phase assemblages of silica saturated igneous rocks petrogenetic implications of magmatic cordierite garnet and aluminosilicate abstract many peraluminous igneous rocks are characterized by the presence of al rich minerals such as cordierite garnet and aluminosilicate polymorphs clearly one of the important thermodynamic intensive variables controlling the stability of these minerals relative to al poor phases such as ortho and clinopyroxene must be the activity of al 2 o 3 a al 2 o 3 in the melt calculated phase relationships of anhydrous mineral assemblages in equilibrium with quartz saturated liquids show that at crustal pressures less than about 10 kbar garnet cordierite and aluminosilicate are only likely to crystallize from melts in which a al 2 o 3 is up to one order of magnitude higher than that required for orthopyroxene clinopyroxene crystallization in spite of this peraluminous granitoid liquids saturated with orthopyroxene garnet or orthopyroxene cordierite do not ordinarily contain significantly larger amounts of excess al al in excess of that required to form feldspars than peraluminous liquids saturated with orthopyroxene clinopyroxene these observations indicate that there must be profound compositional controls on the activity coefficient of al in silicic melts and the strongest of such controls appears to be the total alkali content of the melt the activity coefficient of al in silicic melts varies directly with total alkali oxide content so that alkali poor melts can become markedly peraluminous without ever becoming saturated in aluminous phases whereas alkali rich melts of similar normative corundum content can crystallize garnet cordierite or even aluminosilicate because activity of alumina in a silicic melt is not a simple function of excess al content classification schemes for silicic igneous rocks based on model mineralogical assemblages which reflect the value of the thermodynamic effective concentration of al during crystallization convey more petrologic information than those based on chemical parameters such as normative corundum content or alumina saturation index

thermal evolution of upper crustal magmatic systems from the sierra de velasco nw argentina abstract detailed mapping of the huaco intrusive complex in nw argentina documents a shift in the emplacement style of successive magma additions in an upper crustal environment as the system evolves thermally the units of the huaco intrusive complex exhibit a change from a tabular to funnel shaped subhorizontal body huaco unit with stoping and brittle fracturing as main emplacement mechanisms to a cylindrical body sanagasta unit emplaced in a heated host rock which shows local ductile behaviour as the system cools the youngest intrusion la chinchilla unit emplaces again as a subhorizontal tabular to funnel shaped body furthermore magmatic fabrics recorded a regional strain field that changed from a sw ne to wsw ene shortening as well as boundary strain increments and stress reorientation our results demonstrate that the evolution of magma chambers involves the emplacement of magma batches in a stiff crust changing its thermal conditions and maturing the system for following intrusions the latter ascend as visco elastic elongated bodies using the same magmatic plumbing system leading to a shift in the emplacement style the described evolution is similar to that reported for the contemporaneous san blas intrusive complex located in the same range this allows us to affirm that these processes are recurrent in the upper crustal section and that the thermal and rheological conditions of the crust are locally modified by the formation and evolution of long lived magma reservoirs

radiometric dating 210pb 137cs 241am of recent ombrotrophic peat accumulation and evidence for changes in mass balance the paper summarizes and evaluates the results of 210pb 137cs and 241am analysis on 37 recent ombrotrophic peat profiles from these and previously published data it is concluded that radiometric measurements alone cannot be relied on to give an accurate chronology of peat accumulation where constrained by profiles of 241am activity in the upper part and one or more pollen dated horizons below this 210pb can provide good chronologies of peat accumulation provided the constraint does not displace the calculated 210pb dates by more than c 10 profiles of 137cs activity in most cases fail to contribute significantly to dating recent peat at somewhat degraded sites the residual dry mass of peat representing the last c 150 years is reduced further analysis of well dated profiles suggests that this may reflect loss of catolelm peat newly exposed to aerobic decomposition processes after a fall in water table several mechanisms are proposed for the displacement and net loss of lead including 210pb

cordieritite and leucogranite formation during emplacement of highly peraluminous magma the el pilon granite complex sierras pampeanas argentina cordieritites and highly peraluminous granites within the el pilon granite complex sierras pampeanas argentina were emplaced during a medium p high t metamorphic event during the initial decompression of a cambrian orogen along the southwestern margin of gondwana very fresh orbicular and massive cordieritite bodies with up to 90 cordieritite are genetically associated with a cordierite monzogranite pluton and a larger body of porphyritic granodiorite the petrogenesis of this association has been studied using petrographical mineralogical thermobarometric geochemical geochronological and isotope methods the granitic magmas were formed by anatexis of mid crustal metamorphic rocks formed earlier in the pampean orogeny the cordieritites appear at the top of feeder conduits that connected the source region located at 6 kbar with the pluton emplacement level at 3 7 0 3 kbar a fall in the liquidus temperature of the melt during emplacement was produced by a marked increase in fluid activity owing to rapid decompression and assimilation of surrounding hydrous metapelitic schists followed by isobaric crystallization high mg cordierite crystallized early on biotite sillimanite restitic mineral assemblages of the assimilated schists or at the wall of the feeder conduits strong convection in the small magma chamber caused flow segregation of cumulate cordierite and restite developing leucogranites and highly evolved pegmatoids that are in isotopic equilibrium rapid ascent of highly peraluminous magmas might explain why emplacement of these granites was simultaneous with the metamorphic peak registered in neighbouring high grade migmatite and granulite terranes

petrology geochemistry and genesis of high al tonalite and trondhjemites of the cornucopia stock blue mountains northeastern oregon tonalitic and trondhjemitic compositions span a narrow range of trondhjemite sio2 content 65 74 wt and exhibit characteristics of a highal tonalite trondhjemite granitoid ttg suite including light rare earth element lree enrichment low y nb and rb sr and high al2o3 and sr these compositions are consistent with an

composition and origin of garnet from the antarctic peninsula volcanic group of trinity peninsula garnet is widely found as a minor constituent in rocks of the antarctic peninsula volcanic group apvg of trinity peninsula it occurs as conspicuous megacrysts or in xenoliths within volcanic rocks of andesitic rhyolitic composition and as detrital grains in the associated terrestrial sediments it is also found as an accessory mineral in many plutonic rocks from the e coast of the antarctic peninsula evidence is presented to show that the garnet can be divided petrographically and chemically into two main groups type a almandine rich primary igneous garnet and type b less almandine and more pyrope rich garnet as xenocrysts or included in xenoliths within the volcanic rocks comparison with published experimental data on garnet occurrence in acidic igneous rocks suggests that high almandine low spessartine garnet in volcanic rocks is a remnant phase of high pressure crystallization from magma at pressures of 7 kbar garnet with a higher pyrope content is regarded as xenocrystal in origin having been derived from garnet bearing country rocks at depth either as accidental inclusions or through direct partial melting restite of the lower crust and implies that a considerable thickness 25 km of crustal material was in existence before the generation of the mesozoic magmatic arc the origin of these calc alkaline magmas may therefore be due at least in part to partial melting of pre existing sialic crustal material

solubility of excess alumina in hydrous granitic melts in equilibrium with peraluminous minerals at 700 800 c and 200 mpa and applications of the aluminum saturation index we have studied the controls on the alumi num saturation index asi molec al2o3 cao na2o k2o and the concentration of normative corundum of granitic liquids saturated in alumina by equilibrating peraluminous minerals with initially meta luminous haplogranitic minimum composition liquids at 700 800 c and 200 mpa at and below h2o saturation the asi and normative corundum increase with increasing h2o concentration in the melt 0 04 to 0 10 moles excess al2o3 per mole of h2o temperature and with addition of the non haplogranitic components fe mg and b the asi parameter and concentration of normative corundum cannot be used to monitor aal2o3 between different mineral assemblages and melt because other components that affect the solubility of alumina including h2o fe mg and b do not appear in their formulations asi and normative corundum however provide petrogenetic information about magmas gener ated by partial melting of strongly peraluminous proto liths by virtue of their regular and predictable variation with melt composition e g h2o concentration and temperature for the application of these data to natural rocks it is necessary to choose as an analogue system the asi solubility or normative corundum solubility relations of the most chemically complex peraluminous mineral present in the rock comparison of asi values of anatectic leucosomes and allochthonous leucogranites with experimentally predicted values suggests low h2o concentrations in melt during crustal partial melting rapid melt segregation before equilibration with restitic peraluminous phases is also suggested in some cases

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

sedimentology of the changuinola peat deposit organic and clastic sedimentary response to punctuated coastal subsidence an extensive peat deposit on the caribbean coast near changuinola panama has developed in an area subject to periodic earthquake driven coseismic subsidence thick low ash low sulfur peat is accumulating immediately behind an aggrading and prograding barrier system and adjacent to a flood prone sediment laden river measurements of changes in local sea level as a result of a recent april 1991 earthquake reveal 30 50 cm of subsidence greatest at the southeastern extent of the study area where the peat surface is submerged to a depth of 3 m beneath the shallow waters of almirante bay transgression is proceeding from southeast to northwest parallel to the trend of the coast and the long axes of both the peat deposit and the major sand bodies in the eastern part of the deposit the effects of sea level rise are evident in the degree of humification mineral matter and sulfur content of mangrove and back mangove peats offshore and immediately adjacent to the marine margin and in peats associated with brackish tidal channels that drain the deposit however most of the deposit shows no indications of marine influence even though 40 of the peat is below present sea level the western section of the deposit has evolved from low lying palm swamps which originated in swales on the barrier bar into an oligotrophic bog plain with a water table elevated 6 75 m above sea level as the mire evolved transitions in vegetation resulted in transitions in peat types highly humified forest swamp and palm swamp peats underlie and surround well preserved fibric sedge peats and create a partial hydrological bounding surface that restricts subsurface drainage from the central bog the high water table and elevated topography of the mire and the low permeability and erosion resistance of the dense woody peat effectively insulate most of the deposit from both clastic influx and the extensive intrusion of rising marine waters it is evident that thick peat and hence coal deposits can accumulate due to tectonically driven punctuated subsidence without leaving a record of high clastic input within the peat even immediately adjacent to environments of active clastic deposition

the significance of garnet in the permo carboniferous volcanic rocks of the pyrenees garnet occurs within rhyolitic to dacitic lavas ignimbrites and volcaniclastic sediments of the pyrenean calc alkaline permo carboniferous volcanic rocks on the basis of texture and major and trace element composition the garnet is interpreted as having a phenocrystal as opposed to a xenocrystal or restite origin the pyrenean silicic volcanics are strongly peraluminous consistent with derivation as a partial melt from a sedimentary source they were erupted contemporaneously with the hercynian thermal anomaly which produced regional granulite facies metamorphism and anatectic granites the late hercynian granodiorites were also intruded at about this time geochemical data suggest that the petrogenesis of the volcanic rocks differed from that of the granodiorites ree garnet matrix partition coefficients for garnets from a welded rhyolitic ignimbrite are similar to those reported for other phenocrystal garnets in rhyolites ree abundances imply that the garnets crystallized from a hree depleted melt and this is considered to reflect the effects of garnet in the restite experimental constraints suggest that the garnet crystallized at pressures 4 5kb implying that the rhyolites originated from 14 17km depth and were erupted rapidly from these depths in order for the garnet to survive these figures place constraints on the thermal regime prevailing in the late stages of the hercynian orogeny

the role of manganese in the paragenesis of magmatic garnet an example from the old woman piute range california a reply strongly peraluminous plutons of the old woman piute range southeastern california display the differentiation sequence muscovite biotite granite rightarrow muscovite garnet biotite granite rightarrow garnet muscovite granite rightarrow garnet muscovite aplite and pegmatite the garnets show no evidence of reaction but biotites in garnet bearing rocks are rimmed by muscovite metamorphic assemblages in the country rocks suggest pressures of emplacement less than 4 5 kb the plutons become relatively enriched in manganese with differentiation whole rock ratios of molar mn fe mg increase from 0 01 in least felsic garnet free rocks to 0 3 in most felsic garnet rich rocks mno concentrations show a corresponding rise in biotites 0 4 to 1 8 wt and less clearly in garnets 9 to 25 the differentiation sequence textural relations and mineral and whole rock compositions suggest that garnet paragenesis was controlled by the reaction biotite mn rich liquid rightarrow garnet

dissolution of quartz albite and orthoclase in h2o saturated haplogranitic melt at 800 c and 200 mpa diffusive transport properties of granitic melts at crustal anatectic conditions we have conducted experiments on dissolution of quartz albite orthoclase and corundum into h2o saturated haplogranite melt at 800 c and 200mpa over a duration of 120 1488h with the aim of ascertaining the diffusive transport properties of granitic melts at crustal anatectic temperatures cylinders of anhydrous starting glass and a single mineral phase quartz or feldspar were juxtaposed along flat and polished surfaces inside gold or platinum capsules with 10 wt added h2o concentration profiles in glass quenched melt perpendicular to the mineral glass interfaces and comparison with relevant phase diagrams suggest that melts at the interface are saturated in the dissolving phases after 384h and with longer durations the concentration profiles are controlled only by diffusion of components in the melt the evolution of the concentration profiles with time indicates that uncoupled diffusion in the melt takes place along the following four linearly independent directions in oxide composition space sio2 n a2o and k2o axes si na and k eigenvectors respectively and a direction between the al2o3 na2o and k2o axes al eigenvector such that the al na molar ratio is equal to that of the bulk melt and the al na k molar ratio is equal to the equilibrium asi mol al2o3 na2o k2o of the melt experiments in which a glass cylinder was sandwiched between two mineral cylinders quartz and albite quartz and k feldspar or albite and corundum tested the validity of the inferred directions of uncoupled diffusion and explored longrange chemical communication in the melt via chemical potential gradients the application of available solutions to the diffusion equations for the experimental quartz and feldspar dissolution data provides diffusivities along the directions of the si eigenvector and al eigenvector of 2 0 2 8 10 15 m 2 s and 0 6 2 4 10 14 m 2 s respectively minimum diffusivities of alkalis 3 9 10 11 m 2 s are orders of magnitude greater than the tetrahedral components of the melt the information provided here determines the rate at which crustal anatexis can occur when sufficient heat is supplied and diffusion is the only mass transport mixing process in the melt the calculated diffusivities imply that a quartzo feldspathic source rock with initial grain size of 2 3mm undergoing hydrostatic h2o saturated melting at 800 c infinite heat supply could produce 20 30 vol of homogeneous melt in less than 1 10 years slower diffusion in h2o undersaturated melts will increase this time frame

origin of peraluminous granites and granodiorites iberian massif spain an experimental test of granite petrogenesis the discrimination between potential source materials involved in the genesis of iberian granites and granodiorites as well as the role of mantle crust inter actions are examined using constraints imposed by melting experiments melting assimilation experiments and sr nd isotope systematics the sr nd isotope re lationships indicate the existence of diaerent genetic trends in which juvenile mantle materials are involved by diaerent mechanisms 1 a source trend traced by a particular evolution of the pre hercynian basement and indicating mantle participation at the time of sedimen tation 2 a set of magmatic trends traced by gabbro tonalite enclave granodiorite associations implying the incorporation of new mantle material at the time of granite generation these relationships strongly support a pure crustal origin for the peraluminous leucogranites derived from partial melting of crustal protoliths and a hybrid origin for the peraluminous granodiorites these granodiorites are the most abundant granitic rocks of the central iberian zone ciz of the iberian massif implying that processes of hybridisation by assimilation and or magma mixing played an important role in granitoid production during the hercynian orogeny these hypotheses have been tested by means of melting and assimilation experiments melting experiments in the range 800 900 c and at pressures of 3 6 10 and 15 kbar indicate that 1 several potential source materials such as bt ms gneisses and metagreywackes are suitable for the production of peraluminous leucogranite melts 2 the melt compositions are always leucogranitic re gardless of pressure 3 pressure exerts a strong influ ence on the fertility of the source experiments at 3 kbar produce more than 20 vol of melt compared with less than 5 vol of melt produced at 10 and 15 kbar and at the same temperature the melting assimilation experi ments carried out at 1000 c and 4 7 and 10 kbar and using a proportion of 50 gabbro and 50 gneiss give high melt proportions more than 50 vol and noritic residues these melts have the composition of le ucogranodiorites and overlap with part of the compo sitional range of peraluminous granodiorites of the iberian massif the generation of more mafic granodiorites may be explained by the incorporation of some residual orthopyroxene to the granodiorite mag mas the low solubility of fe mg prevents the gen eration of granodiorite melts with more than 3 wt of mgo feo at all crustal pressures the large volumes of peraluminous hybrid granodiorites produced by assimilation of crustal rocks by mantle magmas imply that an important episode of crustal growth took place during the late palaeozoic hercynian orogeny in the iberian massif

vapour absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases implications for anatexis in the deep to very deep continental crust and active continental margins

monitoring and numerical modelling of riverbank erosion processes a case study along the cecina river central italy riverbank retreat along a bend of the cecina river tuscany central italy was monitored across a near annual cycle autumn 2003 to summer 2004 with the aim of better understanding the factors influencing bank changes and processes at a seasonal scale seven flow events occurred during the period of investigation with the largest having an estimated return period of about 1 5 years bank simulations were performed by linking hydrodynamic fluvial erosion groundwater flow and bank stability models for the seven flow events which are representative of the typical range of hydrographs that normally occur during an annual cycle the simulations allowed identification of i the time of onset and cessation of mass failure and fluvial erosion episodes ii the contributions to total bank retreat made by specific fluvial erosion and mass wasting processes and iii the causes of retreat the results show that the occurrence of bank erosion processes fluvial erosion slide failure cantilever failure and their relative dominance differ significantly for each event depending on seasonal hydrological conditions and initial bank geometry due to the specific planimetric configuration of the study bend which steers the core of high velocity fluid away from the bank at higher flow discharges fluvial erosion tends to occur during particular phases of the hydrograph as a result fluvial erosion is ineffective at higher peak discharges and depends more on the duration of more moderate discharges slide failures appear to be closely related to the magnitude of peak river stages typically occurring in close proximity to the peak phase preferentially during the falling limb but in some cases even before the peak while cantilever failures more typically occur in the late phase of the flow hydrograph when they may be induced by the cumulative effects of any fluvial erosion copyright 2008 john wiley sons ltd

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

experimental partitioning of be cs and other trace elements between cordierite and felsic melt and the chemical signature of s type granite the trace element signature that cordierite crd imparts to silicic magmas was evaluated by experiment using metapelite mineral mixtures to produce cordierite bearing peraluminous granitic melts at 200 mpa ph2o from 700 to 850 c most elemental partition coefficients vary with t beryllium is strongly compatible with dbecrd melt values decreasing linearly from 202 0 to 6 7 as t rises from 700 to 850 c manganese is compatible dmncrd melt 7 67 to 1 92 over the same range of t and shows similar values to those reported for biotite in silicic melts incompatible components include li rb b f and p although cs is nearly compatible in cordierite especially at higher t dcscrd melt 0 19 to 0 60 where the large alkalis are better accommodated structurally cordierite appears to be the most effective crystalline reservoir of be and cs in metapelites and their anatectic melts natural data support the hypothesis that crd when present in granitic melts sequesters be cs and in the absence of garnet mn s type granitic rocks containing crd show consistently low be contents mean 0 8 ppm be with an average range of 1 to 1 20 whereas crd free granites e g containing accessory garnet exhibit distinctly higher be contents mean 6 ppm be with an average range of 3 to 12 these values increase further in evolved facies mean 69 ppm be with a an average range of 11 to 145 which commonly give rise to beryl bearing pegmatites whole rock signatures of be discriminate source environments of silicic magmas at a resolution equal to the boundaries of the cordierite stability field e g at the p t x conditions where cordierite gives rise to garnet aluminum silicate cordierite bearing granitic rocks contain low cs contents mean 1 8 ppm cs compared to the crd free equivalents mean 18 ppm cs mn contents also correlate with the presence mean 0 01 wt mno or absence of crd mean 0 09 wt mno depending on its contribution to anatexis cordierite may either give or take s type chemical character from granitic liquids resulting in a distinctive crd associated group of s type elements this signature is different from that of micas high li f and to a lesser degree be and mn whole rock compositions of granites coupled with notable absences of beryl in their associated pegmatites indicate that a sizable population of s type granites originated from crd bearing sources the normative crd component of silicic peraluminous melts is 4 wt to 850 c higher modal contents of cordierite reflect either restite entrainment or peritectic reactions which produce crd after magma ascent to shallow depths the distinctive trace element signature of cordierite now provides improved resolution of the source mineralogy for s type magmas

holocene evolution of the western orinoco delta venezuela the pristine nature of the orinoco delta of eastern venezuela provides unique opportunities to study the geologic processes and environments of a major tropical delta remote sensing images shallow cores and radiocarbon dating of organic remains form the basis for describing deltaic environments and interpreting the holocene history of the delta the orinoco delta can be subdivided into two major sectors the southeast sector is dominated by the rio grande the principal distributary and complex networks of anastomosing fluvial and tidal channels the abundance of siliciclastic deposits suggests that fluvial processes such as overbank flooding strongly influence this part of the delta in contrast the northwest sector is represented by few major distributaries and overbank sedimentation is less widespread relative to the southeast sector peat is abundant and occurs in herbaceous and forested swamps that are individually up to 200 km 2 in area northwest directed littoral currents transport large volumes of suspended sediment and produce prominent mudcapes along the northwest coast mapping of surface sediments vegetation and major landforms identified four principal geomorphic systems within the western delta plain 1 distributary channels 2 interdistributary flood basins 3 fluvial marine transitional environments and 4 marine influenced coastal environments coring and radiocarbon dating of deltaic deposits show that the northern delta shoreline has prograded 20 30 km during the late holocene sea level highstand progradation has been accomplished by a combination of distributary avulsion and mudcape progradation this style of deltaic progradation differs markedly from other deltas such as the mississippi where distributary avulsion leads to coastal land loss rather than shoreline progradation the key difference is that the orinoco delta coastal zone receives prodigious amounts of sediment from northwest moving littoral currents that transport sediment from as far away as the amazon system 1600 km late holocene progradation of the delta has decreased delta plain gradients increased water levels and minimized overbank flooding and siliciclastic sedimentation in the northwest sector these conditions coupled with large amounts of direct precipitation have led to widespread peat accumulation in interdistributary basins because peat forming environments cover up to 5000 km 2 of the delta plain the orinoco may be an excellent analogue for interpreting ancient deltaic peat deposits

petrological evolution of a suite of spinel granulites from vizianagram eastern ghats belt india and genesis of sapphirine bearing assemblages a suite of spinel cordierite granulites from viziangram eastern ghats belt india preserve mineral assemblages and reaction textures indicative of peak metamorphic conditions of 1000 c 8 10 kbar followed successively by near isobaric cooling down to 750 800 c near isothermal decompression to 4 5 kbar and late hydration p t conditions of each stage are evaluated through a combination of petrogenetic grid approach and thermobarometry sapphirine is developed in sillimanite bearing acid pegmatite veins that intruded the spinel cordierite granulite close to peak metamorphic conditions and also in the host rock in immediate contact with the pegmatite both sillimanite and sapphirine in the pegmatite are considered to be magmatic phases field observations and textural characteristics suggest that al metasomatism of the spinel cordierite granulite due to the intrusion of pegmatite was responsible for sapphirine formation in the spinel granulite

soil creep in a mesotidal salt marsh channel bank fast seasonal and water table mediated abstract muddy banks of marsh channels experience soil creep a viscous like slow deformation resulting in a net downslope transport here we present the first field evidence of soil creep in a mesotidal salt marsh using both pole displacement and high precision measurements of soil deformation taken with a vibrating wire extensometer over two years soil extended 2 4 mm m year in the high marsh platform and 20 40 mm m year in the low marsh bank this was equivalent to a soil diffusivity of about 0 2 m2 year which is much higher than the diffusivity on hillslopes and also matches the high value predicted by long term marsh evolution models high precision 0 03 mm soil deformation measurements shed light on the dynamics of the creep process first the low bank compressed and extended by 5 mm m every tidal cycle suggesting that the diurnal changes in water level play a role in the bank movements second net soil extension took place mostly during fall net soil extension preceded the period of ice formation suggesting that freezing and ice rafting are not a leading cause of soil creep net soil extension was likely triggered by an increase in water table during fall which decreased the effective stress and thus destabilized the bank this high water table was caused by a combination of an abiotic factor the increase in atmospheric precipitation and a biotic factor the decrease in evapotranspiration triggered by vegetation senescence this study confirms that creep is a significant process in marsh morphodynamics and that vegetation can reduce mass wasting in marsh channels

early carboniferous sub to mid alkaline magmatism in the eastern sierras pampeanas nw argentina a record of crustal growth by the incorporation of mantle derived material in an extensional setting a recently discovered granitic intrusion at cerro la gloria in western sierra de famatina nw argentina is representative of sub to mid alkaline carboniferous magmatism in the region the main rock type consists of microcline quartz and plagioclase with amphibole magnetite ilmenite biotite epidote zircon allanite and sphene as accessory minerals we report a u pb zircon shrimp age for the pluton of 349 3 ma mswd 1 1 i e tournaisian whole rock chemical composition and nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust endt between 0 58 and 0 46 and tdm values of about 1 1 ga the pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back arc basalts coeval early carboniferous a type granites occur farther east in the sierras pampeanas probably generated during lithospheric stretching overall the early carboniferous granitic rocks show a west to east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west based on the observed patterns of geochronology geochemistry and field relationships we suggest that a type magma genesis in the eastern sierras pampeanas was linked to an andean type margin where the lithospheric mantle played a role in its generation

soil creep the driving factors evidence and significance for biogeomorphic and pedogenic domains and systems a critical literature review abstract soil and regolith creep have been analyzed for at least the last 140 years using many methodological configurations and temporal and spatial scales the general concept of creeping soil and its mechanism first proposed by w m davis and g k gilbert at the end of the 19th century evolved since the 1940s towards theoretical models and precise short and long term field measurements this fruitful epoch continued with results enhanced at the turn of the 20th century by the application of new research methods e g radiometry and a redefinition of the term soil creep to encompass the sum of stochastic shallow subsurface and near surface processes causing net downslope movement of soil or regolith simultaneously another possibility of creep detection was noticed in dendrochronology and since the 1970s in the formally defined discipline of dendrogeomorphology indirect evaluations of creep activity were performed based on tree ring analyses of bent trees this method found many followers but was also heavily criticized as imprecise and lacking in evidence of which kind of tree trunk curvature e g pistol butt like deformation s shape curvature could be ascribed to creep movement from the beginning soil creep was associated with the activity of living organisms on hillslopes however this aspect of creep studies has never been fully developed in spite of the solid foundations and directions of potential studies pointed out by charles darwin at the end of the 19th century in this paper we focus on the historical context of soil creep studies and highlight forest ecosystems as probably the most active environment of biogenic creep mainly due to tree uprooting and other biomechanical effects of living and dead trees root channel infilling tree root mounding etc that are a factors in biotransport in the final sections the position of biogenic creep in the structure of biogeomorphic systems is discussed in relation to such important conceptual frameworks as the biogeomorphic ecosystem biogeomorphic feedback window and ecosystem engineering we also describe several hypotheses that should be carefully tested in the future and propose several research methods that have the ability to further our knowledge about soil creep radiometry laser scanning and soil micromorphology

occurrence and origin of andalusite in peraluminous felsic igneous rocks andalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks including rhyolites aplites granites pegmatites and anatectic migmatites some published stability curves for and sil and the water saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts we examine 108 samples of andalusite bearing felsic rocks from more than 40 localities world wide our purpose is to determine the origin of andalusite including the t p x controls on andalusite formation using eight textural and chemical criteria size compatibility with grain sizes of igneous minerals in the same rock shape ranging from euhedral to anhedral with no simple correlation with origin state of aggregation single grains or clusters of grains association with muscovite with or without rims of monocrystalline or polycrystalline muscovite inclusions rare mineral inclusions and melt inclusions chemical composition andalusite with little significant chemical variation except in iron content 008 171 wt feo compositional zoning concentric sector patchy oscillatory zoning cryptically reflect growth conditions compositions of coexisting phases biotites with high siderophyllite eastonite contents aliv 268 007 atoms per formula unit muscovites with 057 401 wt feo and 002 285 wt tio2 and apatites with 353 018 wt f coexisting muscovite biotite pairs have a wide range of f contents and fbt 1612fms 0015 most coexisting minerals have compositions consistent with equilibration at magmatic conditions the three principal genetic types of andalusite in felsic igneous rocks are type 1 metamorphic a prograde metamorphic in thermally metamorphosed peraluminous granites b retrograde metamorphic inversion from sillimanite of unspecified origin c xenocrystic derivation from local country rocks and d restitic derivation from source regions type 2 magmatic a peritectic water undersaturated t associated with leucosomes in migmatites b peritectic water undersaturated t as reaction rims on garnet or cordierite c cotectic water undersaturated t direct crystallization from a silicate melt and d pegmatitic watersaturated t associated with aplite pegmatite contacts or pegmatitic portion alone type 3 metasomatic water saturated magma absent spatially related to structural discontinuities in host replacement of feldspar and or biotite intergrowths with quartz the great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin of the many possible controls on the formation of andalusite excess al2o3 water concentration and fluid evolution high be b li p high f high fe mn ti and kinetic considerations the two most important factors appear to be excess al2o3 and the effect of releasing water either to strip alkalis from the melt or to reduce alumina solubility in the melt of particular importance is the evidence for magmatic andalusite in granites showing no significant depression of the solidus suggesting that the and sil equilibrium must cross the granite solidus rather than lie below it magmatic andalusite however formed is susceptible to supra or sub solidus reaction to produce muscovite in many cases textural evidence of this reaction remains but in other cases muscovite may completely replace andalusite leaving little or no evidence of its former existence

the arburese igneous complex sw sardinia italy an example of dominant igneous fractionation leading to peraluminous cordierite bearing leucogranites as residual melts abstract the arburese massif sw sardinia italy is a zoned shallow pluton emplaced in the frontal part of a nappe pile at the end of the hercynian orogeny its main lithologies are a pyroxene bearing biotite granodiorites and b cordierite bearing peraluminous leucogranites they form a sequence of sharply bounded multiple intrusions the leucogranites lg form the core of the intrusive complex and are younger than the granodiorites three types of granodiorites gd 1 gd 2 gd 3 have been distinguished on the basis of their field relations and their pyroxene content dark coloured enclaves are common in the granodiorites their compositions range from biotite bearing norite to pyroxene biotite quartzdiorite quartz monzodiorite a small body of olivine bearing quartz monzogabbro norite mg forms an independent intrusion in the border zone of the igneous massif it is crosscut by the granodiorite the whole rock sequence from mg to gd 3 is characterized by the same mafic assemblage opx cpx bt ilm it formed by dominant crystal liquid fractionation processes from a mantle derived olivine bearing magma from gd 3 to three types of leucogranites lg 1 lg 2 and lg 3 the rocks are progressively enriched in al reaching a peraluminous composition in the leucogranites cordierite makes its appearance among the primary mineral phases though confined to the later products of crystallization remarkably cordierite was also observed among the latest crystallization products in some granodiorites and even in some dark monzodioritic enclaves independent whole rock age determinations were made on the mg to gd 3 series and on the leucogranites these gave very similar values of 309 19 and 304 21 ma respectively some textural and mineralogical features common to both gd 3 granodiorite and the leucogranites e g both contain ti biotites characterized by a common trend of al and fe enrichment suggest a genetic linkage between these two rock types the extreme iron rich compositions displayed by the mafic minerals biotite cordierite of the leucogranites suggest that they crystallized from highly evolved melts petrochemical trace element and ree distribution patterns strongly support a common origin for the whole igneous sequence including the leucogranites strontium isotope compositions do not conflict with the above conclusion the oxygen isotopic composition changes progressively with time of emplacement 18 o increases from 7 6 in the granodiorites up to 11 9 in the leucogranites this probably reflects some degree of magma contamination due to assimilation of crustal material the sr and o isotope systematics reveal that the mg gd series is characterized by very small variations of the sr ratio and by pronounced changes in 18 o on account of the rather high sr initial ratios the observed trend is interpreted as due to afc assimilation fractional crystallization processes of a source contaminated parental magma modelling the evolution path from mg to lg in terms of afc processes shows the following a the assumed fractionation steps give realistic mineral assemblages compatible with the observed lithologies b they are able to produce high proportions of both leucogranitic and granodioritic rocks as observed in the field and c they can produce a progressive increase of alumina up to the peraluminous compositions observed in the more evolved members good results were also obtained by modelling the behaviour of trace elements such as rb sr and ba whose distribution is mainly governed by the major mineral phases the geochemical fingerprint of the whole igneous sequence is typical of a k rich calc alkaline series only some rare grey enclaves in the gd 3 granodiorite and in the leucogranites display a typical shoshonitic character their presence probably reflects the open system condition of the deep seated magma reservoir

aluminum in zircon as evidence for peraluminous and metaluminous melts from the hadean to present zircon structurally accommodates a range of trace impurities into its lattice a feature which is used extensively to investigate the evolution of silicate magmas one key compositional boundary of magmas is defined by whether the molar ratio of al2o3 cao na2o k2o is larger or smaller than unity here we report 800 al in zircon concentrations from 19 different rocks from the lachlan fold belt southeastern australia new england usa and arunachal leucogranites eastern himalaya with al2o3 cao na2o k2o whole rock values that range from 0 88 to 1 6 zircons from peraluminous rocks yield an average al concentration of 10 ppm which distinguishes them from crystals found in metaluminous rocks 1 3 ppm this difference is related to the materials involved in the melting assimilation and or magma differentiation processes for example magmas that assimilate al rich material such as metapelites are expected to produce melts with elevated alumina activities and thus zircons with high al concentrations these observations are applied to the archean and hadean jack hills detrital zircon record detrital archean zircons with ages from about 3 30 to 3 75 ga yield al in zircon concentrations consistent with origins in peraluminous rocks in 8 of the cases n 236 a single zircon from the pre 3 9 ga age group n 39 contains elevated al contents which suggests that metaluminous crustal rocks were more common than peraluminous rocks in the hadean weathered material assimilated into these hadean source melts was not dominated by al rich source material

the boon and bane of radiocarbon dating radiocarbon is widely used to determine the age of samples that are less than 50000 years old but the conversion of radiocarbon ages to calendar ages is not always straightforward in their perspective guilderson et al 1 analyze the calibrated age ranges that can be obtained from the current calibration standard the intcall98 curve they take into account not only the inherent limitations of the radiocarbon calendar calibration curve but also analytical uncertainties the results bear directly on our ability to understand the dynamic climate system and the relations between climate and past societies 1 http www sciencemag org cgi content full 307 5708 362

primary melt inclusions in andalusite from anatectic graphitic metapelites implications for the position of the al2sio5 triple point anatectic crustal xenoliths in the miocene volcanic rocks of mazarron southeast spain contain andalusite with melt inclu sions an unprecedented finding microstructures indicate that the melt inclusions were trapped during andalusite growth the vola tile content of the peraluminous inclusions is too low cl 0 4 wt f 0 3 wt p 1500 ppm b 850 ppm to have caused a significant reduction of the wet solidus temperature moreover the presence of graphite as observed during partial melting is expected to have raised the temperature of the wet solidus melting temperatures for the inclusions obtained from quartz albite or thoclase haplogranite system 680 790 8c zr 620 705 8c and light rare earth element 615 725 8c thermometry indicate that the stability of andalusite 1 melt is incompatible with the position of the most commonly used andalusite 5 sillimanite equilibrium and that the al2sio5 triple point must be placed at higher tem peratures and pressures

holocene peatland initiation in the greater everglades the mechanisms involved in the initiation and development of the greater everglades peatland ecosystems remain a topic of discussion in this study we first present an overview of basal ages of peat deposits in south florida which shows two major episodes of peatland initiation between 7 0 4 5 kyr and 3 5 2 0 kyr our analysis of regional climate proxy data sets led to three alternative hypotheses that may explain the timing and duration of these two peatland initiation episodes 1 decreased drainage due to relative sea level rsl rise during the holocene 2 gradual increase in precipitation throughout the holocene and 3 a combination of increasing precipitation rising rsl and oscillations in the climate system we test whether these three hypotheses can explain the pattern of initiation and development of the greater everglades peatlands using models that simulate the nonlinear processes involved in peat production and decomposition the model results suggest that rsl rise could explain the onset of peatland initiation and imply that the climate was wet enough for peat development also during the early holocene the first two hypothesized mechanisms in combination with climate oscillations may explain the onset of peat accumulation at 8 2 kyr b p the two phased character of peatland initiation maybe explained by the spatial distribution of local drainage conditions as peatland development is highly nonlinear our model uncovers a mechanistic way how peats can suddenly shift from a dry high equilibrium to a wet low equilibrium resulting in lake formation as observed in paleoecological studies in the greater everglades

stable isotope constraints on the al2sio5 triple point rocks from the proterozoic priest pluton contact aureole new mexico usa the priest pluton contact aureole in the manzano mountains central new mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1 42 ga priest pluton quartz garnet and quartz sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 c at a distance of 1 km from the pluton to 690 c at the contact with the pluton comparison of calculated temperature estimates with one dimensional thermal modelling suggests that background temperatures between 300 and 350 c existed at the time of intrusion of the priest pluton fibrolite is found within 300 m of the priest pluton in pelitic and aluminous schist metamorphosed at temperatures 580 c coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium suggesting these minerals were stable reaction products during peak metamorphism the fibrolite in isograd is coincident with the staurolite out isograd in pelitic schist and k feldspar is not observed with the first occurrence of fibrolite this suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite bearing pelitic schist muscovite rich aluminous schist locally preserves the al2sio5 polymorph triple point assemblage kyanite andalusite and fibrolite andalusite and fibrolite but not kyanite are in isotopic equilibrium in the aluminous schist co nucleation of fibrolite and andalusite at 580 c in the presence of muscovite and absence of k feldspar suggests that univariant growth of andalusite and fibrolite occurred kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 c lower than andalusite and fibrolite growth quartz muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative suggesting that late isotopic exchange between externally derived fluids and muscovite but not quartz occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz 500 c

carbon and sediment accumulation in the everglades usa during the past 4000 years rates drivers and sources of error 1 tropical and subtropical wetlands are considered to be globally important sources of greenhouse gases but their capacity to store carbon is presumably limited by warm soil temperatures and high rates of decomposition unfortunately these assumptions can be difficult to test across long timescales because the chronology cumulative mass and completeness of a sedimentary profile are often difficult to establish we therefore made a detailed analysis of a core from the principal drainage outlet of the everglades of south florida in order to assess these problems and determine the factors that could govern carbon accumulation in this large subtropical wetland accelerator mass spectroscopy dating provided direct evidence for both hard water and open system sources of dating errors whereas cumulative mass varied depending upon the type of method used radiocarbon dates of gastropod shells nevertheless seemed to provide a reliable chronology for this core once the hard water error was quantified and subtracted long term accumulation rates were then calculated to be 12 1 g m 2 yr 1 for carbon which is less than half the average rate reported for northern and tropical peatlands moreover accumulation rates remained slow and relatively steady for both organic and inorganic strata and the slow rate of sediment accretion 0 2 mm yr 1 tracked the correspondingly slow rise in sea level 0 35 mm yr 1 reported for south florida over the past 4000 years these results suggest that sea level and the local geologic setting may impose long term constraints on rates of sediment and carbon accumulation in the everglades and other wetlands