rutile solubility and tio2 activity in silicate melts an experimental study abstract sixty eight new experiments on rutile solubility in synthetic silicate melts of a wide compositional range have been conducted at ambient pressure p at temperature t from 1230 to 1500 c the results support the previous findings namely 1 at fixed t addition of silica and alumina results in essential decrease of rutile solubility 2 at fixed composition increasing t results in essential increase in rutile solubility based on the new and literature experimental data 350 data points new equations describing rutile solubility in silicate melts as a function of t p and melt composition both empirical and consistent with the thermodynamic model of melts ghiorso and sack 1995 are derived the new equation predicts log xtio2 tio2 mole fraction in the melts with a standard error of 0 066 which is equivalent to ca 15 error in a wide range of melts major element composition and the xtio2 value in the range from 0 0008 to 0 39 tio2 from 0 1 to 44 8 wt covered by the experiments the equations are applied to calculate the tio2 activity values relative to rutile saturation atio2 m rut in a variety of well documented natural magmatic rocks the resulting atio2 m rut values for different tested samples vary from 0 1 to close to 1 simultaneously solving the new equations and the equation for ti in zircon geothermometer ferry and watson 2007 gives more reliable t estimates than those based on the assumption of constant atio2 m rut 0 6 or 1

the effect of phosphorus on manganocolumbite and mangaotantalite solubility in peralkaline to peraluminous granitic melts solubility experiments of mn columbite mnnb2o6 and mn tantalite mnta2o6 were conducted under water saturated conditions in synthetic haplogranitic melts containing different amounts of phosphorus at 800 c and 100 mpa all experiments were carried out in cold seal rapid quenching pressure vessels rqv with water as a pressure medium experimental results show that 1 the solubilities of mnnb2o6 and mnta2o6 in peralkaline melts are higher than those in peraluminous melts 2 phosphorus has strong influence on the solubilities of mnta2o6 and mnnb2o6 in peralkaline melts kspnb and kspta decrease from 104 89 10 4 mol2 kg2 and 107 62 10 4 mol2 kg2 for melts without p2o5 to 16 11 10 4 mol2 kg2 and 7 96 10 4 mol2 kg2 for melts containing 4 00 wt p2o5 respectively 3 phosphorus has less influence on the solubilities of mnta2o6 and mnnb2o6 in peraluminous melt kspnb decrease from 4 50 10 4 mol2 kg2 for melts without p2o5 to 0 73 10 4 mol2 kg2 and kspta from 3 57 10 4 mol2 kg2 to 0 14 10 4 mol2 kg2 for melts contaning 5 00 wt p2o5 taking the structural role of phosphorus into account p decreases the solubility of mn columbite and mn tantalite via competing for network modifiers

chemical differentiation of the bishop tuff and other high silica magmas through crystallization processes re examination of petrochemical data for high silica ash flow tuffs suggests that crystal liquid fractionation plays a major role in the differentiation of high silica magmas for the high silica bishop tuff the enrichment and depletion of 27 major and trace elements correlates with the bulk crystal liquid partition coefficients calculated from the data of hildreth in ways that are consistent with crystal fractionation comparison of the bishop tuff with other high silica tuffs and rhyolites with different compositional variations and different phenocryst assemblages shows the correspondence of chemical gradients with mineralogy crystal zoning studies in the cordillera paine granite in southern chile suggest that the evolution of differentiated liquids similar to those in the bishop tuff results from crystal fractionation the physical mechanisms of separation of liquid and crystals for the paine granite and bishop tuff were quite different however differentiation of the high silica ash flows might result from advection of a less dense liquid from a crystallizing boundary into the top of the magma chamber

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

two lattice models of trace element behavior a response abstract two lattice melt components of bottinga and weill 1972 nielsen and drake 1979 and nielsen 1985 are applied to major and trace element partitioning between coexisting immiscible liquids studied by ryerson and hess 1978 and watson 1976 the results show that 1 the set of components most successful in one system is not necessarily portable to another system 2 solution non ideality within a sublattice severely limits applicability of two lattice models 3 rigorous application of two lattice melt components may yield effective partition coefficients for major element components with no physical interpretation and 4 the distinction between network forming and network modifying components in the sense of the two lattice models is not clear cut the algebraic description of two lattice models is such that they will most successfully limit the compositional dependence of major and trace element solution behavior when the effective partition coefficient of the component of interest is essentially the same as the bulk partition coefficient of all other components within its sublattice

element partitioning between minerals and melt melt composition and melt structure abstract mineral melt trace element partition coefficients d i mineral melt concentration ratio mineral melt can vary by up to two orders of magnitude in the melt composition range of natural magmatic liquids as most geochemically interesting minor and trace elements are network modifiers in silicate melts one would expect as observed positive correlation between mineral melt partition coefficients d i mineral melt and the degree of polymerization of the silicate melt nbo t linear relationships between d i mineral melt and nbo t sometimes exist over restricted nbo t ranges where the type of q n species in the melt does not change and their abundance does not vary greatly other experimental d i mineral melt vs nbo t data suggest log linear or log log relations in some cases whereas in other studies there are no simple relationships between d i mineral melt and nbo t of the melt when relating mineral melt partition coefficients to degree of melt polymerization nbo t it is assumed that a principal melt structural control on d i mineral melt is the activity of nonbridging oxygen in the melt a nbo the a nbo is related to nbo t the nbo t is not however a quantitative measure of a nbo because the nonbridging oxygens in coexisting q n species in melts are energetically non equivalent this is evident in relationships between activity coefficient ratios of melt network modifying cations i j where the ionization potentials of i and j differ and nbo t of the melt such relationships are parabolic with minima or maxima at nbo t near 1 and resemble the relations of abundance ratio x q3 x q2 vs nbo t of the melt interestingly this nbo t value is near that of natural basalt melt these observations suggest that i nbo q 3 and j nbo q 2 bond characteristics differ from one another govern trace element solution behavior in silicate melts and therefore control the effect of melt composition on mineral melt partitioning in silicate systems

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

solution properties of rare earth elements in silicate melts inferences from immiscible liquids abstract the partitioning behavior of rare earth elements ree between immiscible silicate liquids was investigated as a function of temperature and total ree oxide concentration ree preferentially partition into the end member liquids that are rich in network modifying cations with two liquid partition coefficients greater than any other cation the two liquid partition coefficients of ree in a given experiment are essentially identical to one another no evidence was seen to suggest that the solution mechanisms of ree in high silica liquids vary with differences in their ionic radii two lattice models of trace element solution behavior fail to remove the compositional dependence of ree two liquid partition coefficients due to the assumption that all network forming and network modifying cations mix ideally with one another as a result it is probably incorrect to derive physical interpretations from the results of the application of two lattice models the results are applied to the origins of the lree enriched phenocryst matrix partition coefficients seen for some ferromagnesian minerals in high silica rhyolites since liquids with extremely high concentrations of t o t linkages do not strongly discriminate between the ree on the basis of their ionic radii the anomalous ree crystal matrix partition coefficient patterns are best explained as the result of contamination of these minerals by lree accessory phases

extreme fractionation from zircon to hafnon in the koktokay no 1 granitic pegmatite altai northwestern china the koktokay no 1 pegmatite is a li cs ta rich granitic pegmatite located in altai northwestern china zircon is present in most textural zones of this pegmatite and in its contact zone with surrounding metagabbro here we describe the detailed associations of zircon with other minerals and the internal textures and chemistry of the zircons most zircon grains from the contact zone have relatively low hfo 2 2 18 0 18 7 wt zircon grains from the aplite zone contain 2 although their thin and bright rims have higher hfo 2 10 8 13 0 wt most zircon grains from the quartz muscovite zone have complex internal textures and have hfo 2 contents of 2 up to 36 1 wt zircon sl from the cleavelandite quartz spodumene zone can be divided into two types based on petrography and chemistry one group of zircons appears to be typical magmatic zircon and are greater than 100 m in size closely associated with albite and have hfo 2 contents of 13 0 19 5 wt the second group of zircons is typically associated with muscovite and or spodumene is small in size down to a few micrometers and may exhibit zoning or alteration textures the hfo 2 contents of this second zircon group are 19 8 58 9 wt indicating the presence of hafnian zircon to zirconian hafnon large hfo 2 content variations of up to 34 8 wt were also observed within single zoned crystals we suggest that the increase of hfo 2 in the magmatic zircon from 9 4 wt in the contact zone to 19 5 wt in the cleavelandite quartz spodumene zone mainly reflects fractional crystallization of pegmatite magma however the occurrence of hafnian zircon and hafnon in the cleavelandite quartz spodumene zone is likely related to coupled li f fluxing effects in the pegmatite magma

co 2 melts and granulite metamorphism the discovery that granulites contain co2 rich fluid inclusions while amphibolites in the same terrane contain h2o rich or mixed co2 h2o inclusions1 has led to a debate over the processes that produce granulite metamorphism one theory2 4 maintains that granulites form as the result of a massive influx of co2 from lower crustal or mantle sources such an influx could be responsible for the marked depletion many granulite terranes show in large ion lithophile elements2 5 others however maintain that granulite facies metamorphism is a necessary complement of crustal melt ing6 9 recently lamb and valley10 11 have argued that granulite metamorphism may be a product of any one of three processes co2 streaming partial melting or recrystallization of originally dry rocks here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust

cordierite gneisses of southern kerala india petrology fluid inclusions and implications for crustal uplift history the cordierite bearing gneisses occurring as elongate patches in an 8 to 10 km wide zone along the achankovil fault lineament at the northern margin of the southern kerala crustal segment represent an important lithological unit in the archaean granulite terrane of south india the textural relationships in these rocks are consistent with the following main reactions 1 garnet quartz cordierite hypersthene 2 garnet sillimanite quartz cordierite 3 hypersthene sillimanite quartz cordierite 4 sillimanite spinel cordierite corundum and 5 biotite quartz sillimanite cordierite k feldspar many of the mineral associations and reaction textures including the remarkable preservation of symplectites are indicative of partial replacement of high pressure assemblages by cordierite bearing lower pressure ones during an event of rapid decompression temperature estimates from coexisting mineral phases show 710 garnet biotite 791 garnet cordierite and 788 c garnet orthopyroxene pressure estimates from mineral assemblages range from 5 4 to 7 kb detailed fluid inclusion studies in quartz associated with cordierite show high density co2 0 80 0 95 g cm3 as the dominant fluid phase with traces of probable ch4 in the sillimanite bearing rocks the isochore for the higher density fluid inclusions defines a pressure of 5 5 kb the fracture bound co2 and co2 h2o ch4 inclusions indicate simultaneous entrapment at 400 c and 1 7 kb in the cordierite hypersthene assemblage and 340 c and 1 2 kb in the cordierite sillimanite assemblage the p t path delineated from combined solid and fluid data corresponds to the piezothermic array of the gneisses and is characterized by t convex nature indicative of rapid and virtually isothermal crustal uplift probably aided by extensional tectonics

evidence for fluid pathways through archaean crust and the generation of the closepet granite karnataka south india abstract in the archaean amphibolite granulite facies transition zone of southern karnataka anatexis of amphibolite facies peninsular gneiss occurred based on cross cutting relationships this melting event which produced the closepet granite was contemporaneous with the formation of charnockitic rocks this formation of charnockite took place initially due to channelized co 2 rich volatiles moving along an anastamosing network of shears and micro fissures these fluid pathways are now represented by brown veins of charnockite later the fluid became more pervasive and resulted in areas of total inversion to charnockite a lateral change along the fluid pathways from charnockite through incipient charnockite to either a thin granitic vein or a vein of reddening can be traced through the country rocks these different effects along the same laterally continuous vein are interpreted to represent the lateral change of volatile composition which in front of the co 2 rich fluids were increasingly h 2 o dominated

sedimentary fractionations between al ti and zr and the genesis of strongly peraluminous granites sedimentary sorting produces alternating shales and sandstones with complementary chemistries the preferential partitioning of quartz and zircon in the coarse grained fraction of the sediments results in a strong fractionation between sio 2 zr and other components al 2 o 3 tio 2 as a consequence shales may be defined chemically as having higher al 2 o 3 sio 2 and tio 2 zr ratios than their source rocks sandstones have the opposite characteristics and vary in composition according to the maturity of their source and the intensity of sorting processes most sedimentary suites display a good correlation between al 2 o 3 sio 2 and tio 2 zr this result is used to evaluate an average bulk composition for recent clastic materials and to model their composition on a ternary al ti zr diagram because the al 2 o 3 tio 2 ratio of igneous rocks is highly sensitive to magmatic differentiation igneous and sedimentary trends crosscut in the al ti zr di

the occurrence and distribution of mo and molybdenite in unaltered peralkaline rhyolites from pantelleria italy small hexagonal and triangular platelets of molybdenite mos2 5 to 25 m in diameter were identified in phenocrysts and matrix glass of unaltered felsic volcanic rocks from pantelleria italy the mos2 occurs commonly in pantellerites peralkaline rhyolites rarely in pantelleritic trachytes and never in trachytes the occurrence of euhedral mos2 platelets in all phenocryst phases in matrix glass and even in some melt inclusions indicates that mos2 precipitated directly from the peralkaline melt despite mos2 saturation the melt glass contains greater than 95 of the mo in pantellerian rocks x ray fluorescence analyses of 20 whole rocks and separated glasses show that whole rocks consistently contain less mo than corresponding matrix glasses the differences being in proportion to phenocryst abundances the mo contents increase with differentiation from trachytes 2 12 ppm to pantellerites 15 25 ppm and correlate positively with incompatible elements such as th y and nb the mo concentrations as determined by secondary ion mass spectrometry are essentially the same in matrix glasses and melt inclusions showing that mo did not partition strongly into a volatile fluid phase during outgassing the high mo contents of the pantellerites relative to metaluminous magmas with 1 5 ppm may be due to several factors 1 the enhanced stability of highly charged cations such as mo6 u4 and zr4 in peralkaline melts 2 the rarity of fe ti oxides and litanite into which mo might normally partition 3 reduced volatility of mo in low fo2 h2o poor 1 2 wt peralkaline magmas geochemical modeling indicates that the precipitation of mos2 can be explained simply by the drop in temperature during magmatic differentiation the occurrence of mos2 in pantellerites may result from their high mo concentrations and low redox state ni nio 2 5 relative to metaluminous magmas causing them to reach mos2 saturation at magmatic temperatures the apparent absence of mos2 microphenocrysts in more oxidized metaluminous rhyolites may indicate that mo is dissolved primarily as a hexavalent ion in those magmas

crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts abstract accessory minerals commonly occur attached to or included in the major crystalline phases of felsic and some intermediate igneous rocks apatite is particularly common as inclusions but fe ti oxides pyrrhotite zircon monazite chevkinite and xenotime are also known from silicic rocks accessories may nucleate near the host crystal liquid interface as a result of local saturation owing to formation of a differentiated chemical boundary layer in which accessory mineral solubility would be lower than in the surrounding liquid differentiation of this boundary layer would be greatest adjacent to ferromagnesian phenocrysts especially fe ti oxides it is with oxides that accessories are most commonly associated in rocks a boundary layer may develop if the crystal grows more rapidly than diffusion can transport incorporated and rejected elements to and from the phenocryst diffusion must dominate over convection as a mode of mass transfer near the advancing crystal liquid interface in order for a boundary layer to exist accumulation of essential structural constituent elements of accessory minerals owing to their slow diffusion in evolved silicate melt also may force local saturation but this is not a process that applies to all cases local saturation is an attractive mechanism for enhancing fractionation during crystallization differentiation if accessory minerals attached to or included in phenocrysts formed because of local saturation their host phenocrysts must have grown rapidly when accessories nucleated in comparison to lifetimes of magma reservoirs some inconsistencies remain in a local saturation origin for accessory phases that cannot be evaluated without additional information

vapor undersaturated experiments with macusani glass h 2 o at 200 mpa and the internal differentiation of granitic pegmatites vapor undersaturated fractional crystallization experiments with macusani glass macusanite a peraluminous rhyolite obsidian at 200 mpa yield mineralogical fabrics and zonation and melt fractionation trends that closely resemble those found in zoned granitic pegmatites and other granitoids of comparable composition typically peraluminous li be ta rich deposits the zonation from the edge of charges inward is characterized by 1 fine grained sodic feldspar quartz border zones 2 a fringe of very coarse grained graphic quartz feldspar intergrowths that flair radially toward melt and terminate with nearly monophase k feldspar 3 cores of very coarse grained nearly monominerallic quartz or virgilite lialsi5o12 mica and 4 late stage fine grained albite mica intergrowths that are deposited from alkaline na rich interstitial melt at vapor saturation similar experimental products have been observed in compositionally simpler less evolved systems liquid lines of descent from initially h2o undersaturated runs are marked by a decrease in sio2 and increases in na k b p f h2o and a variety of trace lithophile cations these trends are believed to be governed by three factors 1 disequilibrium growth of feldspars quartz via metastable supersaturation 2 fractionation of melt toward sio2 depleted na rich compositions due to increases in b p and f and 3 changes in nucleation and growth rates mostly as a function of the h2o content of melt x w m in contrast experiments that are cooled below the liquidus from the field of melt aqueous vapor london et al 1988 fail to replicate pegmatitic characteristics in most respects on the basis of these and other experiments we suggest that the formation of pegmatite fabrics stems primarily from fractional crystallization in volatile rich melts and that enrichments in normally trace lithophile elements result from melt differentiation trends toward increasingly alkaline silica depleted compositions although vapor saturation at near solidus and subsolidus conditions may promote extensive recrystallization an aqueous vapor phase does not appear to be necessary for the generation of most of the salient characteristics of pegmatites

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

zircon saturation and zr diffusion in rhyolitic melts and zircon growth geospeedometer zircon is a ubiquitous accessory mineral in silicic igneous rocks we have carried out new zircon dissolution experiments to refine our understanding of zr diffusion and zircon solubility in several rhyolitic melts zr diffusivity depends strongly on temperature and h 2 o content and weakly on pressure and anhydrous melt composition the diffusion data for each individual melt follows the arrhenius relation the dependence of zr diffusivity on temperature pressure and melt composition including h 2 o content is modeled for different rhyolitic melts in this study and for the combined literature and our data our data on zr concentration at zircon saturation in silicic melts show strong dependence on temperature and weak dependence on pressure and melt composition and are somewhat off the trend based on existing zircon solubility models the dissolution or growth rate of a freely falling zircon crystal in a specific hydrous rhyolitic melt is modeled the controlling factors are mostly the temperature and zr concentration in the melt typical zircon growth rate in wet rhyolitic melt is 0 01 to 1 0 m yr the size of zircon crystals can be used to place limit on the cooling rate of its hosting magma the presence of large indigenous zircon crystals in bishop tuff requires slow cooling of the bishop tuff magma chamber

application of proton microprobe data to trace element partitioning in volcanic rocks abstract in situ proton microprobe analyses are used to calculate partition coefficients d for mn fe cu zr ga rb sr y zr ba and pb and more restricted data for ni ge nb and as in pyroxenes olivines biotites hornblendes feldspars and fe ti oxides from 32 volcanic rocks all with glassy matrices these range from leucitite and picritic basalt to high silica and peralkaline rhyolites data for pigeonite aenigmatite allanite and leucite are included where these occur as additional phases coefficients have also been rayleigh fractionation corrected this correction being significant for those elements with high coefficients in specific mineral phases and where pronounced crystallisation has occurred partition coefficients are presented as plots vs nbo t ratios of coexisting glasses melts negative correlations are observed based on log transformed data for mn zn fe and y pyroxenes mn zn and fe olivines mn and zn fe ti oxides nb y sr and zr hornblendes sr plagioclase and ba cu zn and fe feldspars these element partition coefficients are not correlated with bulk mineral compositions zr and ga are not correlated with either glass or mineral compositions except zr in hornblende the correlations support the role of melt polymerisation in controlling element partitioning at least for those elements listed although possible effects of temperature cannot be uniquely isolated in the present data set the coefficients presented here are generally consistent with published values for equivalent rock compositions one difference being that some of the higher published coefficients e g sr in plagioclase are not matched in the present data set from a practical viewpoint the relative regularity of the coefficient data presented does facilitate choice of relevant coefficient values that are required for petrogenetic modelling procedures

iron and titanium solution properties in peraluminous and peralkaline rhyolitic liquids the saturation surface of pseudobrookite fe2tio5 was determined for melts in the system sio2 al2o3 k2o feo fe2o3 tio2 at 1400 c and 1 atm the variation in concentrations of fe2o3 tio2 and fe2tio5 in liquids can be used to infer relative changes in activity coefficients of these components with changing k2o k2o al2o3 of the melts saturation concentrations of these components are low and relatively constant in the peraluminous melts and increase with increasing k2o k2o al2o3 in peralkaline liquids the activity coefficients of fe2o3 and tio2 and fe2tio5 therefore are higher in peraluminous liquids than in peralkaline liquids in this system in addition the iron redox ratio was measured as a function of k2o k2o al2o3 for liquids just below the saturation surface f text o text 2 was fixed so all variations in redox ratio are entirely due to changes in melt composition the redox ratio from unsaturated liquids was applied to saturated liquids where redox analysis of the glass is impossible the homogeneous equilibrium experiments indicate that the activity coefficient of fe2o3 relative to that of feo is significantly greater in peraluminous melts than peralkaline melts both the heterogeneous and homogeneous equilibria suggest that in peralkaline liquids k in excess of that required to charge balance tetrahedral al 3 is used to stabilize both fe 3 and ti 4 calculations show that ferric iron and titanium compete equally effectively for charge balancing potassium but neither can outcompete aluminum the observed changes in solution properties of fe2o3 and tio2 in the synthetic melts are used to explain variations in fe ti oxide stabilities in natural peraluminous and peralkaline rhyolites and granites since the activity coefficients of both ferric iron and titanium are significantly higher in peraluminous liquids than in peralkaline liquids fe ti oxides should occur earlier in the crystallization sequence in peraluminous rhyolites than in peralkaline rhyolites in addition iron will be reduced in peraluminous granites and rhyolites relative to peralkaline ones under comparable p t and f text o text 2 finally observed crystallization patterns for minerals containing highly charged cations other than ferric iron and titanium are evaluated in the context of this and other experimental studies

solution behavior of 4 cations in high silica melts petrologic and geochemical implications the saturation surfaces of rutile tio2 zircon zrsio4 and hafnon hfsio4 were determined in anhydrous peraluminous high silica liquids of the system sio2 al2o3 na2o k2o as functions of silica concentration at 1 400 c in air the saturation concentrations of tio2 zro2 and hfo2 in rutile zircon and hafnon saturated liquids respectively decrease smoothly and gradually as functions of increasing silica concentration thermodynamic analyses of the data demonstrate that the activity coefficients of tio2 zro2 and hfo2 increase smoothly and gradually as silica concentration is increased from 67 wt to 80 wt and that changes in sio2 of 1 or 2 wt result in small changes in the saturation concentrations and activity coefficients of 4 cations because the solution behavior of 4 cations in highly siliceous liquids 75 wt sio2 is predictably different than in less siliceous liquids 70 to 75 wt sio2 classification of highly siliceous igneous rocks on the basis of silica concentration alone should not be interpreted to mean that their solution chemistry differs significantly from that of less siliceous rocks the results of this study are compared with other studies of 4 cation solution behavior from this it is concluded that variations in liquid compositions observed in cogenetic suites of high silica rhyolites cannot cause the observed changes in 4 cation concentrations thus even if a large change in solution behavior of 4 cations is inferred from the large variations in their concentrations it cannot be due to changes in bulk composition of the parental liquid in addition the similarity in the solution behavior of zr and hf seen in this study suggests that their solution mechanisms are similar it is thus unlikely that liquid state processes can fractionate one with respect to another and variations in zr hf ratios in suites of extrusive rocks are likely due to crystal liquid equilibria e g zircon fractionation

alkali granites with pan african affinities from kerala s india in the kerala region of the south indian shield alkali granite plutons ranging in age from 550 to 740 ma intrude precambrian charnockites and migmatites all of them are spatially related to regional fault lineaments their geochemistry is similar to pan african post kinematic plutons from the afro arabian shield they are of granitic to adamellitic composition with high sio 2 k 2 o up to 12 and na 2 o and low cao feo and mgo their major element composition together with a low 87 sr 86 sr i 0 703 from one pluton denote a type affinities however local charnockites have such low rb and radiogenic 87 sr that the granites could have formed by crustal melting their overall trace element characteristics are typical of alkaline granites in general but the details show that most resemble more recent volcanic arc granites and one pluton is typical of a within plate setting the presence of co 2 rich fluid inclusions supports the notion that the enrichment of rare earth an

effect of substitution of tio2 for sio2 on afeo in magma abstract using both high and low feo starting materials 167 liquids are equilibrated with metallic fe under controlled oxygen fugacity at 1600 k to contour for a feo three pseudoternaries in the system sio 2 tio 2 kalo 2 ca 0 5 alo 2 feo the pseudoternaries contain synthetic analogs of the most abundant terrestrial and lunar ultramafic and mafic magmas the data are used to test a simple thermodynamic model in which ti and si are assumed to be randomly distributed on the same set of sites in the network of magma these two elements do not mix ideally on these sites instead they experience a strong regular solution type interaction with each other using a short equation the model permits calculation of the activity of feo in ti bearing melts from data in ti free liquids the experimental data along with published results for feo sio 2 tio 2 melts clearly support the model the molar coeffcient for the si ti interaction is 105 kj mole for most of the investigated melts including synthetic analogs of basalts basaltic andesites and picrites the exception is the one set that are both kalo 2 free and ca 0 5 alo 2 bearing containing synthetic analogs of komatiites and lunar basalts in which drops to 42 kj mole this suggests that the mixing of si and ti is a closer approximation to ideality in these alkali poor melts the physical reason for this variation in is unknown and warrants further investigation finally the model permits calculation of the effect of ti substitution for si on the activity of any component in magma that contains feo sio 2 and or tio 2 the short equation for the activity of fayalite is given as an example

a tectonic perspective of incipient charnockites in east gondwana abstract examples of spectacular occurrences of charnockite in an arrested stage of formation in crustal segments of east gondwana comprising india sri lanka and antarctica are evaluated to constrain the broad tectonic framework under which the charnockite formed field structures petrologic features and information including fluids trapped within inclusions in minerals suggest that the conversion of upper amphibolite facies gneisses to charnockites was brought about by the local desiccation induced by the infiltration of co2 rich fluids through faults and shears under brittle quasi brittle conditions the stable isotopic composition of these fluids is consistent with a juvenile sublithospheric source available geochronologic data indicate multiple events of the incipient charnockite formation at 2500 1000 and 500 ma in these crustal domains in each case incipient charnockite formation post dates regional metamorphism hence the process of local desiccation is inferred to have operated in cratonized crustal segments the p t t paths characterized by isothermal decompression are consistent with incipient charnockite development during the waning stages of collisional mobile belts when the tectonic regime changed from compressional to extensional crustal distension triggers the release of co2 rich fluids from depth which are transported to mid crustal levels through magmatic conduits a notion supplemented by the occurrence of alkaline intrusives of pan african age degassing of these magmas could have supplied the co2 required for stabilizing the anhydrous assemblages in the charnockites

coordination chemistry of titanium iv in silicate glasses and melts iv xanes studies of synthetic and natural volcanic glasses and tektites at ambient temperature and pressure abstract the coordination environment of ti iv in seven natural and synthetic glasses of basaltic trachytic rhyolitic composition as well as four tektites has been studied using high resolution ti k edge x ray absorption near edge structure xanes spectroscopy at ambient temperature and pressure pre edge features of ti k edge xanes spectra for these glasses suggest that 5 ti is the dominant ti coordination in all volcanic glasses however in the less polymerized glasses studied basaltic and trachytic 6 ti is also important 30 50 of the total ti but 4 ti was not detected in contrast 4 ti is important in the most polymerized glasses rhyolites and tektites from 30 to 60 of the total ti depending on nbo t with 6 ti below the detection level 10 at the local structure around ti in the natural volcanic glasses is similar to that observed in compositionally similar synthetic silicate glasses and also in ti bearing silicate glass and melts with simpler compositions the presence of f cl and h 2 o does not appear to affect the coordination of ti based on ti k edge xanes measurements of natural glasses bearing these volatile components in contrast the presence of nonbridging oxygens produced by network modifiers favors 5 ti in these glass melts in parallel 4 ti is important when nonbridging oxygens are at small concentrations nbo t 6 ti is detected i e when present 10 of the total ti when alkaline earths are dominant over alkalis in agreement with bond valence predictions for ti bearing silicate glass melts below tio 2 saturation the abundance of 5 ti in these silicate glasses and presumably their melts is in sharp contrast with the rarity of this ti coordination state in common rock forming minerals titanium cannot readily enter the structure of most rock forming minerals because it is present dominantly as titanyl bearing 5 ti o units in most natural magmas in contrast 6 ti and 4 ti present respectively in basic and acidic magmas are better able to enter inosilicates but these coordination states represent only a fraction of the ti in basalts explaining the usually moderate level of incompatibility of ti during magmatic differentiation finally 5 ti transforms to 6 ti during crystallization of ti rich minerals ilmenite rutile pyrochlore

the role of fluids in the development of a granulite facies transition zone in s india the bodinayakkanur area of tamil nadu lies in the transition zone p 5 kbar t 700 c from amphibolite to granulite facies metamorphism in the archaean high grade terrain of s india within the zone 3 closely associated rock units occur 1 precursor grey gneiss 2 pink migmatitic and metasomatic gneiss 3 green charnockitic granulites field observation shows that the complex inter relations between these 3 rock types can be related to influx of fabric and layer guided co 2 and h 2 o k 2 o rich fluid phases derived from a deep seated source

experimental evidence for mo isotope fractionation between metal and silicate liquids stable isotope fractionation of siderophile elements may inform on the conditions and chemical consequences of core mantle differentiation in planetary objects the extent to which mo isotopes fractionate during such metal silicate segregation however is so far unexplored we have therefore investigated equilibrium fractionation of mo isotopes between liquid metal and liquid silicate to evaluate the potential of mo isotopes as a new tool to study core formation we have performed experiments at 1400 and 1600 c in a centrifuging piston cylinder tin was used to lower the melting temperature of the fe based metal alloys to 1400 c while variable fe oxide contents were used to vary oxygen fugacity in graphite and mgo capsules isotopic analyses were performed using a double spike technique in experiments performed at 1400 c the 98mo 95mo ratio of silicate is 0 19 0 03 0 19 0 03 95 confidence interval heavier than that of metal this fractionation is not significantly affected by the presence or absence of carbon molybdenum isotope fractionation is furthermore independent of oxygen fugacity in the range iw 1 79 to iw 0 47 which are plausible values for core formation experiments at 1600 c show that at equilibrium the 98mo 95mo ratio of silicate is 0 12 0 02 0 12 0 02 heavier than that of metal and that the presence or absence of sn does not affect this fractionation equilibrium mo isotope fractionation between liquid metal and liquid silicate as a function of temperature can therefore be described as mometal silicate98 95 4 70 0 59 105 t2 our experiments show that mo isotope fractionation may be resolvable up to metal silicate equilibration temperatures of about 2500 c rendering mo isotopes a novel tool to investigate the conditions of core formation in objects ranging from planetesimals to earth sized bodies

apatite dissolution into peraluminous haplogranitic melts an experimental study of solubilities and mechanisms apatite ap dissolution and solubility in the peraluminous haplogranite system were investigated in three sets of experiments at 750 c and 200 mpa ph2o as functions of ap grain size abundance melt ap ratio and melt asi alumina saturation index mol al2o3 na2o k2o cao between an asi of 1 1 1 metaluminous to mildly peraluminous the solubility of ap as measured by p2o5 in the melt is low 0 1 wt p2o5 which agrees with previous work in equilibrium runs with more strongly peraluminous compositions however ap solubility increases linearly with asi to p2o5 0 63 wt in melt at asi 1 3 and can be denned by a simple equation p2o5 3 4 3 1 asi r 0 833 which agrees within error with previous calculations reported in the literature in experiments where ap grains are sparce and widely separated i e simulating ap distribution in an ap bearing peraluminous protolith undergoing anatexis the dissolution of ap involves transient disequilibrium that promotes higher solubility of p2o5 up to nearly six times the equilibrium values adjacent to ap at a given bulk asi of melt and this increases the al si of melt in the ap dissolution aureole the approach to equilibrium between ap and melt is hastened by increasing the ap melt ratio within the local melt pool e g the capsule or by decreasing ap grain size for a similar melt ap ratio diffusivity calculations using inverse error function methods for the normal diffusion of p and ca yield dp 10 11 cm2 s and dca 10 10 cm2 s confirming prior results that diffusion of p away from ap surfaces is the rate limiting factor in ap dissolution these results combined with previous work at higher temperatures but on similar hydrous melts yield an activation energy e for p diffusion of 16 5 kcal mol and a frequency factor d0 of 1 4 10 3 cm2 s phosphorus diffusion through the melt is accompanied by uphill diffusion of al resulting in an increase of up to 2 wt al2o3 in melt near the ap within the ap dissolution aureole melt contains one additional al cation for every p cation i e al p 1 1 which is suggestive of an associated alpo4 complex the exchange reaction in melt is represented by alpsi 1 in which 2 to4 clusters are created in the place of 1 this exchange should be accompanied by discernable changes in melt properties e g density molar volume viscosity the results have applications to equilibrium and local disequilibrium effects during anatexis of apbearing metapelites and to the saturation limits of ap during crystallization of peraluminous magmas the dissolution of flux bearing minerals such as ap e g those containing p b or f may influence the stability and solubility of other trace element bearing minerals during anatexis of metapelites texture e g grain size and distribution and spacial relationship to other minerals plays an important role in reaction kinetics and at least temporarily in mineral solubilities the disequilibrium behavior of accessory minerals has critical consequences for ree modelling of granites

metamorphic p t conditions of the kerala south india khondalite belt a granulite facies supracrustal terrain the khondalite belt of southernmost india is an immense section of granulite facies metasediments of probable early proterozoic age dominant rock types are migmatized khondalites garnet sillimanite pm cordierite metapelites acid biotite garnet graphitic gneisses and charnockites quartzofeldspathic gneisses with orthopyroxene throughout much of southern kerala the biotite gneisses are in a state of arrested conversion to charnockite partially decrepitated co 2 inclusions in quartz at the ponmudi mixed biotite gneiss charnockite quarry have significant ch 4 and or n 2 and are texturally late the probable sequence of events in metasediments was 1 migmatization 2 charnockite formation 3 symplectitic cordierite formation and 4 late co 2 entrapment phase equilibrium diagrams of anatexis in aluminous systems and fe mg exchange between garnet and biotite indicate metamorphic temperatures of 650 850 c for the khondalites temperatures based on garnet orthopyroxene

refining upon the climatic background of the early pleistocene hominid settlement in western europe barranco leon and fuente nueva 3 guadix baza basin se spain abstract the early pleistocene sites of barranco leon and fuente nueva 3 guadix baza basin se spain have yielded thousands of mode 1 or oldowan lithic artifacts both sites and one tooth in layer d formerly 5 of barranco leon today considered to be some of the earliest evidence of humans in western europe at ca 1 2 1 5 ma previous quantitative paleoclimatic reconstructions based on herpetile assemblages indicated that during the formation of these two sites the mean annual temperature and mean annual precipitation were higher than they are now in the southeastern iberian peninsula with lower continentality here we propose new climatic reconstructions where the mean monthly temperature and precipitation and the difference between the four driest months and the four rainiest months are estimated climatograms are built in order to specify the distribution and variation of temperature and precipitation during the year and the aridity indices of gaussen lautensach mayer dantin revenga and de martonne are used to characterize ombroclimatic differences according to these new climatic parameters rainfall distribution through the year shows considerably higher precipitation in every season except summer and early autumn which remain drier and thus consistent with a mediterranean climate pattern no change is observed in the duration of the aridity period which remains four months long however the value of the aridity index of de martonne is higher than 20 subhumid climate in barranco leon and fuente nueva 3 whereas today it is lower than 20 semi arid climate suggesting major changes in the ombroclimatic type these results yield a more precise scenario for the paleoclimatic conditions that prevailed during the late early pleistocene in the guadix baza basin and permit us to contrast the ages obtained from numerical dating and biochronology the very warm and humid climate reconstructed for both barranco leon and fuente nueva 3 suggests that in accordance with the numerical dating these two sites are contemporaneous with the particularly warm interglacial peaks of marine isotope stages 43 49 i e between 1 35 and 1 47 ma the similarity between reconstructed climates the high overlap between their estimated precipitation and between the difference of the driest from the rainiest season suggest that these sites may correspond to the same part of a climatic glacial interglacial cycle but because the evolutionary degree of the rodent mimomys savini shows a slightly more derived state for fuente nueva 3 than for barranco leon i e a short chronological difference between the two sites they may correspond to two consecutive warm interglacial peaks

fluid inclusions in rocks from the amphibolite facies gneiss to charnockite progression in southern karnataka india direct evidence concerning the fluids of granulite metamorphism fluid inclusion studies of rocks from the late archaean amphibolite facies to granulite facies transition zone of southern india provide support for the hypothesis that co2 rich h2o poor fluids were a major factor in the origin of the high grade terrain charnockites closely associated leucogranites and quartzo feldspathic veins contain vast numbers of large co2 rich inclusions in planar arrays in quartz and feldspar whereas amphibole bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed facies quarries contain no large fluid inclusions inclusions in the northernmost incipient charnockites as at kabbal karnataka occasionally contain about 25 mol of immiscible h2o lining cavity walls whereas inclusions from the charnockite massif terrane farther south do not have visibile h2o microthermometry of co2 inclusions shows that miscible ch4 and n2 must be small probably less than 10mol combined densities of co2 increase steadily from north to south across the transitional terrane entrapment pressures calculated from the co2 equation of state range from 5 kbar in the north to 7 5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750 c in quantitative agreement with mineralogic geobarometry this agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions calculations on the stability of the charnockite assemblage biotite orthopyroxene k feldspar quartz show that an associated fluid phase must have less than 0 35 h2o activity at the inferred p and t conditions which agrees with the petrographic observations high tio2 content of biotite stabilizes it to lower h2o activities and the steady increase of biotite tio2 southward in the area suggests progressive decrease of ah2o with increasing grade oxygen fugacities calculated from orthopyroxene magnetite quartz are considerably higher than the graphite co2 o2 buffer which explains the absence of graphite in the charnockites the present study quantifies the nature of the vapours in the southern india granulite metamorphism it remains to be determined whether co2 flushing of the crust can by itself create large terranes of largeion lithophile depleted granulites or whether removal of h2o bearing anatectic melts is essential

structural environment around th4 in silicate glasses implications for the geochemistry of incompatible me4 elements the structural environment around th4 in several silicate glasses containing 1 3 wt th4 was investigated as a function of melt composition and polymerization using extended x ray absorption fine structure exafs spectroscopy one set of glasses had the bulk chemical composition of monticellite mo diopside di anorthite an albite al olivine basalt o bas tholeiitic basalt t bas calc alkaline dacite dac and calc alkaline rhyolite rhy a second set of th bearing glasses had the bulk al composition but also contained either 5 5 wt f or 2000 ppm cl in glasses containing 3 wt th th coordination varies from 8 d viiith o 2 41 0 02 a in mo and di to a mixture of 8 and 6 coordinations d vi viiith o 2 35 2 36 0 02 a in all other compositions the presence of viiith is related to the thorianite tho2 saturation in the glass in al glasses containing 1 wt th 6 fold coordinated th dominates d vith o 2 32 0 02 a no clear evidence for f or cl complexes around th is observed in any of the halogen containing glasses weak but significant interactions of th with tetrahedral si al network and or alkali elements are observed especially in highly polymerized compositions rhy in which th si ai na k contributions are detected between 3 25 and 3 50 0 05 a these results suggest that in this set of glasses th does not form complexes but rather acts as a weak network modifier the presence of vith in silicate glasses an unusual low coordination for th suggests that the ionic radius of th in magmatic systems is lower than previously reported rth 0 94 a the resulting high th o bond strength 0 7 vu unknown in minerals may explain the observed low crystal melt partition coefficients of th4 i e its incompatible character during magmatic differentiation increasing polymerization is predicted to favor higher coordination around th viii ix as a result of a shortage of nbo to which th preferentially bonds hence dramatic changes in the crystal chemistry of th are predicted i e the incorporation of th in minerals comparison with u4 suggests that vith is less abundant in similar glass melt systems compared with viu this may explain the u th fractionation often observed in zircon as well as the preferential incorporation of th in accessory minerals containing large sites in which th4 can be substituted e g the ca2 site in titanite apatite or zirkelite

partition coefficients for rare earth elements in mafic minerals of high silica rhyolites the importance of accessory mineral inclusions abstract ree concentrations of mafic mineral separates from high silica rhyolites measured by inaa are high and variable compared to electron microprobe analyses of the minerals themselves the mafic phases commonly contain inclusions or have adhering grains of accessory rare earth element ree rich minerals optical and electron microscopic observation revealed discrete grains of chevkinite rare earth titano silicate included within clinopyroxenes from the sierra la primavera mexico rhyolite and monazite grains adhering to ortho and clinopyroxenes from the bishop tuff california during hand picking of mineral separates inclusions are only partly removed as a result the magnitude and variability of true mineral melt partition coefficients for light ree have been overestimated the true ree partition coefficients of la primavera and bishop tuff pyroxenes obtained by microprobe are only slightly higher than they are in lower silica rhyolites and are not as variable as previously thought the partitioning slope is positive as in less silicic systems the relative partitioning behavior of ree in high silica rhyolites is dominated by crystal chemical controls and not by liquid structural effects when a partition coefficient is used for crystallization calculations adhering phases and inclusions should be retained or the minor phases must be accounted for separately alternatively a bulk distribution coefficient can be calculated using whole rock and glass compositions and the glass mode

grenville magmatism in west texas petrology and geochemistry of the red bluff granitic suite zone of regional extension as suggested by recent geophysical and the red bluff granitic suite rbgs underlies part of the franklin petrological studies mountains in west texas it was emplaced into a middle proterozoic shelf sequence at 1120 35 ma the suite is predominantly metaluminous it consists of early granitic to quartz syenitic sills a main mass of alkali feldspar granite small bodies of alkali feldspar syenite leucogranitic dikes and late stage peralkaline

fe ti oxide chronometry with application to granulite formation abstract ilmenites occurring in amphibolite and granulite facies orthogneisses in sri lanka yield precise u pb ages coexisting ilmenite magnetite in the granulite facies rocks indicate temperature conditions of 680 60 c and log o 2 17 0 1 2 the u pb age of 498 2 3 3 ma obtained for ilmenite is indistinguishable from the pb pb age of 501 5 ma derived for coexisting silicate minerals and suggests that oxide silicate equilibration has been preserved in contrast fe ti oxides in adjacent amphibolitefacies rocks indicate a temperature of 830 60 c and log o 2 12 7 0 8 and record u pb ages of 1093 7 7 6 ma and 1095 4 4 2 ma these ages are some 600 ma older than those obtained for coexisting silicate phases in the same sample or for both oxide and silicate minerals in the nearby charnockite and relate either to the time of emplacement of the granitic precursor or to some earlier metamorphic episode these results suggest that temperature o2 and fluid conditions estimated from fe ti oxides should be interpreted with extreme caution