experimental reactions of amphibolite with boron bearing aqueous fluids at 200 mpa implications for tourmaline stability and partial melting in mafic rocks reactions between hornblende plagioclase amphibolite and acidic and alkaline b bearing aqueous fluids have been investigated by experiments at 475 600 c and 200 mpa at 600 c hornblende calcic plagioclase react to form tourmaline danburite clinopyroxene quartz in acidic fluids containing 0 5 1 0 wt b2o3 tourmaline is precipitated directly from acidic fluids and the reaction is driven by neutralization of fluids by na ca derived from the breakdown of reactant solids the concentration of b2o3 in fluids needed to stabilize tourmaline increases as ph increases above approximately 6 0 and tourmaline is unstable in alkaline fluids ph approximately 6 5 7 0 regardless of b concentration in addition to acid base relations tourmaline stability is favored by comparatively higher activity coefficients for al species in acidic fluids the concentrations of al and si in fluid increase with alkalinity with the eventual production of felsic borosilicate melts through partial melting of the plagioclase component of the amphibolite in seeded experiments tourmaline also contributes components to melt partial melting is evident in the range 500 525 c at 200 mpa in experiments with 8wt b2o3 in fluid as na2b4o7 the experimental results are applied primarily to metasomatic reactions between mafic rocks and borate fluids derived from granitic magmas but tourmaline stability and partial melting in mafic regional metamorphic systems are also discussed briefly

geological interpretation of gas content trends walloon subgroup eastern surat basin queensland australia abstract the middle jurassic walloon subgroup is a prolific low rank coal seam gas csg resource in the surat basin australia this study applied a regionally consistent stratigraphic framework model to open file gas desorption and coal property data from 69 cored csg wells to assess spatial trends in gas content and test the influence of large scale geological heterogeneity on present day gas distribution continued successful exploration and production and possible future microbial regeneration of the walloon csg resource require an improved understanding of the controls on gas content distribution across the core region of production in the eastern surat basin the walloon subgroup can be subdivided into upper juandah and lower taroom coal measures separated by a relatively coal barren and sandstone dominated unit tangalooma sandstone within these units lateral variation in coal character is high precluding a regionally agreed coal group or seam correlation while the total dataset shows a general increase in gas content dry ash free d a f with increasing depth as a function of hydrostatic pressure there is considerable scatter in the distribution within individual wells gas content follows one of three trends gas content either 1 increases 2 increases then decreases or 3 decreases with depth trend 2 parabolic wells predominate with the gas content maxima coinciding with the tangalooma sandstone regardless of depth as such gas content layering follows rather than cross cuts the stratigraphy in the majority of wells the change in gas content with depth between the different units and the wells was used to identify domains of different gas gradients which could relate to differing gas saturations and geological setting the well profiles and gas gradients were analysed against available open file coal quality data without strong correlation to rank or type parameters the majority of very high gas gradient values occur in coals within or above the tangalooma sandstone suggesting that it forms a regional scale barrier both to possible gas migration from underlying and higher rank coals and to meteoric and biogenic recharge from above increased ethane values in the upper taroom and tangalooma csg support this interpretation alternatively this distribution may be the result of alternating adsorption desorption cycles linked to burial and uplift the wells containing the highest mean gas gradients span a range of subtle rank and structural domains but generally occur in structural highs under topographic lows with hydrostatic head wells from smaller scale anticlines not associated with topographic surface lows mainly exhibit lower mean gas gradients the study significantly expands the number of published gas content trends and proposes several factors influencing their formation and geographical distribution the results form the basis for future targeted sampling to test for gas origins migration retention and biogenic recharge processes within the surat basin

trace element modelling of pelite derived granites the presence or absence of a vapour phase during incongruent melt reactions of muscovite and biotite together with the composition of the protolith determines the trace element characteristics of the resulting melt provided that equilibrium melting occurs for those phases that host the tracc elements of interest for granitic melts rb sr and ba provide critical constraints on the conditions that prevailed during melting whereas ree are primarily controlled by accessory phase behaviour mass balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction and deplete the restite in feldspar hence the melt will be characterized by low rb sr and high sr ba ratios in contrast vapour absent melting will result in a smaller melt fraction and an increase in the restitic feldspar consequently high rb sr and low sr ba ratios are predicted vapour absent melting will also enhance the negative eu anomaly in the melt granites that result from the incongruent melting of biotite in the source will be characterized by higher rb concentrations than those that result from the incongruent melting of muscovite the himalayan leucogranites provide an example of unfractionated crustally derived eutectic melts that are enriched in rb but depleted in sr and ba relative to their metasedimentary protoliths these compositions may be generated by the incongruent melting of muscovite as a low melt fraction f 0 1 from a pelitic source under vapour absent conditions

phase equilibria of a fluorine rich leucogranite from the st austell pluton cornwall highly evolved leucogranitic rocks in the st austell pluton cornwall of hercynian age contain accessory muscovite topaz and fluorite the authors have studied the h sub 2 o saturated melting behavior of one representative sample its solidus and liquidus pass through the points 663 and 725 sup 0 c respectively at 1 kbar 640 and 665 sup 0 c at 2 kbar 610 and 717 sup 0 c at 4 kbar and 608 and 700 sup 0 c at 8 kbar plagioclase is on the liquidus at low pressure and topaz is on the liquidus at 4 kbar the fluorite is consumed in the formation of the first formed liquid calcium can partition into an evolved granitic melt if complexed by fluorine the fluorite appears to be largely primary in fresh fluorite granite at st austell and not to reflect the albitization of oligoclase in the surrounding biotite granite such fluorine rich leucogranites can be expected to be of subsolvus character

differentiation of strongly peraluminous perphosphorus granites the pedrobernardo pluton central spain the pedrobernardo pluton 295 2 ma initial 87sr 86sr 0 7122 is a layered sheet like granite body with distinct vertical zoning biotite muscovite bearing granites in the lower zone muscovite biotite leucogranites in the middle zone and muscovite bearing aplites and pegmatites in the upper zone the magma was anatectically generated intruded almost free of restite and evolved as a closed system by crystallization in situ geochemical modelling indicates that the lower and middle zones followed an equilibrium crystallization path with no separation between solids and melt and the upper zone followed a fractional crystallization path magmatic fractionation produced leucocratic differentiates with high p concentrations and a p ca ratio higher than in apatite the excess p is located in plagioclase and k feldspar monazite apatite fractionation in the lower and middle zones produced extreme depletions in th u y and the rees and chondrite normalized ree patterns with low lrees hrees and a slight discontinuity between nd and sm about 90 of the th u y and rees reside in accessory minerals especially in monazite biotite and muscovite have very low ree contents with small positive or no eu anomalies k feldspar and plagioclase in both the lower and upper zones have moderate ree contents with positive eu anomalies feldspars from the upper zone in contrast have negative eu anomalies and have very low ree contents compared to available data from literature dsr ba th u y reesbio mineral are lower by one to three orders of magnitude and dreesplag kfsp are also lower by one order of magnitude these huge discrepancies are attributed not only to contamination with ree rich inclusions in mineral concentrates used in previous studies but to real differences in geochemical behavior of trace elements in peraluminous systems with respect to subaluminous systems biotite melt and probably k feldspar melt distribution coefficients for th u y and the rees in peraluminous magmas must be much lower than previously considered

phosphorus and the rare earth elements in felsic magmas an assessment of the role of apatite abstract the solubility of fluorapatite in 17 silica rich melts in the system na 2 o k 2 o al 2 o 3 sio 2 with and without cao or caf 2 was determined at 1 kbar water pressure and 750 900 c apatite saturation occurs at levels of dissolved p 2 o 5 ranging between 0 04 0 02 and 0 28 0 13 wt with only 4 values outside the 0 09 0 20 wt range the results demonstrate not only that apatite is a common liquidus phase in felsic melts but also that under most circumstances it remains in the residue during episodes of partial fusion of the crust given a solubility limit of 0 14 wt dissolved p 2 o 5 the mean of the experimental values a source containing as little as 0 05 p 2 o 5 must be 35 melted before apatite is lost from the residue and no longer buffers the melt p 2 o 5 concentration at the saturation value higher abundances of p 2 o 5 in the source postpone the loss of residual apatite to still higher degrees of melting and if the source p 2 o 5 content exceeds 0 14 wt apatite must be residual for all degrees of melting increasing in abundance as melting proceeds the generally secondary influence of apatite on the rare earth element ree patterns of melt and residue is most apparent when garnet and or amphibole is minor or lacking in the residue fractional crystallization of intermediate e g andesitic magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of ree in the melt

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

chemical variation and significance of tourmaline from southwest england tourmaline is a common and locally abundant mineral in all products of the granite magmatism and associated hydrothermal activity in southwest england particularly in the county of cornwall tourmaline of magmatic origin is homogeneous and is marked by high fe mg high f and high al in the place of divalent cations r2 site the substitution of al for divalent cations such as mg and fe super 2 is charge compensated by a deficit of alkalies and protons in other structural sites tourmaline compositions from the granites clearly reflect the sequence of increased differentiation among the magma types with biotite granites as the least evolved and topaz granites as the most evolved in contrast tourmaline of hydrothermal origin within granites displays fine scale compositional zonation with a general tendency toward more magnesian compositions nearer the schorl dravite solid solution i e little or no al in the r2 site metasomatic tourmaline precipitated in the surrounding pelitic and mafic rocks also possesses fine scale zonation and generally reflects the compositions of the host rocks in addition tourmaline formed in the country rocks has a higher proportion of fe super 3 to fe super 2 which presumably indicates a higher oxidation state of fluids in the metamorphic rocks than in the granites this variation may correlate with the mobilization of tin from the granites and its consequent deposition as cassiterite in the host rocks the abundance of magmatic tourmaline is limited by the initial fe mg content of the magmas to not more than a few modal or weight percent although they contain tourmaline the biotite granites are not the most likely sources of boron for voluminous late stage tourmalinization magmas that contained only tourmaline or no fe mg minerals at all such as the those that formed the topaz granites may have been sources of large quantities of boron the most intense areas of tourmalinization do surround the small stocks and sheets of topaz microgranite high concentrations of tourmaline for example in hydrothermal veins and breccias appear to require mixing of two different chemical reservoirs one a source of boron the magmas or fluids

tourmaline in a low grade clastic metasedimentary rock an example of the petrogenetic potential of tourmaline detrital tourmaline grains and their associated tourmaline overgrowths provide a means to unravel the provenance and petrogenetic history of low grade clastic metasedimentary rocks evidence derives from tourmaline grains found in a lithic wacke metamorphosed to chlorite zone conditions the detrital tourmaline cores are diagnostic indicators of the source rocks of the sediment whereas the overgrowths record both diagenetic and metamorphic reactions in the rock tourmaline grains consist of a detrital core surrounded by asymmetric overgrowths comprised of inner and outer rims abrupt chemical discontinuities between each of these zones implies that volume diffusion within tourmaline was minor under the conditions of formation compositions of the detrital cores vary widely yet can be correlated with source rock types that are consistent with lithic fragments recognizable in the metawacke at either the analogous or antilogous pole inner rim compositions proximal to the detrital cores converge despite the substrate tourmaline composition indicating an approach to chemical equilibrium however significant dufferences in al and x site vacancies at the expense of mg na and ti between the analogous and antilogous poles of the inner rims demonstrate the presence of significant amounts of compositional polarity outer rim compositions at either pole also converge but compositional polarity between the analogous and antilogous poles persists the presence of the inner and outer rims separated by a compositional discontinuity suggests punctuated evolution of the overgrowth this implies that boron was sporadically available during diagenesis and metamorphism based on boron contents of minerals this may correspond to a mechanism such as boron release due to polytypic change of illite or consumption of illite and or muscovite as such tourmaline growth stages may serve as a monitor of chemical reactions in low grade metamorphic rocks

experimental partitioning of rb cs sr and ba between alkali feldspar and peraluminous melt hydrous partial melting experiments performed between 650 and 750 c at 200 mpa h20 on synthetic metapelite compositions quartz albite muscovite biotite mineral mixtures doped with ba sr rb and cs yielded alkali feldspar crystals with a wide range of compositions in equilibrium at their rims with peraluminous melt measured partition coefficients for normally trace lithophile elements between feldspar and melt d m fsp gl m ba sr rb cs do not depend on either temperature or bulk composition of melt for the compositions studied values of d sr fsp gl are between 10 and 14 and appear to be independent of the albite and orthoclase contents of the feldspar crystals in contrast values of d ba fsp gl and d rb fsp glare strongly dependent on the orthoclase content of feldspar relationships that can be expressed by the following linear equations d ba fsp gl 0 07 0 25 orthoclase and d rb fsp gl 0 03 0 0 1 orthoclase where orthoclase is in mole percent these equations reproduce the range of previously reported values for d ba and d rb determined on natural and synthetic samples a single partition coefficient for cs was also determined at d cs fsp gl 0 13 these data can be used in conjunction with recently published partition coefficients for muscovite biotite and plagioclase feldspars blundy and wood 1991 icenhower and london 1995 to model quantitatively the trace element signatures of peraluminous magmas during anatexis and crystallization depending on bulk composition and pressuretemperature trajectories during episodes of partial melting ba and sr and to a lesser extent rb are retained at the source while cs and li are enriched in the first melts these results explain why peraluminous granitic magmas with s type sedimentary sources carry a distinctive enrichment in li and cs

tectonics of the pliocene homs basalts syria and implications for the dead sea fault zone activity timing of activity along the yammuneh segment lebanon syria of the dead sea fault zone and its northward continuation is still a subject of controversy our field structural analysis and observations on radar landsat and digital elevation model imagery of the homs region syria are of concern for plio quaternary tectonics of the whole northern part of the dead sea fault zone we show in this paper that the northern dead sea fault zone has remained active since the onset of the homs basalts at c 6 ma continuing movement in recent times is indicated by the occurrence of quaternary pull apart structures and offset of active ravines along the fault the homs basalts are related to the distinct oval shaped shin volcanic edifice of which the long axis trends nw the volcano was fed through nw striking tension fractures which now form dykes and volcanic ridges these patterns are consistent with a ne sw extension that occurred c 6 ma ago the northwestern end of the shin volcano is left laterally displaced c 20 km yielding a c 3 3 mm a 1 mean rate of relative movement between the arabian and african plates in the northern part of the dead sea fault zone the overall trace of the main active dead sea fault zone is not a single transform but forms an irregular plate boundary composed of transform fault and collision belt segments fine grained mylonite developed in the fault corridor may have favoured aseismic deformation in the shin volcano area

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

evolution of the cornubian ore field southwest england part i batholith modeling and ore distribution the cornubian ore field of southwest england is associated with an s type ilmenite series high heat production hhp two mica tourmaline bearing monzogranite batholith which formed between 270 to 295 ma base metal deposits are mainly in the form of steeply dipping lodes emplaced in the batholith roof and a hornfelsic sequence of upper paleozoic mud rocks sandstones and mafic volcanics the steep sided batholith has a gently sloping base 10 to 15 km below the present surface three dimensional modeling of gravity data indicates that it has a total volume of about 68 000 km 3 and is divisible into distinct western and eastern parts computer aided evaluation of mineral production statistics shows that most of the sn w cu and as production is located within the axial trace of the batholith with the western end enriched in sn cu and zn relative to the eastern end by contrast most of the fe as sb f ba and china clay production is located in the eastern end while pb and ag production is concentrated in the country rock filled south facing embayments on the flank of the batholith all significant wolframite production is from within the granite or within a 700 m distance of an observed or projected granite contact whereas 90 to 100 percent of the sn cu and as production occurs within 1 500 m of a contact by contrast 30 percent of the zn production and over 80 percent of the pb and ag production is from distances of more than 1 500 m from a granite contact pre syn and postbatholith faults and extensional fractures exert a fundamental control on ore distribution with most of the sn w cu as zn ores hosted by fractures trending parallel to the axial trace of the batholith and most of the pb sb ag ba and fluorite mineralization hosted by faults trending approximately normal to the axial trace of the batholith the nature of the sedimentary sequence also influenced metal distribution with most of the sn cu as production coming from areas underlain by devonian argillaceous rocks an important feature of the granite which has a bearing on its metallogenic potential is its high internal heat production due to its high total content of u th and k modeling of the cooling history of the granites reveals that temperatures in the deeper parts of the batholith were maintained close to or above the solidus for 10 to 20 ma after initial emplacement at 280 to 295 ma residual volatile and metal enriched magmas trapped within the batholith provided source materials for younger magmatism 270 280 ma and the main episode of lode formation which was related to the late magmatic events continued high heat flow throughout the mesozoic and cainozoic promoted thermally driven ground water convection and epithermal mineralization

apatite in felsic rocks a model for the estimation of initial halogen concentrations in the bishop tuff long valley and tuolumne intrusive suite sierra nevada batholith magmas

estimating abundances of volatile and other mobile components in evolved silicic melts through mineral melt equilibria to variable degrees of accuracy from source partitioning between and earths residual minerals and anatectic melts through melt fractionation partitioning between igneous minerals and residual melts to the end stages of magma solidification where higher concentrations of normally trace elements may promote saturation in their crystalline phases the magmatic abundance of rare alkalis and alkaline earths

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

behaviour of trace elements during magmatic processes in the crust application to acidic volcanic rocks of tuscany italy abstract plio quaternary acidic rock suites from three volcanic centres of tuscany italy san vincenzo roccastrada and monte amiata are composed predominantly of rhyolitic and rhyodacitic ignimbrites the rocks are characterized by distinct enrichments particularly of k and rb and display considerable compositional differences among the various volcanic centres the volcanic rocks of roccastrada and most of s vincenzo were probably generated by partial melting of metapelitic rocks from the paleozoic basement of tuscany leaving a residue composed of biotite cordierite and quartz and sometimes plagioclase or garnet the enrichment of k and related elements can be attributed to selective contamination by fluids the other volcanic rocks including trachytes of mt amiata have also been affected by interaction with an upper mantle component probably k rich basaltic rocks a similar petrogenetic model can be applied to the granodiorite pluton of mt capanne elba which belongs to the same magmatic cycle since the variation trends observed in the tuscany volcanic rocks are similar to those reported from many acidic plutons it appears that crustal anatexis accompanied by interaction with upper mantle components has frequently played an important role in the genesis of the granitic rocks

apatite solubility in peraluminous liquids experimental data and an extension of the harrison watson model abstract new experimental data at 750 1000 c 2 5 kbar and for peraluminous melt compositions show a dramatic increase of apatite solubilities and phosphorus melt contents when compared to metaluminous or peralkaline compositions harrison and watson 1984 the formation of alumino phosphate units in peraluminous melts qualitatively explains our data a new apatite solubility model incorporating the data for the peraluminous region is presented application of this model to examples of peraluminous felsic suites is discussed

phase equilibria of phlogopite lamprophyres from western mexico biotite liquid equilibria and p t estimates for biotite bearing igneous rocks olivine and augite minette powders have been equilibrated from one bar to nearly 2 0 kbar water saturated and from 900 to 1300 c and then quenched rapidly at oxygen fugacities controlled between the nickel nickel oxide nno and hematite magnetite hm oxygen buffers the liquidus of both samples is suppressed 100 c at water saturated conditions and 1500 bar both lavas contained between 3 and 4 wt water at the stage of phenocryst precipitation the partitioning of ferric and ferrous iron between phlogopite and liquid has been determined on eight samples across 3 log f o2 units when these determinations are combined with previous studies fe2o3 feo total of mg rich biotite can be calculated knowing log fo2 t and xfe thermodynamic modelling of biotite liquid equilibria results in two expressions for calculating activity coefficients for annite and phlogopite in natural biotites based on the partitioning of bao and tio2 between biotite and liquid we have formulated a thermometer and barometer over the range of 400 c tio2 partitioning between phlogopite and liquid is a function of temperature 50 c and is insensitive to pressure and h2o and o2 activities bao partitioning between phlogopite and liquid is a function of both temperature and pressure 4 kbar the latter being most important applying the tio2 and bao partitioning expressions to lamprophyre and lamproite suites shows that mexican minettes equilibrated at low pressures 5 to 15 kbar 4 kbar and temperatures 1090 to 1160 c 50 c while australian lamproites equilibrated at higher p up to 30 kbar 4 kbar and t 1125 to 1400 c 50 c experimental glass compositions and phenocryst fractionation calculations together with the bao and tio2 based pressure calculations indicate that felsic minettes from the mexican suite of lavas can be generated by simple fractionation of a more mafic parent minette at mid to lower crustal pressures

incongruent dissolution of ree and sr rich apatite in peraluminous granitic liquids differential apatite monazite and xenotime solubilities during anatexis the relative solubilities of monazite mnz xenotime xno and apatite ap ree and sr rich and ree and sr poor have been studied in peraluminous granitic liquids in month long experiments at 750 c and 200 mpa ph20 in contrast with the high solubility of the apatite 0 7 wtojop205 in strongly peraluminous liquids monazite and xenotime have much lower solubilities 0 05 wt jop205 in mildly to strongly peraluminous compositions p205 in the liquid is 0 03 0 04 wt joat xenotime saturation and 0 02 0 05 wt jo at monazite saturation in keeping with the low p in the glasses re203 contents are below emp detection thresholds 50 08 wt jo for all conditions of experiments saturation of liquid at equilibrium and local saturation around apatite apatite dissolves incongruently crystallizing ree rich monazite on its surface 1 4 m iong grains resulting in similar low ree contents in liquids monazite precipitation occurs along the margins of dissolving apatite crystals even though the bulk liquid is not monazite saturated the abundance of monazite microcrystals increases with the ree content of the apatite and the degree of apatite dissolution the reaction relationship ap liqi mnz liq2 stemming from differences in relative solubilities greater than an order of magnitude between apatite and monazite results in the dissolution of much smaller amounts of ree into peraluminous liquids than expected by simple evaluation of apatite ree contents the amount of ree contributed from apatite directly to peraluminous granitic liquid is related to the amount of apatite dissolved by simple mass balance only if the total ree content of the apatite is sufficiently low that monazite saturation in liquid 50 100 ppm re203 at 750 c is avoided during dissolution of sr rich apatite the sr partitions into the liquid and at 750 c and 200 mpa ph20 the diffusion coefficient of sr in liquid is 2 x 10 10 cm2js r2 0 659 the reaction relationships described above may have application to some textural features observed in natural igneous rocks for example clusters of monazite microcrystals might be indicators of dissolved apatite monazite morphology can be used to distinguish the source of its ree and the general petrological process rock anatexis or magma crystallization under which the monazite formed in addition monazite microcrystals could serve as nucleation sites for other minerals which might explain their common inclusion in biotite and amphibole within granitoids

experimental study of chlorine behavior in hydrous silicic melts chlorine solubility in silicate melts has been investigated at 830 850 plus minus 5c and at pressures ranging from 50 to 200 mpa using both natural pantellerite rhyolite phonolite and synthetic sio sub 2 al sub 2 o sub 3 k sub 2 o na sub 2 o compositions and a stated h sub 2 o nacl kcl fluid phase near 4 molal at 100 mpa cl contents in pantelleritic melts reach a solubility plateau at initial aqueous phase molality near 2 this plateau coincides with a large immiscibility gap between aqueous and chloride rich fluids with the coexisting cl saturated aqueous phase cl ranges from 2 720 plus minus 120 ppm in rhyolite to 8 960 plus minus 85 ppm in pantellerite and reaches 6 270 plus minus 170 ppm in phonolite at 100 mpa between 50 and 200 mpa the cl content in pantelleritic melt decreases from 9 640 plus minus 200 ppm to 5 040 plus minus 150 ppm although cl solubility increases with increasing feo sup in high sio sub 2 melts it is mainly controlled by the al si and na k al molar ratios of the melt with a minimum at na k al 1 in a series of synthetic rhyolitic to pantelleritic melts the experimental results suggest that chlorine occurs as alkali chloride complexes in high sio sub 2 more melts they also indicate that cl is concentrated in the aqueous fluids in equilibrium with sio sub 2 rich melts the exact value of d depending on melt composition and melt chlorine concentration volcanic degassing will create chlorine rich hydrothermal fluids and decrease chlorine melt content less

partitioning of fluorine and chlorine between biotite and granitic melt experimental calibration at 200 mpa h 2 o experiments from 640 to 680 c 200 mpa h2o at fo2 nno employing a natural f rich vitrophyric rhyolite from spor mountain utah assessed the effect of variable mg 100mg mg mn fe on the partitioning of fluorine and chlorine between biotite bt and melt over this temperature interval bt fluorite quartz is the sole liquidus phase partition coefficients for fluorine between biotite and glass dfbt melt show a strong dependence on the mg of bt dfbt melt varies from 1 5 to 7 2 over the range of mg from 21 to 76 a strong linear correlation between dfbt melt and mg has a slope of 9 4 and extrapolates through the origin i e dfbt melt 0 at mg 0 an annite siderophyllite solid solution in these experiments dclbt melt values 1 to 6 in the same experiments vary inversely with mg the al content of biotite does not vary with the aluminum saturation index asi molar al2o3 alkali and alkaline earth oxides of melt but two exchange mechanisms involving al appear to operate in these micas 1 alvi aliv siiv mgiv and mgiv 2aliv 2siiv iv the effects of other components such as li or other intensive parameters including fo2 have yet to be evaluated systematically at comparable mg of bt however the spor mountain rhyolite yields higher dfbt melt values than an li rich strongly peraluminous melt previously investigated the results indicate that the mg of bt exerts the principal control on halogen partitioning with asi and t as second order variables the experimental partition coefficients compare well with other experimental results but not with most volcanic rocks magmatic bt from most rhyolites records higher dfbt melt due to reequilibration with degassed h2o depleted magma and perhaps with f2o 1 exchange that may accompany oxidation fe3 o fe2 oh 1 this behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near morococala bolivia bt is sharply zoned with f rich rims but bt core melt inclusion pairs fall on our experimental curve for dfbt melt these experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks for plutonic rocks the actual f content of melt not a relative activity ratio involving hf species can be reasonably estimated if the mica has not undergone subsolidus reequilibration this information is potentially useful for some shallow level ca poor magmas that are thought to be rich in f e g a and s type granites but do not conserve f well as rocks

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

trace element sanidine glass distribution coefficients for peralkaline silicic rocks and their implications to peralkaline petrogenesis abstract sanidine glass distribution coefficients for 11 trace elements have been determined on six peralkaline and two subalkaline silicic rocks distribution coefficients for na sc fe cs la ce sm tb and lu from this study and the literature show little variation within analytical uncertainty for silicic rocks of peralkaline and subalkaline affinity distribution coefficients for eu and rb show a marked decrease with increasing peralkalinity this variation may be the result of the decrease in the degree of polymerization from subalkaline to peralkaline silicic melts previous studies involving modelling of peralkaline rocks have selected incorrectly eu and rb sanidine glass distribution coefficients determined from subalkaline silicic rocks

fertile granites of precambrian rare element pegmatite fields is geochemistry controlled by tectonic setting or source lithologies abstract a total of twenty fertile granites and their rare element pegmatite aureoles was examined to correlate their tectonic position relative to the preceding orogenic event with their geochemical signature namely li rb cs be ga sn nb ta y ree sc ti zr be th u f nyf the range of t time difference between metamorphism and or deformation and granite intrusion is 20 to 465 ma for the examined cases there is some tendency toward syn to late orogenic origin of lct s as opposed to post to anorogenic timing of nyf s but the correlation is poor compared to the previously accepted stereotype similarly both metaluminous and peraluminous fertile granites are about equally spread among syn late and anorogenic environments lct systems are largely associated with peraluminous granites whereas all metaluminous to peralkaline plutons bear the nyf signature tectonic control on geochemistry of granite pegmatite suites is poor and secondary to the fundamental influence of the source lithologies yielding granitic melts the lct type suites are derived from undepleted upper crustal lithologies suffering their first anatectic event which mobilizes the most volatile components into low temperature low percentage melts supracrustal sequences as well as ortho and para lithologies of their basement serve as protoliths with variable degrees of involvement the nyf type suites whose plutons have the geochemical signature of a granites may be derived by melting of depleted lower crustal sources residual after preceding anatectic events or possibly by melting of juvenile lithologies with short crustal residence differentiation from mantle derived magmas is highly improbable volatile extraction of nyf elements from the mantle and their pre enrichment in protoliths or melts seems to be required the few mixed systems with variable expressions of a nyf lct signature are derived basically as nyf suites but contaminated by the lct elements from undepleted upper crust via several possible mechanisms

partitioning of lithophile trace elements between h 2 o and h 2 o co 2 fluids and topaz rhyolite melt experiments have been conducted with h 2 o and h 2 o co 2 bearing fluids and melts of a cl f and lithophile trace element enriched vitrophyre associated with topaz rhyolite from spor mountain utah the starting material is mildly peraluminous and contains 1 25 wt percent f and 1 500 ppm cl the experiments were conducted with f sub o 2 controlled near ni nio at pressures of 0 5 to 4 kbars and at temperatures of 770 degrees to 950 degrees c fluid melt trace element distribution coefficients d i were calculated for li be b rb sr y zr nb cs ba ce th and u after ion microprobe analysis of starting material and run product glasses the concentration of element i in the fluid was calculated by mass balance and the fluid melt distribution coefficient d i was calculated as ppm by weight of i in fluid ppm by weight of i in melt the partitioning of li rb sr y nb cs and ce between aqueous fluids and melts of topaz rhyolite composition depends strongly on temperature and pressure at 0 5 kbars li rb sr y nb cs and ce concentrate in the fluid at temperatures or 900 degrees c at 2 kbars this same group of lithophile elements except for sr concentrate in the fluid at t or 900 degrees c be b and ba partition in favor of the melt at 0 5 and 2 kbars and at all temperatures of this study in the presence of co 2 free aqueous fluids at 4 kbars and at temperatures of 770 degrees to 950 degrees c all lithophile elements partition in favor of the melt this behavior for lithophile elements mimics that of published data on the partitioning of pb and zn between rhyolitic melts and cl bearing fluids previous studies have demonstrated an equivalent effect of pressure on the distribution of chalcophile trace elements between granitic melts and cl bearing aqueous fluids fluid melt distribution coefficients for the lithophile trace elements are significantly smaller for topaz rhyolite melts in equilibrium with mixed co 2 h 2 o fluids than for topaz rhyolite melts plus co 2 free aqueous fluids there is no evidence for co 2 complexing with the lithophile trace elements at pressures 6 m the data imply that the alkali elements m dissolve into the fluid as mcl 0 complexes and that ce dissolves into the fluid as a cecl 0 3 complex water saturated magmas of topaz rhyolite composition may become differentiated with respect to li cs and rb or ce y and nb if an aqueous fluid phase is evolved and moves through the magma under the influence of a temperature gradient that has a maximum temperature of at least 850 degrees to 900 degrees c

experimental determination of the solubility of ce monazite in sio2 al2o3 k2o na2o melts at 800 c 2 kbar under h2o saturated conditions the solubility of cerium monazite cepo4 in sio2 al2o3 k2o na2o melts was determined at 800 c 2 kbar under water saturated conditions dissolution experiments using synthetic ce monazite crystals and crystallization experiments using ce and po4 bearing glasses yield results in good agreement microprobe analyses of glasses using a sensitive method detection limit to 130 ppm show that the solubility of monazite is dependent on the melt composition and is especially low for peraluminous compositions application of these data to various geologic occurrences is in good agreement with petrographic and geochemical observations and shows that monazite fractionation is probably a very common phenomenon in two mica granites this provides a good explanation for the light ree and th depletion commonly observed in such rocks

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