oxygen diffusion in amphiboles the kinetics of oxygen isotope self diffusion in natural samples of hornblende tremolite and richterite have been measured samples were run under hydrothermal conditions using 18o enriched water profiles of 18o 16o 18o vs depth into the crystal were obtained using an ion microprobe the depths of sputtered holes were measured using an optical interferometer at 1000 bars 100 mpa water pressure the activation energies q and pre exponential factors d0 for diffusion parallel to c are d0 cm2 sec q kcal gm atom t c hornblende 1 20 1 10 741 6 650 800 tremolite 2 30 2 10 8 39 5 650 800 richterite 3 5 2 10 4 57 2 650 800 the diffusion coefficient d for hornblende at 800 c and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions an increase in water pressure from 200 to 2000 bars increases d by a factor of 2 7 for diffusion parallel to c at 800 c the d value for hornblende at 800 c is about 0 01 that for quartz and 0 001 that for anorthite as a result closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars it is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions

submillimeter scale zonation of δ18o in quartz and feldspar isle of skye scotland abstract laser probe analysis of oxygen isotope ratios in submillimeter domains of quartz and feldspar in hydrothermally altered granite reveals complex intragranular zonation that can be contoured within individual grains two samples collected within 10 m of each other in the maol na gainmhich epigranite mge isle of skye scotland are similar petrographically yet display very different 18o zonation patterns sk1 has isotopically zoned quartz 18 o 1 8 8 2 with intragranular gradients of 18o as great as 5 mm and individually homogeneous grains of k feldspar 0 2 which also display a large range of values 18 o 1 7 to 7 3 oxygen isotope gradients of more than 11 mm were measured in this sample in contrast sk4 has homogeneous quartz 18o 5 7 0 3 n 14 and a zoned k feldspar phenocryst 18 o 3 3 to 6 9 isotopic heterogeneity in quartz correlates with the degree of chlorite alteration in adjacent hornblende grains the isotopic heterogeneity revealed by the laser probe in single grains of quartz and feldspar is greater than the entire range of values displayed by bulk mineral analysis of samples collected throughout the mge pluton although the average values of 18o obtained by laser probe are identical to the average bulk values values in k feldspar suggest exchange at 400 c with infiltrating meteoric water 18 o 10 the marked differences in zonation profiles between samples on the outcrop scale suggest that permeability and isotope exchange during hydrothermal alteration was highly heterogeneous these microanalytical data for quartz and feldspar provide the first oxygen isotope evidence from pervasively altered granites that paleopermeability is caused at least in part by intracrystalline microfractures

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance

the δ18o of primary and secondary waters in hydrous volcanic glass abstract we present analyses of phyllosilicates volcanic glass and a series of experiments and observations on the 18o of water extracted by rapid thermal pyrolysis at 1450 c using the tcea thermal conversion elemental analyzer the study includes the same hydrous glasses that were previously analyzed for their d values of extracted water seligman et al 2016 we utilize natural phyllosilicates where water is present as oh only natural magmatic and experimentally quenched glasses in which water occurs as h2om and oh and glasses that underwent low temperature secondary hydration by meteoric water almost completely as h2om our study documents that 1 thermal extraction and simultaneous pyrolysis of h2ot into co and h2 produces little or no exchange

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

epidote in calcalkaline magmas an experimental study of stability phase relationships and the role of epidote in magmatic evolution experiments on tonalite and granodiorite were performed at conditions ranging from 2 1 to 18 kbar and 550 to 850 c to establish the magmatic stability field of epidote as a function of p t and f02 at water saturated conditions and f02 buffered by nno epidote has a wide magmatic stability field in tonalite at 13 kbar this field extends from the wet solidus at 630 to 790 0c the low pressure intersection of the magmatic epidote crystallization reaction and the h20 saturated tonalite solidus and hence the minimum pressure for magmatic epidote occurs at about 5 kbar the clapeyron slopes of epidote melting reactions are moderately positive in p t space at pressures below the intersection of the epidote melting and the gamet in reactions at 13 kbar 790 c at this intersection epidote reaches its maximal thermal stability 790 c in tonalite h20 in the presence of garnet that is above 14 kbar epidote melting reactions have steep negative clapeyron slopes in p t space at p t conditions near the low pressure intersection of the epidote melting reaction with the water saturated solidus experiments were also performed with f02 buffered by hm at these oxidizing conditions the magmatic stability field of epidote is enlarged down to a pressure of about 3 kbar in granodiorite the stability of epidote has been investigated at pressures from 7 5 to 18 kbar with respect to the tonalite the epidote out reaction is located almost at the same p t conditions at 10 kbar but the maximum thermal stability is about 50 c lower toward higher pressures although few experiments indicate that epidote stability does not increase dramatically with decreasing water activity at present state it is not possible to predict melting and crystallization behavior of epidote at waterundersaturated conditions modal abundances of epidote are low 05 vol in the presence of major 40 vol melt fractions thus epidote in calc alkaline magmatic rocks is not likely to affect major elements strongly by fractional crystallization

oxygen diffusion in quartz diffusional oxygen exchange between quartz and water was studied by determining 18o profiles in the quartz using an ion microprobe inverse error function plots of the data are linear as required for a diffusion mechanism one natural and two synthetic samples were used and although they had different impurity concentrations oh na and al per 106 si atoms are 7 4000 35 835 and 67 127 respectively the d values were the same within experimental uncertainty diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700 c and diffusion normal to the rhombohedron is intermediate the data indicate a change in slope on an arrhenius plot at the boundary at 100 mpa 1 kbar water pressure the preexponential factors do in square centimeters per second and activation energies q in kilojoules per mole for quartz 500 550 c are as follows do 190 and q 284 parallel to c and do 8 10 2 and q 238 normal to the rhombohedron for quartz 600 800 c do 4 10 7 and q 142 parallel to c do 9 10 7 and q 155 normal to the rhombohedron and do 1 10 4 and q 234 normal to c d increases with ph 2 o and between 25 and 350 mpa log d versus log h 2 o has a slope of approximately 1 1 these d values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water free systems

weathering processes and the composition of inorganic material transported through the orinoco river system venezuela and colombia abstract the composition of river borne material in the orinoco river system is related primarily to erosion regime which in turn is related to tectonic setting especially notable is the contrast between material derived from tectonically active mountain belts and that from stable cratonic regions for a particular morpho tectonic region the compositional suites of suspended sediment bed material overback deposits and dissolved phases are fairly uniform are are typically distinct from whose of other regions for each region a consistent set of chemical weathering reactions can be formulated to explain the composition of dissolved and solid loads in developing these formulations erosion on slopes and storage of solids in soils and alluvial sediments are important considerations compositionally verymature sediment is derived from areas of thick soils where erosion is transport limited and from areas where sediments are stored for extended periods of time in alluvial deposits compositionally immature sediments are derived from tectonically active mountain belts where erosion is weathering limited weathering limited erosion also is important in the elevated parts of the guayana shield within areas of sleep topography compared to the mountain belts sediments derived from elevated parts of the shield are more mature a greater degree of chemical weathering seems to be needed to erode the rock types typical of the shield the major element chemistry and mineral composition of sediment delivered by the orinoco river to the ocean are controlled by rivers that have their headwaters in mountain belts and cross the llanos a region of alluvial plains within the foreland basin the composition of sediments in rivers that drain the shield seems to be established primarily at the site of soil formation whereas for rivers that drain the mountain belts additional weathering occurs during s episodes of storage on alluvial plains as sediments are transported across the llanos to the main stem of the orinoco after mixing into the main stem there seems to be little subsequent alteration of sediment

effects of cation substitutions in garnet and pyroxene on equilibrium oxygen isotope fractionations high grade metamorphic rocks were used to explore oxygen isotope fractionations between pyroxene and garnet and to investigate the effects on fractionation factors of the cation substitutions fe3 al 1 and ca fe mg 1 recrystallized granulite facies 725 c wollastonite ores from the northern adirondack highlands contain essentially only the minerals clinopyroxene a di hd solid solution garnet a grs adr solid solution wollastonite and exhibit a systematic dependence of measured fractionations on the fe3 content of calcic garnet cpx cagrt 0 14 0 12 0 78 0 20 xadr and wo cagrt 0 15 0 22 0 57 0 33 xadr in eclogites formed at t 650 c measured compositions of ca poor garnet and omphacite combined with experimental data indicate that ca poor fe rich garnet is enriched in 18o compared to both diopside and grossular extrapolating to 1000 k alm di c 0 2 and alm grs c 0 5 orthopyroxene and clinopyroxene from gore mountain new york show a constant fractionation that is independent of rock type as expected if they have the same closure temperature these data imply opx cpx c 0 7 at 1000 k measured fractionations among ca poor garnet orthopyroxene clinopyroxene and hornblende in the gore mountain rocks further indicate an 18o enrichment in ca poor garnet over grs c 0 5 at 1000 k the new measurements are indistinguishable from expected equilibrium values based on experiments for the minerals enstatite diopside grossular wollastonite and feldspar but consistently indicate a significant isotope effect for the simple octahedral cation substitutions fe3 al 1 grs vs adr and ca fe mg 1 ca poor garnet vs grs opx vs cpx neither cation substitution has been directly investigated for its effect on 18o 16o fractionation with experiments in silicates chemical characterization of minerals is required prior to petrological interpretation of oxygen isotope trends

bulk rock and mineral chemistries and ascent rates of high k calc alkalic epidote bearing magmas northeastern brazil abstract a manifestation of the pan african brasiliano orogeny 700 550 ma in northeastern brazil was the emplacement of widespread neoproterozoic granitoids in diverse tectonic terranes among these plutons are the magmatic epidote bearing conceicao das creoulas caldeirao encantado murici and boqueirao plutons located close to the boundary between the alto pajeu and cachoeirinha salgueiro terranes the plutons are high k calc alkalic granodiorites to monzogranites with tabular k feldspar megacrysts pistacite atomic fe 3 fe 3 al in epidote in these granitoids ranges from 21 to 27 high oxygen fugacity log f o 2 19 to 13 and the preservation of epidote suggest that the magma was oxidized al in hornblende barometry indicates hornblende solidification between 6 and 8 kbar at 620 to 780 c according to the hornblende plagioclase thermometer zircon saturation thermometry attests to a near liquidus temperature range from 794 to 853 c partial corrosion of magmatic epidote in these four plutons occurred during an interval of no more than 10 30 years which corresponds to maximum magma ascent rates of 650 1000 m year diking associated with regional shearing probably facilitated rapid transport of granitic magma through hot continental crust at peak metamorphism and permitted survival of epidote that was out of equilibrium at the low pressure of final emplacement similarities between mineralogical composition chemistry and isotopic compositions end 0 60ga between 2 and 5 t dm from 1 2 to 1 3 ga 18 o values 10 v smow of these four plutons and neoproterozoic magmatic epidote bearing plutons elsewhere in northeastern brazil argue for similar metabasaltic mafic sources that had previously experienced low temperature alteration

isotopic exchange in mineral fluid systems ii oxygen and hydrogen isotopic investigation of the experimental basalt seawater system abstract oxygen and hydrogen isotopic exchange reactions between basalt and seawater at t 300 to 500 c were investigated using oceanic tholeiitic basalt 18 o 5 7 d 70 natural seawater 18 o 1 2 d 5 and artificial seawater 18 o 5 3 as starting materials the starting basalts varied in the crystallinity from holocrystalline to glass but were ground to approximately the same grain size 100 mesh the water rock mass ratios ranged from 1 to 3 and the duration of the experiments ranged from 167 to 576 days in general depletion of 18o and enrichment of d in basalts occur at all temperatures with the magnitudes of change being greater as temperature time and to a lesser degree glass content increases the trends in isotopic shifts are directly related to changes in the style and intensity of mineralogic alterations in the basalt e g smectite at 300 c talc actinoiite at 400 500 c the changes in the 18 o values of basalts and seawater in the experimental systems were observed to follow closely with those expected from a first order rate law rate constants for the oxygen isotopic exchange between rock and water range from 10 9 5 to 10 8 0 moles oxygen m2 of solid surface sec for temperatures of 300 to 500 c the activation energy for the isotopic exchange reaction was calculated to be 11 5 kcal mol an application of our experimental rate data to natural systems suggests that the oxygen isotope equilibrium between basalt and seawater in the mid oceanic ridge may take place within approximately 1000 years at 350 c our experimental data also suggest the equilibrium oxygen isotopic fractionation factors between the altered basalt and seawater to be 3 5 0 5 at 300 c 2 0 0 4 at 400 c and 0 5 0 25 at 500 c the observed hydrogen isotopic fractionation factors between the altered basalts and seawater in our experimental systems were about 74 at 300 c about 62 at 400 c and about 48 at 500 c these fractionation factors are probably attributable to the equilibrium fractionation between fe rich secondary phases and seawater

do s type granites commonly sample infracrustal sources new results from an integrated o u pb and hf isotope study of zircon in contrast to i type granites which commonly comprise infracrustal and supracrustal sources s type granites typically incorporate predominantly supracrustal sources the initial aim of this study was to identify the sources of three scottish caledonian 460 ma s type granites kemnay cove and nigg bay by conducting oxygen u pb and hf isotope analyses in zircon in order to characterise one potential end member magma involved in the genesis of the voluminous late caledonian 430 400 ma i type granites field whole rock geochemical and isotopic data are consistent with the generation of the s type granites by melting their dalradian supergroup country rocks while hf isotope compositions of magmatic zircon u pb data of inherited zircons and high mean zircon 18o values of 9 0 2 7 2sd and 9 8 2 0 for the kemnay and cove granites support this model the nigg bay granite contains zircons with much lower 18o values 6 8 2 1 similar to those found in scottish i type granites this suggests that the nigg bay granite contains low 18o material representing either altered supracrustal material or more likely an infracrustal source component with mantle like 18o mixing trends in plots of 18o vs ehf for s type granite zircons indicate involvement of at least two sources in all three granites this pilot study of scottish caledonian s type granites demonstrates that while field and whole rock geochemical data are consistent with local melting of only supracrustal sources the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic evolution involving two or more magma sources

empirical equations for the prediction of the significant bracketed and uniform duration of earthquake ground motion abstract the complete characterization of earthquake ground motion includes the length of the interval of strong shaking as well as the amplitude and frequency content of the time series there are relatively few published equations available for the prediction of strong motion duration from earthquakes which may in part be a consequence of the fact that the duration of shaking has generally not been considered in structural engineering recognizing that there are many applications for which an estimate of the duration of ground motion is needed this study presents new empirical predictive equations for a number of definitions of strong motion duration using the records from the next generation of attenuation nga global database of accelerograms from shallow crustal earthquakes the equations can be used to estimate ground motion durations from shallow crustal earthquakes of magnitude between m w 4 8 and 7 9 at distances up to 100 km from the source

contrasting sources and p t crystallization conditions of epidote bearing granitic rocks northeastern brazil o sr and nd isotopes abstract the 618 ma curral de cima tonalite and 577 ma lourenco monzodiorite northeastern brazil are magmatic epidote bearing plutons that carry ferrohornblende biotite titanite and epidote major trace and isotope chemistry suggests that the major magmas of the two plutons followed similar differentiation trends but derived from source rocks that differed in age and isotopic composition the mineral phases of the curral de cima tonalite the presence of amphibole rich clots and juvenile component average end 3 55 point to an i type source for these rocks these data and high calculated 18 o w r 10 0 for the tonalite and high 18 o value for a clot 9 3 argue that the clots are fragments of a metabasaltic source rock that has been hydrothermally altered at a low temperature in contrast average calculated 18 o w r for the lourenco monzodiorite 7 8 87 sr 86 sr 0 7083 end 14 6 and t dm 1 92 ga these data are compatible with a long crustal residence time of lower crust amphibolites source epidote in the curral de cima pluton crystallized close to the nno buffer and hornblende chemistry due to al reequilibration yielded sub solidus temperature and pressure in contrast in the lourenco pluton epidote crystallized close to the hm buffer and al in hornblende points to near solidus solidification 685 c around 4 4 kbar this study confirms that magmatic epidote in granitic plutons can crystallize at pressures lower than 5 5 kbar under higher f o 2 as experimentally foreseen rapid magma transportation through hot continental crust during the peak of metamorphism in early stages of an orogenic cycle prevents epidote dissolution

oxygen isotope ratios of zircon magma genesis of low δ18o granites from the british tertiary igneous province western scotland igneous zircons from granites of the british tertiary igneous province btip in western scotland preserve magmatic oxygen isotope ratios in spite of hydrothermal alteration and thus provide new evidence for the genesis of granites co2 laser and mass spectrometer based bulk 1 3 mg oxygen isotope analyses from individual plutons show remarkable homogeneity in zircon 18o regardless of zircon size and magnetism average reproducibility of 18o zrc in a single pluton is 0 2 in addition ion microprobe analysis shows that individual zircons all have the same 18o and thus bulk samples are homogeneous fourteen granite bodies from the isle of skye show extreme variability in zircon 18o values from pluton to pluton 18o zrc 0 6 5 3 vsmow all the granites record zircon 18o values less than would be in high temperature equilibrium with normal mantle 18o zrc 5 except for the beinn an dubhaich granite zircons from one intrusion in the mafic cuillins center also have low 18o 2 5 similar variability is observed in 18o amongst three intrusions on the isle of mull 18o zrc 3 4 5 9 both mull and skye are characterized by intense shallow paleohydrothermal systems and the low and variable 18o values of the magmas likely result from melting or interaction with hydrothermally altered country rock analysis of granites on the isle of arran where the percentage of low 18o country rocks is much less yields higher 18o zrc of 6 8 and 5 4 published trace element and radiogenic isotope data for btip mafic and felsic rocks have supported the hypothesis that the granites are formed primarily by differentiation from the mantle with magmas experiencing minimal crustal input dickin et al phil trans r soc london a 310 1984 755 780 however conservative estimates based on oxygen isotope compositions suggest that some of the btip granites can only be formed from normal magmas 18o wr 6 if they assimilated significant amounts of hydrothermally altered crustal material some granites require up to 40 bulk crustal input and are better explained as the result of wall rock melting by an adjacent mafic pluton whereas pb and sr suggest a general trend towards increasing input of shallow crust for the younger granites dickin et al phil trans r soc london a 310 1984 755 780 the correlation between age and 18o is less apparent suggesting that pb and sr isotopic compositions are decoupled from oxygen isotopes this would be expected of melted country rock that was hydrothermally altered in oxygen isotope ratios but not in pb and sr isotope ratios

distinguishing magmatic from subsolidus epidote laser probe oxygen isotope compositions epidote occurs in four textural varieties in the mount lowe intrusion of southern california euhedra anhedra intergrowths and cross cutting veins of these conspicuous elongate euhedra which range up to 3 cm in length meet most of the established textural criteria for magmatic epidote equant anhedral grains which are 5 mm in diameter are texturally ambiguous although rare allanite cores are suggestive of a magmatic origin epidote intergrowths with hornblende and biotite also meet certain textural criteria for crystallization from a magma notably euhedral faces against biotite finally late stage veins of epidote cross cut all phases in the rock and are likely subsolidus oxygen isotopic compositions of these four textural varieties of epidote determined using the laser probe indicate that the use of textural criteria alone in establishing epidote parageneses can be misleading intracrystalline 18o variations in the euhedra document both magmatic and subsolidus compositions oxygen isotope compositions are bimodal averaging 5 36 0 13 n 11 and 4 66 0 23 n 21 these data combined with plagioclase and zircon 18o values are interpreted to indicate that magmatic oxygen isotopic compositions have been preserved where epidote 18o is greater than 5 significant portions of each euhedral crystal have been affected by subsolidus exchange and are characterized by epidote 18o values less than 5 intracrystalline 18o values of epidote anhedra range from 4 50 to 6 08 and thus also document both magmatic and subsolidus compositions subsolidus exchange is volumetrically less significant in the anhedra than in the euhedra values of 18o for intergrowths and late stage cross cutting veins of epidote average 3 81 0 22 and document clearly subsolidus growth the data confirm that in the mount lowe intrusion large euhedra of epidote are magmatic in origin consistent with textural evidence the data also indicate that equant anhedra of epidote are also magmatic in origin and thus the absence of good crystal faces does not necessarily indicate subsolidus growth the subsolidus origin of intergrowths of epidote with euhedral faces against biotite indicates that well developed crystal faces do not require growth from a magma finally the subsolidus origin of the vein epidote is consistent with textural evidence the increased resolution available with laser based oxygen isotope measurements offers an important test for documenting whether epidote is of magmatic or subsolidus origin given the barometric significance of magmatic epidote oxygen isotope compositions can be used to aid in the interpretation of epidote bearing plutons

oxygen isotope geothermometers for metamorphic rocks the chicago mineral carbonate oxygen isotope fractionation curves have been combined with mineral water fractionation data for jadeite zoisite and rutile and new data for grossular water to provide a set of self consistent mineral pair calibrations the a coefficients in the equation 1000 in a 106t 2 of the new mineral pair fractionations are jadeite zoisite grossular rutile quartz 1 69 2 00 3 03 5 02 jadeite 0 31 1 34 3 33 zoisite 1 03 3 02 grossular 1 99 the isotopic fractionation properties of natural pyralspite garnet ca fe mg mn 3al2si3o12 can be approximated by those of the grossular end member appropriate substitutions also yield coefficients for the solid solution minerals sodic pyroxene and epidote e g aquartz sodic pyroxene 2 75 1 06xjd aquartz epidote 2 00 0 75xps where xjd and xps are the mole fractions of the jadeite and pistacite components respectively the new data set is particularly suitable for the geothermometry of metamorphic rocks 18o data from minerals of the high pressure metamorphic rocks of the sesia zone of italy and cyclades complex of greece yield well constrained mean temperatures of 572 and 478d c respectively type iii blueschist metabasalts of the franciscan formation of california give mean quartz garnet temperatures of 354d c

zircon u pb dating of water rock interaction during neoproterozoic rift magmatism in south china abstract a combined study of zircon u pb dating and mineral o isotope analysis was carried out for neoproterozoic granite gabbro and gabbrodiorite from the beihuaiyang zone that marks a rifted margin between the south china block and the supercontinent rodinia the results provide precise constraints on timing of water rock interaction resulting in unusual 18 o depletion in igneous minerals cl imaging reveals that zircons mostly show clear oscillatory and or sector zoning with or without thin alteration rims no metamorphic zircon occurs in the igneous intrusives although they suffered greenschist facies metamorphism during triassic continental collision la icpms zircon u pb dating yields consistent ages of 739 8 to 754 5 ma which are interpreted as timing of magma crystallization mineral separates have highly variable 18 o values of 0 6 to 6 8 for zircon 1 2 to 12 5 for quartz and 0 3 to 8 4 for plagioclase oxygen isotope heterogeneities exist within single intrusions and between different intrusions magmatic zircons with low 18 o values and a weighted mean u pb age of 748 6 ma crystallized from low 18 o magmas that were derived from melting of hydrothermally altered low 18 o rocks both low and normal 18 o intrusives in the beihuaiyang zone are shown to form at the same time contemporaneous with the rodinia breakup thus tectonic evolution from supercontinental rift to breakup is proposed to create conditions for alternating processes of high t hydrothermal alteration and low 18 o magmatism such processes are a characteristic feature of high t water rock interaction in volcanic rift margins associated with localized water saturated melting while the occurrence of low 18 o magmas demonstrates penetration of meteoric water into the rift tectonic zone of crustal melting the existence of normal 18 o magmas indicates that their emplacement may serve as a thermal engine to drive the second stage of meteoric water rock interaction resulting in the unusual 18 o depletion in adjacent rocks the concurrent occurrence of both low and normal 18 o magmas appears to indicate highly heterogeneous high t water rock interaction during rift magmatism

petrogenesis and source rocks of the high k calc alkaline and shoshonitic i type granitoids in the northwestern part of east junggar nw china abstract the metaluminous i type granitoids in the northwestern part of east junggar include the kala an high k calc alkaline granodiorite and the zhalate shoshonitic quartz monzonite which were emplaced at 331 ma and 323 ma respectively they represent formations from two independent magmatic events the i type granitoids have arc geochemical features verified by the following characteristics 1 enrichments in large ion lithophile element lile and light rare earth element lree relative to high field strength element hfse with strong positive pb and negative ti anomalies 2 ehf t values 13 8 to 12 7 that are similar to juvenile crust generated in modern island arcs 3 87sr 86sr i 0 7035 0 7043 and end t 5 8 6 7 values and ree pattern that are consistent with arc volcanic rocks in east junggar granodiorite and quartz monzonite show mantle like weighted mean 18o values of 5 3 and 5 9 respectively which precludes stagnant oceanic crust as the source rock the metamorphosed underplating mafic intrusion buried at depth most probably served as the source rock for the granodiorite and the quartz monzonite the quartz monzonite exhibits remarkably higher contents of k2o na2o al2o3 ree and other incompatible elements and lower contents of feo mgo and cao than the granodiorite these distinct chemical compositions and geochemical features of the quartz monzonite and the granodiorite were most likely inherited from differences between their source rocks which were jadeite and phengite bearing garnet amphibolite respectively the coeval occurrence of high k calc alkaline granodiorite and shoshonitic quartz monzonite in the northwestern part of east junggar suggests a post collisional setting and the subduction must has ceased before 331 323 ma

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

mineralogical fluid inclusion and stable isotope characteristics of birimian orogenic gold mineralization at the morila mine mali west africa abstract gold mineralization at morila is hosted in flat lying meta graywacke associated with a birimian aged massigui schist belt in mali three major events of deformation metamorphism fluid flow d 1 d 2 and d 3 are recognized at morila with mineralization associated with various increments of deformation and fluid flow during d 2 three distinct vein sets v 1 v 3 associated with d 1 d 3 respectively have also been recognized v 1 vein sets are thin poorly defined quartz feldspar metamorphic segregations broadly parallel to the regional fabric v 2 veins are generally boudinaged foliation parallel quartz feldspar veins or occur as felsic melt segregations in the form of quartz k feldspar muscovite biotite blebs located essentially within the ore zone v 3 veins occur as sub parallel to high angle biotite arsenopyrite gold veins that appear to be extensional in nature the ore zone is characterized by compositionally distinct biotite higher mgo and lower feo and more calcic plagioclase than otherwise texturally identical minerals in the footwall and hanging wall of the mineralized zone suggesting that prograde metamorphic minerals were modified by post metamorphic mg and ca rich mineralizing fluids gold forms discrete grains in the v 2 veins or in the adjacent host rock and is closely associated with pyrrhotite and arsenopyrite gold also occurs with sb bi ore minerals in rare microfractures numerous thin biotite arsenopyrite gold veinlets cut d 1 and d 2 fabrics and often carry significant gold associated with kink bands in the d 2 fabric fluid inclusion and oxygen isotope study provide evidence that morila fluids are intermediate salinity 12 16 wt nacl equivalent nacl cacl 2 mgcl 2 h 2 o ch 4 n 2 fluids of magmatic origin 18 o water 10 9 0 3 the ore forming fluid subsequently unmixed to form a compositional range of vapor rich ch 4 n 2 dilute nacl ch 4 n 2 aqueous and liquid rich brine associated with the opening of a micro fracture network and vein development quartz biotite isotope geothermometry indicates that mineralization occurred between 350 to 450 c gold mineralization at morila was an integral part of coeval plutonism deformation and regional metamorphism while the modification of the ore fluid by unmixing was the most important factor associated with the concentration of economically viable gold grades at morila

geochemistry and petrogenesis of the marginal zone of the mount lowe intrusion central san gabriel mountains california the marginal zone of the mount lowe intrusion mli is composed of a mineralogically and geochemically distinctive suite of quartz poor alkali enriched monzodiorites and quartz monzodiorites low initial 87sr 86sr rb and rb sr coupled with high ab or k rb ba and sr suggest that feldspathic continental crust did not contribute significantly to the magma the inferred original magma composition is most consistent with experimental data for about 20 batch modal melting of eclogite or quartz eclogite of altered oceanic tholeiite composition the high absolute ba and sr abundances require in addition an enriched component perhaps derived from dehydration processes within a subducted slab at the level of exposure the range of observed compositions resulted from fractionation of hornblende plagioclase sphene apatite and magnetite fractionation resulted in formation of a peraluminous mn enriched residual liquid which crystallized garnet the grandite enriched composition of the garnet is inferred to be due to high pressure ca 6 7 kb high f text o text 2 crystallization relative to typical plutonic garnets the composition of the marginal zone of mli has many chemical similarities to early mesozoic plutonism in the cordilleran continental margin magmatic arc of western north america

crystal chemical controls on rare earth element partitioning between epidote group minerals and melts an experimental and theoretical study we have experimentally determined the partitioning of ree rare earth elements between zoisite and hydrous silicate melt at 1 100 c and 3 gpa all ree behave moderately compatible in zoisite with respect to the melt and all d ree zo melt show a smooth parabolic dependence on ionic radius the partitioning parabola peaks at nd left d nd zo melt 4 9 right and the compatibility slightly decreases towards la left d la zo melt 3 9 right and decreases by half an order of magnitude towards yb left d yb zo melt 1 1 right application of the elastic strain model of blundy and wood 1994 to the available zoisite and allanite ree mineral melt partitioning data and comparison with partitioning pattern calculated from a combination of structural and physical data taken from the literature with the elastic strain model suggest that in zoisite ree prefer the a1 site and that only la and ce are incorporated into the a2 site in significant amounts in contrast in allanite all ree are preferentially incorporated into the large and highly co ordinated a2 site as a result zoisite fractionates the mree effectively from the hree and moderately from the lree while allanite fractionates the lree very effectively from the mree and hree consequently the presence of either zoisite or allanite during slab melting will lead to quite different ree pattern in the produced melt

magmatic evolution and crustal recycling for neoproterozoic strongly peraluminous granitoids from southern china hf and o isotopes in zircon zircons can retain a grain scale record of granitoid compositional evolution that is accessible through microanalysis in situ u pb hf and o isotope data yield new insights into the petrogenesis and evolution of the neoproterozoic strongly peraluminous granitoids of the jiangnan orogen jo southern china a negative correlation of th u versus d 18 o is found for most analyses some zircons from eastern jo granitoids show d 18 o variations of 3 6 from core to rim indicating a dramatic shift toward higher oxygen isotope values by voluminous partial melting of supracrustal rocks and signaling a transition from i type like to s type like magmas during the later stage of magmatic evolution this mechanism provides a reasonable explanation why some granitoids have intermediate geochemical features between s type and i type granites hf isotope trends indicate that a larger proportion of mature continental crust was incorporated into the magma sources of the western jo granitoids whereas more juvenile arc crust was incorporated into the eastern jo magmas no significant depleted mantle derived mafic magma was injected into the jo granitoid magmas instead radiogenic hf and nd signatures in jo granitoids reflect incorporated juvenile arc crust and document crustal growth in southern china during the early neoproterozoic ca 900 ma thus our zircon data suggest that strongly peraluminous granitoids which are widely regarded as the products of orogenesis that primarily recycle evolved crust can also record important information about early crustal growth

geology of the post collisional porphyry copper molybdenum deposit at qulong tibet porphyry deposits are usually thought to form from subduction related calc alkaline magmas in magmatic arc settings although some porphyry deposits also occur in post collisional extensional settings the post collisional deposits remain poorly understood here we describe the igneous geology alteration mineralogy and mineralization history of qulong a newly discovered porphyry cu mo deposit in southern tibet that belongs to the post collisional class the deposit is associated with miocene monzogranite granodiorite intrusions and is hosted partly by jurassic andesitic dacitic volcanic rocks the deposit contains 7 1 mt cu and 0 5 mt mo metal the 19 5 ma granodioritic monzogranitic rongmucuola pluton with diorite enclaves is the earliest miocene intrusive unit it was intruded by a regularly shaped stock p porphyry and then thin dikes x porphyry of monzogranite at about 17 7 ma the main cu mo mineralization is associated with the p porphyry a barren diorite porphyry intruded the p and x porphyries around 15 7 ma petrologic trends of the intrusions suggest that the miocene intrusions have similar origins and probably formed by fractionation of a deeper magma chamber emplacement of miocene porphyries controlled by the qulong anticline is in direct response to the rapid uplift erosion of the gangdese arc batholiths in southern tibet miarolitic cavities and unidirectional solidification textures key evidence for volatile separation have been recognized in the p and x porphyries respectively early potassic alteration characterized by quartz k feldspar anhydrite pervades the p porphyry and rongmucuola pluton laterally this alteration grades into quartz biotite anhydrite k feldspar which affects all miocene intrusions except the latest dioritic porphyry wall rocks of rongmucuola pluton and jurassic andesitic dacitic volcanics within 1 1 5 km of the porphyries are dominated by pervasive potassic alteration an outer halo of propylitic alteration epidote chlorite calcite extends up to 2 km away from the deposit feldspar destructive alteration sericite chlorite clay minerals has overprinted most of the potassic and part of the propylitic alteration the alteration is strongly pervasive in the interior of the porphyry bodies and occurs as vein halos away from the porphyry bodies the earliest quartz k feldspar alteration and veins are barren whereas approximately 60 of the cu reserves is associated with slightly later quartz biotite anhydrite alteration barren assemblages are related to irregular quartz k feldspar anhydrite veins which are truncated by the x porphyry cu sulfide bearing assemblages are associated with discontinuous chalcopyrite biotite and continuous quartz anhydrite chalcopyrite molybdenite veins deposition of cu mo with abundant anhydrite occurred during or between emplacement of closely related porphyries from high temperature magmatically derived fluids and was probably caused by the disproportionation of so

emplacement petrological and magnetic susceptibility characteristics of diverse magmatic epidote bearing granitoid rocks in brazil argentina and chile abstract magmatic epidote mep bearing granitoids from five neoproterozoic tectonostratigraphic terranes in northeastern ne brazil early palaeozoic calc alkalic granitoids in northwestern nw argentina and from three batholiths in chile have been studied the elongated shape of some of these plutons suggests that magmas filled fractures and that dyking was probably the major mechanism of emplacement textures reveal that in many cases epidote underwent partial dissolution by host magma and in these cases may have survived dissolution by relatively rapid upward transport by the host magma in plutons where such a mechanism is not evident unevenly distributed epidote at outcrop scale is armoured by biotite or near solidus k feldspar aggregates which probably grew much faster than epidote dissolution preventing complete resorption of epidote by the melt al in hornblende barometry indicates that in most cases amphibole crystallized at p 5 kbar kyanite bearing thermal aureoles surrounding plutons that intruded low grade metamorphic rocks in ne brazil support pluton emplacement at intermediate to high pressure mep show overall chemical variation from 20 to 30 mol mole percent pistacite ps and can be grouped into two compositional ranges ps20 24 and ps27 30 the highest ps contents are in epidotes of plutons in which hornblende solidified under p 3 0 10 3 si typical of magnetite series granitoids crystallized under higher oxygen fugacity in ne brazil argentina and chile it seems that mep is more common in precambrian to palaeozoic ilmenite series granitoids while its occurrence in magnetite series granitoids is more restricted to mesozoic to tertiary granitoids

the unregenerate sao rafael pluton borborema province northeastern brazil abstract the late neoproterozoic sao rafael pluton located in the serido belt of the borborema geologic province northeastern brazil is an unfoliated metaluminous epidote bearing high k calc alkalic granitoid three facies are present in its northern lobe i a dominant massive granite to quartz monzonite porphyritic facies containing microcline phenocrysts ii subordinate fine grained facies similar to the porphyritic facies except lacking amphibole that crop out as inselbergs serras and iii rare pillows of microgranular mafic enclaves mmes that are present within the porphyritic facies chemical analyses include 19 bulk rock data and 33 microprobe data of epidote and amphibole beginning with bulk chemistry and postulated mineral compositions an iterative procedure of solving matrices characterizes a rock sample derived information includes i identifying what minerals are present ii their relative abundances and iii their chemical compositions even for minerals such as biotite or amphibole with complexly variable compositions calculated mineral compositions are compatible with microprobe data the matrix solutions further indicate alternative mineral assemblages and the compositions of phantom minerals that could have existed in the magma source although absent in the pluton they establish that olivine but not clinopyroxene was a possible phantom mineral and that mmes are of unrelated magmatic origin according to matrix analysis amphibole in an outer zone of the porphyritic facies is solely of neoformed origin whereas amphibole in an inner zone of the porphyritic facies is of mixed origin partly neoformed and partly residual i e xenocrystic patchy amphibole derived from mmes a qualified geothermometer provides tsolidus 730 6 c for the outer subfacies and a qualified geobarometer provides p 0 78 0 06 kbar amphibole in mmes crystallized at depth at p 3 3 kbar corroded epidote attests to the pressure sensitive instability of that mineral during rapid ascent decompression cathodoluminescence reveals complexly structured zircons with magmatic overgrowths and inherited cores 207pb 206pb ages of inherited components cluster at 2 0 ga and 2 4 ga these ages being prominent in the local basement oscillatory zoned magmatic components yield a concordia upper intercept age of 580 9 ma interpreted as the age of igneous crystallization values of 18o in zircon are only slightly higher than 18o in mantle like zircon at 0 58 ga ehf in magmatic zircon was notably unradiogenic 22 to 26 indicating a magma source in old continental crust as adduced by ages of inherited zircon a u pb titanite date is around 0 55 ga for the rb sr and sm nd whole rock systems isotopic inheritance from the magma protolith is so extreme and variable that isochrons are actually scatterchrons that provide no useable age information rb sr cooling ages of biotite are 498 1 ma most sm nd depleted mantle ages lie in the range of 2 6 0 13 ga sao rafael magma was unregenerate not isotopically homogenized when emplaced

evaluating the potential of sem eds analysis for fingerprinting suspended sediment derived from two contrasting topsoils sediment source fingerprinting can provide information on the contribution of various sediment sources to the suspended load of a stream this study was conducted to evaluate the potential of sem eds analysis scanning electron microscopy energy dispersive spectrometry to fingerprint two types of topsoil in a research basin a black chernozemic farmland soil and a grey podzolic forest soil simulated suspended sediment samples were prepared from each of the two soils and sem eds analysis of these samples provided information about the size distribution mineralogy and morphology of the particles the two simulated suspended sediment samples were similar in particle size distribution mineralogy and total chemistry so that these characteristics did not provide a distinctive fingerprint for the two topsoils an important difference between the two simulated suspended sediment samples was the morphology of the clay class particles consisting of water stable clay aggregates clay class particles derived from the grey forest soil were irregular whereas those derived from the black farmland soil were rounded this difference in particle morphology can be explained by the difference in land use and may provide a distinct fingerprint for each topsoil

oxygen isotope constraints on metamorphic fluid flow townshend dam vermont u s a fluid rock interaction during amphibolite facies metamorphism has been investigated for rocks exposed in a single 400 m long lithologically heterogeneous outcrop near townshend vermont usa oxygen isotopic compositions have been measured in profiles across single garnet crystals from thirteen samples and from hornblende and garnet separates from thirty three fine grained samples this outcrop was previously studied who measured large oxygen isotope gradients 3 per thousand in garnets from one sample and inferred large amounts of pervasive fluid flow all of the garnets that we have analyzed show less isotopic zonation le 1 per thousand twelve of them have zonation 0 5 per thousand and the mineral separate data imply both a strong correlation of isotopic composition with rock type and large gradients in peak metamorphic fluid isotopic compositions although devolatilization reactions in these rocks must have produced metamorphic fluids the data preclude cross foliation time integrated fluid fluxes greater than 300 600 cm sup 3 cm sup 2 during prograde amphibolite facies metamorphism the isotopic trends can all be interpreted in terms of closed system behavior channeled fluid flow or diffusive exchange of oxygen in an interconnected grain boundary fluid and within uncertainty cross strike advective fluxes could have been zero any significant flow at this locality was dominantly either layer parallel or channeled out of the system in veins more if all fluid flow occurred in veins then their spacing must have been less than 300 600 m the data are inconsistent with massive and pervasive metamorphic fluid flow across strike but do not address layer parallel flow less

geochemical evidence for origin of magma mixing for the triassic monzonitic granite and its enclaves at mishuling in the qinling orogen central china abstract whole rock major trace elements and sr nd isotopes mineral chemistry and zircon u pb and lu hf isotope compositions are reported for i type monzogranite and its mafic microgranular enclaves from the triassic mishuling granite in the qinling orogen central china zircon u pb dating yields an age of 212 5 ma for the mafic enclaves and 213 3 ma for the host monzogranite whole rock major trace elements sr nd isotopes and zircon hf isotopes preclude simple crystal fractionation or restite unmixing for a genetic link between the enclaves and the monzogranite mixing between mafic and felsic magmas is compatible with the geochemical data the monzogranite displays an enriched lile and lree pattern high 87sr 86sr i ratios of 0 7068 to 0 7071 and low end t values of 9 2 to 5 7 suggesting a crustal origin its high mg 47 6 to 50 7 cr and ni contents clearly indicate its derivation from mafic refractory lower crust most of the zircons in the monzogranite have negative ehf t values of 12 5 to 0 1 and two stage hf model ages of 1049 to 1683 ma and some zircons in the host monzogranite have positive ehf t values of 0 3 to 4 8 and single stage hf model ages of 665 to 845 ma this indicates that the monzogranite is most likely to be formed by partial melting of a proterozoic lower continental crust the mafic enclaves have fine grained igneous textures and contain abundant acicular apatite that indicates rapid cooling they also have a high 87sr 86sr i ratio of 0 7071 and a low end t value of 6 5 suggesting a derivation from ancient subcontinental lithosphere mantle sclm zircons in the mafic microgranular enclaves have ehf t values of 1 6 to 5 1 and single stage hf model ages of 700 to 910 ma suggesting a possible origin from partial melting of the neoproterozoic sclm that may form in association with contemporaneously supercontinental rifting in the south china block therefore the mishuling i type granite may form by anatexis of meso neoproterozoic lower crust with mixing between the mafic and felsic magmas in particular we interpret the positive ehf t values as being derived from reworking of neoproterozoic juvenile lithosphere in response to tectonic extension in the late stage of the triassic continental collision between the south china block and the north china block

chemical and thermal constraints on focussed fluid flow in the lower oceanic crust the mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems we have investigated the role of zones of focussed fluid flow in the plutonic complex of the oman ophiolite in the near axis cooling of the oceanic crust lineaments identified on aerial photographs that occur at 1 km spacing show evidence for extensive hydrothermal fluid flow through regions 10 to 50 m wide fluid flow is initiated in these regions at 800 c and continues at least into the lower greenschist facies strontium isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the moho computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are 1x108 kgm2 modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the sr isotopic composition of samples from the focussed fluid flow zones the combination of independent estimates of the fluid and heat fluxes such as these can provide more rigorous constraints on the thermal history than either approach used in isolation our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid ocean ridges this is because fluids flowing through channels become chemically rock buffered at smaller fluid fluxes than those flowing pervasively through a rock mass thus if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought

oxygen isotope ratios of archean plutonic zircons from granite greenstone belts of the superior province indicator of magmatic source zircons commonly preserve primary magmatic oxygen isotope ratios even through high grade metamorphic events unlike quartz which more readily loses its primary 18o value by subsolidus exchange intrusive bodies from the superior province canada have an average 18o zircon of 5 8 0 6 n 59 rocks those from granite greenstone belts have an average 18o zrc 5 8 0 6 n 55 in contrast to those from metasedimentary belts that average 18o zrc 6 7 0 2 n 4 the 18o zircon values from granite greenstone belts indicate a maximum allowable input of 14 supracrustal high 18o material into archean magmas of the superior province relative to mantle 18o values igneous bodies of the superior province have been separated into two groups based on geochemistry unenriched plutons and sanukitoids magmatic oxygen isotope ratios inferred from these zircons indicate up to a subtle but significant division of 1 between the two geochemical sets of rocks this distinction is apparent only in the 18o zrc and not in more commonly analyzed less refractory minerals such as quartz and feldspar oxygen isotope data correlate with geochemical modeling of sanukitoids the average 18o zrc for sanukitoids is 6 4 0 2 in comparison to 5 5 0 4 for unenriched plutons there is almost no overlap in values the elevation of sanukitoid 18o zrc indicates input from a high 18o source such as fluids from the subducting oceanic crust melts of subducted sediments or continental crust all of the plutons analyzed in this study have been dated by u pb zircon techniques and the combination of u pb ages and 18o zrc indicates little variability of 18o versus age during archean plutonism in the superior province therefore recycling of sediments and other high 18o sources was not a dominant process during crustal growth from 3003 to 2670 ma in the superior province

epidote phenocrysts in dacitic dikes boulder county colorado epidote ps21 crystallized early as elongate phenocrysts in late cretaceous rhyodacitic dikes in the vicinity of ward boulder county colorado other unusual phenocryst phases are garnet gr17 24 and muscovite in a xenolith containing kyanite corundum biotite and plagioclase magmatic garnet grew as a rim around xenocrystic pyrope rich py37 garnet the xenolith was derived from a granulite facies zone not represented at the present day erosion surface which is composed of upper amphibolite facies cordierite and sillimanite bearing gneisses the dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8 13 kilobars phenocrysts cystallized in the temperature range 800 to 700 c under h2o and o2 activities greater than normal for silicic magmas this occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early crystallization phase in silicic magmas

empirical calibration of oxygen isotope fractionation in zircon new empirical calibrations for the fractionation of oxygen isotopes among zircon almandine rich garnet titanite and quartz are combined with experimental values for quartz grossular the resulting a coefficients k2 are zrc alm grs ttn qtz 2 64 2 71 3 03 3 66 zrc 0 07 0 39 1 02 alm 0 32 0 95 grs 0 63 full size table table options view in workspace download as csv for the relation 1000 ln y x ay x 106 t2 the fractionation of oxygen isotopes between zircon and coexisting minerals can provide otherwise unavailable evidence of magmatic processes including crystallization remelting and assimilation fractional crystallization

oxygen isotope compositions and magmatic epidote from two contrasting metaluminous granitoids ne brazil oxygen isotope compositions of mineral separates were determined from two metaluminous granitoids emas and sao rafael plutons from northeastern brazil the i type emas pluton has high 18o wr values 11 5 11 8 whereas the sao rafael pluton has low 18o wr values 7 5 8 1 but sr and nd are characteristics of s type granitoids measured mineral mineral fractionations suggest continuous sub solidus inter mineral isotope exchange among all minerals except zircon there is a large and consistent quartz epidote fractionation that gives apparent temperatures that are much lower than anticipated closure temperatures for epidote oxygen isotope fractionation between natural zircon and magmatic epidote is opposite to that predicted from theoretical determinations as 18o epidote 18o zircon an empirical calibration based on these results would suggests a closure t for oxygen in epidote of 500 c and qtz epi 5 19 at 500 c

the rapid heating defocused beam technique a co2 laser based method for highly precise and accurate determination of δ18o values of quartz abstract we describe a laser fluorination technique specifically for quartz that allows both high accuracy and high precision determination of oxygen isotope ratios regardless of grain size the rapid heating defocused beam technique rhd utilizes a defocused 32 w co 2 laser at full power which delivers 17 w 20 w mm 2 to the sample to quickly react quartz in contrast other recent laser fluorination studies use a tightly focused laser beam and slow heating i e sharp and kirschner 1995 fouillac and girard 1996 kirschner and sharp 1997 and find laser analysis of fine grained quartz to be inaccurate three quartz standards nbs 28 qz bra and qz cwru were analyzed using the rhd technique at the university of wisconsin to test for grain size effects rhd analyses show no correlation between grain size and 18 o values and are in excellent agreement with 18 o values obtained using conventional fluorination techniques additional analyses of the same quartz standards performed at university of wisconsin using slow heating and a focused beam yield 18 o values that are both less precise and significantly up to 0 8 lower than the accepted values attempts to use the rhd technique with a 20 w co 2 laser which delivers 8 w 10 w mm 2 to the sample at brgm were unsuccessful probably due to insufficient power density of the defocused beam we conclude that the rhd technique yields excellent accuracy and precision but that power densities of 15 20 w mm 2 may be necessary across a large part of the sample surface we recommend the use of lasers with at least 30 w of power

the source magmatic contamination and alteration of the idaho batholith zircons from cretaceous and tertiary granitic rocks of the idaho batholith preserve the best record of magmatic oxygen isotope ratios the coupling of oxygen isotope data from refractory minerals such as zircon and garnet with that from less refractory minerals such as quartz reveals the complex evolution and alteration history of the idaho batholith values of 18o zircon throughout the batholith are relatively homogeneous at 6 9 0 9 1 sd n 21 rocks despite variations in age and chemistry samples from the bitterroot lobe have an average 18o zircon of 7 1 0 3 n 7 rocks and samples from the atlanta lobe have an average 18o zircon of 7 0 1 0 n 9 rocks four epizonal tertiary plutons have an average 18o zircon of 7 2 0 2 n 4 rocks these data indicate a similar magmatic source for all granitic rocks of the idaho batholith except the tertiary epizonal casto pluton which has a 18o zircon of 4 0 0 1 n 1 rock the zircon data show that the range of normal magmatic 18o is considerably more restricted than was previously inferred despite some samples containing two micas and or garnet or having a mildly peraluminous character quartz is more variable than zircon in 18o average 18o quartz for the entire batholith is 10 4 1 1 1 sd n 65 rocks with a range of 7 7 to 12 4 excluding the casto pluton the spread of quartz values is 2 7 larger and typically higher than that calculated relative to zircon quartz data from the bitterroot lobe and epizonal tertiary plutons suggest the alteration of primary 18o during metamorphic events quartz data from the atlanta lobe suggest closed system diffusional exchange during cooling igneous garnet 18o values are higher than and out of equilibrium with those of coexisting zircon by 0 1 0 9 assuming an average 18o for unaltered wall rock metasediments the incorporation of 10 metasediment into a magma after crystallization of zircon raises the 18o magma by 0 9 the largest measured garnet zircon these quartz and garnet 18o from the atlanta and bitterroot lobes suggest magmatic contamination with a high 18o rock after the crystallization of zircon and before that of garnet and quartz

constraints on rates of granitic magma transport from epidote dissolution kinetics epidote is stable only at high pressure in granitic magmas but is found in magmas crystallized at shallow crustal levels this observation suggests qualitatively that magma transport is sufficiently rapid to prevent complete epidote resorption experimental measurements of epidote dissolution showed that the presence of epidote implies that magma transport from the deep crust in these systems was rapid and that the transport was by dikes rather than diapirs

fluid inclusions muscovite ar ar age and fluorite trace elements at the baiyanghe volcanic be u mo deposit xinjiang northwest china implication for its genesis abstract the baiyanghe be u mo deposit is located in the late paleozoic xuemisitan kulankazi island arc of the northwestern margin of the junggar plate northwest china it is the largest be deposit 2 2 m tons of ore with grades ranging from 0 2 to 1 4 in asia orebodies in the deposit occur as fractures along contact zones between the yangzhuang granite porphyry intrusion and devonian pyroclastic country rocks and within the porphyry itself muscovite fluorite veins are closely associated with the be u mo mineralization a new ar ar dating of the muscovite in this study yields a plateau age of 303 0 1 6 ma which constrains the timing of the be u mo mineralization of the deposit three stages of fluorite of different colors have been identified at the deposit with the earliest dark purple fluorite more closely associated with the mineralization microthermometry of fluid inclusions obtained from the three stages of fluorite suggests that the fluorites were precipitated as veins from low temperature 120 150 c hydrothermal fluids with salinity ranging from 4 7 to 19 7 wt nacl eqv based on the trace elemental concentrations and ree patterns of the fluorite the style of veining and the low salinity and low temperature characters of the fluid inclusions it is suggested that be and u were most likely transported as fluoride complexes and mo as hydroxyl complexes pb isotopic compositions of the ores and country rocks as well as o and h isotopic characters of the ore related muscovite indicate mixing between magmatic and meteoric waters both contributed to formation of the ore forming fluids metallic be u and mo were most likely leached out from the granite porphyry by the fluids the fluid mixing led to the reduction of u mo and be and their precipitation at the deposit

thermodynamics and lattice vibrations of minerals 5 applications to phase equilibria isotopic fractionation and high pressure thermodynamic properties in previous papers in this series a model that uses available elastic structural and spectroscopic data on minerals has been used to predict the thermodynamic functions cv heat capacity s entropy e internal energy and f helmholtz free energy in this paper four applications to problems of current geochemical and geophysical interest are presented 1 interpretation of complex trends of calorimetric data 2 calculation of phase equilibria 3 calculation of oxygen isotopic fractionation factors and 4 estimation of the effect of pressure on thermodynamic functions the model demonstrates that trends in high temperature thermodynamic properties of silicates are determined by the position and relative numbers of high frequency modes generally antisymmetric si al o stretching modes the position of these modes varies systematically with degree of polymerization of tetrahedra and therefore high temperature calorimetric behavior is relatively systematic as a function of crystal structure and mineral composition trends at low frequency are much more complex because the low frequency optic modes that most strongly influence the low temperature thermodynamic functions depend in a complex way on the size coordination and mass of cations and various polyhedra in the minerals the heat capacity curves of kyanite andalusite and sillimanite and of quartz coesite and stishovite show crossovers that cannot be explained by debye theory which accounts only for acoustic mode behavior but can be explained by the model spectra proposed because proper account is taken of the changing low and high frequency optic modes upon polymorphic transformations the proposed model is sufficiently accurate that phase equilibrium problems can be addressed the quartz coesite stishovite equilibrium curves the kyanite andalusite sillimanite triple point and the breakdown of albite to jadeite plus quartz are cited as specific examples for each example predicted slopes of equilibrium curves agree moderately well to excellently with slopes determined experimentally the calculated slopes are sensitive to spectroscopic parameters particularly to the distribution of optic modes in the far infrared this sensitivity is discussed in detail for the albite breakdown reaction the model can be used for prediction of isotopic fractionation factors if spectra of the isotopic forms of the mineral are known or postulated a simple set of rules for generating hypothetical spectra of 18o minerals from measured spectra of the 16o forms is given reduced partition functions are calculated for 13 minerals at 298 k the model values of reduced partition function 1000 ln of these minerals decrease in the order quartz calcite albite muscovite clinoenstatite anorthite diopside pyrope grossular zircon forsterite andradite rutile in good agreement with experimental data at 1000 k the first six minerals show small crossovers so that the order becomes calcite muscovite albite quartz anorthite and clinoenstatite the differences in 1000 ln at high temperature for these minerals are so small that the model probably cannot address the deviations from experimental trends the model clearly defines the region in which the fractionation factors do not follow a 1 t trend and should be useful for extrapolation of experimental data to low temperatures finally a modified gruneisen parameter model is proposed for shift of the lattice vibrational frequencies under compression and thermodynamic properties to 1000 kbar 1000 k are given for nine minerals at 1 mbar the predicted decrease in entropy at 298 k ranges from 54 of the 1 bar value for periclase to 25 for stishovite

volume self diffusion of oxygen in biotite muscovite and phlogopite micas the kinetics of oxygen self diffusion have been measured in natural samples of biotite muscovite and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses all experiments were run at 1000 bars 100 mpa water pressure except those designed to measure water pressure dependence which ranged from 200 to 2000 bars the arrhenius relations obtained for layer parallel diffusive transport in the three micas by bulk exchange experiments fit to an infinite cylinder model are mineral biotite muscovite phlogopite 34 2 39 5 42 3 9 1 10 6 7 7 10 5 1 4 10 4 t c 500 800 512 700 600 900 the results of ion microprobe sims depth profiling perpendicular to the layers parallel to c axis yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers a single layer parallel step profiling experiment yields a diffusion coefficient identical within uncertainty to those obtained from bulk exchange experiments a slight dependence of diffusion rate on water pressure may obtain in phlogopite however the difference is approximately within the uncertainty of the measurement diffusion rates are similar within about a half an order of magnitude for a range of biotite composition from annite4 to annite63 as well as for muscovite comparison of oxygen diffusion rates in biotite from this study with ar k and rb diffusion from published data supports the idea that water is the oxygen bearing species in hydrothermal diffusion experiments the implication of these results for natural systems is that the micas especially biotite will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300 c the rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions full size table table options view in workspace download as csv

empirical determination of oxygen isotope fractionation factors for titanite with respect to zircon and quartz abstract measurements of oxygen isotope fractionations between coexisting igneous titanite ttn and zircon zrc have been used to formulate a self consistent empirical calibration of equilibrium oxygen isotope partitioning 1000 ln zrc ttn 1 02 10 6 t 2 this calibration is based on the average measured zrc igneous ttn 1 2 0 3 n 27 rocks and a closure temperature of titanite to oxygen diffusion of approximately 650 c the average measured fractionation between zircon and metamorphic titanite is 2 1 0 4 n 5 rocks these results show that zrc ttn can be used to distinguish igneous vs metamorphic or hydrothermal titanite the new zircon titanite fractionation has been combined with published experimental and empirical data to provide a new quartz titanite fractionation curve 1000 ln qtz ttn 3 57 10 6 t 2 this new calibration is consistent with qtz igneous ttn data from coexisting mineral pairs in granitic rocks modeling diffusional exchange of oxygen during cooling in a typical granitic rock using the new calibration results in the same qtz igneous ttn as measured 4 5 0 43 n 25 rocks both measured zrc igneous ttn and qtz igneous ttn differ from fractionations calculated using the semiempirical increment method the consistency of empirical zrc igneous ttn in a range of geologic environments allows the calculation of a more accurate fractionation factor for titanite

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

role of tectonism and fractional crystallization in the origin of lower paleozoic epidote bearing granitoids northwestern argentina in the northern pampean ranges a large group of peraluminous two mica ordovician granitoids commonly contains epidote bearing facies this granitoid suite exhibits a wide compositional range within the calc alkaline series although the mineralogical and chemical characteristics do not fit the typical features of either i or s type granites many of these plutons show asymmetric compositional zoning which is interpreted to be the product of fractional crystallization under tectonic stress tonalites occur only in the more fractionated plutons and are interpreted as cumulates magmatic epidote in the granitoids suggests a protracted crystallization period the pressure and temperature conditions at the emplacement level are inferred to be moderate widespread tourmaline bearing pegmatites indicate that the later facies crystallized from a water and volatile rich liquid however relict anhydrous minerals and a relatively low initial 87 sr 86 sr ratio 0 705 suggest that the granitoids were derived from a water undersaturated parental magma of deep origin

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance comment

comments on stable isotope geothermometry the system quartz water we show that the results of quartz water partial oxygen exchange experiments under hydrothermal conditions are determined primarily by exchange between water and dissolved silica species and not by diffusion in the solid hence these experiments do not provide an accurate value of the variation of fractionation with temperature but only a rough indication of it we conclude that the bottinga and javoy 1 equation relying on this type of information and on the precise calculation of the coefficient a 3 7 in the thermometric equation 1000 ln a b 106 t2 is not seriously questioned by these17o 18o partial exchange experiments

mineralogy and geothermobarometry of magmatic epidote bearing dikes front range colorado epidote phenocrysts ps 19 24 in laramide age porphyritic dacite dikes in the front range of colorado prove that epidote can crystallize directly from a magma the dikes are high k calc alkaline dacites and rhyodacites sio 2 64 70 phenocryst assemblages consist of combinations of epidote plagioclase an 53 19 biotite fe 3 fe total 0 25 mg mg fe 2 0 42 0 55 quartz aluminous amphibole al total 2 4 3 0 per formula unit igneous garnet and 2 6 gros 13 27 spess 2 13 pyr 7 26 alm 51 60 igneous muscovite and sanidine holocrystalline aphanitic groundmasses consisting mainly of quartz plagioclase and alkali feldspar comprise 60 75 of the dike rocks and require that epidote grew early in the crystallization sequence petrographic and microprobe data distinguish three types of magmatic epidote phenocrysts with different petrogenetic connotations implied by each type type i epidote occurs as euhedral phenocrysts up to 8 mm long with low content of allanite component thermobarometric calculations provide empirical evidence that epidote is stable in silicic calc alkaline magmas at high total pressure high water content and high oxygen fugacity the dacitic magmas originated by partial melting at temperatures above 800 c temperatures 50 c ranged from 800 to 880 c during early phenocryst growth down to 620 to 670 c during groundmass crystallization phenocrysts formed at pressures between 7 2 1 0 and 12 2 0 kbar oxygen fugacity was within the magnetite stability field at two log units 1 0 below the hematite magnetite buffer h 2 o fugacity calculations give values ranging from 3 4 to 16 kbar depending on how annite activity in biotite is modeled the occurrences of types i and iii epidote combined with geobarometry give a minimum pressure of epidote stability in the dike magmas of 8 0 1 0 kbar these results agree with experimental evidence and with conditions inferred for the crystallization of magmatic epidote in large cordilleran plutons phenocrysts in the dikes were preserved in conditions outside their stability limits by rapid emplacement and quenching in shallow level dikes

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

caltepec fault zone an early permian dextral transpressional boundary between the proterozoic oaxacan and paleozoic acatlan complexes southern mexico and regional tectonic implications 1 the tectonic boundary between the grenville age oaxacan and paleozoic acatlan crystalline complexes in southern mexico named the caltepec fault zone cfz is characterized for the first time as a dextral transpressional nnw trending and ene dipping ductile fault zone of early permian age the complexes are welded by a syntectonic magmatic epidote bearing granite along the entire length of the cfz from east to west the 2 6 km wide cfz consists of disrupted and retrograded banded gneisses of the oaxacan complex quartz feldspar mylonite and the syntectonic magmatic epidote bearing cozahuico granite czg with huge xenoliths up to several kilometers long and up to 600 m wide of the proterozoic gneisses thrust westward over metasedimentary tectonites of the acatlan complex the czg shows magmatic fabrics that represent a transition to solid state deformation characterized by subvertical foliation subhorizontal nne and sse dipping mineral stretching lineation and dextral kinematics the megaxenoliths underwent partial melting developing banded migmatites with layers of epidote bearing granitic neosome the parallelism of fabrics in these anatexitic rocks and in the enclosing deformed granite suggests that ductile deformation migmatization of xenolithic gneisses and granite emplacement along the cfz were coeval the neosome yielded a u pb zircon concordant age of 275 6 1 ma probably dating the peak of the tectonothermal event we interpret the cfz as a major terrane boundary accommodating transpressional interaction between the acatlan and oaxaquia blocks which were amalgamated in an oblique convergent setting by early permian time as the leading edge of gondwana impinged onto the southern margin of laurentia along the marathon ouachita suture to form pangea

conditions for early cretaceous emerald formation at dyakou china fluid inclusion ar ar and stable isotope studies the dyakou emerald occurrence is located in malipo county in the province of yunnan southern china the occurrence lies in the northern part of the laojunshan song chay metamorphic core complex which is exposed in an area of approximately 2 000 km2 and extends across the border between china and vietnam emerald mineralization is hosted by pegmatite and associated quartz veins that intrude deformed proterozoic biotite muscovite granofels and schist hydrogen and oxygen isotope results from the emerald channel waters and emerald respectively are consistent with an igneous fluid source the 18o fractionation between emerald and quartz yields vein temperatures of 365 to 420 c fluid inclusions indicate that the emerald precipitated from saline brines ranging from almost pure water to 10 5 mass percent nacl equiv fluid inclusion isochores intersected with 18o data yield pressures changing along the geothermal gradient from 1 500 to 3 300 bars ar ar geochronology of biotite and muscovite from the emerald veins yields consistent ages of 124 1 ma these constraints combined with field observations indicate that the dyakou emerald deposit is consistent with the igneous related model for emerald formation

stable and radiogenic isotope systematics in epidote group minerals epidote minerals the epidote group together with the orthorhombic polymorph zoisite occur in a wide range of igneous metamorphic and sedimentary lithologies although often present in only small quantities epidote group minerals can nonetheless be used to generate important quantitative constraints on processes such as regional metamorphism deep seated pluton emplacement and uplift hydrothermal fluid flow and sedimentary provenance e g zen and hammarstrom 1984 brandon et al 1996 cartwright et al 1996 keane and morrison 1997 spiegel et al 2002 petrologic studies on epidote group minerals formed by these and other processes are reviewed in bird and spieler 2004 grapes and hoskin 2004 enami et al 2004 and schmidt and poli 2004 in this chapter stable and radiogenic isotope studies of epidote group minerals will be reviewed although stable and or radiogenic isotope data exist for epidote group minerals from a number of different geologic settings such data have produced particularly important insights into 1 fluid flow associated with variable grades of metamorphism particularly ultra high pressure uhp metamorphism 2 intrusion of deep seated plutonic systems 3 the nature of hydrothermal alteration in geothermal systems 4 age relations in hydrothermal systems and 5 sedimentary provenance based on the chemical composition of naturally occurring epidote minerals oxygen hydrogen and chlorine stable isotopes all have the potential to be important petrogenetic indicators in systems involving epidote minerals oxygen is present in epidote minerals bound in isolated to4 tetrahedra t2o7 groups and in the hydroxyl site as both oh and o 2 hydrogen and chlorine are present in the hydroxyl site as oh and cl respectively although cl is present in only very small quantities frei et al 2004 to date oxygen and hydrogen isotope systematics have been studied both

stable isotope thermometry at high temperatures determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by urey in theory this should be simple calibrated mineral pairs are common analysis is rapid and there is no significant pressure correction however in spite of widespread application the promise of reliable thermometry has been elusive stable isotope temperatures in metamorphic and igneous rocks are often controversial is the fractionation between two phases a thermometer speedometer rate dependant hygrometer p h2o dependant or chimera a number of factors have contributed to this uncertainty including incomplete and sometimes conflicting calibration of isotope fractionation factors limited understanding of the kinetics of mineral exchange and the lack of microanalytical techniques this situation has improved dramatically over the past decade permitting reliable and detailed thermal histories to be inferred in a landmark paper bottinga and javoy 1975 empirically calibrated the oxygen isotope fractionation factors among ten rock forming minerals at t 500 c their compilation shows that 84 of analyzed samples contain at least three minerals meeting a concordance test for equilibrium they concluded the great majority of igneous and metamorphic rocks has conserved a state of oxygen isotope exchange equilibrium if correct this bodes well for thermometry however this conclusion was contested by deines 1977 who critically examined the existing data and concluded that less than half of the rocks analyzed to date would yield concordant 18o derived temperatures while seeming irreconcilable both conclusions are based on fact all rocks contain some degree of disequilibrium but many also preserve a thermometric record it is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted the organization of this paper will be to discuss the assumptions

an investigation into earthquake ground motion characteristics in eastern north america abstract a random vibration model of the hanks mcguire type is used to predict peak ground motions at rock sites in eastern north america the assumed geometric decay and distance dependent duration approximate the propagation of direct body waves at short distances and lg waves at regional distances the model predicts peak acceleration and velocities as well as response spectra and magnitude for a given seismic moment and corner frequency to predict ground motions for a given lg magnitude m lg the model is first used to calculate the seismic moment corresponding to that magnitude assuming a source scaling law then knowing the moment and corner frequency the model is used to calculate peak ground motions available data from strong motion recordings and from the ectn and lrsm networks modified to estimate horizontal ground motions on rock where appropriate are used to verify the model9s assumption and predictions modified mercalli intensity data are also used ground motions predicted by the model with a stress drop of 50 to 200 bars agree with the ground motion data but the m lg values computed by the model for given seismic moments do not agree with nuttli9s 1983 moment versus m lg data for large earthquakes resolution of the latter disagreement awaits the collection of instrumental data from large events in eastern north america

hydrothermal alteration of oxygen isotope ratios in quartz phenocrysts kidd creek mine ontario magmatic values are preserved in zircon zircons from rhyolites hosting the kidd creek volcanogenic massive sulfide deposit preserve magmatic oxygen isotope ratios 18 o 5 4 0 3 n 11 that are indistinguishable from average values of other plutonic and volcanic rocks of the superior province oxygen isotope ratios of petrographically pristine primary quartz phenocrysts however are greatly elevated in 18 o and heterogeneous 18 o 14 2 to 16 1 such phenocrysts are not a good monitor of magmatic processes the presence of healed microcracks in quartz phenocrysts as seen in cathodoluminescence images and the variability of quartz oxygen isotope ratios suggest that recrystallization is the process of 18 o elevation in contrast to the variability of quartz the 18 o zircon values do not vary regardless of the degree of magnetic susceptibility the 18 o values of zircon are interpreted to reflect the primitive magmatic oxygen consistent with the rhyolite geochemistry for magmas derived by partial melting of a tholeiitic source our data show that the 18 o values of quartz and whole rock at kidd creek are a good monitor of hydrothermal exchange and are 5 higher than the magmatic value that is preserved in zircon due to its refractory nature

the oxygen isotope anatomy of a slowly cooled metamorphic rock the distribution of o isotope ratios in a slowly cooled metamorphic rock is investigated by a combination of ion microprobe and laser probe techniques the sample has unusually simple geometry centimeter size domains of feldspar and garnet separated by a one millimeter to one centimeter thick layer of millimeter size magnetite grains isotopic zonation at two different scales is documented a sharp decrease in 18 o in the outer 10 100 m of individual magnetite grains is in agreement with modeling of isotopic reequilibration during cooling by interdiffusion between feldspar and magnetite the presence of this zonation in the outer portions of magnetite grains that are adjacent to garnet indicates extensive transport along grain boundaries during diffusive exchange millimeter scale zonation in the polygranular layer of magnetite is caused by the interaction of volume diffusion and grain boundary transport the effects of which depend upon local textures grain boundaries enhance bulk diffusion in polygranular aggregates and allow exchange between nontouching grains the results document a complex textural control to millimeter scale o isotope zonation and geothermometry in slowly cooled rocks and document the importance of fast grain boundary transport in controlling diffusive exchange