ion microprobe analysis of oxygen isotopes in garnets of complex chemistry abstract accurate ion microprobe analysis of oxygen isotope ratios in garnet is possible if appropriate standards are employed to correct for instrumental bias a component of which depends on the cation chemistry of the analyzed mineral in this study 26 garnet standards including 14 new standards that span the compositional range of pyrope almandine grossular spessartine and andradite were analyzed repeatedly by ion microprobe to develop a new method of correcting for instrumental bias in garnets all analyses were normalized to a single master garnet standard uwg 2 before bias from cation composition was considered bias due to cation composition in garnet was found to correlate with grossular content in pyralspite garnets and with andradite in ugrandite garnets bias is correlated with molar volume in garnets of all compositions in this study although this correlation is suitable as a correction scheme for bias a more accurate correction scheme based on the grossular and andradite compositions of garnet is proposed this method reproduces the bias of all but one standard to within a range of 0 4 an accuracy that is on the same order as the reproducibility 0 3 2 s d of the master garnet standard uwg 2 but that remains an independent source of error the new correction scheme is used to successfully reproduce laser fluorination analyses along a traverse of a polymetamorphic zoned skarn garnet from the adirondack mountains while previous analyses were at the mm scale the new data resolve a gradient of 18 o of 2 1 over 16 m if experimentally derived diffusion coefficients are correct these new results show that granulite facies metamorphism was significantly faster than previously assumed and the thermal peak was less than 5 myr

the essence of time fertile skarn formation in the variscan orogenic belt abstract extending from northern america to central china the variscan belt is a paleozoic orogen exceptionally well endowed in magmatic hydrothermal ore deposits including skarn deposits yet the genesis of fertile skarns and their distinction from barren equivalents in orogenic zones is only poorly constrained here we present innovative u pb laser ablation inductively coupled plasma mass spectrometry geochronology of garnet from different skarns in the erzgebirge a classic metallogenic province in central europe garnet ages obtained not only constrain the timing of fertile skarn formation and associated sn w fe zn cu and in mineralization but also clearly distinguish these from barren skarn bodies we show that barren skarns formed during times of peak regional metamorphism at 340 ma whereas mineralized skarns are temporally associated with late orogenic magmatism at 325 313 ma as well as post orogenic magmatism at 308 295 ma the recognition of discrete mineralization events associated with the largest and economically most important skarn deposits provides valuable insight into the punctuated evolution of magmatic hydrothermal systems in ancient collisional orogens on a regional scale this has important implications to direct future mineral exploration

why metasomatic fronts are really metasomatic sides back scattered electron microscope images of metasomatic calc silicate rocks demonstrate complex mineral growth patterns often oscillatory zoning defines a euhedral or subhedral morphology we infer from this the existence of an appreciable transient porosity up to a few percent during metasomatic calc silicate growth and argue that porous reacting calc silicate layers become conduits for irreversible metamorphic fluid loss because the layers are too permeable to sustain a steep gradient in hydraulic head downdip fluid pressure in the calc silicate must generally be lower than in adjacent low permeability lithologies so that steep local gradients in the hydraulic head will drive fluid from surrounding rocks up down or sideways into the transiently porous reacting layer so called metasomatic fronts separate transiently permeable layers that have undergone metasomatism from little altered low permeability rocks thus they develop parallel to the flow path at its edge and should be termed metasomatic sides

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

hydrous basalt limestone interaction at crustal conditions implications for generation of ultracalcic melts and outflux of co 2 at volcanic arcs abstract high degassing rates for some volcanoes typically in continental arcs e g colli albani volcanic district etna vesuvius italy merapi indonesia popocatepetl mexico are thought to be influenced by magma carbonate interaction in the crust in order to constrain the nature of reaction and extent of carbonate breakdown we simulated basalt limestone wall rock interactions at 0 5 1 0 gpa 1100 1200 c using a piston cylinder and equal mass fractions of calcite caco3 and a hydrous 4 wt h2o basalt in a layered geometry contained in aupd capsules all experiments produce melt fluid calcite clinopyroxene plagioclase calcic scapolite spinel with increasing t plagioclase is progressively replaced by scapolite clinopyroxene becomes cats rich and fluid proportion as inferred from vesicle population increases at 1 0 gpa 1200 c our hydrous basalt is superliquidus whereas in the presence of calcite the experiment produces calcite clinopyroxene scapolite melt with the consumption of calcite with increasing t and decreasing p melt on a volatile free basis becomes silica poor 58 1 wt at 1 0 gpa 1100 c to 34 9 wt at 0 5 gpa 1200 c and cao rich 6 7 wt at 1 0 gpa 1100 c to 43 7 wt at 0 5 gpa 1200 c whereas al2o3 drops e g 19 7 at 1100 c to 12 8 wt at 1200 c at 1 0 gpa as clinopyroxene becomes more cats rich high t or low p melt compositions are ultracalcic potentially presenting a new hypothesis for the origin of ultracalcic melt inclusions in arc lava olivines wall rock calcite consumption is observed to increase with increasing t and decreasing p at 0 5 gpa our experiments yield carbonate assimilation from 21 6 to 47 6 between 1100 and 1200 c using measured co2 outflux rates for mts vesuvius merapi etna and popocatepetl over a t variation of 1100 to 1200 c at 0 5 gpa we calculate 6 92 of magmatic input estimates undergo this extent of assimilation suggesting that up to 3 of the current global arc co2 flux may be crustally derived application of the assimilation extent bracketed in this study to the estimated elevated number of carbonate assimilating arc magmatic systems active during the late cretaceous to early paleogene suggests that magma induced upper plate decarbonation alone had the potential to contribute up to 2 7 10 14 5 6 10 15 g y co2 assuming no dilution and complete gaseous release of all assimilated carbon using an estimated assimilation extent averaged from current systems gives a slightly lower though still significant value of 5 5 10 14 g y of excess co2 being released into the atmosphere

origin and evolution of skarn fluids empire zinc skarns central mining district new mexico u s a abstract garnet pyroxene sphalerite skarns in the empire mine replace paleozoic carbonates adjacent to the tertiary hanover fierro granodiorite skarn geometry suggests that fluids migrated up pre ore dikes faults and the igneous contact and were deflected laterally into the permeable tierra blanca limestone beneath the relatively impermeable parting shale silicates associated with propylitically altered pre ore dikes are enriched in deuterium d and depleted in18o relative to the hanover fierro pluton and post ore igneous rocks early skarn silicates are also depleted in18o with respect to the pluton while later skarn minerals are depleted in both d and18o variations in isotope composition of alteration and skarn minerals indicate isotope heterogeneities in mineralizing fluids even at the small scale of centimeters isotope thermometry indicates that there is some degree of subsolidus re equilibration of igneous and alteration minerals several possible fluid flow regimes may have operated to produce the fluids calculated to be in exchange equilibrium with the various rocks and minerals of the empire skarn system and mixing of end member meteoric formation and magmatic fluids in different proportions can produce observed d 18o trends an end member magmatic fluid could produce the d enrichment observed for early skarn fluids but this would require isolating magmatic fluids from external fluid sources during cooling of the system from magmatic temperatures of 700 c to skarn temperatures of the order of 400 c the d enrichment may also be explained by the mixing of magmatic and formation waters lower d values however require that a large proportion of late stage skarn fluids must be a d depleted tertiary meteoric water and magmatic water is restricted to a relatively minor component the end member mixing approach indicates significant changes in fluid flow systematics over a relatively narrow range in temperature alternatively observed trends in both d and 18o for skarn fluids can also be reproduced by interacting a d depleted meteoric water with the hanover fierro pluton at low and variable system water rock ratios and temperatures between 250 and 400 c during migration along the long fluid flow paths implied by the low system water rock ratios 0 1 the salinity of dilute meteoric waters could increase through interaction with minerals or leaking fluid inclusions in the country rock correlation of isotope depletions of the carbonate wallrocks with inferred fluid flow conduits suggests significant amounts of fluid rock exchange at relatively high local water rock ratios during focusing of flow by critical structures although different c sources might require smaller values it is clear that large 1 local water rock ratios are required to produce depletions observed in both18o and13c in hydrothermal calcites stable isotope evidence does not require the presence of a significant magmatic fluid component and suggests that the bulk of the skarn fluids could instead be derived predominantly from a d depleted meteoric water

mechanisms of oxygen isotopic exchange and isotopic evolution of 18 o 16 o depleted periclase zone marbles in the alta aureole utah insights from ion microprobe analysis of calcite oxygen isotope ratios have been measured by ion microprobe and millimeter scale dental drill along detailed sampling traverses across the boundary between periclase bearing 18o 11 8 and periclase free 18o 17 2 marble layers in the periclase per zone of the alta stock aureole utah these data define a steep coherent gradient in 18o that is displaced a short distance 4 cm into the periclase free cal fo layer sem and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale clear well polished calcite grains are isotopically homogeneous within analytical uncertainty 0 27 2sd more poorly polished pitted texturally retrograde turbid looking calcite has lower and more variable 18o values and replaces clear calcite along fractures cleavage traces or grain boundaries despite significant lowering of the 18o values in calcite throughout both layers during prograde metamorphism ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in 18o diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion 30 000 62 000 years at 575 600 c exceed significantly the timescale 1 250 years estimated for the prograde development of the 18o gradient at the boundary between these two marble layers the ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains thus the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization

coal top detection by conductively guided borehole radar wave imaging for open cut blast hole drilling abstract damage to the top of coal seams caused by incorrect blast stand off distances results in coal losses of up to 10 15 to the australian open cut coal mining operations this is a serious issue to be addressed here we propose to use a new forward looking imaging technique based on the borehole radar technology to predict the coal seam top in real time while drilling blast holes this is achieved by coupling the conventional borehole radar waves on to a steel drill rod to induce a guided wave along the axial drill rod the drill rod ahead of the borehole radar behaves as a forward looking antenna for the guided waves both numerical modelling and field trials simulating a drill rod as an antenna are used to investigate the feasibility of the proposed technique for prediction of the coal top under typical open cut environments numerical modelling demonstrated that conductivity of the overburden is the most important factor affecting our ability to see coal seams ahead of the drill bit the guided borehole radar waves could be used for top coal prediction and a theoretical prediction error less than 10 cm and a forward looking capability of 4 6 m can be achieved field trials at australian open cut coal mines also demonstrated that guided borehole radar waves can be observed and used for prediction of coal top ahead of drill bit during blast hole drilling in resistive open cut environments the average resistivity should be higher than 75 m

zonation patterns of skarn garnets records of hydrothermal system evolution chemically zoned skarn garnets provide a continuous record of hydrothermal processes in lower paleozoic sedimentary rocks within the contact aureole around the drammen granite in the oslo rift southern norway major and trace element zonation profiles the latter obtained using a scanning high resolution proton microprobe reveal early infiltration controlled growth of relatively grossular rich garnets the major and trace elements compositions being buffered by local mineral dissolution subsequent rapid epitaxial growth of andradite rich garnet on grossular rich cores marks the onset of vigorous and focused fluid flow along high permeability zones during this later stage the hydrothermal fluid composition was to a large extent externally controlled and the andradite precipitating from these fluids was characterized by high as and w contents the zonation patterns of the andradite rich garnets record at least five intermittent growth periods with rapid andradite precipitation from fluid batches with high f o 2 and progressively decreasing as and w contents thin layers poor in fe as and w but relatively high in al and mn represent periods of slow growth rates between the major pulses of hydrothermal fluids the marked rimward decrease in the as and w contents of the garnets may reflect influx of meteoric waters or exhaustion of these elements in the hydrothermal fluid reservoir caused by boiling controlled distillation processes at depth

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

andradite skarn garnet records of exceptionally low δ 18 o values within an early cretaceous hydrothermal system sierra nevada ca skarn garnets in the mineral king roof pendant of the south central sierra nevada within sequoia national park california usa reveal variable fluid chemistry with a significant component of meteoric water during metasomatism in the early cretaceous sierra nevada batholith we focus on andradite garnet associated with pb zn mineralization in the white chief mine laser fluorination oxygen isotope analyses of 18o of garnet 18o grt from sites along the skarn show a large range of values 8 8 to 4 6 vsmow ion microprobe sims analyses elucidate that individual andradite crystals are strongly zoned in 18o grt up to 7 of variation total rare earth element concentrations ree across individual garnets show progressive depletion of skarn forming fluids in these elements during garnet growth these findings support a skarn model of earliest red high 18o grandite garnet consistent with a magmatic dominated equilibrium fluid 18o h2o as high as 8 later green andradite crystallized in equilibrium with a low 18o fluid indicating a significant influx of meteoric fluid 18o h2o 6 to 5 following a hiatus in garnet growth associated with late stage pb zn mineralization latest orange overprint rims have higher 18o values 18o h2o 0 2 and depleted total rees suggesting influx of high 18o trace element depleted fluid derived from regional metamorphism of the carbonate host remarkably low 18o grt values in the white chief canyon skarn require a significant proportion of meteoric fluid available during 400 c andradite forming metasomatism fluid flow was channelized at the pluton wallrock contact evidenced by the narrow extent of skarn

the rare earth elements and uranium in garnets from the beinn an dubhaich aureole skye scotland uk constraints on processes in a dynamic hydrothermal system garnets from skarns in the beinn an dubhaich granite aureole isle of skye scotland have a large range of concentrations of uranium 0 2 358 ppm and the rare earth elements ree 23 4724 ppm variations in these concentrations correlate with major element zonation within the garnets and with changes in the shape of ree patterns typical patterns in most garnets display light ree lree enrichment flat heavy ree hree distribution and a negative eu anomaly these patterns are interpreted to represent equilibrium trace element exchange between pre existing pyroxene hydrothermal fluid and calcic garnets iron rich zones are characterized by positive eu anomalies and an increase in the abundance of the lree relative to the hree these patterns are interpreted as resulting from changes in ree speciation related to the introduction of externally buffered fluid to the skarn system relatively fe poor zones show strongly hree enriched patterns with negative eu anomalies and in some instances depletions in y relative to ho and dy which are interpreted as resulting from surface sorption of the ree during rapid disequilibrium garnet growth strong correlations between u abundance and the ree patterns indicate that the same processes have affected u distribution both types of pattern can be modified by the effects of closed system crystallization on ree abundance in the fluid and changes in fluid major element chemistry

unraveling histories of hydrothermal systems via u pb laser ablation dating of skarn garnet abstract skarns are one of the few direct genetic links between magma and base metals in ore deposits yet temporal relationships between skarns and their assumed causative plutons are rarely confirmed with radiometric formation ages skarns have been notoriously difficult to date directly due to a limited primary mineral assemblage generally garnet and pyroxene discrepancies in timescales of skarn formation versus pluton emplacement emphasize our tenuous understanding of temporal relationships between plutons and hydrothermal systems including the duration of hydrothermal activity and skarn mineral growth rates here we apply a new method for u pb dating grossular andradite grandite garnet via la icp ms and present the first comprehensive study to focus on u pb dates of primary skarn garnet we present u pb age constraints on garnets from multiple skarns within the jurassic and cretaceous arcs of the north american cordillera two case studies darwin and empire mountain skarns illustrate the effectiveness of the u pb grandite method by demonstrating that garnet u pb ages agree within error of the pluton and skarn hosted u pb zircon ages two additional study localities black rock and mojave skarns demonstrate the ability of grandite u pb ages to 1 directly identify a causative igneous event and 2 record timing of major tectonic events specifically extensional to trans tensional shifts in the jurassic mojave of california

ion microprobe survey of the grain scale oxygen isotope geochemistry of minerals in metamorphic rocks abstract the oxygen isotope compositions of calcite diopside dolomite forsterite garnet k feldspar kyanite plagioclase quartz and wollastonite were analyzed in suites of contact and regional metamorphic rocks using an ion microprobe spatial resolution was 10 m precision measured as the standard deviation of working standards averaged over the entire project was 0 13 0 18 for three carbonate standards and 0 11 0 12 for two silicate standards a total of 1176 analyses excluding standards were made of 73 minerals in 23 samples both intercrystalline and intracrystalline variability in 18 o is greater in contact than in regional metamorphic rocks of 27 minerals analyzed in contact metamorphosed rocks 70 exhibit statistically significant grain to grain variability in 18 o over areas 1 41 cm 2 with the largest range in silicates and carbonates in a single sample of 7 4 forsterite and 10 6 dolomite of 88 grains analyzed in two or more places in contact metamorphosed rocks 32 exhibit statistically significant intracrystalline variability in 18 o with the largest range in a single silicate and carbonate grain of 3 1 forsterite and 10 1 dolomite in contrast 44 of 45 minerals in regional metamorphic rocks exhibit significant grain to grain variability in 18 o over areas 1 17 cm 2 with the largest range in silicates and carbonates in a single sample of only 1 1 plagioclase and 0 9 calcite only 6 of 144 grains analyzed in two or more places in regional metamorphic rocks exhibit significant intracrystalline variability in 18 o with the largest range in a single silicate and carbonate grain of only 1 5 diopside and 0 7 calcite the difference in intercrystalline and intracrystalline variability in 18 o between contact and region metamorphic rocks is explained by the longer duration and slower reaction rates of regional metamorphism rather than to differences in temperature there is no significant difference in intercrystalline and intracrystalline variability in 18 o in regional metamorphic rocks among samples from the biotite garnet and kyanite zones calcite inclusions in forsterite and calcite and quartz inclusions in garnet either have 18 o that is statistically indistinguishable from 18 o of the same mineral occurring as nearby matrix grains or have statistically significant lower 18 o no reversed isotope fractionations were measured between coexisting mineral pairs minerals in individual samples however exhibit a wide range in the degree to which they attained and preserve oxygen isotope fractionations consistent with metamorphic temperatures recorded by mineral equilibria processes that account for grain scale departures from isotope exchange equilibrium include a overstepping of prograde mineral reactions b growth zoning in low diffusivity minerals c interaction of rocks with fluids at the peak of metamorphism and or during cooling d retrograde mineral reactions and e closed system isotope exchange between coexisting minerals during cooling this study provides new information about 1 the degree to which a variety of textural changes experienced by rocks during metamorphism are associated with changes in 18 o 2 oxygen isotope homogenization at the outcrop scale among contrasting lithologies 3 changes in 18 o with increasing grade of regional metamorphism and 4 time scales of metamorphic process

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

new constraints on fluid sources in orogenic gold deposits victoria australia fluid inclusion microthermometry raman spectroscopy and noble gas plus halogen geochemistry complemented by published stable isotope data have been used to assess the origin of gold rich fluids in the lachlan fold belt of central victoria south eastern australia victorian gold deposits vary from large turbidite hosted orogenic lode and disseminated stockwork gold only deposits formed close to the metamorphic peak to smaller polymetallic gold deposits temporally associated with later post orogenic granite intrusions despite the differences in relative timing metal association and the size of these deposits fluid inclusion microthermometry indicates that all deposits are genetically associated with similar low salinity aqueous co2 bearing fluids the majority of these fluid inclusions also have similar 40ar 36ar values of less than 1500 and 36ar concentrations of 2 6 58 ppb by mass that are equal to or much greater than air saturation levels 1 3 2 7 ppb limited amounts of nitrogen rich fluids are present at a local scale and have the highest measured 40ar 36ar values of up to 5 700 suggesting an external or distinct source compared to the aqueous fluids the predominance of low salinity aqueous carbonic fluids with low 40ar 36ar values in both orogenic and intrusion related gold deposits is attributed to fluid production from common basement volcano sedimentary sequences and fluid interaction with sedimentary cover rocks turbidites aqueous fluid inclusions in the stawell magdala deposit of western victoria including those associated with n2 preserve mantle like br cl and i cl values in contrast fluid inclusions in deposits in the eastern structural zones which contain more abundant shales have elevated molar i cl ratios with maximum values of 5 170 10 6 in the melbourne zone br i ratios in this zone range from 0 5 to 3 0 that are characteristic of fluid interaction with organic rich sediments the maximum i cl and characteristic br i ratios provide evidence for organic br and i released during metamorphism of the shales therefore the regional data provide strong evidence for the involvement of sedimentary components in gold mineralisation but are consistent with deeper metamorphic fluid sources from basement volcano sedimentary rocks the overlying sediments are probably involved in gold mineralisation via fluid rock interaction

magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of california effects of assimilation crustal thickness and depth of crystallization in order to better understand geologic fac tors controlling pronounced regional variations in whole rock chemistry mineralogy and mineral chemistry in the batholiths of california we calculate the magmatic intensive variables f hf f h 2 o f hf f hcl and f o 2 regional scale west to east increases in f oh in mafic silicates corresponding to the systematic i wc to i mc and i sc progression reflect orders of magnitude increase in f hf f h 2 o attending pluton crystallization low f hf f h 2 o of formation of western i wc types is consistent with their derivation from low fluorine source rocks in subducted oceanic slabs or the upper mantle in contrast higher f hf f h 2 o of crystallization of i mc and i sc types to the east implies the involvement of 1 progressively greater amounts of continental crustal source material such as biotite bearing metamorphic rocks their unweathered sedimentary derivatives retaining f rich mafic minerals or their fusion products and or 2 source materials which become more f rich toward the continental interior the regional distribution of i mc and i sc types suggests that the precambrian craton of western north america or derivative sediments may extend farther north in california and be morphologically more complex than previously thought from seemingly out of place occurrences of i wc plutons on the eastern slopes of the sierra nevada batholith we infer the existence of regions where precambrian basement was thin or absent in mesozoic time which prevented extensive cratonal contamination of subducted slab or upper mantle derived magmas new methods for estimating t f o 2 relations in the magmas demonstrate that i scr granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of i wc i mc and i sc types under conditions at or below the maximum stability limit of graphite in equilibrium with a c o h s gas phase the local scale formation of i scr granites in plutonic belts within specific wall rock terranes containing highly reducing sediments or metasediments may occur by contamination of i types with graphitic pelite or in some cases by the direct fusion of this reducing pelitic wall rock the spatial distribution of i scr granite provinces therefore is controlled simply by wall rock lithology amphibole geobarometry demonstrates a general west to east decrease in crystallization pressure across the sierra nevada batholith in contrast the peninsular ranges batholith displays a west to east crystallization pressure increase the bulk of the california batholiths crystallized at pressures less than 4 5 kb and depths less than about 15 19 km in the southern sierra nevada batholith the san gabriels and the eastern peninsular ranges however plutons crystallized at pressures exceeding 6 kb at deep crustal levels 23 km reconstruction of the pre erosion top of the batholith shows that in an east west cross section the central sierra nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one

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

oxygen isotope and trace element zoning in hydrothermal garnets windows into large scale fluid flow behavior single garnet crystals from a large multicyclic hydrothermal system in far east russia are strongly growth zoned microscale ion microprobe analysis reveals consistent intracrystalline patterns in oxygen isotope composition and boron and iron contents reflecting evolution of the hydrothermal system from magmatic to meteoric dominated conditions and repetitive influx of magmatic fluids large intracrystalline variations in oxygen isotope composition 12 18 o delimit temporal evolution of magmatic and meteoric fluid input within different portions of the hydrothermal system and suggest that fluid flow regimes differed between central and distal portions of the system these results provide compelling evidence that microanalytical data can be extrapolated from intracrystalline scales to study macroscale processes in complex large scale fluid rock systems

a detailed record of shallow hydrothermal fluid flow in the sierra nevada magmatic arc from low δ18o skarn garnets garnet from skarns exposed at empire mountain sierra nevada california united states batholith have variable 18o values including the lowest known 18o values of skarn garnet 4 0 in north america such values indicate that surface derived meteoric water was a significant component of the fluid budget of the skarn forming hydrothermal system which developed in response to shallow emplacement 3 3 km of the 109 ma quartz diorite of empire mountain values of 18o measured in situ across single garnet crystals by secondary ion mass spectrometry vary considerably up to 7 and sometimes abruptly indicating variable mixing of meteoric magmatic and metamorphic water brecciation in the skarns and alteration of the empire mountain pluton suggests that fracture enhanced permeability was a critical control on the depth to which surface waters penetrated to form skarns and later alter the pluton compared to other sierran systems much greater volumes of skarn rock suggest an exceptionally vigorous hydrothermal system that saw unusually high levels of decarbonation reaction progress likely a consequence of the magma intruding relatively cold wallrocks inboard of the main locus of magmatism in the sierran arc at that time

formation of jasper and andradite during low temperature hydrothermal seafloor metamorphism ongeluk formation south africa andradite and andradite grossular solid solutions have repeatedly been described as minor constituents of weakly metamorphosed ocean floor metabasites and associated calcareous or siliceous sediments the conditions of garnet formation however have remained rather elusive here wedescribe the discovery of andradite as a minor but widespread constituent of chert and jasper closely associated with only very weakly metamorphosed basaltic andesites of the early paleoproterozoic ongeluk formation transvaal supergroup south africa the andradite is most commonly fine grained and forms skeletal crystals that poikilotopically enclose and replace quartz and hematite macroscopically coarse grained andradite is scarce and was recognized at one locality only up to 3 mm largeandradite grains occur closely associated with quartz in short veins that crosscut thin beds of andradite bearing jasper the cogenetic association of quartz and andradite was studied in detail to constrain the conditions of garnet formation our results indicate that andradite crystallized at temperatures well below 200 c in the sub seafloor environment in siliceous sediments of hydrothermal origin that provided an ample source of sio2 and fe2o3 andradite formed when hydrothermally modified seawater introduced ca that was leached when seawater interacted with plagioclase and clinopyroxene in the surrounding volcanic pile other important preconditions for garnet formation were the lack of mixing between the hydrothermal fluid and cold seawater and low fco2 it appears reasonable to extend the findings of this study to other recorded occurrences of andradite in modern and ancient ocean floor metabasites

middle cretaceous greenhouse hydrologic cycle of north america we present a paleolatitudinal precipitation reconstruction for the greenhouse setting of mid latitude north america based on the oxygen isotopic composition of sphaerosiderites found in middle cretaceous wetland paleosols our reconstructed middle cretaceous d 18 o values of precipitation are 4 less than values from comparable modern low elevation coastal settings free of monsoons the data fit a conceptual model in which the precipitation source for the eastern margin of the cretaceous western interior seaway of north america is an 18 o enriched oceanic coastal jet in this subtropical tropical setting midcretaceous precipitation rates are interpreted to range from 2500 to 4100 mm yr