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

ordering and exsolution processes in or rich alkali feldspar megacrysts from the eldzhurtinskiy granite caucasus the extremely young 2 5 ma i type eldzhurtinskiy granite complex central caucasus is uniform with respect to modal composition major and trace element chemistries of bulk rocks and mineral phases in contrast it reveals two types of alkali feldspar megacrysts differing in tetrahedral al content 2t1 and exsolution microtextures

petrological modifications in granitic rocks from the siljan impact structure evidence from cathodoluminescence abstract cathodoluminescence cl and microprobe analyses were performed on quartz plagioclase and k feldspar from granitic rocks in the siljan impact area central sweden in the upper 2200 m and in the vicinity of the gravberg 1 well secondary reddish luminescing quartz and brownish luminescing albite ab 98 and k feldspar or 95 are the dominant minerals primary magmatic quartz plagioclases oligoclase andesine and k feldspar show blue yellowish and bluish luminescence colours respectively the reddish luminescence colour of quartz is interpreted to have been induced by the shock heating episode of the impact and subsequent elevated temperature in the granites the impact shock pressure effects as shown by shatter cones and planar features were retained within granite dominated by blue luminescing quartz most alteration of the feldspar took place during the cooling episode that followed the impact heating post impact fracturing facilitated the infiltration of meteoric water which has further enhanced alteration of feldspars

turbid alkali feldspars from the isle of skye northwest scotland partially turbid alkali feldspars from hydrothermally altered tertiary granites on the isle of skye the red hills granites were studied using light microscopy scanning and transmission electron microscopies and energy dispersive x ray spectroscopy limpid cores and turbid rims of individual crystals were compared to determine the causes of the turbidity the limpid cores were cryptoperthitic with lamellar widths of 0 1 0 3 m in contrast the turbid rims contained k rich and narich areas coarsened to 0 5 m turbid regions contained abundant inclusions whereas limpid regions did not two generations of turbidity were recognized feldspars from the beinn an dubhaich granite a granite with near normal values for 18o 16o possessed limpid cores surrounded by turbid rims that cast a reddish brown hue in transmitted light when viewed in darkfield light microscopy the regions with the reddish brown turbidity were blue this is consistent with the hypothesis that the cloudy appearance of these turbid regions arises from the scattering of light by micrometerto submicrometer sized inhomogeneities in refractive index caused by fluid filled cavities feldspars from the loch ainort granite a granite with low values for 18o 16o possessed limpid and reddish brown turbid cores surrounded by turbid rims that cast a blackish hue in transmitted light ion thinning of the turbid areas produced an abundance of small holes 1 2 m apparently the remains of fluid inclusions transmission electron microscopy revealed that some holes from regions of reddish brown turbidity contained non feldspar material including halite and metal rich phases of various compositions in contrast blackish turbid regions contained cavities filled with alteration products such as kaolinite hence the feldspars from granites on the isle of skye apparently record interactions with at least two fluids a saline fluid possibly a late stage magmatic fluid and a meteoric fluid

redox effects on calcite portlandite fluid equilibria at forearc conditions carbon mobility methanogenesis and reduction melting of calcite 2014 by walter de gruyter berlin boston oxygen fugacity fo2 is a fundamental parameter that controls carbon mobility in aqueous fluids in geological environments such as subduction zones where reduced serpentinite fluids have the potential to infiltrate oxidized carbonate bearing lithologies using experiments and calculations we describe how mineral fluid equilibria evolve as fo2decreases in the model ca c o h system at forearc conditions 300 700 c and 2 10 kbar experimental calcite solubility was constant at fo2values from quartz fayalite magnetite qfm to hematite magnetite hm at lower fo2values of iron magnetite im or wustite magnetite wm calcite reacted with h2to form methane plus portlandite or melt these results were consistent with thermodynamic calculations and indicate that carbon mobility as parameterized by total aqueous carbon ctot is strongly dependent on fo2 at constant pressure and temperature carbon mobility is minimized at oxidizing conditions where ctot is controlled by calcite solubility carbon mobility is maximized at the most reducing conditions because all the carbon in the system is present as ch4 an intermediate region of carbon mobility exists in which calcite is stable with a ch4 bearing fluid as pressure increases from 2 to 10 kbar the fo2range over which calcite is stable with a methane rich fluid shifts to more reducing conditions the variety of geological conditions with the potential for redox enhancement of carbon mobility becomes more restricted with depth reduction melting was observed at 700 c and 6 kbar and at 650 c and 10 kbar due to the partial reaction of calcite to portlandite at conditions above the hydrous melting curve of calcite portlandite although likely metastable in the present experiments reduction melting may occur in nature whenever h2partially reduces carbonate minerals at pressures and temperatures above the hydrous melting curve of calcite portlandite whether it causes melting or not calcite reduction is likely an important mechanism for abiotic methanogenesis in natural systems such as subduction zone forearcs or similar environments with the potential for interaction of reduced fluids with carbonate minerals because calcite solubility at oxidized conditions is already known to increase substantially with pressure the additional increase in carbon mobility provided by calcite reduction implies that subduction zones may host some of the most carbon rich aqueous fluids on earth

hydrothermal alteration of tertiary igneous rocks from the isle of skye northwest scotland hydrothermal alteration of tertiary granites from skye involed the reaction of igneous amphibole biotite ferroaugite fayalite alkali feldspar plagioclase feldspar magnetite and ilmenite with aqueous fluid primarily to combinations of secondary alkali feldspar chlorite and montmorillonite lesser amounts of muscovite calcite epidote prehnite zeolite and pyrite also developed during mineralogical alteration of the granites there was a net addition to rock of h and a net loss of ca fe and probably na there is no positive evidence for a significant change in their al si mg or k contents hydrothermal alteration of skye granite involved not only large scale migration of 18o 16o d and h but also of ca fe and probably na mineral thermometry combined with textural observations suggests that chemical isotopic and most mineralogical alteration were synchronous and occurred at 350 450 c textural observations further suggest that the mechanism of isotopic exchange between granite and hydrothermal fluid primarily was solution reprecipitation

zeolites in hydrothermally altered rocks over the past 40 years two universally accepted maxims of geology dealing with zeolite minerals have been shown to be incorrect the first is that zeolites occur primarily as cavity fillings in basaltic igneous rocks and the second is that these zeolites crystallized from so called hydrothermal solutions i e thermal waters from depth we recognize today that enormous quantities of zeolites occur as low temperature low pressure alteration products of pyroclastic material in sedimentary rocks and that many of the zeolites filling vugs and cavities in basalts and traprocks are actually diagenetic in origin having precipitated directly from ground water percolating through the rock mass see walker 1951 1959 1960a see walker b nashar and davies 1960 this is not to say that zeolite deposits of hydrothermal origin do not exist far from it the extensive zones of zeolites surrounding hot spring and geothermal activity in yellowstone national park wyoming wairakei new zealand reykjavik iceland pauzhetsk kamchatka russia lardello italy and takenoyu japan and in the vicinity of magmatic ore deposits such as the kuroko deposits of northern japan suggest that waters arising and or heated from below have indeed played an important role in their formation hydrothermal alteration can be defined as the crystallization of mineral material or the alteration of preexisting mineral material that has taken place in the presence of hot solutions the composition and temperature of which were wholly or partially the result of a deep seated magmatic body these reactions usually take place under comparatively high solution solid ratios over a wide temperature range and under low pressure conditions hydrothermal alteration is controlled by many factors including solution chemistry temperature rock permeability and the composition and stability of the starting material although hydrothermal solutions are very diverse in composition alkaline solutions are generally most favorable for formation of zeolites the temperature

routine characterization and interpretation of complex alkali feldspar intergrowths almost all alkali feldspar crystals contain a rich inventory of exsolution twin and domain microtextures that form subsequent to crystal growth and provide a record of the thermal history of the crystal and often of its involvement in replacement reactions sometimes multiple microtextures strongly influence the subsequent behavior of feldspars at low temperatures during diagenesis and weathering they are central to the retention or exchange of trace elements and of radiogenic and stable isotopes this review is aimed at petrologists and geochemists who wish to use alkali feldspar microtextures to solve geological problems or who need to understand how microtextures influence a particular process we suggest a systematic approach that employs methods available in most well founded laboratories the crystallographic relationships of complex feldspar intergrowths were established by the 1970s mainly using single crystal x ray diffraction but such methods give limited information on the spatial relationships of the different elements of the microtexture or of the mode and chronology of their formation which require the use of microscopy we suggest a combination of techniques with a range of spatial resolution and strongly recommend the use of orientated sections sections cut parallel to the perfect 001 and 010 cleavages are the easiest to locate and most informative techniques described are light microscopy scanning electron microscopy using both backscattered and secondary electrons including the use of surfaces etched in the laboratory electron probe microanalysis and analysis by energy dispersive spectrometry in a scanning electron microscope transmission electron microscopy we discuss the use of cathodoluminescence as an auxiliary technique but do not recommend electron backscattered diffraction for feldspar work we review recent publications that provide examples of the need for great care and attention to pre existing work in microtextural studies and suggest several topics for future work

magnetic polarity stratigraphy of the tertiary igneous rocks of skye scotland summary palaeomagnetic results are given for 115 sites mainly from the central igneous intrusive complex and also radiometric dates by the 40ar 39ar step heating method these are combined with published dates and palaeomagnetic results for the lavas and regional dyke swarms with the aim of establishing the magnetic polarity stratigraphy igneous activity spanned at least the approximate interval 61 54 ma and a polarity sequence of at last r n r n r but could span seven polarity intervals r n r n r n r depending on the age of the loch ainort epigranite however these polarities cannot be identified with specific chrons of either the harland et al 1982 or berggren et al 1985 polarity time scales the skye igneous activity therefore spans more polarity intervals than in mull ardnamurchan or rhum and this is consistent with the radiometric ages interpretations of the results for the coire uaigneich and related granites and for the loch ainort epigranite present problems and partial solutions are offered

the geology and geochronology of the annandagstoppane granite western dronning maud land antarctica the annandagstoppane granite is exposed at three nunataks in western dronning maud land antarctica it comprises medium to coarse grained granite crosscut by veins of pegmatite and graphic granite and has many s type characteristics such as containing normative corundum greater than 1 1 molecular al2o3 cao k2o na2o greater than 1 1 and very little zircon hydrothermal alteration in the granite is variably developed and has affected only certain minerals in any phase r sr and pb whole rock and mineral isotopic data suggest 1 that sr isotopes within it were nearly homogenized on a whole rock scale about 2823 ma ago by this hydrothermal alteration 2 that the pb isotopic system was also disturbed at that time and 3 that the granite may have been was emplaced sometime during the interval 3115 ma to 2945 ma ago the granite was probably intruded by the annandagstoppane gabbro about 1200 ma ago resetting the rb sr system in biotite the annandagstoppane granite may form part of a basement complex to the proterozoic sedimentary volcanic and mafic igneous rocks exposed to the east in the ahlmannryggen and the borgmassivet its chemical composition and geologic history appears to be unique in antarctica and in the kaapvaal craton of southern africa consistent with the possibility that the annandagstoppane granite is part of a crustal fragment that joined antarctica relatively late in the history of that continent

eight phase alkali feldspars low temperature cryptoperthite peristerite and multiple replacement reactions in the klokken intrusion eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the klokken intrusion the processes leading to the formation of the first four phases have been described previously the feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition between 750 and 600 c depending on bulk composition to give fully coherent strain controlled braid cryptoperthites with sub m periodicities below 500 c in the microcline field these underwent a process of partial mutual replacement in a deuteric fluid producing coarse up to mm scale turbid incoherent patch perthites we here describe exsolution and replacement processes that occurred after patch perthite formation both or and ab rich patches underwent a new phase of coherent exsolution by volume diffusion or rich patches began to exsolve albite lamellae by coherent nucleation in the range 460 340 c depending on patch composition leading to film perthite with 1 m periodicities below 300 c misfit dislocation loops formed which were subsequently enlarged to nanotunnels ab rich patches bulk composition ab91or1an8 in one sample exsolved giving peristerite with one strong modulation with a periodicity of 17 nm and a pervasive tweed microtexture the ab rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at 450 c this is the first time peristerite has been imaged using tem within any perthite and the first time peristerite has been found in a relatively rapidly cooled geological environment the lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion all the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity we term this material ultra porous late feldspar uplf it is composed predominantly of regions of microporous very or rich feldspar mean ab2 5or97 4an0 1 associated with very pure porous albite ab97 0or1 6an1 4 implying replacement below 170 90 c depending on degree of order in tem uplf has complex irregular diffraction contrast similar to that previously associated with low temperature albitization and diagenetic overgrowths replacement by uplf seems to have been piecemeal in character ghost like textural pseudomorphs of both braid and film parents occur formation of patch perthite film perthite and peristerite occurred 104 105 year after emplacement but there are no microtextural constraints on the age of uplf formation

a chlorine disinfectant for excess argon released from k felsspar during step heating abstract the release of excess 40 ar 40 ar e from virtually all alkali feldspars at low temperatures 40 ar 39 ar step heating experiments obscures potentially valuable age and thermal history information we report a method that takes advantage of the differential release of 40 ar e between contiguous isothermal heating steps due to the decrepitation of fluid inclusions and which permits derivation of a correction for the contaminating argon differences in age and cl k between successive temperature pairs commonly yield a single well correlated array on a 40 ar k versus cl k plot that identifies the cl correlated composition of 40 ar e 40 ar e cl affecting that sample eighteen of the twenty k feldspar samples we have measured yield a single 40 ar e cl component that varies from 1 1 10 3 to 4 2 10 6 making possible recovery of otherwise unobtainable thermochronological information the upper bound on the 40 ar e cl distribution of 10 3 may reflect ar saturation of pore fluids or the maximum duration between deuteric alteration and cooling below ar closure in k feldspar

alteration and mass transfer in cataclasites and mylonites in 6 6 km of granitic crust at the siljan impact structure central sweden granitic rocks deformed by cataclasis and mylonitization on macro a few meters and micro thin section scales are found at depths down to 6 6km in the siljan impact structure in central sweden granites near fault planes exhibit 1 fracturing kinking fragmentation and recrystallization of feldspars into pure k and na endmember varieties 2 fragmentation polygonization and development of undulose extinction in quartz and 3 kinking appearance of wavy extinction and alteration of biotite chlorite amphibole and alteration of ilmenite and magnetite whole rock chemical analyses of deformed and undeformed rocks show that deformed rocks are enriched in sio2 by about 5 wt and depleted in other oxides by variable percentages apart from rb and co the concentrations of other trace elements including ba sr zn zr pb cd cu cr ni v u th la and li are lower in deformed relative to undeformed rocks mass balance calculations for a 1000 cm3 model granite which were based on modal mineralogy whole rock chemistry and mineral analyses suggest that the break down of primary biotite chlorite and amphibole in deformed zones released elements to circulating fluids these calculations also indicate liberation of water and a doubling of porosity from 1 to 2 during the deformation episodes later precipitation of minerals in shear and tension fractures reduced this porosity within the upper 2000 m of the gravberg 1 well the formation of fracture filling minerals smectite calcite hematite chlorite and albite is impact related and was favored by active circulation of meteoric water fracture filling minerals in the upper 2000 m of the borehole formed at temperatures of 70 to 200 c between depths of 2000 and 3500 m fracture filling mineral assemblages dominated by fe mg chlorite sphene and epidote suggest formation temperatures in the range of 150 to 300 c occurrence of pumpellyite and prehnite in some altered biotite and chlorite of the deformed zones between 3500 and 5500 m suggest preimpact metamorphism and formation temperature above 150 c below 5500 m the mineral assemblages in the fractures are dominated by quartz sphene epidote and some muscovite and chlorite indicating a temperature range between 300 and 450 c one of the possible origins for the ch4 and h2 gases detected in the gravberg 1 well is a combination of hydrogen ions released by decomposition of hydrated silicates biotite chlorite hornblende with carbon the presence of iron in the deformed granitic rocks prevented the resulting ch4 from being oxidized

phyllosilicate and rutile compositions as indicators of sn specialization in some southeastern australian granites biotites from unaltered sn granites in southeastern australia are highly ferroan fe fe mg mn 0 75 whereas biotites from barren granites are less ferich fe fe mg mn 0 65 similar distinctions between sn specialized and barren granites can be observed in the other phyllosilicates especially chlorite biotites and muscovites from sn granites have greater be cs f li mo rb sc sn tl y and zn and lesser ba abundances than corresponding micas from barren granites in the same district alteration of barren granites also results in similar enrichments in micas of these elements sn and zn because of their abundance and retention during degradation of biotite to chlorite are the best trace element discriminants between barren granites and sn granites altered granites with the sn content of phyllosilicates being a better indicator than zn rutile inclusions within phyllosilicates from unaltered sn granites have nb2o5 contents up to 26 the ta content tends to increase with nb content but especially high ta contents occur in the rutile inclusions of granites that give rise to pegmatitic deposits the rutile inclusions in sn granites may also have substantial sn and w contents the rutiles of barren granites have low nb ta sn and w contents but sn and w increase with alteration together the ratio fe fe mg mn and sn contents in phyllosilicates and rutile compositions can be used to identify the sn mineralization potential of a granite

a stable isotope study of retrograde alteration in sw connemara ireland dalradian metamorphic rocks lower ordovician meta igneous rocks mgs and caledonian granites of the connemara complex in sw connemara all show intense retrograde alteration alteration primarily involves sericitization and saussuritization of plagioclase the alteration of biotite and hornblende to chlorite and the formation of secondary epidote the alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs and was the result of a phase of fluid influx into these rocks in hand specimen k feldspar becomes progressively reddened with increasing alteration mineralogical alteration in the mgs and caledonian granites took place at temperatures 275 15 c and in the mgs pfluid is estimated to be 1 5 kbar during alteration the d values of alteration phases are 18 to 29 fluid inclusions 47 to 61 chlorites and 11 to 31 epidotes chlorite 18o values are 0 2 to 4 3 while 18o values for quartz k feldspar pairs show both positively sloped mgs and highly unusual negatively sloped caledonian granites arrays diverging from the normal magmatic field on a plot the stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200 140 c the retrograde fluid had d 20 to 30 in all lithologies but the fluid 18o varied both spatially and temporally within the range 4 to 7 the fo2 of the fluid that deposited the epidotes in the mgs varied with its 18o value with the most 18o depleted fluid being the most oxidizing the d values together with low 0 18o values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin this meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration the variation in retrograde fluid 18o values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid rock interaction infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the galway granite soon after intrusion at 400 ma however convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later

dislocation formation and albitization in alkali feldspars from the shap granite orthoclase rich alkali feldspars in the lower devonian shap granite northwest england contain two generations of albite rich feldspar these have partially replaced earlier exsolution microtextures consisting of albite lamellae coarse semicoherent albite films and fine coherent albite platelets in tweed orthoclase the earlier generation of replacive albiterich feldspar ab90an9ori occurs together with orthoclase rich feldspar ab10or90 i n veins that crosscut exsolution microtextures throughout grain interiors this episode of recrystallization was mediated by magmatic fluids at 410 8c estimated from two feldspar geothermometry and was driven by stored elastic strain energy which was relatively homogeneously distributed throughout the microtextures the later generation of replacive albite rich feldspar which is restricted to grain margins and is compositionally pure ab 99 was produced by magmatic hydrothermal fluids at 370 8c this generation of albite rich feldspar does not crosscut exsolution microtextures and has selectively replaced volumes of highly elastically strained feldspar surrounding edge dislocations along semicoherent albite films marked differences in controls of the localization of the two generations of replacive albite rich feldspar by pre existing exsolution microtextures indicate that significant numbers of edge dislocations developed along albite films after the first phase of fluid feldspar interaction and associated albitization but before the second phase this relation indicates that edge dislocations formed between 410 and 370 8c these observations have important implications for understanding the factors that control the interaction of alkali feldspars with fluids both in cooling igneous rocks and in clastic sedimentary rocks during diagenesis

hydrothermal alteration and ree th mineralization at the rodeo de los molles deposit las chacras batholith central argentina ree rare earth element and th mineralization at the rodeo de los molles deposit occurs within an elliptical body of hydrothermally altered rocks fenite located in a biotite monzogranite of the las chacras batholith ore assemblages are found as isolated patches of intergrown britholite allanite apatite bastnaesite fluorite sphene quartz and aegirine augite as well as nodules of uranothorite and late stage veins of calcite fluorite and bastnaesite composition volume computations suggest that the fenite was produced by alteration of the biotite monzogranite by addition of k and na and loss of ca and sr petrographic evaluations indicate that microcline and plagioclase have been replaced by perthite and biotite was converted to aggregates of clinochlore anatase kaolinite and hematite relict biotite is characterized by lower fe fe mg and ti values with progressive alteration fluorine rich phlogopite is present in mineralized areas but textural evidence suggests that it was not produced via biotite alteration mass balance constraints also show that ca and mg in ore zones may result from redistribution rather than their being a result of external derivation the 18o values of quartz 8 6 11 1 and feldspar 7 8 10 6 suggest that feldspar continued to exchange oxygen isotopes with a fluid to lower temperatures than did quartz feldspars equilibrated with a fluid of 18o 8 at a fluid rock ratio less than 1 the 18o values of quartz and aegirine augite that crystallized during ree mineralization also suggest equilibration with a fluid of 18o 8 the d values of biotite 83 to 120 are relatively low for igneous rocks and are thought to have resulted from exsolution of a d enriched magmatic vapor the d values of both mineralized and barren fenites are consistent with equilibration with fluid of magmatic origin meteoric water was involved in the production of calcite and clinochlore alteration and late stage calcite fluorite bastnaesite veins the 13c values of calcite and bastnaesite 7 8 to 13 5 suggest that carbon was derived by leaching of carbon from igneous and or enclosing metamorphic rock types and that a majority of carbon ultimately was derived from sedimentary organic meterial

progressive metamorphism of the chert bearing durness limestone in the beinn an dubhaich aureole isle of skye scotland a reexamination

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

reddish metagranites from the gennargentu igneous complex sardinia italy insight into metasomatism induced by magma mingling the recrystallization of quartz and plagioclase to higher calcium concentrations the progressive increase in the mgo mgo feo of chlorite closer to the contact indicates that this phase also recrystallized the iron released during chlorite recrystallization was buffered by hematite formation in the pores of metasomatic k feldspar the gennargentu metagranites provide evidence that metasomatic fluids can play a major role in driving metamorphic reactions in contact aureoles for instance the expected increase of ca in plagioclase owing to thermal equilibration was not achieved in the high t zone of the aureole because of fluid assisted removal of cations we conclude that caution should be taken when interpreting the processes that underpin contact metamorphism in terms of thermally driven ionic diffusion alone because the role of fluids may be significant if not overwhelming in the domains closest to the magmatic source

magmatic evolution of the skye igneous centre western scotland modelling of assimilation recharge and fractional crystallization the skye igneous centre forming part of the british tertiary magmatic province developed over a 7 myr period 61 54 ma and is characterized by a complex suite of lavas hypabyssal and intrusive rocks of picritic to granitic composition the intrusion of magma from mantle to crust at 2 10 3 km 3 yr 6 mt yr advected magmatic heat of roughly 0 2 gw averaged over the period of magmatism supporting an excess heat flux of about 130 mw m 2 or about twice the present day average continental heat flow the volume of new crust generated at skye 15 000 km 3 spread over the present day area of skye corresponds to 9 km of new crust the geochemical evolution of the skye magmatic system is constrained using the energy constrained recharge assimilation and fractional crystallization ecrafc model to understand variations in the sr and pb isotopic and sr trace element composition of the exposed magmatic rocks with time the character composition and specific enthalpy of both assimilant and recharge magma appears to change systematically up section suggesting that the magma reservoirs migrated to progressively shallower levels as the system matured the model of the magma transport system that emerges is one in which magma batches are stored initially at lower crustal levels where they undergo rafc evolution residual magma from this stage then migrates to shallower levels where mid crustal wall rock is assimilated the recharge magma at this level is characterized by an increasingly crustal signature for some of the stratigraphically youngest rocks the data suggest that the magma reservoirs ascended into and interacted with upper crustal torridonian metasediments

contact metamorphism hydrothermal alteration of tertiary basalts from the isle of skye northwest scotland alkali olivine basalts from skye were simultaneously contact metamorphosed by tertiary gabbro and granite intrusions and altered by the hydrothermal convection system that the plutons induced four metamorphic zones were mapped around the plutons furthest from the intrusions in the primary olivine zone metabasalts are composed of combinations of igneous olivine augite plagioclase titaniferous magnetite ilmenite zeolites gyrolite sulfides and chlorite smectite intergrowths closer to the plutons in the smectite zone saponite and carbonate appear primary olivine and gyrolite disappear and zeolites decrease dramatically in abundance still closer to the plutons in the amphibole zone actinolite edenite chlorite sphene epidote andradite and quartz appear and saponite and chlorite smectite intergrowths disappear along parts of the contact between gabbro and basalt in the orthopyroxeneolivine zone orthopyroxene metamorphic olivine and biotite appear and amphibole chlorite sphene epidote andradite carbonate and quartz disappear whole rock chemical data indicate only minor change in the major element chemical composition of the metabasalts during progressive metamorphism hydrothermal alteration two pyroxene eothermometry and various mineral fluid equilibria suggest the range of peak temperatures attained in the metamorphic zones orthopyroxene olivine zone 900 1 030 c amphibole zone 400 900 c smectite and primary olivine zones 5 smow mineralogically more altered basalts from the smectite zone have whole rock 18o 2 to 5 still more mineralogically altered basalts from the amphibole zone with one exception have 18o 2 completely recrystallized hornfelses from the orthopyroxene olivine zone have 18o 0 the principal mechanism of isotope exchange between basalt and metamorphic hydrothermal fluid probably was heterogeneous mineralfluid reaction

development of microporosity diffusion channels and deuteric coarsening in perthitic alkali feldspars turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the klokken syenite using sem and tem it is caused by the presence of myriads of tubular micro inclusions either fluid filled micropores or sites of previous fluid inclusions and is associated with coarsening of microperthite and development of sub grains micropores are abundant in coarsened areas in which porosities may reach 4 5 but are almost absent from uncoarsened pristine braind microperthite areas the coarsening is patchy and involves a scale increase of up to 103 without change in the composition of the phases low albite and low microcline or in the bulk composition of the crystal it occurs abruptly along an irregular front within individual crystals which retain their original shapes the coherent braid microperthite gives way across the front to an irregular semi coherent film perthite over a few m and then to a highly coarsened irregular patch perthite containing numerous small sub grains on scales of a few hundred nm in both phases the coarsening and micropore formation occured at a t 400 450 c and it is inferred to have been driven by the release of coherent strain energy low angle grain boundary migration being favoured by a fluid the patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin possibly arising in part through exsolution of water from the feldspar the sub grain texture and microporosity modify profoundly the permeability of the rock and greatly enhance the subsequent reactivity of the feldspars

ion microprobe analysis of oxygen isotope ratios in quartz from skye granite healed micro cracks fluid flow and hydrothermal exchange quartz grains in hydrothermally altered granites from the isle of skye are highly heterogeneous and not equilibrated in oxygen isotope ratio at the 20 m scale ion microprobe analysis of one grain shows a gradient of 13 over 400 m and a greater range in 18o than all quartz previously analyzed on the isle of skye other crystals from the same outcrop are homogeneous digitized cathodoluminescence images reveal patterns of magmatic zoning and brittle fracturing not otherwise detectable the ion probe analysis correlates low 18o values on a micro scale to one set of healed cracks thus quartz exchanges oxygen isotopes primarily by solution and reprecipitation along fractures in contrast to more reactive feldspar that appears to exchange from the grain boundary inward macroscopic models of isotope exchange are not realistic for these rocks the flow of hydrothermal fluids was heterogeneous anisotropic and crack controlled

contact zone interaction of metabasites with metapelites amphibolite facies mineral assemblages chemical profiles and their origin the puolankajarvi formation finland abstract the amphibolite facies iron rich metapelites garnet staurolite mica schists of the early proterozoic puolankajarvi formation are intercalated with metamorphosed basic rocks which vary from iron rich amphibolites to magnesium rich layered sills and serpentinites garnet cummingtonite hornblende assemblages are abundant at the contacts of the metapelites with amphibolites while garnet chlorite garnet gedrite chlorite garnet gedrite cummingtonite chlorite and chlorite cummingtonite hornblende assemblages occur at their contacts with the layered magnesium rich sill the bulk composition profiles of these contacts normalised with respect to chromium and or aluminium show conspicuous enrichment in fe s ga and co and depletion in si k ba and rb some elements e g mg mn ni p zr and hf are depleted or enriched in only small amounts while others e g ca na sr and the rare earth elements may show both enrichment and depletion in the course of a profile although early infiltration during the hydration of metabasites and tectonic reworking may have produced some interaction of the basites with the pelites the major metasomatic event is considered to have bile elements the composition of the metapelites for example approached that of magnesium rich ultramafites through potassium depletion while the ultramafites became markedly enriched in iron to conserve the volume silica was depleted from the contact which underwent iron enrichment

the influence of nacl h2o fluids on reactions between olivine and plagioclase an experimental study at 0 8 gpa and 800 900 c abstract the lower to middle continental and oceanic crust is dominated by mafic gabbroic rocks which are variably metamorphosed at amphibolite granulite facies conditions forming reaction rims or veins containing secondary pyroxenes amphibole and phlogopite metamorphic fluids in the mid crust are typically nacl rich brines in this study olivine and plagioclase were reacted with varying amounts of fluid 0 2 wt added that had compositions ranging from pure h2o to nacl solutions 0 2 mol in a series of experiments using the piston cylinder apparatus at 0 8 gpa and temperatures from 800 to 900 c coronas of clinopyroxene clinopyroxene amphibole and clinopyroxene phlogopite were formed the thicknesses of reaction rims i e the relative amount of reaction rim products increase with the relative amount of fluid present in the system and are therefore linked to the level of fluid saturation along grain boundaries and interphase boundaries samples with no added h2o show little to no signs of reaction indicating the necessity for fluids in initiating a reaction between olivine and plagioclase the concentration of nacl in the fluid also increases the rates of rim growth at 900 c and 2 wt added fluid cl rich hydrous dacite trachyte melt compositions formed and are similar to low degree partial melts of oceanic crust the cl contents of secondary amphiboles are positively correlated with the feo and k2o contents of the amphibole consumption of fluid during the formation of hydrous rim phases may lead to high cl within transient fluid filled pores along grain boundaries which may lead to locally cl rich amphiboles

a fluid inclusion and stable isotope study of 200 ma of fluid evolution in the galway granite connemara ireland fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the caledonian galway granite these fluids were examined by petrography microthermometry chlorite thermometry fluid chemistry and stable isotope studies the earliest fluid was a h2o co2 nacl fluid of moderate salinity 4 10 wt nacl eq that deposited late magmatic molybdenite mineralised quartz veins v1 and formed the earliest secondary inclusions in granite quartz this fluid is more abundant in the west of the batholith corresponding to a decrease in emplacement depth within veins and to the east this fluid was trapped homogeneously but in granite quartz in the west it unmixed at 305 390 c and 0 7 1 8 kbar homogeneous quartz 18o across the batholith 9 5 0 4 n 12 suggests v1 precipitation at high temperatures perhaps 600 c and pressures 1 3 kbar from magmatic fluids microthermometric data for v1 indicate lower temperatures suggesting inclusion volumes re equilibrated during cooling the second fluid was a h2o nacl kcl low moderate salinity 0 10 wt nacl eq moderate temperature 270 340 c high d 18 2 low 18o 0 5 2 0 fluid of meteoric origin this fluid penetrated the batholith via quartz veins v2 which infill faults active during post consolidation uplift of the batholith it forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende sericitization and saussuritization of plagioclase and reddening of k feldspar the salinity was generated by fluid rock interactions within the granite within granite quartz this fluid was trapped at 0 5 2 3 kbar having become overpressured this fluid probably infiltrated the granite in a meteoric convection system during cooling after intrusion but a later age cannot be ruled out the final fluid to enter the granite and its host rocks was a h2o nacl cacl2 kcl fluid with variable salinity 8 28 wt nacl eq temperature 125 205 c d 17 to 45 18o 3 to 1 2 13cco2 19 to 0 and 34ssulphate 13 23 that deposited veins containing quartz fluorite calcite barite galena chalcopyrite sphalerite and pyrite v3 correlations of salinity temperature d and 18o are interpreted as the result of mixing of two fluid end members one a high d 17 to 8 moderate 18o 1 2 2 5 high 13cco2 4 low 34ssulphate 13 high temperature 205 230 c moderate salinity 8 12 wt nacl eq fluid the other a low d 61 to 45 low 18o 5 4 to 3 low 13c 10 high 34ssulphate 20 23 low temperature 80 125 c high salinity 21 28 wt nacl eq fluid geochronological evidence suggests v3 veins are late triassic the high d end member is interpreted as a contemporaneous surface fluid probably mixed meteoric water and evaporated seawater and or dissolved evaporites whereas the low d end member is interpreted as a basinal brine derived from the adjacent carboniferous sequence this study demonstrates that the galway granite was a locus for repeated fluid events for a variety of reasons from expulsion of magmatic fluids during the final stages of crystallisation through a meteoric convection system probably driven by waning magmatic heat to much later mineralisation concentrated in its vicinity due to thermal tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated

alkali feldspar microtextures as provenance indicators in siliciclastic rocks and their role in feldspar dissolution during transport and diagenesis abstract alkali feldspars usually exhibit optical or sub optical intergrowths of albite and k feldspar known generically as perthite which have morphologies and crystallographic characteristics that are distinctive of the igneous or metamorphic environment in which they grew and cooled to surface temperatures we review the current terminology and understanding of how the microtextures form and discuss their role in feldspar dissolution during weathering transport and diagenesis we show how microtextures can be used as provenance indicators in arkosic siliciclastic rocks using examples from the fulmar formation a reservoir rock in the upper jurassic humber group in the central north sea in which feldspar dissolution is a major source of secondary porosity we describe the most effective techniques for routine characterization of which the most practical is back scattered electron imaging in a scanning electron microscope often coupled with cathodoluminescence we provide an atlas of the main types of perthitic intergrowth likely to be encountered in igneous and metamorphic rocks worldwide and show how many of the microtextures can be matched in clastic grains in the fulmar formation grains which are preferentially preserved have microtextures that make them relatively unreactive in aqueous fluids at low temperatures and we conclude that provenance has a major impact on reservoir quality detrital grains in the fulmar exhibit authigenic albitization k feldspar overgrowths and replacement and we discuss how diagenetic features can be distinguished from replacive features developed in the protolith

quality of ptrm acquisition in pyrrhotite bearing contact metamorphic limestones possibility of a continuous record of earth magnetic field variations abstract the rockmagnetic and palaeomagnetic signal in pyrrhotite bearing limestones of different contact metamorphic settings were investigated related to intrusions ranging from small sills to large magmatic complexes magnetic susceptibility the pyrrhotite magnetite ratio and thermal modelling serve as an investigative tool to define three metamorphic zonations a contact zone of a mixed magnetic assemblage and low susceptibilities a pyrrhotite bearing transitional zone where full thermoremanent magnetisations trms are acquired due to temperatures above the curie temperature of pyrrhotite t c po and a marginal zone containing pyrrhotite and magnetite generated at temperatures below t c po the fact that trms can consist of independent ptrms is successfully tested by modified thellier experiments it is shown that a metamorphic environment with low fluid circulation provides a scenario for the recording of independent ptrms multicomponent behaviour of the nrm residing in samples from the transitional zone can therefore be interpreted as a continuous record of earth magnetic field variations

microtextural controls of weathering of perthitic alkali feldspars the relationship between the microtexture and dissolution behaviour of fresh hf acid etched and naturally weathered alkali feldspar phenocrysts from the lower devonian shap granite has been investigated by sem and tem a novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface further electron microscope work suggests that shap phenocrysts are representative of the alkali feldspar in the protolith of many soils fresh and unweathered shap feldspars have a complex microtexture comprising areas of pristine cryptoperthite and lamellar microperthite cross cut by volumes of microporous altered feldspar or patch perthites cryptoperthites are made up of 75 nm wide albite films the platelets are coherent but albite films have numerous edge dislocations along their interface with orthoclase 001 and 010 cleavage surfaces intersect 2 3 edge dislocations m2 in three dimensions these edge dislocations form an orthogonal net in the murchison plane of easy fracture close to 601 patch perthites are irregular semicoherent to incoherent intergrowths of albite and irregular microcline subgrains with 0 65 0 70 sub m to m sized pores m2 microporous patch perthites form by dissolution reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime dissolution of shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture especially by dislocations and exsolution lamellae the core and strain energy associated with dislocation outcrops on 001 and 010 cleavage surfaces promotes rapid dissolution at those sites and formation of nn sized etch pits after 30 s of laboratory etching with hf acid vapour with progressive hf etching crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite naturally weathered feldspars were collected from the glacial erratic boulders fine gravels surrounding exposed granite surfaces and from peat soil overlying the granite during natural weathering etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate 15 m below the cleavage surface forming an interconnecting ladder like grid of submicrometer wide channels in the murchison plane coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly but faster than orthoclase this is probably because the albite lamellae have significant elastic coherency strain but this is much less per unit volume of albite than the core and strain energy associated with edge dislocations patch perthites etch in hf vapour and weather rapidly in nature to produce a honeycomb like texture of interconnecting nanometer to micrometer sized pits which nucleate at preexisting micropores or incoherent subgrain boundaries the size and density of etch pits on microperthite surfaces which is determined by rates of growth and coalescence may be a useful progress variable for natural and experimental dissolution all alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale these microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains the controls on absolute dissolution rates and why they commonly change over time the nonlinear variation of dissolution rate with grain size the ratio of alkali ions released into solution and disparities between laboratory dissolution rates and those observed in the field

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

imprints of multiple fluid regimes on episyenites in the bohus granite sweden abstract a number of episyenite occurrences within the neoproterozoic bohus granite southwestern sweden were investigated in order to elucidate their genesis and past associated fluid regimes fluid inclusion microthermometry and o h isotope systematics integrated with information from detailed mineralogy reveal a paragenetic succession within individual episyenites including 1 albitization of plagioclase accompanied by dissolution of quartz with a vuggy feldspathic reservoir rock as a result 2 resumed infilling and replacement of igneous fe ti phases by various phyllosilicates muscovite chlorite and or illite hematite assemblages and quartz due to rift related reactivation and 3 sporadic argillitic alteration and precipitation of fracture calcite although sparse owing to later overprinting there are a few remnants that provide arguments for the involvement of meteoric water shifted to a more 18 o rich composition during the initial stage of dissolution and albitization the main vug filling phase is proposed to be associated with the ingress of surface derived fluids with negative 18 o values during the regional thermal anomaly accompanying the development of the nearby permo carboniferous oslo rift at 250 ma the fluids involved during this event show consistently a marked but variable 18 o depletion and resemble the modern day shield brines with modal salinities of 11 24 wt nacl eq low nacl nacl cacl 2 weight ratios and no discernible co 2 even though no p t path can be defined for this phase the fluid pressure can be restricted to 0 3 0 8 kbar and temperature to 160 280 c assuming a sedimentary overburden of 3 km an overall variability in the 18 o and d data can mainly be ascribed to 1 precipitation from fluids affected by isotopic exchange with the host rock and or sedimentary overburden under highly variable water rock ratios and to some extent with 2 varying degrees of partial re equilibration during subsequent hydrothermal interaction at lower temperatures as emphasized by the presence of fluid inclusions of suspected secondary origin in quartz

deep drilling at the siljan ring impact structure oxygen isotope geochemistry of granite the siljan ring is a 362 ma old impact structure formed in 1700 ma old i type granites a 6 8 km deep borehole provides a vertical profile through granites and isolated horizontal diabase sills fluid inclusion thermometry and oxygen isotope compositions of vein quartz granite diabase impact melt and pseudotachylite reveal a complex history of fluid activity in the siljan ring much of which can be related to the meteorite impact in granites from the deep borehole 18o values of matrix quartz increase with depth from near 8 0 at the surface to 9 5 at 5760 m depth in contrast feldspar 18o values decrease with depth from near 10 at the surface to 7 1 at 5760 m forming a pattern opposite to the one defined by quartz isotopic compositions values of 18o for surface granites outside the impact structure are distinct from those in near surface samples from the deep borehole in the deep borehole feldspar coloration varies from brick red at the surface to white at 5760 m and the abundances of crack healing calcite and other secondary minerals decrease over the same interval superimposed on the overall decrease in alteration intensity with depth are localized fracture zones at 4662 5415 and 6044 m depth that contain altered granites and which provided pathways for deep penetration of surface water the antithetic variation of quartz and feldspar 18o values which can be correlated with mineralogical evidence of alteration provides evidence for interaction between rocks and impact heated fluids 100 300 c in the upper 2 km of the pluton penetration of water to depths below 2 km was restricted by a general decrease in impact fracturing with depth and by a 60 m thick diabase sill at 1500 m depth that may have been an aquitard at depths below 4 km in the pluton where water rock ratios were low oxygen isotopic compositions preserve evidence for limited high temperature 500 c exchange between alkali feldspar and fluids the high temperature exchange may have been a post impact event involving impact heated fluids or a post magmatic event

regional 18o depletion of neoproterozoic igneous rocks from avalonia cape breton island and southern new brunswick canada neoproterozoic igneous rocks of the avalonian mira terrane cape breton island and caledonia terrane southern new brunswick have experienced regional 18 o depletion the majority of these rocks have 18 o wr values between 1 and 6 markedly different from igneous rocks of the inboard ganderian terranes which have normal high 18 o wr values of 7 to 12 the 18 o depletion of these avalonian terranes resulted from pervasive hydrothermal alteration all of the igneous rocks contain classic propylitic alteration assemblages of turbid feldspar chlorite epidote sericite and calcite and host multiple generations of veins containing the same phases the 18 o values of the rock forming minerals show strong disequilibrium with qtz fs ranging from 2 5 to 11 0 and kfs plg from 0 5 to 1 0 such values are indicative of post magmatic hydrothermal alteration the hydrothermal alteration occurred at ca 560 550 ma during initial transcurrent rifting of avalonia at the gondwanan margin the development of rift wrench basins with associated bimodal volcanism allowed large volumes of hydrothermal fluids to penetrate and circulate within mira and caledonia terrane crust before its submergence in the early cambrian ca 540 530 ma the hydrothermal fluids were predominantly meteoric although the presence of a significant seawater component cannot be ruled out the almost ubiquitous 18 o depletion exhibited by the neoproterozoic rocks in these avalonian terranes is absent in the associated ganderian terranes suggesting that avalonia remained separate from ganderia until at least the cambrian ordovician this 18 o depletion provides an additional geochemical tool for identification of avalonian crust in other areas of the appalachians and europe

epidote bearing veins in the state 2 14 drill hole implications for hydrothermal fluid composition epidote bearing veins in state 2 14 drill core from 900 to 2960 m depth were examined using backscattered electron microscopy and electron probe microanalysis to characterize the mineralogy parageneses texture and composition of vein minerals in order of decreasing abundance minerals in epidote bearing veins are pyrite calcite k feldspar quartz anhydrite hematite chlorite fe cu zn sulfides actinolite titanite and allanite the downhole distribution of minerals in epidote bearing veins pyrite and quartz varies as a function of depth and includes 1 calcite above 2000 m 2 k feldspar between 1700 and 2745 m 3 anhydrite between 2195 and 2745 m 4 hematite sulfides above 2773 m and 5 actinolite below 2890 m where present k feldspar was the first mineral to precipitate in veins followed by epidote in all other veins epidote was the earliest vein mineral to form calcite quartz anhydrite hematite and sulfides were paragenetically later compositional zoning common in most vein epidotes is typically symmetric with al rich cores and fe3 rich rims the minimum mole fraction of ca2fe3si3o12 oh xps in vein epidotes decreases systematically with increasing depth from 0 33 at 906 m to 0 21 at 2900 m and the maximum xps at any given depth is greater than 0 33 thermodynamic analyses of phase relations among vein filling minerals and aqueous solutions at depths near 1867 m and 300 c indicate that the modern reservoir fluid in the salton sea geothermal system is in equilibrium with calcite hematite quartz epidote xps 0 33 anhydrite the predicted fugacity of co2 14 bars for the modern salton sea brine is in close agreement with the calculated value of fco2 for the 1867 m production fluid theoretical phase diagrams in the system cao k2o fe2o3 al2o3 sio2 h2o o2 s2 co2 demonstrate that the mineralogies and mineral parageneses recorded hi epidote bearing veins and the observed variations in al fe3 content of vein epidotes may result from only minor changes in the fugacity of co2 o2 and s2 of the geothermal fluid

tertiary meteoric hydrothermal systems and their relation to ore deposition northwestern united states and southern british columbia tertiary meteoric hydrothermal systems have altered the rocks exposed over more than 5 of the land surface of the northwestern united states and southern british columbia including at least 25 000 km 2 in idaho the systems typically involved convective circulation of fluid derived from ordinary meteoric groundwaters around crystallizing calc alkaline epizonal plutons emplaced into coeval volcanic cover rocks these individual systems had widely ranging lifetimes of 10 3 to 10 6 years and operated locally throughout the cenozoic although the most profound development of such activity occurred during eocene time individual systems varied in size from a few tens of square kilometers yankee fork idaho or less to several thousand square kilometers sawtooth and castro ring zones idaho typically regional propylitization aacompanied the fluid circulation although the higher temperature alteration assemblages were developed locally as were intense alteration effects e g silicification sericitization etc near some veins and in mining districts a significant amount probably 25 50 of the mineral production and potential in the region is closely related to tertiary meteoric hydrothermal systems oxygen and hydrogen isotopic data clearly demonstrate the close geologic association of meteoric hydrothermal systems and mineralization in 1 the paleocene cu zn pb mn main stage mineralization at butte montana 2 numerous eocene epithermal deposits principally valued for au and ag but also including significant deposits of cu pb zn f sb etc as at republic washington and in several mining districts in the idaho batholith and the challis volcanic field 3 several eocene skarn deposits valued for w ima idaho and cu mackay idaho 4 important lead silver vein and replacement deposits of tertiary bluebell british columbia and of probable cretaceous and early tertiary age wood river idaho 5 several potentially economic mo be and u bearing eocene porphyry plutons and 6 miocene epithermal deposits most prominently the au and ag bearing veins at silver city and delamar idaho the hg deposits at the mcdermitt caldera nevada and oregon and at weiser idaho and au deposits in the western cascade range and lake county oregon a close spatial association has been demonstrated between ore deposits and rocks having anomalous 18 o values and low d values the most important deposits are associated with relatively small generally 5 300 km 2 zones of low 18 o values and they are particularly closely linked with zones of very steep 18 o 16 o gradients in the altered rocks these associations hold much promise for the use of 18 o and d contour maps in future exploration efforts

high temperature fluid rock interactions in a layered syenite pluton variation in mineral composition with stratigraphie height in layered igneous intrusions is generally taken to indicate progressive fractional crystallization and following pioneering work on the skaergaard complex1 petrologists have used this variation in modelling the evolution of magma bodies in cooling plutons however major chemical changes may occur in circulating deuteric fluids2 3 in the klokken south greenland layered syenite we have discovered that mineral compositions vary both along and normal to the strike of individual layers and we argue that major chemical changes have occurred in a circulating aqueous fluid constrained to move along relatively permeable high temperature aquifers in the layered series

fluid rock interaction in the miocene post caldera tejeda intrusive complex gran canaria canary islands insights from mineralogy and o and h isotope geochemistry the intra caldera volcaniclastic deposits of the miocene tejeda caldera on gran canaria host an similar to 12 km diameter intrusive complex comprising a peralkaline trachytic to phonolitic cone sheet swarm surrounding a central core of hypabyssal syenite stocks both intrusive rock types display textural and mineralogical features indicative of secondary fluid rock interaction including 1 deuteric mineral phases e g aegirine alkali amphibole analcime 2 turbid alkali feldspars and 3 hydrothermal mineral phases phyllosilicates fe ti oxides mn oxides and quartz altered cone sheets have whole rock delta o 18 values ranging from 0 center dot 1 to 10 center dot 0 parts per thousand n 22 and whole rock delta d values between 62 and 149 parts per thousand n 28 three altered syenite samples have whole rock delta o 18 values of 2 center dot 5 1 center dot 5 and 0 center dot 9 parts per thousand and corresponding delta d values of 91 99 and 121 parts per thousand the h2o concentrations of the altered cone sheets range from 0 center dot 4 to 0 center dot 8 wt n 28 and the altered syenites have h2o concentrations of 0 center dot 5 0 center dot 5 and 0 center dot 6 wt respectively the majority of altered samples are depleted in o 18 relative to the typical delta o 18 range for unaltered trachytes and syenites delta o 18 6 8 parts per thousand indicative of interaction with local meteoric water delta o 18 c 8 parts per thousand at temperatures epsilon 150 degrees c only one cone sheet sample appears petrographically unaltered and has a typical igneous isotopic composition delta o 18 7 center dot 1 parts per thousand delta d 48 parts per thousand and a relatively high h2o concentration 2 center dot 2 wt a weak correlation r 0 center dot 55 between delta d and h2o is observed in the cone sheets reflecting the combined effects of magmatic h2o exsolution and subsequent deuteric and hydrothermal alteration no systematic variation in delta o 18 or delta d was detected across the cone sheet swarm most likely reflecting overprinting of isotopic compositions during successive intrusive events however the highest delta o 18 values 8 center dot 2 10 center dot 0 parts per thousand occur in clay bearing cone sheets from the central part of the intrusive complex suggesting enhanced infiltration of relatively cool meteoric water in this area overall at least three phases of fluid rock interaction can be distinguished 1 deuteric alteration c 300 500 degrees c by late magmatic fluids expelled from a solidifying crystal mush 2 hydrothermal alteration epsilon 150 300 degrees c by meteoric water during the final stages of crystallization and or immediately following solidification of the intrusive complex 3 retrograde alteration related to the influx of relatively cool 150 degrees c meteoric waters

the lac des iles palladium deposit ontario canada part i the effect of variable alteration on the offset zone the recently discovered offset zone of the mine block intrusion of the lac des iles complex hosts palladium mineralization with unusually high pd pt and pd ir ratios in rocks that range from relatively unaltered norite to amphibolites and chlorite actinolite talc schist quantitative assessment of the effect of progressive alteration using mineral modes was done using total silicate h2o as a monitor of reaction progress moles h2o added to form alteration minerals per 100 g of rock major mineral modal variations define three reaction regions 1 0 00 0 03 characterized by epidote clinozoisite formation and some amphibole 2 0 03 0 23 characterized by formation of chlorite amphibole quartz muscovite sericite and calcite after plagioclase pyroxene and 3 0 23 0 28 characterized by the formation of talc after earlier formed amphibole epidote occurs as an incongruent product from the destruction of plagioclase that is itself lost as the reaction proceeds pyroxene is altered at about twice the rate of plagioclase resulting in pyroxene rich protoliths to be more altered than those relatively enriched in plagioclase major elements variations largely reflect variations in the plagioclase pyroxene ratio of the protolith but compositional trends suggest a loss of na with reaction progress the base metal sulfides chalcopyrite pyrrhotite and pentlandite show decreasing abundance with reaction progress forming pyrite magnetite as an intermediate reaction product that also is lost as the reaction proceeds millerite is overall low but increases slightly a more limited data set on the platinum group minerals suggests that platinum group element pge arsenides increase whereas pge sulfides and pge bi tellurides decrease with reaction progress assuming ore element concentrations in the protolith were constant and similar to relatively fresh norites pd increases modestly by 5 whereas pt decreases by about 65 in the most altered rocks similarly cu au and s decrease by 60 82 and 94 respectively in the most altered rocks the antithetical behavior of pd and the fact that pd enrichment is not seen in altered dikes suggest that the pd originally was enriched in the more melanocratic protoliths that are most extensively altered and that pd also was lost in part these results are consistent with the mineralization having been a high temperature event that predated the amphibolite greenschist alteration

the rapakivi granites of south greenland stable isotope characteristics of their black and white facies and the nature of their protolith abstract the rapakivi granites of south greenland were emplaced into the ketilidian orogenic belt at about 1 74 ga during an episode of extensional tectonics petrographically they can be subdivided into a roughly subequal black facies and white facies distinguished by the colour turbidity of the alkali feldspars and by the associated mineralogy which is more hydrous i e amphibole and biotite compared to olivine and pyroxene in the white facies the oxygen isotope composition of these rocks is notably homogeneous from thirteen outcrops sampled over an area extending some 200 km 100 km all alkali feldspars have an average 18 o of 10 2 0 4 this lack of variation is consistent with the consensus petrogenetic model which invokes a very well mixed two component protolith the high abundance of 18 o mean whole rock 18 o 9 4 argues that the major contribution perhaps 90 although originally mantle derived had undergone 18 o enrichment in a low temperature sedimentary cycle there is no oxygen isotopic distinction between alkali feldspars in the black and white facies which is argued to be a consequence of the relatively anhydrous nature of the melt hydrogen isotope compositions vary widely by 30 but are coherent on a hand specimen scale as demonstrated by approximately equal d of biotite and alkali feldspar it is not clear at which stage the d h heterogeneity was introduced

the malanjkhand copper molybdenum deposit india mineralization from a low temperature ore fluid of granitoid affiliation copper and subordinate molybdenum mineralization at malanjkhand occurs within a fracture controlled quartz reef enclosed in a pink granitoid body surrounded by grey granitoids constituting the regional matrix sulfide bearing stringers pegmatites with only quartz microcline and sulfide disseminations all within the pink granitoid represent other minor modes of occurrences despite this diversity in mode of occurrence the mineralogy of ores is quite consistent and conform to a common paragenetic sequence comprising an early ferrous stage of precipitation of magnetite i and pyrite i and the main stage chalcopyrite mineralization with minor sphalerite pyrite ii magnetite ii molybdenite and hematite both stages witnessed continuous precipitation of quartz microcline chlorite biotite and epidote the enclosing pink granitoid and the regional grey granitoids display alteration features such as saussuritization of plagioclase breakdown of hornblende and chloritization of biotite on a regional scale indicating interaction with a pervasive fluid quartz and microcline precipitation mostly restricted within the pink granitoid postdates this alteration four types of primary inclusions were encountered in quartz from ore samples 1 type i aqueous biphase l v inclusions the commonest variety in all ore types 2 type ii aqueous carbonic laq lcarb vcarb 3 type iii pure carbonic lcarb vcarb type ii and iii being restricted to stringer and pegmatitic ores and 4 rare polyphase laq vaq calcite gypsum inclusions quartz in granitoids contain primary type i inclusions only type i inclusions from ore samples furnish a temperature range after a rough pressure correction to the th maxima of 140 180 c of 150 275 c and a moderately low salinity of 4 12 wt nacl equivalent this is inferred to represent the signature of the major component f2 of the ore fluid a few type i inclusions of higher tm up to 380 c and low salinity and density represent the other f1 identifiable component of the ore fluid present in low proportion the th maxima and the total range in salinity of type i inclusions in quartz from granitoids are strikingly similar to those from the ore samples composition of syn ore chlorites furnished a temperature range of 185 327 c which conforms to the fluid inclusion microthermometric data pressure estimates using standard fluid inclusion geobarometric methods vary from 550 to 1790 bar in the stringer ores observed temperature salinity density relationships are best explained by a two stage evolution model of the ore fluid the first stage witnessed mixing of the two components f1 and f2 in unequal proportion bringing about mineralization the second stage of evolution was marked by the separation of a carbonic component on continued sulfide precipitation and attendant increase in salinity of the fluid the f1 component emerged as a distinct heated and co2 s charged entity due to steam heating and contamination of the early ingressed f2 fluid at the fracture zone the pervasive fluid phase in the surrounding granitoids contributed the f2 component

argon diffusion domains in k feldspar i microstructures in mh 10 samples of mh 10 a k feldspar thought to contain discrete diffusion domains for argon have been examined by light and tem microsopy to search for domain boudaries we examined both unheated k feldspar separates and samples heated in vacuum between 750 and 1150 c in order to characterize the initial microstructure as well as any changes resulting from laboratory heating the mh 10 rock shows almost no evidence of plastic deformation and the k feldspar does not possess networks of dislocations which might define a subgrain structure derived from external stresses three classes of substructure are present in the k feldspar but only the third appears to have been affected by laboratory heating 1 cross hatched extinction in light microscope is common but variably developed tem reveals almost no albite pericline twinning but tweed microstructure is ubiquitous 2 turbid zones occur with about 5 volume abundance micropores 1 to 2 m in diameter characterize these regions which often also contain bleds of albite up to 40 m indiameter tem shows these turbid zones to be very complex with intricate twin and tweed structures at the sub micron scale and numerous dislocation and strain features 3 albitic exsolution lamellae 1 m thick 20 m long and separated by 1 m are rarely seen in the light microscope but tem shows that 0 01 m thick by 0 2 1 m long lamellae separated by 0 1 to 0 5 m occur in about 20 of the sample these lamellae are disk shaped have a semi coherent or coherent relationship to the host k feldspar and seem to disappear upon heating to 750 c extensive diffusion studies have been conducted on hm 10 and the results analyzed in terms of distribution of sizes of diffusion domains the preferred result of the analyses was a distribution consisting of three distinct sizes the largest domain size identified in this way is approximately 50 to 100 m and this corresponds to blocks of k feldspar defined by the network of fractured turbid zones the smallest domain size inferred from diffusion analysis is 0 1 m which corresponds to the spacing between albite exsolution lamellae that may represent fast diffusion pathways alternatively the modulated argon distribution accompanying the lamellar variation in feldspar composition may result in an overestimation of the initial diffusion coefficients even though a promising agreement exists between domain analyses based upon diffusional and microstructural analyses at least two questions remain unanswered firstly no clear candidate has been identified for a domain of intermediate size secondly the diffusion analysis has been formulated with independent gas release from all domains whereas the observed structures are likely to be nested i e small domains exist inside bigger domains with the network of semi coherent lamellar boundaries having connectivity and permeability which can provide only a modest increase in argon diffusivity over bulk values

structure of a hypovolcanic acid complex inferred from magnetic susceptibility anisotropy measurements the western red hills granites skye scotland thulean igneous province the geochemistry and the injection mechanism of hypovolcanic ring dykes have been extensively studied but such is not the case for their internal fabric the tertiary western red hills epigranites of the isle of skye are a classic example of such intrusions using anisotropy of magnetic susceptibility measurements we present the first structural data of their internal magmatic fabric the magnetic foliations equated with the magmatic flow planes have strikes which roughly follow the walls of the different intrusions they dip steeply toward the convex wall of each intrusion the lineations or maximal magnetic susceptibility axes generally have shallow plunges except in the latest granite intrusion these structures appear to be related to the compressional deformation of each intrusion toward the end of its crystallization this shortening would be a consequence of a radial and compressive stress field acting after each injection of magma this radial stress field is interpreted as the effect of high magma pressures originating from the acid magma chamber underlying the ring dyke complex at a shallow depth

hydrothermally altered peraluminous archaean granites as a provenance model for witwatersrand sediments hydrothermal alteration of peraluminous granitic rocks occurring at varkenskraal northwest of the witwatersrand basin consists of two major styles the first style pervasive in nature is represented by two stages i a propylitic alteration and ii a disseminated sericitization both alteration stages were accompanied by a deposition of disseminated sulphides the second style is represented by a vein related alteration in four stages including potassium feldspar minor carbonate and minor sulphide mineralization nodules of uraniferous carbonaceous matter and small amounts of particulate gold could possibly be associated with either one or both styles hydrothermally altered rocks of similar nature have been observed in a number of other localities close to the witwatersrand basin their abundance large volumes and low grade disseminated pyrite gold mineralization indicate that rocks of this type could have been the provenance of the witwatersrand gold bearing sediments

evolution and modulation of a poleward propagating anticyclonic eddy along the japan and kuril kamchatka trenches to investigate the relationships between the movement of an eddy and its interior structure and water properties four profiling floats were deployed in an anticyclonic eddy in the western north pacific in 2013 april october daily float profiles showed rapid changes in temperature and salinity corresponding to strong interactions between eddies north of the subtropical kuroshio extension after the first interaction with a warm core eddy in april the isolation of the winter mixed layer from the surface was observed forming a subsurface remnant layer another interaction with a cold fresh eddy at middepths in may resulted in the formation of a multilayer structure the eddy then moved poleward along the japan and kuril kamchatka trenches indicating changes in its propagation pattern coupled to its interior structure the eddy then moved northward june july stalled july august and moved eastward august october in addition to a general declining trend the properties of the warm saline core changed over a short time period coinciding with changes in propagation a density anomaly at middepths of the eddy changed location during the stalled period however denser waters were continuously observed in the southeast part of the eddy during its northward and eastward movement this unidirectional density anomaly pattern was consistent with the structure of the poleward propagating eddy which interacted with the western topographic boundary meridional exchanges of heat and material were potentially elevated by the eddy s advection and movement as well as by water modifications in the eddy associated with exchanges along its perimeter

patterns and processes of oxygen isotope exchange in a fossil meteoric hydrothermal system cuillins gabbro complex isle of skye scotland oxygen isotope compositions of whole rock specimens and mineral separates from the cuillins gabbro complex isle of skye scotland are employed to determine the patterns and processes of18o depletion in the outer unlayered gabbro oug and associated dikes whole rock 18o values range from 4 8 to 1 1 smow and dike 18o values range from 4 7 to 2 8 mineral separates from three oug samples yield 18o values from 5 3 to 4 8 for augite and 4 1 to 0 8 for plagioclase an early small scale hydrothermal circulation system was initiated by the oug prior to the large scale hydrothermal convection established by the later layered cuillins complex lcc dikes were emplaced in the oug after intrusion of the lcc and had only a minor effect on hydrothermal circulation in the oug there is evidence of enhanced fluid flow along dike gabbro contacts isotopic compositions of augite separates demonstrate a normal 18o value for the oug magma with all18o depletion in the oug due to subsolidus exchange processes including diffusion and surface reaction the mineral separates yield a pattern of18o depletion consistent with a diffusion mechanism the bulk of the exchange having occurred in the plagioclase secondary mineral formation played a subordinate role in the18o depletion of the oug the calculated water to rock mass ratio necessary to effect the observed18o depletion in the oug is on the order of 0 2 although a much greater amount of water circulation probably occurred the cooling duration required to explain the measured18o depletion in the oug by diffusion is very short 140 years at 750 c 2400 years at 550 c compared to the duration necessary for pure conductive cooling 105 to 106 years rapid local cooling rates in the oug due to meteoric water convection are consistent with the observed18o depletion in oug samples

alteration of the oceanic lower crust at a slow spreading axis insight from vein related zoned halos in olivine gabbro from atlantis massif mid atlantic ridge incipient stage alteration products in relatively fresh oceanic gabbros from deep boreholes provide critical information on hydration processes in the oceanic lower crust and their effect on lithosphere dynamics we present the results of a petrographic study on the alteration of olivine bearing gabbroic rocks recovered from the deeper parts of integrated ocean drilling program iodp hole u1309d in the atlantis massif near the mid atlantic ridge at 308n in these rocks alteration is localized in proximity to fluid infiltration veins or igneous contacts it is most conspicuous in halos surrounding amphibole chlorite veins or leucocratic veins in olivine bearing gabbros where coronitic fringes of tremolite chlorite and talc occur around discrete olivine grains many of the halos exhibit a zonal pattern with systematic changes in mineral assemblage generally consisting of three zones tremolite chlorite around relict olivine plagioclase contacts talc pseudomorphs after olivine and tremolite pseudomorphs after olivine the tremolite chlorite assemblage appears in increasing amounts and talc grows unevenly with increasing thickness toward the veins the alteration minerals have highly magnesian compositions reflecting the compositions of the precursor igneous phases within the zone closest to the veins green hornblende with a relatively high al content occurs showing textures suggestive of its later formation than the coronitic tremolite and chlorite considering the mode of occurrence and chemical composition of the minerals combined with thermodynamic calculations of silica and water activities in a simplified system we conclude that the zoned halos were caused by metasomatism owing to protracted or sequential infiltration of hydrothermal fluids at amphibolite facies conditions 450 7508c 1 5 2 kbar textural relationships clearly indicate that zoned halos formed earlier than serpentinization and clay mineral formation and suggest that the high temperature amphibolite facies alteration took place in a near axis region before the exhumation of the lower crustal rocks recent results of seafloor drilling have provided supporting evidence for the predominance of gabbroic rocks in oceanic core complexes the similarity in mineral association between zoned halos and schistose fault rocks suggests that preferential formation of talc and or chlorite rather than serpentine at contacts between gabbroic rocks and peridotite plays an essential role in detachment faulting and tectonic exhumation of oceanic core complexes from lower crustal levels

reply to comments on the kinetics of textural equilibrium in forsterite marbles by m b holness m j bickle and c m graham

stable isotope relations in an open magma system laacher see eifel frg 18o 16o and d h ratios have been measured for matrix glasses and phenocrysts from the zoned phonolitic laacher see tephra sequence 11000 y b p east eifel volcanic field frg to study open system behaviour of the associated magma system mineral and glass 18o values appear to be largely undisturbed by low temperature secondary alteration record isotopic equilibrium and confirm previous conclusions based on radiogenic isotope evidence of early small scale crustal assimilation during differentiation of parental magmas in a crustal magma chamber one sanidine glass pair possibly documents the late stage influx of meteoric fluids into the topmost magma layer prior to eruption a sealing carapace of chilled magma which itself was strongly contaminated prevented large scale fluid exchange up to the point prior to eruption when this carapace was fractured and meteoric water gained access to parts of the magma system d h measurements of various glass types glass inclusions dense and pumiceous glass and amphiboles gave conflicting results suggesting a combination of degassing volatile exchange with country rocks and hydration stable isotope ratios for primitive parental magmas 18o 5 5 to 7 0 and mantle megacrysts 18o 5 5 to 6 0 d 21 to 38 for amphiboles and phlogopite resp suggest a rather variable fluid composition for the sub eifel mantle

oxygen isotope geochemistry of hydrothermally altered synmetamorphic granitic rocks from south central maine usa hydrothermally altered mesozonal synmetamorphic granitic rocks from maine have whole rock 18o smow values 10 7 to 13 8 constituent quartz feldspar and muscovite have 18o in the range 12 4 to 15 2 10 0 to 13 2 and 11 1 to 12 0 respectively mean values of q f 18oquartz 18ofeldspar 2 4 and q m 18oquartz 18omuscovite 3 3 are remarkably uniform standard deviations of both are 0 2 measured q f and q m values demonstrate that the isotopic compositions of the minerals are altered from primary magmatic 18o values but that the minerals closely approached oxygen isotope exchange equilibrium at subsolidus temperatures analyzed muscovites have d smow values in the range 65 to 82 feldspars in the granitic rocks are mineralogically altered to either a muscovite calcite b muscovite calcite epidote c muscovite epidote or d muscovite only a consistent relation exists between the assemblage of secondary minerals and the oxygen isotope composition of whole rocks quartz and feldspar rocks with assemblage a have whole rock 18o 12 1 and contain quartz and feldspar with 18o 13 8 and 11 4 respectively rocks with assemblages b c and d have whole rock 18o 11 4 and contain quartz and feldspar with 18o 13 1 and 11 0 respectively the correlation suggests that the mineralogical alteration of the rocks was closely coupled to their isotopic alteration three mineral thermometers in altered granite suggest that the hydrothermal event occurred in the temperature range 400 150 c 100 150 c below the peak metamorphic temperature inferred for country rocks immediately adjacent to the plutons calculations of mineral fluid equilibria indicate that samples with assemblage a coexisted during the event with co2 h2o fluids of text x text co text 2 0 03 0 13 and 18o 10 8 to 12 2 while samples with assemblages b c or d coexisted with fluids of text x text co text 2 leqslant 0 03 and 18o 9 4 to 10 1 compositional variations of the hydrothermal fluids were highly correlated fluids enriched in co2 were also enriched in 18o because co2 was added to the granites during hydrothermal alteration and because fluids enriched in co2 were enriched in 18o some or all of the variation in 18o of altered granites may have been caused by addition of 18o to the rocks during the hydrothermal event the source of both the co2 and 18o could have been high 18o metasedimentary country rocks the inferred change in isotopic composition of the granites is consistent with depletion of the metacarbonate rocks in 18o close to the plutons and with large volumes of fluid that were inferred from petrologic data to have infiltrated the metacarbonate rocks during metamorphism a close approach of minerals to oxygen isotope exchange equilibrium in altered mesozonal rocks from maine is in marked contrast to hydrothermally altered epizonal granites whose mineral commonly show large departures from oxygen isotope exchange equilibrium the difference in oxygen isotope systematics between altered epizonal granites and altered mesozonal granites closely parallels a differences between their mineralogical systematics both differences demonstrate the important control that depth exerts on the products of hydrothermal alteration deeper hydrothermal events occur at higher temperature and are longer lived minerals and fluid have sufficient time to closely approach both isotope exchange and heterogeneous chemical equilibrium shallower hydrothermal events occur at lower temperatures and are shorter lived generally there is insufficient time for fluid to closely approach equilibrium with all minerals