Variation in river multi-element chemistry related to bedrock buffering: an example from the Adirondack region of northern New York, USA

Water from four north-flowing rivers (Oswegatchie, Grasse, Raquette, and West Branch of the St. Regis) traversing three distinct geologic terranes (Adirondack Highlands, Adirondack Lowlands, St. Lawrence Valley), originating in the acidified northern Adirondack region, display rapid spatial changes in downriver multi-element chemistry and physical parameters. Downriver, most soluble elements increase (Ba, Ca, Cu, K, Mg, Na, Rb, S, and Sr) while statistically significant decreases in insoluble elements (Al, Ce, Dy, Er, Fe, Gd, La, Mn, Nd, Pr, and Y) also occur. Lithium, Si, and Zr did not show a consistent increasing or decreasing trend. Concentrations of most elements measured on June 5, 2008 were greater than 7?weeks later; however, greater discharge and lower pH enhanced Al concentrations occurred during the later sampling date. Elemental ratios track changes in lithology, anthropogenic influence, and pH. The Raquette River shows the least variation in elemental concentrations because of storage in numerous hydropower reservoirs and has non-detectable concentrations of some redox sensitive elements such as Co, Ni, and V. Oswegatchie tributaries display similar geochemical trends dependent upon their location and local bedrock and show geochemical trends along the trunks of major rivers that are also evident in smaller drainage basins. Comparison with analytical results from the Western Adirondack Stream Survey also indicates acidification is prevalent in the Adirondack Highlands, particularly in the Oswegatchie headwaters during periods of high flow, but acidity is rapidly buffered downriver due to interaction with marble in the Adirondack Lowlands.     

Debris slide hazards in the Adirondack Province of New York state

More than 400 debris slide scars have been identified in the Adirondack Province through aerial photographic interpretation. Debris slides are concentrated in the high relief, east-central portion of the province, although some isolated scars occur in the north and southwest. Intense rainfall, relief, slope angle, slope form, and slope exposure are recognized as factors in Adirondack slide formation and distribution. Vegetational, pedological, and geological characteristics also influence slope stability conditions, but complexities in after-the-fact analyses obscure their exact function in slide activity. On the basis of slope angle and slope form, more than 600 major valley systems which exhibit slide potential were identified in the Adirondacks. Approximately 25 percent of these sites of potential slides are on private land. The prospect for higher land-use in these areas warrants serious consideration of the hazards of debris slides. Debris slides cannot be prevented, or even accurately predicted, but certain measures may possibly be taken to avert extensive economic damage or loss of life.     

Genesis of metapelitic migmatites in the Adirondack Mountains, New York

Oxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. Values of δ¹⁸O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (～1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite facies NW Adirondacks and the granulite facies SE Adirondacks, δ¹⁸O_(Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is typically unzoned and δ¹⁸O_(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ¹⁸O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ¹⁸O_(Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ¹⁸O_(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ¹⁸O metasedimentary rocks.     

Origin of biotite-hornblende-garnet coronas between oxides and plagioclase in olivine metagabbros,Adirondack region,New York

Complex multivariant reactions involving Fe-Ti oxide minerals, plagioclase and olivine have produced coronas of biotite, hornblende and garnet between ilmenite and plagioclase in Adirondack olivine metagabbros. Both the biotite (6–10% TiO₂) and the hornblende (3–6% TiO₂) are exceptionally Titanium-rich. The garnet is nearly identical in composition to the garnet in coronas around olivine in the same rocks. The coronas form in two stages:

1. Plagioclase+Fe-Ti Oxides+Olivine+water =Hornblende+Spinel+Orthopyroxene±Biotite +more-sodic Plagioclase
2. Hornblende+Orthopyroxene±Spinel+Plagioclase =Garnet+Clinopyroxene+more-sodic Plagioclase

The Orthopyroxene and part of the clinopyroxene form adjacent to olivine. Both reactions are linked by exchange of Mg²⁺ and Fe²⁺ with the reactions forming pyroxene and garnet coronas around olivine in the same rocks. The reactions occur under granulite fades metamorphic conditions, either during isobaric cooling or with increasing pressure at high temperature.     

Carbon isotope thermometry in marbles of the Adirondack Mountains, New York

Abstract Carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the Adirondack Mountains, New York. Eighty-nine calcite-graphite pairs from the amphibolite grade NW Adirondacks change systematically in temperature north-westwards from 680 to 640 to 670° C over a 30-km distance, reflecting transitions from amphibolite facies towards granulite facies to the north-west and to the south-east. Temperature contours based on calcite-graphite thermometry in the NW Adirondacks parallel mineral isograds, with the orthopyroxene isograd falling above 675° C, and indicate that regional metamorphic temperatures were up to 75° C higher than temperatures inferred from isotherms based on cation and solvus thermometry (Bohlen et al. 1985). Fifty-five calcite-graphite pairs from granulite grade marbles of the Central Adirondacks give regional metamorphic temperatures of 670–780° C, in general agreement with cation and solvus thermometry. Data for amphibolite and granulite grade marbles show a 12%oo range in δ¹³C_cal and δ¹³C_gr. A strong correlation between carbon isotopic composition and the abundance of graphite (C_gr/C_rock) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in δ¹³C _vs. C_gr/C_rock and δ¹³C_cal vs. δ¹³C_gr could not have been preserved if significant amounts of CO₂-rich fluid had pervasively infiltrated the Adirondacks at any time. The close fit between natural data and calculated trends of δ¹³C vs. C_gr/C_rock indicates a biogenic origin for Adirondack graphites, even though low δ¹³C values are not preserved in marble. Delamination of 17 graphite flakes perpendicular to the c-axis reveals isotopic zonation, with higher δ¹³C cores. These isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature, and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite. Samples located >2km from anorthosite show a decrease of 0.5-0.8%oo in the outer 100 μ of the grains, while samples at distances over 8 km show smaller core-to-rim decreases of c.0.2%oo. Correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism. Core graphite compositions indicate contact metamorphic temperatures were 860–890° C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (Δ(_cal-gr) <3.2%oo) are not included, then the remaining 38 granulite facies samples define the relation Δ¹³C(_cal-gr) = 3.56 ± 10⁶T^-2 (K).     

The origin of olivine-plagioclase coronas in metagabbros from the Adirondack Mountains, New York

Olivine-plagioclase coronas in metagabbros from the Adirondack Mountains, New York (USA) are spatially well-organized reaction textures consisting most commonly of sequential layers of orthopyroxene, clinopyroxene, plagioclase, and garnet; the textures are characteristic of diffusion-controlled reaction kinetics. Although similar coronas have been interpreted by previous workers in terms of an isochemical steady-state diffusion model, petrographical relations and material-balance calculations establish that coronas in the Adirondack metagabbros cannot be treated as isochemical and do not form in a single-stage steady-state process; instead they evolve through time in a complex open-system reaction. In this study, the isochemical diffusion model is modified to account for elemental fluxes across the outer boundaries of the coronal reaction band, thereby approximating the open-system behaviour of the coronas. The sequence and relative proportions of product minerals calculated by the open-system steady-state model correspond closely to those observed in coronas of the Adirondacks, over a wide range of values for the relative diffusivities of chemical components involved in the reaction, regardless of the particular method used to determine material balance in the reaction texture. Despite this correspondence, petrographical evidence for successive replacement of coronal product layers reveals that the Adirondack coronas evolved through one or more transient states, rather than forming in a single-stage steady-state process. There is no evidence that the successive replacement of coronal product layers resulted from changes in pressure or temperature, but there is petrographical evidence that these changes resulted from modification of the composition of reactant plagioclase as the corona-forming reaction proceeded. This is confirmed by the fact that the evolution of the coronas over time can be replicated with the open-system diffusion model by simulating the effect of the gradual exhaustion of plagioclase as a source of the Ca and Si components required for reaction. These simulations suggest that successive stages in the evolution of the coronas are characterized by these product sequences: (i) orthopyroxene-clinopyroxene-plagioclase-garnet; (ii) orthopyroxene-clinopyroxene-garnet; and (iii) orthopyroxene-garnet. All of these stages, and the transitions between them, are observed petrographically. Coronas in Adirondack metagabbros appear, therefore, to have originated in a complex, open-system, diffusion-controlled reaction in which the product assemblages changed as the reaction progressed.     

Application of the titanium-in-quartz thermometer to pelitic migmatites from the Adirondack Highlands, New York

The 'TitaniQ' (Ti-in-quartz) solubility thermometer was applied to migmatitic metapelites from the southern and western Adirondack Highlands, New York, to examine the effect of granulite facies metamorphism on the distribution of Ti in quartz. Both cathodoluminescence imaging and quantitative traverses revealed that individual grains of Adirondack quartz are highly zoned with respect to Ti, and that core-to-rim decreases of Ti are common. Large ranges in calculated temperature were observed within each sample. One sample, not considered to be saturated with respect to TiO₂, gave maximum temperatures more than 100 °C lower than previously estimated peak temperatures. Rutile-saturated southern and western Adirondack samples yielded peak estimates of ≥803 ± 11 °C and ∼860–870 °C, respectively, which are similar to previous estimates from major phase thermometry. Minimum Ti-in-quartz matrix temperatures from rutile-saturated samples are 630 °C, which is interpreted as the closure temperature for Ti diffusion in quartz in these samples. This study demonstrates that Ti-in-quartz thermometry can yield details of rock evolution if the textural setting and reaction history of the quartz is clear, and can yield near-peak metamorphic temperatures in some cases, if care is taken to test for post-peak diffusional resetting.     

Geochemistry and origin of albite gneisses,northeastern Adirondack Mountains,New York

Albite gneisses containing up to 8.7 percent Na₂O and as little as 0.1% K₂O comprise a significant part of the Proterozoic Lyon Mountain Gneiss in the Ausable Forks Quadrangle of the northeastern Adirondacks, New York State. Two distinct types of albite gneisses are present. One is a trondhjemitic leucogneiss (LAG) consisting principally of albite (Ab95–Ab98) and quartz with minor magnetite and, locally, minor amounts of amphibole or acmiterich pyroxene. LAG probably originated by metamorphism of a rhyolitie or rhyodacitic ash-flow tuff with A-type geochemical affinities, following post-depositional analcitization in a saline or saline-alkaline environment. The other type is a mafic albite gneiss (MAG) containing albite and pyroxene along with 0–45 percent quartz, minor amphibole, and titanite. MAG locally displays pinstripe banding and contains albite (Ab98) megacrysts up to 5 cm across. Its precursor may have been a sediment composed of diagenetic analcite or albite, dolomite, and quartz. Both types of albite gneiss are interlayered with granitic gneisses (LMG) of variable composition derived from less altered tuffs. A potassium-rich (up to 9.7% K₂O) microcline gneiss facies may have had a protolith rich in diagenetic K feldspar. We propose that the albite gneisses and associated granitic gneisses are the granulite-facies metamorphic equivalent of a bimodal, dominantly felsic, volcanic suite with minor intercalated sediments, probably including evaporites. The volcanics were erupted in an anorogenic setting, such as an incipient or failed intracontinental rift. Deposition took place in a closed-basin, playa lake environment, where diagenetic alteration resulted in redistribution of the alkalis and strong oxidation.     

The Petrology and Geochemistry of Mafic Igneous Rocks in the Anorthosite-Bearing Adirondack Highlands, New York

Igneous rocks of broadly basaltic composition are widely distributedin the anorthosite-bearing Adirondack Highlands of New York.They constitute a mafic series of rocks that occurs on the marginsof the anorthosite series and up to 50 km away from the anorthosite.On an Sr reference diagram, the mafic series has an apparentinitial ⁸⁷Sr/⁸⁶Sr value of 0.7036, which is consistent with1100 Ma subcontinental mantle. The series was probably emplacedin the interval 1150–1100 Ma, during which time anorthositeand granitic magmas were also emplaced. The rocks were metamorphosedshortly thereafter ({small tilde} 1088 Ma) under upper-amphiboliteand granulite facies conditions. Individual bodies of maficrocks range in thickness from small enclaves and layers >>1 m thick to large lensoid masses several tens of meters thick.Despite deformation and metamorphic recrystallization, manyof the rocks retain chiHed margins, cross-cutting relationships,and relict igneous textures. Selected samples of the mafic series have been analyzed fortheir major and trace element compositions. Metamorphism didnot significantly alter the igneous geochemical relationships,and the rocks retain mantle-like values for Zr/Nb, K/Zr, K/Rb,Rb/Sr, and ⁸⁷Sr/⁸⁶Sr. The most primitive rocks of the seriesare silica-undersaturated gabbroic troctolites, and the moreevolved rocks are basaltic in composition. The mafic seriesas a whole has high abundances of A1₂O₃, FeO, the light rareearth elements (LREE), and other incompatible trace elements.Even the most geochemically primitive compositions have highFeO contents. The Fe enrichment and Si depletion that are shownby chemically evolved compositions are consistent with a Fennertrend of fractionation. Low levels of normative di indicatethat high Fe is not a result of the extensive fractionationof cpx. The geochemical trends that are defined by the traceelements, including the REE, suggest the basaltic rocks maybe differentiates of a parental magma of gabbroic troctolitecomposition. The main compositional trend of the mafic seriescan be simulated by 61% crystallization of olivine and plagioclaseof a gabbroic troctolite, followed by 13% crystallization ofolivine, plagioclase, clinopyroxene, and titanomagnetite atthe final stages. The modelled ratio of olivine to plagioclasecrystallization changes from 1–8: 1 to 0–64: 1.These non-cotectic ratios may reflect a delay in the crystallizationof plagioclase relative to olivine, possibly as a result oflow nucleation rates. At later stages of differentiation, plagioclasewas more important in the crystallization of the series. Delayedcrystallization of plagioclase may also have resulted in thehigh A1₂O₃ contents and enhancement of Eu and Sr relative toother trace elements at early stages of differentiation. The mafic series and the silica-saturated anorthosite seriestogether form an anorthosite-norite-troctolite (ANT) suite.More than one mantle composition may have been involved in generatingthe Adirondack mafic magmas. The rocks retain geochemical evidenceof a source that was depleted in basaltic components (cpx) butenriched in Fe, Ti, K, and the LREE. Previously documented evidenceof anticlockwise cooling paths (Bohlen, 1987) and of a regionalgravity high centered beneath the Adirondack region (Simmons,1964) suggests that of the continental crust by basaltic magmasmay have underplating of the continental crust by basaltic magmasmay have been an important feature of the tectonic evolutionofthe region. A model of mantle upwelling beneath thinning continentalcrust explains the geochemically hybrid nature of the maficseries magmas. It is also consistent with a tectonic settingof incipient or failed continental rifting, to which the generationof the anorthosites is commonly attributed.     

Chemistry and transport of soluble humic substances in forested watersheds of the Adirondack Park,New York

Studies were conducted in conjunction with the Integrated Lake-Watershed Acidification Study (ILWAS) to examine the chemistry and leaching patterns of soluble humic substances in forested watersheds of the Adirondack region. During the summer growing season, mean dissolved organic carbon (DOC) concentrations in the ILWAS watersheds ranged from 21–32 mg C l^?1 in O/A horizon leachates, from 5–7 mg C l^?1 in B horizon leachates, from 2–4 mg C l^?1 in groundwater solutions, from 6–8 mg C l^?1 in first order streams, from 3–8 mg C l^?1 in lake inlets, and from 2–7 mg C l^?1 in lake outlets. During the winter, mean DOC concentrations dropped significantly in the upper soil profile. Soil solutions from mixed and coniferous stands contained as much as twice the DOC concentration of lysimeter samples from hardwood stands. Results of DOC fractionation analysis showed that hydrophobia and hydrophilic acids dominate the organic solute composition of natural waters in these watersheds. Charge balance and titration results indicated that the general acid-base characteristics of the dissolved humic mixture in these natural waters can be accounted for by a model organic acid having an average_pK_a of 3.85, an average charge density of 4–5 μeq mg^?1 C at ambient pH, and a total of 6–7 meq COOH per gram carbon.     

Quartz–garnet isotope thermometry in the southern Adirondack Highlands (Grenville Province, New York)

Quartz–garnet oxygen isotope thermometry of quartz‐rich metasedimentary rocks from the southern Adirondack Highlands (Grenville Province, New York) yields metamorphic temperatures of 700–800 °C, consistent with granulite facies mineral assemblages. Samples from the Irving Pond quartzite record Δ¹⁸O(Qtz–Grt) = 2.68 ± 0.21‰ (1 s.d. , n = 15), corresponding to peak metamorphic conditions of 734 ± 38 °C. This agrees well with the estimates from garnet–biotite exchange thermometry. Similar temperature estimates are obtained from Swede Pond (682 ± 47 °C, n = 3) and King's Station (c. 700 °C). The Whitehall area records higher temperatures (798 ± 25 °C, n = 3). All of these temperatures are higher than previous regional temperature estimates. The c. 800 °C temperatures near Whitehall are consistent with preservation of pre‐granulite contact temperatures adjacent to anorthosite. The preservation of peak metamorphic temperatures in garnet of all sizes is consistent with slow oxygen diffusion in garnet, and closure temperatures of at least 730 °C. Peak metamorphic fractionations are preserved in rocks with varying quartz:feldspar ratios, indicating that the modal percentage of feldspar does not affect retrograde oxygen exchange in these rocks. The lack of this correlation suggests slow rates of oxygen diffusion in quartz and feldspar, consistent with the results of anhydrous oxygen diffusion experiments.     

Pollution records from sediments of three lakes in New York State

Sediment core segments from Sylvan Lake, Lake Champlain and Lake Canadarago were dated radiometrically with ²¹⁰Pb and ¹³⁷Cs. Their respective sedimentation rates were determined to be 0.11, 0.14 and 0.52 g cm^?2 yr^?1. For the two lakes of lower sedimentation the variations of selected elemental abundances as function of depth were analyzed. Two groupings were found: Al, K, Ti, Rb and Zr were correlated among themselves but reflected different variations in the input of terrigenous erosion material to the lakes. The Cu, Zn and Pb correlated among themselves showed similar depth dependence with increasing concentrations toward the top which can be attributed to cultural pollution. Recent ‘excess’ fluxes to the sediments above the natural contribution by clastic material were derived for the location of the cores, which for Cu, Zn and Pb amounted to 3.8, 24 and 16 μg cm^?2 yr^?1 respectively for Sylvan Lake and 4.9, 20 and 16 μg cm^?2 yr^?1 for Lake Champlain. The corresponding ²¹⁰Pb flux was 3.3 and 2.3 dpm cm^?2 yr^?1, respectively for the two lakes.Approximate residence times in the water column were obtained for trace metals at the Lake Champlain location. Short residence times estimated for Pb (< 0.15 yr) and Cu (< 0.4 yr) indicate fast removal, whereas those for Zn (1.0 ± 0.3 yr) and Cr (2.0 ± 0.5 yr) appeared to be dominated by the water residence time.     

Evidence for Differential Unroofing in the Adirondack Mountains, New York State, Determined by Apatite Fission-Track Thermochronology

Apatite fission-track ages of 168-83 Ma for 39 samples of Proterozoic crystalline rocks, three samples of Cambrian Potsdam sandstone, and one Cretaceous lamprophyre dike from the Adirondack Mountains in New York State indicate that unroofing in this region occurred from Late Jurassic through Early Cretaceous. Samples from the High Peaks section of the Adirondack massif yielded the oldest apatite fission-track ages (168-135 Ma), indicating that it was exhumed first. Unroofing along the northern, northwestern, and southwestern margins of the Adirondacks began slightly later, as shown by younger apatite fission-track ages (146-114 Ma) determined for these rocks. This delay in exhumation may have resulted from burial of the peripheral regions by sediment shed from the High Peaks. Apatite fission-track ages for samples from the southeastern Adirondacks are distinctly younger (112-83 Ma) than those determined for the rest of the Adirondack region. These younger apatite fission-track ages are from a section of the Adirondacks dissected by shear zones and post-Ordovician north-northeast-trending normal faults. Differential unroofing may have been accommodated by reactivation of the faults in a reverse sense of motion with maximum compressive stress, sigma1, oriented west-northwest. A change in the orientation of the post-Early Cretaceous paleostress field is supported by a change in the trend of Cretaceous lamprophyre dikes from east-west to west-northwest.     

Fluid flow and mass transport at the Valentine wollastonite deposit, Adirondack Mountains, New York State

The Valentine wollastonite skarn in the north-west Adirondack Mountains, New York, is a seven million ton deposit which resulted from channellized infiltration of H₂O-rich, silica-bearing fluids. The wollastonite formed by reaction of these fluids with non-siliceous calcite marble. The skarn formed at the contact of the syenitic Diana Complex and was subsequently overprinted by Grenville-age granulite facies metamorphism and retrograde hydrothermal alteration during uplift. Calcite marbles adjacent to the deposit have generally high δ¹⁸O values (c. 21‰), typical of Grenville marbles which have not exchanged extensively with externally derived fluids. Carbon isotopic fractiona-tions between coexisting calcite and graphite in the marbles indicate equilibration at 675d? C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14‰ variation (-1‰ to 13‰). Some isotopic heterogeneity is preserved from skarn formation, and some represents localized exchange with low-δ¹⁸O retrograde fluids. Detailed millimetre- to centimetre-scale isotopic profiles taken across skarn/marble contacts reveal steep δ¹⁸O gradients in the skarn, with values increasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high δ¹⁸O calcite to form wollastonite. Calcite in the marble preserves high δ¹⁸O values to within <5 mm of the skarn contact. The preservation of high δ¹⁸O values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during both the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts. Although these steep isotope gradients resemble fluid infiltration fronts, they actually represent the sides of the major flow system. Because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock, such models are not applicable to this hydrothermal system and, by extension, to many other metamorphic systems where low-permeability rocks restrict fluid migration pathways. Minimum time-integrated fluid fluxes have been calculated at the Valentine deposit using oxygen isotopic mass balance, reaction progress of fluid buffering reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silica mass balance calculations yield the largest minimum flux and are hence the most realistic.     

Pressure, Temperature and C-O-H Fluid Fugacities across the Amphibolite-Granulite Transition, Northwest Adirondack Mountains, New York

Pressures, temperatures, water activities (a_H2O) and fugacitiesof the other C-O-H fluid species have been estimated on a traverseacross the amphibolite-granulite facies boundary in the MajorParagneiss, northwest Adirondacks, N.Y. Two-feldspar pairs givetemperatures ranging from 650?C in the central portion of theunit to 760?C towards the northeast. Biotite-garnet pairs giveerratic temperatures compared to two-feldspar temperatures.This discrepancy appears to be due to retrograde resetting asdetermined from compositional zoning patterns in biotites andgarnets. Some of the discrepancy may also be due to non-idealityof pyrope-almandine mixing or to non-ideality from other components.Pressures ranging from 5?4 kb for the southwestern portion ofthe unit to 8?0 kb in the northeast were determined from anorthite-grossular-sillimanite-quartzbarometry. Minimum pressures of 5?8 kb were also determinedfrom coexisting garnet + rutile. Values of a_H2O of 0?08-0?5estimated from biotite and muscovite dehydration reactions showno correlation with grade. The variability in a_H2O suggeststhat it is locally controlled and that a homogeneous, pervasivefluid was not present during high grade metamorphism. Graphiteequilibria indicate that f_O2 was less than 0?5 log units belowQFM and that if a fluid was present, it was rich in CO₂ andH₂O. P-T-a_H2O values suggest that partial melting did not occurduring metamorphism. Pervasive flooding with CO₂ does not appearto have occurred. The amphibolite-granulite transition at thislocality is characterized by increasing temperature and pressure.     

Hornblende geothermometry of amphibolite layers of the Popple Hill gneiss, north-west Adirondack Lowlands, New York, USA

The exchange reaction tschermakite+2 diopside+2 quartz=tremolite+2 anorthite (HPCQ), in combination with the thermodynamic database TWQ (version 1.02, Berman, 1991), has been evaluated for its usefulness as a geothermometer. This reaction, which is both water conserving (independent of water fugacity) and which does not require the presence of garnet, is well suited for studying pyroxene‐bearing amphibolites. As an application of this geothermometer, we have re‐examined the amphibolites occurring in the Popple Hill gneiss of the Adirondack Lowlands of New York, USA, to better understand the magnitude of temperature variation preserved in the amphibolites themselves in this classic locality. At an assumed constant pressure of 7 kbar, the temperatures range from 619 to 682 °C from Edwards to Pierrepont and are uncorrelated with either distance along the strike of the region or with modal mineralogical variations. Hornblende exhibits a narrow compositional range suggesting that there has been little or no thermal gradient along the strike of the Lowlands. Temperatures recorded just north of Colton are, however, distinctly higher (694–758 °C). Although it is likely that the Popple Hill gneiss amphibolites experienced some effects of progressive metamorphism, particularly in the vicinity of Colton, the variations in modal mineralogy are most likely the result of such factors as local variations in the bulk chemistry of the protolith and in the fugacity of H₂O due to infiltration of diluting species (e.g. CO₂, CH₄), rather than a regional temperature variation. Temperatures recorded by the HPCQ geothermometer reported here are similar in magnitude and geographic trend to those reported for graphite–calcite carbon‐isotope thermometry by Kitchen & Valley (1995), suggesting that peak metamorphism in the Adirondack Lowlands involved laterally extensive and fairly uniform isotherms.     

Pressure, temperature and cooling rates of granulite facies migmatitic pelites from the southern Adirondack Highlands, New York

Detailed petrographic analysis was performed on samples from five localities within the southern Adirondacks. Textures and zoning patterns in garnet from all samples provide evidence for dehydration melting of biotite. Zoning of grossular in garnet – providing a record of prograde growth – shows both increasing and decreasing trends in garnet from the same sample. However, Ca concentrations at the garnet rims of most samples are identical (grossular = 3.4%). These observations have been interpreted as evidence for the differential timing of garnet nucleation and growth. All Fe/(Fe + Mg) and some spessartine distributions are consistent between samples, displaying diffusive profiles established largely upon cooling. Only one sample, in which retrogression was minimal, contains garnet with flat Fe/(Fe + Mg) profiles. A general pelitic pseudosection constructed in the system MnNCKFMASH reveals a maximum for Ca in garnet where the plagioclase‐out isopleth intersects the solidus (muscovite = 0). The pseudosection predicts bell‐shaped core‐to‐rim profiles of grossular during anatexis, similar to those observed in the rocks. Garnet–biotite thermometry and GASP barometry indicate peak temperatures of at least 790 °C at about 7–9 kbar, similar to conditions determined for the central Adirondacks. Cooling rates determined from finite difference modelling of spessartine and Fe/(Fe + Mg) diffusional profiles indicate a multi‐stage cooling history in which some period of rapid cooling (>200 °C Myr^?1) is required.     

Compositional controls on spinel clouding and garnet formation in plagioclase of olivine metagabbros,Adirondack Mountains,New York

Olivine metagabbros from the Adirondacks usually contain both clear and spinel-clouded plagioclase, as well as garnet. The latter occurs primarily as the outer rim of coronas surrounding olivine and pyroxene, and less commonly as lamellae or isolated grains within plagioclase. The formation of garnet and metamorphic spinel is dependent upon the anorthite content of the plagioclase. Plagioclase more sodic than An_38±2 does not exhibit spinel clouding, and garnet rarely occurs in contact with plagioclase more albitic than An_36±4. As a result of these compositional controls, the distribution of spinel and garnet mimics and visually enhances original igneous zoning in plagioclase. Most features of the arrangement of clear (unclouded) plagioclase, including the shells or moats of clear plagioclase which frequently occur inside the garnet rims of coronas, can be explained on the basis of igneous zoning. The form and distribution of the clear zones may also be affected by the metamorphic reactions which have produced the coronas, and by redistribution of plagioclase in response to local volume changes during metamorphism.Authors listed alphabeticallyPublished by permission of the Director, New York State Museum, Journal Series Number 299     

Experimental examination of two-pyroxene graphical thermometers using natural pyroxenes with application to metaigneous pyroxenes from the Adirondack Mountains,New York

Clinopyroxene with exsolved orthopyroxene and coexisting orthopyroxene with exsolved clinopyroxene (inverted pigeonite) in metaigneous rocks from the Adirondacks, New York, were experimentally homogenized at temperatures near those inferred for their original crystallization. The purposes were several: (1) to test the graphical two-pyroxene geothermometer of Lindsley (1983); (2) to test the hypothesis of Bohlen and Essene (1978) that these were originally igneous pyroxenes; and (3) to test whether modal recombination of complexly exsolved pyroxenes yields realistic compositions. Experiments on Fe-rich compositions at 930° and 870° C (1 GPa) are compatible with the graphical thermometer of Lindsley (1983); however, this graphical thermometer yields apparent temperatures approximately 50° C too high for experiments at 1050° C and 1100° C (0 MPa). This suggests that at intermediate Mg/Fe the augite isotherms for these temperatures lie at lower wollastonite compositions than shown by Lindsley. The results are, however, in good agreement with isotherms derived from the solution model of Davidson (1985). When these isotherms are applied to a variety of terrestrial and lunar igneous rocks and the metaigneous rocks from the Adirondacks, temperatures given by augite and pigeonite compositions from coexisting pairs are similar. Comparison of the experimentally homogenized compositions with modally recombined compositions of Bohlen and Essene (1978) show that discrepancies between augite and pigeonite temperatures may nevertheless arise if pyroxene grains formed by granular exsolution are not correctly reintegrated.