Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone,Adirondack Mountains,NY, USA

Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral ¹⁸O/¹⁶O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ¹⁸O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S₁) is crosscut by near‐vertical ultramylonitic shear zones (S₂), which are locally reoriented by a NNW‐dipping mylonitic fabric (S₃). Texturally early titanite (pre‐S₂) shows diffusion‐dominated δ¹⁸O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ¹⁸O profiles for cooling rates of 50 ± 20 °C Ma^?1, which are considerably faster than the 1–5 °C Ma^?1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S₂) has higher overall δ¹⁸O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ¹⁸O zoning, indicating infiltration of elevated‐δ¹⁸O fluids along S₂ shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S₃) has growth‐dominated δ¹⁸O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.     

Paleolimnological reconstruction of recent changes in assemblages of Cladocera from acidified lakes in the Adirondack Mountains (New York)

Remains of Cladocera were examined in short sediment cores from three Adirondack lakes with mean pHs below 5 and a fourth with a mean pH of 6.5. These cores were collected as part of the Paleoecological Investigation of Recent Lake Acidification (PIRLA I) project. Historical and paleolimnological evidence suggests that pH has decreased in each of the acid lakes in recent decades. In all of the study cores, the greatest changes in net accumulation rates, assemblage composition, and species richness occurred in recently deposited sediments. The similar timing of events in all lakes suggests that a regional disturbance was responsible. In the three acid lakes, there was a strong association of changes in cladoceran assemblages and diatom, chrysophyte, and geochemical evidence of acidification. The occurrence of recent changes in non-acid Windfall Pond indicates that other factors may also have affected Cladocera in the study lakes.This is the fifteenth of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D. F. Charles and D. R. Whitehead are guest editors for this series.     

Interpretation of210Pb profiles and verification of the CRS dating model in PIRLA project lake sediment cores

This paper reports results and analysis of²¹⁰Pb-activity measurements in 51 lake-sediment cores from 32 lakes in the four PIRLA (Paleoecological Investigations of Recent Lake Acidification) project regions (Adirondack Mountains [New York], Northern New England, Northern Florida, and the Northern Great Lakes States). General application of the Constant Rate of Supply (Constant Flux) model for²¹⁰Pb dating is valid for lakes in the PIRLA study, although application of the model is equivocal in a few lakes.²¹⁰Pb inventories and profiles are replicable among closely spaced cores within a lake. Specific²¹⁰Pb activity in surface sediments is negatively correlated with bulk sediment accumulation rate in seepage lakes, but not in drainage lakes. Drainage lakes with lower pH have lower unsupported²¹⁰Pb inventories in sediments, but the relationship does not occur in seepage lakes.²¹⁰Pb profiles in only seven of the cores, all from either the Adirondacks or the northern Great Lakes states, exhibit exponential decay curves. Deviations from an exponential profile include a flattening of the profile in the top few cm or excursions of one or a few measurements away from an exponential curve.²¹⁰Pb dates typically agree with other chronostratigraphic markers, most of which are subject to greater uncertainty. Several hypotheses, including sediment mixing, hydrologic regime, sediment focusing, and acidification, are proposed to explain variation of²¹⁰Pb distribution among lakes and regions. Hydrologic factors exert control on unsupported²¹⁰Pb inventories in PIRLA lakes, and there is a strong focusing effect in drainage lakes but a weak focusing effect in seepage lakes.This is the third of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

POSTFIRE SUCCESSION IN AN ADIRONDACK FOREST*

ABSTRACT. Landscape diversity has increased with the surprising postfire establishment of aspen at upper elevations (700–945 meters above sea level) in the High Peaks of Adirondack Park in upstate New York. Tree seedlings returned quickly to the charred slopes west of Noonmark Mountain after an accidental fire consumed the forest in 1999. Aspen stands have replaced the spruce‐fir‐birch forests in the burned area even though mountain paper birch is expected to colonize burned sites at these elevations. Environmental conditions, historical events, and unique circumstances help explain why quaking aspen and bigtooth aspen rather than paper birch blanket the burned mountainside. Climate change over the past century to warmer, wetter conditions may have fostered this marked shift in species composition. In the unburned firebreak that people cleared to contain the flames, pin cherry has regenerated from seeds stored in the soil for nearly a century. The history of pin cherry on the site suggests that large fires or severe windthrow may have been more common in the region than was previously documented.     

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.     

WINTER LAND BREEZE IN A HIGH LATITUDE COMPLEX COASTAL AREA

The winter land breeze (WLB) was studied in an area with complex topography on the Swedish West Coast (58°N). Methods used were mapping directions of smoke plumes, temperature soundings by tethered balloon and helicopter, three years of monitoring temperature and wind at three levels on a suspension bridge and on a mast, and meteorological observations from three stations located at different distances from the coast. To initiate the development of the WLB, the necessary temperature difference between sea and land was 5°C, but the magnitude of the difference appeared to have little influence. The direction of the typical WLB was at an oblique angle to the coast. The WLB started with a sudden wind shift as a result of frictional decoupling from the prevailing synoptic wind caused by increased stability. The flow, with a depth of about 100 m, developed within the nocturnal inversion at the level of the plateau-shaped mountains and overrode the cold air in the valleys and followed the uplifted inversion over the city of Göteborg. The different layers in the inversion were tilted toward the sea with an inclination similar to the slope of the terrain. In a layer above the WLB but also within the inversion, there was an onshore wind that might be the return flow to the WLB. [Key words: winter land breeze (WLB), decoupling, overriding, urban heat island circulation (UHIC), high latitude, Sweden.]     

The Buck Creek-Boreas River Adirondack watershed monitoring program

The Buck Creek-Boreas River Adirondack Watershed Monitoring Program, located in the Adirondack region of New York State, United States, combines the monitoring of headwater streams, soils, and vegetation based on a watershed design. Continuous monitoring of six watersheds is linked to the sampling of more than 400 additional Adirondack streams between 2003 and 2019 for chemical analysis of 14 constituents throughout the highly valued Adirondack ecoregion that covers an area of more than 24 000 km². Much of this landscape has a low capacity for acid buffering, but due to spatial variation in geologic features, some areas are moderately to well acid buffered. This program includes data that extends back to the early 1980s and is ongoing. The focus of the program is on the watersheds of headwater Adirondack streams. Soil, vegetation and stream data are used to better understand environmental effects on the linkages of these ecosystem components. Documentation of the long-term responses of Adirondack ecosystems to environmental disturbances such as acid rain, climate change and other unforeseen factors is the primary objective of the program.     

Granulite facies metamorphism, palaeo-isotherms and disturbance of the U-Pb systematics of zircon in anorogenic plutonic rocks from the Adirondack Highlands

Abstract In the Adirondack Highlands of New York State, the effect of granulite facies metamorphism on the physical and isotopic characteristics of zircon from anorogenic plutonic rocks has a distinct geographical pattern. The location of zircon populations which appear to have been altered describes a roughly circular area where metamorphic palaeotemperatures have been determined to be in excess of 750° C. Zircons from anorogenic plutonic rocks outside this area were undisturbed during metamorphism and yield well constrained ages. Granitic, charnockitic and mangeritic anorogenic plutonic rocks peripheral to the Marcy anorthosite massif have large, euhedral, prismatic zircons that display fine, internal, magmatic growth zonations and abundant, randomly orientated, mineral inclusions. Co-genetic zircon fractions yield linear discordant arrays and well constrained upper intercepts of 1125–1157 Ma. Metamorphic zircon is limited to sporadically developed and volumetrically insignificant, clear, low-U overgrowths or protuberances. In marked contrast, zircons from petrographically and geochemically identical rocks adjacent to, or within, the Marcy anorthosite massif are typically large, limpid, anhedral to subhedral crystals or crystal fragments lacking internal features except for tubular cavities and CO-₂-rich inclusions. Co-genetic zircon fractions yield nearly concordant, non-linear clusters with ²⁰⁷Pb/²⁰⁶Pb minimum ages of 1073–1095 Ma. Metamorphic overgrowths cannot be readily identified by optical or cathodoluminscence techniques; however, many grains show complex and unusual external boundaries suggestive of post-crystallization modification. These data indicate that temperatures as low as 750° C, in combination with other factors, may have been sufficient to facilitate recrystallization, and diffusion of radiogenic Pb from the zircon crystal structure, during the complex, protracted metamorphism of the Adirondack Highlands.     

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.     

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).