Susan M. Swapp

Acidic dissolution of plagioclase: In-situ observations by hydrothermal atomic force microscopy

Hydrothermal atomic force microscopy (HAFM) provides in situ access to the surfaces of dissolving crystals at temperatures above the ambient boiling point of water. Here, we applied HAFM to the (001) surfaces of labradorite and anorthite at temperatures up to 125°C. In HCl solutions (pH 2) we observed the formation of a rough and soft surface layer on both minerals. By applying high loading forces to the scanning tip, the soft layer can be removed and the underlying interface (between the fresh solid and the altered layer) can be observed. In this way, in situ information about the thickness of the altered layer on plagioclase and the morphology of the underlying interface can be obtained. On labradorite, the thickness of this layer does not exceed about 30 nm within the first 5 hr of exposure to acidic solution at 125°C, but on anorthite thicknesses of up to about 300 nm were observed. The uncovered interface on anorthite shows a nonuniform morphology and either appears rough in AFM images or shows a step-like pattern. On anorthite, etch pits spread underneath the altered layer. This suggests that material must be released and transported through the layer without obvious changes in morphology of the layer’s surface. Based on the rate of spreading of etch pits, the dissolution rate was calculated to be about 2 × 10−6 mol m−2 s−1 at 125°C. This value agrees reasonably well with literature data and supports the suggestion that dissolution mainly takes place underneath the altered layer and not on its surface.

Late Archean structural and metamorphic history of the Wind River Range: Evidence for a long-lived active margin on the Archean Wyoming craton

The Archean rocks of the Wind River Range in western Wyoming record a Late Archean history of plutonism that extends for more than 250 m.y. The range is dominated by granitic plutons, including the 2.8 Ga Native Lake gneiss, the 2.67 Ga Bridger batholith, the 2.63 Ga Louis Lake batholith, and late 2.54 Ga granites. These plutons provide a means of distinguishing the complex metamorphism and deformation that affected the range in the Late Archean. Five deformation events are recorded. D1 is a penetrative deformation that occurred during the earliest granulite-facies metamorphism; D2 is a folding event, probably in amphibolite facies, that deforms porphyritic dikes that cut the D1 fabrics. Both D1 and D2 predate the intrusion of the ca. 2.8 Ga Native Lake gneiss. D3 is a folding event, accompanied by upper amphibolite to granulite metamorphism, that deformed the Medina Mountain sequence, a sequence of rocks that was either deposited or thrust upon the Native Lake gneiss. D4 is a fabric-forming event associated with the Mount Helen structural belt (MHSB). It is represented by mylonites in the MHSB, a penetrative fabric in the Bridger batholith, and folding of the D3 structures in the Medina Mountain sequence. We consider D3 and D4 to be coeval with the emplacement of the Bridger batholith, and hence to date at ca. 2.67 Ga. The latest structures (D5) are fabrics associated with the folding and thrusting of the 2.65 Ga South Pass sequence. We recognize at least four metamorphic events. M1 is associated with the D1 fabrics and occurred at high T (>750°C) and high P (∼7–8 kilobars). M2 (650–750 °C and 4–5.5 kilobars) is associated with the intrusion of the Bridger batholith and formation of the D3 and D4 structures. The D5 structures of the South Pass sequence record M3, which is low P (∼2–3 kilobars) and low T (∼500°C). The final metamorphism, M4, is a contact metamorphism around the Louis Lake batholith. In the south against the South Pass sequence, the metamorphism occurred at ∼3 kilobars and at temperatures <700 °C. In contrast, in the north where the Louis Lake batholith is charnockitic, the metamorphism occurred at 6 kilobars and 800°C. This pressure gradient is probably a reflection of tilting of the Wind River block during the Laramide orogeny. The composition of the plutons and the structural and metamorphic history of the Wind River Range indicate that during the Late Archean this area occupied the active margin of the Wyoming province. This tectonic environment is similar to the long-lived Phanerozoic margins of North America. The Wind River Range represents the best-documented active margin of Archean age.

Catalytic regeneration of mercury sorbents

Traditionally, mercury sorbents are disposed of in landfills, which may lead to contamination of soil and groundwater. In this work, the regeneration of activated carbon (AC) as a mercury sorbent was investigated. The decomposition of HgCl2 on the surface of pure AC was studied, as well as sorbent which has been treated with FeCl3 or NaCl. In all cases, the sorbent is found to be structurally stable through a single regeneration, which is verified through BET, XRD, and XPS analysis. The desorption of mercury from the sorbent is found to follow Henrys law. Additionally, a kinetic analysis suggests that although the presence of activated carbon lowers the energy requirement for the desorption of mercury, it significantly decreases the rate by decreasing the concentration of the HgCl2. FeCl3 and NaCl both promoted the decomposition of HgCl2, but FeCl3 did so more significantly, increasing the rate constants by a factor of 10 and decreasing the activation energy for the decomposition of HgCl2 by 14% to 40%.

Hadean origins of Paleoarchean continental crust in the central Wyoming Province

The scarce remnants of Earth’s earliest history make it challenging to describe the crust-forming processes that operated during that time, and all evidence that survived subsequent tectonism and recycling deserves to be studied closely. We present geologic, petrologic, geochemical, and isotopic descriptions of Paleoarchean gneisses in the central Wyoming Province. We identify two groups of gneisses: a bimodal suite of amphibolite and tonalite-trondhjemite-granodiorite (TTG) layered gneisses (3385−3450 Ma), and a suite of massive trondhjemite and granite gneisses (3300−3330 Ma). Here, 3.82 Ga inherited zircon components are present in several samples. Negative bulk rock initial e Nd values also indicate that older crust was involved. Oxygen isotopic compositions of zircon mainly fall within the range of mantle zircon δ 18 O values, but several analyses extend up to ∼6.5‰. Initial Hf isotopic compositions of 3.82 Ga zircon are negative and require derivation from Hadean crustal sources. Our data reveal the record of a 3.82 Ga differentiation event during which Hadean crust was partially melted, and zircon crystallized from those melts. Hadean crust must have persisted in the central Wyoming Province until 3.45−3.37 Ga, when mafic crust partially melted to form the TTG layered gneisses, which also incorporated the 3.82 Ga zircons. Subsequent intracrustal recycling at 3.33−3.30 Ga produced calc-alkalic granites. The central Wyoming Province provides a significant addition to the sparse record of Hadean crust being magmatically reworked to form the abundant quartzofeldspathic gneisses common in Archean terrains worldwide, which effectively transformed Earth’s evolving continental crust from mafic to felsic in composition.

Neoarchean tectonic history of the Teton Range: Record of accretion against the present-day western margin of the Wyoming Province

Although Archean gneisses of the Teton Range crop out over an area of only 50 × 15 km, they provide an important record of the Archean history of the Wyoming Province. The northern and southern parts of the Teton Range record different Archean histories. The northern Teton Range preserves evidence of 2.69–2.68 Ga high-pressure granulite metamorphism (>12 kbar, ~900 °C) followed by tectonic assembly with isotopically juvenile quartzofeldspathic metasedimentary rocks under high-pressure amphibolite-facies conditions (~7 kbar, 675 °C) and intrusion of extensive leucogranites. Together, these events record one of the oldest continent-continent collisional orogenies on Earth. Geochemical, thermobarometric, and geochronological data from the gneisses of the southern Teton Range show that this part of the uplift records a geologic history that is distinct from the northern part. It contains a variety of quartzofeldspathic gneisses, including a 2.80 Ga granodioritic orthogneiss and the 2.69–268 Ga Rendezvous Gabbro. None of these preserves evidence of the granulite metamorphism seen in the northern Teton Range. Instead, they have affinities with rocks elsewhere in the Wyoming Province. The boundary between the northern and southern areas is occupied by the Moran deformation zone, a broad zone of high strain along which the northern and southern areas were assembled at ca. 2.62 Ga under moderate pressures and temperatures (T = 540–600 °C and P < 5.0 kbar). The final Archean event of the Teton Range was the emplacement at 2.55 Ga of the Mount Owen batholith, a peraluminous leucogranite that intrudes the Moran deformation zone. The rocks of the northern Teton Range record events that are not present elsewhere in the Wyoming Province. We propose that they formed at 2.70–2.67 Ga some place distal to the Wyoming Province and that they were accreted from the west against the Wyoming Province along the Moran deformation zone at ca. 2.62 Ga. This date is coeval with deformation and metamorphism in the southern accreted terranes and indicates that at this time, accretion was taking place along both the southern margin and western margins of the Wyoming Province. INTRODUCTION The Wyoming Province is an Archean craton that occupies most of Wyoming and portions of Montana, and adjacent states. The Archean rocks are exposed in the cores of basement-involved Laramide uplifts. The early mafic crust appears to have been Hadean (Frost et al., 2017), though most of the exposed area consists of Paleoarchean to Neoarchean quartzofeldspathic orthogneisses that retain an isotopic signature of that ancient crust (Frost, 1993). The Wyoming craton is subdivided into three main provinces (Fig. 1; Mueller and Frost, 2006). The northwestern province is the Montana metasedimentary province, which is an area composed of quartzite, pelitic schist, and carbonate rock associations that are structurally interleaved with quartzofeldspathic gneiss, all of which were accreted at ca. 2.55 Ga. The Beartooth-Bighorn magmatic zone, which occupies the core of the craton, is dominated by orthogneisses. Most of the Beartooth-Bighorn magmatic zone contains rocks that were last deformed between 2.86 Ga and 2.71 Ga. On the southern and western margins of the Beartooth-Bighorn magmatic zone, these older gneisses were overprinted by deformation that is as young as 2.63 Ga. The southern margin of the craton contains the southern accreted terranes, consisting of various fragments of arcs and continents that were accreted to the Wyoming Province at ca. 2.63 Ga. The Teton Range, a small range of spectacular mountains in northwestern Wyoming, exposes some of the westernmost outcrops of the Archean Wyoming Province (Fig. 2). The northern portion of the range, described by B. Frost et al. (2006), Frost et al. (2016a), and Swapp et. al. (2018), contains some of the oldest high-pressure granulites in the world. In this paper, we summarize the past work on the northern Teton Range, identify a deformation zone that marks the contact between gneisses of the northern and southern Teton Ranges, and discuss how this structural belt relates to the final Neoarchean assembly of the Wyoming Province.

2.7 Ga high-pressure granulites of the Teton Range: Record of Neoarchean continent collision and exhumation

Continent-continent collisional orogens are the hallmark of modern plate tectonics. The scarcity of well-preserved high-pressure granulite facies terranes minimally obscured by later tectonic events has limited our ability to understand how closely Archean tectonic processes may have resembled betterunderstood modern processes. Here we describe Neoarchean gneisses in the Teton Range of Wyoming, USA, that record 2.70 Ga high-pressure granulite facies metamorphism, followed by juxtaposition of gneisses with different protoliths, and then by intrusion of leucogranites generated through decompression melting in response to post-collisional uplift. This evidence is best explained as the result of a 2.70–2.68 Ga Himalayan-style orogeny, and suggests that, although subduction may have been occurring earlier in the Archean, doubling of continental thickness by continent-continent collisions may date back to at least 2.7 Ga.