Robert P. Wintsch

D/H isotope ratios of kerogen, bitumen, oil, and water in hydrous pyrolysis of source rocks containing kerogen types I, II, IIS, and III

Abstract Immature source rock chips containing different types of kerogen (I, II, IIS, III) were artificially matured in isotopically distinct waters by hydrous pyrolysis and by pyrolysis in supercritical water. Converging isotopic trends of inorganic (water) and organic (kerogen, bitumen, oil) hydrogen with increasing time and temperature document that water-derived hydrogen is added to or exchanged with organic hydrogen, or both, during chemical reactions that take place during thermal maturation. Isotopic mass-balance calculations show that, depending on temperature (310–381°C), time (12–144 h), and source rock type, between ca. 45 and 79% of carbon-bound hydrogen in kerogen is derived from water. Estimates for bitumen and oil range slightly lower, with oil–hydrogen being least affected by water-derived hydrogen. Comparative hydrous pyrolyses of immature source rocks at 330°C for 72 h show that hydrogen in kerogen, bitumen, and expelled oil/wax ranks from most to least isotopically influenced by water-derived hydrogen in the order IIS > II ≈ III > I. Pyrolysis of source rock containing type II kerogen in supercritical water at 381°C for 12 h yields isotopic results that are similar to those from hydrous pyrolysis at 350°C for 72 h, or 330°C for 144 h. Bulk hydrogen in kerogen contains several percent of isotopically labile hydrogen that exchanges fast and reversibly with hydrogen in water vapor at 115°C. The isotopic equilibration of labile hydrogen in kerogen with isotopic standard water vapors significantly reduces the analytical uncertainty of D/H ratios when compared with simple D/H determination of bulk hydrogen in kerogen. If extrapolation of our results from hydrous pyrolysis is permitted to natural thermal maturation at lower temperatures, we suggest that organic D/H ratios of fossil fuels in contact with formation waters are typically altered during chemical reactions, but that D/H ratios of generated hydrocarbons are subsequently little or not affected by exchange with water hydrogen at typical reservoir conditions over geologic time. It will be difficult to utilize D/H ratios of thermally mature bulk or fractions of organic matter to quantitatively reconstruct isotopic aspects of paleoclimate and paleoenvironment. Hope resides in compound-specific D/H ratios of thermally stable, extractable biomarkers (“molecular fossils”) that are less susceptible to hydrogen exchange with water-derived hydrogen.

Dissolution and replacement creep: a significant deformation mechanism in mid-crustal rocks

Abstract Zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and S–C fabrics in these rocks were produced by dissolution, precipitation, and replacement processes, even at epidote–amphibolite facies metamorphic conditions. The metamorphic fabric is defined by alternating layers and folia dominated by quartz, feldspars, and biotite+epidote. Zoning patterns in most metamorphic plagioclase, orthoclase, epidote, and sphene are truncated at boundaries normal to the shortening direction, suggesting dissolution. Interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite, epidote, and myrmekitic intergrowths of plagioclase and quartz. Metamorphic plagioclase grains are also replaced by epidote. We interpret these microstructures to reflect strain-enhanced dissolution. The cores of many grains show asymmetric overgrowths with at least two generations of beards, all oriented on the ends of grains that face the extension direction. We interpret these textures to reflect precipitation of components dissolved by deformation-enhanced dissolution. While biotite and quartz probably deformed by dislocation creep, the overall deformation was accommodated by dissolution perpendicular to the shortening direction, and precipitation parallel to it. These chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks. Thus wet crust is likely to be weaker than calculated from these experimental studies.

U–Pb geochronology of zircon and polygenetic titanite from the Glastonbury Complex, Connecticut, USA: an integrated SEM, EMPA, TIMS, and SHRIMP study

Abstract U–Pb ages for zircon and titanite from a granodioritic gneiss in the Glastonbury Complex, Connecticut, have been determined using both isotope dilution thermal ionization mass spectrometry (TIMS) and the sensitive high resolution ion microprobe (SHRIMP). Zircons occur in three morphologic populations: (1) equant to stubby, multifaceted, colorless, (2) prismatic, dark brown, with numerous cracks, and (3) elongate, prismatic, light tan to colorless. Cathodoluminescence (CL) imaging of the three populations shows simple concentric oscillatory zoning. The zircon TIMS age [weighted average of 207Pb/206Pb ages from Group 3 grains—450.5±1.6 Ma (MSWD=1.11)] and SHRIMP age [composite of 206Pb/238 U age data from all three groups—448.2±2.7 Ma (MSWD=1.3)], are interpreted to suggest a relatively simple crystallization history. Titanite from the granodioritic gneiss occurs as both brown and colorless varieties. Scanning electron microscope backscatter (BSE) images of brown grains show multiple cross-cutting oscillatory zones of variable brightness and dark overgrowths. Colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning. Electron microprobe analysis (EMPA) clearly distinguishes the two populations. Brown grains contain relatively high concentrations of Fe2O3, Ce2O3 (up to ∼1.5 wt.%), Nb2O5, and Zr. Cerium concentration is positively correlated with total REE+Y concentration, which together can exceed 3.5 wt.%. Oscillatory zoning in brown titanite is correlated with variations in REE concentrations. In contrast, colorless titanite (both as discrete grains and overgrowths on brown titanite) contains lower concentrations of Y, REE, Fe2O3, and Zr, but somewhat higher Al2O3 and Nb2O5. Uranium concentrations and Th/U discriminate between brown grains (typically 200–400 ppm U; all analyses but one have Th/U between about 0.8 and 2) and colorless grains (10–60 ppm U; Th/U of 0–0.17). In contrast to the zircon U–Pb age results, SHRIMP U–Pb data from titanite indicate multiple growth episodes. In brown grains, oscillatory zoned cores formed at 443±6 Ma, whereas white (in BSE) cross-cutting zones are 425±9 Ma. Colorless grains and overgrowths on brown grains yield an age of 265±8 Ma (using the Total Pb method) or 265±5 Ma (using the weighted average of the 206Pb/238U ages). However, EMPA chemical data identify zoning that suggests that this colorless titanite may preserve three growth events. Oscillatory zoned portions of brown titanite grains are igneous in origin; white cross-cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite. Colorless titanite replaced and grew over the magmatic titanite during the Permian Alleghanian orogeny. These isotopic data indicate that titanite, like zircon, can contain multiple age components. Coupling SHRIMP microanalysis with EMPA and SEM results on dated zones as presented in this study is an efficient and effective technique to extract additional chronologic data to reveal the complexities of igneous crystallization and metamorphic growth.

Ages and origins of rocks of the Killingworth Dome, south-central Connecticut: implications for the tectonic evolution of southern New England

The Killingworth dome of south-central Connecticut occurs at the southern end of the Bronson Hill belt. It is composed of tonalitic and trondhjemitic orthogneisses (Killingworth complex) and bimodal metavolcanic rocks (Middletown complex) that display calc-alkaline affinities. Orthogneisses of the Killingworth complex (Boulder Lake gneiss, 456 ± 6 Ma; Pond Meadow gneiss, ∼460 Ma) were emplaced at about the same time as eruption and deposition of volcanic-sedimentary rocks of the Middletown complex (Middletown Formation, 449 ± 4 Ma; Higganum gneiss, 459 ± 4 Ma). Hidden Lake gneiss (339 ± 3 Ma) occurs as a pluton in the core of the Killingworth dome, and, on the basis of geochemical and isotopic data, is included in the Killingworth complex. Pb and Nd isotopic data suggest that the Pond Meadow, Boulder Lake, and Hidden Lake gneisses (Killingworth complex) resulted from mixing of Neoproterozoic Gander terrane sources (high 207Pb/204Pb and intermediate εNd) and less radiogenic (low 207Pb/204Pb and low εNd) components, whereas Middletown Formation and Higganum gneiss (Middletown complex) were derived from mixtures of Gander basement and primitive (low 207Pb/204Pb and high εNd) sources. The less radiogenic component for the Killingworth complex is similar in isotopic composition to material from Laurentian (Grenville) crust. However, because published paleomagnetic and paleontologic data indicate that the Gander terrane is peri-Gondwanan in origin, the isotopic signature of Killingworth complex rocks probably was derived from Gander basement that contained detritus from non-Laurentian sources such as Amazonia, Baltica, or Oaxaquia. We suggest that the Killingworth complex formed above an east-dipping subduction zone on the west margin of the Gander terrane, whereas the Middletown complex formed to the east in a back-arc rift environment. Subsequent shortening, associated with the assembly of Pangea in the Carboniferous, resulted in Gander cover terranes over the Avalon terrane in the west; and in the Middletown complex over the Killingworth complex in the east. Despite similarities of emplacement age, structural setting, and geographic continuity of the Killingworth dome with Oliverian domes in central and northern New England, new and published isotopic data suggest that the Killingworth and Middletown complexes were derived from Gander crust, and are not part of the Bronson Hill arc that was derived from Laurentian crust. The trace of the Ordovician Iapetan suture (the Red Indian line) between rocks of Laurentian and Ganderian origin probably extends from Southwestern New Hampshire west of the Pelham dome of northcentral Massachusetts and is coverd by Mesozoic rocks of the Hartford basin.

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks—in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas—provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

Chlorite-illite/muscovite interlayered and interstratified crystals: A TEM/STEM study

Phyllosilicates in rocks which are transitional from mudstone to slate from Lehigh Gap, Pa., have been studied by a variety of techniques, including high resolution Transmission Electron Microscopy and Analytical Electron Microscopy. The principal minerals are “white mica” which is transitional from illite in mudstone to ordered twolayer mica in slate, and chlorite. 7Å berthierine occurs more rarely. Dioctahedral and trioctahedral layers are shown to be interleaved in individual crystals at all scales between the following two end members: (1) both random and regular 1∶1 interlayering at the scale of individual layers, as shown, in part, by lattice fringe images. (2) packets of trioctahedral and dioctahedral layers up to a few thousand Ångstroms or microns in thickness, detectable with ordinary optical techniques. The complete range of intermediate structures is represented in samples which are in transition to slate. Bulk analytical (EMPA), X-ray diffraction or other measurements are shown to result in averages over both kinds of layers when TEM techniques are not used.

Open-system, constant-volume development of slaty cleavage, and strain-induced replacement reactions in the Martinsburg Formation, Lehigh Gap, Pennsylvania

No important changes in the volume of mudrocks occur across the mudstone-to-slate transition in the Martinsburg Formation at Lehigh Gap, Pennsylvania. Mass-balance calculations based on chemical analyses and specific-gravity measurements of 48 mudstones and 26 graywackes from the Martinsburg Formation across this 130-m strain gradient show constant ratios of Al2O3, TiO2, FeOT, MgO, MnO, Y, V, and Zr. This reflects the relatively low differential mobility of these components during diagenesis and slaty cleavage development, as well as the uniformity of compositions of the mudstones and graywackes at deposition. Using minimum-strain samples and Al2O3 as references, the calculations show no loss in SiO2, but large losses of CaO, Na2O, Ba, and Sr from the outcrop that are directly proportional to strain, and reflect an open system on the scale of kilometers. Analyses of graywacke-mudstone metagraywacke-slate) pairs show that losses in SiO2, Na2O, and volume, and gains in K2O and Ba in the graywacke/metagraywacke are balanced by opposite changes in the adjacent mudstone/slate. This documents a local mobility of SiO2 on the scale of centimeters. The inertness of most major components over the 100-m outcrop (especially SiO2), however, requires (1) that the composition of the fluid passing through the rocks was closely buffered by the quartz-albite-muscovite-chlorite assemblage, (2) that the volume of the fluid was small, and (3) that the P-T gradient down which it traveled was gentle. The changes in chemical composition identified can be explained by the inferred replacement reaction plag + 1M musc + K+ = 2M musc + qz + Ca+2 + Na+ + Sr+2 + Ba+2 The strong correlation of minor-element chemistry and strain indicates that this reaction was strain induced and that changes in bulk chemical composition are defined by the progress of this reaction.

Contrasting P‐T‐t paths: Thermochronologic evidence for a Late Paleozoic final assembly of the Avalon Composite Terrane in the New England Appalachians

Strongly contrasting pressure-temperature-time paths for the Avalon composite terrane and the structurally overlying Putnam-Nashoba zone in eastern New England obtained from thermochronologic and thermobarometric data are best explained by a late Paleozoic underthrusting of cover rocks by the Avalon composite terrane. We present new Ar and U-Pb thermochronologic data that show that in the southern Hope Valley zone, Permian (280 Ma) anatectic metamorphic conditions of 700°C and 6 kbar were quenched by relatively rapid cooling (12°C/m.y.) and exhumation (0.5 km/m.y.) for ∼40 m.y. In contrast, peak metamorphic conditions in the Putnam-Nashoba zone predate Silurian intrusions, and slower cooling (3.5°C/m.y.) began at about 400 Ma. One-dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the Permian metamorphism of the Avalon composite terrane. Because of the absence of high-grade Alleghanian metamorphism in rocks overlying the Avalon terrane, we conclude that high-grade Alleghanian metamorphism in the Avalon terrane occurred east of rocks now overlying it and that significant motion between Avalon and this cover occurred after peak Alleghanian metamorphism. Similarly contrasting metamorphic histories between Avalon inliers (Willimantic window, Massabesic complex gneiss, Pelham dome) and their cover rocks reveals the regional significance of this boundary. The core rocks all show Permian cooling, but the cover rocks show post-Acadian cooling ages decreasing from east to west to the Pelham area, where hornblende ages in Avalon and cover differ by only 35 rather than 80 m.y. Model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years. Consequently, underthrusting of Avalon is constrained to be middle Mississippian or younger. Because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover, core rocks of the Pelham dome must have been relatively close to this leading edge. Thus Carboniferous to Permian underplating from a generally eastward direction best explains these thermochronologic relationships.

Evidence for syntectonic crystallization for the mudstone to slate transition at Lehigh Gap, Pennsylvania, U.S.A.*

Data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well-developed slaty cleavage at Lehigh Gap, Pennsylvania, and for slates from quarries in the same area. Samples were studied by optical microscopy, powder X-ray diffraction and electron microprobe analysis, with emphasis on transmission and analytical electron microscopy. Mineral grains are virtually free of deformation-induced strain. Concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed: (1) the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage; (2) crystal imperfections as expressed in layer terminations, low angle grain boundary-like features and other defects decrease in density; (3) complex mixed layering is replaced by homogeneous packets of layers of single phases and (4) illite transforms to muscovite, with increase in K + A1 and change from a lMd to 2M polytype. Slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding, mass transport of components, and crystallization to form new grains parallel to cleavage. It reflects transitions from imperfect, metastable phases toward ordered stable phases in a low temperature (-225°C) metamorphic environment.

Early Cretaceous Normal Faulting in Southern New England: Evidence from Apatite and Zircon Fission‐Track Ages

New apatite and zircon fission‐track (AFT, ZFT) ages from Mesozoic sediments and adjacent crystalline rocks from southern New England reveal age gradients from Middle Jurassic to Early Cretaceous. These gradients reflect the rotation of crustal blocks after the setting of the youngest AFT ages (∼140 Ma). The AFT ages of 168–98 Ma for 32 samples of Paleozoic metamorphic rocks east and west and within the Hartford Basin of Massachusetts and Connecticut indicate that unroofing in these regions occurred from Late Jurassic through Early Cretaceous. In both the Hartford Basin and rocks from crystalline terranes east of the basin, AFT ages show a regional trend of increasing age to the east, suggesting a down‐to‐the‐east rotation of ∼10° in each area. The ZFT ages from the Hartford Basin arkoses (167–238 Ma) and northern Bronson Hill (147–196 Ma) rocks support this gradient of eastward increase in AFT age. A south‐to‐north gradient of decreasing AFT age (139–107 Ma) for the sedimentary rocks in the Hartford and Deerfield Basins of Connecticut and Massachusetts suggests a 1°‐hinged uplift to the north that postdates the youngest AFT age of ∼100 Ma. Final juxtaposition of AFT ages across the eastern Border fault between the Hartford Basin and Bronson Hill terrane in Massachusetts and Connecticut indicates displacement younger than the youngest AFT ages of ≤100 Ma (Late Cretaceous). Thus, the age of the graben structure of the Hartford Basin is Cretaceous, and this structure cannot be cited as evidence that these basins are Early Mesozoic “rift” basins.