Danny M. Rye

Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece

Abstract At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C. The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ 18 O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ 18 O H 2 O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite. The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD H 2 O values for the metamorphic fluid decrease from −5‰ in the glaucophane zone to an average of about −70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend. Theδ 18 O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism. Detailed data on 20 marble units show systematic variations of δ 18 O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ 18 O gradients at the margins and large δ 18 O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ 18 O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates. Almost all the δ 13 C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ 13 C values of most marble units show a general correlation with δ 18 O values. The CO 2 H 2 O mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ 13 C values of the CO 2 range from −8.0 to 3.6‰ and indicate that at some localities CO 2 in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.

A model for the kinetic control of quartz dissolution and precipitation in porous media flow with spatially variable permeability: Formulation and examples of thermal convection

We present the formulation and model results of kinetically controlled quartz dissolution and precipitation in a two-dimensional heterogeneous permeable medium. The quartz matrix is modeled as a partially occluded spherical close pack, with dissolution and precipitation occurring on the exposed faces of the grains. This formulation yields larger permeabilities and lower surface area to fluid volume ratios in regions of larger grain radii, and thus allows us to investigate the influence of crack-like regions on the flow and silica exchange. We use the kinetic data for quartz dissolution of Rimstidt and Barnes [1980]. Thermal convection results indicate that channelizing of fluid flow in high-permeability zones is enhanced in the transient regime by buoyancy effects arising from the advection of heat. The highly permeable zones are most out of chemical equilibrium, owing to their lower surface areas and to more rapid advection. Porosity changes are most pronounced in regions of high surface area often downstream of high-permeability zones. Oscillatory convection is observed, accompanied by saturation state reversals peripheral to the high-permeability zones. Such internally generated oscillatory regimes provide a mechanism for quartz zonation down to the nanometer scale. When the thermal forcing is strong enough, recurrent plumes emanate from the thermal boundary layers and plunge through the high-permeability zones. When departures from equilibrium become significant, we observe some regions of undersaturated upwelling fluid moving down temperature, and regions of oversaturated downwelling fluid moving up temperature, both cases opposing the conventional wisdom based on equilibrium.

OXYGEN ISOTOPE RECORD OF FLUID INFILTRATION AND MASS TRANSFER DURING REGIONAL METAMORPHISM OF PELITIC SCHIST, CONNECTICUT, USA

Abstract We present petrologic and oxygen isotopic evidence for the interaction of deep crustal fluids with kyanite zone pelitic schist during amphibolite facies metamorphism of the Wepawaug Schist, south-central Connecticut. We focus on a sample of schist (sample MBW-1) cut by a 2–6 cm wide quartz vein. The vein is surrounded by zones of wallrock alteration (selvages) that are rich in micas relative to quartz and feldspar, have low Si Al and Na Al , contain staurolite and kyanite, and vary in thickness from about 1–5 cm. Staurolite and kyanite are rare or absent beyond the selvage margins. We have measured the δ18O of quartz, plagioclase, muscovite, garnet, kyanite, staurolite, garnet, and biotite along several mm-scale resolution traverses across the quartz vein and the adjacent schist. Garnets in the selvages record core-to-rim increases in δ18O of nearly 2%. Modeling of prograde reaction histories indicates that this zonation requires the infiltration of external fluids. Beyond the selvage margins, isotopic zonation in garnet is about 0.8% from core-to-rim and is consistent with prograde reaction with little or no infiltration. We suggest, therefore, that the selvages were zones of significant fluid infiltration and that the region now occupied by the quartz vein was the major fluid conduit. Earlier petrologic studies (Ague, 1994b) indicated that quartz veins and adjacent selvages were conduits for major down-temperature flow of H2O-rich fluids with time-integrated fluid fluxes of ∼3 × 105 m3 m−2. Isotopic modeling of advective flow suggests that down-temperature fluxes of this magnitude would have increased bulk δ18O by ∼1‰, consistent with the isotopic record preserved by zoned selvage garnets. Quartz in veins surrounded by selvages from five other localities throughout the amphibolite facies have δ18O that is statistically indistinguishable from that of the bulk of the quartz in MBW-1. Thus, we conclude that the amphibolite facies portion of the Wepawaug Schist was a zone of major, channelized outflow of metamorphic fluids down the regional temperature gradient. During the latter stages of amphibolite facies metamorphism subsequent to the bulk of vein and selvage formation, MBW-1 was infiltrated by isotopically light fluids that were probably derived from synmetamorphic igneous intrusions. This infiltration modified the isotopic composition of plagioclase throughout the rock and, therefore, we suggest that the infiltration was pervasive. Muscovite retains its pre-infiltration isotopic composition, however, which suggests short timescales of fluid-rock interaction on the order of 103–104 years. The total duration of flow may have been longer than this because our calculations do not take episodic flow into account. Modeling of possible isotopic shifts resulting from diffusion of oxygen isotopes between matrix phases during slow cooling indicates that MBW-1 must have been dry for most of its retrograde cooling history.

Early life history of Nautilus: evidence from isotopic analyses of aquarium-reared specimens

Specimens of Nautilus species caught in the wild show a marked increase in oxygen isotopic composition between embryonic and postembryonic septa. The significance of this increase in terms of the early life history of Nautilus has been unclear. To help explain this pattern, we analyzed the isotopic composition of the septa of three specimens of Nautilus belauensis raised in aquariums under controlled temperature conditions. Our results indicate that both embryonic and postembryonic septa are secreted with the same temperature-dependent fractionation of aragonite relative to water as that of other aragonite-secreting molluscs (Grossman and Ku 1986). The δ 18 O values of the septa thus provide a reliable means of determining the water temperature in which the septa form. Calculated temperatures based on oxygen isotopic data from specimens caught in the wild reveal that embryonic development occurs at 22°-24° corresponding to a depth of 100-200 m depending on the location. The increase in δ 18 O in postembryonic septa reflects a migration into colder, deeper water after hatching. In Cretaceous nautilids, a systematic shift in δ 18 O is not present, indicating that these animals probably did not change their habitat after hatching. This is consistent with the likelihood that they lived in shallower environments than that of modern Nautilus.

Use of stable sulfur isotope systematics for evaluating oxidation reaction pathways and in-cloud-scavenging of sulfur dioxide in the atmosphere

Sulfur dioxide injected into the atmosphere is most likely oxidized into sulfate. Two major oxidation pathways are possible: (1) a homogeneous pathway involving gas reaction with hydroxyl radicals and (2) a heterogeneous pathway involving aqueous dissolution or aerosol reactions. The relative importance of these reaction pathways conditions is controversial. Sulfur isotope ratios can be used to quantify the relative importance of these reaction pathways. However, its application was severely hampered by the fact that the isotope fractionation factor for the homogeneous pathway was not known. A significant isotope fractionation in the homogeneous SO[sub 2] oxidation is identified for the first time using an ab initio quantum mechanical calculation. By using the sulfur isotope fractionation factors the authors demonstrate a technique that uses measurements of the sulfur isotope ratio in gaseous SO[sub 2], aerosol SO[sub 4] and sulfate in wet precipitation to quantify the relative importance of the homogeneous and heterogeneous reaction pathways as well as the in-cloud scavenging of sulfur dioxide for a set of isotopic observations at New Haven, CT, USA. 15 refs., 1 fig., 2 tabs.

Regional metamorphic dehydration and seismic hazard

We use a one-dimensional model of coupled heat transport, fluid transport, porosity feedback, and metamorphism to investigate how dehydration reactions, proceeding during burial of hydrous rocks in the footwall of thrusts or the hanging wall of normal faults, may influence the temporal evolution of pore fluid pressure (Pf). Based on a new interpretation of existing kinetic data for the dehydration of model serpentinite, we advance the hypothesis that Pf buildup to tensile failure may be possible within decades of the onset of dehydration in low permeability metamorphic rocks of the middle crust. This model contrasts with both: 1) typical earthquake nucleation models which involve preexisting fluids that play a passive role, responding to tectonic stresses rather than generating them and 2) conventional treatments of Pf generation based on equilibrium thermodynamics and constant rates of dehydration, which require 10³–106 years to produce significant Pf increases. Our results suggest that repeated cycles dehydration-induced seismicity are plausible in low permeability areas of the mid-crust, and can be sustained throughout an orogenic episode if fresh hydrous minerals descend into the metamorphic zone along the downgoing face of a thrust- or normal-fault system.

Early ontogeny of Eutrephoceras compared to Recent Nautilus and Mesozoic ammonites: evidence from shell morphology and light stable isotopes

Observations on the morphology of the early whorls of Eutrephoceras dekayi (Morton), a widespread Cretaceous nautilid, are supplemented with oxygen and carbon isotopic analyses (δ 18 O and δ 13 C) of the early septa of five well-preserved specimens to help identify the point of hatching on the shell. Septa 4 and 5 are more closely spaced than preceding septa and probably correspond in time of formation with a constriction or first broken aperture on the outer shell one-third whorl forward of the fourth septum. In modern Nautilus , morphologic, isotopic, and observational data suggest that similar features mark hatching. Between the fourth and fifth septa in E. dekayi , δ 18 O values show a shift of variable magnitude from heavy to lighter values followed by a return to heavier values over the next one to three septa. This isotopic shift is compatible with a hatching interpretation and may be explained as the result of kinetic and equilibrium effects on emergence from an egg capsule. Eutrephoceras dekayi hatched at about 9 mm in diameter, one-third the hatching size of modern Nautilus. Like Nautilus, E. dekayi probably produced few young, all of which were active swimmers at hatching. In contrast, Mesozoic ammonoids produced numerous offspring ranging from 0.5 to 1.5 mm in diameter which may have spent some time in the plankton. These differences in life history may correlate with differences in ecologic specialization, environmental tolerance, and habitat between ammonoids and nautilids and may have contributed to their disparate rates of evolution during the Mesozoic.

Basement geology and tectonic evolution of Ireland as deduced from Pb isotopes

In Ireland, lead isotope data collected on Carboniferous-hosted Zn-Pb deposits can and has been interpreted to indicate that lead and other metals were derived from the underlying basement rocks. In order to test this hypothesis, whole rock lead isotope measurements were made on greywackes from the Longford-Down Inlier, which represent the along strike continuation of the northern part of the Caledonian basement underlying the base metal deposits. The lead isotope data define a linear array on a 207pb/204pb versus 206Pb/204Pb plot, with end points at (207Pb/204Pb) = 15.542, (206Pb/204Pb) = 18.456 and (207Pb/204Pb) = 15.461, (206Pb/204Pb) = 17.973. No evidence of lead isotope overprinting by the mineralizing event could be found in the samples studied. When the lead isotope data are corrected for addition of radiogenic lead since the time of base metal mineralization (c. 350 Ma ago), there is a 1:1 correlation between the lead isotopic composition of galenas in the base metal deposits and the Longford-Down grewyackes. These results support the hypothesis that lead and other metals were derived from the greywackes underlying the base metal mineralization. On a 208Pb/204Pb versus 206Pb/204Pb plot, neither whole rock greywacke nor galena lead isotope data define a linear array. The 208Pb/204Pb-206Pb/204Pb trend is best explained by tectonic or sedimentological mixing of three isotopically distinct lead isotope end members. These end members are: (1) old unradiogenic basement in NW Ireland (Lewisian complex precursor?), (2) Ordovician arc material from NW Ireland and (3) Ordovician arc material form SE Ireland. Isotopic modelling of these end members shows that the present-day mixing line seen in the Ordovician and Silurian sediments can be calculated back to values inferred for the Ordovician arc end members using the same values of μ as were used to back calculate the lead isotopic composition of the greywackes to the time of mineralization. This information is further confirmation of the sediment-mixing model presented. A tectonic model developed from these and established geological data suggest a late Ordovician closure of the Iapetus Ocean, and that the Silurian greywackes are a by-product of eroded Ordovician arc material from NW and SE of the Iapetus suture.

Hydrogen isotope exchange kinetics between H2O and H4SiO4 from ab initio calculations

Abstract Oxygen isotope exchange between H 2 O and H 4 SiO 4 was modeled with ab initio calculations on H 4 SiO 4 + 7H 2 O. Constrained optimizations were performed with the B3LYP/6–31+G(d,p) method to determine reactants, transition states, and intermediates. Long-range solvation was accounted for using self-consistent reaction field calculations. The mechanism for exchange involves two steps, a concerted proton transfer from H 4 SiO 4 forming a 5-coordinated Si followed by a concerted proton transfer from the 5-coordinated Si forming another H 4 SiO 4 . The 5-coordinated Si intermediate is C2 symmetric. At 298K and with implicit solvation included, the Gibbs free energy of activation from transition state theory is 66 kJ/mol and the predicted rate constant is 16 s −1 . Equilibrium calculations between 298K and 673K yield α H4SiO4-H2O that are uniformly less than, but similar to, α qtz-H2O , and therefore α qtz-H4SiO4 is expected to be relatively small in this temperature range.

Dissolution and precipitation via forced‐flux injection in a porous medium with spatially variable permeability: Kinetic control in two dimensions

We model kinetically controlled dissolution and precipitation in a porous medium. Using quartz as the reactive mineral, our specific focus is on the spatial distribution of flow velocities and on changes in solute concentration and porosity introduced by heterogeneities in the initial permeability field and by differing the solute concentration of the infiltrating fluid. Upon a background permeability we impose isolated “crack-like” zones of high permeability and low surface area to fluid volume ratios. Our two-dimensional modeling of forced-flux injection of oversaturated and undersaturated fluid reveals features inaccessible to previous homogeneous permeability studies. Although realistic velocities can give rise to narrow boundary layers in the solute concentration, disequilibrium is favored in the high-permeability zones yielding plume-like distributions of solute concentration. Aside from the rapid changes in porosity near the injection level, the other regions of rapid porosity change occur just downstream from the crack-like zones, where fluid furthest from equilibrium with respect to quartz encounters regions of higher surface area to fluid volume ratios. Flow rates are significantly enhanced between isolated high-permeability zones, an effect which is even more dramatic for both closer crack spacing and higher permeability contrasts. Undersaturated injection leads to permeability homogenization along the flow direction due to dissolution at the trailing edges of the cracks. Oversaturated injection tends to increase permeability heterogeneities along the flow direction, due to precipitation at the crack trailing edges. We also discuss the scaling properties of flow lines and reaction rates.