V11E-01 INVITED
Mineral-Water Reaction Kinetics: From Molecules to Mountains
The fate of mountains is decided at the nanometer space- and femto-second time-scale. Consequently, our understanding of mineral-water reaction kinetics requires an in depth understanding of the underlying molecular processes and – in addition – demands an ability to connect the microscopic information with the macroscopic geologic bodies formed over millions of years. Today, petrologists can build on a wealth of existing data and observations, utilize sophisticated analytical tools and supercomputers for their studies. Still, the critical question is this: Is there actually enough information stored in a rock to decipher the kinetics that governed its formation? While this critical question remains yet unanswered, we have the opportunity to study water-rock reaction kinetics in the laboratory and develop a fundamental understanding of the reaction kinetics that control rock-forming processes. A combination of microscopic experimental techniques and quantum chemistry-based computer simulations provides the basis for new geochemical hypotheses. These may than be explored and tested in natural rock samples. http://cohesion.rice.edu/naturalsciences/earthscience/facultyDetail.cfm?riceid=1026
V11E-02
Dehydration Behavior of Metapelites and Metabasites at Very low to low Grade Metamorphic Conditions
Thermodynamic calculations have been undertaken in the system Na-Ca-K-Fe-Mg-Al-Si-Ti-H-O with the PERPLE_X software package (Connolly, 1990 and updates) for a better understanding of the dehydration behavior of metapelites and metabasites during prograde metamorphism. To obtain reasonable results for the temperature range 150-450° C at pressures up to 25 kbar, the subsequent solid solution models were introduced being compatible with the applied thermodynamic data set of Holland & Powell (1998 and updates): a three component model for Mg-Fe2+-Fe3+-pumpellyite, a two component model for Fe2+-Mg- stilpnomelane, a four component amphibole model (tremolite - Fe2+-tremolite - glaucophane - Mg- riebeckite), and a four component Na-pyroxene model (acmite - jadeite - diopside - hedenbergite). The water contents released by prograde metamorphism up to 450° C from MORB and psammopelitic compositions on top of oceanic crust, were obtained by calculating P-T pseudosections. Metabasite contains 6-7 wt% H2O bound to minerals at 150° C depending on the oxidation state. Along geotherms lower than 7° C/km typical for young subduction zones, no water is released up to 400° C. However, reduction of the rock causes release of small amounts of water. Metapsammopelitic rocks also store about 6 wt% H2O in minerals at 150° C. Considerable amounts of water are liberated by mineral reactions already in the temperature range 150-250° C also at the above mentioned low geotherms. This behavior determines the rheological characteristics of the upper oceanic crust during early subduction. If water is exclusively released in the sedimentary portion of the downgoing crust only this material gets weakened to be scraped off to form accretionary wedges. At geotherms of 15-20° C/km both lithologies show significant dehydration at very low metamorphic grade. For instance, in cold frontal paleoaccretionary prisms of the Chilean Coastal Cordillera metapelites by far dominate whereas in hotter basal accretionary prisms both low grade basic oceanic crust and continental sediments occur. We also hypothesize that accretionary wedge complexes with a clear dominance of sediments should not have formed in hot subduction zones typical for Precambrian Earth.
V11E-03
The Subsurface Structure of a Submarine Hydrothermal System, ODP/IODP Hole 1256D
Hole 1256D penetrates a full upper oceanic crustal section through lavas, sheeted dikes, and ~100 m into upper plutonic rocks that formed at a superfast spreading rate at the East Pacific Rise 15 m.y. ago. Electron microprobe analyses of secondary minerals plus oxygen isotope analyses of vein minerals document the thermal structure of the hydrothermal system. The ~800 m volcanic section consists of mainly sheet and massive flows, partly altered to saponite and celadonite, typical of altered submarine basalts but with little oxidation effects. Saponite and Celadonite (n=9) have δ18O values of 13.2-17.8‰, and quartz/chalcedony (n=12) has values of 20.1-30.9‰. These indicate temperatures of 50-110°C in the volcanic section, generally increasing downward. There is a stepwise increase in alteration grade downward across the 60 m lithologic transition from lavas to dikes, which are variably altered to chlorite and other greenschist minerals at temperatures ~250-350°C. Alteration intensity and grade increase downward in the 350 m sheeted dikes, with more amphibole than chlorite below 1300 mbsf, and common retrograde effects. The lower 60 m of sheeted dikes are variably recrystallized to granoblastic textures (Ca-plagioclase, clinopyroxene, orthopyroxene, ilmenite, magnetite). Plagioclase-amphibole and two-pyroxene thermometry indicate temperatures of up to >800°C for this contact metamorphism. Oxygen isotope data indicate formation of quartz veins in the dikes at temperatures of 300°C up to 400-500°C, depending on fluid compositions. Two fractionated gabbro units intrude into granoblastic sheeted dikes beginning at 1407 mbsf. The gabbroic rocks are moderately to highly altered to amphibole, secondary plagioclase, epidote, chlorite, prehnite and laumontite. Plagioclase-amphibole thermometry indicates maximum temperatures of 400- >800°C in the plutonic section, whereas oxygen isotope analyses of quartz veins in these rocks indicate temperatures of 260°C up to 400-500°C, depending on fluid compositions. The stepped thermal gradient in the upper section is similar to that in Hole 504B, but the thermal gradient in the thin sheeted dike complex at Site 1256 was steep. The transition to the plutonic section reveals a complex history of repeated cycles of magmatism and hydrothermal alteration, with contact metamorphism influenced by prior hydrothermal effects.
V11E-04
Chrysotile Nanotubes: Potential Host of Insoluble Chlorine in Serpentinitized Oceanic Crust.
Numerous studies have shown that seafloor serpentinites contain significant concentrations of chlorine (up to 1 wt% Cl) and hence represent an important reservoir of Cl that can potentially be subducted into the mantle. Chlorine in serpentinites is clearly present in both water-soluble and insoluble phases, but a longstanding and as yet unresolved question is the exact identity of the mineralogic hosts of the chlorine. One possibility is that insoluble Cl may be present in the serpentine structure substituting for hydroxyl. To address this question we have used EPMA and TEM to study an ocean floor serpentinite sampled by the Ocean Drilling program (Kane Fracture Zone - Mid Atlantic Ridge). The ODP sample 153-920D-8R-2 is a serpentinized harzburgite, containing water-soluble and insoluble Cl contents of 0.09 wt% and 38 ppm, respectively. This sample has undergone extensive serpentinization (~85%) resulting in the development of both mesh and bastitic replacement textures. A significant portion of the serpentine consists of chrysotile nanotubes with diameters of 20-35 nm. Using a combination of FEG HRTEM and energy filtered TEM, we have examined the spatial distribution of chlorine at the nanoscale within regions of these nanotubes. This process is challenging because the nanotubes damage quite rapidly in the electron beam and so acquisition of HRTEM and EFTEM images must be carried out quickly in order to avoid significant degradation of the sample. Our data show that Cl is clearly concentrated in both the cores of the nanotubes and in amorphous or poorly ordered materials around the periphery of the nanotubes. This observation has been confirmed from images taken both normal and parallel to the nanotube axes. The contrast in HRTEM images indicates that the nanotube cores may be partially filled by an amorphous material that is the carrier of the chlorine. We cannot rule out the possibility that some Cl is present substituting for hydroxyl in the chrysotile structure. However, it is clear that the levels of structurally-bound Cl are below the detection limits for EFTEM analysis and are significantly lower than in the nanotube cores. The amorphous material that is pervasive surrounding the chrysotile nanotubes generally contains lower apparent concentrations of Cl than the nanotube cores. These data suggest that the cores of chrysotile nanotubes maybe an important and previously unrecognized carrier of Cl in serpentinized oceanic crust. During subduction, Cl contained within the nanotube cores may be released into fluids at relatively shallow levels, prior to complete dehydration of chrysotile.
V11E-05 INVITED
Modeling stable isotope transport in metamorphic and hydrothermal systems
Stable isotopes are powerful tools for deciphering the fluid flow histories of metamorphic terrains. The nature of fluid flow, fluid sources, and fluid fluxes can be delineated in well constrained studies. Continuum mechanics models for stable isotope fluid-rock exchange were developed and used over the last three decades in an attempt to accurately interpret the signatures left behind by fluid flow in the earths crust. The efforts have been hampered by the realization that the exchange of many stable isotopes, e.g. oxygen and carbon, by intracrystalline diffusion, hence without re-organization of the crystal lattice, appears to be too slow to achieve significant exchange. This should lead to relatively flat isotopic exchange profiles on hand-, outcrop, or aureole scale. Nevertheless, isotopic fronts are typically sharp (sub mm to cm scale), when measured in the field. This has lead to the suggestion that these sharp fronts correspond to the sides of infiltration fronts, implying the data to have been collected at a high angle to the infiltration direction. Nevertheless, the fact that the oxygen and carbon fronts are located at the same place is not explained by this. A review of published carbon and oxygen data reveals that many contact aureoles show linear trends in oxygen-carbon isotope ratio diagrams for carbonate sample suits. This implies that the fluid composition infiltrating the aureoles had essentially an X(CO2) of 0.5. This is in contrast to skarn mineralogy developed, which requires a water-rich fluid, in agreement with the general notion that igneous fluids are water-rich. These and other observations indicate that the mass transport equation used for stable isotope exchange needs to be improved to model appropriately the actual isotope kinetics during fluid-rock exchange. Detailed isotope studies on systems where net transport reactions are driven by mass transport have led us to identify different exchange mechanisms, including: a) the stable isotope exchange is given by instantaneous mass balance written for the isotope during reaction; b) equilibrium precipitation of products, but slow exchange kinetics for reactants. These observations require that the reactive term in the stable isotope reactive transport equation is re-written to include the net transfer reactions, which in turn implies the solution of the transport equation for the elements driving the reaction.
V11E-06
Mechanisms of Oxygen Isotopic Exchange and Isotopic Evolution of 18O/16O-Depleted Periclase Zone Marbles in the Alta Aureole, Utah, USA—Insights From ion Microprobe Analysis of Calcite
Infiltration of water-rich fluids during prograde metamorphism has produced significant but variable 18O/16O depletion in dolomitic marbles within the periclase (Per) zone of the Alta Stock aureole, Utah. At one location, a marble layer containing calcite (Cal) and forsterite (Fo) and an adjacent layer containing Cal + Per (replaced by brucite) + humite are depleted to δ18O values of 17.2 and 11.8 permil, respectively, from original protolith values (>25 permil). Detailed ion microprobe and millimeter-scale dental drill sampling traverses across the boundary between the two layers define a steep, coherent gradient in δ18O that is displaced a short distance (4 cm) into the higher δ18O Cal + Fo layer. Textural studies (optical, SEM) and ion microprobe analyses show that there are two isotopically and texturally distinct types of calcite at the grain scale in this traverse. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty; ±0.2 to 0.45 permil, two SD) and their δ18O values confirm the basic geometry and location of the gradient defined by the dental drill samples. More poorly polished (pitted), texturally retrograde ‘turbid'-looking calcite has lower and more variable δ18O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Within the interiors of both layers there is no systematic spatial variation of δ18O in either ion microprobe or dental drill results. However there are systematic differences (up to 0.8 permil) between the ion microprobe analyses and the dental drill samples in these layer interiors, which suggest that minor amounts of retrograde calcite are incorporated at the sub-mm scale into these dental drill samples. Despite significant and pervasive depletion of 18O/16O in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that clear calcite grains are now isotopically homogeneous within analytical uncertainty (±0.2 to 0.45 permil, two SD) across the entire gradient in δ18O, not just within the relatively homogeneous interiors of the two layers. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion, 2000 to 62,000 years at 575-600oC, exceed significantly the timescale (approx. 1250 yrs) estimated for the prograde development of the δ18O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms (e.g., dissolution-reprecipitation) accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains. The ion microprobe data are consistent with, but do not require that metamorphic calcite forms in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization. However the presence of retrograde calcite-dolomite temperatures (450- 550oC) recorded in calcite from the periclase zone suggests that both Mg exchange and oxygen isotope homogenization accompanied recrystallization during the early stages of retrograde cooling.
V11E-07
Lithium and Oxygen Isotopic Composition of the Oceanic Crust formed at a Superfast Spreading Ridge, Hole 1256D
Oxygen and lithium isotopic compositions of whole rocks from ODP/IODP Hole 1256D were investigated. The upper about 1000 m extrusive basalts have higher oxygen isotope values(\begin{math}δ{}\end{math}18O= 6.1- 9.2\textperthousand{})reflecting low temperature alteration at \begin{math}<\end{math} 200-250\textdegree{}C. Below 1100 mbsf the rocks are dominated by depleted \begin{math}δ{}\end{math}18O values from 3.0 to 6.0\textperthousand{}, which indicates alteration at high temperature (\begin{math}>\end{math}250\textdegree{}C). In the sheeted dike complex the whole rock \begin{math}δ{}\end{math}18O values gradually decrease to a minimum of 3.0\textperthousand{} at \textasciitilde{}1350 mbsf and then increase toward fresh MORB \begin{math}δ{}\end{math}18O value at the top of plutonic section. A down-hole profile of lithium concentrations mimics the oxygen isotope variation trend. Above the depth of \textasciitilde{} 900 mbsf the oceanic crust is dominated by rocks with enriched lithium content relative to fresh MORB (3ppm) and below that depth lithium concentrations gradually decrease with depth and then slowly increase toward the value for unaltered MORBs with a kink at \textasciitilde{} 1350 mbsf. Li isotopes behave differently. The upper volcanic zone is characterized by a spread of \begin{math}δ{}\end{math}7Li from depleted to enriched values. Cooper et al.(2008) showed that most of the whole-rocks in this zone are within the range of \begin{math}δ{}\end{math}7Li for fresh EPR samples(3.1-5.2 \textperthousand{}), with a few of the samples having lower or slightly higher \begin{math}δ{}\end{math}7Li, although the vein minerals from Leg 206 samples all have high\begin{math}δ{}\end{math}7Li. The majority of the massive flows and sheeted dikes show enriched \begin{math}δ{}\end{math}7Li values. In the deeper sheeted dike complex, \begin{math}δ{}\end{math}7Li values show a steep gradient from enrichment to depletion and then return to values for fresh MORBs after several tens of meters in the plutonic section. This indicates a rapid change of the hydrothermal system from water domination (w/r\begin{math}>\end{math}1) to rock domination(w/r\begin{math}<\end{math}1) and a restricted seawater penetration in the plutonic section. A systematic shift from enriched to depleted \begin{math}δ{}\end{math}7Li values of whole rocks occurs at a much deeper depth at \textasciitilde{} 1300 mbsf. Whole rocks at the kink position around 1350 mbsf - defined by the down-hole variation of \begin{math}δ{}\end{math}18O values - also show the lowest \begin{math}δ{}\end{math}7Li values in the lower sheeted dike complex. The kink position probably marks the transition from a fluid dominated system to a rock dominated system, which may indicate the presence of a zone of hydrothermal upwelling at the top of plutonic rocks.
V11E-08
Silicon Isotope Fractionation During Acid Water-Igneous Rock Interaction
Silica enrichment by metasomatic/hydrothermal alteration is a widespread phenomenon in crustal environments where acid fluids interact with silicate rocks. High-sulfidation epithermal ore deposits and acid-leached residues at hot-spring settings are among the best known examples. Acid alteration acting on basalts has also been invoked to explain the relatively high silica contents of the surface of Mars. We have analyzed basaltic-andesitic lavas from the Kawah Ijen volcanic complex (East Java, Indonesia) that were altered by interaction with highly acid (pH~1) sulfate-chloride water of its crater lake and seepage stream. Quantitative removal of major elements during this interaction has led to relative increase in SiO2 contents. Our silicon isotope data, obtained by HR-MC-ICPMS and reported relative to the NIST RM8546 (=NBS28) standard, show a systematic increase in &δ&&30Si from -0.2‰ (±0.3, 2sd) for unaltered andesites and basalts to +1.5‰ (±0.3, 2sd) for the most altered/silicified rocks. These results demonstrate that silicification induced by pervasive acid alteration is accompanied by significant Si isotope fractionation, so that alterered products become isotopically heavier than the precursor rocks. Despite the observed enrichment in SiO2, the rocks have experienced an overall net loss of silicon upon alteration, if Nb is considered as perfectly immobile. The observed &δ&&30Si values of the alteration products appeared to correlate well with the inferred amounts of silicon loss. These findings would suggest that &28Si is preferentially leached during water-rock interaction, implying that dissolved silica in the ambient lake and stream water is isotopically light. However, layered opaline lake sediments, that are believed to represent precipitates from the silica-saturated water show a conspicuous &30Si-enrichment (+1.2 ± 0.2‰). Because anorganic precipitation is known to discriminate against the heavy isotope (e.g. Basile- Doelsch et al., 2006), the &δ&&30Si value of dissolved silicon in the lake water must be even higher. We infer that progressive cation removal alone is inadequate to describe rock dissolution and silicification by acid fluid. Exchange of silicon between the solution and mineral phases probably accompanied the alteration process. This hypothesis is qualitatively consistent with the idea that elements in solution take part in the formation of altered silica-rich layers at mineral-solution interfaces, as invoked to interpret surface reactions during silicate mineral weathering (e.g., Adriaens et al., 1999; Hellmann et al., 2003). References Adriaens et al., 1999. Surf. Interface Anal., 27: 8-23 Basile-Doelsch et al., 2006. Nature, 433: 399-402. Hellmann et al., 2003. Phys. Chem. Minerals, 30: 192-197.