Volcanology, Geochemistry, Petrology [V]

V31C   MCW:Level 1   Wednesday  0800h

The Dynamic Reaction: Interactions of Metamorphic Reactions and Deformation in Nature, Experiments, and Models I Posters

Presiding: D L Whitney, Geology and Geophysics,University of Minnesota; C Holyoke III, Brown University


The Effects of Carbonic Fluids on Quartz Deformation: An Experimental Investigation

* Chernak, L J (Linda_Chernak@Brown.edu) , Department of Geological Sciences, Brown University, Box 1846 324 Brook St., Providence, RI 02912, United States
Tullis, J (Jan_Tullis@Brown.edu) , Department of Geological Sciences, Brown University, Box 1846 324 Brook St., Providence, RI 02912, United States

It has long been known that trace amounts of water significantly decrease the dislocation creep strength of quartz but there has been little research on the effects of other fluids. Recent observations of naturally deformed quartz- rich rocks indicate that carbonic fluids cause strengthening and/or embrittlement. We have undertaken an experimental study to examine the effects of carbonic fluids on quartz deformed in the dislocation creep regime. An initial suite of constant displacement rate, axial compression experiments at $900\deg$C, 1.5 GPa and 10$^{- 5}$/s have been done on 4 materials: Black Hills quartzite (BHQ; both aqueous and carbonic fluid inclusions (FIs) with an FTIR-determined sub-optical scale water content of ~0.06 wt %); Heavitree quartzite (HQ; dominantly methane FIs; FTIR water content of ~0.03 wt %); and 2 samples of vein quartz, AL3-2 (dominantly aqueous FIs with an FTIR water content of ~0.0035 wt %) and AL3-7 (dominantly carbonic FIs with an FTIR water content of ~0.111 wt %). For the quartzites, ~50% shortening removes most FIs from grain centers. Water from the aqueous FIs becomes invisibly distributed along grain boundaries, whereas the carbonic fluids re-form distinct FIs along grain boundaries sub-parallel to compression, consistent with the non-wetting behavior of carbonic fluids in quartz. As-is BHQ is weaker than as-is HQ (yield stress of 475 vs. 550 MPa, respectively). Similarly, vein quartz AL3-7 is weaker than AL3-2 (400 vs. 675 MPa, respectively). These results indicate that the sub-optical scale water content is most important for the dislocation creep strength. For BHQ buffered by Ni-NiO, the addition of ~0.05 wt % water reduces the flow stress by ~125 MPa and produces �higher temperature' dislocation creep microstructures. Preliminary experiments on BHQ with a thin central layer of dolomite powder, which reacts with quartz to produce diopside and CO$_{2}$, gave puzzling results. A sample at P & T for only 1 hr before deformation started, with only minor reaction, had comparable strength to the as-is sample but showed extra high concentrations of CO$_{2}$ FIs on quartz grain boundaries close to the dolomite layer. However, a sample held at P & T for 85 hrs before deformation started was weaker than the water-added sample (150 vs. 350 MPa, respectively); the quartz near the reacted layer was highly deformed with more and larger recrystallized grains than the water-added sample and abundant but small and dispersed CO$_{2}$ FIs, whereas the quartz grains toward the ends of the sample were little deformed or recrystallized, with high concentrations of irregular large CO$_{2}$ FIs along their boundaries. These data show that cause-and-effect relationships between the release of metamorphic fluids and deformation are complex and not easily predicted.


Reaction-Enhanced Strain and Fabric Development Following Pluton Emplacement in Ductile Shear Zones

* Marsh, J H (jeff.marsh@umit.maine.edu) , Department of Earth Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, ME 04469, United States
Johnson, S E (johnsons@maine.edu) , Department of Earth Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, ME 04469, United States
Koons, P O (peter.koons@maine.edu) , Department of Earth Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, ME 04469, United States

We investigate the role of metamorphic hydration reactions and fluid advection in grain-scale rheological weakening processes in plutonic rocks emplaced into active ductile shear zones in the northeastern Appalachian Mountains of North America. Ductile shear zones may act as self-organized, dissipative systems, in which the localization of strain weakens host rocks sufficiently to generate fluid/melt pathways that may further weaken the zone. This may facilitate continued accumulation of strain and/or subsequent rejuvenation. In the presence of pressurized fluids/melt, strain may be partitioned heterogeneously within the shear zones as melts crystallize and host rocks are injected with fluids/melt. Locally, diffusion and metamorphic reaction rates are increased due to strain, thermal adjustment and activity of a liquid phase. In this study, two Paleozoic ductile shear zones have been chosen for the investigation of the transient, non-linear coupling of metamorphic reactions, deformation, diffusion and fluid advection. Detailed mapping of sheared plutonic rocks within the Washington Shear Zone (WSZ) of south-central Maine and the Cape Freels Shear Zone (CFSZ) of the eastern Gander Zone, Newfoundland has identified large strain and fabric gradients within each unit. At each of the localities, transition from a weak tectonic �LS� fabric to a strongly mylonitic/schistose fabric proximal to the host rock contact is accompanied by metamorphic hydration reactions. In the WSZ, grain-size reduction and development of a strong planar fabric (schistosity) near the contact with the host rocks coincides with the reaction of clinopyroxene + H20 => hornblende. In the CFSZ, a sharp transition to a mylonitic fabric within 25m of the host rock contact is accompanied by the reaction of K-feldspar + H20 => muscovite + quartz. Thus, approaching the pluton margins, mylonitization is facilitated by the rheological weakening that results from the addition of smaller, relatively weak phases in place of the stronger clinopyroxene and K-feldspar grains. Microstructural and crystallo-chemical data obtained through the use of Electron Microprobe and Electron Back-Scatter Diffraction techniques provide quantitative evaluation of the grain-scale processes at work during reaction-enhanced shearing at these locations.


An Effect of Brittle Deformation on Metamorphic Reactions as Seen in the Albite to Jadeite + Coesite Transition

* Gleason, G (gleasong@cortland.edu) , Geology Dept., SUNY Cortland, Cortland, NY 13045, United States
Green, H W (harry.green@ucr.edu) , IGPP, Dept. Earth Sciences, UC Riverside, Riverside, CA 92521, United States

We present an experimental study in which brittle faults in albitite provide localization sites for jadeite and coesite nucleation once the rock is in the conditions of the high pressure-phase stability field. Cores of natural albitite (grain size of ~120 x 40 x 40 $\mu$m, vacuum dried at 110$\deg$C for 12 hrs, sealed in Pt cans) were used in three types of experiments. In the first type, a sample was pressurized to 2.6 GPa at room temp. and developed faults 30$\deg$ to the length of the sample, corresponding to the maximum compressive stress direction during pressurization. In the second type, samples were pressurized to 3.5 GPa, then annealed at 800$\deg$C for 3 hrs, and developed zones of new phases along the pre-existing (and still visible) faults. In the third type, a sample was pressurized to 1.3 GPa, faulting occurred, then held at 800$\deg$C for 24 hrs in order to anneal the brittle structures in the albite field. Temp. was then lowered to 500$\deg$C and pressurization continued to 2.8 GPa. Once at 2.8 GPa, the temperature was raised to 800$\deg$C, bringing the metastable albite into a temp. range which promoted the kinetics of the transformation. The specimen was annealed for 1.75 hrs prior to deformation at a strain rate of 10$^{-4}$s#^{-1}$ for .75 hrs resulting in a total natural strain of 0.45. While under load, the experiment was quenched to room temperature in less than one minute. In the second type of samples, the new phases are heterogeneously distributed. Within the fault zones, 40 to 60% of the material is new phases: jadeite + coesite, whereas outside the fault zones, less than 10% of the material is jadeite + coesite. In the third type of sample, only 1% of the material is new phases: some in the healed fault and some outside of it. Transmission electron microscopy of the faulted-only sample (the first type) reveals the extremely fine grain size (~40 nm dia.) associated with the fault. This microstructural data combined with the differences in amount of new phases in the second and third types of samples, indicate that the presence of the faults, and the reduced grain size associated with them, promoted nucleation of the denser phases.


Forward Analyses of Retrogressive Rehydration Reactions of Eclogite

* Kuwatani, T (tatsu@eps.s.u-tokyo.ac.jp) , Dept. Earth and Planetary Sci., Univ. Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
Toriumi, M (tori@k.u-tokyo.ac.jp) , Dept. Complexity Sci. and Engi., Univ. Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8561 Japan
Okamoto, A (okamoto@mail.kankyo.tohoku.ac.jp) , Dept. Environmental Stud., Tohoku Univ, Aoba 6-6-20 Aoba-ku Aramaki, Sendai, 980-8579 Japan
Katayama, I (katayama@gaea.k.u-tokyo.ac.jp) , Dept. Complexity Sci. and Engi., Univ. Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8561 Japan

Recent studies have shown that metamorphic belts underwent pervasive retrogressive hydration metamorphism (ex. Ota et al. 2004; Okamoto and Toriumi 2005 etc. in the Sambagawa metamorphic belt). It is very important to develop the methodology which can deal quantitatively with the progress of metamorphic rehydration reactions. Retrogressive rehydration reactions (1) require the enough supply of external water, and (2) proceed heterogeneously. These make it impossible for the conventional methodologies, which inevitably assume the excess of water and perfect equilibrium in the whole system, to analyze metamorphic rehydration. In this study, we attempted to develop the new methodology of forward analyses of retrogressive rehydration reactions by improving the differential thermodynamic forward modeling (Spear 1993). In differential thermodynamic forward modeling, three types of equations (thermodynamic equilibrium constraints, stoichiometric constraints and mass balance constraints) are written in total differential forms. The variables in this system of equations are the changes of temperature (dT), pressure (dP), chemical compositions of minerals (dXs) and molar amounts of minerals (dMs). The thermodynamic system with fixed bulk chemical composition is divariant as specified in Duhem�fs theorem. Therefore, applying a series of dP) and dT as independent parameters from the initial conditions, we can trace continuous dXs and dMs as dependent variables, in other words, the progress (evolution) of the metamorphic reactions, along the arbitrary P-T trajectory. The above forward model can apply directly to dehydration metamorphism due to the independence of the amount of water in the system and perfect equilibrium, so we can calculate the amount of dehydrated water (dMwater) as dependent variables along the arbitrary P-T path (Kuwatani et al. 2005 AGU Fall Meeting). On the other hand, we have to introduce (1) the dependence of water amount of hydration and (2) partial equilibrium into the forward modeling in order to apply to retrogressive hydration reactions. In this study, we made the following two assumptions: (1) only water is supplied from the outside of the system (partial open system, and we consider this amount of supplied water (dMwater) as the new independent variable as well as dP and dT, and (2) we add the total amount of non-equilibrium mineral (dMnon-equilibrium) to the dependent variables. The system of equations of the forward modeling still remains solvable. Thus, in this model, we can simulate the reduction of the non-equilibrated area (relic minerals and the interior parts in zoned minerals, etc.) and growth of equilibrated minerals, in response to hydration (dMwater) and dP and dT during retrograde metamorphism. From the results of several conditions, we found that the changes of pressure and temperature control the mineral compositions and mode of equilibrated minerals, and that the extent of reactions increases in proportion to hydration (dMwater). In this presentation, we will introduce, for example, the results of forward analyses of retrogressive rehydration reactions of eclogite, and discuss the validity of our model and interdependence between hydration and deformation/ kinetics in natural system by comparing to the observation of natural sample (the Iratsu eclogite in the Sambagawa metamorphic belt). Moreover we will discuss the influence of rehydration metamorphism on the exhumation of high-pressure rocks.


Deformation mode of talc from subsurface to dehydration conditions: implications for the subducting oceanic lithosphere

* Andreani, M (andreani@msem.univ-montp2.fr) , Lab. de Tectonophysique, Place Eugene Bataillon, Montpellier, 34095 France
Escartin, J (escartin@ipgp.jussieu.fr) , IPGP, 4 Place Jussieu, Paris, 75005 France
Hirth, G (ghirth@whoi.edu) , WHOI, McLean 208 MS#8, Woods Hole, MA 02543, United States
Evans, B (brievans@mit.edu) , MIT, 77 Massachussetts Ave., Cambridge, USA 02139 France

Talc forms as an alteration product of the oceanic lithosphere where it is particularly abundant along major oceanic faults and it persists at depth during subduction. Its presence considerably modifies the mechanical properties of the lithosphere that will evolve along the subduction path down to the dehydration field of talc. In order to study the evolution of deformation modes in talc over this wide range of conditions deformation experiments were conducted on intact cores of talc from room temperature (T) to 850�C, at confining pressures (P) from 0 to 300 MPa, and at strain rates of ~10-5 s-1 (We actually tested a range of strain rates) Microstructural analyses were performed by coupling optical and electron microscopy (SEM, TEM). Mechanical tests confirm the very low strength of talc and reveal a modest positive dependence on confining pressure and decreasing temperature. Shear microcracking is dominant at low P-T conditions and accompanies talc plasticity, marked by kinking and folding, activated at higher temperature conditions. Both mechanisms are controlled by the weak van der waals forces bonding (001) planes in talc. Even at high P-T conditions, strain is localized along semi-brittle shear zones, despite the fact that sample strengths remain significantly below the confining pressure. A transition from stable to unstable sliding occurs over 600-700�C, close to the dehydration T of talc. An incipient dehydration of deformed talc at 700�C is suggested under TEM but dehydration products are only clearly identified during deformation at 850�C: Talc = Enstatite + amorphous SiO2. Dehydration is localized in the main deformation band where it is marked by the formation of localized zones of high porosity, with a �cataclastic� microstructure in which broken talc lamellae form a network of thin filaments enclosing nanometric enstatite grains. This microstructure may result from an increase of local fluid pressure similar to what may trigger deep earthquakes along the subducting slab. At shallower levels, also typical of oceanic detachment faults, small amounts of talc (the weakest mineral) should promote localized deformation, even at low differential stress, and provide a stable sliding in the semi-brittle field.


A metamorphic view on earthquakes and fluid pathways in subducting oceanic plates

* John, T (timm.john@fys.uio.no) , Physics of Geological Processes, Oslo University, PO BOX 1048 Blindern, 0316, Oslo Norway

Two main hypotheses have been proposed to explain intermediate-depth (70-300km) intra-slab seismicity; one suggests that high fluid pressures lead to dehydration embrittlement that triggers earthquakes, the other suggests that melt shear instabilities trigger seismic slip and may thereby produce permeabilities. Dehydration embrittlement seems to cause the earthquakes in the hydrous parts of the slab, where e.g., wet blueschists transform to dry eclogites. Whereas the mostly anhydrous lower oceanic crust typically transform to eclogites due to infiltration of external fluids at depths well beyond equilibrium because of the kinetic hindrance of solid-solid transformation. The investigated field region consists of fragments of a paleo-subduction zone exposed in Zambia. Outcropping gabbros and eclogites have been interpreted to be cogenetic, representing relics of subducted lower oceanic crust. Gabbros and eclogites occur in close proximity and gradual stages of the prograde gabbro-to-eclogite transformation are preserved by disequilibrium textures of incomplete reactions. Eclogitization took place at conditions of about 630-690C and 2.6-2.8 GPa. Eclogitization was accompanied by channelized fluid flow that produced veins of eclogite-facies minerals. At one outcrop, it is possible to directly investigate rocks that experienced an earthquake during their burial in a subduction zone. Field evidence indicates that intermediate-depth earthquakes produce frictional melts in subducting slabs and that the seismic failure was subsequently followed, not preceded, by infiltration of external fluid. Shortly after pseudotachylyte formation an external hydrous fluid infiltrated the rocks, which implies that regions of seismic faulting became preferred higher permeability zones for subsequent fluid infiltration. This subsequent fluid flow led to continuous vein formation during ongoing burial. The initial frictional failure may have been caused by hydraulic fracturing due to external fluids, or have been a response to nearby volume reduction, or a combination of both. The findings, however, indicate that once rupture begins in a dry 'metastable' oceanic gabbro, frictional melting can promote intermediate-depth earthquakes under eclogite-facies conditions and this seismic event can produce permeabilities for external fluids. The passing fluids mobilize those trace elements from the eclogites, which are characteristic of a 'hydrous slab component' in arc magmas. Since the fluids released by dehydrating slabs are believed to be the primary trigger for arc magmatism, it is proposed that intermediate-depth earthquakes have the potential to produce fluid-pathways within and out of the slab.


Growth and Characterization of Complex Mineral Surfaces

Jettestuen, E (espen.jettestuen@fys.uio.no) , Physics of Geological Processes, University of Oslo, PO Box 1048, Blindern, Oslo, 0316 Norway
* Jamtveit, B (bjorn.jamtveit@geo.uio.no) , Physics of Geological Processes, University of Oslo, PO Box 1048, Blindern, Oslo, 0316 Norway
Podladchikov, Y (Podladchikov@fys.uio.no) , Physics of Geological Processes, University of Oslo, PO Box 1048, Blindern, Oslo, 0316 Norway
DeVilliers, S (v.s.d.de@fys.uio.no) , Physics of Geological Processes, University of Oslo, PO Box 1048, Blindern, Oslo, 0316 Norway
Amundsen, H (Hans.Amundsen@geo.uio.no) , Physics of Geological Processes, University of Oslo, PO Box 1048, Blindern, Oslo, 0316 Norway
Meakin, P (Paul.Meakin@inl.gov) , Physics of Geological Processes, University of Oslo, PO Box 1048, Blindern, Oslo, 0316 Norway
Meakin, P (Paul.Meakin@inl.gov) , Center for Advanced Modeling and Simulation, Idaho National Laboratory, Idaho Falls, ID 83415, United States

Precipitation of mineral aggregates near the Earths surface or in subsurface fractures and cavities often produces complex microstructures and surface morphologies. Here we demonstrate how a simple surface normal growth (SNG) process may produce microstructures and surface morphologies very similar to those observed in some natural carbonate systems. A simple SNG model was used to fit observed surfaces, thus providing information about the growth history and also about the frequency and spatial distribution of nucleation events during growth. The SNG model can be extended to systems in which the symmetry of precipitation is broken, for example by fluid flow. We show how a simple modification of the SNG model in which the local growth rate depends on the distance from a fluid source and the local slope or fluid flow rate, produces growth structures with many similarities to natural travertine deposits.


Quantification of Phase Distribution: Multiple Area Density Maps Using a Bitmap Image

* Kim, S (neogeo94@korea.ac.kr) , Department of Earth and Environmental Sciences, Korea University, Anam-dong, Seoul, 136-701 Korea, Republic of
Ree, J (reejh@korea.ac.kr) , Department of Earth and Environmental Sciences, Korea University, Anam-dong, Seoul, 136-701 Korea, Republic of

The spatial distribution pattern of constituent phases of rocks has important information for understanding nucleation processes of phases in igneous and metamotphic rocks, timing relationship between partial melting and deformation in migmatites, and effect of one phase on static or dynamic grain growth of another phase in metamorphic rocks. For the quantification of the spatial distribution of a phase, the most widely used is the ��distance-to-nearest-neighbor' method. However, the effects of local clustering, size and shape of a phase are not conisdered in this method. Although several methods have been proposed to overcome this problem, they do not provide a simple numerical result or show a complex relationship between the factors affecting the phase distribution. Here, we introduce a new method for much simpler quantification of the phase distiribution using image analysis techinques. In a bitmap image, the ��area density map' (ADM) is constructed by calculating pixel density of the objects (grains of a phase) at each pixel position for a unit area and the area of the objects within the unit area. In this single ADM, the density of an object increases continuously toward the center of the object reflecting the object size. By overlaying single ADM's with progessively increasing the size of the unit area, we can produce a synthesized map (or multiple ADM) having an average area density of all ADM's overlaid. In this multiple ADM, the average area density at each pixel position represents a value combining four factors; total fraction, clustering pattern, size and shape of the objects on an image. In a bitmap image containing objects on a null background, a pixel is used as a basic element. Pixels of input images have specific gray scale values to distinguish a grain from the background and individual grains. To construct the multiple ADM, we substitute an average density value calculated from number of pixels for the gray scale value at each position (i.e. attribute of each pixel). To quantify the effect of the size and shape of the objects, pixels only on indivudual objects are used for a statistical calculation. In simple cases having a constant shape, size and fraction of objects, our total area density of input images gives a specific numerical value representing the distribution pattern of objects (e.g. clustered, random and uniform distribution). Also, our multiple ADM can single out a numerical value for the effect of local clustering of the objects and has a potential to numerically analyze the effect of the size and shape of the objects.