Oliver Plümper

Effect of Secondary Phase Formation on the Carbonation of Olivine

Large-scale olivine carbonation has been proposed as a potential method for sequestering CO(2) emissions. For in situ carbonation techniques, understanding the relationship between the formation of carbonate and other phases is important to predict the impact of possible passivating layers on the reaction. Therefore, we have conducted reactions of olivine with carbonated saline solutions in unstirred batch reactors. Altering the reaction conditions changed the Mg-carbonate morphology. We propose that this corresponded to changes in the ability of the system to precipitate hydromagnesite or magnesite. During high-temperature reactions (200 degrees C), an amorphous silica-enriched phase was precipitated that was transformed to lizardite as the reaction progressed. Hematite was also precipitated in the initial stages of these reactions but dissolved as the reaction proceeded. Comparison of the experimental observations with reaction models indicates that the reactions are governed by the interfacial fluid composition. The presence of a new Mg-silicate phase and the formation of secondary products at the olivine surface are likely to limit the extent of olivine to carbonate conversion.

The interface-scale mechanism of reaction-induced fracturing during serpentinization

Peridotite serpentinization has first-order effects on geochemical and petrophysical processes in the lithosphere. This process induces intensive fracturing, generating fluid pathways to facilitate the hydration of vast amounts of originally impermeable rocks, but the mechanism linking interfacial reaction processes with fracture propagation has not been understood. By combining microstructural characteristics of olivine lizardite-serpentinization with fundamental aspects of interface-coupled dissolution-precipitation and crack growth theory, we propose a microstructurally consistent, self-propagating fracturing mechanism. Fracturing is driven by stress generated from the growth and transformation of a metastable amorphous proto-serpentine phase, where stress is localized within surface perturbations (etch pits and coalesced etch pits) that originate from the anisotropic dissolution of olivine. Water migration into fractures reiterates the process, resulting in hierarchical olivine grain segmentation. Our results indicate that the advancement of serpentinization at the grain scale is independent of solid-state diffusion and does not rely on external forces.

Superplastic nanofibrous slip zones control seismogenic fault friction

Understanding the internal mechanisms controlling fault friction is crucial for understanding seismogenic slip on active faults. Displacement in such fault zones is frequently localized on highly reflective (mirrorlike) slip surfaces, coated with thin films of nanogranular fault rock. We show that mirror-slip surfaces developed in experimentally simulated calcite faults consist of aligned nanogranular chains or fibers that are ductile at room conditions. These microstructures and associated frictional data suggest a fault-slip mechanism resembling classical Ashby-Verrall superplasticity, capable of producing unstable fault slip. Diffusive mass transfer in nanocrystalline calcite gouge is shown to be fast enough for this mechanism to control seismogenesis in limestone terrains. With nanogranular fault surfaces becoming increasingly recognized in crustal faults, the proposed mechanism may be generally relevant to crustal seismogenesis. Nanogranular microstructures found in simulated carbonate faults control the physical sliding mechanism during rupture. Nanofibers involved in fault rupture Changing fault properties during rupture dictates the size and extent of an earthquake. Faulting leads to well-known microstructures that may play a role in how natural faults slip during rupture. Verberne et al. investigated tiny, nanogranular fibers found in microstructures generated on simulated carbonate faults. A microphysical model was able to account for how the small and aligned fiber produced runaway fault slip, similar to that seen in natural faults. These small structures play a role in carbonate faulting and similar microstructures could control fault rupture in other types of rocks. Science, this issue p. 1342

Experimental investigations into the silicification of olivine: Implications for the reaction mechanism and acid neutralization

Abstract Neutralization of acidic fluids by means of fluid-olivine interactions is important in volcanic environments and has been proposed as a practical scheme for the neutralization of acidic sulfaterich fluids. To understand the interaction of olivine with highly acidic fluids we have reacted whole olivine crystals and a dunite cube with different sulfuric acid solutions at temperatures ranging from 60-120 °C. Reaction of olivine with 2 and 3.6 M acid concentrations produced a layered amorphous silica pseudomorph of the original olivine grain. The mechanism of pseudomorphic replacement was studied by reacting olivine with an 18O-enriched acid solution and examining the products using Raman spectroscopy. Peak shifts in the Raman spectra show that 18O was incorporated into the silica rim, including the siloxane ring structures. The formation of a layered silica pseudomorph, the incorporation of 18O into the silica rim and the dependence of the replacement rim strength on the acid concentration indicate that the pseudomorphic replacement occurred by means of an interface-coupled dissolution-reprecipitation mechanism. When olivine was reacted with 1 M sulfuric acid amorphous silica was produced but no longer formed a pseudomorph of the olivine grain. Reaction with 0.1 M acid, or solutions containing Na, encouraged the formation of hematite as well as amorphous silica. From the known Fe-phase stabilities for our experimental conditions and the dependence of hematite formation on the presence of Na we propose that initially jarosite phases precipitated, which transformed into hematite during the experiment.

Nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation: Crystal plasticity at sub-seismic slip rates at 18–150 °C

A central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic-seismic transitions in fault gouges, and to identify microstructural indicators for such transitions. We present new data on the slip stability of calcite fault gouges, and on microstructural development down to the nanometer scale. Our experiments consisted of direct shear tests performed dry at slip rates of 0.1–10 μm/s, at a constant normal stress of 50 MPa, at 18–150 °C. The results show a transition from stable to (potentially) unstable slip above ~80 °C. All samples recovered showed an optical microstructure characterized by narrow, 30–40-μm-wide, Riedel and boundary shear bands marked by extreme grain comminution, and a crystallographic preferred orientation (CPO). Boundary shear bands, sectioned using FIB-SEM (focused ion beam scanning electron microscopy), revealed angular grain fragments decreasing from 10 to 20 μm at the outer margins to ~0.3 μm in the shear band core, where dense aggregates of nanograins also occurred. Transmission electron microscopy, applied to foils extracted from boundary shears using FIB-SEM, combined with the optical CPO, showed that these aggregates consist of calcite nanocrystals, 5–20 nm in size, with the (104)[201] dislocation glide system oriented parallel to the shear plane and direction. Our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity. Because crystal plasticity is strongly thermally activated, we infer that the transition to velocity-weakening slip is likely due to enhanced crystal plasticity at >80 °C. This implies that tectonically active limestone terrains will tend to be particularly prone to shallow-focus seismicity.

Magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes

Serpentinization of ultramafic rocks during hydrothermal alteration at mid-ocean ridges profoundly changes the physical, chemical, rheological, and magnetic properties of the oceanic lithosphere. There is renewed interest in this process following the discovery of widespread exposures of serpentinized mantle on the seafloor in slow spreading oceans. Unroofing of mantle rocks in these settings is achieved by displacement along oceanic detachment faults, which eventually results in structures known as oceanic core complexes (OCCs). However, we have limited understanding of the mechanisms of serpentinization at the seafloor and in particular their relationship with the evolution of OCCs. Since magnetite is a direct product of serpentinization, the magnetic properties of variably serpentinized peridotites can provide unique insights into these mechanisms and their evolution in the oceanic lithosphere. Here we present new results from an integrated, rock magnetic, paleomagnetic, and petrological study of variably serpentinized peridotites from the first fossil OCC recognized in an ophiolite. Integration with existing data from mid-ocean ridge-related abyssal peridotites recovered from several scientific ocean drilling sites yields the first magnetic database from peridotites extending across the complete range (0–100%) of degrees of serpentinization. Variations in a range of magnetic parameters with serpentinization, and associated paleomagnetic data, provide: (i) key constraints on the mechanism(s) of serpentinization at mid-ocean ridges; (ii) insights on the potential for serpentinized peridotites to contribute to marine magnetic anomalies; and (iii) evidence that leads to a new conceptual model for the evolution of serpentinization and related remanence acquisition at OCCs.

Dynamics of intraoceanic subduction initiation: 1. Oceanic detachment fault inversion and the formation of supra‐subduction zone ophiolites

Subduction initiation is a critical link in the plate tectonic cycle. Intraoceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid-ocean ridges, as in e.g., the Neotethys Ocean during the Jurassic, remains a matter of debate. In recent years, extensional detachment faults have been widely documented adjacent to slow-spreading and ultraslow-spreading ridges where they cut across the oceanic lithosphere. These structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration, and can therefore effectively localize deformation. Here, we show geochemical, tectonic, and paleomagnetic evidence from the Jurassic ophiolites of Albania and Greece for a subduction zone formed in the western Neotethys parallel to a spreading ridge along an oceanic detachment fault. With 2-D numerical modeling exploring the evolution of a detachment-ridge system experiencing compression, we show that serpentinized detachments are always weaker than spreading ridges. We conclude that, owing to their extreme weakness, oceanic detachments can effectively localize deformation under perpendicular far-field forcing, providing ideal conditions to nucleate new subduction zones parallel and close to (or at) spreading ridges. Direct implication of this, is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge. Our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context, and calls for future research on three-dimensional modeling of subduction initiation and how upper plate extension is associated with that.

Dynamics of intraoceanic subduction initiation : 2. Suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions

Analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle. Here we focus on suprasubduction zone (SSZ) ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at fore-arc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off-ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab pull due to, e.g., eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere, and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far-field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle.

Fragmentation of wall rock garnets during deep crustal earthquakes

Deep crustal earthquakes cause fragmentation of wall rocks. Fractures and faults riddle the Earth’s crust on all scales, and the deformation associated with them is presumed to have had significant effects on its petrological and structural evolution. However, despite the abundance of directly observable earthquake activity, unequivocal evidence for seismic slip rates along ancient faults is rare and usually related to frictional melting and the formation of pseudotachylites. We report novel microstructures from garnet crystals in the immediate vicinity of seismic slip planes that transected lower crustal granulites during intermediate-depth earthquakes in the Bergen Arcs area, western Norway, some 420 million years ago. Seismic loading caused massive dislocation formations and fragmentation of wall rock garnets. Microfracturing and the injection of sulfide melts occurred during an early stage of loading. Subsequent dilation caused pervasive transport of fluids into the garnets along a network of microfractures, dislocations, and subgrain and grain boundaries, leading to the growth of abundant mineral inclusions inside the fragmented garnets. Recrystallization by grain boundary migration closed most of the pores and fractures generated by the seismic event. This wall rock alteration represents the initial stages of an earthquake-triggered metamorphic transformation process that ultimately led to reworking of the lower crust on a regional scale.

Fast-moving dislocations trigger flash weakening in carbonate-bearing faults during earthquakes

Rupture fronts can cause fault displacement, reaching speeds up to several ms−1 within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicate-bearing rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening). Flash weakening is also observed in experiments performed in carbonate-bearing rocks but evidence for melting is lacking. To unravel the micro-physical mechanisms associated with flash weakening in carbonates, experiments were conducted on pre-cut Carrara marble cylinders using a rotary shear apparatus at conditions relevant to earthquakes propagation. In the first 5 mm of slip the shear stress was reduced up to 30% and CO2 was released. Focused ion beam, scanning and transmission electron microscopy investigations of the slipping zones reveal the presence of calcite nanograins and amorphous carbon. We interpret the CO2 release, the formation of nanograins and amorphous carbon to be the result of a shock-like stress release associated with the migration of fast-moving dislocations. Amorphous carbon, given its low friction coefficient, is responsible for flash weakening and promotes the propagation of the seismic rupture in carbonate-bearing fault patches.