Nina S. C. Simon

The origin of garnet and clinopyroxene in “depleted” Kaapvaal peridotites

A detailed petrographic, major and trace element and isotope (Re–Os) study is presented on 18 xenoliths from Northern Lesotho kimberlites. The samples represent typical coarse, low-temperature garnet and spinel peridotites and span a P–T range from f60 to 150 km depth. With the exception of one sample (that belongs to the ilmenite–rutile– phlogopite–sulphide suite (IRPS) suite first described by [B. Harte, P.A. Winterburn, J.J. Gurney, Metasomatic and enrichment phenomena in garnet peridotite facies mantle xenoliths from the Matsoku kimberlite pipe, Lesotho. In: Menzies, M. (Ed.), Mantle metsasomatism. Academic Press, London 1987, 145–220.]), all samples considered here have high Mg# and show strong depletion in CaO and Al2O3. They have bulk rock Re depletion ages (TRD) >2.5 Ga and are therefore interpreted as residua from large volume melting in the Archaean. A characteristic of Kaapvaal xenoliths, however, is their high SiO2 concentrations, and hence, modal orthopyroxene contents that are inconsistent with a simple residual origin of these samples. Moreover, trace element signatures show strong overall incompatible element enrichment and REE disequilibrium between garnet and clinopyroxene. Textural and subtle major element disequilibria were also observed. We therefore conclude that garnet and clinopyroxene are not co-genetic and suggest that (most) clinopyroxene in the Archaean Kaapvaal peridotite xenoliths is of metasomatic origin and crystallized relatively recently, possibly from a melt precursory to the kimberlite. Possible explanations for the origin of garnet are exsolution from a high-temperature, Al- and Ca-rich orthopyroxene (indicating primary melt extraction at shallow levels) or a majorite phase (primary melting at >6 GPa). Mass balance calculations, however, show that not all garnet observed in the samples today is of a simple exsolution origin. The extreme LREE enrichment (sigmoidal REE pattern in all garnet cores) is also inconsistent with exsolution from a residual orthopyroxene. Therefore, extensive metasomatism and probably re-crystallization of the lithosphere after melt-depletion and garnet exsolution is required to obtain the present textural and compositional features of the xenoliths. The metasomatic agent that modified or perhaps even precipitated garnet was a highly fractionated melt or fluid that might have been derived from the asthenosphere or from recycled oceanic crust. Since, to date, partitioning of trace elements between orthopyroxene and garnet/clinopyroxene is poorly constrained, it was impossible to assess if orthopyroxene is in chemical equilibrium with garnet or clinopyroxene. Therefore, further trace element and isotopic studies are required to

Modeling of craton stability using a viscoelastic rheology

Archean cratons belong to the most remarkable features of our planet since they represent continental crust that has avoided reworking for several billions of years. Even more, it has become evident from both geophysical and petrological studies that cratons exhibit deep lithospheric keels which equally remained stable ever since the formation of the cratons in the Archean. Dating of inclusions in diamonds from kimberlite pipes gives Archean ages, suggesting that the Archean lithosphere must have been cold soon after its formation in the Archean (in order to allow for the existence of diamonds) and must have stayed in that state ever since. Yet, although strong evidence for the thermal stability of Archean cratonic lithosphere for billions of years is provided by diamond dating, the long-term thermal stability of cratonic keels was questioned on the basis of numerical modeling results. We devised a viscoelastic mantle convection model for exploring cratonic stability in the stagnant lid regime. Our modeling results indicate that within the limitations of the stagnant lid approach, the application of a sufficiently high temperature-dependent viscosity ratio can provide for thermal craton stability for billions of years. The comparison between simulations with viscous and viscoelastic rheology indicates no significant influence of elasticity on craton stability. Yet, a viscoelastic rheology provides a physical transition from viscously to elastically dominated regimes within the keel, thus rendering introduction of arbitrary viscosity cutoffs, as employed in viscous models, unnecessary.

Spontaneous formation of fluid escape pipes from subsurface reservoirs

Ubiquitous observations of channelised fluid flow in the form of pipes or chimney-like features in sedimentary sequences provide strong evidence for significant transient permeability-generation in the subsurface. Understanding the mechanisms and dynamics for spontaneous flow localisation into fluid conductive chimneys is vital for natural fluid migration and anthropogenic fluid and gas operations, and in waste sequestration. Yet no model exists that can predict how, when, or where these conduits form. Here we propose a physical mechanism and show that pipes and chimneys can form spontaneously through hydro-mechanical coupling between fluid flow and solid deformation. By resolving both fluid flow and shear deformation of the matrix in three dimensions, we predict fluid flux and matrix stress distribution over time. The pipes constitute efficient fluid pathways with permeability enhancement exceeding three orders of magnitude. We find that in essentially impermeable shale (10−19 m2), vertical fluid migration rates in the high-permeability pipes or chimneys approach rates expected in permeable sandstones (10−15 m2). This previously unidentified fluid focusing mechanism bridges the gap between observations and established conceptual models for overcoming and destroying assumed impermeable barriers. This mechanism therefore has a profound impact on assessing the evolution of leakage pathways in natural gas emissions, for reliable risk assessment for long-term subsurface waste storage, or CO2 sequestration.

Shear-induced Dilation and its Implications for Chimney Flow in Porous Rocks

new model of shear-induced dilation and shear-enhanced compaction in brittle (elastic) and ductile (viscous) rocks is proposed. The essential feature of the model is the dependence of the porosity equation on the equivalent shear stress. This allows dilation of the pore space even at nominally compressive effective pressures in agreement with experimental data. The implications for the formation of fluid- or gas-filled chimneys are considered. Spontaneous self-localization of Darcy flow in a deforming porous rock due to preferential dilation of the pore space is a viable mechanism for chimney formation.

High Permeability Pathways Triggered by Subsurface Storage Operations

High permeability fluid flow pathways are widely observed in nature, and their formation occurs on both geological and human timescales. Outcrop study as well as interpretation of seismic cross-section show clear evidences of these multi-scale vertical pipe features, where unconsolidated to loose material is present inside. Even if well documented, their formation process remains still not answered yet. We propose a physically consistent 3D two-phase model of focusing fluid flow that allow the formation of such vertical high permeability channels. Viscous or creep rheology is the key feature to explain this formation process. Our result show that the proposed mechanism triggers pipe formation where permeability increases over two orders of magnitude in impermeable shale, and with propagation speed close to 2 meters per year in these usual ceiling rocks. Our results are in good accordance with the values needed to explain the fast vertical breakthrough of CO2 plume in the layered Sleipner saline aquifer.

The Importance of Poromechanics for CO2 Storage Integrity

Understanding deformation related to CO2 injection into subsurface reservoirs is crucial to ensure storage integrity, but our understanding is hampered by the lack of models that capture the non-linear interaction between fluid flow and complexly deforming rocks. We developed new fully coupled (i.e. the solid feels the fluid pressure and fluid flow is affected by solid stresses and deformation of the porous rock) models that allow the simulation of CO2 injection and flow in a stressed crust that may deform visco-elastically, including non-linear rheology leading to weakening. These simulations reproduce features observed in CO2 injection operations, such as localization of flow into chimneys or channels and flow that is (locally and periodically) faster than predicted by Darcy flow. The model also allows investigating the effect of injection on the over-, under- and sideburden of the reservoir and to predict the effect of stress changes in the neighbouring formations. Comparison of a fully coupled with an incompletely coupled simulation illustrates the necessity of proper coupling.

Dynamic Permeability due to Physical Coupling of Reactive CO2-flow and Deformation

Low permeability reservoirs often show variations of permeability over time, which are conventionally explained by the reactivation or opening of existing fractures or the formation of a new network of (hydro-)fractures due to high fluid pressures. Altern