[1] New thermal-petrologic models of subduction zones are used to test the hypothesis that intermediate-depth intraslab earthquakes are linked to metamorphic dehydration reactions in the subducting oceanic crust and mantle. We show that there is a correlation between the patterns of intermediate-depth seismicity and the locations of predicted hydrous minerals: Earthquakes occur in subducting slabs where dehydration is expected, and they are absent from parts of slabs predicted to be anhydrous. We propose that a subductingoceanicplatecanconsistoffourpetrologicallyandseismicallydistinctlayers:(1) hydrated, fine-grained basaltic upper crust dehydrating under equilibrium conditions and producing earthquakes facilitated by dehydration embrittlement; (2) coarse-grained, locally hydrated gabbroic lower crust that produces some earthquakes during dehydration but transformschieflyaseismicallytoeclogiteatdepthsbeyondequilibrium;(3)locallyhydrated uppermost mantle dehydrating under equilibrium conditions and producing earthquakes; and (4) anhydrous mantle lithosphere transforming sluggishly and aseismically to denser minerals. Fluid generated through dehydration reactions can move via at least three distinct flowpaths:percolationthroughlocal,transient,reaction-generatedhigh-permeabilityzones; flow through mode I cracks produced by the local stress state; and postseismic flow through fault zones. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7230 Seismology: Seismicity and seismotectonics; 8123 Tectonophysics: Dynamics, seismotectonics; 8135 Tectonophysics: Evolution of the Earth: Hydrothermalsystems (8424); 3660 Mineralogyand Petrology: Metamorphicpetrology;

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JB001129

Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?

In northern Italy in 1997, two earthquakes of magnitudes 5.7 and 6 (separated by nine hours) marked the beginning of a sequence that lasted more than 30 days, with thousands of aftershocks including four additional events with magnitudes between 5 and 6. This normal-faulting sequence is not well explained with models of elastic stress transfer, particularly the persistence of hanging-wall seismicity that included two events with magnitudes greater than 5. Here we show that this sequence may have been driven by a fluid pressure pulse generated from the coseismic release of a known deep source of trapped high-pressure carbon dioxide (CO2). We find a strong correlation between the high-pressure front and the aftershock hypocentres over a two-week period, using precise hypocentre locations and a simple model of nonlinear diffusion. The triggering amplitude (10-20 MPa) of the pressure pulse overwhelms the typical (0.1-0.2 MPa) range from stress changes in the usual stress triggering models. We propose that aftershocks of large earthquakes in such geologic environments may be driven by the coseismic release of trapped, high-pressure fluids propagating through damaged zones created by the mainshock. This may provide a link between earthquakes, aftershocks, crust/mantle degassing and earthquake-triggered large-scale fluid flow.

/pdf/aftershocks-driven-by-a-high-pressure-co2-source-at-depth-zdqs706kqs.pdf

Aftershocks driven by a high-pressure CO2 source at depth.

In the upper crust, where hydraulic gradients are typically 10 MPa km−1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ∼10−16 m² to ∼10−18 m² between 5- and 12-km depth Below ∼12 km, which broadly corresponds to the brittle-plastic transition, mean k is effectively independent of depth at ∼10−185±1 m² Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi-exponential decay of permeability with depth of log k ≈ −32 log z - 14, where k is in meters squared and z is in kilometers At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat Such conditions may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling

/pdf/permeability-of-the-continental-crust-implications-of-2k5x9p64pc.pdf

Permeability of the continental crust: Implications of geothermal data and metamorphic systems

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

The decline and fate of an iron-induced subarctic phytoplankton bloom

At subduction zones, fluid flow, pore pressure, and tectonic processes are tightly interconnected. Excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration, and it has profound effects on fault and earthquake mechanics through its control on effective stress. The egress of these overpressured fluids, which is in part governed by the presence of permeable fault zones, is a primary mechanism of volatile and solute transport to the oceans. Recent field measurements, new constraints gained from laboratory studies, and numerical modeling efforts have led to a greatly improved understanding of these coupled processes. Here, we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs, and focus on recent advances that have quantified permeability architecture, fluxes, the nature and timing of transience, and pressure distribution, thus providing new insights into the connections between fluid, metamorphic, mechanical, and fault slip proc...

Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure

Fluid is released by dehydration reactions during prograde metamorphism. If the dilation of the pore space is insufficient to provide storage for all the released fluid, then pore pressure excess is generated. Whether the excess can be maintained over long duration hinges on the hydraulic transport properties of the rock. Motivated by recent experimental and microstructural observations, we developed a theoretical model which incorporates dehydration and porosity production rates as source terms in the hydraulic diffusion equation. The permeability was assumed to be sensitively dependent on the porosity. The finite difference technique was used to analyze the generation and maintenance of pore pressure excess for several types of boundary conditions of importance in laboratory and crustal scales. Analytic estimates of the pore pressure anomaly were also obtained. The model is in reasonable agreement with experimental observations on dehydration-induced weakening and transient buildup of pore pressure in a nominally drained sample. It provides hydrogeological constraints on the development of pore pressure excess in metamorphic and tectonic settings. The maintenance of a nearly lithostatic pore pressure requires the permeability to be below a critical value which increases with increasing dehydration rate and thickness of the dehydrating layer, and with decreasing porosity production rate. If these constraints are not met, the pore pressure excess can only occur as a transient pulse, the amplitude of which may approach lithostatic for sufficiently large dehydration rate and layer thickness, or sufficiently low permeability.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JB02484

Generation and maintenance of pore pressure excess in a dehydrating system 2. Theoretical analysis

The generation and maintenance of excess pore pressures in dehydrating gypsum aggregates were investigated using experiments and microstructural analyses. A triaxial deformation apparatus, was equipped with a pore fluid system connected directly to the dehydrating sample. This system was operated in constant fluid volume mode to monitor pore pressure increase under undrained conditions, and in constant pore pressure mode to monitor fluid expulsion under drained conditions. X ray diffraction and backscatter scanning electron microscopy were used to characterize the spatial relationship among gypsum, the product phase bassanite, and the pores. In addition, we measured the permeability and pore compressibility of the starting material and explored the influence of effective and pore pressures, temperature, and axial load on fluid expulsion. Three stages of fluid expulsion and microstructural evolution during dehydration of an initially low-porosity, low-permeability gypsum aggregate are defined: (1) Initially, fluid released by the reaction is trapped in isolated or discontinuous pore networks and high pore pressures are possible. (2) An interconnected pore network eventually develops and fluid readily escapes. (3) Fluid expulsion slows down drastically as the reaction nears completion. As a result of coupling between dehydration and porosity production, both the cumulative volume of fluid expelled and the expulsion rate increase with increasing temperature, effective pressure, and axial load and with decreasing pore pressure. Our hydrological and microstructural data, combined with previous mechanical data, provide a better understanding of the relationships among changes in fluid volume, porosity, and pore pressure excess, and the deformation behavior of a dehydrating system where drainage evolves with time.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JB02485

Generation and maintenance of pore pressure excess in a dehydrating system 1. Experimental and microstructural observations

Deformation and pore pressure in dehydrating gypsum under transiently drained conditions

Dehydration reactions can induce excess pore-fluid pressures that are high enough to affect the mechanical strength and ductility of rocks. Laboratory experiments have been conducted to investigate this phenomenon in dehydrating rocks with porosities that are initially negligible but increase as the reaction progresses. Polycrystalline gypsum samples were first heated under pressure to induce dehydration, then axially loaded after different amounts of reaction. Within the first 1% of reaction, the ultimate strength of the sample decreased and embrittlement was observed. The ultimate strength recovered within 3% of reaction and eventually exceeded that of pure gypsum. Our results indicate that the initial weakening and embrittlement correspond to a pulse of excess pore pressure in the sample interior. Subsequent strengthening is caused by a decrease in pore pressure and the higher strength of the product phase.

Suz-chung Ko

Papers

Generation and maintenance of pore pressure excess in a dehydrating system 2. Theoretical analysis

Generation and maintenance of pore pressure excess in a dehydrating system 1. Experimental and microstructural observations

Deformation and pore pressure in dehydrating gypsum under transiently drained conditions

The transition from weakening to strengthening in dehydrating gypsum: Evolution of excess pore pressures