James A. D. Connolly

Metamorphic devolatilization of subducted marine sediments and the transport of volatiles into the Earth's mantle

Volatiles, most notably CO2, are recycled back into the Earths interior at subduction zones. The amount of CO2 emitted from arc volcanism appears to be less than that subducted, which implies that a significant amount of CO2 either is released before reaching the depth at which arc magmas are generated or is subducted to deeper depths. Few high-pressure experimental studies have addressed this problem and therefore metamorphic decarbonation in subduction zones remains largely unquantified, despite its importance to arc magmatism, palaeoatmospheric CO2 concentrations and the global carbon cycle. Here we present computed phase equilibria to quantify the evolution of CO2 and H2O through the subduction-zone metamorphism of carbonate-bearing marine sediments (which are considered to be a major source for CO2 released by arc volcanoes). Our analysis indicates that siliceous limestones undergo negligible devolatilization under subduction-zone conditions. Along high-temperature geotherms clay-rich marls completely devolatilize before reaching the depths at which arc magmatism is generated, but along low-temperature geotherms, they undergo virtually no devolatilization. And from 80 to 180 km depth, little devolatilization occurs for all carbonate-bearing marine sediments. Infiltration of H2O-rich fluids therefore seems essential to promote subarc decarbonation of most marine sediments. In the absence of such infiltration, volatiles retained within marine sediments may explain the apparent discrepancy between subducted and volcanic volatile fluxes and represent a mechanism for return of carbon to the Earths mantle.

Metamorphic devolatilization of subducted oceanic metabasalts: implications for seismicity, arc magmatism and volatile recycling

Subducted oceanic metabasalts are believed to be a primary source of volatiles for arc magmatism and fluid-induced seismicity. From phase equilibria computed for an average oceanic metabasalt we present a model for subduction zone devolatilization for pressures up to 6 GPa (∼180 km). Along high temperature geotherms complete dehydration occurs under forearcs, whereas dehydration does not occur along low temperature geotherms. For intermediate geotherms, major dehydration occurs under subarcs and provides a subjacent H2O source for arc volcanism. Decarbonation is negligible along cold and intermediate geotherms and limited along high temperature geotherms. Because decarbonation is limited for all subducted carbonate-bearing lithologies, transfer of CO2 from subducted slabs to arc magmas may be triggered by aqueous fluid infiltration. Metabasalt devolatilization could induce seismicity in forearcs (high temperature geotherms) and subarcs (intermediate geotherms); however, because of the lack of devolatilization, metabasalts would not be a fluid source for seismicity with low temperature geotherms. Along low temperature geotherms, limited devolatilization of subducted oceanic metabasalts and marine sediments in forearcs and subarcs provides a mechanism for return of volatiles to the deeper mantle.

Melting of the continental crust: Some thermal and petrological constraints on anatexis in continental collision zones and other tectonic settings

Useful constraints on the depth-temperature range and degree of melting of lower crustal rocks have been deduced by combining the results from experimental petrology with plausible PTt paths calculated for continental collision zones. H2O is required to generate melts at the temperatures of crustal orogenesis. Our modeling has shown that residual free water from subsolidus dehydration reactions produces less than 0.5% granitic melt at the H2O-saturated solidus. Breakdown of hydrous minerals results in fluid-absent (dehydration)- melting. For average geotherms such melting of muscovite and biotite in metapelite will generate at most 25% granitic melt. Because of much higher solidus temperatures, fluid-absent melting of hornblende in metabasalt (amphibolite) will not occur in collision zones without augmented heat input from the mantle. Fluid-absent melting of epidote in metabasalt, and low Ca amphibole and biotite in metavolcanics, can produce up to 10% melt in continental collision zones. Minor melt production from dehydration-melting of chlorite, staurolite, chloritoid, or talc will occur at pressures greater than ∼0.6 GPa. The calculated paths envelope small regions or PT space, thus identifying which mineral reactions are important for crustal anatexis and at which PT conditions future experiments should be made. In a dynamically evolving crust during uplift, thermal buffering by melting reactions is a minor component of the heat bucket. Extension of thickened continental crust will not promote dehydration-melting without the incursion of mantle heat. Delamination of thickened eclogitic lower crust, or decompression of the asthenosphere in response to lithosphere thinning are the most promising mechanisms to induce extensive lower crustal melting including amphibolite.

Are the regional variations in Central American arc lavas due to differing basaltic versus peridotitic slab sources of fluids

Central American arc volcanism shows strong regional trends in lava chemistry that result from differing slab contributions to arc melting. However, the mechanism that transfers slab-derived trace elements into the mantle wedge remains largely unknown. By using a dynamic model for mantle flow and fluid release, we model the fate of three different slab-fluid sources: sediment, ocean crust, and serpentinized mantle. In the open subarc system, sediments lose almost all their highly fluid mobile elements by ∼50 km depth, so other fluid sources are necessary to explain the slab signal in arc-lava compositions. The well-documented transition from lavas with a strong geochemical slab signature (i.e., high Ba/La ratios) found in Nicaragua to lavas with a weaker slab signature (i.e., low Ba/La ratios) erupted in Costa Rica seems easiest to produce by a higher fraction of serpentine-hosted fluids released from the deeply faulted, highly serpentinized lithosphere subducting beneath Nicaragua than from the less deeply faulted, thicker, amphibolitic oceanic-crust and oceanic-plateau lithosphere subducting beneath Costa Rica.

Devolatilization‐generated fluid pressure and deformation‐propagated fluid flow during prograde regional metamorphism

The obstruction to fluid flow formed by the rocks overlying a metamorphic devolatilization front causes the fluid pressure gradient in the reacting rocks to diverge from lithostatic. This drives deformation in tandem with the fluid pressure anomaly generated by the volume change of the reaction. Numerical simulations show that once the vertical extent of the reacted rocks is comparable to the compaction length, compaction processes caused by the difference between confining and fluid pressure gradients generate a positive fluid pressure anomaly (effective pressure < 0) above the reaction front, irrespective of the reaction volume change. Consequent dilational deformation propagates the anomaly upward, leading to underpressuring and densification at the reaction front and detachment of a wave of anomalous fluid pressure and porosity. Creep is a viable mechanism for such wave propagation for crustal viscosities < 10 15 MPa s. Continuous upward strengthening of the crust increases the wavelength and amplitude of the fluid pressure waves and thereby the likelihood of hydrofracture. Order of magnitude strength contrasts are adequate to arrest wave propagation, forming water sills that become increasingly stable in the absence of deviatoric stress. Although the fluid pressure gradient within a wave may be near hydrostatic, Rayleigh convection is unlikely. Thus, in the absence of lateral perturbations, fluid flow is upward and episodic, despite continuity of devolatilization. Porosity waves provide a mechanism for temporal focusing of metamorphic fluid fluxes with the potential to increase the efficacy of heat and mass transport.

Compaction-driven fluid flow in viscoelastic rock

Abstract Compaction driven fluid flow is inherently unstable such that an obstruction to upward fluid flow (i.e. a shock) may induce fluid-filled waves of porosity, propagated by dilational deformation due to an effective pressure gradient within the wave. Viscous porosity waves have attracted attention as a mechanism for melt transport, but are also a mechanism for both the transport and trapping of fluids released by diagenetic and metamorphic reactions. We introduce a mathematical formulation applicable to compaction driven flow for the entire range of rheological behaviors realized in the lithosphere. We then examine three first-order factors that influence the character of fluid flow: (1) thermally activated creep, (2) dependence of bulk viscosity on porosity, and (3) fluid flow in the limit of zero initial connected porosity. For normal geothermal gradients, thermally activated creep stabilizes horizontal waves, a geometry that was thought to be unstable on the basis of constant viscosity models. Implications of this stabilization are that: (1) the vertical length scale for compaction driven flow is generally constrained by the activation energy for viscous deformation rather than the viscous compaction length, and (2) lateral fluid flow in viscous regimes may occur on greater length scales than anticipated from earlier estimates of compaction length scales. In viscous rock, inverted geothermal gradients stabilize vertically elongated waves or vertical channels. Decreasing temperature toward the earths surface can induce an abrupt transition from viscous to elastic deformation-propagated fluid flow. Below the transition, fluid flow is accomplished by short wavelength, large amplitude waves; above the transition flow is by high velocity, low amplitude surges. The resulting transient flow patterns vary strongly in space and time. Solitary porosity waves may nucleate in viscous, viscoplastic, and viscoelastic rheologies. The amplitude of these waves is effectively unlimited for physically realistic models with dependence of bulk viscosity on porosity. In the limit of zero initial connected porosity, arguably the only model relevant for melt extraction, travelling waves are only possible in a viscoelastic matrix. Such waves are truly self-propagating in that the fluid and the wave phase velocities are identical; thus, if no chemical processes occur during propagation, the waves have the capacity to transmit geochemical signatures indefinitely. In addition to solitary waves, we find that periodic solutions to the compaction equations are common though previously unrecognized. The transition between the solutions depends on the pore volume carried by the wave and the Darcyian velocity of the background fluid flux. Periodic solutions are possible for all velocities, whereas solitary solutions require large volumes and low velocities.

Why is terrestrial subduction one-sided?

Subduction of the lithosphere at convergent-plate boundaries takes place asymmetrically—the subducted slab sinks downward, while the overriding plate moves horizontally (one-sided subduction). In contrast, global mantle convection models generally predict downwelling of both plates at convergent margins (two-sided subduction). We carried out two-dimensional (2-D) numerical experiments with a mineralogical-thermomechanical viscoelastic-plastic model to elucidate the cause of one-sided subduction. Our experiments show that the stability, intensity, and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab. Two-sided subduction occurs at low slab strength (sin[φ] 0.15). The weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism. The resulting weak interplate zone localizes deformation at the interface and decouples the strong plates, facilitating asymmetric plate movement. Our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self-sustaining one-sided subduction processes.

Modeling open system metamorphic decarbonation of subducting slabs

Fluids derived from the devolatilization of subducting slabs play a critical role in the melting of the mantle wedge and global geochemical cycles. However, in spite of evidence for the existence and mobility of an aqueous fluid phase during subduction metamorphism, the effect of this fluid on decarbonation reactions in subducting lithologies remains largely unquantified. In this study we present results from thermodynamic modeling of metamorphic devolatilization of subducted lithologies for pressures up to 6 GPa using an approach which considers fluid fractionation from source lithologies and infiltration from subjacent lithologies. This open system approach in which fluid flow is an intrinsic component of the chemical model offers an alternative to closed system models of subduction zone decarbonation. In general, our models simulating pervasive fluid flow in subducting lithologies predict CO2 fluxes measured from volcanic arcs more closely than models which assume purely channelized flow. Despite the enhanced effect of H2O-rich fluid infiltration on subduction decarbonation, our results support the hypothesis that CO2 is returned to the deep mantle at convergent margins, particularly in cool and intermediate subduction zones. Our results demonstrate that for most subduction zones, a significant proportion of the CO2 derived from the slab is lost beneath the fore arc, and therefore CO2 flux estimates based on measurements within the volcanic arc alone may significantly underestimate the slab-derived CO2 flux for individual margins. Nevertheless, our predicted global slab-derived CO2 flux from convergent margins of 0.35–3.12 × 1012 mols CO2/yr is in good agreement with previous estimates of global arc volcanic flux. Because our predicted global slab-derived CO2 flux is significantly less than atmospheric CO2 drawdown by chemical weathering, significant CO2 emission from other geologic regimes (e.g., hot spots) would be required to balance the global carbon cycle.

Serpentinization of Oceanic Peridotites: Implications for Geochemical Cycles and Biological Activity

Ultramafic rocks are a major component of the oceanic lithosphere and are commonly exposed near and along slow- and ultra-spreading ridges and in other tectonically active environments. The serpentinization of mantle material is a fundamental process that has significant geophysical, geochemical and biological importance for the global marine system and for subduction zone environments. Mineral assemblages and textures are typically complex and reflect multiple phases of alteration, deformation and veining during emplacement, hydrothermal alteration, and weathering. In this paper, we review mineralogical and geochemical consequences of serpentinization processes in oceanic upper mantle sequences in different tectonic environments and discuss the relationship between serpentinization and fluid chemistry. We present phase equilibria that provide models for interpreting mineral-fluid relationships in oceanic serpentinites and allow the simultaneous evaluation of the conditions for redox, hydration and carbonation processes. These models predict that serpentinization reactions are sensitive to Si content of ultramafic rocks and that serpentine phases have an upper stability limit of ∼450°C, where H 2 O-rich fluids will be dominant. More pervasive serpentinization commences with olivine breakdown reactions below ∼425°C and leads to progressively more reduced fluids with decreasing temperature. Our calculations indicate that carbonates may have extensive stability fields in CH4-rich fluids in Si-deficient systems and that they may be significant in generating reducing conditions. If methane formation driven by serpentinization is common, its contribution to the carbon cycle in submarine biogeochemical systems may be substantial. Serpentinization may thus be an important process in sustaining diverse microbial communities in subsurface and near-vent environments and has consequences for the existence of a deep biosphere.

Metamorphic controls on seismic velocity of subducted oceanic crust at 100–250 km depth

Abstract Most circum-Pacific subduction zones at 100–250 km depth contain layers in which seismic velocities are ca. 5% slower than in the adjacent mantle. We compute seismic velocities from thermodynamic data for equilibrium metabasalt mineralogies, determined by free energy minimization, at subduction zone conditions. Lawsonite stability has a profound effect on seismic velocities of subducted oceanic metabasalts. Velocity reductions of 3–7% are estimated for lawsonite–eclogites derived by metamorphism of hydrothermally altered oceanic basalt subducted along relatively cool geotherms, whereas a 2–4% velocity increase is characteristic of anhydrous eclogites within the coesite stability field. The restricted depth extent of low-velocity layers is explicable through the influence of the coesite–stishovite transition, which reduces lawsonite stability at high pressure. This transition also increases the positive velocity anomaly in anhydrous eclogites to 4–6%, an effect that may account for deep high-velocity layers. The quality of the match between the properties of lawsonite–eclogite and low-velocity layers supports the contention that significant quantities of volatiles are retained within the oceanic crust beyond sub-arc depths. Because the velocity anomalies are explicable in terms of equilibrium phase relations, we find no reason to invoke metastability of metamorphic reactions to explain the low-velocity layers.