Robert P. Lowell

Seafloor hydrothermal systems

The discovery of seafloor hydrothermal systems approximately two decades ago has led to a major reassessment of the Earths thermal and geochemical budgets and has revolutionized our understanding of biological processes. This review traces the development of the study of seafloor hydrothermal systems from the indirect evidence provided by conductive heat flow anomalies to the discovery of ≈ 350°C black smoker vents on the East Pacific Rise at 21°N. Although the review focuses on physical characteristics and processes, it outlines some key characteristics of vent fluid chemistry that provide constraints on physical models. Ridge crest systems have thermal power outputs ranging from 10 to 104 MW. They are transient systems, driven by magmatic heat sources, but episodic events such as megaplumes, the interplay between focused and diffuse venting, and other aspects related to their thermal, chemical, and biological evolution remain poorly understood. Advances will be made by continuing exploration and discovery to determine the full range of possible phenomena both on and off axis and in different tectonic settings. In order to understand the complete, integrated ridge system, however, future studies must include long-term monitoring of an active system, deep drilling into the reaction zone, and mathematical modeling that incorporates both physical and chemical constraints.

Silica precipitation in fractures and the evolution of permeability in hydrothermal upflow zones

Analytical models are used to compare the rates at which an isolated fracture and vertical, parallel fracture sets in hydrothermal upflow zones can be closed by silica precipitation and thermoelastic stress. Thermoelastic sealing is an order of magnitude faster than sealing by silica precipitation. In vertical fracture sets, both the amount of silica precipitation resulting from cooling and the total thermal expansion of the country rock may be insufficient to seal cracks at depth. These crack systems may ultimately close because the pressure dependence of silica solubility maintains precipitation during upflow even after the temperature gradient vanishes.

On the temporal evolution of high-temperature hydrothermal systems at ocean ridge crests

The results of heat balance calculations for a single-pass hydrothermal system overlying an axial magma body, based on a given rate of heat output of 102–103 MW at a time t0, predict that vent temperatures should decay rapidly for t > t0, as the magma freezes and the boundary layer between the hydrothermal system and liquid magma thickens. The model may describe a declining phase of high-temperature, high-heat-output hydrothermal activity. The model shows that for systems with heat output ∼100 MW or greater, the boundary layer between the magma and hydrothermal system must remain thin, if vent temperatures remain relatively constant on a decadal timescale. A thin boundary layer can be maintained as a result of downward migration of the hydrothermal system into freshly frozen magma or by some mechanism that maintains high heat flux from liquid magma to the base of the boundary layer. Some combination of these factors is likely to operate. Downward migration of the hydrothermal system into freshly frozen magma may occur in conjunction with fracturing resulting from dike injection and the propagation of these cracks laterally away from the dike as a result of thermal stresses. High heat flux from liquid magma to the base of the hydrothermal system cannot be maintained simply by convection within the magma chamber. High heat flux might be maintained as a result of magma chamber replenishment or by latent heat transfer during the formation of a cumulate mush at the base of the magma chamber, however. A hydrothermal system, in which the permeability decreases with time, can maintain relatively constant vent temperatures even though the thermal output declines. Better time series data on thermal output of the vents, and not just on the vent temperature, could help distinguish whether the permeability is decreasing or whether heat flux as well as vent temperatures are relatively constant.

Modeling continental and submarine hydrothermal systems

Continental and submarine hydrothermal systems are an integral part of the Earths thermal regime. Continental systems account for less than 1% of the global heat loss, whereas submarine systems account for nearly 25%. Naturally heated waters of continental hydrothermal systems have been utilized for centuries, yet mathematical models of these systems have been developed only during the past 40 years. By contrast, direct observation of seafloor hydrothermal venting occurred just slightly more than a decade ago. However, mathematical models of submarine hydrothermal heat transfer were developed to explain conductive heat flow anomalies at ocean ridge crests even before the vents were found. Both cellular convection in fluid-saturated porous media and single-pass, pipe model flow provide a conceptual framework for the construction of mathematical models hydrothermal systems. Recent numerical simulations of low- and high-temperature hydrothermal systems in continental and submarine settings highlight the role of permeability in controlling the style of hydrothermal circulation. Future modeling will likely focus on the complexities of heat transfer between magma and the hydrothermal system as well as on the tectonic, thermal, and chemical processes that affect rock permeability in space and time. Future work must also address the three-dimensional nature of hydrothermal systems and two-phase flow in submarine hydrothermal systems. Whether hydrothermal systems might record climatic changes has yet to be investigated.

Dike injection and the formation of megaplumes at ocean ridges.

A simple hydrologic model of seawater circulation at ocean ridge axes implies that the transient occurrence of large volumes of buoyant, heated water in the oceanic water column (megaplumes) can be attributed to the emplacement of dikes in oceanic crust. For dikes to generate megaplume flow, the permeability of both the recharge areas and the upflow zone must be greater than that required for ordinary black smokers. An increase in permeability in the upflow zone by several orders of magnitude results from dike emplacement, and megaplume discharge ceases as the dike cools. Vigorous black smoker venting may not persist very long at a megaplume site after the event occurs.

Effect of seafloor temperature and pressure variations on methane flux from a gas hydrate layer: Comparison between current and late Paleocene climate conditions

We investigate the response of a methane hydrate layer in marine sediments to cyclic seafloor perturbations of temperature and pressure in order to determine the change in seafloor methane flux resulting from gas hydrate dissociation or accumulation. By using a one-dimensional model describing mass, energy, and methane transport through porous sediments we show that seafloor pressure changes have negligible effect on methane transport to the seafloor. The effect of seafloor temperature perturbations is more pronounced than that of pressure. With an initial seafloor temperature of 3°C, which corresponds to current conditions on Earth, a +4°C seafloor temperature perturbation occurring over 104 years does not significantly effect methane transport. Thus such a perturbation is not likely to have a significant impact on the current global climate or to give rise to an event such as the δ13C excursion during the late Paleocene thermal maximum (LPTM). If the initial seafloor temperature is assumed to be 11°C, which corresponds to the conditions of the late Paleocene, a +4°C temperature perturbation over a period of 104 years could result in complete dissociation of methane hydrate layers situated at water depths around 1200 m. In this case, the calculations show that the change in methane flux might be able to explain the δ13C excursion of marine carbonate fossils during the LPTM. This result is weakened because the simplifications in the model tend to yield overestimates of the change of methane flux. The principal point is that strong coupling between methane transport and seafloor temperature occurs because significant hydrate accumulation and dissociation take place near the seafloor only when the seafloor temperature is relatively high. This was the case during the late Paleocene, but it is not the case at present.

Origin of elevated sediment permeability in a hydrothermal seepage zone, eastern flank of the Juan de Fuca Ridge, and implications for transport of fluid and heat

Elevated sediment porosity and permeability may help to focus upward fluid seepage observed over a buried basement high on the eastern flank of the Juan de Fuca Ridge. Consolidation and permeability tests of fine-grained hemipelagic and turbidite sediments from the seepage site and from nearby sites that are not experiencing seepage indicate that sediments at the seepage site (primarily hemipelagic) have 10–15% higher porosity and 10× higher permeability at a given depth and 10–100× lower hydraulic impedance for the 40-m-thick sediment column. The correlation of consolidation and permeability properties with sediment type rather than location indicates that the relatively high porosity and permeability do not result from fluid flow but are intrinsic properties of the hemipelagic sediment. On the basis of measured sediment properties, fluid pressure at the top of basement is <5 kPa greater than hydrostatic. A simple circulation model that incorporates this estimate and proximity to the nearest basement outcrops (4–20 km) suggests that local basement permeability is between 3×10−13 and 3×10−11 m2, within the range measured in nearby boreholes. The measured sediment properties combined with other published data indicate that a few tens of meters of fine-grained terrigenous, hemipelagic, or calcareous marine sediment effectively seal the basement aquifer, whereas much thicker siliceous or pelagic sediment may support thermally and chemically significant fluid flow. The large contrast in hydraulic impedance among sediment columns of different types could cause regional variation in the evolution of ridge flank hydrothermal systems and in the contribution of seepage to ridge flank fluxes.

Stress‐dependent permeability and the formation of seafloor event plumes

We address the question of formation of event plumes following dike emplacement in a hydrothermal upflow zone at a mid-ocean ridge. We assume a preexisting low- to moderate-temperature single-pass hydrothermal system and suggest that dike emplacement provides a damaged zone of high permeability along its margins as well as the heat required to drive the event plume. We also consider the role of thermoelastic stresses in limiting the heat output of the event plume. Our calculations show that event plumes can result from dike emplacement into a preexisting, moderate-temperature hydrothermal system, provided the local permeability generated by the dike is ∼10−8-10−10 ° m2. These values are consistent with our limited field observations from the Susanville ophiolite, suggesting that the permeability near dike margins attributed to dike emplacement results mainly from open fractures mostly aligned parallel to the margins of dike. We estimate the porosity in the damaged zone to be of the order 0.1–1%. If the high-permeability zone has relatively low porosity (∼0.1%), thermoelastic stresses close the fractures sufficiently to reduce the heat output, giving rise to a weak event plume (∼1014 J). If the porosity is higher (≥ 1%), however, thermoelastic stresses become unimportant, and a similar dike can generate an event plume of ∼1017 J. Following event plume emission, the circulation decays rapidly to its original temperature; however, the heat output from the chronic plume is greater because of the increased permeability resulting from dike emplacement. The decay of heat output to preevent plume levels requires that the newly created permeability be sealed, perhaps as a result of chemical precipitation in the cracks.

The mechanism of phreatic eruptions

We investigate the mechanism for initiating phreatic eruptions following the emplacement of a shallow magmatic intrusion into water-saturated permeable rock which contains subsidiary low-permeability crack networks and disconnected cracks. Heat from the intrusion causes the local groundwater to boil and ascend through the main permeable crack network. As the ascending superheated steam heats the overlying rock, the water in the subsidiary networks and disconnected cracks will boil. The pressure exerted by the vapor in the subsidiary and disconnected cracks can lead to rapid horizontal crack propagation, resulting in an increase in crack length by more than an order of magnitude. According to the model, the eruption process starts near a free surface and migrates rapidly along thermoelastic isostresses as a result of multiple breakage of the thin surface layers above the cracks. For certain crack and rock parameters, however, the crack propagation mechanism, instead of leading to a dynamic eruption, may generate a highly cracked zone that may be removed later by fluid transport processes. The proposed mechanism gives rise to precursory phenomena observed in conjunction with many phreatic eruptions. According to the model developed here, phreatic eruptions are most likely to occur only for a rather restricted set of rock parameters. For example, the country rock should not be too strong (σt ≅ 10 MPa) and should be characterized by two-scale permeability structure involving a main crack network of relatively high permeability (≳10−12 m2) and a subsidiary crack network with much lower permeability ( 10−2). These restrictions may explain indirectly why phreatic eruptions are not ubiquitous in volcanic regions.

Percolation Theory, Thermoelasticity, and Discrete Hydrothermal Venting in the Earth's Crust.

As hydrothermal fluid ascends through a network of cracks into cooler crust, heat is transferred from the fluid to the adjacent rock. The thermal stresses caused by this heating close cracks that are more or less vertical. This heating may affect network connections and destroy the permeable crack network. Thermoelastic stresses caused by a temperature difference of ∼1000�C can decrease the interconnectivity of a crack network to the percolation threshold. If the temperature is slightly less, thermoelastic stresses may focus the discharge in hydrothermal systems into discrete vents.