H. Paul Johnson

Earthquake-induced changes in a hydrothermal system on the Juan de Fuca mid-ocean ridge

Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to seismic disturbances, with observed changes in vent temperature. But these disturbances resulted from submarine volcanic activity; until now, there have been no observations of the response of a vent system to non-magmatic, tectonic events. Here we report measurements of hydrothermal vent temperature from several vents on the Juan de Fuca ridge in June 1999, before, during and after an earthquake swarm of apparent tectonic origin. Vent fluid temperatures began to rise 4–11 days after the first earthquake. Following this initial increase, the vent temperatures oscillated for about a month before settling down to higher values. We also observed a tenfold increase in fluid output from the hydrothermal system over a period of at least 80 days, extending along the entire ridge segment. Such a large, segment-wide thermal response to relatively modest tectonic activity is surprising, and raises questions about the sources of excess heat and fluid, and the possible effect on vent biological communities.

Fluxes of fluid and heat from the oceanic crustal reservoir

Abstract Recent discoveries define a global scale fluid reservoir residing within the uppermost igneous oceanic crust, a region of seafloor that is both warm and may harbor a substantial biosphere. This hydrothermal fluid reservoir formed initially within volcanic rocks newly erupted at mid-ocean ridges, but extends to the vastly larger and older ridge flanks. Upper oceanic crust is porous and permeable due to the presence of lava drainbacks, fissuring, and inter-unit voids, and this porosity and permeability allows active fluid circulation to advect measurable quantities of lithospheric heat from the crust to an average age of 65 Myr. A compilation of crustal porosities shows that this fluid reservoir contains nearly 2% of the total volume of global seawater. Heat flow and sediment thickness data allow calculation of reservoir temperatures, predicting 40°C mean temperatures in Cretaceous crust. Utilizing these temperature estimates, heat flow measurements and models for the thermal structure and evolution of the oceanic lithosphere, we have computed mean hydrothermal fluxes into the deep ocean as a function of plate age. The total hydrothermal volume flux into the oceans approaches 20% of the total riverine input and may contribute to the global seawater mass balance.

Crustal magnetization reveals subsurface structure of Juan de Fuca Ridge hydrothermal vent fields

A near-bottom geophysical survey on the Endeavour segment of the northern Juan de Fuca Ridge shows that regions of well-defined low crustal magnetization are strongly correlated with both active and extinct submarine hydrothermal vent sites. In particular, at the Main Endeavour Field, we find discrete magnetization lows associated with each cluster of vents. Magnetization lows are directly centered beneath the vent clusters and have diameters of ∼100 m, which implies a near-vertical, narrow, pipe-like source region located directly beneath the surface expression of the vent edifices. Lows are also separated from each other by only 200 m, which further implies highly focused zones. Magnetization lows are also associated with inactive and extinct vent areas, which indicates that alteration of the magnetic minerals in the crust rather than (necessarily temporary) thermal demagnetization is the primary process responsible for the low magnetization. These narrow pipe-like bodies are highly characteristic of alteration pipes found in ophiolites and are indicative of hydrothermal fluid up-flow zones. Thus, each magnetization low may define an individual upwelling zone, with distinct subsurface plumbing and thermal structure. The crustal-magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems. At the Main Endeavour Field, magnetization lows are distributed along the trend of the rift valley in a semiregular pattern with a spacing of ∼200 m, arguing that upward flow may be partitioned into regularly spaced intervals along the axis of the rift valley.

On modeling the thermal evolution of the oceanic upper mantle: An assessment of the cooling plate model

The plate cooling model for the thermal evolution of the oceanic upper mantle has been widely accepted to explain observed variations of depth to oceanic basement and conductive heat flow with the age of the seafloor. Several estimates of “best fitting” plate model parameters, derived from depth, heat flow, and age data, have been proposed, but the viability of the plate model itself has not been rigorously evaluated. We have used published mean depths and depths to basement at Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) drilling sites to test the plate cooling model based on two criteria: First, viable plate models must have coefficients that are consistent with the slope of the corresponding root t line because the half-space (or root t) subsidence of young seafloor is implicit in the plate model (i.e., the slope of the root t line can be calculated directly from the coefficients of the plate subsidence model). Second, any viable physical model must fit the data with an acceptable degree of systematic misfit; large systematic misfits indicate that the model cannot explain the observations. Fits of half-space (root t) models to depth versus age data for young (< ∼80 Ma) seafloor indicate basal temperatures in the range 1300 to 1370°C. Based on the age at which the depths deviate from the root t line, the minimum plate thickness that is compatible with the best fitting half-space models for young seafloor is 120 km. In contrast, all best fitting plate models yield systematically higher temperatures (1450 to 1470°C) and thinner plates (102–118 km). The plate model can explain the depth to basement at DSDP/ODP drill sites with a satisfactory degree of systematic misfit, but we find that there is no plate cooling model that can explain the variation of the mean depths (derived from the DBDB5 database) with age over the entire range of ages (0 to 165 Ma). Models that “best fit” the entire data set have unacceptably large systematic misfits over the entire range of seafloor age, whereas models that minimize the systematic misfit for young seafloor (0 to 81 Ma) fail for older seafloor. The plate model clearly fails to explain the observations. The cooling history of old seafloor is not simply an extension of the cooling history of young seafloor according to a simple plate cooling model. Previous studies have suggested, as an alternative to the plate model, that observed variations of basement depth with age are best explained by the combined effects of a “normal” half-space cooling process and the dynamic and/or thermal effects of hot spots or mantle plumes. Our results are entirely consistent with the half-space cooling model, and we find that the best reference model for “normal” subsidence is d(t) = (2600±20)m + (345±3)m (m.y.)−½ t½. We also find that the heat flow predicted by this half-space model is consistent with the most reliable average heat flow values from the Pacific.

Variations in oceanic crustal magnetization: Systematic changes in the last 160 million years

The amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age. Globally, the amplitudes decrease with age over the first 20 to 30 million years, a phenomenon that has been attributed to the low-temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks. In oceanic crust older than 40 million years, however, the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years. In order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust, we compiled all of the existing rock magnetic data from relevant Deep Sea Drilling Project and Ocean Drilling Program sites that sampled extrusive rocks from “normal” ocean crust. This compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes, both in the decrease due to oxidation in the first 20 to 30 million years, and in the increase in magnetization in older (>80 million years) crust. This positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks. Further, the Curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years, and does not increase significantly beyond that point. Finally, the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust. Specifically, we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic FeTi oxides in the older crustal rocks. This large increase in abundance of the FeTi oxides may be related to a systematic increase in bulk FeTi content in older tholeiitic basalts, or, more likely, is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases.

Hydrothermal vents of Explorer Ridge, northeast Pacific

Abstract The first submersible exploration of Explorer Ridge found extensive hydrothermal fields in relatively old pillow basalts. Massive coalescing spires formed the basis for 25 m high mounds atop which chimneys emit grey ‘moke’ and water over 300°C. These sulfides are among the largest such ocean deposits found to date. Three types of vents were found: (1) abiotic iron- and zinc-rich vents; (2) low temperature biotic vents; and (3) high temperature H 2 S-rich vents. Coordinated suites of water, rock and animal samples indicated a basic similarity to vents on Juan de Fuca Ridge while demonstrating a wide range of variation.

Near-axis heat flow measurements on the northern Juan De Fuca Ridge: Implications for fluid circulation in oceanic crust

Present models of the cooling of oceanic crust suggest that convection of hydrothermal fluid is a major component of the process. In axial regions, abundant faults and open fissures are associated with the venting of high temperature hydrothermal fluid. In older crust, where the insulating sediment cover is thick, previous studies have shown that basement topography is the dominant forcing factor for within-crust fluid circulation. In the intermediate region, where young crust is lightly sedimented, heat flow data are difficult to obtain with traditional techniques. To determine whether topography or permeability is the dominant process controlling fluid circulation in the near-axis region, we conducted a profile of heat flow measurements using the submersible ALVIN, on the Endeavour Segment of the Juan de Fuca Ridge. Our data indicate that topographic forcing is responsible for the long wavelength variations, with high heat flow at the ridge summits, and low values in the inter-ridge valleys. The locations of the extreme values of heat flow taken within the context of subsurface faulting are consistent with a model where a ridge-valley topographic pair comprises a single circulation cell. This model predicts that the source area for the high temperature axial vents may be in the flanking inter-ridge valleys.

Alteration processes at Deep Sea Drilling Project/Ocean Drilling Program Hole 504B at the Costa Rica Rift: Implications for magnetization of oceanic crust

Magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504B during Ocean Drilling Program leg 111. Our results, combined with those from previous studies, show abrupt first-order changes in magnetic properties at alteration boundaries. Within the 504B crustal section, changes in style and degree of alteration fall near the boundaries of the three well-defined lithologic units: the extrusive basalts, the transition zone, and the sheeted dike complex. This postemplacement alteration heavily influences magnetic properties and is observed to change, in both style and degree, with depth within each lithologic unit. Our results indicate that, at 504B, the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration. The upper, more permeable basalts above this critical depth have not experienced this reheating. Within the sheeted dike complex, the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth. The changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth. Overall, these observations suggest that the effective penetration of water into layer 2C decreased with increasing depth and resulted in a lower rate of convective cooling. Magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration, the upper dikes have become magnetically more stable than the lower dikes. A review of all magnetic properties indicate that the dike section at 504B carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict. We suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions. Despite a magnetization which is lower than expected, it appears that the sheeted dike complex at hole 504B is capable of making a substantial contribution to the overlying marine magnetic anomaly. Because of the systematic decrease in hydrothermal alteration of magnetic minerals, the ability of the dike section to contribute decreases as a function of depth.

Do layer 3 rocks make a significant contribution to marine magnetic anomalies? In situ magnetization of gabbros at Ocean Drilling Program hole 735B

We estimate the average magnetization of a vertical section of oceanic gabbros using paleomagnetic and downhole magnetic logging techniques and evaluate the ability of these crustal rocks to contribute to marine magnetic anomalies. Results from Ocean Drilling Program Hole 735B show that the drilled crustal section has a mean effective remanent magnetization of 2.5 A/m. Olivine gabbros, which make up 60% of the section, have an average effective magnetization between 1 and 2 A/m. We used two-dimensional forward modeling and the average effective magnetization of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface. These results indicate that if polarity boundaries within oceanic layer 3 are near-vertical, crustal sections similar to 735B would contribute from 25% to 75% of the overlying marine magnetic anomalies at the sea surface. The shape of the polarity boundaries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitude and skewness of marine magnetic anomalies at individual spreading centers.

High-resolution bathymetric surveys using scanning sonars: Lava flow morphology, hydrothermal vents, and geologic structure at recent eruption sites on the Juan de Fuca Ridge

The CoAxial and Cleft segments of the Juan de Fuca Ridge have isolated, chronic, high-temperature, and focused hydrothermal vent sites. Both segments also have experienced recent volcanic eruptions which produced extensive, ephemeral, low-temperature, and diffuse hydrothermal venting. To study the geologic setting of these sites, high-resolution bathymetric surveys at eight locations on the CoAxial and Cleft segments were collected between 1993 and 1999. Two 675-kHz scanning sonar systems were used, Mesotech on the submersible Alvin and Imagenex on the remotely operated vehicle Jason. The bathymetry from these surveys can be gridded at a scale of 2–4 m and contoured at 1 m and thus can resolve many fine-scale features on the seafloor that are indistinguishable in multibeam bathymetry collected at the sea surface. Bathymetric data at this resolution are particularly useful for identifying geologic features related to diking, faulting, and lava flow emplacement. For example, the high-resolution bathymetric maps show that submarine fissure eruptions that form pillow lavas last long enough to become localized and to produce point source constructs along their length, and their extrusion rate is low enough that no significant drainback occurs. In contrast, lobate sheet flows are formed by short-lived, high-effusion rate eruptions in which no localization of output occurs along the eruptive fissure, and inflation is quickly followed by drainback, resulting in extensive collapse features. However, if the process of submarine lava flow inflation occurs at a slower rate and over a longer period of time, it can create lava rises up to 25 m high with distinctive structure and morphology. The scanning sonar data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites. The fine-scale bathymetry establishes that all the hydrothermal vent sites studied at the CoAxial and Cleft segments are located along prominent volcanic or tectonic extensional structures which provide the physical pathway for fluids from the subsurface to the seafloor. Furthermore, the fine-scale morphology of recent lava flows can be used as a qualitative indication of eruption duration.