Ridge-flank Hydrothermal Circulation Research Articles

The influence of permeability anisotropy in the upper ocean crust on advective heat transport by a ridge-flank hydrothermal system

In this study, we highlight the importance of permeability anisotropy on the hydrogeological regime of a ridge-flank hydrothermal system. Our study site, North Pond, is a marine sediment pond on ∼8 Ma seafloor in the North Atlantic, and represents a low-temperature, end-member ridge-flank hydrothermal system. Previous simulations of North Pond elucidated long-standing hypotheses concerning hydrothermal fluid and heat transport in the upper volcanic crust but failed to fully explain observed patterns of seafloor heat flux in this area. Here we use variography, a geostatistical method, to quantify relations between seafloor heat-flux measurements, and coupled numerical simulations of fluid and heat flow to simulate the hydrogeologic regime. Directional variography shows that heat-flux observations are correlated along-strike of the regional crustal fabric. Three-dimensional simulations that include permeability anisotropy are able to replicate seafloor heat-flux patterns across North Pond. The simulations that result in the best match to thermal data incorporate permeability anisotropy in the horizontal plane. We find that the feedback between permeability anisotropy and the asymmetric geometry of North Pond combine to promote advective removal of heat and mass within the crustal aquifer. These findings suggest that permeability anisotropy in the oceanic crust may influence ridge-flank hydrothermal circulation more broadly.

The role of outcrop‐to‐outcrop fluid flow in off‐axis oceanic hydrothermal systems under abyssal sedimentation conditions

It has been proposed that ridge flank hydrothermal circulation by outcrop‐to‐outcrop (lateral) flow may be the dominant mode of oceanic hydrothermal circulation globally. In this model, the upper igneous crust is an aquifer overlain by low permeability sediments, and aquifer‐ocean fluid exchange occurs through basement outcrops. Thermally induced pressure gradients drive fluid laterally from recharge outcrops to discharge outcrops. To test the global applicability of outcrop‐to‐outcrop flow, models of synthetic basement bathymetry representative of crust formed at different spreading rates, pelagic sediment supply and post depositional transport, and sediment hydraulic impedance are used to quantify the time‐varying distribution of sediment and basement outcrops globally. Results suggest that basement outcrops may be 40–50% closer together than previously estimated. The modeled sediment and outcrop results are coupled with a two‐dimensional model of outcrop‐to‐outcrop fluid flow and heat exchange in a vertically isothermal crustal aquifer to predict the spatial distributions of seafloor heat flow within the simulation region under this mode of hydrothermal circulation. It is found that both the time‐varying average and standard deviation of modeled seafloor heat flow required by outcrop‐to‐outcrop flow simultaneously fit the global heat flow data if the average aquifer permeability decreases from ∼10−9 m2 to ∼10−11 m2 over the duration of the global heat flow deficit (to ∼65 Myr). This permeability range is consistent with other estimates of upper crustal permeability on comparable spatial scales, and supports the proposition that outcrop‐to‐outcrop fluid flow may be the dominant mode of off‐axis hydrothermal circulation globally.

Hydrothermal circulation within topographically rough basaltic basement on the Juan de Fuca Ridge flank

We use coupled heat and mass transport models to investigate the basement permeability distribution and hydrothermal circulation patterns in an area with large basement relief below flat seafloor on the eastern flank of the Juan de Fuca Ridge. If heat transport around Ocean Drilling Program Sites 1026 and 1027 were purely conductive, the top of basement at Site 1027 (below ∼600 m of sediment) would be ∼50°C warmer than at Site 1026 (below ∼250 m of sediment), but the top of basement in this area is nearly isothermal. Vigorous fluid circulation in hydrothermal (uppermost) basement is required in order to maintain a near isothermal sediment‐basement contact. We estimate the magnitude of hydrothermal basement permeability, test the reliability of conductive‐proxy models used to estimate basement permeability for this setting, and examine the evolution of the hydrothermal circulation system as sediment accumulates. We find hydrothermal basement permeabilities ≥10−10 m2 are required for the top of basement near Sites 1026 and 1027 to be nearly isothermal, consistent with results from the conductive‐proxy models. Modelled fluid circulation in the direction that allows both the observed sign of temperature difference between Sites 1026 and 1027 (warmer in the basement high) and a close approximation of the observed relative basement pressures is only achieved with a heterogeneous permeability distribution, connected high permeability conduits hosted within low permeability rock. This suggests that ridge flank hydrothermal circulation in this setting occurs dominantly within narrow zones of elevated permeability, consistent with observed, scale‐dependent variations in permeability in this region, and conceptual models of heterogeneous hydrogeologic systems in general.

Evidence for basaltic Sr in midocean ridge-flank hydrothermal systems and implications for the global oceanic Sr isotope balance

Previous models and calculations of the global mass balance of Sr in the oceans have shown that the input of unradiogenic basaltic Sr from on-axis midocean ridge hydrothermal systems is much less than needed to balance the input of radiogenic Sr delivered to the oceans by rivers. The implication is that either the oceans are far from steady state with respect to Sr isotope balance (and that the 87Sr/86Sr ratio of seawater is increasing at unprecedented rates) or that there is a significant missing source of basaltic Sr. It has long been recognized that off-axis hydrothermal fluxes might significantly affect the mass and isotopic balance of Sr and other elements in the oceans, but nearly all previous work has concluded that the 87Sr/86Sr ratio of pore fluids in ridge-flank hydrothermal areas is virtually indistinguishable from the seawater ratio or is dominated by authigenic carbonates. In contrast, we report here the 87Sr/86Sr ratios of warm springs, sediment pore fluids, and basement reservoir fluid with a clear basaltic signature from the eastern flank of the Juan de Fuca ridge (JFR). Fluids venting from Ocean Drilling Program Hole 1026B on the Juan de Fuca east flank have relatively stable Sr isotope and major element composition for the 3 yr following drilling. These results and similar results recently reported by Elderfield et al. (1999) indicate that low-temperature ridge-flank hydrothermal circulation has an important effect on the Sr isotope balance in the oceans. If published values for the other major sources of Sr input to the oceans (rivers and axial hydrothermal flux) are accurate, then the rate of increase of the 87Sr/86Sr ratio in seawater (∼0.000054 per million years) can be accommodated if ridge flanks on a global scale deliver fluids to the ocean with Δ (87Sr/86Sr)/heat ratios one third to one half of the ratio found in warm JFR basement fluids. Based on published Sr and O isotope signatures of calcite veins in the uppermost basaltic ocean crust, the average Δ (87Sr/86Sr)/heat ratio of low-temperature fluids is in the range required to balance the oceanic Sr isotope budget. Although the 87Sr/86Sr ratios of the JFR flank fluids in this study overlap with fluid properties inferred from some calcite veins in the upper oceanic crust, the magnitudes of the Δ (87Sr/86Sr)/heat ratios of nearly all of the JFR flank fluids are too large to be representative of the average global flank fluid flux; the same has been argued on the basis of the extremely high implied Mg flux.

Osmium isotopes in hydrothermal fluids from the Juan de Fuca Ridge

We present Os data for axial high-temperature and off-axial low-temperature hydrothermal solutions from the Juan de Fuca Ridge. The high-temperature, H 2S-bearing axial fluids have unradiogenic Os isotopes pointing to a nearly complete domination of osmium isotopes from the basalts during hydrothermal circulation. The ridge axis fluids typically do not show large enrichment in osmium concentration over seawater although one high-temperature fluid has an Os concentration enhanced by a factor of 4 above seawater. It appears that the Os concentration of high-temperature hydrothermal fluids is typically buffered at roughly the seawater concentration. We suggest that subseafloor precipitation of pyrite from the high-temperature hydrothermal fluids controls the osmium transportation. The axial hydrothermal activity does not supply significant amounts of unradiogenic osmium to the deep oceans. In contrast, a low-temperature off-axis fluid is enriched in non-radiogenic osmium over seawater by a factor of 9, showing much less precipitation of osmium at low temperature. Because a large fraction of the cooling of oceanic lithosphere occurs on ridge flanks, Os from low-temperature, ridge-flank hydrothermal circulation may be a significant contributor to the balance of Os in the oceans. A detailed balance between Os contributions from dissolution of cosmic dust and hydrothermal fluids is still not possible. The low-temperature hydrothermal sample gives 187Os/ 188Os=0.110±0.001. This extremely unradiogenic osmium can only come from a source that underwent depletion of Re over 2.6 Ga ago. Assuming no contamination during sample collection, this result suggests that the convecting upper mantle contains ancient depleted material that imparted unradiogenic osmium to intruding basaltic melts.

CO 2-depleted fluids from mid-ocean ridge-flank hydrothermal springs

Hydrothermal spring fluids were collected from Baby Bare, a basement outcrop on the sediment-covered eastern flank of the Juan de Fuca Ridge overlying 3.5 Ma-old crust. These waters are venting at 25°C but have cooled from 64°C within the upper 200–300 m of the upflow zone during their ascent. Unlike ridge-axis hydrothermal vent fluids previously sampled, the total CO 2 content of the endmember spring fluids (0.85 μmol kg −1) is depleted with respect to bottom seawater. The very low alkalinities (0.43 meq L −1) and high Ca 2+ concentrations (55 mmol kg −1) of endmember spring fluids suggest that the removal of C in the spring fluids is associated with carbonate precipitation in the igneous basement. Assuming that 8–20% of the total ridge-flank heat loss rate of 50 × 10 18 cal yr −1 (6.6 TW) is removed by porewater advecting from the sediment at ≥25°C (an upper limit dictated by the global input rate of Mg +2 to the oceans; Mottl and Wheat, 1994), a maximum global carbon sink of 1.0–2.6 × 10 11 mol yr −1 ( 1.2–3.1 Mton-C yr −1) and a maximum global alkalinity sink of 140–340 × 10 9 eq yr −1 are calculated for warm (≥25°C) ridge-flank hydrothermal circulation. This carbon flux is only 2–5% of current estimates of subaerial and submarine volcanic CO 2 emissions (∼50 × 10 11 mol yr −1), indicating that hydrothermal alteration of oceanic crust on young mid-ocean ridge flanks at temperatures of a few tens of degrees Celsius is only a relatively minor sink for carbon on a global basis. It is still possible, however, that ridge-flank alteration at lower temperatures (<25°C) is an important component of the global carbon budget.

Petrological investigations of low temperature hydrothermal alteration of the upper crust, Juan de Fuca Ridge, ODP Leg 168

Abstract Ten sites were drilled across the eastern flank of the Juan de Fuca Ridge during ODP Leg 168 to investigate the nature of ridge-flank hydrothermal circulation and crustal evolution in a region of varying sediment-basement topography. The sites were divided into: (i) the Hydrothermal Transition (HT) transect, a zone of gradually thickening sedimentary cover, extending from the ridge-flank boundary for a distance of 20 km to the east; (ii) the Buried Basement (BB) transect, which consists of uniformly, flat lying basement, capped by a regionally continuous thick sedimentary sequence; and (iii) the Rough Basement (RB) transect, represented by a region of basement highs and troughs, located ∼100 km east of the ridge axis, and capped by a variable thickness of sediment. Across the flank, a progressive sequence of alteration was recognized, produced as the hydrothermal system evolved from open oxidative conditions involving a slightly modified seawater derived fluid, to a more restricted/closed, non-oxidative environment in which more strongly modified seawater derived fluids interacted with the basement. Alteration commenced with the formation of chlorite at relatively high temperatures in non-oxidative to oxidative conditions, followed by the first wide spread stage of low temperature oxidative alteration with the formation of iron oxyhydroxides and celadonite. Progressive burial, along with continued alteration of the basement, restricted fluid circulation resulting in the formation of saponite ± pyrite over a range of temperatures under non-oxidative conditions. The final stage of alteration is represented by carbonate, which formed under non-oxidative conditions by interaction of strongly modified seawater and the crust.