Modelling the Lost City hydrothermal field: influence of topography and permeability structure

Abstract

The Lost City hydrothermal field (LCHF) is hosted in serpentinite at the crest of the Atlantis Massif, an oceanic core complex close to the mid-Atlantic Ridge. It is remarkable for its longevity and for venting low-temperature (40-91°C) alkaline fluids rich in hydrogen and methane. IODP Hole U1309D, 5 km north of the LCHF, penetrated 1415 m of gabbroic rocks and contains a near-conductive thermal gradient close to 100°C km À1 . This is remarkable so close to an active hydrothermal field. We present hydrothermal modelling using a topographic profile through the vent field and IODP site U1309. Long-lived circulation with vent temperatures similar to the LCHF can be sustained at moderate permeabilities of 10 À14 to 10 À15 m 2 with a basal heatflow of 0.22 W m À2 .Seafloor topography is an important control, with vents tending to form and remain in higher topography. Models with a uniform permeability throughout the Massif cannot simultaneously maintain circulation at the LCHF and the near-conductive gradient in the borehole, where permeabilities <10 À16 m 2 are required. A steeply dipping permeability discontinuity between the LCHF and the drill hole is required to stabilize venting at the summit of the massif by creating a lateral conductive boundary layer. The discontinuity needs to be close to the vent site, supporting previous inferences that high permeability is most likely produced by faulting related to the transform fault. Rapid increases in modelled fluid temperatures with depth beneath the vent agree with previous estimates of reaction temperature based on geochemical modelling.

Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversion of streamer data and borehole measurements

“…The vertical velocity structure is similar both north and south of the projection of Hole U1309D, but on average, the velocities south of the hole are reduced, Figures c and d. This additional velocity reduction would be consistent with pervasive, uplift‐related deformation and a concomitant increased permeability, the plumbing of the Lost City hydrothermal field is sensitive to both the topography and permeability structure of the Southern Ridge [ Titarenko and McCaig , ].…”

“…By extrapolating from the borehole, we infer that zone 1 traces the interval of most intense damage in the footwall of the detachment, while zones 1 and 2 combined cover the total depth of detachment damage and the current limit of advective cooling. Hydrothermal models with enhanced permeability over a ∼750 thick layer on the Southern Ridge shoulder and Central Dome produce depth limited circulation cells that can match both the borehole temperature profile and vent fluid temperatures at the Lost City hydrothermal field [ Titarenko and McCaig , ].…”

A fault‐driven circulation model for the Lost City Hydrothermal Field

“…Using high‐temperature values of α f = 10 −3 °C −1 and ν = 10 −7 m 2 /s, one obtains k ~ 10 −14 m 2 . This is similar to the estimates of Titarenko and McCaig [] derived from numerical models. This result is a linear function of the area A , which could be somewhat larger or smaller than assumed in equation .…”

The geochemistry of methane isotopologues

“…Models of convective hydrothermal circulation at Lost City indicate that the bulk of actively-venting fluids have migrated along a narrow range of flow paths that are surrounded by already fully-serpentinized rock with little additional potential for H 2 generation, suggesting that H 2 in vent fluids may have instead formed by serpentinization occurring in meandering flow paths away from the main flow channels, and later diffused or mixed into the ascending fluid (Titarenko and McCaig, 2016). Assuming that equilibration of the methane isotopologues proceeds through CH 4 -H 2 O exchange, ∆ 13 CH 3 D temperatures for CH 4 from the present study (red bar in Fig.…”

“…Cooling of these rocks along an f O 2 trajectory parallel to those of typical oxygen buffers may allow for respeciation of mantle-derived CO 2 to CH 4 to occur in the presence of mafic minerals (olivine and orthopyroxene) deep within the oceanic crust (Mathez et al, 1989;Kelley and Früh-Green, 1999). Serpentinization occurring distal to the rocks from which CH 4 -rich fluids are extracted may explain why CH 4 and H 2 concentrations are not tightly correlated across seafloor hydrothermal systems (Keir, 2010;Kawagucci et al, 2013).Models of convective hydrothermal circulation at Lost City indicate that the bulk of actively-venting fluids have migrated along a narrow range of flow paths that are surrounded by already fully-serpentinized rock with little additional potential for H 2 generation, suggesting that H 2 in vent fluids may have instead formed by serpentinization occurring in meandering flow paths away from the main flow channels, and later diffused or mixed into the ascending fluid (Titarenko and McCaig, 2016). Assuming that equilibration of the methane isotopologues proceeds through CH 4 -H 2 O exchange, ∆ 13 CH 3 D temperatures for CH 4 from the present study (red bar in Fig.…”