Hydrogeologic Properties, Processes, and Alteration in the Igneous Ocean Crust

Fisher, A. T.; Alt, Jeffrey C; Bach, Wolfgang

doi:10.1016/b978-0-444-62617-2.00018-9

Cited by 17 publications

(18 citation statements)

References 104 publications

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“…While variations in b shift these curves by up to an order of magnitude in k A , the similarity of the Q S versus b response makes it difficult to constrain the aquifer's thickness on the basis of these results alone. Borehole logs and results from packer testing show evidence for high permeability to a depth of ~320 m into the crust but also suggest that the upper extrusive ocean crust may comprise a series of thin, highly permeable units [ Becker et al , ; Fisher et al , ]. While we did not explicitly model separate zones within a single aquifer, the fact that a number of simulations involving thin aquifers (100 m, 200 m) resulted in active hydrothermal siphons and were consistent with the field constraints suggests that thin zones are capable of supporting observed flows without violating field constraints.…”

Section: Modeling Constraints On Crustal Properties and Flow Geometrymentioning

confidence: 79%

Three‐dimensional modeling of outcrop‐to‐outcrop hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge

et al. 2016

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We present three‐dimensional simulations of coupled fluid and heat transport in the ocean crust, to explore patterns and controls on ridge‐flank hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge. Field studies have shown that there is large‐scale fluid flow in the volcanic ocean crust in this region, including local convection and circulation between two basement outcrops separated by ~50 km. New simulations include an assessment of crustal permeability and aquifer thickness, outcrop permeability, the potential influence of multiple discharging outcrops, and a comparison between two‐dimensional (profile) and three‐dimensional representations of the natural system. Field observations that help to constrain new simulations include a modest range of flow rates between recharging and discharging outcrops, secondary convection adjacent to the recharging outcrop, crustal permeability determinations made in boreholes, and the lack of a regional seafloor heat flux anomaly as a consequence of advective heat loss from the crust. Three‐dimensional simulations are most consistent with field observations when models use a crustal permeability of 3 × 10−13 to 2 × 10−12 m2, and the crustal aquifer is ≤300 m thick, values consistent with borehole observations. We find fluid flow rates and crustal cooling efficiencies that are an order of magnitude greater in three‐dimensional simulations than in two‐dimensional simulations using equivalent properties. Simulations including discharge from an additional outcrop can also replicate field observations but tend to increase the overall rate of recharge and reduce the flow rate at the primary discharge site.

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Section: Modeling Constraints On Crustal Properties and Flow Geometrymentioning

confidence: 79%

Three‐dimensional modeling of outcrop‐to‐outcrop hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge

et al. 2016

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“…The only realistic way to produce multipass flow is thermal convection, which generally requires permeabilities >5 × 10 –16 m 2 to operate 47 , 48 . Fracturing is the most likely way to achieve such permeabilities in gabbro and peridotite at Moho depths 49 . Fault zone permeability is generally dominated by the damage zone of a fault typically 100–400 m wide 50 , 51 .…”

Section: Resultsmentioning

confidence: 99%

No significant boron in the hydrated mantle of most subducting slabs

et al. 2018

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Boron has become the principle proxy for the release of seawater-derived fluids into arc volcanics, linked to cross-arc variations in boron content and isotopic ratio. Because all ocean floor serpentinites so far analysed are strongly enriched in boron, it is generally assumed that if the uppermost slab mantle is hydrated, it will also be enriched in boron. Here we present the first measurements of boron and boron isotopes in fast-spread oceanic gabbros in the Pacific, showing strong take-up of seawater-derived boron during alteration. We show that in one-pass hydration of the upper mantle, as proposed for bend fault serpentinisation, boron will not reach the hydrated slab mantle. Only prolonged hydrothermal circulation, for example in a long-lived transform fault, can add significant boron to the slab mantle. We conclude that hydrated mantle in subducting slabs will only rarely contribute to boron enrichment in arc volcanics, or to deep mantle recycling.

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“…Different distributions of pillows and sheets, along with variation in the paleo-seafloor topography and the associated variations in timing of sedimentation, mean the paleo-hydrology of these crustal sections was variable (e.g. Bach et al, 2004;Fisher et al, 2014). To help in understanding the paleo-hydrology, calcite amygdales (and occasional vugs and veins) were sampled (Fig.…”

Section: Field Constraintsmentioning

confidence: 99%

Temperature dependence of chemical exchange during seafloor weathering: Insights from the Troodos ophiolite

Coogan

Gillis

2018

Geochimica et Cosmochimica Acta

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Chemical fluxes associated with low-temperature, off-axis, alteration of the upper oceanic crust (seafloor weathering) may play an important role in controlling the composition of the ocean and the long-term carbon cycle. However, it is challenging to quantify these fluxes and how they change with changing bottom water temperature over geological timescales. Here we study the exchange of major elements associated with seafloor weathering in the exceptionally preserved Cretaceous Troodos ophiolite and compare them to less well constrained data from drill cores from the modern ocean basins. Calcite O-isotope thermometry from four traverses through the lavas reveals a well-ventilated region at the top of the lava pile where alteration occurred at nearconstant temperatures similar to that of bottom water. The lithological makeup of the crust appears to control the thickness of this region, with increased abundance of sheet flows marking the base of this zone. Maintaining low-temperatures in the well-ventilated region requires large fluid fluxes facilitated by the high permeability of the pillow lavas. Large-scale addition of CO 2 to the crust only occurs in this well-ventilated region indicating that the alkalinity producing reactions required for calcite precipitation occurred at temperatures sensitive to bottom water temperature. Comparison of whole rock geochemical data for samples from the well-ventilated zone with volcanic glass compositions shows that the changes in rock compositions due to seafloor weathering are large (roughly -4 wt% SiO 2 , +0.5 to 1 wt% MgO, -5 wt% CaO, -0.5 wt% Na 2 O, +3.5 wt% K 2 O and + 2.5 wt% CO 2 ). Comparison to data from drill cores from modern oceanic crust that was also altered under well-ventilated conditions suggests that bottom water temperature plays a major role in controlling the chemical exchange between the ocean and lavas; parameterizations for the associated fluxes are provided. Because elemental exchange between the ocean and oceanic crust globally depends strongly on bottom water temperature, fixed values for these fluxes cannot be used in models of the evolution of seawater chemistry or subduction fluxes. INTRODUCTIONThe major ion composition of seawater is important in many aspects of the Earth system from its effect on the carbon cycle (via alkalinity) to modulating biomineralization (e.g. Sundquist, 1991;Porter, 2010). Additionally, because the major ion composition of the ocean reflects the integrated fluxes of ions into and out of the ocean, an understanding of the controls on ocean chemistry, along with a history of ocean chemistry, may allow the history of these fluxes to be unraveled (e.g. Spencer and Hardie, 1991;Demicco et al., 2005). Of the fluxes that play important roles in controlling ocean chemistry, perhaps the least well-constrained is that associated with lowtemperature alteration of the upper oceanic crust (lavas) in off-axis hydrothermal systems. These systems operate across the abyssal plains starting close to the ridge axis and continuing until...

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Hydrogeologic Properties, Processes, and Alteration in the Igneous Ocean Crust

Cited by 17 publications

References 104 publications

Three‐dimensional modeling of outcrop‐to‐outcrop hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge

Three‐dimensional modeling of outcrop‐to‐outcrop hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge

No significant boron in the hydrated mantle of most subducting slabs

Temperature dependence of chemical exchange during seafloor weathering: Insights from the Troodos ophiolite

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