Age dependent porosity of young upper oceanic crust: Insights from seafloor gravity studies of recent volcanic eruptions

Pruis, Matthew J.; Johnson, H. Paul

doi:10.1029/2001gl013977

Cited by 10 publications

(7 citation statements)

References 36 publications

(38 reference statements)

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“…As the ocean crust ages, alteration of the extrusive volcanic rocks and precipitation of alteration minerals reduces the size of the smaller voids (Alt 1995), while mechanical collapse during migration through the near‐axis tectonic zone reduces the size of the larger voids (Gregg & Chadwick 1996; Chadwick et al. 1999a; Pruis & Johnson 2002).…”

Section: The Oceanic Upper Crustal Reservoirmentioning

confidence: 99%

“…For scale, 34% is the approximate porosity of a container of randomly ordered spheres of any diameter, and permeabilities of 10 )10 to 10 )12 m 2 lie within the range for coarse and fine sand beds; properties that allow both large fluid storage volumes and circulation that can be driven by relatively low thermal gradients. As the ocean crust ages, alteration of the extrusive volcanic rocks and precipitation of alteration minerals reduces the size of the smaller voids (Alt 1995), while mechanical collapse during migration through the near-axis tectonic zone reduces the size of the larger voids (Gregg & Chadwick 1996;Chadwick et al 1999a;Pruis & Johnson 2002).…”

Section: The Oceanic Upper Crustal Reservoirmentioning

confidence: 99%

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On sampling the upper crustal reservoir of the NE Pacific Ocean

2006

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The oceanic upper crustal reservoir is a 600-m thick layer of porous and permeable basaltic rock that forms the uppermost igneous basement underlying the global ocean. Pore spaces within this fluid aquifer contain a significant fraction of the global seawater, and active circulation through this reservoir has profound influence on the chemical composition of the ocean, strongly impacting the biological environment near the sea surface. Because of the relative inaccessibility of the deep seafloor, where hydrothermal fluid discharges and seawater re-charges the oceanic crustal aquifer, our understanding of the dynamic physical, chemical and biological processes is strongly dependent on our ability to obtain uncontaminated samples from this challenging environment. Recent technological advances have addressed some, but certainly not all of these sampling problems, providing new data and samples that test our current hypotheses about the crustal fluid reservoir. Current scientific interest in the sub-seafloor biosphere has focused on the uppermost igneous oceanic crust as likely to be one of the most habitable environments, because of its porosity and locus of hydrothermal circulation of chemical nutrients. Recent observations indicate that sub-seafloor crustal environments harbor novel CO 2 -utilizing bacteria (primary producers) that could be a significant source of carbon-fixation in the ocean, thus broadening possible habitable zones both on Earth and elsewhere where microbial life could exist independent of nutrient input from photosynthesis.

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Section: The Oceanic Upper Crustal Reservoirmentioning

confidence: 99%

Section: The Oceanic Upper Crustal Reservoirmentioning

confidence: 99%

On sampling the upper crustal reservoir of the NE Pacific Ocean

2006

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“…The exponent in Archie's law is 1.2. The profiles are shown only at depths ranging from 50 to 400 m, which is the depth range of the conductive anomaly porosity, which was estimated from outcrop rock samples at the northern Gorda Ridge (Pruis and Johnson 2002), is a plausible porosity for the uppermost crust. The hypothetical profile is just double of the porosity profile from Evans et al (1998), and the highest porosity of the hypothetical profile is 34 % in the uppermost layer (Fig 19.8a).…”

Section: Discussionmentioning

confidence: 99%

Electrical Resistivity Structure of the Snail Site at the Southern Mariana Trough Spreading Center

Matsuno

Kimura

Seama

2014

Subseafloor Biosphere Linked to Hydrothermal Systems

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The electrical resistivity of the oceanic crust is sensitive to the porosity of the crust and the fluid temperature within crustal fractures and pores. The spatial variation of the crustal porosity and the fluid temperature that is related to a hydrothermal circulation can be deduced by revealing an electrical resistivity structure of the oceanic crust involving a hydrothermal site. We carried out a magnetometric resistivity experiment using an active source to reveal an electrical resistivity structure of the oceanic crust at the Snail site on the ridge crest of the Southern Mariana Trough. Active source electric currents were transmitted along and across the ridge axis in a 4,000 m 2 area including the Snail site. Five ocean bottom magnetometers were deployed around the Snail site as receivers to measure the magnetic field induced by the transmission of the active source electric currents. The amplitude of the induced magnetic field was calculated by maximizing data density and the signal to error ratio in the data, and locations of the transmissions were determined using several types of calibration data. An optimal 1-D resistivity structure of the oceanic crust, averaged over the experimental area, was deduced by least squares from the data of the amplitude of the magnetic field and the location of the transmission. After calculating magnetic field anomalies, which are deviations of the observed amplitude from the prediction of the optimal 1-D resistivity model, an optimal 3-D resistivity structure was deduced from the magnetic field anomalies through trial and error 3-D forward modeling. The optimal 1-D resistivity structure is a two-layer model, which consists of a 5.6 Ω-m upper layer having a 1,500 m thickness and a 0.1 Ω-m underlying half-space. Using Archie's law and porosity profiles of the oceanic crust, the resistivity of 5.6 Ω-m at depths ranging from 800 to 1,500 m suggests the presence of hightemperature fluid related to the hydrothermal circulation. The resistivity of 0.1 Ω-m below 1,500 m depth may represent a magma mush that is a heat source for the hydrothermal circulation. The optimal 3-D resistivity structure includes a conductive anomaly (0.56 Ω-m in approximately 300 m 2 area down to 400 m depth) immediately below the Snail site, two resistive anomalies (56 Ω-m with slightly larger volumes than the conductive anomaly) adjacent to the conductive anomaly on the across-ridge side, and three conductive anomalies away from the Snail site. The conductive anomaly immediately below the Snail site suggests hydrothermal fluid, and the adjacent resistive anomalies suggest areas of low porosity. The size and distribution of the conductive and resistive anomalies near the Snail site constrains the size and style of the hydrothermal circulation.

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“…Our identification of sites of seawater recharge cannot distinguish between seawater inflow that feeds either the high‐temperature fields or the low‐temperature diffuse vents that surround them. The upper crust in the axial region of Endeavour Ridge is suggested by gravity measurements to be both porous (>10%) and likely permeable [ Holmes and Johnson , 1993; Johnson et al , 2000; Pruis and Johnson , 2002; Gilbert and Johnson , 1999], which would reduce any influence of along‐axis faults to produce strong anisotropy in subsurface fluid flow. Although the present conductive heat flow measurements are the most extensive seafloor survey conducted to date on bare rock seafloor, our necessarily two‐dimensional results do not constrain the circulation pathways in the critical third dimension of depth.…”

Section: Measurement Practice and Errorsmentioning

confidence: 99%

Hydrothermal circulation within the Endeavour Segment, Juan de Fuca Ridge

Johnson

Tivey

Bjorklund

et al. 2010

Geochem Geophys Geosyst

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[1] Areas of the seafloor at mid-ocean ridges where hydrothermal vents discharge are easily recognized by the dramatic biological, physical, and chemical processes that characterize such sites. Locations where seawater flows into the seafloor to recharge hydrothermal cells within the crustal reservoir are by contrast almost invisible but can be indirectly identified by a systematic grid of conductive heat flow measurements. An array of conductive heat flow stations in the Endeavour axial valley of the Juan de Fuca Ridge has identified recharge zones that appear to represent a nested system of fluid circulation paths. At the scale of an axial rift valley, conductive heat flow data indicate a general cross-valley fluid flow, where seawater enters the shallow subsurface crustal reservoir at the eastern wall of the Endeavour axial valley and undergoes a kilometer of horizontal transit beneath the valley floor, finally exiting as warm hydrothermal fluid discharge on the western valley bounding wall. Recharge zones also have been identified as located within an annular ring of very cold seafloor around the large Main Endeavour Hydrothermal Field, with seawater inflow occurring within faults that surround the fluid discharge sites. These conductive heat flow data are consistent with previous models where high-temperature fluid circulation cells beneath large hydrothermal vent fields may be composed of narrow vertical cylinders. Subsurface fluid circulation on the Endeavour Segment occurs at various crustal depths in three distinct modes: (1) general east to west flow across the entire valley floor, (2) in narrow cylinders that penetrate deeply to high-temperature heat sources, and (3) supplying low-temperature diffuse vents where seawater is entrained into the shallow uppermost crust by the adjacent high-temperature cylindrical systems. The systematic array of conductive heat flow measurements over the axial valley floor averaged ∼150 mW/m 2 , suggesting that only about 3% of the total energy flux of ocean crustal formation is removed by conductive heat transfer, with the remainder being dissipated to overlying seawater by fluid advection.

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Age dependent porosity of young upper oceanic crust: Insights from seafloor gravity studies of recent volcanic eruptions

Cited by 10 publications

References 36 publications

On sampling the upper crustal reservoir of the NE Pacific Ocean

On sampling the upper crustal reservoir of the NE Pacific Ocean

Electrical Resistivity Structure of the Snail Site at the Southern Mariana Trough Spreading Center

Hydrothermal circulation within the Endeavour Segment, Juan de Fuca Ridge

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