Comment on “The petrologic case for a dry lower crust” by Bruce W. D. Yardley and John W. Valley

Wannamaker, Philip E.

doi:10.1029/1999jb900324

Cited by 97 publications

(42 citation statements)

References 73 publications

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“…A clear example of this is the ongoing debate over the reason for the high conductivities observed in the lower crust. The available evidence has been taken to support both graphite or aqueous fluids as an explanation for the elevated conductivity (Yardley and Valley 1997;Wannamaker 2000). This is discussed in detail later in this chapter.…”

Section: Non-uniqueness In Interpreting Electrical Resistivity Measurmentioning

confidence: 88%

Mapping the Distribution of Fluids in the Crust and Lithospheric Mantle Utilizing Geophysical Methods

Unsworth

Rondenay

2012

Lecture Notes in Earth System Sciences

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Geophysical imaging provides a unique perspective on metasomatism, because it allows the present day fluid distribution in the Earth's crust and upper mantle to be mapped. This is in contrast to geological studies that investigate midcrustal rocks have been exhumed and fluids associated with metasomatism are absent. The primary geophysical methods that can be used are (a) electromagnetic methods that image electrical resistivity and (b) seismic methods that can measure the seismic velocity and related quantities such as Poisson's ratio and seismic anisotropy. For studies of depths in excess of a few kilometres, the most effective electromagnetic method is magnetotellurics (MT) which uses natural electromagnetic signals as an energy source. The electrical resistivity of crustal rocks is sensitive to the quantity, salinity and degree of interconnection of aqueous fluids. Partial melt and hydrogen diffusion can also cause low electrical resistivity. The effects of fluid and/or water on seismic observables are assessed by rock and mineral physics studies. These studies show that the presence of water generally reduces the seismic velocities of rocks and minerals. The water can be present as a fluid, in hydrous minerals, or as hydrogen point defects in nominally anhydrous minerals. Water can further modify seismic properties such as the Poisson's ratio, the quality factor, and anisotropy. A variety of seismic analysis methods are employed to measure these effects in situ in the crust and lithospheric mantle and include seismic tomography, seismic reflection, passive-source converted and scattered wave imaging, and shear-wave splitting analysis. A combination of magnetotelluric and seismic data has proven an effective tool to study the fluid

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Section: Non-uniqueness In Interpreting Electrical Resistivity Measurmentioning

confidence: 88%

Mapping the Distribution of Fluids in the Crust and Lithospheric Mantle Utilizing Geophysical Methods

Unsworth

Rondenay

2012

Lecture Notes in Earth System Sciences

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“…Some authors have proposed that interconnected graphite films may be responsible for this effect (Yardley and Valley, 1997) but this hypothesis has a number of weaknesses (Wannamaker, 2000). An alternative proposal is that aqueous saline fluids are responsible for the low resistivity of the lower crust (Wannamaker, 2000;Jones, 1992b). If the lower crust has a significant fluid content, then shearing could enhance the porosity and permeability, which would further lower the electrical resistivity.…”

Section: Imaging Resistivity Structure Of Shear Zones In the Mid And mentioning

confidence: 99%

On the geoelectric structure of major strike-slip faults and shear zones

Unsworth

Bedrosian

2014

Earth Planet Sp

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Magnetotelluric imaging of the San Andreas Fault has shown that seismically-active segments are characterized by a zone of low resistivity in the upper crust. Similar resistivity features are observed on other major strike-slip faults, and may have a common origin in a region of fractured rock, partially or fully saturated with groundwater. Other strike-slip faults show possible zones of reduced resistivity in the mid and lower crust that may be related to zones of ductile shear. Additional MT surveys are required to elucidate the role of fluids in controlling the seismic behaviour of major faults, both in and below the seismogenic zone. A set of synthetic inversions show that MT data is sensitive to the geoelectric structure of a shear zone at mid-crustal depths.

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“…Whittington and others (1999) estimate that melting begins at 720°C and 8 kbars with 4 weight percent water in the melt. A water content of 4 weight percent for the melt means that a water activity of 0.6 to 0.7 and therefore a resistivity of 0.2 ohm-m can be expected (Wannamaker, 1986). There is sufficient water in the melt to lower its resistivity substantially.…”

Section: Magnetotelluric Studiesmentioning

confidence: 99%

“…Wannamaker (1986) shows that the resistivity of a silicic melt is dependent on its water content and can vary from 0.1 ohm-m for water-saturated melting to 1000 ohm-m for dry partial melting. Whittington and others (1999) estimate that melting begins at 720°C and 8 kbars with 4 weight percent water in the melt.…”

Section: Magnetotelluric Studiesmentioning

confidence: 99%

Overview of hydrothermal activity associated with active orogenesis and metamorphism: Nanga Parbat, Pakistan Himalaya

Chamberlain¹

2002

American Journal of Science

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ABSTRACT. The Nanga Parbat Massif is located in northwestern Pakistan and occurs in the interior of the western syntaxis of the Himalaya. Recent studies have shown that the massif exposes an active metamorphic anomaly developed in IndianPlate rocks exhumed from beneath and surrounded by rocks of the Kohistan/Ladakh island-arc. Geochemical and geophysical data from Nanga Parbat show that an active hydrothermal system exists within the massif. In the upper 5 to 6 kilometers of the crust, above a shallow brittle/ductile transition constrained by microseismic data, there is a shallow hydrothermal system with meteoric fluids channelized along active fault zones. In contrast, below the brittle/ductile transition fluids are metamorphic in origin and restricted to unconnected packets along grain boundaries.The shallow system is stratified with water-rich fluid inclusions near the surface grading to vapor-rich inclusions at the brittle/ductile transition, as evidenced from fluid inclusion studies. These fluids are meteoric in origin and enter the massif along shear and fracture zones and exit the massif along the bordering fault zones. Evidence for these channelized meteoric fluids comes from stable isotopic values of minerals and rocks collected from fault zones that show that these rocks have interacted with meteoric fluids, as well as magnetotelluric (MT) data that show that the fault zones are electrically conductive and embedded in very resistive rocks.The deeper portions of the hydrothermal system consist of unconnected metamorphic fluids. Stable isotopic and fluid inclusion data show that fluids were present during the recent metamorphism but the rocks did not extensively interact with these fluids. Existence of an unconnected fluid system is supported by the MT studies showing that below the brittle/ductile transition the rocks are relatively resistive (values ranging from 1000 -10,000 ohm-m), ruling out an interconnected fluid network. Seismic and MT data also show no evidence of a large magma body at depth.We suggest that the type of hydrothermal system observed at Nanga Parbat can be an important feature of late-stage orogenic evolution, especially near the tectonically complex edges of orogens. The tectonism in these areas exposes deep, relatively dry rocks that have previously lost their volatile content during the earliest stages of tectonic thickening and metamorphism. These relatively dry rocks are rapidly uplifted and exposed within the massifs where they interact with surface waters at high temperatures and shallow depths.

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Comment on “The petrologic case for a dry lower crust” by Bruce W. D. Yardley and John W. Valley

Cited by 97 publications

References 73 publications

Mapping the Distribution of Fluids in the Crust and Lithospheric Mantle Utilizing Geophysical Methods

Mapping the Distribution of Fluids in the Crust and Lithospheric Mantle Utilizing Geophysical Methods

On the geoelectric structure of major strike-slip faults and shear zones

Overview of hydrothermal activity associated with active orogenesis and metamorphism: Nanga Parbat, Pakistan Himalaya

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