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Measuring mantle hydrogen content to map ore-forming fluids and model plate tectonics

General

Organisation
Project start
01.01.2018
Project end
31.12.2019
Type of project
ARMAP/NSF
Project theme
Geoscience
Project topic
Geology

Fieldwork / Study

Fieldwork country
Greenland (DK)
Fieldwork location

Geolocation is 0, 0

Fieldwork start
14.07.2018
Fieldwork end
26.07.2018

SAR information

Fieldwork / Study

Fieldwork country
Greenland (DK)
Fieldwork location

Geolocation is 0, 0

Fieldwork start
15.07.2018
Fieldwork end
22.07.2018

SAR information

Project details

02.12.2019
Science / project plan

.

Science / project summary
This project uses the geophysical method magnetotellurics (MT), which images the electrical conductivity of the Earth to depths of ~400 km. In Greenland, the aim of this work is to constrain mantle viscosity and improve ice-loss calculations. The electrical conductivity of the Earth’s upper mantle is most strongly controlled by temperature, hydrogen content and the presence of melt. These three factors also control its viscosity (resistance to flow), so MT data are one of the best ways to measure mantle viscosity. This is particularly important for Greenland since viscosity is one of the most important and poorly constrained inputs into glacial isostatic adjustments models. These models seek to account for the mantle’s slow response to past changes in ice sheet thickness and to remove this long-term signal from current measurements of ice loss. Greenland’s mantle viscosity is likely to be particularly complex. Mantle rocks from northern and southern Greenland are ancient, with cold temperatures and depleted compositions that would be expected to have high viscosity. However, the Iceland Plume passed beneath central Greenland ~40 to 100 million years ago, and that section of mantle is likely to have been heated and hydrated, giving it a lower viscosity. This project’s long-term aim is to collect MT data over several areas on Greenland to image the mantle both over the Iceland Plume track and distal from it. Summit Station is ideally located to image the Greenland mantle that has been impacted by the Iceland Plume. They will model the data and interpret the results in terms of mantle viscosity and then feed these viscosity calculations into improved glacial isostatic adjustment models. The MT data will also image any sub-ice melt layers, so they will also be able to test the hypothesis that increased surface heat flow over the Iceland Plume track is leading to increased sub-ice melting. In the work at Summit in 2018, researchers plan to carry out an initial study in which they will: (1) Deploy two long-period instruments in the vicinity of Summit to provide information on mantle conductivity, electrical dimensionality and strike direction, in order to plan the geometry of future, deep-field campaigns. (2) Deploy a network of broad-band instruments in the vicinity of Summit to investigate the presence of basal melt layers. (1) Test different instruments and electrodes in side-by-side tests to compare data quality and determine optimal instrument configurations. MT instruments are passive (only recording naturally occurring electric and magnetic fields) and are powered by batteries and solar panels. They consist of a magnetometer or magnetometers, which must be shallowly buried for stability, a logger unit, and two electric dipoles, which consist of metal plates shallowly buried in snow/ice, separated by ~100 m. The whole area for a station deployment is therefore ~100 x 100 m. The sensors are extremely sensitive so stations must be deployed at least ~1 km from electrical noise and physical vibrations.
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