January 4, 2021

Team Profile:
The Curie Point Depth: where rocks lose their magnetisation
contributor(s)

Craig Miller

photo credit:
Witter & Miller, 2017

Evaluating the Curie point depth is part of GNG research seeking to characterise New Zealand’s supercritical geothermal resources.

What is the Curie point & the Curie depth?

The Curie point is the temperature at which rocks lose their permanent magnetisation, typically at about 580°C, depending on rock type.

The Curie point depth is the depth within the Earth’s crust at which this temperature occurs.

Why is this Important?

The Curie depth provides information about the thermal structure of the crust, such as how quickly temperature increases with depth, as well as  properties such as rheological strength that determines if the crust is brittle or ductile. Mapping the Curie depth helps us locate areas of higher heat flow, where geothermal reservoirs and heat sources are closer to the surface.

Shallow Curie depth means higher heat flow

Curie-point depths are usually shallower in regions that have geothermal potential, young volcanism and thinned crust – like the Taupō Volcanic Zone (TVZ) in New Zealand’s central North Island, as it indicates hotter rocks are closer to the surface.

In the TVZ, the geothermal gradient (the rate of increasing temperature with depth towards the Earth's interior) is about 50°C/km. Away from tectonic plate boundaries, this gradient is about 25–30°C/km.

Heat flow gradient in the TVZ geothermal areas (red line) is around twice that of regions  away from volcanic and tectonic settings(black line).

Mapping heat flow & determining Curie depth

We can estimate this Curie depth using spectral analysis of aeromagnetic data. This geophysical technique uses a magnetometer to measure the strength of Earth’s magnetic field, and how it varies on a local scale, due to geology. Measurements are usually conducted from an aircraft, resulting in detailed maps of the magnetic field.  

By analysing the frequency content of these magnetic maps, we can determine the depth at which magnetisation disappears, the Curie depth.  A single Curie depth is calculated for an approximately 100 cubic km block of data. The analysis is then repeated for a neighbouring data block to build a map of Curie point depth.

The maps look something like this:

Example of a heat flow map showing Curie point depth from the Yukon, Canada (Witter & Miller, 2017). In this map contours are the CPD and colours are the heat flow from well measurements.  Higher heat flow is coincident with shallow CPD.
Rough evaluation of the curie point depth in the Central North Island of New Zealand from satellite magnetic data (Li and Wang, 2017). Future work will improve the accuracy of this map by compiling low level aeromagnetic data, delivering greater resolution.  

Sources & References:

Þ  Witter, J. and Miller, C.,2017. Curie point depth mapping in Yukon. Yukon Geological Survey, Open File 2017-3, 37p.

Þ  Li,C.-F., Lu, Y., and Wang, J. (2017). A global reference model of Curie-point depths based on EMAG2. Scientific Reports, 7(1), 45129. https://doi.org/10.1038/srep45129

 

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