Heat-producing Granite Bodies

The Radiogenic Model

Optimum sites for low-temperature (< 150°C) geothermal resources in the tectonically stable eastern United States will probably be associated with crustal igneous rocks that contain relatively high concentrations of the heat-producing radioactive isotopes of uranium, thorium, and potassium. Moderate amounts of heat-producing isotopes occur in all crystalline basement rocks, but the principal geothermal targets in the southeastern U.S. are the relatively young (257-330 Ma) syn- and postmetamorphic U- and Th-bearing, heat-producing granitoid bodies that were intruded into the crystalline basement of the now-exposed Piedmont. They also occur in the basement beneath the sediments of the Atlantic Coastal Plain. The sediments, because of their low thermal conductivity, act as a thermal insulator, like a sweater. Granitoids crop out over a large area of the central and southern Appalachian Piedmont and Blue Ridge, and extend eastward in the basement rocks concealed beneath the sediments of the Atlantic Coastal Plain. A conspicuous negative Bouguer gravity anomaly is generally associated with the granitoid. The combination of relatively high heat flow from a heat-producing granitoid concealed beneath sediments of relatively low thermal conductivity was defined by Costain and others (1980) as the radiogenic model illustrated below.

In order to illustrate the effect on subsurface temperatures of the Atlantic Coastal Plain water-saturated sedimentary wedge, we show the effect of varying thickness of sediment overlying heat-producing and non-heat-producing crust, as shown in the above figure. The figure below summarizes how the subsurface temperature in the sediments is determined by:

1. The thickness of Atlantic Coastal Plain (ACP) sediments that overlie and conceal the igneous intrusions in the heat-producing crystalline basement. The sedimentary wedge varies in thickness from zero at the Fall Line to about 3 km at Cape Hatteras, NC,
2. The thermal conductivity of the ACP sediments that overlie the heat-producing basement intrusions (mcal/cm-sec-^oC in the figure below),
3. The amount of heat given off by the radiogenic intrusive rocks in the crystalline basement.as a result of the radioactive decay of U, Th, and K (A_oin the figure below.

This simple model has been called the "Radiogenic Model".

Confirmation of the Radiogenic Model

The model was confirmed at the Portsmouth, VA, drill site, where a -40 mgal Bouguer gravity anomaly near Portsmouth, Virginia was believed to be caused by a granite body beneath the sediments of the Atlantic Coastal Plain. Drill-site CP25 was located near the center of the circular gravity anomaly at latitude 36° 5l.01' and longitude 76° 29.83' (see geothermal well in the illustration above). The hole was drilled through the coastal plain sediments to 557 m (1828 ft.) by Gruy Federal, Inc. during December 1978. From January to April 1979, the hole was deepened to 611 m (2005 ft.) and a continuous, 1-1/2 inch diameter core was obtained from 557 to 611 m (1828-2005 ft.). The presence of a heat-producing granite body at this location was confirmed. Hole C-25 (Portsmouth) was drilled into a late Alleghanian, unmetamorphosed, heat-producing granite. Hole C-26 (Isle of Wight) was drilled into the non-granitic, non-heat-producing, metamorphosed country rock into which the granite was intruded. The temperatures adjacent (C-26) to the granite and over (C-25) the granite body are shown below. Notice the higher temperatures in Hole C-25 at any depth. For example, at a depth of about 500 meters, the temperature in Hole C-25 is about 8º C higher than in C-26. The higher temperatures in C-25 are a direct result of the extra heat produced by the radioactive decay of U, Th, and K (about 80% of the heat comes from U and Th) in the granite beneath C-25. The optimum sites for geothermal resource development are therefore over such granite bodies because higher temperatures are reached at shallower depths. There are numerous such optimum locations in the southeastern United States. Where the granites are concealed beneath Coastal Plain sediments, or where they do not reach the top of crystalline basement, for example at Crisfield, MD, they can be located by geophysical exploration using gravity and magnetics. Click on the hole locations on the map below for the details of the data at each site.

References