Measurements of radiogenic heat production on basement samples from Sites 897 and 900

Abstract

Concentrations of radionuclides were measured on six samples each from Sites 897 and 900 of Leg 149 in the Iberia Basin and four samples each from three dredge sites located off Galicia Bank. They are used to calculate radiogenic heat production for rocks located in a rifted, nonvolcanic continental margin setting. Results can be separated into three distinct groups: Galicia dredge samples, primarily from granodiorites (Aav = 1.67 ± 0.31 µW/m 3 ); Site 900 samples, primarily from metamorphosed gabbros (Aav = 0.21 + 0.19 µW/m 3 ); and Site 897 samples of serpentinized peridotites (Aav < 0.01 µW/m 3 ). Values for the Galicia Bank granodiorites are lower than typical granitic values observed in Brittany. Values for Site 900 metamorphosed gabbros are higher than typical values for tholeiitic basalts and similar to values for the lower continental crust. Some reduction is probably caused by radionuclide mobility during weathering. However, it is not possible to distinguish these rocks as lower continental crust, rather than lower oceanic crust, based solely on these values. The low values for the serpentinite samples from Site 897 are consistent with low concentrations in mantle peridotites and additional reduction during hydration. For simple models, heat flow across older rifted margins such as Iberia and Galicia Banks should increase from ocean to continent, in proportion to the increased volume of radionuclides in the continental rocks. These results, therefore, suggest that heat-flow observations over the thinned continental crust of the Galicia Bank Margin should be elevated with respect to measurements on the thinned continental (or oceanic) crust of the Iberia Basin. However, a preliminary comparison between seafloor measurements off Galicia Bank and deeper Leg 149 measurements from Sites 897, 898, and 900 indicate just the reverse. Comparable values for both locations are observed in the oceanic/serpentinite ridge domain, but landward values on transitional crust at Site 900 in the Iberia Basin are 16-23 mW/m 2 higher than values landward of the serpentinite ridge on Galicia Bank.