SUMMARY

Measurements of radionuclide concentrations on samples from Sites 897 and 900 of Leg 149 in the Iberia Basin and from three dredge site locations off Galicia Bank yield values of radiogenic heat production which can be separated into three distinct groups: Galicia dredge samples, primarily granodiorites (A_av= 1.67 0.31 µW/m³); Site 900 samples, primarily metamorphosed gabbros (A_av = 0.21 0.19 µW/m³); and Site 897 serpentinized peridotites (A_av < 0.01 µW/m³). Values for the Galicia Bank granodiorites are lower than typical granitic values observed in Brittany and Cornwall. Values for Site 900 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 probably premature 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 typical for mantle peridotites and additional reduction during hydration.

These results 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. A preliminary comparison between previous shallow-marine measurements off Galicia Bank and deeper measurements from Sites 897, 898, and 900 of Leg 149 in the Iberia Basin indicate just the reverse. Comparable values for both margins 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/m² higher than values landward of the serpentinite ridge on Galicia Bank. The large uncertainties in heat flow for the Leg 149 sites and the limited number of determinations of radiogenic heat production, however, means that this inconsistency requires further confirmation from additional measurements of radiogenic heat production in basement landward of Site 900 and additional heat flow stations in its vicinity.