MEASUREMENT OF RADIOGENIC HEAT PRODUCTION

The location and generalized values for marine and land heat flow are shown in Figure 1 for the regions adjacent to the Leg 149 sites. This region has the largest number of heat flow measurements relevant to the study of passive margins, with numerous values on land (Lucazeau and Vasseur, 1989) and three detailed transects at sea across Goban Spur, Galicia Bank, and the Amorican Margins (Foucher and Sibuet, 1980; Louden et al., 1991). Values of radiogenic heat production in the Paleozoic metamorphosed cratons that border these margins are generally high (e.g., Richardson and Oxburgh, 1978; Jolivet et al., 1989), and there is a clear decrease from higher continental values (often >90 mW/m²) to lower oceanic values (generally <60 mW/m²). However, continental heat flow and radiogenic heat production measurements are lacking within the Paleozoic section of northwestern Spain, immediately adjacent to the Leg 149 drill sites and to previous drill sites off Galicia Bank.

The location of the Leg 149 sites in the Iberia Basin and dredge sample locations adjacent to Galicia Bank are shown in Figure 2. Also shown are the location of three seismic profiles (Lusigal 12, Resolution 3, and Sonne 16) that have been used to construct a seismic transect along the drill sites (Shipboard Scientific Party, 1993). Six samples each from Sites 897 and 900 were analyzed for the concentration of radiogenic heat-producing elements; four samples were analyzed from each of the dredge sites GAL 02,07/08, and 11. A general description of the sampled rock types and the location of each sample are given in Table 1.

The technique used for measuring the concentrations of radioelements in rocks is described by Mareschal et al. (1989). Samples were crushed to a fine powder and neutron activated in the "slowpoke" reactor at École Polytechnique (Montréal). The samples were subjected to a flux of 5 10¹¹ neutrons/cm²-s for 2 hr. After irradiation, the concentrations of U, Th, and K were measured by gamma-ray spectrometry, following the semi-absolute method of Bergerioux et al. (1979). Gamma rays were measured with planar low-energy photon scintillometers and coplanar GeLi detectors. For U²³⁸, gamma rays at 277 kev from the disintegration of Np²³⁹ (half life 2.35 days) were counted. For Th²³², the gamma rays at 312 kev were produced by disintegration of Pa²³³ (half life 27 days). For K⁴¹, gamma rays at 1525 kev from the disintegration of K⁴² (half life 12 hr) were counted. For some samples, the K concentration was also measured with the standard X-ray spectrometry technique (Schroeder et al., 1980). The overall reproducibility was verified by measuring different aliquots of the same sample and was better than 5%.

Concentrations of the radioelements and the resulting radiogenic heat production are given in Table 1. The heat production values are shown as histograms in Figure 3. Except for one lower value of 0.307 µW/m³ for a dolomite sample from Galicia Bank, values can be separated into three distinct groups: Galicia dredge samples, primarily granodiorites (A_av = 1.67 0.31 µW/m³); Hole 900A samples, primarily metamorphosed gabbros (A_av = 0.21 0.19 µW/m³); and serpentinized peridotites from Holes 897C and 897D (A_av< 0.01 µW/m³). The average heat production for the Galicia Bank granodiorites are lower than the average for Paleozoic granites observed in Brittany (3.3 0.6 µW/m³; Jolivet el al., 1989) and Wales and Cornwall (3.1 2.1 µW/m³; Richardson and Oxburgh, 1978), as might be expected. Values for Site 900 are higher than typical values of 0.08 µW/m³ for tholeiitic basalts (Stacey, 1992) and similar to values of 0.4 µW/m³ for the lower continental crust (Galson, 1983; Ashwal et al., 1987; Fountain et al., 1987; Mareschal et al, 1989). The serpentinite samples from Site 897 have negligible radionuclides. This result is not surprising as typical mantle peridotites have low concentrations, and these concentrations (particularly U²³⁸) would be further reduced during hydration. Similar weathering could also have reduced the concentrations of radionuclides at Site 900.