Borehole heat flow along the eastern flank of the Juan de Fuca Ridge, including effects of anisotropy and temperature dependence of sediment thermal conductivity

Abstract

The thermal conductivities of 15 whole‐round sediment samples collected during Ocean Drilling Program (ODP) Leg 168 between 17 and 440 m below the seafloor on the eastern flank of the Juan de Fuca Ridge were tested to document their anisotropy and temperature dependence using the divided bar technique. Tests over a temperature range of 5°C to 60°C reveal variations in conductivity of up to ±15%. The sign and amplitude of these variations depend on the thermal conductivity at laboratory (room) temperature (λrt): if λrt ≈ 0.8 W m−1 K−1 (high porosity), conductivity increases with temperature; if λrt ≈ 1.2 W m−1 K−1 (moderate porosity), conductivity does not change with temperature; if λrt ≈ 1.6 Wm−1K−1 (low porosity), conductivity decreases with temperature. This behavior results from a positive temperature coefficient for seawater (λ ∝ T) and a negative coefficient for rock matrix (λ ∝ 1/T). A special sampling technique for unconsolidated sediments made it possible to measure horizontal (λHOR) and vertical (λVER) components of thermal conductivity independently and to determine a mean anisotropy value (λHOR/λVER) of 1.2. Corrections, which are <10% for anisotropy and <1% for temperature, were applied using in situ temperatures, shipboard line source thermal conductivities, porosities, and the geometric mean mixing model that accounts for matrix and porewater constituent conductivities. On the basis of these corrected conductivities and a harmonic averaging of values weighted according to the lithologic division of the sediment sections into clay‐rich and sand‐rich units, values of heat flow estimated from borehole temperature measurements are lower than those previously estimated by ∼10% except for one site where the section is dominated by sand. Shallow seafloor heat flow measured with gravity‐driven probes may also need to be corrected for anisotropy, although the degree of anisotropy in the unconsolidated, high‐porosity sediments within a few meters of the seafloor is poorly constrained.