Geothermal Measurements on DSDP Leg 26

Abstract

Nine downhole in situ temperature estimates were obtained from five holes on DSDP Leg 26. The heat-flow determinations were reliable at four sites. Closely spaced temperature measurements in two holes provided a good confirmation of the reproducibility of the measured temperatures using the downhole recorder. A number of temperature-versus-time records clearly shows the decay of penetration heating of the probe, permitting theoretical extrapolation of in situ temperature to equilibrium. Heat flux is nearly constant with depth at the two sites, each with four temperature points (including sea-bottom-water temperature). There is close general agreement between the Leg 26 sites and the results of the nearest shallow ocean probe. The Leg 26 sites show no systematic variation with age.

Site U1422

“…Thermal conductivity in fine-grained sediment is, as a first approximation, a linear combination of the conductivities of the sedimentary grains and the interstitial water and therefore depends upon porosity, water content, and lithology (Hyndman et al, 1974). Overall, thermal conductivity values range from 0.7 to 1.0 W/(m·K) without a clear increase with depth.…”

“…Thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndman et al (1974) (see "Physical properties" in the "Methods" chapter [Tada et al, 2015b]). The calculated in situ values are as much as 2.6% below the measured laboratory values.…”

A record of spontaneous subduction initiation in the Izu–Bonin–Mariana arc

“…At these depths, the sediment was poorly consolidated, so a needle probe was inserted into the sediment through a 2 mm hole, drilled into the plastic core liner, to measure the thermal conductivity. All measurements were subsequently corrected for in situ conditions using the method of Hyndman et al 4 in which the temperatures were determined from the interpolated APCT-3 measurements ( A preliminary estimate of the sediment load-corrected depth of the basement at Site U1438 gives a depth of ~5527 m. A residual depth of ~550 m is calculated from an expected depth of 5000 m for a 50 Ma age plate 6 . This is consistent with other estimates for the west Philippine Sea Plate and within the range observed for the Shikoku and Parece Vela basins, which have excess depths of 500-1000 m 7 .…”

Site U1390

“…Thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndman et al (1974) (see "Physical properties" in the "Methods" chapter [Expedition 339 Scientists, 2013b]). The calculated in situ values average 1.5% higher than the measured laboratory values.…”