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Figure F11. Measured and estimated temperatures with depth in Hole 1256D (Wilson, Teagle, Acton, et al., 2003; Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006). Thermal profile estimated using the measured conductive heat flow and appropriate thermal conductivities for the lava, dikes, and gabbros. Thermal data for Hole 1256D is compared against the stratigraphy and equilibrium temperature log for Hole 504B located on 6.9 Ma crust south of the intermediate spreading rate Costa Rica Rift. At the depth achieved in Hole 1256D by the end of Expedition 312, that site is estimated to be ~50° cooler (~114°C) than the temperature measured at the equivalent depth in Hole 504B (~163°C). The highest temperature recorded in Hole 504B was 189°C at 1979 mbsf (Alt, Kinoshita, Stokking, et al., 1993). We highlight the 120°C isotherm, thought to be close to the limit of prokaryotic life (e.g., Heberling et al., 2010) for Holes 1256D and 504B. Hole 1256D is yet to reach this threshold, but Hole 504B is above this temperature below ~800 mbsf. TAP = Temperature/Acceleration/Pressure tool.

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