9:20 AM - 9:35 AM
[MGI28-02] Estimation of thermal diffusivity in the upper oceanic crust
Keywords:thermal diffusivity, IODP - International Ocean Discovery Program, inversion, oceanic crust
The thermal evolution of planetary crust and lithosphere is largely governed by how much heat is transferred and at what rate. On the other hand, the physical properties used in today's geoscience, such as thermal diffusivity beneath the seafloor, are estimated from laboratory measurements of core samples and so on. The actual measurements and changes with depth on a crustal scale have not yet been clarified. In this study, the thermal diffusivity was estimated from measured temperature data in the upper oceanic crust, showing that the thermal diffusivity changes with depth in the upper oceanic crust.
The data used to estimate the thermal diffusivity were temperature data from the oceanic crust borehole at different time stamps, measured with reciprocating instruments in Hole 1256D in ODP Leg 206 (2002) and IODP Exp 309, 312 (2005). The borehole is 1450m deep, and the data obtained corresponds to the upper part of the oceanic crust. The temperature profiles obtained during the survey recorded the temperature disturbance around the borehole caused by the cooling water during drilling and the recovery of the temperature after drilling was completed. Borehole temperatures changed up to 50 °C due to cooling and recovery from drilling.
The thermal diffusivity was estimated by using an axisymmetric two-dimensional thermal diffusion simulation that follows the two-dimensional thermal diffusion equation in cylindrical coordinates. This simulation assumes no horizontal heat flux across the central axis and maintains fixed equilibrium temperature at the top, bottom, and outer edge boundaries. The thermal diffusivity structure is modeled as a one-dimensional layered structure. This approach reproduces the cooling and recovery of the upper oceanic crust, and the thermal diffusivity was estimated by comparing the calculated values with the measured temperature data from the IODP. The difference between the borehole temperature calculated using the estimated thermal diffusivity and the measured temperature data from IODP was about 0.67 °C RMS.
The profile of the thermal diffusivity estimated by the inversion shows that the thermal diffusivity varies with depth by about 50% on a scale of about 200 m. In addition, the result of the estimation on the scale of tens of meters is 0.47 ± 0.2 mm2/s, which is approximately half of the thermal diffusivity of crustal rocks measured in the laboratory. This indicates that the upper part of the oceanic crust is more insensitive to thermal fluctuations than previously thought, and suggests that the shallow crust may act as an insulator.
The data used to estimate the thermal diffusivity were temperature data from the oceanic crust borehole at different time stamps, measured with reciprocating instruments in Hole 1256D in ODP Leg 206 (2002) and IODP Exp 309, 312 (2005). The borehole is 1450m deep, and the data obtained corresponds to the upper part of the oceanic crust. The temperature profiles obtained during the survey recorded the temperature disturbance around the borehole caused by the cooling water during drilling and the recovery of the temperature after drilling was completed. Borehole temperatures changed up to 50 °C due to cooling and recovery from drilling.
The thermal diffusivity was estimated by using an axisymmetric two-dimensional thermal diffusion simulation that follows the two-dimensional thermal diffusion equation in cylindrical coordinates. This simulation assumes no horizontal heat flux across the central axis and maintains fixed equilibrium temperature at the top, bottom, and outer edge boundaries. The thermal diffusivity structure is modeled as a one-dimensional layered structure. This approach reproduces the cooling and recovery of the upper oceanic crust, and the thermal diffusivity was estimated by comparing the calculated values with the measured temperature data from the IODP. The difference between the borehole temperature calculated using the estimated thermal diffusivity and the measured temperature data from IODP was about 0.67 °C RMS.
The profile of the thermal diffusivity estimated by the inversion shows that the thermal diffusivity varies with depth by about 50% on a scale of about 200 m. In addition, the result of the estimation on the scale of tens of meters is 0.47 ± 0.2 mm2/s, which is approximately half of the thermal diffusivity of crustal rocks measured in the laboratory. This indicates that the upper part of the oceanic crust is more insensitive to thermal fluctuations than previously thought, and suggests that the shallow crust may act as an insulator.