Thermodynamics of the Upper Mantle

Abstract

Summary An analysis is made of the energy input from the upper mantle to the crust since Palaeozoic times under the Japanese islands. It is argued that significant changes in this rate have occurred. These changes in energy input are interpreted in terms of unsteady mantle convection. The effect of the C layer on convection is discussed. The upper mantle is a source of both energy and materials for the development of the crust. Therefore analysis of geological phenomena (tectogenesis) from the thermodynamical point of view is useful to understand the present state and evolutionary path of the upper mantle. The energy analysis is carried out for the formation process of the Japan Islands. It is concluded that the rate of energy supply from the upper mantle during the late Palaeozoic to the late Mesozoic (first active period) is 0.8-1.6fical/cmzs in excess of the normal (stationary) flow. During the Miocene (another significant active period) energy supply reached several pcai/cmzs (4). The latter activation has been continuing up to the present. The above abnormal (unsteady) energy supply is realized only by enthalpy transfer. The activation of upper mantle might be possible either by the ,thermal convection or by two phase convection, of which the latter corresponds to a local generation of magmas. Two possible types of activation are compared with concept of the thermodynamical engine (3). A study of thermodynamical coupling of the mantle and crust is carried out by a method of numerical experiment. As an example the unsteady mantle convection in Newtonian viscous fluid is discussed in relation to the thermal and mechanical response to the crust. The thermal convection is assumed to be generated locally by the horizontal temperature gradient within the mantle. Considering the time scale of actual geological phenomena the effective viscosity of the upper mantle is estimated as 1021-1022 P. The maximum speed reaches an order of cm/year. A sequence of epiorogenic volcanism and the following plutonism is obtained (5). In many cases the activation of the upper mantle took place at the ocean-continent boundary. For the activation of the mantle in a specified region there exists two types of possibility. The first is that the upper mantle was already inhomogeneous and the geological processes are regarded as the reflection of the mantle inhomogeneity. However, the thermal stress due to heat source inhomogeneities is shown to be insufficient to generate the mantle differentiation. The second possibility is the information transfer of surface inhomogeneities into the deeper part. The generation of thermal convection described above is a possible mechanism. When the heat flow through the Moho is unsteady, the thermal state of the crust also becomes unsteady. In addition, variation of crustal thickness due to orogenism and sedimentation-erosion, redistribution of radiogenic heat source, and the progress of regional metamorphism affect the thermal state of the crust. Secular variation of