地球物質情報に基づく地球熱史解明：到達点の概観と今後の展開

Abstract

Approaches to elucidate thermal history of the earth based on information of earth materials are reviewed. Limitations of these approaches are examined, and ways for the improvement and additional approaches to better constrain the thermal history of the earth are proposed. A short note of the current thermal status of the earth is followed by examination of earth's thermal history based on geophysical modeling of mantle convection, combination of which with material information is essential to deepen our understanding. There are several proxies of earth materials for secular changes of the thermal state of the earth's interior. Those often used so far are: (1) chemical composition of magmas, from which ‘potential temperatures’ of the ambient mantle are estimated, (2) pressure and temperature conditions of crustal materials (metamorphic rocks), from which ‘metamorphic geothermal gradients’ are estimated, (3) thickness of the crust and lithosphere, from which thermal gradients of the crust and lithosphere are estimated along with the temperature estimation of the bottoms, and (4) pressure and temperature of mantle materials, from which ‘mantle geothermal gradients’ of the lithosphere are estimated. Each method has problems to be resolved for quantitative estimation of the secular variation of the earth's thermal state. The following approaches are proposed: (1) coupling thickness of oceanic crust and depletion zone of residual mantle and major element composition of volcanic rocks, (2) high-resolution analysis of thermal history of crust and mantle materials to better constrain steady-state geotherms, and (3) simultaneous estimation of ambient pressure and temperature as well as mantle potential temperature from analysis of magma intrusions in the crust. Finally, the importance of extraterrestrial materials and earth-like exoplanets to reveal thermal history of the early earth, for which direct information is not available, is remarked.