On thermal features within the transition zone

Abstract

The negative gradient of specific entropy with depth, multiplied by the thermal Grüneisen parameter, and the quotient of temperature and gravity acceleration defines the dimensionless parameter Δ as a function of the depth z. It proves tantamount to the relative deviation of the effective compressibility from that which would occur under isentropic conditions at z. In the latter form, Δ( z) is accessible to computation by the PREM Earth model. The values of Δ in the core and lower mantle are rather more close to zero as compared with those in the transition zone, TZ, which according to PREM is three-layered and reaches from 220 to 670 km depth. In the intermediate layer, 400 < z [km] < 600, Δ is between 0.7 and 1.0, while in the upper and lower layers of TZ the values of Δ are about −0.2 and −0.6, respectively. It is shown that Δ + 1 is a lower bound of C P C V throughout, where C P and C V are the principal specific heats. The comparatively great values of C P C V to be expected within the TZ refer to inhomogeneities due to phase transitions occurring at several depths. PREM allows the occurrence of only one first order transition at z = 400 km with a negative Clausius-Clapeyron slope, and of a second order transition with a slope estimated at 0.02 GPa K −1. Further transitions at depths intermediate between 400 and 600 km must be of second order. The corresponding slopes are approximately 0.07 GPa K −1. The two principal phase transitions are strongly negatively correlated.