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Thermal Gradient in the Earth's Interior

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Thermal Gradient in the Earth's Interior Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Solid Earth Discuss., 7, 2501–2525, 2015 www.solid-earth-discuss.net/7/2501/2015/ doi:10.5194/sed-7-2501-2015 SED © Author(s) 2015. CC Attribution 3.0 License. 7, 2501–2525, 2015 This discussion paper is/has been under review for the journal Solid Earth (SE). Thermal gradient in Please refer to the corresponding final paper in SE if available. the Earth’s interior On the thermal gradient in the M. Tirone Earth’s deep interior Title Page M. Tirone Abstract Introduction Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Conclusions References 44780 Bochum, Germany Tables Figures Received: 19 August 2015 – Accepted: 20 August 2015 – Published: 8 September 2015 Correspondence to: M. Tirone ([email protected]) J I Published by Copernicus Publications on behalf of the European Geosciences Union. J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 2501 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract SED Temperature variations in large portions of the mantle are mainly controlled by the re- versible and irreversible transformation of mechanical energy related to pressure and 7, 2501–2525, 2015 viscous forces into internal energy along with diffusion of heat and chemical reactions. 5 The simplest approach to determine the temperature gradient is to assume that the Thermal gradient in dynamic process involved is adiabatic and reversible, which means that entropy re- the Earth’s interior mains constant in the system. However heat conduction and viscous dissipation during dynamic processes effectively create entropy. The adiabatic and non-adiabatic tem- M. Tirone perature variation under the influence of a constant or varying gravitational field are 10 discussed in this study from the perspective of the Joule–Thomson (JT) throttling sys- tem in relation to the transport equation for change of entropy. The JT model describes Title Page a dynamic irreversible process in which entropy in the system increases but enthalpy Abstract Introduction remains constant (at least in an equipotential gravitational field). A comparison is made between the thermal gradient from the JT model and the thermal gradient from two Conclusions References 15 models, a mantle convection and a plume geodynamic model coupled with thermo- Tables Figures dynamics including a complete description of the entropy variation. The results show that the difference is relatively small and suggests that thermal structure of the astheno- J I spheric mantle can be well approximated by an isenthalpic model when the formulation includes the effect of the gravitational field. For non-dynamic or parameterized mantle J I 20 dynamic studies the JT formulation provides a better description of the thermal gradient Back Close than the classic isentropic formulation. Full Screen / Esc 1 Introduction Printer-friendly Version In the Earth’s deep interior dynamic processes involving large pressure variations in- Interactive Discussion duce temperature changes that often are approximately described by an adiabatic gra- 25 dient. The transformation of pressure–volume mechanical work into thermal heat is strictly considered adiabatic when there is no exchange of heat between the system 2502 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and the surroundings, δq = 0. The common simplification applied to solid Earth prob- lems, is that the process is also reversible, hence the transformation is isentropic (Lewis SED and Randall, 1961; Denbigh, 1971; Sandler, 1988; Turcotte and Schubert, 1982; Schu- 7, 2501–2525, 2015 bert et al., 2001). The reversible condition is in most cases an approximation, in fact 5 spontaneous or natural process are intrinsically irreversible, therefore dS is usually not zero (Lewis and Randall, 1961; Denbigh, 1971; Zemansky et al., 1975; Sandler, Thermal gradient in 1988). The irreversible entropy production in the mantle comes from various sources, the Earth’s interior mainly from heat conduction transformation of mechanical energy into internal energy by viscous forces (viscous dissipation) and chemical reactions. While determination of M. Tirone 10 the entropy production from all these effects may require a full scale dynamic thermal model (e.g Bird et al., 2002), an alternative is given by the description of the throttling Title Page process designed by the Joule–Thomson (JT) experiment) (e.g. Zemansky et al., 1975) which is usually referred as a typical example of an isenthalpic process and has been Abstract Introduction extensively discussed in the geological context by Ganguly(2008). Conclusions References 15 Waldbaum(1971) considered the adiabatic expansion in the Earth interior assuming essentially the formulation derived from the JT experiment. Ramberg(1971) included Tables Figures in the treatment of irreversible decompression the effect of the gravitational potential and Spera(1981) applied a time-dependent dynamic version to metasomatic fluid flow. J I A detailed study of the isenthalpic formulation for irreversible mantle upwelling also J I 20 extended to melting processes was presented by Ganguly(2005). However some questions remain in particular whether the JT model is better suited Back Close than the isentropic formulation to describe the thermal gradient in the mantle. In addi- Full Screen / Esc tion it has not been quantified yet the difference in terms of thermal or entropy change with the complete entropy description from a dynamic thermal model. Printer-friendly Version 25 In Sect.2 the thermodynamic and heat transport formulation is presented with fo- cus on the entropy change and the connection to the JT formulation. In Sects.3 and Interactive Discussion 4 a 2-D mantle convection and a 2-D thermo-chemical plume model combined with a chemical equilibrium approach (Gibbs free energy minimization) are used to deter- mine the entropy-related term and the irreversible thermal contribution. The result is 2503 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | then compared with the JT thermal model in order to understand whether the JT for- mulation could provide a reasonable description of the thermal structure of the mantle. SED The general outline of the numerical computation of the adiabatic thermal gradient 7, 2501–2525, 2015 (reversible and irreversible) in combination with a chemical equilibrium approach is de- 5 scribed in Sect.5 along with a series of mantle geotherms to illustrates the e ffect of the irreversible entropy production based on the JT formulation. Numerical details can be Thermal gradient in found in the appendix. the Earth’s interior M. Tirone 2 Thermodynamic and transport formulation of thermal change This section presents the formulation for the entropy change in relation to heat conduc- Title Page 10 tion and viscous dissipation in transport models and the relation that need to be fulfilled for a process to be assimilated to the JT model. The general expression for the entropy Abstract Introduction change of a moving fluid in a quasi-equilibrium state is given by Bird et al.(2002): Conclusions References DS q Tables Figures ρ = −∇ · + σ (1) Dt T J I where S is the entropy per unit mass, D=Dt is the substantial derivative, the first term 15 on the right hand side (rhs) is the rate of entropy increase by heat conduction and J I the second term σ is the entropy production. The first law of thermodynamics dU = Back Close T dS − P dV for a small mass moving with the fluid is: Full Screen / Esc DU DS DV = T − P (2) Dt Dt Dt Printer-friendly Version and the transport equation for the internal energy is given by the following (Bird et al., Interactive Discussion 20 2002): DU ρ = −∇ · q − P r · v − τ : rv (3) Dt 2504 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | where the first term on the rhs is the rate of energy change by heat conduction, the second term is the reversible energy change by volume compression/expansion and SED the last term is irreversible energy increase by viscous dissipation assuming a Newto- 7, 2501–2525, 2015 nian fluid with velocity v and viscous stress tensor τ. Inserting DS=Dt from Eq. (1) in 5 Eq. (2) and combine the relation for DU=Dt with Eq. (3): Thermal gradient in q DV −T r · + T σ − ρP = −∇ · q − P r · v − τ : rv. (4) the Earth’s interior T Dt M. Tirone Using the relation P r · v = ρP DV=Dt (Sandler, 1988) and the relation: q 1 1 r · = r · q + q · r (5) Title Page T T T Abstract Introduction in Eq. (4), the entropy production σ can be defined: Conclusions References 1 1 10 σ = q · r − τ : rv (6) T T Tables Figures The effect of chemical transformations and other potential sources of entropy are ig- J I nored for simplicity or because negligible in the mantle (electric resistance, magnetic field, non-newtonian fluid). The equation of change for temperature can be obtained J I from Eq. (1) by replacing σ with the expression given in Eq. (5) and the thermodynamic Back Close 15 relation for dS as a function of P , T , dS = Cp=T dT − αV dP for a mass moving with the fluid: Full Screen / Esc DT DP ρC = −∇ · q − τ : rv + αT , (7) Printer-friendly Version p Dt Dt Interactive Discussion where ρ = 1=V has been applied. This is the standard description of temperature change that, for an inviscid fluid under adiabatic and time and space invariant con- 20 ditions, describes an isentropic thermal gradient (dT=dP jS = αV T=Cp). 2505 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | In the well known Joule–Thomson experiment (e.g. Zemansky et al., 1975; Callen, 1985; Ganguly, 2008) for which only a brief description is given here, a gas is steadily SED flowing from one sub-system to another sub-system through a porous plug. The plug 7, 2501–2525, 2015 avoids turbulent flow and inhomogeneous pressure distribution in both sub-systems.
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