Evolution of the interior of Mercury influenced by coupled magmatism-mantle convection system and heat flux from the core

Abstract

To discuss mantle evolution in Mercury, I present two‐dimensional numerical models of magmatism in a convecting mantle. Thermal, compositional, and magmatic buoyancy drives convection of temperature‐dependent viscosity fluid in a rectangular box placed on the top of the core that is modeled as a heat bath of uniform temperature. Magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through a matrix. Widespread magmatism caused by high initial temperature of the mantle and the core makes the mantle compositionally stratified within the first several hundred million years of the 4.5 Gyr calculated history. The stratified structure persists for 4.5 Gyr, when the reference mantle viscosity at 1573 K is higher than around 1020 Pa s. The planet thermally contracts by an amount comparable to the one suggested for Mercury over the past 4 Gyr. Mantle upwelling, however, generates magma only for the first 0.1–0.3 Gyr. At lower mantle viscosity, in contrast, a positive feedback between magmatism and mantle upwelling operates to cause episodic magmatism that continues for the first 0.3–0.8 Gyr. Convective current stirs the mantle and eventually dissolves its stratified structure to enhance heat flow from the core and temporarily resurrect magmatism depending on the core size. These models, however, predict larger contraction of the planet. Coupling between magmatism and mantle convection plays key roles in mantle evolution, and the difficulty in numerically reproducing the history of magmatism of Mercury without causing too large radial contraction of the planet warrants further exploration of this coupling.

Mantle control of the geodynamo: Consequences of top‐down regulation

“…In terms of temporal variability, considerations of the evolution of the core based on its composition and the thermodynamics of the geodynamo generally indicate a monotonic increase in core heat flux going backward in time [Driscoll and Bercovici, 2014;Nimmo, 2015]. This smooth increase with age is partly due to Geochemistry, Geophysics, Geosystems 10.1002/2016GC006334 the predictable rate of increase in radioactive heat production and partly due to the fact that thermal evolution models usually assume only monotonic variations in the surface heat flow.…”

“…(2) the outer core mixes heat and composition on time scales far shorter than its evolutionary time scale, so the core is always in approximate thermal equilibrium with the CMB, and in thermal and compositional equilibrium with the inner core at the ICB, and (3) apart from latent heat, the only significant internal heating is radioactive. Although estimates of the amount of radioactive heat in the core vary considerably [Nimmo, 2015], even the highest of these estimates barely changes the main story.…”

“…Mercury may be an exception. It evidently has a weak dynamo‐generated field [ Anderson et al ., ] and it has been proposed that planet's relatively thin mantle plays a role in its maintenance [ Ogawa , ]. However, the relatively feeble external magnetic field of Mercury would not provide much of a shield for the Earth.…”

Evaluating an impact origin for Mercury's high‐magnesium region

“…Large volcanic provinces on Mercury such as the northern smooth plains demonstrate that the emplacement of large igneous provinces took place at least ~3.8 Ga [ Head et al ., ; Denevi et al ., ; Ostrach et al ., ]. A younger, hotter Mercury would have experienced large‐scale volcanism as well [e.g., Ogawa , ]. Chemically heterogeneous mantle . Because Mercury's silicate shell is thin relative to its core at a thickness of only ~420 km [ Hauck et al ., ], it is difficult to laterally homogenize the mantle.…”

Thermal effects of late accretion to the crust and mantle of Mercury

“…We note the starting absolute temperature profile in the mantle, and the resulting (deep) thermal gradient, strongly influences the amount of komatiite production. A steep hermeotherm would seem to mitigate much mantle convection on either present or past Mercury under reasonable viscosity regimes and with a thick thermal boundary layer (e.g., Redmond and King, 2007;Ogawa, 2016). We thus consider two cases for the temperatures at the lithosphere base and CMB, with accompanying hermeotherm: (i) 1575 -1650 °C from the lithosphere base to the CMB (an average of each temperature range above), which gives an approximate 0.3 °C km -1 13 hermeotherm; and (ii) 1475 -1825 °C (the maximum temperature range of the above estimates), with 1.2 °C km -1 .…”