Differentiation of planetesimals and the thermal consequences of melt migration

Abstract

Abstract-We model the heating of a primordial planetesimal by decay of the short-lived radionuclides 26 Al and 60 Fe to determine (1) the time scale on which melting will occur, (2) the minimum size of a body that will produce silicate melt and differentiate, (3) the migration rate of molten material within the interior, and (4) the thermal consequences of the transport of 26 Al in partial melt. Our models incorporate results from previous studies of planetary differentiation and are constrained by petrologic (i.e., grain-size distributions), isotopic (e.g., 207 Pb-206 Pb and 182 Hf-182 W ages), and mineralogical properties of differentiated achondrites. We show that formation of a basaltic crust via melt percolation was limited by the formation time of the body, matrix grain size, and viscosity of the melt. We show that low viscosity (<1 Pa AE s) silicate melt can buoyantly migrate on a time scale comparable to the mean life of 26 Al. The equilibrium partitioning of Al into silicate partial melt and the migration of that melt acts to dampen internal temperatures. However, subsequent heating from the decay of 60 Fe generated melt fractions in excess of 50%, thus completing differentiation for bodies that accreted within 2 Myr of CAI formation (i.e., the onset of isotopic decay). Migration and concentration of 26 Al into a crust results in remelting of that crust for accretion times less than 2 Myr and for bodies >100 km in size. Differentiation would be most likely for planetesimals larger than 20 km in diameter that accreted within approximately 2.7 Myr of CAI formation.

Differentiation and core formation in accreting planetesimals

“…In their model, any melt that rises towards the surface would cool and crystallize as crust intrusions. Their core formation model resembles that of Moskovitz & Gaidos (2011), though.…”

“…Ghosh & McSween Jr. (1998), Merk et al (2002), Moskovitz & Gaidos (2011), Sramek et al (2012 do not consider sintering of an initially porous planetesimal. Similarly, accretion is not modelled in Ghosh & McSween Jr. (1998), Hevey & Sanders (2006) and Moskovitz & Gaidos (2011). Velocities of melt migration are not calculated self-consistently but are either assumed to be constant (Sahijpal et al 2007) or do not depend on radius, i.e.…”

“…Miyamoto et al 1981;Grimm & McSween Jr. 1993;Sahijpal et al 1995;Bennett & McSween Jr. 1996;Ghosh & McSween Jr. 1998Merk et al 2002;Ghosh et al 2003;Yoshino et al 2003;Sahijpal & Soni 2005;Bizzarro et al 2005;Hevey & Sanders 2006;Sahijpal et al 2007;Moskovitz & Gaidos 2011). The aim of these works is the study of the thermal metamorphism and/or melting of asteroids and planetesimals.…”

“…Therefore, segregation of iron is found to occur only after the crust formed when the decay of the remaining 60 Fe generates melt fractions in excess of 50% (Taylor 1992). In the model by Moskovitz & Gaidos (2011) -as before in the model by Sahijpal et al (2007) -it is assumed that silicate melt can rise toward the surface by porous flow and form a basaltic crust. An assumption that differs from another recent model by Elkins-Tanton et al (2011), who argue that silicate melt would not be able to reach the surface because of its higher density in comparison to that of the primordial crust.…”

“…Recently, Moskovitz & Gaidos (2011) studied how the migration of silicate melt and in particular the redistribution of 26 Al from the interior into a crustal layer would affect the thermal evolution of a planetesimal. They conclude that this process can significantly dampen the increase of the interior temperature and strongly reduce the melting rate.…”