Thermal evolution of anhydrous asteroids

Data sources given in Haack and McCoy (2004). Values in parentheses are calculated initial liquid compositions for elements like Ir and Re with solid/liquid distribution coefficients that are far from unity, meteorite composition may be different from that of the initial liquid core. 

Elemental abundances in group IIIAB iron meteorites (dots) compared to calculated fractional crystallization trends that assume complete separation of crystals and melt (solid curve) and incomplete separation of those phases (dashed curve). Modified from Haack and McCoy (2004). 

The peak temperatures that asteroids experienced can be estimated from chemical exchange reactions between minerals - so-called geothermometers. For example, the exchange of calcium between coexisting orthopyroxene and clinopyroxene in highly metamorphosed chondrites has been used to estimate their equilibration temperatures (Slater-Reynolds and McSween, 2005). For ordinary chondrites, these temperatures range up to -1175 K. The experimental conditions at which achondrites melt provide minimum temperatures for their parent bodies. Melting of achondrites typically requires temperatures of 1200 K. 

Metamorphic reactions in anhydrous chondrites mostly involved recrystallization, although a few mineral changes occurred. Of particular interest is an oxidation reaction 

The rates at which parent bodies cooled also provide constraints on thermal models. A method for determining the cooling rates for iron meteorites is described in Box 11.2. A similar method for chondrite cooling rates is also based on the compositions of metal grains. Cooling rates can also be estimated from knowing the blocking temperatures of various radioisotope systems. 

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