Chondrite thermal histories constrained by experimental annealing of Quenggouk orthopyroxene

Abstract

The meteorites called ‘Ordinary chondrites’ preserve a record of processes in the formation of their parent bodies (asteroids or similar objects) in the early Solar System. An important aspect yet to be explained is a series of ‘petrologic types’¹, numbered 3 to 6, with increasingly homogeneous minerals and annealed textures. The annealing (which postdates the highest-temperature events that produced chondrules as droplets of melt) is usually regarded as a prograde metamorphism, that is, reheating after cold accretion of the parent bodies², but evidence has been presented for origin of the textures and mineral compositions during primary cooling (see, for example, ref. 3). Here we report the first constraint on chondrite ‘metamorphic’ history from experimental annealing of a silicate mineral in natural meteoritic specimens. ‘Striated’ orthopyroxene was converted almost entirely to normal orthopyroxene in 1 week at approximately 800 °C, without detectable change of composition. This result tends to support the primary cooling hypothesis.