Thermal metamorphic evolution of the Pułtusk H chondrite breccia – compositional and textural properties not included in petrological classification

Abstract

The thermal history that chondrites experienced on their parent body is an aspect of their petrological classification. However, in the classification scheme, metamorphic conditions are generally limited to the peak metamorphic temperature attained, while it is known that reconstruction of the genuine thermal evolution of any rock requires identification of various metamorphic factors, definition of the temperature-time path during metamorphism and characterization of the processes responsible for heating. Study of the brecciated Pułtusk H chondrite shows that the meteorite comprises both low and high petrologic type material and should be classified as a H3.8–6 chondrite. Based on the textures and mineral and chemical composition, the thermal metamorphic history of the breccia is reconstructed and it is shown to be inconsistent with the petrologic classification; the textural maturation and degree of compositional equilibrium in the meteorite do not correspond to the temperatures attained. The metamorphic conditions are shown to be a function of the primary composition of the accreted minerals and of two metamorphic phases, progressive and retrogressive. First, a prograde phase led to textural maturation and equilibration of the chemical composition of silicates and oxides. The peak metamorphic temperatures were at least ~700ºC for the type 3.8. and 4 material, and up to ~1000ºC in H6 clasts i.e., sufficient to locally give rise to partial melting. The following retrograde metamorphism led to compositional re-equilibration of minerals and textural re-equilibration of minerals with partial melts. The cooling rate during retrograde metamorphism down to at least ~700ºC was low, which allowed potassium feldspar to form patches in Na-plagioclase and pseudobrookite-armalcolite breakdown to form an association of ilmenite and rutile. The two-phase metamorphic evolution of the Pułtusk breccia was the most likely the result of impact heating, which affected the parent body in its very early history.