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

Agata M Krzesińska

doi:10.7306/gq.1267

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 Pultusk 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 Pultusk breccia was the most likely the result of impact heating, which affected the parent body in its very early history.

Highlights

The parent bodies of ordinary chondrites, during accretion and ~60 Ma later, were commonly affected by thermal metamorphism (e.g., Brearley and Jones, 1998; Huss et al, 2006)
The most distinct aspect of brecciation of the meteorite is the presence of black-coloured zones rich in broken and crushed, cataclased lithic fragments (Fig. 1), formed due to late, Thermal metamorphic evolution of the Pu3tusk H chondrite breccia – compositional and textural properties
Comparison of this record with standard petrological classification (Van Schmus and Wood, 1967) of the same material shows inprecision the classificatory parameters reflect accretional metamorphic conditions affecting the meteorite on its parent body

Summary

Introduction

The parent bodies of ordinary chondrites, during accretion and ~60 Ma later, were commonly affected by thermal metamorphism (e.g., Brearley and Jones, 1998; Huss et al, 2006). To gain insight into the connection between petrological classification and the early thermal evolution of chondrites, scientific studies have been undertaken by such means as thermochronological constraints (e.g., Pellas and Storzer, 1981; Trieloff et al, 2003; Bouvier et al, 2007), Ar-Ar gas retention measurements (Swindle et al, 2009; Bogard, 2011) or quantifications of metallographic cooling rates (e.g., Scott et al, 2014 and references therein) None of these provided a clear link and all they show that petrologic types never systematically correlate with any metamorphic factor (e.g., Grimm, 1985; Scott et al, 2014)

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Discussion

Conclusion