Response of silicate chemical composition variation on thermal metamorphism of ordinary chondrites and classification of petrologic types:the case of L chondrites from Grove Mountains,Antarctica

摘要

Analysis of the thermal metamorphism of the ordinary chondrites is a key premise for gaining insights into the accretion and heating of rocky bodies in the early solar system.Such an analysis also represents an essential condition for constraining the early thermal and evolutionary histories of asteroids and terrestrial planets.Classifying ordinary chondrites into petrologic type(type 3–6)is the criterion for studying the thermal metamorphism of their parent bodies.However,the boundary between the unequilibrated(type 3)and equilibrated(type 4–6)chondrites is ambiguous at present,thus,limiting the understanding of their thermal metamorphism.In this study,the petrology,mineralogy and chemical composition of a set of seven ordinary chondrites with different degrees of thermal metamorphism collected from Grove Mountains(Antarctica)have been studied.The results demonstrated that these chondrite samples were L3.7,L3.8,L3.9,L3.9/4,L4,L5 and L6 type meteorites,with optimal correlations of Si,Mg,Fe,Mn and Ca with equilibrium degree of the olivine and low-calcium pyroxene and petrologic type.In this respect,the multi-parameter classification standard PMD(SiO2)-PMD(MgO)-PMD(MnO)-PMD(CaO)based on the percent mean deviation(PMD)of the chemical compositions of the olivine and low-calcium pyroxene was proposed to distinguish between the unequilibrated and equilibrated meteorites.The proposed standard exhibited high“resolution”in terms of classification,thus,also deepening the understanding of the effect of the silicate mineral composition in the thermal metamorphism of chondrites.Highlights The chemical groups and petrologic types of the selected seven Antarctic chondrites were L3.7,L3.8,L3.9,L3.9/4,L4,L5 and L6.A new method for petrologic type classification is proposed to distinguish the unequilibrated and equilibrated chondrites.The above developed multi-parameter system exhibited high“resolution”in terms of classification.