Textural Evidence for Shock-related Metasomatic Replacement of Olivine by Phosphates in the Chelyabinsk Chondrite

Introduction: Phosphate minerals are thought to occur in chondritic parent bodies mainly as a result of secondary alteration processes [1]. Jones et al., [2] investigated the textural occurrences of phosphates in a range of Ordinary Chondrites (OCs) (petrologic types 3.9–6) and found support for a model involving progressive solid-state recrystallisation and equilibration in a closed parent body system. However, the nature of the phosphate source materials and their mode of incorporation into mineral form is not fully understood. Here we investigate the textural relationships in the recent ‘Chelyabinsk’ meteorite (LL5, S5) fall and provide supporting evidence for the phosphate minerals being secondary, specifically following olivine replacement. Background: Typically, OC’s contain two prominent phosphate minerals: merrilite (Ca9NaMg(PO4)7) and apatite (Ca5(PO4)3(OH,F,Cl)). Prevailing models of phosphate growth involve the initial generation of merrilite during mild heating on the asteroidal parent body and the subsequent formation of apatite by reaction of merrillite with halogen-rich aqueous fluids. Chelyabinsk is a brecciated and highly shocked chondrite which contains both merrilite and apatite. Isotopic and petrological studies are yet to fully provide a definitive model of formation [3-5]. We have undertaken a comprehensive textural study to assess these models. Methods: Phosphates in the sample were characterised utilising petrographic and scanning electron microscopy (SEM). The SEM is equipped with an Energy Dispersive Spectrometer (EDS) and was used to construct major elemental maps of the sample. We targeted individual phosphate grains for detailed imaging and textural examination. Point-and-Identification analysis was used to distinguish the phosphate phases and define their relationships to coexisting mineral phases. Results: Relative frequencies for key textural associations of phosphates in Chelyabinsk are shown in Figure 1. Associations with olivine, fractures and chromite/plagioclase (maskelynite) are dominant; these are, predominantly, grain boundary contacts (regular, irregular and embayed) as well as, more rarely, inclusions of given phases within phosphates and the hosting of phosphate grains entirely within a given phase. Figure 2 is an example showing key observed phosphate textural associations. Figure 1: Textural associations of phosphates with other minerals in Chelyabinsk. The relative frequencies in each class are similar between apatite and merrillite; figures given are averages across all grains observed.