Alkali‐halogen metasomatism of theCMcarbonaceous chondrites

M R Lee,A J King,B E Cohen

doi:10.1111/maps.13405

Abstract

AbstractMeteorite Hills (MET) 01075 is unique among theCMcarbonaceous chondrites in containing the feldspathoid mineral sodalite, and hence it may provide valuable evidence for a nebular or parent body process that has not been previously recorded by this meteorite group.MET01075 is composed of aqueously altered chondrules and calcium‐ and aluminum‐rich inclusions (CAIs) in a matrix that is predominantly made of serpentine‐ and tochilinite‐rich particles. The chondrules have been impact flattened and define a foliation petrofabric. Sodalite occurs in a 0.6 mm sizeCAIthat also contains spinel, perovskite, and diopside together with Fe‐rich phyllosilicate and calcite. By analogy with feldspathoid‐bearingCAIs in theCVandCOcarbonaceous chondrites, the sodalite is interpreted to have formed by replacement of melilite or anorthite during alkali‐halogen metasomatism in a parent body environment. While it is possible that theCAIwas metasomatized in a precursor parent body, then excavated and incorporated into theMET01075 parent body, in situ metasomatism is the favored model. The brief episode of relatively high temperature water–rock interaction was driven by radiogenic or impact heating, and most of the evidence for metasomatism was erased by subsequent lower temperature aqueous alteration.MET01075 is very unusual in sampling aCMparent body region that underwent early alkali‐halogen metasomatism and has retained one of its products.

Highlights

All of the CM carbonaceous chondrites have undergone extensive parent body aqueous alteration, which has been dated to 4563.4 +0.4/–0.5 Ma using the carbonate 53Mn-53Cr system (Fujiya et al 2012)
We describe sodalite in the CM carbonaceous chondrite Meteorite Hills (MET) 01075. This mineral has not been previously found in a CM, but feldspathoids occur in CV and CO carbonaceous chondrites where they are interpreted to have formed during relatively high temperature (>~100 °C) Fe-alkali-halogen metasomatism within the parent body
The X-ray diffraction (XRD) pattern and mineral assemblage of MET 01075 are typical for CM carbonaceous chondrites (e.g., Howard et al 2015)

Summary

Introduction

All of the CM carbonaceous chondrites have undergone extensive parent body aqueous alteration, which has been dated to 4563.4 +0.4/–0.5 Ma using the carbonate 53Mn-53Cr system (Fujiya et al 2012). We describe sodalite in the CM carbonaceous chondrite Meteorite Hills (MET) 01075. This mineral has not been previously found in a CM, but feldspathoids occur in CV and CO carbonaceous chondrites where they are interpreted to have formed during relatively high temperature (>~100 °C) Fe-alkali-halogen metasomatism within the parent body

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Conclusion