Color Cathodoluminescence and minor element zonation of forsterite in Mukundpura chondrite

Abstract

Mukundpura is a carbonaceous chondrite (CM2) recently fell in Rajasthan, India (June 6, 2017). A typical fine–grained, clast–dominant matrix contains a few isolated forsterite and FeO–rich olivine grains. In this study, forsterite–rich olivines were investigated using color cathodoluminescence (CL) and Raman spectroscopy in order to explain the primitive stages of asteroidal aqueous alteration. Isolated forsterite (Fo₉₉) in Mukundpura emits bright CL of varying color and shows CL zonation in different patterns accounting the structural defects and chemical inhomogeneity. Blue luminescence (also distinguished by enriched CaO and TiO₂) is common in cores of the relict forsterite attributing refractory nature of the olivine. Electron Probe Micro Analyzer (EPMA) line scan across the CL–active forsterite grains shows minor elements zonation especially for activator elements and thus provides a correlation of color of the emitted luminescence with diffusible ions. The red CL zonation (also characterized by enriched FeO, Cr₂O₃, and MnO content) is common along the majority of forsterite rims suggesting aqueous activity in the parent asteroid. The strongest doublet Raman peaks corresponding to 821 and 854 cm⁻¹ are due to SiO₄ tetrahedral vibrational modes, and other peaks are often related to infer pure crystalline state of the forsterite. Thus, a combination of CL imaging and Raman spectroscopy is useful to explain the chemical–structural properties of luminescent pure forsterite and also helps in understanding the aqueous alteration of CM chondrite.