Color Cathodoluminescence and minor element zonation of forsterite in Mukundpura chondrite

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 (Fo<inf>99</inf>) 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 TiO<inf>2</inf>) 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, Cr<inf>2</inf>O<inf>3</inf>, 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 SiO<inf>4</inf> 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.