Case study of chondrule alteration with IR spectroscopy in NWA 2086 CV3 meteorite

Abstract

Analyzing the alteration in an olivine chondrule of the NWA 2086 CV3 meteorite, infrared spectral, electron microprobe and optical microscopic observations were correlated to each other. The intensity and wavelength positions of olivine peaks changed characteristically with the progression of alteration and related Fe/Mg substitution inward of the chondrule. Moderate to good correlations were identified between Fo% composition and positions of 830 and 860cm−1 IR peaks. The disappearance of 1020cm−1 peak by structural change happens already at a low level alteration without changing the optical appearance of the mineral. The existence of the 980cm−1 peak is found to be an indicator of the intact phase of olivine. While profiles perpendicular to the chondrule׳s perimeter showed that the alteration progressed 15–20μm distance inward without observable fractures (probablly by some diffusion related process), the “alteration distance” from various obvious fractures inside the chondrule was only 3–5μm distance. These observations suggest that the substitution was more effective close to the matrix, and also related to some fluids that although were able to circulate along the large internal fractures too, did not produce such strong substitution there, like what happened close to the matrix. It was also demonstrated that the poorly exploited contact mode observations with ATR based reflection method in infrared spectroscopy provide a useful tool to analyze the alteration at micrometer scale without much sample preparation, and enable identifying alterations already at such a low level where the olivines still look optically intact.