Nanoscale infrared imaging analysis of carbonaceous chondrites to understand organic-mineral interactions during aqueous alteration

Significance

Spatial relationships between organic matter and minerals are necessary for understanding the formation and evolution of organic matter during aqueous and thermal alteration in their parent bodies, as well as preaccretional history. Infrared spectroscopy is a powerful tool to analyze the molecular structures of organic matter and identification of minerals. However, its spatial resolution is limited due to the diffraction limit. Recently, the atomic force microscopy (AFM) based IR nanospectroscopy was developed and applied in various scientific fields to overcome the diffraction limit of IR. We applied the AFM-based IR nanospectroscopy to carbonaceous chondrites and studied organic-mineral associations at the ∼30 nm spatial resolution.

Abstract

Organic matter in carbonaceous chondrites is distributed in fine-grained matrix. To understand pre- and postaccretion history of organic matter and its association with surrounding minerals, microscopic techniques are mandatory. Infrared (IR) spectroscopy is a useful technique, but the spatial resolution of IR is limited to a few micrometers, due to the diffraction limit. In this study, we applied the high spatial resolution IR imaging method to CM2 carbonaceous chondrites Murchison and Bells, which is based on an atomic force microscopy (AFM) with its tip detecting thermal expansion of a sample resulting from absorption of infrared radiation. We confirmed that this technique permits ∼30 nm spatial resolution organic analysis for the meteorite samples. The IR imaging results are consistent with the previously reported association of organic matter and phyllosilicates, but our results are at much higher spatial resolution. This observation of heterogeneous distributions of the functional groups of organic matter revealed its association with minerals at ∼30 nm spatial resolution in meteorite samples by IR spectroscopy.