Formation of Coesite in a Shocked Amygdule from Lonar Crater: A Nano-IR and Raman Tomography Analysis

Abstract:

Coesite is a high-pressure, high-temperature polymorph of SiO₂ that has been identified as a shock feature in 30 of the 186 known craters on Earth. Coesite is most frequently considered to be the result of a solid-state transformation process, but its formation in impact settings is a subject of considerable debate. It likely can form from a melt at high temperature or via solid-state transformation.

Jaret et al. (2016) identified coesite in a shocked amygdule from a vesicular basalt from Lonar Crater, India. Lonar is a 570 ka, 1.8 km diameter impact crater in the 65 Ma Deccan basalts. The target rocks have been hydrothermally and aqueously altered, resulting in zeolite, chalcedony, opal, and quartz amygdules. The impact then generated shocked silica phases from these secondary minerals. Jaret et al. (2016) used optical petrography, nuclear magnetic resonance (NMR) spectroscopy, and micro-Raman spectroscopy to identify aggregates of coesite occurring with two distinct silica-rich glass phases. They interpreted their results as evidence for crystallization of coesite directly from a melt. Here, we use near-field infrared (nano-IR) imaging and spectroscopy and 3-D Raman tomography to further characterize the Lonar coesite.

We used a neaspec neaSNOM nano-IR system to acquire nano-IR images and spectra at ~20 nm/pixel of a thin section of a shock class 2 Lonar basalt. 3D Raman tomography data (~600 nm/pixel) were generated using a WiTEC alpha300 confocal Raman microscope. We created hyperspectral Raman maps with the Raman laser focused to variable depths within the sample and combined the data using visualization software.

Figure 1 shows a representative nano-IR image and spectra of a coesite aggregate. The image was acquired with a broadband IR laser centered at 1250 cm^-1, providing a strong contrast between two phases. Point spectra confirm the presence of coesite crystallites surrounded by amorphous SiO₂ glass. 3D Raman tomography further demonstrates that the coesite aggregates are generally flat or disc-shaped. The textures of the coesite crystallites and shapes of the aggregates support the hypothesis that the coesite crystallized from a melt that resulted from localized shock heating upon the collapse of vesicles.