CAN ORGANIC-WALLED MICROFOSSILS SURVIVE HIGH METAMORPHIC HEATING? CHARACTERIZATION OF EXPERIMENTALLY-HEATED ACRITARCHS USING RAMAN SPECTROSCOPY

It is generally perceived that highly metamorphosed rocks are not conducive to fossil preservation, and thus rocks metamorphosed beyond greenschist grade have typically not been a target of paleontological research. However, the potential for microfossil preservation in high-grade metamorphic rocks has not been extensively tested. From Archean-Paleoproterozoic amphibolite-grade carbonaceous quartzites collected in the Wutaishan area of North China, we have extracted circular graphite discs characterized by features related to graphitization and distinct biological morphologies (i.e. circular to elliptical shapes, marginal concentric folds, surficial wrinkles, and complex nanostructures). These discs are interpreted as graphitized, compressed organic vesicles. Petrographic analysis of the quartzite samples reveals two populations of graphite particles, hosted in the matrix and in the clasts, respectively. The matrix- and clast-hosted graphite are interpreted to be of different origins, which is confirmed by Raman microprobe analysis. Raman spectra of matrix graphite show higher variability in D:G (“disordered” band to “graphite” band) ratio as compared to the consistent spectra of the clast-hosted graphite. Moreover, Raman spectra of extracted graphite discs are highly consistent with those of clast-hosted graphite, but differ from matrix graphite, suggesting that they may have been derived from the clast-hosted population. To experimentally examine whether organic-walled microfossils graphitize and retain biological morphology during metamorphic heating, Mesoproterozoic Ruyang Group rock samples containing dense populations of acritachs (i.e. Dictyosphaera delicata and Shiuyousphaeridium macroreticulatum) have been heated to 500°C (± 2.5°C) for varying lengths of time. These samples were subsequently palynologically macerated, and extracted acritarchs were examined via Raman spectroscopy and scanning electron microscopy. The morphologies of the heated acritarchs are retained, and no features related to graphitization have been observed. Preliminary Raman analysis reported here illustrates a decrease in D:G ratios of heated acritarchs as compared to spectra of unheated acritarchs, suggesting that the heated samples are becoming more graphitized.