Paper No. 5
Presentation Time: 2:45 PM
CELLULAR ULTRASTRUCTURE PRESERVATION IN PLANT ORGANIC FOSSILS IN HIGH-PRESSURE METAMORPHIC ROCKS
GALVEZ, Matthieu E., Laboratoire de Géologie, Département des Géosciences, Ecole Normale Supérieure-CNRS, Ulm, 24 rue Lhomond, Paris, 75005, France, BEYSSAC, Olivier, Institut de Mineralogie et de Physique des Milieux Condenses, CNRS and IPGP, 140 rue de Lourmel, Paris, 75015, France, BENZERARA, Karim, Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie, Sorbonne Universités - UPMC, 4 Place Jussieu, Paris, 75005, France, BERNARD, Sylvain, Deutsches Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ Sektion 4.3, Organische Geochemie, Telegrafenberg, B 456, Potsdam, D-14473, Germany, MENGUY, Nicolas, Institut de Mineralogie et de Physique des Milieux Condenses, CNRS and IPGP, Campus Jussieu Case courrier 115, 4 place jussieu, Paris, 75015, France, COX, Simon, GNS Science, 1 Fairway Drive, Avalon Lower Hutt 5010 PO Box 30368, Lower Hutt, 5040, New Zealand and BROWN Jr, Gordon E., Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, matt_aussies@hotmail.com
The preservation of biological information in the geological record is fundamental for the study of life origins and evolution on Earth. The burial history of fossils is generally restricted to the shallowest part of the earth crust, although few cases of fossil preservation in deeply buried and highly metamorphosed rocks have been reported. Metamorphism is actually considered as highly detrimental to the preservation of biological information in rocks. We describe here macro-fossils of plant tissues in blueschist metamorphic rocks (greywackes) from New Zealand. These fossils are not shielded in any mineralized concretion. They display remarkable morphological and chemical preservation regarding their blueschist (~350°C & 6kbars) metamorphic history which is attested by the spectacular presence of lawsonite. We provide a complete description of both the carbonaceous material (CM) and the mineral components of both the fossils and host rocks by combining electron microscopy and spectromicroscopy analyses.
The petrographic study of the mineral assemblage of the host rock shows the presence of detrital albite, quartz and phengite/chlorite, as well as secondary phases such as hydrated alumino-silicates (lawsonite and epidotes), Ti-rich silicate (titanite) and minor apatites. SEM microscopy shows that leaf organic remains were composed of small carbonaceous matter flakes enclosed within a phyllosilicate grid forming geometrical patterns with dimensions in the range 10 to 100 µm. Raman microspectroscopy as well as HRTEM and STXM analyses reveal the graphitic ordering of the fossil CM. The aromatic planes are oriented parallel to the bedding plane of the rock and locally, the sp2 hybridized C atoms are associated with carboxylic and other oxygenated radicals.
We interpret the geometrical patterns as a mineral template of pre-existing leaf cells. The organic matter enclosed within this phyllosilicate template is probably a graphitized remnant of the original organic material. Chemical modification that might have occurred during the whole geological history will be discussed, as well as the possible role of phyllosilicates in the preservation process. Exploring metamorphic rocks might be a new route to investigate the archives and evolution of life on Earth.
