T215 Lagerstätten through Time: An Examination of Exceptional Preservational Pathways from the Terminal Proterozoic through Today II

Tuesday, 2 November 2010
1:30 PM - 5:30 PM
Room 603

CELLULAR ULTRASTRUCTURE PRESERVATION IN PLANT ORGANIC FOSSILS IN HIGH-PRESSURE METAMORPHIC ROCKS

Paper number: T215-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

  • Matthieu E. Galvez Presenting Author
    Laboratoire de Géologie, Département des Géosciences, Ecole Normale Supérieure-CNRS, Ulm, Paris, N/A
  • Olivier Beyssac
    Institut de Mineralogie et de Physique des Milieux Condenses, CNRS and IPGP, Paris, N/A
  • Karim Benzerara
    Sorbonne Universités - UPMC, Paris, N/A
  • Sylvain Bernard
    Deutsches Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ Sektion 4.3, Organische Geochemie, Potsdam, N/A
  • Nicolas Menguy
    Institut de Mineralogie et de Physique des Milieux Condenses, CNRS and IPGP, Paris, N/A
  • Simon Cox
    Land & Water, Clayton South VIC
  • Gordon E. Brown
    Stanford University, Stanford, CA

Abstract

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.