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[SMP22-02] Evaluation of the biogenicity of 2.72 Ga titanite filled microtubular texture
Keywords:Titanite microtubes, TEM, Stromatolites, Pilbara Craton
The 3.5 Ga titanite-filled microtubes protruding from healed fractures in metamorphosed basaltic glass (pillow lava, hyaloclastites) are thought to be the oldest trace fossil (Banerjee et al., 2006). Similar microtubular occurrences have been reevaluated as metamorphic growth texture (lepot et al., 2011; Grosch and McLoughlin, 2014). The ambiguity in the mode of formation of microtubes and their preservation by titanite still exists.
In this study, micro- to nano-scale structural and chemical analyses were performed to evaluate the process behind the formation of titanite-filled microtubular structures in 2.72 Ga Tumbiana Formation stromatolite, Pilbara Craton, Australia. The presence of carbonaceous material (CM) was found within and along the boundaries of titanite tubes. The EBSD data suggests that the titanites have not been subjected to any deformation and have a single crystal orientation for the microtubular structure. The high-resolution TEM images and selected area diffraction (SAED) pattern reveal the amorphous nature of CM indicating biogenic origin. The titanite tubes have several branches with smaller diameters compared to the main tube extend mainly into the grain boundaries and penetrate to quartz and calcite. The tubular structure with branches having a smaller diameter with terminal particles (pyrite, CM) is often considered as ambient inclusion trail (AIT), but the absence of terminal particles at the tip of branches cannot exclude the formation of tubes by microbial tunneling. The SAED pattern of the titanite shows that the main tube and branches have a single crystal orientation with a slight misalignment of less than 2 degrees. The scanning TEM-EDS map reveals the presence of anatase nanocrystals and apatite closely associated with the titanite tubes might be precipitated by microorganisms. The lack of Ti-rich minerals available for supplying titanium to fill titanite implies that microbial processes may have played a role in segregating titanium. Moreover, the Fe/Al ratio of titanites shows an off-trend to common titanite composition compared to the global occurrences. The single crystal orientation of the microtubes suggests that the titanite might have precipitated from a highly supersaturated solution (rich in Ti, Ca, and Si) produced by the interaction between organic acid released from the microorganism with the host mineral due to the burial pressure.
References
Banerjee et al. (2006) Earth Planet. Sci. Lett. 241, 707-722; Grosch and McLoughlin (2014) Proc. Natl. Acad. Sci. U. S. A. 111, 8380–8385; Lepot et al. (2011) Earth Planet. Sci. Lett. 312, 37-47.
In this study, micro- to nano-scale structural and chemical analyses were performed to evaluate the process behind the formation of titanite-filled microtubular structures in 2.72 Ga Tumbiana Formation stromatolite, Pilbara Craton, Australia. The presence of carbonaceous material (CM) was found within and along the boundaries of titanite tubes. The EBSD data suggests that the titanites have not been subjected to any deformation and have a single crystal orientation for the microtubular structure. The high-resolution TEM images and selected area diffraction (SAED) pattern reveal the amorphous nature of CM indicating biogenic origin. The titanite tubes have several branches with smaller diameters compared to the main tube extend mainly into the grain boundaries and penetrate to quartz and calcite. The tubular structure with branches having a smaller diameter with terminal particles (pyrite, CM) is often considered as ambient inclusion trail (AIT), but the absence of terminal particles at the tip of branches cannot exclude the formation of tubes by microbial tunneling. The SAED pattern of the titanite shows that the main tube and branches have a single crystal orientation with a slight misalignment of less than 2 degrees. The scanning TEM-EDS map reveals the presence of anatase nanocrystals and apatite closely associated with the titanite tubes might be precipitated by microorganisms. The lack of Ti-rich minerals available for supplying titanium to fill titanite implies that microbial processes may have played a role in segregating titanium. Moreover, the Fe/Al ratio of titanites shows an off-trend to common titanite composition compared to the global occurrences. The single crystal orientation of the microtubes suggests that the titanite might have precipitated from a highly supersaturated solution (rich in Ti, Ca, and Si) produced by the interaction between organic acid released from the microorganism with the host mineral due to the burial pressure.
References
Banerjee et al. (2006) Earth Planet. Sci. Lett. 241, 707-722; Grosch and McLoughlin (2014) Proc. Natl. Acad. Sci. U. S. A. 111, 8380–8385; Lepot et al. (2011) Earth Planet. Sci. Lett. 312, 37-47.