Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p34b..04m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P34B-04
Physics
[3625] Mineralogy And Petrology / Petrography, Microstructures, And Textures, [3665] Mineralogy And Petrology / Mineral Occurrences And Deposits, [3954] Mineral Physics / X-Ray, Neutron, And Electron Spectroscopy And Diffraction, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Iron (oxyhydr)oxides, abundant on the Martian surface, can be formed via magmatic, hydrothermal, diagenetic or weathering processes. At present their formation are poorly understood. The study of Precambrian samples may be key to better understanding the Martian environmental conditions, past and present. Our FIB-TEM study of iron oxides from a banded iron formation (Gandarela Formation (2.4 Ga), Minas Supergroup, Minas Gerais, Brazil) provides a Precambrian terrestrial analogue for the formation of diagenetic ferrihydrite. A detailed mineralogical, petrological and FIB-TEM study of a carbonate banded iron formation (known locally as carbonate itabirites-CI) has revealed the presence of previously unknown nano-platelets of ferrihydrite, goethite and hematite hosted within dolomite. The CI are millimetrically alternating bands of pink dolomite and grey hematite. The fine sedimentary laminations, micritic sized dolomite crystals and nano (oxyhydr)oxide inclusions in the dolomite grains likely reflect a primary origin during deposition. The pink colour of the dolomite is a result of the nano inclusions of euhedral hematite (≤1 μm) and nano platelets of ferrihydrite and gothite (≥10 nm). The smallest ferrihydrites (~10 nm) are mostly rounded with a hint of a euhedral shape whereas the larger ferrihydrites (400 nm) are euhedral platelets. Clinochlore and talc indicate that the CI have experienced greenschist facies metamorphism at two major intervals; the Transamazonian at 2.1 Ga and the Brasiliano at 0.8 Ga. The ferrihydrites occur either as individual droplets or as clusters of up to 30 crystals. A common feature of all the ferrihydrite platelets is their close association with porosity indicating that the ferrihydrite platelets are a result of precipitation from fluid inclusions trapped within the dolomite during its formation. Deciphering the relative time of precipitation of the ferrihydrite is key in understanding the overall history of the rock. Dislocations within the dolomite are pinned by the ferrihydrite crystals and clusters indicating their presence prior to deformation of the dolomite. Since there is no evidence for recrystallization of the dolomite we can assume that the ferrihydrite was precipitated prior to the first deformation event at 2.1 Ga. Qualitative spectra analyses of the ferrihydrite platelets on high-angle annular dark field (HAADF) images show Si and Al in the ferrihydrite structure. Transport of Fe3+ in an oxidising solution can occur via complexing with HCO3. Subsequent interaction and establishment of equilibrium between the dolomite and HCO3 ions would have allowed the precipitation of ferrihydrite during early diagenesis. Ferrihydrite is unstable above 400°C and converts to hematite. However the presence of Si (as observed in our sample) inhibits the conversion to hematite up to temperatures of 800°C. Thus the ferrihydrite would remain stable, possibly since the initial formation of the rock at 2.4 Ga. The presence of euhedral nanometric hematite suggests dehydration from the precursor ferrihydrite.
Morgan Rhonda
Orberger Beate
Wirth Richard
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