HR: 1340h
AN: V43B-1576    [Abstracts]
TI: Raman spectra of carbonaceous material in Archean chert and silica dike: a thermal structure of ancient ocean floor
AU: * Kitajima, K
EM: saburo@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AB: Carbonaceous material (CM) is widespread in metamorphic rocks. CM is also reported from Archean rocks such as sedimentary rock and hydrothermal vein. Raman spectrum of CM changes with the degree of graphitization by metamorphism. The purpose of this study is to determine of the thermal structure of the low-grade zone in the Archean accretionary complex which was not able to determine using metamorphic petrology because of luck of index minerals, and to select of the best sample for analysis of molecular fossil. The North Pole area (3.5 Ga) is one of the best regions in the Archean greenstone belt, because this area had been subjected only to very low-grade metamorphism. A 1/5000 scale mapping was performed in the North Pole area. The mapped area is divided into seven units bounded by layer-parallel thrusts: Units-I, -II, -III, -IV, -V, -VI and -VII, in ascending order. These units are divided into MORB-type (Unit-I and -II) and OIB-type (Unit-III, -IV, -V, -VI and -VII) units by lithology and mode of occurrence. Microfossils are reported from the bedded chert and silica dike which is composed of very fine-grained silica in the Unit-I. We analyzed 20 bedded and silica dike samples collected from ancient seafloor (0 m) to 900 m depth of oceanic crust. The characterization of samples was performed by Raman microspectroscopy in situ using conventional petrologic double polished thin sections. All sample shows ordered peak (O-peak) and disordered peak (D-peak) around 1580-1610 cm^-1 and ~1355 cm^-1, respectively. There is no trend in D/O intensity and D/O area ratio. D/O width ratio and full width at half maximum (FWHM) of D-peak, however, indicate a clear trend, except bedded chert samples. D/O width ratio is decreasing with depth, from 2.2 to ~1, and FWHM of D-peak is also decreasing with depth from 100 cm^-1 to 60 cm^-1 between top of the unit and 180 m depth. It is considered that decreasing of D/O width ratio and FWHM of D-peak occurs with increasing of metamorphic grade. This suggests that ancient geothermal gradient of a seafloor hydrothermal system is preserved in the upper part (<180 m) of Unit-I. The carbonaceous material from the shallowest silica dike is the lesser immature in this unit, and the sample is better suited for paleontological studies.
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 9623 Archean
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005