Mikroraman-Labor
Zusammenfassung der Projektergebnisse
1. Mineral identification (entire Tectonophysics/Geochronology group). The micro-Raman severed as a standard tool to identify minerals, in particular inclusion in other minerals. 2. FTIR and Raman spectroscopic studies of natural apatite irradiated with swift heavy ions. The shifts (ω), widths (Γ) and peak areas (A) of nine intense Raman bands of ion-irradiated (5×10^11 cm-2 11.1 MeV/amu 238U-ions) prismatic apatite sections are offset relative to the corresponding values for unirradiated apatite. The latent ion-track diameters calculated from the ratios of the peak areas before and after irradiation (amorphous fraction) give an average of 5.8±0.7 nm. Combined with data from other experiments, it is shown that there exists a linear energy transfer (dE/dx) threshold for track registration in apatite, estimated at 3.3±0.5 MeV/µm, not much lower than the threshold for track etching (4.5± 0.5 MeV/µm). Above the threshold, the track diameter increases with the ion mass (M) and dE/dx of the incident ions, converging to a maximum (±10 nm at dE/dx ≥35 MeV /µm; M ≥ 200 amu). These observations are consistent with those of Jaskierowicz et al. (2004) and support the compound spike mechanism of track formation (Chadderton, 2003). 3. Routine application of micro-Raman spectrometry to fission-track analysis (entire particle-track group). The anion ratio (Cl/F) and other lattice substitutions exert control over the annealing rate of fission tracks in apatite and so influences the paleotemperature estimates from length measurements of fossil tracks. These are accounted for by measuring Cl or the etch pit size. Raman measurements perpendicular to the c-axis shown that the shifts of the ν2(E2g) and ν3(Ag) bands at 448 and 1053 cm-1 correlate with existing kinetic parameters and permit to estimate the unit cell size and so assess the overall effect of lattice substitutions with high spatial resolution. We mounted and polished plane sections cut from a Durango apatite at 0, 30, 60 and 90° to the c-axis. The first interval includes at least 5 known bands at 430, 448, 580, 590 and 607 cm-1. Fitting of the spectra is problematic because the variable intensities and proximities of neighbouring bands make it difficult to obtain reproducible estimates of their exact shifts and widths and of the related lattice and compositional parameters. In a next step, these measurements will be extended to apatites of different composition. We remain confident that, with proper standardization and some limitations, routine application of micro-Raman spectrometry to fission-track analysis will be practical. 4. Methodological studies in mineral separation (entire Geochronology group). Micro-Raman and fission-track investigation of apatites obtained via mechanical and electrical (SelFrag) rock fragmentation methods, revealed no significant differences between both. Testing the influence of high-voltage mineral liberation on grain size, shape and yield, and on fission track and 40Ar/39Ar dating. 5. A micro-Raman investigation of zircons from the Kontinentale Tiefbohrung (Germany). The parallel downhole trends of the shifts (ω1, ω3) and widths (Γ1, Γ3) of the ν1(SiO4) and ν3(SiO4) internal stretching bands of 1650 zircons from the KTB indicate total retention of self-irradiation damage down to ~3 km depth, partial annealing from ~3 to ~5 km and complete annealing from ~5 km. At the present geothermal gradient, the partial annealing zone for damage accumulation and isothermal holding for a period of ~100 Ma ranges from 90-145 °C. This gives a closure temperature of ~120 °C for average geological cooling rates. This radiation-damage partial annealing zone overlaps with the zircon (U,Th)-He partial retention zone. After recalculation with a revised Γ3-baseline, based on measurements of zircons from the total annealing zone, the provisional "radiation damage ages" (effective lossless damage accumulation times) calculated from the sample-mean Γ3-values and (U,Th)-concentrations and the calibration curve of Nasdala et al. (2001), fall within the age range of low-temperature thermochronometers down to ~4 km depth and drop to 0 Ma at ≤6 km depth. 6. Raman spectroscopy of carbonaceous material (Tectonopyhsics group). This method is used to determine metamorphic peak temperatures. The reproducibility is still low because of biases related to the measurement and spectrum-fitting procedure and sample characteristics. The Raman group at the University of Göttingen initiated a series of calibration experiments, in which our group in Freiberg is taking part. Measurements of reference samples covering the entire temperature range, provided by Göttingen, are in progress. The aim is that a set of reference materials, combined with standardised measurement and spectrum-fitting procedures help produce more comparable metamorphic temperature estimates.
