The Raman spectra of carbonaceous material (CM) from 19 metasediment samples collected from six widely separated areas of Southwest Japan and metamorphosed at temperatures from 165 to 655°C show systematic changes with metamorphic temperature that can be classified into four types: low-grade CM (c. 150-280°C), medium-grade CM (c. 280-400°C), high-grade CM (c. 400-650°C), and well-crystallized graphite (> c. 650°C). The Raman spectra of low-grade CM exhibit features typical of amorphous carbon, in which several disordered bands (D-band) appear in the first-order region. In the Raman spectra of medium-grade CM, the graphite band (G-band) can be recognized and several abrupt changes occur in the trends for several band parameters. The observed changes indicate that CM starts to transform from amorphous carbon to crystallized graphite at around 280°C, and this transformation continues until 400°C. The G-band becomes the most prominent peak at high-grade CM suggesting that the CM structure is close to that of well-crystallized graphite. In the highest temperature sample of 655°C, the Raman spectra of CM show a strong G-band with almost no recognizable D-band, implying the CM grain is well-crystallized graphite. In the Raman spectra of low-to medium-grade CM, comparisons of several band parameters with the known metamorphic temperature show inverse correlations between metamorphic temperature and the full width at half maximum (FWHM) of the D1-and D2-bands. These correlations are calibrated as new Raman CM geothermometers, applicable in the range of c. 150-400°C. Details of the methodology for peak decomposition of Raman spectra from the low to medium temperature range are also discussed with the aim of establishing a robust and user-friendly geothermometer.

The degree of graphitization of carbonaceous material (CM) has been widely used as an indicator of metamorphic grade. Previous work has demonstrated that peak metamorphic temperature (T) of regional metamorphic rocks can be estimated by an area ratio (R2) of peaks recognized in Raman spectra of CM. The applicability of this method to low-pressure (<3 kbar) contact metamorphism was tested using Raman spectroscopic analyses of samples from two contact-metamorphic aureoles in Japan (Daimonji and Kasuga areas). A suitable measurement procedure allows the dependence of the geothermometer on sample type (thin section, chip) and incident angle of laser beam relative to the c-axes of CM to be tested. Two important general results are: (i) in addition to standard thin sections, chips are also suitable for spectral analysis; and (ii) the incident angle of the laser beam does not significantly affect the temperature estimation, i.e. spectral measurements for the geothermometer can be carried out irrespective of the crystallographic orientation. A laser wavelength of 532 nm was used in this study compared with 514.5 nm in an independent previous study. A comparison shows that the use of a 532-nm laser results in a slightly, but systematically larger R2 ratio than that of a 514.5-nm laser. Taking this effect into account, our results show that there is a slight but distinct difference between the R2-T correlations shown by contact and regional metamorphic rocks: the former are slightly bettercrystallized (have slightly lower R2 values) than the latter at the same temperature. This difference is interpreted as due to the degree of associated deformation. Despite the slight difference, the results of this study coincide within the estimated errors of ±50°C with those of the previously proposed Raman CM geothermometer, thus demonstrating the applicability of this method to contact metamorphism. To facilitate more precise temperature estimates in regions of contact metamorphism, a new calibration for analyses using a 532-nm laser is derived. Another important observation is that the R2 ratio of metamorphosed CM in pelitic and psammitic rocks is highly heterogeneous with respect to a single sample. To obtain a reliable temperature estimate, the average R2 value must be determined by using a substantial number of measurements (usually N > 50) that adequately reflects the range of sample heterogeneity. Using this procedure (with 532-nm laser) and adapting our new calibration, the errors of the Raman CM geothermometer for contact metamorphic rocks decrease to $ ±30°C.

Published experimental data including garnet and clinopyroxene as run products were used to develop a new formulation of the garnet-clinopyroxene geothermometer based on 333 garnet-clinopyroxene pairs. Only experiments with graphite capsules were selected because of difficulty in estimating the Fe 3+ content of clinopyroxene. For the calibration, a published subregular-solution model was adopted to express the non-ideality of garnet. The magnitude of the Fe-Mg excess interaction parameter for clinopyroxene (W FeMg Cpx ), and differences in enthalpy and entropy of the Fe-Mg exchange reaction were regressed from the accumulated experimental data set. As a result, a markedly negative value was obtained for the Fe-Mg excess interaction parameter of clinopyroxene (W FeMg Cpx = ) 3843 J mol )1 ). The pressure correction is simply treated as linear, and the difference in volume of the Fe-Mg exchange reaction was calculated from a published thermodynamic data set and fixed to be )120.72 (J kbar )1 mol )1 ). The regressed and obtained thermometer formulation is as follows:Tð CÞ ¼f2784 þ 14:52 P þ ð2601 þ 1:44 PÞð2X grs X prp À AÞ þ ð1183 þ 6:98 PÞðX grs 2 À AÞÀ 105ð2X grs X alm þ BÞ þ ð814:6 þ 3:61PÞðX grs 2 þ BÞ À ð254:6 þ 8:42 PÞð2X prp X alm À X alm 2 þ CÞ À 83:6ðX prp 2 À 2X prp X alm þ CÞ þ 1388 X sps À 462ðX Mg Cpx À X Fe Cpx Þg=flnK D þ 1:431 þ 0:695ð2X grs X prp þ X grs 2 À 2AÞ þ 0:203ðX grs 2 À 2X grs X alm Þ þ 0:922X sps g À 273;where T = temperature, P = pressure (kbar), A = 0.5 X grs (X prp ) X alm ) X sps ), B = 0.5 X grs (X prp ) X alm + X sps ), C = 0.5 (X grs + X sps ) (X prp ) X alm ), X prp = Mg ⁄ (Fe 2+ + Mn + Mg + Ca) Grt , X alm = Fe ⁄ (Fe 2+ + Mn + Mg + Ca) Grt , X sps = Mn ⁄ (Fe 2+ + Mn + Mg + Ca) Grt , X grs = Ca ⁄ (Fe 2+ + Mn + Mg + Ca) Grt , X Mg Cpx = Mg ⁄ (Al + Fe total + Mg) Cpx , X Fe Cpx = Fe 2+ ⁄ (Al + Fe total + Mg) Cpx , K D = (Fe 2+ ⁄ Mg) Grt ⁄ (Fe 2+ ⁄ Mg) Cpx , Grt = garnet, Cpx = clinopyroxene. A test of this new formulation to the accumulated data gave results that are concordant with the experimental temperatures over the whole range of the experimental temperatures (800-1820°C), with a standard deviation (1 sigma) of 74°C. Previous formulations of the thermometer are inconsistent with the accumulated data set; they underestimate temperatures by about 100°C at >1300°C and overestimate by 100-200°C at <1300°C. In addition, they tend to overestimate temperatures for high-Ca garnet (X grs % 0.30-0.50). This new formulation has been tested against previous formulations of the thermometer by application to natural eclogites. This gave temperatures some 20-100°C lower than previous formulations.

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Equilibrium pressure-temperature (P-T) conditions were estimated for kyanite-bearing eclogite from Nove´Dvory, Czech Republic, by using garnet-clinopyroxene thermometry and garnet-clinopyroxenekyanite-coesite (or quartz) barometry. The estimated P-T conditions are 1050-1150°C, 4.5-4.9 GPa, which are mostly the same as previously estimated values for garnet peridotite from Nove´Dvory (1100-1250°C, 5-6 GPa). Such very high-P conditions, which correspond to about 150-km depth, have been obtained for some garnet peridotites in the Gfo¨hl Unit of the Bohemian Massif, but pressure conditions of eclogites associated with the garnet peridotites have not been so well constrained. This is the first substantial finding of eclogite that gives such very high-P conditions in the Gfo¨hl Unit of the Bohemian Massif. The Gfo¨hl Unit mainly consists of felsic granulite or migmatitic gneiss, but these rock types do not display high-P (>2.5 GPa) evidence. It is unclear whether both the peridotite body and surrounding felsic rocks in the Gfo¨hl Unit were buried to very deep levels, but at least some garnet peridotites and associated eclogites in the Gfo¨hl Unit have ascended from about 150-km depth.

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X-ray diffraction (XRD) analyses of carbonaceous materials were carried out in conjunction with petrological studies for selected metamorphic rocks in order to compare the structural state of carbonaceous materials between contact and regional metamorphic rocks. The most extensive study was done for the Daimonji contact aureole in the eastern part of Kyoto city, Japan. The Daimonji contact aureole can be divided into three mineral zones using mineral parageneses of pelitic rocks: chlorite, biotite and cordierite zones. The cordierite zone can be further subdivided into lower-and higher-grade subzones. Petrological considerations allow the two isograd reactions that define the lower-and higher-grade cordierite subzones to be determined and suggest these reactions occurred at 510-560°C and 560-590°C per 2.0-2.3 kbar, respectively. A combination of the petrological studies and the XRD data of carbonaceous materials suggest that fully ordered graphite (FG; defined by d(002) s 3.360 A following the convention used by many workers), appears around 560-590°C in the Daimonji contact aureole. This data and refinement of geothermometer for published data confirmed that the FG appears a t 400-500°C in regional metamorphic rocks, but a t higher than 530°C in contact aureoles. One possible explanation for such a temperature difference is the duration of heating. However, the width at half height (WH) of the graphite peak attains a similar value of 0.30" at around 500°C both in contact and regional metamorphic rocks, suggesting that WH value is a more reliable indicator of metamorphic grade than the change of d(002) value. Furthermore, the depressed d(002) data of graphite was observed locally in the higher grade part (3 500°C) of the Ryoke regional metamorphic belt, where granitic intrusions exist within a few km distance. These facts indicate that the duration of heating is not an important factor controlling the change of d(002) value. It is possible that interlayered impurities, such as chlorine, which was derived from igneous intrusions, may be an important factor in suppressing the reduction in d(002) at temperatures greater than 500°C.