Graphitization in a high-pressure, low-temperature metamorphic gradient: a Raman microspectroscopy and HRTEM study

Beyssac O., Rouzaud J.N., Goffé B., Brunet F. and Chopin C., Contributions to Mineralogy and Petrology, 143, 19-31.(2002).

Abstract:
The graphitization of carbonaceous material (CM) in a high-pressure metamorphic gradient is characterized along a cross section in the Schistes Lustres formation, Western Alps. Along this 25-km cross section, both the CM precursor and the host-rock lithology are homogeneous, and the prograde evolution of the pressure-temperature metamorphic conditions from the lower blueschist-facies (13 kbar, 330°C) to the eclogite-facies (20 kbar, 500°C) is tightly constrained by literature data. Raman microspectroscopy shows that at the micrometre scale, this process is progressive and continuous with increasing metamorphic grade, and that the structure of CM is very sensitive to temperature variations. At the nanometre scale (HRTEM), the CM is composed of a mixture of a microporous phase and an onion-ring like phase, both known as non-graphitizing under the effect of temperature at ambient pressure. The HP-LT graphitization produces structurally and microtexturally heterogeneous CM. With increasing metamorphic grade, the graphitization of the two types of CM proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of CM. The microporous material is progressively transformed into graphite through a macroporous transitional stage. In this case, graphitization mainly occurs on the pore walls as a result of pore growth. In the case of concentric onion-ring like material, graphitization occurs in the regions with the largest radius of curvature, i.e. on the outer part of the ring. In comparison with 1-bar experiments, pressure seems to induce microtextural changes, which allows the subsequent structural modifications of the starting material.

Raman spectra of carbonaceous material from metasediments : a new geothermometer

Beyssac O., Goffé B., Chopin, C. and Rouzaud J.N. Journal of Metamorphic Geology, 20, 859-871 (2002)

Abstract:
Metasedimentary rocks generally contain carbonaceous material ( CM) deriving from the evolution of organic matter originally present in the host sedimentary rock. During metamorphic processes, this organic matter is progressively transformed into graphite s.s. and the degree of organisation of CM is known as a reliable indicator of metamorphic grade. In this study, the degree of organisation of CM was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts. This degree of organisation, including within-sample heterogeneity, was quantified by the relative area of the defect band (R2 ratio). The results from the Schistes Lustres (Western Alps) and Sanbagawa (Japan) cross-sections show that (1) even through simple visual inspection, changes in the CM Raman spectrum appear sensitive to variations of metamorphic grade, (2) there is an excellent agreement between the R2 values calculated for the two sections when considering samples with an equivalent metamorphic grade, and (3) the evolution of the R2 ratio with metamorphic grade is controlled by temperature (T). Along the Tinos cross-section (Greece), which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks, the R2 ratio is nearly constant. Consequently, the degree of organisation of CM is not affected by the retrogression and records peak metamorphic conditions. More generally, analysis of 54 samples representative of high-temperature, low-pressure to high-pressure, low-temperature metamorphic gradients shows that there is a linear correlation between the R2 ratio and the peak temperature [T(°C) = -445 R2 + 641], whatever the metamorphic gradient and, probably, the organic precursor. The Raman spectrum of CM can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism. Temperature can be estimated to +/-50°C in the range 330-650°C. A few technical indications are given for optimal application.

On the characterization of disordered and heterogeneous carbonaceous materials using Raman spectroscopy

Beyssac O., Goffé, B., Petitet, J.P., Froigneux, E., Moreau, M. and Rouzaud, J.N. (2003) Spectrochimica Acta A, 59, 2267-2276.

Abstract:
The applicability of Raman spectroscopy to characterize disordered and heterogeneous carbonaceous materials (CM) is discussed, by considering both natural and synthetic CM. First, different analytical mismatches during the measurement are discussed and technical indications are provided in order to eliminate them. Second, the accuracy and relevance of the different parameters obtained by the decomposition of spectra by conventional fitting procedure, is reviewed. Lastly, a new Raman technique (Raman area mode micro spectroscopy) giving an homogeneous repartition of power within a large laser beam is presented, this technique being powerful to study strongly heterogeneous CM and/or photosensitive samples.

Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material

Beyssac O., Bollinger L., Avouac J.P. et Goffé B. Earth and planetary Science Letters, 225, 233-241.

The determination of metamorphic conditions is critical to the understanding of the formation of mountain belts. However, all collisional mountain belts contain large volumes of accreted sediments generally lacking metamorphic index minerals and are therefore not amenable to conventional petrologic investigations. By contrast, these units are often rich in carbonaceous material, making it possible to determine thermal metamorphism through Raman spectroscopy of carbonaceous material (RSCM method), a technique that has been recently calibrated [Beyssac et al., J. Metamorph. Geol. 20 (2002) 859-871]. The Lesser Himalaya (LH) is one of these problematic cases with a very poor mineralogy, but a key structural position within the Himalayan system that makes LH considered as diagnostic of the overall thermal behaviour of the orogen. This work demonstrates the performance of the RSCM technique and shows that this technique might thus be used to detect inter-sample variations as small as ~10-15°C , but absolute temperatures can only be determined to +/- 50°C due to the uncertainty on the calibration. This study reveals that the LH has undergone a large-scale thermal metamorphism, with temperature decreasing progressively from about 540°C at the top to less than 330°C within the deepest exhumed structural levels.

Experimental study of the microtextural and structural transformations of carhonaceous materials under pressure and temperature

Beyssac O., Brunet F., Petitet J.P., Goffé B. and Rouzaud J.N.European Journal of Mineralogy, 15, 937-951. (2003)

Abstract:
A coal and two reference synthetic-cokes, a lamellar graphitizing anthracene-based coke (AC) and a microporous nongraphitizing saccharose-based coke (SC), were held at pressures up to 8 GPa and temperatures up to 1473 K under dry and hydrous conditions. Their subsequent structural and microtextural modifications were characterized by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy (microspectroscopy and area-mode spectroscopy). No significant transformation was observed in products held up to 95 hours at temperatures below 1273 K. At 1273 K and above, for constant run duration, the higher the pressure, the better is the carbonaceous material organization. The effect of pressure on the graphitization mechanisms is most obvious in the case of the non-graphitizing SC for which the main role of pressure is to transform the microporous microtexture towards a lamellar one. Graphitization is initiated in pore walls as a result of pore growth and thus appears as a local phenomenon. Triperiodic graphite was detected by HRTEM in all SC samples synthesized at 2 GPa and above (1273 K, 95 hours). In the case of the AC material, microtexture remains lamellar and the graphitization results in the reorientation and the in-plane growth of the aromatic layers and rather appears as a bulk phenomenon. The evolution of the coal is intermediate between the two synthetic cokes, as this precursor was microtexturally heterogeneous, composed of an intermediate microtexture between lamellar and microporous ones. Graphitization under pressure appears to be a progressive and continuous process that proceeds heterogeneously through the carbonaceous matrix. The products recovered from high-pressure experiments are structurally and microtexturally heterogeneous, and this heterogeneity raises important problems with respect to the characterization scale and transformation rate in the experimental products. Indeed, in comparison to graphitization temperature in natural samples, it is expected that longer run duration would have led to triperiodic graphite under most experimental conditions achieved here. However, our experimental results show that pressure mainly speeds up the graphitization process, or even makes it possible in the case of the SC, by inducing microtextural and subsequent structural transformations. In all cases, whatever the pressure, temperature defines the highest structural state.