Raman Spectroscopy Of Carbonaceous Material Research Articles

Distribution and intensity of High-Temperature Low-Pressure metamorphism across the Pyrenean-Cantabrian belt: constraints on the thermal record of the pre-orogenic hyperextension rifting

Whereas a straightforward link between crustal thinning and geothermal gradients during rifting is now well established, the thermal structure of sedimentary basins within hyperextended domains remains poorly documented. For this purpose, we investigate the spatial distribution of rift-related High-Temperature Low-Pressure (HT/LP) metamorphism recorded in the preserved hyperextended rift basins inverted and integrated in the Pyrenean-Cantabrian belt. Based on Vitrinite Reflectance (Ro) data measured in 169 boreholes and more than 200 peak-metamorphic temperatures (Tmax) data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM) added to ∼425 previously publishedTmaxdata, we propose a new map depicting the spatial distribution of the HT/LPmetamorphism of the Pyrenean-Cantabrian belt. We also provide three regional-scale geological cross-sections associated with RoandTmaxdata to constrain the distribution of paleo-isograds at depth. Based on these results, we show that the impact of rift-related metamorphism is restricted to the pre- and syn-rift sequence suggested by the depth profiles of Rovalues measured in different tectonostratigraphic intervals (pre-, syn- and post-rift and syn-convergence sediments). However, a small strip of early orogenic sediments (Santonian in age) appears also affected by high temperatures along the North Pyrenean Frontal Thrust and above the Grand Rieu ridge, which we attribute to the percolation of hot hydrothermal fluids sourced from the dehydration of underthrust basement and/or sedimentary rocks at depth during the early orogenic stage. The map shows that the HT/LPmetamorphism (reaching ∼500 °C) is recorded with similar intensity along the Pyrenean-Cantabrian belt from the west in the Basque-Cantabrian Basin to the east in the Boucheville and Bas-Agly basins, for similar burial and rift-related structural settings. This thermal peak is also recorded underneath the northern border of the Mauléon Basin (calibrated by wells). It suggests that the high temperatures were recorded at the basement-sediment interface underneath the most distal part of the hyperextended domain. At basin-scale, we observe in the Basque-Cantabrian, Mauléon-Arzacq and Tarascon rift segments an asymmetry of the thermal structure revealed by different horizontal thermal gradients, supporting an asymmetry of the former hyperextended rift system. Using our results, we compare the Pyrénées to the Alps that also recorded hyperextension but no HT/LPmetamorphic event and suggest that the high-temperature record within the basins depends on high sedimentation rate promoting a thermal blanketing effect and circulation of hydrothermal fluids.

Evaluation of Deformation Temperatures in Carbonate Mylonites at Low Temperature Thrust-Tectonic Settings via Micro-Raman Spectroscopy

Carbonaceous materials (CMs) have been widely used to assess temperatures in sedimentary and metamorphic carbonate rocks. The use of Raman spectroscopy of carbonaceous material (RSCM) is largely devoted to the study of deformed rocks hosted in thrust-tectonic settings. Raman spectroscopy of carbonaceous material successfully allows the study of carbonate rocks at a temperature as high as 650 °C. In this study, a set of carbonate-mylonite rocks (Italian Alps) were investigated using micro-Raman spectroscopy, in order to infer the deformation conditions associated with the Alpine thrusts, expected to occur at T < 350 °C. Micro-Raman spectra were collected using two sources: green (532 nm) and red (632.8 nm) lasers. Several deconvolution procedures and parameters were tested to optimize the collected spectrum morphologies for the laser sources, also in accordance with the low temperature expected. The obtained temperatures highlight two clusters: one at 340–350 °C for the samples collected in the axial part of the Alpine chain, and the other at 200–240 °C for those collected in the external thrust-and-fold belt. These results agree with the independent geological and petrological constraints. Consistent results were obtained using 532 and 632.8 nm laser sources when the appropriate deconvolution approach was used.

Paleogeothermal Gradients Across an Inverted Hyperextended Rift System: Example of the Mauléon Fossil Rift (Western Pyrenees)

AbstractThe fossil rift in the North Pyrenean Zone, which underwent high temperature‐low pressure metamorphism and alkaline magmatism during Early Cretaceous hyperextension, was studied to explore the geothermal regime at the time of rifting. In this work, we combined Raman lab analysis and thermal numerical modelling to shed light on the distribution of geothermal gradients across the inverted hyperextended Mauléon rift basin during Albian and Cenomanian time, its period of active extension. Data were acquired from a set of 155 samples from densely spaced outcrops and boreholes, analyzed using Raman spectroscopy of carbonaceous material. The estimated paleogeothermal gradient is strongly related to the structural position along the Albian‐Cenomanian rift, increasing along a proximal‐distal margin transect from ~34°C/km in the European proximal margin to ~37–47°C/km in the two necking zones and 57–60°C/km in the hyperextended domain. This pattern of the paleogeothermal gradient induced a complex interaction between brittle and ductile deformation during crustal extension. A numerical model reproducing the thermal evolution of the North Pyrenees since 120 Ma suggests that mantle heat flow values may have reached 100 mW/m2 during the rifting event. This model reveals that above the thermal pulse, the temperature gradient varied within a small range of 55 to 62°C/km, as inferred from RSCM peak temperatures. We demonstrate that the style of reactivation during subsequent convergence influenced the thermal structure of the inverted rift system.

Subduction-collisional processes between the Eurasian and Philippine Sea plates: Constraints from thermal-age paths of the Taiwan Orogen

Records of the thermal-age path of orogenic rocks are crucial in understanding subduction and collisional processes between a continent and an arc. Tectonic events and thermochronological data of the Taiwan Orogen have been extensively investigated. We use the zircon UPb youngest ages as the (initial) subduction age, the Raman Spectroscopy of Carbonaceous Material, 40Ar/39Ar age of mica porphyroblasts, whole-rock LuHf isochron age and thermal results as prograde peak metamorphic condition during subduction and basal accretion, and the apatite fission-track data for cooling age of the Taiwan Orogen to illuminate the subduction and collisional processes between the Eurasian Plate and the Luzon Arc. The Hsuehshan Range Nappe in the west of the Taiwan Orogen, whose subduction was initiated at 12 Ma, reached peak metamorphism with a temperature of ~250 °C to 490 °C at a depth of 8 km during the interval 6–2.5 Ma, mainly by basal superimposed accretion. The isothermal dwelling during peak metamorphism condition of the Hsuehshan Range suggests that tectonic underplating through basal superimposed accretion contribute significantly to the formation of the Taiwan Orogen, and led to a sudden increase of subduction and décollement depths, and slow downward thickening of the orogenic wedge and mountain root growth. Located in the east of the Taiwan Orogen, the Yuli Beltsubduction was initiated at 15.4–16 Ma, and attained peak metamorphism at a depth of 40 km, with a temperature of 550–560 °C and a pressure of 10.5–13 kbar during the interval 12.2–4.4 Ma due to basal superimposed accretion. The exhumation of the Hsuehshan Range commenced at ~2.5 Ma, which is later than that of the Yuli Belt at ~3.4 Ma. Such distinct and different thermal-age paths of the Hsuehshan Range and the Tananao Complex indicate that different tectonic wedges in the Taiwan Orogen are formed through different subduction and accretional processes.

Sinistral strike-slip shearing along the Jiali shear zone around the Eastern Himalaya syntaxis region: Evidences for oligocene eastward limited translation of Tibet

In and around the Eastern Himalaya Syntaxis (EHS), the metamorphic complex and its structural boundaries are key tectonics to investigate the kinematics and thermal evolution of the continental collision. We provide new structural, kinematic and geochronological data along the northeastern boundary of the EHS, namely the Jiali strike-slip shear zone. Structures and various kinematic indicators record sinistral strike-slip shearing in the zone. Based on the microstructural features, crystallographic preferred orientation patterns of quartz and their slip systems, coupled with Raman Spectroscopy of Carbonaceous Materials (RSCM) geothermometry, the ductile sinistral shearing fabrics formed at upper greenschist and amphibolite facies (>500 °C). Two hornblende, one muscovite and five biotite separates from the mylonites in the shear zone were dated by the 40Ar/39Ar method. A hornblende plateau age of 23.7 Ma is interpreted as representing or being close to the time of sinistral movement. Muscovite and biotite 40Ar/39Ar ages from 22.9 to 16.9 Ma represent cooling ages following sinistral shearing. It is concluded on the basis of these data that the Jiali sinistral strike-slip shear zone was active in latest Oligocene. This sinistral movement on the Jiali shear zone is considered to link with the Ailao Shan-Red River shear zone, and form a unique, continuous structural boundary around the EHS. The Jiali-Ailao Shan-Red River sinistral shear zone and Gaoligong dextral shear zone formed a huge conjugate strike-slip shear pair around the EHS to accommodate the N–S shortening corresponding to northward indentation of the folded Indian continent. The conjugate strike-slip shear systems also contribute to transporting Tibet southeastward in a limited scale.

Structure and origin of the gold mineralization in the Nacimiento Block: The Los Burros deposits (Central California)

The Los Burros gold deposit is located within the Franciscan complex of the Nacimiento block, California. Although this deposit was mined during the late 19th and early 20th centuries, its origin and its relationship with the regional metamorphism and deformation are unclear. In order to better understand the mode of formation of the deposit, and its possible link with a post-subduction thermal anomaly, we performed a detailed geological and structural mapping within the Los Burros district, coupled with a (U-Th)/He thermochronological study on apatite and zircon. Fluid-inclusion and stable isotope (O and C) analyses of the ore deposit provide constraints on the temperature and potential source of the fluid(s) responsible for the Los Burros gold formation. Our results demonstrate that the Los Burros Au deposit formed from a metamorphic aqueous CO2-CH4 fluid at temperatures of ~300 °C. Results from Raman Spectroscopy of Carbonaceous Materials confirm the presence of a thermal anomaly in the range 260–320 °C near the Los Burros Mining District as previously inferred by vitrinite reflectance techniques. Our structural and preliminary thermochronological analyses show that the Los Burros Au deposit consists of a series of meter-wide, quartz-calcite-sulfide “en-echelon” veins, formed during a late transpressive event related to San Andreas fault activity and the subduction of a slab-window during the past 30 Ma. Our new data suggest that the thermal anomaly recorded in the vicinity of the Los Burros Mining District is not associated with a magmatic-derived hydrothermal system such as described within the epithermal McLaughlin deposit in Northern California, but rather with a late transpressive deformation event that uplifted deeply buried metamorphic rocks, possibly connected to a local metamorphic overprint.

Organic metamorphism as a key for reconstructing tectonic processes: a case study from the Austroalpine unit (Eastern Alps)

At the northwestern margin of the Gurktal Alps (Eastern Alps), Eoalpine (Cretaceous) thrusting of carbonaceous material (CM) bearing metasediments formed a very low- to low-grade metamorphic nappe stack above higher-grade metamorphic basement nappes. Sedimentary burial as well as progressive metamorphism transformed the enclosed CM to anthracite, metaanthracite and semigraphite. In a kinematically well-constrained section at the northwestern frontal margin of the nappe stack, this transformation has been investigated by vitrinite reflectance measurements and Raman spectroscopy of carbonaceous materials (RSCM). Automated, interactive fitting of Raman spectra estimates the metamorphic peak temperatures in a complete section through the upper part of the Upper Austroalpine unit. A RSCM trend indicates a temperature profile of ca. 250–600 °C. The top part of the gradient is reconstructed by one-dimensional thermal modeling. The certainty of ca. ± 25 °C at a confidence level of 0.9 resembles the data variability within a sample location. Due to the large calibration range, the method is able to reconstruct a thermal crustal profile in space and time. The study highlights the versatility of RSCM, which characterizes almost 250 Ma of a complex and polyphase tectonic history. RSCM data characterize the Variscan metamorphic grade in nappes now imbricated in the Eoalpine nappe stack. They additionally constrain a numerical model which emphasizes the significance of an increased thermal gradient in a continental margin towards the western Neotethyan ocean during Permo-Triassic lithospheric extension. It finally characterizes the Eoalpine metamorphic gradient during nappe stacking and a significant metamorphic jump related to exhumation and normal faulting.

Conodont thermometry by Raman spectroscopy on carbonaceous material: a case study from the Northern Calcareous Alps (Mürzalpen Nappe, Eastern Alps)

Abstract Carnian metapelites from the southeastern segment of the Mürzalpen Nappe (Northern Calcareous Alps, Eastern Alps) were heated to 280-310 °C, estimated by Raman spectroscopy of carbonaceous material (RSCM). This temperature range is correlated to a Color Alteration Index of 5.0-6.5, determined on conodonts from adjacent Anisian to Norian carbonates. Average RSCM temperatures estimated on the conodonts are biased towards higher temperatures. The spectral characteristics of the conodont apatite suggest a composition altered during progressive recrystallization, influencing the band parameters of the included carbonaceous matter. Consequently, accurate conodont RSCM thermometry needs an assessment of apatite alteration.

The relation between peak metamorphic temperatures and subsequent cooling during continent\u2013continent collision (western Central Alps, Switzerland)

The maximum temperature (Tmax) and subsequent exhumation reflect the relations between advective and conductive heat transport, which in turn depend on the tectonic evolution. To unravel these relations in an orogen, precise Tmax data need to be combined with relative time information for the displacements of adjacent units. We present new Tmax data based on Raman spectroscopy of carbonaceous material (RSCM) and zircon fission track (FT) data, which are combined with previous data and then discussed jointly. We follow this approach in the Central Alps at the western edge of the Lepontine dome. Our analysis indicates two main tectono-metamorphic domains in this area: domain A comprises the Lower Helvetic units involving the Aar Massif; domain B is situated south of the Helvetic main thrust, in the footwall of the Simplon line. In domain A, thrusted Helvetic units were overprinted mainly by reverse faulting in the Aar Massif. The thermal evolution is related to the inversion of the former Doldenhorn basin. Tectonic transport during inversion brought into contact units with substantially different Tmax. Temperature gradients were then reduced by conductive heat transfer, but thermal overprinting during cooling involved subsequent vertical movements as well. Zircon FT data yield apparent ages between 12 and 18 Ma in the external part, but 8–9 Ma in the internal part of the Aar Massif. The youngest ages are taken as the cooling at a given temperature, whereas the other data are discussed as being only partially resetted along a temperature path in the partial annealing zone of the zircon FT. When combined with age data for Tmax and apatite FT data from the literature, the youngest group exhibits exhumation rates between 0.5 and 1.2 km/Ma in the time range between 20 Ma and today. In all of domain B, Tmax was significantly higher than in domain A. In domain B the estimated rates of exhumation are 0.8–1.0 km/Ma for the post-20 Ma time interval. Despite of different temperature evolution, the exhumation rates are similar in both domains. The study shows the necessity to combine detailed tectonic data to interpret the T–t evolution of such an area.

Deciphering the metamorphic evolution of the Pulo do Lobo metasedimentary domain (SW Iberian Variscides)

Abstract. The Pulo do Lobo domain is one of the units exposed within the orogenic suture zone between the Ossa-Morena and the South Portuguese zones in the SW Iberian Variscides. This metasedimentary unit has been classically interpreted as a Rheic subduction-related accretionary prism formed during pre-Carboniferous convergence and eventual collision between the South Portuguese Zone (part of Avalonia) and the Ossa-Morena Zone (peri-Gondwanan terrane). Discrete mafic intrusions also occur within the dominant Pulo do Lobo metapelites, related to an intra-orogenic Mississippian transtensional and magmatic event that had a significant thermal input. Three different approaches have been applied to the Devonian–Carboniferous phyllites and slates of the Pulo do Lobo domain in order to study their poorly known low-grade metamorphic evolution. X-ray diffraction (XRD) was used to identify the mineralogy and measure crystallographic parameters (illite “crystallinity” and K-white mica b-cell dimension). Compositional maps of selected samples were obtained from electron probe microanalysis, which allowed for processing with XMapTools software, and chlorite semiempirical and thermodynamic geothermometry was performed. Thermometry based on Raman spectroscopy of carbonaceous material (RSCM) was used to obtain peak temperatures. The microstructural study shows the existence of two phyllosilicate growth events in the chlorite zone, the main one (M1) related to the development of a Devonian foliation S1 and a minor one (M2) associated with a crenulation cleavage (S2) developed in middle–upper Carboniferous times. M1 entered well into epizone (greenschist facies) conditions. M2 conditions were at lower temperature, reaching the anchizone–epizone boundary. These data accord well with the angular unconformity that separates the Devonian and Carboniferous formations of the Pulo do Lobo domain. The varied results obtained by the different approaches followed, combined with microstructural analysis, provide different snapshots of the metamorphic history. Thus, RSCM temperatures are higher in comparison with the other methods applied, which is interpreted to reflect a faster re-equilibration during the short-lived thermal Mississippian event. Regarding the metamorphic pressure, the data are very homogeneous: very low celadonite content (0 %–10 %) in muscovite (and low values of K-white mica b-cell dimension; 8.995 Å mean value), indicating a low pressure–temperature gradient, which is unexpected in a subduction-related accretionary prism. Highlights A multidisciplinary approach has been applied to study the metamorphism of the Pulo do Lobo metapelites. Devonian metamorphism entered epizone conditions. Carboniferous metamorphism reached the anchizone–epizone boundary. The inferred low-pressure gradient is incompatible with a subduction-related accretionary prism.

Applications of Raman Spectroscopy in Metamorphic Petrology and Tectonics

Raman spectroscopy is widely applied in metamorphic petrology and offers many opportunities for geological and tectonic research. Minimal sample preparation preserves sample integrity and microtextural information, while use with confocal microscopes allows spatial resolution down to the micrometer level. Raman spectroscopy clearly distinguishes mineral polymorphs, providing crucial constraints on metamorphic conditions, particularly ultrahigh-pressure conditions. Raman spectroscopy can also be used to monitor the structure of carbonaceous material in metamorphic rocks. Changes in structure are temperature-sensitive, so Raman spectroscopy of carbonaceous material is widely used for thermometry. Raman spectroscopy can also detect and quantify strain in micro-inclusions, offering new barometers that can be applied to understand metamorphic and tectonic processes without any assumptions about chemical equilibrium.

Deformational Temperatures Across the Lesser Himalayan Sequence in Eastern Bhutan and Their Implications for the Deformation History of the Main Central Thrust

AbstractWe postulate that the inverted metamorphic sequence in the Lesser Himalayan Sequence of the Himalayan orogen is a finite product of its deformation and temperature history. To explain the formation of this inverted metamorphic sequence across the Lesser Himalayan Sequence with a focus on the Main Central Thrust (MCT) in eastern Bhutan, we determined the metamorphic peak temperatures by Raman spectroscopy of carbonaceous material and established the deformation temperatures by Ti‐in‐quartz thermobarometry and quartz c axis textures. These data were combined with thermochronology, including new and published 40Ar/39Ar ages of muscovite and published apatite fission track, and apatite and zircon (U‐Th)/He ages. To obtain accurate metamorphic, deformation, and closure temperatures of thermochronological systems, pressures and cooling rates for the period of interest were derived by inverse modeling of multiple thermochronological data sets, and temperatures were determined by iterative calculations. The Raman spectroscopy of carbonaceous material results indicate two temperature sequences separated by a thrust. In the external sequence, peak temperatures are constant across the structural strike, consistent with the observed hinterland‐dipping duplex system. In the internal temperature sequence associated with the MCT shear zone, each geothermometer yields an apparent inverted temperature gradient although with different temperature ranges, and all temperatures appear to be retrograde. These observations are consistent with the quartz microfabrics. Further, all thermochronometers indicate upward younging across the MCT. We interpret our data as a composite peak and deformation temperature sequence that formed successively and reflects the broadening and narrowing of the MCT shear zone in which the ductile deformation lasted until ~11 Ma.

The 3‐D Thermal Structure of the Helvetic Nappes of the European Alps: Implications for Collisional Processes

AbstractUnderstanding the rheology of orogenic wedges requires the knowledge of the structural and thermal evolution of collisional units. In this study, we document the maximum temperature reached by the sedimentary cover nappes of the External Crystalline Massif (Western and central Alps) by Raman spectroscopy of carbonaceous material, between the Belledonne (France) and the Aar (Switzerland) Massifs. These cover units form the Helvetic/Dauphinois nappe complex. Maximum temperatures reached by the Upper Helvetic nappes lie in a range spanning from below 220 and 350 °C ± 50 °C. For the Lower Helvetic nappes, the temperatures spread between 226 and 358 °C ± 50 °C. These temperatures were projected on two structural cross sections in order to constrain the 3‐D thermal structure. From these data, we propose that the Helvetic nappes were deformed and emplaced before and/or during the thermal peak, which supports recent findings that shortening in the External Crystalline Massif was mainly accommodated during a 5‐ to 10‐Myr‐long thermal peak before deformation localized along crustal thrusts, which exhumed and cooled down the wedge. During this late exhumation, the isotherms corresponding to the thermal peak were passively folded.

The Nappe des Marbres Unit of the Basque‐Cantabrian Basin: The Tectono‐thermal Evolution of a Fossil Hyperextended Rift Basin

AbstractThe architecture of the Pyrenean‐Cantabrian belt results from the inversion of a series of former Cretaceous rift basins. A HT‐LP metamorphic event dated at 105 to 85 Ma ago is commonly associated with an Albo‐Cenomanian episode of hyperextension of the continental crust. This metamorphism is well known in the eastern Basque‐Cantabrian Basin within the Nappe des Marbres Unit (NMU) that is preserved from intense compressional deformation during the Pyrenean orogeny. Based on a structural study at the scale of the eastern Basque‐Cantabrian Basin and on a dense sampling for TRSCM estimates with the Raman spectroscopy of carbonaceous material (RSCM) method (Raman Spectrometry on Carbonaceous Material), we show following results: (1) the NMU has recorded two major phases of deformation, related to a localized extensional ductile foliation (S1) during the Cretaceous rifting and later by the regional N‐S shortening, recorded by a regional cleavage (S2) observed only in shale sediments. (2) The NMU is affected by early salt tectonics related to several diapirs and salt walls fed by the Upper Triassic evaporitic layer. (3) The NMU recorded maximum temperatures exceeding 550°C, which represent some of the highest temperatures along the Pyrenees. These new data demonstrate that there is no E‐W lateral metamorphic gradient across the belt and that hyperextension rifting occurred consistently within a high‐thermal regime; (4) metamorphic isograds are oblique in respect of the main structures, suggesting that the metamorphic event postdates most of the early salt‐related deformations prior to or coeval with early stages of rifting. As it represents one of the best preserved example of preorogenic hyperextended basin, the eastern Basque‐Cantabrian Basin record may be used regionally to better understand the rift to orogen evolution of the Pyrenean Internal Metamorphic Zone but also more generally as an analog of salt‐bearing hyperextended rift record.

Thermal History of the Northern Taiwanese Slate Belt and Implications for Wedge Growth During the Neogene Arc‐Continent Collision

AbstractA significant issue in the study of orogenic systems concerns the roles played by frontal and basal accretion in the construction of orogenic wedges. These different accretion mechanisms result in different thermal histories, with underplated materials experiencing significant heating and deformation during tectonic burial. This work provides new thermal data from Raman spectroscopy of carbonaceous material in combination with structural and stratigraphic observations of the northern Taiwan slate belt to address these questions of wedge development. Sedimentary rocks of the Northern slate belt were deposited on the Chinese continental margin immediately before the onset of the Neogene Taiwan arc‐continent collision. In the slates of the northern Hsüehshan Range, a large‐scale pop‐up structure on the prowedge of the Taiwan Orogen, synorogenic metamorphism has been investigated through analyses of peak temperatures and metamorphic field gradients. Results indicate underthrusting of the margin sediments to ~8‐km depth with significant folding in two major duplexes occurring before underplating. Such basal accretion is considered responsible for the distinct culmination of the Hsüehshan Range in central Taiwan and its relative uplift with respect to the Backbone Range to the east along the Lishan Fault. A similar underthrusting scenario is also suggested for the Backbone Range Slate Belt. We propose that basal accretion is the predominant mechanism in the growth and evolution of the Taiwan orogenic wedge and may have been achieved through inversion of a graben system on the ancient passive margin during continental subduction.

Thermal maturity of carbonaceous material in conodonts and the Color Alteration Index: Independently identifying maximum temperature with Raman spectroscopy

The Color Alteration Index (CAI) of conodont specimens is commonly used for identifying the maximum temperature to which units of sedimentary rock, particularly carbonates, have been heated. Observable color variations in these fossils are thought to be a result of the thermally-induced structural evolution of organic carbonaceous matter (CM). Such temperature history information is extremely valuable for applications in hydrocarbon exploration as well as for constraining other temperature-related geological processes in sedimentary systems. However, the identification of CAI depends on the qualitative visual assessment of color, which, along with a host of other potential complications, may yield inaccurate determinations of maximum temperature. Raman spectroscopy allows thermally-induced structural changes in CM to be quantified, and it has been used to estimate the thermal maturity in metasedimentary rocks for almost two decades. Here, we use Raman spectroscopy of carbonaceous material (RSCM) in conodont specimens and their Mississippian to Upper Triassic host rocks from British Columbia, Canada, to estimate maximum temperatures based on the transformation (structural reorganization) of disordered carbon to graphite. This study demonstrates that the maximum temperatures experienced by conodont specimens, as calculated from RSCM using the Iterative Fitting of Raman Spectra (IFORS) technique, correlate well with CAI but lie outside the suggested ranges in some instances. This may be due to complex thermal histories of these conodont specimens or the influence of diagenetic alteration, not thermal histories, on conodont color. We recommend the application of Raman spectroscopic analyses of CM in conodonts and their host rocks to obtain more confident, accurate, and precise estimations of maximum temperature that are independent of CAI.

Provenance and Variscan low-grade regional metamorphism recorded in slates from the basement of the (SW Hungary)

A metapelitic sequence with Silurian protolith from the Horváthertelend Unit (Tisza Mega-unit, Hungary) has a K-white mica + chlorite + quartz + albite + anatase ± tourmaline mineral assemblage. Moderately developed disjunctive and well-developed continuous foliations are present. Geochemical results reflect a dominantly felsic source of the protoliths, suggesting an intermediate to acidic volcanic arc in the provenance area. Metasandstone pebbles in the metaconglomerate indicate a recycled sedimentary source. Raman spectroscopy of carbonaceous material indicates a ~ 350‒370 °C peak metamorphic temperature. The Kübler Index (KIBasel) values of the phengitic K-white mica indicate epizonal metamorphism (0.22 ± 0.04 Δ°2θ). The Chlorite ‘Crystallinity’ Index (ChCCIS) suggests metamorphic alteration near to the anchizone‒epizone boundary (0.31 ± 0.06 Δ°2θ). K–Ar ages of K-white mica are interpreted as a result of Variscan metamorphism (c. > 310 Ma) and post-Variscan uplift (c. 290 Ma). The predominance of lydite and slate of Llandoverian age and the overlying coarse-grained metagreywacke and metaconglomerate beds of the Szalatnak Slate Formation show strong lithological similarities with the proximal Silurian sequences in the Małopolska Massif (Kielce Region, Holy Cross Mountains, Poland). The original position of the Horváthertelend Unit is presumably to the northeast from the Bohemian Massif, next to the Upper Silesian Block (Moravo-Silesian Zone) and the Małopolska Terrane.

Carbonaceous material export from Siberian permafrost tracked across the Arctic Shelf using Raman spectroscopy

Abstract. Warming-induced erosion of permafrost from Eastern Siberia mobilises large amounts of organic carbon and delivers it to the East Siberian Arctic Shelf (ESAS). In this study Raman spectroscopy of carbonaceous material (CM) was used to characterise, identify and track the most recalcitrant fraction of the organic load: 1463 spectra were obtained from surface sediments collected across the ESAS and automatically analysed for their Raman peaks. Spectra were classified by their peak areas and widths into disordered, intermediate, mildly graphitised and highly graphitised groups and the distribution of these classes was investigated across the shelf. Disordered CM was most prevalent in a permafrost core from Kurungnakh Island and from areas known to have high rates of coastal erosion. Sediments from outflows of the Indigirka and Kolyma rivers were generally enriched in intermediate CM. These different sediment sources were identified and distinguished along an E–W transect using their Raman spectra, showing that sediment is not homogenised on the ESAS. Distal samples, from the ESAS slope, contained greater amounts of highly graphitised CM compared to the rest of the shelf, attributable to degradation or, more likely, winnowing processes offshore. The presence of all four spectral classes in distal sediments demonstrates that CM degrades much more slowly than lipid biomarkers and other traditional tracers of terrestrial organic matter and shows that alongside degradation of the more labile organic matter component there is also conservative transport of carbon across the shelf toward the deep ocean. Thus, carbon cycle calculations must consider the nature as well as the amount of carbon liberated from thawing permafrost and other erosional settings.

Contrasting degrees of recrystallization of carbonaceous material in the Nelson aureole, British Columbia and Ballachulish aureole, Scotland, with implications for thermometry based on Raman spectroscopy of carbonaceous material

AbstractThe degree of recrystallization of carbonaceous material (CM), as monitored by Raman microspectroscopy, was examined as a function of metamorphic grade in two well‐studied contact aureoles containing carbonaceous pelites: the Nelson aureole, British Columbia and the Ballachulish aureole, Scotland. Here, we use (a) the R2 ratio extracted from the Raman spectrum of CM as a proxy for the degree of graphitization (0.0 in perfect graphite then increasing with structural defects) and (b) the second‐order S1 band (~2,700 cm−1) as a marker for the tridimensional ordering of CM. The Nelson aureole (garnet–staurolite–andalusite–sillimanite–K‐feldspar sequence, ~550–650°C, 3.5–4.0 kbar) was developed in rocks that were unmetamorphosed prior to contact metamorphism, whereas the Ballachulish aureole (cordierite–andalusite–K‐feldspar–sillimanite sequence, ~550–700°C, ~3.0 kbar) was developed in rocks that had been metamorphosed to garnet grade conditions (~7 kbar, ~500°C) c. 45 Ma before contact metamorphism. Thirty‐one samples were examined from Nelson and 29 samples from Ballachulish. At Nelson, the R2 ratio steadily decreases from ~0.25 to 0.0 as the igneous contact is approached, whereas at Ballachulish, the R2 ratio remains largely unchanged from regional values (~0.20–0.25) until less than 100 m from the igneous contact. The second‐order S1 band reveals that carbonaceous material (CM) was transformed to highly “ordered” locally tridimensional graphitic carbon at Ballachulish by regional metamorphism prior to contact metamorphism, whereas CM was still a disordered turbostratic (bidimensional) material before contact metamorphism in the case of Nelson. Pretexturation of CM likely induced sluggish recrystallization of CM and delayed graphitization in the Ballachulish aureole. Temperatures of recrystallization of the CM in the two aureoles were estimated using different published calibrations of the thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM), with differences among the calibrations being minor. In the Nelson aureole, temperatures are in reasonable agreement with those indicated by the metapelitic phase equilibria (all within 50°C, most within 25°C). In the Ballachulish aureole, the retarded crystallization noted above results in increasing underestimates of temperatures compared to the metapelitic phase equilibria (up to ~75°C too low within 200 m of the igneous contact). Our study calls for careful attention when using RSCM thermometry in complexly polymetamorphosed rocks to assess properly the meaning of the calculated temperature.

Carbonaceous Materials in the Fault Zone of the Longmenshan Fault Belt: 2. Characterization of Fault Gouge from Deep Drilling and Implications for Fault Maturity

In recent works on the determination of graphitization of carbonaceous materials (CM) within the principal slip zone (PSZ) of the Longmenshan fault (China), we demonstrated that the formation of graphite, resulted from strain and frictional heating, could be evidence of past seismic slip. Here we utilize Raman Spectroscopy of CM (RSCM) on the CM-bearing gouges in the fault zone of the Longmenshan fault belt, at the borehole depth of 760 m (FZ760) from the Wenchuan earthquake Fault Scientific Drilling project-1 (WFSD-1), to quantitatively characterize CM and further retrieve ancient fault deformation information in the active fault. RSCM shows that graphitization of CM is intense in the fault core with respect to the damage zone, with the graphitized carbon resembling those observed on experimentally formed graphite that was frictionally generated. Importantly, compared to the recognized active fault zone of the Longmenshan fault, the RSCM of measured CM-rich gouge shows a higher degree of graphitization, likely derived from high-temperature-perturbation faulting events. It implies that FZ760 accommodated numerous single-event displacement and/or at higher normal stresses and/or in the absence of pore fluid and/or along a more localized slip surface(s). Because graphite is a well-known lubricant, we surmise that the presence of the higher degree graphitized CM within FZ760 will reduce the fault strength and inefficiently accumulate tectonic stress during the seismic cycle at the current depth, and further infer a plausible mechanism for fault propagation at the borehole depth of 590 m during the Mw 7.9 Wenchuan earthquake.