Terra Nova, 22, 354–360, 2010
The Raman spectrum of carbonaceous material (CM) from advanced diagenesis (∼200 °C) to low‐grade metamorphism (∼320 °C) is documented in the Helvetic flysch of the ...Glarus Alps (Switzerland). The spectrum is complex, with several defect bands at ∼1200 (D4), ∼1350 (D1), ∼1500 (D3) and ∼1620 (D2) cm−1. We document the evolution of these bands relative to the ‘graphite’ G band with increasing metamorphic grade, and we show that this qualitative evolution may be used as a proxy for temperature in the Glarus Alps. We develop a robust peak‐fitting method and propose quantitative parameters that may be used as proxies for thermal metamorphism in this region. Further work in other geological contexts is needed to assess whether the spectral evolution observed in the Glarus Alps may be generalized and to critically assess the potential for calibrating a general, empirical and quantitative thermometer based on the Raman spectrum of CM in low‐grade rocks.
Graphitizing anthracene-based cokes and non-graphitizing saccharose-based chars were processed at temperatures from 450°C to 2900°C at ambient pressure. This offers a whole set of samples that ...greatly differ in structure. Here, their structural evolution was monitored by combining XRD and visible (green) Raman spectroscopy as well as, for the first time, near-infrared Raman and synchrotron-based C-XANES spectroscopies. These different techniques provide complementary information especially regarding the spatial resolution they achieved. However, despite its importance, the quantitative comparison between the structural parameters extracted from these techniques is difficult. Based on a new signal deconvolution procedure to extract quantitative structural information from C-XANES data and the achievement of a new dataset on a complete series of graphitization, the reliability and the precision of the information which can be retrieved from each technique are discussed. C-XANES spectroscopy appears to provide reliable proxies for the extent of aromatic layers of graphitic compounds and an empirical calibration is proposed.
Although lawsonite‐bearing rocks are rare in exhumed high‐pressure (HP) terranes, they are considered to exert a primary role in subduction dynamics. Recent observations in natural settings have ...shown that fluid–rock interaction at HP conditions, including metasomatism, may lead to unusually high lawsonite amounts even in rocks that originally contained little or no lawsonite. This process may therefore bear important implications for element recycling in subduction zones. A detailed characterization of the geochemical fingerprints associated with lawsonite metasomatism is presented in this contribution. The studied rocks belong to the HP terranes of Alpine Corsica (France), which is the largest documented exposure for lawsonite metasomatism. Metasomatic lawsonite displays complex compositional zoning, including high trace element, Cr and Ti content. The trace element content is much higher compared with the average of non‐metasomatic lawsonite, and is in line with the re‐incorporation of large amounts of trace elements (e.g. REE, Sr, Pb, Th) in the rock during metasomatism, as shown by mass transfer calculations. Our data suggest that serpentinites represented the main fluid source for the metasomatism, with concurrent contribution of other, possibly Ca‐rich lithologies, such as mafic or meta‐sedimentary rocks. We propose that the breakdown of metasomatic lawsonite may contribute to the genesis of magmas and their characteristic geochemical signatures.
The Western Alps are a classic subduction‐related collisional orogen with well‐preserved, deeply subducted ophiolitic remnants of oceanic lithosphere. Some (e.g. Monviso, Voltri) were recognized as a ...palaeo‐subduction channel, with tectonic blocks showing a wide range of pressure–temperature conditions. We herein evaluate for the first time the metamorphic homogeneity of the extensive Zermatt‐Saas ophiolite. Zermatt‐Saas peak eclogitic assemblages are represented by omphacite–garnet ± phengite ± epidote ± lawsonite ± glaucophane in MORB‐derived metabasalts and garnet–chloritoid–talc ± lawsonite ± phengite in hydrothermalized metabasalts. Thermobarometric estimates with thermocalc and Raman Spectroscopy of carbonaceous material reveal homogeneous peak burial conditions at around 540 ± 20 °C and 23 ± 1 kbar. P–T paths indicate that the whole of the ophiolite, at least 60 km across, strikingly underwent similar burial and exhumation patterns and detached from the slab at depths around 80 km. The Zermatt‐Saas ophiolite thus appears to be the world’s largest oceanic lithosphere fragment exhumed from such depths, which provides important constraints on interplate coupling mechanisms.
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 Lustrés (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.
Laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy are complementary techniques providing respectively chemical and structural information on the sample target. These techniques are ...increasingly used in Earth and Planetary sciences, and often together. LIBS is locally destructive for the target, and the laser-induced effects due to LIBS laser shots on the structure and on the Raman fingerprint of a set of geological samples relevant to Mars exploration are here investigated by Raman spectroscopy and electron microscopy. Experiments show that the structure of samples with low optical absorption coefficients is preserved as well as the structural information carried by Raman spectra. By contrast, minerals with high optical absorption coefficient can be severely affected by LIBS laser shots with local amorphization, melting and/or phase transformation. Thermal modeling shows that the temperature can reach several thousands of degrees at the surface for such samples during a LIBS laser shot, but decreases rapidly with time and in space. In 2020, NASA Mars 2020 mission will send a rover equiped with a combined LIBS/Raman instrument for remote analysis (SuperCam) as well as proximity science instruments at fine scale for X-ray fluorescence called PIXL for Planetary Instrument for X-ray Lithochemistry, and deep UV Raman spectroscopy called SHERLOC for Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals. We discuss the implications of our results for the operation of these instruments and show that (i) the SuperCam analytical footprint for Raman spectroscopy is many times larger than the LIBS crater, minimizing any effects and (ii) SHERLOC and PIXL analysis may be affected if they analyze within a LIBS crater created by SuperCam LIBS.
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•Laser-induced heating during LIBS analysis depends on the sample optical properties.•Structure of transparent minerals is not affected by LIBS laser shots.•Opaque minerals can be severely affected by LIBS shots due to absorption.
The Southern Alps orogen of New Zealand results from late Cenozoic convergence between the Indo-Australian and Pacific plates and is one of the most active mountain belts in the world. Metamorphic ...rocks carrying a polymetamorphic legacy, ranging from low-greenschist to high-grade amphibolites, are exhumed in the hanging wall of the Alpine Fault. On a regional scale, the metamorphic grade has previously been described in terms of metamorphic zones and mineral isograds; application of quantitative petrology being severely limited owing to unfavorable quartzofeldspathic lithologies. This study quantifies peak metamorphic temperatures (T) in a 300×20km area, based on samples forming 13 transects along-strike from Haast in the south to Hokitika in the north, using thermometry based on Raman spectroscopy of carbonaceous material (RSCM). Peak metamorphic T decreases across each transect from ≥640°C locally in the direct vicinity of the Alpine Fault to less than 330°C at the drainage divide 15–20km southeast of the fault. Thermal field gradients exhibit a degree of similarity from the southernmost to the northernmost transects, are greater in low-grade semischist than high-grade schist, are affected by folding or discontinuous juxtaposition of metamorphic zones, and contain limited information on crustal-scale geothermal gradients. Temperatures derived by RSCM thermometry are slightly (≤50°C) higher than those derived by traditional quantitative petrology using garnet–biotite thermometry and THERMOCALC modeling. The age of RSCM T appears to be mostly pre-Cenozoic over most of the area except in central Southern Alps (Franz Josef–Fox area), where the amphibolite facies schists have T of likely Cenozoic age. The RSCM T data place some constraints on the mode of exhumation along the Alpine Fault and have implications for models of Southern Alps tectonics.
•RSCM thermometry quantifies peak metamorphic temperatures in Alpine Fault hanging wall.•RSCM T gradients across sub-greenschist to amphibolite facies rocks are systematic.•Field gradients of RSCM T are affected by faults and folds.•RSCM T constrain Southern Alps uplift and exhumation of a greywacke-schist transition.•Peak temperatures for Alpine Schist are mostly Mesozoic, but locally Cenozoic.
Laser induced breakdown spectroscopy (LIBS), as performed by the ChemCam instrument, provides a new technique to measure hydrogen at the surface of Mars. Using a laboratory replica of the LIBS ...instrument onboard the Curiosity rover, different types of hydrated samples (basalts, calcium and magnesium sulfates, opals and apatites) covering a range of targets observed on Mars have been characterized and analyzed. A number of factors related to laser parameters, atmospheric conditions and differences in targets properties can affect the standoff LIBS signal, and in particular the hydrogen emission peak. Dedicated laboratory tests were run to identify a normalization of the hydrogen signal which could best compensate for these effects and enable the application of the laboratory calibration to Mars data. We check that the hydrogen signal increases linearly with water content; and normalization of the hydrogen emission peak using to oxygen and carbon emission peaks (related to the breakdown of atmospheric carbon dioxide) constitutes a robust approach. Moreover, the calibration curve obtained is relatively independent of the samples types.
•Hydrated samples were analyzed using laser induced breakdown spectroscopy.•Experimental parameters were varied to test different types of signal normalization.•We obtain a calibration of the H signal applicable to a large range of martian data.
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Understanding 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.
Key Points
Maximum temperatures attained in the Helvetic nappes increase from their core to their front, from 220 to 358 °C
The 3‐D thermal structure is constrained by combining the maximum temperature distribution in map view and in cross sections
Helvetic nappes were deformed and emplaced before and/or during the thermal peak of Alpine collision
This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U–Pb dating of titanite to delineate the metamorphic history ...of a well‐exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of ∼340 °C. Temperatures increase down‐section, reaching ∼650 °C at the base of the shear zone, defining an apparent metamorphic field gradient of ∼310 °C km−1 across the entire structure. U–Th–Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1‐km‐thick zone of distributed top‐down‐to‐the‐northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo‐isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up‐section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.
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