Search

In this study, we propose a new approach based on Raman hyperspectral imaging and chemometrics for the analysis of chemically heterogeneous surfaces of weathered minerals. A pyrite sample showing a heterogeneous surface with different alteration products, is used to test the validity of the technique. Principal Component Analysis (PCA) was initially used to evaluate data structure and identify Raman phases and vibrational modes related to minor and major weathering features. Afterwards, Multivariate Curve Resolution-alternating least squares (MCR-ALS) was carried out to identify the specific chemical components of the major weathering phases. Finally, for the analysis of the minor weathering phases, K-means clustering was carried out to identify groups of similar pixels of low percentage of variance in the image. Three major components are found in the analyzed sample, corresponding to pyrite and hematite vibrational modes, as well as features of an amorphous alteration product patina. Four additional spectral signatures revealing the presence of sulfates were found after the analysis of minor components. The proposed method enables a semi-quantitative threshold-based characterization of chemical features and provides a visual representation of the phase distribution on the surface of the sample.

Despite their strong Raman scattering and importance in several applications in the geological and biological sciences, Mg-calcites have not been thoroughly investigated by Raman spectroscopy. In this study, we investigated whether Raman spectra of carbonates are sensitive to the structural and chemical changes occurring when Mg2+ substitutes Ca2+ in the calcite lattice. Different carbonate samples with variable Mg content (from 0 to 20 mol% MgCO3) of biological and inorganic origin have been first beamed under a Raman spectrometer and then analysed by scanning electron microscopy-energy dispersive spectrometer and electron microprobe analysis-wavelength dispersive spectrometer microprobes to determine their chemistry. The biological samples (foraminifers, algae, barnacles and echinoderms) were collected from areas situated at different latitudes and water temperature and saturation, factors affecting the chemical and isotopic composition of shells in marine organisms. The Raman band positions are directly linked to the amount of magnesium present in the calcite lattice, and all peaks of Mg-calcite spectra show a consistent and linear increase in Raman shifts according to their Mg content, which is a consequence of the decreased inter-atomic distances following the substitution of Ca2+ with the smaller Mg2+ ion. This study demonstrates that Raman spectroscopy provides an innovative work perspective in marine biology, provenance studies and oil exploration when estimates of mineralogical and chemical changes are the focus of the study. Raman spectroscopy is even more attractive – for the mentioned fields of research – than many other methods because of the non-destructive nature and its very short analytical time

Supplementary Item 2: Methods

Raman spectroscopy is an innovative tool with tremendous potential, serving as a fundamental complement to a variety of provenance methods including heavy-mineral analysis and detrital geochronology. Because of its accuracy, efficiency and versatility, the results of the Raman technique are indispensable for fully reliable identification of heavy minerals in grain mounts or thin sections. Thorny long-standing problems that cannot be solved confidently with a polarizing microscope alone, such as the determination of opaque and altered heavy minerals, of detrital grains as small as a few microns, or of colourless crystals with uncertain orientation and rounded morphology, can finally be addressed. Although the method can be highly automatized, the full ability and experience of the operator is required to combine Raman data with the optical information obtained under the microscope on the same grains, which is essential for the efficient application of the method in provenance studies. This article provides exemplary Raman spectra useful for the comparison and determination of over 70 different opaque and transparent heavy-mineral species commonly found in sediments, conveying specific information on the genesis of their source rocks, and thus is particularly useful in provenance diagnoses and palaeotectonic reconstructions.

This article investigates landscape characteristics and sediment composition in the western Greater Caucasus by using multiple methods at different timescales. Our ultimate goal is to compare short-term versus long-term trends in erosional processes and to reconstruct spatio-temporal changes in sediment fluxes as controlled by partitioning of crustal shortening and rock uplift in the orogenic belt. Areas of active recent uplift are assessed by quantitative geomorphological techniques [digital elevation model (DEM) analysis of stream profiles and their deviation from equilibrium] and compared with regions of rapid exhumation over longer time intervals as previously determined by fission-track and cosmogenic-nuclide analyses. Complementary information from petrographic and heavy-mineral analyses of modern sands and ancient sandstones is used to evaluate erosion integrated throughout the history of the orogen. River catchments displaying the highest relief, as shown by channel-steepness indices, correspond with the areas of most rapid exhumation as outlined by thermochronological data. The region of high stream gradients is spatially associated with the highest topography around Mount Elbrus, where sedimentary cover strata have long been completely eroded and river sediments display the highest metamorphic indices and generally high heavy-mineral concentrations. This study reinforces the suggestion that the bedrock-channel network can reveal much of the evolution of tectonically active landscapes, and implies that the controls on channel gradient ultimately dictate the topography and the relief along the Greater Caucasus. Our integrated datasets, obtained during a decade of continuing research, display a general agreement and regularity of erosion patterns through time, and consistently indicate westward decreasing rates of erosional unroofing from the central part of the range to the Black Sea. 2014 John Wiley & Sons, Ltd.

Garnet is one of the most widespread heavy minerals in sediments derived from orogenic systems. Its chemical composition varies systematically with temperature and pressure conditions, and thus provides information on the metamorphic evolution of source areas that is crucial in tectonic and geodynamic reconstructions. Garnet is easily identified in mineral grain mounts and is relatively stable during burial diagenesis. Raman spectroscopy allows rapid determination of garnet compositions in grain mounts or thin sections of sand and sandstone samples, and can be used to assess their density and chemical composition quite accurately (“MIRAGEM” method of Bersani et al., 2009; Andò et al., 2009). In the D’Entrecastreaux Islands of southeastern Papua New Guinea, the world’s youngest (U)HP rocks are exposed. There, mafic rocks and their felsic host gneisses were metamorphosed under eclogite facies conditions from late Miocene to Pliocene, before being exhumed from depths of 90 km (Baldwin et al., 2004, 2008). The eclogite preserves a peak assemblage of garnet, omphacite, rutile, phengite and Si02 (Hill and Baldwin, 1993). A coesite-eclogite has been found in one small island outcrop. In order to sample garnet populations representative of a larger geographical area, we sampled and studied a heavy-mineral-dominated placer sand (HMC 80) from a beach from SE Goodenough Island. Garnet grains in beach sand are associated with blue-green to subordinately green-brown amphibole and minor epidote, omphacitic clinopyroxene, titanite, apatite and rutile. The subordinate low-density fraction is feldspatho-quartzose with high-rank metamorphic rock fragments and biotite (Q62 F35 Lm2; MI 360). Detrital garnets are mostly classified as almandine with relatively high Mg and Ca and lacking Mn, typical of the eclogite facies (Win et al., 2007; type Ci garnets of Mange and Morton 2007; Andò et al., 2013). In well-described stratigraphic sequences within syn-and post-tectonic basins adjacent to orogenic systems, Raman-assisted heavy-mineral studies allow us to detect the first arrival of eclogitic garnet, and thus to assess the minimum age of exhumation and final unroofing of high-pressure rocks (Malusà et al., 2011; Malusà and Garzanti, 2012). However, in the (U)HP terrane of southeastern Papua New Guinea, sediments derived from the actively exhuming D’Entrecasteaux Island core complexes are still being deposited offshore, are rarely preserved sub-aerially, and as such stratigraphic constraints are limited. Raman analysis of detrital garnets from placer sand thus provides invaluable constraints to compare with mineral assemblages preserved in exhumed eclogites

Amphibole and garnet are among the most widespread heavy minerals in orogenic sediments. Their chemical composition and optical properties vary markedly and systematically with temperature and pressure conditions during growth, and thus provide important information on the metamorphic evolution of source areas that is crucial in palaeotectonic and palaeogeodynamic reconstructions. This study investigates the chemical composition of detrital amphiboles and garnets derived from parent rocks of progressively increasing metamorphic grade through a well-studied composite section across the Central and Southern Alps, including the granulite-facies core of the Late Palaeozoic orogen exposed in the Ivrea–Verbano Zone and the amphibolite-facies core of the Cenozoic orogen exposed in the Lepontine Dome. We specifically focus on metamorphic grade because it represents the best proxy for tectono-stratigraphic crustal level, and hence degree of unroofing of source areas. In river sands collected between metamorphic isograds corresponding to crystallization temperatures ranging from c. 500 °C to c. 850 °C, TiO2 gradually increases in detrital amphibole while its colour progressively changes from blue-green in the lower amphibolite-facies where actinolite, hornblende and tschermakite are most abundant, to brown in the granulite facies where pargasite is dominant. Detrital garnets display moderate gradual changes across the amphibolite-facies Lepontine Dome, where low-Mg ‘type B’ garnets predominate. Almandine-spessartine is spatially associated with abundance of pegmatites while entering the zone of anatexis (Southern Steep Belt), where grossular or grossular-andradite-spessartine are occasionally found. A sharp change occurs while reaching granulite-facies in the Ivrea–Verbano Zone, where high-Mn garnets disappear and ‘type A’ almandine-pyrope (from ‘stronalite’ metasediments) and ‘type C’ almandine-pyrope-grossular (from metagabbros of the Mafic Complex) predominate. Also redefined in this article are a series of numerical indices based on amphibole colour and relative abundances of diverse key minerals (chloritoid, staurolite, andalusite, kyanite, fibrolitic and prismatic sillimanite), useful to accurately assess the average metamorphic grade of meta-igneous and metasedimentary source rocks.

Subduction complexes large enough to be exposed subaerially and to become significant sources of terrigenous detritus are formed by tectonic accretion above trenches choked with thick sections of remnant-ocean turbidites. They thus need to be connected along strike to a large Alpine-type or Andean-type orogen, where huge volumes of orogenic detritus are produced and conveyed via a major fluvio-deltaic system to the deep sea (Ingersoll et al., 2003).

Raman mineral analysis (RaMAn) is an innovative, efficient and user-friendly technique representing the ideal tool to perform provenance analysis of silt-sized sediments, which represent most of the sediment flux in river systems and the predominant grain size in large deltas and submarine fans (Andò et al., 2011). RaMAn allows in fact the reliable recognition of detrital grains down to 5 mum in size.

Raman spectroscopy is an innovative tool with tremendous potential in provenance studies, serving as a complement to classical petrographic and mineralogical analysis as well as to other techniques including detrital geochronology. Thorny long-standing problems such as the identification of opaque and weathered grains can be finally addressed. Even more important, the Raman technique allows us to reliably determine silt-sized detrital grains, thus opening up new frontiers in sedimentary petrology.

Compositional evolution of orogenic sediment can be viewed as resulting from downstream mixing of detritus shed by several end-member sources to constitute the polygenic product deposited in foreland basins. Such a model of simple mixing, which does not consider modifications caused by physical and chemical processes through the sedimentary cycle, is applied here to first-cycle detritus presently produced in the Alps to assess sediment budgets and erosion patterns across the orogen. Using the same actualistic end members, the model is applied next to the recent past to reconstruct paleodrainage changes during major Pleistocene glaciations, and finally to a more remote past to tentatively infer erosional evolution and dispersal paths in the earliest Oligocene stages of Alpine morphogeny. A decisive factor of success in such an approach is the use of large multiple datasets displaying sufficiently distinct values in diverse end members. Forward mixing models can be made for bulk petrography and heavy-mineral suites as well as for any physical (grain density), geochemical (element concentrations, stable-isotope ratios), or geochronological parameters (apatite fission tracks, zircon U-Pb age spectra), and separately on sediments of various sizes and transport modes (suspended load and bedload). By comparing and integrating several independent estimates thus obtained we progressively gain a more robust insight about diverse geological processes, and may converge eventually toward a truer vision of our past.

Natural feldspars are framework silicates and among the most common rock-forming minerals of planetary crusts. Our aim is to determine the Raman spectral characteristics of feldspar-group minerals, in order to build a consistent set of Raman spectra and to recognize them in all possible specimens of natural sediments and man-made pottery and tools of archaeological value. Sediments represent in fact the principal inorganic raw material employed in ancient times to produce ceramics. The characterization of Na- versus K-rich alkali feldspars in pottery fragments is complex due to heterogeneity of materials used and presence of both crystalline and amorphous phases1,2. Raman spectroscopy is an innovative, efficient, and user-friendly technique representing the ideal complementary tool to classical provenance analysis of sediments and sedimentary rocks. The method is non-destructive, does not require specific preparation, and can be used also directly on thin sections and polished rock. Detrital feldspars are determined by comparison of Raman peaks observed on our samples with reference spectra reported in the literature calibrated spectra obtained from minerals of specific chemistry and mineralogical databases available online. It is often convenient to photograph and label each single grain on the thin section or grain mount, in order to observe optical properties under the microscope before and after identification with Raman spectroscopy. Raman spectroscopy is a powerful tool to confidently identify dubious grains independently of their size and orientation. Last but not least, it allows routine identification of minerals as fine grained as fine silt6, and it is consequently an unrivalled tool for quantitative analysis of silt and siltstone, which represent a huge fraction of fluvial, turbiditic and eolian transport. A comprehensive study of vibrational mode behaviour in a wide variety of natural feldspar-group minerals is useful for geologist, archeologist and industry.

Sourced as the Nile in distant basaltic rift highlands, the Orange River is the predominant ultimate source of sand for the Namib desert dunes, as proved independently by bulk-petrography, heavy-mineral, pyroxene-chemistry, and U/Pb zircon-age datasets. Additional local entry points of sand do exist at the edges of the desert, and were quantified by comparison with detrital modes and heavy-mineral suites of hinterland-river sediments. After long-distance fluvial transport, Orange sand is washed by ocean waves and dragged northward by vigorous longshore currents. Under the incessant action of southerly winds, sand is blown inland and carried farther north to accumulate in the Namib erg, a peculiar wind-dominated sediment sink displaced hundreds of kilometres away from the river mouth. And yet changes in sand mineralogy along the way are minor. After a multistep journey of cumulative 3000 km from their source in Lesotho, volcanic rock fragments and pyroxene are found in unchanged abundance as far as the northern edge of the desert. Only locally is volcanic detritus slightly depleted and minor but regular enrichment in quartz and garnet observed, the sole potential effect of prolonged transport or recycling of Tertiary eolianites. Selective comminution of fragile minerals is thus proved unable to substantially modify sand composition in fluvial, coastal, or eolian settings. Mechanical processes have a much greater effect on the morphology of detrital grains, which in Namib dunes appear commonly shaped into nearly perfect spheres. Eolian sorting concentrates denser minerals locally in placer lags, but such effects can be identified and compensated for. This study demonstrates that mechanical breakdown is unable to markedly affect provenance signatures even during long-distance and prolonged multistep transport in high-energy settings. In arid climates, where chemical processes are negligible, high-resolution bulk-petrography and heavy-mineral analyses are thus powerful techniques to quantitatively reconstruct provenance, and to trace sediment sources and dispersal paths over distances up to thousands of kilometres.

Garbology, the science of finding stuff out about someone or something by digging through their trash, was invented in September 1970 by Alan Jules Weberman, a gadfly journalist who tried to unveil private secrets of Bob Dylan and his wife, the ex-bunny girl Sarah Lowndes, by searching the trash outside their Greenwich Village home in New York. Garbological applications to the Earth Sciences (“provenance analysis”) provide unique information on the hidden secrets of orogenic belts by scrutinizing the detritus they produce (“waste” in old geomorphological literature). This daring and exciting task requires painstaking dedication. First, we need to determine end-member signatures of “waste” shed by the diverse tectonic units incorporated in the mountain belt. This can be done only in modern settings and climatic conditions that grant minimum chemical weathering. Distortions of the original compositional signals by hydrodynamic processes need to be carefully checked and corrected for. Next, by combining the end-member signals as thus determined using forward modelling, we get a grasp of compositions expected to be found in the orogen’s garbage cans (“sedimentary basins”). The comparison between the expected and the observed may eventually lead us to decrypt the garbological record preserved in stratigraphic successions, and to unravel events the traces of which have long been erased by erosion from bedrock exposed in the thrust-belt. Simple statistical procedures allow us to interpret with a good amount of confidence information stored in recent deposits of the Pleistocene, but inferences become more and more uncertain when we try to extrapolate our knowledge of the present backwards in time to reconstruct sceneries of the remote past. By carefully checking diverse alternative hypotheses, however, garbologists may be of help to serious bedrock geologists to reconstruct with unsuspected precision the orogenic history of the Alps since the very beginning of convergence between Africa-Adria and Europe.

This research presents the first multitechnique provenance study of the Siwalik Group in the Himalayan foreland basin in India, using the Jawalamukhi section, magnetostratigraphically dated at 13–5 Ma. Combined with provenance data from a Dharamsala Formation sedimentary section (21–13 Ma) located close by, it forms the longest temporally continuous record of Himalayan erosion in the Indian foreland basin. Sandstone petrography and heavy mineral analysis, conglomerate clast composition, Ar-Ar dating of detrital white micas, and Sm-Nd analyses on siltstones, conglomerate matrix and conglomerate clasts was undertaken to determine (1) shifts in source region through time and (2) changes in detrital lag times related to exhumation rates in the hinterland, together interpreted in the light of thrusting events. We interpret the data to show a slow down in exhumation rate of the Higher Himalaya by 16–17 Ma, after which time the locus of thrusting propagated south of the Main Central Thrust, and erosion of the low grade Haimanta Formation to the south became significant. The nonmetamorphosed Inner Lesser Himalaya breached its Haimanta cover by 9 Ma with the metamorphosed Inner Lesser Himalaya (Lesser Himalayan Crystalline Series) exhuming to surface by 6 Ma. This event caused sufficient disruption to established drainage patterns that all Higher Himalayan material was diverted from this location at this time.

Fission-track dating on detrital apatites from modern sands of the Po Delta is used for a provenance study of sediments in the Po River basin. Analysed samples show a fission-track grain-age distribution characterized by two prominent peaks at 7.7 Ma and 17 Ma. The youngest peak accounts for 46% of the total population of dated grains. This young component in the grain-age distribution is consistent with bedrock cooling ages observed in the Western Alps between the External Massifs and the Houiller unit, as well as in the Lepontine dome of the Central Alps and in the Miocene foredeep units of the Apennines, that overall represent only 12% of the orogenic source area. Results suggest that most of the sediment load in the last 10(2)-10(5) years was supplied by focused erosion of relatively small areas that experienced short-term erosion rates one order of magnitude higher than in the rest of the.

Heavy mineral concentration in sediments depends primarily on the chemistry and tectono-stratigraphic level of rocks eroded within continental-block, arc, or orogenic source terranes. Detritus derived from mantle peridotites and lower crustal gabbros, as well as from high-pressure (oceanic or continental eclogite) and high-temperature (amphibolite, granulite) metamorphic rocks contains one to two orders of magnitude more heavy minerals than detritus derived from upper crustal rocks including granites and sedimentary successions. Concentrated heavy mineral assemblages, however, may result from density-sorting during erosion, transport, or deposition by tractive currents, which can very effectively segregate minerals with even small differences in density within distinct grain-size fractions and sedimentary environments (e.g., fluvial channel versus overbank, beach versus shelf). Conversely, depleted heavy mineral assemblages may result from severe diagenetic dissolution in ancient sandstones, a process that in Alpine and Himalayan foreland basins very extensively affected clastic wedges older than the Pleistocene. Through a series of key modern examples from various geodynamic settings in arid to semi-arid climate, we stress the importance of giving full consideration to heavy mineral concentration while interpreting provenance of terrigenous sediments and sedimentary rocks. The concentration parameters introduced herein (Heavy Mineral Concentration index, Source Rock Density index), coupled with parameters based on heavy mineral species with either contrasting density (% opaque, % ultradense, % ZR) or chemical stability, allow us to reveal and quantify also the effects of hydraulic sorting in the depositional environment and of diagenetic dissolution in ancient terrigenous rocks. We document that in most modern sands, the relative abundance of chemically stable species (e.g., zircon, tourmaline, rutile, apatite, chrome spinel) is considerably less than the experience on ancient sandstones generally induces to believe, and show that the actualistic approach provides crucial insight for a correct interpretation of heavy mineral suites

This study discusses the laws which govern sediment deposition, and consequently determine sizedependent compositional variability. A theoretical approach is substantiated by robust datasets on major Alpine, Himalayan, and African sedimentary systems. Integrated (bulk-petrography, heavy-mineral, X-ray powder diffraction) multiple-window analyses at 0.25ϕ to 0.50ϕ sieve interval of eighty-five fluvial, beach, and eolian-dune samples, ranging from very fine silt to coarse sand, document homologous intrasample compositional trends, revealed by systematic concentration of denser grains in finer-grained fractions (“size– density sorting”). These trends are explained by the settling-equivalence principle, stating that detrital minerals are deposited together if their settling velocity is the same. Settling of silt is chiefly resisted by fluid viscosity, and Stokes' law predicts that size differences between detrital minerals in ϕ units (“size shifts”) are half the difference between the logarithms of their submerged densities. Settling of pebbles is chiefly resisted by turbulence effects, and the Impact law predicts double size shifts than Stokes' law. Settling of sand is resisted by both viscosity and turbulence, the settling-equivalence formula is complex, and size shifts increase – with increasing settling velocity and grain size – from those predicted by Stokes' law to those predicted by the Impact law. In wind-laid sands, size shifts match those predicted by the Impact law; size– density sorting is thus greater than in water-laid fine sands. New analytical, graphical, and statistical techniques for rigorous settling-equivalence analysis of terrigenous sediments are illustrated. Deviations associated with non-spherical shape, density anomalies, inheritance from source rocks, or mixing of detrital species with contrasting provenance and different size distribution are also tentatively assessed. Such integrated theoretical and experimental approach allows us to mathematically predict intrasample compositional variability of water-laid and wind-laid sediments, once the density of detrital components is known.

International audience; 40Ar–39Ar dating of detrital white micas, petrography and heavy mineral analysis and whole-rock geochemistry has been applied to three time-equivalent sections through the Siwalik Group molasse in SW Nepal [Tinau Khola section (12–6 Ma), Surai Khola section (12–1 Ma) and Karnali section (16–5 Ma)]. 40Ar–39Ar ages from 1415 single detrital white micas show a peak of ages between 20 and 15 Ma for all the three sections, corresponding to the period of most extensive exhumation of the Greater Himalaya. Lag times of less than 5 Myr persist until 10 Ma, indicating Greater Himalayan exhumation rates of up to 2.6 mm year−1, using one-dimensional thermal modelling. There are few micas younger than 12 Ma, no lag times of less than 6 Myr after 10 Ma and whole-rock geochemistry and petrography show a significant provenance change at 12 Ma indicating erosion from the Lesser Himalaya at this time. These changes suggest a switch in the dynamics of the orogen that took place during the 12–10 Ma period whereby most strain began to be accommodated by structures within the Lesser Himalaya as opposed to the Greater Himalaya. Consistent data from all three Siwalik sections suggest a lateral continuity in tectonic evolution for the central Himalayas.