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Carbonatites are peculiar magmatic rocks with mantle-related genesis, commonly interpreted as the products of melting of CO2-bearing peridotites, or resulting from the chemical evolution of mantle derived magmas, either through extreme "differentiation or secondary immiscibility. Here we report the first finding of anatectic carbonatites of crustal origin, preserved as calcite-rich polycrystalline inclusions in garnet from low-to-medium pressure migmatites of the Oberpfalz area, SW Bohemian Massif (Central Europe). These inclusions originally trapped a melt of calciocarbonatitic composition with a characteristic enrichment in Ba, Sr and LREE. This interpretation is supported by the results of a detailed microstructural and microchemical investigation, as well as re-melting experiments using a piston cylinder apparatus. Carbonatitic inclusions coexist in the same cluster with crystallized silicate melt inclusions (nanogranites) and COH fluid inclusions, suggesting conditions of primary immiscibility between two melts and a fluid during anatexis. The production of both carbonatitic and granitic melts during the same anatectic event requires a suitable heterogeneous protolith. This may be represented by a sedimentary sequence containing marble lenses of limited extension, similar to the one still visible in the adjacent central Moldanubian Zone. The presence of CO2-rich fluid inclusions suggests furthermore that high CO2 activity during anatexis may be required to stabilize a carbonate-rich melt in a silica-dominated, system. This natural occurrence displays a remarkable similarity with experiments on carbonate-silicate melt immiscibility, where CO2 saturation is a condition commonly imposed. In conclusion, this study shows how the investigation of partial melting through melt inclusion studies may unveil unexpected processes whose evidence, while preserved in stiff minerals such as garnet, is completely obliterated in the rest of the rock due to metamorphic re-equilibration. Our results thus provide invaluable new insights into the processes which shape the geochemical evolution of our planet, such as the redistribution of carbon and strategic metals during orogenesis. (C) 2016 Elsevier B.V. All rights reserved.
Albian-Turonian subduction-accretionary complexes are exposed widely in the Central Pontides. A major portion of the accretionary complexes is made up of a metaflysch sequence consisting of slate/phyllite and metasandstone intercalation with blocks of marble, Na-amphibole bearing metabasite, and serpentinite. The metaflysch sequence represents distal parts of a large Lower Cretaceous submarine turbidite fan deposited on the Laurasian active continental margin that was subsequently accreted and metamorphosed during the Albian. Raman spectra of carbonaceous material of the metapelitic rocks revealed that the metaflysch consists of metamorphic packets with distinct peak metamorphic temperatures. The majority of the metapelites are low-temperature (ca. 330 degrees C) slates characterized by lack of differentiation of the graphite (G) and D2 defect bands. They possibly represent offscraped distal turbidites along the toe of the Albian accretionary wedge. Other phyllites are characterized by a slightly pronounced G band with a D2 defect band occurring on its shoulder. Peak metamorphic temperatures of these phyllites are constrained to 370-385 degrees C. The phyllites are associated with a strip of incipient blueschist facies metabasites and are found as a sliver within the offscraped distal turbidites. We interpret the phyllites as underplated continental sediments together with oceanic crustal basalt along the basal decollement. Tectonic emplacement of the underplated rocks into the offscraped distal turbidites was possibly achieved by out-of-sequence thrusting causing tectonic thickening and uplift of the wedge.
Remnants of hydrous melt formed at mantle depths have been identified and characterized within high-pressure leucogranulites of the Orlica-Snieznik Dome (Bohemian Massif, central Europe). They occur as nanogranites in garnet formed via partial melting of granitoids during the Variscan orogeny. Melt composition and H2O content have been investigated in situ after experimental re-homogenization of the nanogranites, and are consistent with melts produced experimentally from crustal lithologies at mantle depths. This is the first geochemical study of melt inclusions from natural crustal rocks equilibrated close to the stability field of coesite, shedding light on how continental crust melts during deep subduction. Whereas decompressional melting is commonly invoked for deeply subducted crustal lithologies, melting occurred near or at the metamorphic peak pressure in the Orlica-Snieznik granulites. Melting of deeply subducted crustal rocks significantly modifies the rheology and thus promotes fast exhumation: this process has a critical influence on the geodynamic evolution of subduction-collision orogens as well as crustal differentiation at depth.
Ultrahigh-pressure (UHP), coesite-bearing edogites in the Himalaya have been documented from the Kaghan Valley in Pakistan and the Tso Morani area in northwest India. These complexes are part of the northern edge of the Indian plate that has been subducted to, and metamorphosed at, mantle depths of more than 100 km before being exhumed. Both UHP complexes are located today directly adjacent to the Indus-Tsangpo suture zone and are not separated by non-metamorphosed sequences of Tethyan sediments from the Asian margin. Herein, we present new data for one fresh coesite-bearing eclogite from the Tso Moran massif. Therein, garnets are zoned reflecting their growth during prograde and peak metamorphism and showing a thin retrograde overgrowth. Inclusions can be directly correlated to the compositional zoning and are seen as either relicts of the protolith mineral paragenesis and as "snap shots" of the mineral paragenesis during subduction and under peak conditions. Rare earth element concentrations (REE) were obtained for garnet, mineral inclusions in garnet and matrix minerals. The REE pattern in garnet reflects a sequential change in matrix minerals and their proportions due to net transfer reactions during subduction and peak metamorphism. Using conventional geothermobarometry, a peak pressure of ca. 44-48 kbar at 560-760 degrees C followed by an S-shaped exhumation curve has been deduced. Gibbs free energy minimization modelling was used to supplement our analytical findings. (C) 2015 Elsevier B.V. All rights reserved.
This study monitors regional changes in the crystallinity of carbonaceous matter (CM) by applying Micro-Raman spectroscopy to a total of 214 metasediment samples (largely so-called Bundnerschiefer) dominantly metamorphosed under blueschist- to amphibolite-facies conditions. They were collected within the northeastern margin of the Lepontine dome and easterly adjacent areas of the Swiss Central Alps. Three-dimensional mapping of isotemperature contours in map and profile views shows that the isotemperature contours associated with the Miocene Barrow-type Lepontine metamorphic event cut across refolded nappe contacts, both along and across strike within the northeastern margin of the Lepontine dome and adjacent areas. Further to the northeast, the isotemperature contours reflect temperatures reached during the Late Eocene subduction-related blueschist-facies event and/or during subsequent near-isothermal decompression; these contours appear folded by younger, large-scale post-nappe-stacking folds. A substantial jump in the recorded maximum temperatures across the tectonic contact between the frontal Adula nappe complex and surrounding metasediments indicates that this contact accommodated differential tectonic movement of the Adula nappe with respect to the enveloping Bundnerschiefer after maximum temperatures were reached within the northern Adula nappe, i.e. after Late Eocene time.
The pressures required for diamond and coesite formation far exceed conditions reached by even the deepest present-day orogenic crustal roots. Therefore the occurrence of metamorphosed continental crust containing these minerals requires processes other than crustal thickening to have operated in the past. Here we report the first in situ finding of diamond and coesite, characterized by micro-Raman spectroscopy, in high-pressure granulites otherwise indistinguishable from granulites found associated with garnet peridotite throughout the European Variscides. Our discovery confirms the provenance of Europe's first reliable diamond, the "Bohemian diamond," found in A.D. 1870, and also represents the first robust evidence for ultrahigh-pressure conditions in a major Variscan crustal rock type. A process of deep continental subduction is required to explain the metamorphic pressures and the granulite-garnet peridotite association, and thus tectonometamorphic models for these rocks involving a deep orogenic crustal root need to be significantly modified.
Orbitolinids are larger foraminifera widespread in Lower Cretaceous shallow-water carbonates of the Tethyan realm. They are among the most important fossil groups used for Biostratigraphy. Despite this and although the structural features of the group have been described in detail, very little is known about the composition of their agglutinated test and the process by which they selected foreign grains. In this study, the test of Orbitolina d'Orbigny, 1850 (subgenus Mesorbitolina Schroeder, 1962) from Aptian shallow-water carbonate deposits of southern Italy has been studied in detail. We combine petrographic techniques (optical microscope and SEM) with energy-dispersive x-ray spectrometry (EDS), electron probe microanalyzer (EPMA), X-ray diffraction and Raman spectroscopy analyses.
The results show that the test of Mesorbitolina is composed of carbonate and non-carbonate agglutinated grains with the latter distributed across the test with a specific pattern, moving from the marginal to the central zone. In the marginal zone, non-carbonate grains are found only in the epidermis and along the septa which are composed of quartz, with smaller amounts of illite/muscovite and K-feldspar grains. In the central zone of the test, non-carbonate grains are distributed in two ways. Coarse grains of quartz and K-feldspar are abundant and randomly placed in the endoskeleton embedded in a mosaic of minute carbonate grains. Flat grains, mainly of illite/muscovite constitute the external part of the septa. Our observations indicate that Mesorbitolina did select and place agglutinated grains across its test, mainly according to their shape, whereas it did not select particles according to grain size. The distribution of agglutinated particles according to their mineralogical composition shows some contradictory evidence and therefore, at the moment, grain selection in function of mineralogy cannot be completely confirmed or ruled out. Analogies in the test composition of Mesorbitolina specimens from coeval deposits from different areas of southern Italy indicate that the features of their agglutinated test are typical characters of the genus Mesorbitolina. However, it is still unclear what advantage was obtained by the foraminifer by the described test features.
Raman microspectroscopy on carbonaceous material (RSCM) from the eastern Tauern Window indicates contrasting peak-temperature patterns in three different fabric domains, each of which underwent a poly-metamorphic orogenic evolution: Domain 1 in the northeastern Tauern Window preserves oceanic units (Glockner Nappe System, Matrei Zone) that attained peak temperatures (T-p) of 350-480 degrees C following Late Cretaceous to Palaeogene nappe stacking in an accretionary wedge. Domain 2 in the central Tauern Window experienced T-p of 500-535 degrees C that was attained either within an exhumed Palaeogene subduction channel or during Oligocene Barrovian-type thermal overprinting within the Alpine collisional orogen. Domain 3 in the Eastern Tauern Subdome has a peak-temperature pattern that resulted from Eo-Oligocene nappe stacking of continental units derived from the distal European margin. This pattern acquired its presently concentric pattern in Miocene time due to post-nappe doming and extensional shearing along the Katschberg Shear Zone System (KSZS). T-p values in the largest (Hochalm) dome range from 612 degrees C in its core to 440 degrees C at its rim. The maximum peak-temperature gradient (70 degrees Ckm(-1)) occurs along the eastern margin of this dome where mylonitic shearing of the Katschberg Normal Fault (KNF) significantly thinned the Subpenninic- and Penninic nappe pile, including the pre-existing peak-temperature gradient.
Raman spectra of gas hydrates : differences and analogies to ice 1h and (gas saturated) water
(2005)
It is generally accepted that Raman spectroscopic investigations of gas hydrates provide vital information regarding the structure of the hydrate, hydrate composition and cage occupancies, but most research is focused on the vibrational spectra of the guest molecules. We show that the shape and position of the Raman signals of the host molecules (H2O) also contain useful additional information. In this study, Raman spectra (200-4000cm(-1)) of (mixed) gas hydrates with variable compositions and different structures are presented. The bands in the O-H stretching region (3000- 3800cm(-1)), the O-H bending region (1600-1700cm(-1)) and the O-O hydrogen bonded stretching region (100-400cm(-1)) are compared with the corresponding bands in Raman spectra of ice Ih and liquid water. The interpretation of the differences and similarities with respect to the crystal structure and possible interactions between guest and host molecules are presented. (c) 2005 Elsevier B.V. All rights reserved