Daniel Vielzeuf

Experimental determination of the fluid-absent melting relations in the pelitic system

In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als.The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C.We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites.The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.

Partial melting of metagreywackes. Part I. Fluid-absent experiments and phase relationships

Island arcs, active and passive margins are the best tectonic settings to generate fertile reservoirs likely to be involved in subsequent granitoid genesis. In such environments, greywackes are abundant crustal rock types and thus are good candidates to generate large quantities of granitoid magmas. We performed a series of experiments, between 100 and 2000 MPa, on the fluid-absent melting of a quartz-rich aluminous metagreywacke composed of 32 wt% plagioclase (Pl) (An22), 25 wt% biotite (Bt) (XMg45), and 41 wt% quartz (Qtz). Eighty experiments, averaging 13 days each, were carried out using a powder of minerals (≤5μm) and a glass of the same composition. The multivariant field of the complex reaction Bt+Pl+Qtz⇔Grt/Crd/Spl+ Opx+Kfs+melt limited by the Opx-in and Bt-out curves, is located between 810–860°C at 100 MPa, 800–850°C at 200 MPa, 810–860°C at 300 MPa, 820–880°C at 500 MPa, 860–930°C at 800 MPa, 890–990°C at 1000 MPa, and at a temperature lower than 1000°C at 1500 and 1700 MPa. The melting of biotite+plagioclase+ quartz produced melt+orthopyroxene (Opx) +cordierite (Crd) or spinel (Spl) at 100, 200 and 300 MPa, and melt+orthopyroxene+garnet (Grt) from 500 to 1700 MPa (+Qtz, Pl, FeTi Oxide at all pressures). K-feldspar (Kfs) was found as a product of the reaction in some cases and we observed that the residual plagioclase was always strongly enriched in orthoclase component. The P-T surface corresponding to the multivariant field of this reaction is about 50 to 100°C wide. At temperatures below the appearance of orthopyroxene, biotite is progressively replaced by garnet with increasing P. At 850°C, we observed that (1) the modal proportion of garnet increases markedly with P; (2) the grossular content of the garnet increases regularly from about 4 mol% at 500 MPa to 15 mol% at 2000 MPa. These changes can be ascribed to the reaction Bt+Pl+Qtz ⇔ Grt+Kfs+melt with biotite +plagioclase+quartz on the low-P side of the reaction. As a result, at 200 MPa, we observed the progressive disappearance of biotite without production of orthopyroxene. These experiments emphasize the importance of this reaction for the understanding of partial melting processes and evolution of the lower continental crust. Ca-poor Al-metagreywackes represent fertile rocks at commonly attainable temperatures (i.e. 800–900°C), below 700 MPa. There, 30 to 60 vol.% of melt can be produced. Above this pressure, temperatures above 900°C are required, making the production of granitoid magmas more difficult. Thin layers of gneisses composed of rothopyroxene, garnet, plagioclase, and quartz (±biotite), interbedded within sillimanite-bearing paragneisses, are quite common in granulite terrains. They may result from partial melting of metagreywackes and correspond to recrystallized mixtures of crystals (+trapped melt) left behind after removal of a major proportion of melt. Available experimental constraints indicate that extensive melting of pelites takes place at a significantly lower temperature (850°C±20) than in Al-metagreywackes (950°C±30), at 1000 MPa. The common observation that biotite is no longer stable in aluminous paragneisses while it still coexists commonly with orthopyroxene, garnet, plagioclase and quartz, provides rather tight temperature constraints for granulitic metamorphism.

Crustal splitting and the emplacement of Pyrenean lherzolites and granulites

Abstract Pyrenean lherzolites representing fragments of the upper mantle are embedded in Mesozoic sediments along the 500-km-long, 20-km-wide “North Pyrenean Zone”. These ultramafics and the nearby occurrence of granulites, the presence of undersaturated rocks intrusive in the Cretaceous, and the development of a narrow elongate zone of “Pyrenean metamorphism”, confer a fundamental orogenic and geodynamic significance to this zone. These phenomena and the emplacement of the lherzolites may be attributed to an important phase of crustal thinning followed by a succession of compressive stages. The crustal thinning involved in the North Pyrenean Zone is possibly related to a transcurrent movement mainly during Albian and Senonian times, indicating a transform zone [24]. The ascent and emplacement of lherzolites in Albian sediments may have occurred in response to alternating tensional and compressional stresses operating in transcurrent zones. The Pyrenean metamorphism in Mesozoic sediments may be related to convective (emplacement of lherzolites) as well as conductive (thermal anomaly in connection with the rifting) processes. This model is compared with existing transform domains. Finally, it is suggested that crustal thinning may permit the incorporation of lower crust-upper mantle associations in the upper levels of the crust during subsequent compressive stages.

Nano to macroscale biomineral architecture of red coral (Corallium rubrum)

Abstract Different techniques have been used to characterize the physical and chemical structure of the red coral calcitic skeleton. A section normal to the axis of the skeleton shows a medullar zone surrounded by a circular domain composed of concentric rings. Growth rings are revealed by the cyclic variation of organic matter (OM) and Mg/Ca ratio. These growth rings are annual; thus, both OM and Mg/Ca ratio can be used to date red coral colonies. Growth rings display wavelets. The internal structure of each wavelet results from the stacking of layers with tortuous interfaces. Tortuosity is due to the presence of microprotuberances. Interfaces between layers may display sharp discontinuities indicative of interruption of the mineralizing process. SEM and TEM studies show that each layer is made of (1) fibers, organized or not in fan-shaped structures; and (2) submicrometer (apparently mono-) crystalline units. Fibers are superstructures made of submicrometer units possibly assembled by an oriented aggregation mechanism. HRTEM studies show that in spite of displaying single-crystal scattering behavior, the submicrometer crystalline units are made of 2-5 nm nanograins again possibly aggregated by a mechanism of oriented attachment. Thus, submicrometer crystalline units and polycrystalline fibers can be both defined as mesocrystals. The red coral skeleton is a hierarchically organized organic-inorganic composite that exhibits porosity and structural and compositional order on length scales from the nanoscale to the macroscale.

Multilevel modular mesocrystalline organization in red coral

Abstract Biominerals can achieve complex shapes as aggregates of crystalline building blocks. In the red coral skeleton, we observe that these building blocks are arranged into eight hierarchical levels of similarly (but not identically) oriented modules. The modules in each hierarchical level assemble into larger units that comprise the next higher level of the hierarchy, and consist themselves of smaller, oriented modules. EBSD and TEM studies show that the degree of crystallographic misorientation between the building blocks decreases with decreasing module size. We observe this organization down to a few nanometers. Thus, the transition from imperfect crystallographic order at millimeter scale to nearly perfect single crystalline domains at nanometer scale is progressive. The concept of “mesocrystal” involves the three-dimensional crystallographic organization of nanoparticles into a highly ordered mesostructure. We add to this concept the notion of “multilevel modularity.” This modularity has potential implications for the origin of complex biomineral shapes in nature. A multilevel modular organization with small intermodular misorientations combines a simple construction scheme, ruled by crystallographic laws, with the possibility of complex shapes. If the observations we have made on red coral extend to other biominerals, long-range crystallographic order and interfaces at all scales may be key to how some biominerals achieve complex shapes adapted to the environment in which they grow.

Oxygen isotope heterogeneities and diffusion profile in composite metamorphic-magmatic garnets from the Pyrenees

Abstract Unusually large δ18O heterogeneities (≥4‰) within single crystals are reported in garnets from dioritic migmatites in the Pyrenees. These heterogeneities, together with contrasting Ca and P zoning, allow the identification of different growth zones. Garnet cores with high δ18O values (12-14‰) are relatively poor in Ca (7.9 mol% Grs) and rich in P (400-900 ppm P2O5). In contrast, garnet rims with lower δ18O values (7-12 ‰) are richer in Ca (12-14% Grs) and poorer in P (100-200 ppm). These growth zones can be ascribed to a metamorphic event followed by crustal partial melting and contamination by magmas from the mantle. High δ18O intra-crystalline contrasts result from mineral growth in an open magmatic system involving the interaction of partial melts with distinct δ18O signatures. At the garnet core-rim interface, compositional profiles in major divalent cations are consistent with the relaxation of an initial sharp step in Ca, Fe, and Mg by Ca ↔ (Fe, Mg) interdiffusion. At the same interface, an O-isotope profile is documented. The analogy of Ca and O isotope profiles suggests that the δ18O distribution may also result from a diffusion process. In this particular case (temperature, garnet composition, oxygen fugacity), O diffusion appears to be of the same order of magnitude as Ca ↔ (Fe, Mg) interdiffusion. Considering a duration of 10 Ma for the plutono-metamorphic event in the Pyrenees, Ca and O diffusivities in the range 10.22 m2/s (at 850 °C) are retrieved from the measured profiles. Like Ca, O diffusion in garnet at magmatic temperatures (850.900 °C) is both slow enough to preserve large δ18O heterogeneities and fast enough to generate relaxation profiles.

An Improved Experimental Calibration of the Olivine-Spinel Geothermometer

The calibration of the olivine-spinel geothermometer by Fabriès (1979) is commonly adopted by a number of petrologists. But the temperatures calculated in this way for ultramafic rocks are significantly lower than those obtained by the pyroxene geothermometers. These O1-Sp temperatures are also lower than those measured experimentally in the natural system (four-phase lherzolite). Different rates of cation diffusion cannot fully account for these differences. The temperature deviation is actually related to the inconsistencies between natural and experimental data which support the calibration. A re-evaluation of the calibration is proposed on the basis of a set of new experimental data.

Experimental study of argon sorption in quartz: Evidence for argon incompatibility

Abstract We have conducted Ar sorption experiments on two varieties of natural quartz minerals (Q112, Q113) and a synthetic one (QHE37). Runs were performed in an internally heated pressure vessel at 1300°C and pressures of up to 8000 bar for run times between 1 and 12 d, on grain sizes ranging from 11–20 to 60–80 μm. Run products were analysed by gas chromatography (GC), Knudsen cell mass spectroscopy (KMS), electron microprobe (EMP), and scanning electron microscopy (SEM). Release spectra of Ar desorption were monitored by KMS. For sample Q112 and QHE37 two release signals are observed (500–1000°C and 1200–1600°C). When two grain sizes of the same specimen (QHE37) are analysed, the high temperature peak does not vary whereas the low temperature peak is significantly increased with decreasing grain size, suggesting desorption of surface bonded Ar. Argon contents from the high temperature peak indicate an Ar sorption of 28 ppm (QHE37) and 48 ppm (Q112) at 4070 bar. Specimen Q113 does not exhibit low temperature release, nevertheless, its Ar content is higher in smaller grains (11–20 μm: 431 ppm, 60–80 pm: 128 ppm), while increasing the duration of the experiments from ∼ 1 d to ∼ 10 d does not change the Ar content. This apparently erratic behaviour suggests an extrinsic control for Ar sorption. EMP analysis of all samples at the μm scale reveals heterogeneous Ar distribution. A few enriched spots with Ar up to 4000 ppm are observed (e.g., Q113) compared to a background concentration below the detection limit of ∼30 ppm. The average concentration measured by EMP is fairly similar to the high temperature step of bulk analytical methods (KMS or GC). We can conclude that bulk measurements of the sorption of Ar do not document equilibrium dissolution. Assuming the Ar diffusivity to be fast enough to permit saturation of at least the 11–20 μm grain fraction after ∼ 10 d, at 8000 bar and 1300°C, an upper bound of Ar solubility can be given as 30 ppm. In contrast, bulk methods yield variable average Ar concentrations which depend on experimental conditions. This indicates that solubilities measured by bulk methods grossly overestimate the true solubility. A quartz /melt partition coefficient of less than 0.006 can be derived.

Mesoscale twinning and crystallographic registers in biominerals

Abstract Mesocrystals attract increasing interest in the fields of physics, chemistry, materials, and Earth sciences. Yet, structural properties of this new class of solid materials are not fully described. Biominerals often display complex hierarchical mesocrystalline organizations. We report on the crystallography of sclerites, which are small (-80 μm), beautifully shaped calcitic biomineral structures found in the living tissues of octocorals. Despite a layered concentric structure, the submicrometer crystalline units constituting the sclerites display a remarkably simple crystallographic organization of similarly oriented units with only a low degree of misorientation between them. Some sclerites display crystallographic sectors, leading to the concept of “mesotwin.” A mesotwin is to a twin what a “mesocrystal” is to a crystal: an analog with defects. On the basis of EBSD data, we propose a simple conceptual crystallographic model that accounts for the observed features. This model involves different rhombohedral unit blocks, with identical shapes and volumes, but different crystallographic faces. EBSD data show that quite unexpectedly slight misorientations of crystallites in the sclerites are not at random but organized around the three equivalent a axes of the hexagonal unit cell of calcite. In a subtle way, the overall organization and morphologies of the red coral sclerites are connected to the calcite crystallography.

Raman characterization of synthetic magnesian calcites

Abstract Magnesian calcites are important components of sediments and biominerals. Although Raman spectra of calcite, dolomite, and magnesite are well known, those of magnesian calcites deserve further investigation. Nineteen syntheses of magnesian calcites covering the range 0–50 mol% MgCO3 have been carried out at high pressure and temperature (1–1.5 GPa, 1000–1100 ℃). The crystalline run products have been characterized by μ-Raman spectroscopy. For all lattice and internal modes (L, T, ν1, ν4, 2ν2) but ν3, wavenumbers align closer to the calcite– dolomite line than the calcite–magnesite line. The compositional dependence is strong and regression curves with high correlation coefficients have been determined. Full-width at half maximum (FWHM) plot along parabolas that depart from the calcite–dolomite or calcite–magnesite lines. The limited data dispersion of both shifts and FWHM allow using Raman spectral properties of magnesian calcites to determine the Mg content of abiotic calcites. A comparison with Raman data from the literature obtained on synthetic magnesian amorphous calcium carbonate (Mg ACC) shows that the wavenumber position of the ACC ν1 mode is systematically shifted toward lower values, and that their FWHM are higher than those of their crystalline counterparts. The FWHM parameters of crystalline and amorphous materials do not overlap, which allows a clear-cut distinction between crystalline and amorphous materials. In synthetic magnesian calcites, the shift and FWHM of Raman bands as a function of magnesium can be interpreted in terms of changes of metal-O bond lengths resulting from the replacement of calcium by magnesium. The facts that the wavenumber of magnesian calcites are close to the calcite–dolomite line (not calcite-magnesite), that the FWHM of the T, L, and ν4 modes reach a maximum around 30 ±5 mol% MgCO3, and that a peak specific to dolomite at 880 cm–1 is observed in high-magnesian calcites indicate that dolomite-like ordering is present above ~10 mol% MgCO3. Mg atom clustering in cation layers combined with ordering in successive cation basal layers may account for the progressive ordering observed in synthetic magnesian calcites.