Pressure-induced structural modifications in Mg2GeO4-olivine: A Raman spectroscopic study

Raman spectra of Mg₂GeO₄-olivine were obtained from ambient pressure up to 34 GPa at ambient temperature. Under quasi-hydrostatic pressure conditions, the following modifications in the Raman spectra occur as pressure increases: 1) near 11 GPa, two sharp extra bands appear in the 600–700 cm^?1 frequency range, and increase in intensity with respect to the olivine bands; 2) above 22 GPa, these two bands become very intense, and the number, position and relative intensity of the other vibrational bands drastically change; 3) the intensity of sharp bands progressively decreases above 25 GPa. The transformation occurs at lower pressures under non-hydrostatic conditions. During decompression to atmospheric pressure, the high-pressure phase partially reverts to olivine. These observations can be interpreted as the progressive metastable transformation from the olivine structure to a crystalline phase with four-fold coordinated Ge, in which the GeO₄ tetrahedra are polymerized. We propose that the metastable high-pressure phase is a structurally disordered spinelloid close to the hypothethical ω- or ?^*-phase, and forms by a shear mechanism assisted by the development of a dynamical instability in the olivine structure. Implications for the transformations undergone by olivines under far-from-equilibrium conditions (e.g. in subducting lithospheric slabs and in shocks) are discussed.     

Fluid production rate during the regional metamorphism of a pelitic schist

Phase equilibria modeling of the pressure–temperature (P–T) path of regional metamorphism and associated fluid expulsion, combined with constraints on the timescale of garnet growth by Sm–Nd geochronology, elucidates the fluid production rate and fluid flux during Barrovian metamorphism of pelitic rocks from Townshend Dam, VT, USA. This modeling builds on a published companion study that utilized Sm–Nd geochronology of concentric growth zones in multiple garnet grains, to constrain the duration of garnet growth in a large sample of schist at Townshend Dam to 3.8?±?2.2 million years (Gatewood et al., Chem Geol 401:151–168, 2015). P–T pseudosections combined with observed mineral compositions constrain garnet growth conditions, and were utilized to construct P–T path-dependent thermodynamic forward models. These models determine that garnet growth was initiated at ~?0.6 GPa and ~?525 °C, with a roughly linear loading and heating P–T trajectory to >?0.8 GPa and ~?610 °C. Loading and heating rates of 2.4 km·Myear^?1 (with a range of 1.6 to 5.8 km·million year^?1) and 23 °C·million year^?1 (with a range of 14 to 54 °C·million year^?1), respectively, are consistent with model estimates and chronologic constraints for tectono-metamorphic rates during orogenesis. Phase equilibria modeling also constrains the amount of water release during garnet growth to be ~?0.7 wt% (or >?2 vol%), largely resulting from the complete consumption of chlorite. Coupling this estimate with calculated garnet growth durations provides a fluid production rate of 5.2 kg·m^?3·million year^?1 (with a range of 3.2 to 12.2 kg·m^?3·million year^?1) and when integrated over the overlying crustal column, a regional-scale fluid flux of 0.07–0.37 kg·m^?2·million year^?1. This range of values is consistent with those derived by numerical models and theory for regional-scale, pervasive fluid flow. This study signifies the first derivation of a fluid production rate and fluid flux in regional metamorphism using a direct chronology of water-producing (garnet-forming) reactions and can provide a framework for future studies on elucidating the nature and timescales of fluid release.     

Crystal structure, Raman and FTIR spectroscopy, and equations of state of OH-bearing MgSiO3 akimotoite

MgSiO₃ akimotoite is stable relative to majorite-garnet under low-temperature geotherms within steeply or rapidly subducting slabs. Two compositions of Mg–akimotoite were synthesized under similar conditions: Z674 (containing about 550 ppm wt H₂O) was synthesized at 22 GPa and 1,500 °C and SH1101 (nominally anhydrous) was synthesized at 22 GPa and 1,250 °C. Crystal structures of both samples differ significantly from previous studies to give slightly smaller Si sites and larger Mg sites. The bulk thermal expansion coefficients of Z674 are (153–839 K) of a ₁ = 20(3) × 10^?9 K^?2 and a ₀ = 17(2) × 10^?6 K^?1, with an average of α ₀ = 27.1(6) × 10^?6 K^?1. Compressibility at ambient temperature of Z674 was measured up to 34.6 GPa at Sector 13 (GSECARS) at Advanced Photon Source Argonne National Laboratory. The second-order Birch–Murnaghan equation of state (BM2 EoS) fitting yields: V ₀ = 263.7(2) Å³, K _T0 = 217(3) GPa (K′ fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: α _a  = 8.2(3) and α _c  = 10.68(9) (10^?6 K^?1); β _a  = 11.4(3) and β _c  = 15.9(3) (10^?4 GPa). Hydration increases both the bulk thermal expansivity and compressibility, but decreases the anisotropy of structural expansion and compression. Complementary Raman and Fourier transform infrared (FTIR) spectroscopy shows multiple structural hydration sites. Low-temperature and high-pressure FTIR spectroscopy (15–300 K and 0–28 GPa) confirms that the multiple sites are structurally unique, with zero-pressure intrinsic anharmonic mode parameters between ?1.02 × 10^?5 and +1.7 × 10^?5 K^?1, indicating both weak hydrogen bonds (O–H···O) and strong OH bonding due to long O···O distances.     

Effect of grain growth on cation exchange between dunite and fluid: implications for chemical homogenization in the upper mantle

The effect of grain growth on the cation exchange between synthesized forsterite aggregates (i.e., dunite) and nickel-rich aqueous fluid was evaluated experimentally at 1.2 GPa and 1,200°C. The grain boundary (GB) migration caused nickel enrichment in the area swept by the GBs in a fashion similar to that reported for stable isotope exchange in the quartz aggregates. The progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to ~80 times that was calculated for a system having stationary GBs. The overall diffusivity of the nickel along the wet GBs and interconnected fluid networks was found to be 6.5 × 10⁻¹⁹–6.7 × 10⁻¹⁸ m³/s, which is 4–5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite. These results show that the grain growth rate is a fundamental factor in the evaluation of the time scale of chemical homogenization in the upper mantle.     

西藏罗布莎蛇绿岩中不同产出的纯橄岩及成因探讨

罗布莎蛇绿岩中的纯橄岩有三种产出情况,除了与豆荚状铬铁矿伴生的薄壳状纯橄岩外,还有产在方辉橄榄岩底部被认为是堆晶岩的厚层状纯橄岩和方辉橄榄岩中的透镜状纯橄岩。厚层状纯橄岩约700～1000m厚,以橄榄石富镁(Fo93～95),单斜辉石低铝富镁(Al2O30.47%～0.85%,Mg#95～97),铬尖晶石高铬低镁(Cr#值平均77,Mg#平均51)为特征。该纯橄岩中的浸染状铬铁矿也是高铬低镁型,但Mg#值(平均59)高于厚层状纯橄岩的副矿物铬尖晶石。薄壳状纯橄岩与厚层状纯橄岩成分相近,其橄榄石Fo92～94,单斜辉石Al2O3<1%和Mg#95～97;铬尖晶石的Cr#值平均71,Mg#值平均52。与薄壳状纯橄岩伴生的块状铬铁矿为高镁高铬型,但Mg#值(平均68)相对更高些,Cr#值平均79。透镜状纯橄岩的特征是橄榄石Fo(91～92)和铬尖晶石Cr#(60左右)均低于前两类纯橄岩,但单斜辉石的Al2O3(1.41%～1.71%)则高于前两者。透镜状纯橄岩的矿物成分与方辉橄榄岩重叠,两者为渐变过渡关系。研究对比表明,罗布莎厚层状纯橄岩不同于经典的蛇绿岩的超镁铁质堆晶岩,认为将其成因解释为拉斑玄武质熔体与地幔橄榄岩的反应较为合理。透镜状纯橄岩与方辉橄榄岩存在成生联系,可能是地幔橄榄岩高度部分熔融的产物,或熔体和方辉橄榄岩在原位发生反应的产物;薄壳状纯橄岩成因与厚层状纯橄岩相同,但与其相伴的块状铬铁矿是否由拉斑玄武质熔体与方辉橄榄岩反应形成,值得商榷。     

Beryl-II,a high-pressure phase of beryl: Raman and luminescence spectroscopy to 16.4 GPa

The Raman and Cr³⁺ and V²⁺ luminescence spectra of beryl and emerald have been characterized up to 15.0 and 16.4 GPa, respectively. The Raman spectra show that an E _1g symmetry mode at 138 cm^?1 shifts negatively by ?4.57 (±0.55) cm^?1/GPa, and an extrapolation of the pressure dependence of this mode indicates that a soft-mode transition should occur near 12 GPa. Such a transition is in accord with prior theoretical results. Dramatic changes in Raman mode intensities and positions occur between 11.2 and 15.0 GPa. These changes are indicative of a phase transition that primarily involves tilting and mild distortion of the Si₆O₁₈ rings. New Raman modes are not observed in the high-pressure phase, which indicates that the local bonding environment is not altered dramatically across the transition (e.g., changes in coordination do not occur). Both sharp line and broadband luminescence are observed for both Cr³⁺ and V²⁺ in emerald under compression to 16.4 GPa. The R-lines of both Cr³⁺ and V²⁺ shift to lower energy (longer wavelength) under compression. Both R-lines of Cr³⁺ split at ~13.7 GPa, and the V²⁺ R₁ slope changes at this pressure and shifts more rapidly up to ~16.4 GPa. The Cr³⁺ R-line splitting and FWHM show more complex behavior, but also shift in behavior at ~13.7 GPa. These changes in the pressure dependency of the Cr³⁺ and V²⁺ R-lines and the changes in R-line splitting and FWHM at ~13.7 GPa further demonstrate that a phase transition occurs at this pressure, in good agreement with our Raman results. The high-pressure phase of beryl appears to have two Al sites that become more regular under compression. Hysteresis is not observed in our Raman or luminescence spectra on decompression, suggesting that this transition is second order in nature: The occurrence of a second-order transition near this pressure is also in accord with prior theoretical results. We speculate that the high-pressure phase (beryl-II) might be a mildly modulated structure, and/or that extensive twinning occurs across this transition.     

Compression moduli of Cr3+-centered octahedra in a variety of oxygen-based rock-forming minerals

High-pressure electronic absorption spectra at room temperature and at pressures 10^?4<P[GPa]<8 were measured in the spectral range 380<λ[nm] <780(26218>ν?[cm^?1]>12820) on analysed single crystal slabs, about 20?μm thick, of Cr³⁺-bearing spinel (I), kyanite (II), corundum (III), pyrope (IV) and uvarovite (V) using DAC-cell techniques in combination with single-beam microscopespectrometry. Ligand field theoretical evaluation of the spectra yielded following results: (i)?the octahedral crystal field parameter, 10Dq_Cr3+[6], linearly shifts on increasing pressure to higher energies with slopes, (δ10Dq_Cr3+[6]/δP), of 103.1 (I), 99.5 (II), 104.0 (III), 111.7 (IV) and 110.3?[cm^?1/GPa] (V) (reliability parameters r≥0.92), (ii)?The Racah-parameter B_Cr3+[6], reflecting the covalency of the Cr–O bonds, does not significantly change with pressure up to 8?GPa, in those cases where it could be evaluated from the spectra (III, IV, V). This result is contrary to the behaviour of B_Cr3+[6] with increasing temperature (Taran et?al. 1994) and shows that P and T are not inversely correlated parameters with respect to B_Cr3+[6], a decrease of which reflects an increase in covalency. (iii)?This result enabled to extract octahedral compression moduli, k_Cr3+[6], from the pressure slopes of 10Dq_Cr3+[6]: 312+48 (I), 297+70 (II), 298+44 (III), 275+35 (IV), 257+32?GPa (V). Quotients k_cr3+[6]/k_bulk,phase are nearly the same (ca. 1.6) for I, II, IV and V but significantly lower (ca. 1.1) for III. Deviations between spectroscopically determined k_Cr3+[6] and published k_Al[6], obtained by HP-XRD on ruby and pyrope, are interpreted by lattice strain induced by [Cr³⁺,?Al3+?1]^[6] substitution.     

Cumulates of arc magmas incorporated into the Sulu UHP metamorphic belt,eastern China

ABSTRACT

The Hujialin ultramafic complex in the central region of the Sulu ultra-high pressure (UHP) metamorphic belt consists of discontinuous lenses of garnet-bearing clinopyroxenite and dunite surrounded by marginal serpentinite. The clinopyroxenite shows relatively low concentrations of compatible elements, such as Cr (≤1670 ppm) and Ni (≤514 ppm) and Ir-group platinum group elements (IPGE; Ir, Os, and Ru; ≤4.8 ppb in total). They show varying ratios (0.02–2.50) of IPGE to Pd-group PGE (PPGE). Their chondrite-normalized rare earth elements (REE) patterns are convex and the total REE concentrations range from 18 to 63 times that of Cl chondrite. The bulk rocks show a ‘subduction-related’ geochemical signature, with high concentrations of fluid-mobile elements (i.e. Sr, Ba) relative to high-field strength elements (i.e. Nb, Y, Zr). Clinopyroxene is diopside and contains low Al₂O₃ (<2.76 wt.%) and high SiO₂ (54.6–56.9 wt.%). Olivine grains enclosed by clinopyroxene and in the matrix show relatively low Fo (76.6–80.7) and NiO contents (0.18–0.29 wt.%). The bulk rock compositions and mineral chemistry of olivine and clinopyroxene suggest that the unit was a cumulate of a subduction-related melt. On the other hand, dunite and its hydration product, serpentinite, have a different origin. The bulk rock and mineral chemistry suggest that dunite represents a mantle wedge peridotite in a spinel-stable field. Both clinopyroxenite and spinel-bearing dunite were once located in the mantle wedge below the southern margin of the North China craton (NCC), and were dragged by a mantle flow into the continental subduction channel along the interface between the subducting Yangtze craton (YZC) and the overlying NCC. Although clinopyroxenite and dunite are dense (2.8–3.2 g/cm³), the buoyancy-driven exhumation of voluminous granitic rocks of the YZC likely brought clinopyroxenite and dunite to shallow crustal depths. The lack of the evidence for high pressure to ultra-high pressure (HP-UHP) metamorphism in spinel-bearing dunite may be explained by overall low Al and Ca in the bulk rocks. Alternatively, dunite was not subducted to deep levels, but exhumed together with the deeply subducted clinopyroxenites and granite during their exhumation.     

Antigorite equation of state and anomalous softening at 6 GPa: an in situ single-crystal X-ray diffraction study

The compressibility of antigorite has been determined up to 8.826(8) GPa, for the first time by single crystal X-ray diffraction in a diamond anvil cell, on a specimen from Cerro del Almirez. Fifteen pressure–volume data, up to 5.910(6) GPa, have been fit by a third-order Birch–Murnaghan equation of state, yielding V ₀ = 2,914.07(23) Å³, K _T0 = 62.9(4) GPa, with K′ = 6.1(2). The compression of antigorite is very anisotropic with axial compressibilities in the ratio 1.11:1.00:3.22 along a, b and c, respectively. The new equation of state leads to an estimation of the upper stability limit of antigorite that is intermediate with respect to existing values, and in better agreement with experiments. At pressures in excess of 6 GPa antigorite displays a significant volume softening that may be relevant for very cold subducting slabs.     

Petrostructural evolution of ultramafic rocks of the Kalninsky chromite-bearing massif, Western Sayan

The Kalininsky ultramafic massif is a fragment of lower structural zone of the Kurtushiba ophiolitic belt in the extreme northeastern part of the Western Sayan. The massif is composed largely of rocks making up the dunite-garzburgite banded complex. The northeastern part of the massif is composed mainly of dunite with linear NW-trending chromite-bearing zones, the localization of which is controlled by banding of the dunite-harzburgite complex. Harzburgite and dunite are characterized by inhomogeneous structures and textures caused by nonuniform ductile deformation, which is expressed as heterogeneous extinction, kink bands, and syntectonic and annealing recrystallization. The petrostructural patterns of olivine in harzburgite and dunite provide evidence for three stages of ductile deformation. At the first stage under deep mantle-crustal conditions, the ductile flow of ultramafic rocks developed mainly in a regime of axial compression, high temperature (>1000°C), and low strain rate (? < 10^?6 s^?1), which resulted in translational gliding along the (010)[100] and (100)[001] systems in olivine and enstatite, respectively, in combination with a subordinate role of syntectonic recrystallization. Consequently, the rocks acquired a medium-grained (mesogranular) microstructure. At the second stage, related to the thermal effect on ultramafics, the ductile flow developed under the settings of low strain rate (? < 10^?6 s^?1) and rising temperature (>1000°C). The translational gliding in olivine proceeded largely along (010)[100] and was accompanied by diffusion creep. As the temperature rose, ductile deformation gave way to secondary recrystallization of annealing, which facilitated the growth of olivine grains free of dislocations owing to absorption of individual grains oriented adversely relative to the compression axis and deformed grains saturated with dislocations. As a result, dunite and harzburgite with a coarse-grained porphyroblastic microstructure have been formed. The third stage of ductile flow was apparently related to their transport along deep-seated thrust faults under settings of intense shear deformations at a high temperature (～1000°C) and strain rate (? >10^?4 s^?1). The ductile flow in olivine resulted in heterogeneous translational gliding along (010)[100] and accompanied by intense syntectonic recrystallization with the formation of a porphyroblastic microstructure. Chromite mineralization in dunite is controlled by internal banding. Intense ductile flow facilitated the metamorphic separation of linearbanded Cr-spinel segregations. Thus, the results of a petrostructural study show that ultramafic rocks of the Kalninsky massif, ascending to the upper lithosphere, underwent both axial and shear ductile deformations in the mantle and lower crust, and these deformations controlled chromite mineralization.     

Low-temperature evolution of OH bands in synthetic forsterite, implication for the nature of H defects at high pressure

We performed in situ infrared spectroscopic measurements of OH bands in a forsterite single crystal between ?194 and 200 °C. The crystal was synthesized at 2 GPa from a cooling experiment performed between 1,400 and 1,275 °C at a rate of 1 °C per hour under high silica-activity conditions. Twenty-four individual bands were identified at low temperature. Three different groups can be distinguished: (1) Most of the OH bands between 3,300 and 3,650 cm^?1 display a small frequency lowering (<4 cm^?1) and a moderate broadening (<10 cm^?1) as temperature is increased from ?194 to 200 °C. The behaviour of these bands is compatible with weakly H-bonded OH groups associated with hydrogen substitution into silicon tetrahedra; (2) In the same frequency range, two bands at 3,617 and 3,566 cm^?1 display a significantly anharmonic behaviour with stronger frequency lowering (42 and 27 cm^?1 respectively) and broadening (~30 cm^?1) with increasing temperature. It is tentatively proposed that the defects responsible for these OH bands correspond to H atoms in interstitial position; (3) In the frequency region between 3,300 and 3,000 cm^?1, three broad bands are identified at 3,151, 3,178 and 3,217 cm^?1, at ?194 °C. They exhibit significant frequency increase (~20 cm^?1) and broadening (~70 cm^?1) with increasing temperature, indicating moderate H bonding. These bands are compatible with (2H)_Mg defects. A survey of published spectra of forsterite samples synthesized above 5 GPa shows that about 75 % of the incorporated hydrogen belongs to type (1) OH bands associated with Si substitution and 25 % to the broad band at 3,566 cm^?1 (type (2); 3,550 cm^?1 at room temperature). The contribution of OH bands of type (3), associated to (2H)_Mg defects, is negligible. Therefore, solubility of hydrogen in forsterite (and natural olivine compositions) cannot be described by a single solubility law, but by the combination of at least two laws, with different activation volumes and water fugacity exponents.     

Equations of state of magnesium silicates anhydrous B and superhydrous B

High-pressure single crystal X-ray diffraction experiments of phase anhydrous B and superhydrous B have been carried out to 7.3 and 7.7?GPa, respectively, at room temperature. Fitting a third-order Birch-Murnaghan equation of state to the P-V data yields values of V ₀?=?838.86?±?0.04?ų, K_T,0?=?151.5?±?0.9?GPa and K′?=?5.5?±?0.3 for Anhy-B and V ₀?=?624.71?± 0.03?ų, K_T,0?=?142.6?±?0.8?GPa and K′?=?5.8?±?0.2 for Shy-B. A similar analysis of the axial compressibilities in Anhy-B reveals that the c-axis is most compressible (K_c?=?137?±?3?GPa), the b-axis is least compressible (K_b?=?175?±?4?GPa), and the a-axis is intermediate (K_a?=?148?±?1?GPa). In Shy-B, the a-axis is most compressible (K_a?=?135?±?1?GPa), followed by the b- and c-axes which have similar compressibilities (K_b?=?146?±?3?GPa; K_c?=?148?±?3?GPa). The fact that the b-axis of Shy-B is approximately 16% more compressible than Anhy-B is primarily due to differences in the O-T layer in which the H atoms are located and the linkages with the adjacent O layers. The rigid edge-sharing chains of MgO₆ and SiO₆ octahedra in the O layer control compressibility along the a- and c-axes in both structures. The net result is a reduction in the overall anisotropic compression from ～22% in Anhy-B to ～9% in Shy-B.     

Diffusion and solubility of hydrogen and water in periclase

Cylinders of synthetic periclase single crystals were annealed at 0.15–0.5 GPa and 900–1200 °C under water-saturated conditions for 45 min to 72 h. Infrared spectra measured on the quenched products show bands at 3,297 and 3,312 cm^?1 indicating V _OH ^? centers (OH-defect stretching vibrations in a half-compensated cation vacancy) in the MgO structure as a result of proton diffusion into the crystal. For completely equilibrated specimens, the OH-defect concentration, expressed as H₂O equivalent, was calculated to 3.5 wt ppm H₂O at 1,200 °C and 0.5 GPa based on the calibration method of Libowitzky and Rossmann (Am Min 82:1111–1115, 1997). This value was confirmed via Raman spectroscopy, which shows OH-defect-related bands at identical wavenumbers and yields an H₂O equivalent concentration of about 9 wt ppm using the quantification scheme of Thomas et al. (Am Min 93:1550–1557, 2008), revised by Mrosko et al. (Am Mineral 96:1748–1759, 2011). Results of both independent methods give an overall OH-defect concentration range of 3.5–9 (+4.5/?2.6) ppm H₂O. Proton diffusion follows an Arrhenius law with an activation energy E _a = 280 ± 64 kJ mol^?1 and the logarithm of the pre-exponential factor logD_o (m² s^?1) = ?2.4 ± 1.9. IR spectra taken close to the rims of MgO crystals that were exposed to water-saturated conditions at 1,200 °C and 0.5 GPa for 24 h show an additional band at 3,697 cm^?1, which is related to brucite precipitates. This may be explained by diffusion of molecular water into the periclase, and its reaction with the host crystal during quenching. Diffusion of molecular water may be described by logD_H2O (m² s^?1) = ?14.1 ± 0.4 (2σ) at 1,200 °C and 0.5 GPa, which is ~ 2 orders of magnitude slower than proton diffusion at identical P-T conditions.     

Measurements of hillslope debris flow impact pressure on obstacles

We present measurements of hillslope debris flow impact pressures on small obstacles. Two impact sensors have been installed in a real-scale experimental site where 50?m³ of water-saturated soil material are released from rest. Impact velocities vary between 2 and 13?m/s; flow heights between 0.3 and 1.0?m. The maximum impact pressures measured over 15 events represent between 2 and 50 times the equivalent static pressures. The measurements reveal that quadratic velocity-dependent formulas can be used to estimate impact pressures. Impact coefficients C are constant from front to tail and range between 0.4?<?C?<?0.8 according to the individual events. The pressure fluctuations to depend on the sensor size and are between 20% and 60% of the mean pressure values. Our results suggest that hazard guidelines for hillslope debris flows should be based on quadratic velocity-dependent formulas.     

Miocene to recent alkaline volcanism between Al Haruj and Waw an Namous (southern Libya)

Unspiked K–Ar ages, petrological, geochemical and isotopic data are reported on samples from southern Libya (Wan an Namous—Al Haruj area). The Wan an Namous intracaldera cone dated at 0.2?Ma consists of unusually undersaturated foidite, representing the most extreme compositions among Libyan and Tibestian lavas. A basanitic and a basaltic lava flow located north-west of Wan an Namous, and probably belonging to the Al Haruj volcanic field, were dated at 5.1?Ma and 8.1?Ma. These data extend the range of ages previously reported for Al Haruj lavas. REE and multi-element patterns are typical of alkaline intraplate magmas. Sr ratios range from 0.70314 to 0.70812, whereas Nd ratios are very homogeneous (0.51290–0.51293). Pb ratios (19.231?<?²⁰⁶Pb/²⁰⁴Pb?<?19.547, 15.607?<?²⁰⁷Pb/²⁰⁴Pb?<?15.641 and 38.859?<?²⁰⁸Pb/²⁰⁴Pb?<?39.242) are typical of HIMU-FOZO compositions. Such isotope characteristics are very similar to those available on two Gharyan (northern Libya) lavas and largely overlap those of Hoggar and Cameroon Line alkaline rocks. These lavas were produced by low and variable degrees of partial melting of a garnet- and amphibole-bearing mantle source, constraining the depth of melting between 80 and 150?km. Crustal contamination was also probably involved for the oldest sample. Mineral compositions of a dunite–harzburgite xenolith clearly indicate that the lithospheric mantle was affected by partial melting and metasomatic processes by magmatic liquids, probably associated with the genesis of Cenozoic lavas. Lithospheric delamination and asthenospheric upwelling, due to the reactivation of lithospheric megastructures induced by the Africa-Europe convergence, could represent a model for the genesis of Libyan lavas, as in Hoggar.     

Structural and vibrational behaviour of fluorapatite with pressure. Part I: in situ single-crystal X-ray diffraction investigation

Using single-crystal X-ray diffraction from a diamond anvil cell, the compressibility of a synthetic fluorapatite was determined up to about 7?GPa. The compression pattern was anisotropic, with greater change along a than c. Unit cell parameters varied linearly with β _a =3.32(8)?10^?3 and β _c =2.40(5)?10^?3 GPa^?1, giving a ratio β _a :β _c =1.38:1. Data fitted with a third-order Birch-Murnaghan EOS yielded a bulk modulus of K ₀=93(4)?GPa with K′=5.8(1.8). The evolution of the crystal structure of fluorapatite was analysed using data collected at room pressure, at 3.04 and 4.72?GPa. The bulk modulus of phosphate tetrahedron is about three times greater than the bulk modulus of calcium polyhedra. The values were 270(10), 100(4) and 86(3) GPa for P, Ca1 (nine-coordinated) and Ca2 (seven-coordinated) respectively. While the calcium polyhedra became more regular with pressure, the distortion of the phosphate tetrahedron remained unchanged. The size of the channel extending along the [001] direction represented the most compressible direction. The Ca2–Ca2 distance decreased from 3.982 to 3.897?Å on compression from 0.0001 to 4.72?GPa. The anisotropic compressional pattern may be understood in terms of the greater compressibility of the channel size over the polyhedral units. The reduction of the channel volume was measured by the evolution of the trigonal prism, having the Ca2–Ca2–Ca2 triangle as its base and the c lattice parameter as its height. This prism volume changed from 47.3?ų at room pressure to 44.78?ų at 4.72?GPa. Its relatively high bulk moduli, 86(3) GPa, indicated that the channel did not collapse with pressure and the apatite structure could remain stable at very high pressure.     

Asymmetric dynamics at subduction zones derived from plate kinematic constraints

The lithospheric sinking along subduction zones is part of the mantle convection. Therefore, computing the volume of lithosphere recycled within the mantle by subducting slabs quantifies the equivalent amount of mantle that should be displaced, for the mass conservation criterion. The rate of subduction is constrained by the convergence rate between upper and lower plates and the motion of the subduction hinge H that may either converge or diverge relative to the upper plate. Here, starting from the analysis of the slab hinge kinematics, we evaluate the subduction rate at 31 subduction zones worldwide, useful to compute volumes of sinking lithosphere into the mantle. Our results show that ∼190 km³/yr and ∼88 km³/yr of lithospheric slabs are currently subducting below H-divergent and H-convergent subduction zones, respectively. We also propose supporting numerical models providing asymmetric volumes of the subducted lithosphere, using the subduction rate instead of plate convergence, as boundary condition. Furthermore, H-divergent subduction zones appear to be coincident with subductions having “westward”-directed slabs, whereas H-convergent subduction zones are mostly compatible with those that have “eastward-to-northeastward”-directed slabs. On the basis of this geographical polarity, our lithospheric volume estimation gives ∼214 km³/yr and ∼88 km³/yr of subducting lithosphere, respectively. This entails that W-directed subduction zones contribute more than twice in lithospheric sinking into the mantle with respect to E-to-NE-directed ones. In accordance with the conservation of mass principle, this volumetric asymmetry in the mantle suggests a displacement of ∼120 km³/yr of mantle material from west to east, providing a constraint for global asymmetric mantle convection.     

Ductile faulting, dynamic recrystallization and grain-size-sensitive flow of olivine

Twiss (1976) has suggested that the “ductile faulting” events observed by Post (1973) during high temperature creep of dunite are due to a transition from creep by dislocation movement to a diffusion accommodated, grain-boundary sliding mechanism following a reduction in grain size by dynamic recrystallization. Similarly, Goetze (1978) has explained both ductile faulting and water weakening of dunite by transition to a “nonlinear Coble” creep mechanism. However, the fundamental assumption made by Twiss (1976) that the stress exponent, n, reduces to unity during ductile faulting events is questionable. If the stress exponent remains high, (n≥3), then a diffusion-accomodated grain-boundary sliding mechanism is excluded. “Nonlinear Coble” creep would remain a viable alternative; however, this model fails to adequately explain the water weakening phenomenon, and the available data do not constrain us to this model. Assuming that the water-weakening phenomenon can be explained by other models (e.g., Blacic, 1972), it will be shown (by analogy with the behavior of metals) that a third model, also consistent with the available data, also qualitatively explains the observations associated with ductile faulting without appeal to a transition in creep mechanisms. The model is similar to one for metals undergoing deformation by dislocation movement and recovery by dynamic recrystallization, which commonly exhibit behavior virtually identical to that observed in dunite during ductile faulting events without transition to grain-size-sensitive creep mechanisms.