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The major and trace elemental compositions of clinopyroxene from basalt were used to characterize the nature of the primitive magma and structural environment beneath the southern Okinawa Trough (SOT), which is an initial back-arc basin at a continental margin. The clinopyroxenes in the basalt were augite with variable Mg# contents (73.37–78.22). The regular variations in major oxide contents (i.e., CaO, FeO, TiO2, and Cr) with decreasing Mg# implied that the clinopyroxenes evolved from being enriched in Mg, Ca, and Cr to being enriched in Fe and Ti. The clinopyroxenes had relatively low rare earth element concentrations (7.51 × 10−6 to 12.68 × 10−6) and negative Eu anomalies (δEu = 0.67–0.95). The Kdcpx values of clinopyroxenes (0.2–0.26), which were used to examine whether the clinopyroxene was equilibrated with its host basalt, demonstrate that these clinopyroxene phenocrysts were not captured crystals but were instead produced by crystallization differentiation of the magma. The calculated clinopyroxene crystallization temperatures showed a narrow range of 990–1061°C, and their crystallization pressures ranged from 2.0 to 3.2 kbar. The geochemistry features of these clinopyroxenes indicated that the parent magma belonged to the subalkaline tholeiitic magma series and suggested that the magma experienced crystallization differentiation of olivine, plagioclase, and clinopyroxene, where the crystallization of plagioclase occurred earlier than that of clinopyroxene. Combined with geophysical data, this research on primitive magma and its crystallization differentiation from clinopyroxene indicates that the SOT is in the stage of 'seafloor spreading' and that basaltic rocks produced from tholeiitic magma represent the generation of oceanic crust. [ABSTRACT FROM AUTHOR]

Pre-edge X-ray absorption features are commonly used to derive redox states for transition metal oxides in crystals and glasses. Several calibrations for Fe2+ and Fe3+ in silicate glasses have utilized the general relationships among pre-edge peak intensity, energy, and redox state. However, absorption variations complicate those relationships in anisotropic crystals. Although absorption anisotropy at and above the energy of the rising edge adheres to the typical cos2 relationship observed in absorption spectroscopies at other energies, the anisotropy of the pre-edge is far more complicated. Prior studies focusing on pre-edge absorption anisotropy demonstrate a 1-cos4φ dependence of absorption magnitudes with rotation. Experiments presented here show that absorption magnitudes of the pre-edge vary as a function of both electric field vector orientation and wave vector direction. However, rotations around the field vector axis or wave vector axis individually result in cos2 dependence of absorption magnitudes. Rotations where both wave vector and field vector orientation are varied are not well fit by either model in the pre-edge. The resulting anisotropy complicates the process of measuring characteristic absorption in the pre-edge, making valence state determinations challenging for strongly anisotropic crystal structures such as pyroxene. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lherzolites form through ∼ 8–16 % near-fractional melting beneath the ocean ridges. • Porphyroclastic harzburgites recorded 8–12% hydrous melting of a depleted source. • Equigranular harzburgites are metasomatized by eclogite-derived melt. • The Luobusa ophiolite has experienced a two-stage tectonic evolution. The petrogenesis of the Luobusa mantle peridotites (Southern Tibet) has remained unclear. Here, a suit of the Luobusa peridotites is presented to trace their petrogenetic history. These peridotites contain protogranular lherzolites, porphyroclastic harzburgites and equigranular harzburgites, which span most compositional spectrum of global mid-ocean ridge peridotites (MORP) and supra-subduction zone peridotites (SSZP). The petrogenesis of these peridotites was linked to a two-stage mantle evolution from the MOR to the SSZ. The lherzolites are least refractory with comparable compositions to the MORP such as high whole-rock Al 2 O 3 (0.90–1.72 wt%) and low spinel Cr# (20–40), which represent mantle residues after 8–16 % near-fractional melting of a depleted MORB mantle (DMM) source beneath anhydrous MOR. The porphyroclastic harzburgites are highly depleted with an SSZP affinity indicated by extremely low whole-rock Al 2 O 3 (<0.50 wt%) and high spinel Cr# (57–65). They represent residual mantle wedge fluxed by the slab-fluids above a subduction zone after subduction initiation. Their trace element compositions of clinopyroxenes are consistent with the additional 8–12 % flux melting of a depleted harzburgite source that previously experienced ∼ 10 % depletion of an initial DMM source. The equigranular harzburgites are characterized by the enrichments of Na, Cr, Ti and MREE in amphiboles and clinopyroxenes and Ti in spinels, which might result from the metasomatism of a slab-derived eclogite melt. Therefore, the heterogeneous Luobusa mantle peridotites in this study recorded a multi-stage tectonic evolution from the MOR to the SSZ. [ABSTRACT FROM AUTHOR]

The pressure-temperature (PT) conditions and position of different groups of eclogites in the sub-cratonic lithospheric mantle (SCLM) worldwide were established using clinopyroxene Jd-Di and garnet thermobarometry. Beneath Siberia, Fe-eclogites found within the 3.0–4.0 GPa formed in Early Archean times. In the Middle and Late Archean, eclogites were melted during and after subduction. High-Mg eclogites (partial melts or arc cumulates) are related to low-T (LT) geotherms. Melt-metasomatized eclogites trace a high-temperature (HT) geotherm. Eclogitic diamond inclusions from Siberia mostly belong to the middle SCLM (MSCLM) part. Ca-rich eclogites from Precambrian Indian kimberlites are located in the MSCLM. In Phanerozoic time, they were located in the lithosphere base. In Proterozoic South Africa, Ca-rich eclogites and grospydites occur within 4.0–5.0 GPa and HT eclogite and diamond inclusions from the Premier pipe trace a HT geotherm at depths of 7.0–4.0 GPa, showing an increase in Fe upwards in the mantle section. Similar trends are common for eclogites worldwide. In the Wyoming craton, kimberlites captured eclogite xenoliths from the 4.0–2.5 GPa interval. Mantle eclogites have clinopyroxenes and garnet trace element patterns with high (La/Yb)n determined by KDs with melts and are magmatic. Flatter and bell-like REE patterns with Eu anomalies, HFSE troughs, and U and Pb peaks, are common for clinopyroxenes from MORB-type "basaltic" eclogites. High-Mg eclogites show less fractionated incompatible element branch in patterns. LILE-enrichments and HFSE troughs are typical for kyanite-bearing eclogites. Clinopyroxenes from diamond-bearing eclogites show lower REE, troughs in Nb and Zr, and peaks in Pb and U concentrations, compared to barren eclogites with round smooth trace element patterns and small depressions in Pb and Ba. [ABSTRACT FROM AUTHOR]

Solid solution compounds along the LiNaFeGeO clinopyroxene series have been prepared by solid state ceramic sintering and investigated by bulk magnetic and calorimetric methods; the Na-rich samples with x(Na) > 0.7 were also investigated by low-temperature neutron diffraction experiments in a temperature range of 4-20 K. For samples with x(Na) > 0.76 the crystal structure adopts the C2/ c symmetry at all measuring temperatures, while the samples display P2/ c symmetry for smaller Na contents. Magnetic ordering is observed for all samples below 20 K with a slight decrease of T with increasing Na content. The magnetic spin structures change distinctly as a function of chemical composition: up to x(Na) = 0.72 the magnetic structure can be described by a commensurate arrangement of magnetic spins with propagation vector k = (½, 0 0), an antiferromagnetic (AFM) coupling within the FeO octahedra zig-zag chains and an alternating AFM and ferromagnetic (FM) interaction between the chains, depending on the nature of the tetrahedral GeO chains. The magnetic structure can be described in magnetic space group P 2/ c. Close to the structural phase transition for sample with x(Na) = 0.75, magnetic ordering is observed below 15 K; however, it becomes incommensurately modulated with k = (0.344, 0, 0.063). At 4 K, the magnetic spin structure best can be described by a cycloidal arrangement within the M1 chains, the spins are within the a- c plane. Around 12 K the cycloidal structure transforms to a spin density wave (SDW) structure. For the C2/ c structures, a coexistence of a simple collinear and an incommensurately modulated structure is observed down to lowest temperatures. For 0.78 ≤ x(Na) ≤ 0.82, a collinear magnetic structure with k = (0 1 0), space group P 2/ c and an AFM spin structure within the M1 chains and an FM one between the spins is dominating, while the incommensurately modulated structure becomes dominating the collinear one in the samples with x(Na) = 0.88. Here the magnetic propagation vector is k = (0.28, 1, 0.07) and the spin structure corresponds again to a cycloidal structure within the M1 chains. As for the other samples, a transition from the cycloidal to a SDW structure is observed. Based on the neutron diffraction data, the appearance of two peaks in the heat capacity of Na-rich samples can now be interpreted as a transition from a cycloidal magnetic structure to a spin density wave structure of the magnetically ordered phase for the Na-rich part of the solid solution series. [ABSTRACT FROM AUTHOR]

The heavy minerals and the varietal characteristics of clinopyroxene grains (CPx) in alluvial deposits and other surficial formations located on the Loire-Seine interfluve show that no Loire-Seine connection has occurred since at least the Upper Pliocene. High percentage of green CPx in the NNW of Gien testifies of an old Loire terrace prior to the Pleistocene Ft terrace where brown CPx are prevalling. The rapid decreasing to the north of grenat and CPx is explained by removed Loire material and not by direct fluvial Loire transport.

Seven clinopyroxene compositions along the join M2LiM1AlTSi2O6 (spodumene) to M2LiM1Fe3+TSi2O6 (ferri-spodumene) were synthesized at 2 GPa, 800 °C under highly oxidizing conditions (using H2O2 fluid) in an end-loaded piston cylinder. In addition, the LiFe3+Si2O6 composition was also synthesized under the intrinsically reducing conditions in a piston cylinder, to check the effect of fO2 on iron speciation. The run products were characterized by field emission scanning electron microscope (FE-SEM), Rietveld refinements on XRPD synchrotron data, and space groups were assigned using SAED-TEM patterns. Run products are composed mainly of lithium clinopyroxene (Li-Cpx), plus minor amounts of hematite (magnetite under reducing condition) and corundum, as independently detected by image analysis (area%) and Rietveld refinements (wt%); moreover, Rietveld results were used to derive cell parameters, M1-site occupancy (Al vs. Fe3+), atomic positions, and average bond lengths of all these Li-Cpx indexed in the C2/c space groups according to SAED-TEM. Li-Cpx with Al and Fe3+ amounts close to 50:50 are actually slightly richer in Al apfu than nominal; the LiFe3+Si2O6 grown under very oxidized and reducing conditions have very similar cell parameters, indicating that fO2 is unable to induce a significant incorporation of Fe2+ in these Li-Cpx. The replacement of Al with Fe3+ induces a linear (%) increase of the cell edges following b > a > c, whereas β is roughly constant and the cell volume increases linearly. Furthermore, the substitution of Al with Fe3+ only weakly affects the T-O average length (<1%), whereas M2-O and M1-O bonds increase linearly of 2.3 and 5.0%, respectively. These new experimental data have been compared with other available on Li-, Na-, and Ca-Cpx, i.e., M2(Li,Na,Ca,Mg,Fe2+)M1(Mg,Fe2+Al,Ni,Cr,Ga,V,Fe3+,Mn,Sc,In)TSi2O6, to model lattice strain, bond lengths, steric effects, and phase transitions behaviors. The replacement of Al with progressively larger cations in LiM3+Si2O6 Cpx (M3+: Ni, Cr, Ga, V, Fe3+, Ti, Sc, and In) results in a linear increase following V > b > a > c, whereas β is roughly constant except for Ti-end-member and P21/c compositions. Lattice strains induced by X, T, and P for Li-Cpx in the C2/c stability field show that when the M1 site is progressively filled with a large cation, ε1 axis (ε1 > ε2 > ε3) increases along b, whereas ε2 and ε3 are nearly parallel to a and at about 30° from c. Conversely, T will provoke a similar enlargement of ε1 and ε2 along b and a edges, respectively, whereas ε3 is again oriented at about 30° from c; the increasing of P will instead shorten all strain tensor components (ε1, ε2, and ε3) with a similar percentage amount; notably, high-P is the only stress that induces a strain component to be almost parallel to c edge. Moreover, finite lattice strains and orientation in C2/c LiMe3+Si2O6 Li-Cpx induced by Me3+: Al-Fe3+, Fe3+-Sc, Sc-In are slightly different, with ε1 invariably lying along b; conversely, Li-Na cation substitution is completely different with the highest and lowest deformations on the ac plane and ε2 along b; ε3 vector is negative and oriented at about 30° from T-chains. The ideal replacement of Al with larger cations up to In in Li-Cpx induces the M1-O, M2-O, and T-O average bond lengths to increase by 10.6, 4.3, and <0.5%. Steric effects in LiM1Me3+Si2O6 and NaM1Me3+Si2O6 Cpx are significant and very similar, whereas several other Me1+ and Me2+ substitutions in Cpx at both the M1 and M2 site, keeping fixed the other site, display less or even the absence of steric effects. Our new data also better elucidate relationships between Li-Cpx composition, symmetry at room and non-ambient conditions and Tc. The aggregate cation radii at the M1 site does not exclusively control the stability of C2/c and P21/c polymorphs; instead valence electrons can profoundly favor the stabilization of a polymorph. Finally, Li can be easily and accurately detected (0.1/0.2 apfu) in natural clinopyroxenes by cell parameters, especially using the β angle. [ABSTRACT FROM AUTHOR]

Pyroxene-type compounds along the solid solution series LiFeGeO ( P2/ c space group symmetry) and NaFeGeO ( C2/ c) have been synthesized at 1273-1373 K and investigated by single-crystal X-ray diffraction at 298 K and between 105 and 298 K for LiNaFeGeO in order to study structural variations associated with the chemical- and temperature-driven P2/ c ⇔ C2/ c phase transition. Fe Mössbauer spectroscopy, differential thermal analysis, and determination of thermal expansion coefficients from high-temperature powder X-ray diffraction data complete the investigations. The C2/ c phase is stabilized with increasing Na content and temperature, and the phase boundary is found at 789 K in pure LiFeGeO, decreasing to 109 K in the compound LiNaFeGeO. From the available data, a T- X structural phase diagram is constructed. The P2/ c ⇔ C2/ c phase transition is accompanied by distinct variations in lattice parameters, rearrangements in M2 site coordination as well as changes in tetrahedral site chain kinking. Also, chemical strain and the thermal expansion tensor change in size and orientation with Li by Na replacement. The study will provide a detailed description of the observed variations in bond lengths, bond angles, and distortion parameters. [ABSTRACT FROM AUTHOR]

Petrological and geochemical studies performed with invoking data on the compositions of clinopyroxenes have clarified the conditions of formation of Vendian-Cambrian basaltic complexes in the Dzhida zone of the Paleoasian Ocean (northern Mongolia and southwestern Transbaikalia). The research was based on a comparative analysis with reference igneous basaltic associations. Of special importance are our microprobe data on trace and rare-earth elements in clinopyroxenes from igneous rocks of different present-day geodynamic settings, namely, N-MORB (Mid-Atlantic Ridge, Central Atlantic), OIB (Bouvet Island, South Atlantic), WPB (within-plate tholeiitic plateau basalts of the Siberian Platform), and boninites of ensimatic arcs (Izu-Bonin island arc, Pacific). The studies have shown that the paleo-oceanic structures in the district of the Urgol guyot formed during geodynamic processes under the impact of mantle plumes on oceanic spreading crust, which resulted in oceanic basaltic plateaus and within-plate oceanic islands. All these structures were later superposed by typical island-arc structure-lithologic associations. Formation of basalt complexes in the Dzhidot guyot district proceeded with a stronger effect of enriched plume melts of within-plate oceanic islands as compared with the Urgol guyot. This is evidenced from petrochemical and geochemical data showing the development of OIB-type magmatic systems on the oceanic basement. Data on clinopyroxenes confirm the participation of mantle plume in this process, which led to the evolution of magmas from typical oceanic basalts (MORB) to plateau basalts and OIB. [ABSTRACT FROM AUTHOR]

Abstract: Regularities of the mantle structure beneath the Siberian Craton were determined using the monomineral thermobarometry and common Opx-Gar methods. Samples were taken from 80 pipes from the Siberian Craton and in comparison 70 pipes from worldwide kimberlites. The largest pipes contain several dunite layers in the lower part of lithospheric mantle which are responsible for the diamond grade. The lithospheric mantle consists of two major parts divided at a depth of 4.0GPa by a pyroxenite layer. Major intervals determined for the mantle beneath Udachnaya and Mir are: 1) 8.0–6.5GPa harzburgites, eclogites and dunitic veins; 2) 6.5–5.5GPa sheared peridotites, low-Cr pyroxenites, dunites; 3)in 5.5–4.0GPa interval there are 4–6 layers of harzburgitic paleoslabs; 4) 4.0–3.5GPa the pyroxenites lens; 5) upper layered Sp-Gar peridotite sequence including a trap of basaltic and other silicate melt cumulates at 3.0–2.0GPa. The lithospheric mantle beneath seven different tectonic terrains in Siberia is characterized by TRE geochemistry and major elements of peridotitic clinopyroxenes. The mantle in Magan terrain contains more fertile peridotites in the South (Mir pipe) than in North (Alakit) which were metasomatized by subduction-related melts producing Phl and Cpx about 500–800Ma ago. Daldyn terrain is essentially harzburgitic in the west part (abyssal peridotite) but in the east in Upper Muna (East Daldyn terrain) the mantle is more differentiated and in general more oxidized. The Markha terrain (Nakyn) contains depleted but partly refertilized harzburgites, subducted pelitic material and abundant eclogites. Circum-Anabar mantle is ultradepleted in the lower part but in the upper regions it has been fertilized by fluid-rich melts very enriched in incompatible elements. The P-Fe# diagrams (and other components) reveal different structure of mantle columns in each terrain. They are subvertical for the mantle sampled by Devonian pipes. Beneath Mesozoic pipes the mantle has been affected by melt percolation caused by the Siberian Superplume which created continuous Fe-enrichment in the upper part. The models of continent growth and evolution are briefly discussed. In general the geothermal regime and mantle heating is negatively correlated with the thickness of lithosphere. The sheared peridotites under Udachnaya and other kimberlite pipe are likely to have formed after the intrusion of protokimberlite volatile rich (hydrous) melts and hydraulic fracturing. This mechanism is responsible for the origin of asthenospheric lenses. Progressive melting especially in the pervasive zones may be responsible for the creation of 3-4 upper asthenospheric lens near mostly before 4.0GPa which may be accompanied by mantle diapirism. Such a lens is the trap for the kimberlites in Siberia in Mesozoic time and in rifted intracontinental areas and margins. [Copyright &y& Elsevier]

Abstract: A natural crystal with NaMnSi2O6 composition (space group C2/c) belonging to the family of clinopyroxene structures was investigated by in-situ high-pressure single-crystal X-ray diffraction up to 9 GPa in order to verify if the compressibility can be predicted from the unit-cell volume V 0 at 1 bar applying an established volume–compressibility relationship. Together with data for isostructural analogue phases the equation-of-state parameters of NaMnSi2O6 showed that such simplified relationship can not be longer applied. The findings demonstrate that prediction models for isostructural series are consistent not only for the aspect of structural isotopy, and valid relationships presuppose similar character of the electronic structure. In case of pyroxenes containing transition elements the established relationship is only valid for series of compounds with cations of identical oxidation state according to isovalent electronic character. [Copyright &y& Elsevier]

Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSiO and LiCrSiO clinopyroxenes, evidences as vivid similarities, as noticeable differences. The similarities reflect the fact that in all cases Cr enters the small octahedral M1-site of the clinopyroxene structure. The differences are due to some iron content in the natural samples causing broad intense near infrared bands of electronic spin-allowed dd transitions of Fe(M1, M2) and intervalence Fe/Fe charge-transfer transition, and by different symmetry and different local crystal fields strength of Cr in the crystal structures. The positions of the spin-allowed bands of Cr, especially of the low energy one caused by the electronic A → T transition, are found to be in accordance with mean M1-O distances. The local relaxation parameter ε calculated for lim → 0 from the spectra and interatomic $$ \left\langle {Cr - O} \right\rangle $$ and $$ \left\langle {Mg - O} \right\rangle $$ distances yields a very high value, 0.96, indicating that in the clinopyroxene structure the local lattice relaxation around the 'guest' ion, Cr, deviates greatly from the 'diffraction' value, ε = 0, than in any other known Cr-bearing systems studied so far. Under pressure the spin-allowed bands of Cr shift to higher energies and decrease in intensity quite in accordance with the crystal field theoretical expectations, while the spin-forbidden absorption lines remain practically unshifted, but also undergo a strong weakening. There is no evident dependence of the Racah parameter B of Cr reflecting the covalence of the oxygen-chromium bond under pressure: within the uncertainty of determination it may be regarded as practically constant. The values of CrO octahedral modulus, $$ k_{\text{poly}}^{\text{loc}} $$, derived from high-pressure spectra of natural chromium diopside and synthetic NaCrSiO kosmochlor are very close, ~203 and ~196 GPa, respectively, being, however, nearly twice higher than that of MgO octahedron in diopside, 105(4) GPa, obtained by Thompson and Downs (). Such a strong stiffening of the structural octahedron, i.e. twice higher value of $$ k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} $$ comparing with that of $$ k_{{{\text{Mg}}^{2 + } }}^{\text{loc}} $$, may be caused by simultaneous substitution of Ca by larger Na in the neighboring M2 sites at so-called jadeite-coupled substitution Mg + Ca → Cr + Na. It is also remarkable that the values of CrO octahedral modulus of NaCrSiO kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high-pressure X-ray structural refinement by Origlieri et al. (). Taking into account that the overall compressibility of the clinopyroxene structure should mainly be due to the compressibility of M1- and M2-sites, our $$ k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} $$-value, ~196 GPa, looks much more consistent with the bulk modulus value, 134(1) GPa. [ABSTRACT FROM AUTHOR]

The core structures of dislocations in diopside have been calculated within the Peierls model, which assumes a planar core. 1/2<110> dislocations can dissociate into two collinear partial dislocations. We show that [001] glide is very difficult in (010) and that a non-collinear dissociation of [001](100) (modelled within a Peierls-Nabarro-Galerkin approach) makes glide equally easy in (100) and {110}. A widely spread core structure corresponding to a low lattice friction has been found for [100](010) and [010](100) dislocations which is not supported by mechanical data and, together with TEM observations, suggests that another, probably non-planar core structure is possible for these dislocations. [ABSTRACT FROM AUTHOR]

Abstract: The metaperidotites found in the Happo-O''ne region of central Japan are mostly lherzolites to harzburgites with subordinate dunites. Their protoliths originated as a series of refractory residues by near-fractional melting, 15–25% melting in the spinel field, which is confirmed by a HREE melting model of the apparently primary clinopyroxene. These metaperidotites display U-shaped primitive mantle (PM)-normalized REE patterns (0.02–0.5 times PM) and are highly enriched in LILE (0.2–20 times PM) relative to HFSE (<0.2 times PM), providing evidence for mantle-wedge metasomatism. In-situ analyses confirmed that clinopyroxenes and tremolite are enriched in fluid-mobile elements (B, Sr, Pb, Li, Cs, Ba and Rb; 0.1–100 times PM) coupled with depletion of HFSE (Ta, Hf, Th, Zr, Ti and Nb; <0.7 times PM)+U; these chemical features of clinopyroxenes and tremolite are similar to those of their whole rocks, and reflect slab-fluid metasomatism. We recognized three stages of clinopyroxenes that are different in morphology and REE patterns, but show the same major-element chemistry. Nearly euhedral clinopyroxene (Cpx1) displays a spoon-shaped REE pattern (0.2–3 times PM), reflecting the first stage (stage 1) of metasomatism. Acicular clinopyroxene (Cpx2) after tremolite+olivine shows a U-shaped REE pattern (0.06–1 times PM) (stage 2), whereas fine-grained clinopyroxene (Cpx3) derived from orthopyroxene shows very low concentrations of REE (stage 3). These three successive stages of retrogressive clinopyroxene formation possibly denote a retrogressive chemical change of the involved fluids during multi-stage metasomatism. The fluid of stage 1 is highly enriched in LREE (0.2–2 times PM), MREE, Pb, Sr, Li and Rb relative to the stage-3 fluid that is very low in MREE, Pb, LREE and Sr, but has high Na, Mn, Ba, B and Cs. The stage-2 fluid, which is low in LREE (0.06–1 times PM), Li and Pb, and very low in Cs, Rb and Hf relative to the fluid of stage 1, is considered to be a transitional fluid between stages 1 and 3. The retrogressive change of the fluid composition is possibly equivalent to a transversal change of the slab-derived fluid within the mantle wedge due to a change of metamorphic condition, depth of the subducting slab and an episode of metasomatism during the exhumation of the Happo-O''ne peridotites. [Copyright &y& Elsevier]

This work presents new binary and ternary diagrams based on the chemical composition of volcanic clinopyroxenes that distinguish the tectonic environments in which their host rocks were formed. The present work identifies the following tectonic environments: intraplate calc-alkaline volcanism (IP), alkaline volcanism in a thinned continental crust (CI), alkaline volcanism in islands over an oceanic crust (OI), calc-alkaline volcanism in subduction zones (SU), calc-alkaline volcanism originating in crustal anatexis (AX), and ulrapotassic volcanism (UP). The SU field was subdivided into andesitic (AN) and trachyandesitic (TA) compositions. [ABSTRACT FROM AUTHOR]

Abstract: This work focuses on petro-chemical characterisation of the volcanic inclusions often found within the fabric of many ceramic remains from various archaeological sites in eastern Sicily. Petrographic observations and EMP analyses revealed that volcanic glass is strongly influenced by chemical modifications during the firing process, whereas clinopyroxenes keep their original composition unaltered thus constituting an effective tool for provenance attributions. Multivariate statistical analysis of the latter can in fact unambiguously discriminate between different productive areas by comparisons with pyroxenes from volcanics belonging to distinct magmatic provinces. The method proposed here was tested on ceramic remains of different ages and typologies found in southern Italy. [Copyright &y& Elsevier]

Abstract: Steady-state deformation experiments were carried out in a deformation-DIA (D-DIA) high-pressure apparatus on oriented diopside single crystals, at pressure (P) ranging from 3.8 to 8.8GPa, temperature (T) from 1100 to 1400°C, and differential stress (σ) between 0.2 and 1.7GPa. Three compression directions were chosen in order to test the activity of diopside dislocation slip systems, i.e., ½<110>{} systems activated together, both [100](010) and [010](100) systems together, or [001] dislocation slip activated in (100), (010) and {110} planes. Constant applied stress and specimen strain rates () were monitored in situ using time-resolved synchrotron X-ray diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products revealed that dislocation creep was responsible for sample deformation. Comparison of the present high-P data with those obtained at room-P by Raterron and Jaoul (1991) – on similar crystals deformed at comparable T–σ conditions – allows quantifying the effect of P on ½<110>{} activity. This translates into the activation volume V*=17±6cm3/mol in the corresponding creep power law. Our data also show that both ½<110> dislocation slips and [001] have comparable slip activities at mantle P and T, while [100](010) and [010](100) slip systems remain marginal. These results show that P has a significant effect on high-T dislocation creep in diopside, the higher the pressure the harder the crystal, and that this effect is stronger on ½<110> slip than on [001] slip. [Copyright &y& Elsevier]

Ultramafic (dunite‐clinopyroxenite) complexes with platinum mineralization are common in different tectonic settings. Among them are zoned complexes of the Ural‐Alaskan type, zoned complexes of platforms (Konder massif, Siberia), as well as layered intrusion complexes (Gulin massif, Siberia). In determining the genesis of these formations, an important role belongs to the study of melt inclusions and rock‐forming minerals, including clinopyroxene. Clinopyroxenes of the ultramafic complexes exhibit a general trend of evolution, expressed as a decrease in magnesium content (Mg# = 0.95 to 0.7) and an increase in the contents of titanium and aluminum oxide (0–4.5% and 0.5–7.0%, respectively) in the dunite ‐ clinopyroxenite series. In the coordinates of Ti ‐ AL (IV) the clinopyroxenes of the Ural‐Alaskan complexes of the Platinum belt form a linear trend of compositions with a slight increase (0–0.03 f.u.) in the titanium contents. In comparison with them the clinopyroxenes of the zoned platform complexes (Konder massif) demonstrate a shift in the trend of compositions, due to an increase in the titanium content to the moderate (0.05 f.u.) values. For the layered intrusions (Gulin pluton), it is typical another trend of clinopyroxene compositions with significantly higher titanium contents (0.04–0.12 f.u.) at the similar level of aluminum. Clinopyroxene compositions of zoned complexes of the Platinum belt are "controlled" by the area of boninites and tholeites of island arcs, and the zoned complexes of platforms by the areas of island arc tholeites and sub‐alkaline intraplate basalts. Clinopyroxenes of the layered intrusions are located in the field of compositions of alkaline intraplate or MORB basalts. The geochemical study of clinopyroxenes, performed by laser ablation method (LA‐ICP‐MS) at the University of Kanazawa (Japan) and the Institute of Geology and Geochemistry of the UB RAS (Zaitseva et al., 2018), established the following features: (1) In ultrabasites of zoned complexes of the Urals and Siberia (Konder massif), clinopyroxene is characterized by an arc‐shaped distribution of rare earth elements with increasing contents from dunites to pyroxenites at a relative deficit of heavy REE (LaN/YbN = 1.2–5.4). In magnetite clinopyroxenites (enriched REE), its composition is close or identical to that in orthoclase‐bearing pyroxenites (tylaites). Negative anomalies of lead, zirconium, hafnium, and titanium, as well as positive anomalies of strontium, are typical for clinopyroxene zoned complexes. (2) In ultrabasites of layered intrusions (Gulin massif) clinopyroxene is characterized by higher level of light lanthanides with a steeper slope of the REE distribution spectra (LaN/YbN = 8–13). Lead, zirconium and hafnium anomalies are also inherent in the compositions; unlike the zoned complexes a weak negative anomaly of strontium and a positive anomaly of titanium are observed. Using the partition coefficients between clinopyroxene and the melt, the compositions of parental melts for ultramafic complexes were calculated. In general, these model melts are characterized by oblique spectra of compositions typical of island arc and intraplate environments (Fig. 1). Model melts corresponding to the Ural‐Alaskan‐type zoned complexes (Platinum belt) show low levels of REE with a noticeable deficiency of heavy lanthanides (LaN/YbN = 8–25). By their high (≥ 2) CaO/Al2O3 ratio, they are close to ankaramite type island arc melts, differing by a steeper slope of the REE distribution spectra. Model melts corresponding to the zoned platform complexes (Konder massif) demonstrate a higher level of REE content and a deficit of heavy REE (LaN/YbN = 38), with a similar distribution pattern. At a lower Ca/Al ratio (0.4–1.9), they are close to melts of the picritic type. In comparison with them, model melts corresponding to the layered intrusions (Gulin massif) demonstrate a significantly higher level of REE and a steeper slope of the distribution spectra (LaN/YbN = 67–103), typical of alkaline mechmitic melts. In general, the results of the mineralogical and geochemical study of clinopyroxenes of ultramafic complexes correlate well with the data on melt inclusions in spinels from dunites (Simonov et al., 2017). [ABSTRACT FROM AUTHOR]

A mantle xenolith suite from two Late Tertiary necks on Sal Island (Cape Verde Archipelago) consists of nearly equivalent amounts of anhydrous spinel-bearing lherzolites and harzburgites, in which secondary metasomatic textural domains are superimposed on the original protogranular textures. Detailed petrographic studies, coupled with in situ major and trace element analyses of the constituent minerals and interstitial glasses, reveal the complex evolutionary history of the Cape Verde lithospheric mantle, from depletion in the garnet facies to re-equilibration and re-enrichment in the spinel stability field. Low CaO (16·4–18·0 wt %) and heavy rare earth element (HREE; Ybn = 2·4–4·8), and high Cr2O3 (1·06–1·84 wt %) contents in the clinopyroxenes of the lherzolites can be quantitatively accounted for by (1) low-degree (∼4%) partial melting of a Primitive Mantle-like garnet lherzolite followed by (2) partial re-equilibration of the melting residuum from the garnet to the spinel stability field. This model is further supported by thermobarometric estimates (T = 975–1210°C; P = 1·3–2·1 GPa), which cluster around the spinel–garnet boundary in the peridotite system. Secondary parageneses, regardless of the primary lithologies, are characterized by (1) two clinopyroxenes, cpx2-O and cpx2-C, respectively related to orthopyroxene and clinopyroxene destabilization after reaction with metasomatic fluids, and (2) glasses with anomalously high, even for continental settings, K2O contents (up to 8·78 wt %), together with K-feldspar. Major and trace element mass balance calculations between the primary and secondary parageneses suggest infiltration of a kimberlite-like metasomatizing agent (on volatile-free basis, MgO 17–27 wt %; K2O/Na2O 1·6–3·2 molar; (K2O + Na2O)/Al2O3 1·1–3·0 molar; Rb 91–165 ppm; Zr 194–238 ppm). The kimberlite-like metasomatism in the Cape Verde lithospheric mantle, together with the presence of lherzolitic domains, partially re-equilibrated from the garnet to the spinel stability field, may suggest the presence of subcontinental mantle lithosphere relicts left behind by drifting of the African Plate during the opening of the Central Atlantic Ocean. [ABSTRACT FROM AUTHOR]

The composition of the mantle plumes that created large oceanic plateaus such as Ontong Java or the Caribbean is still poorly known. Geochemical and isotopic studies on accreted portions of the Caribbean plateau have shown that the plume source was heterogeneous and contained isotopically depleted and relatively enriched portions. A distinctive feature of samples from the Caribbean plateau is their unusual Sr isotopic compositions, which, at a given Nd isotopic ratio, are far higher than in samples from other oceanic plateaus. Sr, O and He isotopic compositions of whole rocks and magmatic minerals (clinopyroxene or olivine) separated from komatiites, gabbros and peridotites from Gorgona Island in Colombia were determined to investigate the origin of these anomalously radiogenic compositions. Sequentially leached clinopyroxenes have Sr isotopic compositions in the range 87Sr/86Sr=0.70271–0.70352, systematically lower than those of leached and unleached whole rocks. Oxygen isotopic ratios of clinopyroxene vary within the range δ18O=5.18–5.35‰, similar to that recorded in oceanic island basalts. He isotopic ratios are high (R/Ra=8–19). The lower 87Sr/86Sr ratios of most of the clinopyroxenes shift the field of the Caribbean plateau in Nd–Sr isotope diagrams toward more ‘normal’ values, i.e. a position closer to the field defined by mid-ocean ridge basalts and oceanic-island basalts. Three clinopyroxenes have slightly higher 87Sr/86Sr ratios that cannot be explained by an assimilation model. The high 87Sr/86Sr and variations of 143Nd/144Nd are interpreted as a source characteristic. Trace-element ratios, however, are controlled mainly by fractionation during partial melting. We combine these isotopic data in a heterogeneous plume source model that accounts for the diversity of isotopic signatures recorded on Gorgona Island and throughout the Caribbean plateau. The heterogeneities are related to old recycled oceanic lithosphere in the plume source; the high 3He/4He ratios may indicate that the source material once resided in the lower mantle. [Copyright &y& Elsevier]

Mantle xenoliths included in the alkaline basic lavas from the Paleogene Veneto Volcanic Province (VVP) consist of predominant spinel lherzolites (21–6% clinopyroxene) and minor spinel harzburgites (4–2% clinopyroxene), mainly protogranular textured. Most of the xenoliths show superimposed textural evidence of metasomatic processes, consisting of reaction patches and spongy clinopyroxenes, variably associated with glass and secondary olivine, clinopyroxene, spinel and feldspar. Whole-rock and mineral major and trace element data indicate a complex history of depletion and enrichment processes undergone by the continental lithospheric mantle beneath a within-plate region. Protogranular-textured clinopyroxenes from lherzolites show heavy rare earth element (HREE) contents ∼10 times chondrite and strong light REE (LREE) depletion [(La/Yb)N = 0·002–0·025)], whereas those in harzburgites vary from slightly LREE depleted to LREE enriched [(La/Yb)N = 0·64–4·00], with much lower HREE contents (∼3 times chondrite). Most HREE patterns can be reproduced by a simple fractional melting model and extraction of ∼5–22% basic melts, starting from the most fertile VVP spinel lherzolite. The anomalously low HREE contents of clinopyroxenes in a single lherzolite sample, however, require more complex processes possibly involving multistage melting of garnet- to spinel-bearing sources, or diffusion-controlled melt–peridotite interactions. The slightly LREE-depleted to LREE-enriched patterns of protogranular-textured clinopyroxenes in harzburgites and spongy clinopyroxenes in lherzolites [(La/Yb)N = 0·27–5·67] can be accounted for by metasomatic enrichment events. Major and trace element mass balance calculations were successfully carried out to quantitatively model the metasomatic parageneses by the addition of 1–6% of Na-alkaline basic melt/s. Magmatic analogues of the modelled metasomatizing agent/s occur in the South Alpine domain as late Cretaceous lamprophyric dykes, as well as VVP alkaline basic lavas. An estimate of timing of the enrichment processes was achieved by a REE inward diffusion model in primary and spongy clinopyroxenes, assuming LREE-enriched secondary clinopyroxenes as the boundary surface composition. Model calculations imply a complete chemical rehomogenization of a 1 mm crystal in a time span of 4·8–16 kyr. Consequently, the observed REE zoning of clinopyroxenes, together with the presence of glassy patches, indicates that the most recent metasomatic processes occurred shortly before the entrainment of mantle xenoliths by the host lavas. [ABSTRACT FROM PUBLISHER]

Major and trace element data on basanitic to phonolitic lavas of different ages and from different parts of Tenerife (Canary Islands), and their mafic silicates have been used to obtain more detailed information about processes taking place in crustal magma chambers associated with ocean island magmatism in Tenerife. Clinopyroxene phenocrysts in basanitic to phonolitic lavas consist of diopside-salite (referred to as Al-salite) with alternating normal and reverse zoning, and commonly contain a rounded or corroded core of homogeneous Na-rich diopside-salite (referred to as Na-salite). In general Al-salite contains lower amounts of rare earth elements (REE) and Y, and more Mg, Al, Ti, Cr, Sc and Ni than Na-salite. Variations in trace element concentrations and ratios are only weakly related to variations in mg-number. Petrographic and compositional relations among the lavas and mafic silicates are interpreted as the results of fractional crystallization in periodically refilled magma chambers (FCM processes). The FCM processes took place at temperatures of 1040-1260°C and pressures of about 0·2-0·5 GPa; that is, partly within the old oceanic crust and partly within the overlying sequence of Canary Islands lavas. FCM processes may lead to significant fractionation between incompatible trace elements whose ratios in mafic magmas are used to characterize their mantle source(s). Melts subjected to FCM processes will, furthermore, produce significantly larger masses of cumulates than melts of similar mg-number that have only been subjected to simple fractional crystallization. [ABSTRACT FROM PUBLISHER]