Spinel Lherzolite Research Articles

Melt–peridotite interaction in the shallow lithospheric mantle of the North China Craton: evidence from melt inclusions in the quartz-bearing orthopyroxene-rich websterite from Hannuoba

To better understand the origin, migration, and evolution of melts in the lithospheric mantle and their roles on the destruction of the North China Craton (NCC), we conducted a petrological and geochemical study on a quartz-bearing orthopyroxene-rich websterite xenolith from Hannuoba, the NCC, and its hosted melt and fluid inclusions. Both clinopyroxene and orthopyroxene in the xenolith contain lots of primary and secondary inclusions. High-temperature microthermometry of melt inclusions combined with Raman spectroscopy analyses of coexisting fluid inclusions shows that the entrapment temperature of the densest inclusions was ~1215°C and the pressure ~11.47 kbar, corresponding to a depth of ~38 km, i.e. within the stability of the spinel lherzolite. Intermediate pressure inclusions probably reflect progressive fluid entrapment over a range of depths during ascent, whereas the low-pressure inclusions (P < 2 kbar) may represent decrepitated primary inclusions. In situ laser-ablation ICP-MS analyses of major and trace elements on individual melt inclusions show that the compositions of these silicate melt inclusions in clinopyroxene and orthopyroxene are rich in SiO2, Al2O3, and alkalis but poor in TiO2 and strongly enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), with negative anomalies of high-field strength elements (HFSEs). These characteristics suggest that the silica-rich melts could be derived from the partial melting of subducted oceanic slab. Therefore, this kind of quartz-bearing orthopyroxene-rich websterite may be produced by interaction between the slab-derived melts with the mantle peridotite. This study provides direct evidence for the origin, migration, and evolution of melts in the lithospheric mantle, which may play an important role in the destruction of the NCC.

Geochemical and Sr–Nd–Pb isotopic compositions of lithospheric mantle: Spinel lherzolites in alkaline basalts from the northwestern Ethiopian plateau

The geochemical characteristics of mantle xenoliths (spinel lherzolites) in Quaternary alkaline basalts from the northwestern Ethiopian plateau provide new insight into the conditions of melting associated with the Afar plume. The major element compositions of the spinel lherzolites are in the range of those of the primitive mantle. The high modal content of the clinopyroxene (14–23%) indicate that the mantle xenoliths are fertile spinel lherzolites that have experienced insignificant partial melting. However, enrichment in highly incompatible elements (LREE, Ba, Pb, Th, and U) and the absence of any typical metasomatic minerals indicate that the spinel lherzolites underwent an event of later cryptic metasomatic enrichment induced by plume–related, hydrous fluid–rich silicate melts. The 87Sr/86Sr (0.70184–0.70324), 143Nd/144Nd (0.51313–0.51338), and 206Pb/ 204Pb (17.17–19.06) values of the clinopyroxene separate from the spinel lherzolites display systematic variations between those of depleted MORB mantle (DMM) and enriched compositions interpreted as related to the Afar mantle plume. The mineral compositions yield an estimated temperature and pressure in the range of 921– 973 °C and 13–17 kbar, respectively, indicating that the mantle xenoliths originated at shallow depths of 40 to 55 km above the Afar mantle plume.

Melting and metasomatism in the lithospheric mantle of NE Spain: Geochemical and Sr[sbnd]Nd isotopic characteristics

The study of major and trace element compositions of whole rocks and minerals, along with SrNd isotopes on separated clinopyroxene crystals, from mantle xenoliths in Neogene–Quaternary basaltic rocks, serves to evaluate the processes which have affected the subcontinental lithospheric mantle in the Catalan Volcanic Zone of NE Spain, and to compare it to other mantle domains in Europe. These xenoliths are mainly anhydrous spinel lherzolites and harzburgites, with rare olivine websterites. Concentrations of Al2O3, CaO, TiO2 and mildly incompatible trace elements decrease gradually from lherzolites to harzburgites. Light rare earth elements (LREE) and other incompatible trace elements display scatter vs. depletion indexes and usually increase from depleted lherzolites to much enriched harzburgites, indicating that metasomatism affected mainly the latter. Clinopyroxene rare earth element (REE) patterns show slight LREE depletion in most lherzolites and LREE–middle rare earth element (MREE) enrichment in harzburgites. The exceptions are a few clinopyroxene patterns from lherzolites with significant LREE–MREE depletion or LREE enrichment and u-shapes. The SrNd isotopic compositions of clinopyroxene also show a gradual variation from depleted MORB mantle (DMM) lherzolites to very enriched mantle (EM) harzburgites (87Sr/86Sr: 0.702486–0.709772; 143Nd/144Nd: 0.513359–0.512411). Most of these compositions match those for off-craton xenoliths. They are also similar to compositions observed in xenoliths of anhydrous spinel lherzolites and harzburgites from the neigbouring subcontinental lithospheric mantle of SE Spain and the southern part of the Massif Central, France. Fractional melting estimates from a Primitive Mantle source indicate that melt extraction was under 12% and 26% for lherzolites and harzburgites respectively. Melting took place in the spinel lherzolite field except for a few lherzolites that may have started to melt in the presence of garnet and continued in the spinel field. The formation of DMM lherzolites as refertilisation products, via percolation and reaction of N-MORB type basalts with refractory and ancient isotopically enriched harzburgites, cannot be ruled out. However, there is no definitive compositional evidence of such a process. It is suggested that harzburgites would have undergone multi-stage metasomatism. An earlier episode would have been caused by subduction-related hydrous fluids or melts related to the Variscan orogeny or an earlier orogenic episode. This is deduced from the time integrated low 143Nd/144Nd and very enriched 87Sr/86Sr values of harzburgite clinopyroxene. A later metasomatic episode is mainly cryptic and would have been related to the percolation of alkaline silicate melts and carbonatite derivatives. It would have affected lherzolites and especially harzburgites, causing opposite isotopic trends in the former compared to the latter. The SrNd isotopic compositions of the metasomatic agents would be close to that of a websterite xenolith. The age of this second metasomatism is also uncertain, since magmatic alkaline episodes have been registered since the Permo-Triassic periods.

The occurrences of Mg-hydroxycarbonates in serpentinites of the western section of the South Aegean volcanic arc (West Attica peninsula-Northeastern Argolis peninsula), Greece

In the area of West Attica Peninsula, Greece, ophiolitic rocks which outcrop mainly at the Geraneia Mountains, overlie Triassic-Jurassic limestone and covered by Plio - Pleistocene sedimentary rocks. The relatively widespread ophiolite occurrences are mainly serpentinized peridotites, considered as spinel lherzolites and dunites. In the Northeastern Argolis Peninsula several small remnants of an ophiolitic nappe outcrop which consists from serpentinized and tectonically deformed peridotites, overthrusted on the Eocene flysch. Both regions are characterized by manifestations of volcanic activity. Recently, white mineral aggregates have been discovered in several places in highly altered and serpentinized ultrabasic rocks of the above regions, with the form of irregular accumulations. The identified minerals are hydromagnesite, which predominates, and also magnesite, huntite, and minerals of pyroaurite group. For the study of the Mg-hydroxycarbonate minerals assemblages and the hosting material, PXRD mineralogical analysis, SEM investigations and SEM-EDS microprobe and chemical analyses has been performed. It is suggested that the origin of the secondary Mg-rich hydroxycarbonates was controlled by the alteration of the ophiolites due to a combining actions of intense hydrothermal activity with CO2 -rich fluids, tectonism, as well as the groundwater chemistry that is characterized by high Mg/Ca ratio and alkaline pH values.

Fluid and deformation induced metamorphic processes around Moho beneath continent collision zones: Examples from the exposed root zone of the Caledonian mountain belt, W-Norway

Abstract Exposed High Pressure (HP) and Ultra High Pressure (UHP) metamorphic terrains have been studied in order to assess the metamorphic processes and their role in changing petrophysical properties near Moho depth in continental root zones. The investigation points to a critical role of fluid and deformation in metamorphic transformation in the deep crust and upper mantle. This applies to a) formation of granulite facies areas, b) transformation of granulites to eclogites, c) retrogression of eclogite facies rocks to amphibolite and green schist facies rocks and d) the spinel lherzolite to garnet lherzolite transition. Dry rocks both feldspar bearing and ultramafic remain with their Pre-HP and UHP structures and anhydrous mineralogy preserved while reactions occur where fluid has been introduced along deformation zones. A mixture of metamorphic facies formed at widely variable times on metre to km scale will be present throughout the crust and upper mantle. Pseudotachylytes (frictional melts or ultracomminuted material) are observed in both ultramafic and feldspar bearing lithologies spatially associated with HP and UHP rocks suggesting that rock properties at Moho depth allow earthquakes. Seismicity enhances the metamorphic and metasomatic transitions through fragmentation and by opening the rock for fluid influx. Ductile eclogite facies shear zones nucleate along the brittle structures. These observations point to Moho as a rock processing zone with the following facets: 1. A metastable dry and strong lower crust and upper mantle. 2. Earthquakes and tremors result in fluid flow and HP metamorphism. 3. A pronounced weakening of the hydrated and transformed rocks allows flow of material and the development of new fabrics (LPO) in the transformed rocks. Deep tremor and earthquakes at Moho depth may record ongoing metamorphic transitions.

Melts of garnet lherzolite: experiments, models and comparison to melts of pyroxenite and carbonated lherzolite

Phase equilibrium experiments on a compositionally modified olivine leucitite from the Tibetan plateau have been carried out from 2.2 to 2.8 GPa and 1,380–1,480 °C. The experiments-produced liquids multiply saturated with spinel and garnet lherzolite phase assemblages (olivine, orthopyroxene, clinopyroxene and spinel ± garnet) under nominally anhydrous conditions. These SiO2-undersaturated liquids and published experimental data are utilized to develop a predictive model for garnet lherzolite melting of compositionally variable mantle under anhydrous conditions over the pressure range of 1.9–6 GPa. The model estimates the major element compositions of garnet-saturated melts for a range of mantle lherzolite compositions and predicts the conditions of the spinel to garnet lherzolite phase transition for natural peridotite compositions at above-solidus temperatures and pressures. We compare our predicted garnet lherzolite melts to those of pyroxenite and carbonated lherzolite and develop criteria for distinguishing among melts of these different source types. We also use the model in conjunction with a published predictive model for plagioclase and spinel lherzolite to characterize the differences in major element composition for melts in the plagioclase, spinel and garnet facies and develop tests to distinguish between melts of these three lherzolite facies based on major elements. The model is applied to understand the source materials and conditions of melting for high-K lavas erupted in the Tibetan plateau, basanite–nephelinite lavas erupted early in the evolution of Kilauea volcano, Hawaii, as well as younger tholeiitic to alkali lavas from Kilauea.

Geochemistry and petrogenesis of the 595 Ma shoshonitic Qunai monzogabbro, Jordan

The last stage in the formation of the Arabian Nubian Shield in Jordan was dominated by post-orogenic igneous activity of the ∼610–542Ma Araba Suite, including a monzogabbroic stock intruding the Saramuj Conglomerate, near the southeastern corner of the Dead Sea. The geological setting, petrography, geochemistry and geothermometry of the monzogabbro and other cogenetic varieties are used to shed light on the petrogenesis of this stock and reveal its magma source. The monzogabbro, megaporphyry dikes, and scattered syenite pockets are co-magmatic and alkaline, potassic and shoshonitic in nature. REE and trace elements patterns indicate that these magmas were produced from a mantle that had been modified by subduction-related metasomatism. The parental mafic magma could have been derived by 10% partial melting of LILE-enriched phlogopite-bearing spinel lherzolite, probably lithospheric mantle, in association with post-collisional extension. Fractional crystallization of this parental magma by olivine and pyroxene gave rise to the monzogabbroic magma.The megaporphyry dikes with their giant labradorite plagioclase megacrysts represent feeders of a voluminous volcanic activity that could have lasted for about 105 years.Thermodynamic modeling applying the MELTS software indicates crystallization of this suite in the temperature range of 1184–760°C at a pressure of 2kbars, agreeing with olivine-pyroxene, pyroxene, and two-feldspar thermometry. The modeled mineralogy and sequence of crystallization of constituent minerals using MELTS is in remarkable agreement with the observed modal mineralogy of the monzogabbro. Furthermore, a great degree of congruity exists between the modeled and observed chemistry of the major minerals with only minor discrepancies between modeled composition of biotite and olivine.

Back-arc Paleo-Tethys related blueschist from Central Iran, south of Chupanan, Isfahan Province

Late Paleozoic blueschists present good exposures in the Pateyar metamorphic complex (south of Chupanan, Isfahan Province). They are formed by metamorphism of primitive basaltic lavas. Petrography and microprobe analyses show that the studied rocks consist of glaucophane, actinolite, actinolitic hornblende, plagioclase (albite), sphene, magnetite, quartz and apatite. Secondary minerals are epidote, chlorite, pyrite, hematite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300–400°C and 7–11 kbar. Chemical signatures of the studied metamorphic rocks conclude that they retain main geochemical characteristics of the protoliths, which allow the petrochemical interpretations. Geochemical analyses of these blueschists show that they were originally tholeiitic basalts. Evident negative anomalies of Nb, Ta and Ti relative to Th, La and Ce, in the primitive mantle normalized spider-gram, reveal subduction role in their petrogenesis. The studied metavolcanics exhibit an intermediate chemistry between the N-MORB (normal mid-ocean ridge basalt) and IATB (island arc tholeitic basalt). Enrichment in LREE (light rare earth elements) and LILE (large ion lithophile elements) and relative depletion in HFSE (high field strength elements) suggest a back-arc basin paleotectonic setting for the Chupanan samples. The primitive magma of the analyzed samples possibly have been produced by 8–13% melting of a spinel lherzolite. The field and petrological data propose that the studied Paleozoic metavolcanics were formed in a back-arc basin above the northward subduction of Paleo-Tethys oceanic lithosphere in Central Iran. The chemical criteria of the LILE/HFSE ratio suggests that the subduction zone was young and immature during the volcanism.

Indicator minerals of diamond in the lamproitic diatreme, Kostomuksha region, Karelia

The mineralogy of a new lamproitic diatreme 200–250 m in diameter and 3 ga in area is studied in detail. The chemical and 3-D mineralogical analysis identify the diatreme rocks as strongly altered olivine lamproites with a large volume (50–60%) of xenoliths of strongly altered spinel (garnet) lherzolites and harzburgites-dunites. Numerous grains-xenocrysts of indicator minerals of diamond have been extracted from the heavy concentrates (the weight of the initial product is 742 g and the size is 100–500 μm) as a result of hydroseparation: (1) subcalcium (CaOav. 2.6 wt %) high-Cr (Cr2O3 av. 5.3 wt %) pyrope (50 grains); (2) chrome diopside (7 and 8 mol % of kosmochlor and jadeite components, respectively, >40 grains); (3) high-Cr chromite (Cr2O3 > 62 wt %); and (4) picroilmenite (MgO 12–13.8 wt %) and Cr-rutile (Cr2O3 1.1 wt %). Xenocrysts prove the mantle endogene (the level of garnet lherzolites) source of the magmatic center of lamproites and forecast the diamond potential of the new diatreme in the Kostomuksha ore district.

Melting of plagioclase + spinel lherzolite at low pressures (0.5 GPa): An experimental approach to the evolution of basaltic melt during mantle refertilisation at shallow depths

The presence of plagioclase+spinel lherzolites among ocean floor samples and in some ophiolite complexes invites speculation on their origin and relationships to processes of magmatism and lithosphere refertilisation beneath mid-ocean ridges. In an experimental approach to their petrogenesis, we have determined the compositions of liquids and co-existing minerals in the six phase assemblage [liquid+olivine+orthopyroxene+clinopyroxene+plagioclase+spinel] at 0.5GPa and 1100°C to 1200°C. In our experimental approach we maintained the olivine Mg# [Mg/(Mg+Fe)] close to 90 (i.e., 88.8–95.5) but varied plagioclase from anorthite to albite. The major variations in liquid compositions are related to plagioclase composition. Liquids have much lower MgO and FeO and higher SiO2 and Al2O3 than liquids in the 6-phase plagioclase+spinel lherzolite at 0.75GPa and 1GPa. Liquids are quartz-normative (silica-oversaturated) for plagioclase that are more calcic than An40 but nepheline-normative (critically silica-undersaturated) for plagioclase that are more sodic than An25. Liquid compositions are quite unlike natural MORB glasses with similar Mg# (i.e., compatible with parental magmas from lherzolitic mantle with Mg#≈90). Our study provides no support for models of MORB petrogenesis which suggest extraction of near-solidus melts from plagioclase lherzolite at low pressure. Similarly, referring to numerical models of melting volumes beneath mid-ocean ridges (Langmuir et al., 1992; McKenzie and Bickle, 1988) in which melt increments are calculated for different sites and these increments pooled to form MORB, our data argue that melts equilibrated with plagioclase±spinel lherzolite at <1GPa cannot be significant components of such ‘pooled melt’ focussed from within the melting volume. The compositions of minerals from plagioclase±spinel lherzolite at Lanzo (northern Italy; Piccardo et al., 2007) are compared with our experimental assemblages at 0.5, 0.75 and 1GPa, leading to the conclusion that the Lanzo plagioclase±spinel lherzolites equilibrated at pressures between 0.75 and 1GPa, at temperatures ~100–200°C below the solidus. Field, petrological and geochemical studies argue that the Lanzo plagioclase±spinel lherzolites are ‘refertilised’ by the reaction of residual harzburgite or lherzolite with percolating intergranular basaltic magma (Piccardo et al., 2007). The experimental study suggests that the process of refertilisation took place at depths of 25–30km. Our experimental data also define the co-variance of Na2O in coexisting plagioclase (An25 to An94) and clinopyroxene at 0.5 and 0.75GPa. From these data, the Na2O content of clinopyroxene can be used as a predictor for the co-existing plagioclase composition in the very common occurrences of partially serpentinised peridotite in which plagioclase is completely saussuritised.

The provenance of sub-cratonic mantle beneath the Limpopo Mobile Belt (South Africa)

Petrological, whole rock major element and mineral chemical analysis of mantle xenoliths from the Venetia kimberlite pipes (533Ma) in South Africa reveals an apparently stratified cratonic mantle beneath the Central Zone of the Limpopo Mobile Belt (LMB) that separates the Kaapvaal and Zimbabwe Cratons. Combined pressure–temperature (P–T) data and petrographic observations indicate that the mantle consists of an upper layer of Low-T coarse-equant garnet+spinel lherzolite (~50 to ~130km depth). This layer is underlain by a region of mixed garnet harzburgites and garnet lherzolites that are variably deformed (~130 to ~235km depth). An equilibrated geotherm did not exist at the time of kimberlite eruption (533Ma) and a localised heating event involving the introduction of asthenospheric material to the High-T lithosphere below 130km is inferred.Low-T garnet–spinel lherzolites are highly melt depleted (40% on average). In contrast, the High-T lithosphere (mostly at diamond stable conditions) consists of a mixed zone of variably sheared and melt depleted (30% on average) garnet harzburgite and mildly melt depleted (20% on average) garnet lherzolite. The chemistry of the High-T xenoliths contrasts with that of minerals included in diamond originating from the same depth. Inclusions suggest diamond crystallisation in a more melt depleted lithosphere than represented by either Low- or High-T xenoliths. High-T xenoliths are proposed to represent formerly melt depleted lithosphere, refertilised by asthenosphere-derived melts during the diapiric rise of a proto-kimberlitic melt pocket. This process is coupled to the positive temperature perturbation observed in the High-T xenoliths and may represent a common process in the lower lithosphere related to localised but intense tectono-magmatic events immediately preceding kimberlite eruption.The presence of clinopyroxene, garnet and abundant orthopyroxene in the Low-T lherzolite implies a history of melt depletion followed by metasomatic addition of Si–Al–Ca, forming high-temperature orthopyroxene from which clinopyroxene and garnet exsolved. Si enrichment is a characteristic feature of the majority of the Kaapvaal Craton to the south of the LMB but not of the Zimbabwe Craton to the north, implying a Kaapvaal origin. The provenance of the High-T lithosphere beneath the LMB is less well constrained as it is intensely modified by kimberlitic magmatism and diamond inclusion chemistry does not show significant systematic variation across the cratons. The presence of rare, mildly silica enriched high-temperature harzburgites suggests that a Kaapvaal origin for the entire lithosphere beneath the LMB is most likely.

Thermodynamic model for partial melting of peridotite by system energy minimization

AbstractWe present a new straightforward algorithm that calculates the energy minimization of a melt‐present system and incorporates newly calibrated silicate melt thermodynamic parameters. This algorithm searches for equilibrium phase assemblages, fractions, and compositions that lead to the system having a global minimum of total Gibbs free energy (G). It calculates changes in G with respect to minimal amounts of dissolution or solidification of melt and solid end‐member components using a constant bulk composition constraint. In addition, we have formulated a set of solid‐melt end‐member components and dissolution‐precipitation stoichiometry that enables the modeling of a melt‐present system. Melt thermodynamic properties are calibrated based on an ideal mixing model using ΔCp and ΔV (the differences in molar specific heat and volume between the corresponding melt and solid end‐member components, respectively), based on solid properties established during previous studies. We also describe the application of the energy minimization algorithm and thermodynamic melt parameters to melting of spinel lherzolite at 1 GPa, in a SiO2–Al2O3–FeO–Fe3O4–MgO–CaO system, including olivine, clinopyroxene, orthopyroxene, and spinel. Our calculations agree well with experimentally determined melting phase relations, and temperature and phase fraction relationships, including solidus temperatures, indicating that direct calibration of thermodynamic melt parameters at pressures and temperatures corresponding to melting conditions is a useful approach. The energy minimization algorithm and thermodynamic configuration presented here will allow the modeling of mantle melting in a variety of geodynamic settings.

Heterogeneidades do Manto Litosférico Subcontinental ao extremo Sul da Placa Sul-Americana: influência da subducção atual e interações litosfera-astenosfera sob o Campo Vulcânico de Pali Aike

O Campo Vulcânico de Pali Aike (CVPA) está localizado no extremo sul da América do Sul em uma área complexa devido à interação das placas oceânicas de Nazca, Antártica e Scotia com a placa continental Sul-Americana, e a proximidade com a zona de subducção da dorsal do Chile (Junção Tríplice do Chile). É formado por basanitos, basaltos alcalinos e rochas piroclásticas que hospedam xenólitos mantélicos compostos essencialmente por granada-espinélio peridotitos e espinélio peridotitos, com presença de pargasita e flogopita. Os xenólitos mantélicos estudados neste trabalho pertencem ao maar Laguna Timone, em Pali Aike, e são predominantemente granada-espinélio lherzolitos, espinélio lherzolitos, granada-espinélio hazburgitos e um glimerito. A paragênese mineralógica granada+espinélio+Ti-flogopita+K-pargasita, presente nas amostras, indica que as rochas provêm da zona de transição granada-espinélio peridotito no manto superior. Eventos de fusão parcial são indicados pela correlação negativa definida entre os elementos maiores contra o #Mg, bem como pelo padrão com enriquecimento das ETR leves com relação às pesados dos granada-espinélio harzburgitos, e pelo empobrecimento dos ETR pesados nos espinélio lherzolitos, submetidos a modelamento que sugere até 16% de fusão parcial (fusão em equilíbrio não modal). Os granada-espinélio lherzolitos possuem valores similares ao do Manto Primitivo, caracterizando-se como um manto “fértil” comparado às demais amostras. Todos os xenólitos possuem expressivo enriquecimento nos elementos calcófilos (W, Pb, Mo, Sn), devido ao metassomatismo pela subducção da placa de Antártica sob a placa Sul-Americana. A presença de um glimerito, de peridotitos com minerais hidratados (flogopita e pargasita) e as similaridades com peridotitos metassomatisados por líquidos astenosféricos (e.g. peridotitos do distrito de Manzaz, Argélia e peridotitos do campo vulcânico Vitim, Baikal, Sibéria) com baixas razões Ba/Nb, Ba/La e U/Nb, indicam metassomatismo por fluidos astenosféricos devido ao soerguimento da astenosfera durante a passagem da Junção Tríplice do Chile pela região de Pali Aike.

Mantle refertilization by melts of crustal-derived garnet pyroxenite: Evidence from the Ronda peridotite massif, southern Spain

Geochemical studies of primitive basalts have documented the presence of crustal-derived garnet pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet pyroxenite at ∼1.5GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al2O3, CaO, Sm/Yb and FeO⁎ at a given SiO2. Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled pyroxenite to hybridized peridotite. Garnet pyroxenite melting and melt–peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustal-derived garnet pyroxenite to extruded lavas depends on the reactivity of pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by pyroxenite–peridotite interaction, such as secondary spinel lherzolite in Ronda, may nucleate magmatic channels that remain chemically isolated from the ambient mantle and act as preferential pathways for melts with the signature of recycled crust.

A REE-in-two-pyroxene thermometer for mafic and ultramafic rocks

A REE-in-two-pyroxene thermometer for mafic and ultramafic rocks has been developed on the basis of the temperature and pyroxene composition dependent rare earth element (REE) partitioning between coexisting orthopyroxene and clinopyroxene. This trace element based two-pyroxene thermometer is built on two parameterized lattice strain models for REE partitioning between pyroxene and basaltic melt that were independently calibrated using data from pyroxene-melt partitioning experiments (Sun and Liang, 2012; Yao et al., 2012). By treating REE and Y as a group in temperature calculations, one can reduce analytical uncertainties in trace element analysis through least squares inversion of orthopyroxene–clinopyroxene partitioning data, and identify and exclude variations induced by secondary or metasomatic processes. For convenience, a simple robust regression program is provided for temperature inversion. Application of the REE-in-two-pyroxene thermometer to well-equilibrated spinel lherzolite and harzburgite xenoliths from the continental lithosphere demonstrates the internal consistency of the two pyroxene-melt REE partitioning models and the trace and major element based pyroxene thermometers: temperatures derived from the REE-in-two-pyroxene thermometer agree very well with temperatures calculated using major element based pyroxene thermometers. Applications of the REE-in-two-pyroxene thermometer to abyssal peridotites and mafic cumulates reveal a significant difference in the calculated temperatures between the REE and major element based pyroxene thermometers: temperatures derived from the REE-in-two-pyroxene thermometer are consistently higher than those derived from the major element based pyroxene thermometers. This discrepancy in temperature likely results from a difference in diffusion rate and hence closure temperature between the 2+ and 3+ cations in the pyroxenes. An independently calibrated REE-in-two-pyroxene thermometer may offer new insight into the thermal history of mafic and ultramafic rocks from the upper mantle and lower crust.

57Fe Mössbauer spectroscopy of mineral assemblages in mantle spinel lherzolites from Cenozoic alkali basalt, eastern China: Petrological applications

Mineral assemblages in spinel lherzolite xenoliths from the Cenozoic alkali basalt of eastern China were analyzed by 57Fe Mössbauer spectroscopy for Fe3+/ΣFe and distribution of Fe2+/Fe3+ in non-equivalent crystal cites. Orthopyroxene, clinopyroxene and spinel have 0.08–0.13, 0.19–0.31 and 0.13–0.23 Fe3+/ΣFe, respectively. The cation-exchange equilibrium temperatures, Fe3+ partition coefficient and oxygen fugacity of the upper mantle peridotites were calculated. The equilibrium temperatures are between 1103K and 1405K consistent with those reported elsewhere. The oxygen fugacities of spinel lherzolites calculated with olivine–orthopyroxene–spinel (ΔlogƒO2 from −1.1 to 0) and clinopyroxene–olivine–orthopyroxene oxybarometers (ΔlogƒO2 from −2.0 to 0.7) are consistent with previously reported upper mantle values.

Late Neoproterozoic volcanics and associated granitoids at Wadi Ranga, south Eastern Desert, Egypt: A transition from subduction related to intra-arc magmatism

The late Neoproterozoic of Wadi Ranga area, south Eastern Desert was characterized by extensive magmatism consisting predominantly of calc-alkaline older granitoids, Ranga volcanics and granite porphyry intrusions forming a belt of volcanic-intrusive complexes. Geological studies of the Ranga volcanics revealed two sequences of volcanic suites: the older mafic sequence composed essentially of basalt/basaltic andesite lava flows and the associated pyroclastics, while the younger felsic one represented by pitchstone porphyry. Geochemically, the older granitoids are exhibit low K-calc-alkaline characters and most likely formed in a convergent margin (arc) tectonic environment. Basalt/basaltic andesites are compositionally uniform with tholeiitic affinity. They have trace element characteristics of arc related volcanics with low Nb (0.2–0.9ppm), Zr/Nb ratios of 40–81 with an average 54 compared to 32 in average N-MORB, signify a convergent margin setting with a depleted mantle wedge. Low Mg# (45 to 60 with an average 51), Co and Ni contents may be indicative of a non-plume-influenced mantle wedge. Compositionally, these mafic volcanics are commensurate with modern intra-arc basalts. REE modeling indicates that the magma from which these mafic volcanics were formed was generated by about 17–30% partial melting of a depleted mantle that composed mainly of spinel lherzolite in an extensional marginal basin. Geochemical characters of major, trace and REE patterns of the granite and pitchstone porphyries proved their formation from the same magma source which is originally different from the magma producing the mafic volcanics. The relative enrichment in K2O and LREE of the pitchstone porphyry along with the depletion in CaO and MgO relative to granite porphyry probably resulted from crustal contamination processes.

Low oxygen fugacity dependency for the deformation of partially molten lherzolite

We present the results of an experimental investigation of the influence of oxygen fugacity on the deformation of a partially molten spinel lherzolite using a 0.1MPa gas-media creep rig under temperatures of 1160–1190°C, stresses of 4–74MPa and well-controlled oxygen fugacities of 10−7 to 10−11MPa. The partially molten spinel lherzolite was deformed by a dislocation-mediated creep process such as dislocation creep or dislocation-accommodated grain boundary sliding (DGBS) (n=3.5±0.3) as well as in the diffusion creep regime (n=1.2±0.2). The average oxygen fugacity exponent of lherzolite is 0.04±0.02, which is significantly smaller than the values measured for olivine single crystals (0.10–0.36) and dunite rocks (0.20) deformed by dislocation creep. We attribute the low oxygen fugacity exponent of partially molten lherzolite samples either to the operation of grain boundary sliding, if DGBS dominates the deformation, or to the presence of pyroxenes whose deformation has a weak or no dependency on oxygen fugacity, if dislocation creep dominates the deformation. In the latter case, the oxygen fugacity exponent decreases rapidly with decreasing volume fraction of olivine. The low oxygen fugacity exponent for our partially molten lherzolite samples implies that dislocation creep or DGBS of mantle peridotite will most likely be insensitive to variations of oxygen fugacity in the upper mantle.

Juvenile subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt

Mineral compositions, clinopyroxene trace-element and Sr–Nd–Hf isotopic compositions, whole‐rock major‐element and Platinum Group Element (PGE) contents, and Re–Os isotopes of peridotite xenoliths in the Cenozoic alkali basalts from Abaga, Inner Mongolia (China) help to constrain the nature of the subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt. The mantle xenoliths are mainly spinel lherzolites, with minor olivine websterite and spinel harzburgite. Clinopyroxene–orthopyroxene equilibration temperatures are higher than 840°C. An olivine websterite (sample A2-2) has the lowest MgO and highest Al2O3, CaO and TiO2 contents, whereas a harzburgite (sample A2-1) has the highest MgO and low Al2O3, CaO and TiO2 contents. The spinel lherzolites have major-element concentrations transitional between the harzburgites and websterites. According to the chemical characteristics and Sr–Nd–Hf isotopic compositions of clinopyroxenes, two groups of spinel lherzolites can be identified. Group 1 spinel lherzolites are moderately depleted in Light Rare Earth Elements (LREEs) and with flat Heavy Rare Earth Element (HREE) patterns, indicating a low degree of melt extraction and minimal metasomatism. They have clinopyroxenes with homogeneous 87Sr/86Sr (0.7030−0.7033), 143Nd/144Nd (0.5131–0.5132) and 176Hf/177Hf (0.2834–0.2835) ratios. They have flat chondrite-normalized PGE patterns and radiogenic 187Os/188Os ratios (0.1232–0.1256), within the range of modern convective mantle. Group 2 spinel lherzolites have low HREE contents with variable LREE concentrations, suggesting complex melt extraction and mantle metasomatism processes. They have relatively heterogeneous Sr–Nd–Hf isotopic compositions with 87Sr/86Sr of 0.7040–0.7054, εNd values of +6.1 to +12.1 and εHf values of +19.2 to +474. The Group 2 spinel lherzolites have flat chondrite-normalized PGE patterns and radiogenic 187Os/188Os ratios (0.1231–0.1252). The websterite has a magmatic origin with weakly decoupled Nd–Hf isotopes and the lowest 187Os/188Os ratio of 0.1219. The harzburgite has a 187Os/188Os ratio of 0.1280 similar to that of the modern convective mantle. All these characteristics indicate that the Group 2 spinel lherzolites represent ancient lithospheric mantle that experienced high‐degree melt extraction and subsequent extensive silicate metasomatism. Thus, it is proposed that the SCLM beneath the Abaga region of the eastern Central Asian Orogenic Belt (CAOB) is mostly juvenile with minor relicts of old depleted lithospheric mantle, which cooled from near-adiabatic to near-lithospheric temperatures to form new lithospheric mantle during the formation of the eastern CAOB.

Carbonated alkali-silicate metasomatism in the North Africa lithosphere: Evidence from Middle Atlas spinel-lherzolites, Morocco

Mantle xenoliths from Pliocene-Quaternary volcanic diatremes in the Azrou area (Middle Atlas, Morocco) include protogranular to porphyroclastic spinel lherzolites with superimposed metasomatic textures involving pyroxenes and spinel as the main reacting phases.Thermobarometric estimates on these xenoliths show pressure (P)–temperature (T) equilibrium conditions in the range 1.1–1.4 GPa and 900–1090 °C.Bulk rocks have flat heavy rare earth element (HREE) patterns (0.6–2.2 times chondrite) and are variably enriched in light REE (LREE; LaN/YbN between 1.1 and 15.6).The constituent clinopyroxenes are characterized by flat HREE distributions (5.1–11.9 times chondrite) and variable LREE enrichment with LaN/YbN from 0.4 to 25, which generally conform to the bulk rock chemistry. Trace elements characteristics of the metasomatised clinopyroxenes suggest that the metasomatasing agents were highly alkaline carbonate-rich melts such as nephelinites/melilitites or, as extreme, silico-carbonatites.Sr–Nd isotopic composition analyses carried out on clinopyroxene separates yield 87Sr/86Sr = 0.70243–0.70335, 143Nd/144Nd = 0.51273–0.51325. The helium isotope composition of olivines ranges from 6.2 to 6.8 Ra. These values, coupled with the radiogenic lead isotopic composition available in the literature, suggest that the Middle-Atlas lithospheric mantle interacted with HIMU-like metasomatic components. These are in turn related to local mantle upwellings along pre-existing tectonic lineaments – located at the northern border of the West African Craton – that were reactivated as far-field foreland reaction of the Africa-Europe collisional system.