Peridotite Matrix Research Articles

Deformation of a crystalline olivine aggregate containing two immiscible liquids: Implications for early core–mantle differentiation

Deformation-assisted segregation of metallic and sulphidic liquid from a solid peridotitic matrix is a process that may contribute to the early differentiation of small planetesimals into a metallic core and a silicate mantle. Here we present results of an experimental study using a simplified system consisting of a polycrystalline Fo90-olivine matrix containing a small percentage of iron sulphide and a synthetic primitive MORB melt, in order to investigate whether the silicate melt enhances the interconnection and segregation of FeS liquid under deformation conditions at varying strain rates. The experiments have been performed at 2 GPa, 1450 °C and strain rates between 1×10−3 s−1 to 1×10−5 s−1. Our results show that the presence of silicate melt actually hinders the migration and segregation of sulphide liquid by reducing its interconnectivity. At low to moderate strain rates the sulphide liquid pockets preserved a roundish shape, showing the liquid behavior is governed mainly by surface tension rather than by differential stress. Even at the highest strain rates, insignificant FeS segregation and interconnection were observed. On the other hand the basaltic melt was very mobile during deformation, accommodating part of the strain, which led to its segregation from the matrix at high bulk strains leaving the sulphide liquid stranded in the olivine matrix. Hence, we conclude that deformation-induced percolation of sulphide liquid does not contribute to the formation of planetary cores after the silicate solidus is overstepped. A possible early deformation enhanced core–mantle differentiation after overstepping the Fe–S solidus is not possible between the initial formation of silicate melt and the formation of a widespread magma ocean.

“Roots” of magmatic systems in the large continental igneous provinces

Large igneous provinces (LIPs) are known from theArchean and were widely abundant in the Phanerozoic, when they formed large fields of flood basalts(traps) on the continents and in the oceans [10].According to the data available, the evolution of thePhanerozoic LIPs in most cases started from the continental riftogenesis accompanied by eruptions of midalkaline Fe–Ti basalt and picrobasalt, which are replacedby tholeiitic basalt at the second stage [5, 10, 11].The appearance of a LIP results from the rise ofmantle superplumes formed at the liquid core–mantleboundary [3, 10, 14]. However, the concrete state oftheir material and the conditions of its adiabatic melting are not clear. The main source of objective information may be provided by the mantle xenoliths inbasalt and basanite, which are fragments of the material of the front part of the plume above the zone ofadiabatic melting, where the primary magmas areformed. Our study is mainly aimed at concretizationof the ideas on the structure of the front parts of mantleplumes, their material, the level of penetration into thelithosphere, and the conditions of magma generationfrom an example of mantle xenoliths in basalt of theQuaternary TellDanun Volcano, Harrat Ash Samahvolcanic plateau adiabatic, South Syria [8].STRUCTURE OF THE FRONT PART OF THE PLUMEAs is evident from the data available on the structure of Phanerozoic LIPs, melting was localized in thefront parts of secondary plumes, “protuberances” onthe surface of the mantle superplume. Most likely, theirrise resulted from the local accumulation of fluid components providing additional floatage of the material.According to the data of [7], the diffluent plumehead should have a triangular crosssectional shapewith the appearance of a chamber of adiabatic meltingin its upper part (Fig. 1). The intruding newly formedmagma mostly removes fragments of the upper, cooledmarginal part of the plume. These are mantle xenolithsobserved in alkaline (subalkaline) Fe–Ti basalt andbasanite in the modern rift systems [6]. This materialdoes not come from the melting zone, but carriesimportant information on the material of mantleplumes, its phase composition, and components participating in melting. Most populations do not containxenoliths of the lower crustal rocks, which distinguishes these rocks from kimberlite pipes [6] and mayshow that the frontal parts of the local (secondary)mantle plumes reached the base of the sialic uppercrust.MANTLE XENOLITHS IN BASALT AND BASANITE OF SOUTH SYRIAThe populations of mantle xenoliths in basalt arecharacterized by surprising sameness and are dividedinto two major types: (1) the most abundant rocks ofthe “green, or Crdiopside” series and (2) more occasional rocks of the “black, or Al–Tiaugite” series,which often form veins in rocks of the “green” series[6]. The composition of these populations practicallydoes not depend on a concrete region [6], which provides evidence for the uniform nature of melting mantle substrates across the Earth. Most likely, this is thematerial of the mantle thermochemical plumes risingfrom the liquid core–mantle boundary and formed bythe silicate mantle material, as well as by fluid components of the core [1, 3, 14]. We studied these xenolithsin the example of the abovementioned QuaternaryTellDanun Volcano [8].Xenoliths of the “green” series are characterized bythe presence of Crdiopside and a very constant highMg composition of olivine and pyroxenes (Mg# =88–92). They are mostly represented by spinel peridotite with cataclastic, rarely protogranular texturewith the composition varying from predominant spinellherzolite to depleted spinel harzburgite and occasional spinel pyroxenite (mostly websterite). The latterforms veins in the peridotite matrix and is often characterized by the presence of thin kelyphytic rims of

Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province

The great majority of continental flood basalts (CFBs) have a marked lithospheric geochemical signature, suggesting derivation from the continental lithosphere, or contamination by it. Here we present new Pb and Os isotopic data and review previously published major element, trace element, mineral chemical, and Sr and Nd isotopic data for geochemically unusual mafic and ultramafic dikes located in the Antarctic segment (Ahlmannryggen, western Dronning Maud Land) of the Karoo CFB province. Some of the dikes show evidence of minor contamination with continental crust, but the least contaminated dikes exhibit depleted mantle – like initial εNd (+9) and 187Os/188Os (0.1244–0.1251) at 180 Ma. In contrast, their initial Sr and Pb isotopic compositions (87Sr/86Sr=0.7035–0.7062, 206Pb/204Pb=18.2–18.4, 207Pb/204Pb=15.49–15.52, 208Pb/204Pb=37.7–37.9 at 180 Ma) are more enriched than expected for depleted mantle, and the major element and mineral chemical evidence indicate contribution from (recycled) pyroxenite sources. Our Sr, Nd, Pb, and Os isotopic and trace element modeling indicate mixed peridotite–pyroxenite sources that contain ∼10–30% of seawater-altered and subduction-modified MORB with a recycling age of less than 1.0 Ga entrained in a depleted Os-rich peridotite matrix. Such a source would explain the unusual combination of elevated initial 87Sr/86Sr and Pb isotopic ratios and relative depletion in LILE, U, Th, Pb and LREE, high initial εNd, and low initial 187Os/188Os. Although the sources of the dikes probably did not play a major part in the generation of the Karoo CFBs in general, different kind of recycled source components (e.g., sediment-influenced) would be more difficult to distinguish from lithospheric CFB geochemical signatures. In addition to underlying continental lithosphere, the involvement of recycled sources in causing the apparent lithospheric geochemical affinity of CFBs should thus be carefully assessed in every case.

Crystal chemistry of amphiboles: implications for oxygen fugacity and water activity in lithospheric mantle beneath Victoria Land, Antarctica

Amphibole is the hydrous metasomatic phase in spinel-bearing mantle xenoliths from Baker Rocks, Northern Victoria Land, Antarctica. It occurs in veins or in disseminated form in spinel lherzolites. Both types derive from reaction between metasomatic melts and the pristine paragenesis of the continental lithospheric mantle beneath Northern Victoria Land. To determine the effective role of water circulation during the metasomatic process and amphibole formation, six amphibole samples were fully characterized. Accurate determination of the site population and the state of dehydrogenation in each of these amphiboles was carried out using single-crystal X-ray diffraction, electron microprobe and secondary ion mass spectroscopy on the same single crystal. The Fe3+/ΣFe ratio was determined by X-ray absorption near edge spectroscopy on amphibole powder. The degree of dehydrogenation determined by SIMS is 0.870–0.994 O3(O2−) a.p.f.u., primary and ascribed to the Ti-oxy component of the amphibole, as indicated by atom site populations; post-crystallization H loss is negligible. Estimates of aH2O (0.014–0.054) were determined from the dehydration equilibrium among end-member components assuming that amphiboles are in equilibrium with the anhydrous peridotitic phases. A difference up to 58 % in determination of aH2O can be introduced if the chemical formula of the amphiboles is calculated based on 23 O a.p.f.u. without knowing the effective amount of dehydrogenation. The oxygen fugacity of the Baker Rocks amphibole-bearing mantle xenoliths calculated based upon the dissociation constant of water (by oxy-amphibole equilibrium) is between −2.52 and −1.32 log units below the fayalite–magnetite–quartz (FMQ) buffer. These results are systematically lower and in a narrow range of values relative to those obtained from anhydrous olivine–orthopyroxene–spinel equilibria (fO2 between −1.98 and −0.30 log units). A comparative evaluation of the two methods suggests that when amphibole is present in mantle peridotites, the application of oxy-amphibole equilibrium is preferred, because ol–opx–sp oxy-calibrations are not “sensitive” enough in recording the effects (if any) of amphibole in the peridotite matrix. Amphibole acts as the main H acceptor among the peridotite minerals and may prevent fluid circulation and buffer oxygen fugacity. The important conclusion of this study is that amphibole within the lithospheric mantle does not always means high water activity and oxidizing conditions.

Percolation of enriched melts during incremental open-system melting in the spinel field: A REE approach to abyssal peridotites from the Southwest Indian Ridge

The effects of melting in an open-system scenario are here explored looking to the rare earth element (REE) distribution in mantle residues. We consider a peridotite matrix equilibrated in the spinel field accounting for melt inflow during partial melting. The fertility of the source, inflowing melt composition and melt addition rate as well as the effects of varying the critical mass porosity in an incremental scenario are tested. When a relatively enriched melt enters the system, residual clinopyroxene REE normalized patterns apparently rotate around a light to intermediate REE due to concomitant increase of the light REEs and decrease of the heavy REEs. This effect is enhanced when the critical mass porosity is large with respect to the degree of melting. In these cases the system approaches batch more than fractional melting behaviour because the liquid is preferentially retained in the matrix. This geometry is suggestive of melt accumulation at depth. Four sample suites from the Southwest Indian Ridge are considered. Spinel field equilibrated clinopyroxenes in lherzolites and harzburgites show dredge-scale REE compositional trends that crosscut model fractional melting trajectories. Observed local trends correspond to rotations of the REE patterns attesting for near-batch episodes in the subridge melting history and infiltration of enriched liquids whose composition resemble that of garnet field-generated melts.

Carbonatites and primary carbonates in the Rio Apa and Amambay regions, NE Paraguay

In the Rio Apa and Amambay regions, north-eastern Paraguay (NPAA), potassic, alkaline-carbonatitic rocks (138–139Ma) predate the eruption of tholeiitic flood basalts (133±1Ma). These rocks, mainly outcropping as dykes or ring-like complexes, intrude a Cambro-Ordovician carbonate platform and Precambrian metamorphic rocks along with their Silurian and Permo-Carboniferous sediments. The main rock-types range from basanite to trachyte and trachyphonolite (and intrusive equivalents) to carbonatite, in addition to glimmeritic and pyroxenitic veins. Geological and geophysical evidence indicate that the NPAA magmatism is related to extensional tectonics, like the Early Cretaceous alkaline-carbonatitic complexes from central-eastern Paraguay (127±1Ma) and the eastern magmatic occurrences of the Paraná Basin.Oxygen and carbon isotope compositions (whole rocks and carbonates) vary from values close to the field of continental lithospheric mantle, or that of primary carbonatites, up to values typical of a hydrothermal environment. An isotope exchange model implies that the main isotope variations, ranging from a magmatic (e.g., 1200°C) to a low temperature (<400°C) environment, involved fluids with CO2/H2O ratios between 0.8 and 1.0. In particular, the OC isotopic variations, in combination with the La vs La/Yb ratios, suggest an increasingly higher level of carbonate in the silicate liquids, with increasing evolution, i.e., basanite→phonotephrite→trachyphonolite–phonolite (trachyte)→carbonatite. SrNd isotopes show that the carbonatites are mantle derived without significant crustal contamination and that they can be related to isotopically enriched sources where newly formed veins (enriched component) and peridotite matrix (depleted component) underwent differing isotopic evolution.TDM model ages for NPAA range from 1.2 to 2.3Ga. Considering that in the whole Paraná Basin isotopically distinct K-alkaline and tholeiitic magmas were generated following the enrichment of the subcontinental mantle mainly between 1.0 and 2.3Ga (Paleo-Mesoproterozoic events), the mantle sources preserved the isotopic heterogeneities over long time periods, suggesting a non-convective, i.e., lithospheric, mantle source beneath different cratonic or intercratonic areas.

Geochemical variations at ridge-centered hotspots caused by variable melting of a veined mantle plume

We model the dynamics and melting of a ridge-centered mantle plume, and predict the geochemical composition of magma at the surface. The mantle source is a fine-scale mixture of a small fraction of hydrous peridotite that is relatively enriched in incompatible elements (“EC”) and is embedded in a drier peridotite (“DC”) matrix. We assume all magma erupts at the ridge and calculate the contribution of EC and DC to the pooled composition along the ridge. If viscosity increases as melting dehydrates the mantle, EC contributes more to the pooled magma at the hotspot center than anywhere else along the ridge. The magnitude of this EC anomaly increases with Rayleigh number, and the along-axis distance to normal ridge composition increases with Rayleigh number, plume radius, and thermal buoyancy flux. A subset of model calculations designed to simulate the Iceland hotspot and Mid-Atlantic Ridge predict variations in crustal thickness, 87Sr/86Sr, and La/Sm with magnitudes and widths along the ridge that are comparable to, but less than, those observed. Improved fits to the observations require the innermost plume mantle to be compositionally distinct from the ambient asthenosphere; for example, by having a slightly higher mass fraction of EC (13–16%), or with DC having slightly higher 87Sr/86Sr and La/Sm. The inferred bulk plume 87Sr/86Sr composition, however, is within the predicted range of the source of normal mid-ocean ridge basalts worldwide. The broader implication is that the source of the Iceland plume is more similar in composition to the ambient upper mantle than previously thought, as a large part of the variation in ridge basalt composition can be attributed to the dynamics of mantle flow and melting.

Redox state of subcontinental lithospheric mantle and relationships with metasomatism: insights from spinel peridotites from northern Victoria Land (Antarctica)

Rift-related Cenozoic alkaline mafic lavas from northern Victoria Land (Antarctica) carry abundant mantle xenoliths whose oxygen fugacities (fO2) were determined to assess how the metasomatism, related to Cenozoic magmatism, affected the state of oxidation of the lithospheric mantle. The xenoliths used for this study are anhydrous spinel peridotites sampled in two localities, Greene Point and Baker Rocks, that show different extents of metasomatism: these are limited to incompatible element enrichments in Greene Point and to enrichments in major, minor and trace elements at Baker Rocks. The data set includes a composite xenolith from Baker Rocks, formed by a depleted lherzolite crosscut by an amphibole-bearing vein. Mossbauer spectroscopy was used to accurately determine the Fe3+/Fetot ratios in spinel and amphibole minerals. Amphiboles were also characterized by Single-Crystal X-ray Diffraction, and the crystallographic data were used to calculate the dehydrogenation. The oxidation state recorded by the xenoliths ranges from 0.2 to 1.5 log-bar units below the fayalite–magnetite–quartz (FMQ) buffer (ΔlogfO2) with the highest values observed in the metasomatized samples from Greene Point. For the vein of composite Baker Rocks xenolith, Δlog fO2 was estimated on the basis of the amphibole in -1.7 log-bar units, a value close to those calculated for all Baker Rocks xenoliths (ΔlogfO2 = −1.5 to −1.1 log-bar units). These results indicate a similar oxidation state for lithospheric mantle prior to the metasomatic event at Greene Point and Baker Rocks (ΔlogfO2 ~ −1.3 log-bar units). Metasomatism produced different effects in the shallow mantle at the two sites. At Greene Point, an oxidizing metasomatic melt caused the rise of fO2 in peridotite portions close to melt conduits up to FMQ. In contrast, at Baker Rocks, a metasomatizing melt with fO2 similar to that of the peridotite matrix produced chemical changes in the surrounding mantle rocks and amphibole crystallization without significantly modifying the local oxidation state. The origin of such different metasomatic melts, as well as the variety of primary magmas produced during the magmatic phase of Cenozoic rifting, is linked to the geodynamic evolution of the rift and probably involved the melting of a heterogeneous mantle source composed of a peridotite veined by pyroxene-bearing veins formed from an earlier amagmatic phase of the rift.

Exsolution of Garnet and Clinopyroxene from High-Al Pyroxenes in Xugou Peridotite, Eastern China

Serpentinized massif peridotite in the Xugou, Su-Lu ultrahighpressure (UHP) metamorphic belt, eastern China, preserves texturally old (porphyroclastic) orthoand clinopyroxene with up to two generations of lamellae of garnet, clinopyroxene and Mg-chromite. Their crystallographic orientation with respect to the host pyroxene is consistent with an origin by solid-state exsolution. Comparison of integrated mineral chemistry with simplified and natural chemical datasets suggests that both aluminous precursor pyroxenes were in equilibrium at a minimum pressure of 4 GPa and within a temperature range of about 1300^15008C. Steep isopleths of Ca in orthopyroxene imply that exsolution occurred during cooling. Al diffusion modelling suggests growth of widely spaced lamellae in orthopyroxene down to about 9008C. Integrated Al contents between wide lamellae record a minimum of 4 GPa pressure during cooling. Compositionally uniform exsolved minerals were formed at 4·3 0·3 GPa and 730 308C and reflect a cratonic geotherm with about 33 mWm 2 surface heat flow. The peridotite matrix mineral assemblage of olivineþ orthopyroxene garnet Mg-chromite clinopyroxene phlogopite records strain-induced recrystallization that partially to completely replaced precursor porphyroclasts. The recrystallized minerals lack lamellar exsolution. Recrystallized orthopyroxene, with Al2O3 at 0·13 wt %, indicates conditions of 5·5 0·3 GPa and 760 308C, which are higher-grade metamorphic conditions than those preserved in the chemically equilibrated exsolution microstructures. Both estimates overlap with the range reported for the Early Mesozoic UHP metamorphism in the region (4·0^ 6·7 GPa and 760^9708C).Major element melt models applied to previously published Xugou peridotite data suggest high degrees of melt extraction (30^35 %) in the garnet peridotite stability field (3^ 4·5 GPa) until garnet and clinopyroxene exhaustion. Coincidence in pressure and in the order of temperature of equilibration of precursor pyroxenes and peridotite melting implies that peridotite formation occurred at 135 km depth in the subcontinental lithospheric mantle (SCLM) beneath the Archaean North China Craton. Subsequent refertilization, mineral exsolution and chemical re-equilibration during long-term cooling in the SCLMoccurred prior to deformation and incorporation of the mantle fragment into the continental crust during UHP metamorphism at a minimum depth of 170 km. Because the Xugou precursor pyroxenes and peridotite formed at depths greater than the regional SCLM (c. 90 km), we infer that the orogenic peridotite massif formed part of the former hanging wall of the Archaean SCLM, which delaminated after the Late Mesozoic.

Heterogeneous mantle sources feeding the volcanic activity of Mt. Karacadağ (SE Turkey)

The volcanic activity of Mt. Karacadağ (SE Anatolia) is divided into three major stages: Siverek Stage (∼11–2.7Ma), Karacadağ Stage (∼1.9–1.0Ma) and Ovabağ Stage (0.4–0.01Ma). The magmas are mildly alkaline mafic rocks, mostly basanites, hawaiites and alkali basalts. Detailed geochemical investigation indicates a continuous variation of composition with time, with the oldest products (Siverek Stage) being characterized by average lowest HFSE (Ti, Hf, Zr, Nb, Ta), 143Nd/144Nd, Nb/U, Ta/Yb, Nb/Nb* and the highest ΔQ, La/Nb, Ti/Nb, Zr/Nb, Ba/Nb, Th/Ta, K/La, and the youngest products (Ovabağ Stage) at the opposite end of the trend. The overall incompatible element content of the Karacadağ volcanic rocks resembles closely average HIMU-OIB compositions, with the oldest samples deviating more strongly from typical compositions of “anorogenic” magmas.We interpret these geochemical variations with a process of partial melting of a chemically and mineralogically heterogeneous mantle source rather than with process of variable crustal contamination at shallow depths. During the first stages of mantle melting the volumes with lowest solidus temperature (e.g., the amphibole and phlogopite-rich metasomes, particularly abundant in the Arabia mantle xenolith suite) contributed significantly to the partial melts. “Average” peridotitic matrix is involved in partial melting processes only when these metasomatic volumes start to be exhausted, producing mantle melts with geochemical composition resembling average “anorogenic” mildly alkaline sodic rocks, common in the circum-Mediterranean area.

Deformation of a crystalline aggregate with a small percentage of high-dihedral-angle liquid: Implications for core–mantle differentiation during planetary formation

Core–mantle differentiation of small planetesimals has been suggested to be initiated by deformation-assisted segregation of molten Fe–S through a crystalline peridotite matrix. In this study we used two different experimental approaches to investigate the effect of varying the strain rate and of varying the dihedral angle of the liquid phase on liquid interconnectivity. (i) High pressure experiments were performed on simplified systems consisting of olivine plus a small percentage of liquid FeS (dihedral angle ~ 75°) or gold (dihedral angle ~ 150°) at a temperature above the FeS/Au solidus but below the silicate solidus. Samples were deformed at high pressure in a deformation-DIA multianvil press at strain rates between 10 − 3 and 10 − 6 s − 1 . (ii) Optical in situ analogue experiments on norcamphor plus H 2O (dihedral angle ~ 86°) were performed at room temperature to complement the d-DIA experiments. The crystalline matrix in both the d-DIA and analogue experiments deforms by grain boundary migration assisted dislocation creep under the experimental conditions. Furthermore, the liquid pockets display a similar strain rate dependent behaviour in both sets of experiments: At high strain rates deformation is strongly localised by the liquid phase, and liquid interconnection and segregation occurs. At low strain rates the geometry of liquid pockets is controlled predominantly by surface tension and little elongation and interconnection occurs. We propose a new empirical scaling approach using the d-DIA results, which indicates that some degree of high dihedral angle liquid interconnection and segregation may be possible down to a few percent residual liquid under natural conditions for moderate dihedral angles (60° < Θ < 90°). This, however, would not suffice for efficient core–mantle segregation and an additional mechanism, such as a magma ocean, would be required.

Redox freezing and melting in the Earth’s deep mantle resulting from carbon–iron redox coupling

Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth's mantle.

Mantle xenoliths from Tallante (Betic Cordillera): Insights into the multi-stage evolution of the south Iberian lithosphere

Extreme compositional heterogeneities have been recorded in mantle xenoliths from Tallante, attracting an intense petrological interest that is reflected in the impressive number of scientific studies. In this contribution we present new isotopic analyses carried out by MC-ICP-MS and TIMS on minerals separated from samples previously characterized by Beccaluva et al., 2004 (Lithos 75, 67–87). The new Sr–Nd–Hf isotopic analyses, combined with the previously published data, highlight that anhydrous cpx-poor peridotites display enrichment trends towards Enriched Mantle (EM) components, related to metasomatic interaction between alkaline melt and a depleted peridotite matrix that suffered melt extraction in the garnet stability field. In contrast, anhydrous cpx-rich lherzolites (containing up to 5% plagioclase) cluster around the Depleted Mantle (DM) component and are possibly related to a pervasive refertilization induced by tholeiitic melts that permeated the Betic lithosphere. These metasomatic reactions possibly occurred during the Mesozoic tectono-magmatic cycle that ultimately led to the opening of the central Atlantic and the westward propagation of the Neotethys rifting and oceanization. Further metasomatic reactions within the lithosphere occurred in the Neogene, during the Alboran back-arc development in connection with the Betic orogenic event. This cycle is documented by composite peridotite xenoliths crosscut by plagioclase and orthopyroxene ± quartz ± phlogopite ± amphibole veins that are characterized by crustal Sr–Nd–Hf isotopic signatures. This indicates hydrous silica-oversaturated melts as causative metasomatic agents, in turn implying the recycling – via subduction – of continental crust components within the mantle.

Identification of Source Lithology in the Hawaiian and Canary Islands: Implications for Origins

Results are reported of an exploration of mantle source lithology for intraplate magmas using whole-rock and olivine phenocryst compositions. This analysis includes modern mid-ocean ridge basalts and Archean komatiites as low- and high-temperature reference frames. It is shown that the Ni, Ca, Mn, and Fe/Mn contents of olivine phenocrysts in modern mid-ocean ridge basalts and Archean komatiites are consistent with a normal peridotite source. In contrast, olivine phenocrysts in shield-building lavas on Hawaii are higher in Ni and Fe/Mn, and lower in Mn and Ca than those expected to crystallize from melts of a normal peridotite source, and point to the importance of pyroxenite as proposed by Sobolev and co-workers. Hawaiian shield stage lavas and their olivine phenocrysts are similar to those expected from partial melts of a 100% stage 2 pyroxenite source. Such a source might form from a variety of melt–rock, melt–melt, and rock–rock reactions. Primary pyroxenite-derived magmas have a range of SiO2 contents that are positively correlated with 187Os/188Os and negatively correlated with 3He/4He. These results are consistent with a Hawaiian plume containing recycled crust within a peridotite matrix. Variable amounts of free silica are inferred in Hawaiian pyroxenite sources, which contribute to the production of SiO2-rich magmas. In contrast, peridotite and olivine pyroxenite melting are inferred to produce SiO2-poor pre-shield magmas at Loihi. The interaction of SiO2-rich and -poor magmas in the Hawaiian plume will trigger crystallization, not mixing. Mixing is permitted at low pressures in melt conduits and magma chambers, and work on olivine-hosted melt inclusions will be useful to evaluate its importance. In contrast to Hawaii, many ocean island basalts in localities such as the Canary Islands are deficient in SiO2, and may have been generated by partial melting of olivine pyroxenites that formed by solid-state reaction between recycled crust + peridotite in the lower mantle. There is likely to be a wide range of whole-rock pyroxenite compositions in the mantle, as well as significant variability in Mn and Fe/Mn in both peridotite partial melts and their olivine phenocrysts. In general, there are not likely to be well-defined end-member peridotite and pyroxenite sources in the mantle. Nevertheless, taxonomical difficulties encountered in source lithology identification may yield rich rewards, such as a better understanding of the relationship between lithological diversity in the lower mantle and its petrological expression in intraplate magmatism.

Helium and argon isotopic compositions of mantle xenoliths from Tallante and Calatrava, Spain

We have analyzed by single-step crushing helium and argon isotopes in olivine and orthopyroxene from mantle xenoliths of Calatrava (CLV) in central Spain and Tallante (TL) in southeast Spain. The investigation focused on carefully selected samples previously characterized in terms of major and trace elements on both bulk rock and constituent minerals, and Sr and Nd isotopes on clinopyroxene separates. Six analyses were performed on protogranular spinel lherzolites from CLV, and 17 were performed on spinel harzburgites, lherzolites, and orthopyroxenites from TL. The 40Ar/ 36Ar ratio was between 296 and 622, indicating atmospheric contamination, which probably occurred during exposure to the surface. The helium-isotope ratio ( 3He/ 4He) ranged between 3.6 and 6.5 Ra in CLV samples and between 1.4 and 5.7 Ra in TL samples. There was a positive correlation between the 3He/ 4He and 4He/ 40Ar* ratios, possibly reflecting diffusive fractionation between 3He, 4He, and 40Ar within mantle sections interacting with ascending melts. However, the difference between the maximum 3He/ 4He ratios measured in CLV and TL appears to be related to significant differences in the metasomatic melts that affected the two sectors of the lithospheric mantle. In agreement with the findings of previous studies, the helium isotopes at CLV are compatible with metasomatism due to ascending HIMU-type asthenospheric melts. In contrast, the lower 3He/ 4He values recorded at TL suggest subduction-related metasomatic components that are possibly related to the Cenozoic subduction of the Betic system. Such event plausibly introduced crust-derived fluids that metasomatized the mantle wedge, slightly decreasing its 3He/ 4He value. Noble gases appear decoupled from other elements during these mantle processes, since comparatively low 3He/ 4He values have been recorded also in samples that are relatively unmetasomatized in terms of incompatible lithophile elements. We hypothesize a role for volatile-dominated, CO 2-rich fluids progressively decoupling from the ascending metasomatic melts and migrating in the surrounding peridotite matrix to form a diffuse aureola enriched in noble gases.

Small-scale mantle heterogeneity on the source of the Gran Canaria (Canary Islands) Pliocene–Quaternary magmas

New chemical and Sr–Nd–Pb isotopic data of the Plio-Quaternary mafic lavas of Gran Canaria are used to investigate their mantle source composition. The most prominent aspects of the new dataset are the slight isotopic differences between the Plio-Quaternary (Post-Roque Nublo Group) and the older Pliocene (Roque Nublo Group) mafic parental magmas, which reflect small-scale mantle heterogeneities. Melting of two mantle materials, one isotopically more depleted and similar to the Depleted Mantle (DM) and the other with more radiogenic Pb-isotope ratios comparable to a mantle with high U/Pb ratio (HIMU), accounts for the isotopic and trace element composition of the Pliocene–Quaternary magmas of Gran Canaria. Geochemical variations show that the Pliocene–Quaternary mantle source is compositionally and lithologically heterogeneous and supports the presence of a silica-deficient pyroxenite mantle component. The contribution of the pyroxenite component in the generation of the Roque Nublo and Post-Roque Nublo magmas is estimated to be in the range from 50 to 70%. Trace element ratios support mixing between the two mantle components (pyroxenite veins in a peridotite matrix) which obscure the original chemical and isotopic composition of these two end-members.

Osmium isotopes and Fe/Mn ratios in Ti-rich picritic basalts from the Ethiopian flood basalt province: No evidence for core contribution to the Afar plume

Variations in the Fe/Mn ratio and Osmium isotopes in magnesium-rich mafic rocks from plume-related volcanic provinces have been exploited to imply the entrainment of core material in mantle plumes and the involvement of ancient recycled oceanic lithosphere. Here we present new major and trace element, 187Os/ 188Os ratios and precise Fe/Mn ratios on a suite of MgO-rich basalts, picritic basalts and ankaramites from the 30 Ma Ethiopian flood basalt province that shed new light on these arguments. The lavas show a range of compositions with MgO varying from 5 to 20 wt.% although the primary magma is inferred to have an MgO content of 15–16 wt.%. The lavas are also characterised by low Al 2O 3 contents (7–9 wt.% at 15 wt.% MgO), implying an origin from ∼ 150 km depth at a temperature in excess of 1600 °C, consistent with an origin in the early phases of Afar mantle plume activity. Osmium isotopes in samples with > 10 wt.% MgO are unradiogenic with 187Os/ 188Os < 0.127 while those with lower MgO define a positive correlation with Re/Os and appear to have been contaminated by crustal material. Fe/Mn ratios determined by ICP-MS vary from 65.4 to 78.5 in rocks with > 10 wt.% MgO and show greater variation to both higher and lower values in less magnesian samples. These values are high compared with MORB and Icelandic basalts, comparable with the Fe/Mn ratios of Hawaiian and other selected ocean island basalts, and are a characteristic of the primary magma. There is no evidence to suggest that Fe/Mn is fractionated during peridotite melting at low pressures less than 5 GPa, implying in agreement with previous studies that high Fe/Mn ratios are a compositional feature of the magma source region. The lack of association of radiogenic 187Os/ 188Os with the high Fe/Mn ratios of the Ethiopian picritic basalts calls into question the link to possible entrainment of core material in the source of the Afar mantle plume. Similarly, the unradiogenic 187Os/ 188Os ratios preclude a significant contribution from ancient recycled oceanic lithosphere. An alternative model is suggested in which melts generated at high pressures (> 7 GPa) during the initial turbulent ascent of the Afar plume head form pyroxene rich veins with high Fe/Mn ratios and high incompatible element contents in a peridotite matrix. These highly fertile source regions contribute significantly to melt compositions during the early phases of plume emplacement.

Post-deformational impregnation of depleted MORB in Nain lherzolite (Central Iran)

Plagioclase lherzolites of Nain melange, Iran, show peculiar textures that indicate melt impregnation: (1) droplet or bleb-like grains of plagioclase distributed in the peridotite matrix, (2) plagioclase-bearing clinopyroxenite seams, and (3) trails of plagioclase crosscutting pyroxene porphyroclasts. The textural characteristics show post-deformational igneous formation of plagioclase, and possibly, associated clinopyroxene, from the impregnating melt. The melt has precipitated the clinopyroxenite seams and chemically modified all the peridotite minerals. Highly refractory compositions of the precipitated minerals suggest involvement of a highly depleted MORB-like melt. The melt was an increment of partial melt produced by 8% to 10% fractional melting from the MORB source. This is in contrast to the involvement of ordinary MORB in melt impregnation in abyssal plagioclase peridotites. Integration of increments of mantle partial melts to form MORB was possibly incomplete in the very incipient mid-ocean ridge as in the short-lived Nain back-arc basin.

Asthenospheric percolation of alkaline melts beneath the St. Paul region (Central Atlantic Ocean)

Two peridotite suites collected by submersible in the equatorial Atlantic Ocean (Hekinian et al., 2000) were studied for textures, modes, and in situ major and trace element compositions in pyroxenes. Dive SP12 runs along the immersed flank of the St. Peter and Paul Rocks islets where amphibole-bearing, ultramafic mylonites enriched in alkalies and incompatible elements are exposed (Roden et al., 1984), whereas dive SP03 sampled a small intra-transform spreading centre situated about 370 km east of the St. Peter and Paul Rocks. Both suites are characterized by undeformed, coarse-grained granular textures typical of abyssal peridotites, derived from residual mantle after ∼ 15% melting of a DMM source, starting in the garnet stability field. Trace element modelling, textures and lack of mineral zoning indicate that the residual peridotites were percolated, reacted and refertilized by ∼ 2.6% partially aggregated melts in the uppermost level of the melting region. This relatively large amount of refertilization is in agreement with the cold and thick lithosphere inferred by previous studies. Freezing of trapped melts occurred as the peridotite entered the conductive layer, resulting in late-stage crystallization of olivine, clinopyroxene, spinel, ± plagioclase. Chondrite-normalized REE patterns in clinopyroxenes from SP03 indicate that they last equilibrated with (ultra-) depleted partial melts. In contrast, REE concentrations in clinopyroxenes from SP12 display U and S shaped LREE-enriched patterns and the calculated compositions of the impregnating melts span the compositional range of the regional basalts, which vary from normal MORB to alkali basalt sometimes modified by chromatographic fractionation with no, or very limited, mineral reaction. Thus the mylonitic band forming the St. Peter and St. Paul Rocks ridge is not a fragment of subcontinental lithospheric mantle left behind during the opening of the Central Atlantic, nor the source of the alkaline basalts as previously suggested. Rather, dive SP12 sampled residual peridotites of normal MORB mantle that were located close to channels transporting alkali basalts. Reacted melts escaping from these channels, infiltrated, and locally equilibrated with, the peridotite matrix by ion exchange reactions. Relicts of the source of the alkaline basalts were not sampled but our study suggests that it was a component of the MORB mantle underlying the St. Paul region.

Centrifuge assisted percolation of Fe–S melts in partially molten peridotite: Time constraints for planetary core formation

The mechanism which segregates molten Fe–S into metallic cores of planetary bodies is still not fully understood. Due to the high interfacial energy and wetting angle between Fe–S melts and silicate mantle minerals, the continuous percolative flow of such melts cannot be efficient for the core segregation in planetary bodies. A series of percolation experiments has been realized on a partially molten fertile garnet peridotite, employing a centrifuging piston cylinder. A high temperature garnet peridotite with Mg# ~0.90 composed of 60 vol.% olivine, 15 vol.% orthopyroxene, 6 vol.% clinopyroxene and 19 vol.% garnet has been used as the silicate matrix. Peridotite powders with the 100–200 or 20–30 µm grain size were mixed with 5–30 vol.% Fe–S of eutectic composition Fe 70S 30. The aggregates were centrifuged at 500–700 g at temperatures below and above the melting point of the peridotite. The centrifuge experiments revealed a negligible percolation of Fe–S melts through the unmolten peridotite matrix. Only at T > 1260 °C, i.e. above the solidus of the peridotite, and starting with 5 vol.% of Fe 70S 30 the vertical melt gradient achieved 1–2 vol.%/mm. In samples with 15 vol.% Fe 70S 30 the vertical separation achieved 2–2.5 vol.%/mm after 10 h of centrifuging at 500 g. An increase in the degree of partial silicate melting in the peridotite leads to an increase of the Fe–S separation rate from the peridotite matrix. Fe–S contents >10 vol.% cause an increase of the Fe–S melt droplet size and of the effective percolation velocity of Fe–S melt. A threshold dividing fast (>10 cm per year) and slow percolations (<1 mm per year) of Fe –S melt is found around 14–15 vol.% of Fe 70S 30. The experimentally determined permeabilities of Fe–S melt in the unmolten peridotite with 7–10 vol.% of Fe 70S 30 melt are 10 − 18 –10 − 19 m 2, which is 2–3 orders of magnitude lower than the values calculated previously from static experiments. The presence of the silicate melt increases the segregation velocity of Fe–S melt in a partially molten peridotite by more than one order of magnitude with respect to the unmolten peridotite matrix. This could provide an effective segregation of Fe–S melt in a planetary mantle down to 2.5 vol.% of residual Fe–S melt. The extremely slow percolation of Fe–S melt in the absence of the partial silicate melting precludes a scenario of metallic core formation via percolation before temperatures allow a substantial partial melting of mantle silicates in planetary bodies.