Geochemistry and Evolution of Subcontinental Lithospheric Mantle in Central Europe: Evidence from Peridotite Xenoliths of the Kozakov Volcano, Czech Republic

Neogene basanite lavas of Kozákov volcano, located alongthe Lusatian fault in the northeastern Czech Republic, containabundant anhydrous spinel lherzolite xenoliths that providean exceptionally continuous sampling of the upper two-thirdsof central European lithospheric mantle. The xenoliths yielda range of two-pyroxene equilibration temperatures from 680°Cto 1070°C, and are estimated to originate from depths of32–70 km, based on a tectonothermal model for basalticunderplating associated with Neogene rifting. The sub-Kozákovmantle is layered, consisting of an equigranular upper layer(32–43 km), a protogranular intermediate layer that containsspinel–pyroxene symplectites after garnet (43–67km), and an equigranular lower layer (67–70 km). Negativecorrelations of wt % TiO₂, Al₂O₃, and CaO with MgO and clinopyroxenemode with Cr-number in the lherzolites record the effects ofpartial fusion and melt extraction; Y and Yb contents of clinopyroxeneand the Cr-number in spinel indicate 5 to 15% partial melting.Subsequent metasomatism of a depleted lherzolite protolith,probably by a silicate melt, produced enrichments in the largeion lithophile elements, light rare earth elements and highfield strength elements, and positive anomalies in primitivemantle normalized trace element patterns for P, Zr, and Hf.Although there are slight geochemical discontinuities at theboundaries between the three textural layers of mantle, theretends to be an overall decrease in the degree of depletion withdepth, accompanied by a decrease in the magnitude of metasomatism.Clinopyroxene separates from the intermediate protogranularlayer and the lower equigranular layer yield ¹⁴³Nd/¹⁴⁴Nd valuesof 0·51287–0·51307 (_Nd = +4·6 to+8·4) and ⁸⁷Sr/⁸⁶Sr values of 0·70328–0·70339.Such values are intermediate with respect to the Nd–Srisotopic array defined by anhydrous spinel peridotite xenolithsfrom central Europe and are similar to those associated withthe present-day low-velocity anomaly in the upper mantle beneathEurope. The geochemical characteristics of the central Europeanlithospheric mantle reflect a complex evolution related to Devonianto Early Carboniferous plate convergence, accretion, and crustalthickening, Late Carboniferous to Permian extension and gravitationalcollapse, and Neogene rifting, lithospheric thinning, and magmatism. KEY WORDS: xenoliths; lithospheric mantle; REE–LILE–HFSE; Sr–Nd isotopes; Bohemian Massif     

The Geochemistry of the Arabian Lithospheric Mantle--a Source for Intraplate Volcanism?

We present trace element and Sr–Nd–Hf–Pb isotopecompositions for clinopyroxenes from anhydrous spinel peridotiteand garnet ± spinel pyroxenite xenoliths of Pan-Africanlithospheric mantle from Jordan, including the first high-precisiondouble-spike Pb isotope measurements of mantle clinopyroxene.Clinopyroxenes from the peridotites are variably Th–U–LILE–LREEenriched and display prominent negative Nb, Zr and Ti anomalies.MREE–HREE abundances can generally be modelled as partialmelting residues of spinel lherzolite with primitive-mantle-likecomposition after extraction of 5–10% melt, whereas theenrichments in Th–U–LILE–LREE require a Pan-Africanor later metasomatic event. The large range of Nd, Sr, Pb andHf isotope ratios in both peridotites and pyroxenites (e.g.Nd 1·4–17·5; ²⁰⁶Pb/²⁰⁴Pb 17·2–20·4;Hf 0·6–164·6) encompasses compositionsmore radiogenic than mid-ocean ridge basalt (MORB), and Pb isotopescover almost the entire range of oceanic basalt values. Hf valuesare some of the highest ever recorded in mantle samples andare decoupled from Nd in the same samples. Marked correlationsbetween Sr–Nd–Pb isotopes, LILE–LREE enrichmentsand HFSE depletion suggest that the metasomatizing agent wasa carbonatitic-rich melt and isotopic data suggest that metasomatismmay have been related to Pan-African subduction. The metasomaticmelt permeated depleted upper mantle (<16 kbar) during Pan-Africansubduction at 600–900 Ma, and the variably metasomatizedmaterial was then incorporated into the Arabian lithosphericmantle. There is no evidence for recent metasomatism (<30Ma) related to the Afar plume like that postulated to have affectedsouthern Arabian lithospheric mantle. Hf isotopes in the mantleclinopyroxenes are unaffected by metasomatism, and even somestrongly overprinted lithologies record ancient (>1·2Ga) pre-metasomatic Lu–Hf signatures of the depleted uppermantle that was the protolith of the Arabian lithospheric mantle.The ‘resistance’ of the Lu–Hf isotopic systemto later metasomatic events resulted in the development of extremelyheterogeneous Hf isotopic signatures over time that are decoupledfrom other isotopic systems. No mantle sample in this studyexactly matches the chemical and isotopic signature of the sourceof Jordanian intraplate basalts. However, the xenolith compositionsare broadly similar to those of the source of Arabian intraplatebasalts, suggesting that the numerous Cenozoic intraplate volcanicfields throughout Arabia may be the product of melting uppermantle wedge material fertilized during Pan-African subductionand incorporated into the Arabian lithospheric mantle. We proposea model whereby the proto-Arabian lithospheric mantle underwenta major melting event in early Proterozoic–late Archeantimes (at the earliest at 1·2 Ga). Island-arc volcanismand major crust formation occurred during the Pan-African orogeny,which liberated fluids and possibly small-degree melts thatmigrated through the mantle creating zones of enrichment forcertain elements depending upon their compatibility. Immobileelements, such as Nb, were concentrated near the base of themantle wedge providing the source of the Nb-rich Jordanian volcanicrocks. More mobile elements, such as LILE and LREE, were transportedup through the mantle and fertilized the shallow mantle sourceof the Jordanian xenoliths. Following subduction, the mantlewedge became fossilized and preserved distinct enriched anddepleted zones. Lithospheric rifting in the Miocene triggeredpartial melting of spinel-facies mantle in the lower lithosphere,which mixed with deeper asthenospheric garnet-facies melts asrifting evolved. These melts entrained segments of variablycarbonatite-metasomatized shallow lithospheric mantle en routeto the surface. KEY WORDS: Arabian lithospheric mantle; Jordan; mantle xenoliths; Sr–Nd–Hf–Pb isotopes     

Mantle Mush Compaction: a Key to Understand the Mechanisms of Concentration of Kimberlite Melts and Initiation of Swarms of Kimberlite Dykes

Kimberlite pipes or dykes tend to occur in clusters (a few kilometresin diameter) within fields 30–50 km in diameter. Theyare generally considered to originate from low degrees of partialmelting of carbonated peridotite within zones of ascending mantle.Numerical modelling shows that at the depth of formation ofkimberlite melts (>>200 km), mantle compaction processescan result in the formation of melt pockets a few tens of kilometresacross, with melt concentrations up to 7%. The initiation ofswarms of kimberlite dykes at the top of these melt pocketsis inevitable because of the large excess pressure between themelt and the surrounding solid, which exceeds the hydraulicfracturing limit of the overlying rocks. After their initiationat mantle depth the swarm of dykes may reach the surface ofthe Earth when the entire cratonic lithosphere column is inextension. We propose that kimberlite fields represent the surfaceenvelope of dyke swarms generated inside a melt pocket and thatkimberlite clusters represent the discharge of melt via dykesoriginating from sub-regions of the pocket. This model reproducesthe worldwide average diameter of kimberlite fields and is consistentwith the observation that some of the main kimberlite fieldsdisplay age ranges of c. 10 Myr. It is deduced that, at thescale of the Kaapvaal craton, different fields such as Kimberley,N. Lesotho and Orapa, dated at 80–90 Ma, probably resultfrom synchronous melt pockets forming inside an ascending mantleflow. The same model could apply to the fields of the Rietfontein,Central Cape and Gibeon districts dated at 60–70 Ma. Itis suggested that the same mantle flow that produced the Kimberley,N. Lesotho and Orapa fields migrated over 20–30 Myr afew hundred kilometres westward to form the Rietfontein, CentralCape and Gibeon fields. KEY WORDS: kimberlites; mantle; compaction; convection; volcanism     

Magmatic Evolution and Ascent History of the Aries Micaceous Kimberlite, Central Kimberley Basin, Western Australia: Evidence from Zoned Phlogopite Phenocrysts, and UV Laser 40Ar/39Ar Analysis of Phlogopite-Biotite

The Neoproterozoic Aries kimberlite was emplaced in the centralKimberley Basin, Western Australia, as a N–NNE-trendingseries of three diatremes infilled by lithic-rich kimberlitebreccias. The breccias are intruded by hypabyssal macrocrysticphlogopite kimberlite dykes that exhibit differentiation toa minor, high-Na–Si, olivine–phlogopite–richteritekimberlite, and late-stage macrocrystic serpentine–diopsideultramafic dykes. Mineralogical and geochemical evidence suggeststhat the high-Na–Si, olivine–phlogopite–richteritekimberlite was derived from the macrocrystic phlogopite kimberliteas a residual liquid following extended phlogopite crystallizationand the assimilation of country rock sandstone, and that themacrocrystic serpentine–diopside ultramafic dykes formedas mafic cumulates from a macrocrystic phlogopite kimberlite.Chemical zonation of phlogopite–biotite phenocrysts indicatesa complex magmatic history for the Aries kimberlite, with theearly inheritance of a range of high-Ti phlogopite–biotitexenocrysts from metasomatized mantle lithologies, followed bythe crystallization of a population of high-Cr phlogopite phenocrystswithin the spinel facies lithospheric mantle. A further oneto two phlogopite–biotite overgrowth rims of distinctcomposition formed on the phlogopite phenocrysts at higher levelsduring ascent to the surface. Ultra-violet laser ⁴⁰Ar/³⁹Ar datingof mica grain rims yielded a kimberlite eruption age of 815·4± 4·3 Ma (95% confidence). ⁴⁰Ar/³⁹Ar laser profilingof one high-Ti phlogopite-biotite macrocryst revealed a radiogenic⁴⁰Ar diffusive loss profile, from which a kimberlite magma ascentduration from the spinel facies lithospheric mantle was estimated(assuming an average kimberlite magma temperature of 1000°C),yielding a value of 0·23–2·32 days for thenorth extension lobe of the Aries kimberlite. KEY WORDS: ⁴⁰Ar/³⁹Ar; diamond; kimberlite; mantle metasomatism; phlogopite–biotite     

Chemical and Isotopic Studies of Ultramafic Inclusions from the San Carlos Volcanic Field, Arizona: A Bearing on their Petrogenesis

Compositions of the principal minerals and Pb, Nd, and Sr isotopeanalyses of clinopyroxene (cpx) separates are reported for TypeI spinel peridotite xenoliths from the Peridot Mesa vent ofthe San Carlos Volcanic Field. The principal phases are in chemicalequilibrium within each inclusion. Systematic changes in mineralcomposition accompany lithological changes from fertile lherzolitesto infertile harzburgites. These changes are consistent witha fusion residue origin for the major element component of thexenoliths, as noted previously by Frey & Prinz (1978). ExcessFe is additionally present in some inclusions. Pyroxene equilibrationtemperatures calculated using the Wells (1977) geothermometerfall in the narrow range of 1022?34?C (1 s.d.). Equilibrationpressures poorly limit corresponding depths to anywhere between30 and 65 km within the lithospheric mantle. The geotherm is‘advective’ and elevated by 500?C at the depth ofsampling over a reference conductive shield geotherm. The highheat flow measured at the surface results from a combinationof extension and magmatism, with the temperature perturbationextending into the lithospheric mantle. ¹⁴³Nd/¹⁴⁴Nd ratios (0?51251–0?51367) and ⁸⁷Sr/⁸⁶Sr ratios(0?70190–0?70504) in cpx demonstrate gross isotopic heterogeneitybeneath the Peridot Mesa vent. This largely overlaps the oceanicmantle array, although four inclusions have Nd greater thanmid-ocean ridge basalts (MORB). PM-228J with _Nd = +20 is themost extreme yet reported for a spinel Iherzolite. Pb abundancesin cpx (generally <0?03ppm) are far lower than previouslyreported values. ²⁰⁶Pb/²⁰⁴Pb ratios (17?5–19?1) overlapoceanic basalts and do not correlate with ⁸⁷Sr/⁸⁶Sr ratio. However,some of the inclusions exhibit MORB-like ²⁰⁶Pb/²⁰⁴Pb ratiosbut much higher ⁸⁷Sr/⁸⁶Sr ratios, which suggests a possiblegenetic link of detached lithospheric mantle with certain oceanicislands. Metasomatic trace element enrichment processes are most widespreadin the infertile (Al-poor, Cr-rich) inclusions, as noted byFrey & Prinz (1978). This systematic relationship is a localfeature of the mantle and suggests that some degree of meltingoccurs commensurately with incompatible element addition. Inparticular, anhydrous peridotite above its volatile-presentsolidus that was flushed with C-O-H fluids containing incompatibleelements would melt and form an enriched infertile fusion residue.The ascending magmas responsible for forming Type II peridotiteveins are the most probable source of the volatiles and mayin some cases react to produce chemical gradients in the wall-rock.Prior metasomatism is also evident isotopically in some inclusions.Overall, the lithospheric mantle beneath Peridot Mesa has suffereda multi-stage history of enrichment, depletion and melting atvarious times since it became attached to the crust above.     

Interaction between Continental Lithosphere and the Iceland Plume--Sr-Nd-Pb Isotope Geochemistry of Tertiary Basalts, NE Greenland

Volcanic rocks associated with Atlantic opening in northerneast Greenland (73–76N) form a 1-km thickness of basalticlavas located on the coast some 400 km north of the major basaltaccumulations of the Blosseville Coast (<70N). The LowerLava Series, which makes up the lower half of the sequence atHold with Hope and all of that at Wollaston Forland, is composedof homogeneous quartz tholeiites (5–8% MgO). These aremildly light rare earth element (LREE) enriched (La/Yb_N 2.060.45,1 S.D.) and show strong chemical and Pb-Nd-Sr isotopic similaritiesto Icelandic tholeiites. They are distinguished from Atlanticmid-ocean ridge basalt (MORB) in having less radiogenic Pb andNd, higher 8/4 and lower 7/4, and depletion in K and Rb relativeto other incompatible elements, and show no evidence of a MORBasthenosphere component in their source. A single nephelinitein the Lower Series has essentially similar isotopic characteristicsand K, Rb depletion. The tholeiites were derived from the hothead of the Iceland plume, which had spread laterally withinthe upper mantle, and represent large melt fractions (15–20%)from spinel-facies mantle combined with small melt fractions(2.2%) from the garnet facies. Pb isotopic data indicate thatthe Iceland plume contains no MORB asthenospheric component,and is therefore most unlikely to arise from enriched streaksin the convecting upper mantle. The K, Rb depletion is sharedwith the HIMU ocean islands, and suggests a similar origin forthe Iceland plume in subduction-processed oceanic crust. Therelatively low ²⁰⁶Pb/²⁰⁴Pb ratios, and near-MORB Sr-Nd isotopes,suggest that Iceland overlies an immature HIMU plume. The conformably overlying upper half of the Hold with Hope sequence(the Upper Lava Series) is extremely heterogeneous, being mainlyolivine and quartz tholeiites (4.5–9.5% MgO in inferredmelt compositions, and up to 27% in accumulative lavas), withoccasional undersaturated compositions. The latter are concentratednear the base of the Upper Series, and are associated with stronglyincompatible-element-enriched tholeiites. These enriched sampleshave La/Yb_N from 7.3 to 28.5, with most tholeiites 13, and theundersaturated rocks >23. They are isotopically heterogeneous,with a basanite resembling Icelandic compositions, and an alkalibasalt having much less radiogenic Pb and Nd. The bulk of theUpper Series tholeiites has a limited La/Yb_N range (4.7–7.3)but a wide range in isotope ratios, from almost Icelandic valuesto ⁸⁷Sr/⁸⁶Sr₅₀=0.7100, ²⁰⁶Pb/²⁰⁴Pb₅₀=18.7, and ¹⁴³Nd/¹⁴⁴Nd₅₀=0.51247.This isotopic range is well correlated with SiO₂, Ce/Pb, andK/Nb, in a manner suggesting crustal assimilation-fractionalcrystallization (AFC) relationships. The mantle-derived end-memberof the Upper Series is displaced to slightly less radiogenicNd than the Lower Series samples, perhaps through mixing witha small component from the subcontinental lithospheric mantle.A larger proportion of this melt was derived from garnet-faciesmantle than for Lower Series samples, and melt fractions weresmaller in both garnet and spinel stability fields. As isotopic compositions similar to those of Icelandic lavasare found in each of the three stratigraphic groups (Lower Series,basal enriched Upper Series, and normal Upper Series tholeiites),the Upper Series were derived from this mixed source, but stillhad a very dominant plume isotopic signature. The continuedpresence of a lithospheric ‘lid’ is indicated bythe smaller melt fractions in both garnet and spinel facies(0.01 and 0.1, respectively) than those responsible for theLower Series lavas. The thicker crust in the region allowedstagnation of the magmas in the plumbing system of a centralvolcano and consequent extensive accumulation, fractionation,and assimilation of crustal rocks.     

Lithospheric Mantle Evolution beneath the Eifel (Germany): Constraints from Sr-Nd-Pb Isotopes and Trace Element Abundances in Spinel Peridotite and Pyroxenite Xenoliths

The Pb isotope compositions of amphiboles and clinopyroxenesin spinel peridotite and pyroxenite mantle xenoliths from theintra-plate Quaternary volcanic fields of the Eifel province(Germany) are strongly correlated with their Sr–Nd isotopeand trace element compositions. High-temperature anhydrous xenolithsfrom a depth of around 60 km have trace element and Sr–Nd–Pbisotope compositions similar to the depleted source of mid-oceanridge basalts (Depleted MORB Mantle, DMM). Amphibole-bearingxenoliths from shallower depths (<45 km) provide evidencefor three temporally distinct episodes of mantle metasomatismin the subcontinental lithosphere: (1) aqueous fluids from anisotopically enriched (EM-like) mantle reservoir caused amphiboleformation during deformation in the shallow continental lithosphericmantle and may be subduction related, probably associated withthe last major tectonic event that influenced the area (Hercynianorogeny). (2) During a second phase of mantle metasomatism theEM-like lithospheric mantle was affected by melts from an ancient,HIMU-like (high time-integrated µ = ²³⁸U/²⁰⁴Pb) mantlesource. The HIMU-like component introduced by these fluids hada much more radiogenic Pb isotope composition than the asthenosphericsource of the widespread Cenozoic magmatism in Europe and maybe linked to reactivation of ancient subducted crustal domainsduring the Hercynian orogeny or to early Cretaceous deep-sourcedmantle plumes. (3) During a brief final stage the heterogeneouslyenriched EM–HIMU subcontinental lithosphere was locallymodified by basaltic melts migrating along fractures and veinsthrough the upper mantle as a consequence of the Cenozoic Eifelvolcanism. Although a DMM component is completely lacking inthe metasomatic fluids of the metasomatic episodes 1 and 2,the vein melts of episode 3 and the Cenozoic Eifel lavas requiremantle sources containing three end-member components (DMM–HIMU–EM).Thus, mobilization of the more depleted mantle material occurredat the earliest in the Tertiary, contemporaneously with thedevelopment of the extensive rift system and main melt generationin Europe. Alternatively, the variety of Sr–Nd–Pbisotope signatures of the metasomatic agents may have been producedby melting of isotopically distinct mantle domains in a heterogeneousuprising mantle plume. KEY WORDS: Eifel; Europe; mantle xenoliths; metasomatism; Pb isotopes     

Sr-Nd-Pb Isotopic Compositions of Peridotite Xenoliths from Spitsbergen: Numerical Modelling Indicates Sr-Nd Decoupling in the Mantle by Melt Percolation Metasomatism

Several spinel peridotite xenoliths from Spitsbergen have Sr–Ndisotopic compositions that plot to the right of the ‘mantlearray’ defined by oceanic basalts and the DM end-member(depleted mantle, with low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to Sr–Nd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. ‘Chromatographic’effects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing Sr–Nd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, Sr–Nd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling     

FTIR Spectrum of Phenocryst Olivine as an Indicator of Silica Saturation in Magmas

Fourier Transform infrared (FTIR) absorption spectra of hydroxylwere measured on olivine phenocrysts from hydrous basaltic meltsthat originated in island-arc tectonic settings. The basalticmelts encompass a wide range of silica activities from orthopyroxene-saturatedhypersthene-normative to nepheline-normative compositions. Theintensities and wavenumber placement of hydroxyl absorptionbands correlate with the degree of silica saturation of theparent melt from which the olivine crystallized. Olivines fromsilica-undersaturated nepheline-normative melts absorb IR radiationin the wavenumber range 3430–3590 cm^–1 (Group 1).In contrast, olivines from orthopyroxene-saturated boniniticmelts exhibit hydroxyl absorption bands in the wavenumber range3285–3380 cm^–1 (Group 2). Olivines crystallizedat intermediate silica activities exhibit a combination of thetwo groups of hydroxyl IR bands, where the proportion of Group2 bands increases with increasing silica saturation of the parentmelt. The positions of hydroxyl absorption peaks observed herefor natural samples are consistent with previous measurementson experimentally annealed olivines. Thus protonation experimentscan be employed to make spectroscopically dry olivine structuresvisible by IR, yielding information on the silica saturationof the parental magmas. Hydroxyl concentrations in the studiedolivines were estimated to be 1–2 ppm, corresponding toan olivine–melt partition coefficient of (1·0 ±0·3) x 10^–4. KEY WORDS: nominally anhydrous minerals; olivine; water; mantle; silica activity; melt inclusions     

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa

The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (¹⁴³Nd/¹⁴⁴Nd = 0·51267–0·51283,⁸⁷Sr/⁸⁶Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar     

Dissolution Rates of Upper Mantle Minerals in an Alkali Basalt Melt at High Pressure: An Experimental Study and Implications for Ultramafic Xenolith Survival

The dissolution rates of the major upper mantle minerals olivine,orthopyroxene, clinopyroxene, spinel, and garnet have been determinedin an alkali basalt melt at superliquidus temperatures and 5,12, and 30 kb. At low pressure where olivine is the liquidusphase of the basalt, olivine has a slower dissolution rate thanclinopyroxene; however, at higher pressure where clinopyroxeneis the liquidus phase, clinopyroxene has a slower dissolutionrate than olivine. The relative rates of dissolution of olivineand clinopyroxene at each pressure are, therefore, governedby their relative stabilities in the melt and hence by the structureof the melt. As the degree of superheating above the liquidusincreases at each pressure, the dissolution rates of olivineand clinopyroxene converge, suggesting that the melt undergoestemperature-induced structural changes. Orthopyroxene has a dissolution rate similar to olivine at highpressure and similar to clinopyroxene at low pressure. Spinelhas the slowest dissolution rate at each pressure. Garnet dissolvesvery rapidly at 12 kb and at a comparable rate of olivine at30 kb. The dissolution rates determined in the experiments varyfrom 9.21 ? 10^–9cm s^–1 for spinel at 5 kbar and1250?C to 3.83 ? 10^–5cm s^–1 for garnet at 30 kband 1500?C. Textures produced during the dissolution experiments are relatedto mineral stability in the melt at each pressure and are independentof the degree of superheating. The mineral phases that are stableon or near the liquidus exhibit no reaction; whereas complexreaction textures and crystallization characterize dissolutionof minerals that are relatively unstable in the melt. Concentration profiles in the melt adjacent to the same crystalfor different experimental durations are identical, indicatingthat dissolution is time-independent and a steady-state process.However, cation diffusion coefficients calculated for single-componentoxides in the melt reveal that dissolution may not be completelycontrolled by diffusion of cations away from the crystal/meltinterface. The apparent diffusivities positively correlate withthe dissolution rate, which suggests that the stability of themineral is an important factor to consider when deriving diffusioncoefficients from these experiments. Other factors that maybe involved are multi-component effects and the nature of thediffusing species in the melt. A simple model has been constructed that predicts the survivalof ultramafic xenoliths in alkali basalt magmas as a functionof xenolith radius, magma ascent time and superheating. Theresults of the model suggest that the relative proportions ofperidotite and pyroxenite xenoliths brought to the surface inalkali basalts are generally representative of their proportionsas constituents of the upper mantle. Further experiments usingdifferent melt compositions are required to extend the model.     

The Evolution of the Upper Mantle beneath the Canary Islands: Information from Trace Elements and Sr isotope Ratios in Minerals in Mantle Xenoliths

Laser ablation microprobe data are presented for olivine, orthopyroxeneand clinopyroxene in spinel harzburgite and lherzolite xenolithsfrom La Palma, Hierro, and Lanzarote, and new whole-rock trace-elementdata for xenoliths from Hierro and Lanzarote. The xenolithsshow evidence of strong major, trace element and Sr isotopedepletion (⁸⁷Sr/⁸⁶Sr 0·7027 in clinopyroxene in themost refractory harzburgites) overprinted by metasomatism. Thelow Sr isotope ratios are not compatible with the former suggestionof a mantle plume in the area during opening of the AtlanticOcean. Estimates suggest that the composition of the originaloceanic lithospheric mantle beneath the Canary Islands correspondsto the residues after 25–30% fractional melting of primordialmantle material; it is thus significantly more refractory than‘normal’ mid-ocean ridge basalt (MORB) mantle. Thetrace element compositions and Sr isotopic ratios of the mineralsleast affected by metasomatization indicate that the upper mantlebeneath the Canary Islands originally formed as highly refractoryoceanic lithosphere during the opening of the Atlantic Oceanin the area. During the Canarian intraplate event the uppermantle was metasomatized; the metasomatic processes includecryptic metasomatism, resetting of the Sr–Nd isotopicratios to values within the range of Canary Islands basalts,formation of minor amounts of phlogopite, and melt–wall-rockreactions. The upper mantle beneath Tenerife and La Palma isstrongly metasomatized by carbonatitic or carbonaceous meltshighly enriched in light rare earth elements (REE) relativeto heavy REE, and depleted in Zr–Hf and Ti relative toREE. In the lithospheric mantle beneath Hierro and Lanzarote,metasomatism has been relatively weak, and appears to be causedby high-Si melts producing concave-upwards trace element patternsin clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-richharzburgites/lherzolites, dunites, wehrlites and clinopyroxenitesformed from mildly alkaline basaltic melts (similar to thosethat dominate the exposed parts of the islands), and appearto be mainly restricted to magma conduits; the alkali basaltmelts have caused only local metasomatism in the mantle wall-rocksof such conduits. The various metasomatic fluids formed as theresults of immiscible separations, melt–wall-rock reactionsand chromatographic fractionation either from a CO₂-rich basalticprimary melt, or, alternatively, from a basaltic and a siliceouscarbonatite or carbonaceous silicate melt. KEY WORDS: mantle xenoliths; mantle minerals; trace elements; depletion; carbonatite metasomatism     

Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands

The volcanic history of Santo Antão, NW Cape Verde Islands,includes the eruption of basanite–phonolite series magmasbetween 7·5 and 0·3 Ma and (melilite) nephelinite–phonoliteseries magmas from 0·7 to 0·1 Ma. The most primitivevolcanic rocks are olivine ± clinopyroxene-phyric, whereasthe more evolved rocks have phenocrysts of clinopyroxene ±Fe–Tioxide ± kaersutite ± haüyne ± titanite± sanidine; plagioclase occurs in some intermediate rocks.The analysed samples span a range of 19–0·03% MgO;the most primitive have 37–46% SiO₂, 2·5–7%TiO₂ and are enriched 50–200 x primitive mantle in highlyincompatible elements; the basanitic series is less enrichedthan the nephelinitic series. Geochemical trends in each seriescan be modelled by fractional crystallization of phenocrystassemblages from basanitic and nephelinitic parental magmas.There is little evidence for mineral–melt disequilibrium,and thus magma mixing is not of major importance in controllingbulk-rock compositions. Mantle melting processes are modelledusing fractionation-corrected magma compositions; the modelssuggest 1–4% partial melting of a heterogeneous mantleperidotite source at depths of 90–125 km. Incompatibleelement enrichment among the most primitive magma types is typicalof HIMU OIB. The Sr, Nd and Pb isotopic compositions of theSanto Antão volcanic sequence and geochemical characterchange systematically with time. The older volcanic rocks (7·5–2Ma) vary between two main mantle source components, one of whichis a young HIMU type with ²⁰⁶Pb/²⁰⁴Pb = 19·88, 7/4 =–5, 8/4 0, ⁸⁷Sr/⁸⁶Sr = 0·7033 and ¹⁴³Nd/¹⁴⁴Nd= 0·51288, whereas the other has somewhat less radiogenicSr and Pb and more radiogenic Nd. The intermediate age volcanicrocks (2–0·3 Ma) show a change of sources to two-componentmixing between a carbonatite-related young HIMU-type source(²⁰⁶Pb/²⁰⁴Pb = 19·93, 7/4 = –5, 8/4 = –38,⁸⁷Sr/⁸⁶Sr = 0·70304) and a DM-like source. A more incompatibleelement-enriched component with 7/4 > 0 (old HIMU type) isprominent in the young volcanic rocks (0·3–0·1Ma). The EM1 component that is important in the southern CapeVerde Islands appears to have played no role in the petrogenesisof the Santo Antão magmas. The primary magmas are arguedto be derived by partial melting in the Cape Verde mantle plume;temporal changes in composition are suggested to reflect layeringin the plume conduit. KEY WORDS: radiogenic isotopes; geochemistry; mantle melting; Cape Verde     

Origin of Paleoproterozoic Komatiites at Jeesiorova, Kittila Greenstone Complex, Finnish Lapland

Komatiites from the 2 Ga Jeesiörova area in Finnish Laplandhave subchondritic Al₂O₃/TiO₂ ratios like those in Al-depletedkomatiites from Barberton, South Africa. They are distinct inthat their Al abundances are higher than those of the Al-depletedrocks and similar to levels in Al-undepleted komatiites. Moderatelyincompatible elements such as Ti, Zr, Eu, and Gd are enriched.Neither majorite fractionation nor hydrous melting in a supra-subductionzone setting could have produced these komatiites. Their highconcentrations of moderately incompatible elements may haveresulted from contamination of their parental melt through interactionwith metasomatic assemblages in the lithospheric mantle or enrichmentof their mantle source in basaltic melt components. Re–Osisotope data for chromite from the Jeesiörova rocks yieldan average initial ¹⁸⁷Os/¹⁸⁸Os of 0·1131 ± 0·0006(2), _Os(I) = 0·1 ± 0·5. These data, coupledwith an initial _Nd of +4, indicate that melt parental to thekomatiites interacted minimally with ancient lithospheric mantle.If their mantle source was enriched in a basaltic component,the combined Os–Nd isotopic data limit the enrichmentprocess to within 200 Myr prior to the formation of the komatiites.Their Os–Nd isotopic composition is consistent with derivationfrom the contemporaneous convecting upper mantle. KEY WORDS: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment     

Roles of Melting and Metasomatism in the Formation of the Lithospheric Mantle beneath the Leizhou Peninsula, South China

This study characterizes the nature of fluid interaction andmelting processes in the lithospheric mantle beneath the Yingfenglingand Tianyang volcanoes, Leizhou Peninsula, South China, usingin situ trace-element analyses of clinopyroxene, amphibole andgarnet from a suite of mantle-derived xenoliths. Clinopyroxenesfrom discrete spinel lherzolites exhibit large compositionalvariations ranging from extremely light rare earth element (LREE)-depletedto LREE-enriched. Trace-element modelling for depleted samplesindicates that the Leizhou lherzolites are the residues of amantle peridotite source after extraction of 1–11% meltgenerated by incremental melting in the spinel lherzolite fieldwith the degree of melting increasing upwards from about 60km to 30 km. This process is consistent with gradational meltingat different depths in an upwelling asthenospheric column thatsubsequently cooled to form the current lithospheric mantlein this region. The calculated melt production rate of thiscolumn could generate mafic crust 5–6 km thick, whichwould account for most of the present-day lower crust. The formationof the lithospheric column is inferred to be related to Mesozoiclithosphere thinning. Al-augite pyroxenites occur in compositexenoliths; these are geochemically similar to HIMU-type oceanisland basalt. These pyroxenites postdate the lithospheric columnformation and belong to two episodes of magmatism. Early magmatism(forming metapyroxenites) is inferred to have occurred duringthe opening of the South China Sea Basin (32–15 Ma), whereasthe most recent magmatic episode (producing pyroxenites withigneous microstructures) occurred shortly before the eruptionof the host magmas (6–0·3 Ma). Trace-element traversesfrom the contacts of the Al-augite pyroxenite with the spinelperidotite wall-rock in composite xenoliths record gradientsin the strength and nature of metasomatic effects away fromthe contact, showing that equilibrium was not attained. Significantenrichment in highly incompatible elements close to the contacts,with only slight enrichment in Sr, LREE and Nb away from thecontact, is inferred to reflect the different diffusion ratesof specific trace elements. The observed geochemical gradientsin metasomatic zones show that Sr, La, Ce and Nb have the highestdiffusion rates, other REE are intermediate, and Zr, Hf andTi have the lowest diffusion rates. Lower diffusion rates observedfor Nb, Zr, Hf and Ti compared with REE may cause high fieldstrength element (HFSE) negative anomalies in metasomatizedperidotites. Therefore, metasomatized lherzolites with HFSEnegative anomalies do not necessarily require a carbonatiticmetasomatizing agent. KEY WORDS: China; lithosphere; mantle xenoliths; clinopyroxene trace elements; mantle partial melting; mantle metasomatism; trace-element diffusion rates     

Garnet Peridotite Xenoliths from the Vitim Volcanic Field, Baikal Region: the Nature of the Garnet--Spinel Peridotite Transition Zone in the Continental Mantle

A suite of large and fresh peridotite xenoliths from a picritetuff deposit in the Cenozoic Vitim volcanic field, {small tilde}200km east of Lake Baikal, shows a continuous gradation from protogranularspinel through garnet–spinel to very abundant garnet peridotites.This includes composite nodules in which all these lithologiescoexist on the scale of a few centimeters. Garnet and many spinellherzolites are remarkably fertile in terms of their ‘basaltic’major element contents (CaO 30–37%, MgO 37–40%,Ca/Al=11, Cr/Al<013), whereas some garnet–spineland spinel peridotites are moderately depleted (Cr/Al 014–045).T estimates are 850–880C for the fertile spinel lherzolitesapparently brought up from shallow depths of 40–50 km.This contrasts with 980–1030C for depleted spinel peridotitesand 1000–1150C for the garnet-bearing peridotites forwhich equilibration pressures between 16 and 23 kbar are inferred.The data suggest that garnet and spinel peridotites coexistin the sub-Vitim mantle at a pressure of {small tilde}18 kbarover an interval of {small tilde}2 kbar, with the appearanceof garnet, and with the garnetto-spinel ratio in this transitionalzone primarily being controlled by bulk rock contents of Ca,Al, Cr, and Cr/Al ratios, in addition to P–T conditions. The Vitim peridotites show little evidence for metasomatic enrichment:they commonly show depletion of LREE compared with intermediateREE; this includes also rare amphibole-bearing veins. The fertilespinel and garnet lherzolites have very similar bulk rock majoroxide contents and REE distribution patterns; these featuresindicate a lack of significant chemical vertical mantle stratificationin that region. Garnet peridotites from Vitim show large differencesin modal and chemical composition from garnet peridotite xenolithsfrom Yakutian and South African kimberlites, suggesting distinctlithospheric mantle structure and composition in Archean cratonsand post-Archean mobile belts. ^* Present address: School of Earth Sciences, Macquarie University, N.S.W. 2109, Australia     

Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise

A comprehensive model for the activity of the elementary accretion segment at fast‐spreading ridges relies on integration of structural data from the Oman ophiolite and geophysical results from the East Pacific Rise (EPR) around 9°N, which are of comparable size and spreading rates. The axial melt lens at shallow crustal level provides a link between Deval segmentation at the seafloor and a lower melt sill at Moho level, imaged at the EPR as a crustal melt zone (CMZ) and mapped in Oman as the Moho transition zone (MTZ). Both are attached to a mantle upwelling at the EPR, and to a frozen diapir in Oman. The physical link between diapiric mantle uprising at the Moho and Devals segmentation at the seafloor is the melt being injected from the mantle into the lower MTZ, ponding there, and then being released by powerful injections into the upper melt lens. The magma chamber covers the diapir at a distance of 5 km from the ridge axis.     

Dehydration Melting of Metabasalt at 8-32 kbar: Implications for Continental Growth and Crust-Mantle Recycling

We report the results of partial melting experiments between8 and 32 kbar, on four natural amphibolites representative ofmetamorphosed Archean tholeiite (greenstone), high-alumina basalt,low-potassium tholeiite and alkali-rich basalt. For each rock,we monitor changes in the relative proportions and compositionof partial melt and coexisting residual (crystalline) phasesfrom 1000 to 1150C, within and beyond the amphibole dehydrationreaction interval. Low percentage melts coexisting with an amphiboliteor garnet amphibolite residue at 1000–1025C and 8–16kbar are highly silicic (high-K₂O granitic at 5%; melting, low-Al₂O₃trondhjemitic at 5–10%). Greater than 20% melting is onlyachieved beyond the amphibole-out phase boundary. Silicic tointermediate composition liquids (high-Al₂O₃ trondhjemitic-tonalitic,granodioritic, quartz dioritic, dioritic) result from 20–40%melting between 1050 and 1100C, leaving a granulite (plagioclase+ clinopyroxene orthopyroxene olivine) residue at 8 kbarand garnet granulite to eclogite (garnet + clinopyroxene) residuesat 12–32 kbar. Still higher degrees of melting ( 40–60%)result in mafic liquids corresponding to low-MgO, high-Al₂O₃basaltic and basaltic andesite compositions, which coexist withgranulitic residues at 8 kbar and edogitic or garnet granulitic(garnet + clinopyroxene + plagioclase orthopyroxene) residuesat higher pressures (12–28 kbar). As much as 40% by volumehigh-Al₂O₃ trondhjemitic-tonalitic liquid coexists with an eclogiticresidue at 1100–1150C and 32 kbar. The experimental datasuggest that the Archean tonalite-trondhjemite-granodiorite(TTG) suite of rocks, and their Phanerozoic equivalents, thetonalite-trondhjemite-dacite suite (including ‘adakites’and other Na-rich granitoids), can be generated by 10–40%melting of partially hydrated metabasalt at pressures abovethe garnet-in phase boundary (12 kbar) and temperatures between1000 and 1100C. Anomalously hot and/or thick metabasaltic crustis implied. Although a rare occurrence along modern convergentplate margins, subductionrelated melting of young, hot oceaniccrust (e.g. ocean ridges) may have been an important (essential)element in the growth of the continental crust in the Archean,if plate tectonic processes were operative. Coupled silicicmelt generation-segregation and mafic restite disposal may alsooccur at the base of continental or primitive (sub-arc?) crust,where crustal overthickening is a consequence of underplatingand overaccretion of mafic magmas. In either setting, net growthof continental crust and crustmantle recycling may be facilitatedby relatively high degrees of melting and extreme density contrastsbetween trondhjemitictonalitic liquids and garnet-rich residues.Continuous chemical trends are apparent between the experimentalcrystalline residues, and mafic migmatites and garnet granulitexenoliths from the lower crust, although lower-crustal xenolithsin general record lower temperatures (600–900C) and pressures(5–13 kbar) than corresponding residual assemblages fromthe experiments. However, geo-thermobarometry on eclogite xenolithsin kimberlites from the subcontinental mantle indicates conditionsappropriate for melting through and beyond the amphibole reactioninterval and the granulite-eclogite transition. If these samplesrepresent ancient (eclogitized) remnants of subducted or otherwisefoundered basaltic crust, then the intervening history of theirprotoliths may in some cases include partial melting. KEY WORDS: dehydration melting; metabasalt; continental growth; crust–mantle recycling ^*Corresponding author. Present address: Mineral Physics Institute and Center for High Pressure Research, Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794, USA     

Lower Crustal Magma Genesis and Preservation: a Stochastic Framework for the Evaluation of Basalt-Crust Interaction

We present a quantitative assessment of the thermal and dynamicresponse of an amphibolitic lower crust to the intrusion ofbasaltic dike swarms in an arc setting. We consider the effectof variable intrusion geometry, depth of intrusion, and basaltflux on the production, persistence, and interaction of basalticand crustal melt in a stochastic computational framework. Distinctmelting and mixing environments are predicted as a result ofthe crustal thickness and age of the arc system. Shallow crustal(30 km) environments and arc settings with low fluxes of mantle-derivedbasalt are likely repositories of isolated pods of mantle andcrustal melts in the lower crust, both converging on daciticto rhyodacitic composition. These may be preferentially rejuvenatedin subsequent intrusive episodes. Mature arc systems with thickercrust (50 km) produce higher crustal and residual basaltic meltfractions, reaching 0·4 for geologically reasonable basaltfluxes. The basaltic to basaltic andesite composition of bothcrustal and mantle melts will facilitate mixing as the networkof dikes collapses, and Reynolds numbers reach 10^–4–1·0in the interiors of dikes that have been breached by ascendingcrustal melts. This may provide one mechanism for melting, assimilation,storage and homogenization (MASH)-like processes. Residual mineralassemblages of crust thickened by repeated intrusion are predictedto be garnet pyroxenitic, which are denser than mantle peridotiteand also generate convective instabilities where some of thecrustal material is lost to the mantle. This reconciles thethinner than predicted crust in regions that have undergonea large flux of mantle basalt for a prolonged period of time,and helps explain the enrichment of incompatible elements suchas K₂O, typical of mature arc settings, without the associatedmass balance problem. KEY WORDS: crustal anatexis; delamination; lower crust; magma mixing; thermal model     

Thermal Emplacement Model for the Alpine Lherzolite Massif at Balmuccia, Italy

The spinel lherzolite massif at Balmuccia, northwest Italy,forms an elongate north-south trending lens (4.5 x 0.5 x 1.1km) within the pre-Alpine granulite basement complex of theIvrea zone. The western contact is a mylonite fault zone formedduring late emplacement cataclastic flow near the Insubric line;to the east the lherzolite massif is separated from the granulitesby a magmatic sheath of layered pyroxenites, pyroxene pegmatitesand meta-gabbros. Pyroxene reaction zones on gabbro dikes indunite pods which lie east of the main lherzolite massif showthat emplacement occurred at pressures >9 kb, based on peridotiteequilibria studies. Phase chemistry calculations on pyroxenitesand granulites show ambient P–T conditions to have been850 °C (Cpx–Opx equilibria) and 10–13 kb (Opx–Gt;Plg–Gt–Sill–Qtz) during emplacement of thelherzolite massif. Temperature calculations on 12 peridotitesfrom throughout the massif suggest an earlier high-T stage (1200°C; Ol–Px–Sp) followed by partial re-equilibrationat lower T (850–950 °C; Cpx–Opx). The areaswithin the lherzolite massif with the highest calculated Ol–Px–Sptemperatures have the lowest Cpx–Opx temperatures, suggestingthat the apparent Cpx–Opx temperatures are due to re-equilibrationduring emplacement. The spinel lherzolite probably originatedat 12 and 20 kb, based on the mineral assemblage Ol + Opx +Cpx + Sp + Hnbd. The inferred P–T ranges put both themassif and the granulites on a geotherm that is high for continentalcrust and implies a high surface heat flow at the time of emplacement(2.2 µcal/cm² sec). The Balmuccia area later became thelocus of early Mesozoic rifting between the North and SouthAlpine plates. These relationships at Balmuccia are similarto the Great Basin of the western United States, where mantlexenoliths in young basalts that show P–T conditions of1100–1300 °C at 17–20 kb, occur in an area ofhigh heat flow (2.0 µCal/cm² sec average) and extension.This suggests an association between up-welling of mantle peridotitesbelow continents and ensialic tensional tectonics.