Petrogenesis of Cogenetic Silica-Oversaturated and -Undersaturated Syenites by Periodic Recharge in a Crustally Contaminated Magma Chamber: the Kangerlussuaq Intrusion, East Greenland

The Palaeogene Kangerlussuaq Intrusion (50 Ma) of East Greenlanddisplays concentric zonation from quartz-rich nordmarkite (quartzsyenite) at the margin, through pulaskite, to foyaite (nephelinesyenite) in the centre; modal layering and igneous laminationare locally developed but there are no internal intrusive contacts.This is an apparent violation of the phase relations in Petrogeny'sResidua System. We propose that this intrusion is layered, gradingfrom quartz syenite at the bottom to nepheline syenite at thetop. Mineral and whole-rock major and trace element data andSr–Nd–Hf–Pb isotope data are presented thatprovide constraints on the petrogenesis of the intrusion. Radiogenicisotope data indicate a continuously decreasing crustal componentfrom the quartz nordmarkites (⁸⁷Sr/⁸⁶Sr = 0·7061; _Ndi= 2·3; _Hfi = 5·2; ²⁰⁶Pb/²⁰⁴Pb_meas = 16·98)to the foyaites (⁸⁷Sr/⁸⁶Sr = 0·7043–0·7044;_Ndi = 3·8–4·9; _Hfi = 10·7–11·1;²⁰⁶Pb/²⁰⁴Pb_meas = 17·78–17·88); the foyaitesare dominated by a mantle isotopic signature. The average Mg-numberof amphibole cores becomes increasingly primitive, varying from26·4 in the nordmarkites to 57·4 in the pulaskites.Modal layering, feldspar lamination and the presence of hugebasaltic xenoliths derived from the chamber roof, now restingon the transient chamber floor, demonstrate bottom-upwards crystallization.The intrusion cannot, therefore, have formed in a system closedto magmatic recharge. The lack of gneissic xenoliths in thenordmarkites suggests that most contamination took place deeperin the crust. In the proposed model, the nordmarkitic magmaformed during crustal assimilation in the roof zone of a large,silica-undersaturated alkali basaltic/basanitic, stratifiedmagma chamber, prior to emplacement in the uppermost crust.The more primitive syenites, terminating with foyaite at thetop of the intrusion, formed as a consequence of repeated rechargeof the Kangerlussuaq Intrusion magma chamber by tapping lesscontaminated, more primitive phonolitic melt from deeper partsof the underlying chamber during progressive armouring of theplumbing system. KEY WORDS: Kangerlussuaq; East Greenland; syenite; crustal contamination; magma mixing     

The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition

Re-examination of the Skaergaard intrusion in the context ofits regional setting, combined with new data from explorationdrilling, has resulted in a revised structural model for theintrusion. It is modelled as an irregular box, c. 11 km fromnorth to south, up to 8 km from east to west, and 3·4–4km from the lower to the upper contact. The walls of the intrusionare inferred to follow pre-existing and penecontemporaneoussteep faults, and the floor and roof seem largely controlledby bedding planes in the host sediments and lavas, similar toregional sills. The suggested shape and volume are in agreementwith published gravimetric modelling. Crystallization alongall margins of the intrusion concentrated the evolving meltin the upper, central part of the intrusion, best visualizedas an ‘onion-skin’ structure inside the box. Thetotal volume is estimated to c. 280 ± 23 km³, of which13·7% are referred to the Upper Border Series (UBS),16·4% to the Marginal Border Series (MBS) and 69·9%to the Layered Series (LS). In the LS, the Lower Zone (LZ) isestimated to constitute 66·8%, the Middle Zone (MZ) 13·5%and the Upper Zone (UZ) 19·7%. The new volume relationshipsprovide a mass balance estimate of the major and trace elementbulk composition of the intrusion. The parental magma to theSkaergaard intrusion is similar to high-Ti East Greenland tholeiiticplateau basalts with Mg number c. 0.45. The intrusion representsthe solidification of contemporary plateau basalt magma trappedand crystallized under closed-system conditions in a crustalreservoir at the developing East Greenland continental margin. KEY WORDS: bulk composition; emplacement; mass proportions; Skaergaard intrusion; structure     

The Zoned Plagioclase of the Skaergaard Intrusion, East Greenland

The variation in zoning and An-content of plagioclases in theSkaergaard intrusion was investigated using microprobe analysisand optical methods on material from the surface of the intrusionas well as from a drill core from the hidden zone of the layeredseries. The plagioclases display cryptic variation and prominentvariations in type of zoning with structural height: oscillatoryzoned and resorbed plagioclase cores are predominant in thehidden zone, unzoned cores are typical of the lower zone, andplagioclases of the upper zone display skeletal zoning. Thereverse, oscillatory and skeletal zoning of plagioclase coresis ascribed to supercooled crystallization, and not to convectionor other petrogenetic factors. The variations in type of zoningand An-content of plagioclases from the hidden zone suggestonset of convection at the same time in the cooling historyof the intrusion as is indicated by the transition between thetranquil division and the banded division of the marginal bordergroup. A crystallization model is suggested, which accountsfor the observed variations. The variation in An-content ofplagioclases from the hidden zone suggests a limited extensionof some hundred metres of the intrusion at depth.     

Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanicrock characterized by spinifex texture, there is a growing recognitionthat similar textures can also form in high-level dykes andsills. Here, we report the results of a petrological and geochemicalinvestigation of a 5 m thick komatiite sill in Dundonald Township,Ontario, Canada. This unit forms part of a series of komatiitesand komatiitic basalts, some of which clearly intruded unconsolidatedsediments. The komatiite sill is differentiated into a spinifex-texturedupper part and an olivine cumulate lower part. Features characteristicof the upper sections of lava flows, such as volcanic brecciaand a thick glassy chilled margin, are absent and, instead,the upper margin of the sill is marked by a layer of relativelylarge (1–5 mm) solid, polyhedral olivine grains that gradesdownwards over a distance of only 2 cm into unusually large,centimetre-sized, skeletal hopper olivine grains. This is underlainby a 1 m thick zone of platy spinifex-textured olivine and coarse,complex, dendritic, spinifex-textured olivine. The texture ofthe olivine cumulate zone in the overlying unit is uniform rightdown to the contact and a lower chilled margin, present at thebase of all lava flows, is absent. The textures in the silland the overlying unit are interpreted to indicate that thesill intruded the olivine cumulate zone of the overlying unit.Thermal modelling suggests that soon after intrusion, a narrowinterval of the overlying cumulate partially melted and thatthe liquid in the upper part of the sill became undercooled.The range of olivine morphologies in the spinifex-textured partof the sill was controlled by nucleation and crystallizationof olivine in these variably undercooled liquids. KEY WORDS: komatiite; intrusion; spinifex texture; olivine     

Partial Crystallization of Mid-Ocean Ridge Basalts in the Crust and Mantle

Pressures at which partial crystallization occurs for mid-oceanridge basalts (MORB) have been examined by a new petrologicalmethod that is based on a parameterization of experimental datain the form of projections. Application to a global MORB glassdatabase shows that partial crystallization of olivine + plagioclase+ augite ranges from 1 atm to 1·0 GPa, in good agreementwith previous determinations, and that there are regional variationsthat generally correlate with spreading rate. MORB from fast-spreadingcenters display partial crystallization in the crust at ridgesegment centers and in both mantle and crust at ridge terminations.Fracture zones are likely to be regions where magma chambersare absent and where there is enhanced conductive cooling ofthe lithosphere at depth. MORB from slow-spreading centers displayprominent partial crystallization in the mantle, consistentwith models of enhanced conductive cooling of the lithosphereand the greater abundance of fracture zones through which theypass. In general, magmas that move through cold mantle experiencesome partial crystallization, whereas magmas that pass throughhot mantle may be comparatively unaffected. Estimated pressuresof partial crystallization indicate that the top of the partialmelting region is deeper than about 20–35 km below slow-spreadingcenters and some ridge segment terminations at fast-spreadingcenters. KEY WORDS: MORB; olivine gabbro; partial crystallization; partial melting; ridge segmentation; fracture zones; crust; mantle; lithosphere     

Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions

Textural maturity describes the extent to which a rock has evolvedfrom the initial reaction-controlled texture towards texturalequilibrium controlled by the minimization of interfacial energy.Solidification in a magma chamber results in the formation ofan impingement texture by the random juxtaposition of planar-sidedgrains. Orthocumulates, in which the initial melt-filled poresare pseudomorphed by later-crystallizing phases, have an ophiticor intersertal texture immediately after complete solidification,which then evolves towards solid-state equilibrium by roundingof initially planar grain boundaries and an increase in themedian dihedral angle subtended at the junctions of two primocrysticgrains with the interstitial phase. The bulk of the increasein angle occurs just below the solidus temperature in kilometre-scalemafic plutons. Quantification of textural maturity via measurementof dihedral angle populations in troctolitic and gabbroic cumulatesfrom the Rum Eastern Layered Intrusion and the Skaergaard Intrusiondemonstrates that the rocks preserve a record of thermal eventsrelated to magma chamber replenishment and the onset of chamber-wideconvection. Textural maturity is also a function of the liquidusphase assemblage: for systems in which only olivine and plagioclaseare liquidus (i.e. cumulus) phases in the main magma body abovethe crystal mush, the texture is significantly less mature thanthat in systems in which clinopyroxene is an additional liquidusphase. The difference in textural maturity reflects differencesin the cooling and solidification rate, and demonstrates directlythat the liquidus phase assemblage plays a role in determiningthe thermal history of plutons. KEY WORDS: cumulates; dihedral angles; Rum; Skaergaard; textures     

The Isotopic Composition of Strontium in the Skaergaard Intrusion, East Greenland

The isotopic composition of strontium in the different partsof the differentiated Skaergaard intrusion has been determined.The average Sr⁸⁷/Sr⁸⁶ ratio for the basic rocks was found tobe 0?7065?0?002. Higher values, between 0?7101 and 0?7303, wererestricted to the late-stage acid granophyres. The Sr⁸⁷/Sr⁸⁶ratios for the basic Skaergaard rocks are similar to those foundby previous workers. The enrichment in Sr⁸⁷ expressed in theSr⁸⁷/Sr⁸⁶ ratio is taken to indicate contamination of the acidgranophyres by a source enriched in Sr⁸⁷. From considerationsbased upon circumstantial evidence the average country rock,composed of old Precambrian acid to intermediate gneiss, isnot sufficiently enriched in.Sr⁸⁷ to account for the Sr⁸⁷ enrichmentobserved in the acid granophyres by a simple assimilation process.At the present stage of the investigation the enrichment ofthe acid granophyres in Sr⁸⁷ is unexplained.     

The Cocos and Carnegie Aseismic Ridges: a Trace Element Record of Long-term Plume-Spreading Center Interaction

The aseismic Cocos and Carnegie Ridges, two prominent bathymetricfeatures in the eastern Pacific, record 20 Myr of interactionbetween the Galápagos hotspot and the adjacent GalápagosSpreading Center. Trace element data determined by inductivelycoupled plasma-mass spectrometry in >90 dredged seamountlavas are used to estimate melt generation conditions and mantlesource compositions along the ridges. Lavas from seamount provinceson the Cocos Ridge are alkalic and more enriched in incompatibletrace elements than any in the Galápagos archipelagotoday. The seamount lavas are effectively modeled as small degreemelts of a Galápagos plume source. Their eruption immediatelyfollows the failure of a rift zone at each seamount province'slocation. Thus the anomalously young alkalic lavas of the CocosRidge, including Cocos Island, are probably caused by post-abandonmentvolcanism following either a ridge jump or rift failure, andnot the direct activity of the Galápagos plume. The seamountshave plume-like signatures because they tap underlying mantlepreviously infused with Galápagos plume material. Whereasplume heterogeneities appear to be long-lived, tectonic rearrangementsof the ridge plate boundary may be the dominant factor in controllingregional eruptive behavior and compositional variations. KEY WORDS: mantle plume; mid-ocean ridge; Galápagos; abandoned rift; partial melting of the mantle     

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     

Volcanism in the Vitim Volcanic Field, Siberia: Geochemical Evidence for a Mantle Plume Beneath the Baikal Rift Zone

The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km³ of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. ⁸⁷Sr/⁸⁶Sr_i(0·7039–0·7049), ¹⁴³Nd/¹⁴⁴Nd_i (0·5127–0·5129)and ¹⁷⁶Hf/¹⁷⁷Hf_i (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na₂O, P₂O₅, K₂O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]_n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting     

Experimental Petrology of the Kiglapait Intrusion: Cotectic Trace for the Lower Zone at 5 kbar in Graphite

The inferred crystallization history of the troctolitic LowerZone of the Kiglapait Intrusion in Labrador is tested by meltingmineral mixtures from the intrusion, made to yield the observedcrystal compositions on the cotectic trace of liquid, plagioclase,and olivine. Melting experiments were made in a piston-cylinderapparatus, using graphite capsules at 5 kbar. Lower Zone assemblagescrystallized from 1245°C, 5% normative augite in the liquid,to 1203°C, 24% normative augite in the liquid at saturationwith augite crystals. This transit is consistent with modaldata and the large volume of the Lower Zone. The 1245°Ccotectic composition matches the average Inner Border Zone composition.Quenched troctolitic liquid from the Upper Border Zone, andothers from nearby Newark Island, plot on or near our experimentalcotectic, supporting a common fractionation history. Olivine–plagioclaseintergrowths from cotectic troctolitic melt show mosaic texturesreflecting the differing barriers to nucleation of these twophases. The linear partitioning of X_Ab in plagioclase–meltyields an intercept constant K_D = 0·524 for these maficmelts. Observed subsolidus exchange of Ca between plagioclaseand olivine elucidates the loss of Ca from plutonic olivines.The bulk composition of the intrusion is revised downward inFo and An. KEY WORDS: experimental; olivine; plagioclase; Kiglapait; partitioningAbbreviations: AP, MT, IL, OR, AB, AN, DI, HY, OL, FO, NE, Q, FSP, AUG: (Oxygen) Normative components; Ap, Aug, Ilm, Ol, Pl: Phases; Ab, An, Di, Fa, Fo, Or, Wo: Phase components; also ternary endmembers; BSE: Back-scattered electron; CaTs: Calcium Tschermak's component, CaAlAlSiO₆; D: Partition coefficient; f: Fugacity; F_L: Fraction of the system present as liquid = 1 – (PCS/100); FMQ: Fayalite = magnetite + quartz buffer; IBZ: Inner Border Zone; IW: Iron = wüstite buffer; kbar: kilobar, 10⁸ pascal; K_D: Exchange coefficient; KI: Kiglapait Intrusion; L: Liquid phase; LLD: Liquid line of descent; Ma: Mega-annum, age; Myr: Mega-year, time; OLHY: Normative OL + HY; OLRAT: The ratio OLHY/(OLHY + AUG); P: Pressure; P: Phosphorus; PCS: Percent solidified (volume); SMAR: South Margin average composition; T: Temperature, °C; UBZ: Upper Border Zone; WM: Wüstite = magnetite buffer; Wo: Wollastonite component of pyroxene; X: Mole fraction; X_Mg: Molar ratio Mg/(Mg + Fe²⁺); , X_Mg(0): Initial X_Mg before MT is formed in the norm calculation; X: Coordinate, horizontal axis; Y: Coordinate, vertical axis     

High-pressure Partial Melting of Mafic Lithologies in the Mantle

We review experimental phase equilibria associated with partialmelting of mafic lithologies (pyroxenites) at high pressuresto reveal systematic relationships between bulk compositionsof pyroxenite and their melting relations. An important aspectof pyroxenite phase equilibria is the existence of the garnet–pyroxenethermal divide, defined by the enstatite–Ca-Tschermakspyroxene–diopside plane in CaO–MgO–Al₂O₃–SiO₂projections. This divide appears at pressures above 2 GPa inthe natural system where garnet and pyroxenes are the principalresidual phases in pyroxenites. Bulk compositions that resideon either side of the divide have distinct phase assemblagesfrom subsolidus to liquidus and produce distinct types of partialmelt ranging from strongly nepheline-normative to quartz-normativecompositions. Solidus and liquidus locations are little affectedby the location of natural pyroxenite compositions relativeto the thermal divide and are instead controlled chiefly bybulk alkali contents and Mg-numbers. Changes in phase volumesof residual minerals also influence partial melt compositions.If olivine is absent during partial melting, expansion of thephase volume of garnet relative to clinopyroxene with increasingpressure produces liquids with high Ca/Al and low MgO comparedwith garnet peridotite-derived partial melts. KEY WORDS: experimental petrology; mantle heterogeneity; partial melting; phase equilibrium; pyroxenite     

The Petrology and Geochemistry of Volcanic Rocks on Jeju Island: Plume Magmatism along the Asian Continental Margin

The incompatible element signatures of volcanic rocks formingJeju Island, located at the eastern margin of the Asian continent,are identical to those of typical intraplate magmas. The sourceof these volcanic rocks may be a mantle plume, located immediatelybehind the SW Japan arc. Jeju plume magmas can be divided intothree series, based on major and trace element abundances: high-aluminaalkalic, low-alumina alkalic, and sub-alkalic. Mass-balancecalculations indicate that the compositional variations withineach magma series are largely governed by fractional crystallizationof three chemically distinct parental magmas. The compositionsof primary magmas for these series, using inferred residualmantle olivine compositions, suggest that the low-alumina alkalicand sub-alkalic magmas are generated at the deepest and shallowestdepths by lowest and highest degrees of melting, respectively.These estimates, together with systematic differences in traceelement and isotopic compositions, indicate that the upper mantlebeneath Jeju Island is characterized by an increased degreeof metasomatism and a change in major metasomatic hydrous mineralsfrom amphibole to phlogopite with decreasing depth. The originalplume material, having rather depleted geochemical characteristics,entrained shallower metasomatized uppermost mantle material,and segregated least-enriched low-alumina alkalic, moderatelyenriched high-alumina alkalic, and highly enriched sub-alkalicmagmas, with decreasing depth. KEY WORDS: Jeju Island; magma genesis; mantle plume; subcontinental mantle     

Olivine in the Udachnaya-East Kimberlite (Yakutia, Russia): Types, Compositions and Origins

Olivine is the principal mineral of kimberlite magmas, and isthe main contributor to the ultramafic composition of kimberliterocks. Olivine is partly or completely altered in common kimberlites,and thus unavailable for studies of the origin and evolutionof kimberlite magmas. The masking effects of alteration, commonin kimberlites worldwide, are overcome in this study of theexceptionally fresh diamondiferous kimberlites of the Udachnaya-Eastpipe from the Daldyn–Alakit province, Yakutia, northernSiberia. These serpentine-free kimberlites contain large amountsof olivine (50 vol.%) in a chloride–carbonate groundmass.Olivine is represented by two populations (olivine-I and groundmassolivine-II) differing in morphology, colour and grain size,and trapped mineral and melt inclusions. The large fragmentalolivine-I is compositionally variable in terms of major (Fo_85–94)and trace element concentrations, including H₂O content (10–136ppm). Multiple sources of olivine-I, such as convecting andlithospheric mantle, are suggested. The groundmass olivine-IIis recognized by smaller grain sizes and perfect crystallographicshapes that indicate crystallization during magma ascent andemplacement. However, a simple crystallization history for olivine-IIis complicated by complex zoning in terms of Fo values and traceelement contents. The cores of olivine-II are compositionallysimilar to olivine-I, which suggests a genetic link betweenthese two types of olivine. Olivine-I and olivine-II have oxygenisotope values (+ 5·6 ± 0·1 VSMOW, 1 SD)that are indistinguishable from one another, but higher thanvalues (+ 5·18 ± 0·28) in ‘typical’mantle olivine. These elevated values probably reflect equilibriumwith the Udachnaya carbonate melt at low temperatures and ¹⁸O-enrichedmantle source. The volumetrically significant rims of olivine-IIhave constant Fo values (89·0 ± 0·2 mol%),but variable trace element compositions. The uniform Fo compositionsof the rims imply an absence of fractionation of the melt'sFe²⁺/Mg, which is possible in the carbonatite melt–olivinesystem. The kimberlite melt is argued to have originated inthe mantle as a chloride–carbonate liquid, devoid of ‘ultramafic’or ‘basaltic’ aluminosilicate components, but becameolivine-laden and olivine-saturated by scavenging olivine crystalsfrom the pathway rocks and dissolving them en route to the surface.During emplacement the kimberlite magma changed progressivelytowards an original alkali-rich chloride–carbonate meltby extensively crystallizing groundmass olivine and gravitationalseparation of solids in the pipe. KEY WORDS: kimberlite; olivine; partial melting; carbonatitic melt; oxygen isotopes; H₂O     

Pyroxenes from the Late Stages of Fractionation of the Skaergaard Intrusion, East Greenland

The pyroxenes of the upper zone of the Skaergaard layered seriesconsist of an iron-rich series of brown and green clinopyroxenes.Five new analyses are presented, together with a revised trendline which includes the full range of clinopyroxenes believedrepresentative of the Skaergaard fractionation sequence. Therange for the augite-ferroaugite-ferrohedenbergite series isfrom Ca_42?4 Mg_47?9 Fe_9?7 to Ca_42?5 Mg_0?4 Fe_57?1, the most ferriferousvariety coexisting with a pure fayalite. A re-study of the compositionsand textures of certain green ferrohedenbergites supports thebelief that they are the product of sub-solidus inversion offerriferous ß-wollaston?tes, which crystallized asa temporary phase between the periods when brown ferrohedenbergitescrystallized directly from the magma. A consideration of thecompositional and textural relationships between green and brownpyroxenes, and the significance of the mosaic inversion texture,have led to an interpretation of the crystallization and inversionsequence for these minerals. Four new analyses of ferrohedenbergitesfrom the downward-crystallized upper border group rocks provideevidence for a trend which differs slightly from that for thelayered series ferrohedenbergites.     

Garnet-bearing Xenoliths from Salt Lake Crater, Oahu, Hawaii: High-Pressure Fractional Crystallization in the Oceanic Mantle

The focus of this study is a suite of garnet-bearing mantlexenoliths from Oahu, Hawaii. Clinopyroxene, olivine, and garnetconstitute the bulk of the xenoliths, and orthopyroxene is presentin small amounts. Clinopyroxene has exsolved orthopyroxene,spinel, and garnet. Many xenoliths also contain spinel-coredgarnets. Olivine, clinopyroxene, and garnet are in major elementchemical equilibrium with each other; large, discrete orthopyroxenedoes not appear to be in major-element chemical equilibriumwith the other minerals. Multiple compositions of orthopyroxeneoccur in individual xenoliths. The new data do not support theexisting hypothesis that all the xenoliths formed at 1 6–22GPa, and that the spinel-cored garnets formed as a consequenceof almost isobaric subsolidus cooling of a spinel-bearing assemblage.The lack of olivine or pyroxenes in the spinel–garnetreaction zones and the embayed outline of spinel grains insidegarnet suggest that the spinel-cored garnets grew in the presenceof a melt. The origin of these xenoliths is interpreted on thebasis of liquidus phase relations in the tholeiitic and slightlysilica-poor portion of the CaO–MgO–Al₂O₃–SiO₂(CMAS) system at pressures from 30 to 50 GPa. The phase relationssuggest crystallization from slightly silica-poor melts (ortransitional basaltic melts) in the depth range 110–150km beneath Oahu. This depth estimate puts the formation of thesexenoliths in the asthenosphere. On the basis of this study itis proposed that the pyroxenite xenoliths are high-pressurecumulates related to polybaric magma fractionation in the asthenosphere,thus making Oahu the only locality among the oceanic regionswhere such deep magmatic fractional crystallization processeshave been recognized. KEY WORDS: xenolith; asthenosphere; basalt; CMAS; cumulate; oceanic lithosphere; experimental petrology; mantle; geothermobarometry; magma chamber     

Two different pyroxene crystallisation trends in the trough bands of the Skaergaard Intrusion,East Greenland

Pyroxenes and olivines from the trough bands in the Upper Zone (UZa) of Skaergaard Intrusion have been investigated, together with previously analysed pyroxenes (Brown, 1957; Brown and Vincent, 1963) and olivines from the Layered Series ferrodiorites. The electron microprobe, electron microscope, and analytical electron microscope EMMA-4 were used. Results show a striking difference between the cumulus and intercumulus trends of the trough-band pyroxenes. The cumulus trend follows that of the main Layered Series whereas the intercumulus trend shows a shrinking of the miscibility gap together with great enrichment in the Fs molecule, the miscibility gap being symmetrical about ～Wo₂₄. The shrinking appears to be a function of the different crystallisation conditions in the intercumulus liquid which was closed off in “cells” from the main mass of supernatant liquid. Enrichment in the Fs molecule is due to the much lower crystallisation temperatures of the intercumulus pyroxenes. Iron enrichment is also reflected in the intercumulus olivines. For the cumulus trend, Brown's calcium-poor pyroxene trend (1957) has been extended into more iron-rich parts of the pyroxene quadrilateral, well after olivine has reappeared and subsequent to the increase in calcium of the ferroaugites. The subsolidus trend for pyroxenes in the Fs-rich region has also been established.     

Geochemical Evidence for Slab Melting in the Trans-Mexican Volcanic Belt

Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo–Zitácuaro volcanic field (VBZ) incentral Mexico indicate that slab melting plays a key role inthe petrogenesis of the Trans-Mexican Volcanic Belt. Rocks fromthe VBZ are typical arc-related high-Mg andesites, but two differentrock suites with distinct trace element patterns and isotopiccompositions erupted concurrently in the area, with a traceelement character that is also distinct from that of other Mexicanvolcanoes. The geochemical differences between the VBZ suitescannot be explained by simple crystal fractionation and/or crustalassimilation of a common primitive magma, but can be reconciledby the participation of different proportions of melts derivedfrom the subducted basalt and sediments interacting with themantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlateinversely with Pb and Sr isotopic compositions, indicating thatthe Sr and Pb budgets are strongly controlled by melt additionsfrom the subducted slab. In contrast, an inverse correlationbetween Pb(Th)/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios, which extend to lowerisotopic values than those for Pacific mid-ocean ridge basalts,indicates the participation of an enriched mantle wedge thatis similar to the source of Mexican intraplate basalts. In addition,a systematic decrease in middle and heavy rare earth concentrationsand Nb/Ta ratios with increasing SiO₂ contents in the VBZ rocksis best explained if these elements are mobilized to some extentin the subduction flux, and suggests that slab partial fusionoccurred under garnet amphibolite-facies conditions. KEY WORDS: arcs; mantle; Mexico; sediment melting; slab melting     

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