Experimental Petrology of a Highly Potassic Magma

The melting behaviour of a highly potassic biotite mafuriteof the Central African olivine leucitite kindred has been studiedexperimentally as a function of pressure (to 30kb) temperature,and water content (0%, 5%, 15%, 25%, and 40% H₂O). Olivine isthe liquidus phase up to 30 kb for all water contents studiedexcept for anhydrous (clinopyroxene on the liquidus) and 15%H₂O (phlogopite on the liquidus) conditions. Analyses of phasescrystallizing from the biotite mafurite show that pressure hasvery little effect on the composition of clinopyroxene whichis extremely calcium-rich, and low in Al₂O₃ and TiO₂ for allconditions investigated. Phlogopite has low TiO₂ content andtitanphlogopite cannot be a refractory phase in the upper mantlecausing Ti-depletion in partial melts in equilibrium with titanphlogopite.There are apparently no conditions where the extremely potassicbiotite mafurite could be a partial melt from pyrolite but derivationfrom an olivine+clinopyroxene+phlogopite+ilmenite assemblageoccurring as ‘enriched’ patches in the upper mantle,is possible. Liquids in equilibrium with phlogopite as a residualphase at 30 kb would be olivine nephelinites with approximately5% K₂O, Na₂O/K₂O 1 and TiO₂ > 5+. Crystal elutriation withtransported residual phlogopite reacting (phlogopite+liquid1 olivine+liquid 2) at lower pressures provides a mechanismfor K-enrichment and generating Na₂O/K₂O < 1.     

Petrology and Petrogenesis of Cumulate Peridotites and Gabbros from the Marum Ophiolite Complex, Northern Papua New Guinea

The Marum ophiolite complex in northern Papua New Guinea includesa thick (3–4 km) sequence of ultramafic and mafic cumulates,which are layered on a gross scale from dunite at the base upwardsthrough wehrlite, lherzolite, plagioclase lherzolite, pyroxenite,olivine norite-gabbro and norite-gabbro to anorthositic gabbroand ferrogabbro at the top. Igneous layering and structures,and cumulus textures indicate an origin by magmatic crystallizationin a large magma chamber(s) from magma(s) of evolving composition.Most rocks however show textural and mineralogical evidenceof subsolidus re-equilibration. The cumulate sequence is olivine and chrome spinel followedby clinopyroxene, orthopyroxene and plagioclase, and the layeredsequence is similar to that of the Troodos and Papuan ophiolites.These sequences differ from ophiolites such as Vourinos by thepresence of cumulus magnesian orthopyroxene, and are not consistentwith accumulation of low pressure liquidus phases of mid-oceanridge-type olivine tholeiite basalts. The cumulus phases show cryptic variation from Mg- and Ca-richearly cumulates to lower temperature end-members, e.g. olivineMg_93–78, plagioclase An_94–63. Co-existing pyroxenesdefine a high temperature solidus with a narrower miscibilitygap than that of pyroxenes from stratiform intrusions. Re-equilibratedpyroxene pairs define a low-temperature, subsolidus solvus.Various geothermometers and geobarometers, together with thermodynamiccalculations involving silica buffers, suggest the pyroxene-bearingcumulates crystallized at 1200 °C and 1–2 kb pressureunder low f_O2. The underlying dunites and chromitites crystallizedat higher temperature, 1300–1350 °C. The bulk of thecumulates have re-equilibrated under subsolidus conditions:co-existing pyroxenes record equilibration temperatures of 850–900°C whereas olivine-spinel and magnetite-ilmenite pairs indicatefinal equilibration at very low temperatures (600 °C). Magmas parental to the cumulate sequence are considered to havebeen of magnesian olivine-poor tholeiite composition (>50per cent SiO₂, 15 per cent MgO, 100 Mg/(Mg + Fe²⁺) 78) richin Ni and Cr, and poor in TiO₂ and alkalies. Fractionated examplesof this magma type occur at a number of other ophiolites withsimilar cumulate sequences. Experimental studies show that suchlavas may result from ial melting of depleted mantle lherzoliteat shallow depth. The tectonic environment in which the complexformed might have been either a mid-ocean ridge or a back-arebasin.     

Melting experiments on SiO2-rich lamproites to 6.4 GPa and their bearing on the sources of lamproite magmas

Summary Supra-solidus phase relations at temperatures and pressures ranging from 800 to 1700 °C and 2 to 6.4 GPa have been determined experimentally for three silica-rich lamproites: hyalo-leucite phlogopite lamproite (Oscar, West Kimberley); sanidine richterite lamproite (Cancarix, Murcia-Almeria); and phlogopite transitional madupitic lamproite (Middle Table Mountain, Wyoming). All samples have extended melting intervals (500–600 °C). Bulk composition has a significant control on the nature of the initial liquidus phases, with orthopyroxene occurring at low pressures (<4 GPa) in the relatively calcium-poor Oscar and Cancarix lamproites. At higher pressure (>6 GPa) orthopyroxene is replaced by garnet plus clinopyroxene as near-liquidus phases in the Oscar lamproite and by orthopyroxene plus clinopyroxene in the Cancarix sample. Clinopyroxene is a near-liquidus phase at all pressures in the Middle Table Mountain lamproite. Near-solidus phase assemblages at high pressure (>5 GPa) are: clinopyroxene + phlogopite + coesite + rutile + garnet (Oscar); clinopyroxene + garnet + coesite + K–Ti-silicate (Cancarix); clinopyroxene + phlogopite + apatite + K–Ti-silicate (Middle Table Mountain). In all compositions olivine is never found as a liquidus phase at any of the temperatures or pressures studied here. The phase relationships are interpreted to suggest that silica-rich lamproites cannot be derived by the partial melting of lherzolitic sources. Their genesis is considered to involve high degrees of partial melting of ancient metasomatic veins within a harzburgitic-lherzolitic lithospheric substrate mantle. The veins are considered in their mineralogy to be similar to the experimentally-observed, high pressure, near-solidus phase assemblages. The composition of silica-rich primary lamproite magmas differs between cratons as a consequence of differing mineralogical modes of the source veins and different relative contributions from the veins and wall-rocks to the partial melts. Received February 21, 2000; revised version accepted July 3, 2001     

Petrological, Geochemical and Isotopic Constraints on the Origin of the Harzburg Intrusion, Germany

We present mineralogical, petrological and geochemical datato constrain the origin of the Harzburg mafic–ultramaficintrusion. The intrusion is composed mainly of mafic rocks rangingfrom gabbronorite to quartz diorite. Ultramafic rocks are veryrare in surface outcrops. Dunite is observed only in deepersections of the Flora I drill core. Microgranitic (fine-grainedquartz-feldspathic) veins found in the mafic and ultramaficrocks result from contamination of the ultramafic magmas bycrustal melts. In ultramafic and mafic compositions cumulatetextures are widespread and filter pressing phenomena are obvious.The order of crystallization is olivine pargasite, phlogopite,spinel plagioclase, orthopyroxene plagioclase, clinopyroxene.Hydrous minerals such as phlogopite and pargasite are essentialconstituents of the ultramafic cumulates. The most primitiveolivine composition is Fo_89·5 with 0·4 wt % NiO,which indicates that the olivine may have been in equilibriumwith primitive mantle melts. Coexisting melt compositions estimatedfrom this olivine have mg-number = 71. The chemical varietyof the rocks constituting the intrusion and the mg-number ofthe most primitive melt allow an estimation of the approximatecomposition of the mantle-derived primary magma. The geochemicalcharacteristics of the estimated magma are similar to thoseof an island-arc tholeiite, characterized by low TiO₂ and alkalisand high Al₂O₃. Geochemical and Pb, Sr and Nd isotope data demonstratethat even the most primitive rocks have assimilated crustalmaterial. The decoupling of Sr from Nd in some samples demonstratesthe influence of a fluid that transported radiogenic Sr. Leadof crustal origin from two isotopically distinct reservoirsdominates the Pb of all samples. The ultramafic rocks and thecumulates best reflect the initial isotopic and geochemicalsignature of the parent magma. Magma that crystallized in theupper part of the chamber was more strongly affected by assimilatedmaterial. Petrographic, geochemical and isotope evidence demonstratesthat during a late stage of crystallization, hybrid rocks formedthrough the mechanical mixing of early cumulates and melts withstrong crustal contamination from the upper levels of the magmachamber. KEY WORDS: Harzburg mafic–ultramafic intrusion; Sr–Nd–Pb isotopes; magma evolution; crustal contamination     

Petrology of Lamproites from Smoky Butte, Montana

Hyalo-armalcolite-phlogopite lamproites and sanidine-phlogopitelamproites occurring at Smoky Butte, Montana are rocks formedfrom rapidly quenched, high temperature, uncontaminated lamproiticmagma. Petrographic variations are attributable to differentcooling histories of several batches of compositionally identicalmagma. Compared with other occurrences of lamproite, the rocksare unusually rich in TiO₂ and are characterized by the presenceof abundant armalcolite and the most TiO₂-rich phlogopites yetfound in this paragenesis. Compositional data are given fortitanian phlogopite, olivine, diopside, titanian potassian richterite,armalcolite, sanidine, analcite, and glass. The mafic mineralsare Al-deficient and exhibit very little compositional variation.Original leucite has been pseudomorphed by sanidine or analcite.The latter mineral was probably formed at the same time thatthe glass lost K, and gained Na, during alteration by groundwater.All of the lamproites are strongly enriched in Ta, Hf, and thelight REE (La /Yb = 162–280), and have high MgO and Crcontents. Mineralogical, geochemical, and previously publishedisotopic data are combined in developing a petrogenetic modelwhich suggests that these lamproites were derived from an ancient(2.5 Ga) doubly metasomatized harzburgitic source, and thatthey represent relatively primitive lamproites which were intrudedat near-liquidus temperatures.     

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     

Melting Experiments on a Sanidine Phlogopite Lamproite at 4-7 GPa and their Bearing on the Sources of Lamproitic Magmas

Suprasolidus phase relations at pressures from 4 to 7 GPa andtemperatures from 1000 to 1700C have been determined experimentallyfor a sanidine phlogopite lamproite from North Table Mountain,Leucite Hills, Wyoming. The lamproite is silica rich and hasbeen postulated to be representative of the magmas which wereparental to the Leucite Hills volcanic field. Near-liquidusphases above 5 GPa are pyrope-rich garnet and jadeite-rich pyroxene.Below 5 GPa, jadeite-poor pyroxene is the only near-liquidusphase. Near-solidus assemblages consist of clinopyroxene, titanianpotassium richterite and titanian phlogopite with either potassiumtitanian silicate above 5 GPa or potassium feldspar below 5GPa. The potassium titanian silicate is a newly recognized high-pressurephase ranging in composition from K₄Ti₂Si₇O₂₀ to K₄TiSi₈O₂₀.It coexists with coesite at pressures above 6 GPa at 1100–1400C.A previously unrecognized K-Ba-phosphate is a common near-solidusphase. The phase relationships are interpreted to suggest thatlamproites cannot be derived by the partial melting of simplelherzolitic sources. However, it is proposed that sanidine phlogopitelamproites an derived by high degrees of partial melting ofancient metasomatic veins within a harzburgitic–lherzoliticlithospheric substrate mantle. The veins are considered to consistof phlogopite, K–Ti-richterite, K–Ba-phosphate andK–Ti-silicates. KEY WORDS: lamproilte; experimental petrology; upper mantle ^*Corresponding author     

Contact Metamorphosed Ultramafic Rocks in the Western Sierra Nevada Foothills, California

Within the western Sierra Nevada metamorphic belt, linear bodiesof alpine-type ultramafic rock, now composed largely of serpentineminerals, parallel the regional strike and commonly coincidewith major fault zones. Within this metamorphic belt, east ofSacramento, California, ultramafic rocks near a large maficintrusion, the Pine Hill Intrusive Complex, have been emplacedduring at least two separate episodes. Those ultramafic rocks,evidently unaffected by the Pine Hill Intrusive Complex andcomposed largely of serpentine minerals, were emplaced alonga major fault zone after emplacement of the Pine Hill IntrusiveComplex. Those ultramafic rocks, contact metamorphosed by thePine Hill Intrusive Complex, show a zonation of mineral assemblagesas the igneous contact is approached: olivine+antigorite+chlorite+tremolite+Fe-Cr spinel olivine+talc+chlorite+tremolite+Fe-Crspinel olivine+anthophyllite+chlorite+tremolite+Fe-Cr spinel olivine+orthopyroxene+aluminous spinel+hornblende+Fe-Cr spinel.Superimposed on these mineral assemblages are abundant secondaryminerals (serpentine minerals, talc, chlorite, magnetite) whichformed after contact metamorphism. Correlation of observed mineralassemblages with the experimental systems, MgO-SiO₂-H₂O andMgO-Al₂O₃-SiO₂-H₂O suggests an initial contact temperature of775±25 °C for the Pine Hill Intrusive Complex assumingPtotal P_fluid PH₂O. The pressure acting on the metamorphic rockduring emplacement of the intrusion is estimated to be a minimumof 1.5 kb.     

Petrogenesis of Carbonate-orthopyroxenites (Sagvandites) and related rocks from Troms, Northern Norway

Carbonate-orthopyroxenites from Troms consist of enstatite andmagnesite with variable amounts of olivine, talc, serpentine,chlorite, and phlogopite and include the ore minerals chromite,magnetite, pentlandite, pyrite, and in some cases pyrrhotite,heazlewoodite, millerite, and maucherite. Related rocks compriseolivine-magnesite-, talc-magnesite and olivine-orthopyroxene(saxonite)-assemblages. Allochemical replacement reactions,involving mobile CO₂, H₂O, and SiO₂, are shown to comprise themain petrogenetic mechanism. Saxonite, however, may representa possible source material. Discussion of the model system MgO-SiO₂-CO₂-H₂O for P_flukl =2 and 7 kb, respectively, indicates that best agreement withpetrographic evidence is reached assuming elevated pressuresand both gas-excess and gas-deficiency conditions by means oflocal equilibria. The gas-deficient assemblage enstatite+talc+forsterite+magnesiteis presumed to be stable at pressures above 5 kb. Recalculationof whole-rock analyses to a CO₂-free basis by several alternativemethods suggests that rock evolution could have followed thetrend dunite saxonite orthopyroxenite sagvandite+related rocks.A simple geometric method is used to outline possible schemesof rock evolution, involving gas-deficient phase assemblages.     

Experimental Crystallization of a High-K Arc Basalt: the Golden Pumice, Stromboli Volcano (Italy)

The near-liquidus crystallization of a high-K basalt (PST-9golden pumice, 49·4 wt % SiO₂, 1·85 wt % K₂O,7·96 wt % MgO) from the present-day activity of Stromboli(Aeolian Islands, Italy) has been experimentally investigatedbetween 1050 and 1175°C, at pressures from 50 to 400 MPa,for melt H₂O concentrations between 1·2 and 5·5wt % and NNO ranging from –0·07 to +2·32.A drop-quench device was systematically used. AuPd alloys wereused as containers in most cases, resulting in an average Feloss of 13% for the 34 charges studied. Major crystallizingphases include clinopyroxene, olivine and plagioclase. Fe–Tioxide was encountered in a few charges. Clinopyroxene is theliquidus phase at 400 MPa down to at least 200 MPa, followedby olivine and plagioclase. The compositions of all major phasesand glass vary systematically with the proportion of crystals.Ca in clinopyroxene sensitively depends on the H₂O concentrationof the coexisting melt, and clinopyroxene Mg-number shows aweak negative correlation with NNO. The experimental data allowthe liquidus surface of PST-9 to be defined. When used in combinationwith melt inclusion data, a consistent set of pre-eruptive pressures(100–270 MPa), temperatures (1140–1160°C) andmelt H₂O concentrations is obtained. Near-liquidus phase equilibriaand clinopyroxene Ca contents require melt H₂O concentrations<2·7–3·6 and 3 ± 1 wt %, respectively,overlapping with the maximum frequency of glass inclusion data(2·5–2·7 wt % H₂O). For olivine to crystallizeclose to the liquidus, pressures close to 200 MPa are needed.Redox conditions around NNO = +0·5 are inferred fromclinopyroxene compositions. The determined pre-eruptive parametersrefer to the storage region of golden pumice melts, which islocated at a depth of around 7·5 km, within the metamorphicarc crust. Golden pumice melts ascending from their storagezone along an adiabat will not experience crystallization ontheir way to the surface. KEY WORDS: basalt; pumice; experiment; phase equilibria; Stromboli     

Klyuchevskoy Volcano, Kamchatka, Russia: The Role of High-Flux Recharged, Tapped, and Fractionated Magma Chamber(s) in the Genesis of High-Al2O3 from High-MgO Basalt

Mineral Chemistry, and major and trace element variations ofthe basalts from Klyuchevskoy, the world's most active islandare volcano, are most consistently explained by the persistenceof a non-steady state, erupting, recharging, and fractionatingmagma chamber in which fractionation of a parental high-MgObasalt melt produces high-Al₂O₃ basalt. Although fractionalcrystallization is the dominant controlling mechanism, periodicrecharge with a more primitive high-MgO basalt is also an importantprocess contributing to the chemical evolution of the magmas.Hybrid basalts are the mixed product of high-Al₂O₃ basalt rechargedwith high-MgO basalt. The lavas range in composition from high-MgO, low-Al₂O₃ ( 12wt. % MgO, 14 wt. % Al₂O₃) to high-Al₂O₃, low-MgO ( 18 wt. %Al₂O₃, 4 wt. % MgO). The high-MgO lavas are characterized byphenocrysts of olivine (cores FO_90–80 and rims FO_85–75)with chromite inclusions [Cr/(Cr + Al)0.7], clinopyroxene (Wo_46–42En_48–42Fs_15–7),and the rare occurrence of orthopyroxene (En_72–70). Allthe phenocrysts are normally zoned and set in a groundmass ofplagioclase, pigeonite, clinopyroxene, magnetite, orthopyroxene.The high-Al₂O₃ basalts contain plagioclase (An_85–55),olivine (Fo_80–65), clinopyroxene (Wo_45–30En_50–38Fs_23–11), orthopyroxene (En_72–70) phenocrysts, that preserve bothnormal and reverse zoning in a groundmass of plagioclase, pigeonite,olivine, clinopyroxene, magnetite, orthopyroxene. Hybrid basaltshave characteristics of both high-MgO basalts and high-Al₂O₃basalts and preserve complicated normal-to-reverse, reverse-to-normal,and normally zoned phenocrysts. No hydrous minerals are presentin any of the lavas. The varied basaltic magmas erupted from Klyuchevskoy are derivedfrom a magma chamber(s) located near the base of the Kamchatkacrust (pressures 0.5–0.9 GPa) and characterized by relativelyhigh crystallization temperatures, some in excess of 1150C.Under these conditions, the fractionation of a parental high-MgOmagma, produced principally from the melting of a fluid-fluxed,peridotitic mantle wedge, results in the production of a chemicallydiverse spectrum of basalts ranging from high-MgO, low-Al₂O₃to high-Al₂O₃, low-MgO basalt, traversing the relatively primitiveend of both the calc-alkalic and tholeiitic differentiationtrends.     

Amphibole-bearing Cumulate Inclusions, Grand Canyon, Arizona and their bearing on Silica-Undersaturated Hydrous Magmas in the Upper Mantle

Rare inclusions in Holocene basanite within the western GrandCanyon are comprised of poikilitic titaniferous amphibole togetherwith variable proportions of relatively Fe-rich clinopyroxene,orthopyroxene, olivine, Cr-poor spinel, and pyropic garnet,magnesian ilmenite, and titaniferous phlogopite. No feldsparhas been found in the 219 inclusions investigated. Availableexperimental data suggest crystallization at approximately 20kb (65 km depth) in a region where the crust is 30–40km thick. On the basis of their fabric, the inclusions appear to representcumulates, but other modes of origin cannot be completely ruledout. Anhydrous grains, including some considered to be postcumulusprecipitates, experienced extensive resorption into the interstitialhydrous melt before it ultimately crystallized, perhaps 100?C below liquidus temperatures, as the poikilitic amphibole.In spite of these crystal-melt reactions, and some probablesubsolidus recrystallization as well, systematic variationsin cumulus phase compositions exist and indicate one main precipitationsequence was ol+sp, ol+sp+cpx, cpx+cpx+sp, cpx+sp, cpx+sp+ilm.The local pyropic garnet appears postcumulus in the last threecumulus assemblages. The igneous bodies represented by the inclusions comprised arelatively small portion of the upper mantle sampled by theascending basanitic magma. But in contrast to the thin amphibole-bearingveins in the mantle-derived massif at Etang de Lherz, the igneousbodies beneath the Grand Canyon are considered to be substantiallylarger in dimension, on the order of at least meters ratherthan a few centimeters. Primary nephelinite-basanite melts produced by variable butsmall degrees of partial melting of hydrous upper mantle arenot represented by the poikilitic amphiboles because of complexprocesses at the site of emplacement, including reactions betweenmelt and chromian-spinel peridotite wall rocks.     

A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland

The clinopyroxene–plagioclase–plagioclase dihedralangle, _cpp, in gabbroic cumulates records the time-integratedthermal history in the sub-solidus and provides a measure oftextural maturity. Variations in _cpp through the Layered Seriesof the Skaergaard intrusion, East Greenland, demonstrate thatthe onset of crystallization of clinopyroxene (within LZa),Fe–Ti oxides (at the base of LZc) and apatite (at thebase of UZb) as liquidus phases in the bulk magma is recordedby a stepwise increase in textural maturity, related to an increasein the contribution of latent heat to the total heat loss tothe surroundings and a reduction in the specific cooling rateat the crystallization front of the intrusion. The onset ofboth liquidus Fe–Ti oxide and apatite crystallizationis marked by a transient increase in textural maturity, probablylinked to overstepping before nucleation. Textural maturationat pyroxene–plagioclase–plagioclase triple junctionseffectively ceases in the uppermost parts of the Layered Seriesas a result of the entire pluton cooling below the closure temperaturefor dihedral angle change, which is 1075°C. Solidificationof the Layered Series of the Skaergaard intrusion occurred viathe upwards propagation of a mush zone only a few metres thick. KEY WORDS: magma; partial melting; asthenosphere; olivine; mantle     

Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy)

Garnet clinopyroxenite and garnet websterite layers occur locallywithin mantle peridotite bodies from the External Liguride Jurassicophiolites (Northern Apennines, Italy). These ophiolites werederived from an ocean–continent transition similar tothe present-day western Iberian margin. The garnet clinopyroxenitesare mafic rocks with a primary mineral assemblage of pyrope-richgarnet + sodic Al-augite (Na₂O 2·5 wt %, Al₂O₃ 12·5wt %), with accessory graphite, Fe–Ni sulphides and rutile.Decompression caused Na-rich plagioclase (An_50–45) exsolutionin clinopyroxene porphyroclasts and extensive development ofsymplectites composed of secondary orthopyroxene + plagioclase(An_85–72) + Al-spinel ± clinopyroxene ±ilmenite at the interface between garnet and primary clinopyroxene.Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under syn-kinematicconditions, at the expense of two-pyroxenes + Al-spinel. Mg-richgarnet has been also found in the websterite layers, which arecommonly characterized by the occurrence of symplectites madeof orthopyroxene + Al-spinel ± clinopyroxene. The enclosingperidotites are Ti-amphibole-bearing lherzolites with a fertilegeochemical signature and a widespread plagioclase-facies myloniticfoliation, which preserve in places a spinel tectonite fabric.Lu–Hf and Sm–Nd mineral isochrons (220 ±13 Ma and 186.0 ± 1·8 Ma, respectively) have beenobtained from a garnet clinopyroxenite layer and interpretedas cooling ages. Geothermobarometric estimates for the high-pressureequilibration have yielded T 1100°C and P 2·8 GPa.The early decompression was associated with moderate cooling,corresponding to T 950°, and development of a spinel tectonitefabric in the lherzolites. Further decompression associatedwith plagioclase–olivine growth in both peridotites andpyroxenites was nearly isothermal. The shallow evolution occurredunder a brittle regime and led to the superposition of hornblendeto serpentine veining stages. The garnet pyroxenite-bearingmantle from the External Liguride ophiolites represents a raretectonic sampling of deep levels of subcontinental lithosphereexhumed in an oceanic setting. The exhumation was probably accomplishedthrough a two-step process that started during Late Palaeozoiccontinental extension. The low-pressure portion of the exhumationpath, probably including also the plagioclase mylonitic shearzones, was related to the Mesozoic (Triassic to Jurassic) riftingthat led to continental break-up. In Jurassic times, the studiedmantle sequence became involved in an extensional detachmentprocess that resulted in sea-floor denudation. KEY WORDS: garnet pyroxenite; ophiolite; non-volcanic margin; mantle exhumation; Sm–Nd and Lu–Hf geochronology     

Effect of Water on the Composition of Magmas Formed at High Pressures

Portions of the system MgO-CaO-Na₂O-Al₂O₃-SiO₂-H₂O have beenstudied in the pressure range 13–35 kb at near-liquidustemperatures. The liquidus field of forsterite relative to thatof orthopyroxene is considerably wider under anhydrous thanunder anhydrous conditions and it covers part of the plane ofsilica-saturation in a wide pressure range. Partial meltingof simple garnet lherzolite (= forsterite+orthopyroxene+clinopyroxene+garnet)with water produces quartz-normative liquids at pressures upto at least 25 kb regardless of water content. Hydrous mineralsare not encountered at or near the solidus temperatures exceptin a Na-rich part of the system. Microprobe analysis of therun products in this synthetic system shows that the liquid(glass) in equilibrium with the lherzolite mineral assemblageis silica and alumina-rich at 20 kb under vapor-present conditions.With increasing degree of partial melting, the liquid changesits composition, passing into a ‘vapour-absent region’and becoming less silicic. Fractional crystallization of olivinetholeiitic magma under hydrous conditions also produces silica-richmagmas at high pressures. If the system is open to water, andwater pressure is less than total pressure, the compositionof the liquid varies from quartz-normative to olivine (±nepheline)-normativedepending on water pressure. It is suggested that in the presenceof water, silica-rich magmas such as those of calc-alkalic andesiteor dacite may be formed by direct partial melting of the peridotiticupper mantle at depths down to about 80 km. A large degree ofpartial melting of lherzolite under hydrous conditions wouldproduce SiO2 and MgO-rich magmas. The clinoenstatite rock fromCape Vogel, Papua, may have been formed by such a process. Peridotiteswith low CaAl2SiO5/jadeite ratios in the clinopyroxene couldproduce nepheline-normative magma by small degree of partialmelting and tholeiitic magma by large degree of partial meltingunder hydrous conditions.     

Experimental studies on three potassium-rich ultramafic rocks from Damodar Valley, East India

Summary An experimental study on the phase relationships of three potassium-rich ultramafic rocks from the Damodar Valley, Gondawana basins, has been performed under upper mantle P–T conditions (1.0–2.5 GPa, 700–1200 °C). The Mohanpur lamproite and Satyanarayanpur minette, both from the Raniganj basins, have been investigated with the addition of 15 wt% H₂O. No water was added in the experiments done on an olivine minette from the Jarangdih coal mine, Bokaro Basin, which originally contains 15 wt% CO₂ and 2.86 wt% H₂O. In all cases, olivine is the liquidus phase followed by phlogopite. The subsolidus assemblage for the three rocks is a phlogopite-bearing harzburgite, associated with apatite, Mg-ilmenite and carbonates for the Jarangdih rock; apatite, chromian spinel and carbonates and priderite (only between 1.0 and 1.2 GPa) in the case of the Mohanpur lamproite, and finally apatite, chromian spinel, rutile, and carbonate in the Satyanarayanpur sample. Although orthopyroxene is absent in the natural potassium-rich ultramafic rocks, its presence in the run products of the Jarangdih rock is possibly related to a reaction between olivine and a CO₂-bearing fluid phase. The presence of orthopyroxene in the run products of Mohanpur and Satyanarayanpur rocks may be due to a reaction between K-feldspar, olivine and a vapour phase to produce phlogopite and orthopyroxene. On the basis of present experimental investigation and isotopic studies made by previous investigators, it has been suggested that these K-rich rocks have crystallized from melts derived by vein-plus-wall-rock melting of a phlogopite-bearing harzburgite source rock. Received December 15, 1999; revised version accepted June 17, 2001     

Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton

Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withU–Pb perovskite emplacement ages of 590–555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyre–carbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but Cr–Mg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping ⁸⁷Sr/⁸⁶Sr_i ratios (0·70369–0·70662)and show a narrow range in initial _Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (15–20wt %) and Ni (200–574 ppm) content; the abundant groundmasscarbonate has ¹³C_PDB between –5·7 and –5,similar to primary mantle-derived carbonates, and ¹⁸O_SMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 4–7 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (¹³C_PDB –3·7 to –2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, Sr–Nd isotopes; U–Pb geochronology     

Geobarometry for Peridotites: Experiments in Simple and Natural Systems from 6 to 10 GPa

Experiments with peridotite minerals in simple (MgO–Al₂O₃–SiO₂,CaO–MgO–SiO₂ and CaO–MgO–Al₂O₃–SiO₂)and natural systems were conducted at 1300–1500°Cand 6–10 GPa using a multi-anvil apparatus. The experimentsin simple systems demonstrated consistency with previous lowerpressure experiments in belt and piston–cylinder set-ups.The analysis of spatial variations in pyroxene compositionswithin experimental samples was used to demonstrate that pressureand temperature variations within the samples were less than0·4 GPa and 50°C. Olivine capsules were used in natural-systemexperiments with two mineral mixtures: SC1 (olivine + high-Alorthopyroxene + high-Al clinopyroxene + spinel) and J4 (olivine+ low-Al orthopyroxene + low-Al clinopyroxene + garnet). Theexperiments produced olivine + orthopyroxene + garnet ±clinopyroxene assemblages, occasionally with magnesite and carbonate-richmelt. Equilibrium compositions were derived by the analysisof grain rims and evaluation of mineral zoning. They were comparedwith our previous experiments with the same starting mixturesat 2·8–6·0 GPa and the results from simplesystems. The compositions of minerals from experiments withnatural mixtures show smooth pressure and temperature dependencesup to a pressure of 8 GPa. The experiments at 9 and 10 GPa producedandradite-rich garnets and pyroxene compositions deviating fromthe trends defined by the lower pressure experiments (e.g. higherAl in orthopyroxene and Ca in clinopyroxene). This discrepancyis attributed to a higher degree of oxidation in the high-pressureexperiments and an orthopyroxene–high-P clinopyroxenephase transition at 9 GPa. Based on new and previous resultsin simple and natural systems, a new version of the Al-in-orthopyroxenebarometer is presented. The new barometer adequately reproducesexperimental pressures up to 8 GPa. KEY WORDS: garnet; mineral equilibrium; multi-anvil apparatus; orthopyroxene; geobarometry     

Melting and phase relations in the plane tholeiite-lherzolite-nepheline basanite to 40 kilobars with geological implications

Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated.The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb.The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb.In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb.Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P _Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks.The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable.Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite.Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable.The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature.During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine orthopyroxene clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO₂. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.Publication No. 681, Institute of Geophysics and Planetary Physics, University of California, Los Angeles.     

Experimental Constraints on the Role of Garnet Pyroxenite in the Genesis of High-Fe Mantle Plume Derived Melts

The anhydrous phase relations of an uncontaminated (primitive),ferropicrite lava from the base of the Early Cretaceous Paraná–Etendekacontinental flood basalt province have been determined between1 atm and 7 GPa. The sample has high contents of MgO (14·9wt %), FeO^* (14·9 wt %) and Ni (660 ppm). Olivine phenocrystshave maximum Fo contents of 85 and are in equilibrium with thebulk rock, assuming a of 0·32. A comparison of our results with previous experimental studiesof high-Mg rocks shows that the high FeO content of the ferropicritecauses an expansion of the liquidus crystallization field ofgarnet and clinopyroxene relative to olivine; orthopyroxenewas not observed in any of our experiments. The high FeO contentalso decreases solidus temperatures. Phase relations indicatethat the ferropicrite melt last equilibrated either at 2·2GPa with an olivine–clinopyroxene residue, or at 5 GPawith a garnet–clinopyroxene residue. The low bulk-rockAl₂O₃ content (9 wt %) and high [Gd/Yb]_n ratio (3·1)are consistent with the presence of residual garnet in the ferropicritemelt source and favour high-pressure melting of a garnet pyroxenitesource. The garnet pyroxenite may represent subducted oceaniclithosphere entrained by the upwelling Tristan starting mantleplume head. During adiabatic decompression, intersection ofthe garnet pyroxenite solidus at 5 GPa would occur at a mantlepotential temperature of 1550°C and yield a ferropicriteprimary magma. Subsequent melting of the surrounding peridotiteat 4·5 GPa may be restricted by the thickness of theoverlying sub-continental lithosphere, such that dilution ofthe garnet pyroxenite melt component would be significantlyless than in intra-oceanic plate settings (where the lithosphereis thinner). This model may explain the limited occurrence offerropicrites at the base of continental flood basalt sequencesand their apparent absence in ocean-island basalt successions. KEY WORDS: continental flood basalt; ferropicrite; mantle heterogeneity; mantle melting; phase relations; pyroxenite