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1.
The common upper mantle assemblage olivine-orthopyroxene-spinelmay be used to calculate the oxygen fugacity at which mantle-derivedperidotites have equilibrated. The equilibrium has been calibratedusing the large amount of existing data on the thermodynamicproperties of each phase in this assemblage. A by-product ofthis procedure is a new calibration of the olivine-spinel Mg-Fe2+exchange geothermometer. Application of the equilibrium to avariety of peridotite xenoliths indicates that the oxygen fugacityof the upper mantle lies between the quartz-fayalite-magnetite(QFM) and w?stite-magnetite (WM) oxygen buffers; the few apparentexceptions to this rule may be due to analytical error, particularlyin the Fe3+ content of the spinet phase. In fact, the determinationof Fe3+ in spinet is at present the limiting factor in the accurateapplication of the method: within this limitation, the presentlyavailable evidence suggests that the oxygen fugacity of themantle may be laterally homogeneous over wide regions, but mayalso show small differences between these regions. The fluidspecies in the system C-H-O at such oxygen fugacities are predominantlyCO2 and/or H2O, and not CH4/H2 The minimum possible oxygen fugacity of the mantle is givenby the nickel content of olivine in equilibrium with orthopyroxene;for typical mantle compositions this minimum curve is virtuallycoincident with the iron-w?stite (IW) oxygen buffer.  相似文献   

2.
Unusually alumina-poor orthopyroxene is found in a spinel peridotitefrom the Horoman Peridotite Complex, Japan. Al2O3, Cr2O3 andCaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter)are <0·25 wt %, <0·04 wt % and <0·3wt %, respectively, and are distinctively lower than those inorthopyroxene porphyroclasts. The Low-Al OPX occurs in two modes,both at the margin of olivine. The first mode of occurrenceis as the rim of a large orthopyroxene porphyroclast in contactwith olivine. This type of Low-Al OPX occurs only locally (15µm x 45 µm), and the orthopyroxene rim in contactwith olivine more commonly has normal Al2O3 contents (>2wt %). In the second mode of occurrence, the Low-Al OPX occursas a thin film, 5 µm x 50 µm in dimension, at agrain boundary between olivine and clinopyroxene. Trace elementcompositions of porphyroclast clinopyroxene in the sample indicatethat the sample having the Low-Al OPX underwent metasomatismalthough there are no hydrous minerals around the Low-Al OPX.Petrographic observations and trace element compositions ofclinopyroxene combined with an inferred P–T history ofthe Horoman peridotite suggest that the Low-Al OPX was formedthrough a very local reaction between peridotite and invasivefluids, probably formed by dehydration of a subducted slab,in a late stage of the history of the Horoman peridotite. Crystallizationof orthopyroxene, representing addition of silica to mantlelherzolite via a CO2 + H2O-bearing fluid phase, is a mechanismfor metasomatic alteration of mantle wedge peridotite. KEY WORDS: Horoman Peridotite Complex; low-Al orthopyroxene; metasomatism; mantle wedge  相似文献   

3.
Mantle xenoliths from Tenerife show evidence of metasomatismand recrystallization overprinting the effects of extensivepartial melting. The evidence includes: recrystallization ofexsolved orthopyroxene porphyroclasts highly depleted in incompatibletrace elements into incompatible-trace-element-enriched, poikiliticorthopyroxene with no visible exsolution lamellae; formationof olivine and REE–Cr-rich, strongly Zr–Hf–Ti-depletedclinopyroxene at the expense of orthopyroxene; the presenceof phlogopite; whole-rock CaO/Al2O3 >> 1 (Ca metasomatism) inrecrystallized rocks; and enrichment in incompatible elementsin recrystallized rocks, relative to rocks showing little evidenceof recrystallization. The ‘higher-than-normal’ degreeof partial melting that preceded the metasomatism probably resultsfrom plume activity during the opening of the Central AtlanticOcean. Sr–Nd isotopic compositions are closely similarto those of Tenerife basalts, indicating resetting from theexpected original mid-ocean ridge basalt composition by themetasomatizing fluids. Metasomatism was caused by silicic carbonatitemelts, and involved open-system processes, such as trappingof elements compatible with newly formed acceptor minerals,leaving residual fluids moving to shallower levels. The compositionsof the metasomatizing fluids changed with time, probably asa result of changing compositions of the melts produced in theCanary Islands plume. Spinel dunites and wehrlites representrocks where all, or most, orthopyroxene has been consumed throughthe metasomatic reactions. KEY WORDS: Canary Islands; Tenerife; mantle xenoliths; geochemistry; Ca metasomatism; open-system processes; lithosphere; ocean islands  相似文献   

4.
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  相似文献   

5.
Experimental Melting of Carbonated Peridotite at 6-10 GPa   总被引：2，自引：0，他引：2  
Partial melting of magnesite-bearing peridotites was studiedat 6–10 GPa and 1300–1700°C. Experiments wereperformed in a multianvil apparatus using natural mineral mixesas starting material placed into olivine containers and sealedin Pt capsules. Partial melts originated within the peridotitelayer, migrated outside the olivine container and formed poolsof quenched melts along the wall of the Pt capsule. This allowedthe analysis of even small melt fractions. Iron loss was nota problem, because the platinum near the olivine container becamesaturated in Fe as a result of the reaction Fe2SiO4Ol = FeFe–Ptalloy + FeSiO3Opx + O2. This reaction led to a gradual increasein oxygen fugacity within the capsules as expressed, for example,in high Fe3+ in garnet. Carbonatitic to kimberlite-like meltswere obtained that coexist with olivine + orthopyroxene + garnet± clinopyroxene ± magnesite depending on P–Tconditions. Kinetic experiments and a comparison of the chemistryof phases occasionally grown within the melt pools with thosein the residual peridotite allowed us to conclude that the meltshad approached equilibrium with peridotite. Melts in equilibriumwith a magnesite-bearing garnet lherzolite are rich in CaO (20–25wt %) at all pressures and show rather low MgO and SiO2 contents(20 and 10 wt %, respectively). Melts in equilibrium with amagnesite-bearing garnet harzburgite are richer in SiO2 andMgO. The contents of these oxides increase with temperature,whereas the CaO content becomes lower. Melts from magnesite-freeexperiments are richer in SiO2, but remain silicocarbonatitic.Partitioning of trace elements between melt and garnet was studiedin several experiments at 6 and 10 GPa. The melts are very richin incompatible elements, including large ion lithophile elements(LILE), Nb, Ta and light rare earth elements. Relative to theresidual peridotite, the melts show no significant depletionin high field strength elements over LILE. We conclude fromthe major and trace element characteristics of our experimentalmelts that primitive kimberlites cannot be a direct productof single-stage melting of an asthenospheric mantle. They rathermust be derived from a previously depleted and re-enriched mantleperidotite. KEY WORDS: multianvil; carbonatite melt; peridotite; kimberlite; element partitioning  相似文献   

6.
Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO2 calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO2. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO2-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass  相似文献   

7.
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8.
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 XAb in plagioclase–meltyields an intercept constant KD = 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; FL: 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; KD: 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; XMg: Molar ratio Mg/(Mg + Fe2+); , XMg(0): Initial XMg before MT is formed in the norm calculation; X: Coordinate, horizontal axis; Y: Coordinate, vertical axis  相似文献   

9.
Low-Pressure Experimental Constraints on the Evolution of Komatiites   总被引：1，自引：0，他引：1  
THY  P. 《Journal of Petrology》1995,36(6):1529-1548
Melting experiments were performed on a komatiitic basalt with17 wt% MgO from Munro Township, Ontario, at I-atm pressure andan oxygen fugacity controlled approximately to the fayalite-magnetite-quartzbuffer. The experiments showed that olivine appears at 1344±5°C,spinel at 1334±6°C plagioclase at 1185±5°C,augite at 1176±5°C and pigeonite at 1154±6°C.Compositionally, olivine varies from Fo90 to Fo74 and displaysan average KFe/MgD (ol/liq) of 0•32. The spinels are chromitesand chromian spinels with Mg/(Mg + Fe2+) ratios between 0•66and 0•;32, which show a marked correlation with meltingtemperature. The pyroxenes show an average KFe/MgD (px/liq)of 0•26, identical for augite and pigeonite. Plagiodaseranges compositionally between An82 and An72 Plotted in thepseudo-quaternary basalt phase diagram, the liquid line of descentis similar to that observed for quartz tholeiitic magmas. Therefore,the low-pressure, late-stage evolution products of komatiiteand basaltic komatiite parental magmas will chemically and mineralogicallybe ferrobasaltic quartz tholeiites. High-temperature and high-pressuremodeling suggests that the main observed compositional variationof Munro komatiites can be explained by low-pressure crystalfractionation and accumulation of olivine into komatiite liquidswith below 21•5–23•5 wt% MgO and eruptive temperaturesbelow 1435–1465°C for oxygen fugacities between thefayalite-magnetite quartz (FMQ) and iron-wiistite (IW) buffers.The maximum magnesium content of liquid komatiites, assumingequilibrium Fo94 olivine, is 27–29 wt% MgO and eruptivetemperatures are between 1515 and 1540°C. KEY WORDS: komatiites; experimental petrology; Munro Township; Ontario  相似文献   

10.
Peridotites in the Ulten Zone (Upper Austroalpine, Eastern Alps),occur as small bodies within lower-crustal rocks (gneisses andmigmatites) subducted at eclogite-facies conditions during theVariscan orogeny. They record a complex metamorphic and deformationevolution as indicated by the transition from coarse-grainedspinel-bearing peridotites to fine-grained garnet + amphibole-bearingperidotites, and are interpreted as portions of mantle wedgethat were incorporated in a downgoing slab of cold continentalcrust. The transition from spinel- to garnet-bearing assemblagewas accompanied by significant input of metasomatic agents,as shown by the crystallization of abundant amphibole. Herewe present trace-element mineral chemistry data for selectedUlten peridotites, with the aim of unravelling the nature ofthe metasomatic processes. Amphiboles display significant lightrare earth element (LREE) enrichment [CeN/YbN = 3·90–11·50;LREE up to (20–50) x C1], high Sr (150–250 ppm),K (1910–7280 ppm) and Ba (280–800 ppm) contents,and low concentrations of high field strength elements (HFSE)(Zr = 14–25 ppm, Y = 6·7–16 ppm, Ti = 1150–2500ppm, Nb = 2–7 ppm). On the basis of (1) the evidence formodal orthopyroxene decrease as a result of the garnet-formingreaction rather than abundant orthopyroxene crystallization,(2) the high modal amounts of amphibole (up to 23%) in the mostmetasomatized peridotites and (3) the strong large ion lithophileelement (LILE)/HFSE fractionation in amphiboles, we infer thatthe metasomatic agent was an H2O–CO2 fluid with a lowCO2/H2O ratio. Petrological investigations and geochronologicaldata indicate that the host metapelites experienced in situpartial melting and migmatization concomitantly with the garnet+ amphibole-facies recrystallization in the enclosed peridotites.We infer that the metasomatizing hydrous fluids could representthe residual fluids left after the crystallization of leucosomes,starting from water-undersaturated melts produced during migmatizationof the host gneisses. KEY WORDS: garnet peridotite; crustal metasomatism; amphibole; hydrous fluids  相似文献   

11.
Distribution of Ferric Iron in some Upper-Mantle Assemblages   总被引：16，自引：5，他引：11  
The distribution of ferric iron among the phases of upper-mantlerocks, as a function of pressure (P), temperature (T) and bulkcomposition, has been studied using 57Fe Mssbauer spectroscopyto determine the Fe3+/Fe ratios of mineral separates from 35peridotite and pyroxenite samples. The whole-rock Fe3+ complementof a peridotite is typically shared approximately evenly amongthe major anhydrous phases (spinel and/or garnet, orthopyroxeneand clinopyroxene), with the important exception of olivine,which contains negligible Fe3+. Whole-rock Fe3+ contents areindependent of the T and P of equilibration of the rock, butshow a well-defined simple inverse correlation with the degreeof depletion in a basaltic component. Fe3+ in spinel and inboth pyroxenes from the spinel Iherzolite facies shows a positivecorrelation with temperature, presumably owing to the decreasein the modal abundance of spinel. In garnet peridotites, theFe3+ in garnet increases markedly with increasing T and P, whereasthat in clinopyroxene remains approximately constant. The complexnature of the partitioning of Fe3+ between mantle phases resultsin complicated patterns of the activities of the Fe3+ -bearingcomponents, and thus in calculated equilibrium fO2, which showlittle correlation with whole-rock Fe3+ or degree of depletion.Whether Fe3+ is taken into account or ignored in calculatingmineral formulae for geothermobarometry can have major effectson the resulting calculated T and P. For Fe-Mg exchange geothermometers,large errors must occur when applied to samples more oxidizedor reduced than the experimental calibrations, whose fO2 conditionsare largely unknown. Two-pyroxene thermometry is more immuneto this problem, and probably provides the most reliable P—Testimates. Accordingly, the convergence of P—T valuesderived for a given garnet peridotite assemblage may not necessarilybe indicative of mineral equilibrium. The prospects for thecalculation of accurate Fe3+ contents from electron microprobeanalyses by assuming stoichiometry are good for spinel, uncertainfor garnet, and distinctly poor for pyroxenes. KEY WORDS: mantle; oxidation; partitioning; peridotite; thermobarometry *Corresponding author. Present address: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2, Canada  相似文献   

12.
Macquarie Island is an exposure above sea-level of part of thecrest of the Macquarie Ridge. The ridge marks the Australia–Pacificplate boundary south of New Zealand, where the plate boundaryhas evolved progressively since Eocene times from an oceanicspreading system into a system of long transform faults linkedby short spreading segments, and currently into a right-lateralstrike-slip plate boundary. The rocks of Macquarie Island wereformed during spreading at this plate boundary in Miocene times,and include intrusive rocks (mantle and cumulate peridotites,gabbros, sheeted dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow lavas, picrites, breccias, hyaloclastites),and associated sediments. A set of Macquarie Island basalticglasses has been analysed by electron microprobe for major elements,S, Cl and F; by Fourier transform infrared spectroscopy forH2O; by laser ablation–inductively coupled plasma massspectrometry for trace elements; and by secondary ion mass spectrometryfor Sr, Nd and Pb isotopes. An outstanding compositional featureof the data set (47·4–51·1 wt % SiO2, 5·65–8·75wt % MgO) is the broad range of K2O (0·1–1·8wt %) and the strong positive covariation of K2O with otherincompatible minor and trace elements (e.g. TiO2 0·97–2·1%;Na2O 2·4–4·3%; P2O5 0·08–0·7%;H2O 0·25–1·5%; La 4·3–46·6ppm). The extent of enrichment in incompatible elements in glassescorrelates positively with isotopic ratios of Sr (87Sr/86Sr= 0·70255–0·70275) and Pb (206Pb/204Pb =18·951–19·493; 207Pb/204Pb = 15·528–15·589;208Pb/204Pb = 38·523–38·979), and negativelywith Nd (143Nd/144Nd = 0·51310–0·51304).Macquarie Island basaltic glasses are divided into two compositionalgroups according to their mg-number–K2O relationships.Near-primitive basaltic glasses (Group I) have the highest mg-number(63–69), and high Al2O3 and CaO contents at a given K2Ocontent, and carry microphenocrysts of primitive olivine (Fo86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo90·5). Fractionated, Group II, basaltic glasses aresaturated with olivine + plagioclase ± clinopyroxene,and have lower mg-number (57–67), and relatively low Al2O3and CaO contents. Group I glasses define a seriate variationwithin the compositional spectrum of MORB, and extend the compositionalrange from N-MORB compositions to enriched compositions thatrepresent a new primitive enriched MORB end-member. Comparedwith N-MORB, this new end-member is characterized by relativelylow contents of MgO, FeO, SiO2 and CaO, coupled with high contentsof Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements,and has the most radiogenic Sr and Pb regional isotope composition.These unusual melt compositions could have been generated bylow-degree partial melting of an enriched mantle peridotitesource, and were erupted without significant mixing with commonN-MORB magmas. The mantle in the Macquarie Island region musthave been enriched and heterogeneous on a very fine scale. Wesuggest that the mantle enrichment implicated in this studyis more likely to be a regional signature that is shared bythe Balleny Islands magmatism than directly related to the hypotheticalBalleny plume itself. KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry  相似文献   

13.
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy  相似文献   

14.
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  相似文献   

15.
The alpine-type peridotite at Burro Mountain is a partiallyserpentinized harzburgite-dunite body approximately 2 km indiameter. It lies in a chaotic mélange derived from theFranciscan Formation (Upper Jurassic to Upper Cretaceous) ofthe southern Coast Ranges of California. The peridotite is boundedon the east by a vertical fault in the Nacimiento fault zonethat brings sedimentary rocks of Taliaferro's (1943b) AsuncionGroup (Upper Cretaceous) into contact with the peridotite. Theperidotite appears to be one of a number of tectonic lenses,having a wide range in size, that make up the mélange.These lenses include metagraywacke, metachert, greenstone, amphibolite,and blueschist, as well as ultramafic rocks, and represent awide range of pressure-temperature environments. The outer shell of the peridotite is a sheared serpentinitezone 10–15 m thick. The peridotite was tectonically emplacedat its present level as a cold solid mass and had little effecton the mineral assemblages of the Franciscan Formation. Localdevelopment of lawsonite and aragonite in shear zones may berelated to the peridotite emplacement. Foliated harzburgite forms approximately 60 per cent of theperidotite. It is a lithologically uniform rock that has anolivine: orthopyroxene ratio of approximately 75:25. Accessoryclinopyroxene and chromian spinel generally make up less than5 per cent of the harzburgite. Dunite, composed of olivine,accessory chromian spinel (< 5 per cent), and trace amountsof pyroxene, makes up approximately 40 per cent of the peridotiteand occurs as dikes, sills, and irregular bodies in the harzburgite. Olivine and pyroxene show small but significant compositionalvariations and chromian spinel shows a large range in the cationratio Cr/(Cr+Al+ Fe3+). The compositional variations in theseminerals are related to original differences in bulk chemicalcomposition. The following compositional ranges were determinedfor minerals in the harzburgite: olivine, Fo91.1–Fo91.4;orthopyroxene, En89.8–En91.1; clinopyroxene, Ca47.0Mg50.0Fe3.0–Ca48.7Mg48.2Fe3.1;chromian spinel, Cr/(Cr+Al+Fe3+) 0.37–0.55. The pyroxeneshave a range in A12O3 content of 1.3–3.0 wt per cent.Olivine from dunite ranges from Fo91 to Fo92 7 and the chromianspinel has a range in the Cr/(Cr+Al+Fe3+) ratio of 0.30–0.75.Although all the dunites are lithologically similar, three distincttypes are recognized on the basis of composition of coexistingolivine and chromian spinel. Structural relations between thethree types of dunite suggest three periods of emplacement (possiblyoverlapping) of dunite into harzburgite. The evidence indicatesthat the dunite, and probably also the harzburgite crystallizedfrom an ultramafic magma, probably in the upper mantle. After the magmatic episode and crystallization, the peridotitewas subjected to a deep-seated plastic deformation and recrystallization.The first phase of the deformation produced a pervasive, planarstructural element (S1) that crosscuts many harzburgite-dunitecontacts. It is probable that some of the dunite sills wereemplaced during this deformation. The foliation, S1, is definedby layers of different orthopyroxene content in harzburgite,and by discontinuous layers of chromian spinel in dunite. Flowor slip along S1 produced slip folds in harzburgite—dunitecontacts with axial planes parallel to S1. At a later stage,isoclinal folds developed in S1, and the present olivine microfabricwas probably formed by recrystallization in the stress fieldthat produced the isoclinal folding. In the olivine microfabric,X tends to be perpendicular to the axial planes (S2) of theisoclinal folds and Y and Z tend to form double maxima in S2approximately 90° apart. Mg–Fe2+ distribution betweencoexisting mineral pairs yields a calculated temperature offormation of approximately 1200 °C. Although this temperatureis only a nominal value, it indicates that the mineral pairsequilibrated at a significantly high temperature. In view ofthe deformation and recrystallization, the calculated temperaturepossibly represents subsolidus re-equilibration of the mineralsduring this event. The deformation and recrystallization probablyoccurred shortly after crystallization while the peridotitewas still at a high temperature. A later deep-seated deformation produced small scattered kinkfolds in S1 that tend to disrupt the major olivine microfabric.The kink folding was accompanied or followed by the developmentof kink bands in olivine that reflect intragranular glidingon the system T = [Okl], t = [100]. The kink bands probablyformed at a minimum temperature of 1000 °C. Following the deep-seated deformation, which probably took placein the mantle, the peridotite mass was tectonically detachedand moved upward to its present level in the crust. Cleavages,joints, and faults provided channels for water to pervade theperidotite and allow alteration of the primary minerals.  相似文献   

16.
A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles   总被引：2，自引：0，他引：2  
St Kitts forms one of the northern group of volcanic islandsin the Lesser Antilles arc. Eruptive products from the Mt Liamuigacentre are predominantly olivine + hypersthene-normative, low-Kbasalts through basaltic andesites to quartz-normative, low-Kandesites. Higher-Al and lower-Al groups can be distinguishedin the suite. Mineral assemblages include olivine, clinopyroxene,orthopyroxene, plagioclase and titanomagnetite with rarer amphibole,ilmenite and apatite. Eruptive temperatures of the andesitesare estimated as 963–950°C at fO2 NNO + 1 (whereNNO is the nickel–nickel oxide buffer). Field and mineralchemical data provide evidence for magma mixing. Glass (melt)inclusions in the phenocrysts range in composition from andesiteto high-silica rhyolite. Compositional variations are broadlyconsistent with the evolution of more evolved magmas by crystalfractionation of basaltic parental magmas. The absence of anycovariation between 87Sr/86Sr or 143Nd/144Nd and SiO2 rulesout assimilation of older silicic crust. However, positive correlationsbetween Ba/La, La/Sm and 208Pb/204Pb and between 208Pb/204Pband SiO2 are consistent with assimilation of small amounts (<10%)of biogenic sediments. Trace element and Sr–Nd–Pbisotope data suggest derivation from a normal mid-ocean ridgebasalt (N-MORB)-type mantle source metasomatized by subductedsediment or sediment melt and fluid. The eruptive rocks arecharacterized by 238U excesses that indicate that fluid additionof U occurred <350 kyr ago; U–Th isotope data for mineralseparates are dominated by melt inclusions but would allow crystallizationages of 13–68 ka. However, plagioclase is consistentlydisplaced above these ‘isochrons’, with apparentages of 39–236 ka, and plagioclase crystal size distributionsare concave-upwards. These observations suggest that mixingprocesses are important. The presence of 226Ra excesses in twosamples indicates some fluid addition <8 kyr ago and thatthe magma residence times must also have been less than 8 kyr. KEY WORDS: Sr–Nd–Pb isotopes; U-series isotopes; crystal size distribution; petrogenesis  相似文献   

17.
Fe-rich dunite xenoliths within the Kimberley kimberlites compriseolivine neoblasts with minor elongated, parallel-oriented ilmenite,and rarely olivine porphyroclasts and spinel. Compared withtypical mantle peridotites, olivines in the Fe-rich duniteshave lower forsterite (Fo87–89) and NiO contents (1300–2800ppm), which precludes a restitic origin for the dunites. Chrome-richspinels are remnants of a metasomatic reaction that producedilmenite and phlogopite. Trace element compositions differ betweenporphyroclastic and neoblastic olivine, the latter having higherTi, V, Cr and Ni and lower Zn, Zr and Nb contents, documentingtheir different origins. The dunites have high 187Os/ 188Osratios (0·11–0·15) that result in youngmodel ages for most samples, whereas three samples show isotopicmixtures between Phanerozoic neoblasts and ancient porphyroclasticmaterial. Most Fe-rich dunite xenoliths are interpreted to berecrystallized cumulates related to fractional crystallizationof Jurassic Karoo flood basalt magmatism, whereas the porphyroclastsare interpreted to be remnants from a much earlier (probablyArchaean Ventersdorp) magmatic episode. The calculated parentalmagma for the most primitive olivine neoblasts in the Fe-richdunites is similar to low-Ti Karoo basalts. Modelling the crystalfractionation of the inferred parental magma with pMELTS yieldselement fractionation trends that mirror the element variationof primitive low-Ti Karoo basalts. KEY WORDS: dunite xenoliths; fractional crystallization; Karoo; large igneous province; pMELTS; Re–Os; trace elements  相似文献   

18.
Crystallization experiments were performed at 200 MPa in thetemperature range 1150–950°C at oxygen fugacitiescorresponding to the quartz–fayalite–magnetite (QFM)and MnO–Mn3O4 buffers to assess the role of water andfO2 on phase relations and differentiation trends in mid-oceanridge basalt (MORB) systems. Starting from a primitive (MgO9·8 wt %) and an evolved MORB (MgO 6·49 wt %),crystallization paths with four different water contents (0·35–4·7wt % H2O) have been investigated. In primitive MORB, olivineis the liquidus phase followed by plagioclase + clinopyroxene.Amphibole is present only at water-saturated conditions below1000°C, but not all fluid-saturated runs contain amphibole.Magnetite and orthopyroxene are not stable at low fO2 (QFM buffer).Residual liquids obtained at low fO2 show a tholeiitic differentiationtrend. The crystallization of magnetite at high fO2 (MnO–Mn3O4buffer) results in a decrease of melt FeO*/MgO ratio, causinga calc-alkaline differentiation trend. Because the magnetitecrystallization temperature is nearly independent of the H2Ocontent, in contrast to silicate minerals, the calc-alkalinedifferentiation trend is more pronounced at high water contents.Residual melts at 950°C in a primitive MORB system havecompositions approaching those of oceanic plagiogranites interms of SiO2 and K2O, but have Ca/Na ratios and FeO* contentsthat are too high compared with the natural rocks, implyingthat fractionation processes are necessary to reach typicalcompositions of natural oceanic plagiogranites. KEY WORDS: differentiation; MORB; oxygen fugacity; water activity; oceanic plagiogranite  相似文献   

19.
As a pilot study of the role of water in the attenuation ofseismic waves in the Earth's upper mantle, we have performeda series of seismic-frequency torsional forced-oscillation experimentson a natural (Anita Bay) dunite containing accessory hydrousphases, at high temperatures to 1300°C and confining pressure(Pc) of 200 MPa, within a gas-medium high-pressure apparatus.Both oven-dried and pre-fired specimens wrapped in Ni–Fefoil within the (poorly) vented assembly were recovered essentiallydry after 50–100 h of annealing at 1300°C followedby slow staged cooling. The results for those specimens indicatebroadly similar absorption-band viscoelastic behaviour, butwith systematic differences in the frequency dependence of strain-energydissipation Q–1, attributed to differences in the smallvolume fraction of silicate melt and its spatial distribution.In contrast, it has been demonstrated that a new assembly involvinga welded Pt capsule retains aqueous fluid during prolonged exposureto high temperatures—allowing the first high-temperaturetorsional forced-oscillation measurements under high aqueousfluid pore pressure Pf. At temperatures >1000°C, a markedreduction in shear modulus, without concomitant increase inQ–1, is attributed to the widespread wetting of grainboundaries resulting from grain-scale hydrofracturing and themaintenance of conditions of low differential pressure Pd =Pc – Pf . Staged cooling from 1000°C is accompaniedby decreasing Pf and progressive restoration of significantlypositive differential pressure resulting in a microstructuralregime in which the fluid on grain boundaries is increasinglyrestricted to arrays of pores. The more pronounced viscoelasticbehaviour observed within this regime for the Pt-encapsulatedspecimen compared with the essentially dry specimens may reflectboth water-enhanced solid-state relaxation and the direct influenceof the fluid phase. The scenario of overpressurized fluids andhydrofracturing in the Pt-encapsulated dunite specimen may havesome relevance to the high Q–1 and low-velocity zonesobserved in subduction-zone environments. The outcomes of thisexploratory study indicate that the presence of water can havea significant effect on the seismic wave attenuation in theupper mantle and provide the foundation for more detailed studieson the role of water. KEY WORDS: seismic wave attenuation; water; dunite; hydrous mineral; shear modulus; viscoelasticity; olivine; grain-scale hydrofracturing  相似文献   

20.
Kistufell: Primitive Melt from the Iceland Mantle Plume   总被引：5，自引：2，他引：5  
This paper presents new geochemical data from Kistufell (64°48'N,17°13'W), a monogenetic table mountain situated directlyabove the inferred locus of the Iceland mantle plume. Kistufellis composed of the most primitive olivine tholeiitic glassesfound in central Iceland (MgO 10·56 wt %, olivine Fo89·7).The glasses are interpreted as near-primary, high-degree plumemelts derived from a heterogeneous mantle source. Mineral, glassand bulk-rock (glass + minerals) chemistry indicates a low averagemelting pressure (15 kbar), high initial crystallization pressuresand temperatures (10–15 kbar and 1270°C), and eruptiontemperatures (1240°C) that are among the highest observedin Iceland. The glasses have trace element signatures (Lan/Ybn<1, Ban/Zrn 0·55–0·58) indicative ofa trace element depleted source, and the Sr–Nd–Pbisotopic ratios (87Sr/86Sr 0·70304–0·70308,143Nd/144Nd 0·513058–0·513099, 206Pb/204Pb18·343–18·361) further suggest a long-termtrace element depletion relative to primordial mantle. HighHe isotopic ratios (15·3–16·8 R/Ra) combinedwith low 207Pb/204Pb (15·42–15·43) suggestthat the mantle source of the magma is different from that ofNorth Atlantic mid-ocean ridge basalt. Negative Pb anomalies,and positive Nb and Ta anomalies indicate that the source includesa recycled, subducted oceanic crustal or mantle component. PositiveSr anomalies (Srn/Ndn = 1·39–1·50) furthersuggest that this recycled source component involves lower oceaniccrustal gabbros. The  相似文献   

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