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1.
Suprasolidus phase relations at pressures from 8 to 30 kb andtemperatures from 950 to 1380C have been determined experimentallyfor a glassy armalcolite–phlogopite lamproite from thechilled margin of a medium–grained lamproite from SmokyButte, Montana: The armalcolite-phlogopite lamproite has microphenocrystsof olivine in a groundmass of phlogopite, sanidine, armalcolite,clinopyroxene, chromite, priderite, apatite, and abundant glass.The lamproite is SiO2-rich and has high F/H2O relative to lamproitesthat have been investigated in previous experimental studies.Our data show that with decreasing temperature from the liquidusat pressures above 12 kb, melt coexists successively with:olivine; orthopyroxene + clinopyroxene; orthopyroxene + clinopyroxene+ phlogopite; clinopyroxene +phlogopite; and clinopyroxene +orthopyroxene + K-richterite. Below 12 kb, the assemblage successionis: olivine; olivine + clinopyroxene; olivine + clinopyroxene+ phlogopite; and olivine +clinopyroxene + phlogopite + armalcolite.The main difference from the natural paragenesis is that therock does not contain any orthopyroxene—a feature thatis rather remarkable inasmuch as it has 16% normative hypersthene—andthe rock differs also in that it contains sanidine and priderite.In the experiments, sanidine is observed only as ghostlike domainsin some of the glass and appears to have formed during quenching. The solid phases crystallized experimentally are generally compositionallysimilar to the minerals in the rock. These similarities andthe experimental phase relations support the concept of a rapidinitial magma ascent with only a small temperature drop andcrystallization of olivine, but not of orthopyroxene. At lowerpressures, less than 12 kb, it appears that the magma ascendedmore slowly with a larger temperature drop suggested by thesimilarity of the experimentally determined sequence of assemblagesto the paragenesis of the rock. No quasi-invariant multiphase-saturation point was found suchas might be indicative of pressure and temperature conditionsfor formation of the lamproite magma by eutectic-type partialmelting of a mantle source. The occurrence of olivine, orthopyroxene,and clinopyroxene near the liquidus, and the high proportionof normative hypersthene in the melt suggest that lherzoliteor harzburgite was probable in the magma source rock. The highSiO2 and MgO contents of the Smoky Butte lamproites may indicatethat orthopyroxene was a source mineral even though it did notcrystallize under near-surface conditions. The curve definingthe appearance of phlogopite appears at progressively lowertemperatures from the liquidus as pressure increases, so itwould appear that either phlogopite was not the mantle K-reservoir,or it was entirely consumed during the partial melting process.The composition of the near-liquidus glass in the experimentsis likely to be the composition of the bulk rock less the verysmall amounts of olivine + clinopyroxene + orthopyroxene crystallizedwithin a few degrees below the liquidus. From the inferred compositionof this glass, anhydrous phlogopite is a potential mineral.The principal variable that determines whether phlogopite crystallizesas a near-liquidus mineral is F/H2O; low values of this ratiopromote the presence of phlogopite as a near-liquidus mineralwhereas high values deter its crystallization. The common practiceof adding H2O but not F in experiments to compensate for degassingmay obscure the role of phlogopite in the evolution of lamproitemagmas.  相似文献   

2.
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 Pfluid PH2O. The pressure acting on the metamorphic rockduring emplacement of the intrusion is estimated to be a minimumof 1.5 kb.  相似文献   

3.
OHNMACHT  W. 《Journal of Petrology》1974,15(2):303-324
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 Pflukl =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.  相似文献   

4.
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. olivineMg93–78, plagioclase An94–63. 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 fO2. 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.  相似文献   

5.
Progress () of the infiltration-driven reaction, 4olivine +5CO2 + H2O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m3 volume of rock. Mineral and stableisotope compositions record that XCO2, 18Ofluid, and 13Cfluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusion–dispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress  相似文献   

6.
The system peridotite-H2O–CO2 serves as a simplified modelfor the phase relations of mantle peridotite involving morethan one volatile component. Run products obtained in a studyof phase relations of four mantle peridotites in the presenceof H2O- and (H2O+CO2)- bearing vapors and with controlled hydrogenfugacity (fH2) at high pressures and temperatures have beensubjected to a detailed chemical investigation, principallyby the electron microprobe. Mg/(Mg+Fe) of all phases generally increases with increasingtemperature and with increasing Mg/(Mg+Fe) of the starting material.This ratio appears to decrease with increasing pressure forolivine, and for amphibole coexisting with garnet. DecreasingfH2 from that of IW buffer to that of MH buffer decreases Mg/(Mg+Fe)of the partial melt from approximately 0-85 to approximately0.50, whereas the Fo content of coexisting olivine increasesslightly less than 3 per cent and the Mg/(Mg+Fe) of clinopyroxeneincreases about 4 per cent. However, the variations in Fo contentof olivines are within those observed in olivines from naturalmantle peridotite. The chemistry of other silicate mineralsdoes not significantly reflect variations of fH2. Consequently,the peridotite mineralogy and/or chemistry is not a good indicatorfor the fH2 conditions during crystallization. All crystalline phases, except amphibole, and to some extentgarnet, show increasing Cr content with increasing temperatureand increasing Cr content of the starting material, resultingin a positive correlation with Mg/(Mg+Fe). Partial melts aredepleted in Cr2O3 relative to the crystalline phases. High Mg/Mg+Fe)and Cr2O3 are thus expected in crystal residues after partialmelting. The absolute values depend on degree of melting andthe composition of the parent peridotite. Liquids formed by anatexis of mantle peridotite are andesiticunder conditions of XH2Ov > 0.6 to at least 25 kb total pressureand to more than 200?C above the peridotite solidus. This observationsupports numerous suggestions that andesite genesis in islandarcs may result from partial melting of underlying peridotitemantle. In contrast to basaltic rocks, the absence of amphibole(paragasitic hornblende) does not affect the silica-saturatednature of the liquids. Increasing K2O content of the startingmaterial (up to 1 wt. per cent K2O) results in increasing potassiumcontent of the amphibole (1 wt. per cent K2O) as well as theappearance of phlogopite. The liquid under these conditionsis relatively K20-poor (less than 1 wt. per cent K2O). Partial melts are olivine normative with XH2O 0.5, and initialliquids contain normative ol and ne at XH2O 0.4. The alkalinityof these liquids increases with decreasing XH2O below valuesof 0.5. The (ol+opx)-normative liquids resemble oceanic basaltswhereas (ol+ne)-normative liquids resemble olivine nepheliniteand melilite basalt. Low aHlo and high aCo2 conditions may bethose under which kimberlites and related rocks are formed inthe mantle.  相似文献   

7.
Sapphirine occurs with humite-group minerals and forsteritein Precambrian amphibole-facies rocks at Kuhi-lal, SW PamirMountains, Tajikistan, a locality also for talc+kyanite magnesiohornblendewhiteschist. Most of these sapphirine-bearing rocks are graphiticand sulfidic (pyrite and pyrrhotite) and contain enstatite,clinohumite or chondrodite, spinel, rutile, gedrite, and phlogopite.A phlogopite schist has the assemblage with XFe = Fe/(Fe+Mg)increasing as follows: chlorite (0-003)<phlogopite (0.004–0.005)sapphirine (0.004–0.006) enstatite (0-006)forsterite (0-006–0-007)<spinel (0-014). This assemblage includes the incompatiblepair sapphirine+forsterite, but there is no textural evidencefor reaction. In one rock with clinohumite, XFe increases asfollows: clinohumite (0-002) <sapphirine (0-003) <enstatite(0-004–0-006) <spinel (0-010). Ion microprobe and wet-chemicalanalyses give 0-57–0-73 wt.% F in phlogopite and 0-27wt.% F in chlorite in the phlogopite schist; 0-04, 1.5–1.9,and 4.4 wt.% F in forsterite, clinohumite, and chondrodite,respectively; and 0-0-09 wt.% BeO and 0-05–0-21 wt.% B2O3in sapphirine. Stabilization of sapphirine+clinohumite or sapphirine+chondroditeinstead of sapphirine+phlogopite is possible at high F contentsin K-poor rocks, but minor element contents appear to be toolow to stabilize sapphirine as an additional phase with forsterite+enstatite+spinel.Although sapphirine+forsterite is metastable relative to spinel+enstatitein experiments conducted at aH2O=1 in the MgO-Al2O3-SiO2-H2Osystem, it might be stabilized at aH2O0.5, P4 kbar, T650–700C.Textures in the Kuhi-lal whiteschists suggest a polymetamorphicevolution in which the rocks were originally metamorphosed atT650C, P 7 kbar, conditions under which sapphirine+clinohumiteand sapphirine+chondrodite are inferred to have formed, andsubsequently affected by a later event at lower P, similar T,and lower aH2O. The latter conditions were favorable for sapphirine+forsteriteto form in a rock originally containing chlorite+forsterite+spinel+enstatite.  相似文献   

8.
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/86Sri 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 13CPDB between –5·7 and –5,similar to primary mantle-derived carbonates, and 18OSMOW 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 (13CPDB –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  相似文献   

9.
A new method has been suggested for evaluating the overall basicityof minerals and rocks by using ionization reactions involvingone proton: (sum of cations) + H2O = mineral + H+, (sum of cations) + H2O = (sum of normative minerals of a rock)+ H+. The basicity indicators are expressed as standard free energychanges of these reactions (). At standard water pressure (logPH2O = 0) and chemical activity of the metal ions ( log Mn+= 0), the relationship between and alkalinity of solutions(pH) becomes: = –2.303 RTlog H+ = 2.303 RT pH. The overall basicities of rock-forming oxides, minerals andmajor rocks were calculated from the thermodynamic data on ionsin water solutions and solid compounds.  相似文献   

10.
The Ni-S System and Related Minerals   总被引：1，自引：0，他引：1  
The system Ni-S has been studied systematically from 200? to1, 030? C by means of evacuated, sealed silica-glass tube experimentsand differential thermal analyses. Compounds in the system areNi3S2 (and a high temperature, non-quenchable Ni3?S2 phase),Ni7S6, Ni1–S4 Ni3S4, and NiS2. The geologic occurrenceof the minerals heazlewoodite (Ni2S2), millerite (ßSNi1-2S),polydymite (Ni3S4), and vaesite (NiS2) can now be describedin terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within thelimit of error of the experiments, inverts on heating to a tetragonalor pseudotetragonal phase at 556? C. This high-temperature phase(Ni3 has a wide field of stability, from 23.5 to 30.5 wt percent sulfur at 600? C, and melts incongruently at 806??3? C.The ßNi7S6 phase inverts to Ni78 at 397? C6 when inequilibrium with Ni3S2, and at 400? C when in equilibrium withNiS. Crystals of Ni7S6 break down to Ni3-S2+NiS at 573??3?C.The low-temperature form of Ni1-S1 corresponding to the mineralmillerite, is rhombohedral, and the high-temperature form hasthe hexagonal NiAs structure. Stoichiometric NiS inverts at379??3?C, whereas Ni1-S with the maximum nickel deficiency invertsat 282??5OC. The Ni1-alphS-NiS2 solvus was determined to 985??3?C,the eutectic temperature of these phases. Stoichiometric NiSis stable at 600?C but breaks down to Ni2-S2 and Ni1-S below797?C, whereas Ni1-S with 38.2 wt per cent sulfur melts congruentlyat 992??3?C. Vaesite does not vary measurably from stoichiometricNiS2 composition, and melts congruently at 1.007?5?C. Polydymitebreaks down to aNi-S? vaesite at 356??3?C. Differential thermalanalyses showed the existence of a two-liquid field in the sulfur-richportion of the system above 991?C and over a wide compositionalrange.  相似文献   

11.
Small (<5 km2), lithologically diverse gabbro and dioritestocks make up 2% of the 34 to 2 Ma Chilliwack batholith, andoverlap in age with associated calc-alkaline granitoids. Thesemafic plutons are similar to those in other I-type bath-oliths,and represent basaltic magmas present during batholith formation.Objectives of this study are: (1) to examine the origins ofboth interpluton and intrapluton petrologic diversity, and (2)to compare chemical and Sr-Nd isotopic traits of these gabbroswith those of Cascade are basalts. Mafic rocks in the Chilliwackare divided into a medium-K series (MKS) and a low-K series(LKS). The former contain 0.7–2.4 wt% K20 and are similarin composition to calc-alkaline basalts and basaltic andesites.Inverse REE modeling supports derivation of the MKS by 9–27%melting of a garnet-free, LREE-enriched source (La/bN 2). ChilliwackLKS gabbros have chemical characteristics of low-K olivine tho-eiites,including low K2O (0.3–0.5 wt%) and La/bN (1.7–3.4),and high CaO (8.8–11.3 wt%) and Na2O/K2O (6–22).These traits suggest a source with more clinopyroxene and lowerLa/bN than the MKS source. Differences in Nd(O) between MKSand LKS gabbros suggest that lower Nd/Sm is a long-lived LKSsource characteristic. Lithologic variation within compositeplutons of both series resulted primarily from multiple intrusionof related magmas, in some cases differentiates of a commonparent. Two contrasting examples were studied in detail. AtMt Sefrit, MKS variation (gabbronorite-quartz diorite) is modeledby low-pressure fractionation (ol + plag + cpx), accompaniedby 10% wallrock assimilation. In contrast, chemical and Sr-Ndisotopic variation among LKS gabbro-quartz diorite at CopperLake points to crystallization dominated by clinopyroxene+plagioclaseCr-spinel,indicative of differentiation at pressures 10 kbar, althoughthe assimilant in this case is poorly constrained. Chemicaland isotopic similarities between these mafic plutons and QuaternaryCascade lavas indicate that mafic magmas present during theproduction of Chilli-wack granitoids were low-and medium-K arebasalts. KEY WORDS: are magmatism; Cascades; gabbro; granitoid; trace element *Present address: Department of Physics, Astronomy, & Geology, Valdosta State University, Valdosta, GA 31698, USA. Telephone: (912) 249–4847 or 333–5752 Fax: (912) 333–7389. e-mail: jtepper{at}valdosta.peachnet.edu  相似文献   

12.
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  相似文献   

13.
Klauea historical summit lavas have a wide range in matrix 18OVSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, 18O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine 18O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma 18O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine 18O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination  相似文献   

14.
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 Fo89·5 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  相似文献   

15.
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  相似文献   

16.
An oxygen and hydrogen isotopic study of minerals and wholerocks from the granites of the Mourne Mountains Tertiary complex,and related rocks, shows that whereas a significant circulationof meteoric water was associated with the complex, it had onlyminor and localized effects on the granites themselves. TheSilurian slate and greywacke country rocks, which would havehad 18O(SMOW) values of +10 to +20 before the Tertiary igneousevents, have been depicted 18O to values of –40 to –05Tertiary acid minor intrusions outside the main granite massesare also 18O depleted. l8O whole-rock data on the granites showa range of +6.0 to +9.5, and include values significantly higherthan most of those obtained for the granites of the Tertiarycentral complexes of Skye, Mull, and Ardnamurchan. Many of thelowest whole-rock 18O values are found in samples where theminerals are not in isotopic equilibrium. The mineral oxygenisotopic data can be explained in terms of localized interactionwith meteoric water, resulting in preferential 18O depletionin feldspar(s) and biotite, with quartz being much less affected.The granites all show low values of D(SMOW) for biotite andamphibole separates (–137 to –104). The lowest valuesoccur close to the margins of the plutons, near internal contactsor near greisen localities, and these probably reflect limitedinteraction with meteoric water. The higher D values are fromsamples which show evidence of chloritization. This processappears to have occurred both during interaction with meteoricwater, and also during autometasomatism by an exsolved magmaticfluid in other parts of the plutons, including central locationswhere there is little or no evidence for the penetration ofmeteoric water. Granite samples which exhibit near-equilibriumoxygen isotope fractionations for constituent minerals are characterizedby magmatic O-isotopic compositions. The G2 granite, the largestpluton of the eastern centre, has a magmatic 18O(SMOW) valueof {small tilde}+95; intrusions G3 (eastern centre) and G4(western centre) both have 18O(SMOW) values of {small tilde}+90.The other two main intrusive phases have distinctly lower 18O(SMOW)values: {small tilde}+75 for Gl (the least fractionated graniteof the Mourne Mountains central complex), and from +75 to +85for G5. The oxygen isotopic data rule out simple partial meltingof the country rocks as the origin of the granites and alsopreclude an origin by closed-system fractional crystallizationof basaltic magma typical of that represented by Tertiary basicigneous rocks of the region. * Present address: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham BG12 5GG, UK Present address: School of Engineering Technology, Georgian College, Barrie, Ontario, L4M 3X9, Canada  相似文献   

17.
Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM2 at 1.6 Ga, M2 ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M2 grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:18O(carb) 14–20%; metapelites: 18O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(18O 5–13%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M2 contact metamorphism and fluid—rockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing XCO2with increasing grade from <0.05 (greenschist facies) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology *Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au  相似文献   

18.
A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New 40Ar/39Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO2 and Zr contents, and reinforced by rare earth element (REE),87Sr/86Sr and 143Nd/144Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO2 and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO2 and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, 87Sr/86Sri 0·7035, Ndi 9). However, theyhave very high TiO2 (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO2–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke  相似文献   

19.
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  相似文献   

20.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2=43?7–45?7 wt. percent, Al2O3=1?6O–8?21 wt. per cent, CaO=0?70–8?12wt. per cent,alk=0?10–0?90 wt. per cent and Mg/(Mg+Fe2+)=0?94–0?85)have been investigated in the hypersolidus region from 800?to 1250?C with variable activities of H2O, CO2, and H2. Thevapor-saturated peridotite solidi are 50–200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al2O3. Conversely,lowering CaO/Al2O3 reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H2O in the vapor (XH2O). The data also show that some CO2may dissolve in silicate melts formed by partial melting ofperidotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or coexist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aH2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of (). Such activities of H2Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite.  相似文献   

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