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1.
Experimental Petrology of Melilite Nephelinites 总被引：3，自引：1，他引：3
Experimental study of natural melilite nephelinite lavas ofintermediate K/Na ratio at low pressure (fo2 reveals the presenceof a peritectic ‘point’ of distributary type (1090?C)for liquids saturated with leucite, nepheline, and spinel. Withdecreasing temperature on the olivine + melilite cotectic, botholivine and melilite react with such liquids to produce high-calciumpyroxene at the peritectic. Both the olivine + high-calciumpyroxene and melilite + high-calcium pyroxene cotectics arestable at temperatures below the peritectic. Olivines coexistingwith such liquids are much more magnesian than those in comparabletholeiitic liquids. The olivine-liquid Fe-Mg distribution coefficient is a monotonically increasing function of silica activity over the composition range spannedby melilite nephelinite, ugandite, alkali basalt, and tholeiitebasalt liquids. The analogous Fe-Mg distribution coefficientfor melilite and liquid is effectively constant , while that for high-calcium pyroxene and liquidis highly dependent on the chemistry of high-calcium pyroxene(cf., Sack & Carmichael, 1984). Pseudoternary liquidus projectionsof multiply saturated liquids coexisting with nepheline, leucite,and spinel (?olivine?high-calcium pyroxene?melilite) have beenprepared to facilitate graphical analysis of the evolution oflava compositions during hypabyssal cooling. Major element chemicalanalyses and petrographic features of lavas from Mt. Nyiragongo,East Africa and Oahu, Hawaii (e.g., Denaeyer et al., 1965; Wilkinson& Stolz, 1983) confirm the validity of these diagrams andthe systematics established from the experimental data.
*Reprint requests to R.O. Sack 相似文献
2.
A group of gabbroic anorthosite dykes in northwest Skye containsmegacrysts of calcic plagioclase (An93–87) and olivine(FO87–78.5and plagioclase-rich xenoliths set in a matrixof low-alkali, high Ca olivine tholeiite. Field, mineralogicand petrochemical evidence indicates (1) that the dykes arefilled with crystals concentrated from magmas feeding lava flowsand (2) that, although the megacrysts crystallized from low-alkali,high Ca olivine tholeiite magmas at depths <4 km, the megacrystsand the matrix in each dyke are not cognate. The emplacementtemperature was less than 1230 °C. Low-alkali, high Ca olivinetholeiite is an abundant Hebridean magma type, resembling unfractionatedoceanic tholeiites from the Atlantic. The temporal change fromalkali olivine basalt to low-alkali, high Ca olivine tholeiitein the Hebridean proto-spreading centre may mimic compositionalchanges during initial development of the mid-Atlantic ridge. 相似文献
3.
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
4.
Mineralogy and Petrogenesis of the Deccan Trap Lava Flows around Mahabaleshwar, India 总被引：2，自引：0，他引：2
Electron microprobe analyses of minerals of thirteen DeccanTrap lava flows at Mahabaleshwar have been carried out in thepresent study. All of these flows have tholeiitic bulk compositionsand all, except one (represented by MB-81-17 of Mahoney et al.,1982) contain olivine, plagioclase, two pyroxenes, and Fe-Tioxide minerals. Olivine and plagioclase appear as distinct phenocrystsin all but one flow, and Ca-rich pyroxene joins as a phenocrystphase in the younger flows. Pigeonite and Fe-Ti oxide minerals(titanomagnetite and ilmenite) occur in the groundmass. Olivineoccurs as both groundmass and phenocryst phase in MB-81-17 (whichis the only flow without low-Ca pyroxene phase); in all otherflows olivine appears only as phenocryst phase. In all but one(MB-81-17) flow olivine is completely altered. MB-81-17 olivinegrains are only partly altered, and in this rock the cores ofphenocrysts are rounded and have a composition of Fo77 whereastheir euhedral rims have a composition around Fo67. The groundmassolivine grains in MB-81-17 are Fo41–32. Substantial Fe-enrichmentand zoning trends are shown by the pyroxenes in individual rocks.The cores of Ca-rich pyroxene phenocrysts of some of the flowshave as much as 4 wt. per cent A12O3 and may have crystallizedat higher (crustal) pressures. Pigeonite thermometry (Ishii,1975) suggests an average of 1050?C for crystallization of thegroundmass pigeonite (eruption temperature?). Fe-Ti oxide mineralsare mostly altered in the older flows. In the younger flows,coexisting unaltered titanomagnetite and ilmenite yield maximumtemperature estimates for the crystallization of these phaseof about 1025?C and an oxygen fugacity of 10–11.5 atm.The T-fo2 path followed by these flows seems to have been consistentlysomewhat lower than that defined by the 1 atm. fayalite-magnetitequartz curve. All of the lavas examined have experienced extensivefractional crystallization of olivine and some clinopyroxeneat relatively higher pressures. These lavas were saturated orclose to being saturated with olivine+plagioclase+clinopyroxeneduring eruption. Plagioclase accumulation, although it appearsto have occurred, has not been significant. It is suggestedthat MB-81-17 magma was contaminated by a calcite-rich rock(limestone?) whereas the lower Group 1 magmas may have beenselectively contaminated by quartz-bearing contaminant. Alternately,parental magma of MB-81-1 (with the highest Mg-number and 8= -16) may have been produced in the upper mantle into whichminor masses of old crust was well mixed. Magma mixing, crystalfractionation, and contamination processes of Mahabaleshwarbasalts and possible genetic relationships with other DeccanTrap lavas are discussed. 相似文献
5.
A 525-m-long drill core (DDH-221) through the Partridge Riverintrusion has been divided into four zones on the basis of changesin mineral abundances, compositions and grain size. The igneousrocks in the core consist of cumulate gabbro, troctolite andolivine gabbronorite, in which the original cumulate frameworkof plagioclase and olivine contained varying amounts of trappedintercumulus (pore) liquid. The compositions of the unzoned olivine (Fo31–71) havebeen modified by reaction with Fe-rich in situ intercumulusliquid, but the plagioclase cores (An59–73) have not.The compositions of postcumulus Ca-rich pyroxene, restrictedto En36–44, and the more variable Ca-poor pyroxene (En45–74),follow a downward Fe-enrichment trend similar to the Fe-enrichmentin the olivine. The cumulus olivine expected to be in equilibriumwith plausible parental magmas to these rocks was not preservedin the drill core, nor is the chilled margin to the intrusionsufficiently primitive to account for all the olivine. Revisedmass balance estimates of the primary magmatic compositionsof olivine are Fo67–85. The new limiting value for theprimary olivine is similar to the Fo83–85 olivine expectedto crystallize from the chilled margin to the nearby PigeonPoint olivine diabase sill under equilibrium conditions. Thechanges in the mineral compositions in core DDH-221 do not adequatelydescribe the behavior of parental melts on an equilibrium coolingpath, implying that the cumulus plagioclase and olivine crystallizedelsewhere, and were mixed with varying amounts of intercumulusliquid before introduction to the present crustal site of thePartridge River intrusion. Rock density increases with depth from 2?76 to 3?21, with amean of 2?98 g/cm3. Estimated trapped liquid densities rangefrom 2?56 to 2?92 g/cm3 at high temperatures. This is interpretedto mean that the intercumulus liquid could not have been expelledupward by compaction of the cumulate pile. The dense intercumulusliquid increased downward in abundance to form a series of rocksthat range continuously from variously packed framework cumulatesto chilled non-cumulate rocks in the basal zone. In situ crystallizationis concluded to be the dominant mode of solidification of thePartridge River intrusion, in which infiltration metasomatismis precluded by the high liquid density. 相似文献
6.
Experimental Petrology of the Kiglapait Intrusion: Cotectic Trace for the Lower Zone at 5 kbar in Graphite 总被引：3，自引：2，他引：1
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
7.
Geobarometry for Peridotites: Experiments in Simple and Natural Systems from 6 to 10 GPa 总被引：3，自引：0，他引：3
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
8.
Liquid Immiscibility and the Evolution of Basaltic Magma 总被引：9，自引：0，他引：9
Veksler Ilya V.; Dorfman Alexander M.; Borisov Alexander A.; Wirth Richard; Dingwell Donald B. 《Journal of Petrology》2007,48(11):2187-2210
This experimental study examines relationships between alternativeevolution paths of basaltic liquids (the so-called Bowen andFenner trends), and silicate liquid immiscibility. Syntheticanalogues of natural immiscible systems exhibited in volcanicglasses and melt inclusions were used as starting mixtures.Conventional quench experiments in 1 atm gas mixing furnacesproved unable to reproduce unmixing of ferrobasaltic melts,yielding instead either turbid, opalescent glasses, or crystallizationof tridymite and pyroxenes. In contrast, experiments involvingin situ high-temperature centrifugation at 1000g (g = 9·8m/s2) did yield macroscopic unmixing and phase separation. Centrifugationfor 3–4 h was insufficient to complete phase segregation,and resulted in sub-micron immiscible emulsions in quenchedglasses. For a model liquid composition of the Middle Zone ofthe Skaergaard intrusion at super-liquidus temperatures of 1110–1120°C,centrifugation produced a thin, silicic layer (64·5 wt%SiO2 and 7·4 wt% FeO) at the top of the main Fe-richglass (46 wt% SiO2 and 21 wt% FeO). The divergent compositionsat the top and bottom were shown in a series of static runsto crystallize very similar crystal assemblages of plagioclase,pyroxene, olivine, and Fe–Ti oxides. We infer from theseresults that unmixing of complex aluminosilicate liquids maybe seriously kinetically hampered (presumably by a nucleationbarrier), and thus conventional static experiments may not correctlyreproduce it. In the light of our centrifuge experiments, immiscibilityin the Skaergaard intrusion could have started already at thetransition from the Lower to the Middle Zone. Thus, magma unmixingmight be an important factor in the development of the Fe-enrichmenttrend documented in the cumulates of the Skaergaard LayeredSeries. KEY WORDS: liquid immiscibility; Skaergaard; layered intrusions; experimental petrology 相似文献
9.
Ultra-calcic Magmas Generated from Ca-depleted Mantle: an Experimental Study on the Origin of Ankaramites 总被引：3，自引：1，他引：3
Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitousin arc, ocean-island and mid-ocean ridge settings. They areprimitive in character (XMg > 0·65) and have highCaO contents (>14 wt %) and CaO/Al2O3 (>1·1). Experimentson an ankaramite from Epi, Vanuatu arc, demonstrate that itsliquidus surface has only clinopyroxene at pressures of 15 and20 kbar, with XCO2 in the volatile component from 0 to 0·86.The parental Epi ankaramite is thus not an unfractionated magma.However, forcing the ankaramite experimentally into saturationwith olivine, orthopyroxene and spinel results in more magnesian,ultra-calcic melts with CaO/Al2O3 of 1·21–1·58.The experimental melts are not extremely Ca-rich but high inCaO/Al2O3 and in MgO (up to 18.5 wt %), and would evolve tohigh-CaO melts through olivine fractionation. Fractionationmodels show that the Epi parent magma can be derived from suchultra-calcic experimental melts through mainly olivine fractionation.We show that the experimental ultra-calcic melts could formthrough low-degree melting of somewhat refractory mantle. Thelatter would have been depleted by previous melt extraction,which increases the CaO/Al2O3 in the residue as long as someclinopyroxene remains residual. This finding corrects the commonassumption that ultra-calcic magmas must come from a Ca-richpyroxenite-type source. The temperatures necessary for the generationof ultra-calcic magmas are 相似文献
10.
Vesicular interstitial glass in four kaersutite-bearing spinel–wehrlitexenoliths from Foster Crater, Antarctica has reacted with hostolivine (Fo75–79) and clinopyroxene (Ca47 Mg45 Fe8) andcontains a microphenocryst assemblage of spinel, olivine, andclinopyroxene together with later rhonite and plagioclase. Electronmicroprobe analyses of the glasses have low SiO2 (46–49wt. per cent) and MgO (2.2–3.7) contents and high contentsof alkalis, TiO2 (3.4–4.8), Al2O3 (18.1–20.6) andP2O5 (1.1–1.3). Olivine microphenocryst cores in glassare magnesian (up to Fo88) and must have precipitated from moreprimitive liquids; rim compositions are Fe-rich (Fo75) and inequilibrium with glass. Continuous core to rim zonation in theolivine microphenocrysts indicate that glass compositions havefractionated due to crystallization of the enclosed mineralassemblage. Mass balance addition calculations, using the compositionsand proportions of the crystals in glass, produce melt compositionsappropriate to primary alkali basaltic magmas. Glasses showlight rare earth element (REE) enrichment relative to chondrites(Ce/YbN = 10.5) and, together with Ba, Rb, Cr, Hf, Ta, and Thare similar to many of the basanites from the Erebus VolcanicProvince. Textural relationships of the kaersutite are complex owing tothe instability of kaersutite in the presence of melt. However,in the association with glass we observe textural evidence suchas olivine and clinopyroxene microphenocrysts, identical tothe liquidus phases of the glasses, enclosed by kaersutite crystals.We believe that relationships such as this link the crystallizationof kaersutite to mafic melt which infiltrated and reacted withthe host wehrlite. Thus, the melt did not form in situ withinthe xenolith but originated elsewhere in the upper mantle. 相似文献
11.
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
12.
Calculated Phase Relations in the System NCKFMASH (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O) for High-Pressure Metapelites 总被引：5，自引：0，他引：5
Petrogenetic grids in the system NCKFMASH (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O)and the subsystems NCKMASH and NCKFASH calculated with the softwareTHERMOCALC 3.1 are presented for the P–T range 7–30kbar and 450–680°C, for assemblages involving garnet,chloritoid, biotite, carpholite, talc, chlorite, kyanite, staurolite,paragonite, glaucophane, jadeite, omphacite, diopsidic pyroxene,plagioclase, zoisite and lawsonite, with phengite, quartz/coesiteand H2O in excess. These grids, together with calculated compatibilitydiagrams and P–T and T–XCa and P–XCa pseudosectionsfor different bulk-rock compositions, show that incorporationof Ca into the NKFMASH system leads to many of the NKFMASH invariantequilibria moving to lower pressure and/or lower temperature,which results, in most cases, in the stability of jadeite andgarnet being enlarged, but in the reduction of stability ofglaucophane, plagioclase and AFM phases. The effect of Ca onthe stability of paragonite is dependent on mineral assemblageat different P–T conditions. The calculated NCKFMASH diagramsare powerful in delineating the phase equilibria and P–Tconditions of natural pelitic assemblages. Moreover, contoursof the calculated phengite Si isopleths in P–T and P–XCapseudosections confirm that phengite barometry in NCKFMASH isstrongly dependent on mineral assemblage. KEY WORDS: phase relations; metapelites; NCKFMASH; THERMOCALC; phengite geobarometry 相似文献
13.
Subsolidus phase relations have been determined in the systemsSiO2-Cr-0 and MgO-SiO2-Cr-O in equilibrium with metallic Cr,at 1100–1500C and 0–288 kbar. There are no ternaryphases in the SiO2-Cr-O system at these conditions, ie. onlythe assemblage eskolaite + Cr-metal + SiO2 is found. However,in the MgO-containing system, extensive substitution of Cr2+for Mg is observed in (Mg,Cr2+)2SiO4 olivine, (Mg,Cr2+)2Si2O6pyroxene, and (Mg,Cr2+)Cr3+204 spinel. Cr 3+ levels in olivineand pyroxene are below detection limits. The pyroxene is orthorhombicat monoclinic at higher . The structure of the spinels becomestetragonally distorted at is limited by the breakdown of olivine to pyroxene + spinel+ metal. This maximum amount increases strongly with increasingtemperature, reaching >0.2 at 1500C and 48 kbar. Increasing pressure reduces the maximum. Increasing temperature also increases the maximum amounts of Cr2+ which substitute intopyroxene and spinel, indicating that end-member Cr2Si206 andCr3O4 may become stable above 1650C if melting does not intervene.Powder X-ray diffraction analysis of selected runs has beenused to extract molar volumes of the Mg-Cr2+ solid solutionsas a function of composition, which may be extrapolated to predictmolar volumes for Cr2SiO4 (olivine), Cr2Si2O6 (ortho- and clino-pyroxene)and Cr2O4 (cubic spinel) of 477, 680 and 449 cm3, respectively.The experimental data have been fitted to a thermodynamic model,including free energies of formation for end member Cr2SiO4,Cr2Si2O6 and Cr3O4. This model is then used to predict the amountsof Cr2+ which can be expected in olivine in equilibrium withCr-bearing spinel as a function of T, P and fo2. This amountincreases strongly with temperature along standard T-fo2 buffercurves, and is sufficient to explain the observed high Cr contentsof olivine from komatiites and diamond inclusions at reasonableterrestrial fo, values. The lower fo2 of the lunar environmentresults in significant Cr2+ in olivine being stable to muchlower temperatures. The tendency for the oxidation state ofCr, and hence its geochemical properties, to change with temperaturerelative to other redox reactions makes it a potentially usefulmonitor of the temperatures of uppermantle processes, and isa significant factor in the differing styles of igneous differentiationin the Earth and Moon.
Corresponding author 相似文献
14.
Crystallization Pressure and Cooling History of the Giles Layered Igneous Complex, Central Australia 总被引：1，自引：0，他引：1
The Giles Complex, central Australia, consists of a series oflarge layered gabbroic/ultramafic intrusions emplaced in acidicand intermediate granulites of the Middle Proterozoic Musgraveblock. Lithologies range from well-layered dunite, wehrlite,and pyroxenite in the lower primitive series, to massive olivinegabbro, gabbronorite, and anorthosite in the main units, andferrodiorites, vanadife-rous magnetite layers, and granophyresin the upper, most fractionated parts. Unlike many layered intrusions,the Giles Complex is tectonically dismembered to an extent thata reconstruction of the original morphology is difficult. The Complex is believed to be a type example for medium- tohigh-pressure differentiation. (1) Chilled margin samples (wherepreserved) are orthopyroxene-phyric, and liquidus olivine isreplaced by liquidus orthopyroxene at an mg-number of 0.77,suggesting a pressure-related expansion of the orthopyroxenestability field (Goode & Moore, 1975). (2) Tschermaks substitutioninto pyroxene and plagioclase-orthoclase solid solution areextensive, indicating unusually high crystallization temperaturerelated to high pressure; antiperthites in the Giles Complexare amongst the most calcic reported for terrestrial rocks.(3) The lower primitive cumulate units of the Complex are coroniticand feature a variety of subsolidus high-pressure reaction textures;olivine and cumulus chromite have reacted with calcic plagioclaseto orthopyroxene-clinopyroxene-spinel, olivine-spinel, and clinopyroxene-spinelsymplectites. The principal reaction mechanism for the symplectites was continuousmass transfer of alumina from plagioclase toward spinel, asthe Complex passed from the olivine-plagioclase stability fieldinto the pyroxene-spinel field during cooling. Geothermometersapplicable to the cumulates record a wide range of equilibrationtemperatures from late-magmatic to granulite-metamorphic conditions.FeMg1 exchange gives closure temperatures around 600–700?C,whereas Al2Mg1Si1 net-transfer equilibria have preserved highertemperatures around 750–900 ?C. Defocused beam bulk analysesof exsolved cumulus clinopyroxenes and intercumulus plagioclasesrecover magmatic compositions; i. e., two-pyroxene solvus CaMg-1temperatures plot around 1120?50?C, whereas two-feldspar thermometersgive 1200?C. Pressures are calculated from thermochemical data with the heterogeneousequilibria 2 fo + an = en + di + sp, fo + an = di + Mg-Ts, andfo + an = en + Ca-Ts, after correcting spinel activities forselective retrograde FeMg-1 exchange during cooling. These equilibria,combined with orthopyroxene-spinel Al2Mg-1Si-1 temperaturesfor metamorphic assemblages and two-pyroxene temperatures forcumulus phases define a medium-pressure cooling path extendingfrom 1150 ?C (at 6?5 kb) to 750 ?C (at 6?2 kb). The resultssuggest an isobaric cooling path for the Giles Complex, withno evidence for a post-intrusive metamorphic overprint. Themagmas intruded at lower to middle crustal levels after thepervasive deformation in the Musgrave block, and probably afterthe peak metamorphic event. 相似文献
15.
The Petrology of the Lower Old Red Sandstone Lavas of the Eastern Sidlaw Hills, Perthshire, Scotland
The calc-alkaline lava sequence of the eastern Sidlaw Hillsforms a small part of an extensive volcanic province of LowerOld Red Sandstone (Devonian) age in Scotland and N. England.The Sidlaw lavas ranging from olivine basalt to dacite are allporphyritic with combinations of olivine, plagioclase, clinopyroxene,orthopyroxene, and opaque oxide pheno-crysts. Chemically, thelavas are slightly more alkalic than modern calc-alkaline lavas.There is considerable variation in the ‘incompatible elements’.The differentiation of the lavas can be accounted for by fractionationof olivine+plagioclase+minor ore from a chemically variable,immediately parental magma at low pressure (c. 1 kb PH2O). Itis suggested that fractionation of variable amounts of olivineand clinopyroxene from an olivine tholeiite at moderate PH2Ocould give rise to this chemically variable, high alumina, immediatelyparental magma. 相似文献
16.
Unusually iron-rich pyroxene and olivine occur in rocks associatedwith the Nain anorthosite massif, Labrador. Adamellite and granodioritecontain orthopyroxene (inverted from pigeonite) as iron-richas Ca6Fe82Mg12; comparison with experimental data suggests aminimum pressure of crystallization of 5 kb. Some of these iron-richpyroxene crystals have broken down, apparently upon decreasingpressure, to yield intergrowths of less iron-rich orthopyroxene(near Ca7Fe72Mg21), ferroaugite, fayalite (near Fo9), and quartz.Other rocks, monzonites, contain pyroxenes with calcium-poorcores and ferroaugite rims, as well as crystals composed ofbroad lamellae of ferroaugite and orthopyroxene in sub-equalproportions. Analysis of one such crystal with unusually thinand closely spaced lamellae yielded a bulk composition of Ca24Fe58Mg18.Such pyroxenes probably crystallized near or above the crestof the augite-pigeonite two-phase region, probably above 925°C. This high temperature suggests that the monzonites crystallizedfrom relatively dry magmas. If they represent a residual fractionderived from the same magma as the anorthosite, then that magmamust have been nearly anhydrous. Pigeonite rather than orthopyroxene was the primary magmaticCa-poor pyroxene in most of the Nain rocks studied here. Nucleationrates apparently were low under subsolidus conditions, and low-Capigeonite (Ca2Fe78Mg20) is present in grains where orthopyroxenedid not nucleate as pigeonite cooled and exsolved ferroaugite.Iron-rich orthopyroxene (Ca2Fe79M19) crystallized instead ofpigeonite in a Greenland quartz syenite that contains more abundanthydrous phases. 相似文献
17.
The results of synthesis experiments in the system KAlSiO4—Mg2SiO4—SiO2—SiO2H2Ohave been used to outline the melting and sub-solidus phaserelations at temperatures from 700 to 1200 C and pressuresto 3 kilobars. Studies in this system provide a framework withinwhich petrologic features of the near-surface potassic rocks,some lamprophyres, charnockitic granites, kimberlites, and alliedmica peridotites may be discussed. On the basis of the experimentalstudies the pressure-temperature stability limits of coexistingphases are considered. The bivariant phase relations providea means by which the olivine biotite and pyroxene biotitereaction relations observed in potassic rocks may be accountedfor. The phase relations provide a mechanism for crossing the‘equilibrium thermal divides’ forsterite-potashfeldspar and enstatite-potash feldspar, from the silica-undersaturatedto the silica-oversaturated region. The petrologic importanceof water-undersaturatsed magmas is stressed throughout the discussion. 相似文献
18.
The well-preserved ?lower Proterozoic McIntosh intrusion consistsof 96 macro-layers with a total stratigraphic thickness of about6 km. The lowermost rocks in this possible cone-shaped intrusionare hidden, and the roof and the upper layers were removed byerosion. The layered sequence is dominated by 40 bimodal cyclicunits of troctolite and olivine gabbro. Minor gabbronorite layersoccur throughout the sequence, and are more abundant and morefractionated higher in the sequence. Six imperfect megacycicunits are developed in the upper 2700 m, each unit consistingof several troctolite-olivine gabbro cyclic units followed bya Fe-Ti oxide-bearing gabbronorite. The overall cumulus crystallizationorder in each megacyclic unit was plagioclase first, closelyfollowed by olivine, then augite, orthopyroxene, and magnetitesuccessively. Cryptic composition data for troctolites and olivine gabbrosshow a slight overall decrease of 10 mol per cent An and Fofrom the base to the top of the layered sequence (approximateranges An80–70 and Fo78–68). Several major fluctuationsoccur however, and are generally associated with the oxide gabbronorites,which are significantly more fractionated than the adjacentlayers (plagioclase An53–60, orthopyroxene Mg52–69Each fluctuation comprises a marked progressive discontinuity(rapid normal fractionation) followed by a gradual to rapidregressive discontinuity (or reversal) in the overlying troctolitesand olivine gabbros. Apparently, such marked progressive discontinuitieshave not been described in layered intrusions. A chilled margin and the overall composition of the intrusionsuggest an olivine tholeiite parent magma, inferred to havecrystallized at P 6 kb, relatively low PH2O and high fO2 (>NNO buffer). The troctolite-olivine gabbro cyclic units areinferred to have formed by fractional crystallization of periodicadditions of new magma. However, the oxide gabbronorites seemtoo fractionated relative to the underlying layers to have formedby conventional crystal fractionation mechanisms, and they couldhave resulted from a ‘liquid fractionation’ processin which fractionated residual magma, instead of rising, periodicallybecame denser and ponded on the temporary floor (a density crossover).Gradual, reversed cryptic trends in the cyclic units above theoxide gabbronorite layers may reflect mixing of this fractionatedmagma with successive magma additions. 相似文献
19.
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
20.
This paper reports experiments carried out at 1-atm under conditions of controlled oxygen fugacity, using natural andesites and andesite mixed with augite+synthetic pigeonite or augite+orthopyroxene. The experimental results are used (1) to investigate the controls of Mg# (Mg/[Mg+Fe2+]) and temperature on low-Ca pyroxene stability (pigeonite vs orthopyroxene), (2) to quantify the effects of variations in bulk composition on the position of multiple saturation boundaries in mineral component projection schemes and (3) to develop a thermodynamic model for silica activity for melts saturated with olivine and pyroxene. Over the Mg# range of 0.80–0.30 the minimum temperature of pigeonite stability in natural compositions is equivalent to the Lindsley (1983) boundary determined for pure Ca-Mg-Fe pigeonites. For the low variance, 5-phase assemblage oliv-aug-low-Ca pyroxene-plag-liquid, expressions involving liquid (Na2O+K2O)/(Na2O+K2O+CaO),Mg# and TiO2 content predict temperature and the movement of multiple saturation boundaries in pseudoternary projections in response to changing melt composition. The equilibrium for the low pressure melting of low-Ca pyroxene to olivine+liquid is formulated as a geothermometer and monitor of silica activity. Equilibrium constants estimated from thermochemical data and activities calculated for experimentally produced olivine and pyroxenes are used to develop a model for silica activity in liquid. 相似文献