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1.
The compositions of multiply saturated partial melts are valuablefor the thermodynamic information that they contain, but aredifficult to determine experimentally because they exist onlyover a narrow temperature range at a given pressure. Here wetry a new approach for determining the composition of the partialmelt in equilibrium with olivine, orthopyroxene, clinopyroxeneand spinel (Ol + Opx + Cpx + Sp + Melt) in the system CaOMgOAl2O3SiO2(CMAS) at 1·1 GPa: various amounts of K2O are added tothe system, and the resulting melt compositions and temperatureare extrapolated to zero K2O. The sandwich experimentalmethod was used to minimize problems caused by quench modification,and Opx and Cpx were previously synthesized at conditions nearthose of the melting experiments to ensure they had appropriatecompositions. Results were then checked by reversal crystallizationexperiments. The results are in good agreement with previouswork, and establish the anhydrous solidus in CMAS to be at 1320± 10°C at 1·1 GPa. The effect of K2O is todepress the solidus by 5·8°C/wt %, while the meltcomposition becomes increasingly enriched in SiO2, being quartz-normativeabove 4 wt % K2O. Compared with Na2O, K2O has a stronger effectin depressing the solidus and modifying melt compositions. Theisobaric invariant point in the system CMASK2O at whichOl + Opx + Cpx + Sp + Melt is joined by sanidine (San) is at1240 ± 10°C. During the course of the study severalother isobaric invariant points were identified and their crystaland melt compositions determined in unreversed experiments:Opx + Cpx + Sp + An + Melt in the system CMAS at 1315 ±10°C; in CMASK2O, Opx + Cpx + Sp + An + San + Meltat 1230 ± 10°C and Opx + Sp + An + San + Sapph +Melt at 1230 ± 10°C, where An is anorthite and Sapphis sapphirine. Coexisting San plus An in three experiments helpdefine the AnSan solvus at 12301250°C. KEY WORDS: feldspar solvus; igneous sapphirine; mantle solidus; partial melting; systems CMAS and CMASK2O 相似文献
2.
Chromium as Cr3+ substitutes for octahedrally coordinated Alin upper-mantle minerals, thereby reducing the activity of Al2O3in the system and hence the concentration of Al2O3 in partialmelts. The effect of Cr2O3 on melt compositions multiply saturatedwith the spinel lherzolite phase assemblage has been quantifiedin the system CaOMgOAl2O3SiO2Cr2O3at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel(Cr#sp). The decrease of Al2O3 in the melt with increasing Cr#spis accompanied by increasing MgO and SiO2, whereas CaO remainsalmost constant. Consequently, the CaO/Al2O3 ratio of the meltincreases with Cr#sp, and the melt becomes richer in normativediopside, hypersthene and quartz. The effect may explain certainmantle melts with unusually high CaO/Al2O3 ratios. The concentrationof Cr2O3 in the melt remains low even at high Cr#sp, which meansthat the strong effect of Cr2O3 on partial melting equilibriais not readily apparent from its concentration in the melt itself.The existence of a highly refractory major component such asCr2O3 nullifies simplified conclusions from the inverseapproach in the experimental study of basalt petrogenesis,as there is insufficient information in the composition of thepartial melt to reconstruct the conditions of melting. KEY WORDS: basalt petrogenesis; partial melting; reversal experiment; spinel lherzolite; system CMASCr2O3; CaO/Al2O3 of melt; effect of Cr2O3 相似文献
3.
Using experimental results at 1·0 GPa for the systemsCaOSiO2, MgOSiO2, CaMgSi2O6SiO2 and CaMgSi2O6Mg2SiO4,and all the currently available phase equilibria and thermodynamicdata at 1 bar, we have optimized the thermodynamic propertiesof the liquid phase at 1·0 GPa. The new optimized thermodynamicparameters indicate that pressure has little effect on the topologyof the CaOSiO2, CaMgSi2O6SiO2, and CaMgSi2O6Mg2SiO4systems but a pronounced one on the MgOSiO2 binary. Themost striking change concerns passage of the MgSiO3 phase fromperitectic melting at 1 bar to eutectic melting at 1·0GPa. This transition is estimated to occur at 0·41 GPa.For the CaMgSi2O6SiO2 and CaMgSi2O6Mg2SiO4 pseudo-binaries,the size of the field clinopyroxene + liquid increases withincreasing pressure. This change is related to the shift ofthe piercing points clinopyroxene + silica + liquid (from 0·375mol fraction SiO2 at 1 bar to 0·414 at 1·0 GPa)and clinopyroxene + olivine + liquid (from 0·191 molfraction SiO2 at 1 bar to 0·331 at 1·0 GPa) thatbound the clinopyroxene + liquid field in the CaMgSi2O6·SiO2and CaMgSi2O6·Mg2SiO4 pseudo-binaries, respectively. KEY WORDS: CaOSiO2; CaMgSi2O6Mg2SiO4; CaMgSi2O6SiO2; experiments; MgOSiO2 相似文献
4.
Multianvil melting experiments in the system CaOMgOAl2O3SiO2CO2(CMASCO2) at 38 GPa, 13401800°C, involvingthe garnet lherzolite phase assemblage in equilibrium with CO2-bearingmelts, yield continuous gradations in melt composition betweencarbonatite, kimberlite, melilitite, komatiite, picrite, andbasalt melts. The phase relations encompass a divariant surfacein PT space. Comparison of the carbonatitic melts producedat the low-temperature side of this surface with naturally occurringcarbonatites indicates that natural magnesiocarbonatites couldbe generated over a wide range of pressures >2·5 GPa.Melts analogous to kimberlites form at higher temperatures alongthe divariant surface, which suggests that kimberlite genesisrequires more elevated geotherms. However, the amount of waterfound in some kimberlites has the potential to lower temperaturesfor the generation of kimberlitic melts by up to 150°C,provided no hydrous phases are present. Compositions resemblinggroup IB and IA kimberlites are produced at pressures around56 GPa and 10 GPa, respectively, whereas the compositionsof some other kimberlites suggest generation at higher pressuresstill. At pressures <4 GPa, an elevated geotherm producesmelilitite-like melt in the CMASCO2 system rather thankimberlite. Even when a relatively CO2-rich mantle compositioncontaining 0·15 wt % CO2 is assumed, kimberlites andmelilitites are produced by <1% melting and carbonatitesare generated by even smaller degrees of melting of <0·5%. KEY WORDS: carbonatite; CO2; kimberlite; melilitite; melt generation 相似文献
5.
FeMg exchange is the most important solid solution involvedin partial melting of spinel lherzolite, and the system CaOMgOAl2O3SiO2FeO(CMASF) is ideally suited to explore this type of exchange duringmantle melting. Also, if primary mid-ocean ridge basalts arelargely generated in the spinel lherzolite stability field bynear-fractional fusion, then Na and other highly incompatibleelements will early on become depleted in the source, and themelting behaviour of mantle lherzolite should resemble the meltingbehaviour of simplified lherzolite in the CMASF system. We havedetermined the isobarically univariant melting relations ofthe lherzolite phase assemblage in the CMASF system in the 0·72·8GPa pressure range. Isobarically, for every 1 wt % increasein the FeO content of the melt in equilibrium with the lherzolitephase assemblage, the equilibrium temperature is lower by about35°C. Relative to the solidus of model lherzolitein the CaOMgOAl2O3SiO2 system, melt compositionsin the CMASF system are displaced slightly towards the alkalicside of the basalt tetrahedron. The transition on the solidusfrom spinel to plagioclase lherzolite has a positive Clapeyronslope with the spinel lherzolite assemblage on the high-temperatureside, and has an almost identical position in PT spaceto the comparable transition in the CaOMgOAl2O3SiO2Na2O(CMASN) system. When the compositions of all phases are describedmathematically and used to model the generation of primary basalts,temperature and melt composition changes are small as percentmelting increases. More specifically, 10% melting takes placeover 1·52°C, melt compositions are relativelyinsensitive to the degree of melting and bulk composition, andequilibrium and near-fractional melting yield similar melt compositions.FeO and MgO are the oxides that exhibit the greatest changein the melt with degree of melting and bulk composition. Theamount of FeO decreases with increasing degree of melting, whereasthe amount of MgO increases. The coefficients for FeMgexchange between the coexisting crystalline phases and melt,KdFeMgxlliq, show a relatively simple and predictablebehaviour with pressure and temperature: the coefficients forolivine and spinel do not show significant dependence on temperature,whereas the coefficients for orthopyroxene and clinopyroxeneincrease with pressure and temperature. When melting of lherzoliteis modeled in the CMASF system, a strong linear correlationis observed between the mg-number of the lherzolite and themg-number of the near-solidus melts. Comparison with meltingin the CMASN system indicates that Na2O has a strong effecton lherzolite melting behaviour only at small degrees of melting. KEY WORDS: CMASF; lherzolite solidus; mantle melting 相似文献
6.
We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO2) from experiments at 3 GPa and comparethe compositions of partial melts with those of alkalic andhighly alkalic oceanic island basalts (OIBs). The solidus islocated at 10501075 °C and the liquidus at 1415 °C.The sub-solidus assemblage consists of clinopyroxene, garnet,ilmenite, and calcio-dolomitic solid solution and the near solidusmelt is carbonatitic (<2 wt % SiO2, <1 wt % Al2O3, and<0·1 wt % TiO2). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (3443 wt % SiO2)with high TiO2 (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO2). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO2-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO2systems, carbonate and silicate melt coexist over a wide temperatureinterval for partial melting of SLEC1 carbonated eclogite at3 GPa. Silicate melts generated from SLEC1, especially at highmelt fraction (>20 wt %), may be plausible sources or contributingcomponents to melilitites and melilititic nephelinites fromoceanic provinces, as they have strong compositional similaritiesincluding their SiO2, FeO*, MgO, CaO, TiO2 and Na2O contents,and CaO/Al2O3 ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO2 and FeO* and low Al2O3, that have not been observedfrom partial melting of peridotite ± CO2. In upwellingmantle, formation of carbonatite and silicate melts from eclogiteand peridotite source lithologies occurs over a wide range ofdepths, producing significant opportunities for metasomatictransfer and implantation of melts. KEY WORDS: carbonated eclogite; experimental phase equilibria; partial melting; liquid immiscibility; ocean island basalts 相似文献
7.
Calculated Phase Relations in the System Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O with Applications to UHP Eclogites and Whiteschists 总被引:2,自引:0,他引:2
Pressuretemperature grids in the system Na2OCaOK2OFeOMgOAl2O3SiO2H2O and its subsystems have been calculatedin the range 1545 kbar and 550900°C, usingan internally consistent thermodynamic dataset and new thermodynamicmodels for amphibole, white mica, and clinopyroxene, with thesoftware THERMOCALC. Minerals considered for the grids includegarnet, omphacite, diopside, jadeite, hornblende, actinolite,glaucophane, zoisite, lawsonite, kyanite, coesite, quartz, talc,muscovite, paragonite, biotite, chlorite, and plagioclase. Compatibilitydiagrams are used to illustrate the phase relationships in thegrids. Coesite-bearing eclogites and a whiteschist from Chinaare used to demonstrate the ability of pseudosections to modelphase relationships in natural ultrahigh-pressure metamorphicrocks. Under water-saturated conditions, chlorite-bearing assemblagesin Mg- and Al-rich eclogites are stable at lower temperaturesthan in Fe-rich eclogites. The relative temperature stabilityof the three amphiboles is hornblende > actinolite > glaucophane(amphibole names used sensu lato). Talc-bearing assemblagesare stable only at low temperature and high pressure in Mg-and Al-rich eclogites. For most eclogite compositions, talccoexists with lawsonite, but not zoisite, in the stability fieldof coesite. Water content contouring of pressuretemperaturepseudosections, along with appropriate geotherms, provides newconstraints concerning dehydration of such rocks in subductingslabs. Chlorite and lawsonite are two important H2O-carriersin subducting slabs. Depending on bulk composition and pressuretemperaturepath, amphibole may or may not be a major H2O-carrier to depth.In most cases, dehydration to make ultrahigh-pressure eclogitestakes place gradually, with H2O content controlled by divariantor higher variance assemblages. Therefore, fluid fluxes in subductionzones are likely to be continuous, with the rate of dehydrationchanging with changing pressure and temperature. Further, eclogitesof different bulk compositions dehydrate differently. Dehydrationof Fe-rich eclogite is nearly complete at relatively shallowdepth, whereas Mg- and Al-rich eclogites dehydrate continuouslydown to greater depth. KEY WORDS: dehydration; eclogites; phase relations; THERMOCALC; UHP metamorphism; whiteschists 相似文献
8.
Fluid-saturated subsolidus experiments from 2·0 to 6·5GPa, and from 680 to 800°C have been performed on threemodel peridotites in the system Na2OCaOFeOMgOAl2O3SiO2H2O(NCFMASH). Amphibole and chlorite coexist up to 2·4 GPa,700°C. Chlorite persists to 4·2 GPa at 680°C.Starting from 4·8 GPa, 680°C a 10 Å phase structurereplaces chlorite in all compositions. The 10 Å phasestructure contains significant Al2O3 (up to 10·53 wt%) deviating from the MgOSiO2H2O 10 Å phase(MSH 10 Å phase). A mixed layered structure (chloriteMSH10 Å phase) is proposed to account for aluminium observed.In the Tinaquillo lherzolite amphibole breakdown occurs viathe reaction
Thermal stabilityof chlorite (chlorite + orthopyroxene = forsterite + garnet+ H2O) is shifted towards lower temperatures, compared withthe system MASH. Furthermore, the chlorite thermal breakdownis also related to the degenerate reaction
Chlorite and the Al-10 Å phase structurecontribute significantly to the water budget in subduction zonesin the depth range relevant for arc magmatism, whereas amphibole-relatedfluid release is restricted to the forearc region. Chloriteand Al-10 Å phase breakdowns might explain the occurrenceof a double seismic zone by dehydration embrittlement. KEY WORDS: amphibole; chlorite; high pressure; peridotites; subduction zones 相似文献
9.
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 (Na2OCaOK2OFeOMgOAl2O3SiO2H2O)and the subsystems NCKMASH and NCKFASH calculated with the softwareTHERMOCALC 3.1 are presented for the PT range 730kbar and 450680°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 PT and TXCa and PXCa 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 PT conditions. The calculated NCKFMASH diagramsare powerful in delineating the phase equilibria and PTconditions of natural pelitic assemblages. Moreover, contoursof the calculated phengite Si isopleths in PT and PXCapseudosections confirm that phengite barometry in NCKFMASH isstrongly dependent on mineral assemblage. KEY WORDS: phase relations; metapelites; NCKFMASH; THERMOCALC; phengite geobarometry 相似文献
10.
Calculated Phase Relations in High-Pressure Metapelites in the System NKFMASH (Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O) 总被引:6,自引:0,他引:6
Using an internally consistent thermodynamic dataset and updatedmodels of activitycomposition relation for solid solutions,petrogenetic grids in the system NKFMASH (Na2OK2OFeOMgOAl2O3SiO2H2O)and the subsystems NKMASH and NKFASH have been calculated withthe software THERMOCALC 3.1 in the PT range 536kbar and 400810°C, involving garnet, chloritoid,biotite, carpholite, talc, chlorite, kyanite/sillimanite, staurolite,phengite, paragonite, albite, glaucophane, jadeite, with quartz/coesiteand H2O in excess. These grids, together with calculated AFMcompatibility diagrams and PT pseudosections, are shownto be powerful tools for delineating the phase equilibria andPT conditions of Na-bearing pelitic assemblages for avariety of bulk compositions from high-P terranes around theworld. These calculated equilibria are in good agreement withpetrological studies. Moreover, contours of the calculated phengiteSi isopleths in PT pseudosections for different bulkcompositions confirm that phengite barometry is highly dependenton mineral assemblage. KEY WORDS: phase relations; HP metapelite; NKFMASH; THERMOCALC; phengite geobarometry 相似文献
11.
High-pressure Partial Melting of Mafic Lithologies in the Mantle 总被引:17,自引:2,他引:15
We review experimental phase equilibria associated with partialmelting of mafic lithologies (pyroxenites) at high pressuresto reveal systematic relationships between bulk compositionsof pyroxenite and their melting relations. An important aspectof pyroxenite phase equilibria is the existence of the garnetpyroxenethermal divide, defined by the enstatiteCa-Tschermakspyroxenediopside plane in CaOMgOAl2O3SiO2projections. This divide appears at pressures above 2 GPa inthe natural system where garnet and pyroxenes are the principalresidual phases in pyroxenites. Bulk compositions that resideon either side of the divide have distinct phase assemblagesfrom subsolidus to liquidus and produce distinct types of partialmelt ranging from strongly nepheline-normative to quartz-normativecompositions. Solidus and liquidus locations are little affectedby the location of natural pyroxenite compositions relativeto the thermal divide and are instead controlled chiefly bybulk alkali contents and Mg-numbers. Changes in phase volumesof residual minerals also influence partial melt compositions.If olivine is absent during partial melting, expansion of thephase volume of garnet relative to clinopyroxene with increasingpressure produces liquids with high Ca/Al and low MgO comparedwith garnet peridotite-derived partial melts. KEY WORDS: experimental petrology; mantle heterogeneity; partial melting; phase equilibrium; pyroxenite 相似文献
12.
In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structuralmicrotexturalrelations and thermobarometry indicate that syn-deformationalsegregationcrystallization of in situ stromatic and diatexiteleucosomes occurred at 800°C and 8 kbar. The protolith,the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase,hornblende, biotite, sphene, apatite, zircon, and ilmenite,but the modal mineralogy differs widely. The protolith compositionis straddled by element abundances in the leucosome and themesosome. The leucosomes are characterized by lower CaO, FeO+MgO,mg-number, TiO2 , P2O5 , Rb, Zr and total rare earth elements(REE), and higher SiO2 , K2O, Ba and Sr than the protolith andthe mesosome, whereas Na2O and Al2O3 abundances are similar.The protolith and the mesosome have negative Eu anomalies, butprotolith-normalized abundances of REE-depleted leucosomes showpositive Eu anomalies. The congruent melting reaction for leucosomeproduction is inferred to be 0·325 quartz+0·288K-feldspar+0·32 plagioclase+0·05 biotite+0·014hornblende+0·001 apatite+0·001 zircon+0·002sphene=melt. Based on the reaction, large ion lithophile element,REE and Zr abundances in model melts computed using dynamicmelting approached the measured element abundances in leucosomesfor >0·5 mass fraction of unsegregated melts withinthe mesosome. Disequilibrium-accommodated dynamic melting andequilibrium crystallization of melts led to uniform plagioclasecomposition in migmatites and REE depletion in leucosome. KEY WORDS: migmatite; REE; trace element; partial melting; PT conditions 相似文献
13.
Phase Relations in Peridotitic and Pyroxenitic Rocks in the Model Systems CMASH and NCMASH 总被引:1,自引:1,他引:0
Alpine-type peridotites and associated pyroxenites are foundas lenses in the continental crust in many different orogens.The reconstruction of the pressuretemperature (PT)evolution of these rocks is, however, difficult or even impossible.With geothermobarometry, usually one point on the overall PTpath can be obtained. To use the different mineral assemblagesobserved in ultramafic rocks as PT indicators, quantitativePT phase diagrams are required. This study presents newcalculated phase diagrams for peridotitic and pyroxenitic rocksin the model systems CaOMgOAl2O3SiO2H2O(CMASH) and Na2OCaOMgOAl2O3SiO2H2O(NCMASH), which include the respective solid solutions as continuousexchange vectors. These phase diagrams represent applicablepetrogenetic grids for peridotite and pyroxenite. On the basisof these general petrogenetic grids, phase diagrams for particularperidotite and pyroxenite bulk compositions are constructed.In an example of pyroxenite from the Shackleton Range, Antarctica,the different observed mineral assemblages are reflected bythe phase diagrams. For these rocks, a high-pressure metamorphicstage around 18 kbar and an anticlockwise PT evolution,not recognized previously, can be inferred. KEY WORDS: Antarctic; high-pressure metamorphism; peridotite; phase diagrams; pyroxenite 相似文献
14.
Liquidus relations in the four-component system Na2OAl2O3SiO2F2O1were studied at 0· 1 and 100 MPa to define the locationof fluoridesilicate liquid immiscibility and outlinedifferentiation paths of fluorine-bearing silicic magmas. Thefluoridesilicate liquid immiscibility spans the silicaalbitecryoliteand silicatopazcryolite ternaries and the haplogranite-cryolitebinary at greater than 960°C and 0· 1100 MPa.With increasing Al2O3 in the system and increasing aluminum/alkalication ratio, the two-liquid gap contracts and migrates fromthe silica liquidus to the cryolite liquidus. The gap does notextend to subaluminous and peraluminous melt compositions. Forall alkali feldsparquartz-bearing systems, the miscibilitygap remains located on the cryolite liquidus and is thus inaccessibleto differentiating granitic and rhyolitic melts. In peralkalinesystems, the magmatic differentiation is terminated at the albitequartzcryoliteeutectic at 770°C, 100 MPa, 5 wt % F and cation Al/Na =0· 75. The addition of topaz, however, significantlylowers melting temperatures and allows strong fluorine enrichmentin subaluminous compositions. At 100 MPa, the binary topazcryoliteeutectic is located at 770°C, 39 wt % F, cation Al/Na 0·95, and the ternary quartztopazcryolite eutecticis found at 740°C, 32 wt % F, 30 wt % SiO2 and cation Al/Na 0· 95. Such location of both eutectics enables fractionationpaths of subaluminous quartz-saturated systems to produce fluorine-rich,SiO2-depleted and nepheline-normative residual liquids. KEY WORDS: silicate melt; granite; rhyolite; fluorine; liquid immiscibility 相似文献
15.
Partial Melting Experiments of Peridotite + CO2 at 3 GPa and Genesis of Alkalic Ocean Island Basalts
We document compositions of minerals and melts from 3 GPa partialmelting experiments on two carbonate-bearing natural lherzolitebulk compositions (PERC: MixKLB-1 + 2·5 wt% CO2; PERC3:MixKLB-1 + 1 wt% CO2) and discuss the compositions of partialmelts in relation to the genesis of alkalic to highly alkalicocean island basalts (OIB). Near-solidus (PERC: 1075–1105°C;PERC3: 1050°C) carbonatitic partial melts with <10 wt%SiO2 and 40 wt% CO2 evolve continuously to carbonated silicatemelts with >25 wt% SiO2 and <25 wt% CO2 between 1325 and1350°C in the presence of residual olivine, orthopyroxene,clinopyroxene, and garnet. The first appearance of CO2-bearingsilicate melt at 3 GPa is 150°C cooler than the solidusof CO2-free peridotite. The compositions of carbonated silicatepartial melts between 1350 and 1600°C vary in the rangeof 28–46 wt% SiO2, 1·6–0·5 wt% TiO2,12–10 wt% FeO*, and 19–29 wt% MgO for PERC, and42–48 wt% SiO2, 1·9–0·5 wt% TiO2,10·5–8·4 wt% FeO*, and 15–26 wt% MgOfor PERC3. The CaO/Al2O3 weight ratio of silicate melts rangesfrom 2·7 to 1·1 for PERC and from 1·7 to1·0 for PERC3. The SiO2 contents of carbonated silicatemelts in equilibrium with residual peridotite diminish significantlywith increasing dissolved CO2 in the melt, whereas the CaO contentsincrease markedly. Equilibrium constants for Fe*–Mg exchangebetween carbonated silicate liquid and olivine span a rangesimilar to those for CO2-free liquids at 3 GPa, but diminishslightly with increasing dissolved CO2 in the melt. The carbonatedsilicate partial melts of PERC3 at <20% melting and partialmelts of PERC at 15–33% melting have SiO2 and Al2O3 contents,and CaO/Al2O3 values, similar to those of melilititic to basaniticalkali OIB, but compared with the natural lavas they are moreenriched in CaO and they lack the strong enrichments in TiO2characteristic of highly alkalic OIB. If a primitive mantlesource is assumed, the TiO2 contents of alkalic OIB, combinedwith bulk peridotite/melt partition coefficients of TiO2 determinedin this study and in volatile-free studies of peridotite partialmelting, can be used to estimate that melilitites, nephelinites,and basanites from oceanic islands are produced from 0–6%partial melting. The SiO2 and CaO contents of such small-degreepartial melts of peridotite with small amounts of total CO2can be estimated from the SiO2–CO2 and CaO–CO2 correlationsobserved in our higher-degree partial melting experiments. Thesesuggest that many compositional features of highly alkalic OIBmay be produced by 1–5% partial melting of a fertile peridotitesource with 0·1–0·25 wt% CO2. Owing to verydeep solidi of carbonated mantle lithologies, generation ofcarbonated silicate melts in OIB source regions probably happensby reaction between peridotite and/or eclogite and migratingcarbonatitic melts produced at greater depths. KEY WORDS: alkali basalts; carbonated peridotite; experimental petrology; ocean island basalts; partial melting 相似文献
16.
ACOSTA-VIGIL ANTONIO; LONDON DAVID; MORGAN GEORGE B. VI; DEWERS THOMAS A. 《Journal of Petrology》2006,47(2):231-254
We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H2O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 1201488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H2O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineralglassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO2, Na2O, and K2O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al2O3, Na2O, andK2O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al2O3/[Na2O + K2O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylindersquartz and albite, quartzand K-feldspar, or albite and corundumtested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·02·8) x 1015m2/s and (0·62·4) x 1014 m2/s, respectively.Minimum diffusivities of alkalis [(39) x 1011m2/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 23 mm undergoinghydrostatic, H2O-saturated melting at 800°C (infinite heatsupply) could produce 2030 vol. % of homogeneous meltin less than 110 years. Slower diffusion in H2O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis 相似文献
17.
Al2SiO5 reaction textures in aluminous schist and quartziteof the northern Picuris range, north-central New Mexico, recorda paragenetic sequence of kyanite to sillimanite to andalusite,consistent with a clockwise PT loop, with minor decompressionnear the Al2SiO5 triple-point. Peak metamorphic temperaturesare estimated at 510525°C, at 4·04·2kbar. Kyanite and fibrolite are strongly deformed; some prismaticsillimanite, and all andalusite are relatively undeformed. Monaziteoccurs as inclusions within kyanite, mats of sillimanite andcentimetre-scale porphyroblasts of andalusite, and is typicallyaligned subparallel to the dominant regional foliation (S0/S1or S2) and extension lineation (L1). Back-scatter electron imagesand X-ray maps of monazite reveal distinct core, intermediateand rim compositional domains. Monazitexenotime thermometryfrom the intermediate and rim domains yields temperatures of405470°C (±50°C) and 500520°C(±50°C), respectively, consistent with the progradeto peak metamorphic growth of monazite. In situ, ion microprobeanalyses from five monazites yield an upper intercept age of1417 ± 9 Ma. Near-concordant to concordant analyses yield207Pb206Pb ages from 1434 ± 12 Ma (core) to 1390± 20 Ma (rim). We find no evidence of older regionalmetamorphism related to the 1650 Ma Mazatzal Orogeny. KEY WORDS: Al2SiO5; metamorphism; monazite; thermochronometry; triple-point 相似文献
18.
HARLOV DANIEL E.; JOHANSSON LEIF; VAN DEN KERKHOF ALFONS; FORSTER HANS-JURGEN 《Journal of Petrology》2006,47(1):3-33
A localized dehydration zone, Söndrum stone quarry, Halmstad,SW Sweden, consists of a central, 1 m wide granitic pegmatoiddyke, on either side of which extends a 2·53 mwide dehydration zone (650700°C; 800 MPa; orthopyroxeneclinopyroxenebiotiteamphibolegarnet)overprinting a local migmatized granitic gneiss (amphibolebiotitegarnet).Whole-rock chemistry indicates that dehydration of the graniticgneiss was predominantly isochemical. Exceptions include [Y+ heavy rare earth elements (HREE)], Ba, Sr, and F, which aremarkedly depleted throughout the dehydration zone. Systematictrends in the silicate and fluorapatite mineral chemistry acrossthe dehydration zone include depletion in Fe, (Y + HREE), Na,K, F, and Cl, and enrichment in Mg, Mn, Ca, and Ti. Fluid inclusionchemistry is similar in all three zones and indicates the presenceof a fluid containing CO2, NaCl, and H2O components. Water activitiesin the dehydration zone average 0·36, or XH2O = 0·25.All lines of evidence suggest that the formation of the dehydrationzone was due to advective transport of a CO2-rich fluid witha minor NaCl brine component originating from a tectonic fracture.Fluid infiltration resulted in the localized partial breakdownof biotite and amphiboles to pyroxenes releasing Ti and Ca,which were partitioned into the remaining biotite and amphibole,as well as uniform depletion in (Y + HREE), Ba, Sr, Cl, andF. At some later stage, H2O-rich fluids (H2O activity >0·8)gave rise to localized partial melting and the probable injectionof a granitic melt into the tectonic fracture, which resultedin the biotite and amphibole recording a diffusion profile forF across the dehydration zone into the granitic gneiss as wellas a diffusion profile in Fe, Mn, and Mg for all FeMgsilicate minerals within 100 cm of the pegmatoid dyke. KEY WORDS: charnockite; fluids; CO2; brines; localized dehydration; Söndrum 相似文献
19.
The solubility mechanisms of H2O in peralkaline sodium aluminosilicatequenched melts (anhydrous NBO/T = 0.5) have been studied withRaman spectroscopy as a function of Al/(Al + Si) (003)and H2O content (07.5 wt.%). The coexisting structuralunits in the anhydrous quenched melts are TO2 (Q4), T2O5(Q3),and TO3 (Q2). In Al-free Na2Si4O9 (NS4) melt, H2O forms complexes with Na+(NaOH bonds) and with Si4+ (SiOH bonds). MolecularH2O is also detected. TO3 structural units are not detectedin this composition. In the H2O concentration range between0 and 4 wt.%, there is an approximately 20% increase in NBO/Tresulting from the increased abundance ratio, T2O5/TO2. Withfurther increments in water activity, the NBO/T of hydrous NS4melt is reduced. The depolymerization results from hydroxylationof the silica tetrahedra, whereas polymerization is due to formationof complexes with NaOH bonding. In Al-bearing compositions on the Na2Si4O9Na2(NaAl)4O9join, there is evidence for AlOH bonding in additionto NaOH and SiOH bonds. Among these complexes,the relative abundance of those with SiOH bonds diminisheswith increasing Al/(A1 + Si), whereas complexes with AlOHand NaOH bonds become more important. Complexes withNaOH bonds dominate for H2O4 wt.%, whereas complexeswith AlOH dominate at higher water content. The threestructural units, TO3, T2O5, and TO2, were observed in bothanhydrous and hydrous peralkaline sodium aluminosilicate melts.Their abundance varies, however, with the H2O concentrationin the melts. The NBO/T decreases to a minimum (a 3050%lowering of NBO/T relative to anhydrous materials) for low H2Ocontents (34 wt.% H2O), and increases as the H2O contentis increased further. 相似文献
20.
Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating 总被引:1,自引:0,他引:1
Metapelitic rock samples from the NE Shackleton Range, Antarctica,include garnet with contrasting zonation patterns and two agespectra. Garnet porphyroblasts in K-rich kyanitesillimanitestaurolitegarnetmuscovitebiotite schistsfrom Lord Nunatak show prograde growth zonation, and give SmNdgarnet, UPb monazite and RbSr muscovite ages of518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively.Geothermobarometry and PT pseudo-section calculationsin the model system CaONa2OK2O TiO2MnOFeOMgOAl2O3SiO2H2Oare consistent with garnet growth during prograde heating from540°C/7 kbar to 650°C/7·5 kbar, and partial resorptionduring a subsequent PT decrease to <650°C at <6kbar. All data indicate that rocks from Lord Nunatak were affectedby a single orogenic cycle. In contrast, garnet porphyroblastsin K-poor kyanitesillimanite staurolitegarnetcordieritebiotite-schistsfrom Meade Nunatak show two growth stages and diffusion-controlledzonation. Two distinct age groups were obtained. Laser ablationplasma ionization multicollector mass spectrometry in situ analysesof monazite, completely enclosed by a first garnet generation,yield ages of c. 1700 Ma, whereas monazite grains in open garnetfractures and in most matrix domains give c. 500 Ma. Both agegroups are also obtained by UPb thermal ionization massspectrometry analyses of matrix monazite and zircon, which fallon a discordia with lower and upper intercepts at 502 ±1 and 1686 ± 2 Ma, respectively. SmNd garnet datingyields an age of 1571 ± 40 Ma and RbSr biotiteanalyses give an age of 504 ± 1 Ma. Integrated geochronologicaland petrological data provide evidence that rocks from MeadeNunatak underwent a polymetamorphic Barrovian-type metamorphism:(1) garnet 1 growth and subsequent diffusive garnet annealingbetween 1700 and 1570 Ma; (2) garnet 2 growth during the RossOrogeny at c. 500 Ma. During the final orogenic event the rocksexperienced peak PT conditions of about 650°C/7·0kbar and a retrograde stage at c. 575°C/4·0 kbar. KEY WORDS: garnet microtexture; PT pseudosection; geochronology; polymetamorphism; Shackleton Range; Antarctica 相似文献