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1.
We have performed time series experiments for periods rangingfrom 3 min to 44 h on the interaction of granite melt and partiallymolten basalt at 920C and 10 kbar, in the presence of 5 wt.%water. With time, the assemblage of the basalt domain changesfrom predominantly amphibole+plagioclase to clinopyroxene+garnet;the melt fraction increases from {small tilde}25 to 40%;and between the two domains, the melt compositions progressivelyequilibrate. Initially in each run, melts of the basalt domainhave uniform plateau concentrations for SiO 2, Al 2O 3, CaO, MgO,and FeO because the activities of these components are regulatedby the mineral assemblage, but at advanced stages of reaction,no such control is evident. We have derived analytical expressionsto describe and simulate the diffusion profiles. The concentrationprofiles for SiO 2, Al 2O 3, CaO, and Na 2O in the granite, emanatingfrom the basaltgranite interface, have been used to estimateeffective diffusivities. The values from the shorter runs arecompared with those of the experiment of longest duration forwhich we assumed finite couples in our calculations. In thediffusion calculations for K 2O the difference in melt fractionbetween the two domains is accounted for. The resulting values(in cm 2/s) are: DNa2O=6 10 7, DK2O=3 10 7, DMgO=9 10 8, DCaO=(46) 10 8, and DSiO2 and DAl2O3=(306) 10 8. They are in reasonable agreement with values fromother studies. On the basis of our experiments we calculatethat mafic enclaves of magmatic origin should equilibrate toa large degree with their host magma in slowly cooling non-convectinggranitic plutons. Enclaves approaching complete re-equilibrationretain distinctly higher modal amounts of mafic minerals. Theydo not compositionally resemble binary magma mixtures, but aremore like host magma with accumulated crystals. We show thatthe modal differences between enclave and host are indicativeof the temperature of homogenization and that, in principle,this temperature can be deduced from equilibrium phase diagrams.
* Present address: Mineralogisch-Petrologisches Institut, Universitt Gttingen, Goldschmidtstrasse 1, 3400 Gttingen, Germany 相似文献
2.
In arid and semiarid regions from the southwestern USA and vast areas of northwestern Mexico, Santa Ana wind events modify the environment with high temperatures, very low humidity, and dust storms representing a recurrent phenomenon that triggers asthma and other respiratory diseases. While research has emphasized Santa Ana wind effects on the USA side, northwestern Mexico has been less investigated. Numerical modeling of a severe dust storm in November 2018, applying the Weather Research and Forecasting model coupled with a chemistry module (WRF-Chem), revealed that erosion, transport, and dust storms extend along the peninsula and the Gulf of California. Santa Ana winds eroded large areas, transported desert conditions to urban zones, causing high dust concentrations and reducing the relative humidity below 10%, deteriorating climatic conditions favorable to wellness. In Tijuana, Mexicali, Ensenada, San Diego, and Los Angeles, PM10 and PM2.5 concentrations (particle matter with diameter below 10 µm and 2.5 µm) reached values over 2000 µg/m3 for PM10, with daily mean concentrations well above national standards, leading to poor air quality and representing a health threat even in short-term exposure. This Santa Ana event transported dust particles several hundreds of kilometers over urban areas, the Gulf of California, and the Pacific Ocean. Severe soil deterioration was simulated within the study area, reaching dust emissions above 700,000 t, including croplands from the northern part of Baja California and Sonora's coastal area. 相似文献
3.
We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO 2) 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 % SiO 2, <1 wt % Al 2O 3, and<0·1 wt % TiO 2). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (3443 wt % SiO 2)with high TiO 2 (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO 2). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO 2-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO 2systems, 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 SiO 2, FeO *, MgO, CaO, TiO 2 and Na 2O contents,and CaO/Al 2O 3 ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO 2 and FeO * and low Al 2O 3, that have not been observedfrom partial melting of peridotite ± CO 2. 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
4.
A correlation between the crossite component (NaM 4) in Ca-amphiboleand pressure of metamorphism has long been recognized (Shido& Miyashiro, 1959), but only recently has the reaction beenidentified which buffers this aspect of amphibole composition(Brown, 1974): Ca-amphibole+iron oxide+albite+chloriteI+H 2O (±stilp,qtz) = crossite+epidote (±muscovite, qtz). The exact stoichiometry of the reaction depends on compositionalvariables in the minerals, especially Fe 2+/Mg and Fe 3+/Al. Ca-amphiboleshould have fixed NaM 4, at any given T and P, where it coexistswith iron oxide, albite, and chlorite. Comparison of Ca-amphibole composition with mineral assemblage,in rocks from Otago, N.Z., and elsewhere, supports this hypothesis.In any terrane NaM 4 is nearly constant at a particular metamorphicgrade where amphibole exists in the buffering assemblage, butvaries widely outside of this assemblage. Variations in Fe 2+/Mgand Fe 3+/Al in the amphibole have relatively little effect onNaM 4, but in high pressure amphiboles NaM 4 varies inverselywith Al iv. Ca-amphiboles from high pressure areas have substantially moreNaM 4 (Otago, 0.6 of 2.0) than those from lower pressure areas(Sierra contact aureoles, 0.1). These relations suggest thatin the buffering assemblage, the NaM 4 content of Ca-amphiboleshould be a useful relative barometer for low to medium grademetamorphic rocks. 相似文献
5.
The Alxa Plateau has one of the highest frequencies of dust events in China and one of the greatest contributions to East
Asian dust. We compiled climate and dust storm data for the Alxa Plateau based on observational data from ten meteorological
stations from 1960 to 2005. Our analysis showed that Guaizihu and Minqin dust centers had >26 days per year with dust storms
versus 7–13 days for other desert and Gobi regions on the plateau. Variations in dust storm frequency during the study period
showed that dust storms increased during the 1960s (until 1972), decreased until the late 1990s, and then increased slightly
until 2002. About 75.6% of dust storms occurred in March, April, May, June, and July. Between 78.2 and 88.1% of the dust storms
occurred during the daytime and 28.9% of the dust storms occurred between 13:00 and 16:00. The mean durations of dust storms
in the Alxa Plateau ranged from 715 to 3,462 min. The annual number of minutes of dust storms averaged >1,800 min in the western
Alxa Plateau. Dust storm frequency was inversely related to duration: the longer the average duration, the lower the frequency
of such storms. 相似文献
6.
Major and trace element, SrNdPb isotope and mineralchemical data are presented for newly discovered ultrapotassiclavas in the Tangra YumcoXuruco graben in southern Tibet.The ultrapotassic lavas are characterized by high MgO, K 2O andTiO 2, low Al 2O 3 and Na 2O contents, and also have high molarK 2O/Al 2O 3, molar (K 2O + Na 2O)/Al 2O 3 and K 2O/Na 2O ratios. Theirhigh abundances of incompatible trace elements such as largeion lithophile elements (LILE) and light rare earth elements(LREE) reach the extreme levels typical of lamproites. The lamproitesshow highly radiogenic 87Sr/ 86Sr (0· 71660·7363) and unradiogenic 143Nd/ 144Nd (0· 5117960·511962), low 206Pb/ 204Pb (18· 45918· 931),and elevated radiogenic 207Pb/ 204Pb (15· 673215·841) and 208Pb/ 204Pb (39· 55740· 058) ratios.On the basis of their geochemical and isotopic systematics,the lamproites in south Tibet have a distinct magma source thatcan be differentiated from the sources of potassic lavas inthe east Lhasa and Qiangtang blocks. Their high Nb/Ta ratios(17· 1019· 84), extremely high Th/U ratios(5· 7013· 74) and distinctive isotope compositionsare compatible with a veined mantle source consisting of partialmelts of subducted Tethyan oceanic sediments and sub-continentallithospheric depleted mantle. Identification of the lamproitesand the delineation of their mantle source provide new evidencerelevant for models of the uplift and extension of the Tibetanplateau following the Indo-Asia collision. Metasomatism by partialmelts from isotopically evolved, old sediment subducted on theyoung Tethyan slab is an alternative explanation for PrecambrianNd and Pb model ages. In this model, differences in isotopiccomposition along-strike are attributed to differences in thetype of sediment being subducted, thus obviating the need formultiple metasomatic events over hundreds of million years.The distribution of lamproites, restricted within a northsouth-trendinggraben, indicates that the initiation of eastwest extensionin south Tibet started at 25 Ma. KEY WORDS: lamproites; subducted oceanic sediment; Tibetan active continental collision belt 相似文献
7.
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-rockTiO 2 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 TiO 2 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 TiO 2 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/ 86Sr i 0·7035, Nd i 9). However, theyhave very high TiO 2 (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 MgFe 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 TiO 2Zr contents and are the most enriched magmagroup of the KarooAntarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENEWSW) 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 NNESSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke 相似文献
8.
The solubility mechanisms of H 2O in peralkaline sodium aluminosilicatequenched melts (anhydrous NBO/T = 0.5) have been studied withRaman spectroscopy as a function of Al/(Al + Si) (003)and H 2O content (07.5 wt.%). The coexisting structuralunits in the anhydrous quenched melts are TO2 (Q4), T2O5(Q3),and TO3 (Q2). In Al-free Na 2Si 4O 9 (NS4) melt, H 2O forms complexes with Na +(NaOH bonds) and with Si 4+ (SiOH bonds). MolecularH 2O is also detected. TO3 structural units are not detectedin this composition. In the H 2O concentration range between0 and 4 wt.%, there is an approximately 20% increase in NBO/Tresulting from the increased abundance ratio, T 2O 5/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 Na 2Si 4O 9Na 2(NaAl) 4O 9join, 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 H 2O4 wt.%, whereas complexeswith AlOH dominate at higher water content. The threestructural units, TO 3, T 2O 5, and TO 2, were observed in bothanhydrous and hydrous peralkaline sodium aluminosilicate melts.Their abundance varies, however, with the H 2O concentrationin the melts. The NBO/T decreases to a minimum (a 3050%lowering of NBO/T relative to anhydrous materials) for low H 2Ocontents (34 wt.% H 2O), and increases as the H 2O contentis increased further. 相似文献
9.
Equilibrium crystallization experiments at atmospheric pressureand over a range of oxygen fugacity ( fO 2) have been carriedout on a ferro-basaltic composition similar to liquids proposedto have been parental to much of the exposed portion of theSkaergaard intrusion. Before Fe-Ti oxide saturation the liquidline of descent is little affected by fO 2. However, the appearancetemperatures of the magnetite-ulvspinel solid solution (Mt)and the ilmenite-haematite solid solution (Ilm) depend stronglyon fO 2. Above the fayalite-magnetite-quartz (FMQ) buffer Mtis the first oxide phase to appear on the liquidus, but belowthe FMQ buffer Ilm is the first oxide to crystallize. The appearancetemperature of Mt is 1100C at FMQ and the Mt liquidus slopeis 30C/log fO 2 unit between FMQ;2 and FMQJ+1. The Ilmliquidus is at 1100C between FMQ and FMQ2, but movesto lower temperature at higher fO 2 where Mt is the first oxidephase. The results indicate that the ferric iron content ofMt-saturated melts varies linearly with inverse temperature,and that Ilm saturation is closely related to melt TiO 2 content.Mt saturation produces an immediate enrichment of SiO 2 and depletionin FeO * in the melt phase, whereas Ilm saturation produces similarenrichment in SiO 2, but inn enrichment may continue for 10Cbelow the ilmenite liquidus. The experimental liquids reacha maximum of 18 wt% FeO *, at 48 wt% SiO 2 for ilmenite-saturatedmelts at low f O2, more differentiated melts having lower ironand higher silica. Cotectic proportions, derived from mass balancecalculations, are in good agreement with data from natural samplesand other experimental studies. Olivine resorption is inferredat all fO 2, with the onset of resorption occurring 10C higherthan the appearance of magnetite. The effect of fO 2 on silicatemineral compositions, and partitioning of elements between coexistingmineral-melt pairs, is small. Thermodynamic considerations suggestthat variations of Fe-Mg partitioning between the iron-richolivines, pyroxenes and melts produced in this study may beexplained by known non-idealities of Fe-Mg mixing in the crystallinephases, rather than nonidealities in the coexisting melts. Theseexperiments also provide insights into many features commonto natural tholeiitic series of volcanic and plutonic rocks,and provide experimental data required for modelling of fractionalcrystallization and crystallization closed to oxygen, processeswhich are not easily investigated experimentally. KEY WORDS: ferro-basalt; Fe-Ti oxides; oxygen fugacity; Skaergaard intrusion; iron enrichment
*Corresponding author. Present address: Bayerisches Geoinstitut, Univerritt Bayreuth, D-95440 Bayreuth, Germany 相似文献
10.
Experiments were conducted to determine the solubilities ofH 2O and CO 2 and the nature of their mixing behavior in basalticliquid at pressures and temperature relevant to seqfloor eruption.Mid-ocean ridge basaltic (MORB) liquid was equilibrated at 1200°Cwith pure H 2O at pressures of 176717 bar and H 2OCO 2vapor at pressures up to 980 bar. Concentrations and speciationof H 2O and CO 2 dissolved in the quenched glasses were measuredusing IR spectroscopy. Molar absorptivities for the 4500 cm 1band of hydroxyl groups and the 5200 and 1630 cm 1 bandsof molecular water are 067±003, 062±007,and 25±3 l/mol-cm, respectively. These and previouslydetermined molar absorptivities for a range of silicate meltcompositions correlate positively and linearly with the concentrationof tetrahedral cations (Si+Al). The speciation of water in glass quenched from vapor-saturatedbasaltic melt is similar to that determined by Silver &Stolper ( Journal of Petrology 30, 667709, 1989) in albiticglass and can be fitted by their regular ternary solution modelusing the coefficients for albitic glasses. Concentrations ofmolecular water measured in the quenched basaltic glasses areproportional to f H 2O in all samples regardless of the compositionof the vapor, demonstrating that the activity of molecular waterin basaltic melts follows Henry's law at these pressures. Abest fit to our data and existing higher-pressure water solubilitydata (Khitarov et al., Geochemistry 5, 479492, 1959;Hamilton et al., Journal of Petrology 5, 2139, 1964),assuming Henrian behavior for molecular water and that the dependenceof molecular water content on total water content can be describedby the regular solution model, gives estimates for the Vo, mH2Oof 12±1 cm 3/mol and for the 1-bar water solubility of011 wt%. Concentrations of CO 2 dissolved as carbonate in the melt forpure CO 2-saturated and mixed H 2O-CO 2-saturated experiments area simple function of fCO2 These results suggest Henrian behaviorfor the activity of carbonate in basaltic melt and do not supportthe widely held view that water significantly enhances the solutionof carbon dioxide in basaltic melts. Using a V o, mr of 23 cm 3/mol(Pan et al., Geochimica et Cosmochimica Acta 55, 15871595,1991), the solubility of carbonate in the melt at 1 bar and1200°C is 05 p.p.m. Our revised determination of CO 2solubility is 20% higher than that reported by Stolper &Holloway ( Earth and Planetary Science Letters 87, 397408,1988). KEY WORDS: mid-ocean ridge basalts; water and carbon dioxide solubility; experimental petrology 相似文献
11.
The equilibrium phase relations of two volcanic rocks from thesubduction-related Mexican Volcanic Belt have been determinedwith an argon-pressurized internally heated vessel. One rockis the trachyandesite erupted from El Chich?n Volcano in 1982;the other is a primitive basalt erupted from Jorullo Volcanoin 1759. A simplified synthetic equivalent to the trachyandesitewas also investigated in lesser detail. All charges were saturatedwith hydrous vapor and a sulfur-bearing mineral. Temperatureranged from 800 to 1000?C, pressure from 1 to 4 kb, and fo2was controlled by four different solid oxygen buffers in a doublegold capsule configuration: fayalite-magnetite-quartz (FMQ),Ni-NiO (NNO), manganosite-hausmanite (MNH), and magnetite-hematite(MTH). Pyrrhotite was the only sulfur-bearing mineral observed in chargesbuffered under FMQ and NNO, whereas anhydrite crystallized underthe more oxidizing MNH or MTH; both of these observations areconsistent with those of earlier workers. With increasing temperatureand pressure, SiO 2 and K 2O decreased in the experimental melts,whereas Al 2O 3 and CaO increased. Sulfur solubility in silicatemelts was low (<0?1 wt% equivalent SO t3) for pyrrhotite-saturatedcharges, but significantly greater (to 1?3 wt.% SO t3) when anhydritewas present. Sulfur solubility in anhydrite-saturated meltsshowed strong positive dependence on both temperature and P vapor. Sulfur amounted to some 2?5 wt.% (SO t3) of the total ejectaduring the 1982 El Chich?n eruptions, and the original magmaticsulfur content was in the range 1?2525 wt% SO t3.Extrapolations of experimental temperature and pressure dependencesfor sulfur solubility indicate that such concentrations couldbe contained in a hydrous, oxidized, basaltic parent melt generatedunder Benioff zone conditions. During ascent through the uppermantle and crust, the sulfur solubility limit of the melt wouldcontinuously decrease; in response, most of the sulfur wouldbe transferred from the melt to anhydrite crystals and a separategas phase. Trachyandesite pumices erupted from El Chich?n in1982 contained both pyrrhotite and anhydrite at a temperatureof 800?C. The composition of the natural pyrrhotite yieldedan fo2 estimate 1 log unit above the NNO buffer. Based on compositionalvariations in the experimental melts with temperature and pressure,the composition of the matrix glass in the 1982 pumices indicatesequilibration of the magmatic liquid at about Ptotal= Pvapor=2kb just before eruption. At that time, sulfur in El Chich?ntrachyandesite was about equally partitioned between anhydritemicrophenocrysts and some 20 vol.% gas phase in which H 2S wasprobably the dominant sulfur-bearing species. The melt thencontained only 0?05 wt.% SO t3, consistent with experimentalsolubility limits at 800?C and P vapor=2 kb. 相似文献
12.
Electron microprobe analyses are presented for new-formed mineralsfrom a small exposure of semi-schistose Taveyanne Formationof the pumpellyite-actinolite facies near Lo?che, Valais. Comparisonsare drawn with minerals of other low-grade metamorphic areas,especially in southern New Zealand. Sphene shows considerablesubstitution of Ca(Al,Fe)SiO 4(OH) for CaTiSiO 5. Epidotes aresharply divided into early pistacitic (Ps = 0.280.37)and later clinozoisitic varieties (Ps = 0.110.19). Pumpellyitesrange from pumpellyite-(Fe) to pumpellyite-(Al) and are generallyless Fe-rich than those of zeolite and prehnite-pumpellyitefacies. Pumpellyite inclusions in albitized plagioclase areparticularly low in Mg. Actinolites are low in A1 2O 3, TiO 2,and Na 2O, essentially identical compositions being nucleatedon detrital augite, hornblende, and in the matrix. Phengitesare also extremely low in Na 2O and TiO 2. Chlorites are ripidolites.Albitized clastic plagioclase has the composition An0.71.6and albite in clinozoisite-calcite-albite-phengite-chloriteveins An2.12.3. Calcites carry minor Mn > Fe ? Mg.New-formed iron oxides are absent, whereas pyrrhotite and minorpyrite occur in one rock, buffering fs2 and indicating low fo2. Ratios Mg: Fe * (Fe * = total Fe) in coexisting chlorites andA1, Na-poor actinolites vary sympathetically both in the Lo?cheand southern New Zealand rocks here considered, giving K D =(Mg/Fe *) actlnolIte/(Mg/Fe *) chlorle = 1.72. Mg/Fe * ratios inpumpellyites tend to vary sympathetically with those of coexistingchlorites and actinolites but are more variable. Substitutionof (Fe, Mg)Si for A1 2 in phengitic micas and chlorites variessympathetically in the same suites between mafic volcanic andmore pelitic extremes. Various minor elements also behave ina consistent fashion, indicating an encouraging tendency towardsequilibrium. Variable (though small) A1 2O 3 contents of actinolite,Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe * ratiosin phengites, even within a single grain, are evidence of short-rangedisequilibrium; metamorphic equilibration is evidently easierbetween some crystal structures and structural sites than betweenothers. In phase rule analysis of assemblages in such rocks it is commonlynecessary to treat Fe 2O 3, FeO, and MgO as separate componentsand it may also be necessary to regard CO 2 as an inert componentand/or to interpret observed assemblages as of low variance.The presence of the Ca-Al silicates and sphene indicates verylow X co2 in the metamorphic fluids in all rocks examined exceptan albite-chlorite-calcite-quartz-anatase assemblage. But higherAn in albites than in isofacial and in greenschist facies rocksof southern New Zealand can be ascribed to significantly higher Xco2 at Lo?che, especially in the veins, than in New Zealand. Pumpellyite and epidotes of the pumpellyite-actinolite faciestend to be lower in Fe and richer in Al than those of lowergrade facies. Important reactions include those of the formpumpellyite-(Fe 3+)+chlorite+quartz+H 2=pumpellyite-(Al)+actinolite,and pumpellyite+chlorite+quartz- epidote+actinolite+water.Careful selection of pumpellyite and chlorite compositions isrequired for experimental and chemographic analysis of pumpellyitestability. In the absence of critical data, temperatures ofabout 250350? and pressures of several kilobars are provisionallysuggested for the Lo?che metamorphism. 相似文献
13.
Sediment mixing and recycling through a subduction zone canbe detected in lead isotopes and trace elements from basaltsand sediments from the Kermadec-Hikurangi Margin volcanic arcsystem and their coupled back-arc basins. Sr, Nd and Pb isotopesfrom the basalts delineate relatively simple, almost overlapping,arrays between back-arc basin basalts of the Havre Trough-NgatoroBasin ( 87Sr/ 86Sr = 0.70255; Nd=+9.3; 206Pb/ 204Pb = 18.52; 208Pb/ 204Pb= 38.18), island arc basalts from the Kermadec Arc togetherwith basalts from Taupo Volcanic Zone ( 87Sr/ 86Sr 0.7042; Nd= +5; 206Pb/ 204Pb= 18.81; 208Pb/ 204Pb = 38.61), and sedimentsderived from New Zealand's Mesozoic (Torlesse) basement ( 87Sr/ 86Sr 0.715; Nd 4; 206Pb/ 204Pb 18.86; 208Pb/ 204Pb 38.8).Basalts from the arc front volcanoes have high Cs, Rb, Ba, Th,U and K, and generally high but variable Ba/La, Ba/Nb ratios,characteristic of subduction-related magmas, relative to typicaloceanic basalts. These signatures are diluted in the back-arcbasins, which are more like mid-ocean ridge basalts. Strongchemical correlations in plots of SiO 2 vs CaO and loss on ignitionfor the sediments (finegrained muds) are consistent with mixingbetween detrital and biogenic (carbonate-rich) components. Otherdata, such as Zr vs CaO, are consistent with the detrital componentcomprising a mixture of arc- and continent-derived fractions.In chondrite-normalized diagrams, most of the sediments havelight rare earth element enriched patterns, and all have negativeEu anomalies. The multielement diagrams have negative spikesat Nb, P and Ti and distinctive enrichments in the large ionlithophile elements and Pb relative to mantle. Isotopic measurementsof Pb, Sr and Nd reveal restricted fields of Pb isotopes butwide variation in Nd and Sr relative to other sediments fromthe Pacific Basin. Rare K-rich basalts from Clark Volcano towardthe southern end of the oceanic Kermadec Island Arc show unusualand primitive characteristics ( 2% K 2O at 50% SiO 2, Ba 600p.p.m., 910% MgO and Ni > 100 p.p.m.) but have highlyradiogenic Sr, Nd and Pb isotopes, similar to those of basaltsfrom the continental Taupo Volcanic Zone. These oceanic islandarc basalts cannot have inherited their isotope signatures throughcrustal contamination or assimilationfractional crystallizationtype processes, and this leads us to conclude that source processesvia bulk sediment mixing, fluid and/or melt transfer or somecombination of these are responsible. Although our results showclear chemical gradients from oceanic island arc to continentalmargin arc settings (Kermadec Arc to Taupo Volcanic Zone), overlapbetween the data from the oceanic and continental sectors suggeststhat the lithospheric (crustal contamination) effect may beminimal relative to that of sediment subduction. Indeed, itis possible to account for the chemical changes by a decreasenorthward in the sediment flux into the zone of magma genesis.This model receives support from recent sediment dispersal studiesin the Southern Ocean which indicate that a strong bottom current(Deep Western Boundary Current) flows northward along the easterncontinental margin of New Zealand and sweeps continental derivedsediment into the sediment-starved oceanic trench system. Thetrace element and isotopic signatures of the continental derivedcomponent of this sediment are readily distinguished, but alsodiluted in a south to north direction along the plate boundary. KEY WORDS: subduction zone basalts; sediments; Sr-, Nd-, Pb-isotopes; trace elements
*Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia. 相似文献
14.
Seven new analyses of kaersutites and.two of oxykaersutitesfrom Japan and surrounding areas are given, together with theiroptical properties. The type formula of kaersutite can be writtenas (Na,K)Ca 2Mg 3Fe 2+(Ti,Fe 3+)Al iv2Si 6O 22(OH) 2 and that of oxykaersutiteas (Na,K) Ca 2Mg 3(Ti,Fe 3+) 2Al iv2Si 6O 24. Transformation from kaersutiteto oxykaersutite must have taken place when the ratio of Fe 3+/Fe 2+was about 2. 相似文献
15.
The Edgecumbe volcanics, which range from basalt through rhyodacite,have Sr contents between 125 and 370 p.p.m., Rb contents of1 to 70 p.p.m., Ba ranging from 50550 p.p.m. and initial 87Sr/ 86Sr ratios between 0.70291 and 0.70404. No simple correlationexists between these components and silica. The highest 87Sr/ 86Srvalues occur in a group of intermediate lavas (5560 wt.per cent SiO 2) while the rhyodacites have initial ratios between0.7035 and 0.7038. With increasing silica, Sr increases to amaximum in the andesites and then steadily decreases; Ba andRb increase over the same compositional range. The highest 87Sr/ 86Srlavas have major and trace element concentrations which departfrom trends defined by most of the lavas. The variation in strontiumisotopic compositions suggests interaction between parentalbasaltic magma and crustal material. Attempts to model the assimilationprocess using fixed end-member assimilation and assimilation-fractionalcrystallization models have failed to produce the observed chemicaltrends. Because the parental basaltic liquid underwent littlefractionation, the variability in hybrid lavas is attributedto variation in contaminant composition. Initial melts werelow in CaO, A1 2O 3, MgO and Sr and enriched in SiO 2. K 2O, Na 2O,Rb and Ba. As melting progressed, melts became enriched in themore refractory components. Because hybrid strontium isotopiccomposition is a function of Sr concentration as well as isotopiccompositions, the Sr content of the assimilant strongly influencesresultant isotopic systematics. The development of the assimilantssuggests plagioclase was a residual phase during early melting.This model of crustal assimilation represents one end-memberin the spectrum of processes responsible for the generationof continental volcanic suites. 相似文献
16.
Phase relations for the bulk compositions 3CaO·2FeO x·3SiO 2+excessH 2O and CaO·FeO x·2SiO 2+excess H 2O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO 2. Andradite, Ca 3Fe 3+2Si 3O 12, synthesized above 550 °C hasan average unit cell edge, ao, of 12.055±0.001 Å,and an index of refraction, n, of 1.887±0.003. Belowthis temperature, ao increases whereas n decreases, indicatingthe formation of a member of the andradite-hydroandradite solidsolution. At 2000 bars P fluid andradite is stable above an fO2of 10 15 bar at 800 °C and 10- 32 bar at 400 °C. At lower fO2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe 2+SiO 4)+ wollastonite and kirschsteinite+xonotlite (Ca 6Si 6O 17(OH) 2). Synthetic hedenbergite, CaFe 2+Si 2O 6, has average unit cell dimensionsof ao = 9.857± 0.004 Å, bo = 9.033±0.002Å, co = 5.254±0.002 Å and ß = 104.82°±0.03°,and refractive indices of n = 1.731±0.003 and n = 1.755±0.005.At 2000 bars P fiuid, hedenbergite is stable below an fO2 of10- 13 bar at 800 °C and 10- 28 bar at 400 °C. Above these fO2 values, hedenbergite+O 2 breaks down to andradite+magnetite+quartz. The mineral pair andradite +hedenbergite thus limit the fO2range possible for their joint formation under equilibrium conditions. The hydration of wollastonite to xonotlite occurs at much lowertemperatures than previous experimental work indicated. A tentativehigh temperature limit for this reaction is set at 185°±15°C and 5000±25 bars and 210°±15 °Cand 2000±20 bars. Inasmuch as the growth of xonotlitefrom wollastonite + H 2O was never accomplished, this high temperaturelimit does not represent an equilibrium univariant curve. Nine phases were encountered in the study of andradite and hedenbergite.They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite,xonotlite, quartz, ilvaite, and vapor (fluid). An invariantpoint analysis using the method of Schreinemakers shows thetopologic relations of the reactions involved. The resultinggrid can be used to interpret natural occurrences. 相似文献
17.
Volume diffusion rates for five rare earth elements (La, Ce, Nd, Dy, and Yb) have been measured in single crystals of natural diopside at pressures of 0.1 MPa to 2.5 GPa and temperatures of 1,050 to1,450 °C. Polished, pre-annealed crystals were coated with a thin film of rare earth element oxides, then held at constant temperature and pressure for times ranging from 20 to 882 h. Diffusion profiles in quenched samples were measured by SIMS (secondary ion mass spectrometry) depth profiling. At 1 atm pressure, with the oxygen fugacity controlled near the quartz-fayalite-magnetite buffer, the following Arrhenius relations were obtained for diffusion normal to (001) (diffusion coefficient D in m 2/s): log 10DYb=(-4.64ǂ.42)-(411ᆠ kJ/mol/2.303 RT); log 10DDy=(-3.31ǃ.44)-(461ᆽ kJ/mol/2.303 RT); log 10DNd=(-2.95DŽ.64)-(496ᇡ kJ/mol/2.303 RT); log 10DCe=(-4.10ǃ.08)-(463ᆳ kJ/mol/2.303 RT); log 10DLu=(-4.22DŽ.66)-(466ᇢ kJ/mol/2.303 RT). Diffusion rates decrease significantly with increasing ionic radius, with La a factor of ~35 slower than Yb. The relationship between diffusivity and ionic radius is consistent with a model in which elastic strain plays a critical role in governing the motion of an ion through the crystal lattice. Activation volumes for Yb and Ce diffusion, at constant temperature and oxygen fugacity, are 9.0DŽ.0 cm 3/mol and 8.9Dž.2 cm 3/mol, respectively, corresponding to an order of magnitude decrease in diffusivity as pressure is increased from 0 to 3 GPa at 1,200 °C. Diffusion of Nd is such that grain-scale isotopic equilibrium in the mantle can be achieved in ~1 My under conditions near the peridotite solidus (~1,450 °C at 2.5 GPa). The equilibration time is much longer under P, T conditions of the lithospheric mantle or at the eclogite solidus (~1 Gy at 1.5 GPa and 1,150 °C). Because of the relatively strong decrease in diffusivity with pressure (two orders of magnitude between 2.5 and 15 GPa along an adiabatic temperature gradient), Nd transport in clinopyroxene will be effectively frozen at pressures approaching the transition zone, on time scales less than 100 My. Rare earth element diffusion rates are slow enough that significant disequilibrium uptake of REE by growing clinopyroxene phenocrysts may be preserved under natural conditions of basalt crystallization. The relative abundances and spatial distributions of REE in such crystals may provide a sensitive record of the cooling and crystallization history of the host lava. 相似文献
18.
We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H 2O-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 H 2O. 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: SiO 2, Na 2O, and K 2O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al 2O 3, Na 2O, andK 2O 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. Al 2O 3/[Na 2O + K 2O])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 10 15m 2/s and (0·62·4) x 10 14 m 2/s, respectively.Minimum diffusivities of alkalis [(39) x 10 11m 2/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, H 2O-saturated melting at 800°C (infinite heatsupply) could produce 2030 vol. % of homogeneous meltin less than 110 years. Slower diffusion in H 2O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis 相似文献
19.
The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 616 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km 3 of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·314·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. 87Sr/ 86Sr i(0·70390·7049), 143Nd/ 144Nd i (0·51270·5129)and 176Hf/ 177Hf i (0·28290·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na 2O, P 2O 5, K 2O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu] n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting 相似文献
20.
The distribution of Ti 4+, V 3+, Cr 3+, Mn 2+, Fe(total), Co 2+ andNi 2+ among synthetic olivine, calcic clinopyroxene and maficsilicate liquid has been studied between 1125 and 1250 ?C underanhydrous conditions at 1 bar total pressure. The distributionof iron and magnesium among the three phases was concluded tobe independent of temperature and may be described by the twoequations
Titanium and vanadiumdid not enter olivine in significant amounts. The mean valueof the ratio (wt. per cent TiO 2 in Cpx)/(wt. per cent TiO 2 inL) was 0.29?0.04 for assemblages in which the liquid had botholivine and hypersthene in the norm but the ratio was greaterif the liquid was nepheline normative. Vanadium was concentratedin the pyroxene in some experiments and in the liquid in others,but it was not possible to conclude whether the change in distributionbehavior was due to varying temperature or changing liquid composition.Equilibrium partitioning of chromium was not achieved but theresults indicate that Cr 2O 3 was most strongly enriched in clinopyroxeneand showed a slight preference for olivine over the liquid.The divalent transition elements were each enriched in olivinerelative to clinopyroxene and the degree of enrichment increasedin the order predicted by crystal field theory. The mean (wt.per cent oxide in 01)/(wt. per cent oxide in Cpx) ratios were2.0 for MnO, 2.4 for FeO, 3.9 for CoO and 5.6 for NiO. Manganesewas enriched in olivine relative to the liquid and in the liquidrelative to the clinopyroxene. Cobalt and nickel were more concentratedin the crystalline phases than in the liquid but the degreeof enrichment was markedly less in the experiments in whichthe liquids were more mafic. 相似文献
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