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1.
Disequilibrium phenocryst assemblages in the Younger Andesitesand Dacites of Iztacc?huatl, a major Quaternary volcano in theTrans-Mexican Volcanic Belt, provide an excellent record ofepisodic replenishment, magma mixing, and crystallization processesin calc-alkaline magma chambers. Phenocryst compositions andtextures in ‘mixed’ lavas, produced by binary mixingof primitive olivine-phyric basalt and evolved hornblende dacitemagmas, are used to evaluate the mineralogical and thermal characteristicsof end-members and the physical and chemical interactions thatattend mixing. Basaltic end-members crystallized olivine (FO90–88) andminor chrome spinel during ascent into crustal magma chambers.Resident dacite magma contained phenocrysts of andesine (An45–35),hypersthene (En67–61), edenitic-pargasitic hornblende,biotite, quartz, .titanomagnetite, and ilmenite. On reachinghigh-level reservoirs, basaltic magmas were near their liquidiat temperatures of about 1250–1200?C according to theolivine-liquid geothermometer. Application of the Fe-Ti-oxidegeothermometer-oxygen barometer indicates that hornblende dacitemagma, comprising phenocrysts (<30 vol. per cent) and coexistingrhyolitic liquid, had an ambient temperature between 940 and820?C at fO2s approximately 0?3 log units above the nickel-nickeloxide buffer assemblage. Mixing induced undercooling of hybridliquids and rapid crystallization of skeletal olivine (Fo88–73),strongly-zoned clinopyroxene (endiopside-augite), calcic plagioclase(An65–60); and orthopyroxene (bronzite), whereas low-temperaturephenocrysts derived from hornblende dacite were resorbed ordecomposed by hybrid melts. Quartz reacted to form coronas ofacicular augite and hydroxylated silicates were heated to temperaturesabove their thermal stability limit ({small tilde}940?C foramphibole, according to clinopyroxene-orthopyroxene geothermometry,and {small tilde}880?C for biotite). Calculations of phenocrystresidence times in hybrid liquids based on reaction rates suggestthat the time lapse between magma chamber recharge and eruptionwas extremely short (hours to days). It is inferred that mixing of magmas of diverse compositionis driven by convective turbulence generated by large differencesin temperature between end-members. The mixing mechanism involves:(1)rapid homogenization of contrasting residual liquid compositionsby thermal erosion and diffusive transfer (liquid blending);(2) assimilation of phenocrysts derived from the low-temperatureend-member; and (3) dynamic fractional crystallization of rapidlyevolving hybrid liquids in a turbulent boundary layer separatingbasaltic and dacitic magmas. The mixed lavas of lztacc?huatlrepresent samples of this boundary layer quenched by eruption.  相似文献   

2.
The phase relations of three basalts, the Picture Gorge tholeiite,the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkalibasalt, were studied at 5 kb water pressure, 680–1000°C,at the oxygen fugacities of the quartz-fayalite-magnetite (QFM)and hematite-magnetite (HM) buffers. In the range 680–850 °C, the crystalline assemblageon the QFM buffer is dominantly hornblende+ plagioclase, ±ilmenite, magnetite, sphene, fayalitic olivine, and phlogopiticmica. From 875 to 1000 °C the crystalline assemblage ishornblende+ olivine± augite+ ilmenite± magnetite.A melt phase is present from 700 to 1000 °C; a vapor phasewas present in all charges. The hornblendes formed on the QFM buffer range in compositionfrom common green hornblendes at low temperatures to kaersutitichornblendes at 1000 °C. A1(IV) and Ti increase temperature.AI(VI) passes through a maximum near 825 °C, decreasingboth above and below this temperature. AI(IV) is proportionalto the sum A1(VI)+2Ti. There is a positive linear correlationof approximately 3 : 1 between AI(IV) and the number of cationsin the A-site. The most likely explanation for this correlationat present is that the substitution of AI(VI) or Ti+4for a divalentcation creates local charge imbalances in the amphibole structurewhich can be compensated only by further A-site substitution.There also appears to be a correlation between the a-cell dimensionof hornblende and the A-site occupancy. Above a thresh holdvalue of approxmately 0.5 cations in A, a increases as A-siteoccupancy increases. Phase relations on the hematite-magnetite buffer are considerablysimpler. The hornblendes show relatively little change in compositionas temperature increases, and in the tholelitic compositionsbreak down at or below 970 °C 35–60 °C above thefirst appearance of augite±olivine. The melting of hornblendeis incongruent in all cases. The Fe-Ti oxides are pseudo-brookiteand titanohematite; at 1000 °C these oxides make up 10 percent by weight of the assemblage and contain most of the Tio2and FeO in the charge. The patterns of hornblende variation observed in this studycompare closely with those reported in a wide range of experimentaland field data. The appearance of high-TiO2 kaersutitic hornblendesin the tholeities at 1000° C, PH2O= 5 kb on the QFM bufferimplies that the restricted occurence of kaersutite in nature(where it is associated only with mafic to intermediate alkalicrocks) is controlled by volatile content (H2O,F2)rather thanby differences in condensed bulk composition.  相似文献   

3.
High-temperature–pressure experiments were carried outto determine the chlorine–hydroxyl exchange partitioncoefficient between hornblende and melt in the 1992 Unzen dacite.Cl in hornblende and melt was analyzed by electron microprobe,whereas OH in hornblende and melt was calculated assuming anionstoichiometry of hornblende and utilizing the dissociation reactionconstant for H2O + O = 2(OH) in water-saturated melt, respectively.The partition coefficient strongly depends on the Mg/(Mg + Fe)ratio of hornblende, and is expressed as ln K1 = (Cl/OH)hb/(Cl/OH)melt= 2·37 – 4·6[Mg/(Mg + Fe)]hb at 2–3kbar and 800–850°C. The twofold variation in Cl contentin the oscillatory zoned cores of hornblende phenocrysts inthe 1991–1995 dacite cannot be explained by the dependenceof the Cl/OH partition coefficient on the Mg/(Mg + Fe)hb ratio,and requires c. 80% variation of the Cl/OH ratio of the coexistingmelt. Available experimental data at 200 MPa on Cl/OH fractionationbetween fluid and melt suggest that c. 1·2–1·8wt % degassing of water from the magma can explain the required80% variation in the Cl/OH ratio of the melt. The negative correlationbetween Al content and Mg/(Mg + Fe) ratio in the oscillatoryzoned cores of the hornblende phenocrysts is consistent withrepeated influx and convective degassing of the fluid phasein the magma chamber. KEY WORDS: chlorine; element partitioning; hornblende; oscillatory zoning; Unzen volcano  相似文献   

4.
The near-liquidus crystallization of a high-K basalt (PST-9golden pumice, 49·4 wt % SiO2, 1·85 wt % K2O,7·96 wt % MgO) from the present-day activity of Stromboli(Aeolian Islands, Italy) has been experimentally investigatedbetween 1050 and 1175°C, at pressures from 50 to 400 MPa,for melt H2O concentrations between 1·2 and 5·5wt % and NNO ranging from –0·07 to +2·32.A drop-quench device was systematically used. AuPd alloys wereused as containers in most cases, resulting in an average Feloss of 13% for the 34 charges studied. Major crystallizingphases include clinopyroxene, olivine and plagioclase. Fe–Tioxide was encountered in a few charges. Clinopyroxene is theliquidus phase at 400 MPa down to at least 200 MPa, followedby olivine and plagioclase. The compositions of all major phasesand glass vary systematically with the proportion of crystals.Ca in clinopyroxene sensitively depends on the H2O concentrationof the coexisting melt, and clinopyroxene Mg-number shows aweak negative correlation with NNO. The experimental data allowthe liquidus surface of PST-9 to be defined. When used in combinationwith melt inclusion data, a consistent set of pre-eruptive pressures(100–270 MPa), temperatures (1140–1160°C) andmelt H2O concentrations is obtained. Near-liquidus phase equilibriaand clinopyroxene Ca contents require melt H2O concentrations<2·7–3·6 and 3 ± 1 wt %, respectively,overlapping with the maximum frequency of glass inclusion data(2·5–2·7 wt % H2O). For olivine to crystallizeclose to the liquidus, pressures close to 200 MPa are needed.Redox conditions around NNO = +0·5 are inferred fromclinopyroxene compositions. The determined pre-eruptive parametersrefer to the storage region of golden pumice melts, which islocated at a depth of around 7·5 km, within the metamorphicarc crust. Golden pumice melts ascending from their storagezone along an adiabat will not experience crystallization ontheir way to the surface. KEY WORDS: basalt; pumice; experiment; phase equilibria; Stromboli  相似文献   

5.
This work reports new data on phenocryst composition and carbonate-silicate equilibria in the volcanic rocks of kamafugitic affinity in the Toro-Ankole Province (East African rift zone). Inclusions of primary carbonates (calcite and dolomite) were found in olivines from ugandite and mafurite of the Bunyaruguru volcanic field. The initial compositions of melt inclusions in olivine from the ugandite were calculated from microprobe analyses and correspond to carbonatites. The find of barite and dolomite-barite inclusions in the olivine from the mafurite indicates that the melt contained sulfate sulfur, which is typical of oxidizing conditions. The calculation of the olivine-spinel equilibrium (T-$ f_{O_2 } $ f_{O_2 } ) showed that crystallization of phenocrysts in mafurite occurred at oxygen fugacities above the NNO buffer (ΔQFM ∼ 2.5) in a wide temperature range (1230–750°C).  相似文献   

6.
Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H2O. This reflectsthe fact that conditions of low aH2O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH2O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt  相似文献   

7.
Crystallization experiments were conducted on dry glasses fromthe Unzen 1992 dacite at 100–300 MPa, 775–875°C,various water activities, and fO2 buffered by the Ni–NiObuffer. The compositions of the experimental products and naturalphases are used to constrain the temperature and water contentsof the low-temperature and high-temperature magmas prior tothe magma mixing event leading to the 1991–1995 eruption.A temperature of 1050 ± 75°C is determined for thehigh-temperature magma based on two-pyroxene thermometry. Theinvestigation of glass inclusions suggests that the water contentof the rhyolitic low-temperature magma could be as high as 8wt % H2O. The phase relations at 300 MPa and in the temperaturerange 870–900°C, which are conditions assumed to berepresentative of the main magma chamber after mixing, showthat the main phenocrysts (orthopyroxene, plagioclase, hornblende)coexist only at reduced water activity; the water content ofthe post-mixing dacitic melt is estimated to be 6 ± 1wt % H2O. Quartz and biotite, also present as phenocrysts inthe dacite, are observed only at low temperature (below 800–775°C).It is concluded that the erupted dacitic magma resulted fromthe mixing of c. 35 wt % of an almost aphyric pyroxene-bearingandesitic magma (1050 ± 75°C; 4 ± 1 wt % H2Oin the melt) with 65 wt % of a phenocryst-rich low-temperaturemagma (760–780°C) in which the melt phase was rhyolitic,containing up to 8 ± 1 wt % H2O. The proportions of rhyoliticmelt and phenocrysts in the low-temperature magma are estimatedto be 65% and 35%, respectively. It is emphasized that the strongvariations of phenocryst compositions, especially plagioclase,can be explained only if there were variations of temperatureand/or water activity (in time and/or space) in the low-temperaturemagma. KEY WORDS: Unzen volcano; magma mixing; experimental study  相似文献   

8.
A 1500 m thick sheet-like body of ferroaugite syenite is divisiblemineralogically into an upper and lower series of syenites.The lower syenites are characterized by well developed igneouslayering defined by mafic cumulus minerals. The syenites aresaturated to oversaturated and contain as cumulus phases alkalifeldspar, olivine (Fa83–93), ferroaugite (Di50Hd45Ac5–Di5Hd90Ac5)and ilmeno-magnetite. Amphiboles which crystallized from theintercumulus liquid range in composition from ferrohastingsitichornblende to ferroedenitic hornblende to ferroactinolitic edenite.The upper series are coarse grained cumulates with poorly definedlayering and abundant patch pegmatites. Cumulus phases are alkalifeldspar, olivine (Fa93), and acmitic-hedenbergite (Di5Hd50Ac5–Ac50Hd50).Intercumulus liquids are peralkaline and crystallized to aenigmatiteand amphiboles which range in composition from ferrorichteritickatophorite to ferrorichterite, Patch pegmatites are peralkalinerocks composed of ferrorichterite, ferroactinolite, alkali feldspar,aenigmatite, quartz and zircon. Extreme differentiation of ferroaugitesyenite magma generates residua which are ironrich, oversaturatedand peralkaline. Initial and final temperatures of crystallizationare estimated from mineral stability data to be 800–900°C to 500–550 °C respectively. Thermodynamic dataand mineral compositions indicate that during crystallizationthe oxygen fugacity of the magma decreased from approximately10–15 to 10–23–10–24 bars. Ferroaugitesyenite pyroxene compositional trends are similar to those ofundersaturated peralkaline syenites (llimaussaq) and demonstratethat acmite enrichment trends are independent of silica activityand take place under decreasing oxygen fugacities.  相似文献   

9.
Boninites are an important ‘end-member’ supra-subductionzone magmatic suite as they have the highest H2O contents andrequire the most refractory of mantle wedge sources. The pressure–temperatureconditions of boninite origins in the mantle wedge are importantto understanding subduction zone initiation and subsequent evolution.Reaction experiments at 1·5 GPa (1350–1530°C),2 GPa (1400–1600°C) and 2·5 GPa (1450–1530°C)between a model primary high-Ca boninite magma composition anda refractory harzburgite under anhydrous and H2O-undersaturatedconditions (2–3 wt % H2O in the melt) have been completed.The boninite composition was modelled on melt inclusions occurringin the most magnesian olivine phenocrysts in high-Ca boninitesfrom the Northern Tongan forearc and the Upper Pillow Lavasof the Troodos ophiolite. Direct melting experiments on a modelrefractory lherzolite and a harzburgite composition at 1·5GPa under anhydrous conditions (1400–1600°C) havealso been completed. Experiments establish a P, T ‘meltinggrid’ for refractory harzburgite at 1·5, 2 and2·5 GPa and in the presence of 2–3 wt % H2O. Theeffect of 2–3 wt % dissolved H2O produces a liquidus depressionin primary boninite of  相似文献   

10.
Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications   总被引：16，自引：5，他引：16  
The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO2relative to the Ni–NiO solid buffer), between 800 and660°C and for melt H2O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H2O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H2O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO2 lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H2O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H2O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO2 at or below the fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H2O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria  相似文献   

11.
K-feldspar–plagioclase–quartz mineral textures aswell as biotite and hornblende compositions are compared forsuites of metamorphosed mafic rocks from two widely separatedtraverses. A portion of either traverse has experienced a high-gradedehydration event transforming it from an H2O-rich, hornblende-bearingzone to an H2O-poor, hornblende-free, orthopyroxene-bearing,‘granulite facies’ zone at 700–800°C and7–8 kbar. In the Kigluaik Mountains, Seward Peninsula,Alaska, dehydration took place over an 85 cm thick layer ofmetatonalite in contact with a marble during regional metamorphismand involved a CO2-rich fluid, whereas for the Val Strona diOmegna traverse, Ivrea–Verbano Zone, northern Italy, dehydrationtook place over a 3–4 km thick sequence of metabasitesinterlayered with metapelites in a contact metamorphic eventinvolving basaltic magmas intruded at the base of the sequence.Orthopyroxene-bearing samples from both dehydration zones showmicro-veins of K-feldspar along quartz and plagioclase grainboundaries as well as replacement antiperthite in plagioclase.K came primarily from the breakdown of hornblende + quartz toorthopyroxene ± clinopyroxene, feldspar and fluid. Biotiteeither was stabilized or formed in the dehydration zones andis enriched in Ti, Mg, F and Cl relative to biotite in the amphibolitefacies zone. KEY WORDS: KCl–NaCl brines; metasomatism; granulite facies metamorphism; charnockite–enderbite; orthopyroxene; K-feldspar; biotite; hornblende  相似文献   

12.
AUDETAT  A.; PETTKE  T. 《Journal of Petrology》2006,47(10):2021-2046
The magmatic processes leading to porphyry-Cu mineralizationat Santa Rita are reconstructed on the basis of petrographicstudies, thermobarometry, and laser-ablation inductively-coupled-plasmamass-spectrometry analyses of silicate melt and sulfide inclusionsfrom dikes ranging from basaltic andesite to rhyodacite. Combinedresults suggest that magma evolution at Santa Rita is similarto that of sulfur-rich volcanoes situated above subduction zones,being characterized by repeated injection of hot, mafic magmainto an anhydrite-bearing magma chamber of rhyodacitic composition.The most mafic end-member identified at Santa Rita is a shoshoniticbasaltic andesite that crystallized at 1000–1050°C,1–3 kbar and log fO2 = NNO + 0·7 to NNO + 1·0,whereas the rhyodacite crystallized at 730–760°C andlog fO2 = NNO + 1·3 to NNO + 1·9. Mixing betweenthe two magmas caused precipitation of 0·1–0·2wt % magmatic sulfides and an associated decrease in the Cucontent of the silicate melt from 300–500 ppm to lessthan 20 ppm. Quantitative modeling suggests that temporal storageof ore-metals in magmatic sulfides does not significantly enhancethe amount of copper ultimately available to ore-forming hydrothermalfluids. Magmatic sulfides are therefore not vital to the formationof porphyry-Cu deposits, unless a mechanism is required thatholds back ore-forming metals until late in the evolution ofthe volcanic–plutonic system. KEY WORDS: porphyry-Cu; sulfur; sulfides; magma mixing; LA-ICP-MS  相似文献   

13.
Crystallization temperatures (T) and oxygen fugacities (fO2)of kimberlite magma are estimated from oxides included in olivinephenocrysts from the Leslie, Aaron, Grizzly and Torrie kimberlitepipes in the central Slave Province, Canada. Crystallizationtemperatures recorded by olivine–chromite pairs at anassumed pressure of 1·0 GPa are 1030–1170°C± 50°C, with a mean of  相似文献   

14.
After 11·5 ka of quiescence (24·5–13 ka),the Nevado de Toluca volcano started a 2500 year period of activity.This period was characterized by a dome destruction event at13 ka, a small Plinian event at 12·1 ka, and a largePlinian eruption at 10·5 ka. About 10 km3 of magma waserupted that was homogeneous in composition (63·3–65·7SiO2 wt % whole-rock) and in mineralogy. Pumice consists ofplagioclase (An30–59) > orthopyroxene (En56–59)> hornblende >> Fe–Ti oxides + rare apatite (inopx) + biotite, set in a rhyolitic matrix (72–76 SiO2wt %). 40Ar/39Ar analysis of single biotite crystals yieldedages (0·81–4·7 Ma), that do not correspondto eruption ages. The biotite represents partially assimilatedxenocrysts, which could have resided in the magma for only ashort period of time. Mineral chemical data, coupled with hydrothermalexperiments, indicate that prior to eruption the dacitic magmastagnated at a depth of 4·5–6 km below the summitat water pressures of 160–210 MPa and a temperature of824 ± 12°C on the basis of Fe–Ti oxide thermometry,and under water-saturated conditions. To stabilize a homogeneousmagma body of >10 km3 at 824°C in the upper crust, wepropose that reheating of the dacitic reservoir by hotter magmabatches was able to maintain the equilibrium between the temperatureof the magma and the assimilation of wall-rock over a periodof 2500 years. Based on similarities among the juvenile products,we suggest that the three eruptions were fed from the same magmabody. KEY WORDS: explosive volcanism; xenocrystic contamination; Nevado de Toluca, Mexico  相似文献   

15.
In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structural–microtexturalrelations and thermobarometry indicate that syn-deformationalsegregation–crystallization 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; P–T conditions  相似文献   

16.
We present an experimental and petrological study aimed at estimatingthe pre-eruptive conditions of a Holocene dacitic lava fromVolcán San Pedro (36°S, Chilean Andes). Phase-equilibriumexperiments were performed at temperatures (T) from 800 to 950°C,and mainly at 200 MPa, but also at 55, 150, and 406 MPa. Oxygenfugacity (fO2) ranged from the Ni–NiO buffer (NNO) to3·5 log units above (NNO + 3·5), and water contentsfrom  相似文献   

17.
Uncontaminated volcanic rocks from the 60 Ma Vaigat Formation,West Greenland, contain 6·5–30 wt % MgO, averaging15·5 wt % MgO. Olivine (mg-number 77·4–93·3)forms diverse assemblages of zoned phenocrysts and xenocrystsshowing evidence for equilibrium and fractional crystallization,oxidation, partial to complete re-equilibration, as well asmagma mixing. The olivine crystals contain glass inclusionsand have high contents of Ca and Cr, indicating that all olivineswith up to mg-number 93·0 crystallized from melts. Associatedchromites (mg-number 45·4–77·2) are essentiallyunzoned and in equilibrium with the olivines. Matrix glassesfrom pillow breccias have 6·7–8·8 wt % MgOand quenched close to 1200°C with oxidation states one log-unitabove the NNO (nickel–nickel oxide) buffer. Compositionaldifferences between the glasses from different volcanic membersare inherited from the primary melts. The magmas erupted ascrystal-charged melts, and liquids with more than  相似文献   

18.
This paper describes the melting relations of three basalts,a Picture Gorge tholeiite, the 1921 Kilauea olivine tholeiite,and the 1801 Hualalai alkali basalt, at 5 kb water pressure,680–1045 °C, at the oxygen fugacities of the quartz-fayalite-magnetite(QFM) and hematite-magnetite (HM) buffer. All melts producedwithin the hornblende stability field are strongly quartzo-feldspathic.All are quartz-normative, including those from the alkali basalt,and all but two of the melts are corundum-normative. Melt compositionshows very little dependence on oxygen fugacity within the hornblendestability field, as MgO and FeO contents are very low. Whenhornblende begins to melt extensively (1000°–1045°C), the TiO2, FeO, and MgO contents of the melt increaseabruptly. In this range, melts formed on the HM buffer havemuch higher Mg/Fe ratios and lower TiO2 than melts formed onthe QFM buffer. Melt composition is also quite insensitive to changes in basaltcomposition, within the hornblende stability field. The chiefexception is the Na/Ca ratio, which varies directly with Na/Cain the starting basalt. When projected into the Ab-An-Or-Qzquaternary system, all melts produced follow a rather narrowspiral path through the tetrahedron; they descend from the Ancorner, moving toward Qz at constant Ab/Or, moving toward Oronly when plagioclase± quartz begin to precipitate. The melting behavior of hornblende, plagioclase, and augitein these experiments has been examined closely, with the followingresults: successive partial melts may differ from each otherby compositional increments which are very different in compositionfrom the minerals contributing to the melt in the temperatureinterval under consideration. These increments can almost neverbe expressed solely in terms of members of the one or two mineralsolid solutions from which they are actually derived. In a fewcases the increments cannot be expressed in terms of any reasonablecombination of minerals. This pattern contrasts markedly withthat observed in fractional crystallization, in which the differencebetween successive melts must always correspond to present orpossible phenocryst minerals. The contrast implies that magmaseries generated by any kind of melting process, equilibriumor fractional, should be recognizably different from seriesgenerated by fractional crystallization, if minerals like hornblendeor pyroxene are involved.  相似文献   

19.
Low-Pressure Experimental Constraints on the Evolution of Komatiites   总被引：1，自引：0，他引：1  
THY  P. 《Journal of Petrology》1995,36(6):1529-1548
Melting experiments were performed on a komatiitic basalt with17 wt% MgO from Munro Township, Ontario, at I-atm pressure andan oxygen fugacity controlled approximately to the fayalite-magnetite-quartzbuffer. The experiments showed that olivine appears at 1344±5°C,spinel at 1334±6°C plagioclase at 1185±5°C,augite at 1176±5°C and pigeonite at 1154±6°C.Compositionally, olivine varies from Fo90 to Fo74 and displaysan average KFe/MgD (ol/liq) of 0•32. The spinels are chromitesand chromian spinels with Mg/(Mg + Fe2+) ratios between 0•66and 0•;32, which show a marked correlation with meltingtemperature. The pyroxenes show an average KFe/MgD (px/liq)of 0•26, identical for augite and pigeonite. Plagiodaseranges compositionally between An82 and An72 Plotted in thepseudo-quaternary basalt phase diagram, the liquid line of descentis similar to that observed for quartz tholeiitic magmas. Therefore,the low-pressure, late-stage evolution products of komatiiteand basaltic komatiite parental magmas will chemically and mineralogicallybe ferrobasaltic quartz tholeiites. High-temperature and high-pressuremodeling suggests that the main observed compositional variationof Munro komatiites can be explained by low-pressure crystalfractionation and accumulation of olivine into komatiite liquidswith below 21•5–23•5 wt% MgO and eruptive temperaturesbelow 1435–1465°C for oxygen fugacities between thefayalite-magnetite quartz (FMQ) and iron-wiistite (IW) buffers.The maximum magnesium content of liquid komatiites, assumingequilibrium Fo94 olivine, is 27–29 wt% MgO and eruptivetemperatures are between 1515 and 1540°C. KEY WORDS: komatiites; experimental petrology; Munro Township; Ontario  相似文献   

20.
Within the Zitácuaro–Valle de Bravo (ZVB) regionof the central Mexican Volcanic Belt (MVB), three lava serieshave erupted during the Quaternary: (1) high-K2O basaltic andesitesand andesites; (2) medium-K2O basaltic andesites, andesitesand dacites; (3) high-TiO2 basalts and basaltic andesites. Thedominant feature of the first two groups is the lack of plagioclaseaccompanying the various ferromagnesian phenocrysts (olivine,orthopyroxene, augite, and hornblende) in all but the dacites.This absence of plagioclase in the phenocryst assemblages ofthe high-K2O and medium-K2O intermediate lavas is significantbecause it indicates high water contents during the stage ofphenocryst equilibration. In contrast, the high-TiO2 group ischaracterized by phenocrysts of plagioclase and olivine. Thespatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280–330 km fromthe Middle America Trench (MAT), is characterized by high-K2Omelts that are relatively enriched in fluid-mobile elementsand have the highest 87Sr/86Sr ratios. Medium-K2O basaltic andesiteand andesite lavas are present throughout the transect, butthose closest to the MAT are MgO-rich (3·5–9·4wt %) and have phenocryst assemblages indicative of high magmaticwater contents (3·5–6·5 wt % water) andrelatively low temperatures (950–1000°C). In markedcontrast, the northern section of the ZVB transect (380–480km from the MAT) has high-TiO2, high field strength element(HFSE)-enriched magmas that have comparatively dry (< 1·5wt % magmatic water) and hot (1100–1200°C) phenocrystequilibration conditions. The central section of the ZVB transect(330–380 km from the MAT) is a transition zone and producesmoderately light rare earth element (LREE) and large ion lithophileelement (LILE)-enriched, medium-K2O lavas with phenocryst assemblagesindicative of intermediate (1·5–3·5 wt %)water contents and temperatures. The high-K2O series compositionsare the most enriched in LILE and LREE, with a narrow rangeof radiogenic 87Sr/86Sr from 0·704245 to 0·704507,143Nd/144Nd values ranging from 0·512857 to 0·512927(Nd = 4·27–5·63), and 208Pb/204Pb valuesfrom 38·248 to 38·442, 207Pb/204Pb values from15·563 to 15·585, and 206Pb/204Pb values from18·598 to 18·688. The medium-K2O series compositionsare only moderately enriched in the LILE and LREE, with a broaderrange of 87Sr/86Sr, but similar 143Nd/144Nd and 208Pb/204Pbvalues to those of the high-K2O series. In contrast, the high-TiO2series compositions have little enrichment in LILE or LREE andinstead are enriched in the HFSE and heavy rare earth elements(HREE). The high-TiO2 lavas are isotopically distinct in theirlower and narrower range of 143Nd/144Nd. The isotopic variationsare believed to reflect the upper mantle magma source regionsas the low content of phenocrysts in most lavas precludes significantupper crustal assimilation or magma mixing, other than thatrepresented by the presence of quartz xenocrysts (< 2 vol.%) with rhyolitic glass inclusions, which are found in manyof these lavas. The systematic spatial variation in compositionof the three lava series is a reflection of the underlying subduction-modifiedmantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas  相似文献   

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