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1.
The petrography, mineralogy, and geochemistry of a suite oflavas from the northwestern part of Epi Island in the VanuatuArc, southwest Pacific Ocean, are described. The more primitivemembers of this suite are rich in clinopyroxene phenocrystsand are strikingly similar to primitive lavas from MerelavaIs. in the same arc. These primitive, clinopyroxene-rich lavasare designated arc ankaramites to differentiate them from primitive,olivine-rich arc picrites which also occur in this arc system.The primitive Epi lavas are shown to have evolved from low-Kprimary melts which were saturated in both olivine and clinopyroxene.The most Mg-rich olivine (mg-number 92?2) and clinopyroxene(mg-number 94?4) in the ankaramites represent cotectic crystallizationwith Cr-rich spinels. Initial plagioclase (An94) crystallizedin equilibrium with olivine (mg-number 78–80) and theplagioclase-olivine cotectic path extends to mg-number 50 andAn58. The ankaramitic parent magma composition is calculated fromthe most primitive olivine phenocryst composition and the liquidline of descent, and has 14?5% MgO, 11% A12O3, 14?8%CaO, 0?29%K2O, and flat REE patterns. The origin of this parent magmahas been modelled with Ghiorso & Carmichael's (1985) programSILMIN. An assimilation model involving a clinopyroxenite orwehrlite assimilate and a low-K picrite host requires ca. 90%assimilate to match the phase chemistry and bulk-rock chemistryof the parental ankaramite. The required degree of superheatingnecessary to achieve this, and the apparent restriction of low-Kpicrites to Anatom Island in the far south of the arc, rendersthis model unsatisfactory. Partial melting models involvingtypical upper mantle lherzolite also fail to give satisfactoryresults, but partial melting of a wehrlite source (mg-number87-88) with < 10% normative (mol.) orthopyroxene, at 5?10kband 1325?C, closely matches the parental ankaramite composition.These results can be reconciled with melting of lower crustalcumulates by an ascending peridotite diapir, a hypothesis whichaccounts for the very low Ni contents of the parental meltsand primitive phenocrysts. The more evolved lavas define two distinct assemblages: a relativelytight grouping of high-K andesites straddling the high-K-‘shoshonite’boundary, characterized by low Zr/Rb (2?2) and high K2O/Na2Oratios (1?3–0?9), and a relatively coherent fractionationpathway to dacites straddling the ‘calc-alkaline’-high-Kboundary, with Zr/Rb = 2?9 and K2O/Na2O=0?6. Numerical modellingdemonstrates that the dacite trend is compatible with fractionationfrom an ankaramite parent, whereas the high-K andesites areincompatible with open- or closed-system fractionation fromankaramitic or picritic sources and may represent fractionated,hybrid magmas, largely derived from melting of lower crustalgabbros.  相似文献   

2.
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   

3.
Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitousin arc, ocean-island and mid-ocean ridge settings. They areprimitive in character (XMg > 0·65) and have highCaO contents (>14 wt %) and CaO/Al2O3 (>1·1). Experimentson an ankaramite from Epi, Vanuatu arc, demonstrate that itsliquidus surface has only clinopyroxene at pressures of 15 and20 kbar, with XCO2 in the volatile component from 0 to 0·86.The parental Epi ankaramite is thus not an unfractionated magma.However, forcing the ankaramite experimentally into saturationwith olivine, orthopyroxene and spinel results in more magnesian,ultra-calcic melts with CaO/Al2O3 of 1·21–1·58.The experimental melts are not extremely Ca-rich but high inCaO/Al2O3 and in MgO (up to 18.5 wt %), and would evolve tohigh-CaO melts through olivine fractionation. Fractionationmodels show that the Epi parent magma can be derived from suchultra-calcic experimental melts through mainly olivine fractionation.We show that the experimental ultra-calcic melts could formthrough low-degree melting of somewhat refractory mantle. Thelatter would have been depleted by previous melt extraction,which increases the CaO/Al2O3 in the residue as long as someclinopyroxene remains residual. This finding corrects the commonassumption that ultra-calcic magmas must come from a Ca-richpyroxenite-type source. The temperatures necessary for the generationof ultra-calcic magmas are  相似文献   

4.
Dehydration melting experiments of alkali basalt associatedwith the Kenya Rift were performed at 0·7 and 1·0GPa, 850–1100°C, 3–5 wt % H2O, and fO2 nearnickel–nickel oxide. Carbon dioxide [XCO2 = molar CO2/(H2O+ CO2) = 0·2–0·9] was added to experimentsat 1025 and 1050°C. Dehydration melting in the system alkalibasalt–H2O produces quartz- and corundum-normative trachyandesite(6–7·5 wt % total alkalis) at 1000 and 1025°Cby the incongruent melting of amphibole (pargasite–magnesiohastingsite).Dehydration melting in the system alkali basalt–H2O–CO2produces nepheline-normative tephriphonolite, trachyandesite,and trachyte (10·5–12 wt % total alkalis). In thelatter case, the solidus is raised relative to the hydrous system,less melt is produced, and the incongruent melting reactioninvolves kaersutite. The role of carbon dioxide in alkalinemagma genesis is well documented for mantle systems. This studyshows that carbon dioxide is also important to the petrogenesisof alkaline magmas at the lower pressures of crustal systems.Select suites of continental alkaline rocks, including thosecontaining phonolite, may be derived by low-pressure dehydrationmelting of an alkali basalt–carbon dioxide crustal system. KEY WORDS: alkali basalt; alkaline rocks; carbon dioxide; dehydration melting; phonolite  相似文献   

5.
The distinctive island-arc ankaramites exemplified by the activeVanuatu arc may be produced by melting of refractory lherzoliteunder conditions in which melting is fluxed by H2O + CO2. Parentalpicritic ankaramite magmas with maximum CaO/Al2O3 to  相似文献   

6.
Experiments defining the distribution of H2O [Dw = wt % H2O(melt)/wt% H2O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H2O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 3–7 kbar and 800–1000°C.H2O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H2O vapour-saturatedconditions Dw ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedDw decreases to minimum values of 2·6–5·0at moderate to low cordierite H2O contents (0·6–1·1wt %). At very low aH2O, cordierite contains less than 0·2–0·3wt % H2O and Dw increases sharply. The Dw results are consistentwith melt H2O solubility models in which aH2O is proportionalto Xw2 (where Xw is the mole fraction of H2O in eight-oxygenunit melt) at Xw  相似文献   

7.
Macquarie Island is an exposure above sea-level of part of thecrest of the Macquarie Ridge. The ridge marks the Australia–Pacificplate boundary south of New Zealand, where the plate boundaryhas evolved progressively since Eocene times from an oceanicspreading system into a system of long transform faults linkedby short spreading segments, and currently into a right-lateralstrike-slip plate boundary. The rocks of Macquarie Island wereformed during spreading at this plate boundary in Miocene times,and include intrusive rocks (mantle and cumulate peridotites,gabbros, sheeted dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow lavas, picrites, breccias, hyaloclastites),and associated sediments. A set of Macquarie Island basalticglasses has been analysed by electron microprobe for major elements,S, Cl and F; by Fourier transform infrared spectroscopy forH2O; by laser ablation–inductively coupled plasma massspectrometry for trace elements; and by secondary ion mass spectrometryfor Sr, Nd and Pb isotopes. An outstanding compositional featureof the data set (47·4–51·1 wt % SiO2, 5·65–8·75wt % MgO) is the broad range of K2O (0·1–1·8wt %) and the strong positive covariation of K2O with otherincompatible minor and trace elements (e.g. TiO2 0·97–2·1%;Na2O 2·4–4·3%; P2O5 0·08–0·7%;H2O 0·25–1·5%; La 4·3–46·6ppm). The extent of enrichment in incompatible elements in glassescorrelates positively with isotopic ratios of Sr (87Sr/86Sr= 0·70255–0·70275) and Pb (206Pb/204Pb =18·951–19·493; 207Pb/204Pb = 15·528–15·589;208Pb/204Pb = 38·523–38·979), and negativelywith Nd (143Nd/144Nd = 0·51310–0·51304).Macquarie Island basaltic glasses are divided into two compositionalgroups according to their mg-number–K2O relationships.Near-primitive basaltic glasses (Group I) have the highest mg-number(63–69), and high Al2O3 and CaO contents at a given K2Ocontent, and carry microphenocrysts of primitive olivine (Fo86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo90·5). Fractionated, Group II, basaltic glasses aresaturated with olivine + plagioclase ± clinopyroxene,and have lower mg-number (57–67), and relatively low Al2O3and CaO contents. Group I glasses define a seriate variationwithin the compositional spectrum of MORB, and extend the compositionalrange from N-MORB compositions to enriched compositions thatrepresent a new primitive enriched MORB end-member. Comparedwith N-MORB, this new end-member is characterized by relativelylow contents of MgO, FeO, SiO2 and CaO, coupled with high contentsof Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements,and has the most radiogenic Sr and Pb regional isotope composition.These unusual melt compositions could have been generated bylow-degree partial melting of an enriched mantle peridotitesource, and were erupted without significant mixing with commonN-MORB magmas. The mantle in the Macquarie Island region musthave been enriched and heterogeneous on a very fine scale. Wesuggest that the mantle enrichment implicated in this studyis more likely to be a regional signature that is shared bythe Balleny Islands magmatism than directly related to the hypotheticalBalleny plume itself. KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry  相似文献   

8.
In this work we investigate the olivine-phyric basalt suiteof the Aphanasey Nikitin Rise, an intraplate volcanic structureformed during the Late Cretaceous in the Indian Ocean. The parentalmelt of the basalt suite has a hypersthene-normative tholeiiticcomposition with low H2O content (0·3–0·5wt %) and high SiO2/Al2O3 (3·5). The basalt suite ischaracterized by Nb, Ta, Th and U depletion, and uniquely low206Pb/204Pb and 143Nd/144Nd among the Cretaceous tholeiiticbasalts of the Indian Ocean. Our modelling demonstrates thatfractional crystallization of depleted mantle-derived melt andlower continental crust assimilation is a suitable model forthe genesis of the parental magma of this suite. The continentalcrustal material involved is characterized by long-term Rb,U and Th depletion and probably remained isolated for >109years in cratonic Gondwanan lithosphere. On a broader scale,two geochemical groups can be distinguished among tholeiitesformed in the Indian Ocean basin during the period 115–75Ma, from the Aphanasey Nikitin Rise, the southern Kerguelenand Naturaliste plateaux and the Broken Ridge. Both groups havea compositional range from hypersthene-normative basalt to basalticandesite and are characterized by Nb–Ta depletion, extremelylow  相似文献   

9.
Sediment Melts at Sub-arc Depths: an Experimental Study   总被引：14，自引：0，他引：14  
The phase and melting relations in subducted pelites have beeninvestigated experimentally at conditions relevant for slabsat sub-arc depths (T = 600–1050°C, P = 2·5–4·5GPa). The fluid-present experiments produced a dominant paragenesisconsisting of garnet–phengite–clinopyroxene–coesite–kyanitethat coexists with a fluid phase at run conditions. Garnet containsdetectable amounts of Na2O (up to 0·5 wt%), P2O5 (upto 0·56 wt%), and TiO2 (up to 0·9 wt%) in allexperiments. Phengite is stable up to 1000°C at 4·5GPa and is characterized by high TiO2 contents of up to 2 wt%.The solidus has been determined at 700°C, 2·5 GPaand is situated between 700 and 750°C at 3·5 GPa.At 800°C, 4·5 GPa glass was present in the experiments,indicating that at such conditions a hydrous melt is stable.In contrast, at 700°C, 3·5 and 4·5 GPa, asolute-rich, non-quenchable aqueous fluid was present. Thisindicates that the solidus is steeply sloping in P–T space.Fluid-present (vapour undersaturated) partial melting of thepelites occurs according to a generalized reaction phengite+ omphacite + coesite + fluid = melt + garnet. The H2O contentof the produced melt decreases with increasing temperature.The K2O content of the melt is buffered by phengite and increaseswith increasing temperature from 2·5 to 10 wt%, whereasNa2O decreases from 7 to 2·3 wt%. Hence, the melt compositionschange from trondhjemitic to granitic with increasing temperature.The K2O/H2O increases strongly as a function of temperatureand nature of the fluid phase. It is 0·0004–0·002in the aqueous fluid, and then increases gradually from about0·1 at 750–800°C to about 1 at 1000°C inthe hydrous melt. This provides evidence that hydrous meltsare needed for efficient extraction of K and other large ionlithophile elements from subducted sediments. Primitive subduction-relatedmagmas typically have K2O/H2O of 0·1–0·4,indicating that hydrous melts rather than aqueous fluids areresponsible for large ion lithophile element transfer in subductionzones and that top-slab temperatures at sub-arc depths are likelyto be 700–900°C. KEY WORDS: experimental petrology; pelite; subduction; UHP metamorphism; fluid; LILE  相似文献   

10.
A mantle xenolith suite from two Late Tertiary necks on SalIsland (Cape Verde Archipelago) consists of nearly equivalentamounts of anhydrous spinel-bearing lherzolites and harzburgites,in which secondary metasomatic textural domains are superimposedon the original protogranular textures. Detailed petrographicstudies, coupled with in situ major and trace element analysesof the constituent minerals and interstitial glasses, revealthe complex evolutionary history of the Cape Verde lithosphericmantle, from depletion in the garnet facies to re-equilibrationand re-enrichment in the spinel stability field. Low CaO (16·4–18·0wt %) and heavy rare earth element (HREE; Ybn = 2·4–4·8),and high Cr2O3 (1·06–1·84 wt %) contentsin the clinopyroxenes of the lherzolites can be quantitativelyaccounted for by (1) low-degree (4%) partial melting of a PrimitiveMantle-like garnet lherzolite followed by (2) partial re-equilibrationof the melting residuum from the garnet to the spinel stabilityfield. This model is further supported by thermobarometric estimates(T = 975–1210°C; P = 1·3–2·1 GPa),which cluster around the spinel–garnet boundary in theperidotite system. Secondary parageneses, regardless of theprimary lithologies, are characterized by (1) two clinopyroxenes,cpx2-O and cpx2-C, respectively related to orthopyroxene andclinopyroxene destabilization after reaction with metasomaticfluids, and (2) glasses with anomalously high, even for continentalsettings, K2O contents (up to 8·78 wt %), together withK-feldspar. Major and trace element mass balance calculationsbetween the primary and secondary parageneses suggest infiltrationof a kimberlite-like metasomatizing agent (on volatile-freebasis, MgO 17–27 wt %; K2O/Na2O 1·6–3·2molar; (K2O + Na2O)/Al2O3 1·1–3·0 molar;Rb 91–165 ppm; Zr 194–238 ppm). The kimberlite-likemetasomatism in the Cape Verde lithospheric mantle, togetherwith the presence of lherzolitic domains, partially re-equilibratedfrom the garnet to the spinel stability field, may suggest thepresence of subcontinental mantle lithosphere relicts left behindby drifting of the African Plate during the opening of the CentralAtlantic Ocean. KEY WORDS: Cape Verde; mantle metasomatism; garnet signatures; clinopyroxenes; kimberlites  相似文献   

11.
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy  相似文献   

12.
WALLACE  PAUL J. 《Journal of Petrology》2002,43(7):1311-1326
Submarine pillow basalts (34 Ma) recovered from the NorthernKerguelen Plateau at ODP Site 1140 contain abundant unalteredglass, providing the first opportunity to measure the volatilecontents of tholeiitic basaltic magmas related to the Kerguelenmantle plume. The glasses have La/Sm and Nb/Zr ratios that varyfrom values similar to Southeast Indian Ridge (SEIR) MORB (Unit1), to slightly more enriched (Unit 6), to values transitionalbetween SEIR MORB and basaltic magmas formed by melting of theKerguelen plume (Units 2 and 3). Volatile contents for glassesin Units 1 and 6 are similar to depleted mid-ocean ridge basalt(MORB) values (0·25–0·27 wt % H2O, 1240–1450ppm S, 42–54 ppm Cl). In contrast, H2O contents are higherfor the enriched glasses (Unit 2, 0·44 wt % H2O; Unit3, 0·69 wt %), as are S (1500 ppm) and Cl (146–206ppm). Cl/K ratios for all glasses are relatively low (0·03–0·04),indicating that assimilation of hydrothermally altered materialdid not occur during shallow-level crystallization. H2O/Ce forthe enriched glasses (Units 2 and 3) is significantly lowerthan Pacific and South Atlantic MORB values, suggesting thatlow H2O/Ce may be an inherent characteristic of the Kerguelenplume source. Vapor saturation pressures calculated using theH2O and CO2 contents of the glasses indicate that  相似文献   

13.
Experimental studies were carried out to evaluate phase relationsinvolving titanite–F–Al-titanite solid solutionin the system CaSiO3–Al2SiO5–TiO2–CaF2. Theexperiments were conducted at 900–1000°C and 1·1–4·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and XAl ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO2+]–1[AlF2+]1.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in XAl, whereasthe maximum XAl of titanite solid solution occurs with fluoriteand either anorthite or Al2SiO5. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO4O is very close to the ideal molecular activity model(XTi) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain  相似文献   

14.
The 456 ± 4 Ma Skattøra migmatite complex in thenorth Norwegian Caledonides consists of migmatitic nepheline-normativemetagabbros and amphibolites that are net-veined by numerousnepheline-normative anorthositic and leucodioritic dykes. Plagioclase(An20–50) is the dominant mineral (85–100%) in thedykes and the leucosome, but amphibole is generally presentin amounts up to 15%. The following observations strongly suggestformation of the anorthositic magma by anatexis of the surroundinggabbro in the presence of an H2O-bearing fluid phase: (1) themigmatites have plagioclase-rich (anorthositic) leucosomes andamphibole-rich restites; (2) crystallization of amphibole inthe anorthositic and leucodioritic dykes suggests high H2O activity;(3) the presence of coarse-grained to pegmatitic dykes and miaroliticcavities indicates a fluid-rich magma; (4) hydration zones thatsurround many anorthosite dykes suggest that the magma probablyexpelled H2O-rich fluids during crystallization. Water-saturatedmelting experiments at 0·5–1·5 GPa and temperaturesfrom 800 to 1000°C have been performed on a nepheline-normativegabbro to test the proposed petrogenesis of the Skattøraanorthosites. The glasses produced close to the solidus aretonalitic in composition, but they become richer in plagioclaseat higher temperatures. At and below 1·0 GPa, the residuesare composed of amphibole. Experiments above 1·0 GPaproduced residual garnet and/or zoisite in addition to amphibole,suggesting that the anorthositic dykes in the Skattøramigmatite complex formed below 1·25 GPa. The experimentsshow that the high Na2O content of the anorthosite dykes canonly be produced if Na is added to the charges. The glass thatbest fits the composition of the Skattøra dykes was producedat 1·0 GPa and 900°C with 2 wt % Na(OH) added. KEY WORDS: anorthosite; dyke swarm; anatexis; experimental petrology  相似文献   

15.
La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km3. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (76–77 wt % SiO2) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO2), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 wt% H2O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (87Sr/86Sri 0·7094–0·7131;143Nd/144Nd 0·51222–0·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H2O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber  相似文献   

16.
Phase relations for the bulk compositions 3CaO·2FeOx·3SiO2+excessH2O and CaO·FeOx·2SiO2+excess H2O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO2. Andradite, Ca3Fe3+2Si3O12, 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 Pfluid andradite is stable above an fO2of 1015 bar at 800 °C and 10-32 bar at 400 °C. At lowerfO2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe2+SiO4)+ wollastonite and kirschsteinite+xonotlite (Ca6Si6O17(OH)2). Synthetic hedenbergite, CaFe2+Si2O6, 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 Pfiuid, hedenbergite is stable below an fO2 of10-13 bar at 800 °C and 10-28 bar at 400 °C. Above thesefO2 values, hedenbergite+O2 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 + H2O 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.
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   

18.
Anhydrite solubility in H2O–NaCl solutions was measuredat 6–14 kbar, 600–800°C and NaCl mole fractions(XNaCl) of 0–0·3 in piston–cylinder apparatus.Solubilities were determined by weight changes of natural anhydritein perforated Pt envelopes confined with fluid in larger Ptcapsules. In initially pure H2O at 10 kbar and 800°C, CaSO4concentration is low (0·03 molal), though much largerthan at the same temperature and 1 kbar. Hematite-buffered experimentsshowed slightly lower solubilities than unbuffered runs. CaSO4solubility increases enormously with NaCl activity: at 800°Cand 10 kbar and XNaCl of 0·3, CaSO4 molality is 200 timeshigher than with pure H2O. Whereas CaSO4 solubility in pureH2O decreases with rising T at low T and P, the high-P resultsshow that anhydrite solubility increases with T at constantP at all XNaCl investigated. The effects of salinity and temperatureare so great at 10 kbar that critical mixing between sulfate-richhydrosaline melts and aqueous salt solutions is probable at900°C at XNaCl 0·3. Recent experimental evidencethat volatile-laden magmas crystallizing in the deep crust mayevolve concentrated salt solutions could, in light of the presentwork, have important implications regarding such diverse processesas Mount Pinatubo-type S-rich volcanism, high-f O2 regionalmetamorphism, and emplacement of porphyry Cu–Mo ore bodies,where anhydrite–hematite alteration and fluid inclusionsreveal the action of very oxidized saline solutions rich insulfur. KEY WORDS: anhydrite; sulfur; solubility; metamorphic brines; granulites  相似文献   

19.
Mineral and melt inclusions in olivines from the most Mg-richmagma from the southern West Sulawesi Volcanic Province indicatethat two distinct melts contributed to its petrogenesis. Thecontribution that dominates the whole-rock composition comesfrom a liquid with high CaO (up to 16 wt %) and low Al2O3 contents(CaO/Al2O3 up to 1), in equilibrium with spinel, olivine (Fo85–91;CaO 0·35–0·5 wt %; NiO 0·2–0·30wt %) and clinopyroxene. The other component is richer in SiO2(>50 wt %) and Al2O3 (19–21 wt %), but contains significantlyless CaO (<4 wt %); it is in equilibrium with Cr-rich spinelwith a low TiO2 content, olivine with low CaO and high NiO content(Fo90–94; CaO 0·05–0·20 wt %; NiO0·35–0·5 wt %), and orthopyroxene. Boththe high- and low-CaO melts are potassium-rich (>3 wt % K2O).The high-CaO melt has a normalized trace element pattern thatis typical for subduction-related volcanic rocks, with negativeTa–Nb and Ti anomalies, positive K, Pb and Sr anomalies,and a relatively flat heavy rare earth element (HREE) pattern.The low-CaO melt shows Y and HREE depletion (Gdn/Ybn 41), butits trace element pattern resembles that of the whole-rock andhigh-CaO melt in other respects, suggesting only small distinctionsin source areas between the two components. We propose thatthe depth of melting and the dominance of H2O- or CO2-bearingfluids were the main controls on generating these contrastingmagmas in a syn-collisional environment. The composition ofthe low-CaO magma does not have any obvious rock equivalent,and it is possible that this type of magma does not easily reachthe Earth's surface without the assistance of a water-poor carriermagma. KEY WORDS: melt inclusions; mineral chemistry; olivine; syn-collisional magmatism; ankaramites; low-Ca magma  相似文献   

20.
Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H2O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO2, two-pyroxenes+ plagioclase + Fe–Ti oxides + apatite, 3 wt % H2O, P= 1·5 kbar, fO2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO2, olivine + two-pyroxenes, 3 wt % H2O,P = 1–4 kbar). Magma mixed at 4–6 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H2O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H2O. Degassing from 0·031 km3 of eruptedmagma accounts for only 0·7 wt % of the observed SO2emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles  相似文献   

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