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1.
In the system CaO-MgO-Al2O3-SiO2-Na2O-H2O under 5 kb pressurethe invariant equilibrium forsterite-orthopyroxene-Ca-rich clinopyroxene-amphibole-plagioclase-liquid-vapourhas been identified at 960?12 ?C. A similar invariant assemblagewith spinel replacing Ca-rich clinopyroxene exists at 950?8?C. The liquid in the former equilibrium contains 16.5 per cent(wt.) normative quartz and 3 per cent Na2O; the plagioclaseis more calcic than An87; the pyroxenes contain about 3 percent Al2O3 and the amphibole is hypersthene-normative. Two anhydrousthermal maxima, the olivine-Ca-rich clinopyroxene-plagioclaseand the orthopyroxene-Ca-rich clinopyroxene-plagioclase dividezones are not encountered in this system, and nepheline-normativeliquids may crystallize amphibole?olivine?Ca-rich clinopyroxeneto produce quartz-normative residual liquids of andesite-typecomposition. A thermal maximum involving amphibole-olivine-Ca-richclinopyroxene-liquid-vapour exists for liquids containing approximately11 per cent normative nepheline and liquids more undersaturatedthan this will crystallize these phases to produce extremelynephelinitic liquids. Phase diagrams are presented which facilitate the predictionof crystallization sequences and liquid evolution paths forany basic or intermediate composition under the conditions employedhere. 相似文献
2.
Basaltic liquids, saturated at one-atmosphere in augite, olivine, and plagioclase, show a strong linear trend in pseudo-quaternary projections from nepheline to slightly quartz normative compositions. Coexisting augites show a corresponding variation from slightly nepheline to hypersthene normative compositions. Consequently, alkalic and mildly alkalic liquids coexist with nepheline normative augites; tholeiitic liquids coexist with hypersthene normative augites; and transitional liquids coexist with augites compositionally intermediate between those crystallizing in alkalic and tholeciitic melts. The variation in augite composition is due to a decrease in wollastonite and non-quadrilateral components, going from mildly alkalic, to transitional, and tholeiitic augites. The observed liquid and augite trends lack correlations between normative composition and crstallization temperature. This is an artifact of plotting unrelated liquid lines of descent. Individual liquid lines of descent, in contrast, display good correlation between normative composition and crystllization temperature, which, for both liquids and coexisting augites, are highly oblique to nearly perpendicular to the general trend. Because the composition of the first augite to appear on the liquidus varies as a function of the bulk system, the location of the thermal divide shows corresponding variations. As a consequence, unifying cotectics and pseudo-quaternary normative projections should not, without specific experimental knowledge, be used to address liquid lines of descent and common magmatic parentages. 相似文献
3.
Constraining arc magma sources at continental arc settings is a delicate task, because chemical signatures from crustal processes obscure the slab and mantle signatures. Here, we present major, trace, and volatile element compositions of olivine-hosted melt inclusions (Fo82–89) selected from the most primitive lavas (Mg# >?60) from two Ecuadorian volcanoes (Puñalica and Sangay) situated at the southern termination of the Andean Northern Volcanic Zone. Melt inclusions (MI) from Puñalica are nepheline normative and have basaltic-to-basaltic-andesite compositions (45–56 wt% SiO2) similar to peridotite-derived melts. Sangay MI is also nepheline normative, with high CaO (up to 16 wt% and CaO/Al2O3?<?1) and low silica contents (41.9–44.5 wt%) pointing out an amphibole-bearing clinopyroxenite source. Both volcanoes display volatile-rich compositions (up to 6100 ppm Cl, 2200 ppm F, and 6700 ppm S). These MI cannot be related to their host lavas by fractional crystallization, implying that they represent true primitive liquids. The source of Puñalica MI was metasomatized by slab-derived melts that imprints its low Ba/Th, Sr/Th, and high Th/La (average values of 66, 129, and 0.22, respectively). On the contrary, the slab component added to the source of Sangay MI has a higher Ba/Th, Sr/Th, and low Th/La (average values of 261, 517, and 0.11, respectively) which could suggest a relative contribution of aqueous fluids. This dichotomy is related to the presence of the Grijalva Fracture Zone that separates a younger and hotter oceanic crust to the north (below Puñalica) from a colder and older oceanic crust to the south (below Sangay). 相似文献
4.
Peter Thy 《Contributions to Mineralogy and Petrology》1983,82(2-3):232-251
Basaltic glasses from the three alkalic areas of Iceland (Snaefellsnes Volcanic Zone, Sudurland Volcanic Zone and Vestmannaeyjar Volcanic Area) contain plagioclase, olivine, clinopyroxene, chromian spinel and titanomagnetite as phenocryst phases. The glasses are hypersthene to nepheline normative alkali basaltic with FeO/ MgO ratios between 1.4–4.7. Olivine ranges in composition from Fo90 to Fo55, plagioclase from An90 to An50 and clinopyroxene from En45Fs10Wo45 to En40Fs17Wo43. Clinopyroxene reveals systematic Ti:Al metastable crystallization trends related to the composition of the enclosing glass. Two types of phenocryst are present in most glasses and show a bimodality in size and composition. Microphenocryst phases are those most likely to have crystallized from the enclosing glass, while macrophenocrysts may have crystallized from a liquid of slightly less evolved composition. The glasses show complex phenocryst-glass relations which can be related to a polybaric effect. The normative glass compositions are related to 2-phase cotectic surfaces in the basalt tetrahedron and define the position of the 3-phase cotectic line. In general with increasing FeO/MgO in the glass the phenocryst assemblages vary from clinopyroxene, olivine and plagioclase along a clinopyroxene-olivine surface to olivine and plagioclase along an olivine-plagioclase surface. The normative glass compositions show a deflection from clinopyroxene-bearing to clinopyroxene-free glasses. The appearance of plagioclase together with clinopyroxene and olivine can be explained in the light of experimental investigations of the effect of pressure on phase relations. The major element variation of the glasses is interpreted as representing mantle derived magma batches of primary liquids, modified to some degree by high (6 kbar) and intermediate to low pressure (below 3 kbar) crystal fractionation towards equilibrium phase relations during ascent and residence in crustal magma chambers. The observed deflection in normative compositions of the glasses marks the position of the high pressure 3-phase cotectic line. The bimodality in size and composition of plagioclase and olivine phenocrysts can be related to high pressure crystal fractionation in the melt. The Fe-Ti basalt glasses from Sudurland are believed to be quenched high pressure compositions. 相似文献
5.
The genesis of basaltic magmas 总被引：29，自引：2，他引：29
This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.These experimental and analytical results are used to determine the directions of fractionation of basaltic magmas during crystallization over a wide range of pressures. At pressures corresponding to depths of 35–70 km separation of aluminous enstatite from olivine tholeiite magma produces a direct fractionation trend from olivine tholeiites through olivine basalts to alkali olivine basalts. Co-precipitation of sub-calcic, aluminous clinopyroxene with the orthopyroxene in the more undersaturated compositions of this sequence produces derivative liquids of basanite type. Magmas of alkali olivine basalt and basanite type represent the lower temperature liquids derived by approximately 30% crystallization of olivine-rich tholeiite at 35–70 km depth. At depths of about 30 km, fractionation of olivine-rich tholeiite with separation of both olivine and low-alumina enstatite, joined at lower temperatures by sub-calcic clinopyroxene, leads to derivative liquids with relatively constant SiO2 (48 to 50%) increasingly high Al2O3 (15–17%) contents and retaining olivine + hypersthene normative chemistry (5–15% normative olivine). These have the composition of typical high-alumina olivine tholeiites. The effects of low pressure fractionation may be superimposed on magma compositions derived from various depths within the mantle. These lead to divergence of the alkali olivine basalt and tholeiitic series but convergence of both the low-alumina and high-alumina tholeiites towards quartz tholeiite derivative liquids.The general problem of derivation of basaltic magmas from a mantle of peridotitic composition is discussed in some detail. Magmas are considered to be a consequence of partial melting but the composition of a magma is determined not by the depth of partial melting but by the depth at which magma segregation from residual crystals occurs. Magma generation from parental peridotite (pyrolite) at depths up to 100 km involves liquid-crystal equilibria between basaltic liquids and olivine + aluminous pyroxenes and does not involve garnet. At 35–70 km depth, basaltic liquids segregating from a pyrolite mantle will be of alkali olivine basalt type with about 20% partial melting but with increasing degrees of partial melting, liquids will change to olivine-rich tholeiite type with about 30% melting. If the depth of magma segregation is about 30 km, then magmas produced by 20–25% partial melting will be of high-alumina olivine tholeiite type, similar to the oceanic tholeiites occurring on the sea floor along the mid-oceanic ridges.Hypotheses of magma fractionation and generation by partial melting are considered in relation to the abundances and ratios of trace elements and in relation to isotopic abundance data on natural basalts. It is shown that there is a group of elements (including K, Ti, P, U, Th, Ba, Rb, Sr, Cs, Zr, Hf and the rare-earth elements) which show enrichment factors in alkali olivine basalts and in some tholeiites, which are inconsistent with simple crystal fractionation relationships between the magma types. This group of elements has been called incompatible elements referring to their inability to substitute to any appreciable extent in the major minerals of the upper mantle (olivine, aluminous pyroxenes). Because of the lack of temperature contrast between magma and wall-rock for a body of magma near to its depth of segregation in the mantle, cooling of the magma involves complementary processes of reaction with the wall-rook, including selective melting and extraction of the lowest melting fraction. The incompatible elements are probably highly concentrated in the lowest melting fraction of the pyrolite. The production of large overall enrichments in incompatible elements in a magma by reaction with and highly selective sampling of large volumes of mantle wall-rock during slow ascent of a magma is considered to be a normal, complementary process to crystal fractionation in the mantle. This process has been called wall-rock reaction. Magma generation in the mantle is rarely a simple, closed-system partial melting process and the isotopic abundances and incompatible element abundances of a basalt as observed at the earth's surface may be largely determined by the degree of reaction with the mantle or lower crustal wall-rocks and bear little relation to the abundances and ratios of the original parental mantle material (pyrolite).Occurrences of cognate xenoliths and xenocrysts in basalts are considered in relation to the experimental data on liquid-crystal equilibria at high pressure. It is inferred that the lherzolite nodules largely represent residual material after extraction of alkali olivine basalt from mantle pyrolite or pyrolite which has been selectively depleted in incompatible elements by wall-rock reaction processes. Lherzolite nodules included in tholeiitic magmas would melt to a relatively large extent and disintegrate, but would have a largely refractory character if included in alkali olivine basalt magma. Other examples of xenocrystal material in basalts are shown to be probable liquidus crystals or accumulates at high pressure from basaltic magma and provide a useful link between the experimental study and natural processes. 相似文献
6.
The two-liquid field between alkali-carbonate liquids and phonolite or nephelinite magmas from the Oldoinyo Lengai volcano has been determined between 0.7 and 7.6 kb and 900°–1,250° C. The miscibility gap expands with increase in
and decrease in temperature. Concomitantly there is a rotation of tie-lines so that the carbonate liquids become richer in CaO. The element distribution between the melts indicates that a carbonate liquid equivalent in composition to Oldoinyo Lengai natrocarbonatite lava would have separated from a phonolitic rather than a nephelinitic magma. CO2-saturated nephelinites coexist with carbonate liquids much richer in CaO than the Lengai carbonatites, but even so these liquids have high alkali concentrations. If the sövites of hypabyssal and plutonic ijolite-carbonatite complexes originated by liquid immiscibility, then large quantities of alkalis have been lost, as is suggested by fenitization and related phenomena. The miscibility gap closes away from Na2O-rich compositions, so that the tendency to exsolve a carbonatite melt is greater in salic than in mafic silicate magmas. The two-liquid field does not approach kimberlitic compositions over the range of pressures studied, suggesting that the globular textures observed in many kimberlite sills and dykes may be the result of processes other than liquid immiscibility at crustal pressures. 相似文献
7.
Compositions of immiscible liquids in volcanic rocks 总被引：8，自引：5，他引：8
A. R. Philpotts 《Contributions to Mineralogy and Petrology》1982,80(3):201-218
Immiscible liquids, preserved as chemically distinct, glassy globules, (Si-rich and Fe-rich) occur in many tholeiitic basalts and some alkaline and calcalkaline lavas. The glasses typically form part of a dark mesostasis containing skeletal magnetite crystals. In thick flows, the Si-rich liquid may crystallize to granophyric patches, and the Ferich one to aggregates of hedenbergite, magnetite, and accessory phases. The mesostases containing these immiscible phases constitute from 20% of a primitive olivine tholeiite (MgO=7.5%) to 50% of a highly fractionated quartz tholeiite (MgO=2.8%), but may be less if the rock is oxidized. Abundant ferric iron promotes early crystallization of magnetite and prevents the iron enrichment necessary to reach the immiscibility field; thus, aa flows rarely exhibit immiscibility, whereas the more reduced pahoehoe ones do.Alumina and alkalis are concentrated in the Si-rich liquid, whereas the remainder of the major elements are concentrated in the Fe-rich melt; but the partitioning of Fe, Mg, Ca, and P is less pronounced in alkaline rocks than in tholeiites. Conjugate liquids have compositions of granite and Fe-rich pyroxenite, though the Si-rich melt in alkaline rocks is more syenitic and the Fe-rich one contains considerable normative alkali feldspar. The liquids coexist with plagioclase and augite of, respectively, An50 and Ca34Mg19Fe47 compositions in tholeiites, and An40 and Ca42Mg29Fe29 in alkaline rocks. Immiscibility is not restricted to K-rich residual liquids, but the miscibility gap is narrower for Na-rich compositions. In tholeiitic basalts with 52% SiO2, the Na2O/K2O ratios in conjugate liquids are equal, but at lower silica contents the Si-rich liquid is relatively more sodic, whereas at higher silica contents it is relatively more potassic. This may explain the association of sodic granites with mid ocean ridge basalts.Immiscible liquids are present in sufficient amounts in so many volcanic rocks that magma unmixing should be considered a viable means of differentiation during the late stages of fractionation of common magmas, at least at low pressures. 相似文献
8.
The variant rock types of an Alkaline-Carbonatite Complex (ACC) comprising alkali pyroxenite, nepheline syenite, phoscorite, carbonatite, syenitic fenite and glimmerite along with REE and Nb-mineralization are found at different centres along WNW-ESE trending South Purulia Shear Zone (SPSZ) in parts of Singhbhum Crustal Province. The ACC occurs as intrusions within the Mesoproterozoic Singhbhum Group of rocks. Alkali pyroxenite comprises of aegirine augite, magnesiotaramite, magnesiokatophorite as major constituents. Pyrochlore and eucolite are ubiquitous in nepheline syenite. Phoscorite contains fluorapatite, dahllite, collophane, magnetite, hematite, goethite, phlogopite, calcite, sphene, monazite, pyrochlore, chlorite and quartz. Coarse fluorapatite shows overgrowth of secondary apatite (dahllite). Secondary apatite is derived from primary fluorapatite by solution and reprecipitation. The primary fluorapatite released REE to crystallize monazite grains girdling around primary apatite. Carbonatite is composed dominantly of Srcalcite along with dolomite, tetraferriphlogopite, phlogopitic biotite, aegirine augite, richterite, fluorapatite, altered magnetite, sphene and monazite. The minerals comprising of the carbonatite indicate middle stage of carbonatite development. Fenite is mineralogically syenite. Glimmerite contains 50–60% tetraferriphlogopite. An alkali trend in the evolution of amphiboles (magnesiotaramite-magnesiokatophorite-richterite) and chinopyroxenes (aegirine augite, aegirine) during the crystallization of the suite of rocks is noted. Monazite is the source of REE in phoscorite and carbonatite. Fluorapatite has low contents of REE, PbO, ThO2 and UO2. Pyrochlore reflects Nb-mineralization in nepheline syenite and it is enriched in Na2O, CaO, TiO2, PbO and UO2. Pyrochlore containing UO2 (6.605%) and PbO (0.914%) in nepheline syenite has been chemically dated at 948 ± 24 Ma by EPMA. 相似文献
9.
Interstitial glass and pyroxene in a nepheline syenite described by Wolff (1987) have been more precisely and accurately re-analysed using a cryogenic microprobe technique. The glass compositions, with Na2O = 13.4−17.0 wt.% and (Na+K)/Al = 1.81−2.26 at SiO2 = 53−56.5 wt.% (on a 100% normalised basis) are more extreme than any known erupted magma. ZrO2 contents of the glasses are 1.4%–1.8% by weight, while F ranges between 1% and 1.4%. The high Zr content is due to the failure of a Zr-silicate phase to precipitate, despite predicted zircon saturation of the liquids above their probable quench temperatures. Partitioning of Zr between pyroxene and liquid is highly variable and apparently controlled by sector zoning; Zr is preferentially incorporated into aegirine {001} growth sectors. The glasses demonstrate the extent to which peralkaline phonolitic liquids can be fractionated in closed magma systems. 相似文献
10.
The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit. 相似文献
11.
Nature of alkalic volcanic rock series 总被引：15，自引：0，他引：15
Akiho Miyashiro 《Contributions to Mineralogy and Petrology》1978,66(1):91-104
The alkalic rocks are here regarded as a category in a classification of igneous rock series (rock associations) and not as a class in petrographic systematics. The alkalic series as a whole are characterized by higher Na2O+K2O content than the subalkalic series in the alkali vs. SiO2 diagram.At least three different trends (types) of differentiation appear to exist in large-scale alkalic volcanic associations. One (here designated as the Kennedy trend) starts from weakly nepheline-normative basalt and shows increasing normative nepheline with advancing fractionation to reach a phonolitic composition. Another (here called the Coombs trend) starts from hypersthene-normative basalt and shows increasing normative hypersthene and then normative quartz with advancing fractionation to reach a comenditic composition. Besides these two trends, it seems that many alkalic associations exist which show a differentiation trend starting from nepheline-normative basaltic composition and leading to hypersthenenormative, and then to quartz-normative compositions (here designated as the straddle-B type).Alkalic rocks of these three trends are higher not only in Na2O+K2O but also in Rb, Ba, Sr and Zr than subalkalic rocks. The alkalic basalts as a whole are characterized by higher contents of such elements and not by any degree of silica undersaturation. It is widely believed that alkalic rocks are characterized by the presence of normative nepheline as well as by the absence of orthopyroxene and pigeonite. Indeed such a relationship holds for the Kennedy trend, but it is not always valid for other types of alkalic associations. Some alkalic rocks of the Coombs trend and straddle-B type have quartz (or other silica minerals) and orthopyroxene and pigeonite. 相似文献
12.
D. J. Ellis 《Contributions to Mineralogy and Petrology》1976,58(2):149-180
High pressure pyroxene- and amphibole-rich inclusions are found in a number of Victorian Newer Volcanics volcanoes. The host lavas range from nepheline basanite to nepheline hawaiite and nepheline mugearite. The wide variation in chemistry and mineralogy of the inclusions is explained by crystallization from basaltic magmas under varying P-T and PH2O conditions at depth. At moderate pressure wehrlite inclusions (ol+cpx) form, whereas at higher pressures pyroxenites (opx+cpx) and genetically related megacrysts form. Under relatively anhydrous conditions the clinopyroxene megacrysts show a trend of Ca enrichment whereas under hydrous conditions, when amphibole is also stable, the pyroxene shows a trend to greater iron enrichment. The trend nepheline basanite to nepheline mugearite has developed by extensive fractionation of amphibole at elevated pressures under hydrous conditions. Under less hydrous conditions where clinopyroxene assumes the dominant role during crystal fractionation, derivative liquids display a trend of increasing K2O/Na2O ratio, with little modification of their level of undersaturation. Olivine plays a decreasing role in crystal fractionation processes with increasing pressure. The available evidence indicates that the only magma which could have been parental to the observed basanites was a more picritic basanite. 相似文献
13.
Fawzy F. Abu El-Ela 《Journal of African Earth Sciences》1997,24(4):473-496
The Wadi Dabr intrusive complex, west of Mersa-Alam, Eastern Desert, Egypt ranges in composition from gabbro to diorite, quartz diorite and tonalite. The gabbroic rocks include pyroxene-horn blend e gabbro, hornblende gabbro, quartz-hornblende gabbro, metagabbro and amphibolite. Mineral chemistry data for the gabbroic rocks indicate that the composition of clinopyroxenes ranges from diopside to augite and the corresponding magma is equivalent to a volcanic-arc basalt. Plagioclase cores range from An75 to An34 for the gabbroic varieties, except for the metagabbro which has An 11–18. The brown amphiboles are primary phases and classified as calcic amphiboles, which range from tschermakitic hornblende to magnesiohornblende. Green hornblende and actinolite are secondary phases. Hornblende barometry and hornblende-plagioclase themometry for the gabbroic rocks estimate crystallisation conditions of 2–5 kb and 885–716°C.The intrusive rocks cover an extensive silica range (47.86–72.54 wt%) and do not exhibit simple straight-line variation on Harker diagrams for many elements (e.g. TiO2, Al2O3, FeO*, MgP, CaO, P2O5, Cr, Ni, V, Sr, Zr and Y). Most of these elements exhibit two geochemical trends suggesting two magma sources.The gabbroic rocks are relatively enriched in large ion lithophile elements (K, Rb, Sr and Ba) and depleted in high field strength elements (Nb, Zr, Ti and Y) which suggest subduction-related magma. Rare earth element (REE) data demonstrate that the gabbroic rocks have a slight enrichment of light REE [(La/Yb)N=2.67−3.91] and depletion of heavy REE ((Tb/Yb)N=1.42−1.47], which suggest the parent magma was of relatively primitive mantle source.The diorites and tonalites are clearly calc-alkaline and have negative anomalies of Nb, Zr, and Y which also suggest subduction-related magma. They are related to continental trondhjemites in terms of Rb---Sr, K---Na---Ca, and to volcanic-arc granites in terms of Rb---and Nb---Y.The Wadi Dabr intrusive complex is analogous to intrusions emplaced in immature ensimatic island-arcs and represents a mixture of mantle (gabbroic rocks) and crustal fusion products (diorites and tonalites) modified by fractional processes. 相似文献
14.
One-atmosphere, melting experiments, controlled at the fayalite-magnetite-quartz oxygen buffer, on mildly alkalic and transitional basalts from Iceland show that these begin to crystallize Fe-Ti oxide minerals (magnetite and/or ilmenite) at 1105±5°C, apparently independently of bulk composition and the order of silicate and oxide mineral crystallization. Most samples crystalline plagioclase and olivine as the first two crystalline phases, augite as the third phase, and an Fe-Ti oxide mineral as the fourth phase. The main effects of Fe-Ti oxide crystallization are a marked decrease in FeO and TiO2 in the liquid, and a notable increase in SiO2 and Al2O3, and the minor oxides K2O and P2O5, with decreasing temperature. The most silicic glasses are compositionally mugearitic and shoshonitic basaltic andesites. Because the smallest amount of glass that could be analyzed with the microprobe represents 20–55 percent liquid remaining, it can be expected that more silicic liquids will occur at lower temperatures. On normative, pseudoternary projections, the general effect of Fe-Ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in normative quartz. This is caused by a strong systematic convergence, with the appearance of Fe-Ti oxides, of the bulk solid precipitates toward the liquid compositions, as projected on the triangle plagioclase-diopside-olivine. For alkalic basalts, the bulk solid precipitate shows an increase in normative diopside with falling temperature and Fe-Ti oxide crystallization. This causes the liquids to move toward decreasing normative diopside and relatively little variation in nepheline. The experimental observations imply that mildly alkalic and transitional magmas, without stabilizing a Fe-Ti oxide mineral, will not evolve toward early silica saturation. 相似文献
15.
Experimental study of clinopyroxenite partial melting and the origin of ultra-calcic melt inclusions 总被引：14，自引：0，他引：14
Ultra-calcic melt inclusions (UCMI: CaO>13.5 wt% and/or CaO/Al2O3>0.9) are magnesian and near-primary liquids trapped in volcanic phenocrysts from mid-ocean ridges, arcs, back-arcs, and ocean islands. UCMI can be subdivided into two classes based on tectonic association and degree of silica saturation: those from arcs are nepheline normative and those from all other localities (silicic UCMI) are hypersthene normative. Silicic UCMI share a number of common features, including primitive host minerals, low alkali contents, and variable ratios of K2O/TiO2 ranging to high values. Their compositions are not easily derived by partial melting of mantle lherzolite. Accordingly, we have performed a series of partial melting experiments on three clinopyroxenite compositions at 1.0 to 2.0 GPa to investigate the role of partial melting of clinopyroxene-rich lithologies in silicic UCMI genesis. Estimated solidus temperatures for all three compositions are similar to those of normal peridotites, but 1.0 GPa isobaric melt productivities are higher for clinopyroxenite than for peridotite. High degree partial melts of the clinopyroxenites are ultra-calcic and have similarities to silicic UCMI, but the experiments produce ultra-calcic liquids only at melt fractions greater than 30% and temperatures higher than 1,350 °C at 1.0 GPa. Such temperatures are higher than those likely to be prevailing beneath normal mid-ocean ridges, which suggests that some or all silicic UCMI may originate by a process other than simple partial melting of clinopyroxene-rich lithologies. We consider a possible role for partial melting of depleted harzburgite in the genesis of silicic UCMI. 相似文献
16.
Insights into the origin of primitive silica-undersaturated arc magmas of Aoba volcano (Vanuatu arc) 总被引：1，自引：1，他引：0
Fanny Sorbadere Pierre Schiano Nicole Métrich Esline Garaebiti 《Contributions to Mineralogy and Petrology》2011,162(5):995-1009
Aoba picrites in Vanuatu arc (Southwestern Pacific) offer the opportunity to address the question of the origin of Si-undersaturated
arc magmas, through the geochemical study of their olivine-hosted melt inclusions. These latter delineate a differentiation
trend of calc-alkaline silica-undersaturated basalts, with typical trace-element patterns of arc magmas. The most primitive
melt inclusions, preserved in olivines with Fo ≥ 89, have normative nepheline compositions with CaO/Al2O3 > 0.8, but belong to three distinct populations differing in their enrichment or depletion in LILE, Cl, and alkalis (Rb,
K). The dominant population is characterized by medium-LILE concentrations (La/Yb ~ 7–8) and represents the parental magma
of the Aoba lavas. The two others (La/Yb ~ 20 and 2) are either significantly enriched or extremely depleted in LILE, Cl,
and alkalis. This compositional variability of primitive magma batches requires the multi-stage mixing between melts generated
by partial melting of both peridotite and clinopyroxene-rich lithologies. Medium-LILE magma derives from the mixing between
peridotite- and clinopyroxenite-derived melts, whereas the high- and low-LILE melts involve amphibole-bearing and amphibole-free
clinopyroxenite sources, respectively. 相似文献
17.
Jean Klerkx Sarah Deutsch Paul De Paepe 《Contributions to Mineralogy and Petrology》1974,45(2):107-118
The Cape Verde islands are characterized by the presence of very strongly alkalic lavas. Cenozoic volcanics—covering the broadest compositional range present in the archipelago—and ranging from alkali-basalts to phonolites, associated with plutonic essexites and nepheline syenites, were analyzed for Sr isotopic compositions and concentrations in K, Rb and Sr. The close values of the Sr87/Sr86 ratios (ranging from 0.7029 to 0.7033) indicate a comagmatic origin for the different rock types; no correlation appears between the Sr isotopic composition and the K-content of the lavas, thus indicating that the lavas with high K2O/K2O + Na2O ratio are generated from a primary magma by differentiation at shallow depths. The values of the Sr isotopic composition are distinctly lower than most values obtained for lavas of other oceanic islands. The origin of the magma type is discussed on the basis of these isotopic compositions and the K/Rb and Rb/Sr ratios: it is suggested that the primary magma has a nephelinitic composition and was formed by partial melting of a small fraction of undepleted mantle peridotite, containing phlogopite; the deeper part of the mantle where this nephelinitic magma generates would have a strontium isotopic ratio of about 0.703 and a Rb/Sr ratio lower than that of the upper part. 相似文献
18.
T. H. Green 《Contributions to Mineralogy and Petrology》1977,65(1):59-67
Melting experiments on a model pelitic composition yield low-spessartine garnet as an important residual phase at pressures above 7 kb. The K
D
values for distribution of iron and magnesium between coexisting garnet and liquid in the pelitic composition are mainly sensitive to temperature, but also have a small pressure dependence. At temperatures above 950 ° C garnet has a higher
value than coexisting liquid, but below 950 ° C the garnet
value is lower than that of the coexisting liquid. Thus at temperatures below 950 ° C silicic magmas may fractionate garnet and produce more magnesian derivative liquids.Reconnaissance experiments with added MnO content in the model pelite demonstrate that spessartine-rich garnets are stable in silicic liquids to pressures as low as 3 kb. The MnO and CaO contents of the experimentally crystallized garnets show an antipathetic relation. Also, the grossular content of near-liquidus garnets crystallizing from a range of compositions increases with increasing pressure. The spessartine and grossular contents of most natural garnets in eastern Australian granitic rocks suggest that these garnets formed at pressures greater than 5 kb. Increased spessartine content allows crystallization of garnet in equilibrium with a silicic magma well within the pressure limit of stability of cordierite, provided the garnet contains 10 mol.% spessartine. Thus the depth range over which garnet and cordierite may coexist in a silicic melt is broadened, subject to the availability of MnO. The effect of increased Mn content on the low-pressure stability limit of garnet may also explain the lack of resorption of some garnets in granitic magmas, as these magmas rise to shallower levels. These euhedral garnets characteristically show zoning from an Mn-poor core (typically <4 % MnO) to an Mn-richer rim (typically >4 % MnO) and may reflect continued growth of the garnet in a low pressure regime, stabilized by Mn concentrated in the residual liquid fractions of the crystallizing magma. 相似文献
19.
Summary ¶The Campanian Ignimbrite rock samples include two compositionally distinct populations of clinopyroxene phenocrysts, and the entrapped MI (melt inclusions) are also different in composition. The cores of the more MgO-enriched phenocrysts carry basaltic trachyandesite MI that contain >6wt.% MgO, whereas other phenocrysts contain MI with <4wt.% MgO. The MgO-enriched MI also contain comparatively greater abundances of F, CaO, TiO2, P2O5, SO2, and Sr and show marginally higher ratios of (CaO/Al2O3) than the low-MgO MI. Most of the high-MgO MI also contain comparatively more H2O. The MgO-enriched MI are restricted to diopsidic clinopyroxenes and show minimal compositional variability, demonstrating that they were derived from a common magmatic source or sources. We interpret these MI to represent primary, mafic magma. In contrast, the more evolved, low-MgO melt inclusions, which are restricted to salitic clinopyroxenes, span the compositional range of trachyandesite to trachyte. The low-MgO fractions of Campanian Ignimbrite magma evolved via fractional crystallization with or without mingling or mixing with more primitive, high-MgO magma.Interestingly, the MI from the Giugliano sample also cluster into low-MgO and high-MgO fractions, and the evolutionary trends for major, minor, and trace elements mirror those exhibited by the Campanian Ignimbrite MI, suggesting that both magmas were derived from similar or the same source(s) and that the processes of magma evolution were equivalent for both magmas.The MI also indicate that the Campanian Ignimbrite and Giugliano magmas did not form by evolution of Taurano magma, because the geochemical trends expressing melt evolution of the former and latter magmas are too dissimilar. Most Taurano MI show higher (CaO/Al2O3) and contain less SiO2, (Na2O+K2O), Cl, Li, Rb, Cs, Sr, Nb, Th, and U than the high-MgO and low-MgO Campanian Ignimbrite and Giugliano MI, indicating that the Taurano MI represent magmas which were much more primitive.Received July 15, 2002; revised version accepted March 27, 2003 相似文献
20.
Analyses of coarse-grained refractory inclusions typically do not have the solar CaO/Al2O3 ratio, probably reflecting nonrepresentative sampling of them in the laboratory. Many previous studies, especially those done by instrumental neutron activation analysis (INAA), were based on very small amounts of material removed from those restricted portions of inclusions that happened to be exposed on surfaces of bulk meteorite samples. Here, we address the sampling problem by studying thin sections of large inclusions, and by analyzing much larger aliquots of powders of these inclusions by INAA than has typically been done in the past. These results do show convergence toward the solar CaO/Al2O3 ratio of 0.792. The bulk compositions of 15 coarse-grained inclusions determined by INAA of samples >2 mg have an average CaO/Al2O3 ratio of 0.80 ± 0.18. When bulk compositions are obtained by modal recombination based on analysis of thin sections with cross-sections of entire, large, unbroken inclusions, the average of 11 samples (0.79 ± 0.15) also matches the solar value. Among those analyzed by INAA and by modal recombination, there were no inclusions for which both techniques agreed on a CaO/Al2O3 ratio deviating by >∼15% from the solar value. These results suggest that: individual inclusions may have the solar CaO/Al2O3 ratio; departures from this value are due to sample heterogeneity and nonrepresentative sampling in the laboratory; and it is therefore valid to correct compositions to this value. We present a method for doing so by mathematical addition or subtraction of melilite, spinel, or pyroxene. This yields a set of multiple, usually slightly different, corrected compositions for each inclusion. The best estimate of the bulk composition of an inclusion is the average of these corrected compositions, which simultaneously accounts for errors in sampling of all major phases. Results show that Type B2 inclusions tend to be more SiO2-rich and have higher normative Anorthite/Gehlenite component ratios than Type B1s. The inclusion bulk compositions lie in a field that can result from evaporation at 1700-2000K of CMAS liquids with solar CaO/Al2O3, but with a wide range of initial MgO (30-60 wt%) and SiO2 (15-50 wt%) contents. 相似文献