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1.
Melting experiments have been performed on a primitive, mildly alkalic glassy lava (10 wt.% MgO) from the 1965 eruption of the Surtsey volcano located at the tip of the south-eastern propagating rift zone of Iceland. At atmospheric pressure, approximately on the FMQ oxygen buffer, olivine (Fo81) crystallizes from 1240°C, followed by plagioclase (An70) from 1180°C and augite from 1140°C. The experimental glasses coexisting with olivine, plagioclase and augite are ferrobasaltic enriched in FeO (13.6–14.2 wt.%) and TiO2 (4.0–4.4 wt.%). In high pressure, piston-cylinder, graphite-controlled runs, olivine occurs as the liquidus phase until 14 kbar, above which augite is the liquidus phase. Low-Ca pyroxene is not a liquidus phase at any pressure. The high pressure liquids are, relative to the one atmosphere liquids, significantly enriched in Al2O3 and Na2O and depleted in CaO as a result of changes in the crystallizing assemblages. Furthermore, liquidus augite is dominantly subcalcic and shows significant enrichment in Al and depletion in Ti. Subliquidus plagioclase is enriched in sodium relative to low pressure phase compositions. Evaluated in normative projections, contrasting liquid lines of descent are revealed as a function of pressure. At one atmosphere, the multisaturated liquids are located close to the thermal divide defined by the plane olivine-plagioclase-augite, but appear, with advanced degrees of crystallization, to be moving away from the thermal divide toward normative quartz. The augites crystallizing in the one atmosphere experiments are calcic and slightly nepheline normative. In the 10 and 12.5 kbar experiments, the augites become subcalcic and dominantly hypersthene normative. Because of this shift in augite compositions, transitional basaltic liquids may at high pressure evolve from the tholeiitic side of the olivine-plagioclase-diopside normative divide onto the alkalic side. With increasing pressure above 15 kbar, the liquidus augite compositions move back toward the olivine-plagioclase-diopside normative divide. 相似文献
2.
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
3.
The south-eastern propagating rift zone of Iceland shows a progression from tholeiitic, to transitional, and mildly alkalic basalts going toward the front of the propagator. A petrogenetic model has been formulated based on low and high pressure melting experiments. The evolution of the magmas behind the front of the propagating rift is dominated by near surface processes (e.g., Katla and Hekla volcanic systems). Compared with the one atmosphere liquid line of descent, the Vestmannaeyjar lavas, erupted at the front of the propagating rift, have systematically higher Al2O3 and Na2O contents consistent with the experimentally determined effects of high pressure, Labradoritic plagioclase megacrysts, which occur in the early phase of the Surtsey eruption at the front of the propagator, are consistent with the predicted effect of high pressure on plagioclase composition. On the other hand, augites similar to those of the high pressure experiments are unknown among the phenocryst and xenolith assemblages of the Vestmannaeyjar lavas (and other Icelandic lavas). Petrographic evidence points toward a high water content in the evolved lavas of the Vestmannaeyjar. A relatively high water activity and high pressure in the magma chambers at the front of the propagator could have caused a significant suppression of the liquids temperature, in particular for plagioclase. Seismic and magnetotelluric evidence suggest that magma chambers behind the propagating front occur at the depth equivalent to 2–3 kbar of pressure. At the front, magma chambers have been located by geophysical evidence at significantly greater depths equivalent to 3–8 kbar. The lavas erupted at the front of the propagator are located to the alkalic side of the thermal divide and, therefore, can be expected to evolve toward nepheline saturation under slightly hydrous conditions. The most evolved of these lavas are of ferrobasaltic compositions and may be saturated with augite. The lavas erupted behind the front are located to the transitional side of the divide and evolve toward quartz saturation under essentially anhydrous conditions. In contrast to the Vestmannaeyjar lavas, the lavas behind the propagator often contain augite as a phenocryst. It is concluded that the chemical variation observed along the south-eastern propagating rift of Iceland is an effect of primary chemical features and that water plays a significant role on the liquid lines of descent at the front of the propagator. 相似文献
4.
Feldspar phenocrysts, microphenocrysts, groundmass feldspar, interstitial material of feldspar composition, and residual SiO2-K2O-rich glass in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively with the electron microprobe. Rocks studied include tholeiite, olivine tholeiite, oceanite, alkalic olivine basalt, alkalic basalt, hawaiite, mugearite, trachyte, basanite, and basanitoid. Results and conclusions: i) In all rocks studied, An decreases and Or increases from phenocrysts to microphenocrysts to groundmass feldspar to interstitial material of feldspar composition. ii) Phenocrysts occur in rocks of the tholeiitic and alkalic suites and, in spite of differences in bulk rock compositions, overlap in composition. iii) Groundmass feldspar in rocks of the tholeiitic suite are nearly identical in composition; the same is true for rocks of the nephelinic suite. However, in the highly differentiated alkalic suite, groundmass feldspar composition ranges from labradorite to sanidine; i.e. the higher the bulk rock CaO, the higher is the An content, and the higher the bulk K2O, the higher is the Or content. iv) In general, rocks with phenocrysts have groundmass feldspar less An-rich than those without phenocrysts. v) In rocks of the tholeiitic suite, normative feldspar approaches modal feldspar. However, in rocks of the alkalic and nephelinic suites, normative feldspar, because of the presence of highly alkalic interstitial material and the absence of nepheline in the mode but its presence in the norm, is drastically different from modal feldspar. vi) Hawaiites contain labradorite and not andesine, as per definition, and mugearite contains andesine and not oligoclase, as groundmass feldspar. In fact, when considering phenocrysts and interstitial material of feldspar composition, hawaiites range from bytownite to sanidine and mugearite from andesine to sodic sanidine, but normative feldspar plots in the andesine field for hawaiites and the oligoclase field for mugearite. vii) Rocks of the three suites can be distinguished on the basis of Or and An in groundmass feldspar, the presence of thin rims of groundmass composition of phenocrysts of rocks of the alkalic suite, and the presence of interstitial material of anorthoclase to sanidine composition in rocks of the alkalic and nephelinic suites. iix) Rocks transitional between the tholeiitic and alkalic suites are observed and are characterized by transitional mineral compositions.This paper was first presented as a talk before the 68. Annual Meeting of the Cordilleran Section of the Geological Society of America, Honolulu, Hawaii, March 29–April 1, 1972. 相似文献
5.
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
6.
Ian Ridley 《Contributions to Mineralogy and Petrology》1971,32(4):251-266
On Rhum, Eigg, Canna and Muck Tertiary volcanics rest upon a Mesozoic or Pre-Mesozoic basement. Aphyric, olivine-phyric, and plagioclase-phyric basalts are recognized. The aphyric basalts are mildly alkaline or transitional types with either a few percent normative nepheline or normative hypersthene. They have anomalously low concentrations of Rb, Sr and K2O compared to Tertiary tholeiites from the same province.Aphyric hawaiites, and mugearties are found on all the islands, but are particularly abundant on Rhum. The volcanics from Bloodstone Hill, Rhum, originally described as mugearites are anomalous in that they are quartz normative and contain both augite and hypersthene, in contrast to the normal one-pyroxene rocks of the alkali basalt-trachyte association (Muir and Tilley, 1961). These volcanics have closer affinities to the icelandites, the presence of basic plagioclase xenocrysts suggesting an hybrid origin.Olivine and plagioclase are involved in the low-pressure fractionation of the transitional basalts, whereas pyroxene and titanomagnetie play only minor roles. Consequently, the suppression of titanomagnetite crystallization results in an initial trend towards iron enrichment. The presence of both oversaturated and undersaturated derivitives following the hawaiite stage of differentiation, reflects variation in the amount of extracted pyroxene and titanomagnetite.Felsites and pitchstones intrude the volcanic pile on Eigg. The felsites carry corroded quartz crystals and rare alkali feldspar. The more crystal rich pitchstones generally contain augite, hypersthene, zoned plagioclase and titanomagnetite. One from Rudh an Tancaird contains alkali feldspar, titanomagnetite and ferrohedenbergite.Whole rock analyses and microprobe analyses of feldspars and pyroxenes indicate that the acid volcanics are not genetically related to the basalt-hawaiite-mugearite lineage. The felsites appear to have been derived from Torridonian arkose by partial melting, but the pitchstones could only be derived by anatexis of Lewisian gneiss basement (see Dunham, 1968) substantially more basic than that outcropping on Rhum.It is suggested that the low concentrations of Rb, Sr, and K2O in the alkaline and transitional basalts, mitigates against extensive pre-eruptive differentiation. Possibly the basalts could have been derived by partial melting of a mantle depleted in these elements. 相似文献
7.
The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite and augite of phenocryst in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser. It is shown that there is a compositional gap of about 25 mole per cent CaSiO3 between groundmass pigeonite and augite, compared with 35 per cent CaSiO3 between phenocrystic augite and bronzite. Subcalcic augite or pigeonitic augite was not found. The groundmass augite, which occurs only as thin rims of pigeonite and hypersthene, is less calcic and more iron-rich than the phenocryst augite. It is also shown that the groundmass pigeonite is 3–4 mole per cent more CaSiO3-rich than the coexisting groundmass hypersthene. The Fe/(Mg + Fe) ratios of these coexisting hypersthene and pigeonite are about 0.31 and 0.33, respectively. It is suggested from these results that a continuous solid solution does not exist between augite and pigeonite of the Fe/(Mg + Fe) ratio at least near 0.3 under the conditions of crystallization of groundmass of the tholeiitic andesite. It is suggested from the Mg-Fe partition and the textural relation that the groundmass augite crystallized from a liquid more iron-rich than that from which groundmass hypersthene and pigeonite crystallized. 相似文献
8.
本文研究的峨眉山玄武岩的辉石均为单斜辉石。东川碱性火山岩中以透辉石为主,显示碱性岩系辉石的特征,但相对贫 LREE、Ti 等不相容元素,结晶时的 fO2较高;攀枝花岩带主要是透辉石质普通辉石;其他剖面和地区多为普通辉石,且具拉斑质岩系富钙辉石的成分和演化特点。攀枝花带和西岩区辉石在化学成分上比较原始,而东岩区和中岩区辉石的成分演化程度相对较高。含 Ti 的“其他”阳离子对在各岩区（带）的辉石中均限重要,暗示它们的形成与火山弧环境无关。 相似文献
9.
The Mineralogy and Petrology of Mount Suswa, Kenya 总被引：1，自引：0，他引：1
Mount Suswa, a Quaternary volcano in the Rift Valley of Kenya,is composed of sodalite-trachytes, sodalite-phonolites, andphonolites, the majority of which are mildly peralkaline. Thelavas are predominantly feldspathic with phenocrysts of alkalifeldspar and, less commonly, slightly sodic augite and fayaliticolivine. The groundmass typically contains alkali feldspar,augite, titanomagnetite, and may in addition contain sodalite,nepheline, alkali amphibole, aenigmatite, and glass. The lavasof four stratigraphically distinct episodes can be distinguishedon the basis of mineralogy and chemical composition. These lavasare the products of at least three parental magmas, none ofwhich appears to be a derivative of the other. Each ‘magmatype’ represents an independent episode of magma generation,emplacement, and eruption. The order of eruption in the finalepisode corresponds to increasing peralkalinity and undersaturationwith respect to silica, and indicates that these lavas weregenerated via the tapping of a differentiating magma, with thefirst lavas being the least differentiated. Utilizing coexistingfeldspar, residual glass, and bulk rock compositions, the derivationof peralkaline phonolitic residual liquids from a trachyticparent is shown to be a process controlled by feldspar fractionation. 相似文献
10.
A Model of Magmatic Crystallization 总被引：2，自引：0，他引：2
A computer model simulating fractional crystallization at oneatmosphere pressure incorporates nine broadly-defined minerals—magnetite,olivine, hypersthene, augite, quartz, plagioclase, orthoclase,leucite, and nepheline. The crystallization temperature of eachmineral is considered to be a smooth function of the compositionof the magmatic liquid. These mineral temperature equationsare obtained by multiple linear regression analysis of informationfrom published silicate systems and rock melting experiments.The nine equations are solved for any primary liquid, withinthe broad range of common magma types, to select the crystallizingmineral or minerals. Partition ratios from published experimentsand analyses of lavas and phenocrysts permit calculation ofthe composition of the crystallizing mineral assemblage. Subtractionof a small amount of that composition from the primary liquidyields a new liquid, which may be recycled to yield a sequenceof liquids during fractional crystallization. The crystallizationmodel handles assemblages of co-precipitating minerals, andcan trace progressive saturation in new minerals, substitutionof a new mineral for an old mineral, and cessation of crystallizationof a mineral. The sequences of minerals and liquids derivedfrom a broad set of primary liquids are geologically realistic,so the model is useful in predicting phenocrysts in volcanicrocks and events during crystallization of shallow intrusions. 相似文献
11.
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
12.
The paper deals with the petrography, mineralogy, chemistry,and genesis of selected calcalkaline volcanics from the Carboniferousof New South Wales. The investigated rocks, mainly vitrophyrictypes, range in composition from hypersthene basalt throughhypersthene-augite dacite and ignimbritic hypersthene-hornblendedacite to ignimbrific biotite rhyodacite. Chemical and opticaldata are provided for the principal phenocryst phases whichinclude hypersthene, augite, iron-titanium oxides (cation-deficientspinels and ilmenite), hornblende, biotite, plagioclase, and,more rarely, alkali feldspar. The available data do not favourthe derivation of the dacites from basaltic magma by low pressurefractional crystallization, sialic contamination, or mixingof magmas. The origin of these rocks, particularly the hypersthene-augitedacites, has been referred to the Green-Ring wood model of drypartial melting of high-alumina quartz ecologite in the uppermantle. The chemistry of whole rock groundmass pairs indicatesthat some of the more salic eruptives can be interpreted aslow pressure differentiates of dacitic parental melts. A characteristicfeature of the more mafic volcanics is their pronounced modaland normative excess of hypersthene over augite. The early andextensive precipitation of orthopyroxene from strongtly hypersthenenormativemafic and intermediate liquids relatively low in Fe and Mg playedan important role in maintaining the calc-alkaline characterof derivative liquids. 相似文献
13.
A phase diagram for mid-ocean ridge basalts: Preliminary results and implications for petrogenesis 总被引：3，自引：0，他引：3
Edward Stolper 《Contributions to Mineralogy and Petrology》1980,74(1):13-27
Samples of a primitive mid-ocean ridge basalt (MORB) glass were encapsulated in a mixture of ol (Fo90) and opx (En90) and melted at 10, 15, and 20 kbar. After quenching, the basaltic glass was present as a pool within the ol+opx capsule, but its composition had changed so that it was saturated with ol and opx at the conditions of the experiment. By analyzing the quenched liquid, the location of the ol+opx cotectic in the complex, multicomponent system relevant to MORB genesis was determined.As pressure increases from 1 atm to 10 kbar, the dry ol+opx cotectic moves from quartz tholeiitic to olivine tholeiitic compositions. With further increases in pressure, the cotectic continues to move toward the ol-di-plag join (i.e., toward alkalic compositions). Between 15 and 20 kbar, ol+opx+di-saturated liquids change from tholeiitic to alkalic in character, although part of the ol+opx cotectic is still in the tholeiitic (i.e, hy-normative) part of composition space. At pressures of 10–15 kbar, tholeiitic liquids may be able to fractionate to alkalic liquids on the ol+di cotectic.Primitive MORB compositions come close to but do not actually lie on the ol+opx cotectic under any conditions studied. This suggests that not even the most primitive of known MORBs are primary melts of the mantle. The correspondence of most MORBs to the 1 atm ol+di+plag cotectic suggests that low pressure fractionation was involved in their genesis from parent liquids. Picritic liquids that have been proposed as parents to the MORB suite could equilibrate with harzburgite (or Iherzolite) at 15–20 kbar and thus could be primary. Fractionation of ol from these liquids could yield primitive MORB liquids, but other primary liquids or more complex fractionation paths involving others phases in addition to ol cannot be ruled out. The possibility that these picritic liquids could equilibrate with ol+opx at 25–30 kbar cannot be ruled out. 相似文献
14.
The MD dyke swarm is composed of four generations of large basictholeiite dykes which cut the entire Archaean craton of southernWest Greenland. The four successive generations (MD1, MD2, MD3a,MD3b) are characterized by their orientation and cross-cuttingrelationships and by their mineralogy, texture and progressivelyevolved tholeiitic chemistry. Rare-earth element (REE) abundancessuggest that the dykes may have a fairly complex petrogeneticevolution. The suite varies from early (MD1) heteradcumulatenorites to ophitic and sub-ophitic gabbroic and doleritic rocks(MD2 and MD3) and the youngest generation (MD3b) comprises plagioclase-phyricdolerites. The pyroxene chemistry parallels the geochemical evolution ofthe dykes showing an overall Fe-enrichment trend. However, theclinopyroxenes are enigmatic in that, although they occur predominantlyas part of medium and coarse-grained holocrystalline textures,they are chemically highly variable and calcium-poor, many plottingin the metastable field in the system MgSiO3 (En)-CaSiO3 (Wo)-FeSiO3(Fs). Many individual grains are extremely complex and may beregularly or irregularly zoned. Along with more typical pyroxene forms, the MD1 dykes containpyroxene dendrites poikilitically enclosed by plagioclase. Thedendrites vary compositionally from hypersthene bases to branchesof pigeonite and subcalcic augite and terminate in augite branchtips. The MD2 and MD3a dyke pyroxenes are the most complex.The majority of them are sub-ophitic grains, many with successivezones of orthopyroxene, pigeonite, subcalcic augite, augiteand ferroaugite. However, Ca-enrichment or Ca-depletion, Fe-enrichmentor Fe-depletion and apparently opposing zoning trends can occurin neighbouring grains. Even small interstitial pyroxenes showa very wide range of compositions. Morphologically unusual andcomplex clinopyroxene ‘cylinders’ occur in someof the MD3a dykes. They are chemically relatively uniform andare normal tholeiitic augites. The MD3b rocks have small concentricallyzoned sub-ophitic pyroxenes which show Ca-enrichment with arelatively constant Fs component (29 to 39 mol. per cent). Themost extremely zoned grains have hypersthene cores with successivecoronas of pigeonite and subcalcic augite and have margins ofaugite or ferroaugite. The present ‘coexistence’of such compositionally widely variable pyroxenes and the extremeand often irregular nature of their chemical zoning make thedetermination of true original coexisting pyroxene phases andthe use of a two pyroxene geothermometer very difficult andof limited significance. The presence of a wide variety of pyroxenes of apparently bothstable and metastable compositions in these holocrystallinedykes suggests that these rocks have undergone a complex andrather unusual cooling history. The principal genetic factorswhich could have influenced their crystallization are (1) supercooling,(2) the evolution of discrete interstitial liquid cells, (3)augite-pigeonite peritectic reactions and (4) plagioclase growthand delay of pyroxene nucleation during supercooling of liquidto below the basalt liquidus. 相似文献
15.
H. G. Huckenholz 《Contributions to Mineralogy and Petrology》1973,40(4):315-326
Fassaitic augite (augite 3) occurs in clinopyroxenite fragments with cumulus textures or as anhedral crystals in alkali basalts and nepheline basanites of the Hocheifel Area. Rimming of augite 3 by phenocrystic augite (augite 2) followed by groundmass augite (augite 4) defines the sequence of the clinopyroxene crystallization. Fassaitic augites from other alkali-basalt series reveal clinopyroxene crystallization trends of increasing ferri Tschermak's molecule and concomitant acmite as fractionation proceeds. This trend appears to be much more common than previously assumed.Dedicated to K. Jasmund in honor of his sixtieth birthday. 相似文献
16.
The Mio-Pliocene Shirahama Group, Izu Peninsula, Central Japan,a well-exposed submarine volcanic arc complex of lava flows,pyroclastic rocks and associated shallow intrusives, is characterizedby a tholeiitic series (basalt to dacite) and a calc-alkalineseries (andesite to dacite). Chemical variations in the tholeiiticseries and calc-alkaline series are consistent with crystalfractionation from basalt and magnesian andesite (boninite),respectively. Crystal–liquid phase relations of thesemagmas have been investigated by study of sample suites fromthese two series. Compositions of liquids in equilibrium withphenocrysts were determined by microprobe grid analyses, inwhich 49 points were averaged in 03 mm 03 mm groundmassareas. The liquid compositions, coupled with the phenocrystmineralogy of the same samples, define the liquid lines of descentof these volcanic arc magmas. Major findings include the following:(1) Crystallization of the tholeiitic series magma is consistentwith early stage crystallization in the simple system Fo–Di–Silica–H2O,with olivine having a reaction relation to augite and the tholeiiticliquid. (2) The later stage products of the tholeiitic seriesmagma are, however, crystal-poor (<10%) dacites with no maficminerals, suggesting that tholeiitic liquids, hypersthene andaugite were no longer on the cotectic (3) A characteristic ofthe calc-alkaline series magmas is the development of rhyoliticliquids. Hypersthene, augite, plagioclase and Fe–Ti oxideoccur in most calc-alkaline rocks studied, and hornblende andquartz can be found in about half of these. However, their differentiationpaths show that the cotectic relation between quartz and liquidended at a later stage, resulting in the resorption of quartzphenocrysts and ultimately in the formation of quartz-free magmas.(4) The late-stage liquids of both the tholeiitic and calc-alkalineseries have deviated from their cotectics, which cannot be explainedby fractional crystallization alone. The addition of H2O froman outside system is probably required to explain the differentiationpaths. (5) The formation of chilled margins, the in situ crystallizationof a magma chamber in the solidification zone, and/or the migrationof groundwater into the magma chamber are thought to be likelyprocesses affecting magmas during their migration and intrusioninto the crust. An extreme effect of H2O addition would be tolower the liquidus temperatures of all precipitating silicatephases far below their restorable range before eruption, resultingin the production of aphyric magmas. Even when a temperaturedecrease in the magma chamber causes a liquid to intersect theliquidus of a pre-existing phase, the addition of H2O shiftsthe cotectic toward SiO2, resulting in quartz being the lastphase to crystallize. The resorption of quartz is interpretedto be the result of a liquidus boundary shift caused by theaddition of H2O. The genesis of aphyric rhyolites is thereforeinferred to result from fractional crystallization followingaddition of H20. KEY WORDS: Shirahama Group; Japan; island arc; rhyolite; magma series 相似文献
17.
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
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Formation of shoshonites from calcalkaline basalt magmas: geochemical and experimental constraints from the type locality 总被引：4，自引：2，他引：4
James K. Meen 《Contributions to Mineralogy and Petrology》1987,97(3):333-351
Independence volcano, Montana is a major center of the Absaroka volcanic field, from which absarokite, shoshonite, and banakite were originally defined. One magmatic trend at Independence volcano, from high-alumina tholeiitic basalt through shoshonite to high-K dacite, may be modeled by fractional crystallization of observed phenocryst phases (plagioclase, hypersthene, augite, and magnetite). Trace-element and Sr and Nd isotopic compositions of rocks are consistent with this model.Compositions of partial melts from experiments on four rocks at 1 atm and at 10 kbar demonstrate that rock compositions represent a nearly-anhydrous liquid line of descent at a pressure much closer to 10 kbar than to 1 atm. The line of descent involves crystallization of orthopyroxene, not olivine, resulting in strong enrichment in K2O with little increase in SiO2. Crystallization at either lower pressures or with water present, involving olivine, results in enrichment in both SiO2 and K2O.High-pressure (10 kbar) fractional crystallization of basaltic magma, resulting in formation of shoshonites, may occur at the base of thick crust (e.g., in continental interiors or in very mature arcs). At least a portion of the relationship between K2O content of arc-related magmas and depth to the Benioff Zone may be attributed to thickening of crust towards the back-arc, resulting in higher pressures of fractionation in Moho-level chambers. 相似文献
20.
Experimental melting studies were conducted on a nepheline mugearitecomposition to pressures of 31 kbar in the presence of 0–30%added water. A temperature maximum in the near-liquidus stabilityof amphibole (with olivine) was found for a water content of3·5 wt % at a pressure of 14 kbar. This is interpretedto have petrogenetic significance for the derivation of nephelinemugearite magmas from nepheline hawaiite by amphibole-dominatedfractional crystallization at depth within the lithosphericmantle. Synthetic liquids at progressively lower temperaturesrange to nepheline benmoreite compositions very similar to thoseof natural xenolith-bearing high-pressure lavas elsewhere, andsupport the hypothesis that continued fractional crystallizationcould lead to high-pressure phonolite liquids. Independent experimentaldata for a basanite composition modeled on a lava from the sameigneous province (the Newer Basalts of Victoria) permit theinference that primary asthenospheric basanite magmas undergopolybaric fractional crystallization during ascent, and mayevolve to liquids ranging from nepheline hawaiite to phonoliteupon encountering cooler lithospheric mantle at depths of 42–50km. Such a model is consistent with the presence in some evolvedalkalic lavas of both lithospheric peridotite xenoliths indicativeof similar depths and of megacryst suites that probably representdisrupted pegmatitic segregations precipitated from precursoralkalic magmas in conduit systems within lithospheric mantle. KEY WORDS: experiment; high pressure; alkalic magmas; amphibole; nepheline mugearite; basanite; lithosphere 相似文献