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1.
The 168 m-thick Shiant Isles Main Sill is a composite body, dominated by an early, 24 m-thick, picrite sill formed by the intrusion of a highly olivine-phyric magma, and a later 135 m-thick intrusion of olivine-phyric magma that split the earlier picrite into a 22 m-thick lower part and a 2 m-thick upper part, forming the picrodolerite/crinanite unit (PCU). The high crystal load in the early picrite prevented effective settling of the olivine crystals, which retain their initial stratigraphic distribution. In contrast, the position of the most evolved rocks of the PCU at a level ~80% of its total height point to significant accumulation of crystals on the floor, as evident by the high olivine mode at the base of the PCU. Crystal accumulation on the PCU floor occurred in two stages. During the first, most of the crystal load settled to the floor to form a modally and size-sorted accumulation dominated by olivine, leaving only the very smallest olivine grains still in suspension. The second stage is recorded by the coarsening-upwards of individual olivine grains in the picrodolerite, and their amalgamation into clusters which become both larger and better sintered with increasing stratigraphic height. Large clusters of olivine are present at the roof, forming a foreshortened mirror image of the coarsening-upwards component of the floor accumulation. The coarsening-upwards sequence records the growth of olivine crystals while in suspension in a convecting magma, and their aggregation into clusters, followed by settling over a prolonged period (with limited trapping at the roof). As olivine was progressively lost from the convecting magma, crystal accumulation on the (contemporaneous) floor of the PCU was increasingly dominated by plagioclase, most likely forming clusters and aggregates with augite and olivine, both of which form large poikilitic grains in the crinanite. While the PCU is unusual in being underlain by an earlier, still hot, intrusion that would have enhanced any driving force for convection, we conclude from comparison with microstructures in other sills that convection is likely in tabular bodies >100 m thickness. 相似文献
2.
Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers
nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the
flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world.
In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India
has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which
occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III
of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically
corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized
by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine
and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo61). Plagioclase phenocrystal composition
ranges from An 51 to An 71 whereas the same variation of the groundmass plagioclase composition corresponds to An 31 to An
62. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH2O condition.
The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field,
however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned
pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are
found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO2 condition
in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst
them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental
magma. Trace element modeling indicates that equilibrium batch-melting of plume source followed by fractionation of olivine,
clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis
of the mafic sill and the associated basaltic flows. 相似文献
3.
Petrology of the Basistoppen Sill, East Greenland: A Calculated Magma Differentiation Trend 总被引：1，自引：3，他引：1
The 660 m thick Basistoppen sill is an Eocene, tholeiitic, layeredintrusion emplaced in the upper part of the Skaergaard complexshortly after solidification of the Skaergaard magma. Despiteits small size, the Basistoppen sill has one of the most extensivedifferentiation sequences known. The ranges of the solid solutionsin olivine, plagioclase, and pyroxene from the Basistoppen arecomparable to those in the Skaergaard and Bushveld intrusions.The rocks of the sill are orthocumulates composed of approximately35% trapped liquid and 65% cumulus minerals and can be dividedinto zones based on changes in the cumulus mineral assemblage.From the base upward those zones are: a Gabbro Picrite Zonecontaining cumulus olivine, Fe-Cr spinel, and minor biotite;a Bronzite Gabbro Zone containing cumulus orthopyroxene, Ca-richclinopyroxene, plagioclase, and minor Fe-Cr spinel; a PigeoniteGabbro Zone containing cumulus plagioclase, Ca-rich clinopyroxene,pigeonite, magnetite, and minor ilmenite; and a Fayalite DioriteZone containing cumulus plagioclase, Ca-rich clinopyroxene,magnetite, ilmenite, apatite, and olivine. The Basistoppen isoverlain by a zoned granophyre sill that was most likely derivedin part from the Basistoppen magma and in part from melted Precambriangneiss. The excellent exposure, uncomplicated structure, goodchilled margin, and lack of strong modal layering facilitatethe calculation of a differentiation trend for the Basistoppensill. During crystallization the Basistoppen magma became progressivelyricher in Fe, P, Na, K, Zn, Rb, Zr, La, Sm, and Th, became progressivelypoorer in Mg, Ca, Al, Cr, and Ni, and remained relatively unchangedin Si, Sc, and Sr through at least the first 90% of crystallization. 相似文献
4.
The compositional variation of chromite and associated olivine in chromite-rich and chromitepoor cumulus layers of the Panton Sill is described and a diffusion-controlled crystallization mechanism is proposed to explain this variation. By this mechanism, chromite initially precipitates with a fairly uniform composition, irrespective of the relative proportions of coprecipitating olivine and chromite, and is modified by continued growth during the postcumulus stage. The effect of postcumulus overgrowth of chromite, K
d
=(Mg/Fe2+)liquid/(Mg/Fe2+) chromite6, is to deplete the surrounding magma in chromium and decrease Fe2+ relative to Mg such that a chemical gradient exists between the overlying magma, through which the cumulus grains settled, and the magma in contact with settled chromite grains near the magma/crystal pile interface. Postcumulus equilibration of olivine and chromite with the surrounding magma results in higher Mg/(Mg + Fe2+) ratios of both olivine and chromite and higher Al content of chromite. The extent of this postcumulus modification is directly related to the proportion of chromite to olivine in a particular layer. This model can be extended to stratiform intrusions elsewhere in which chromite coprecipitates with olivine, orthopyroxene or plagioclase and displays similar compositional trends. 相似文献
5.
Basalts dredged from the south wall of a fracture zone transecting the southern Mid-Atlantic Ridge (SMAR) at 54° S are unusual in that they include a suite of highly olivine phyric basalts, sampled along with more normal sparsely plagioclase phyric basalts, and a highly plagioclase phyric basalt. Four basalt types (olivine phyric, sparsely plagioclase phyric, evolved sparsely plagioclase phyric and highly plagioclase phyric) are readily distinguished on the basis of petrography, mineralogy and bulk composition. They range from primitive to evolved, with the olivine phyric basalts having elevated MgO (up to 15.5%) and the plagioclase phyric basalt having elevated Al2O3 (19.3%) and CaO (13.1%) contents. Compositional variations are extremely consistant, with the olivine phyric basalts and the sparsely plagioclase phyric basalts defining coherent linear trends. On the basis of the ratios and covariation of the incompatible trace elements Zr, Nb, Y and Ba, distinct parental magmas for each basalt type are required. An investigation of Fe-Mg and Mg-Ni distribution coefficients between olivine and magma indicates that olivines from the olivine phyric basalts are on average too forsteritic and too Ni poor to have crystallized in a magma corresponding to the host bulk rock composition. This implies that these basalts are enriched in xenocrystic olivine. Olivines from the other basalt types are mostly of equilibrium composition, although there are some exceptions. Petrogenetic models for the formation of the different basalt types are quantitatively evaluated in terms of fractional crystallization/crystal accumulation processes. These indicate that (1) the olivine phyric basalts are the products of olivine and minor Cr-spinel accumulation and do not represent analogues of primary magma, or a liquid fractionation trend; (2) that the sparsely plagioclase phyric basalts were formed by polybaric fractional crystallization of olivine, plagioclase and clinopyroxene; and (3) that the evolved sparsely plagioclase phyric basalts are not readily related to one another. The single highly plagioclase phyric basalt is unrelated to the other basalt types and is cumulus enriched in plagioclase.The different basalt types are unrelated to one another and document the presence of at least four distinct magma types erupted in close proximity at this ridge/transform intersection on the southern end of the Mid-Atlantic Ridge. 相似文献
6.
The Marginal Border Series (MBS) of the Skaergaard intrusionconsists of rocks formed by in situ crystallization againstthe walls of the intrusion. Most of these rocks are productsof fractional crystallization, though samples believed to representchilled liquid occur locally at the intrusive contact. The MBScomprises only 5% of the exposed volume of the intrusion, butwithin its thickness, the order of crystallization and the compositionsof fractionated rocks and minerals vary systematically withdistance inward from the intrusive contact in largely the samemanner as rocks and minerals upward through the Layered Series(LS). Earliest differentiates are cumulates of olivine and plagioclase.The most basic compositions of cumulus plagioclase (An72) andolivine (Fo84) in these rocks indicate that the amount of fractionationpreceding formation of the exposed LS was substantially lessthat previously believed. Field and compositional data indicatethat picritic blocks are xenoliths rather than cumulates ofthe Skaergaard magma. Xenoliths of gneiss in all stages of reactionare locally abundant; however, there is no evidence that uppercrustal material contaminated the magma from which the MBS cumulatesformed. Compositions of cumulus minerals in the MBS differ fromthose in comparable LS rocks. Cumulates in the lower marginscontain more calcic plagioclase, more magnesian augite in allbut the late differentiates, and more iron-rich olivine. Thecompositions of cumulus olivine and to a lesser degree thoseof other mafic silicates, were modified to more iron-rich compositionsby re-equilibration with relatively large amounts of interstitialliquid. The lower MBS and LS crystallized from the same magma, but fractionationoccurred at different rates on the walls and floor of the intrusion.The upper margin may have crystallized from a magma of modifiedcomposition and fractionated at rates different from that inthe lower margin and Upper Border Series (UBS). Crystals onthe floor and roof of the intrusion accumulated faster or moreefficiently than on the walls. At any given stage of fractionation,crystals also accumulated against all sides of the magma chamberat about the same rate. Either the rates of cooling, crystallization,and crystal retention affected accumulation rates locally asfunctions of rock type and geometry of the walls, or these rateswere largely independent of wall rock owing to buffering ofconductive heat loss possibly to an envelope of hydrothermalfluid circulating around the crystallizing magma. The appearanceor disappearance of cumulus minerals in the lower MBS occursat higher structural levels than in the LS and at lower structurallevels than in the UBS. These relationships together with cumulusmineral compositions indicate that magma at the margins wasalways somewhat less fractionated than that at the floor androof of the chamber. It is proposed that these relationshipsreflect the combined effects of liquid and crystal fractionationof the magma within largely independent convection systems inthe lower and upper parts of the chamber. 相似文献
7.
Two Karroo dolerite sills display chemical and mineralogical variation compatible with cumulus enrichment. The Blaauwkrans sill is an olivine tholeiite and contains a central zone slightly enriched in olivine, plagioclase and clinopyroxene. The thicker Hangnest sill is a quartz tholeiite and shows evidence of crystal settling and has a lower zone enriched in cumulus orthopyroxene and plagioclase.The two sills differ quite markedly in their trace element compositions, with the Hangnest magma enriched by a factor of two in LIL elements (Rb, Ba, Nb, Zr, Y) relative to the Blaauwkrans magma. The Hangnest magma contained extremely low Ni contents (3–5 ppm), whereas the Blaauwkrans magma contained higher but more normal Ni (100–110ppm). Such contrasting trace element compositions preclude any simple genetic relationship between the two Karroo magmas but they may be related either through a common parent or are derivatives from separate parental magmas.South African Contribution No. 24 to the International Geodynamics Project 相似文献
8.
The Plagioclase-Magma Density Paradox Re-examined and the Crystallization of Proterozoic Anorthosites 总被引：4，自引：1，他引：3
Intermediate-composition plagioclase (An40–60) is typicallyless dense than the relatively evolved basaltic magmas fromwhich it crystallizes and the crystallization of plagioclaseproduces a dense residual liquid, thus plagioclase should havea tendency to float in these magmatic systems. There is, however,little direct evidence for plagioclase flotation cumulates eitherin layered intrusions or in Proterozoic anorthosite complexes.The layered series of the Poe Mountain anorthosite, southeastWyoming, contains numerous anorthosite–leucogabbro blocksthat constrain density relations during differentiation. Allblocks are more mafic than their hosting anorthositic cumulates,their plagioclase compositions are more calcic, and each blockis in strong Sr isotopic disequilibrium with its host cumulate.Associated structures—disrupted and deformed layering—indicatethat (1) a floor was present during crystallization and thatplagioclase was accumulating and/or crystallizing on the floor,(2) compositional layering and plagioclase lamination formeddirectly at the magma–crystal pile interface, and (3)the upper portions of the crystal pile contained significantamounts of interstitial melt. Liquid densities are calculatedfor proposed high-Al olivine gabbroic parental magmas and Fe-enrichedferrodioritic and monzodioritic residual magmas of the anorthositestaking into account pressure, oxygen fugacity, P2O5, estimatedvolatile contents, and variable temperatures of crystallization.For all reasonable conditions, calculated block densities aregreater than those of the associated melt. The liquid densities,however, are greater than those for An40–60 plagioclase,which cannot have settled to the floor. Plagioclase must eitherhave been carried to the floor in relatively dense packets ofcooled liquid plus crystals or have crystallized in situ. Asloping floor, possibly produced by diapiric ascent of relativelylight plagioclase-rich cumulates, is required to allow for drainingand removal of the dense interstitial liquid produced in thecrystal pile and may be a characteristic feature during thecrystallization of many Proterozoic anorthosites and layeredintrusions. KEY WORDS: magma; density; Proterozoic anorthosites; blocks; plagioclase 相似文献
9.
Don Elthon 《Geochimica et cosmochimica acta》1984,48(4):753-768
Certain petrological features of oceanic volcanic and plutonic rocks are not completely consistent with previously proposed models of crystal fractionation or magma mixing. For example, Sr is often higher in the differentiated basalts of a suite of aphyric rocks than in the relatively primitive basalts even though the differentiated basalts have apparently been produced by crystallization of large amounts of plagioclase with olivine and clinopyroxene. Additionally, oceanic basalts and gabbroic rocks often contain plagioclase crystals in excess of the appropriate cotectic proportions. Certain differentiated oceanic basaltic glasses and aphyric rocks crystallize plagioclase as the liquidus mineral, which would seem inconsistent with the strongly cotectic nature of the olivine + plagioclase + liquid surface.It is proposed here that plagioclase in mid-ocean ridge magma chambers separates from the basaltic liquid that it crystallizes in at a slower rate than does co-crystallizing olivine or pyroxene. Magma mixing in which a portion of the plagioclase remains suspended in the liquid during crystallization results in much more complex liquid lines of descent in mixed magmas and appears to resolve the apparent discrepancies noted above. 相似文献
10.
Stephen H. Richardson 《Geochimica et cosmochimica acta》1979,43(9):1433-1441
Chemical relations in an extensive Karroo dolerite sheet are consistent with large-scale phenocryst redistribution during intrusion of a tholeiitic magma. Flow differentiation provides a mechanism for inward migration of early olivine, plagioclase and augite, resulting in a total phenocryst population concentrated in and roughly symmetrically distributed about the centre of the sill. The inferred early phenocryst content of the magma varies by a factor of two from 23 wt% at the margins to 50 wt% in the centre of the sill. Chilled dolerite from the sill margins is the most phenocryst-poor material and is not representative of the bulk magma. Integration over height in the sill provides the best approximation to the bulk composition. No significant lateral variation in bulk sill chemistry is detected over a distance of 20 km. 相似文献
11.
火成岩中可以包含多种晶体群这一发现具有重要意义,使得成因矿物学重新成为揭示岩浆系统演化的基本指导思想。但是,这种重要性在许多文献中都没有得到反映,其典型实例就是镁铁质层状侵入体中堆晶岩的成因。争论在于堆晶矿物是循环晶还是母岩浆的液相线相。因此,本文致力于探讨四川攀西地区镁铁质层状侵入体中堆晶岩的形成过程,重申成因矿物学的重要意义。显微镜观察表明,堆晶单斜辉石富含Fe-Ti氧化物出溶叶片(含叶片辉石),表明其形成环境明显不同于与斜长石呈共结关系的单斜辉石(无叶片辉石);无叶片辉石和斜长石中的橄榄石包裹体呈浑圆状,表明了橄榄石与结晶环境间的热力学不平衡。橄榄石与熔体间Fe-Mg分配关系分析表明,根据母岩浆成分推测的橄榄石Fo值远低于岩体中观测橄榄石化学成分变化范围(Fo61-Fo81)的高限,表明至少部分橄榄石不是寄主侵入体的液相线相。橄榄石的Mg#值(100×Mg/(Mg+Fe))与微量元素(特别是Ni)的相关关系表明存在多种橄榄石晶体群,它们形成于不同的热力学环境中。晶体沉降过程分析表明,寄主岩浆析出的晶体几乎不可能发生快速重力沉降来形成堆晶岩。所有这些证据都表明,形成堆晶岩的矿物主要来自岩浆系统深部不同的岩浆房中,是被岩浆携带输运到终端岩浆房的循环晶。 相似文献
12.
Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications 总被引：3，自引：1，他引：3
R. Stephen J. Sparks Herbert E. Huppert 《Contributions to Mineralogy and Petrology》1984,85(3):300-309
The dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes. In this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization. Removal of olivine and pyroxene, whose values of fractionation density are larger than the density of the magma, causes the density of residual liquid to decrease. Removal of plagioclase, with fractionation density less than the magma density, can cause the density of residual liquid to increase. During the progressive differentiation of basaltic magma, density decreases during fractionation of olivine, olivine-pyroxene, and pyroxene assemblages. When plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum. Calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects.In the closed-system evolution of basaltic magma, several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization, as well as on the geometry of the chamber. In an early stage of the evolution, where olivine and/or pyroxenes are the fractionating phases, compositional stratification can occur due to side-wall crystallization and replenishment by new magma, with the most differentiated magma tending to accumulate at the roof of the chamber. When plagioclase becomes a fractionating phase a zone of well-mixed magma with a composition close to the density minimum of the system can form in the chamber. The growth of a zone of constant composition destroys the stratification in the chamber. A chamber of well-mixed magma is maintained while further differentiation occurs, unless the walls of the chamber slope inwards, in which case dense boundary layer flows can lead to stable stratification of cool, differentiated magma at the floor of the chamber.In a basaltic magma chamber replenished by primitive magma, the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma. Successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber. In a chamber containing well-mixed, plagioclase — saturated magma, the primitive magma can be either denser or lighter than the resident magma. In the first case, the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma. Mixing then occurs between magmas of different temperatures and compositions. In the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma. 相似文献
13.
<正>GRV 020175 is an Antarctic mesosiderite,containing about 43 vol%silicates and 57 vol% metal.Metal occurs in a variety of textures from irregular large masses,to veins penetrating silicates, and to matrix fine grains.The metallic portion contains kamacite,troilite and minor taenite.Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides.Silicate phases exhibit a porphyritic texture with pyroxene,plagioclase,minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass.The matrix is ophitic and consists mainly of pyroxene and plagioclase grains.Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth;others are characterized by many fine inclusions of plagioclase composition.Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims.Olivine occurs as subhedral crystals,surrounded by a necklace of tiny chromite grains(about 2-3μm).Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature(～1000℃) and a closure temperature(～875℃).Molar Fe/Mn ratios(19-32) of pyroxenes are consistent with mesosiderite pyroxenes(16-35) and most plagioclase compositions(An_(87.5_96.6)) are within the range of mesosiderite plagioclase grains(An_(88-95)).Olivine composition(Fo_(53.8)) is only slightly lower than the range of olivine compositions in mesosiderites(Fo_(55-90)).All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase,it can be further classified as a type 3A mesosiderite.Mineralogical, penological,and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion.Following primary igneous crystallization,the silicate underwent varying degrees of reheating.It was reheated to 1000℃,followed by rapid cooling to 875℃.Subsequently,metal mixed with silicate,during or after which,reduction of silicates occurred;the reducing agent is likely to have been sulfur.After redox reaction,the sample underwent thermal metamorphism,which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts,and homogenized matrix pyroxenes.Nevertheless,metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials. 相似文献
14.
新疆东天山黄山东岩体橄榄石成因意义探讨 总被引：12，自引：6，他引：6
黄山东岩体位于新疆东天山造山带中段,由四次岩浆侵入形成:第一次侵入形成了岩体上部的橄榄辉长岩、角闪辉长岩和闪长岩,构成岩体的主体;第二次侵入形成辉长苏长岩,呈岩墙状分布于岩体西端和西北部;第三次侵入岩石为斜长二辉橄榄岩,位于岩体下部,为主要的赋矿岩石;第四次侵入岩石为底部角闪辉长岩。橄榄石为第三次侵入的斜长二辉橄榄岩和第一次侵入的橄榄辉长岩主要造岩矿物之一,橄榄石的镁橄榄石牌号(Fo)值介于68.5～82.5之间。其中含硫化物斜长二辉橄榄岩中的橄榄石具有较高的Fo值(79.7～82.5);斜长二辉橄榄岩中橄榄石的Fo值为78.3～79.9;而基性程度较低的橄榄辉长岩中橄榄石具有较低的Fo值(68.5～72.2)。利用橄榄石矿物成分计算得出黄山东岩体母岩浆Mg#(Mg2+/(Mg2++Fe2+))为0.59,为原生玄武质岩浆经历结晶分异作用形成。模拟计算结果显示黄山东岩体不含矿岩石中橄榄石是母岩浆经过2%的橄榄石结晶分异且硫达到饱和后,在硫化物熔离的同时岩浆发生橄榄石结晶而形成,并且橄榄石︰硫化物≈50︰1,部分橄榄石成分投点在橄榄石结晶和硫化物熔离的模拟曲线右下侧,指示它们可能受到晶间硅酸盐熔浆作用的影响。含硫化物斜长二辉橄榄岩中Fo值与Ni含量呈负相关关系,说明橄榄石与硫化物熔体之间发生了Fe-Ni交换反应。 相似文献
15.
Adcumulate formation in mafic layered intrusions is attributed either to gravity-driven compaction, which expels the intercumulus
melt out of the crystal matrix, or to compositional convection, which maintains the intercumulus liquid at a constant composition
through liquid exchange with the main magma body. These processes are length-scale and time-scale dependent, and application
of experimentally derived theoretical formulations to magma chambers is not straightforward. New data from the Sept Iles layered
intrusion are presented and constrain the relative efficiency of these processes during solidification of the mafic crystal
mush. Troctolites with meso- to ortho-cumulate texture are stratigraphically followed by Fe–Ti oxide-bearing gabbros with
adcumulate texture. Calculations of intercumulus liquid fractions based on whole-rock P, Zr, V and Cr contents and detailed
plagioclase compositional profiles show that both compaction and compositional convection operate, but their efficiency changes
with liquid differentiation. Before saturation of Fe–Ti oxides in the intercumulus liquid, convection is not active due to
the stable liquid density distribution within the crystal mush. At this stage, compaction and minor intercumulus liquid crystallization
reduce the porosity to 30%. The velocity of liquid expulsion is then too slow compared with the rate of crystal accumulation.
Compositional convection starts at Fe–Ti oxide-saturation in the pore melt due to its decreasing density. This process occurs
together with crystallization of the intercumulus melt until the residual porosity is less than 10%. Compositional convection
is evidenced by external plagioclase rims buffered at An61 owing to continuous exchange between the intercumulus melt and the main liquid body. The change from a channel flow regime
that dominates in troctolites to a porous flow regime in gabbros results from the increasing efficiency of compaction with
differentiation due to higher density contrast between the cumulus crystal matrix and the equilibrium melts and to the bottom-up
decreasing rate of crystal accumulation in the magma chamber. 相似文献
16.
Anton P. le Roex A. J. Erlank H. D. Needham 《Contributions to Mineralogy and Petrology》1981,77(1):24-37
Bulk rock major and trace element variations in selected basalts from the Famous area, in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. For a given Mg/Mg+Fe2+ atomic ratio of the mineral, the NiO content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts. The Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and, in addition, distinguishes the picritic basalts from the other basalt types. These differences indicate that the different magma groups could not have been processed through the same magma chamber, and preclude any direct inter-relationship via open or closed system fractional crystallization.The Fe-Mg partitioning between olivine and host rock suggests that the picritic basalts represent olivine (±Cr-spinel) enriched magmas, derived from a less MgO rich parental magma. The partitioning of Fe and Mg between olivine, Cr-spinel and coexisting liquid is used to predict a primary magma composition parental to the picritic basalts. This magma is characterized by relatively high MgO (12.3%) and CaO (12.6%) and low FeO* (7.96%) and TiO2 (0.63%).Least squares calculations indicate that the plagioclase phyric basalts are related to the plagioclase-pyroxene basalts by plagioclase and minor clinopyroxene and olivine accumulation. The compositional variations within the olivine basalts can be accounted for by fractionation of plagioclase, clinopyroxene and olivine in an open system, steady state, magma chamber in the average proportions 453223. It is suggested that the most primitive olivine basalts can be derived from a pristine mantle composition by approximately 17% equilibrium partial melting. Although distinguished by its higher Zr/Nb ratio and lower NiO content of phenocryst phases, the magma parental to the picritic basalts can be derived from a similar source composition by approximately 27% equilibrium partial melting. It is suggested that the parental magma to the plagioclase-pyroxene and plagioclase phyric basalts might have been derived from greater depth resulting in the fractionation of the Zr/Nb ratio by equilibration with residual garnet.C.O.B. Contribution No. 722 相似文献
17.
Christopher I. Chalokwu Norman K. Grant Alexei A. Ariskin Galina S. Barmina 《Contributions to Mineralogy and Petrology》1993,114(4):539-549
In order to describe the composition and crystallinity of the initial (parental) magma of the Partridge River intrusion of the Keweenawan Duluth Complex, and thereby understand the mode of emplacement and solidification of the intrusion, we have applied a numerical simulation technique called geochemical thermometry (Frenkel et al. 1988). The parental magma was a low-alumina, high-Ti-P olivine tholeiite similar to typical Keweenawan low-alumina, high-Ti-P basalts associated with the Duluth Complex and from the nearby Portage Lake area of the Lake Superior region. The parental magma was emplaced as a crystal-liquid suspension, followed by chilling of an evolved, leading edge ferrodioritic liquid in the basal zone of the intrusion. The conditions of emplacement at the present crustal location were 1,150°C, 2 kbar, and f
O
2 slightly above the wustite-magnetite (WM) buffer. The main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine. Calculated crystallization sequence for the parental magma is olivine+plagioclase (1,240°C)olivine+plagioclase+magnetite (1,146°C, WM+0.5)olivine+plagioclase+magnetite+augite (1,140°C, WM+0.5). The calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1,150°C are Fo66.7±1.1 and An64.5±2.5, respectively, and are similar to the estimated average composition of primary olivine (Fo69.1±2.8) and the average composition of plagioclase core (An66.3±2.8) measured in drill core samples through the intrusion (Chalokwu and Grant 1987). 相似文献
18.
Investigations in the Stillwater Complex: Part II. Petrology and Petrogenesis of the Ultramafic Series 总被引：5，自引：1，他引：5
The Ultramafic series of the Stillwater Complex has been dividedinto two major zones: a Peridotite zone formed of 20 macro-rhythmicunits of dunite-harzburgite-orthopyroxenite, and an overlyingOrthopyroxenite zone. The stratigraphic section has been determinedat Mountain View (2065 m) and at Chrome Mountain (840 m). TheMountain View section apparently formed in a subsiding basinwhereas the rocks at Chrome Mountain accumulated in a relativelystable, higher area of the chamber floor. In both sections,Mg/(Mg + Fe) in cumulus mafic minerals increases with stratigraphicheight in the lower 400 m, then remains relatively constantthrough the rest of the series. The base of the series is marked by the first appearance oflaterally extensive olivine-rich cumulates. The accretion ofthe cumulates and the growth of the chamber proceeded throughperiodic injections of olivine-saturated mafic magma. The lowercontact of the cycles represents a hiatus in crystallizationand a return to a more primitive magma composition. Althoughhotter, the primitive magma was more dense, so it entered thechamber at or near the floor and did not immediately mix withthe more differentiated orthopyroxene-saturated magma alreadypresent. As it cooled by transfer of heat across its upper surface,the primitive magma crystallized olivine and differentiatedin situ to form the lower dunite. With the accumulation of olivinenear the base, the crystal/liquid ratio, and thus the density,decreased at the top of the layer eventually resulting in mixingand the formation of harzburgite. After removal of olivine byresorption and settling from the hybrid magma, orthopyroxenealone crystallized forming an orthopyroxenite. Chromitite layersprobably formed by the mixing of primitive olivine± chromite-saturatedmagma and narrow layers of orthopyroxene-saturated magma trappedunderneath. The Mg-enrichment trend in the lower 400 m resulted from reactionof cumulus olivine and/or orthopyroxene with progressively decreasingvolumes of intercumulus liquid. As heat loss through the floordecreased, accumulation rate approached a steady state, thefraction of trapped liquid remained more or less constant andvariation in Mg/(Mg + Fe) was governed dominantly by cumulusprocesses. The constant NiO abundances in olivine throughoutthe section are consistent with the model for the formationof the macro-rhythmic units. Depletion of NiO was dampened byrepeated additions of parental magma, localized equilibriumcrystallization, mixing, and the effect of postcumulus equi-librationwith varied amounts of trapped liquid. Discordant dunite bodies, which are common at Chrome Mountain,formed by the replacement by olivine of earlier formed cumulates.The replacement involved the incongruent dissolution of ortho-pyroxeneat near-solidus temperatures by a late-stage, hydrous vaporprobably derived from the magma. The vapor phase migrated alongfractures formed by the readjustment of the cumulate pile. 相似文献
19.
Reconciling the diverse records of magmatic events preserved by multiple crystals and minerals in the same sample is often challenging. In the case of basaltic–andesites from Volcán Llaima (Chile), Mg zoning in olivine is always simpler than Ca zoning in plagioclase. A model that explains a number of chemical patterns is that Llaima magmas stall in the upper crust, where they undergo decompression crystallization and form crystal-mush bodies. Frequent magma inputs from deeper reservoirs provide the potential for remobilization and eruption. The records of multiple recharge events in Llaima plagioclase versus an apparent maximum of one such event in coexisting olivine are addressed by using trace element zoning in olivine phenocrysts. We have integrated elements that (1) respond to changes in magma composition due to recharge or mixing (Mg, Fe, Ni, Mn, ±Ca), with (2) elements that are incorporated during rapid, disequilibrium crystal growth (P, Ti, Sc, V, Al). A more complex history is obtained when these elements are evaluated considering their partition coefficients, diffusivities, and crystal growth rates. The olivine archive can then be reconciled with the plagioclase archive of magma reservoir processes. Olivine (and plagioclase) phenocrysts may experience up to three or more recharge events between nucleation and eruption. Diffusion modeling of major and trace element zoning in two dimensions using a new lattice Boltzmann model suggests that recharge events occur on the order of months to a couple of years prior to eruption, whereas crystal residence times are more likely to be on the order of a few years to decades. 相似文献
20.
Detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India. The picrite basalts show bulk compositional variation generated by the fractionation of olivine and chromite. Evolved picrite basalt magma appears to have given rise to basalt by the fractionation of olivine+clinopyroxene, despite the presence of abundant plagioclase phenocrysts. It is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature. The high degree of equilibrium crystallisation which many of the lavas have apparently undergone is interpreted in terms of the mechanism of compensated crystal settling (Cox and Bell, 1972). Experimentally determined atmospheric pressure phase relations are used to model dyke-like magma chambers in some detail. Finally volumetric and age relationships are used to argue that the picrite basalts, despite their porphyritic nature, crystallised from ultramafic liquids containing in some cases at least 16% MgO. 相似文献