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1.
The Ardnamurchan net-veined complex consists of three rock types: a porphyritic mafic rock, an aphyric intermediate rock and a silicic rock. Pillows of mafic and intermediate rock are included in the silicic rock and contain crenulated and some chilled margins. Liquid-liquid relationships are inferred for these three magmas. The trace element data, using ratio-ratio plots, are consistent with magma mixing being the dominant process and are inconsistent with any process that is dominated by crystal fractionation or melting. The major element data, using multiple linear regression techniques, are consistent with magma mixing of high-silica silicic magma and primitive mafic magma, along with about 35 percent crystal fractionation to form the intermediate rock type. All of the data taken together are consistent with a magma mixing model with some fractionation to produce the variation observed. The simplest model is that a fractionating basaltic magma comes into contact with a silicic magma and limited mixing produces the intermediate magma.This net-veined complex may be the only evidence available for interaction of mafic and silicic melts that occurred in the Ardnamurchan high-level magma chamber before the silicic magma was lost to eruptive and surface processes. In general the chemical and field relationships are consistent with Smith's model for the evolution of high-level, magma chambers.  相似文献   

2.
浙东早白垩世火山岩组合的地球化学及其成因研究   总被引：25，自引：6，他引：25  
浙东早白垩世含中性岩的火山岩组合和双峰式火山岩组合的主量元素和微量元素特征对比研究表明，中性火山岩的地球化学特征明显受到伴生的基性和酸性火山岩的影响。它们的微量元素协变关系证实：中性火山岩是由基性岩浆和酸性岩浆混合形成的。双峰式火山岩具有与含中性岩的火山岩组合相类似的主量元素特征，但两者的微量元素特征相差较大。根据东南沿海在早白垩世时期的构造背景是由挤压向伸展拉张转变，以及各种元素具有不同的扩散速度，提出了浙东早白垩世火山岩组合的形成模式：若基性岩浆和酸性岩浆仅进行了部分微量元素的交换，没有来得及发生主量元素的交换，就在伸展拉张的构造背景下喷出地表，则形成双峰式火山岩组合；若基性岩浆与酸性岩浆在地壳深处共存的时间较长，发生一定程度的主量元素交换，则形成偏基性和偏酸性的中性岩浆，若进一步发生化学成分的交换，则可形成典型的安山质岩浆，喷出地表就形成含中性岩的火山岩组合。  相似文献   

3.
Magma mixing is common in the Earth. Understanding the dynamics of the mixing process is necessary for dealing with the likely consequences of mixing events in the petrogenesis of igneous rocks and the physics of volcanic eruptive triggers. Here, a new apparatus has been developed in order to perform chaotic mixing experiments in systems of melts with high viscosity contrast. The apparatus consists of an outer and an inner cylinder, which can be independently rotated at finite strains to generate chaotic streamlines. The two cylinder axes are offset. Experiments have been performed for ca. 2 h, at 1,400°C under laminar fluid dynamic conditions (Re ~ 10−7). Two end-member silicate melt compositions were synthesized: (1) a peralkaline haplogranite and (2) a haplobasalt. The viscosity ratio between these two melts was of the order of 103. Optical analysis of post-experimental samples reveals a complex pattern of mingled filaments forming a scale-invariant (i.e. fractal) distribution down to the μm-scale, as commonly observed in natural samples. This is due to the development in space and time of stretching and folding of the two melts. Chemical analysis shows strong non-linear correlations in inter-elemental plots. The original end-member compositions have nearly entirely disappeared from the filaments. The generation of thin layers of widely compositionally contrasting interfaces strongly enhances chemical diffusion producing a remarkable modulation of compositional fields over a short-length scale. Notably, diffusive fractionation generates highly heterogeneous pockets of melt, in which depletion or enrichment of chemical elements occur, depending on their potential to spread via chemical diffusion within the magma mixing system. Results presented in this work offer new insights into the complexity of processes expected to be operating during magma mixing and may have important petrological implications. In particular: (1) it is shown that, in contrast with current thinking, rheologically contrasting magmas can mix (i.e. with large proportions of felsic magmas and high viscosity ratios), thus extending significantly the spectrum of geological conditions under which magma mixing processes can occur efficiently; (2) the mixing process cannot be modeled using the classical linear two-end-member mixing model; and (3) the chemical compositions on short-length scales represent snapshots within the process of mixing and therefore may not reflect the final composition of the magmatic system. This study implies that microanalysis on short-length scales may provide misleading information on the parental composition of magmas.  相似文献   

4.
Several lines of evidence suggest that the melt generation and segregation regions of the mantle are heterogeneous, consisting of chemically and lithologically distinct domains of variable size and dimension. Partial melting of such heterogeneous mantle source regions gives rise to a diverse range of basaltic magmas. In order to better assess the role of source heterogeneity during mantle melting, we have undertaken a theoretical study of trace element distribution and fractionation during concurrent melting and melt migration in an upwelling, chemically heterogeneous, two-porosity double lithology melting column. Analytical solutions for the abundance of a trace element in the matrix and channel were obtained under the assumptions that the porosity, melt and solid velocities, and solid-melt partition coefficients are constant and uniform. For simplicity, we neglected diffusion and dispersion in the melt. Chemical source heterogeneities of arbitrary size and shape were integrated into the simple melting models by allowing trace element abundance in the source region to vary as a function of time and space. Concurrent melting and melt migration in an upwelling heterogeneous mantle may be approximated as a quasi-steady state problem in which time-dependent concentration patterns produced by melting of heterogeneous source regions are superimposed on a reference steady-state concentration distribution established by melting of the ambient or background mantle. Chromatographic fractionation is especially important for the matrix melt and solid when chemical heterogeneities are involved during melting and melt migration in the mantle, giving rise to significant phase-shift between two incompatible trace elements in the matrix melt and scattered correlations among incompatible trace elements in residual peridotites. Mixing is the chief mass transfer process in the dunite channel where the chromatographic effect is negligible for most of the incompatible trace elements. The lack of chromatographic fractionation among incompatible trace elements and isotopic ratios in MORB suggests either most MORB are channel melts or mixing in magma conduit and chamber is very efficient such that the phase-shift is averaged out during magma transport and storage processes. Advection brings melt produced by smaller-degree of melting in the deeper part of the melting column to the overlying melting region, increasing the incompatible trace element abundance in the matrix and the channel. This advection-induced self-enrichment is especially important when heterogeneous sources are involved and may account for some of the enriched incompatible trace element patterns observed in residual peridotite that were previously interpreted to be a result of mantle metasomatism. Systematic studies of high-resolution spatially correlated mantle samples may help to constrain the melting history and the size and nature of chemical heterogeneities in the mantle.  相似文献   

5.
对主要岩浆作用过程中痕量元素协变方程和实例的分析表明,正确选择元素做图是应用协交图形判别岩浆作用类型的前提,并非任意元素-元素、比值-元素和比值-比值协变图形都能有效判别岩浆作用过程。文中给出了不同形式的协交图形中元素的选择标准,指出强相容元素与强不相容元素的元素-元素协变图形对判别岩浆分离结晶作用和部分熔融作用更为有效和实用,同时也是判别岩浆同化混染作用的重要图解。  相似文献   

6.
Lavas and pyroclastic rocks throughout the volcanic stratigraphy of the Tertiary-Quaternary volcanic complex of Thera in the Aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures, interpreted as indicative of magma mixing. Plagioclase zoning characteristics studied by Nomarski and laser interferometry techniques establish three main categories of plagioclase: (i) inherited plagioclase (nucleated in endmember prior to initial mixing event) (ii) in situ plagioclase (nucleated in mixed or hybrid magma) and (iii) xenocrystic plagioclase. Nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases, depending on the composition of the lava in which they are hosted. These differences reflect the contrasting effects of changes in physical-chemical parameters in basic vis-a-vis more acidic melts during magma mixing and/or influx of new magma into the subvolcanic magma chamber, as well as the influence of magma chamber dynamics on plagioclase equilibration. Variations in bulk major and trace element abundances of Thera volcanic products reflect the dominant overprint of crystal fractionation, but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes, consistent with compositional and textural zoning in plagioclases.  相似文献   

7.
青城子矿集区印支期岩浆作用形成岩基状双顶沟岩体及岩脉状石英二长斑岩等,双顶沟岩体根据其岩相学特征可划分为主体相和中心相两个岩相带,主体相内含有大量暗色微粒包体,并常见矿物不平衡结构,显示岩浆混合的特征。在岩石地球化学方面,包体与寄主岩石的主要氧化物之间具有良好的线性关系,寄主岩石和包体的稀土元素配分曲线和微量元素蛛网图形态相似,指示寄主岩石与包体在岩石形成过程中发生过成分交换及均一化,也显示岩浆混合特征。双顶沟岩体主体相具有高Ba-Sr花岗岩特征,可能为加厚下地壳部分熔融形成的熔体与富集地幔岩浆混合作用的产物,中心相则为主体相经过长石、角闪石、黑云母等的分离结晶作用而形成。石英二长斑岩脉可能为双顶沟岩体演化的浅成相,两者具有相同的源区和成因,矿集区内铅锌、金银矿床的形成可能与岩浆混合作用演化形成的石英二长斑岩相关。  相似文献   

8.
桂东北里松花岗岩中暗色包体的岩浆混合成因   总被引：25，自引：0，他引：25  
在桂东北姑婆山地区的里松花岗岩中,暗色包体广泛分布,包体的形貌、结构构造和矿物学特征表明,它们是岩浆快速冷凝结晶产物;主元素和微量元素组成说明它们属钾玄岩系列,其源岩具OIB型微量元素特征;包体与寄主花岗岩中锆石U-Pb年龄的相同性,排除了来源于深部固体岩石熔融残留体或浅部围岩捕虏体的可能性,而两种岩石化学成分、岩石结构暗色包体和寄主花岗岩在岩石结构和全岩Sr-Nd同位素组成方面的明显差别性,又排除了同源包体或析离体、堆积体的可能性。里松花岗岩在很多地质-地球化学特征上都介于里松暗色包体和姑婆山主体花岗岩之间,里松暗色包体的总体特征显示了岩浆混合成因,是里松暗色包体岩浆与姑婆山主体花岗岩岩浆发生混合时不完全混合的残留物。  相似文献   

9.
The Cenozoic volcanic rock of Shandong Province are mainly alkalic and strongly alkalic basaltic rocks.The Contents of major and trace elements including transitional,incompatible and rare-earth elements were determined.The chemical characterisitics of major and trace elements indicate that these basaltic rocks were derived from a mantle source and probably represent a primary magma,I,e.,unmodifiecd partical melts of mantle peridotite in terms of Mg values,correlatione between P2O5 and Ce,Sr,Ni and Rb concentrations,mantle xenoliths,etc.The abundances of trace elements vary systematically from west to east.The compatible transition elements such as Co,Ni,and Cr show a remarkable depletion,whereas the incompatible and rare-earth elements are abundant as viewed from the chondrite-nor-malized patterns.The chemical composition and correlation are consistent with the tectonic setting.According to the batch and fractional partial melting theory,the trace element contents of Shandong volcanic rocks can be calculated from the two-component mixing model.  相似文献   

10.
Süphan is a 4,050 m high Pleistocene-age stratovolcano in eastern Anatolia, Turkey, with eruptive products consisting of transitional calc-alkaline to mildly alkaline basalts through trachyandesites and trachytes to rhyolites. We investigate the relative contributions of fractional crystallization and magma mixing to compositional diversity at Süphan using a combination of petrology, geothermometry, and melt inclusion analysis. Although major element chemistry shows near-continuous variation from basalt to rhyolite, mineral chemistry and textures indicate that magma mixing played an important role. Intermediate magmas show a wide range of pyroxene, olivine, and plagioclase compositions that are intermediate between those of basalts and rhyolites. Mineral thermometry of the same rocks yields a range of temperatures bracketed by rhyolite (~750°C) and basalt (~1,100°C). The linear chemical trends shown for most major and trace elements are attributed to mixing processes, rather than to liquid lines of descent from a basaltic parent. In contrast, glassy melt inclusions, hosted by a wide range of phenocryst types, display curved trends for most major elements, suggestive of fractional crystallization. Comparison of these trends to experimental data from basalts and trachyandesites of similar composition to those at Süphan indicates that melt inclusions approximate true liquid lines of descent from a common hydrous parent at pressures of ~500 MPa. Thus, the erupted magmas are cogenetic, but were generated at depths below the shallow, pre-eruptive magma storage region. We infer that chemical differentiation of a mantle-derived basalt occurred in the mid- to lower crust beneath Süphan. A variety of more and less evolved melts with ≥55 wt% SiO2 then ascended to shallow level where they interacted. The presence of glomerocrysts in many lavas suggests that cogenetic plutonic rocks were implicated in the interaction process. Blending of diverse, but cogenetic, minerals, and melts served to obscure the true liquid lines of descent in bulk rocks. The fact that chemical variation in melt inclusions preserves deep-seated chemical differentiation indicates that inclusions were trapped in phenocrysts prior to shallow-level blending. Groundmass glasses evolved after mixing and display trends that are distinct from those of melt inclusions.  相似文献   

11.
锆石是地质学研究中最重要的副矿物,其分布广泛、物理、化学性质稳定,记录了结晶时的年龄、温度、氧逸度以及O-Hf-Si-Zr-Li等多元同位素和微量元素信息,被广泛运用于地球科学的研究中。近年来,随着分析技术的发展,研究者在获取锆石年龄的同时也获取了大量锆石微量元素数据,这些数据的积累推动着研究者对锆石微量元素理论研究的不断深入,并取得了一系列重要进展,如发现锆石微量元素组成受锆石本身的晶格特点主导,符合晶格应变模型和类质同象替代机制;发现锆石微量元素组成受到熔体成分演化影响,锆石结晶时的熔体微量元素组成往往不等同于全岩;发现锆石内部的微量元素不均一特征(矿物包裹体、热点、蜕晶化作用等)可能会严重影响锆石的微量元素组成,继而建立了"干净锆石"的判别指标和筛选机制。此外,锆石微量元素的应用研究也取得了长足进展,研究者们不断尝试通过各类锆石微量元素指标、图解、分配系数,识别母岩浆物理化学性质、反演母岩浆组成,大大推动了锆石微量元素在示踪岩浆源区和岩浆过程中的应用。然而,由于锆石微量元素组成受控于多种因素,使得锆石微量元素在实际应用当中常常面临着多解性问题、重叠问题和分配系数的选择问题,在一定程度上影响了锆石微量元素应用研究的可靠性。未来的锆石微量元素研究将不满足于使用传统的低维指标和图解以及分配系数,而将在充分吸收传统方法精华的基础上,从海量数据与更高的维度中寻找元素之间相关性,基于热力学定律揭示新原理,基于更高空间分辨率揭示动力学因素的影响,从数据驱动和理论驱动的全新视角下深入揭示隐藏在锆石微量元素中的信息。  相似文献   

12.
相容元素——不相容元素协变图在岩石成因研究中的意义   总被引：7，自引：1，他引：6  
笔者根据前人提出的各种岩浆作用模式,如分离结晶作用、部分熔融作用、岩浆混合作用等,通过纯数学推导,提出一套判别岩浆岩形成机制的相容元素—不相容元素协变图。 从几何图形的讨论,指出相容元素i（Di〉2）和不相容元素j的CTi—CTi和IogCTi—logCTi图解上可以区分不同成因的岩石,包括分离结晶作用、平衡结晶作用、实比和非实比分离熔融作用,实比和非实比分批熔融作用以及岩浆混合作用。不同成因岩石在协变图上其成分点的分布有明显差别,这在解释山西临县紫金山碱性环状杂岩体的成因中获得满意的效果,从而确认该者体是多次分批部分熔融作用的产物。  相似文献   

13.
Most rhyolite eruption episodes of Tarawera volcano have emitted several physiochemically distinct magma batches (∼1–10 km3). These episodes were separated on a millennial timescale. The magma batches were relatively homogeneous in temperature and composition at pumice scale (>4 cm), but experienced isolated crystallisation histories. At the sub-cm scale, matrix glasses have trace element compositions (Sr, Ba, Rb) that vary by factors up to 2.5, indicating incomplete mixing of separate melts. Some quartz-hosted melt inclusions are depleted in compatible trace elements (Sr, Ti, Ba) compared to enclosing matrix glasses. This could reflect re-melting of felsic crystals deeper in the crystal pile. Individual quartz crystals display a variety of cathodoluminescence brightness and Ti zoning patterns including rapid changes in melt chemistry and/or temperature (∼50–100°C), and point to multi-cycle crystallisation histories. The Tarawera magma system consisted of a crystal-rich mass containing waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

14.
The Pb isotopic composition of rocks is widely used to constrain the sources and mobility of melts and hydrothermal fluids in the Earth's crust. In many cases, the Pb isotopic composition appears to represent mixing of multiple Pb reservoirs. However, the nature, scale and mechanisms responsible for isotopic mixing are not well known. Additionally, the trace element composition of sulphide minerals are routinely used in ore deposit research, mineral exploration and environmental studies, though little is known about element mobility in sulphides during metamorphism and deformation. To investigate the mechanisms of trace element mobility in a deformed Witwatersrand pyrite(FeS_2), we have combined electron backscatter diffraction(EBSD) and atom probe microscopy(APM). The results indicate that the pyrite microstructural features record widely different Pb isotopic compositions, covering the entire range of previously published sulphide Pb compositions from the Witwatersrand basin. We show that entangled dislocations record enhanced Pb, Sb, Ni, Tl and Cu composition likely due to entrapment and short-circuit diffusion in dislocation cores. These dislocations preserve the Pb isotopic composition of the pyrite at the time of growth(~3 Ga) and show that dislocation intersections, likely to be common in deforming minerals, limit trace element mobility. In contrast, Pb, As, Ni, Co, Sb and Bi decorate a highangle grain boundary which formed soon after crystallisation by sub-grain rotation recrystallization.Pb isotopic composition within this boundary indicates the addition of externally-derived Pb and trace elements during greenschist metamorphism at ~2 Ga. Our results show that discrete Pb reservoirs are nanometric in scale, and illustrate that grain boundaries may remain open systems for trace element mobility over 1 billion years after their formation.  相似文献   

15.
Clinopyroxene megacrysts from young melanephelinitic lavas were divided into Cr-rich and Cr-poor suites. Sr, Nd, and Pb isotopic ratios of leached megacrysts and host lava are indistinguishable from each other and indicate a depleted source. Host lavas do not display chemical evidence for significant fractional crystallization, which is required to explain the compositional range of the megacrysts. This rules out a simple cognate genetic relationship between the two, and strictly defines megacrysts as xenocrysts. The well-defined correlations of trace elements with the Mg-numbers in the megacrysts are interpreted as the result of extensive fractional/equilibrium crystallization of magma over a large temperature range at near isobaric condition in the upper mantle. Trace element variations in megacrysts are consistent with fractional crystallization of clinopyroxene alone for the Cr-rich suite, and clinopyroxene + garnet for the Cr-poor suite from at least two bathes of related melts. Megacrysts parent magma might represent mantle melts, which were never erupted in their initial composition.  相似文献   

16.
The concentrations and ratios of the major elements determine the physical properties and the phase equilibria behavior of peridotites and basalts in response to the changing energy contents of the systems. The behavior of the trace elements and isotopic features are influenced in their turn by the phase equilibria, by the physical character of the partial melting and partial crystallization processes, and by the way in which a magma interacts with its wall rocks. Concentrating on the trace element and isotope contents of basalts to the exclusion of the field relations, petrology, major element data, and phase equilibria is as improvident as slaughtering the buffalo for the sake of its tongue. The crust is a cool boundary layer and a density filter, which impedes the upward transfer of hot, dense “primary” picritic and komatiitic liquids. Planetary crusts are sites of large-scale contamination and extensive partial crystallization of primitive melts striving to escape to the surface. Escape of truly unmodified primitive melts to the surface is a rare event, requiring the resolution of daunting problems in chemical and mechanical engineering. Primary status for volumetrically abundant basalts such as mid-ocean ridge basalt, ocean island basalt, and continental flood basalts is denied by their low-pressure cotectic character, first remarked upon on petrological grounds in 1928 and on experimental grounds in 1962. These basalt liquids are products of crystal-liquid separation at low pressure. Primary status for these common basalts is further denied by the phase equilibria of such compositions at elevated pressures, when the required residual mantle mineralogy (magnesian olivine and orthopyroxene) is not stable at the liquidus. It is also denied by the picritic or komatiitic nature of partial melts of candidate upper-mantle compositions at high pressures—a conclusion supported by calculation of the melt composition, which would need to be extracted in order to explain the chemical variation between fertile and residual peridotite in natural ultramafic rock suites. The subtleties of magma chamber partial crystallization processes can produce an astounding array of “pseudospidergrams,” a small selection of which have been explored here. Major modification of the trace element geochemistry and trace element ratios, even those of the highly incompatible elements, must always be entertained whenever the evidence suggests the possibility of partial crystallization. At one extreme, periodically recharged, periodically tapped magma chambers might undergo partial crystallization by ∼95% consolidation of a succession of small packets of the magma. Refluxing of the 5% residual melts from such a process into the main body of melt would lead to eventual discrimination between highly incompatible elements in that residual liquid comparable with that otherwise achieved by 0.1 to 0.3% liquid extraction in equilibrium partial melting. Great caution needs to be exercised in attempting the reconstruction of more primitive compositions by addition of troctolite, gabbro, and olivine to apparently primitive lava compositions. Special attention is focussed on the phase equilibria involving olivine, plagioclase (i.e., troctolite), and liquid because a high proportion of erupted basalts carry these two phases as phenocrysts, yet the equilibria are restricted to crustal pressures and are only encountered by wide ranges of basaltic compositions at pressures less than 0.5 GPa. The mere presence of plagioclase phenocrysts may be sufficient to disqualify candidate primitive magmas. Determination of the actual contributions of crustal processes to petrogenesis requires a return to detailed field, experimental, and forensic petrologic studies of individual erupted basalt flows; of a multitude of cumulate gabbros and their contacts; and of upper-mantle outcrops.  相似文献   

17.
The Kangâmiut dike swarm in West Greenland contains numerous composite dikes with mafic margins and andesitic centers. Internal chilled margins show that the andesitic centers intruded into the middle of the mafic dikes. Major element systematics indicate that the fractionation of olivine, clinopyroxene, plagioclase and Fe–Ti oxides drove the evolution of the Kangâmiut parental magma during its transition from mafic to andesitic compositions. Incompatible trace elements show a marked relative decrease in middle and heavy rare-earth elements (REE) between the mafic margins and the andesitic centers. The decrease in the REE is not explicable by olivine, clinopyroxene, plagioclase and Fe–Ti oxide fractionation or by the fractionation of the accessory phases apatite, zircon or garnet. Rb–Sr and Sm–Nd isotopes from margin and center pairs from these composite dikes are nearly identical indicating that crustal contamination had little to no affect on their evolution. Trace element modeling utilizing the mixing of evolved Kangâmiut magmas and low degree melts derived from partial melting of garnet lherzolite produce excellent fits with the trace element patterns for the andesitic centers. These models suggest that the late-stage evolution of the Kangâmiut dikes included input of mantle melts produced during the end stages of rifting.  相似文献   

18.
Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption.  相似文献   

19.
The Neogene quartz andesites from the Oa? and Gutâi Mountains (Romania) are mid-K calc-alkaline rocks and contain plagioclase-orthopyroxene-clinopyroxene-amphibole-magnetite phenocrysts as well as quartz crystals. They are associated with a volcanic sequence ranging from basalts and basaltic andesites to dacites and rhyolites, but form a separate magma group, mostly in respect to the trace elements. Based on the geochemical data combined with inferences from complex zoning patterns in plagioclase and pyroxene, the evolution of quartz andesites is interpreted in terms of fractional crystallization, AFC and magma mixing. A parental magma deriving from a MORB- or OIB-type source modified by fluids and melts originating from sediments is envisaged.  相似文献   

20.
In plutonic systems, magma mixing is often modelled by mass balance based on whole-rock geochemistry. However, magma mixing is a chaotic process and chemical equilibration is controlled by non-linear diffusive–advective processes unresolved by the study of bulk samples. Here we present textural observations, LA-(MC-)ICP-MS trace element and Sr–Nd isotopic data of accessory apatites and titanites from a hybrid granodiorite of the Neoarchean Matok pluton (South Africa), collected in a zone of conspicuous mixing between mafic and felsic magmas. Apatite grains mostly show a pronounced zoning in CL images, corresponding to abrupt changes in REE and HFSE concentrations recording their transfer through compositionally different melt domains during mixing. These grains crystallized early, at temperatures of 950–1000 °C. Titanite grains crystallized at temperatures of 820–900 °C (Zr-in-sphene thermometry). They show limited intra-grain chemical variations but huge inter-grain compositional scatter in REE and HFSE, pinpointing crystallization within a crystal mush, from isolated melt pockets having different composition from one another owing to incomplete chemical homogenization and variable Rayleigh fractionation. These chemical–textural characteristics, in combination with partitioning models and Polytopic Vector Analysis, point to “self-mixing” between co-genetic dioritic and granodioritic/granitic magmas. Both resulted from differentiation of mantle-derived mafic melts, showing that mixing does not necessarily involve magmas from contrasted (crust vs. mantle) sources. Systematic variations in εNd t (?4.5 to ?2.5) and 87Sr/86Sr(i) (0.703–0.707) of titanite and apatite grains/domains crystallized from the two magmas point to an isotopically inhomogeneous mantle source, which is not resolved by bulk-rock isotopic data. Interaction between the two magmas must have occurred at relatively high temperatures (ca. 900°C) so that their viscosity contrast remained low, allowing efficient mechanical mixing. Despite this, chemical homogenization was incomplete, as recorded by diffusive fractionation between REE–HFSE and Sr. Modelling thereof reveals that chemical exchange between the liquid phases of the two mixed magmas did not last more than a few tens to hundreds of years. The chemical equilibration between mixed magmas thus strongly depends on the considered elements and observational length scales.  相似文献   

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