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1.
Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis 总被引：1，自引：0，他引：1
In an effort to obtain information about mineral/melt trace element partitioning during the high pressure petrogenesis of basic rocks, we determined rare earth and other trace element abundances in megacrysts of clinopyroxene, orthopyroxene, amphibole, mica, anorthoclase, apatite and zircon, and in their host basalts. In general, the ranges of mineral/melt partition coefficients established from experimental partitioning studies and phenocryst/matrix measurements overlap with the ranges of megacryst/host abundance ratios. Our data for Hf, Sc, Ta and Th partitioning represent some of the only estimates available. Consideration of phase equilibria, major element partitioning and isotopic ratios indicate that most of the pyroxene and amphibole megacrysts may have been in equilibrium with their host magmas at high pressures (mostly 10–25 kb). In contrast, it is unlikely that mica, anorthoclase, apatite and zircon megacrysts formed in equilibrium with their host basalts; instead, we conclude that they were precipitated from more evolved magmas and have been mixed into their present host magmas. Consequently, the trace element abundance ratios for megacryst/host should not be interpreted as partition coefficients, but only as guides for understanding trace element partitioning during high pressure petrogenesis. With this caveat, we conclude that the megacryst/ host trace element abundance data indicate that mineral/melt partition coefficients in basaltic systems during high pressure fractionation are not drastically different from partition coefficients valid for low pressure fractionation. 相似文献
2.
Carbonatites are believed to have crystallized either from mantle-derived primary carbonate magmas or from secondary melts
derived from carbonated silicate magmas through liquid immiscibility or from residual melts of fractional crystallization
of silicate magmas. Although the observed coexistence of carbonatites and alkaline silicate rocks in most complexes, their
coeval emplacement in many, and overlapping initial87Sr/86Sr and143Nd/144Nd ratios are supportive of their cogenesis; there have been few efforts to devise a quantitative method to identify the magmatic
processes. In the present study we have made an attempt to accomplish this by modeling the trace element contents of carbonatites
and coeval alkaline silicate rocks of Amba Dongar complex, India. Trace element data suggest that the carbonatites and alkaline
silicate rocks of this complex are products of fractional crystallization of two separate parental melts. Using the available
silicate melt-carbonate melt partition coefficients for various trace elements, and the observed data from carbonatites, we
have tried to simulate trace element distribution pattern for the parental silicate melt. The results of the modeling not
only support the hypothesis of silicate-carbonate melt immiscibility for the evolution of Amba Dongar but also establish a
procedure to test the above hypothesis in such complexes. 相似文献
3.
华北晚中生代和新生代玄武岩中单斜辉石巨晶来源及其对寄主岩岩浆过程的制约:以莒南和鹤壁为例 总被引：3，自引：0，他引：3
本文对华北克拉通晚中生代和新生代碱性玄武质岩石中的单斜辉石巨晶进行了主、微量元素和Sr-Nd同位素的综合研究,发现晚中生代和新生代单斜辉石巨晶存在明显的主、微量元素和同位素组成上的差异。新生代单斜辉石巨晶有Al-普通辉石和次透辉石两类;而中生代单斜辉石巨晶只有Al-普通辉石。新生代单斜辉石SiO_2含量高、REE配分型式为上凸型、LILE和放射性元素含量高,并具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成;而中生代单斜辉石SiO_2含量低、REE配分型式为LREE富集型、LILE和部分HFSE以及放射性元素含量低,并具有比寄主碱性玄武岩稍富集的Sr和Nd同位素组成;巨晶的结构、矿物成分和地球化学特征,以及Mg-Fe在熔体与单斜辉石间的分配状况皆说明,新生代碱性玄武岩中单斜辉石巨晶是碱性玄武岩浆在高压下结晶的,因此二者是同源的;而中生代单斜辉石巨晶是被寄主岩浆偶然捕获的捕虏晶,是不同源的。华北新生代单斜辉石巨晶存在于碱性玄武岩和拉斑玄武岩中,它们具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成,说明即使是碱性玄武岩也不能完全代表软流圈来源的原始岩浆,其在上升过程中或多或少存在同位素组成富集的物质的混入。同时,拉斑玄武岩不是碱性玄武质岩浆直接结晶分异的产物,亦不是完全由部分熔融程度的不同造成的。拉斑玄武岩中存在岩石圈地幔物质的贡献或是岩浆房内碱性玄武质岩浆受地壳混染作用的结果。 相似文献
4.
肖森宏 《中国地球化学学报》1991,10(1):11-20
Based on REE abundances in megacrysts and host basalts and their equilibrium conditions,it has proved that megacrysts may have been produced from the magma derived from the host rocks or of more basic composition.The REE ratios of megacrysts to host rocks may be taken as partition coefficients when both are equilibrium with each other.The crystal fractionation of megacrysts has caused the evolution of REE in the magma.It is obvious that some host basalts are the product of magma evolution after crystal fractionation.According to REE abundances in the host rocks and the partition coefficients between crystal and liquid,the history of crystal fractionation of magma can be traced. 相似文献
5.
Amphibole megacrysts as a probe into the deep plumbing system of Merapi volcano,Central Java,Indonesia 总被引：1，自引：0，他引：1
Amphibole has been discussed to potentially represent an important phase during early chemical evolution of arc magmas, but is not commonly observed in eruptive arc rocks. Here, we present an in-depth study of metastable calcic amphibole megacrysts in basaltic andesites of Merapi volcano, Indonesia. Radiogenic Sr and Nd isotope compositions of the amphibole megacrysts overlap with the host rock range, indicating that they represent antecrysts to the host magmas rather than xenocrysts. Amphibole-based barometry suggests that the megacrysts crystallised at pressures of >500 MPa, i.e., in the mid- to lower crust beneath Merapi. Rare-earth element concentrations, in turn, require the absence of magmatic garnet in the Merapi feeding system and, therefore, place an uppermost limit for the pressure of amphibole crystallisation at ca. 800 MPa. The host magmas of the megacrysts seem to have fractionated significant amounts of amphibole and/or clinopyroxene, because of their low Dy/Yb ratios relative to the estimated compositions of the parent magmas to the megacrysts. The megacrysts’ parent magmas at depth may thus have evolved by amphibole fractionation, in line with apparently coupled variations of trace element ratios in the megacrysts, such as e.g., decreasing Zr/Hf with Dy/Yb. Moreover, the Th/U ratios of the amphibole megacrysts decrease with increasing Dy/Yb and are lower than Th/U ratios in the basaltic andesite host rocks. Uranium in the megacrysts’ parent magmas, therefore, may have occurred predominantly in the tetravalent state, suggesting that magmatic fO2 in the Merapi plumbing system increased from below the FMQ buffer in the mid-to-lower crust to 0.6–2.2 log units above it in the near surface environment. In addition, some of the amphibole megacrysts experienced dehydrogenation (H2 loss) and/or dehydration (H2O loss), as recorded by their variable H2O contents and D/H and Fe3+/Fe2+ ratios, and the release of these volatile species into the shallow plumbing system may facilitate Merapi’s often erratic eruptive behaviour. 相似文献
6.
7.
Ni、Cu和PGE具有不同于其他微量元素的特殊的地球化学性质,这些特殊的性质使得它们在幔源岩浆起源和演化以及岩浆硫化物矿床的成因研究中具有不可替代的作用。在S不饱和的条件下,Ni、Os、Ir和Ru具有相容元素的特性,而Cu和Pd是强不相容元素,因此,它们在玄武岩浆分离结晶过程中常常发生分异。一旦体系达到S饱和,这些元素则会强烈地进入硫化物熔浆,特别是PGE具有极高的硫化物熔浆/硅酸盐熔浆分配系数,极微量的硫化物熔离便可导致残余岩浆中PGE的显著亏损,因此,PGE是玄武岩浆硫化物熔离作用最敏感的示踪元素。硫化物熔离和成矿实质上是幔源岩浆特殊演化过程的结果,所以,Ni,Cu和PGE的特殊性质可用来探讨岩浆硫化物成矿的关键控制因素。Ni、Cu和PGE具有不同的单硫化物固溶体/硫化物熔浆分配系数,因此,它们也是硫化物熔浆结晶分异的重要示踪元素。本文试图从Ni、Cu和PGE地球化学性质和行为入手,并借助一些研究实例,对它们在幔源岩浆起源和演化以及岩浆硫化物矿床成因研究中的示踪意义进行系统介绍。 相似文献
8.
Thomas Pettke Andreas Audtat Urs Schaltegger Christoph A. Heinrich 《Chemical Geology》2005,220(3-4):191-213
The Sn–W mineralized Mole Granite in Eastern Australia hosts zircon populations that crystallized at several stages during a protracted magmatic to hydrothermal evolution. Thirty-four elements have been quantified by laser-ablation inductively-coupled-plasma mass-spectrometric microanalysis with the aim of relating the chemistry of zircon to its growth environment. Trace element contents are highly variable for all textural occurrences. Zircon inclusions in earliest quartz phenocryst suggest that zircon was a liquidus phase that crystallized probably deep in the crust. Trace element contents are conspicuously high, showing only a slight positive Ce anomaly but a pronounced negative Eu-anomaly. Successive crystallization stages of magmatic zircon are characterized by progressive depletion in trace element contents, notably the rare earth elements, with an increasingly important positive Ce-anomaly. This evolution reflects saturation of REE accepting minerals such as monazite, thorite, xenotime and possibly apatite and is affected little by the exsolution of a magmatic–hydrothermal fluid. Zircon that is interpreted to have precipitated from aqueous fluids in Sn–W-bearing quartz veins shows REE patterns indistinguishable from those of late magmatic zircon. When combined with experimental evidence on the fluid–melt partitioning of REE, it indicates that the REE distribution coefficients for zircon/melt and zircon/fluid are largely comparable.
The second example of hydrothermal zircon crystallized some 2 My after the host granite. These crystals reveal an intragranular zonation of increasing trace element concentrations from core to rim. Therefore, REE abundances and patterns alone are not conclusive indicators of the geological environment in which zircon crystallized. Nevertheless, variations in trace element contents of zircon that relate to the chemistry of the melt or fluid from which zircon crystallized, as measured in cogenetic melt and fluid inclusions, are promising for future petrogenetic modeling.
Lead and Cs are strongly incompatible in hydrothermal zircon, with estimated zircon–fluid distribution coefficients D ≤ 0.001, while Sn and Li are moderately incompatible, DSn 0.6 and DLi 0.1, and Ce is compatible, DCe 14. Moreover, hydrothermal zircon has a more pronounced negative Eu-anomaly and higher Ta/Nb and U/Th ratios than the magmatic zircons of the Mole Granite. 相似文献
9.
Understanding the geochemical behavior of chalcophile elements in magmatic processes is hindered by the limited partition coefficients between sulfide phases and silicate melt, in particular at conditions relevant to partial melting of the hydrated, metasomatized upper mantle. In this study, the partitioning of elements Co, Ni, Cu, Zn, As, Mo, Ag, and Pb between sulfide liquid, monosulfide solid solution (MSS), and hydrous mantle melt has been investigated at 1200 °C/1.5 GPa and oxygen fugacity ranging from FMQ−2 to FMQ+1 in a piston-cylinder apparatus. The determined partition coefficients between sulfide liquid and hydrous mantle melt are: 750–1500 for Cu; 600–1200 for Ni; 35–42 for Co; 35–53 for Pb; and 1–2 for Zn, As, and Mo. The partition coefficients between MSS and hydrous mantle melt are: 380–500 for Cu; 520–750 for Ni; ∼50 for Co; <0.5 for Zn; 0.3–6 for Pb; 0.1–2 for As; 1–2 for Mo; and >34 for Ag. The variation of the data is primarily due to differences in oxygen fugacity. These partitioning data in conjunction with previous data are applied to partial melting of the upper mantle and the formation of magmatic-hydrothermal Cu–Au deposits and magmatic sulfide deposits.I show that the metasomatized arc mantle may no longer contain sulfide after >10–14% melt extraction but is still capable of producing the Cu concentrations in the primitive arc basalts, and that the comparable Cu concentrations in primitive arc basalts and in MORB do not necessarily imply similar oxidation states in their source regions.Previous models proposed for producing Cu- and/or Au-rich magmas have been reassessed, with the conclusions summarized as follows. (1) Partial melting of the oxidized (fO2 > FMQ), metasomatized arc mantle with sulfide exhaustion at degrees >10–14% may not generate Cu-rich, primitive arc basalts. (2) Partial melting of sulfide-bearing cumulates in the root of thickened lower continental crust or lithospheric mantle does not typically generate Cu- and/or Au-rich magmas, but they do have equivalent potential as normal arc magmas in forming magmatic-hydrothermal Cu–Au deposits in terms of their Cu–Au contents. (3) It is not clear whether partial melting of subducting metabasalts generates Cu-rich adakitic magmas, however adakitic magmas may extract Cu and Au via interaction with mantle peridotite. Furthermore, partial melting of sulfide-bearing cumulates in the deep oceanic crust may be able to generate Cu- and Au-rich magmas. (4) The stabilization of MSS during partial melting may explain the genetic link between Au-Cu mineralization and the metasomatized lithospheric mantle.The chalcophile element tonnage, ratio, and distribution in magmatic sulfide deposits depend on a series of factors. This study reveals that oxygen fugacity also plays an important role in controlling Cu and Ni tonnage and Cu/Ni ratio in magmatic sulfide deposits. Cobalt, Zn, As, Sn, Sb, Mo, Ag, Pb, and Bi concentrations and their ratios in sulfide, due to their different partitioning behavior between sulfide liquid and MSS, can be useful indices for the distribution of platinum-group elements and Au in magmatic sulfide deposits. 相似文献
10.
我国东部苏鲁皖地区新生代碱性玄武岩中,除了含有大量地幔橄榄岩类捕虏体以外,尚含有一定数量的石榴石、普通辉石和歪长石巨晶。这些巨晶是在地幔不同深度上从玄武岩中晶出的。巨晶组合的分离结晶作用对熔体稀土元素含量有很大影响。赋存巨晶的碱性玄武岩所具有的LREE富集、HREE亏损的稀土元素分配型式是由地幔橄榄岩类部分熔融程度、石榴石巨晶和普通辉石巨晶的早期高压熔离和玄武岩的结晶分异作用等综合因素造成的。 相似文献
11.
Ya-Dong Liu 《International Geology Review》2020,62(15):1845-1861
ABSTRACT Clinopyroxene megacrysts in volcanic rocks can provide substantial information on the evolution of the magmatic system at depth. Although considerable attention has been paid to these crystals, their origin is not yet completely resolved. The clinopyroxene megacrysts worldwide can be divided into two major types in general: the green Cr-diopside type and the black Al-augite type. There is a consensus view that the Cr-diopside megacrysts are mantle xenocrysts, whereas two contrasting opinions exist regarding the origin of the black Al-augite megacrysts. One favours a cognate origin, viewing them as crystallization products of the host magmas under high-pressure; while the other argues that they are xenocrysts crystallized from previous alkali basalts or fragments of mantle peridotites, pyroxenites or pegmatite veins. A review study on the clinopyroxene megacrysts in Meso-Cenozoic volcanic rocks from the North China Craton (NCC) and their comparison with those worldwide provides new constraints on their origin, namely, the Cr-diopside megacrysts, as previously thought, are all xenocrysts, representing disaggregated clinopyroxene crystals from clinopyroxene-rich mantle rocks. Contrary to the formerly proposed cognate origin, the Al-augite megacrysts are also xenocrysts, having no direct genetic link to their host rocks. They crystallized from melts that have formed earlier than the host magmas, and probably accumulated in a magma chamber or occurred as sheets or veins filling a fracture network surrounding a magma chamber in the upper mantle. During the subsequent eruption of the host lavas, these previously formed crystals were incorporated into the magma and were brought up to the surface. 相似文献
12.
Upper mantle xenoliths from Wikieup, AZ, provide abundant evidence for magmatic modification of the uppermost mantle beneath
the Transition Zone between the Colorado Plateau and the southern Basin and Range province. Upper mantle lithologies in this
xenolith suite are represented by spinel peridotite, wehrlite, plagioclase peridotite, and Al-augite group pyroxenites. Isotopic
data for these xenoliths yield relatively uniform values and suggest a common petrogenesis. Al-augite-bearing gabbro and pyroxenite
xenoliths from this locality are interpreted to have formed by crystal fractionation processes from parent alkali basalts
similar to the Wikieup host basalt. Mineral and whole rock compositions show consistent trends of increasing incompatible
element contents (Fe, Al, Ca, Na, K, LIL, and LREE), and decreasing compatible element contents (Mg, Cr, Ni) from spinel peridotite
to wehrlite to plagioclase peridotite to the host basalt composition. These compositional trends are interpreted as resulting
from varying degrees of magma-mantle wall rock interaction as ascending mafic magmas infiltrated upper mantle peridotite.
Small degrees of melt infiltration resulted in slightly modified spinel peridotite compositions while moderate degrees metasomatized
spinel peridotite to wehrlite, and the highest degrees metasomatized it to plagioclase peridotite. Whole rock compositions
and clinopyroxene, plagioclase, and whole rock isotopic data suggest that the infiltrating magmas were the same as those from
which the gabbros and pyroxenites crystallized, and that they were alkalic in composition, similar to the Wikieup host alkali
olivine basalts. Relatively uniform 143Nd/144Nd for the mineral separates and whole rocks in spite of the significantly wide range in their 147Sm/144Nd (0.71–0.23 in clinopyroxene) suggests that the Wikieup xenoliths including gabbro, pyroxenite, peridotite, wehrlite, and
plagioclase peridotite, are all relatively young rocks formed or metasomatized by a relatively recent magmatic episode.
Received: 21 May 1996 / Accepted: 23 December 1996 相似文献
13.
Trace and rare earth element contents were determined by SIMS technique in clinopyroxene and mica crystals from minette lamprophyric enclaves in a potassic syenite host. This co-mingled system was crystallized at high pressures, which varied about 3–5 GPa, as indicated by the presence of K-clinopyroxenes and pyrope-rich garnet with measurable amounts of K2O and Na2O, among the near-liquidus phases. Major and trace element composition of these lamprophyric enclaves is quite similar to those observed in silica-rich lamproites, suggesting that similar sources were involved in their origin. In a general view, the concentrations of most trace and rare earth elements in clinopyroxene of the studied enclaves are higher than those referred to by other authors. Clinopyroxene/melt partition coefficient for most trace elements are close to determinations in alkali-basalts and lamproites from Leucite Hills, with considerable differences relative to Gaussberg lamproites. Furthermore, these partition data are completely different from those determined for potassic lavas crystallized under crustal pressures. Spidergrams for clinopyroxenes exhibit negative-Sr anomalies relative to LREE, which have been associated by most authors to crystallization under low-pressures, out of garnet stability field. The presence of pyrope together with K-clinopyroxene excludes such hypotheses for the studied enclaves. Y and HREE are concentrated in clinopyroxene, whilst the other trace elements have Kd<1. LIL elements, except Rb, have incompatible (Kd<1) behavior in phlogopite. The high partition coefficient for Nb (Kd>3) determined in the studied phlogopite is unusual in lamproites, lamprophyres, and basalts, but frequently observed in phlogopite from metasomatic mantle samples, as well as in acid magmas. This partition value may indicate the lack of other mineral phase with high partition for this element during crystallization, and may be enhanced by the liquid composition progressively closer to alkali feldspar, an unsuitable structure for six-coordinated cations. Ce/Yb, Rb/Sr, and Zr/Hf ratios in clinopyroxene and mica suggest that the minettic magma could produce the host Piquiri potassic syenite by fractional crystallization. This hypothesis is not consistent with Ba concentrations in clinopyroxene and mica, which suggest that a Ba-bearing phase (e.g. alkali feldspar) should be among the fractionated phases in order to produce the potassic syenites. 相似文献
14.
吐哈盆地早二叠世玄武岩原始岩浆性质:来自熔融包裹体成分的制约 总被引：3，自引：3，他引：0
东天山地区的二叠纪玄武岩沿着区域的北东东向断裂呈脉状分布,吐哈盆地玄武岩的40Ar-39Ar坪年龄为298.2±3.8Ma,为早二叠世,与前人的玄武岩年龄结果在误差范围内一致。可能与东天山地区二叠纪岩浆铜镍矿床镁铁-超镁铁岩有密切的成因联系。吐哈玄武岩的主微量成分显示其为岛弧拉斑、大陆弧玄武岩,轻稀土富集和Nb、Ta负异常,指示源区可能经历过俯冲作用的改造。吐哈盆地二叠纪玄武岩含有新鲜的橄榄石和长石斑晶,橄榄石斑晶中熔融包裹体较发育。熔融包裹体为玻璃质、气相和玻璃质、气相、固相两种类型。包裹体中不透明矿物主要为磁铁矿,说明捕获包裹体时岩浆的氧逸度和Fe含量较高。熔融包裹体分为高MgO和低MgO含量两种。高MgO含量的包体同时具有低SiO_2、低微量和稀土元素含量的特征,可能为地幔高部分熔融的产物,且经历过深部演化程度较弱。该高MgO熔体的微量元素显示Nb、Ta亏损的特征,具有N-MORB特征的微量和稀土元素分配模式,预示该熔体为受到俯冲交代的地幔熔融形成。熔融包裹体相对玄武岩具有低的Th和Ta含量、相对弱的Nb和Ta的负异常的特征,指示熔融包裹体的成分经受改造程度低于玄武岩,暗示可能为经历过较少后期作用改造的相对原始的熔体。熔体中Cu含量(12.4×10~(-6)~299×10~(-6))在正常玄武质岩浆含量范围内,而Ni含量(236×10~(-6)~697×10~(-6))高于高镁溢流科马提岩和洋中脊玄武岩。该Cu、Ni含量略显解耦的熔体可能代表了经历过深部少量的硫化物熔离,带走小部分Cu和Ni等成矿元素之后所捕获的岩浆。如果将该熔体视为东天山地区二叠纪岩浆铜镍硫化物矿床的母岩浆,该母岩浆中Ni含量相对较高可能是岩浆铜镍硫化物矿床中矿石的Ni/Cu比值大多大于1.0的主要因素。 相似文献
15.
Continental flood basalts (CFBs), thought to preserve the magmatic record of an impinging mantle plume head, offer spatial and temporal insights into melt generation processes in large igneous provinces (LIPs). Despite the utility of CFBs in probing mantle plume composition, these basalts typically erupt fractionated compositions, suggestive of significant residence time in the continental lithosphere. The location and duration of residence within the lithosphere provide additional insights into the flux of plume-related magmas. The NW Ethiopian plateau offers a well-preserved stratigraphic sequence from flood basalt initiation to termination, and is thus an important target for study of CFBs. This study examines modal observations within a stratigraphic framework and places these observations within the context of the magmatic evolution of the Ethiopian CFB province. Data demonstrate multiple pulses of magma recharge punctuated by brief shut-down events, with initial flows fed by magmas that experienced deeper fractionation (lower crust). Broad changes in modal mineralogy and flow cyclicity are consistent with fluctuating changes in magmatic flux through a complex plumbing system, indicating pulsed magma flux and an overall shallowing of the magmatic plumbing system over time. The composition of plagioclase megacrysts suggests a constant replenishing of new primitive magma recharging the shallow plumbing system during the main phase of volcanism, reaching an apex prior to flood basalt termination. The petrostratigraphic data sets presented in this paper provide new insight into the evolution of a magma plumbing system in a CFB province. 相似文献
16.
The study of melt microinclusions in olivine megacrysts from meimechites and alkali picrites of the Maimecha–Kotui alkali ultramafic and carbonatite province (Polar Siberia) revealed that the melt compositions corrected for loss of olivine due to post-entrapment crystallization of olivine on inclusion walls (differentiates of primary meimechite magma) match well to the composition of nephelinites and olivine melilitites belonging to carbonatite magmatic series. Modeling of fractional crystallization of meimechite magmas results in the high-alkali melt compositions corresponding to the silicate–carbonate liquid immiscibility field. The appearance of volatile-rich melts at the base of magma-generating plume systems at early stages of partial melting can be explained by extraction of incompatible elements including volatiles, by near-solidus melts at low degrees of partial melting, and meimechites are an example of such magmas. Subsequent accumulation of CO2 in the residual melt results in generation of carbonate magma. 相似文献
17.
Effect of melt composition on the partitioning of trace elements between titanite and silicate melt 总被引：7，自引：0，他引：7
Isobaric and isothermal experiments were performed to investigate the effect of melt composition on the partitioning of trace elements between titanite (CaTiSiO5) and a range of different silicate melts. Titanite-melt partition coefficients for 18 trace elements were determined by secondary ion mass spectrometry (SIMS) analyses of experimental run products. The partition coefficients for the rare earth elements and for Th, Nb, and Ta reveal a strong influence of melt composition on partition coefficients, whereas partition coefficients for other studied monovalent, divalent and most quadrivalent (i.e., Zr, Hf) cations are not significantly affected by melt composition. The present data show that the influence of melt composition may not be neglected when modelling trace element partitioning.It is argued that it is mainly the change of coordination number and the regularity of the coordination space of trace elements in the melt structure that controls partition coefficients in our experiments. Furthermore, our data also show that the substitution mechanism by which trace elements are incorporated into titanite crystals may be of additional importance in this context. 相似文献
18.
Partition coefficients for trace elements in silicic magmas 总被引：2，自引：0，他引：2
Trace element partition coefficients for 29 elements, including the rare earths, have been calculated for augite, hypersthene, sanidine, plagioclase, quartz, biotite, titanomagnetite and ilmenite from rhyodacite and rhyolite at Twin Peaks, Utah. Partition coefficients are intermediate to those reported from dacite and high silica rhyolite. At Twin Peaks, as in less silicic magmas, partitioning between phenocrysts and melt is governed primarily by crystal structure constraints as opposed to very high silica systems where the structure and volatile content of the melt become the dominant control of trace element partitioning. 相似文献
19.
I. T. Rass S. S. Abramov V. A. Utenkov V. M. Kozlovskii D. I. Korpechkov 《Geochemistry International》2006,44(7):636-655
In most alkaline-ultrabasic-carbonatite ring complexes, the distribution of trace elements in the successive derivatives of mantle magmas is usually controlled by the Rayleigh equation of fractional crystallization in accordance with their partition coefficients, whereas, that of late derivatives, nepheline syenites and carbonatites, is usually consistent with trends characteristic of silicate-carbonate liquid immiscibility. In contrast to the carbonatites of ring complexes, carbonatites from deep-seated linear zones have no genetic relation with alkaline-ultrabasic magmatism, and the associated alkaline rocks are represented only by the nepheline syenite eutectic association. The geochemical study of magmatic rocks from the Vishnevye Gory nepheline syenite-carbonatite complex (Urals), which is assigned to the association of deep-seated linear zones, showed that neither differentiation of a parental melt nor liquid immiscibility could produce the observed trace element distribution (Sr, Rb, REE, and Nb) in miaskites and carbonatites. Judging from the available fragmentary experimental data, the distribution patterns can be regarded as possible indicators of element fractionation between alkaline carbonate fluid and alkaline melt. Such trace element distribution is presumably controlled by a fluid-melt interaction; it was also observed in carbonatites and alkaline rocks of some ring complexes, and its scarcity can be explained by the lower density of aqueous fluid released from magma at shallower depths. 相似文献
20.
五大连池钾质火山岩中发现具有反环带结构的单斜辉石斑晶。反环带辉石的核部为次透辉石,与边部相比具有较低 的Mg#值（68~77）、TiO2 （0.23~0.50 wt.%）、Cr2O3 （<0.06 wt.%）和较高的Al2O3 （3.4~5.0 wt.%）、Na2O（0.43~0.78 wt.%）、 FeO（8.8~11.0 wt.%）、MnO含量。核部的稀土元素标准化曲线较为平坦,且具有非常显著的Eu和Sr的负异常（Eu/Eu*= 0.35~0.63,Sr/Sr* = 0.03~0.17）,指示其为曾与斜长石平衡的变质岩辉石。在Mg#-TiO2相关图上,核部总体成分落入华北下 地壳低Mg麻粒岩中单斜辉石的成分范围,因此核部应该是来自下地壳麻粒岩的捕掳晶。反环带辉石边部与正常辉石斑晶成 分一致,具有较高的Mg#值（81~85）、TiO2（0.40~1.65 wt.%）、Cr2O3（0.03~0.25 wt.%）和较低的Al2O3（2.1~3.4 wt.%）、Na2O （0.34~0.63 wt.%）、FeO（4.6~6.6 wt.%）、MnO含量,轻重稀土分馏明显（La/Yb） N = 3.23~7.89,与玄武岩全岩的特征吻合。 利用主量和微量元素的分配系数进行的模拟计算均表明,反环带辉石边部与寄主岩浆已达到成分平衡,说明边部是在熔蚀 核部的基础上再生长而成的岩浆成因辉石。五大连池正常辉石斑晶的Mg#值与TiO2具有负相关性,指示岩浆在地壳深度经历 了一定程度的演化。反环带辉石斑晶边部由里到外Al2O3和Na2O含量逐渐增加的趋势支持岩浆经历演化的结论。核部辉石 的麻粒岩属性表明岩浆演化主要发生在下地壳。温压计计算结果也指示正常辉石斑晶和反环带辉石的边部都结晶于下地壳 深度的系列岩浆房。晚期结晶的辉石斑晶总是比早期结晶的辉石更富集不相容元素,说明分离结晶作用导致五大连池玄武 岩的不相容元素更富集。平衡计算表明,与辉石斑晶平衡的岩浆与玄武岩全岩在微量元素特征上高度相似。考虑到全岩高 度一致的微量元素特征,研究认为五大连池玄武岩的成分变化主要受下地壳岩浆房中的结晶分离作用控制,地壳混染（包 括壳源岩浆混合）可以忽略不计。 相似文献