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1.
Patches of glass with a second generation of small crystals of olivine, clinopyroxene, and spinel are abundant in hydrous peridotite mantle xenoliths with tabular equigranular textures from two maar-type volcanoes, Meerfelder Maar and Dreiser Weiher (West Eifel, Germany). The patches are similar in size to the main phases of the hosting peridotite. Their central part is often occupied by relics of pargasitic amphibole. Mass-balance calculations show that the patches were formed by surface controlled incongruent thermal breakdown of amphibole according to the reaction: amphibole olivine + clinopyroxene + spinel + melt. Simultaneously with the decomposition of amphibole, small crystals of olivine, clinopyroxene, and spinel grew radially from the patch/peridotite interface toward the centre of the patch. Apart from sector zoning of clinopyroxene, the crystals are virtually homogeneous and are separated from the amphibole by a seam of melt (glass). Secondary olivines reveal higher Mg-numbers, secondary clinopyroxenes higher Cr2O3 concentrations than olivines and clinopyroxenes, respectively, of the host peridotite. The silica contents of melts produced by the above breakdown reaction range from 48 to 52% SiO2 as a function of the composition of the parent amphiboles. Patches surrounded by primary olivines only reveal no reaction with the host peridotite. The variation of SiO2, MgO and CaO in melts from these patches is the result of minor precipitation of olivine and clinopyroxene during fast cooling. If patches are in contact with primary olivine and orthopyroxene, melts are additionally modified by the reaction liquid 1 + orthopyroxene liquid 2 + olivine + clinopyroxene resulting in more silica-rich compositions between 54 and 58%. For the rare glasses richer in silica, a more complex formation is required. Veinlets along grain boundaries are filled with glasses which are chemically identical to those from nearby patches. This suggests that the veinlets were filled by melts formed by amphibole breakdown during entrainment of the xenoliths to the host magmas. 相似文献
2.
Junji Akai 《Contributions to Mineralogy and Petrology》1982,80(2):117-131
Clinopyroxene transforms to triple chain silicate, double chain silicate (amphibole) and sheet silicate (talc) in the metasomatic process of the Akatani ore deposit. The triple chain silicate is contained in fibrous amphibole-like phase (“amphibole”). It is of electron microscopic size of 1,000 Å at maximum width in b-direction, and is a calciferous analogue of clinojimthompsonite. Various kinds of fine textures formed in metasomatic reaction process were found in clinopyroxenes. A large amount of triple chains and a small amount of double chains were transformed from single chains in the host clinopyroxene, maintaining topotactic relation. The kinetics of the structural change of biopyriboles at the Akatani ore deposit was discussed from the viewpoint of the mode of occurrence of triple and double chain silicates. The nucleation of triple chain structure slab with one triple chain width is apt to occur rather than the nucleation of double chain structure slab with width of two double chains in clinopyroxene host. Various fine textures in clinopyroxenes and amphibole-like phase were interpreted as corresponding to the propagation of metasomatic reactions. 相似文献
3.
In situ trace element analyses of constituent minerals in mantle xenoliths occurring in an alnöite diatreme and in nephelinite plugs emplaced within the central zone of the Damara Belt have been determined by laser ablation ICP-MS. Primitive mantle-normalized trace element patterns of clinopyroxene and amphibole indicate the presence of both depleted MORB-like mantle and variably enriched mantle beneath this region. Clinopyroxenes showing geochemical depletion have low La/Smn ratios (0.02–0.2), whereas those showing variable enrichment have La/Smn ranging up to 3.8 and La/Ybn to 9.1. The most enriched clinopyroxenes coexist with amphibole showing similar REE patterns (La/Smn = 1.3–4.1; La/Ybn = 4.5–9). Primitive mantle-normalized trace element patterns allow further groups to be distinguished amongst the variably enriched clinopyroxenes: one having strong relative depletion in Rb–Ba, Ta–Nb and relative enrichment in Th–U; another with similar characteristics but with additional strong relative depletion in Zr–Hf; and one showing no significant anomalies. Amphiboles show similar normalized trace element patterns to co-existing clinopyroxene. Clinopyroxene and amphiboles showing LREEN enrichment have high Sr and low Nd isotope ratios compared to clinopyroxene with LREE-depleted patterns. Numerical simulation of melt percolation through the mantle via reactive porous flow is used to show that the chromatographic affect associated with such a melt migration process is able to account for the fractionation seen in La–Ce–Nd in cryptically metasomatized clinopyroxenes in Type 1 xenoliths, where melt–matrix interactions occur near the percolation front, whereas REE patterns in clinopyroxenes proximal to the source of metasomatic melt/fluid match those found in modally metasomatized Type 2 xenoliths. The strong fractionation between Rb–Ba, Th–U and Ta–Nb shown by some cryptically metasomatized xenoliths can be also accounted for by reactive porous flow, provided amphibole crystallizes from the percolating melt/fluid close to its source. The presence of amphibole in vein-like structures in some xenoliths is consistent with this interpretation. The strong depletion in Zr–Hf in clinopyroxene and amphibole in some xenoliths cannot be accounted for by melt migration processes and requires metasomatism by a separate carbonate-rich melt/fluid. When taken together with published isotope data on these same xenoliths, the source of metasomatic enrichment of the previously depleted (MORB-like) sub-Damaran lithospheric mantle is attributed to the upwelling Tristan plume head at the time of continental breakup. 相似文献
4.
在中国东部,北起黑龙江、南到海南岛,广泛地分布着新生代碱性玄武质岩石,例如碱性橄榄玄武岩、碧玄岩、霞石玄武岩等。它们含有丰富的超镁铁岩包体和各种巨晶。已发现的有辉石、角闪石、石榴石、歪长石、钛铁矿和云母等巨晶。Harte(1978)把巨晶定义为大的单矿物单个晶体,它比超镁铁岩团块中的矿物粗得多(2—20cm)。然而有的地质学家把巨晶的粒度下限定在1cm(Eggler 等,1979)。笔者遵循这一定义和采纳后一界线,在此基础上着重对中国东部部分地区的辉石和角闪石巨晶进行研究,研究情况见表1。 相似文献
5.
Metasomatism in the subcontinental mantle beneath the Eastern Carpathians (Romania): new evidence from trace element geochemistry 总被引：9，自引：1，他引：9
Françoise Chalot-Prat Anne-Marie Boullier 《Contributions to Mineralogy and Petrology》1997,129(4):284-307
A wide range of trace elements have been analysed in mantle xenoliths (whole rocks, clinopyroxene and amphibole separates)
from alkaline lavas in the Eastern Carpathians (Romania), in order to understand the process of metasomatism in the subcontinental
mantle of the Carpatho-Pannonian region. The xenoliths include spinel lherzolites, harzburgites and websterites, clinopyroxenites,
amphibole veins and amphibole clinopyroxenites. Textures vary from porphyroclastic to granoblastic, or equigranular. Grain
size increases with increasing equilibrium temperature of mineralogical assemblages and results from grain boundary migration.
In peridotites, interstitial clinopyroxenes (cpx) and amphiboles resulted from impregnation and metasomatism of harzburgites
or cpx-poor lherzolites by small quantities of a melt I with a melilitite composition. Clinopyroxenites, amphibole veins and
amphibole clinopyroxenites are also formed by metasomatism as a result of percolation through fracture systems of large quantities
of a melt II with a melanephelinite composition. These metasomatic events are marked by whole-rock enrichments, relative to
the primitive mantle (PM), in Rb, Th and U associated in some granoblastic lherzolites and in clinopyroxene and amphibole
veins with enrichments in LREE, Ta and Nb. Correlations between major element whole-rock contents in peridotites demonstrate
that the formation of interstitial amphibole and clinopyroxene induced only a slight but variable increase of the Ca/Al ratio
without apparent modifications of the initial mantle composition. Metasomatism is also traced by enrichments in the most incompatible
elements and the LREE. The Ta, Nb, MREE and HREE contents remained unchanged and confirm the depleted state of the initial
but heterogeneous mantle. Major and trace element signature of clinopyroxene suggests that amphibole clinopyroxenites and
some granoblastic lherzolites have been metasomatized successively by melts I and II. Both melts I and II were Ca-rich and
Si-poor, somewhat alkaline (Na > K). Melt I differed from melt II in having higher Mg and Cr contents offset by lower Ti,
Al, Fe and K contents. Both were highly enriched in all incompatible trace elements relative to primitive mantle, showing
positive anomalies in Rb, Ba, Th, Sr and Zr. They contrasted by their Ta, Nb and LREE contents, lower in melt I than in melt
II. Melts I and II originate during a two-stage melting event from the same source at high pressure and under increasing temperature.
The source assemblage could be that of a metasomatized carbonated mantle but was more likely that of an eclogite of crustal
affinity. Genetic relationships between calc-alkaline and alkaline lavas from Eastern Carpathians and these melts are thought
to be only indirect, the former originating from partial melting of mantle sources respectively metasomatized by the melts
I and II.
Received: 17 March 1997 / Accepted: 14 July 1997 相似文献
6.
J. F. A. DIENER R. POWELL R. W. WHITE T. J. B. HOLLAND 《Journal of Metamorphic Geology》2007,25(6):631-656
A recent thermodynamic model for the Na–Ca clinoamphiboles in the system Na2 O–CaO–FeO–MgO–Al2 O3 –SiO2 –H2 O–O (NCFMASHO), is improved, and extended to include cummingtonite–grunerite and the orthoamphiboles, anthophyllite and gedrite. The clinoamphibole model in NCMASH is adopted, but the extension into the FeO- and Fe2 O3 -bearing systems is revised to provide thermodynamic consistency and better agreement with natural assemblage data. The new model involves order–disorder of Fe–Mg between the M2, M13 and M4 sites in the amphibole structure, calibrated using the experimental data on site distributions in cummingtonite–grunerite. In the independent set of end-members used to represent the thermodynamics, grunerite (rather than ferroactinolite) is used for FeO, with two ordered Fe–Mg end-members, and magnesioriebeckite (rather than ferritschermakite) is used for Fe2 O3 . Natural assemblage data for coexisting clinoamphiboles are used to constrain the interaction energies between the various amphibole end-members. For orthamphibole, the assumption is made that the site distributions and the non-ideal formulation is the same as for clinoamphibole. The data set end-members anthophyllite, ferroanthophyllite and gedrite, are used; for the others, they are based on the clinoamphibole end-members, with the necessary adjustments to their enthalpies constrained by natural assemblage data for coexisting clino- and orthoamphiboles. The efficacy of the models is illustrated with P – T grids and various pseudosections, with a particular emphasis on the prediction of mineral assemblages in ferric-bearing systems. 相似文献
7.
矿物出溶结构保存有早期母体矿物的物理化学条件信息,对其开展研究不仅有助于了解寄主岩石的来源深度,而且有助于研究减压折返的动力学演化过程.在世界许多高压-超高压带的榴辉岩和石榴辉石岩中,人们普遍发现单斜辉石中有定向排列的针状或棒状SiO2析出物,其矿物相主要为α石英,有时会伴生钙质角闪石等含水矿物.这些定向针状或棒状体通常平行于单斜辉石c[001]轴方向延伸,石英长轴可以为其c[0001]轴或a[1120]轴.电子背散射衍射(EBSD)测试结果表明,多数石英(96%)析出物与寄主单斜辉石具有结晶学取向关系:(1) 50%的石英c轴平行,并且[0001]Qz//[001]Cpx;(2)35%的石英至少有一个a轴平行,并且[1120]Qz//[001]Cpx;(3)11%的石英至少有一个s{1121}面平行,并且(1121)Qz//(100)Cpx.钙质角闪石析出物与寄主单斜辉石也具有密切结晶学取向关系:(100)Amp//(100)Cpx、[010]Amp//[010]Cpx、[001]Amp//[001]Cpx、[100]Amp∧[100]Cpx≈32°.上述定量显微构造证据表明,单斜辉石中定向石英析出物是由出溶作用所形成,并且多数石英出溶体形成于α石英稳定域.已有高温高压实验研究数据表明,单斜辉石中空位的形成和钙埃斯科拉组分(CaEs)的含量均受化学组成、压力、温度等多种因素综合影响:单斜辉石中CaEs含量对化学组成非常敏感,并受到共生矿物体系中自由SiO2相和蓝晶石的共同缓冲;相同化学组成和等压条件下,CaEs含量总体上随温度升高缓慢降低;相同化学组成和等温条件下,CaEs含量在＜6 GPa区间随压力升高而增加,在＞6 GPa区间随压力升高而降低.单斜辉石定向SiO2析出物的形成可能涉及多种因素,高压只是其中必要条件之一.榴辉岩质单斜辉石中“石英±角闪石”析出物很可能形成于开放体系,与熔流体活动密切相关,涉及多阶段物质扩散、晶体成核生长、重结晶、退变质反应等复杂作用过程.单斜辉石中定向SiO2析出物的显微结构特征并非超高压岩石的必要条件,这种特殊显微结构也不能作为证明超高压的充分条件. 相似文献
8.
A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.
Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO2. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship. 相似文献
9.
山东昌乐位于华北克拉通东部,郯庐断裂带中段,广泛出露挟裹幔源捕虏体的新生代碱性玄武岩.为深入了解华北克拉通演化,依据岩相学特征、矿物化学成分特征,把昌乐碱性玄武岩中单斜辉石主要分为三大类:第1类为幔源捕虏体中原生单斜辉石,其主、微量元素含量呈现趋势不一致的解耦现象,且LREE、HREE各异特征表明其经历了多期、不同程度的地幔富集交代和部分熔融作用（低于10%的尖晶石相部分熔融）,交代熔体包括地幔富碱（K）富Al硅酸盐熔/流体,可能有碳酸盐熔体的贡献;第2类为幔源捕虏体中筛状单斜辉石,是部分熔融和熔体交代作用的共同产物,从无筛孔部位到筛孔周围远离筛孔部位和紧挨着筛孔部位,呈现出受熔体作用逐渐加强趋势,离筛孔较远部位记录了玄武质熔浆作用之前的一次富碱（K）富Al熔体交代的特征,而紧挨着筛孔的部位受晚期玄武质熔浆作用的影响最强;第3类为幔源捕虏体中反应边单斜辉石及玄武岩中斑晶单斜辉石,两者的化学成分及形成条件相似,显示HFSE正异常,LILE负异常,受OIB特征寄主玄武岩浆强烈的影响.昌乐新生代碱性玄武岩地幔捕虏体中单斜辉石显示经历多期"熔体作用"和不同程度的部分熔融特征,是研究区岩石圈地幔不均一性的体现. 相似文献
10.
蒙库铁矿床是一个以下泥盆统斜长角闪变粒岩(原岩为火山岩)为围岩的大型矽卡岩型矿床,矽卡岩矿物组合为辉石、石榴子石和方柱石,退化蚀变岩的组成矿物为角闪石、绿帘石、绿泥石、磷灰石等。电子探针分析结果表明,矽卡岩矿物中单斜辉石以透辉石为主,仅存在少量普通辉石;石榴子石端员组分以钙铁榴石为主,伴以少量钙铝榴石和锰铝榴石;角闪石属于单斜角闪石中的阳起石。蒙库铁矿床的矽卡岩与正常的矽卡岩矿床形成方式不同,不是中酸性岩浆与碳酸盐地层接触交代的产物,而是由热流体沿裂隙交代火山变质岩形成的。 相似文献
11.
B. A. Wyatt 《Contributions to Mineralogy and Petrology》1977,61(1):1-9
The anhydrous solidus of a natural clinopyroxene-ilmenite intergrowth from the kimberlite pipe at Monastery was found to be 1300° C at 20 kb increasing to 1470 ° C at 47 kb. The slope of the solidus is 6 ° C/kb, with a constant melting interval of approximately 140 ° C. Clinopyroxene was the liquidus phase in all but one case, and the ilmenite-out curve coincided with the beginning of melting. Controlled cooling experiments resulted in liquidus clinopyroxene crystals surrounded by intergrowths of clinopyroxene and ilmenite. The experimental and natural intergrowths are texturally, crystallographically and chemically similar. These results support the view that the natural clinopyroxene-ilmenite intergrowths found in kimberlites are a product of eutectic crystallisation in the upper mantle. Compositional similarities between the liquidus clinopyroxene and discrete clinopyroxene nodules from kimberlite suggest that the latter are possibly related to the clinopyroxene-ilmenite intergrowths by fractional crystallisation. Differences between the temperatures of equilibration obtained for clinopyroxenes from the natural intergrowths, and the beginning of melting on the clinopyroxene-ilmenite join, may be the result of hydrous melting in the former case, or as a consequence of the natural intergrowths existing as mantle veins or pegmatites at the ambient temperature prior to their incorporation in the kimberlite. 相似文献
12.
河北矾山杂岩体中单斜辉石的研究 总被引：3，自引：1，他引：2
河北矾山杂岩体属于二氧化硅不饱和的超钾质碱性-过碱性岩浆系列,由单斜辉石岩、辉石正长岩和碱长正长岩等不同类型岩石组成。各类型岩石主要组成矿物为单斜辉石、黑云母、石榴石和正长石。本文利用电子探针对单斜辉石进行了详细研究,发现所有单斜辉石属于高钙透辉石,随着岩浆的演化,主要表现为Fe2+对Mg2+的替代关系,结晶趋势为透辉石→钙铁辉石,这揭示矾山杂岩体岩浆体系的特点是高温、中等大小的氧逸度、贫硅、富碱（尤其是钾）。辉石中的AlIV含量取决于岩浆的硅饱和度,硅越不饱和,AlIV含量越高;AlVI含量则与体系中的Al含量呈正相关关系。由Al对Si的替代引起的电荷不平衡主要由八面体位置的Fe3+来补偿,其次为少量的Ti4+和更少量的Al3+。体系中的Ti含量与体系的温度呈正相关关系,而Na含量则与辉石中的Ti和Fe3+含量成正相关关系。不同类型岩石中单斜辉石从核部到边部随着Mg#的降低,Al、Ti含量呈现不同的演化趋势,这是因为它们于岩浆演化的不同阶段开始结晶,经历了不同的岩浆演化史。碱性岩中高钙辉石的出现和成分环带的普遍发育是由岩浆的贫硅富钾特征决定的。单斜辉石的成分不仅受控于结晶时的温度、压力条件,也受控于岩浆的总成分及其变异。 相似文献
13.
Suzanne Y. Wass 《Lithos》1979,12(2):115-132
Geochemical and textural data on clinopyroxenes in individual alkali basaltic flows from provinces in eastern Australia and the Massif Central can be used to differentiate four different modes of origin (three at high pressure) for these clinopyroxenes. Many single flows from the two provinces contain clinopyroxenes of three, or even four, of these origins. Rare flows contain core clinopyroxene with overgrowths of clinopyroxenes of two distinct generation. Each of the overgrowth is compositionally analogous to clinopyroxenes occurring in xenoliths or as discrete crystals in the same host lava. Such rimming relationships provide evidence that the host magma has undergone high pressure crystallisation and confirm that some xenoliths and megacrysts are cognate. With decreasing pressure the major changes in clinopyroxene chemistry are an increase in the ratio Allv/Alvi, a linear increase of atomic proportions of Ti and Al with decreasing Si, and an increase in the Ti: (100 Mg/(Mg + Σ Fe)) ratio of the pyroxenes. Al2O2 wt, % is an unreliable potential geobarometric indicator; consideration of tetrahedral and octahedral site occupancies by Al (Allv/Alvl ratio) is necessary. High-pressure fractionation dominated by clinopyroxene is postulated for some basaltic-composition, resulting in SiO2 depletion, alkali enrichment and decrease in the Mg/(Mg + Fe2+) value of the host magma. 相似文献
14.
Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia 总被引：12，自引：0，他引：12
Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet–spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser–ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field. 相似文献
15.
U. Wiechert Dmitri A. Ionov Karl Hans Wedepohl 《Contributions to Mineralogy and Petrology》1997,126(4):345-364
Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing
harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant
fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and
spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths.
Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope
composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were
also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace
elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable
degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted
clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in
LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to
the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals,
ubiquitous CO2-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized
trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic
enrichment. Low Cu and S contents and high δ34S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from
subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge
basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched
clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation
from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources.
Received: 18 January 1996 / Accepted: 23 August 1996 相似文献
16.
Slab melt and intraplate metasomatism in Kapfenstein mantle xenoliths (Styrian Basin, Austria) 总被引：1，自引：0，他引：1
Anhydrous and amphibole-bearing mantle peridotite xenoliths from Kapfenstein (Styrian Basin) have been studied with the aim of understanding both the processes responsible for amphibole formation and the nature of metasomatizing agents which affected this portion of lithosphere. This area of the Pannonian Basin underwent a subduction event which was followed after about 15 Ma, by alkaline intraplate magmatism. Primary clinopyroxene (cpx1) in four-phase lherzolite xenoliths is characterized by LREE-depleted to slightly LREE-enriched patterns. LREE-depleted cpx1 have low Th and U contents and Zr (and Hf) anomalies varying from slightly negative to positive. LREE-enriched cpx have high Th and U contents and remarkable positive anomalies of Zr and Hf. Primary clinopyroxenes in amphibole-bearing lherzolites present a comparable compositional variation from LREE (and Th, U, Zr, Hf)-depleted type to LREE (and Th, U, Zr, Hf)-enriched type. LREE-depleted cpx1, with strong negative Zr and Ti anomalies, are also recognized in the peridotite matrix of a composite sample cut by a large amphibole vein. Textural and geochemical evidence indicates that amphibole disseminated within the matrix grew at the expense of primary spinel and clinopyroxene, mimicking the trace element patterns of the latter. As a consequence, the geochemical features of amphibole vary in relation to those of clinopyroxene, from enriched to depleted. On the other hand, the composition of vein amphibole in the composite xenolith compares well with amphibole megacrysts and microphenocrysts, suggesting that it represents a fractionation product of alkaline melt that passed through the lithosphere. Two kinds of metasomatism, superimposed on a slightly depleted lithospheric mantle, were identified. A slab-derived melt (proto-adakite?) metasomatic agent was responsible for the first enrichment in Th, U, Zr and Hf observed in clinopyroxene, whereas an alkaline within-plate metasomatic agent caused the formation of the Nb (and Ta)- rich disseminated amphibole. The final process was the alkaline magmatism, which was responsible for the formation of the large amphibole vein and megacrysts. It is proposed that the Nb-poor and Nb-rich amphiboles record the transition between the suprasubduction slab melt-related and the intraplate alkaline metasomatism.
These geochemical features are consistent with a lithospheric portion enriched in slab melt components which was subsequently metasomatized by alkaline melt. Alternatively an asthenospheric uprising could have scavenged a previously slab melt-enriched region of the lithosphere. 相似文献
17.
Many of the coarse-grained peridotite inclusions in basanitesfrom Nunivak Island, Alaska, contain amphibole and a smallerfraction also contain phlogopite and apatite. All of these peridotiteshave light REE/heavy REE abundance ratios greater than chondritesand many have abundances of K, Rb, Sr, Ba and light REE whichexceed estimates for primitive mantle. On the basis of mineraltextures and compositions we infer that the clinopyroxene, amphibole,phlogopite and apatite equilibrated with a metasomatic fluid.Isotopic (Sr and Nd) ratios and parent-daughter abundance datafor the coarse-grained peridotites constrain the age of themetasomatism to be less than 200 million years. Associated amphibole pyroxenite inclusions are not metasomatized;these inclusions probably formed as crystal segregates froman alkalic magma. Both pyroxenites and coarse-grained peridotitesare isotopically similar to basalts from Nunivak Island. Usingthese data, we propose a model in which the metasomatized peridotiteswere wallrocks located adjacent to the pyroxenites, and thatmetasomatism of these peridotites was caused by the infiltrationof a residual silicate melt or volatile-rich fluid derived fromthe parental magma of the pyroxenites; i.e. the metasomatismwas a consequence of basaltic magmatism. Furthermore, the parentalmagma of the pyroxenites was probably petrogenetically relatedto the Nunivak volcanism. REE modelling of fluids in equilibriumwith clinopyroxenes from the coarse-grained peridotites is consistentwith this model. 相似文献
18.
A clinopyroxene suite from Somma-Vesuvius clinopyroxenite cjectaand mcgacrysts has been investigated by single crystal X-raydiffraction combined with electron probe microanalysis. Theaim of the study was to characterize the intracrystalline configurationof the clinopyroxenes and delineate their possible parentalliquid(s). Vesuvius megacrysts and nodule clinopyroxenes show the initial‘signature’ of the leucitite (sensu lato) magmawhile Somma clinopyroxenes are characteristic of leucite phonolitemagma. The crystal chemical variations demonstrate that thespecimens investigated had originated by shallow-level crystallizationof leucitite and its derivatives, under reducing conditions.T-site (Si + Al) deficiency, typical of clinopyroxene from lamproitesis reported in some specimens from the Roman Volcanic Regionfor the first time. Intracrystalline Mg-Fe2+ ordering in M1-M2 appears to be configuration-dependent,yielding the largest KD variation in the clinopyroxene fromphonolite. This is contrary to expectation. 相似文献
19.
Jian Wang Kéiko H. Hattori Rolf Kilian Charles R. Stern 《Contributions to Mineralogy and Petrology》2007,153(5):607-624
Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel
harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths
have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain
websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine.
These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence
of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich
parental magma of the websterite veinlets. The fO2 values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric
mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous
fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO2 ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate
minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches
of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO2 (<0.56 wt%), Cr2O3 (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic
agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in
clinopyroxenes. F-type xenoliths show relatively low fO2 (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying
the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments
likely produced CH4-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths
support the proposed interpretation. 相似文献
20.
Geert-Jan L. de Haas Timo G. Nijland Peter J. Valbracht Cees Maijer Rob Verschure Tom Andersen 《Contributions to Mineralogy and Petrology》2002,143(5):537-550
An SEM, REE, and mineral Sm-Nd isotope study on olivine-plagioclase coronas in the Vestre Dale gabbro, Norway, was carried out in order to solve the temporal relationships within these coronas. It is demonstrated that in contrast to common models, corona formation is a multistage, late-magmatic process, starting with the formation of orthopyroxene by (partial) dissolution of olivine, followed by the nucleation and rapid outward growth of orthopyroxene+spinel symplectites at its outer margin, and concluded by replacement of this precursory outer shell by calcic amphibole. Isotopic equilibrium between inner shell orthopyroxene and the magmatic assemblage, and the REE contents of orthopyroxene and outer shell amphibole also provide strong arguments for a magmatic origin, with the local availability of fractionated magma acting as a limiting factor. The absence of replacement of intercumulus clinopyroxene by amphibole also favours a late stage magmatic rather than a metamorphic origin for corona amphibole. 相似文献