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1.
大别造山带祝家铺辉长岩的铂族元素特征   总被引：8，自引：2，他引：8  
采用镍锍火试金法结合ICP-MS分析了祝家铺14个辉长岩样品中的Ir，Ru，Rh，Pt和Pd的含量。结果显示其PGE的含量较低，原始地幔标准化后的PGE分布模式呈正斜率型，PPGE相对原始地幔略微亏损，而IPGE强烈亏损，Pd／Ir值(22—138)远高于相应的地幔比值，表明铂族元素发生了分异。对祝家铺辉长岩的铂族元素研究表明，在其源区发生过硫化物的分异作用，地壳的混入可能促进了硫化物的饱和。祝家铺辉长岩中铂族元素的分异是因为在地幔部分熔融和岩浆演化过程中，PPGE主要受硫化物控制，而Ir则存在于非硫化物相如尖晶石、可能还有合金之中。  相似文献   

2.
采用镍锍火试金法结合ICP-MS分析了12个北大别白垩纪镁铁-超镁铁岩样品的Ir,Ru,Rh,Pt和Pd的含量,结果显示铂族元素（PGE）的含量较低,原始地幔标准化后的PGE分布模式呈正斜率型,PPGE相对原始地幔略微亏损,而IPGE强烈亏损,Pd/Ir值远高于相应的地幔比值。这些镁铁-超镁铁岩中PGE的强烈分异是地幔低程度的部分熔融过程中,PPGE主要受硫化物控制,而Ir则存在于非硫化物相如尖晶石,可能还有合金之中造成的。同时,铂族元素的分布特征表明这些镁铁-超镁铁岩是岩浆结晶分异的产物。  相似文献   

3.
云南鸡街碱性超基性岩铂族元素地球化学特征   总被引：1，自引：1，他引：0  
赵正  漆亮  黄智龙  严再飞  许成 《岩石学报》2010,26(3):938-946
本文首次报道了攀西裂谷南段云南省鸡街碱性超基性岩中铂族元素(PGE)的地球化学特征。采用改进的Carius管法测定了霞霓钠辉岩、霓霞岩和磷霞岩中的低含量PGE。PPGE与IPGE呈现强烈分馏,推断幔源岩在低程度部分熔融过程中Pt、Pd表现为相似的不相容性,而Ir、Ru表现为相容性,这种分馏效应随着结晶分异作用的进行而逐渐增强。3种岩石均出现Ru的负异常及Pt、Pd的解耦,说明母岩浆经历了早期的橄榄石晶出,在结晶分异过程中Pd较Pt更不相容。由于岩浆上升过程中的压力减小和结晶分异作用导致的成份变化使岩浆可以达到硫的局部饱和而熔离,表现为部分样品中Cu/Pd远高于原始地幔值。本文通过碱性超基性岩与金伯利岩、煌斑岩和邻区碱性、过碱性玄武岩PGE特征的对比,探讨了其岩浆源区及演化特征。  相似文献   

4.
四川攀枝花地区出露有新元古代苦橄质岩脉。本文研究表明这些苦橄质岩脉的铂族元素(PGE)含量较高(19.7~29.0 ng/g),原始地幔标准化后的PGE分布模式呈Pt-Pd富集型,Pd/Ir值(5.64~11.33)与高镁玄武岩和科马提岩相似。同时,这些岩石显示在形成过程中没有经历硫化物和PGE合金矿物的熔离,其原始岩浆起源于地幔较高程度的部分熔融,可能与地幔柱的影响有关。通过扣除铬尖晶石和橄榄石结晶分异对PGE造成的影响,得到原始岩浆的PGE组成特征为Ir、Ru、Rh相对于Pt、Pd明显亏损,在源区已无硫化物存在的条件下,这很可能是由于地幔部分熔融过程中有IPGE合金矿物残留在地幔源区。攀枝花地区苦橄质岩脉可能与该地区冷水箐Cu-Ni硫化物矿床具有相似的原始岩浆组成。  相似文献   

5.
白鑫滩铜镍硫化物矿床位于东天山镁铁-超镁铁质岩带西段,受大草滩深大断裂控制,岩体由辉长岩、橄榄辉石岩、辉石橄榄岩组成。白鑫滩岩体的铂族元素(PGE)含量相对较低,其中IPGE(Os、Ir、Ru)与PPGE(Ru、Pt、Pd)含量相近,PPGE略高于IPGE。岩石∑PGE含量3.82×10~(-9)～5.36×10~(-9),平均4.53×10~(-9)。岩石具相似的原始地幔标准化分布型式,PPGE和IPGE之间分异很弱。通过铂族元素源区示踪分析认为,形成白鑫滩岩体的母岩浆为Mg O含量较高的PGE不亏损的拉斑玄武质岩浆,上升过程中遭地壳物质混染及橄榄石等矿物的分离结晶作用,引起该矿床硫饱和并发生硫化物熔离作用而成矿,深部岩浆通道仍具较好的找矿空间。  相似文献   

6.
陕西太白金矿含金角砾岩中铂族元素特征   总被引：1，自引：2，他引：1  
采用硫镍火法试金(NiS-FA)结合电感耦合等离子质谱(ICP-MS)分析了太白金矿硫化物和含金角砾岩中铂族元素的含量,结果显示,与秦岭地区八卦庙相比铂族元素含量较高,而低于原始地幔,其中铂(Pt)、钯(Pd)、钌(Ru)富集,并结合前人研究资料对铂族元素的来源和迁移机制进行探讨。铂族元素可能受深源的影响,IPGE(Ir、Os、Ru)可能主要以硫化物形式存在而PPGE(Rh、Pt、Pd)可能主要以单质存在。  相似文献   

7.
大别造山带毛屋超镁铁岩的铂族元素研究   总被引：1，自引：1，他引：1       下载免费PDF全文
采用镍锍火试金法结合ICP-MS分析了毛屋斜方辉石岩和石榴二辉岩样品中的Ir、Ru、Rh、Pt和Pd的含量,结果显示其铂族元素(PGE)的含量随岩石类型无规律性的变化,原始地幔标准化后的铂族元素分布模式呈负斜率,Pd、Ir发生了分异。毛屋超镁铁岩铂族元素特征的形成受岩石中铂族元素的存在相制约,PPGE富集在富Cu硫化物,而IPGE以类似残留相、不熔的单硫化物固熔体形式存在,其中地壳混染也起了一定的作用;同时,成岩过程中流体的存在造成了Pt和Pd的活化。因此,单硫化物固熔体和流体的共同作用形成了毛屋超镁铁岩类似残留地幔岩的铂族元素分布特征。  相似文献   

8.
采用镍硫火试金ICP-MS法分析了白马寨铜镍硫化物矿床矿石及其主要围岩铂族元素(PGE)含量,结果表明铂族元素总量(ΣPGE)普遍较低,且ΣPGE由矿床核部的块状矿石(78.15×10-9~555.92×10-9)向外围矿化橄榄岩(6.77×10-9~70.95×10-9)至辉长岩(3.02×10-9)逐步降低。煌斑岩脉岩的ΣPGE也较低,为6.86×10-9~17.03×10-9。ΣPGE与Ni含量呈明显的正相关。不同种类岩石和矿石的PGE原始地幔配分模式明显不同,块状矿石为Pt-Pd配分型,Ir较明显亏损;矿化橄榄岩配分曲线较平坦,IPGE和PPGE分异不明显,但均出现Ru正异常。辉长岩样品和煌斑岩PGE配分模式几乎一致,也均出现明显的Ru正异常。矿石和围岩的Cu/Pd值远高于原始地幔,表明原生岩浆发生了较强的硫化物熔离作用。块状富镍矿石的(Pt+Pd)/(Os+Ir+Ru)和Pd/Ir分别为138.24和24.7,指示其母岩浆为演化较为彻底的玄武质岩浆,而矿化橄榄岩的相应值分别为3.02和16.09,具有科马提质岩浆的特征参数,因此白马寨块状矿石和矿化橄榄岩之间可能没有成因联系。块状硫化物熔体侵位前,其母岩浆经历过长时间强烈的硫化物熔离作用,硫化物熔浆作为独立的侵入相而侵入于III号岩体核部。这个过程反映了峨眉山地幔柱尾部岩浆演化和成矿可能经历过高度的S饱和过程。白马寨矿床形成过程中可能受到地壳物质的混染。  相似文献   

9.
作为塔里木大火成岩省形成最晚的火成岩,新疆巴楚瓦吉里塔格霞石岩的岩浆源区性质的确定对于揭示塔里木大火成岩省的深部地质过程具有重要的约束作用。对瓦吉里塔格霞石岩的铂族元素地球化学特征进行了研究,铂族元素(PGE)分析结果显示,原始地幔标准化的PGE呈正斜率型分布,且Pd/Ir值高于原始地幔比值,说明霞石岩的铂族元素发生了分异。霞石岩全岩的PGE与Mg O呈正相关,Pd/Ir、Cu/Pd与Mg O则呈负相关,说明PGE的分异主要受到橄榄石的结晶分异作用控制,也是其Cu/Pd值极高及岩浆S饱和的因素之一,同时Cu/Pd值说明霞石岩岩浆为硫饱和岩浆,但是没有因素导致岩浆S过饱和进而发生硫化物的熔离。与其他大火成岩省岩石相比,瓦吉里塔格霞石岩极度亏损PGE,SCSS(硫承载量)计算结果表明母岩浆在形成之初就发生S过饱和,主要是地幔低程度部分熔融造成的,据此认为地幔源区的部分熔融程度在塔里木大火成岩省Cu-Ni硫化物铂族元素矿床形成过程中起着至关重要的作用。  相似文献   

10.
对位于雅鲁藏布江缝合带西段北亚带错不扎地幔橄榄岩铂族元素地球化学特征进行研究,旨在对其岩石成因和构造环境进行限定。错不扎地幔橄榄岩以方辉橄榄岩和含单辉方辉橄榄岩为主。含单辉方辉橄榄岩具有较高的铂族元素(PGEs)含量,w(PGEs)=22.31×10~(-9)~36.12×10~(-9),平均值为29.60×10~(-9),高于原始地幔含量;而方辉橄榄岩的w(PGEs)处于17.34×10~(-9)~25.18×10~(-9)之间,平均值为21.08×10~(-9),略低于原始地幔值。相对岩浆成因超基性岩,两岩类皆强烈富Os和Ir,为部分熔融后残余的地幔体。含单辉-方辉橄榄岩及部分方辉橄榄岩含较高的Pd,(Pd/Ir)m比值远大于1,具左倾型PGE组成模式,而且两者都具有低于原始地幔的Cu/Pd比值。综合研究表明错不扎方辉橄榄岩是熔融程度较低的含单辉-方辉橄榄岩继续部分熔融后的残余;两者在部分熔融过程中可能存在部分赋存Pd的硫化物未完全溶解进入硅酸盐熔体,此后又受到俯冲带岩浆渗滤形成的富硫化物熔体的改造。  相似文献   

11.
Platinum group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd) are important geochemical and cosmochemical tracers. Depending on physical and chemical behaviour the PGEs are divided into two subgroups: IPGE (Ir, Os, Ru) and PPGE (Pd, Pt, Rh). Platinum group elements show strong siderophile and chalcophile affinity. Base metal sulfides control the PGE budget of the Earth’s mantle. Mantle xenoliths contain two types of sulfide populations: (1) enclosed within silicate minerals, and (2) interstitial to the silicate minerals. In terms of PGE characters the included variety shows IPGE enriched patterns — similar to the melt-depleted mantle harzburgite, whereas the interstitial variety shows PPGE enriched patterns — resembling the fractionated PGE patterns of the basalt. These PGE characters of the mantle sulfides have been interpreted to be representative of multi-stages melting process of the mantle that helped to shape the chemical evolution of the Earth.  相似文献   

12.
The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block.In the present study,the Wuguiqing profile in thickness of about 1440 m is mainly composed of high-Ti basalts,with minor picrites in the lower part and andesites,trachytes,and rhyolites in the upper part.The picrites have relatively higher platinum-group element（PGE） contents（ΣPGE=16.3-28.2 ppb）,with high Cu/Zr and Pd/Zr ratios,and low S contents（5.03-16.9 ppm）,indicating the parental magma is S-unsaturated and generated by high degree of partial melting of the Emeishan large igneous province（ELIP） mantle source.The slightly high Cu/Pd ratios（11 000-24 000） relative to that of the primitive mantle suggest that 0.007%sulfides have been retained in the mantle source.The PGE contents of the high-Ti basalts exhibit a wider range（ΣPGE=0.517-30.8 ppb）.The samples in the middle and upper parts are depleted in PGE and haveεNd（260 Ma） ratios ranging from -2.8 to -2.2,suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%-0.554% sulfides,and the sulfide segregation process may also provide the ore-forming material for the magmatic Cu-Ni-PGE sulfide deposits close to the studied basalts.The samples in this area show Pt-Pd type primitive mantle-normalized PGE patterns,and the Pd/Ir ratios are higher than that of the primitive mantle（Pd/Ir=1）,indicating that the obvious differentiation between Ir-group platinum-group elements（IPGE） and Pd-group platinum-group elements（PPGE） are mainly controlled by olivine or chromites fractionation during magma evolution.The Pd/Pt ratios of most samples are higher than the average ratio of mantle（Pd/Pt=0.55）,showing that the differentiation happened between Pt and Pd.The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt-alloy phase in the mantle;whereas the differentiation in the high-Ti basalts is probably associated with the fractionation of Fe-Pt alloys,coprecipitating with Ir-Ru-Os alloys.Some high-Ti basalt samples exhibit negative Ru anomalies,possibly due to removal of laurite collected by the early crystallized chromites.  相似文献   

13.
Thirty-three whole-rock drill core samples and thirteen olivine, chromite, and sulfide separates from three differentiated komatiite lava flows at Alexo and Pyke Hill, Canada, were analyzed for PGEs using the Carius tube digestion ID-ICP-MS technique. The emplaced lavas are Al-undepleted komatiites with ∼27% MgO derived by ∼50% partial melting of LILE-depleted Archean mantle. Major and minor element variations during and after emplacement were controlled by 30 to 50% fractionation of olivine Fo93-94. The emplaced lavas are characterized by (Pd/Ir)N = 4.0 to 4.6, (Os/Ir)N = 1.07, and Os abundances of ∼2.3 ppb. Variations in PGE abundances within individual flows indicate that Os and Ir were compatible (bulk DOs,Ir = 2.4-7.1) and that Pt and Pd were incompatible (bulk DPt,Pd < 0.2) during lava differentiation, whereas bulk DRu was close to unity. Analyses of cumulus olivine separates indicate that PGEs were incompatible in olivine (DPGEsOl-Liq = 0.04-0.7). The bulk fractionation trends cannot be accounted for by fractionation of olivine alone, and require an unidentified Os-Ir-rich phase. The composition of the mantle source (Os = 3.9 ppb, Ir = 3.6 ppb, Ru = 5.4 ppb, Pt and Pd = 5.7 ppb) was constrained empirically for Ru, Pt, and Pd; the Os/Ir ratio was taken to be identical to that in the emplaced melt, and the Ru/Ir ratio was taken to be chondritic, so that the absolute IPGE abundances of the source were determined by Ru. This is the first estimate of the PGE composition of a mantle source derived from analyses of erupted lavas. The suprachondritic Pd/Ir and Os/Ir of the inferred Abitibi komatiite mantle source are similar to those in off-craton spinel lherzolites, orogenic massif lherzolites, and enstatite chondrites, and are considered to be an intrinsic mantle feature. Bulk partition coefficients for use in komatiite melting models derived from the source and emplaced melt compositions are: DOs,Ir = 2.3, DRu = 1.0, DPt,Pd = 0.07. Ruthenium abundances are good indicators of absolute IPGE abundances in the mantle sources of komatiite melts with 26 to 29% MgO, as Ru fractionates very little during both high degrees of partial melting and lava differentiation.  相似文献   

14.
The peridotites of the Manipur Ophiolite Complex (MOC) have been examined based on mineral chemistry, major elements and PGE contents. They represent high-magnesian cumulates with Mg# > 0.90 (Mg/Mg+Fe) in olivine and Cr# > 0.12 (Cr/Cr+Al) in spinel. High Mg* contents of the olivine show that these rocks are most likely derived from partial melting of the residual upper mantle. The peridotites contain higher concentration of Palladium Group PGE (PPGE) (Rh=4.4−6.6ppb; Pd=336−458ppb and Pt=14.6−36.4ppb) than the Iridium Group PGE (IPGE) (Os=2.4−5.8ppb; Ir=3.2−4.16ppb and Ru=5.2−7ppb). These are characterized by overall enrichment of PGE concentration (σPGE=365.8 − 516.6 ppb) and high ratio of (Pt+Pd)/(Os+Ir+Ru). This suggests that the rocks are formed by partial melting and crystal fractionation of olivine-rich (picritic) magma.  相似文献   

15.
Summary The distribution of platinum group elements (PGE) within individual lithological units of the dismembered ophiolite of the Great Serpentinite Belt in New South Wales displays distinctive patterns. Within the ophiolite the PGE are mainly magmatic in origin, although the whole sequence has been extensively metamorphosed and deformed. The PGE in this ophiolite demonstrate fractionation resulting from magmatic processes.Harzburgite is characterised by a flat normalised PGE pattern, with only a slight depletion in PPGE. The minor PGE differentiation in the residual mantle rocks is probably due to the control on the PGE distribution by residual alloys and sulfides. This implies that the primary magma, generated from partial melting, was S-saturated.Cumulates of the overlying magmatic sequence show a positively sloped PGE pattern, favouring PPGE enrichment. PGE distribution in the cumulate sequence was controlled by immiscible sulfides, resulting in a similar PGE pattern for individual members of the cumulates. The highest PGE content in the magmatic section is recorded in the banded chromitite where the PGE enrichment probably results from upward-migrating magmatic fluids.Podiform chromitite is the earliest fractionated product from ascending partial melts within narrow magma conduits that channeled melts from the mantle source up to the overlying magma chamber. Such a process operated at high temperatures, hence the high melting-point IPGE was preferentially crystallised along with the chromites so that podiform chromitite displays a negatively sloped PGE pattern. Normally, sulfide saturation in the ascending melt does not take place until the melt enters the crustal magma chamber. However, immiscible sulfide liquids might have been present temporarily in some high-level podiform chromitite to generate a Pt- and Pd-enriched pod. Chromite in this pod is less in both Cr/(Al + Cr) and Mg/(Mg + F2+) than in those of other podiform chromitites that are dominated by IPGE and, therefore, the composition of chromite is of significance in identifying the potential Pt- and Pd-rich chromitites in this ophiolite belt.
Verteilung der Platingruppen-Elemente im Great Serpentinite Belt von New South Wales, Ost-Australien
Zusammenfassung Die Verteilung der Platingruppen-Elemente (PGE) innerhalb der einzelnen lithologischen Einheiten des zerbrochenen Ophiolites des Great Serpentinite Belt in New South Wales zeigt charakteristische Verteilungsmuster. Die PGE sind überwiegend magmatischen Ursprungs, obwohl der gesamte Komplex intensiv metamorphosiert und deformiert worden ist. Innerhalb des Ophiolites zeigen die PGE Fraktionierung, die das Resultat magmatischer Prozesse ist.Der Harzburgit ist durch flache, normierte PGE Verteilungskurven charakterisiert, die lediglich eine schwache Verarmung an PPGE zeigen. Die geringe PGE Differenzierung in den residualen Mantelgesteinen wird durch die Steuerung der PGE Verteilung durch residuale Legierungen und Sulfide kontrolliert. Dies bedeutet, daß das durch Teilaufschmelzung entstandene Magma S-gesättigt gewesen ist.Die Kumulate der hangenden, magmatischen Abfolge zeigen positive PGE Verteilungskurven, die auf eine Anreicherung der PPGE hinweisen. Die PGE Verteilung in der Kumulat-Sequenz wurde durch entmischte Sulfide kontrolliert, weshalb die einzelnen Schichtglieder der Kumulat-Abfolge ähnliche PGE Verteilungsmuster aufweisen. Die gebänderten Chromitite zeigen die höchsten PGE Gehalte der magmatischen Abfolge, die Anreicherung der PGE ist vermutlich auf aufsteigende, magmatische Fluida zurückzuführen.Der podiforme Chromitit ist das früheste Fraktionierungsprodukt der vom Mantel durch enge Kanäle in die Magmakammer aufsteigenden Teilschmelzen. Ein derartiger Prozeß findet bei hohen Temperaturen statt, weshalb die IPGE, die hohe Schmelzpunkte aufweisen, zusammen mit dem Chromit zur Kristallisation gelangten, podiforme Chromitite zeigen daher negative PGE Verteilungskurven. Normalerweise findet eine Schwefel-Sättigung der aufsteigenden Schmelze nicht vor dem Eintritt in die krustale Magmenkammer statt. Temporär könnte jedoch eine entmischte Schwefel-Schmelze bereits in einigen high level podiformen Chromititen existiert haben, sodaß ein Pt- und Pd-angereicherter Pod entsteht. Der Chromit in diesem Pod zeigt niedere Cr/(Al + Cr) und Mg/(Mg + Fe2+) Verhältnisse als jene in anderen podiformen Chromititen, die von IPGE dominiert sind. Die Zusammensetzung des Chromites ist daher signifikant, um Pt-und Pd-reiche Chromitite innerhalb dieses Ophiolit-Gürtels zu identifizieren.

With 9 Figures  相似文献   

16.
本文对塔里木盆地二叠纪大火成岩省中瓦吉里塔格地区超镁铁质隐爆角砾岩进行了铂族元素(PGE)和主、微量及稀土元素分析和研究。结果显示,隐爆角砾岩中Os、Ir、Ru、Rh、Pt和Pd含量分别为0.36×10-9～1.08×10-9、0.23×10-9～0.44×10-9、0.29×10-9～0.92×10-9、0.11×10-9～0.18×10-9、1.88×10-9～3.16×10-9和1.39×10-9～3.52×10-9,均低于原始地幔,与夏威夷苦橄岩相似。该岩石的Pd/Ir比值在3.6～11.9之间,PGE分配模式呈一条正倾斜的曲线,表现出一定程度的分异,具有非俯冲背景下产生的基性-超基性岩的PGE配分特点。略高于原始地幔但变化较小的Cu/Pd比值(5.1×104～12.1×104)表明其岩浆在上升侵位过程中并没有发生明显的饱和硫化物熔离作用,而岩浆源区在部分熔融过程中可能有少量残留的硫化物存在。隐爆角砾岩全岩的IPGE元素与MgO之间基本上呈正相关,而PPGE元素与MgO之间则略成负相关或无明显相关性,指示PGE的分异主要受到橄榄石结晶分异作用的控制。地球化学特征显示隐爆角砾岩的稀土元素总量高度富集(964.1×10-6～1299×10-6)和轻、重稀土强烈分馏((La/Yb)N=45.88～64.90),且微量元素蛛网图上大离子亲石元素富集和Nb、Ta的轻微亏损以及Zr、Hf的明显贫化,表明岩石可能遭受一定程度的地幔交代作用影响。但是,角砾与胶结物具有相近的PGE特征表明交代作用对PGE的影响并不大,暗示PGE可能主要赋存于禁锢在硅酸盐矿物内的硫化物包裹体中。  相似文献   

17.
The Bulqiza ultmafic massif, which belongs to the eastern Mirdita ophiolite of northern Albania, is world renowned for its high‐Cr chromite resource. The high‐Cr chromitites commonly host in the mantle section, while high‐Al chromitites also present in massive dunite of the mantle‐crust transition zone (MTZ) in this massif. Chromian‐spinel in the MTZ high‐Al chromitites and MTZ dunites have much lower Cr# values [Cr/(Cr+Al)×100] (47.7–55.1 and 46.5–51.7, respectively) than those of chromian‐spinel in the high‐Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). The high‐Cr chromitites are rich in IPGE relative to PPGE with 0.10–0.45 PPGE/IPGE ratios, whereas the high‐Al chromitites have higher PPGE/IPGE ratios between 1.20 and 7.80. The partial melting degrees of parental magmas for the high‐Cr chromitites are beyond the critical interval (> 25%) and thus prevented sulfide saturation and diluted Pt and Pd in melts, producing high‐Cr chromitites barren of Pt and Pd. However, the degrees for the high‐Al chromitites just enter the critical interval (20–25%) for the effective extraction of PGE from mantle sulfides, which may account for the enrichments of PPGE in high‐Al chromitites. The parental melts of the high‐Cr chromitites have Al2O3 and TiO2 contents of ～10.6–11.4 wt.% and 0.14–0.31 wt.%, whereas the calculated Al2O3 and TiO2 for the high‐Al chromitites are ～14.9–15.9 wt.% and 0.07–0.61 wt.%, respectively. The calculated melts in equilibrium with the high‐Cr chromitites are boninitic‐like, and those with high‐Al chromitites are MORB‐like but with hydrous, oxidized and TiO2‐poor affinities. To make a compromise between the inconsistence above, we proposed that coexistence of both types of chromitites in the Bulqiza ultramafic massif may reflect that their magma compositions transited from MORB‐like to boninitic‐like in a proto‐forearc setting during subduction initiation. Key words: Chromian‐spinel, Platinum‐group elements, high‐Cr and high‐Al chromitite, Mirdita ophiolite, Albania.  相似文献   

18.
Fourteen peridotite xenoliths collected in the Massif Central neogene volcanic province (France) have been analyzed for platinum-group elements (PGE), Au, Cu, S, and Se. Their total PGE contents range between 3 and 30 ppb and their PGE relative abundances from 0.01 to 0.001 × CI-chondrites, respectively. Positive correlations between total PGE contents and Se suggest that all of the PGE are hosted mainly in base metal sulfides (monosulfide solid solution [Mss], pentlandite, and Cu-rich sulfides [chalcopyrite/isocubanite]). Laser ablation microprobe-inductively coupled plasma mass spectrometry analyses support this conclusion while suggesting that, as observed in experiments on the Cu-Fe-Ni-S system, the Mss preferentially accommodate refractory PGEs (Os, Ir, Ru, and Rh) and Cu-rich sulfides concentrate Pd and Au. Poikiloblastic peridotites pervasively percolated by large silicate melt fractions at high temperature (1200°C) display the lowest Se (<2.3 ppb) and the lowest PGE contents (0.001 × CI-chondrites). In these rocks, the total PGE budget inherited from the primitive mantle was reduced by 80%, probably because intergranular sulfides were completely removed by the silicate melt. In contrast, protogranular peridotites metasomatized by small fractions of volatile-rich melts are enriched in Pt, Pd, and Au and display suprachondritic Pd/Ir ratios (1.9). The palladium-group PGE (PPGE) enrichment is consistent with precipitation of Cu-Ni-rich sulfides from the metasomatic melts. In spite of strong light rare earth element (LREE) enrichments (Ce/YbN < 10), the three harzburgites analyzed still display chondrite-normalized PGE patterns typical of partial melting residues, i.e., depleted in Pd and Pt relative to Ir and Ru. Likewise, coarse-granular lherzolites, a common rock type in Massif Central xenoliths, display Pd/Ir, Ru/Ir, Rh/Ir, and Pt/Ir within the 15% uncertainty range of chondritic meteorites. These rocks do not contradict the late-veneer hypothesis that ascribes the PGE budget of the Earth to a late-accreting chondritic component; however, speculations about this component from the Pd/Ir and Pt/Ir ratios of basalt-borne xenoliths may be premature.  相似文献   

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