The role of apatite fractionation and REE distribution in alkaline rocks from Mt. Etna, Sicily

Summary Melt inclusions in olivine and apatite, and REE distribution of apatite were studied in one of the least differentiated members of the oldest alkaline succession of Mt. Etna. Apatite occurs both as microphenocrysts and as inclusions in olivine crystals, even in the most Mg-rich ones (Fo₈₂). In addition phenocrysts and groundmass are composed of plagioclase, clinopyroxene, olivine and magnetite. Apatite is fluor-apatite, with rather homogeneous major element (measured by electron microprobe, EMP) and REE (measured by laser-ablation microprobe, LAM, and by secondary ion mass spectrometer, SIMS) contents. REE are enriched when compared to the whole rock, with contents in olivine-hosted apatite lower than or the same as those of the microphenocryst cores; these in turn show lower REE values than their edges. Distribution coefficients, calculated from LAM data of microphenocryst edges and whole rock analyses, are higher for LREE (8–12) than for HREE (5–4). In the SiO₂ vs. P₂O₅ diagram melt inclusions and whole rock samples define a trend that is consistent with continuous apatite extraction from a “high P” basalt magma. Finally, whole rock data show LREE/HREE (La/Lu)_n enrichment ratios from hawaiites to mugearites (=1.14), consistent with apatite fractionation, lower than those documented for lavas of the “low P” type (enrichment ratio = 1.34–1.37), where conditions for apatite saturation were not established. Received January 2, 2000; revised version accepted April 2, 2001     

贵州织金磷块岩型稀土矿含矿岩系REE地球化学特征与稀土富集

贵州织金下寒武统戈仲武组(?1gz)磷矿富集大量稀土,但是磷块岩中REE富集机制问题尚不清楚。本文对贵州织金不同稀土含量区间(262×10~(-6)、262×10~(-6)～527×10~(-6)、527×10~(-6)～761×10~(-6)、761×10~(-6))的磷块岩的研究表明,磷块岩的稀土配分均显示出Ce负异常、Eu无明显异常、MREE富集以及富集重稀土元素Y的特征。(La/Sm)N-δCe及δPr-δCe图解显示织金磷块岩Ce异常为真实的Ce异常,代表磷块岩形成在氧化环境。但当时海水的氧化环境并不是控制MREE富集的决定因素。(La/Sm)N-SmN、(Gd/Yb)N-YbN散点图显示MREE富集是稀土在富集过程中稀土发生分异的结果。(Dy/Sm)N-δEu散点图以及Eu无明显异常说明织金磷块岩形成过程中可能无热水作用的参与。地史时期"老磷块岩"普遍存在重稀土亏损的特征。     

REE Geochemical Characteristics of Volcanic Rocks in Zhejiang and Jiangxi Provinces and Their Gological Significance

Based on the data of 64 samples ,the REE geochemical characteristics of volcanic rocks in northern Zhejiang and eastern Jiangxi provinces are discussed in this paper.The REE distribution patterns in acid and intermediate-acid volcanic rocks in these areas display some similarities,as indicated by rightward-inclined V-shaped curves with negative Eu anomalies,which are parallel to earch other.In addi-tion,their REE parameters(ΣREE,ΣLREE/ΣHREE,δEu,Ce/Yb,La/Sm,La/Yb,etc)also va-ry over a narrow range with small deviations.HREE are particularly concentrated in the volcanic rocks as-sociated with uranium mineralization.The initial ^87Sr/^86Sr ratio in the volcanic rocks is about 0.7056-0.7139.All these features in conjunction with strontium isotopic data indicate that the rock-forming materials come from the sialic crust.The REE distribution patterns and REE geochemical parameters of the volcanic rocks ,as well as La/Sm-La and Ce/Yb-Eu/Yb diagrams may be applied to the sources of rock-forming and ore-forming materials.     

REE geochemical characteristics of the two types of granitic rocks in Jiangxi Province and their metallogenic significance

Preliminary studies have shown that REE geochemical characteristics are useful criteria to discriminate the two types of granitic rocks in Jiangxi Province. They are also of significant indication of metallization. The principal REE geochemical indices are ΣREE, ΣCe/ΣY andδEu, as well as REE distribution pattern. Each of the two types of granitic rocks has a metallogenic evolutionary series of its own with a successive decrease inδEu. The application of REE geochemistry in the study of petrogenesis and ore genesis has been receiving increasing attention. As will be shown in this paper, REE geochemical characteristics, especially ΣREE, ΣCe/ΣY,δEu and REE distribution pattern, are efficient indicators for distinguishing between the two series of granitic rocks in Jiangxi, and thus enable us to have a better understanding of the ore-forming processes in this region.     

滇西龙陵-瑞丽断裂带早白垩世火山岩LA-ICP-MS锆石U-Pb定年和地球化学特征

通过对龙陵-瑞丽断裂与主高黎贡断裂夹持区内构造混杂岩进行地质填图和剖面研究,在原划的三叠纪扎多组中发现沿龙陵-瑞丽断裂带呈透镜状断续分布的中酸性火山岩,并获得LA-ICP-MS锆石U-Pb年龄130.0Ma±1.7Ma,首次证实滇西龙陵-瑞丽断裂带存在早白垩世火山岩。结合岩石学特征和地球化学特征判断该火山岩形成于岛弧环境,可能是早白垩世怒江洋闭合过程中岩浆活动的产物。     

Isotope and geochemical characteristics of rocks from the Oshurkovo apatite-bearing massif,Western Transbaikalia

We present the results of a study on gabbroic rocks, syenites, pegmatites, carbonatites, and hydrothermal products of the Oshurkovo apatite-bearing massif. The results include Nd and Sr isotope ratios; the isotope compositions of carbon and oxygen in calcite; oxygen in apatite, magnetite, and silicate minerals (phlogopite, titanite, diopside, amphibole, K-feldspar, and quartz); sulfur in barite; and hydrogen in mica. The isotopic data are close to the EM-1 enriched mantle values and confirm a comagmatic relationship between the gabbros and carbonatites. The binary plot ?_Nd vs. ⁸⁷Sr/⁸⁶Sr demonstrates strong differentiation between silicate rocks and carbonatites, as is the case with the other Late Mesozoic carbonatite occurrences of southwestern Transbaikalia. The oxygen isotope composition of all comagmatic phases also falls within the range of mantle values. A clear trend toward heavier oxygen and lighter carbon isotope compositions is observed in all successively emplaced phases, which is consistent with a trend defined by hydrothermal products formed under the influence of the parent magma chamber. Carbonates formed during the greenstone alteration of gabbroic rocks are enriched in the light oxygen isotope (δ¹⁸O from ?2.8 to ?7.3‰), suggesting a contribution of vadose water.     

湘江悬浮物REE地球化学特征

在湘江长、株、湘潭河段采集悬浮样,利用ICP-MS进行REE分析.结果表明,湘江长株潭断及各河段的河心与两岸水体悬浮物中稀土分布不均匀;稀土分布模式基本相同,均为LREE相对HREE富集,曲线呈明显的右倾状,且与长江、黄河的稀土分布模式相似;Eu、Ce亏损在各河段基本相同,表明湘江悬浮物中的REE主要来源于沉积岩的风化产物,外源性稀土的污染不是很显著.     

REE distribution in zircon from reference rocks of the Arctic region: Evidence from study by the LA-ICP-MS method

The results of the LA-ICP-MS analysis of the concentrations of REEs, U, Th, and Hf in zircon from Paleo- and Neoarchean reference rocks of the Kola region (garnet–amphibole gneiss, basic and acid granulites, and granite pegmatite) are reported. A new methodology of the study of accessory zircons has been validated and modified. The accuracy of the results is confirmed by analysis of standard zircons Temora 1 and 91 500 and by comparison with the data obtained in other laboratories.     

白山市大横路铜钴矿区稀土元素地球化学特征

本文主要从稀土元素的地球化学特征所反映的地球化学信息,研究变质岩原岩的沉积环境,变质阶段稀土元素的变化规律,成矿物质的迁移规律,变质环境等.     

滇西腾冲地区龙川江河谷上新世火山岩SHRIMP锆石U-Pb年龄及其地球化学特征

报道了应用SHRIMP技术测定云南腾冲地区龙川江河谷附近上新世粗面安山岩和粗面岩的年龄结果。该区粗面安山岩与粗面岩的喷发时代接近,形成时代为2.81Ma±0.11Ma,属上新世。该区火山岩具高钾钙碱性的特点,富集大离子亲石元素、稀土元素和高场强元素（如Zr、Th、Hf）的特征明显不同于典型的岛弧钙碱性火山岩,而与遭受地壳混染的软流圈源大陆玄武岩相类似,是在板内拉张环境下地幔岩浆混染了大量下地壳物质的产物。     

粤北竹山下岩体锆石U-Pb同位素定年及其地球化学特征

竹山下岩体锆石SHRI MP U-Pb年龄为161.0±3.0 Ma,属燕山早期岩浆活动产物。该岩体的主要元素显示富硅(Si O2=76.05%~79.16%)、富碱(K2O+Na2O=7.77%~8.40%)、强过铝质(A/CNK=1.08~1.26)和低CaO/Na2O值(0.04~0.06)等特征。微量元素富集大离子元素而亏损Ba,Sr,Ce和Ti;LREE轻微亏损(LREE/HREE=0.82~1.09),Eu亏损明显(δEu=0.06~0.11),且具有较明显的四分组效应(TE1,3=1.07~1.24)。上述特征表明,竹山下岩体属于典型的壳源型花岗岩,是在地壳伸展-减薄的构造背景下,由泥质变质岩经过低程度部分熔融的方式而形成。     

锆石和磷灰石原位地球化学成分对普朗斑岩铜成矿的指示意义

普朗斑岩铜矿床是云南中甸地区最大的斑岩型铜矿床,其成矿作用与普朗复式岩体存在密切的成因联系。对普朗复式岩体成矿的第1期石英闪长玢岩和第2期石英二长斑岩,以及不成矿的第3期闪长玢岩开展了锆石原位微量元素和磷灰石原位主量元素成分的研究。结果表明,锆石Ce/Ce~*值、Ce~(4+)/Ce~(3+)比值和Ce/Nd比值指示,不成矿的第3期闪长玢岩与成矿的第1期石英闪长玢岩的岩浆相对氧化状态大致相当,且明显高于成矿的第2期石英二长斑岩。磷灰石Cl、OH和SO3成分指示,普朗复式岩体3期斑岩具有相似的岩浆含水量,但成矿的第1期石英闪长玢岩和第2期石英二长斑岩较不成矿的第3期闪长玢岩具有更高的岩浆挥发分Cl含量和硫逸度。岩浆的高氧逸度和硫逸度、富水和富挥发分Cl均是形成斑岩铜矿床的关键因素,普朗不成矿的第3期闪长玢岩与成矿的第1期石英闪长玢岩和第2期石英二长斑岩均具有富水和高氧逸度特征,但第3期闪长玢岩较成矿的第1期石英闪长玢岩和第2期石英二长斑岩具有明显更低的岩浆挥发分Cl含量和硫逸度,这很可能是导致第1期石英闪长玢岩和第2期石英二长斑岩成矿,而第3期闪长玢岩不成矿的重要原因之一。     

Diffusion of helium in zircon and apatite

Diffusion of helium has been characterized in natural zircon and apatite. Polished slabs of zircon and apatite, oriented either normal or parallel to c were implanted with 100 keV ³He at a dose of 5 × 10^{15 3} He/cm². Diffusion experiments on implanted zircon and apatite were run in Pt capsules in 1-atm furnaces. ³He distributions following experiments were measured with Nuclear Reaction Analysis using the reaction ³He(d,p)⁴He. For diffusion in zircon we obtain the following Arrhenius relations:

Although activation energies for diffusion normal and parallel to c are comparable, there is marked diffusional anisotropy, with diffusion parallel to c nearly 2 orders of magnitude faster than transport normal to c. These diffusivities bracket the range of values determined for He diffusion in zircon in bulk-release experiments, although the role of anisotropy could not be directly evaluated in those measurements.In apatite, the following Arrhenius relation was obtained over the temperature range of 148–449 °C for diffusion normal to c:

In contrast to zircon, apatite shows little evidence of anisotropy. He diffusivities obtained in this study fall about an order of magnitude lower than diffusivities measured through bulk release of He through step-heating, and within an order of magnitude of determinations where ion implantation was used to introduce helium and He distributions measured with elastic recoil detection.Since the diffusion of He in zircon exhibits such pronounced anisotropy, helium diffusional loss and closure cannot be modeled with simple spherical geometries and the assumption of isotropic diffusion. A finite-element code (CYLMOD) has recently been created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients. We present some applications of the code in evaluating helium lost from zircon grains as a function of grain size and length to diameter ratios, and consider the effects of “shape anisotropy”, where diffusion is isotropic (as in the case of apatite) but shapes of crystal grains or fragments may depart significantly from spherical geometry.     

冈底斯东段古新世朱拉岩体LA-ICP-MS锆石U-Pb定年和岩石地球化学特征

朱拉岩体位于冈底斯带东部,主要由黑云母二长花岗岩组成,其中发育闪长质包体,二者呈渐变过渡关系。3件寄主岩石LA-ICP-MS锆石U-Pb年龄加权平均值分别为64.6Ma±0.8Ma、64.3Ma±0.8Ma和63.9Ma±0.5Ma,含776Ma的继承岩浆锆石。1件闪长质包体样品年龄加权平均值为66.1Ma±0.3Ma。闪长质包体低Si,中Mg（Mg﹟平均41.1）,属铝质高钾钙碱性系列,Nb/Ta值为22,Sr/Y值为3.69,稀土元素分馏较低,LREE/HREE平均值为1.92,Eu强烈亏损,富集大离子亲石元素（Rb、K、U、Th）,亏损高场强元素（Nb、Ta、Sr、Ti）,暗示岩浆成分主要为幔源。寄主岩石富Si、K,贫P,属铝质—偏铝质钙碱性—高钾钙碱性系列,Nb/Ta值15.9,富集大离子亲石元素（Rb、K、Th）,亏损高场强元素（Nb、Ta、Ba、P、Ti）,具有活动陆缘钙碱性岩系的微量元素分布特征。寄主岩石与闪长质包体具有密切的成生联系,闪长岩形成于富集俯冲带组分的地幔熔体,在上升过程中混染了一定量的壳源物质结晶分异产物,寄主岩石则为底侵作用产生的大量壳源熔体与少量幔源熔体混合并发生一定程度分离结晶后的产物。结合前人的研究成果,认为古新世朱拉岩体与雅鲁藏布江大洋的向北消减有关。     

西藏班戈地区青龙花岗闪长岩SHRIMP锆石U-Pb年龄及其地球化学特征

青龙岩体位于冈底斯带北麓,是班戈-嘉黎岩浆岩带的重要组成部分。该岩体主要岩性为花岗闪长岩,形成于白垩纪早期,SHRIMP锆石U-Pb 206Pb/238U年龄加权平均值为123.1Ma±3.4Ma,该年龄反映了青龙花岗闪长岩体的侵位或结晶年龄。花岗闪长岩地球化学数据表明,该岩体具有正常的SiO2含量,较低的K2O/Na2O比值,为弱过铝质岩石。该岩体轻稀土元素较重稀土元素富集,负Eu异常较弱。在微量元素蛛网图上,该岩体表现出强烈的Nb、Ta、Ti、Y负异常,具有岛弧花岗岩的地球化学特征。通过初步研究,认为青龙花岗闪长岩形成于俯冲背景,为与板片俯冲作用有关的同熔型花岗岩。     

云南个旧锡矿田碱性岩和煌斑岩LA-ICP-MS锆石U-Pb测年及其地质意义

笔者报道了运用LA-ICP-MS锆石U-Pb方法对云南个旧地区碱性岩侵入年龄和煌斑岩的测年结果,即白云山碱性岩的形成时代为76.6±3.6Ma.煌斑岩的形成时代为77.2±2.4 Ma,这两种类型的岩石均为燕山晚期岩浆作用的产物.结合区域地质地球物理资料、岩石学组合的构造指示意义及前人的有关其他研究成果,认为这些岩石形成于岩石圈伸展的动力学环境.     

REE geochemical characteristics of the No. 302 uranium deposit in northern Guangdong, South China

The No. 302 uranium deposit, located in Guangdong Province, is a typical granite-type uranium ore deposit REE geochemical characteristics of the wall rocks, pitchblende, altered rocks, calcite and fluorite from this deposit have been systematically studied in this paper. The result showed that the alkali-metasomatic granites and other altered rocks have the same REE distribution patterns as Indosinian granites. It is indicated that the hydrothermal ore-forming solution had altered the Indosinian granites, and ore-forming materials may directly originate from the Indosinian granites. Calcite and fluorite of different stages are the products derived from the same source but different stages. The evolution and degassing of the mineralizing solution might induce LREE enrichment to varying degree. Mantle fluid and a large volume of mineralizer may be the crucial factors controlling uranium mineralization, and the hydrothermal solution with mineralizer played an important role in U transport and concentration. Meanwhile, the degassing of CO2 might promote U and REE precipitation.