Lu-Hf年代学研究——以大别榴辉岩为例

本文利用多接收电感耦合等离子体质谱仪(MC-ICP-MS)进行了同位素稀释法Lu-Hf年代学研究,建立了全岩Lu、Hf的分离条件。分离后Lu溶液中~(176)Lu/~(176)Yb大于30,分离后Hf溶液中Lu和Yb对Hf的干扰(~(176)Lu/~(176)Hf和~(176)Yb/~(176)Hf分别小于2×10~(-6)和2×10~(-4))采用指数法则进行校正。建立了MC-ICP-MS进行Lu、Hf同位素高精度准确测试的校正方法。天然样品中Hf同住素~(176)Hf/~(177)Hf的内部测试精度优于0.0015%,外部精密度优于0.0010%。应用本文建立的方法获得大别双河榴辉岩石榴子石-全岩Lu-Hf等时线年龄为254±16Ma(2σ)。年龄误差稍大与该样品石榴子石中较低的Lu含量(1.1μg/g)和石榴子石-全岩的未充分分开的~(176)Lu/~(177)Hf比值(分别为0.05和0.01)有关。     

锆石Hf同位素组成的LA-MC-ICP-MS测定

利用多接收器等离子体质谱仪(MC-ICPMS)和193nm准分子激光器联用技术,对GJ-1、Temora、91500和Mud Tank四个标准锆石的Hf同位素组成进行测试,并通过指数方法进行同质异位素干扰校正,测得它们的176Hf/177Hf比值分别为0.282006±24(n=159, 2SD)、0.282684±46(n=20, 2SD)、0.282305±32(n=20, 2SD)和0.282509±25(n=48, 2SD)。测定结果与文献报道的值在误差范围内一致。     

新疆东准噶尔花岗岩的全岩Hf同位素研究

在综合对比已有Hf同位素分析方法的基础上,我们选用Ln-Spec(100-150目,H+)特效树脂和多接收器电感耦合等离子体质谱(MC-ICPMS)对4件标准物质和19件岩石样品(新疆东准噶尔的花岗岩及其包体18件、玄武岩1件)进行了全岩Hf同位素研究。所获得的国际标准物质BCR-1、BHVO-2、W-2的176Hf/177Hf比值与文献报道的结果在误差范围内一致,表明所采用的全岩Hf的化学分离和溶液的Hf同位素质谱测定方法是可靠的。本文首次报道了中国花岗岩标样GRS-1的176Hf/177Hf比值,结果为0.282547?07(2?)。依据新疆东准噶尔花岗岩及其包体样品的全岩176Hf/177Hf比值、全岩样品在等离子体质谱(ICP-MS)上测定的Lu、Hf含量和岩石的形成年龄(300 Ma),得到这些样品的全岩εHf(t)。它们与对应样品的锆石εHf(t)在误差范围内总体上的一致性表明,ICP-MS测定的Lu、Hf含量可以用来计算年龄值约300 Ma的全岩初始Hf同位素组成。玄武岩样品QS08-1的全岩εHf(t)与该蛇绿岩套斜长花岗岩的锆石εHf(t)的高度吻合,为利用全岩176Hf/177Hf比值和ICP-MS测定的全岩Lu、Hf含量,研究没有或难以选出锆石的较年轻岩石的Hf同位素组成提供了很好的实例。     

岩石样品Hf化学分离及MC-ICP-MS同位素分析：离子交换色谱法的应用

介绍的离子交换色谱分离岩石样品Lu-Hf的方法，是对现有方法的改进。整个流程以使用HCl为主，配合少量HF，即可达到有效地分离Hf和Lu的目的，Lu和Hf的回收率在90％以上。经MC—ICP。MS分析，每个国际岩石标样的两个平行样获得了在误差范围内一致^176 Hf／^177 Hf比值，其中BHVO-1、BHVO-2和BCR-2的^176 Hf／^177 Hf比值与国内外文献报道的结果在误差范围内一致。     

鲁西新泰孟家屯2.7Ga变质沉积岩与黑云斜长片麻岩锆石Hf同位素特征

对出露于山东新泰孟家屯2.7Ga的孟家屯岩组中石榴石石英岩(M08)、含十字石石榴石黑云母石英片岩(D242-Y2)和黑云斜长片麻岩(M03)(TTG质片麻岩)进行锆石Lu-Hf同位素分析。石榴石石英岩锆石核部176Lu/177Hf、176Hf/177Hf变化范围为0.001730～0.002546、0.281249～0.281360,锆石变质边部176Lu/177Hf、176Hf/177Hf变化范围为0.000123～0.002070、0.281241～0.281318;含十字石石榴石黑云母石英片岩锆石核部176Lu/177Hf、176Hf/177Hf变化范围为0.001334～0.002169、0.281226～0.281324,锆石变质边部176Lu/177Hf、176Hf/177Hf变化范围为0.000445～0.001375、0.281227～0.281309;黑云斜长片麻岩锆石176Lu/177Hf、176Hf/177Hf变化范围为0.000315～0.000847、0.281186～0.281265。孟家屯岩组石榴石英岩、含十字石石榴石黑去母石英片岩中碎屑(岩浆)锆石和黑云斜长片麻岩中岩浆锆石的εHf(t)分别为3.75～7.26、2.31～7.26和3.21～6.27,同时TDM1与其U-Pb年龄非常接近,表明它们起源于新生地壳的部分熔融。结合区域研究资料认为,2.7Ga是鲁西重要的一期构造岩浆热事件,也可能是华北克拉通重要的构造岩浆活动时期。变质沉积岩中许多变质增生锆石相对于核部岩浆锆石低176Lu/177Hf、高176Hf/177Hf,显示变质作用过程中石榴石的存在对锆石的Lu-Hf同位素体系有很大影响。     

大别地体超高压变质岩石锆石Lu-Hf同位素研究

对大别山南部超高压变质带双河和黄镇地区的榴辉岩、片麻岩和硬玉石英岩中变质锆石进行了原位LA-MC-ICP- MS的Lu-Hf同位素分析。双河和黄镇的榴辉岩及双河的硬玉石英岩有低~(176)Lu/~(177)Hf和低~(176)Hf/~(177)Hf组成,两地的片麻岩有高~(176)Hf/~(177)Hf比和高且分散的~(176)Lu/~(177)Hf组成。锆石Hf同位素分布主要受变质原岩的形成时代控制,增生锆石基本上继承了原岩锆石的Hf同位素特征,既有增生锆石相对有低~(176)Lu/~(177)Hf和高~(176)Hf/~(177)Hf、继承重结晶锆石相对有高~(176)Lu/~(177)Hf和低~(176)Hf/~(177)Hf的特征,也有二者相互重叠没有区别的,它主要受原岩性质和变质过程中锆石遭受的溶蚀程度控制。增生锆石的低Lu/Hf是锆石在变质过程中Lu含量下降和Hf含量增高造成的,增生锆石的高~(176)Hf/~(177)Hf继承自岩石中其它高Lu/Hf比矿物的长期演化。继承锆石的初始Hf同位素组成ε_(Hf)值和亏损地幔模式年龄T_(DM)示踪表明,各超高压变质原岩的时代和成因是复杂的:双河榴辉岩原岩物质源自25亿年的亏损地幔和至少27亿年以上的古老晚太古地壳混合。双河片麻岩原岩年龄相同,但有不同的壳幔混合物源。黄镇榴辉岩原岩主要源于亏损幔源岩浆形成的初生地壳的重循环,很少的地壳混染。黄镇片麻岩和榴辉岩的物源区年龄相同。两地片麻岩原岩物源主要来自弱亏损地幔,存在古老地壳物质和地幔物质的混合。大别地区超高压变质岩锆石的Lu-Hf同位素特征主要反映了7～8亿年和18～19亿年时扬子克拉通北缘地区的岩浆活动特点和大地构造环境。     

赣南西华山成矿花岗岩锆石U-Pb年代学和Hf同位素特征及其地质意义北大核心CSCD

赣南西华山钨矿是与侏罗纪花岗岩有关的大型石英脉型矿床,也是我国南岭成矿带赣南地区典型的钨矿床。本文选取西华山钨矿成矿花岗岩进行了锆石LA-ICP-MS U-Pb定年、微量元素和Hf同位素分析,旨在揭示花岗岩的形成时间并推断岩石成因及其对成矿的指示。锆石U-Pb测年结果表明,西华山中粒黑云母花岗岩锆石206 Pb/238 U加权平均年龄为154.8±6.0 Ma。锆石形态、激光拉曼和地球化学特征表明,该花岗岩受到岩浆热液的影响。锆石的176 Yb/177 Hf和176 Lu/177 Hf比值较高,表明源岩浆有强烈的流体出溶作用。西华山黑云母花岗岩的εHf(t)变化范围较小,呈明显负值(-12.8~-10.8),且集中在华夏地块古老基底范畴内,指示其形成于古老地壳物质的部分熔融。     

磷灰石Lu-Hf同位素年代学研究——以河北矾山磷矿为例

磷灰石是广泛出露于三大类岩石中的副矿物,由于磷灰石具有较高的Lu/Hf比值(最高可达约90,而通常用过Lu-Hf定年的石榴石最高只有28),因而是进行高精度Lu-Hf同位素定年的理想矿物。例如在经常缺乏锆石等常规定年矿物的镁铁质岩浆岩中,或者在角闪岩和大理岩等变质岩石中,磷灰石具有常规定年矿物无法替代的优势。但是,磷灰石中极低的Hf含量使得难以获得准确的176Lu/177Hf和176Hf/177Hf比值,阻碍了构筑高精度Lu-Hf矿物等时线。本次研究笔者将AridusⅡ膜去溶与多接收电感耦合等离子体质谱(MC-ICP-MS)联用,有效降低了基体氧化物和氢化物的干扰,提高了仪器信号强度和稳定性,获得了精准的Lu-Hf同位素比值。运用上述方法,我们对采自华北克拉通北缘矾山碱性杂岩体中不同岩带的磷灰石、榍石和辉石获得了高精度的176Lu/177Hf和176Hf/177Hf比值,矿物内部等时线年龄为(226.5±3.6)Ma和(231.3±3.1)Ma。该结果与同一样品中的副矿物(锆石、斜锆石、钙钛矿和榍石)的U-Pb年代学结果一致,不但证明了我们Lu-Hf同位素年代学方法的可行性与可靠性,也进一步限定了矾山杂岩体形成的时代。同时,结合全岩和单矿物的微量元素及Sr-Nd同位素特征,为矾山岩体的演化过程提供了制约条件。     

北苏鲁仰口地区变辉长岩中锆石U-Pb定年、微量元素和Hf同位素特征及其地质意义

北苏鲁仰口地区出露超高压的变辉长岩.锆石阴极发光图像和其内部矿物包体激光拉曼测试的联合研究结果表明,变辉长岩锆石具有弱发光效应的岩浆韵律环带的核和被改造的强发光效应的边.岩浆韵律环带的核部保存大量而复杂的矿物包体,包括普通辉石(Cpx)+斜方辉石(Opx)+斜长石(P1)+石英(Qtz)+黑云母(Bt)+钛铁矿(Ilm)+磷灰石(Ap);边部保存的矿物包体则相对较少,包括普通辉石(Cpx)+斜方辉石(Opx)+斜长石(Pl)+磷灰石(Ap).尽管岩浆韵律环带核部的稀土元素总合量比被改造的锆石边部明显偏高,但二者稀土元素配分模式具有明显的相似性,主要表现为轻稀土相对亏损,而重稀土明显富集,相应的(La/Yb)N=0.00015～0.00039,并具有明显的负Eu异常(Eu/Eu*=0.20～0.26)、相对明显的正Ce异常(Ce/Ce*=71.5～147.4)和较高的Th/U比值(1.97～2.69).上述特征表明,仰口地区变辉长岩中的锆石均为继承性的岩浆锆石,而没有新生的变质锆石.LA-(MC)-ICP-MS锆石原位U-Pb定年和Lu-Hf同位素分析结果表明,两件锆石样品Y1和Y2的年龄数据所构成的不一致线显示了十分接近的上交点和下交点年龄.其上交点年龄分别为785±15Ma(2σ)和784±12Ma(2σ),应代表原岩的形成时代,表明变辉长岩的原岩与Rodinia超大陆裂解的岩浆事件存在密切的成因关系;而下交点年龄分别为226±24Ma(2σ)和228±26Ma(2σ),与苏鲁其它类型超高压岩石中含柯石英锆石微区记录的变质年龄十分吻合,应代表变辉长岩的超高压变质时代.岩浆结晶锆石的核部具有明显偏高的176Lu/177Hf(0.00044～0.00291)和176Yh/177Hf(0.0165～0.1168)比值,而176Hf/177Hf比值变化于0.281956～0.282048之间,相应的εHf(t)=-8.5～-14.0,tDM2=2.03～2.32Ga,表明仰口地区变辉长岩的原岩起源于古元古代时期的富集地幔或发生部分熔融的下地壳残留体.被改造的岩浆结晶锆石的边部则具有明显偏低的176Lu/177Hf(0.00029～0.00060)和176Yh/177Hf(0.0112～0.0200)比值,而176Hf/177Hf(t)比值变化于0.281953～0.282002之间,相应的εHf(t)=-10.2～-11.9,tDM2=2.12～2.21Ga.与岩浆结晶锆石核部相比,被改造的岩浆锆石边部的176Lu/177Hf、176Yb/177Hf、176Hf/177Hf(t)比值和εHf(t)和tDM2值的变化范围更小,表明中-新三叠纪的超高压变质作用使岩浆结晶锆石边部的Lu-Hf同位素体系发生调整,更趋向于均一化.     

一件可能的Hf同位素测定标准锆石

陕西省西乡县汉南杂岩望江山岩体辉长岩中含有丰富的锆石——从600kg辉长岩样品中分选出结晶良好、内部结构简单、成因和年龄单一、Hf同位素比值均一的锆石7g,锆石粒度多为0.2-0.3mm。分别在三个不同实验室利用三种方法对该锆石样品进行了U-Pb同位素年龄测定,获得了在误差范围内完全一致的年龄：819.8 ± 2.5 Ma（LA-ICP-MS）、821.7 ± 1.7 Ma（LA-MC-ICPMS）和822.1 ± 4.5 Ma（SHRIMP）。在国内4个权威实验室对该锆石进行了Lu-Hf同位素测定,获得了在误差范围内完全一致的176Hf/177Hf同位素比值——全部421个测试点加权平均值为0.282535 ± 0.000003（2σ）。采样点岩体规模巨大,露头良好,岩石新鲜,交通方便。该锆石样品满足作为Hf同位素测定标样的各方面的指标,可能是一个比较理想的Hf 同位素测定标样。Hf同位素测定标准物质的研制,是测定获得准确可靠的Hf同位素数据的基础,具有十分重要的实用价值和科学意义。     

人工合成锆石Lu-Hf同位素标样方法研究

研究了人工合成锆石并加入微量元素的实验方法和条件。人工合成锆石是以氧化锆和硅酸锂为原料,以钼酸锂和氧化钼为助熔剂,并加入其他元素如Hf、Lu、Yb、U、Th、Pb等元素,充分混匀后转入带盖铂金坩埚中,在高温马弗炉中连续加热生长形成锆石晶体。利用激光剥蚀系统与电感耦合等离子体质谱技术分析了合成锆石晶体中微量元素含量和Lu-Hf同位素组成,结果表明合成锆石晶体中Lu、Yb、U、Th、Pb等微量元素含量不均一,而Hf的含量和Lu-Hf同位素组成有很好的均一性。     

Separation of Hf and Lu for high-precision isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS

Potential applications of the Lu-Hf isotope system have long been impeded by the analytical difficulties of obtaining data on a wide variety of geological materials. Many of these limitations will now be eliminated because Hf isotopes can be readily measured with high precision and accuracy on small and/or Hf-poor samples using the newly developed magnetic sector-multiple collector ICP-MS, also known as MC-ICP-MS or the `Plasma 54'. We present here a new method to separate and determine isotopic compositions of both Hf and Lu from various types of geological materials using MC-ICP-MS. The chemical separation of Hf and Lu has been designed to take advantage of the characteristics of this unique instrument. The separation of Hf can be achieved with a straightforward two-step ion-exchange column chemistry, which has a high efficiency (better than 85% recovery) and low blanks (typical total blanks less than 150 pg for the largest samples of 1 g bulk rock). The isolation of Lu is achieved with a single-stage ion-exchange column procedure with near 100% yields and blanks below 20 pg. Hf isotopic compositions can be routinely measured on 50 ng Hf with an internal precision better than 20 ppm in less than 15 min and with an external precision better than 40 ppm. Our value for the ¹⁷⁶Hf/¹⁷⁷Hf ratio of the JMC 475 Hf standard currently is 0.282163 ± 9 (2s). The Lu isotopic ratio is measured rapidly and precisely without isolating Lu from the bulk of Yb, and a mass fractionation correction increases the accuracy of the results compared with TIMS data. Our current reproducibility of the Lu/Hf ratio is ≈1%. Selected Lu-Hf isotope analyses of some modern and ancient geological samples validate the technique we have described here and illustrate the new opportunities for Lu-Hf isotope geochemistry that have opened up with the advent of magnetic-sector ICP mass spectrometry. Received: 12 September 1996 / Accepted: 13 January 1997     

Hafnium isotope homogenization during metamorphic zircon growth in amphibolite-facies rocks: Examples from the Shackleton Range (Antarctica)

Hafnium isotope analyses of a large number of metamorphic zircon grains of two garnet-kyanite-staurolite schist samples from the Shackleton Range yielded ¹⁷⁶Hf/¹⁷⁷Hf of 0.28160 ± 0.00003 and 0.28142 ± 0.00003, respectively. The variations of these analyses are less than ±1.2 epsilon units and indicate that all metamorphic zircon grains in the two rocks formed in environments with nearly homogenous Hf isotopic composition. The metamorphic origin of the zircon grains is constrained by textures as well as by their low Th/U (<0.2), ¹⁷⁶Lu/¹⁷⁷Hf (<0.0003), and ¹⁷⁶Yb/¹⁷⁷Hf ratios (<0.009), indicating that they grew in the presence of garnet. Furthermore, the grains yield Pb-Pb ages of c. 1.7 Ga, which is the time of amphibolite-facies metamorphism. In combination with petrological results, it is suggested that the observed ¹⁷⁶Hf/¹⁷⁷Hf homogeneity was caused by a fluid- and deformation-assisted dissolution of detrital zircon grains, followed by new zircon re-precipitation that was accompanied by Hf transport on at least a hand-specimen scale. This interpretation is supported by results obtained from an additional paragneiss sample that contains zoned zircon grains with xenocrystic cores formed at 2.6-1.8 Ga and metamorphic rims with a U-Pb age of 1.7 Ga. The ¹⁷⁶Hf/¹⁷⁷Hf variation of the zircon rims is mostly at ±0.0003, which is much less than that of the magmatic cores (±0.0019). The metamorphic fluid for the dissolution-homogenization-re-precipitation process most likely resulted from prograde reactions among the minerals chlorite-muscovite-biotite-garnet-staurolite-apatite, in agreement with thin section observations and P-T pseudosection calculations.     

U-Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of ²⁰⁶Pb/²³⁸U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ¹⁸O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ε_Hf (t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between ²⁰⁶Pb/²³⁸U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high-pressure eclogite-facies regime.     

Partition coefficients of Hf,Zr, and REE between zircon,apatite, and liquid

Concentration ratios of Hf, Zr, and REE between zircon, apatite, and liquid were determined for three igneous compositions: two andesites and a diorite. The concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient. Although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients, those for the apatite in the diorite may deviate from the partition coefficients. The HREE partition coefficients between zircon and liquid are very large (100 for Er to 500 for Lu), and the Hf partition coefficient is even larger. The REE partition coefficients between apatite and liquid are convex upward, and large (D=10–100), whereas the Hf and Zr partition coefficients are less than 1. The large differences between partition coefficients of Lu and Hf for zircon-liquid and for apatite-liquid are confirmed. These partition coefficients are useful for petrogenetic models involving zircon and apatite.     

扬子克拉通北缘大洪山地区打鼓石群与神农架地区神农架群的对比:锆石SHRIMP U-Pb年龄及Hf同位素证据

出露于扬子克拉通北缘大洪山地区的打鼓石群一直被认为是一套中元古代沉积地层,其上被花山群不整合覆盖。但是由于缺少可靠的年龄依据,打鼓石群与同属扬子克拉通北缘的神农架群以及其他相关地层的对比关系一直存有争议。本文作者利用高精度高分辨率离子探针质谱仪(SHRIMP)测定了打鼓石群中凝灰岩的锆石U-Pb年龄,并利用激光烧蚀多接收器电感耦合等离子体质谱仪(LA-MC-ICPMS)测定了打鼓石群凝灰岩和基本同时代的神农架地区神农架群上亚群凝灰岩的锆石Lu-Hf同位素。这是迄今为止第一次测得了打鼓石群罗汉岭组凝灰岩年龄(1 225±19) Ma和(1 239±23) Ma;锆石Lu-Hf同位素结果显示打鼓石群罗汉岭组凝灰岩与神农架群上亚群野马河组凝灰岩具有相似的Lu-Hf同位素特征：176Hf/177Hf初始值介于0.282 172~0.282 397,εHf(t)值为4.9~12.0, TCDM=1 150~1 943 Ma。结合现有区域地质资料,我们认为打鼓石群与神农架群上亚群属于同时期沉积物,形成于超级地幔柱主导的哥伦比亚超大陆裂解背景。     

The Hf isotopic composition of zircon reference material 91500

Ten new single zircon fragments, analyzed by solution chemistry and MC-ICP-MS, of the 91500 zircon standard show no evidence to support a recent claim based on in situ data that this reference material is heterogeneous in terms of its radiogenic Hf isotope composition and as a consequence should be abandoned as a reliable interlaboratory standard. Rather, the larger spread in ¹⁷⁶Hf/¹⁷⁷Hf among in situ data compared with solution chemistry data may reflect the uncertainties resulting from prominent isobaric interference corrections inherent to the in situ analytical protocol. The unweighted mean values of ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf measured for the ten zircon fragments of this study are 0.282313 ± 0.000008 (2σ) and 0.000311 ± 0.000136 (2σ), respectively. Pooling the mean ¹⁷⁶Hf/¹⁷⁷Hf of this work with those of published solution chemistry studies results in a value of 0.282308 ± 0.000006 (2σ; relative to ¹⁷⁶Hf/¹⁷⁷Hf = 0.282163 for JMC-475), which is recommended here as the currently best estimate of the Hf isotopic composition of this standard. All published studies agree on a value of ~ 0.0003 for the corresponding ¹⁷⁶Lu/¹⁷⁷Hf ratio.     

Effects of melt fractional crystallization on Sr-Nd and Lu-Hf isotope systems: a case study of Triassic migmatite in the Sulu UHP terrane

The Weihai migmatite in the Sulu ultra-high-pressure (UHP) metamorphic terrane, eastern China, underwent partial melting in the Late Triassic during its exhumation. The primary partial melts experienced a decompressional fractional crystallization (DFC) process to produce plagioclase (Pl)-rich leucosome crystallized under eclogite to granulite facies conditions and K-feldspar (Kfs)-rich pegmatitic veins crystallized under amphibolite-facies conditions. In this study, our results demonstrate that the DFC process can cause decoupling between whole-rock Sr and Nd isotopes. The Pl-rich leucosome has ε_Nd(t) values (–10.4 to ?15.0) and initial (⁸⁷Sr/⁸⁶Sr) ratios (0.708173–0.712476) very similar to those of the melanosome, but the Kfs-rich pegmatitic veins have homogeneous ε_Nd(t) values (?14.8 to ?15.2) and significantly high initial (⁸⁷Sr/⁸⁶Sr) ratios (0.713882–0.716284). Our results also suggest that the DFC process can change zircon ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf isotopic ratios, with no effect on ¹⁷⁶Hf/¹⁷⁷Hf ratios or ε_Hf (t) values. Zircon ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios increase dramatically from the Pl-rich leucosome to the Kfs-rich pegmatitic veins, but zircon ¹⁷⁶Hf/¹⁷⁷Hf ratios (Pl-rich leucosomes = 0.282330 ± 0.000017; Kfs-rich pegmatitic veins = 0.282321 ± 0.000026) and ε_Hf (t) values (Pl-rich leucosomes = ?10.9 ± 0.6; Kfs-rich pegmatitic veins = ?11.6 ± 0.8) remain almost unchanged. We propose that the isotopic decoupling between the Pl-rich leucosome and Kfs-rich pegmatitic vein might be caused by melt fractional crystallization occurring too rapidly to allow complete equilibrium between them.