LA-ICP-MS独居石U-Th-Pb测年方法研究

相比LA-ICP-MS锆石U-Pb测年,独居石在一些年轻地质体或流体作用下的矿物定年中更具优势,具有很好的应用前景。然而,大多数独居石Th含量较高(可达7%),包裹体较多,另外随着独居石定年标样不断消耗,存量越来越少,也限制了独居石U-Th-Pb同位素测年的发展与应用。前人利用LA-ICP-MS探究合适的独居石U-Th-Pb测年实验条件,主要是改变激光器的参数,而未对ICP-MS的参数进行系统研究。本文通过改变激光器参数(束斑直径和激光频率)和ICP-MS参数(~(232)Th驻留时间),分别在束斑直径为24μm、16μm和10μm,激光频率为3Hz、4Hz和5Hz,~(232)Th驻留时间为10ms、6ms、3ms和1ms的条件下进行U-Th-Pb测年。最后以独居石RW-1为标样,对独居石样品Bananeira进行校正,期望得到独居石U-Th-Pb测年的最佳条件。结果表明:束斑直径为16μm,~(232)Th驻留时间为3ms或1ms,能量密度为4J/cm~2,激光频率为5Hz,载气He流速为0.35L/min,载气Ar流速为0.95L/min的实验条件下适合独居石测年,这两种条件下Bananeira的~(207)Pb/~(235)U加权平均年龄分别为510.7±8.6Ma(MSWD=0.87)、513.8±5.7Ma(MSWD=0.38,推荐值507.7±1.3Ma),误差在0.59%和1.20%左右;~(208)Pb/~(232)Th加权平均年龄分别为496.9±8.6Ma(MSWD=0.596)、499.8±5.6Ma(MSWD=0.37,推荐值497.6±1.6Ma),误差在0.14%和0.44%左右。并利用此条件对黄山花岗岩(HS-1)进行独居石U-Th-Pb测年,其~(207)Pb/~(235)U加权平均值在128.3±2.4Ma(MSWD=0.73),与本次测定该岩体的锆石年龄数据(127.0±2.1Ma, MSWD=0.93)在误差范围内一致,验证了本实验建立的独居石U-Th-Pb定年方法可靠。     

云南苍山变质带前寒武纪正变质岩的邓尚贤地球化学特征及其形成时代

元素地球化学研究结果表明，苍山变质带内发育两类正变质基性岩：一类为形成于富集型MORB构造环境的变拉斑玄武岩，另一类为形成于板内构造环境的变碱性玄武质岩墙，Sm-Nd同位素体系研究表明，变拉斑玄武岩形成于晚古元古代至中元古代（2．0－1．4G a );变碱性玄武质岩墙形成于中新元古代（1．3－0．8Ga)，已发表的Sm-Nd等时线年龄（2408Ma)其Sm/Nd比植变化很小，故没有实际的地质意义。     

Study on the Sr,Nd,Pb and O Isotopes of Basic Dyke Swarms in the Wudang Block and Basic Volcanics of the Yaolinghe Group

The Sr,Nd and Pb isotopic characteristics of the Wudang basic dyke swarms and basic volcanics of the Yaolinghe Group show that they were derived from the same multi-component mixing source in the mantle.The Wudang basic dyke swarms have(^87Sr/^86Sr)i=0.6905-0.7061,εNd(t)=-1.9-5.0,△^208Pb/^204Pb=35.49-190.26,△^207Pb/^204Pb=Th/Ta and a wide range of La/Yb ratios;and the basic volcanics of the Yaolinghe Group have(^87Sr/^86Sr)i=0.6487-0.7075,εNd(t)=0.11-3.94，△^208Pb/^204Pb=-81.58-219.95,△^207Pb/^204Pb=4.44-16.68and higher Th/Ta and La/Yb ratios,indicating that their source is a mixture of DM and EMⅡ,and the basic volcanics of the Yaolinghe Group were contaminated by crust materials en rout to the surface.Based on the geochemical features of continental tholeiitic basalts and being products of differen tacies derived from the same source,it can be concluded that an important rifting event in the South Qinling basement block occurred during Neoproterozoic,followed by a setting of oceanic basic in the Early Paleozoic.     

对《上地幔Sm／Nd值的演化》一文的商榷意见

对《上地幔Ｓｍ／Ｎｄ值的演化》一文的商榷意见支霞臣（中国科学技术大学地球与空间科学系，合肥２３００２６）１引言上地幔Ｓｍ／Ｎｄ值的演化，是一个兼有意义和难度的地球化学基础理论问题，已有一些学者作过研究。最近，李曙光在＂上地幔Ｓｍ／Ｎｄ值的演化＂［１］...     

Tectonic Environments of Cenozoic Volcanic Rocks in China and Characteristics of the Source Regions in the Mantle

Extensive volcanism is one of the important features of Cenozoic geology in China.Based on temporal-spatial distribution,the volcanism was associated with three major different geological settings:1)the continental rift basalts in Northeast and North China;2)the tension-fault basalts on the continental margins of Southeast China; and 3) the collision-zone high-K volcanics in the Qinghai-Xizang Plateau and its vicinities.The characteristics of “depletion in the south and enrichment in the north“of the China continental mantle are strongly supported by isotopic evidence.The Cenozoic continental cal characters,into the following geochemical provinces:1)the depleted mantle in South China;2)the primary mantle in Northeast and NorthChina; 3)the hybrid and transi-tional mantle in the region of Shandong ,Anhui,Jiangsu and northern Zhejiang;4)the depleted mantle around the Bohai Bay and the Lower Liaohe River;5)the K-metasomatic enriched mantle in the northern part of Northeast China;and 6)the re-cycled enriched mantle in the ancient subduction zone in the Qinghai-Xizang Plateau and its surround-ings.These geochemical characteristics on a regional scale must be a reflection of the nature of lithosphere evolution.     

苏格兰阿伦第三纪火成岩的Nd同位素及其岩浆演化和地壳…

对苏格兰西部阿伦（Ａｒｒａｎ）第三纪火地的地球化学研究，是对高地边界断层（ＨＢＦ）地区地壳结构的一种探索。新的Ｎｄ同位素数据以及其他地球化学证据表明原始慢源岩浆在地壳内部的分异过程中受到过各种混染，有两种不同的同位素混染趋势。北部花岗岩是基性分异体受类似于本区出露的达拉德（Ｄａｌｒａｄｉａｎ）单元地壳岩石的混染产物。中部花岗岩和ＨＢＦ南部几个其他小侵入体同位素数据变化趋势朝向已报道的米德兰谷订粒岩     

SHRIMP Ion Microprobe Zircon U—Pb Age and Sm—Nd Isotopic Characteristics oof the NE Jiangxi Ophiolite and Its Tectonic Implications

The new SHRIMP zircon U-Pb isotopic study suggests that the crystallization age of a highly fractionated magma in the NE Jiangxi ophiolite suite is 968±23 Ma. Re-calculated Sm-Nd isochron age of 955±44 Ma is within analytical errors. consistent with the zircon U-Pb age. With the exception of two anomalous Sm-Nd data, the remaining 15 analyses so far obtained for the ophiolite gave^εNd (T) values falling into a limited range from +4.3 to +6.7, indicating that the ophiolite was derived from a relatively strongly depleted mantle source. Sm-Nd isotopic systematics in some samples may have been strongly affected by post-magmatic events, such as alteration, deformation and metamorphism, resulting in anomalous^εNd (T) values. Combined with published⁴⁰Ar³⁹Ar age data, it can be concluded that the collision between the Yangtze and South China Blocks occurred during 0.97-0.80 Ga. This work was financially supported by the President Grant of the Chinese Academy of Sciences.     

Partitioning and Hydrolysis of Nb and Ta and Their Implications with Regard to Mineralization

K₂NbOF₅ · H₂O and K₂TaF₇ were prepared through melting Nb₂O₅ and Ta₂O₅ respectively with KHF₂ · 2H₂O, followed by recrystallizing. The hydrolysis properties of K₂NbOF₅ and K₂TaF₇ were determined again by using a rapidly quench vessel. As temperature (from 250 to 550 °C) and pressure (from 500 to 1500 bars) increase, the degree of hydrolysis of both K₂NbOF₅ and K₂TaF₇ will increase. Nb- and Ta-fluorine complex compounds are instable in supercritical aqueous fluids. The degree of hydrolysis of both K₂NbOF₅ and K₂TaF₇ decreases with increasing concentration of HF, independent of the concentration of NaF. The partition coefficients of Nb and Ta between granitic melt and fluid phase are less than 0.15, i.e., most of Nb and Ta are left in granitic melt. The partition coefficient of Ta is more dependent on the concentration of HF than that of Nb. The significance of hydrolysis in Nb- and Ta- mineralization is also discussed in the present paper.