Ages and sources of mantle eclogites: ID-TIMS and in situ MC-ICPMS Pb-Sr isotope systematics of clinopyroxene
Ages and sources of mantle eclogites: ID-TIMS and in situ MC-ICPMS Pb-Sr isotope systematics of clinopyroxene
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DOI：
10.1016/j.chemgeo.2018.10.007
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发表时间：
2019-01-05
期刊：
影响因子：
3.9
通讯作者：
Viljoen, K. S.
中科院分区：
文献类型：
--
作者：
Aulbach, Sonja;Heaman, Larry M.;Viljoen, K. S.
通过高精度同位素稀释热电离质谱法获取了来自三个特征明确的洋壳原岩榴辉岩组（Lace/Kaapvaal 克拉通、Orapa/津巴布韦克拉通和 Koidu/西非克拉通）的选定样品中的单斜辉石 (cpx) 的锶和铅同位素组成 (ID-TIMS) 和原位多接收器激光烧蚀电感耦合等离子体质谱 (MC-LA-ICPMS)。本研究的目的有两个：(1) 评估其在获得形成或重置年龄限制方面的效用，并识别可提高成功年龄测定机会的元素特征，以及 (2) 确认通过新颖的 MC-LA-ICPMS 技术获得的结果的准确性和效用。本研究中测量的榴辉岩 cpx 的锶-铅同位素系统是解耦的，可能反映了海水蚀变期间无支撑的放射性 Sr 的添加，或与俯冲混杂岩中的海洋沉积物相互作用，和/或由于地幔交代作用引起的干扰，更不相容的 Pb 更容易受到干扰。尽管历史复杂，但样本子集产生了有意义的模型日期。来自 Lace 的单斜辉石馏分具有高 Pb 含量 (3-6 ppm)、非放射性 Pb 同位素组成 (Pb-206/Pb-204 = 13.57-13.52) 和低 U-238/Pb-204 (1.0-1.5)，给出单阶段模型 Pb 日期为 2.90-2.84 Ga。相比之下，右侧 Orapa 图中的样品的 Geochron 并不能产生有意义的 Pb 模型年龄。然而，这些数据确实定义了次级等时线，可以对其进行建模，以产生影响克拉通岩石圈的主要事件的最小年龄限制。在不确定性范围内，获得的 Koidu 榴辉岩的最终 2.18 +/- 0.45 Ga 年龄反映了由于与 Eburnean 造山运动相关的克拉通下方俯冲而导致的 Pb 同位素系统的扰动，同时它们保留了非放射性 Sr-87/Sr-86 (0.7016)。同样，奥拉帕样本的年龄 (2.20 +/- 0.54 Ga) 被解释为与克拉通西部边缘古元古代增生相关的叠印年龄。辉长岩榴辉岩 (Eu/Eu* > 1) 含有富含斜长石的原岩，其时间积分 Rb/Sr 和 U/Pb 保留了最少的放射性 Sr，并且部分地， 铅。来自 Lace 的几种榴辉岩的高模型 mu（9.0 至 9.1）反映了 LREE、Th 和 Pb 丰度升高的情况。 3.0 Ga 添加了沉积成分，可能源自高 mu 玄武岩原壳的改造，如在其他克拉通上观察到的那样。我们建议，可以通过快速通量原位 LA-ICPMS 技术有效地指导样品定位，该技术在很大程度上产生与 ID-TIMS 相同的结果，尽管精度较低，并且可以进一步帮助识别用于溶液工作的矿物分离物中的金伯利岩污染。
Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction melanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (3-6 ppm), unradiogenic Pb isotopic compositions (Pb-206/Pb-204 = 13.57-13.52) and low U-238/Pb-204 (1.0-1.5) give single-stage model Pb dates of 2.90-2.84 Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18 +/- 0.45 Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic Sr-87/Sr-86 (0.7016). Similarly, the age for samples from Orapa (2.20 +/- 0.54 Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin.Gabbroic eclogites (Eu/Eu* > 1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model mu (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0 Ga addition of a sedimentary component, possibly derived from reworking of a high-mu basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.