Disequilibrium-induced initial Os isotopic heterogeneity in gram aliquots of single basaltic rock powders: Implications for dating and source tracing
Disequilibrium-induced initial Os isotopic heterogeneity in gram aliquots of single basaltic rock powders: Implications for dating and source tracing
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DOI：
10.1016/j.chemgeo.2015.04.010
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发表时间：
2015-06-16
期刊：
影响因子：
3.9
通讯作者：
Wang, Qiang
中科院分区：
文献类型：
--
作者：
Li, Jie;Wang, Xuan-Ce;Wang, Qiang
幔源岩石中Re-Os同位素的不均一性已得到充分证实。然而，地幔源的Os同位素不均匀性是否以及如何影响幔源熔体的Re-Os同位素定年和源区示踪尚不清楚。在这里，我们报告，重复分析克等份的单一玄武岩粉末（参考材料BHVO-2和哈图玄武岩从西准噶尔地区，中国）显示大的变化，在这两个Os浓度和同位素比值。更重要的是，这些重复数据定义了良好的表观Re-Os等时线相关性，产生的年龄明显早于其形成年龄，并显示了Os-187/Os-188比值与常见Os浓度的倒数之间的显著相关性（即，1/Os-192）。这表明玄武岩浆系统中的初始Os同位素组成可能是不均匀的，不能通过粉碎均匀化。理论和数学推导表明，所观察到的Re-Os表观等时线相关性是在形成时没有完全同位素平衡的二元混合的结果，主要是由于地幔条件下含锇难熔包裹体与寄主矿物之间有限的锇同位素扩散交换。回归的原始Os同位素组成可能会偏离地幔源区的真实值。上升过程中的脉冲式地幔熔融和岩浆混合作用是造成玄武质岩石系统初始Os同位素不均匀性的潜在过程。因此，在使用Re-Os同位素体系定年和追踪玄武质岩石或其他低Os岩石的来源时，有必要考虑初始Os同位素组成是否不均匀或是否达到完全同位素平衡。(C)2015 Elsevier B. V.版权所有。
The Re-Os isotopic heterogeneity of mantle-derived rocks has been well documented. However, it is unclear whether and how the Os isotopic heterogeneity of a mantle source can influence Re-Os isotopic dating and source tracing of mantle-derived melts. Here we report that replicate analyses of gram aliquots of single basaltic powders (one of reference material BHVO-2 and three of the Hatu basalts from the western Junggar region, China) show large variations in both Os concentrations and isotopic ratios. More importantly, these replicate data define good apparent Re-Os isochron correlations, yielding ages significantly older than their formation ages, and display remarkable correlations between the Os-187/Os-188 ratio and the reciprocal of the common Os concentration (i.e., 1/Os-192). This indicates that the initial Os isotopic composition in a basaltic magmatic system may be heterogeneous and cannot be homogenized by pulverizing. Theoretical and mathematical deduction demonstrates that the observed apparent Re-Os isochron correlations are the result of binary mixing without complete isotopic equilibrium at the time of formation, primarily due to limited diffusional exchange of Os isotopes between refractory Os-bearing inclusions and host minerals under mantle conditions. The regressed initial Os isotopic composition may bias the true value of the mantle source. Pulsed mantle melting and magma mixing during ascent are potential processes responsible for such initial Os isotopic heterogeneity in basaltic rock systems. Thus, when using the Re-Os isotopic system to date and to trace the source of basaltic rocks or other rocks with relatively low Os concentrations, it is necessary to consider whether the initial Os isotopic composition was heterogeneous or whether it had reached complete isotopic equilibrium. (C) 2015 Elsevier B.V. All rights reserved.