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Aluminum-dependent trace element partitioning in clinopyroxene

机译：Clinopyroxene中铝相关的微量元素分配

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As technical advances have dramatically increased our ability to analyze trace elements, the need for more reliable data on the compositional dependence of trace element partitioning between minerals and melt has become increasingly important. The late-Cretaceous Carmacks Group of south central Yukon comprises a succession of primitive high-Mg ankaramitic lavas characterized by shoshonitic chemical affinities and containing large complexly zoned clinopyroxene phenocrysts. The compositional zonation of the clinopyroxene phenocrysts is characterized by relatively Fe-rich (Mg~# = Mg/(Mg + Fe) = 0.85), but mottled, cores surrounded by mantles of cyclically-zoned clinopyroxene whose Mg~# varies repeatedly between 0.9 and 0.80. These cyclically zoned clinopyroxene mantles appear to record the repeated influx and mixing of batches of primitive with more evolved magma in a deep sub-crustal (approx 1.2 GPa) magma chamber(s). Laser ablation ICP-MS was used to analyze the trace element variation in these zoned clinopyroxenes. The results indicate more than a threefold variation in the absolute concentrations of Th, Zr, rare earth elements (REE), and Y within individual clinopyroxene phenocrysts, with no apparent change in the degree of REE or high field strength element (HFSE) frac-tionation. The variation in absolute abundances of trace elements correlates closely with the major element composition of the clinopyroxene, with the most enriched clinopyroxene having the lowest Mg~# and highest Al contents. The problem is that the amount of crystal frac-tionation required to explain the major element variation (approx 20 percent ) in these clinopyroxene phenocrysts cannot explain the increase in the abundance of the incompatible trace elements, which would require more than 70 percent crystal fractionation, if constant partition coefficients are assumed. The anomalous increase in incompatible trace elements appears to reflect an increase in their partition coefficients with increasing Al~(IV) in the clinopyroxene; with an increase in Al_2O_3 from 1.5 to 4.0 wt. percent during approx 20 percent crystal fractionation over a temperature decrease of approx 100 deg C being associated with more that a threefold increase in the partition coefficients of Th, Zr, REE, and Y. The magnitude of these increases may indicate that the substitution of these trace elements into clinopyroxene is better modeled in some natural systems by a local charge balance model, rather than the distributed charge model that better replicates the results of annealed experiments. These findings indicate that the effect of Al on the partition coefficients of incompatible trace elements in clinopyroxene may be under appreciated in natural magmatic systems and that the application of experimentally determined clinopyroxene partition coefficients to natural systems must be done with caution.
机译：随着技术的进步极大地提高了我们分析痕量元素的能力，对矿物和熔体之间痕量元素分配的成分依赖性的更可靠数据的需求变得越来越重要。育空地区中南部的晚白垩世卡马克群包括一系列原始的高镁安卡拉密质熔岩，这些熔岩的特征是具有斜辉石化学亲和力，并包含大型复杂的分区的斜辉石苯酚。斜辉石苯晶的组成区带特征是相对富铁（Mg〜＃= Mg /（Mg + Fe）= 0.85），但呈斑驳状，核心被环状带斜辉石的地幔围绕，其Mg〜＃在0.9之间反复变化。和0.80。这些环状分区的斜辉石幔似乎记录了在深亚地壳（约1.2 GPa）岩浆室内多次涌入和混合的原始岩浆与更多演化岩浆的混合。激光烧蚀ICP-MS用于分析这些分区的斜向辉石中痕量元素的变化。结果表明，在单个斜辉石苯酚晶体中，Th，Zr，稀土元素（REE）和Y的绝对浓度变化超过三倍，而REE或高场强元素（HFSE）压裂程度没有明显变化。阳离子。微量元素的绝对丰度的变化与次氯环己烯的主要元素组成密切相关，最富集的次氯环戊烯的Mg〜＃最低，Al含量最高。问题在于，解释这些斜辉石苯酚类晶体中主要元素变化（约20％）所需的晶体碎裂量无法解释不相容痕量元素的丰度增加，这将需要超过70％的晶体分馏，如果假设常数分配系数。不相容的痕量元素的异常增加似乎反映了其在分配物中的分配系数的增加是由于斜py烯中Al〜（IV）的增加。 Al_2O_3从1.5 wt。增加到4.0 wt。在温度降低约100℃时，在大约20％的晶体分馏过程中产生的百分率与Th，Zr，REE和Y的分配系数增加三倍以上有关。这些增加的幅度可能表明这些元素的替代在某些自然系统中，通过局部电荷平衡模型可以更好地对微量元素转化为环吡咯酮进行建模，而不是通过分布式电荷模型更好地复制退火实验的结果。这些发现表明，在自然岩浆系统中可能会认识到Al对斜向辉石中不相容的痕量元素分配系数的影响，并且必须谨慎地将实验确定的斜向辉石分配系数应用于自然系统。

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