Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa
Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa
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DOI：
10.1093/petrology/egv036
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发表时间：
2015-06
影响因子：
3.9
通讯作者：
I. Veksler;D. Reid;P. Dulski;J. Keiding;Mathias Schannor;L. Hecht;R. Trumbull
中科院分区：
文献类型：
--
作者：
I. Veksler;D. Reid;P. Dulski;J. Keiding;Mathias Schannor;L. Hecht;R. Trumbull
布什维尔德火成杂岩的上临界区（UCZ）以循环单元的形式显示出壮观的分层，包括一个基底铬铁矿层，上面覆盖着一系列硅酸盐堆积物，从下到上依次为辉石-苏长岩-斜长岩。电子探针和激光烧蚀电感耦合等离子体质谱分析的铬铁矿和硅酸盐矿物层之间的UG 2铬铁矿和Merensky礁揭示的变化，主要和微量元素的组合物，违背现有的模型的解释累积矿物熔体演化。异常特征在铬铁矿与邻近斜长岩和辉石岩堆晶的尖锐接触处最发育。在这里，铬铁矿的组成突然变化，从高和恒定的镁/（镁铁2 H2）和铁2 H2/铁3 H2的比率在铬铁矿层中的变化和一般较低的值在硅酸盐层中浸染。此外，浸染状铬铁矿的组成取决于宿主硅酸盐组合;例如，在Ti，V和Zn含量。重要的是，在铬铁矿-硅酸盐层接触的铬铁矿组合物的突然变化是独立的铬铁矿层的厚度和估计的质量比例的铬铁矿间积云液体。斜长石中的化学变化也是突然的，有些很难与传统的积云间液体再平衡模型相一致。这些特征之一是碱金属与其他不相容的亲石元素的分离。与堆晶斜长石相比，铬铁矿层中的堆晶嵌晶斜长石富集稀土元素，但强烈亏损同样不相容的Li、K和Rb。在超镁铁堆晶和铬铁矿层的堆晶间辉石中也观察到强的碱贫化。为了解释这些功能，我们提出了一个新的模型后积云重结晶，加强了模态分层的晶体液体糊状物，生产观察到的序列几乎monomineralic层的铬铁矿，辉石和斜长岩，定义的循环单元。这个模型的关键要素是建立氧化还原电位梯度之间的接触富铬累积和相邻的硅酸盐层之间的晶体和积云间熔体的围晶反应。由于玄武岩熔体是以Na+为主要电荷载体的离子电解质，氧化还原电位梯度引起Na+和其它碱金属离子的电化学迁移。碱的选择性迁移可以解释循环单元中斜长石成分的神秘特征。钠迁移预计会导致重熔以前形成的堆积和模态矿物比例的重大变化，这可能最终导致形成急剧分裂的单矿物层。铬铁矿中的铁/亚铁比值与循环单元和斜长石中的铁分布的变化意味着
The Upper Critical Zone (UCZ) of the Bushveld Igneous Complex displays spectacular layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence of silicate cumulates in the order, from bottom to top, pyroxenite–norite–anorthosite. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses of chromite and silicate minerals in layers between the UG2 chromitite and the Merensky Reef reveal variations in major and trace element compositions that defy explanation with existing models of cumulate mineral– melt evolution. The anomalous features are best developed at sharp contacts of chromitite with adjacent anorthosite and pyroxenite cumulates. Here, chromite compositions change abruptly from high and constant Mg/(Mg þ Fe 2þ ) and Fe 2þ /Fe 3þ ratios in chromitite layers to variable and generally lower values in chromite disseminated in silicate layers. Furthermore, the composition of disseminated chromites varies depending on the host silicate assemblage; for example, in Ti, V and Zn contents. Importantly, the abrupt change in chromite composition across the chromitite–silicate layer contacts is independent of the thickness of the chromitite layer and the estimated mass proportions of chromite to intercumulus liquid. Chemical variations in plagioclase are also abrupt and some are hard to reconcile with conventional models of re-equilibration with intercumulus liquid. Among those features is the decoupling of alkalis from other incompatible lithophile elements. In comparison with cumulus plagioclase, intercumulus poikilitic plagioclase in chromitite layers is enriched in rare earth elements but strongly depleted in equally incompatible Li, K and Rb. Strong alkali depletion is also observed in intercumulus pyroxene from ultramafic cumulates and chromitite layers. To explain these features, we propose a new model of post-cumulus recrystallization, which intensifies the modal layering in the crystal–liquid mush, producing the observed sequence of nearly monomineralic layers of chromitite, pyroxenite and anorthosite that define the cyclic units. The crucial element of this model is the establishment of redox potential gradients at contacts between chromite-rich cumulates and adjacent silicate layers owing to peritectic reactions between the crystals and intercumulus melt. Because basaltic melts are ionic electrolytes with Na þ as the main charge carrier, the redox potential gradient induces electrochemical migration of Na þ and other alkali ions. Selective mobility of alkalis can explain the enigmatic features of plagioclase composition in the cyclic units. Sodium migration is expected to cause remelting of previously formed cumulates and major changes in modal mineral proportions, which may eventually result in the formation of sharply divided monomineralic layers. The observed variations in ferric/ferrous iron ratios in chromite from the cyclic units and Fe distribution in plagioclase imply a redox gradient of