Unravelling the processes controlling apatite formation in the Phalaborwa Complex (South Africa) based on combined cathodoluminescence, LA-ICPMS and in-situ O and Sr isotope analyses
Unravelling the processes controlling apatite formation in the Phalaborwa Complex (South Africa) based on combined cathodoluminescence, LA-ICPMS and in-situ O and Sr isotope analyses
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DOI：
10.1007/s00410-020-1671-6
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发表时间：
2020-03-23
影响因子：
3.5
通讯作者：
Baele, Jean-Marc
中科院分区：
文献类型：
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作者：
Decree, Sophie;Cawthorn, Grant;Baele, Jean-Marc
Phalaborwa世界级磷酸盐存款（南非）是由一个古元古代碱性复杂的主要组成的磷铁矿，碳酸盐岩，辉石岩，和次要的fenite。此外，正长岩和粗面岩出现在许多卫星体内。新的岩石学和原位地球化学数据沿着在磷灰石上获得的O和Sr同位素数据表明，磷灰石是在主要的寄主岩石（辉石岩、磷铁矿和碳酸岩）中形成的，这些岩石主要是由幔源碳酸岩岩浆的火成岩过程形成的。早期形成的岩浆磷灰石特别富含轻稀土元素（LREE），稀土元素含量的减少归因于岩浆分异和隔离间隙熔体口袋中的早期磷灰石分馏。瑞利分馏有利于在一个恒定的Sr同位素组成的δ O-18（低于1%）略有增加。新鲜的碳酸岩岩浆侵入早期形成的碳酸岩体局部诱导再平衡的早期磷灰石与稀土富集，但在恒定的O和Sr同位素组成。在长岩、正长岩和粗面岩中，磷灰石显示蚀变结构和轻稀土亏损，反映了与流体的相互作用。正长岩和粗面岩中磷灰石中δ O-18的显著减少表明δ O-18贫化的大气流体的贡献。这与卫星体的浅成就位相一致。这些岩石中磷灰石Sr同位素比值的普遍增加反映了随时间推移与围岩的相互作用。这项研究使得以无与伦比的精度破译地质过程的连续性成为可能，这些地质过程导致了世界上最重要的磷酸盐矿床之一。
The Phalaborwa world-class phosphate deposit (South Africa) is hosted by a Paleoproterozoic alkaline complex mainly composed of phoscorite, carbonatite, pyroxenitic rocks, and subordinate fenite. In addition, syenite and trachyte occur in numerous satellite bodies. New petrological and in-situ geochemical data along with O and Sr isotope data obtained on apatite demonstrate that apatite is in the principal host rocks (pyroxenitic rocks, phoscorite and carbonatite) formed primarily by igneous processes from mantle-derived carbonatitic magmas. Early-formed magmatic apatite is particularly enriched in light rare earth elements (LREE), with a decrease in the REE content ascribed to magma differentiation and early apatite fractionation in isolated interstitial melt pockets. Rayleigh fractionation favored a slight increase in delta O-18 (below 1%) at a constant Sr isotopic composition. Intrusion of fresh carbonatitic magma into earlier-formed carbonatite bodies locally induced re-equilibration of early apatite with REE enrichment but at constant O and Sr isotopic compositions. In fenite, syenite and trachyte, apatite displays alteration textures and LREE depletion, reflecting interaction with fluids. A marked decrease in delta O-18 in apatite from syenite and trachyte indicates a contribution from delta O-18-depleted meteoric fluids. This is consistent with the epizonal emplacement of the satellite bodies. The general increase of the Sr isotope ratios in apatite in these rocks reflects progressive interaction with the country rocks over time. This study made it possible to decipher, with unmatched precision, the succession of geological processes that led to one of the most important phosphate deposits worldwide.