Micro- and nano-scale textural and compositional zonation in plagioclase at the Black Mountain porphyry Cu deposit: Implications for magmatic processes
Micro- and nano-scale textural and compositional zonation in plagioclase at the Black Mountain porphyry Cu deposit: Implications for magmatic processes
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DOI：
10.2138/am-2019-6609
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发表时间：
2019-02
影响因子：
3.1
通讯作者：
Mingjian Cao;N. Evans;S. Reddy;D. Fougerouse;P. Hollings;D. Saxey;B. McInnes;D. Cooke;B. McDonald
中科院分区：
文献类型：
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作者：
Mingjian Cao;N. Evans;S. Reddy;D. Fougerouse;P. Hollings;D. Saxey;B. McInnes;D. Cooke;B. McDonald
摘要：使用电子微探针、激光烧蚀-电感耦合等离子体质谱和原子探针断层扫描技术，对来自新鲜同矿化闪长斑岩（菲律宾黑山斑岩铜金矿床）的斜长石斑晶的结构和成分微米级（10-100 μm）和纳米级（10-100 nm）分带进行了主量元素和微量元素的表征。复杂的斜长石晶体 (3.0 × 5.4 mm) 具有斑片状中长石核 (An41–48 mol%)、侵蚀的城石地幔 (An71–80 mol%) 和振荡中长石边缘 (An39–51 mol%)。大多数主要元素和微量元素（Si、Ca、Al、Na、K、Fe、Mg、Ti、Sr、Ba、Pb、La、Ce 和 Pr）都出现了周期宽度为 50 至 200 μm 的微尺度变化，其核心和边缘的 ΔAn 幅度均为 4-12 mol%。地幔具有独特的元素成分，表明在安山岩浆中添加了更热的镁铁质岩浆。原子探针断层扫描显示中长石边缘不存在纳米级变化，但在副城地幔中交替出现纳米级（25-30 nm）富铝、富钙和富硅、富钠区域，Ca/(Ca+Na)at%振幅约为10。物理化学参数的有限变化（富含 H2O，T = 865 至 895 °C，P = 5.3 至 6.2 kbar；由共沉淀角闪石记录的 fO2 = NNO+0.6 至 NNO+1.1）表明微尺度振荡分区可能由内部晶体生长机制控制，而不是由物理化学条件的周期性变化控制。然而，地幔中CaAl-NaSi相互扩散的均匀扩散时间尺度远小于晶粒从地幔到边缘的结晶时间尺度，表明结晶后出溶形成的菱铁矿地幔中存在纳米级分带。斜长石中微尺度分区的发生表明，假设初始温度为 880 °C、宽度为 50 μm、NaSi-CaAl 在含水条件下相互扩散，结晶过程中的最小冷却速率为 0.0005 °C/年。假设受锆石结晶温度限制，纳米级溶出织构的形成温度约为 675 °C，则保留纳米级分区（~28 nm）需要的最小冷却速率为 0.26 °C/年。鉴于这比在岩浆室中发生的冷却速度要快得多，这种纹理可能记录了结晶后的就位历史。
Abstract Textural and compositional microscale (10–100 μm) and nanoscale (10–100 nm) zoning in a plagioclase phenocryst from a fresh, syn-mineralization diorite porphyry (Black Mountain porphyry Cu-Au deposit, Philippines) was characterized for major and trace elements using electron microprobe, laser ablation-inductively coupled plasma-mass spectrometry, and atom probe tomography. The complex plagioclase crystal (3.0 × 5.4 mm) has a patchy andesine core (An41–48 mol%), eroded bytownite mantle (An71–80 mol%), and oscillatory andesine rim (An39–51 mol%). Microscale variations with a periodic width of 50 to 200 μm were noted for most major and trace elements (Si, Ca, Al, Na, K, Fe, Mg, Ti, Sr, Ba, Pb, La, Ce, and Pr) with a ΔAn amplitude of 4–12 mol% in both the core and rim. The mantle has a distinct elemental composition, indicating the addition of hotter mafic magma to the andesitic magma. Atom probe tomography shows an absence of nanoscale variations in the andesine rim but alternating nanoscale (25–30 nm) Al-rich, Ca-rich, and Si-rich, Na-rich zones with a Ca/(Ca+Na)at% amplitude of ~10 in the bytownite mantle. The restricted variations in physiochemical parameters (H2O-rich, T = 865 to 895 °C, P = 5.3 to 6.2 kbar; fO2 = NNO+0.6 to NNO+1.1 recorded by co-precipitated amphibole) suggest microscale oscillatory zoning was likely controlled by internal crystal growth mechanisms, and not by periodic variations in physiochemical conditions. However, the uniform diffusion timescale for CaAl-NaSi interdiffusion in the mantle is far shorter than the crystallization timescale of the grain from mantle to rim, suggesting nanoscale zonation in the bytownite mantle formed by exsolution after crystallization. The occurrence of micro-scale zoning in plagioclase indicates a minimum cooling rate of 0.0005 °C/yr during crystallization, assuming an initial temperature of 880 °C, the width of 50 μm, and NaSi-CaAl interdiffusion under hydrous conditions. Assuming a formation temperature of ~675 °C for the nanoscale exsolution texture as constrained by zircon crystallization temperatures, the retention of nanoscale zoning (~28 nm) requires a minimum cooling rate of 0.26 °C/yr. Given that this is significantly faster cooling than would occur in a magma chamber, this texture likely records the post-crystallization emplacement history.