Trace element compositions in clinopyroxene and plagioclase, and crystallization temperature estimated after Sun and Liang (2017) for Wadi Somerah, Oman

As largest and best-exposed paleo-fast-spread oceanic crust on land, the Samail ophiolite in the Sultanate of Oman represents an ideal natural laboratory for investigating processes at fast-spreading mid-ocean ridges. We studied a layered gabbro sequence from the middle of the plutonic crust showing dm-scale modal layering with olivine abundance gradually decreasing from layer base to top (Wadi Somerah, Sumail block). Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was performed in order to obtain trace element contents and rare earth elements (REE) in particular. Analyses were performed at the Institut für Geowissenschaften (Christian-Albrechts-Universität zu Kiel, Germany) using a 193 nm ArF excimer laser ablation system (GeoLasPro HD, Coherent) coupled to an Agilent 7900s ICP-MS. Analyses were performed as point measurements with 16 Hz pulse rate, 18 Jcm-2 fluence and a crater diameter of 120 µm on plagioclase, and 10 Hz, 12 Jcm-2 and a crater diameter of 32 µm on clinopyroxene. Differences in core and rim compositions of clinopyroxene reveal significant zonation with higher REE contents in the rim. Crystallization temperatures calculated using the REE-thermometer of Sun and Liang (2017; doi:10.1007/s00410-016-1326-9) vary between 1230 and 1300°C.

作为陆地上出露规模最大、暴露最完好的古快速扩张洋壳，阿曼苏丹国的萨迈勒蛇绿岩（Samail ophiolite）是研究快速扩张洋中脊过程的理想天然实验室。我们研究了来自深成地壳中部的层状辉长岩序列，该序列显示出分米级的模态层理，橄榄石丰度从层底到顶部逐渐降低（Sumail地块的Wadi Somerah）。为获取微量元素含量，尤其是稀土元素（rare earth elements, REE），我们进行了激光剥蚀电感耦合等离子体质谱分析（laser ablation inductively coupled plasma mass spectrometry, LA-ICP-MS）。分析在德国基尔克里斯蒂安-阿尔布雷希特大学地球科学研究所（Institut für Geowissenschaften, Christian-Albrechts-Universität zu Kiel）开展，使用193 nm ArF准分子激光剥蚀系统（GeoLasPro HD，Coherent）耦合至Agilent 7900s电感耦合等离子体质谱仪。对斜长石的分析采用点测量模式，参数为脉冲频率16 Hz、能量密度18 J/cm²、剥蚀坑直径120 μm；对单斜辉石的分析参数为脉冲频率10 Hz、能量密度12 J/cm²、剥蚀坑直径32 μm。单斜辉石的核部与边缘成分差异显示出显著的环带特征，边缘的稀土元素含量更高。利用Sun和Liang（2017；doi:10.1007/s00410-016-1326-9）提出的稀土元素温度计计算得到的结晶温度介于1230至1300°C之间。