Length-scales of chemical and isotopic heterogeneity in the mantle section of the Shetland Ophiolite Complex, Scotland
Length-scales of chemical and isotopic heterogeneity in the mantle section of the Shetland Ophiolite Complex, Scotland
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DOI：
10.1016/j.epsl.2018.02.020
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发表时间：
2018-04
影响因子：
5.3
通讯作者：
B. O’Driscoll;R. Walker;P. Clay;J. Day;R. Ash;J. Daly
中科院分区：
文献类型：
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作者：
B. O’Driscoll;R. Walker;P. Clay;J. Day;R. Ash;J. Daly
根据对蛇绿岩和深海橄榄岩的取样，推断海洋地幔中存在千米至亚米尺度的不均匀性。设得兰群岛蛇绿岩（SOC）包含一个保存完好的地幔部分，主要由哈兹勃拉石（体积分数为70%）组成以前报道有可变的主要和微量元素组成，但占主导地位的南极洲初始187Os/188Os组成。为了评估大洋地幔亚米长度尺度上成分不均匀性的保存情况，对SOC地幔剖面的145平方米区域进行了详细绘图和采样。方辉橄榄岩，纯橄榄岩和辉石岩从这个地区进行了分析的亲石和高度亲铁元素（HSE）的丰度，以及为187 Os/188 Os的比例。大多数岩石的亲石元素数据具有俯冲带交代作用的特征。两种纯橄榄岩具有中等分馏的HSE模式和超球粒陨石γ Os（492 Ma）值（+5.1和+7.5），这也是SSZ熔体-岩石相互作用产生的蛇绿质纯橄榄岩的典型特征。相比之下，六块方辉橄榄岩和四块纯橄榄岩的Os、Ir和Ru丰度近似于南极相对丰度，γ Os（492 Ma）值仅介于− 0.6至+2.7之间;这些特征意味着SSZ过程中没有重大影响。两种方辉橄榄岩的放射成因γ Os（492 Ma）值（− 3.5和− 4）也明显较低，并产生中元古代Re贫化时间（TRD）模式年龄。在所研究的区域的范围内的Os同位素组成的范围是一套样品的整个SOC地幔部分的代表性报告，并接近大洋地幔的总同位素变化，在深海橄榄岩中观察到的。这种异质性可以形成和保存的机制涉及低效和时间上不同的熔体提取事件和这些熔体的强局部通道。
Kilometre to sub-metre scale heterogeneities have been inferred in the oceanic mantle based on sampling of both ophiolites and abyssal peridotites. The ∼492 Ma Shetland Ophiolite Complex (SOC) contains a well-preserved mantle section that is dominated by harzburgite (∼70 vol.%) previously reported to have variable major and trace element compositions, yet dominantly chondritic initial187Os/188Os compositions. To assess the preservation of compositional heterogeneities at sub-metre length-scales in the oceanic mantle, a ∼45 m2area of the SOC mantle section was mapped and sampled in detail. Harzburgites, dunites and a pyroxenite from this area were analysed for lithophile and highly-siderophile element (HSE) abundances, as well as for187Os/188Os ratios. Lithophile element data for most rocks are characteristic of supra-subduction zone (SSZ) metasomatic processes. Two dunites have moderately fractionated HSE patterns and suprachondriticγOs(492 Ma)values (+5.1 and +7.5) that are also typical of ophiolitic dunites generated by SSZ melt–rock interactions. By contrast, six harzburgites and four dunites have approximately chondritic-relative abundances of Os, Ir and Ru, andγOs(492 Ma)values ranging only from −0.6 to +2.7; characteristics that imply no significant influence during SSZ processes. Two harzburgites are also characterised by significantly less radiogenicγOs(492 Ma)values (−3.5 and −4), and yield Mesoproterozoic time of Re depletion (TRD) model ages. The range of Os isotope compositions in the studied area is comparable to the range reported for a suite of samples representative of the entire SOC mantle section, and approaches the total isotopic variation of the oceanic mantle, as observed in abyssal peridotites. Mechanisms by which this heterogeneity can be formed and preserved involve inefficient and temporally distinct melt extraction events and strong localised channelling of these melts.