Tungsten isotope composition of the Acasta Gneiss Complex
Tungsten isotope composition of the Acasta Gneiss Complex
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DOI：
10.1016/j.epsl.2015.02.040
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发表时间：
2015-06
影响因子：
5.3
通讯作者：
M. Willbold;S. Mojzsis;Hart Chen;T. Elliott
中科院分区：
文献类型：
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作者：
M. Willbold;S. Mojzsis;Hart Chen;T. Elliott
摘要高精度钨（182 W/184 W）同位素测量的特点，镁铁质和长英质样品的ca。3960 Ma的Arista片麻岩杂岩（AGC; Northwest Territories，Canada）显示放射成因ε 182 W值在+ 0.06至+ 0.15之间。两个CA。3600 Ma长英质样品的ε 182 W = 0，是迄今为止记录的最古老的样品，其W同位素组成与现代地幔没有区别。ε 182 W数据与ε 142 Nd相关（Roth等人，2014年），我们将这种变化归因于3960 Ma原岩的不完全变质均匀化，其中3370 Ma构造热事件中的材料更新。值得注意的是，在3960 Ma AGC样品中观察到的正ε 182 W异常值受变质均匀化影响最小，与其他早太古代岩石（格陵兰岛Isua表壳带;加拿大Nuvvuagittuq表壳带）和晚太古代科斯托穆克沙科马提岩（卡累利阿）中观察到的值相当。这证明了一个全局常量签名。我们推断，前晚单板地幔的存在代表了对不同时代太古代岩石中ε W182 <$+ 0.15值均匀分布的最直接解释。我们表明，这样的概念与来自镁铁质和超镁铁质太古代幔源岩石中高度亲铁元素丰度的独立约束是相容的。迄今为止，在小于约100年的样品中测得的异常ε 182 W和ε 142 Nd的缺失。2800 Ma表明硅酸盐储层的对流均质化。上地幔中富含晚期交付的陨石物质的向下混合可能是这些综合观测的原因。
Abstract High-precision tungsten (182 W/184 W) isotope measurements on well-characterised mafic and felsic samples of the ca. 3960 Ma Acasta Gneiss Complex (AGC; Northwest Territories, Canada) show radiogenic ε 182 W values between+ 0.06 to+ 0.15. Two ca. 3600 Ma felsic samples from this terrane have ε 182 W∼ 0 and are the oldest samples so far documented to have a W isotopic composition indistinguishable from that of the modern mantle. The ε 182 W data are correlated with ε 142 Nd (Roth et al., 2014) and we attribute this variability to incomplete metamorphic homogenisation of the 3960 Ma protolith with more recent material in a 3370 Ma tectono-thermal event. Notably, the value of the positive ε 182 W anomalies seen in the 3960 Ma AGC samples that are least affected by metamorphic homogenisation is comparable to that observed in other early Archean rocks (Isua Supracrustal Belt, Greenland; Nuvvuagittuq Supracrustal Belt, Canada) and the late Archean Kostomuksha komatiites (Karelia). This demonstrates a globally constant signature. We infer that the presence of a pre-late veneer mantle represents the most straightforward interpretation of a uniform distribution of ε W 182∼+ 0.15 value in Archean rocks of different ages. We show that such a notion is compatible with independent constraints from highly siderophile element abundances in mafic and ultra-mafic Archean mantle-derived rocks. The absence of anomalous ε 182 W and ε 142 Nd so far measured in samples younger than ca. 2800 Ma suggests progressive convective homogenisation of silicate reservoirs. The downward mixing of an upper mantle rich in late-delivered meteoritic material might account for these combined observations.