The origin of highly radiogenic Hf isotope compositions in 3.33 Ga Commondale komatiite lavas (South Africa)
The origin of highly radiogenic Hf isotope compositions in 3.33 Ga Commondale komatiite lavas (South Africa)
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DOI：
10.1016/j.chemgeo.2016.10.010
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发表时间：
2017-04-20
期刊：
影响因子：
3.9
通讯作者：
Wilson, Allan H.
中科院分区：
文献类型：
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作者：
Hoffmann, J. Elis;Wilson, Allan H.
从333 Ga的高铝型Commondale科马提岩南部Kaapvaal克拉通（南非）的钻芯样品进行了分析，主要，微量元素和Hf同位素组成，以放置可能的限制其地幔源和地幔耗尽的过程和时间。初始Hf同位素值为+6.5 ~+9.1，表明地幔源区强烈亏损。这与先前发表的相同地质单元的初始Nd值形成对比，该值仅反映了+2的轻微亏损地幔源。微量元素模拟和地幔源Hf-Nd同位素组成模拟表明，强烈不相容元素亏损微量元素模式可以解释为地幔源，是亏损到高程度（约1000年）。40%）的石榴子石稳定域之前，在尖晶石稳定域的restitic地幔的高度熔融。石榴子石稳定性场的耗竭可能与产生低铝Barberton型科马提岩的羽流事件有关。然而，去耦Hf-Nd同位素组成不能用这样的机制来解释。我们讨论了几种可能的情况下，可以解释微量元素的分布以及同位素组成的Commondale科马提岩。基于微量和同位素模拟，我们考虑了三种不同的模式：（1）亏损地幔源区叠加熔融亏损的石榴辉石岩组分，导致Hf-Nd同位素不耦合，这些石榴辉石岩组分起源于较早的地壳分异事件，并被拆沉到地幔中;（2）俯冲流体叠加亏损的地幔源区并加入Th，Nd和其他轻稀土元素（LREE）在科马提岩喷发前不久，并引发了高度熔融;（3）可能起源于经历了早期岩浆海洋分馏过程的地幔源，随后至少发生了三次地幔亏损事件。康芒代尔科马提岩的微量元素和Hf-Nd同位素组成表明，部分太古代地幔由于科马提岩的提取和下地壳残体可能的拆沉作用而具有高度的不均匀性。(C)2016爱思唯尔B. V.保留所有权利。
Drill core samples from the 333 Ga high-alumina-type Commondale komatiites from the southern Kaapvaal craton (South Africa) were analysed for major, trace elements and Hf-isotope compositions in order to place possible constraints on their mantle sources and the process and timing of mantle depletion. The initial epsilon Hf isotope values range from +6.5 to +9.1 indicating a strongly depleted mantle source. This contrasts with previously published initial epsilon Nd values on the same geologic unit that reflect only minor depleted mantle sources of +2. Trace element modelling and modelling of the Hf-Nd isotope composition of the mantle source indicate that the strongly incompatible element-depleted trace element patterns may be explained by a mantle source that was depleted to high-degrees (ca. 40%) in the garnet stability field prior to high-degree melting of the restitic mantle in the spinel stability field. The depletion in the garnet-stability field may have been related to a plume event that produced the low-alumina Barberton-type komatiites. However, the decoupled Hf-Nd isotope compositions cannot be explained by such a mechanism. We discuss several possible scenarios that may explain the trace element distribution as well as the isotope composition of the Commondale komatiites. Based on trace and isotope modelling, we consider three different models (1) the decoupled Hf-Nd isotope compositions resulted from a depleted mantle source overprint by melt-depleted garnet-pyroxenite components that originated from older crustal differentiation events and were delaminated into the mantle; (2) the decoupling resulted from subduction fluids that overprinted a highly depleted mantle source and added Th, Nd and other light rare-Earth elements (LREE) shortly before eruption of the komatiites and triggered the high degree of melting; (3) a possible origin from a mantle source that experienced fractionation processes in an early magma ocean, followed by at least three mantle depletion events. The trace element and Hf-Nd isotope composition of the Commondale komatiites reveal that parts of the Archean mantle must have been highly heterogeneous due to komatiite extraction and possible delamination processes of lower crustal restites. (C) 2016 Elsevier B.V. All rights reserved.