Evolution of the early to late Archean mantle from Hf-Nd-Ce isotope systematics in basalts and komatiites from the Pilbara Craton
Evolution of the early to late Archean mantle from Hf-Nd-Ce isotope systematics in basalts and komatiites from the Pilbara Craton
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DOI：
10.1016/j.epsl.2020.116627
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发表时间：
2021-01-01
影响因子：
5.3
通讯作者：
Muenker, C.
中科院分区：
文献类型：
--
作者：
Hasenstab, E.;Tusch, J.;Muenker, C.
地球地幔早期演化的推论可以从长寿命的放射成因同位素Lu-176-Hf-176和Sm-147-Nd-143等系统中推断出来，对于这些系统，母元素和子元素在低变质等级中基本上保持不动。然而，它仍然是不明确的何时以及在何种程度上幔壳分异过程已经开始在太古代。为了更好地了解太古宙幔壳演化，我们测定了澳大利亚西北部皮尔巴拉群和福蒂斯丘群中一套太古宙镁铁-超镁铁岩样品的初始Lu-176-Hf-176，Sm-147-Nd-143，并以新的方法测定了La-138-Ce-138同位素组成。这些岩石代表了世界上保存最完好的太古代继承之一，并包含在太古代大部分地区重复和长期火山活动脉冲期间喷发的镁铁质-超镁铁质岩石。从皮尔巴拉火山岩和上覆的Fortescue群的六个主要地层组中采集了幔源镁铁-超镁铁岩样品，以表征熔岩的母源地幔源区，并重建这片火山岩岩石圈下的环境地幔的时间演化。此外，我们分析了同生TTG类火成岩套和互层沉积物，以重建岩石圈演化的皮尔巴拉Palladon. Hf-Nd-Ce同位素数据暗示的开始，幔-壳分异皮尔巴拉Palladonas早在类似4.2 Ga，早于任何保存的地层。在误差范围内，耦合Ce-Nd-Hf同位素阵列都相交于铁磁性值，这意味着地球是广泛的铁磁性组成，也为La-138-Ce-138同位素系统。镁铁质岩通常产生强烈耦合的Hf-（i）、Nd-（i）和Ce-（i）值，形成演化中的亏损上地幔成分和原始地幔值（Hf-（i）ca. 0.0~+ 3.2，Nd ~-（i）ca. +0.2至+1.7和Ce-（i）ca. +0.3至-0.1）。由于所有的古太古代样品都缺乏Nb/Th与Hf-（i）或Nd-（i）的共变，富集地壳的混染不太可能解释这种混合趋势。最原始的类地幔镁铁质样品显示出较高的Gd-N/Yb-N比值（2.2-1.4），这意味着一个根深蒂固的，近原始的，上涌地幔的参与，逐步混合到亏损的上地幔。与基性岩相反，大多数（但不是所有）科马提岩在其初始Hf-Nd-Ce同位素组成中是不耦合的，仅在中等高的Hf-Nd-（i）和低的Hf-Ce-（i）值时具有极高的放射成因Hf-（i）值。这种脱钩是最好的解释同化的地幔域经历了早期熔体耗尽的石榴石稳定场，并在高Lu-176/Hf-176的比例，但在适度的Sm-147/Nd-143和La-138/Ce-138的比例随着时间的推移演变。与去耦Hf-Nd同位素组成的岩石消失后，类似于3.2 Ga可能与地幔温度降低，不再能够熔化这样的耐火地幔域。总的来说，我们的新数据的基性岩从皮尔巴拉火山证实存在长期亏损的地幔域在太古代早期，没有采样的锆石Hf同位素记录在皮尔巴拉火山。(C)2020年，任作家。由爱思唯尔公司出版
Inferences on the early evolution of the Earth's mantle can be deduced from long-lived radiogenic isotope Lu-176-Hf-176 and systems such as Sm-147-Nd-143, for which both parent and daughter elements largely remain immobile at low metamorphic grades. However, it remains ambiguous when and to what extent mantle-crust differentiation processes had started in the Archean. For a better understanding of Archean mantle-crust evolution, we determined the initial Lu-176-Hf-176, Sm-147-Nd-143, and, in a new approach, the La-138-Ce-138 isotope compositions of a suite of Archean mafic-ultramafic rock samples from the 3.53-2.83 Ga old Pilbara Craton and 2.78-2.63 Ga old Fortescue Group in NW Australia. These rocks represent one of the best-preserved Archean successions worldwide and contain mafic-ultramafic rocks that were erupted during repeated and long-lived pulses of volcanism throughout much of the Archean. Mantlederived mafic-ultramafic rock samples were collected from six major stratigraphic groups of the Pilbara Craton and the overlying Fortescue Group in order to characterize the parental mantle source regions of the lavas and to reconstruct the temporal evolution of the ambient mantle beneath this piece of cratonic lithosphere. In addition, we analyzed contemporaneous TTG-like igneous suites and interbedded sediments in order to reconstruct the lithospheric evolution of the Pilbara Craton.The Hf-Nd-Ce isotope data imply the onset of mantle-crust differentiation in the Pilbara Craton as early as similar to 4.2 Ga, well prior to any of the preserved stratigraphy. Within error, coupled Ce-Nd-Hf isotope arrays all intersect chondritic values, implying that the Earth is of broadly chondritic composition, also for the La-138-Ce-138 isotope system. Mafic rocks usually yield strongly coupled epsilon Hf-(i), epsilon Nd-(i) and epsilon Ce-(i) values that form a mixing line between an evolving depleted upper mantle composition and the primitive mantle value (epsilon Hf-(i) ca. 0.0 to + 3.2, epsilon Nd-(i) ca. +0.2 to +1.7 and epsilon Ce-(i) ca. +0.3 to -0.1). As all Paleoarchean samples lack co-variations between Nb/Th with epsilon Hf-(i) or epsilon Nd-(i), contamination with an enriched crust is unlikely to explain this mixing trend. The most primitive mantle-like mafic samples show elevated Gd-N/Yb-N ratios (2.2-1.4), implying the involvement of a deep-rooted, near-primitive, upwelling mantle that was progressively mixed into the depleted upper mantle. In contrast to the mafic rocks, most, but not all komatiites are decoupled in their initial Hf-Nd-Ce isotope compositions, by having extremely radiogenic epsilon Hf-(i) values at only moderately high epsilon Nd-(i) and low epsilon Ce-(i) values. This decoupling is best explained by the assimilation of mantle domains that underwent early melt depletion in the garnet stability field and evolved at high Lu-176/Hf-176 ratios but at moderate Sm-147/Nd-143 and La-138/Ce-138 ratios over time. The disappearance of rocks with decoupled Hf-Nd isotope compositions after similar to 3.2 Ga is likely linked to decreasing mantle temperatures that were no longer able to melt such refractory mantle domains. Collectively, our new data for mafic rocks from the Pilbara Craton confirm the presence of long-term depleted mantle domains in the early Archean that are not sampled by the zircon Hf isotope record in the Pilbara Craton. (C) 2020 The Authors. Published by Elsevier B.V.