Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland
Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland
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DOI：
10.1016/j.gca.2010.09.027
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发表时间：
2010-12
影响因子：
5
通讯作者：
J. Elis Hoffmann;C. Münker;A. Polat;S. König;K. Mezger;M. Rosing
中科院分区：
文献类型：
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作者：
J. Elis Hoffmann;C. Münker;A. Polat;S. König;K. Mezger;M. Rosing
从可获得的地质记录中越来越多的证据表明，地球上的地壳-地幔分化早在4.4Ga就开始了。为了评估太古宙早期地幔的衰竭程度，我们在西格陵兰岛南部的Isua上地壳带获得了蚀变最小、特征较好的类博长岩变质玄武岩和伴生变质沉积岩的176lu - 176hf、147Sm-143Nd和高场强元素（HFSE）浓度数据。沉积的初始εHf（3720）值为−0.7 ~ +1.5，初始εNd（3720）值为+1.6 ~ +2.1。蚀变最小的类白钨矿变质玄武岩的初始εHf（3720）值为+3.5 ~ +12.9，初始εNd（3720）值为−0.3 ~ +3.2。这些初始hf同位素比值与SiO2、Al2O3/ tio2等相对不动元素呈现出一致的趋势，表明富集组分通过同化而受到污染，这些富集组分很可能来自该地区最早的地壳。该模型也与先前报道的一些样品的初始γOs（3720）值一致。除εHf（3720）为正外，扰动最小的样品εNd（3720）为正，εHf（3720）与εΝd（3720）为共变。基于这些观察，我们认为，在石榴石稳定性场中，初始εHf（3720）值高达+12.9、高176lu /177Hf值为~ 0.05 ~ ~ 0.09的最贫样品代表了一个具有长期衰竭历史的高度贫地幔源。Isua样品的高精度高场强元（HFSE）数据证实了污染趋势。即使是最原始的样品也显示Nb - Ta负异常和Nb/Ta升高，表明俯冲带设置和由榴辉岩部分熔融产生的长英质熔体对衰竭地幔源的叠加。总的来说，类似于硼长石的变质玄武岩的数据支持了强烈枯竭的地幔储层的存在，正如之前从冥古宙锆石的Hf同位素数据和早期太古宙岩石的142nd - 143nd同位素数据推断的那样。
Growing evidence from the accessible geological record reveals that crust–mantle differentiation on Earth started as early as 4.4Ga. In order to assess the extent of early Archean mantle depletion, we obtained176Lu–176Hf,147Sm–143Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf(3720)values from −0.7 to +1.5 and initial εNd(3720)values from +1.6 to +2.1. Initial εHf(3720)values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd(3720)values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO2, Al2O3/TiO2and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs(3720)values for some of the samples. In addition to the positive εHf(3720)values, the least disturbed samples exhibit positive εNd(3720)values and a co-variation of εHf(3720)and εΝd(3720)values. Based on these observations, it is argued, that the most depleted samples with initial εHf(3720)values of up to +12.9 and high176Lu/177Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb–Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined142Nd–143Nd isotope data for early Archean rocks.