Enriched Hf Nd isotopic signature of veined pyroxenite-infiltrated peridotite as a possible source for E-MORB
Enriched Hf Nd isotopic signature of veined pyroxenite-infiltrated peridotite as a possible source for E-MORB
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DOI：
10.1016/j.chemgeo.2021.120591
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发表时间：
2021-10
期刊：
影响因子：
3.9
通讯作者：
G. Borghini;E. Rampone;C. Class;S. Goldstein;Y. Cai;A. Cipriani;A. Hofmann;L. Bolge
中科院分区：
文献类型：
--
作者：
G. Borghini;E. Rampone;C. Class;S. Goldstein;Y. Cai;A. Cipriani;A. Hofmann;L. Bolge
意大利北部亚平宁地区外利古里德（EL）侏罗系蛇绿岩的辉石岩-橄榄岩序列是由深部熔融渗透和熔融-橄榄岩反应改造的部分MORB丰富地幔组成的。它们代表了morb样脉状地幔的一个极好的自然例子，包括未变质的橄榄岩、辉石岩层和交代橄榄岩。我们对这些地幔序列进行了空间控制的Hf同位素研究，以探讨辉石岩侵位和熔融橄榄岩相互作用对Nd和Hf同位素系统的影响。现今这些岩性的单键氢同位素组成显示出与Nd同位素组成相关的176lu /177Hf和176hf /177Hf比值的大范围。无辉石岩的橄榄岩圈定了一个单键nd同位素阵列，对应于元古代（> ~ 1.5 Ga），可能与该地幔板块的次大陆岩石圈增生有关。辉石岩中176hf /177Hf同位素组成的非均质性主要与176lu /177Hf比值的显著变化相关，反映了原生模态组合中石榴石丰度的变化。随着时间的推移，辉石岩获得了大范围的εHf值，涵盖了洋脊玄武岩中hf单键nd同位素的全球范围。辉石岩衍生熔体的入渗使主橄榄岩获得较低的Lu/Hf比值，从而形成低于未变质橄榄岩的176hf /177Hf比值，形成相当于富集的地幔组分。这一熔融-橄榄岩相互作用可能发生在430 Ma以前的辉石岩侵位期间，并得到了两个lusingonbondhf局部辉石岩-橄榄岩等时线的证实。随着时间的推移，化学和同位素变化产生了EL脉状地幔的单键nd同位素特征，几乎涵盖了已发表的MORB成分的整个范围。辉石岩侵位和主橄榄岩的局部交代作用形成了3个单键富nd地幔域，使EL脉状地幔成为第一个由辉石岩深侵位和辉石岩-橄榄岩相互作用共同作用而形成的富morb类地幔的自然例子。利用EL脉状地幔的结构和同位素特征，模拟了三组分非均质地幔源减压熔融产生的熔体的同位素组成，为中洋脊喷发EMORB的产生提供了一个额外的场景。我们的研究结果强调了深部辉石岩渗透通过与辉石岩衍生熔体相互作用和形成非均质地幔域来改变寄主橄榄岩的潜在作用。
Pyroxenite-peridotite sequences from the External Liguride (EL) Jurassic ophiolites (Northern Apennines, Italy) consist of portions of fertile MORB mantle that were modified by deep melt infiltration and melt-peridotite reaction. They represent an excellent natural example of a MORB-like veined mantle including unmodified peridotite, pyroxenite layers and metasomatized peridotite. We carried out a spatially controlled Hf isotope study on these mantle sequences to investigate how the Nd and Hf isotopic systems are affected by pyroxenite emplacement and melt-peridotite interactions. Present-day Lusingle bondHf isotopic compositions of these lithologies show a large range of176Lu/177Hf and176Hf/177Hf ratios that are correlated with their Nd isotopic compositions. Pyroxenite-free peridotites delineate a Hfsingle bondNd isotope array that corresponds to a Proterozoic age (> 1.5 Ga) which is likely related to the accretion to the subcontinental lithosphere of this mantle sector. Heterogeneous176Hf/177Hf isotopic compositions in pyroxenites mostly correlate with the significant variations of176Lu/177Hf ratios and reflect variable garnet abundance in the primary modal assemblage. Over time, the pyroxenites acquired a large range of εHf values, which encompass the global range of Hfsingle bondNd isotopes in ocean ridge basalts. Infiltration of pyroxenite-derived melts led the host peridotite to acquire low Lu/Hf ratios with the consequent development of176Hf/177Hf ratios lower than in the unmodified peridotite, generating an equivalent of an enriched mantle component. This melt-peridotite interaction likely occurred during the pyroxenite emplacement 430 Ma ago, as confirmed by two Lusingle bondHf local pyroxenite-peridotite isochrons. The chemical and isotopic changes produced, over time, a spread of Hfsingle bondNd isotopic signatures of the EL veined mantle, covering almost the entire range of published MORB compositions. Pyroxenite emplacement and local metasomatism of the host peridotites thus created Hfsingle bondNd enriched mantle domains, making the EL veined mantle the first reported natural example of an enriched MORB-like mantle that formed through the combined effect of deep emplacement of pyroxenite and pyroxenite-peridotite interaction. The structure and isotopic characteristics of the EL veined mantle were used to model the isotopic compositions of melts produced by decompression melting of three-component heterogeneous mantle sources, providing an additional scenario to the generation of EMORB erupted at mid-ocean ridge settings. Our results emphasize the potential role of deep pyroxenite infiltration in modifying the host peridotites by interaction with pyroxenite-derived melts and creating heterogeneous mantle domains.