He, Ne and Ar isotope signatures of mid-ocean ridge basalts and their implications for upper mantle structure: A case study from the Mid-Atlantic Ridge at 4-12°S

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• 作者：Nicole A. Stroncik ; ^{stroncik@gfz-potsdam.de" class="auth_mail" title="E-mail the corresponding author} ; Samuel Niedermann
• 关键词：Noble gases ; Ultra-depleted mantle domains ; Upper mantle heterogeneity ; Mantle dynamics ; Mantle plumes
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 June 2016
• 年：2016
• 卷：183
• 期：Complete
• 页码：94-105
• 全文大小：726 K

文摘

The geochemical structure of the upper mantle in general and its noble gas isotopic structure in particular have been the subject of countless studies, as both provide important insights into mantle dynamic processes and are essential for the formulation of mantle geodynamic models. This contribution presents a noble gas study of basaltic glasses derived from the Mid-Atlantic Ridge (MAR) between 4° and 12°S, an area well known for its high degree of lithophile isotope heterogeneity and its anomalous crustal thickness. The Sr, Nd, Pb and Hf isotopic systematics along this segment of the MAR range from strongly depleted (i.e. more depleted than N-MORB) to highly enriched. Different concepts have been proposed to explain the observed isotopic signatures. Here we show that the high degree of heterogeneity is not confined to the isotopes of the lithophile elements, but is also shown by the noble gas isotopes and noble gas interelement ratios, such as ³He/²²Ne_M or ⁴He/⁴⁰Ar^∗. Helium isotopic ratios (³He/⁴He), ²¹Ne/²²Ne_extra and ⁴⁰Ar/³⁶Ar range from 7.34 ± 0.06 to 9.38 ± 0.08 R_A, from 0.039 ± 0.020 to 0.075 ± 0.007, and from 346 ± 5 to 37,400 ± 1300, respectively. However, the majority of the Ne isotope data are clearly aligned along a single mixing line in the Ne three-isotope diagram, represented by the equation ²⁰Ne/²²Ne = 70.5 × ²¹Ne/²²Ne + 7.782, with a slope distinctly less than that of the MORB line. This indicates that the strongly depleted material is characterised by a significantly more nucleogenic Ne isotopic composition than the normal depleted mantle. We show, based on covariations between ³He/⁴He and ²¹Ne/²²Ne_extra with ²⁰⁶Pb/²⁰⁴Pb and ¹⁷⁸Hf/¹⁷⁷Hf that the strongly depleted material erupted at this MAR segment was most likely produced by an ancient melting event. This implies that isotopic heterogeneities in the upper mantle are not solely caused by the injection of enriched materials from deep-seated mantle plumes or by crustal recycling but may also be due to differences in the depth and degree of melting of upper mantle material over the lifetime of our planet. In addition, the observed along-ridge and off-axis isotope patterns in conjunction with spatial distribution patterns of off-axis seamounts in the area show that the upper mantle can be extremely heterogeneous in space and time even at small scales. This implies that the mixing efficiency of either large- or small-scale convection processes in the upper mantle might be lower than assumed.