U-series disequilibria in subduction zone lavas: Inherited from subducted slabs or produced by mantle in-growth melting?

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• 作者：Fang Huang ; ^{fhuang@ustc.edu.cn" class="auth_mail" title="E-mail the corresponding author} ; Juan Xu ; Julin Zhang
• 关键词：U-series disequilibria ; Subduction zone lavas ; Convergent margin ; In-growth melting
• 刊名：Chemical Geology
• 出版年：2016
• 出版时间：15 November 2016
• 年：2016
• 卷：440
• 期：Complete
• 页码：179-190
• 全文大小：2297 K

文摘

U-series disequilibria of subduction zone lavas have been used to provide temporal constraints on flux transfer in convergent margins. There are debates on whether the disequilibria are inherited from the source modified by slab materials (fluids or sediments) or produced by in-growth melting of the mantle. After filtering out the effect of crustal modification on U-series data, (²²⁶Ra)/(²³⁰Th) is not positively correlated with Sr/Th and Ba/Th in arc basalts with SiO₂ < 52 wt%. This invalidates the key evidence for addition of slab-fluids to the mantle producing ²²⁶Ra excess in subduction zone lavas. Sediment melts with a zircon-free residue may have ²³¹Pa excess, but it is not consistent with the overwhelming Zr-Hf depletions in subduction zone lavas, which instead require a zircon-saturated residue during sediment melting.

With appropriate parameters applied, in-growth melting in the mantle followed by modification in the crust can provide a self-consistent explanation of the U-series disequilibria and the relationships with other geochemical signatures in most subduction zone lavas except the few Tonga-Kermadec lavas with slight ²³¹Pa deficit. On the basis of melting experiments of hydrous peridotites, we further propose that most mantle-derived melts and thus U-series disequilibria should be mainly produced by melting in the hot zone of the mantle wedge, as controlled by thermal structure of the convergent margins. In order to produce enough short-lived nuclides, the timescales of mass transfer and magmatism in the subduction zone should at least range from a few to a few hundred millennia. Future studies on more lavas, numerical modelling, and partition coefficients of U-series nuclides will help for better application of U-series disequilibria into subduction zone magmatism.