Abstract Recent studies of serpentine-free, spinel peridotite xenoliths from the mantle lithosphere beneath the active Kamchatka and West Bismarck arcs have shown that these rocks are enriched in silica and highly depleted in incompatible elements in comparison with melting residues of either primitive or mid-ocean ridge mantle. It has been suggested that the silica-rich nature of peridotites from the intra-oceanic, fore- and sub-arc mantle lithosphere, collectively referred to as ‘Supra-Subduction Zone (SSZ) peridotites’, is primarily of residual origin and inherited from source processes during partial melting in the SSZ mantle asthenosphere (mantle wedge). However, quantifying the contribution of post-melting processes to the silica-rich nature of SSZ peridotites has remained challenging. Here we report petrological and major and trace element data for a new suite of spinel harzburgite xenoliths from the mantle lithosphere beneath TUBAF seamount, located in the fore-arc region of New Ireland (Papua New Guinea area). All samples are fresh peridotites displaying coarse-grained protogranular textures, and sometimes high orthopyroxene (up to ∼30 wt%) at low clinopyroxene (≤4 wt%) contents, which are typical for SSZ peridotites worldwide. TUBAF peridotites in this study have suffered very little post-melting metasomatism through the formation of ≤1 wt% amphibole, which subsequently experienced decompression-induced breakdown during the xenolith ascent. Otherwise, the rocks display a high degree of inter-mineral equilibration and melting signatures preserved through sub-solidus re-equilibration. The bulk-rock chemistry of TUBAF peridotites record a Fe-Al correlation along the 25-30% melting isopleths from ∼2 to We re-assess earlier interpretations of the origins of TUBAF peridotites by melting at mid-ocean ridges. Instead, we show that these rocks have experienced their last melting event in the mantle wedge, similar to samples from the Izu-Bonin-Mariana fore-arc and the Kamchatka and West Bismarck arcs. We also demonstrate that post-melting metasomatism (including fibrous orthopyroxene that are absent from the samples in this study) is unrelated to the residual SSZ mantle signatures, for which we present the results of polybaric and isothermal flux-melting models including minor element partitioning parameterizations. These models imply that residual SSZ signatures form when previously depleted mantle protoliths are hybridized by hydrous, silica-rich liquids. From the unique Fe-Al correlation in TUBAF peridotites and their low temperatures of equilibration, it appears that fractional hybridization-melting processes forming these rocks occurred in a fore-arc environment with shallow mantle decompression, likely during Oligocene to Miocene subduction along the Manus-Kilinailau trench.

中文翻译：

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洋内超俯冲带地幔分次杂交熔融形成的富含二氧化硅的尖晶石方辉石残渣：来自 TUBAF 海山橄榄岩的新证据

摘要 最近对来自活跃的堪察加半岛和西俾斯麦弧下方地幔岩石圈的不含蛇纹石的尖晶石橄榄岩捕虏体的研究表明，与原始或大洋中部的熔融残留物相比，这些岩石富含二氧化硅和高度贫化的不相容元素。脊地幔。有人提出洋内、弧前和弧下地幔岩石圈橄榄岩的富含二氧化硅的性质，统称为“超俯冲带 (SSZ) 橄榄岩”，主要是残余成因，并继承自SSZ 地幔软流圈（地幔楔）部分熔融过程中的源过程。然而，量化熔化后过程对 SSZ 橄榄岩富含二氧化硅的性质的贡献仍然具有挑战性。在这里，我们报告了位于新爱尔兰（巴布亚新几内亚地区）前弧区 TUBAF 海山下方地幔岩石圈的一组新尖晶石斜方辉石包体的岩石学和主要和微量元素数据。所有样品都是新鲜的橄榄岩，显示出粗粒原粒结构，有时斜辉石含量低（≤ 4 wt%）时斜方辉石含量高（高达 ~ 30 wt%），这是全球 SSZ 橄榄岩的典型特征。本研究中的 TUBAF 橄榄岩通过形成 ≤ 1 wt% 的闪石而几乎没有遭受熔融后交代作用，随后在捕虏体上升过程中经历了减压诱导的分解。否则，岩石显示出高度的矿物间平衡和通过亚固相线重新平衡保留的熔化特征。TUBAF 橄榄岩的大块岩石化学记录了沿 25-30% 熔化等值线的 Fe-Al 相关性，从~2 到我们通过在大洋中脊熔化来重新评估对 TUBAF 橄榄岩起源的早期解释。相反，我们表明这些岩石在地幔楔中经历了最后一次熔化事件，类似于来自伊豆-博宁-马里亚纳前弧以及堪察加半岛和西俾斯麦弧的样本。我们还证明了熔融后交代作用（包括本研究中样品中不存在的纤维状斜方辉石）与残留的 SSZ 地幔特征无关，为此我们提供了多元和等温通量熔融模型的结果，包括微量元素分配参数化. 这些模型表明，当先前耗尽的地幔原岩与含水的富含二氧化硅的液体混合时，会形成残留的 SSZ 特征。从 TUBAF 橄榄岩中独特的 Fe-Al 相关性和它们的低平衡温度来看，形成这些岩石的分次杂化熔化过程似乎发生在具有浅地幔减压的前弧环境中，可能是在渐新世到中新世沿 Manus 俯冲的过程中。基利奈劳海沟。