Preserved Nd-Hf-Os isotope variability in replacive channels from the Lanzo ophiolite: Traces of incomplete melt aggregation in the shallow mantle
Preserved Nd-Hf-Os isotope variability in replacive channels from the Lanzo ophiolite: Traces of incomplete melt aggregation in the shallow mantle
复制标题
DOI：
10.1016/j.chemgeo.2023.121779
复制
发表时间：
2023-10
期刊：
影响因子：
3.9
通讯作者：
A. Sanfilippo;C.Z. Liu;V. Salters;A. Mosconi;A. Zanetti;R. Tribuzio
中科院分区：
文献类型：
--
作者：
A. Sanfilippo;C.Z. Liu;V. Salters;A. Mosconi;A. Zanetti;R. Tribuzio
深海橄榄岩的长寿命放射性同位素（MORB 提取的残留物）表明，软流圈本质上是异质的，这是从地球历史上的古代融化事件和地壳循环过程中继承下来的。然而，中洋脊玄武岩（MORB）的平均成分相当均匀，这表明其地幔源的变化在其上升过程中被隐藏。在这里，我们记录了意大利西部阿尔卑斯山兰佐南地幔地块的高渗透性地幔管道中地幔异质性保存得非常好。十米尺度置换体的Nd-Hf-Os同位素为两代地幔通道的存在提供了证据。第一代由与主橄榄岩的主要叶理一致的沙丘组成。替代纯辉石包括具有类似 MORB 不相容元素特征的单斜辉石，初始 (160 Ma) ƐNd 和 ƐHf 范围分别为 +4 至 +7 和 +10 至 +15。第二代由与主叶理不一致的贫辉石方辉石组成，在地球化学上贫乏了不相容元素，并且其单斜辉石显示出高放射性 Hf 同位素（初始ƐHf 高达 +202）。地幔通道的异质性由Resingle bondOs同位素和铂族元素证实。 MORB 型纯橄榄岩具有较高的 Pt、Pd 和局部 Re，并且具有与主体橄榄岩相似的 187Os/188Os 比率 (0.122–0.128)。另一方面，贫化体具有较低的Pt、Pd和Re，以及187Os/188Os比率，范围从主橄榄岩的比率(0.124)到最难熔样品中的高度非放射性值(0.118)。微量元素、PGE 和 Nd-Hf-Os 同位素中保留的异质性凸显了来自高度异质地幔的熔体的渗透，这些熔体仍然部分保存在这些地幔体内。如果应用于当今的大洋中脊，我们的模型表明，通过替代地幔管道迁移的熔体的同位素变异性远远大于海底喷发的岩浆，这意味着不同的地幔成分主要在壳幔边界上方传递和均质化。
Long-lived radiogenic isotopes of abyssal peridotites, residues of MORB extraction, show that the asthenosphere is intrinsically heterogeneous, which is inherited from ancient melting events and crustal recycling during Earth's history. Yet, Mid Ocean Ridge Basalts (MORB) have a rather uniform average composition, suggesting that the variability of their mantle source is concealed during their ascent. Here we document that mantle heterogeneity is exceptionally well preserved in high permeability mantle conduits from the Lanzo South mantle massif, Western Italian Alps. Nd-Hf-Os isotopes of decametre-scale replacive bodies provide evidence for the existence of two generations of mantle channels. The first generation consists of dunites concordant to the main foliation of host peridotites. The replacive dunites include clinopyroxene with MORB-like incompatible element signature and initial (160 Ma) ƐNd and ƐHf ranging from +4 to +7 and from +10 to +15, respectively. The second generation, made up of pyroxene-poor harzburgites discordant to the main foliation, is geochemically depleted in incompatible elements and its clinopyroxene displays highly radiogenic Hf isotopes (initial ƐHf up to +202). The mantle channel heterogeneity is confirmed byResingle bondOs isotopes and platinum-groups elements. The MORB-type dunites have high Pt, Pd and, locally, Re, and have187Os/188Os ratios similar to the host peridotite (0.122–0.128). On the other hand, the depleted bodies have lower Pt, Pd and Re, and187Os/188Os ratios ranging from those of host peridotites (0.124) to highly unradiogenic values (0.118) in the most refractory sample. The preserved heterogeneity in trace elements, PGE, and Nd-Hf-Os isotopes highlights infiltration of melts from a highly heterogeneous mantle, still partially preserved within these mantle bodies. If applied to present-day Mid Ocean Ridges, our model indicates that the isotopic variability of melts migrating through replacive mantle conduits is by far larger than magmas erupted on the seafloor, which implies that diverse mantle components are mainly delivered and homogenised above the crust-mantle boundary.