近日，自然资源部第一海洋研究所Xun WEI等人，研究了西北太平洋Hemler、Vlinder和Il’ichev海山熔岩成因和地球动力学过程的年代学和地球化学约束过程。相关论文于2024年5月8日发表在《中国科学：地球科学》杂志上。

据悉，具有线性年龄递进的大洋板块内火山通常被认为是由上涌的地幔柱熔化而来。然而，西北太平洋的几个海山群表现出复杂的年龄—距离关系，这很难用经典的“地幔柱假说”来解释，它们的起源也是有争议的。本研究利用西北太平洋Hemler、Vlinder和Il’ichev海山熔岩的⁴⁰Ar-³⁹Ar年龄、地球化学和Sr-Nd-Pb-Hf同位素数据，对其岩石成因和地球动力学过程进行了研究。

研究显示，Hemler、Vlinder和Il’ichev海山的熔岩可分为碱性玄武岩、碧玄岩/霞岩和粗面岩。MgO>8wt.%的熔岩具有较高的CaO、FeO^T和TiO₂含量，与碳酸化榴辉岩衍生熔体与多产橄榄岩反应形成的熔体组成相似。这些熔岩具有较高的Zr/Hf比值(40.6-45.2)和负的Zr和Hf异常，表明地幔中存在碳酸盐成分。它们富含不相容的微量元素，具有丰富的地幔1(EM1)样的Sr-Nd-Pb-Hf同位素组成。Vlinder、Il’ichev玄武岩和Hemler熔岩的同位素组成与Rarotonga热点相似。虽然产于同一海山，但与Il’ichev碧玄岩相比，Il’ichev碱性玄武岩的Sr-Nd-Hf同位素组成更为贫化。

根据板块构造重建结果，Hemler (100.1Ma)、Vlinder前- (100.2Ma)和后-shield(87.5Ma)和Il ''ichev (56.4Ma)熔岩年龄明显偏离Macdonald、Arago、Rarotonga和Samoa热点径迹，表明它们不可能直接起源于地幔柱。研究人员提出，在白垩世中期，太平洋板块经过Rarotonga热点时，Rarotonga地幔柱的熔融形成了Vlinder(主要的shield阶段)、Pako和Ioah海山。Rarotonga(可能还有Samoa)的地幔柱物质通过地幔对流分散到周围的软流圈中。这些扩散的地幔柱物质将在Magellan地区广泛分布的岩石圈裂缝下进行减压熔融，形成非热点相关的Hemler和屏蔽前后的Vlinder熔岩。Il’ichev碱玄武岩和碧玄岩可能是岩石圈断裂引起的软流圈非均质富集组分熔融作用的结果。

附：英文原文

Title: Geochronological and geochemical constraints on the petrogenesis and geodynamic process of Hemler, Vlinder, and Il’ichev seamount lavas in NW Pacific

Author: Xun WEI, Yan ZHANG, Xuefa SHI, Hui ZHANG

Issue&Volume: 2024/05/08

Abstract: Oceanic intraplate volcanoes with linear age progressions are usually accepted to be derived from melting of an upwelling mantle plume. Several seamount groups in NW Pacific, however, show complex age-distance relationships that are difficult to explain using the classic “mantle plume hypothesis”, and thus their origins are controversial. In this study, we present ⁴⁰Ar-³⁹Ar age, geochemical, and Sr-Nd-Pb-Hf isotopic data of lavas from Hemler, Vlinder, and Il’ichev seamounts in NW Pacific, to elucidate their petrogenesis and geodynamic process. The lavas from Hemler, Vlinder, and Il’ichev seamounts are classified as alkali basalt, basanite/nephelinite, and trachyte. Lavas with MgO>8 wt.% exhibit high contents of CaO, FeO^T, and TiO₂, similar to the composition of melts formed from reaction between carbonated eclogite-derived melts and fertile peridotite. These lavas have elevated Zr/Hf ratios (40.6–45.2) and negative Zr and Hf anomalies, indicating the presence of a carbonate component in the mantle source. They are enriched in incompatible trace elements and have enriched mantle 1 (EM1)-like Sr-Nd-Pb-Hf isotopic compositions. The isotopic compositions of Vlinder, Il’ichev basanite, and Hemler lavas in this study are similar to the Rarotonga hotspot. Although occurring at the same seamount, the Il’ichev alkali basalts display more depleted Sr-Nd-Hf isotopic compositions compared to Il’ichev basanite. According to plate tectonic reconstruction results, the ages of Hemler (100.1Ma), Vlinder pre- (100.2Ma) and post-shield (87.5Ma), and Il’ichev (56.4Ma) lavas clearly deviate from the Macdonald, Arago, Rarotonga, and Samoa hotspot tracks, indicating that they cannot directly originate from mantle plumes. We propose that in the mid-Cretaceous, when the Pacific plate passed over Rarotonga hotspot, melting of Rarotonga plume formed the Vlinder (main-shield stage), Pako, and Ioah seamounts. The Rarotonga (and possibly Samoa) plume materials would have been dispersed into the surrounding asthenosphere by mantle convection. These diffuse plume materials would undergo decompression melting beneath lithosphere fractures that are widely distributed in the Magellan area, generating non-hotspot related Hemler and pre- and post-shield Vlinder lavas. The Il’ichev alkali basalts and basanite probably result from lithospheric fracture-induced melting of heterogeneous enriched components randomly distributed in the asthenosphere.

DOI: 10.1007/s11430-024-1327-0

Source: https://www.sciengine.com/SCES/doi/10.1007/s11430-024-1327-0