次大陆岩石圈地幔俯冲沉积物的快速再循环

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Jian Wang, Qiang Wang, Lin Ma, Wan-Long Hu, Jun Wang, Elena Belousova, Gong-Jian Tang
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引用次数: 1

摘要

沉积物俯冲再循环在地球地球化学演化中起着关键作用。上升流柱中陆源再生沉积物的存在被引用来解释海洋岛屿火山的EM2特征,其特征是特别高的87Sr/86Sr (>0.706)。然而,大陆地区这种同位素异常的起源以及俯冲沉积物在次大陆岩石圈地幔(SCLM)中的作用尚不清楚。喜马拉雅-西藏造山带是世界上破译大陆俯冲和地幔中俯冲地壳物质命运的最佳地点之一。本文系统研究了西藏西北西昆仑地区新生代钾-超古典熔岩的年代学、矿物化学(尤其是斜辉石)、全岩化学和Sr-Nd-Pb-Hf-O同位素组成。新的次级离子质谱(SIMS)锆石U-Pb定年,结合已发表的年龄结果,将火山活动的时间限制在~8.3 Ma到现在。这些熔岩的地球化学特征与Samoan热点所代表的EM2地幔端元非常相似。整个岩石和单个岩浆斜辉石均呈现弧形微量元素模式,Sr-Nd-Pb-Hf同位素组成显著富集(87Sr/86Sr≥0.7080;εNd≤−4.8;206Pb/204Pb≥18.704;εHf≤−2.6)。结合高锆石δ18O值(6.3 ~ 10.4‰),表明地幔源区富含再循环沉积物质。地球化学模拟和地球物理证据进一步表明,这些沉积物直接来自印度-欧亚碰撞过程中俯冲的印度大陆岩石圈。假设杂化地幔源含~5%印度大陆地壳的部分熔融模型表明,钾-超太assic熔岩的初级熔体可能是由低熔融度(1-5%)含辉云母的石榴石辉橄榄岩熔融形成的。岩浆地球化学特征与msamuange熔融模式一致,表明俯冲沉积物可能与下行的印度板块分离,并向底向上升进入上覆的地幔岩石圈。与俯冲沉积物进入深部地幔的传统模型不同,西昆仑em2型熔岩表明,俯冲沉积物可以在大陆俯冲期间(可能为50 Myr)迅速再循环到SCLM中。我们认为,scm可能是俯冲沉积的重要储层。这些发现对我们理解地幔循环速率和化学非均质性具有重要意义。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Rapid Recycling of Subducted Sediments in the Subcontinental Lithospheric Mantle
ABSTRACT Subduction recycling of sediments plays a key role in the geochemical evolution of Earth. The presence of recycled terrigenous sediments in upwelling plumes has been cited to explain the EM2 signature in ocean island volcanics, characterized by particularly high 87Sr/86Sr (>0.706). However, the origin of such isotopic anomalies in continental regions and the role of subducted sediments in the subcontinental lithospheric mantle (SCLM) remain unclear. The Himalaya–Tibet orogen is one of the world’s best places for deciphering continental subduction and the fate of subducted crustal materials in the mantle. Here we present a systematic study of the geochronology, mineral chemistry (especially clinopyroxene), whole-rock chemistry and Sr–Nd–Pb–Hf–O isotopic compositions of Cenozoic potassic–ultrapotassic lavas from the western Kunlun area of northwestern Tibet. New secondary ion mass spectrometry (SIMS) zircon U–Pb dating, coupled with published age results, constrain the timing of volcanism from ~8.3 Ma to the present. These lavas show geochemical characteristics that closely resemble the EM2 mantle end-member represented by the Samoan hotspot. Both whole rocks and individual magmatic clinopyroxenes display arc-like trace-element patterns and remarkably enriched Sr–Nd–Pb–Hf isotope compositions (87Sr/86Sr ≥ 0.7080; εNd ≤ −4.8; 206Pb/204Pb ≥ 18.704; εHf ≤ −2.6). Together with high zircon δ18O values (6.3–10.4‰), the data point to a mantle source enriched by recycled sedimentary materials. Geochemical modeling and geophysical evidence further indicate that the sediments were directly derived from the subducted Indian continental lithosphere during India–Eurasia collision. Partial melting models assuming a hybridized mantle source that contains ~5% Indian continental crust suggest that the primary melts of the potassic–ultrapotassic lavas could be formed by melting of a phlogopite-bearing garnet lherzolite at low melting degrees (1–5%). The magma geochemistry is consistent with the model of mélange melting, implying that the subducted sediments may detach from the downgoing Indian slab and rise up diapirically into the overlying mantle lithosphere. Unlike traditional models of subducted sediments entering the deep mantle, the western Kunlun EM2-like lavas reveal that subducted sediments can be rapidly recycled into the SCLM during continental subduction (probably <50 Myr). We suggest that the SCLM could be an important reservoir for subducted sediments. The findings are important to our understanding of mantle circulation rates and chemical heterogeneities.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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