Melting and metasomatism in West Eifel and Siebengebirge Sub-Continental Lithospheric Mantle: Evidence from concentrations of volatiles in fluid inclusions and petrology of ultramafic xenoliths
Melting and metasomatism in West Eifel and Siebengebirge Sub-Continental Lithospheric Mantle: Evidence from concentrations of volatiles in fluid inclusions and petrology of ultramafic xenoliths
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DOI：
10.1016/j.chemgeo.2021.120400
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发表时间：
2021-06
期刊：
影响因子：
3.9
通讯作者：
A. Rizzo;B. Faccini;F. Casetta;L. Faccincani;T. Ntaflos;F. Italiano;M. Coltorti
中科院分区：
文献类型：
--
作者：
A. Rizzo;B. Faccini;F. Casetta;L. Faccincani;T. Ntaflos;F. Italiano;M. Coltorti
通过岩相学、矿物化学和流体包裹体(FI)中挥发分浓度的综合研究来约束次大陆岩石圈地幔(SCLM)特定部分的组成和演化的可能性，是一种新的方法，可以为岩石圈内挥发分的循环提供线索。这一方法在活跃或休眠的火山区更为重要，在那里，地表气体排放的特征可能是下面的岩石圈地幔区域的特征。在这方面，在西埃菲尔(约0.5-0.01 Ma)和西本伯里(约30-6 Ma)火山田(德国)露出地表的超镁铁捕虏体是理想的目标，因为它们提供了关于中欧火山省(CEVP)下最有趣的SCLM部分之一的直接信息。通过岩相学观察、矿物物相分析和对橄榄石、斜方辉石和单斜辉石中He、Ne、Ar和CO2含量的测定，对这些地区的五个不同的群体进行了调查。最难熔的SiebengeBirge岩具有高镁橄榄石、高镁、低铝辉石和高铬尖晶石，反映了高熔体提取程度(高达30%)。相反，西艾菲尔的包体在形态和成分上是不均匀的，橄榄石的大镁橄榄岩范围(Fo83-92)，尖晶石的铬#范围(0.1-0.6)，以及辉石的可变Al和Ti含量表明。SiebengeBirge的平衡温度在870°C到1070°C之间，West Eifel捕虏体的平衡温度在~900°C到~1190°C之间，氧逸度值一般在−0.5FMQ和+11.3FMQΔƒO2之间。在这两个地区，FI组分以CO2为主，单斜辉石，大部分斜方辉石的挥发物浓度最高，而橄榄石是贫气的。稀有气体和二氧化碳的分布表明，橄榄石代表了经历了一次或多次熔体提取幕的残余地幔。经大气污染校正的3He/4He比值(Rc/Ra值)从方辉橄榄岩类型的6.8Ra变化到二辉橄榄岩和堆积岩中的5.5Ra，表明原始的类似MORB的地幔特征随着与地壳相关组分的相互作用而逐渐改变，熔体的3He/4He和4He/40Ar*值与已发表的岩浆气体释放值一致。Ne和Ar同位素系统学表明，大多数数据与循环大气成分和类似MORB的地幔之间的混合一致，这不一定需要在CEVP的这一部分之下有较低的地幔羽流参与。SiebengeBirge和West Eifel矿物相的主要元素分布，以及FI组分的系统变化，辉石中Al的富集度与平衡温度的正相关，以及伴随的Rc/Ra随温度的降低而降低，表明SiebengeBirge下的SCLm代表了CEVP中属于第四纪Eifel火山作用的熔融/流体大量渗入之前的华力西岩石圈。相比之下，西艾菲尔捕虏体反映了约6 Ma至约0.5 Ma之间在区域SCLm发生的多次不均匀交代/参考化事件。
The possibility of constraining the composition and evolution of specific portions of the Sub-Continental Lithospheric Mantle (SCLM) by means of an integrated study of petrography, mineral chemistry, and concentrations of volatiles in fluid inclusions (FI) is a novel approach that can provide clues on the recycling of volatiles within the lithosphere. This approach is even more important in active or dormant volcanic areas, where the signature of the gaseous emissions at the surface can be that of the underlying lithospheric mantle domains. In this respect, the ultramafic xenoliths brought to the surface in West Eifel (~0.5–0.01 Ma) and Siebengebirge (~30–6 Ma) volcanic fields (Germany) are ideal targets, as they provide direct information on one of the most intriguing portions of SCLM beneath the Central European Volcanic Province (CEVP). Five distinct populations from these localities were investigated using petrographic observations, mineral phase analyses and determination of He, Ne, Ar and CO2contents in olivine-, orthopyroxene-, and clinopyroxene-hosted FI. The most refractory Siebengebirge rocks have highly forsteritic olivine, high-Mg#, low-Al pyroxene, and spinel with high Cr#, reflecting high extents (up to 30%) of melt extraction. In contrast, xenoliths from West Eifel are modally and compositionally heterogeneous, as indicated by the large forsterite range of olivine (Fo83–92), the Cr# range of spinel (0.1–0.6), and the variable Al and Ti contents of pyroxene. Equilibration temperatures vary from 870 °C to 1070 °C in Siebengebirge, and from ⁓900 °C to ⁓1190 °C in West Eifel xenoliths, at oxygen fugacity values generally between −0.5 and + 1.3 ΔlogƒO2[FMQ]. In both areas, the FI composition was dominated by CO2, with clinopyroxene, and most of the orthopyroxene had the highest concentrations of volatiles, while olivine was gas-poor. The noble gas and CO2distributions suggest that olivine is representative of a residual mantle that experienced one or more melt extraction episodes. The3He/4He ratio corrected for air contamination (Rc/Ra values) varied from 6.8 Ra in harzburgitic lithotypes to 5.5 Ra in lherzolites and cumulate rocks, indicating that the original MORB-like mantle signature was progressively modified by interaction with crustal-related components and melts having3He/4He and4He/40Ar* values consistent with those published for magmatic gaseous emissions. The Ne and Ar isotope systematics indicated that most of the data were consistent with mixing between a recycled atmospheric component and a MORB-like mantle, which does not necessarily require the involvement of a lower mantle plume beneath this portion of the CEVP. The major element distribution in mineral phases from West Eifel and Siebengebirge, together with the systematic variations in FI composition, the positive correlation between Al enrichment in pyroxene and equilibration temperatures, and the concomitant Rc/Ra decrease with increasing temperature, suggest that the SCLM beneath Siebengebirge represented the Variscan lithosphere in CEVP prior to the massive infiltration of melts/fluids belonging to the Quaternary Eifel volcanism. In contrast, West Eifel xenoliths reflect multiple heterogeneous metasomatism/refertilisation events that took place in the regional SCLM between ~6 and ~ 0.5 Ma.