穆斯堡尔光谱仪在揭示地幔氧逸度研究中的应用

王秋霞; 平先权; 郑建平; 戴宏坤

doi:10.3799/dqkx.2022.354

地球科学 ›› 2023, Vol. 48 ›› Issue (03) : 1217 -1231. DOI: 10.3799/dqkx.2022.354

穆斯堡尔光谱仪在揭示地幔氧逸度研究中的应用

王秋霞 ¹ ,
平先权 ¹ ,
郑建平 ¹^,² ,
戴宏坤 ²

作者信息 +

Mineral Ferric Iron Contents of Peridotite Xenoliths by Mössbauer Spectroscopy: Oxygen Fugacity Applications

Author information +

文章历史 +

PDF (2808K)

摘要

利用地幔橄榄岩包体中平衡矿物对的氧逸度计可以有效限定岩石圈地幔的氧化-还原状态.本文使用中国地质大学（武汉）新购置的WSS-10型常温穆斯堡尔光谱仪对华北地块西北部狼山地区橄榄岩包体的四相矿物开展了铁价态的直接测定.结果显示，斜方辉石Fe³⁺/∑Fe为0.05~0.11，单斜辉石的为0.16~0.25，尖晶石的为0.16~0.22，橄榄石为0.利用橄榄石-斜方辉石-尖晶石氧逸度计，获得狼山地区岩石圈地幔氧逸度为FMQ-0.82至FMQ-0.39（均值为FMQ-0.65）.该值略高于依据电子探针测试数据计算获得的氧逸度值（FMQ-1.49至FMQ-0.8，均值为FMQ-1.25），造成这一差异的原因可能是后者忽略了尖晶石晶格中Fe³⁺过剩和阳离子空位导致的非化学计量比.与全球克拉通岩石圈地幔氧逸度值（均值为FMQ-0.35）相比，华北地块西北缘狼山地区岩石圈地幔整体表现为较还原状态，推测与深部地幔低ƒO₂熔体上升交代有关.

Abstract

The redox state of the lithospheric mantle can be estimated by well equilibrated mineral pairs of mantle peridotite xenoliths. Here we explore the mantle redox state of the northwest North China Craton by measuring the mineral Fe³⁺/∑Fe of the Langshan spinel lherzolite xenoliths using the newly established lab of WSS-10 Mössbauer spectroscopy at China University of Geosciences (Wuhan). The results show that the Fe³⁺/∑Fe ratios are 0.05-0.11 for orthopyroxene, 0.16-0.25 for clinopyroxene, 0.16-0.22 for spinel, and no Fe³⁺ was confirmed in olivine. The corresponding mantle oxygen fugacity using the olivine-orthopyroxene-spinel oxy-barometry is FMQ-0.82 to FMQ-0.39 (averaged at FMQ-0.65), slightly higher than the estimates using electron microprobe data (FMQ-1.49 to FMQ-0.8, averaged at FMQ-1.25). This discrepancy may be explained by the non-stoichiometry Fe³⁺ excess and cation vacancies in the spinel lattice, which is generally ignored in the stoichinometric calculation using electron microprobe data. Compared with global cratonic mantle (averaged at FMQ-0.35), the lithospheric mantle underneath the northwest North China Craton, represent by Langshan mantle xenoliths, is relatively reduced, and likely results from metasomatism by low-ƒO₂ melts from deeper mantle domains.