Chromite-induced magnesium isotope fractionation during mafic magma differentiation

Abstract

a b s t r a c tTo better understand the mechanism of Mg isotopic variation in magma systems, here we report high precision Mg isotopic data of 17 bulk rock samples including dunite, clinopyroxenite, hornblendite and gabbro and 10 pairs of dunite-hosted olivine and chromite separates from the well-characterized Alaskan-type Xiadong intrusion in NW China, which formed by continuous and high degree of lithological differentiation from mafic magmas. Chromite separates have highly variable d 26 Mg values from À0.10‰ to 0.40‰, and are consistently heavier than coexisting olivine separates (À0.39‰ to À0.15‰). Both mineral d 26 Mg values and the degrees of inter-mineral fractionation are well correlated with geochemical indicators of magma differentiation, indicating that these inter-sample and inter-mineral Mg isotope fractionations are caused by magma evolution. The d 26 Mg values range from À0.20‰ to À0.02‰ in the dunite, À0.43‰ in the clinopyroxenite, À0.43‰ to À0.28‰ in the hornblendite, 0.18‰ in the chromite-bearing hornblendite, and À0.56‰ to À0.16‰ in the gabbro. The Mg isotopic variations in different types of rocks are closely related to fractional crystallization and accumulation of different proportions of oxides vs. silicates. Chromite crystallization and accumulation is the most important factor in controlling Mg isotope fractionation during the formation of the Xiadong intrusion. Compared to basaltic and granitic magmas, differentiation of the Alaskan-type intrusions occurs at a relatively high oxygen fugacity, which favors chromite crystallization and consequently significant Mg isotope fractionations at both mineral and whole-rock scales. Therefore, Mg isotope systematics can be used to trace the degree of magma differentiation and related-mineralization.

The Critical Role of Fluid‐Mediated Diffusion in Anomalous Fe‐Mg‐O Isotope Fractionations in Ultramafic Rocks of Ophiolites

“…These effects on olivine compositions overlap those of Fe-Mg diffusion with chromite (Chen et al, 2015;Wang et al, 2021;Xiao et al, 2016). Magnesium isotopic variations of chromite in chromitite and dunite follow the fractional crystallization trend (Su et al, 2017), but the crustal dunites shift away from the trend (Figures 8d and 8e). The lack of correlations between Fe isotopic composition in chromite and Fo and Fe 2+ /(Fe 2+ +Mg) (Figure 8f) is inconsistent with the effects of fractional crystallization.…”

Platinum-Group Mineral Occurrences and Platinum-Group Elemental Geochemistry of the Xiadong Alaskan-Type Complex in the Southern Central Asian Orogenic Belt

“…Platinum-group element deposits are yet to be found in this area, although an increasing number of Ni-Cu sulfide deposits were identified in Permian mafic-ultramafic intrusions [12][13][14]. The Xiadong mafic-ultramafic complex in the Mid-Tianshan Terrane, the southern part of the Eastern Tianshan orogen, differs from the Ni-Cu sulfide deposit-hosting mafic-ultramafic intrusions in terms of its geochronology, petrology, mineralogy and geochemistry [15][16][17][18][19][20]. It is composed of dunite, hornblende clinopyroxenite, hornblendite and hornblende gabbro.…”

“…It is composed of dunite, hornblende clinopyroxenite, hornblendite and hornblende gabbro. These rocks are characterized by cumulate textures, high Mg contents, low trace-element abundances, flat rare-earth element (REE) and arc-magma-type trace-element patterns, and considerable Mg isotopic variations at both mineral and whole-rock scales [15,[17][18][19][20]. The Xiadong complex was formed following multiple magmatic pulses according to zircon U-Pb ages (479, 477, 379, and 313 Ma) of four hornblende gabbros [17].…”

古太平洋板块的遗迹和残片<bold>——</bold>地球化学和地球物理学示踪

“…Ti/Ti* = Ti N / (Nd N −0.055 × Sm N 0.333 × Gd N 0.722 ). Yu et al, 2019;Jin et al, 2020;Sun et al, 2021;Zeng et al, 2021)。尽管目前学术界对于玄武质岩石中低 δ 26 Mg 值成因的解释还存在一些不同的意见， 如铬铁矿的分离结晶作用 (Su et al, 2017a(Su et al, , 2019)、层状岩体中堆积矿物间的不平衡动力学分馏 (Xiao et al, 2016;Chen et al, 2018a)，但是多数学者认为中国东部洋岛型碱性玄武岩中 Mg 同位素异常特征(图 2)是继 承自地幔源区中低 δ 26 Mg 值的碳酸盐化橄榄岩地幔 Sun et al, 2021)或碳酸盐化榴辉岩所致 (Huang et al, 2015;Zeng et al, 2021)。 除了异常的 Mg 同位素以外，大量研究还表明，≤106 Ma 的中国东部碱性玄武岩也具有异常的 Zn 同位 素组成 Wang et al, 2018;Jin et al, 2020;Wang and Liu, 2021;Xu et al, 2022) (Geng et al, 2019;He et al, 2019;Tao and Fei, 2021;Cai et al, 2022) 现今已经有多个全球或区域尺度的层析成像模型，对下地幔结构提供了良好的约束(e.g., Zhao, 2004;Huang and Zhao, 2006;Fukao et al, 2009;Simmons et al, 2010;Fukao and Obayashi, 2013;Obayahi et al, 2013; French…”