The magnesium isotopic compositions of the crust and mantle: A study on the Oman ophiolite
The magnesium isotopic compositions of the crust and mantle: A study on the Oman ophiolite
复制标题
DOI：
10.1016/j.chemgeo.2022.120969
复制
发表时间：
2022-06
期刊：
影响因子：
3.9
通讯作者：
J. Eom;T. Yoshimura;N. Akizawa;S. Wakaki;T. Ishikawa;E. Takazawa;K. Yamaoka;H. Kawahata
中科院分区：
文献类型：
--
作者：
J. Eom;T. Yoshimura;N. Akizawa;S. Wakaki;T. Ishikawa;E. Takazawa;K. Yamaoka;H. Kawahata
镁同位素组成(δ26 mg)可以对地表流体-岩石相互作用中的镁循环提供有价值的见解。大洋岩石圈中镁的同位素变化意味着不同同位素的镁的并入，但其潜在的机制仍不清楚。本文测定了阿曼蛇绿岩在扩张中心和俯冲带经历了岩浆活动的洋壳(n=32)和地幔橄榄岩(n=13)的镁同位素组成。地壳剖面记录的δ为26 mg值，范围从−0.58到−0.04‰。在地壳上部，含镁矿物丰度越高，δ26 mg值越轻。而下地壳剖面与δ26 mg值无相关性，表明在洋壳上部2公里范围内，海水形成的热液镁汇意义重大。在地幔剖面中，δ为26 mg，范围为−0.39‰~−0.07‰。7个橄榄岩样品中镁的损失达5.3%，其δ26 mg值略高于全球地幔，这可能与低温风化作用有关。相比之下，三个熔融度高于围岩的橄榄岩样品(尖晶石Cr1=Cr2+Al2)被认为受到了与俯冲有关的相互作用的影响，形成了较轻的δ26 mg值。同位素轻的镁源为俯冲沉积，但镁含量低需要水/岩比>；>；=1。因此，板片来源镁的贡献和地幔δ26 mg值的修正是局部性的。
Magnesium isotope compositions (δ26Mg) can provide valuable insights into Mg cycling through surface fluid-rock interactions. Isotopic variations of Mg within the oceanic lithosphere imply incorporation of isotopically different Mg, but the underlying mechanisms remain uncertain. In this study, we measure the Mg isotopic composition of the oceanic crust (n= 32) and mantle peridotite (n= 13) from the Oman ophiolite, which has experienced magmatism at the spreading center and subduction zone. The crustal section records δ26Mg values that range from −0.58 to −0.04 ‰. In the upper crustal section, higher Mg-bearing mineral abundance tends to have lighter δ26Mg values. However, the lower crustal section has no correlation with the δ26Mg values, indicating that the hydrothermal Mg sink formed by seawater was significant within the upper 2 km of the oceanic crust. In the mantle section, the δ26Mg values range from −0.39 ‰ to −0.07 ‰. Seven peridotite samples, with a loss of MgO by up to 5.3%, have slightly higher δ26Mg values than the global mantle, which may be attributed to low-temperature weathering. In contrast, three peridotite samples with higher melting degrees (spinel Cr# = Cr/(Cr + Al) atomic ratio) than the adjacent rocks are considered to have been affected by subduction-related interactions, forming lighter δ26Mg values. The isotopically light Mg source was subducted sediments, but its low concentration of Mg requires a water/rock ratio >> 1. Therefore, we conclude that the contribution of slab-derived Mg and modification of mantle δ26Mg values were localized.