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Zinc isotope fractionation between Cr-spinel and olivine and its implications for chromite crystallization during magma differentiation
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2021-08-13 , DOI: 10.1016/j.gca.2021.08.005
Chun Yang 1, 2 , Sheng-Ao Liu 1 , Long Zhang 3, 4 , Ze-Zhou Wang 1, 5 , Ping-Ping Liu 6 , Shu-Guang Li 1
Affiliation  

We report the first zinc isotope data (expressed as δ66Zn relative to the JMC-Lyon standard) for chromian spinels (Cr-spinels) and coexisting olivines in oceanic peridotites. All spinel-olivine pairs fall on the 1:1 fractionation line in the diagram of δ66Znspinel versus δ66Znolivine, suggesting equilibrium isotope fractionation. Cr-spinels are always isotopically lighter than coexisting olivines (Δ66Znspinel-olivine = δ66Znspinel − δ66Znolivine = -0.50‰ to -0.33‰; n = 13), which is opposite to the positive Zn isotope fractionation between Al-spinel and olivine observed in cratonic peridotites. The “inverse” Zn isotope fractionation between Cr-spinel and olivine is unlikely to have been caused by low-temperature alteration of the oceanic peridotites, given the lack of correlation between Δ66Znspinel-olivine values and chemical indices of serpentinization and weathering (e.g., LOIs, MgO/SiO2). Ionic model calculation indicates that even considering that Zn occurs at both tetrahedral and octahedral sites in the crystal lattice of spinel, the magnitude of equilibrium fractionation is still inconsistent with the observed Zn isotope fractionation between Cr-spinel and olivine. We suggest a “chemical effect” in which the Zn-O bond length (tetrahedral site) in spinel increases when Cr substitutes Al in octahedral site, which is corroborated by the striking negative correlation of δ66Zn with Cr# [molar Cr3+/(Cr3++Al)] in natural spinels. Therefore, Zn isotopic compositions of natural spinels can be highly variable depending on their chemical compositions.

During magma differentiation, zinc is moderately incompatible in silicate minerals (olivine and pyroxene) but highly compatible in Cr-spinels/chromites that have Zn contents tens of times higher than those of basaltic melts. Given its light Zn isotopic composition, chromite crystallization–if any–can evidently elevate δ66Zn and lower Zn contents of the residual melts. Lunar mare basalts are typically characterized by higher δ66Zn and lower Zn contents relative to terrestrial basalts, but modelling suggests that chromite crystallization during magmatic differentiation is unlikely to account for Zn isotopic and elemental data of lunar mare basalts. Global ocean island basalts (OIBs) and some intraplate alkali basalts have systematically higher δ66Zn and higher Zn contents than those of the normal mantle-derived melts (e.g., mid-ocean ridge basalts; MORBs), which contradicts with a chromite crystallization model. Instead, such signatures can reflect recycling of surface carbonates into the Earth’s deep mantle, reinforcing the application of zinc isotopes as a tracer of deep carbonate recycling.



中文翻译:

Cr-尖晶石和橄榄石之间的锌同位素分馏及其对岩浆分异过程中铬铁矿结晶的影响

我们报告了大洋橄榄岩中铬尖晶石(Cr-尖晶石)和共存橄榄石的第一个锌同位素数据(相对于 JMC-Lyon 标准表示为 δ 66 Zn)。所有尖晶石-橄榄石对都落在 δ 66 Zn尖晶石与 δ 66 Zn橄榄石图中的 1:1 分馏线上,表明平衡同位素分馏。Cr-尖晶石总是比共存的橄榄石在同位素上轻(Δ 66 Zn尖晶石-橄榄石= δ 66 Zn尖晶石- δ 66 Zn橄榄石= -0.50‰至-0.33‰;n = 13),这与在克拉通橄榄岩中观察到的铝尖晶石和橄榄石之间的正锌同位素分馏相反。鉴于Δ 66 Zn尖晶石-橄榄石值与蛇纹石化和风化化学指标之间缺乏相关性,Cr-尖晶石和橄榄石之间的“反向”Zn同位素分馏不太可能是由海洋橄榄岩的低温蚀变引起的。例如,LOI、MgO/SiO 2)。离子模型计算表明,即使考虑到 Zn 在尖晶石晶格中的四面体和八面体位点同时存在,平衡分馏的大小仍然与观察到的 Cr 尖晶石和橄榄石之间的 Zn 同位素分馏不一致。我们提出了一种“化学效应”,即当 Cr 取代八面体位置的 Al 时尖晶石中的 Zn-O 键长(四面体位置)增加,δ 66 Zn 与 Cr# [摩尔 Cr 3+ /(Cr 3+ +Al)] 在天然尖晶石中。因此,天然尖晶石的 Zn 同位素组成可能会因化学成分的不同而变化很大。

在岩浆分异过程中,锌与硅酸盐矿物(橄榄石和辉石)中度不相容,但在锌含量比玄武岩熔体高数十倍的铬尖晶石/铬铁矿中高度相容。鉴于其轻的 Zn 同位素组成,铬铁矿结晶(如果有的话)可以明显提高 δ 66 Zn 并降低残余熔体的 Zn 含量。月海玄武岩的典型特征是与陆地玄武岩相比,δ 66 Zn 含量较高,Zn 含量较低,但建模表明,岩浆分异过程中的铬铁矿结晶不太可能解释月海玄武岩的 Zn 同位素和元素数据。全球洋岛玄武岩(OIBs)和一些板内碱性玄武岩系统地具有较高的δ 66Zn 和比正常地幔衍生熔体(例如大洋中脊玄武岩;MORB)更高的 Zn 含量,这与铬铁矿结晶模型相矛盾。相反,这种特征可以反映表面碳酸盐循环进入地球深部地幔,加强了锌同位素作为深部碳酸盐循环示踪剂的应用。

更新日期:2021-08-15
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