Extreme Zr stable isotope fractionation during magmatic fractional crystallization
Extreme Zr stable isotope fractionation during magmatic fractional crystallization
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岩浆分馏结晶过程中Zr稳定同位素分馏
DOI：
10.1126/sciadv.aax8648
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发表时间：
2019-12
期刊：
影响因子：
13.6
通讯作者：
M. Ibáñez-Mejia;F. Tissot
中科院分区：
文献类型：
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作者：
M. Ibáñez-Mejia;F. Tissot
Zr 稳定同位素在岩浆条件下表现出 >5‰ 的分馏，为熔体化学演化提供了新的见解。锆是一种常用的硅酸盐分异元素示踪剂，但其稳定同位素系统学仍然知之甚少。富含Zr4+的副相，如锆石和斜锆石，可能会保存岩浆环境中Zr同位素行为的独特记录，既作为同位素分馏的潜在驱动因素，又作为熔体成分演化的记录器。为了测试这种潜力，我们测量了来自特征明确的辉长岩火成岩堆积物的 70 种单锆石和斜锆石晶体的稳定 Zr 同位素组成。我们表明，(i) 封闭系统岩浆结晶可以将 Zr 稳定同位素分馏到 >0.5% 的水平，(ii) 锆石和斜锆石相对于它们结晶的熔体来说同位素重，从而推动化学分化的液体向同位素轻的成分发展。由于这些效应与基于矿物熔体键合环境差异的一阶预期相反，锆石结晶过程中的 Zr 稳定同位素分馏可能不仅仅是封闭系统热力学平衡的结果。
Zr stable isotopes exhibit >5‰ fractionations at magmatic conditions, yielding novel insights into melt chemical evolution. Zirconium is a commonly used elemental tracer of silicate differentiation, yet its stable isotope systematics remain poorly known. Accessory phases rich in Zr4+ such as zircon and baddeleyite may preserve a unique record of Zr isotope behavior in magmatic environments, acting both as potential drivers of isotopic fractionation and recorders of melt compositional evolution. To test this potential, we measured the stable Zr isotope composition of 70 single zircon and baddeleyite crystals from a well-characterized gabbroic igneous cumulate. We show that (i) closed-system magmatic crystallization can fractionate Zr stable isotopes at the >0.5% level, and (ii) zircon and baddeleyite are isotopically heavy relative to the melt from which they crystallize, thus driving chemically differentiated liquids toward isotopically light compositions. Because these effects are contrary to first-order expectations based on mineral-melt bonding environment differences, Zr stable isotope fractionation during zircon crystallization may not solely be a result of closed-system thermodynamic equilibrium.