Temperature and co-crystallization effects on Zr isotopes
Temperature and co-crystallization effects on Zr isotopes
复制标题
DOI：
10.1016/j.gca.2023.05.004
复制
发表时间：
2023-05
影响因子：
5
通讯作者：
H. Kirkpatrick;T. Mark Harrison;M. Ibáñez-Mejia;F. Tissot;S. MacLennan;Elizabeth Bell
中科院分区：
文献类型：
--
作者：
H. Kirkpatrick;T. Mark Harrison;M. Ibáñez-Mejia;F. Tissot;S. MacLennan;Elizabeth Bell
通过对La Posta岩体（peninsula Ranges, southern California）的锆石、钛矿、黑云母、角闪洞和整块岩石进行Zr同位素测量，并结合微量元素分析和U-Pb年龄测量，了解火成岩中Zr同位素分馏的控制因素，包括温度、结晶顺序和动力学效应。我们发现在大致相同温度下形成的钛矿和锆石之间存在较大的（>0.6‰）Zr同位素分馏（以δ94/90Zr表示）。利用离子模型和从头算模型计算的平衡分馏因子，我们推断出Zr同位素演化的控制因素包括液相中出现相的相对顺序，其中钛矿分馏导致同位素较轻的熔体，锆石分馏导致同位素较重的熔体。虽然这些Zr分馏模型可以解释锆石中高达~ 1.5‰的δ94/90Zr变化，但结晶顺序、温度和共晶相的存在并不能解释La Posta岩体中锆石晶内Zr同位素分布的所有方面，也不能解释锆石中Zr同位素值的大范围（bb0 ~ 2‰）。如果没有额外的限制，比如对共结晶相的了解和对Zr同位素分馏的真正原因的更好理解，锆石中的Zr同位素仍然是岩浆演化的模糊代表。
We undertook Zr isotope measurements on zircon, titanite, biotite, amphibole, and whole rocks from the La Posta pluton (Peninsular Ranges, southern California) together with trace element analyses and U-Pb age measurements to understand the controls on Zr isotope fractionation in igneous rocks, including temperature, crystallization sequence, and kinetic effects. We find large (>0.6‰) Zr isotope fractionations (expressed as δ94/90Zr) between titanite and zircon forming at approximately the same temperature. Using equilibrium fractionation factors calculated from ionic and ab initio models, we infer the controls on Zr isotope evolution to include the relative order in which phases appear on the liquidus, with titanite fractionation resulting in isotopically lighter melt and zircon fractionation resulting in isotopically heavier melt. While these models of Zr fractionation can explain δ94/90Zr variations in zircon of up to ∼1.5‰, crystallization order, temperature and presence of co-crystallizing phases do not explain all aspects of the intracrystalline Zr isotopic distribution in zircons in the La Posta pluton or the large range of Zr isotopic values among zircons (>2‰). Without additional constraints, such as knowledge of co-crystallizing phases and a better understand of the true causes of Zr isotope fractionation, Zr isotopes in zircon remains an ambiguous proxy of magmatic evolution.