Abstract The Mg/Ca ratio of seawater fluctuated remarkably over the Phanerozoic. The processes that controlled the temporal variation in seawater Mg/Ca ratio may also have caused the secular changes in Mg isotope ratios of seawater. Therefore, the Mg isotope composition of seawater may be useful to understand the long-term variation in seawater chemistry. Different sedimentary carbonates have been proposed as archives of ancient seawater Mg isotope signatures, but each type of carbonate has its limitations. Halite is a common mineral in evaporite deposits and their fluid inclusions may be used to infer the Mg isotope composition of contemporary seawater. In this study, we developed a method to measure the isotope composition of Mg in halite with an accuracy of better than ±0.1‰ in δ26Mg. The accuracy of the method was verified systematically, using synthetic NaCl solutions, as well as Holocene lacustrine halite samples from the Dead Sea. The δ26Mg values of marine halite samples show a large variation, ranging from −0.1‰ to −1.6‰ relative to the DSM3 international Mg isotope standard. The δ26Mg values of some ancient salt samples are lower than a published model curve for δ26Mg of seawater, and such inconsistency could be explained by 1) isotopic evolution of brines due to mineral precipitation, 2) entrapment of evaporite minerals into halite during halite precipitation or post-depositional deformation, 3) recrystallization and dissolution of pre-existing Mg minerals, and 4) limitations of the model curve of seawater δ26Mg values. Collectively, we suggest that Mg isotopes in halite are a new geochemical proxy to study the secular variations in seawater chemistry, syndepositional and post-depositional processes of evaporite deposits. Our data highlight the importance of understanding the sedimentary background and textural details of evaporites for appropriate interpretations of Mg isotope data from halite.

中文翻译：

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从海洋岩盐中提取古代海水的镁同位素特征：一项侦察

摘要 显生宙期间海水的Mg/Ca比值波动较大。控制海水镁/钙比随时间变化的过程也可能导致海水镁同位素比的长期变化。因此，海水的镁同位素组成可能有助于了解海水化学的长期变化。已提出不同的沉积碳酸盐作为古代海水镁同位素特征的档案，但每种类型的碳酸盐都有其局限性。岩盐是蒸发岩矿床中的常见矿物，它们的流体包裹体可用于推断当代海水的镁同位素组成。在本研究中，我们开发了一种测量岩盐中 Mg 同位素组成的方法，其精度优于 δ26Mg 中的 ±0.1‰。系统验证了该方法的准确性，使用合成 NaCl 溶液，以及来自死海的全新世湖相岩盐样品。海洋岩盐样品的δ26Mg值与DSM3国际镁同位素标准相比变化很大，范围从-0.1‰到-1.6‰。一些古盐样品的 δ26Mg 值低于已发表的海水 δ26Mg 模型曲线，这种不一致的原因可以是 1) 矿物沉淀引起的卤水同位素演化，2) 岩盐沉淀过程中蒸发矿物被包埋到岩盐中或沉积后变形，3) 已有镁矿物的再结晶和溶解，以及 4) 海水 δ26Mg 值模型曲线的局限性。总的来说，我们认为岩盐中的镁同位素是研究海水化学长期变化的新地球化学代表，蒸发岩矿床的同沉积和后沉积过程。我们的数据强调了了解蒸发岩的沉积背景和结构细节对于正确解释岩盐镁同位素数据的重要性。