Oxygen isotope trajectories of crystallizing melts: Insights from modeling and the plutonic record
Oxygen isotope trajectories of crystallizing melts: Insights from modeling and the plutonic record
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DOI：
10.1016/j.gca.2017.03.027
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发表时间：
2017-06
影响因子：
5
通讯作者：
C. Bucholz;O. Jagoutz;J. VanTongeren;J. Setera;Zhengrong Wang
中科院分区：
文献类型：
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作者：
C. Bucholz;O. Jagoutz;J. VanTongeren;J. Setera;Zhengrong Wang
火成岩中氧同位素值的升高通常被用来识别地壳上的蚀变或曾经居住在地球表面附近的物质的同化。然而，熔体的δ 18 O值也可以通过闭合系统分离结晶而增加。为了量化结晶引起的熔体δ 18 O变化，我们开发了一个详细的封闭系统分步结晶质量平衡模型，并将其应用于六个实验和自然确定的液体下降线（LLD），这些下降线覆盖了几乎完整的结晶区间（熔体分数为1至<0.1）。所研究的LLDs从无水拉斑玄武岩到含水高钾和钙碱性玄武岩，其特征在于不同的熔融温度-SiO2轨迹，以及结晶相关系。模型结果表明，结晶熔体的熔体分数-温度-SiO2关系是岩浆水含量的强函数，它将控制结晶熔体的δ 18 O路径。俯冲带典型的含水熔体在结晶早期经历了较大的δ 18 O增加，这是由于它们的岩浆温度较低，SiO2含量初始增加较大，以及低δ 18 O相的高温稳定性，如氧化物、角闪石和钙长石斜长石（相对于钠长石）。相反，相对干燥的拉斑玄武岩熔体只有在结晶度大于80%时，δ 18 O才会显著增加。熔体δ 18 O的总计算增加1.0-1.5‰可归因于结晶从0.50到70 wt.% SiO2的模拟封闭系统结晶熔体组合物。作为一个应用实例，我们比较了我们的封闭系统模型的结果，从两个自然的深成序列，一个相对干燥的拉斑玄武岩序列的上部和上部主带（UUMZ）的布什维尔德复杂（南非）和高钾，从弧相关的Dariv火成岩复杂（蒙古）的含水序列的氧同位素矿物数据。选择这两个序列是因为它们的主要和微量元素组合物似乎主要由封闭系统分步结晶控制，并且它们的LLD已被详细建模。利用实测矿物δ 18 O值和计算的矿物-熔体分馏因子计算了平衡熔体δ 18 O值。Dariv火成杂岩和Bushveld杂岩的UUMZ平衡熔体分别增加了2-3‰和1-1.5‰。闭合系统分离结晶模型的结果再现了在Bushveld UUUMZ的平衡熔体δ 18 O中观察到的1‰的增加，而对于Dariv火成杂岩，高δ 18 O物质的同化是必要的，以解释熔体δ 18 O值的增加。同化的演变表壳物质也证实了Sr和Nd同位素分析的单斜辉石从序列。从5.7‰（“原始”地幔）到107.0 ‰（受俯冲影响严重的地幔）的幔源玄武岩δ 18 O值范围开始，我们的模型结果表明，δ 18 O高达8.5‰的高硅熔体（即花岗岩）可以单独通过分离结晶产生。最后，我们模拟了不同LLD的锆石熔体δ 18 O分馏，强调了它们对给定结晶熔体的特定SiO2-T关系的依赖性。潮湿、相对较冷的花岗岩熔体将具有较大的锆石熔体分馏，与炎热、干燥的花岗岩相比，可能高达1.5‰。因此，当使用锆石δ 18 O值计算熔体δ 18 O时，限制特定于感兴趣体系的锆石熔体分馏是至关重要的。
Elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth. The δ18O value of a melt, however, can also increase through closed-system fractional crystallization. In order to quantify the change in melt δ18O due to crystallization, we develop a detailed closed-system fractional crystallization mass balance model and apply it to six experimentally- and naturally-determined liquid lines of descent (LLDs), which cover nearly complete crystallization intervals (melt fractions of 1 to <0.1). The studied LLDs vary from anhydrous tholeiitic basalts to hydrous high-K and calc-alkaline basalts and are characterized by distinct melt temperature-SiO2trajectories, as well as, crystallizing phase relationships. Our model results demonstrate that melt fraction-temperature-SiO2relationships of crystallizing melts, which are strongly a function of magmatic water content, will control the specific δ18O path of a crystallizing melt. Hydrous melts, typical of subduction zones, undergo larger increases in δ18O during early stages of crystallization due to their lower magmatic temperatures, greater initial increases in SiO2content, and high temperature stability of low δ18O phases, such as oxides, amphibole, and anorthitic plagioclase (versus albite). Conversely, relatively dry, tholeiitic melts only experience significant increases in δ18O at degrees of crystallization greater than 80%. Total calculated increases in melt δ18O of 1.0–1.5‰ can be attributed to crystallization from ∼50 to 70 wt.% SiO2for modeled closed-system crystallizing melt compositions. As an example application, we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences, a relatively dry, tholeiitic sequence from the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex (South Africa) and a high-K, hydrous sequence from the arc-related Dariv Igneous Complex (Mongolia). These two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed-system fractional crystallization and their LLDs have been modeled in detail. We calculated equilibrium melt δ18O values using the measured mineral δ18O values and calculated mineral-melt fractionation factors. Increases of 2–3‰ and 1–1.5‰ in the equilibrium melts are observed for the Dariv Igneous Complex and the UUMZ of the Bushveld Complex, respectively. Closed-system fractional crystallization model results reproduce the 1‰ increase observed in the equilibrium melt δ18O for the Bushveld UUMZ, whereas for the Dariv Igneous Complex assimilation of high δ18O material is necessary to account for the increase in melt δ18O values. Assimilation of evolved supracrustal material is also confirmed with Sr and Nd isotope analyses of clinopyroxene from the sequence. Beginning with a range of mantle-derived basalt δ18O values of 5.7‰ (“pristine” mantle) to ∼7.0‰ (heavily subduction-influenced mantle), our model results demonstrated that high-silica melts (i.e. granites) with δ18O of up to 8.5‰ can be produced through fractional crystallization alone. Lastly, we model the zircon-melt δ18O fractionations of different LLDs, emphasizing their dependence on the specific SiO2-Trelationships of a given crystallizing melt. Wet, relatively cool granitic melts will have larger zircon-melt fractionations, potentially by ∼1.5‰, compared to hot, dry granites. Therefore, it is critical to constrain zircon-melt fractionations specific to a system of interest when using zircon δ18O values to calculate melt δ18O.