Experimentally derived F, Cl, and Br fluid/melt partitioning of intermediate to silicic melts in shallow magmatic systems
Experimentally derived F, Cl, and Br fluid/melt partitioning of intermediate to silicic melts in shallow magmatic systems
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DOI：
10.2138/am-2022-8109
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发表时间：
2022-10-26
影响因子：
3.1
通讯作者：
Pyle, David M.
中科院分区：
文献类型：
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作者：
Cassidy, Mike;Iveson, Alexander A.;Pyle, David M.
卤素相对于共存熔体有利于出溶流体的条件是理解许多岩浆作用的关键，包括火山脱气、地壳熔体的演化和成矿。我们报道了中间到硅质熔体的新的F、Cl和Br流体/熔体分配系数，对于这些熔体来说，F和Br值数据特别缺乏；对于不同的CO2-H2O含量，我们首次评估了改变流体组成(X-H2O)对BR流体/熔体分配的影响。实验在压力50-120兆帕，温度800-1100℃，挥发性成分[摩尔X-H2O=H2O/(H2O+CO2)]为0.55：1的条件下进行，镍-镍氧缓冲器周围的氧化还原条件(积分(O2)接近于NNO)。实验没有掺入氯、溴或氟，而是在接近其各自喷发前状态的条件下对含天然晶体的火山产物进行的。因此，这些实验为自然发生的卤水不饱和条件下的卤素分配提供了现实的约束。用二次离子质谱仪(SIMS)对印度尼西亚凯卢德火山和智利奇扎普火山的13个实验玻璃产品及其天然起始原料进行了溴、氯和氟的测量。流体成分受质量平衡的制约。对于Kelud起始物质(大块玄武岩安山岩)，平均块状卤素流体/熔体分配系数和标准偏差为：D-Cl(流体/熔体)=3.4(+/-3.7 1S.D.)，D-F(流体/熔体)=1.7(+/-1.7)，D-Br(流体/熔体)=7.1(+/-6.4)，以及D-Cl(流体/熔体)=11.1(+/-3.5)，对于基扎普原料(散装英安岩)，D-F(流体/熔体)=0.8(+/-0.8)，D-Br(流体/熔体)=31.3(+/-20.9)。平均分配系数的大范围是X-H2O、压力和温度变化的产物。与对合成熔体的研究一致，我们的数据显示，随着离子半径的增加，卤素D-流体/熔体的分配行为呈指数增加，分配行为由熔体组成控制，具体取决于熔体中形成的络合物的性质(例如，SiF4、NaCl、KBr.)。不同卤素的基本化学成分(不同的离子大小和电负性)控制着分配对熔体组成和其他变量变化的响应方式。实验结果证实，在较高的体积氯含量、较高的熔体钠含量、较高的流体X-H2O比和较低的温度下，较多的氯分配到流体中。溴表现出类似的行为，尽管它似乎对温度更敏感，对钠含量和X-H2O不那么敏感。相反，随着熔体中二氧化硅含量的减少(从72wt%到56wt%二氧化硅)，氟在流体中的分配增加，我们将其归因于熔体中可用于形成F络合物的较低的硅丰度。这些新数据提供了对控制岩浆中卤素脱气的条件和过程的更多洞察，并可能有助于收集和解释熔体包裹体和火山气体数据。
The conditions under which halogens partition in favor of an exsolved fluid relative to the coexisting melt are key for understanding many magmatic processes, including volcanic degassing, evolution of crustal melt bodies, and ore formation. We report new F, Cl, and Br fluid/melt partition coefficients for intermediate to silicic melts, for which F and Br data are particularly lacking; and for varying CO2- H2O contents to assess the efects of changing fluid composition (X-H2O) on Br fluid/melt partitioning for the first time. The experiments were conducted at pressures 50-120 MPa, temperatures 800-1100 degrees C, and volatile compositions [molar X-H2O = H2O/(H2O +CO2)] of 0.55 to 1, with redox conditions around the Nickel-Nickel Oxygen bufer (integral(O2) approximate to NNO). Experiments were not doped with Cl, Br, or F and were conducted on natural crystal-bearing volcanic products at conditions close to their respective pre-eruptive state. The experiments therefore provide realistic constraints on halogen partitioning at naturally occurring, brine-undersaturated conditions. Measurements of Br, Cl, and F were made by Secondary Ion Mass Spectrometry (SIMS) on 13 experimental glass products spanning andesite to rhyolitic compositions, together with their natural starting materials from Kelud volcano, Indonesia, and Quizapu volcano, Chile. Fluid compositions were constrained by mass balance. Average bulk halogen fluid/melt partition coefficients and standard deviations are: D-Cl (fluid/melt) = 3.4 (+/- 3.7 1 s.d.), D-F(fluid/melt) = 1.7 (+/- 1.7), and D-Br(fluid/melt) = 7.1 (+/- 6.4) for the Kelud starting material (bulk basaltic andesite), and D-Cl (fluid/melt) = 11.1 (+/- 3.5), D-F(fluid/melt) = 0.8 (+/- 0.8), and D-Br (fluid/melt) = 31.3 (+/- 20.9) for Quizapu starting material (bulk dacite). The large range in average partition coeficients is a product of changing X-H2O, pressure and temperature. In agreement with studies on synthetic melts, our data show an exponential increase of halogen D-fluid/melt with increasing ionic radius, with partitioning behavior controlled by melt composition according to the nature of the complexes forming in the melt (e.g., SiF4, NaCl, KBr). The fundamental chemistry of the diferent halogens (differing ionic size and electronegativities) controls the way in which partitioning responds to changes in melt composition and other variables. Experimental results confirm that more Cl partitions into the fluid at higher bulk Cl contents, higher melt Na, higher fluid X-H2O ratios, and lower temperatures. Bromine shows similar behavior, though it seems to be more sensitive to temperature and less sensitive to Na content and X-H2O. In contrast, F partitioning into the fluid increases as the melt silica content decreases (from 72 to 56 wt% SiO2), which we attribute to the lower abundance of Si available to form F complexes in the melt. These new data provide more insights into the conditions and processes that control halogen degassing from magmas and may help to inform the collection and interpretation of melt inclusions and volcano gas data.