The distribution of sulfur between haplogranitic melts and aqueous fluids
The distribution of sulfur between haplogranitic melts and aqueous fluids
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DOI：
10.1016/j.gca.2009.10.010
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发表时间：
2010-01-15
影响因子：
5
通讯作者：
Keppler, Hans
中科院分区：
文献类型：
--
作者：
Keppler, Hans
硫在单花岗岩熔体和水流体之间的分配被测量为氧逸度(Co-CoO缓冲液到赤铁矿-磁铁矿缓冲液)、压力(0.5-3kbar)和温度(750-850℃)的函数。硫磺总是强烈地分配到流体中。在给定的氧逸度、压力和温度下，硫在熔体和流体之间的分配可以用一个恒定的分配系数来描述。氧逸度是控制硫分配的最重要参数。在2kbar和850℃的Co-CoO缓冲条件下，硫的液/熔体分配系数为468+/-32，而在相同的压力和温度下，氧逸度高于Ni-NiO 0.5-1个对数单位时，硫的液液分配系数下降到47+/-4。进一步增加对赤铁矿-磁铁矿缓冲液的氧逸度对分配系数(D-流体/熔体=49+/-2)几乎没有影响。在氧逸度大于Ni-NiO 0.5-1个对数单位的条件下，系统地研究了D-流体/熔体与温度和压力的关系。在850℃时，压力对分配系数的影响很小(在0.5kbar时，D-流体/熔体=58+/-3；在1kbar时，D-流体/熔体=94+/-9；在2kbar时，D-流体/熔体=47+/-4；在3kbar时，D-流体/熔体=68+/-5)，温度对分配系数的影响也很小。数据表明，在许多爆炸性火山喷发中观察到的“硫过剩”很容易用喷发前岩浆室中存在的一小部分水合流体来解释。硫过剩可以用硫磺的液/熔体分配系数与喷发物中液/熔体的质量比的乘积来计算。在氧化条件下，合理的流体/熔体分配系数为47，10倍的硫过剩相当于喷发物质中17.6wt.%的流体。只有当存在于岩浆室中的一小部分岩浆喷发时，才会出现大量的硫过剩(10倍或更高)。硫的行为在氧化条件下似乎在很大程度上与压力和温度无关，与氯的行为截然不同，氯的流体/熔体分配系数随着压力的增加而强烈增加。因此，火山气体中SO2/HCl比值的变化，如果它们反映了岩浆室中的主要过程，则提供了岩浆中压力变化的指标。特别是，与长英质岩浆共存的水相流体的S/氯比值的显著增加，表明岩浆房和/或岩浆上升到地表的压力降低。(C)2009爱思唯尔有限公司。保留所有权利。
The distribution of sulfur between haplogranitic melt and aqueous fluid has been measured as a function of oxygen fugacity (Co-CoO-buffer to hematite-magnetite buffer), pressure (0.5-3 kbar), and temperature (750-850 degrees C). Sulfur always strongly partitions into the fluid. At a given oxygen fugacity, pressure and temperature, the distribution of sulfur between melt and fluid can be described by one constant partition coefficient over a wide range of sulfur concentrations. Oxygen fugacity is the most important parameter controlling sulfur partitioning. While the fluid/melt partition coefficient of sulfur is 468 +/- 32 under Co-CoO buffer conditions at 2 kbar and 850 degrees C, it decreases to 47 +/- 4 at an oxygen fugacity 0.5-1 log unit above Ni-NiO at the same pressure and temperature. A further increase in oxygen fugacity to the hematite-magnetite buffer has virtually no effect on the partition coefficient (D-fluid/melt = 49 +/- 2). The dependence of D-fluid/melt on temperature and pressure was systematically explored at an oxygen fugacity 0.5-1 log units above Ni-NiO. At 850 degrees C, the effect of pressure on the partition coefficient is small (D-fluid/melt = 58 +/- 3 at 0.5 kbar; 94 +/- 9 at 1 kbar; 47 +/- 4 at 2 kbar and 68 +/- 5 at 3 kbar) and temperature also has only a minor effect on partitioning.The data show the "sulfur excess" observed in many explosive volcanic eruptions can easily be explained by the presence of a small fraction of hydrous fluid in the magma chamber before the eruption. The sulfur excess can be calculated as the product of the fluid/melt partition coefficient of sulfur and the mass ratio of fluid over melt in the erupted material. For a plausible fluid/melt partition coefficient of 47 under oxidizing conditions, a 10-fold sulfur excess corresponds to a 17.6 wt.% of fluid in the erupted material. Large sulfur excesses (10-fold or higher) are only to be expected if only a small fraction of the magma residing in the magma chamber is erupted.The behavior of sulfur, which seems to be largely independent of pressure and temperature under oxidizing conditions is very different from chlorine, where the fluid/melt partition coefficient strongly increases with pressure. Variations in the SO2/HCl ratio of volcanic gases, if they reflect primary processes in the magma chamber, therefore provide an indicator of pressure variations in a magma. In particular, major increases in the S/Cl ratio of an aqueous fluid coexisting with a felsic magma suggest a pressure reduction in the magma chamber and/or magma rising to the surface. (c) 2009 Elsevier Ltd. All rights reserved.