Formation of extremely F-rich hydrous melt fractions and hydrothermal fluids during differentiation of highly evolved tin-granite magmas: a melt/fluid-inclusion study
Formation of extremely F-rich hydrous melt fractions and hydrothermal fluids during differentiation of highly evolved tin-granite magmas: a melt/fluid-inclusion study
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DOI：
10.1007/s00410-004-0624-9
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发表时间：
2005
影响因子：
3.5
通讯作者：
R. Thomas;H. Förster;K. Rickers;J. Webster
中科院分区：
文献类型：
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作者：
R. Thomas;H. Förster;K. Rickers;J. Webster
德国埃尔兹格伯里金瓦尔德黄玉-锌铀矿-钠长花岗岩中的石英晶体除了含有原生和次生流体包裹体(FIS)外，还含有大量的结晶硅酸盐-熔融包裹体(MIS)，其直径可达200tμm，代表了一个高度演化的熔体系统的不同演化阶段，最终形成了与花岗岩有关的锡钨矿化。将特殊的实验技术与共聚焦激光拉曼微探针谱和EMPA相结合，可以精确地测量再均质的MIS中升高的H2O、F和B的含量。在岩浆分异过程中，H2O和F的含量分别从3%增加到30%和1.9%增加到6.4%。然而，还有第二个MI组，非常富含H2O，其值高达55wt%H2O，F浓度约为3wt%。挥发分H2O、F、B和Cl以及Cs和Rb的持续富集可以用分离结晶引发的岩浆分异来解释，因此认为这反映了元素在天然岩浆中的丰度，而不是边界层熔体。熔体和共生流体之间的分配进一步改变了某些元素的岩浆浓度，特别是锡。两种类型的稀土元素与原生元素共存，指示了岩浆结晶早期的流体饱和，与锡、F和B的持续隔离有关。本研究结果通过降低熔体的粘度和增加熔体中的扩散系数，以及通过在熔体和共存流体中形成各种稳定的氟化物络合物，在岩浆分异过程中，通过降低熔体的粘度和增加扩散系数，提供了H2O、F和B在岩浆分异过程中富集锡的特殊重要性的额外证据。
Quartz crystals from topaz–zinnwaldite–albite granites from Zinnwald (Erzgebirge, Germany) contain, in addition to primary and secondary fluid inclusions (FIs), abundant crystalline silicate-melt inclusions (MIs) with diameters up to 200 μm. These MIs represent various stages of evolution of a highly evolved melt system that finally gave rise to granite-related Sn–W mineralization. The combination of special experimental techniques with confocal laser Raman-microprobe spectroscopy and EMPA permits precise measurement of elevated contents of H2O, F, and B in re-homogenized MIs. The contents of H2O and F were observed to increase from 3 to 30 and 1.9 to 6.4 wt%, respectively, during magma differentiation. However, there is a second MI group, very rich in H2O, with values up to 55 wt% H2O and an F concentration of approximately 3 wt%. Ongoing enrichment of volatiles H2O, F, B, and Cl and of Cs and Rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus, is suggested to reflect elemental abundances in natural magmas, and not boundary-layer melts. Partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements, particularly Sn. The coexistence of two types of MIs with primary FIs indicates fluid saturation early in the history of magma crystallization, connected with a continuous sequestration of Sn, F, and B. The results of this study provide additional evidence for the extraordinary importance of the interplay of H2O, F, and B in the enrichment of Sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid.