Zircon solubility and zirconium complexation in H2O+Na2O+SiO2±Al2O3 fluids at high pressure and temperature
Zircon solubility and zirconium complexation in H2O+Na2O+SiO2±Al2O3 fluids at high pressure and temperature
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DOI：
10.1016/j.epsl.2012.06.054
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发表时间：
2012-10
影响因子：
5.3
通讯作者：
M. Wilke;C. Schmidt;J. Dubrail;Karen Appel;M. Borchert;K. Kvashnina;C. Manning
中科院分区：
文献类型：
--
作者：
M. Wilke;C. Schmidt;J. Dubrail;Karen Appel;M. Borchert;K. Kvashnina;C. Manning
锆石是许多高场强元素（HFSE）的重要寄主矿物，特别是Zr和Hf。因此，它在高温高压地质流体中的溶解度在地壳和地幔过程中这些元素的陆地循环中起着重要作用。采用同步辐射X射线荧光原位分析技术，利用水热金刚石对顶砧装置，对锆石在H_2 O-Na_2Si_3 O_7、H_2 O-Na_2Si_3 O_7 + Al_2O_3、H_2 O-Na_2Si_2 O_5、H_2 O-NaAlSi_3 O_8体系中的溶解度以及斜锆石在H_2 O-NaOH体系中的溶解度进行了原位高温高压测量。用X射线吸收近边结构（XANES）谱研究了流体中锆与锆石的络合作用。向H2O中添加Na 2Si 3 O 7可大大提高锆石的溶解度。随着Na_2Si_3 O_7浓度的增加，增强程度增加。在550 ℃、400 MPa下，含10wt%Na_2Si_3O_7的熔体中Zr含量可达86± 2 ppm。当Na_2Si_3O_7含量为30wt%时，在600 ℃和440 MPa下，Zr的最大含量为997±6ppm。锆石在这些流体中的溶解度随压力显著降低，随温度略有增加。Al 2 O3的加入降低了锆英砂的溶解度。在H2O-NaAlSi 3 O 8流体中，Zr浓度在亚ppm至ppm范围内。在600°C和930 MPa条件下，与斜锆石平衡的NaOH溶液中Zr的浓度可达390± 2 ppm，并随压力和温度的升高而增加。原位XANES光谱收集Zr在H2O-Na 2Si 3 O 7，H2O-Na 2Si 3 O 7 + Al 2 O3，H2O-NaOH，和H2O-HCl流体与锆石平衡提供的证据，这些流体之间的锆络合的强烈差异。XANES光谱与模型化合物的光谱的比较和XANES光谱的从头计算模拟显示[8]Zr用于HCl溶液，[7]Zr用于NaOH溶液，[6]Zr用于Na-Al-硅酸盐溶液。对于后者的解决方案，形成的碱性锆硅酸盐复合物是由强烈的依赖性的锆石溶解度Na/Al和模拟的基础上的局部结构锆周围的碱性锆硅酸盐vlasovite和catapleite的光谱的XANES光谱的相似性。碱金属锆硅酸盐复合物是负责提高锆浓度的钠铝硅酸盐轴承解决方案，并很可能发挥重要作用，动员HFSE在流体-岩石相互作用。由于长石和云母的不一致溶解，在高温高压下的水溶液中可能存在高碱/铝，锆石溶解度沿着NaAlSi 3 O 8-Na 2Si 3 O 7结合面的增加表明下地壳和上地幔中含硅酸盐的水溶液可能对Zr或HFSE进行了大量的迁移。
Zircon is an important host mineral for many high-field strength elements (HFSE), particularly Zr and Hf. Thus, its solubility in geologic fluids at high pressure and temperature plays an important role in terrestrial cycling of these elements during processes in the Earth's crust and mantle. We performed in-situ high-pressure, high-temperature measurements of zircon solubility in H2O–Na2Si3O7, H2O–Na2Si3O7+Al2O3, H2O–Na2Si2O5, H2O–NaAlSi3O8fluids, as well as of baddeleyite solubility in H2O–NaOH fluids, by in-situ synchrotron radiation X-ray fluorescence analysis using hydrothermal diamond-anvil cells. Zirconium complexation in fluids in equilibrium with zircon was constrained by in-situ X-ray absorption near-edge structure (XANES) spectroscopy. Zircon solubility is strongly enhanced by addition of Na2Si3O7to H2O. The degree of enhancement increases with Na2Si3O7concentration. The Zr content of fluids containing 10wt% Na2Si3O7reached up to 86±2ppm Zr at 550°C and 400MPa. At 30wt% Na2Si3O7, the maximum Zr concentration was 997±6ppm at 600°C and 440MPa. Zircon solubility in these fluids decreases considerably with pressure and increases slightly with temperature. Addition of Al2O3decreases the zircon solubility. In H2O–NaAlSi3O8fluids, the Zr concentrations are in the sub-ppm to ppm range. Zr concentrations in NaOH solutions in equilibrium with baddeleyite reached up to 390±2ppm at 600°C and 930MPa and increase with pressure and temperature. In-situ XANES spectra collected on Zr in H2O–Na2Si3O7, H2O–Na2Si3O7+Al2O3, H2O–NaOH, and H2O–HCl fluids in equilibrium with zircon provide evidence for strong differences in the Zr complexation between these fluids. Comparison of XANES spectra to those of model compounds and ab-initio simulation of XANES spectra revealed[8]Zr for the HCl solution,[7]Zr for the NaOH solution, and[6]Zr for the Na–Al-silicate-bearing solutions. For the latter solutions, formation of alkali zircono-silicate complexes is indicated by the strong dependence of zircon solubility on Na/Al and the similarity of the XANES spectra to spectra simulated based on the local structure around Zr in the alkali zircono-silicates vlasovite and catapleite. Alkali zircono-silicate complexes are responsible for the enhancement of Zr concentrations in Na–Al-silicate-bearing solutions and very likely play an important role for mobilization of HFSE during fluid–rock interaction. Because high alkali/Al can be expected in aqueous fluids at high pressure and temperature due to incongruent dissolution of feldspar and mica, the increase of zircon solubility along the NaAlSi3O8–Na2Si3O7join points to potentially considerable Zr or HFSE transport by silicate-bearing aqueous fluids in the lower crust and upper mantle.