An experimental study of element partitioning among biotite, muscovite, and coexisting peraluminous silicic melt at 200 MPa (H2O)
An experimental study of element partitioning among biotite, muscovite, and coexisting peraluminous silicic melt at 200 MPa (H2O)
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DOI：
10.2138/am-1995-11-1214
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发表时间：
1995-12
影响因子：
3.1
通讯作者：
Jonathan Icenhower;David London
中科院分区：
文献类型：
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作者：
Jonathan Icenhower;David London
人工变云母(白云母-石英-钠长石和白云母-石英-钠长石-黑云母+硅酸铝±堇青石)在200 Mpa(H2O)下600-750℃的饱和实验表明，沿反应的介稳延伸，白云母+石英+钠长石+H2O=熔体+硅酸铝在625℃开始部分熔融。黑云母在整个温度区间内是稳定的，尽管它在温度范围的高端逐渐与铁铝尖晶石+熔体发生反应。在700°C至725°C之间的初始熔融过程中，白云母分解为刚玉+或正长石。650°C时存在少量刚玉和硅酸铝，而700°C及以上温度下存在含有大量蓝宝石(Fe+Ti)的刚玉和尖晶石。750°C时，刚玉和尖晶石与富含正长石的长石和剩余黑云母共存。熔体的标准成分最小约为30或15 Ms20Qtz35(重量百分比)，不含其他次要成分(如冰晶石)。白云母及其对应的斜长石+刚玉组合都为熔体贡献了大量过剩的铝，使熔体的铝饱和指数(ASI)达到1.4。由于黑云母和白云母在温度区间内的连续反应和重新平衡，Li、Rb、Cs和F在熔体中强烈富集。铁组分(TiO2+FeO+MgO+MnO)的浓度很低(<；1wt%)，但随着温度的升高，主要是由于FeO在熔体中的溶解度增加所致。计算的M元素在黑云母(Bt)和玻璃(Gl)之间的分配系数D(M)Bt/gl，D(Li)Bt/gl(1.7-1)，D(Ba)Bt/gl(~14-6)和D(F)Bt/gl(2.5-1.5)均随温度升高而减小，而对Sr(≈0.0 4)、Rb(≈2.0)和Cs(≈0.4)的分配系数随温度变化不大。在6 5 0℃、D(M)ms/g≈0.8(Li)、3-6(Ba)、1.6(Rb)、0.0 5(Sr)、0.3(Cs)和1.8(F)条件下，还测定了白云母的分配系数。这些结果与长石的其他数据一起表明，在铝质变质岩的深成作用和随后的熔体结晶过程中，Rb、Cs和Ba彼此之间发生了强烈的分馏。最后，F在云母(黑云母或白云母)和熔体之间的低分配系数表明，铝变质岩的初始含水深熔作用可以产生富F熔体；可能不需要启动脱水原岩重熔来产生这种富F熔体的模型。
Abstract H2O-saturated experiments with synthetic metapelite compositions (muscovite-quartz- albite and muscovite-quartz-albite-biotite ± aluminum silicate ± cordierite) performed over the temperature interval of 600-750 °C at 200 MPa (H2O) reveal that partial fusion commences at 625 °C along the metastable extension of the reaction muscovite + quartz + albite + H2O = melt + aluminum silicate. Biotite is stable over the entire temperature interval, although it reacts progressively to hercynite + melt at the high end of the temperature range. Muscovite that survives initial melting breaks down to corundum + or- thoclase between 700 and 725 °C. Minor corundum and aluminum silicate are present at 650 °C, whereas corundum with a large sapphire (Fe + Ti) component is present at and above 700 °C. Finally, corundum and hercynite exist with orthoclase-rich feldspar and remaining biotite at 750 °C. The normative composition of melt at the minimum is approximately Ab30Or15Ms20Qtz35 (in weight percent) without other minor components (e.g., cryolite). Both muscovite and its equivalent orthoclase + corundum assemblage contribute substantial excess Al to melt, bringing the value of the Al saturation index (ASI) of melt to 1.4. Li, Rb, Cs, and F are strongly enriched in melts because of the continuous reaction and reequilibration of biotite and muscovite over the temperature interval. Concentrations of the femic components (TiO2 + FeO + MgO + MnO) are low (< 1 wt%) but rise with temperature primarily because of increasing solubilities of FeO in melt. Calculated partition coefficients, D(M)Bt/gl, between biotite (Bt) and glass (gl) for the element M show that D(Li)Bt/gl (1.7-1.0), D(Ba)Bl/gl (~ 14-6), and D(F)Bt/gl (2.5-1.5) all decrease with increasing temperature, whereas partition coefficients for Sr (≈0.04), Rb (≈2.0), and Cs (≈0.4) remain constant with temperature over a large range of concentrations. Partition coefficients for muscovite (Ms) were also determined at 650 °C; D(M)Ms/gl ≈ 0.8 (Li), 3-6 (Ba), 1.6 (Rb), 0.05 (Sr), 0.3 (Cs), and 1.8 (F). These results, together with other data for feldspar, suggest that Rb, Cs, and Ba become strongly fractionated from one another during anatexis of aluminous metasediments and the ensuing crystallization of melts. Finally, the low partition coefficients for F between micas (biotite or muscovite) and melt indicate that F-rich melts can be generated by the incipient hydrous anatexis of aluminous metasediments; models that invoke remelting of a dehydrated protolith to generate such F-rich melts may not be necessary.