P–T–X controls on Ca and Na distribution between Mg–Al tourmaline and fluid
P–T–X controls on Ca and Na distribution between Mg–Al tourmaline and fluid
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DOI：
10.1007/s00410-016-1246-8
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发表时间：
2016-03
影响因子：
3.5
通讯作者：
Eleanor J. Berryman;B. Wunder;D. Rhede;G. Schettler;G. Franz;W. Heinrich
中科院分区：
文献类型：
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作者：
Eleanor J. Berryman;B. Wunder;D. Rhede;G. Schettler;G. Franz;W. Heinrich
在0.2 ~ 4.0 GPa、500 ~ 700℃范围内，研究了CaO-Na2O-B2O3-Al2O3-MgO-SiO2-H2O-Cl体系中电气石与共存流体的Ca-Na分配。合成实验产生了电气石、钴矿/石英的矿物组合，在某些情况下还有额外的相，通常占固体产物的比例小于1wt %。所合成的电气石为磁石[NaMg3Al6Si6O18(BO3)3(OH)3(OH)]、“氧-uvite”（即“Ca-Mg-O根名”）[CaMg3Al6Si6O18(BO3)3(OH) 30o]和镁- foiite [5.33 (Mg2Al)Al6Si6O18(BO3)3(OH)3(OH)]的固溶体。起始材料由离子强度恒定（2.00m）的流体和Mg/Al比恒定的氧化物混合物组成。因此，在相同温度下合成的电气石晶体的xsite空位数和Mg/Al比变化很小。主要的固溶溶液是通过交换向量xcawo [XNaW(OH)]−1在xsite进行Ca- na交换，交换向量x (Ca 5.2.2)[XNa2]−1作为次级Ca结合机制。碧玺的x位组成反映了流体的组成，流体中的Ca（或Na）浓度对应于各压力和温度下碧玺中的Ca（或Na）含量。在700°C， 0.2 GPa的温度下，Ca优先分解成电气石，产生了最富Ca的电气石晶体。在压力>1.0 GPa时，Ca优先进入流体，形成以na为主的电气石成分。温度对钙钠分配有次要影响，温度越高，电气石中钙掺入量越大。根据实验结果，预计电气石在低压高温富钙岩石中形成时具有富钙成分，与目前电气石的赋存记录一致。体积Mg/Al比和电气石形成体系的pH值也可能影响Ca在电气石中的掺入，但仍有待实验研究。
Ca–Na partitioning between tourmaline and a coexisting fluid is investigated in the system CaO–Na2O–B2O3–Al2O3–MgO–SiO2–H2O–Cl between 0.2–4.0 GPa and 500–700 °C. The synthesis experiments produced a mineral assemblage of tourmaline, coesite/quartz, and in some cases additional phases, typically comprising <1 wt% of the solid product. The synthesized tourmalines are solid solutions of dravite [NaMg3Al6Si6O18(BO3)3(OH)3(OH)], “oxy-uvite” (i.e. “Ca–Mg–O root name”) [CaMg3Al6Si6O18(BO3)3(OH)3O], and magnesio-foitite [☐(Mg2Al)Al6Si6O18(BO3)3(OH)3(OH)]. Starting materials comprised a fluid of constant ionic strength (2.00m) and an oxide mixture with a constant Mg/Al ratio. As a result, the number of vacancies at theXsite and the Mg/Al ratio of tourmaline crystals synthesized at the same temperature vary only slightly. The major solid solution is Ca–Na exchange at theXsite via the exchange vectorXCaWO[XNaW(OH)]−1, with the exchange vectorX(Ca☐)[XNa2]−1serving as a secondary Ca-incorporation mechanism. Tourmaline’sX-site composition reflects the fluid composition, whereby the Ca (or Na) concentration in the fluid corresponds with the Ca (or Na) content in tourmaline at each pressure and temperature. At 0.2 GPa, 700 °C, Ca preferentially partitions into tourmaline, producing the most Ca-rich tourmaline crystals synthesized here. At pressures >1.0 GPa, Ca partitions preferentially into the fluid, resulting in Na-dominant tourmaline compositions. Temperature has a secondary effect on Ca–Na partitioning, with higher temperatures correlating with increased Ca incorporation in tourmaline. Based on the experimental findings, tourmaline is expected to have Ca-rich compositions when it forms in low pressure, high-temperature Ca-rich rocks, consistent with the current record of tourmaline occurrence. The bulk Mg/Al ratio and the pH of the tourmaline-forming system may also affect Ca incorporation in tourmaline, but remain to be investigated experimentally.