温度、浓度对流体中氟钛络合物水解的影响：对深部地质过程中钛元素活动的制约

The effect of temperature and concentration on hydrolysis of fluorinerich titanium complexes in hydrothermal fluids:Constraints on titanium mobility in deep geological processes

Ti是自然界中丰度最高的高场强元素（HFSE）,其主要赋存矿物——金红石的溶解度决定了俯冲带HFSE的活动与循环。而富氟流体被认为是影响Ti等HFSE能否活化、迁移的重要因素。本文对不同浓度的氟钛络合物（K2Ti F6）在100MPa压力和200～500℃温度下的稳定性进行了研究,结果显示其在热液条件下发生显著水解,水解程度与温度和初始浓度密切相关,即温度越高、初始浓度越低,则水解程度越剧烈。对实验数据进行拟合,首次获得了K2Ti F6的表观水解常数与温度的关系式：-ln K=（8972±788）/T-（4.16428±1.40362）,其中获得的热力学参数为：ΔrHΘ=74.59±6.55k J/mol,ΔrSΘ=34.62±11.67J/（mol·K）。同时,运用上述获得的关系式将温度推广到俯冲带条件,计算了金红石溶解度和流体中Ti的最大迁移量之间的关系。结果显示,当金红石的溶解度大于1000μg/m L时,富F流体能有效迁移的Ti大于1μg/m L;随着金红石溶解度的增加,Ti的有效迁移百分比也逐渐增大,但俯冲流体中Ti的最大迁移量可能不超过6700μg/m L,比之前的预计要低得多。本文的研究证实了富F流体能最大程度活化并迁移Ti等HFSE。其中,对于岩浆-热液体系而言,F主要通过在岩浆中预富集,然后再大量分配进入晚期流体中而形成富F流体;对于俯冲带而言,多硅白云母的脱水或热解是形成富F流体/超临界流体的有效途径。

Titanium is the most abundant high-field-strength element（ HFSE） in the Earth. Solubility of rutile,a major host mineral for Ti,governs the activity and cycling of HFSE in subduction zone while F is proposed to play an important role during that.In this study,the stability of F-rich titanium complexes,K2 Ti F6,was investigated at pressure of 100 MPa and temperatures from 200℃to 500℃. The results show that F-Ti complexes hydrolyze visibly on hydrothermal conditions,dependent on temperature and initial concentration. The degree of F-Ti complex hydrolysis is enhanced by elevated temperature and lower initial concentration. On basis of the experimental data,the function between temperatures and the apparent hydrolysis constants of K2 Ti F6in fluids was fitted for the first time：- ln K =（ 8972 ± 788） / T-（ 4. 16428 ± 1. 40362）,where ΔrHΘ= 74. 59 ± 6. 55 k J / mol,and ΔrSΘ= 34. 62 ± 11. 67 J /（ mol·K）. Furthermore,the amount of migrant Ti in fluids was calculated quantitatively by employing the apparent hydrolysis constants of K2 Ti F6under the conditions of subduction zone and rutile solubility. The results show that only when the rutile solubility is more than1000μg / m L can the fluorine-rich fluid migrate Ti effectively（ 〉1μg / m L）,and that the most migrant Ti in the fluids released from slabs may be no more than 6700μg / m L,much less than previous estimations based on rutile solubility. This study confirms that F-rich fluids can activate and thus migrate HFSE to the most extent in magmatic hydrothermal fluid systems and subduction zones,where F can be enriched in hydrothermal fluids by partitioning between F-rich melts and fluids,also in fluids / supercritical fluids derived from slabs through phengite dehydration or pyrogenation.