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为确定B掺入Mg(OH)2时的形式及掺入过程中硼同位素组成的变化,进行了不同设定pH(pH设定)的人工合成无Mg海水中B掺入Mg(OH)2的实验.结果表明,B掺入Mg(OH)2非常迅速,4h能达到平衡,平衡后Mg(OH)2中B浓度[B]固和固相与溶液相间的分配系数Kd随pH设定的升高而降低.而且最高的[B]固和Kd均远高于B被金属氧化物或黏土矿物吸附时的对应值,表明B具有很强的掺入Mg(OH)2的能力.平衡时溶液相的δ11B液f均低于原始溶液的δ11B液i,计算的Mg(OH)2与平衡溶液间的硼同位素分馏系数α固-液变化范围为1.0186～1.0220,平均值为1.0203.这充分表明,B掺入Mg(OH)2时11B优先进入固相,这是B(OH)3优先掺入的结果.B(OH)3与Mg(OH)2间的沉积反应是B(OH)3掺入的直接原因.B(OH)3与Mg(OH)2间的沉积反应和B(OH)4-在Mg(OH)2上吸附可能同时存在,并以沉积反应为主,它们决定了B掺入Mg(OH)2时硼同位素特征.与B只以B(OH)4-形式掺入生物碳酸盐不同,B以B(OH)3和B(OH)4-两种形式同时掺入Mg(OH)2,并以B(OH)3优先掺入为主,pH设定越低掺入的B(OH)3比例越高.由于Mg(OH)2普遍存在于石珊瑚中,这将严重影响珊瑚的硼同位素组成δ11B与海水pH的定量对应关系,因此会给δ11B作为古海水pH的代用指标带来极大的不确定性.
In order to confirm the change of boron isotope composition during the incorporation of Mg (OH) 2 and the incorporation of boron into Mg (OH) 2, Mg (OH) 2 The results show that B (B) incorporates Mg (OH) 2 very quickly and can reach the equilibrium after 4h. The B concentration in the Mg (OH) 2 equilibrium [B] And the highest [B] Kd and Kd are much higher than the corresponding values when B is adsorbed by metal oxides or clay minerals, indicating that B has a strong ability to incorporate Mg (OH) 2. The equilibrium The δ11B solution f of the solution phase is lower than the δ11B solution i of the original solution. The calculated boron isotope fractionation coefficient α between the calculated Mg (OH) 2 and the equilibrium solution α varies from 1.0186 to 1.0220 with an average value of 1.0203. It is well demonstrated that 11B preferentially enters the solid phase when B is doped into Mg (OH) 2, which results from the preferential incorporation of B (OH) 3. The deposition reaction between B (OH) 3 and Mg ) 3 The direct reason for the incorporation The deposition reaction between B (OH) 3 and Mg (OH) 2 and the adsorption of B (OH) 4 on Mg (OH) 2 may coexist and dominate the deposition reaction Which determines the boron isotope characteristics when B is incorporated into Mg (OH) 2, and B only incorporates B (OH) 4- B was mixed with B (OH) 3 and B (OH) 4 in the form of Mg (OH) 2 at the same time, and B (OH) Of B (OH) 3 is higher, because Mg (OH) 2 is prevalent in stony coral, which will seriously affect the quantitative relationship between boron isotope composition δ11B of coral and seawater pH, Substitution indicators bring great uncertainty.