Multiple batch experiments toto investigate Li and B partitioning and isotope fractionation between rock and water during serpentinization

Multiple batch experiments (100 °C, 200 °C; 40 MPa) were conducted, using Dickson-type reactors, to investigate Li and B partitioning and isotope fractionation between rock and water during serpentinization. We reacted fresh olivine (5 g; Fo90; [B] = <0.02 µg/g; d11BOlivine -14 per mil; [Li] = 1.7 µg/g; d7LiOlivine = +5.3 per mil) with seawater-like fluids (75 ml, 3.2 wt.% NaCl) adjusted with respect to their Li (0.2, 0.5 µg/ml; and d7LiFluid +55 per mil) and B (~10 µg/ml and d11BFluid -0.3 per mil) characteristics. At 200 °C a reaction turnover of about 70% and a serpentinization mineral assemblage matching equilibrium thermodynamic computational results (EQ3/6) developed after 224 days runtime. Characterization of concomitant fluid samples indicated a distinct B incorporation into solid phases ([B]final_200 °C = 55.61 µg/g; DS/FB200 °C = 13.42) and a preferential uptake of the lighter 10B isotope (Delta11BS-F = -3.46 per mil). Despite a low reaction turnover at 100 °C (<12%), considerable amounts of B were again incorporated into solid phases ([B]final_100 °C = 25.33 µg/g; DS/FB100 °C = 24.2) with even a larger isotope fractionation factor (Delta11BS-F = -9.97? per mil. While magnitude of isotope fraction appears anti-correlated with temperature, we argue for an overall attenuation of the isotopic effect through changes in B speciation in saline solutions (NaB(OH)4(aq) and B(OH)3Cl-) as well as variable B fixation and fractionation for different serpentinization product minerals (brucite, chrysotile). Breakdown of the Li-rich olivine and limited Li incorporation into product mineral phases resulted in an overall lower Li content of the final solid phase assemblage at 200 °C ([Li]final_200 °C = 0.77 µg/g; DS/FLi200 °C = 1.58). First order changes in Li isotopic compositions were defined by mixing of two isotopically distinct sources i.e. the fresh olivine and the fluid rather than by equilibrium isotope fraction. At 200 °C primary olivine is dissolved, releasing its Li budget into the fluid which shifts towards a lower d7LiF of +38.62 per mil. Newly formed serpentine minerals (d7LiS = +30.58 per mil) incorporate fluid derived Li with a minor preference of the 6Li isotope. At 100 °C Li enrichment of secondary phases exceeded Li release by olivine breakdown ([Li]final_100 °C = 2.10 µg/g; DS/FLi100 °C = 11.3) and it was accompanied by preferential incorporation of heavier 7Li isotope that might be due to incorporation of a 7Li enriched fluid fraction into chrysotile nanotubes.本研究采用迪克森型（Dickson-type）反应釜，开展了两组（100 ℃、200 ℃，40 MPa）批量实验，以探究蛇纹岩化（serpentinization）过程中锂（Li）与硼（B）在岩石-水体系间的分配行为及同位素分馏效应。实验以新鲜橄榄石（olivine，5 g；Fo90；硼含量[B]<0.02 μg/g，δ¹¹B橄榄石=-14‰；锂含量[Li]=1.7 μg/g，δ⁷Li橄榄石=+5.3‰）为固相反应物，与模拟海水流体（75 mL，3.2 wt.% NaCl）进行反应；该流体的锂特征（0.2、0.5 μg/mL，δ⁷Li流体=+55‰）与硼特征（~10 μg/mL，δ¹¹B流体=-0.3‰）均可通过调控实现。在200 ℃条件下，反应运行224天后，反应转化率约达70%，且形成的蛇纹岩化矿物组合与EQ3/6平衡热力学计算结果吻合。对同步采集的流体样品进行表征后发现，硼明显掺入固相产物中（200 ℃下固相最终硼含量[B]最终_200℃=55.61 μg/g；固相-流体硼分配系数DS/FB_200℃=13.42），且轻同位素¹⁰B优先被固相固溶掺入，对应的Δ¹¹B固相-流体为-3.46‰。尽管100 ℃下反应转化率较低（<12%），但仍有大量硼掺入固相产物中（[B]最终_100℃=25.33 μg/g；DS/FB_100℃=24.2），且同位素分馏程度更大，对应的Δ¹¹B固相-流体为-9.97‰。尽管同位素分馏程度与温度呈负相关，但我们认为，盐溶液中硼的形态变化（即四羟基合硼酸钠水溶液NaB(OH)₄(aq)与三羟基合氯合硼阴离子B(OH)₃Cl⁻）、以及不同蛇纹岩化产物矿物（水镁石brucite、纤蛇纹石chrysotile）的硼固定与分馏差异，会整体减弱同位素分馏效应。富锂橄榄石的分解以及锂在产物矿物相中的有限掺入，使得200 ℃下最终固相产物的整体锂含量更低（[Li]最终_200℃=0.77 μg/g；固相-流体锂分配系数DS/FLi_200℃=1.58）。锂同位素组成的一级变化主要由两种同位素特征迥异的端元——新鲜橄榄石与反应流体——的混合所主导，而非平衡同位素分馏过程。在200 ℃条件下，原始橄榄石发生溶解，将其固有的锂释放至流体中，使得流体的δ⁷Li降至+38.62‰；新形成的蛇纹石矿物（δ⁷Li蛇纹石=+30.58‰）会掺入流体来源的锂，且优先吸附轻同位素⁶Li。在100 ℃条件下，次生矿物相的锂富集量超过橄榄石分解释放的锂量（[Li]最终_100℃=2.10 μg/g；DS/FLi_100℃=11.3），且伴随重同位素⁷Li的优先掺入，这一现象可能源于富含⁷Li的流体组分进入纤蛇纹石纳米管结构中。