科研之友
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摘要
Multiple batch experiments (100 degrees C, 200 degrees 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; Fo(90); [B] = <0.02 mu g/g; delta(11) B-Olivine -14%; [Li] = 1.7 mu g/g; delta Li-7(Olivine) = +5.3 parts per thousand) with seawater-like fluids (75 ml, 3.2 wt.% NaCl) adjusted with respect to their Li (0.2, 0.5 mu g/ml; and delta Li-7(Fluid) +55 parts per thousand) and B (similar to 10 mu g/ml and delta B-11(Fluid) -0.3 parts per thousand) characteristics. At 200 degrees 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 degrees C) = 55.61 mu g/g; (DB200 degrees C)-B-S/F = 13.42) and a preferential uptake of the lighter B-10 isotope (Delta B-11(S-F) = -3.46 parts per thousand). Despite a low reaction turnover at 100 degrees C (< 12%), considerable amounts of B were again incorporated into solid phases ([B](final_ 100 degrees C) = 25.33 mu g/g; (DB100 degrees C)-B-S/F = 24.2) with even a larger isotope fractionation factor (Delta B-11(S-F) = -9.97 parts per thousand). 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)(3)Cl-) 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 degrees C ([Li](final_ 200 degrees C) = 0.77 mu g/g; (DLi200 degrees C)-Li-S/F = 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 degrees C primary olivine is dissolved, releasing its Li budget into the fluid which shifts towards a lower delta Li-7(F) of + 38.62 parts per thousand. Newly formed serpentine minerals (delta Li-7(S) = +30.58 parts per thousand) incorporate fluid derived Li with a minor preference of the Li-6 isotope. At 100 degrees C Li enrichment of secondary phases exceeded Li release by olivine breakdown ([Li](final_ 100 degrees C) = 2.10 mu g/g; (DLi100 degrees C)-Li-S/F = 11.3) and it was accompanied by preferential incorporation of heavier Li-7 isotope that might be due to incorporation of a Li-7 enriched fluid fraction into chrysotile nanotubes.
- 出版日期2017-11-15
