The measurements and application of triple O-isotope system is a relatively new and powerful tool in tracing water-rock interaction, allowing to distinguish the temperature-dependent fractionation from the water amount that interacted with the rock. While the use of 18O/16O fractionation between minerals and fluids is well-established, most natural mineral-water reactions remain uncharacterized in terms of the triple isotope fractionation (17O/16O and18O/16O). The triple O-isotope approach is especially promising when it comes to natural samples, where an array of exchanged samples constrains the fluid endmembers. However, multiple exchange mechanisms are likely to occur between the reacting rock and water, which has not been investigated for triple O-isotopes. This study examines the triple O-isotope exchange between fluid and olivine experimentally, as this is a common reaction in nature and mechanisms/rates of reaction are societally important for CO2 sequestration. Water-olivine reaction in batch experiments was investigated at 275 °C at the saturated water vapor pressure of 59.5 bar. Reactants were loaded at mass ratios 0.6 to 2 and exchanged for periods of time between 44 and 1048 h. Local meteoric water and mantle olivine were taken as the initial reactants. The mineral products are brucite, serpentine and minor magnetite that occur as coatings on unreacted olivine as well as individual crystals. The progress of this dissolution-precipitation reaction was traced with the δD-δ′18O-Δ′17O values of reacted fluids and mineral products. After 1048 h of reaction, ∼50 wt% of unreacted olivine remained. In general, experimental data conforms with the triple O-isotope fractionation factors calculated for the co-existing secondary minerals forming in equilibrium with fluid. However, there is scatter and deviations in the reacted fluid compositions likely driven by the difference in relative rates of secondary mineral formation and changing modal and chemical mineral composition. We observe that brucite and magnetite form abundant aggregates early in the progress of the reaction, while chrysotile forms fine-grained coatings that become abundant later. Since individual mineral-water fractionation 103ln18/16α range over 10 ‰ with θ ranging between 0.51 and 0.53, the dynamic behavior of dissolution-precipitation mechanism during alteration of olivine is the main factor to the “wandering” triple O-isotope trajectories in our relatively short experiments.

中文翻译：

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橄榄石实验改变过程中反应控制的三重 O 同位素交换轨迹


三重 O 同位素系统的测量和应用是追踪水-岩石相互作用的一种相对较新且强大的工具，可以区分与温度相关的分馏与与岩石相互作用的水量。虽然矿物和流体之间 18O/16O 分馏的使用已经得到充分证实，但大多数天然矿泉水反应在三重同位素分馏（17O/16O 和 18O/16O）方面仍未表征。当涉及到天然样品时，三重 O 同位素方法尤其有前途，因为一系列交换的样品限制了流体末端。然而，反应的岩石和水之间可能会出现多种交换机制，这尚未针对三重 O 同位素进行研究。本研究通过实验检查了流体和橄榄石之间的三重 O 同位素交换，因为这是自然界中的常见反应，反应机制/速率对 CO2 封存具有社会重要性。在 275 °C 和 59.5 bar 的饱和水蒸气压下，研究了批量实验中的水-橄榄石反应。反应物以 0.6 比 2 的质量比加载，并在 44 至 1048 小时之间交换一段时间。以局部陨石水和地幔橄榄石为初始反应物。矿物产物是水镁石、蛇纹石和少量磁铁矿，它们以涂层形式出现在未反应的橄榄石以及单个晶体上。该溶解-沉淀反应的进程通过反应流体和矿物产物的 δD-δ′18O-Δ′17O 值进行追踪。反应 1048 小时后，剩余 ∼ 50 wt% 的未反应橄榄石。一般来说，实验数据与为与流体平衡形成的共存次生矿物计算的三重 O 同位素分馏因子一致。 然而，反应流体成分存在分散和偏差，这可能是由次生矿物形成的相对速率的差异以及模态和化学矿物成分的变化引起的。我们观察到，水镁石和磁铁矿在反应进行的早期形成丰富的聚集体，而温石棉形成细粒涂层，后来变得丰富。由于单个矿泉水分馏 103ln18/16α 的范围超过 10 ‰，θ 范围在 0.51 和 0.53 之间，因此橄榄石改变过程中溶解-沉淀机制的动力学行为是我们相对较短的实验中“游荡”三重 O 同位素轨迹的主要因素。