Magnesium isotope ratios of pore-fluids and dolomites@en

Magnesium concentrations in deep-sea sediment pore-fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust. To better characterize and differentiate these processes, we have measured magnesium isotopes in pore-fluids and sediment samples from Ocean Drilling Program sites (1082, 1086, 1012, 984, 1219, and 925) that span a range of oceanographic settings. At all sites, magnesium concentrations decrease with depth. At sites where diagenetic reactions are dominated by the respiration of organic carbon, pore-fluid d26Mg values increase with depth by as much as 2 per mil. Because carbonates preferentially incorporate 24Mg (low d26Mg), the increase in pore-fluid d26Mg values at these sites is consistent with the removal of magnesium in Mg-carbonate (dolomite). In contrast, at sites where the respiration of organic carbon is not important and/or weatherable minerals are abundant, pore-fluid d26Mg values decrease with depth by up to 2 per mil. The decline in pore-fluid d26Mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore-fluid. The identity of this enriched magnesium sink is likely clay minerals. Using a simple 1D diffusion-advection-reaction model of pore-fluid magnesium, we estimate rates of net magnesium uptake/removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites. Independent estimates of magnesium isotope fractionation during dolomite precipitation from measured d26Mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites, suggesting that local exchange of magnesium between sediment and pore-fluid at these sites can be neglected. Our results indicate that the magnesium incorporated in dolomite is 2.0-2.7 per mil depleted in d26Mg relative to the precipitating fluid. Assuming local exchange of magnesium is minor at the rest of the studied sites, our results suggest that magnesium incorporated into clay minerals is enriched in d26Mg by 0 per mil to +1.25 per mil relative to the precipitating fluid. This work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources/sinks in low-temperature aqueous systems.深海沉积物孔隙流体中的镁浓度通常随岩心深度增加而降低，这源于沉积物或下伏地壳内白云石或黏土矿物的净沉淀作用。为更好地表征并区分上述成岩过程，我们对大洋钻探计划（Ocean Drilling Program）编号为1082、1086、1012、984、1219及925的数个站位的孔隙流体与沉积物样品中的镁同位素进行了测定，这些站位覆盖了多样的海洋环境背景。所有站位的镁浓度均随深度增加而下降。在成岩作用以有机碳呼吸为主的站位，孔隙流体的δ²⁶Mg值随深度升高可达2‰。由于碳酸盐矿物优先富集²⁴Mg（对应较低的δ²⁶Mg值），此类站位孔隙流体δ²⁶Mg值的升高与镁碳酸盐（白云石）对镁的移除过程相符。与之相反，在有机碳呼吸作用不显著和/或易风化矿物富集的站位，孔隙流体的δ²⁶Mg值随深度降低可达2‰。此类站位孔隙流体δ²⁶Mg值的下降，对应着一个同位素组成相较于孔隙流体更为富集的镁汇，该镁汇的主体大概率为黏土矿物。我们借助简化的一维孔隙流体镁扩散-平流-反应模型，估算了所有站位的镁净吸收/移除速率，以及对应源汇过程的镁同位素分馏系数。通过对1082和1012站位白云石样品的实测δ²⁶Mg值进行独立分析，得到的白云石沉淀过程中镁同位素分馏系数，与上述站位的模型净分馏系数高度一致，表明此类站位沉积物与孔隙流体间的镁局部交换可忽略不计。我们的研究结果显示，相较于沉淀流体，白云石所结合的镁其δ²⁶Mg值偏低2.0‰~2.7‰。假设其余研究站位的镁局部交换作用均较弱，那么黏土矿物所裹挟的镁相较于沉淀流体，其δ²⁶Mg值富集0‰~+1.25‰。本研究证实了镁同位素可作为示踪剂，用于追踪低温水介质系统中的镁源与镁汇过程。