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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Contrasting lithium and magnesium isotope fractionation during continental weathering
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Contrasting lithium and magnesium isotope fractionation during continental weathering

机译：大陆风化过程中锂和镁同位素分馏的对比

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摘要

Magnesium isotopic compositions of a profile through saprolites developed on a diabase dike from South Carolina have been measured in order to study the behavior of Mg isotopes during continental weathering. As weathering progresses, Mg isotopes are greatly fractionated and are correlated with Mg concentration, clay mineral proportions and density of the saprolites δ~(26)Mg values increase from -0.22 in the unweathered diabase to +0.65 in the most weathered saprolite. These observations are consistent with the release of light Mg to the hydrosphere and formation of isotopically heavy Mg in the weathered products. The loss of Mg during weathering can be modeled by Rayleigh distillation with an apparent fractionation factor between the saprolite and fluid (α) of 1.00005 to 1.0004, i.e., up to 0.4‰fractionation in the ~(26)Mg/~(24)Mg ratio between the saprolite and fluid. The large variation in α value reflects a mineralogical control on Mg isotope fractionation during primary dissolution of Mg-rich minerals and formation of secondary minerals during continental weathering. Like Mg isotopes, Li isotopes in the saprolite profile are also greatly fractionated, with δ~7Li values ranging from -6.7 down to -20. The large Li isotope fractionation and variation in Li concentration, as well as irregularities in the δ~7Li profile with depth, however, cannot be explained by Li loss during weathering alone. Instead, Li can be modeled by a two-step process: (1) equilibrium isotope fractionation during continental weathering, which lowered δ~7Li and Li concentrations and produced a Li concentration gradient in the saprolites like that seen in Mg, and (2) subsequent kinetic isotope fractionation produced by diffusion of Li in the saprolites, possibly across a paleo-water table. The results presented here suggest that continental weathering will shift the Mg isotopic composition of the continental crust to values higher than the mantle value, whereas crustal recycling over the history of the Earth will have no discernible effect on the Mg isotopic composition of the mantle.
机译：为了研究大陆风化过程中Mg同位素的行为，已经测量了通过在南卡罗来纳州的辉绿岩堤上发育的腐泥土形成的镁同位素组成。随着风化的进行，镁的同位素被极大地分离，并与镁的浓度，粘土矿物的比例和腐泥土的δ〜（26）Mg值相关，从未风化的辉绿岩中的-0.22增加到最风化的腐泥土中的+0.65。这些观察结果与轻质镁向水圈的释放以及风化产物中同位素重质镁的形成是一致的。风化过程中的镁损失可以通过瑞利蒸馏法模拟，腐泥土和流体（α）之间的表观分馏因子为1.00005至1.0004，即〜（26）Mg /〜（24）Mg中的分馏率最高为0.4‰。腐泥土和液体之间的比率。 α值的大变化反映了矿物学上对富镁矿物初次溶解过程中的镁同位素分馏和大陆风化过程中次生矿物形成的控制。与镁同位素一样，腐泥土剖面中的锂同位素也被极大地分离，δ〜7Li值范围从-6.7到-20。较大的Li同位素分馏和Li浓度变化以及δ〜7Li剖面随深度的不规则性，不能仅通过风化过程中的Li损失来解释。取而代之的是，Li可以通过两步过程进行建模：（1）大陆风化过程中的平衡同位素分馏，降低了δ〜7Li和Li的浓度，并在腐泥土中产生了与Mg相似的Li浓度梯度，（2）锂在腐泥土中（可能跨整个古水位）的扩散而产生的后续动力学同位素分级分离。此处给出的结果表明，大陆风化将使大陆壳的Mg同位素组成转变为高于地幔值的值，而地球历史上的地壳再循环将对地幔的Mg同位素组成没有明显的影响。

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