Abstract Magnesium (Mg) isotopes fractionate during rock/mineral weathering and leaching, secondary mineral neoformation, adsorption/desorption, and plant-related Mg recycling, but the mechanisms and extent of fractionation are not well understood. Here, we report the fate of Mg and its isotopes during basalt weathering and soil development in the Hawaiian Islands by sampling soils of varying age (0.3, 20, 150, 1400, and 4100 ka) in undisturbed humid rainforests. Magnesium concentrations in bulk soils are variable with depth and age, ranging from 0.07 to 8.79 wt.%, and significant Mg depletions (up to 99%) relative to parent basalts are visible after 20 ka. Bulk soils display a large age-dependent range of δ26Mg values ranging from −0.60 to +0.26‰. A sequential leaching scheme showed that labile Mg is depleted whereas residual Mg is enriched in isotopically heavy Mg. The two youngest soils (0.3 ka) display δ26Mg value similar to basalt for both labile or residual Mg, indicating either that basalt weathering causes little Mg isotope fractionation or that δ26Mg value is overwhelmed by the primary minerals during 0.3 ka. However, in the older soils (≥20 ka), the δ26Mg values of both labile and residual Mg vary non-linearly as a function of time, with an increase in the difference between them. These variations are explained by both plant-related Mg recycling and progressive mineral transformations, evolving from short-range-order (SRO) minerals (allophane and ferrihydrite) to more crystalline products (goethite, gibbsite and kaolin minerals). Indeed, plant-related Mg recycling causes the enrichment of light Mg isotopes in the labile Mg, while secondary phases incorporate more and more heavy Mg isotopes with time. These results reconcile experimental and field studies and highlight a weathering control of Mg isotopes delivered to the oceans.

中文翻译：

---

夏威夷 4 Myr 年代序列土壤中镁同位素地球化学的变化

摘要 镁 (Mg) 同位素在岩石/矿物风化和浸出、次生矿物新形成、吸附/解吸以及与植物相关的镁回收过程中分馏，但分馏的机制和程度尚不清楚。在这里，我们通过对未受干扰的潮湿雨林中不同年龄（0.3、20、150、1400 和 4100 ka）的土壤进行采样，报告了夏威夷群岛玄武岩风化和土壤发育过程中镁及其同位素的归宿。大块土壤中的镁浓度随深度和年龄而变化，范围为 0.07 至 8.79 wt.%，并且在 20 ka 后可见相对于母玄武岩的显着镁消耗（高达 99%）。大块土壤的 δ26Mg 值随年龄变化很大，范围从 -0.60 到 +0.26‰。连续浸出方案表明不稳定的镁被耗尽，而残留的镁富含同位素重的镁。两个最年轻的土壤 (0.3 ka) 的 δ26Mg 值与玄武岩的不稳定或残留 Mg 相似，表明玄武岩风化导致 Mg 同位素分馏很少，或者 δ26Mg 值在 0.3 ka 期间被主要矿物压倒。然而，在较旧的土壤（≥20 ka）中，不稳定和残留 Mg 的 δ26Mg 值作为时间的函数呈非线性变化，它们之间的差异增加。这些变化可以通过与植物相关的镁回收和渐进的矿物转化来解释，从短程 (SRO) 矿物（水铝石和水铁矿）演变为更多结晶产品（针铁矿、三水铝石和高岭土矿物）。确实，与植物相关的 Mg 循环导致不稳定 Mg 中轻 Mg 同位素的富集，而随着时间的推移，第二相包含越来越多的重 Mg 同位素。这些结果协调了实验和现场研究，并突出了对输送到海洋的镁同位素的风化控制。