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Probing silicate weathering reactions in soils with B isotopes
Chemical Geology ( IF 3.9 ) Pub Date : 2020-08-01 , DOI: 10.1016/j.chemgeo.2020.119671
A. Voinot , M.-P. Turpault , S. Rihs , L. Mareschal , F. Chabaux , D. Lemarchand

Abstract The determination of the mechanisms and extent of soil mineral weathering can be challenging, and the caveats reside in 1) difficulty identifying minerals that are actually involved in weathering reactions, 2) non-stoichiometric release of cations during weathering processes due to coupled dissolution, precipitation and transformation reactions and, 3) impact of vegetation activity on elemental cycles in upper soil horizons. To better characterize mechanisms controlling mineral weathering in soils and trace the evolution of B concentration and isotope ratios during chemical weathering, quantitative mineralogical analyses were coupled to B isotopes in a group of minerals (biotite, muscovite, K-feldspar and albite). Samples were selected along an Alocrisol (Alumic Cambisol, WRB FAO) soil profile from the bedrock (at 130 cm depth) up to 20 cm depth, developed on granitic bedrock in the Breuil-Chenue forest (France). The samples consist of residual primary minerals associated with weathering secondary phases (vermiculite, kaolinite…) in varying proportions. The B isotopic compositions of the most pristine minerals span a very narrow range of values (around −31‰), whereas all secondary phases point to a much heavier value (around −16‰), regardless of mineralogy. Our results also show a mineral-dependent evolution of B concentration or isotopic composition as weathering progresses: no variation is observed during dissolution of K-feldspars; B behaves like a very mobile element in micas (biotite and muscovite), whereas it concentrates in weathered products derived from albite. However, rates of B concentrations and changes in isotopic compositions appear to be much faster than those inferred from mineralogy or major element concentrations determined by XRD and bulk chemical analyses, respectively. These results indicate that B is involved in very early weathering reactions and raises the question of its actual location in the structure of the various soil minerals as well as its pathway to solution.

中文翻译:

用 B 同位素探测土壤中的硅酸盐风化反应

摘要 确定土壤矿物风化的机制和程度可能具有挑战性,需要注意的是 1) 难以识别实际参与风化反应的矿物,2) 由于耦合溶解,在风化过程中阳离子的非化学计量释放,降水和转化反应,3) 植被活动对上层土壤元素循环的影响。为了更好地表征控制土壤中矿物风化的机制并追踪化学风化过程中 B 浓度和同位素比率的演变,定量矿物学分析与一组矿物(黑云母、白云母、钾长石和钠长石)中的 B 同位素相结合。沿着 Alocrisol (Alumic Cambisol, WRB FAO) 从基岩(深度 130 厘米)到深度 20 厘米的土壤剖面,在 Breuil-Chenue 森林(法国)的花岗岩基岩上发育。样品由不同比例的与风化次生相(蛭石、高岭石……)相关的残留原生矿物组成。大多数原始矿物的 B 同位素组成范围非常窄(约 -31‰),而所有次生相都指向更重的值(约 -16‰),无论矿物学如何。我们的结果还表明,随着风化作用的进行,B 浓度或同位素组成的矿物依赖性演变:在钾长石的溶解过程中没有观察到变化;B 在云母(黑云母和白云母)中表现得像一种非常易流动的元素,而它集中在来自钠长石的风化产品中。然而,B 浓度和同位素组成变化的速率似乎比分别通过 XRD 和大量化学分析确定的矿物学或主要元素浓度推断出的速率快得多。这些结果表明 B 参与了非常早期的风化反应,并提出了它在各种土壤矿物质结构中的实际位置及其解决途径的问题。
更新日期:2020-08-01
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