The presence of silicate melt may enhance rates of cation diffusion in olivine
The presence of silicate melt may enhance rates of cation diffusion in olivine
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DOI：
10.1016/j.epsl.2023.118370
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发表时间：
2023-09-27
影响因子：
5.3
通讯作者：
Bradley，John
中科院分区：
文献类型：
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作者：
Shea，Thomas;Ruth，Dawn;Bradley，John
橄榄石通常被用作‘水晶时钟’，以提取与镁铁质岩浆系统内喷发前扰动有关的时间尺度。扩散比色法的应用需要精确校准铁-镁或其他通常测量的元素如镍、锰和钙在晶格中扩散的速度。在过去，这些速率主要是通过涉及橄榄石单晶、薄膜或粉末源的固体-固体扩散耦合实验来表征的。尽管天然岩浆系统中存在橄榄石周围的熔体，但涉及岩浆的实验很少，这主要是因为控制界面反应很困难。在这项研究中，我们进行了橄榄石-熔体扩散实验，作为扩散计量学方法的测试，并确定熔体的存在是否影响计算的时间尺度。为了接近自然系统，我们在天然Kī劳亚玄武岩中加入了小的天然Kī劳亚和圣卡洛斯橄榄石种子，并跟踪了随着时间的扩散重新平衡。为了更好地控制界面反应，在1290℃的初始超液相线温度下经过一段平衡时间后，快速冷却形成轮缘，并在不同的最终温度(1200,1220,1240,1255°C)下停留6-84h。测量了Fe-Mg，Mn，Ni，Ca的浓度梯度，并利用P或Al等缓慢扩散元素确定了核心-边缘转变的阶梯性质。当使用公布的扩散系数对这些梯度进行建模时，检索到的时间尺度通常比实际实验持续时间长10倍。因此，测量的扩散系数比之前在橄榄石-固体源实验中获得的扩散系数快一个数量级，但它们与仅有的两个熔体-橄榄石数据集非常吻合。我们探索了为什么熔融橄榄石扩散实验往往产生更快的速度的原因。(1)扩散过程中的生长，(2)任何初始溶解步骤中的扩散，以及(3)界面处扩展的管状或平面缺陷对计算的扩散系数的可能影响都被考虑了，但被发现是不重要的。相反，我们认为，与其他固体耦合扩散源相比，玄武岩熔体中Al或H等元素的浓度较高，可能会在界面处产生额外的点缺陷(空位)。未来扩散比色法在橄榄石中的应用可能需要使用熔融轴承实验配置对已公布的扩散系数进行彻底的重新评估。
Olivine is commonly used as a ‘crystal clock’ to extract timescales relevant to pre-eruptive perturbations within mafic magmatic systems. Diffusion chronometry applications require accurate calibrations for the rates at which Fe-Mg or other commonly measured elements like Ni, Mn, and Ca diffuse through the crystal lattice. In the past, these rates have been mainly characterized using solid-solid diffusion couple experiments involving olivine single crystals, thin films, or powder sources. Despite the presence of melt surrounding olivine in natural magmatic systems, very few experiments involving magma have been performed, largely because controlling interface reactions is difficult. For this study, we carried out olivine-melt diffusion experiments as a test of the diffusion chronometry method, and to determine whether the presence of melt influences the calculated timescales. To approximate a natural system, we incorporated small natural Kīlauea and San Carlos olivine seeds within a natural Kīlauea basalt and tracked diffusive re-equilibration through time. To better control interface reactions, after some equilibration period at an initial superliquidus temperature of 1290 °C, the runs were rapidly cooled to form a rim and left to dwell at various final temperatures (1200, 1220, 1240, 1255 °C) for 6–84 h. Concentration gradients for Fe-Mg, Mn, Ni, Ca were measured, and the step-wise nature of the core-rim transition was ascertained using slow diffusing elements like P or Al. When these gradients are modeled using published diffusivities, the timescales retrieved are typically 10 times longer than the actual experiment durations. Thus, measured diffusivities are an order of magnitude faster than those previously obtained in olivine-solid source experiments, but they are in excellent agreement with the only two other melt-olivine datasets. We explore reasons for why melt-bearing olivine diffusion experiments tend to yield faster rates. The possible effects of (1) growth during diffusion, (2) diffusion during any initial dissolution step, and (3) extended tube or planar defects at the interface on calculated diffusivities are all considered but found to be inconsequential. Instead, we argue that additional point defects (vacancies) are likely created at the interface by higher concentrations in elements like Al or H in the basalt melt compared to other solid couple diffusant sources. Future applications of diffusion chronometry in olivine may require a complete re-evaluation of published diffusivities using melt-bearing experimental configurations.