Improved spatial resolution of elemental maps through inversion of LA-ICP-MS data
Improved spatial resolution of elemental maps through inversion of LA-ICP-MS data
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通过 LA-ICP-MS 数据反演提高元素图的空间分辨率
DOI：
10.1016/j.chemgeo.2017.07.001
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发表时间：
2017
期刊：
影响因子：
3.9
通讯作者：
D. Shuster
中科院分区：
文献类型：
--
作者：
M. Fox;Alka Tripathy;D. Shuster
激光烧蚀电感耦合等离子体质谱 (LA-ICP-MS) 提供晶体内元素的空间分布，因此可以限制地质过程的速率。 LA-ICP-MS的空间分辨率受到烧蚀足够材料以超过仪器检测限的要求的限制：材料太少，无法测量浓度；来自同一空间位置的材料过多，并且浓度依赖于深度的变化的可能性增加。由于这一要求和典型的分析设置，对于 ppm 浓度的元素，通常会将烧蚀“点”尺寸的下限设置为约 20 μm。在这里，我们提出了一种使用逆方法实现子光斑尺寸分辨率的方法。我们将分析中采样的空间离散为像素，并注意到一个点采样的像素的平均浓度等于测量的浓度。当多个重叠点对某些相同像素进行采样时，我们可以将每个点的离散表达式组合为线性方程组。通过具有平滑度约束的线性反演，我们可以求解未知的像素浓度。我们通过两个自然例子来强调这种方法，其中扩散过程很重要：岩浆上升速度和 (U-Th)/He 惰性气体热测时法。在这些示例中，准确的结果要求可以从 LA-ICP-MS 数据中恢复真实的浓度梯度。我们表明，推断岩浆上升速度的能力从几个月提高到几周，并且我们能够解释以前无法解释的热测时数据。
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) provides the spatial distribution of elements within crystals and therefore can constrain the rates of geological processes. Spatial resolution of LA-ICP-MS is limited by the requirement to ablate sufficient material to surpass the detection limit of the instrument: too little material and the concentration cannot be measured; too much material from the same spatial location and the possibility of depth dependent variations in concentration increases. Because of this requirement and typical analytical setup, this commonly places a lower bound on the diameter of an ablation ‘spot’ size of approximately 20 μm for elements with ppm concentration. Here we present a means to achieve sub-spot size resolution using inverse methods. We discretize the space sampled in an analysis into pixels and note that the average concentration of the pixels sampled by a spot equals the measured concentration. As multiple overlapping spots sample some of the same pixels, we can combine discrete expressions for each spot as a system of linear equations. Through linear inversion with smoothness constraints we can solve for unknown pixel concentrations. We highlight this approach with two natural examples in which diffusive processes are important: magmatic ascent speeds and (U-Th)/He noble gas thermochronometry. In these examples, accurate results require that the true concentration gradients can be recovered from LA-ICP-MS data. We show that the ability to infer rapid rates of magma ascent is improved from months to weeks and that we are able to interpret previously un-interpretable thermochronometric data.