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首页> 外文期刊>Quaternary geochronology >Calibration of a QEM-EDS system for rapid determination of potassium concentrations of feldspar grains used in optical dating
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Calibration of a QEM-EDS system for rapid determination of potassium concentrations of feldspar grains used in optical dating

机译：QEM-EDS系统的校准快速测定光学约会中使用的长石谷物浓度的快速测定

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摘要

Potassium (K)-rich feldspars are one of two mineral types typically used for optical dating. Feldspar grains can contain up to similar to 14 wt% K that gives rise to an internal dose rate component. This internal component can comprise a significant proportion of the total environmental dose rate to which a mineral grain is exposed. The environmental dose rate term-the denominator in the optical age equation-determines the rate at which electronic charge is transferred into the crystal lattice of a mineral grain over its period of burial. Not all feldspar grains have the same K concentration, so internal dose rates differ between individual grains. K concentrations can range from 0 to 14 wt% due to the variable compositions of feldspar phases, differing proportions of discrete feldspar phases and/or the presence of other mineral inclusions in grains. Numerous techniques are available for determining the K concentrations of individual grains, but are time-consuming, and either lack the spatial resolution to classify discrete mineral phases within multi-phase grains, or the coverage to obtain whole-of-grain average K concentrations. Quantitative evaluation of minerals using energy dispersive spectroscopy (QEMEDS) is a time-efficient and automated mapping technique that has the spatial resolution to classify most mineral phases, as well as the coverage to determine their area proportions. QEM-EDS can also be used to determine elemental concentrations based on spectral matches to EDS reference spectra. It is, however, difficult to determine accurate elemental concentrations for minerals such as feldspars, where solid solutions exist. To overcome this, we establish a QEM-EDS calibration using EDS spectra from six feldspar reference standards to define five solid solution regions along the alkali and plagioclase feldspar series. We test this calibration through comparisons of the K concentrations of discrete phases of three feldspar varieties (orthoclase/microcline, albite and sanidine), derived using both QEM-EDS and wavelength dispersive spectroscopy (WDS). We assume that the WDS-derived K concentrations represent the 'true' K concentrations of the phases, and calculate, from a best-fit weighted regression of the two data sets, a correction and uncertainty estimate that can be applied to the QEMEDS-derived K concentrations, taking into account instrument irreproducibility and any measurement bias. We also test alternative mapping step sizes to optimise efficiency, and apply this time-efficient technique to individual luminescent feldspar grains from two samples. We propose that QEM-EDS is capable of (1) classifying the range of mineral phases in grains, (2) determining the area proportions of each of these phases, and (3) obtaining accurate whole-of-grain average K concentrations of individual feldspar grains using the calibration and correction presented here.
机译：富钾长石是光学测年常用的两种矿物类型之一。长石颗粒可含有高达14 wt%的K，从而产生内部剂量率成分。这种内部成分可能占矿物颗粒暴露的总环境剂量率的很大一部分。环境剂量率项——光学年龄方程中的分母——决定了电子电荷在掩埋期间转移到矿物颗粒晶格中的速率。并非所有长石颗粒都具有相同的钾浓度，因此不同颗粒的内部剂量率不同。由于长石相的成分不同、离散长石相的比例不同和/或颗粒中存在其他矿物包裹体，钾浓度的范围为0至14 wt%。有许多技术可用于确定单个颗粒的钾浓度，但都很耗时，或者缺乏将多相颗粒中的离散矿物相分类的空间分辨率，或者缺乏获得整个颗粒平均钾浓度的覆盖率。使用能量色散光谱（QEMEDS）对矿物进行定量评估是一种高效、自动化的绘图技术，它具有对大多数矿物相进行分类的空间分辨率，以及确定其面积比例的覆盖率。QEM-EDS还可用于根据EDS参考光谱的光谱匹配确定元素浓度。然而，对于存在固溶体的长石等矿物，很难确定准确的元素浓度。为了克服这一问题，我们使用六种长石参考标准的EDS光谱建立了QEM-EDS校准，以确定碱长石和斜长石系列的五个固溶体区域。我们通过比较三种长石（正长石/微斜长石、钠长石和钠长石）离散相的K浓度来测试这种校准，这三种长石使用QEM-EDS和波长色散光谱（WDS）得出。我们假设WDS衍生的K浓度代表相的“真实”K浓度，并根据两个数据集的最佳拟合加权回归计算可应用于QEMEDS衍生的K浓度的校正和不确定度估计，同时考虑仪器不可重复性和任何测量偏差。我们还测试了可选的映射步长，以优化效率，并将此时间效率技术应用于两个样本中的单个发光长石颗粒。我们认为，QEM-EDS能够（1）对颗粒中矿物相的范围进行分类，（2）确定每个相的面积比例，以及（3）使用本文给出的校准和校正，获得单个长石颗粒的准确全颗粒平均K浓度。

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