Collaborative Research: Development of 40Ar/39Ar Intercalibration Pipettes

钾(K)，地球上最丰富的元素之一？地壳存在于许多造岩矿物中。K的同位素之一，40K是放射性的，衰变为40Ar。这个方案构成了应用于岩石和矿物的最常见的地质年代学技术之一的物理基础。近年来，一种需要中子轰击样品以从39K生成39Ar的特殊技术比传统的？K-Ar？方法，称为40Ar/39Ar方法。事实上，40Ar/39Ar放射性同位素测年法是所有地球计时器中适用范围最广的，从早期太阳系形成（约45.6亿年前）到2000年前的火山物质都很有用。这些研究涵盖了火山和地震灾害、矿化事件的时间和程度、全球气候变化和变异、古地磁反转时间标度、化石记录测年（包括人类进化）等主题。因此，对40Ar/39Ar方法的准确度和精密度的任何改进都将提高我们对地质过程的时间和速率的理解。尽管精度堪称典范，但最近通过EARTHTIME计划进行的实验揭示了实验室间的显著不一致，例如不同实验室确定的年龄可能相差4%，大约比典型报告的测量精度大一个数量级。已经确定了造成这些差异的四种可能原因；1)参与实验室采用的数据缩减方案的差异；2)用于收集数据的质谱仪源/检测器系统的未解释非线性；3)由于某些样品包装中中子辐照剂量的变化而导致的矿物标准品未被识别的异质性或实验伪影；4)在不同的实验室中，从分析的碱基中不完全脱气或同位素分馏出Ar。该研究项目通过开发氩气移液管系统来解决实验室间一致性的问题，该系统将在美国参与的40Ar/39Ar实验室之间传输。移液管系统将输送具有完全相同同位素组成和相似气体体积的气体样品，从而控制与自然样品非均质性相关的变量和问题。此外，还将测量移液器系统的可变气体样本量，以评估惰性气体质谱仪的响应线性。本研究的最终目标是对各个NSF支持的40Ar/39Ar实验室进行相互校准，并允许在0.1%或更高的水平上对参与实验室产生的年龄进行直接比较。移液器系统的长期使用将维持40Ar/39Ar实验室之间的校准。本科生和研究生将携带互校准移液器协助测量。这将为学生提供机会，与来自不同40Ar/39Ar实验室的人员进行观察、工作和互动，并为下一代地球年代学家/同位素地球化学家提供独特的培训机会。

Potassium (K), one of the more abundant elements in the Earth?s crust is found in many rock forming minerals. One of the isotopes of K, 40K is radioactive and decays to 40Ar. This scheme forms the physical basis for one of the most common geochronologic techniques applied to rocks and minerals. In recent years, a special technique requiring neutron bombardment of samples to produce 39Ar from 39K has become more favored than the traditional ?K-Ar? method, referred to as the 40Ar/39Ar method. In fact, the 40Ar/39Ar method of radioisotopic dating has the broadest range of applicability of any geochronometer, being useful from early solar system formation (~4.56 billion years ago) to volcanic materials as young as 2,000 years old. These studies cover topics such as volcanic and seismic hazards, timing and extent of mineralization events, global climate change and variability, paleomagnetic reversal timescale, and dating the fossil record, including human evolution. Thus any improvement in the accuracy and precision 40Ar/39Ar method will improve our understanding of the timing and rates of geologic processes.Despite exemplary precision, recent experiments conducted via the EARTHTIME initiative have revealed significant interlaboratory inconsistencies such that ages determined by different labs may vary by us much as 4 %, approximately an order of magnitude larger than typical reported measurement precision. Four possible causes for these discrepancies have been identified; 1) differences in the data reduction protocols employed by the participating laboratories; 2) unaccounted nonlinearity of the mass spectrometer source/detector systems used to collect the data; 3) unrecognized heterogeneity in the mineral standards or an experimental artifact arising from variations in neutron irradiation dosage in some of the sample packages; and 4) incomplete degassing or isotopic fractionation of Ar from the analyzed sanidines in various laboratories. This research project addresses the issue of interlaboratory consistency by developing an argon pipette system that will travel between the participating 40Ar/39Ar labs within the United States. The pipette system will deliver gas samples with exactly the same isotopic composition(s) and similar gas volume(s), thus controlling the variables and issues associated with heterogeneity of natural samples. In addition, variable gas sample volumes from the pipette system will also be measured to assess the response linearity of noble gas mass spectrometers. The ultimate goal of this study is to intercalibrate the various NSF supported 40Ar/39Ar laboratories and allow for the direct comparison of the ages produced from the participating labs, at the level of 0.1% or better. The long-term use of the pipette system will maintain the calibration between the 40Ar/39Ar laboratories. Undergraduate and graduate students will be traveling with the intercalibration pipette assisting with the measurements. This will provide the students the opportunity to see, work, and interact with personnel from the different 40Ar/39Ar laboratories and serve as a unique training opportunity for the next generation of geochronologist/isotope geochemists.