Contributions to precision and accuracy of monazite microprobe ages
Contributions to precision and accuracy of monazite microprobe ages
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DOI：
10.2138/am.2005.1340
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发表时间：
2005-04
影响因子：
3.1
通讯作者：
J. Pyle;F. Spear;D. Wark;C. Daniel;L. Storm
中科院分区：
文献类型：
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作者：
J. Pyle;F. Spear;D. Wark;C. Daniel;L. Storm
摘要我们使用装有4个PET晶体的JEOL 733 Superprobe，以及1-atm气体流动Ar X射线探测器和密封的Ar X射线探测器，研究了控制独居石微探针年龄准确度和精度的因素。多个PET晶体允许在多达3个检测器上同时测定Pb浓度，并且可以解决不同检测器气体对光谱形式的影响。X射线产生、探测和计数序列中的许多因素都会影响光谱形式，包括：加速电压的选择、衍射晶体d间距的变化、X射线准直狭缝的使用以及探测器气体的类型。方舟α X射线和XeLα X射线之间的能量差异导致对于1-atm Ar探测器，不能使用差模PHA过滤的二阶LREE L线X射线的逃逸峰。二阶轻稀土能量传递到计数器和生产，为140毫米罗兰圆，几个有问题的干扰独居石波长色散（WD）光谱的铅区域。WD独居石光谱产生的双金属探测器是免费的二阶LREE干扰的Pb区域;逃逸峰的二阶LREE是过滤与差分模式PHA，如果双金属探测器。硅、钙、钇、铈、磷、钍、铀和铅（2个光谱仪）作为独居石微探针测年方案的一部分进行测量; ZAF校正固定元素的±2σ变化不会影响分析不确定度之外的年龄。ThMα、UMβ和PbMα是年龄成分的分析线。ThM γ 1，2和YLγ 2，3对PbMα干扰的校正是显著的，但可以精确地进行，并使M α分析的精度降低很小的量。UMβ上的ThMγ、M3-N4和M5-P3干扰也可以校正，但高Th样品中的ThM 5和M4吸收边使UMβ背景的估计存在问题。UMβ峰的背景拟合表明，UMβ的线性拟合与指数拟合一般不会产生微探针年龄的统计学显著差异。然而，PbMα峰的线性与指数背景拟合确实产生了显著不同的年龄，最可能的原因是：（1）相对于U的低Pb浓度;（2）ThM β 1对ThM β 1和PbMβ之间背景的干扰;以及（3）含S独居石中的SKα和Kβ干扰。对于25 keV和200 nA下的6 min分析（3 min峰值，3 min背景），1σ Pb精度在1700 ppm下约为3.4%，在750 ppm下约为9.5%;在15 keV下，精度约降低25 keV值的25%。这些精度是恒定的固定电流，分析时间和浓度，但独居石颗粒（域）的不同人口的统计精度是一个功能的域内的分析总数。仪器误差（电流测量，死区时间，脉冲移位，d间距变化）增加了1.10%的随机误差，但脉冲移位和d间距变化引起的误差可以解释和校正。加速电压从25 keV降低到15 keV，ZAF校正因子相对降低了50%，但Trebilcock独居石在15 keV和25 keV的重复年龄分析在统计上是无法区分的。晶粒取向、计算错误的背景强度、未校正的干扰和表面效应也会引入系统误差。精确的背景插值和干扰校正将系统误差降低到约5.20%，除了随机（计数）误差。佛蒙特州独居石的微探针年龄（~420 Ma）和208 Pb/232 Th西姆斯年龄（~430 Ma）在1000万年内基本一致。当PbMα的高本底测量移到PbMβ的短波长一侧时，U-Th-总Pb微探针年龄与208 Pb/232 Th年龄之间的差异被消除，从而消除了可能的ThM γ 1干扰。
Abstract We examine the factors controlling accuracy and precision of monazite microprobe ages, using a JEOL 733 Superprobe equipped with 4 PET crystals, and both 1-atm gas flow Ar X-ray detectors and sealed Xe X-ray detectors. Multiple PET crystals allow for simultaneous determination of Pb concentration on up to 3 detectors, and the effects of different detector gases on spectral form can be addressed. Numerous factors in the X-ray production, detection, and counting sequence affect spectral form, including: choice of accelerating voltage, changes in d-spacing of the diffraction crystal, use of X-ray collimation slits, and type of detector gas. The energy difference between ArKα X-rays and XeLα X-rays results in, for 1-atm Ar detectors, escape peaks of second-order LREE L line X-rays that cannot be filtered using differential mode PHA. The second-order LREE energies are passed to the counter and produce, for a 140 mm Rowland circle, several problematic interferences in the Pb region of a monazite wavelength-dispersive (WD) spectrum. WD monazite spectra produced with Xe detectors are free from second-order LREE interferences in the Pb region; escape peaks of the secondorder LREE are filterable with differential mode PHA if Xe detectors are employed. Silicon, Ca, Y, Ce, P, Th, U, and Pb (2 spectrometers) are measured as part of the monazite microprobe dating protocol; ±2σ variations in elements fixed for ZAF corrections do not affect the age outside of analytical uncertainty. ThMα, UMβ, and PbMα are the analyzed lines of the age components. Corrections for interference of ThMζ1,2 and YLγ2,3 on PbMα are significant, but can be done precisely, and reduce the precision of theMα analysis by a trivially small amount. ThMγ, M3-N4, and M5-P3 interferences on UMβ can be corrected, as well, but ThM5 and M4 absorption edges in high-Th samples make estimation of UMβ background problematic. Background fits for UMβ peaks show that linear vs. exponential fits for UMβ do not, in general, produce statistically significant differences in microprobe ages. However, linear vs. exponential background fits for PbMα peaks do produce significantly different ages, most likely because of (1) low Pb concentrations relative to U; (2) ThMζ1 interference on backgrounds between ThMζ1 and PbMβ; and (3) SKα and Kβ interference in S-bearing monazite. For 6-min analyses (3 min peak, 3 min background) at 25 keV and 200 nA, 1σ Pb precisions are approximately 3.4% at 1700 ppm and 9.5% at 750 ppm; at 15 keV, precision decreases by roughly 25% of the 25 keV value. These precisions are constant for fixed current, analysis time, and concentration, but the statistical precision of distinct populations of monazite grains (domains) is a function of the total number of analyses within the domain. Instrumental errors (current measurement, dead time, pulse shift, d-spacing change) add 1.10% to random errors, but errors caused by pulse shift and d-spacing changes can be accounted for and corrected. Decreasing accelerating voltage from 25 to 15 keV decreases ZAF correction factors by as much as 50% relative, but replicate age analyses of Trebilcock monazite at 15 and 25 keV are statistically indistinguishable. Grain orientation, miscalculated background intensity, uncorrected interferences, and surface effects also introduce systematic errors. Accurate background interpolation and interference correction reduces systematic error to approximately 5.20% in addition to random (counting) error. Microprobe ages (~420 Ma) and 208Pb/232Th SIMS ages (~430 Ma) of monazite from Vermont are in agreement to within ~10 m.y. The discrepancy between U-Th-total Pb microprobe ages and 208Pb/232Th ages is removed when the high background measurement for PbMα is shifted to the short-wavelength side of PbMβ, removing a possible ThMζ1 interference.