A Robust LG‐SIMS Method for Sr Isotope Determination in Apatite Across a Wide Sr Concentration Range,Geostandards and Geoanalytical Research

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A Robust LG‐SIMS Method for Sr Isotope Determination in Apatite Across a Wide Sr Concentration Range
Geostandards and Geoanalytical Research ( IF 2.7 ) Pub Date : 2021-02-23 , DOI: 10.1111/ggr.12377
Heejin Jeon ₁ , Martin J. Whitehouse ₁

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Apatite provides a powerful means to determine the Sr isotopic composition (⁸⁷Sr/⁸⁶Sr) of ancient crust as it contains enough Sr to be measured accurately and precisely while typically having little post‐crystallisation radiogenic ingrowth due to its low Rb. Although utilising LG‐SIMS is beneficial for small‐volume targets, for example, inclusions, it is technically challenging to measure Sr isotopes in apatite with good accuracy and precision by SIMS, mainly due to molecular isobars interfering on Sr isotopes that have not previously been thoroughly assessed. This study evaluates all theoretically possible interferences affecting Sr isotope analysis of apatite by SIMS and develops a principle protocol for analysis and data processing that can be applied across a wide range of Sr concentration, even lower than 100 µg g⁻¹. With careful control of the peak shape and applying precise mass centring at an intermediate mass resolution (ca. 4500), only interferences from the apatite matrix‐produced Ca dimers and ⁴⁰Ca³¹P¹⁶O (present as a low mass tail) were found to be significant for the low trace element reference apatite used in this study, both of which can be precisely measured and corrected for. Despite this comprehensive approach, the Ca₂ correction remained inadequate, possibly due to as yet unidentified doubly charged molecules interfering at the reference mass 82 peak (⁴⁰Ca⁴²Ca). While this can be reduced by employing an energy offset of at least −25 eV, precision significantly worsens in low Sr apatite (ca. 0.0024 in 1SE compared with ca. 0.0010 without offset for < 100 Sr µg g⁻¹ apatite). Use of an empirical correction for the residual mass 82 interference is therefore preferred to an energy offset analytical approach, especially for low Sr apatite. Lastly, a slightly positive offset (ca. +0.0007) of unknown origin in ⁸⁷Sr/⁸⁶Sr was consistently observed across sessions with different analytical conditions, which may also be empirically corrected. The final corrected ⁸⁷Sr/⁸⁶Sr of the reference materials show good accuracy and precision with repeatability from ca. 0.0010 to 0.0002 in 1SE obtained for Sr contents from < 100 to 1500 µg g⁻¹.

中文翻译：

宽广的Sr浓度范围内可靠的LG-SIMS方法测定磷灰石中Sr同位素

磷灰石为确定Sr同位素组成提供了有力手段（⁸⁷ Sr / ⁸⁶古代地壳的Sr），因为它含有足够的Sr，可以精确地测量，而由于Rb低，通常结晶后的放射源向内生长。尽管利用LG‐SIMS对小体积目标（例如夹杂物）有利，但SIMS准确，准确地测量磷灰石中的Sr同位素在技术上具有挑战性，这主要是由于分子等压线干扰了以前从未有过的Sr同位素经过彻底评估。这项研究评估了影响SIMS磷灰石Sr同位素分析的所有理论上可能的干扰，并开发了可用于宽范围Sr浓度（甚至低于100 µg g ^-1）的分析和数据处理原理协议。通过仔细控制峰形并在中等质量分辨率（约4500）下应用精确的质量中心，仅发现了磷灰石基质产生的Ca二聚体和⁴⁰ Ca ³¹ P ¹⁶ O（呈低质量尾巴）的干扰对于本研究中使用的低痕量元素参考磷灰石具有重要意义，两者均可精确测量和校正。尽管采用了这种综合方法，但Ca ₂校正仍然不足，可能是由于尚未识别的双电荷分子干扰了参考质量82峰（⁴⁰ Ca ⁴²钙）。虽然这可以通过采用能量至少-25伏特的偏移被减小，精度显著恶化在低锶磷灰石（CA。0.0024在1SE相比CA。0.0010而不用于胶版<100微克的Sr克^-1磷灰石）。因此，相对于能量偏移分析方法，尤其是对于低Sr磷灰石而言，最好采用经验校正法对残余质量82的干扰进行校正。最后，在具有不同分析条件的会议中，始终观察到⁸⁷ Sr / ⁸⁶ Sr未知来源的略微正偏移（大约+0.0007），也可以根据经验进行校正。最终更正⁸⁷ Sr / ⁸⁶参考材料的Sr表现出良好的精度和精密度，并且可重复性从ca左右。Sr含量从<100至1500 µg g ^-1，在1SE中获得0.0010至0.0002 。

更新日期：2021-02-23

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