A comparative review of magnesium isotope analysis methods for emerging geological applications: Double-spike vs. standard-sample-bracketing

The sample-standard-bracketing (SSB) method and double-spike (DS) method are two representative techniques in isotopic analysis, widely employed for calibrating the instrumental mass biases and achieving ultra-high precision measurement results across various isotopic systems. The SSB method is a technique that involves positioning a set of standards before and after the test sample during analysis, achieving a precision of ±0.06‰ (2SD). The DS method is a technique involving the introduction of two additional spikes into the unknown sample. By measuring the isotopic compositions of this spiked mixture, the true value of the unknown sample can be accurately calculated, yielding a long-term  precision of ±0.03‰ (2SD). The DS method is less influenced by concentration and memory effects, without the rigorous concentration matching required by the SSB method. The SSB method struggles to precisely match the matrix of unknown samples to the bracketing standards, leading to a potential discrepancy in δ²⁶Mg data between the two methods.

In the conventional DS approach, the double spikes are introduced prior to the chemical purification step. This allows for the correction of mass-dependent fractionation that occurs during the purification process. Alternatively, to minimize the potential accumulation of errors from repeatedly purifying samples in iterative analyses, the Mg double spikes are incorporated after the chromatographic purification stage.

At elevated temperatures (>300 K), isotope fractionation between two phases decreases with increasing temperature. This results in only minor isotopic fractionations during high-temperature geological processes. If two phases are in thermodynamic equilibrium, the phase with a smaller coordination number typically has shorter and stronger bonds with higher vibrational energy. This leads to the enrichment of heavy isotopes in the phases with the smaller coordination number. garnet tends to have lighter Mg isotopes, whereas spinel exhibits heavier Mg isotopes compared to minerals with six-fold coordinated. Given this context, distinct mineral assemblages can serve as a novel thermometric tool, which could provide essential information for the thermal history of rocks and tectonic units. an effective Mg isotopic thermometry should exhibit measurable isotope fractionation between rock-forming minerals relative to analytical precision. The improvement of accuracy can effectively reduce temperature measurement errors. Although the SSB method achieves sufficient precision for analyzing garnet, pyroxene, and olivine, the DS method demonstrates superior precision in measuring inter-mineral isotope fractionation, thereby significantly improving the accuracy and reliability of temperature determination results.

The exceptional capability of the DS method in resolving subtle-scale Mg isotope fractionation establishes a robust foundation for its broader applications across diverse geological systems. More pivotally, extending the DS method applications to more comprehensive standard samples will enable in-depth investigations into the fundamental discrepancies between DS and SSB methods in Mg isotopic analyses. This systematic approach not only provides profound implications for validating the consistency and accuracy of inter-method results, but also drives transformative advancements in high-precision Mg isotope ratio analysis.

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