团簇同位素的基本原理与地质应用

Basic principle of clumped isotopes and geological applications

团簇同位素指的是含有2个及2个以上的重同位素结合在一起形成的同位素体。团簇同位素的数值定义为同位素体的相对丰度偏离随机分布状态的程度。测量该相对丰度较低的同位素体需要高精度的质谱仪,难点在于利用同位素组成已知的参考气体和不同同位素组成的加热气体,以获得绝对参考体系下的数值。团簇同位素体的相对丰度非常低,但是具有非常独特的物理和化学性质。比如碳酸盐矿物中^(13)C^(18)O^(16)O的丰度对温度具有敏感性,而与矿物的全岩同位素以及矿物形成时期的流体性质无关,因此可以利用测量的碳酸盐团簇同位素来获得矿物的生长温度,再利用矿物的氧同位素(δ^(18)O),根据传统的氧同位素温度计原理,可以进一步获得矿物的生长流体(水)的氧同位素。目前,团簇同位素温度计已经在古气候(温度)重建、古高度恢复、碳酸盐岩的成岩作用以及甲烷的成因分析等方面得到了广泛应用。评估深埋高温过程引起的C-O化学键重置对碳酸盐团簇同位素的影响、测试仪器产生对团簇同位素的非线性误差校正、以及其他丰度更低的团簇同位素体或大分子的团簇同位素的测量,是下一步的研究方向。

Clumped isotopes are isotopologues containing two or more than two rare isotopes clumping together. The clumped isotope value is used to denote the deviation extent from the stochastic distribution for a certain isotopologue. It is necessary to use a high resolution mass spectrometry to measure an isotopo- logue with relatively low abundance. The key is to obtain samples＇ clumped isotope values in the absolute reference frame by choosing a worked gas with known isotopic composition, and heated gases with different isotopic compositions. The relative abundances of clumped isotopes are extremely low, but clumped iso- topes have peculiar physical and chemical property. For example, carbonate clumped isotopes （ 13C 18O16O）are sensitive to their growth temperature, in dependent of carbonate bulk isotopic compositions and the ox- ygen isotope of the growth fluid. As a result, the growth temperature can be easily obtained using the measured clumped isotope value, and the oxygen isotope of the growth fluid can also be obtained combined the bulk carbonate oxygen isotope data using the conventional oxygen paleothemometry model. The clumped isotope paleothermometry has been widely used in the paleoclimate reconstruction, the paleoelevation esti- mation, carbonate diagenesis, and distinguishing methane origin. The future research focuses include： E- valuation the effect of carbonate C-O reordering resulting from large burial depth and relatively high tem- perature, the nonlinearity between actual and measured value produced by the mass spectrometry, the method of measuring clumped isotopes with even more lower relatively abundance or clumped isotopes of larger molecules.