Abstract:

Barite, a non-metallic mineral primarily composed of barium sulfate(BaSO₄), is widely distributed in nature. Its electron spin resonance(ESR) signal( {\mathrm{SO}}_3^{-}) has been used to date geological events such as hydrothermal activity and tectonic movements. In recent years, ESR dating of barite has been extensively applied in seafloor hydrothermal systems; however, studies on continental barite remain limited. This gap highlights the need for fundamental research into the properties of ESR signals in continental settings, providing a basis for assessing the feasibility and reliability of ESR dating.

This study investigates sedimentary barite(XBD-B1, LSJ-B1) and fault-related barite(LJF-B1) from continental environments, focusing on the ESR signal at g=2.002 3. Key properties examined include microwave saturation power, photosensitivity, thermal stability, and signal saturation. Equivalent doses were calculated and compared with results obtained from the g=1.99${9}^{}$95 signal. The main findings are as follows:

(1)The microwave saturation power of the ESR signal(g=2.002 3) in continental barite ranges from 0.2 to 1mW, with 0.1mW identified as the optimal power for accurate signal measurement.

(2)The ESR signal(g=2.002 3) can be fully annealed after heating at 380℃ for 15 minutes. Its thermal decay follows a second-order kinetic model, with a thermal lifetime at room temperature(20℃) of at least 10⁶ years. Under irradiation of ~1${0}^{}$0000Gy, both g=1.99${9}^{}$95 and g=2.002 3 signals remain unsaturated, suggesting high saturation doses. Considering thermal stability, signal saturation, and a 10% decay allowance, the maximum measurable ages for the ESR signals are 152Ma(XBD-B1), 72Ma(LSJ-B1), and 1.5Ma(LJF-B1).

(3)For fault-related barite, the thermal lifetimes and equivalent doses obtained at g=1.99${9}^{}$95 and g=2.002 3 are consistent within error, indicating that results from these two measurement positions can complement and validate each other. Integrating both enhances dating reliability.

(4)The ESR signal at g=2.002 3 in continental barite exhibits photosensitivity similar to Al centers in quartz, with partial bleaching possible. Thus, fresh, unexposed samples should be collected, stored in darkness, and sealed. If only sun-exposed samples are available, non-bleachable signal components should be extracted for dating.

In summary, the ESR signal of barite(g=2.002 3) is suitable for dating the crystallization age of fault barite since the middle to late Early-Pleistocene, demonstrating notable reliability. This study identifies barite as a promising material for ESR dating of bedrock fault activity and as a robust absolute dating method for constraining the depositional ages of barite deposits.

Key words: sulfate, barite, ESR dating, decay dynamics, thermal stability, photosensitivity

摘要： 重晶石(主要成分为BaSO₄)是自然界中常见的硫酸盐矿物, 其电子自旋共振(electron spin resonance, ESR)信号( {\mathrm{SO}}_3^{-})可用于测定海底热液活动、 断层运动等地质事件的年代。近年来学者们将ESR方法广泛应用于海底热液场重晶石结晶年代的测定, 但大陆型重晶石ESR测年的相关报道较少, 急需对其ESR信号进行测年基础性质的相关研究, 为大陆环境中重晶石的ESR测年可行性及可靠性提供参考。因此, 文中以大陆环境中沉积重晶石(XBD-B1、 LSJ-B1)及断层重晶石(LJF-B1)为研究对象, 探讨了重晶石ESR信号(g=2.002 3)的微波饱和功率、 光敏感性、 热稳定性、 信号饱和性等基本性质。结果表明: 1)大陆环境中重晶石样品的ESR信号(g=2.002 3)微波饱和功率在0.2~1mW之间, 0.1mW是该信号测量的优选功率。2)大陆环境重晶石样品ESR信号(g=2.002 3)在380℃下加热15min能够完全退火, 信号的热衰减符合二级动力学模型, 常温(20℃)下的热寿命至少能达到10⁶a; 饱和剂量>10 000Gy, 在低环境剂量率的条件下, 综合考虑饱和寿命及热寿命, 2个沉积型重晶石及断层重晶石ESR信号的最老可测年龄分别为152Ma、 72Ma、 1.5Ma。3)断层重晶石ESR信号g=1.999 5和g=2.002 3的热寿命和等效剂量在误差范围内一致, 表明2个测量位置的测年结果具有相互补充和验证的潜力。在实际测年应用中可以综合考虑2个信号测量位置的年龄结果, 以提高结果的可靠性。4)大陆环境中重晶石样品的ESR信号(g=2.002 3)光敏特征类似石英中的Al心, 能够被部分晒退, 应尽量采集未曝光的新鲜样品, 若仅能采集暴露于阳光下的样品, 则应提取不可晒退的信号组分测年。综合来看, 重晶石ESR信号(g=2.002 3)能够用于早更新世中晚期以来断层重晶石结晶年代的测定, 具有一定的可靠性。这为基岩断层活动历史研究提供了一种潜在的测年材料与手段, 也为重晶石矿床的沉积年代测定提供了一种有力的绝对测年手段。

关键词: 硫酸盐, 重晶石 ESR信号, 衰变动力学, 热敏感性, 光敏特性