火山灰年代学:原理与应用

Tephrochronology: Principles and applications

火山灰(<2mm)等时标志层是火山灰年代学研究的基础。火山灰地层的直接或间接测年依赖于火山灰中存在合适的原生矿物(如:透长石、锆石、磷灰石)或外来物质(如:炭化木)。然而大多数火山灰沉积层,尤其是远源火山灰,其主要由岩浆源碎屑构成,开展测年显得十分困难。利用火山玻璃(岩浆)的地球化学,是一种被广泛应用的替代性方法。火山玻璃作为岩浆源物质普遍出现在各类近源、远源可见火山灰层和显微火山灰中,其主量、微量元素含量和特定的同位素组成能被电子、激光和离子探针技术准确测定。近源-远源玻璃化学成分的相互匹配,使得火山灰等时标志层得以在时间和空间上相互关联,构成有力的地层学工具,用以同步各类环境、考古和火山活动记录。近源火山沉积和远源火山灰层中的年龄信息得以实现交叉验证和整合。远源海洋、湖泊沉积中的相对年龄信息(氧同位素地层学,年纹层年代学)也为揭示火山活动、古环境变化和考古学事件的时间顺序提供了可能。火山灰年代学在地球科学中应用广泛,尤其是对于第四纪研究,其在同步各类记录从而评估火山活动、气候/环境变化和人类演化/迁移的因果关系中具有至关重要的作用。

Tephrochronology is based on the study of volcanic ash （〈2mm） layers which are effectively time equivalent horizons. Direct or indirect dating of ash layers is possible if suitable fresh ＂ primary＂ phenocrysts （e. g. sanidine, zircon, apatite） or extrinsic dateable materials （e. g. charcoal） exist. But in the majority of deposits, especially distal tephra deposits, this is not possible as they are dominated by juvenile clasts. A widely used alternative is the geochemistry of volcanic glass （magma） that occurs as juvenile （magmatic） particles found in both visible ash layers （ 〈 2mm） and invisible cryptotephra particles （usually 〈 ~ 125 microns） in proximal and distal deposits. These fresh glasses can be analysed for major, minor aad trace elements and specific isotopes （e. g. Pb） using electron, laser and ion probe technologies. Proximal-distal matching of chemistries allows for time equivalent horizons to be linked in both time and space thus providing a powerful correlative tool that helps synchronise distal and proximal environmental, archaeological and volcanic archives. Vital age information exists in both proximal volcanic deposits （]4C, 4~Ar/39Ar, U-Th） and distal ash layers allowing for cross-validation and integration of age information. The addition of relative age information （ i. e. , stratigraphy, annual layers） in distal marine and lacustrine deposits offers a powerful framework for unravelling the precise ＂order＂ of volcanic, environmental and archaeological event. Tephrochronology is applied across Earth Sciences and is particularly useful in Quaternary research for the synchronisation of multiple archives in the evaluation of causative links between volcanic eruptions, climate/environmental change and human evolution/migration.