Fluid-mineral interactions of apatite minerals — an indication of paleo-environment changes

There are few direct quantitative records of past climate (e.g., temperature, precipitation), so we make use of proxy—a physical, chemical, and/or biological entity that responds to climate—to indicate paleo-environment and paleo-climate. Geochemical data (such as elemental and stable isotope ratios) stored in minerals to infer environmental changes that associated with fluid activity can be used by modern techniques. Apatite is an ideal mineral for dating fluid–rock interaction and the cessation of fluid influence, because it is highly susceptible to fluid containing Ca, SO3, Na, Cl, P2O5, and PO3, causing changes of crystal chemistry and structures. Fractures in apatite can also promote fluid–mineral interaction and replacement of elements. The Liupanshan Basin is a Mesozoic-Cenozoic faulted sedimentary basin that located at the northeastern part of the Tibetan Plateau. Many paleoclimate studies have been carried out on the northeastern margin of the Tibetan Plateau through sedimentology, like pollen, grain size, chroma, and geochemistry such as carbon and oxygens isotopes, and other indicators to discuss the environmental variation history and its driving mechanism. Thermochronological studies have also been performed to limit the uplift and exhumation history, and the AFT ages data show a wide and mixed span, from 168.7 Ma to 7.3 Ma. It is not ruled out that the change of apatite element composition caused by fluid-mineral interaction may affect the AFT ages, hence, the physical and chemical characteristics of apatite collected from the Liupanshan Basin were studied through CL imagine, BSE imaging, and EMPA compositional analysis. Under the microscope, the alteration of minerals such as feldspar and plagioclase and the filling of carbonate veins can be clearly observed, indicating fluid-rock interaction. The apatite particles from the Cretaceous strata are mostly subhedral-anhedral, with developed fractures, and the phenomena of regeneration, transformation and re-transformation can be observed. The preliminary EBSD analysis results show that the content of F element in apatite subjected to fluid and later alteration increases, and the content of Cl element is relatively low. The SO3, Na, and PO3 significantly increased for the new growth apatite and fluid-mineral interacted apatite. Clearly, it shows that the apatite fission track ages (27~8 Ma) mark the new growth or interacted ages of the Cretaceous sedimentation, also show that this area is with the acidic fluid basin, corresponding to the uplift of the Tibetan Plateau and global climate change.