APPLICATION OF THE IN SITU COSMOGENIC NUCLIDES 10Be AND 26Al FOR STUDIES OF FORMATION AND EVOLUTIONARY HISTORIES OF THE EARTH SURFACE
10Be和26Al在地表形成和演化研究中的应用

The surface of the earth is continuously undergoing changes as a result of weathering and erosion, plate tectonics and volcanic processes. Continental denudation with its complex rock-water interactions is the central process of global biogeochemical cycling of elements. Rates of denudation depend on variety of factors,in particular rock properties and chemical composition, climate (especially rainfall),structure, and elevation. Thus they are quite variable on a regional scale. A primary step in most geomorphological studies is to gain understanding of the location-specific weathering and erosional processes. An important recent contribution to the Earth sciences has been made with the development of a cosmogenic nuclide method applicable for the study of divers geomorphic processes and histories. The basic principle of application of the in situ cosmogenic radionuclides ~(10)Be (half-life = 1. 5Ma)and ~(26)Al (half-life = 0.71Ma) in quartz for study of the processes on the earth surface is given in this paper.The basis of the application of the in situ cosmogenic nuclides in the Earth sciences is their production in solids by particles of the cosmic rays. The production rates of the in situ cosmogenic nuclides depend on the cosmic ray flux at their situation as well as composition and density of the target material. They are different under different irradiation conditions, such as altitude, latitude, irradiation geomeny,and type of rock. Relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons~3]. Absolute nuclide production rates cannot generally be predicted with any accufacy because of lack of data on excitation functions of nuclides. Based on a natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for about 11 ka were sampled in Sierra Nevada, USA, the absolute production rates of ~(10)Be and ~(26)Al in quartz have been accurately estimated for different mountain altitudes~11]. Therefore, the absolute production rates of ~(10)Be and ~(26)Al in quartz on surface can be estimated fairly accUrately for any given latitude and altitude~5]. Within a rock, the production rates decrease exponentially with depth. The absorption mean-free-path of cosmic ray in common rock types is of the order of 150 - 170g cm~(-2), about 50 - 60cm in typical rocks. This is the distance in which the cosmic ray flux is reduced by a factor of "e",due to nuclear interactions which absorb the cosmic radiation energy.For a given rock (or mineral) and its location, the concentration of a nuclide produced by cosmic ray depends only on (1) the half-life of the nuclide, (2) the geometry of cosmic ray irradiation of the rock during the rock's exposure history, and (3) the exposure duration.Based on the production rates of ~(10)Be and ~(26)Al and their decay consents, their concentrations can be modeled with rock's exposure duration, erosion and sedimentation ates, and ele