关键词: 磷灰石, 氦扩散参数, 封闭温度

Abstract:

In this paper we present the detailed procedure to calculate the helium diffusion parameter and its closure temperature in apatite. We use the step-heating data of Wolf (1996）to show this procedure and its correction. This method is based on the Arrhenius relationship which describes the diffusivity variation with temperature. This procedure is as follows: (1) Do step-heating experiment to get the cumulative gas release fraction. (2) Calculate the lnD/a² of every step using spherical diffusion equation. (3) Make Arrhenius plot, taking the lnD/a² as y-axis and the 1/T (T is the temperature of every step) as x-axis. In this plot, if the points defined by lnD/a2 values and the 1/T values within some temperature range show linearity, it indicates that within this temperature range the apatite diffusion obeys the Arrhenius relationship and thus we can get the diffusion parameter E from the slope and the LnD0/a ² from the intercept in the Arrhenius plot. (4) Insert the value of E and LnD0/a ² into the closure temperature equation to calculate the closure temperature using iterative method. In addition, when we use the above method to calculate the helium diffusion parameter, we should pay attention to the following aspects: (1) Because 1/T is decimal fraction, for apatite diffusion data, 1/T should be multiplied by 104, making it the same order with ln D/ a². (2) When calculating the closure temperature, the unit of the cooling rate ℃/Ma should be converted to K/s. For the cooling rate, 1 ℃/Ma equals to 1 K/Ma.

Key words: apatite, diffusion parameter, closure temperature