GEOL3020 lab exercise: Evaluating the intensity of chemical weathering

Given: Data set containing depth, density, and chemical analyses for samples collected from a weathering profile formed on the Elberton Granite.

Photo of weathering profile

Photo of Parent rock and saprolite

Photo of B horizon

Click here to download the Excel spreadsheet that contains the data.

Complete the following calculations in the spreadsheet.

1. Assume that all weathering is iso-volumetric (i.e., each unit volume of weathered rock is the same as a unit volume of parent rock) and there is no pore space in the parent rock. How much porosity is formed in each sample? Use bulk density (ρ) values.

% Porosity = 100 x [1 - (ρ _weathered/ ρ _{parent rock})]

=100*(1-C17/$C$17)

2. The strain factor (S) is simply the ratio of parent rock density to the weathered rock density assuming one element is conserved (i.e., the conservative element is not lost or gained by chemical reaction). We will choose Ti to be our conservative element. Why is this our assumption? Calculate the strain for each horizon.

S = [(ρ_{parent rock} x C_{Ti-parent rock}) / (ρ_weathered x C_Ti-weathered)] - 1

=(($C$17*$C$12)/(C17*C12))-1

3. The mass transferred in and out of each volume of weathered rock can be compared relative to the parent rock. The mass transferred for each component (T_i) can evaluated with the following relationship,

T_i = [(ρ_weathered x C_i-weathered) / (ρ_rock x C_Ti-rock)] x (S + 1) - 1

=(((C$17*C4)/($C$17*$C4))*(C$18+1))-1

4. Using the Excel graphing function to plot T_i versus depth of horizon for the following components

1. SiO₂
2. Al₂O₃
3. CaO
4. MgO
5. Na₂O
6. H₂O / 10

5.      Explain your results in terms of what you know about weathering processes and the assumptions made.