Why is the Holms-Houtermans Model Concave Down

For the Tera-Wasserburg plot and conventional Concordia plot, we measure a mineral (e.g., zircon) that is enriched in U and initially contains little unradiogenic (conunon) Pb. 

By dividing Eq. (12.19) over Eq. (12.18), we obtain the first derivative of y with respective to x 

Note that, according to Eq. (12.20), the slope dyjdx) of the line for a 

Since the second derivative is negative everywhere, the plot is concave down everywhere. 

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