(Too long for a comment) Replacing $\xi$ with a variable $x$, Mathematica gives the following:
\begin{align*} \Delta(2)&=-1+x\\ \Delta(3)&=(-1+x)^3 x (1+x)\\ \Delta(4)&=(-1+x)^6 x^4 (1+x)^2 \left(1+x+x^2\right)\\ \Delta(5)&=(-1+x)^{10} x^{10} (1+x)^4 \left(1+x^2\right) \left(1+x+x^2\right)^2\\ \Delta(6)&=(-1+x)^{15} x^{20} (1+x)^6 \left(1+x^2\right)^2 \left(1+x+x^2\right)^3 \left(1+x+x^2+x^3+x^4\right) \end{align*} etc...
There is a nice pattern: for $d\ge3$, $\Delta(d)$ contains as factors exactly the cyclotomic polynomials of degree 1 to $d-1$, and $x$, raised to some powers.