To find the number of neutrons in a dipositive zinc ion (Zn$^{2+}$) with a mass number of 70, we first need to understand what each term means and then perform the calculation:

1. Mass number: This is the total number of protons and neutrons in an atom's nucleus. For this zinc ion, the mass number is given as 70.

2. Zinc's atomic number: The atomic number of an element represents the number of protons in the nucleus of an atom of that element. For zinc, the atomic number is 30, which means every zinc atom has 30 protons.

3. Dipositive zinc ion (Zn$^{2+}$): This indicates a zinc ion that has lost two electrons, becoming positively charged. However, the loss of electrons does not affect the number of protons or neutrons in the nucleus, so the atomic and mass numbers are unaffected by the ion's charge state.

To find the number of neutrons in the zinc ion, we subtract the number of protons (atomic number) from the mass number:

$ \text{Number of neutrons} = \text{Mass number} - \text{Atomic number} $

For the zinc ion in question:

$ \text{Number of neutrons} = 70 - 30 = 40 $

Therefore, the number of neutrons in the dipositive zinc ion with a mass number of 70 is 40, which corresponds to option D.