Cain and Abel, the sons of Adam, committed their own sin in addition to that of their father and thus caused their sparks to become more deeply immersed in the kelipot.

Cain murdered Abel, and according to the Midrash, Abel had (previously) looked at the Divine Presence as it descended to consume his sacrifice on the altar. Gazing such at the Divine Presence was deemed a grave sin and warranted for Abel the penalty of death. Thus, they caused further damage and blemish to the soul-sparks from the soul of Adam that were originally damaged by the sin of the First Man.

However, in each generation some of the sparks reincarnate into the world to become rectified.

That is good news. All do not end up like Cain and Abel. There is tikun.

Cain and Abel, the sons of Adam, increased the damage and blemish of the soul-sparks

The level of the "quarry" of the souls of a particular generation can be either that of the sparks of the head, or the eyes, etc.

The "quarry" [as in a rock quarry] is the place in the soul-structure of Adam from which the soul-sparks derive. This varies by the generation. It is like a spiritual quarry from which the sparks of the souls of a particular generation are "hewn." Some of these sparks reincarnate and become rectified, but some of them reincarnate, become more blemished because of sin, and fall deeper into the kelipot, like the souls of Cain and Abel, the sons of Adam.