In AN1.51-52, we find a description of the luminous mind. The Pali version can be found here.
The description in 51-52 implies that it is pre-existing. It's just that the ordinary worldling doesn't discern that it's actually present. But the noble disciples discern that it's actually present.
"yathābhūtaṃ" means "in truth; in reality; in its real essence."
"pajānāti" means "knows clearly".
We know that Nibbana is not conditioned (sankhara) and not impermanent (anicca).
But from the description, it sounds like the luminous mind, which is pre-existing in both worldlings and noble disciples, is always there, just that it is obscured by incoming defilements (kilesa).
Does this mean that the luminous mind is also not conditioned (sankhara) and not impermanent (anicca)?
“Pabhassaramidaṃ, bhikkhave, cittaṃ. Tañca kho āgantukehi upakkilesehi upakkiliṭṭhaṃ. Taṃ assutavā puthujjano yathābhūtaṃ nappajānāti. Tasmā ‘assutavato puthujjanassa cittabhāvanā natthī’ti vadāmī”ti.
“Pabhassaramidaṃ, bhikkhave, cittaṃ. Tañca kho āgantukehi upakkilesehi vippamuttaṃ. Taṃ sutavā ariyasāvako yathābhūtaṃ pajānāti. Tasmā ‘sutavato ariyasāvakassa cittabhāvanā atthī’ti vadāmī”ti.
“Luminous, monks, is the mind. And it is defiled by incoming defilements. The uninstructed run-of-the-mill person doesn’t discern that as it actually is present, which is why I tell you that—for the uninstructed run-of-the-mill person—there is no development of the mind.”
“Luminous, monks, is the mind. And it is freed from incoming defilements. The well-instructed disciple of the noble ones discerns that as it actually is present, which is why I tell you that—for the well-instructed disciple of the noble ones—there is development of the mind.”