According to the Tantrik scriptures, what is the difference between Mantra-Diksha and Mantra-Chaitanya?
As per Tantra, mantra is fruitless unless Mantra-Chaitanya is obtained.
Mantra-Chaitanya and Mantra-Diksha are both related to the Guru who gives the Diksha. Other than that there is no link between them as such that there will be differences.
In other words those are not comparable entities. However both are required for the Mantra-Japa to be a success.
Lord Shiva says, without Mantra-Chaitanya, Japa bears no fruit.
mantrArtham mantra chaitanyam yonimudrAm na vetti yah |
shatakoti japenApi tasya siddhirna jAyate ||
One who chants mantras without knowing MantrArtha, Mantrachaitanya and YonimudrA, does not get Siddhi even after crores of chants.
KulArnava Tantram 15.60
Commentator, Upendra Kumar Das, explains thus:
MantrArtha - The existence of non-difference among the Guru, the deity and the mantra is called MantrArtha.
Mantrachaitanya - Chaitanya or consciousness remains in a latent state in both the mantra and the disciple [during the time of DikshA]. A Guru has several methods, such as the Dasha SamskAras, to energize the latent consciousness in both the mantra and the disciple.
Lord Shiva further says:
chaitanyarahitA mantrAh proktA varnAntu kevalam |
phalam naiva prajacchati laksha koti japAdapi ||
A mantra devoid of mantra-consciousness is nothing but an aggregate of alphabets; Even Lakhs or crores of Japas of such mantras bear no fruits.
KulArnava Tantram 15.62
There are as many as 60 impurities (or Doshas) that a mantra can have. And, an impure mantra is considered as devoid of consciousness.
To clear the mantra of those Doshas, the Tantras give 10 methods of purification.
kathyante dasha samskArA mantra dosha harAh priye |
jananam jivanam pashchAt tAranam vodhanam tatah ||
avishekohatha vimalikaranApyAyane tathA ||
tarpanam dipanam guptih samskArAh kulanAyike ||
O Beloved! the 10 purificatory methods of removing Mantra Doshas are being spoken of ---- TArana, Vodhana, Abhisheka, Vimalikarna, ApyAyana, Tarpana, Dipana and Gupti --- These are the Dasha SamskAras of Mantras, O Goddess of the Kulas.
KulArnava Tantram 15.72,73
So, a Guru, using the above mentioned 10 methods, and may be by the use of other hidden methods, infuse the mantra consciousness just before imparting it to the disciple.
This is what I know about Mantra Chaitanya.
For Mantra Diksha you can refer to this answer.