Dear Ketan,

3.062.10a    tát savitúr váreṇiyam
3.062.10b    bhárgo devásya dhīmahi
3.062.10c    dhíyo yó naḥ pracodáyāt

You can find this and other IAST version of the metrically restored RV at:
http://www.utexas.edu/cola/centers/lrc/RV/

This is the form of the original Gayatri mantra from the Rg Veda but later gayatri mantras follow a different form.

. . . . . . . . . . .vidmahe

. . . . . . . . . .. dhimahI

tan naḥ X pracodayāt

1) Are the Gayatri mantras from the Taittirīya Āraṇyaka book 10 (Mahānārāyaṇa Upaniṣad) the earliest Gayatri mantras in this later form.?

Given the sacredness afforded to the Gayatri mantra from the Rg Veda, does any of the Sanskrit literature say anything about why the later Gayatri mantras follow this different form?

Thanks,

Harry Spier