“T” in subscript in formulas
Interesting question.
It looks like this is a rather common notation in geology and refers to the total iron(II) and iron(III) oxide (e.g. ferrous and ferric) content.
For a chemist this is probably somewhat confusing as “total” is normally denoted via subscripted abbreviation “tot”.
Flipping through numerous sources, it appears there is no standardized notation and alongside with $\ce{FeO_T},$ $\ce{FeO_t},$ $\ce{FeOt}$ and $\ce{FeO^\mathrm{t}}$ are being used.

An introductory textbook by Shikazono [1, p. 21] explicitly defines $\ce{FeO^\mathrm{t}}$:


  $\ce{FeO^\mathrm{t}}$: Total iron $(\ce{FeO} + \ce{Fe2O3})$


Reference


Shikazono, N. Introduction to Earth and Planetary System Science: New View of Earth, Planets and Humans; Springer: Tokyo; New York, 2012. ISBN 978-4-431-54058-8.