Heating graphene oxides to such high temperatures leads to a loss of almost all oxide functionalities by formation of CO and CO2, a process which already peaks at 625K. That means your annealing completely desoxidized the GO part of your sample and all you have left are defected graphene flakes (unless they were so small that they also desorbed). This process is sometimes referred to as "thermal reduction", however I don't like that term because the carbon grid is not actually reduced but just torn apart at most oxygen binding sites.

During my PhD time I did quite some work on desorption of all kinds of oxide species from various sized PAH films:

Article Formation of Sublimable Nanographene Oxides by Reacting Coro...

Article Graphite Mediated Oxidation of Coronene Adsorbates: A UHV Study

Article Comparative Study of Direct and Graphite-Mediated Oxidation ...

Regarding COx desorption, please refer to the supplement of the first paper in particular. An important XPS reference work in this regard would be this one from Elettra:

Article Atomic Oxygen on Graphite: Chemical Characterization and The...

Addition: Carbon is able to reduce P2O5 and PO43- and will form CO2

Article Reduction Mechanism of P2O5 in Steel Slag

Article Reduction of phosphate ore by carbon: Part II. Rate limiting steps

That means even your apatite might not be apatite anymore.