"...Earth's interior is known for by its high temperatures where solid carbonates melt, creating distinctive seismic patterns."
The whole thing is a bit incoherent.
That certainly would melt lower than carbon at its triple point.
Carbon oxygen molecules shift from carbon dioxide to carbon trioxide (i.e. carbonate), depending on pH and other factors. You could say that carbonate is the salt or acid form of CO2.
Carbonate is an important molecule in our blood. It is produced by metabolism of larger biomolecules. When our blood becomes too acidic, we increase our breathing rate to exhale more CO2 and bring our blood pH back to neutral. When our blood is too caustic, our kidneys excrete more carbonate into the urine to neutralize blood pH.