That sounds like a perfect place doe waste. Just pump it I to the mantle. Put this stuff back in the ground where it belongs. Better than in out air or floating around our water.
Reminds me of one of my favorite colony buildings in Master of Orion 2:
"Core Waste Dumps take man-made toxic and polluting agents and stash them deep within the planet. Since they’re so far below surface water supplies and often destroyed by the intense pressures and temperatures at the fringe of the molten core, this completely eliminates all Pollution on the planet." https://masteroforion.gamepedia.com/Core_waste_dump
What a game! I used to make massive spherical space-ships with annihilation beams and the computer would make about 300 small ship fleets. The turn-based combat meant my 5 ships would kill 5 ships with my huge death beams, then all 295 of their small ships would fire each of their 3 dinky peashooters, then 885 shots later, I'd destroy 5 more, then 90 of their ships... Fast forward to today, and I'm setting my data analyses to run for 2 hours and walking away just like I did back then with battles.
Surely this would result in the high temperatures of the mantle burning the waste; the air pollution associated with this must make this a poor option for waste disposal.
From my understanding when things burn hot enough -- which I think the mantel burns hot enough -- this sort of thing is not that big of an issue.
Also, I am not entirely sure this would raise the temperatures of something so massive. In any case it would probably become the "global mantel worming" debate.
This statement is not strictly true but in context correct. Any volcanic eruption is full of really nasty gasses like carbon monoxide, sulfur, chlorine, and even fluorine, to name just a few. This comes from the content of the earth already (well, all the garbage comes from the earth originally also). But like you implied, adding the tiny amount of garbage generated by humans to the mix to an entire crustal column of rock would have insignificant impact on what comes out on top.
It's sad but it's not the most worrisome plastic in the ocean.