So I have an iron planet, and its name is Randall.
Since Randall is an iron planet, he is basically the core of a planet with no crust/mantle, and very few silicates. Now, I, the creator outside of the universe from which Randall resides in, wants Randall to have plate tectonics, for at least a couple gigayears (billion years). However, iron planets cool pretty quickly, and iron is, relative to silicate, much more durable and has a much higher melting point, so I can't figure out how an iron planet could keep said plate tectonics for long. How could Randall have these plate tectonics for a couple of billion years?
Note: I realize there are other "forms" of tectonics, like pipe tectonics, but the main feature I am looking for is that material deposited on the surface can eventually find its way into the "mantle" and out a volcano, allowing for a "rock cycle"-like cycle, so if you can find an alternative to plate tectonics that can achieve that and work on an iron planet, then that would be fine as well.
Note 2: My definition of an iron planet is that the planet is mostly made of iron all the way up to the surface, but plenty of the pie chart is composed of other metals, like nickel, zinc, etc.
TLDR: How can an iron planet remain geologically active for at least a couple of billion years?