Why are alkali basalts found at rifting centres and on top of 'plumes', whereas MORBs are generally tholeiitic?

Question

So I was essentially wondering why it was that the basalts on top of 'plumes' at 'hotspots' (sometimes called OIBs, ocean island basalts) and at continental rifting centres are enriched in alkaline metals (Na and K) compared to the tholeiitic MORBs. Also, I was wondering if someone could explain to me chemically why it is that alkali basalts are more 'mature' than tholeiites? I.e. Is it that the ions are more incompatible than the calcium ions found in the tholeiites? Why is this?

Answer 1

Hi Asked,

As I conceptualize it, tholeiitic series rocks are differentiated (matured, if you will) from general mantle magma composition by having been first partially melted (and held for a while to allow mineral separation) and then reacted with oceanic crustal rock, which reduces them (turns the iron -> +2 state, etc.). If magma has not been allowed to undergo these two processes, b/c it is rapidly moved to the surface by a hot-spot, then it retains its original bulk composition, which is Na/K rich, and oxidized.

-- Cheers, -- Mr. d.