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

In terms of continental rifting centers being enriched in alkali minerals is because the lithosphere is concentrated with potassium and sodium alike. So as a continental rift or spreading center is forming, it has to melt the continental lithosphere which contains these elements and hence t becomes incorporated in the rock.

The word 'mature' I like to think about as the word 'stable'. When speaking about more mature basalts (High K) they're referring t them being less susceptible to chemical weathering than less mature basalts (High Ca). Less mature basalts are more reactive to their environment and break down more readily, chemically speaking. In nature Ca2+ is more chemically reactive than K+ and therefore less mature basalts have more Ca2+ because they will undergo more chemical alteration over time when compared to K+ rich basalts.