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?
Short answer
Na and K are incompatible in the mantle. Low degree melting that occurs deep in the mantle forms magmas enriched in Na and K. Magmas formed due to hot spots are formed deep in the mantle.
Long answer
Tholeiitic basalts are voluminous magmas that form due to decompression of mantle rocks. The mantle then melts to a rather large degree at low pressures to form a relatively reduced magma (therefore tholeiite and not calc-alkaline). Their relative depletion in incompatible elements stems from the fact that the mantle peridotite being melted is a recycled rock, which lost it's incompatible elements long ago.
On the other hand, alkali basalts are produced in smaller volumes than MORB (mid ocean ridge) tholeiitic basalts. OIB (ocean island basalts) are commonly alkaline, but alkaline rocks are not restricted to oceanic settings. They commonly occur on top of hot spots and rift zones, regardless of the nature of the lithospheric plate which they erupt though (oceanic vs continental).
I.e. Is it that the ions (Na and K) are more incompatible than the Calcium ions found in the tholeiites? Why is this?
Yes. Sodium and potassium are less compatible than calcium in minerals that comprise mantle rocks. This figure, taken from my answer to What are the high field strength and large ion lithophile (HFS or HFSE & LIL or LILE) elements?, shows why:
MRFE stands for Mantle Rock Forming Elements. These elements are compatible in olivine, pyroxene, garnet, spinel and anorthite. Na and K are only monovalent and larger than the MRFE. Thus, they are compatible and in cases of mantle melting, they are partitioned to the liquid (i.e. magma) phase.
In order to understand why this enrichment in Na and K occurs, we need to understand where in the mantle these magmas form. Whereas tholeiitic magmas form in shallow depths, alkaline magmas form much deeper. Experiments show that the silica undersaturated nature of these rocks requires very deep melting (tens of kilometers). Since the melting does not occur by decompression, it has to melt by heating: a hot spot. This hot spot melts mantle rocks that did not experience prior melting events by the "ocean conveyor belt", and thus still retain their incompatible elements, including Na and K.
There are two other possible sources of Na and K enrichment in alkali rocks.
why it is that alkali basalts are more 'mature' than tholeiites?
I'm not exactly sure what do you mean by 'mature' but I will address the timing of alkali magmatic activity. Here is a very simplified depiction of what happens:
At first (A), a young plume will still be deep. The melts that it forms are low degree and deep. Then, when the plume 'matures' (B), it causes shallow melting producing voluminous tholeiitic basalts. The margins of the plumes still produce low degrees alkali melts. When the plume wanes (or just migrates somewhere else), you get only alkali melts again (sorry, I was too lazy to draw C in the figure, but it should look not too different than A). This is similar to what's happening at the moment in Hawaii. The newest (still underwater) volcano, Loihi, erupts alkali magmas. This corresponds to stage A in the figure. This is also what the oldest rock on Hawaii (the island) are made of: alkali rocks. Most of the rocks by volume on Hawaii (again, the island) are tholeiitic basalts erupted some hundred thousands year ago, corresponding to stage B in the figure. Recent (geologically) eruptions are yet again alkaline, similar to stage C.
For some extra reading about alkali rocks and related subjects, see these other questions and answers:
Geology/Petrology has proposed some models for the origin of basalts which are based upon chemical evidence from rocks. Modern origin-models have been constrained by isotopic chemistry for some time. I do not claim to be current in this subject, but I believe the conclusions in this classic paper are still accepted as the most likely origin models.
Models of Earth Structure Inferred from Neodymium and Strontium Isotopic Abundances
G. J. Wasserburg and D. J. DePaolo Proceedings of the National Academy of Sciences of the United States of America Vol. 76, No. 8 (Aug., 1979), pp. 3594-3598 Published by: National Academy of Sciences Article Stable URL: http://www.jstor.org
Free copies of this paper are available in several place from the Web. Here is the brief abstract.
ABSTRACT: A simplified model of earth structure based on the Nd and Sr isotopic characteristics of oceanic and continental tholeiitic flood basalts is presented, taking into account the motion of crustal plates and a chemical balance for trace elements. The resulting structure that is inferred consists of a lower mantle that is still essentially undifferentiated, overlain by an upper mantle that is the residue of the original source from which the continents were derived.
