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