Aspects of Basalt Petrology
Authors: Poldervaart Arie 
Published: November 2024

The tetrahedron silica-nepheline-forsterite-diopside contains the essential elements for the chemical definition of alkali basalts and tholeiites, particularly if ferrous iron is included in olivine and diopside. Within the tetrahedron, a silica-saturation plane (albite-orthopyroxene-diopside) separates tholeiites with normative quartz from silica-deficient basalts, a silica-undersaturation plane (albite-olivine-diopside) separates alkali basalts with normative nepheline from basalts without normative nepheline, and a third plane separates alkali basalts from olivine tholeiites. The third plane passes through the olivine and diopside corners of the tetrahedron, and through a point O that marks the intersection of the join albite-enstatite and the line through forsterite and the peritectic point of the system silica-albite-forsterite. A simple formula is derived for this important plane. Basalts fall approximately in the centre of the tetrahedron; alkali basalts on the nepheline side of the silica-undersaturation plane and also in a narrow wedge-shaped region on the silica side of the plane, tholeiites in the remainder of the tetrahedron. High-alumina basalts include both alkali basalts and tholeiites.

Upon fractional crystallisation, alkali-basaltic magmas trend toward a silica-deficient residue, tholeiitic magmas toward a silica-oversaturated residue. The two most important processes that cause a change in the normal crystallisation trend of alkali-basaltic magmas to a trend of silica enrichment, appear to be assimilation of silicic rocks and oxidation of ferrous to ferric iron. The most important process that causes the normal crystallisation trend of tholeiitic magmas to change to a trend of silica impoverishment involves assimilation of carbonate rocks. Analcite is associated with alkali basalts and with tholeiites. Analcite in alkali basalts is probably silica-deficient and may have formed at higher temperatures and pressures than the silica-saturated analcite associated with tholeiites.

Tholeiitic magmas are identified as the primitive basaltic magmas that formed in the mantle throughout geologic time. Alkali-basaltic rocks show circumscribed time-space relationships toward tholeiitic rocks in Hawaii, Japan, South Africa, and India. They also do not seem to extend back in geologic time beyond about 1000 × 106 years, but this conclusion is as yet only tentative. Experimental work indicates that the various division planes in the tetrahedron do not exist at high pressures, due to the formation of diopside-jadeite solid solutions. At depths of 50-150 km, tholeiitic or alkali-basaltic magmas may form by partial melting of the same peridotitic source rocks, depending solely on temperature-pressure conditions. Alkali-basaltic magmas form at higher pressures and higher or lower temperatures than tholeiitic magmas. This interpretation appears adequate to account for the distribution of the two types of basalts in the examples cited. If it is true that there are no ancient alkali basalts, it may indicate that temperatures in the mantle were higher prior to 1000 × 106 years, or that activity in the mantle was shallower than in later geologic time.