Compositional range of primary tholeiitic magmas evaluated from major-element trends

The major-element trend for Hawaiian tholciites is well defined and may be represented by straight lines in a Bowen diagram. The trend can neither be related to olivine accumulation nor to olivine fractionation. Other phenocryst control of the trend is also unlikely. It is suggested that the range in magnesia content for primary Hawaiian tholeiites is from at least 13% MgO to above 20%, MgO. The major-element trend for abyssal tholeiites suggests that abyssal tholeiites with 8–9% MgO are primary magmas. The total possible range in magnesia content for primary tholeiitic magmas is considered to be from 8–9% MgO to about 20% MgO.