Constraints on the origin of Hawaiian lavas

Abstract

Geochemical and geophysical data can be used to constrain models of Hawaiian lava generation. Geochemical data, including radiogenic isotopes, major and trace elements, and transition metals, indicate that the source region of Hawaiian tholeiites must be within the stability field of garnet. The source material consists of at least three isotopically distinguishable components: depleted, enriched, and primitive mantle. Generation of parental Hawaiian magma by extensive fractional crystallization of high‐magnesium primary mantle melts is difficult to reconcile with high transition metal abundances of Hawaiian tholeiites. Rare earth element modeling indicates that the source of at least some Hawaiian tholeiites does not require a pre‐enrichment (metasomatic) event to generate melt. Where incompatible element abundances increase in lavas with no change in radiogenic isotopes (such as in the Waianae range or in the late shield‐building stage at Kohala), the degree of melting of the tholeiite source is decreasing. Where chemical changes are correlated with isotopic variations (as at Haleakala and Kauai), mixing between tholeiite magma and melt from depleted mantle is indicated. Geophysical evidence suggests that melting occurs at depths greater than 60 km. Thus extensive melting of shallow lithosphere to generate Hawaiian tholeiites is invalid. Melting of deep lithosphere is unlikely because the source of late shield‐building alkalic basalts would be severely depleted in incompatible elements. The origin of the Hawaiian Swell is still open to conjecture and cannot constrain whether melting of lithosphere or emplacement of low‐density material beneath the lithosphere occurs. The orientation of volcanic rift zones and the continuity of tholeiitic magmatism from one center to the next imply that the so‐called “periodicity” of melting in the source region is not critical but that the lithosphere itself exerts strong control over the spacing of volcanoes. Calculated eruption rates for the late shield‐building stage at Kohala indicate an inverse exponential dependence on melt fraction. A model consistent with the preceding constraints involves the partial melting of an upwelling deep‐mantle plume at about 60–100 km. The plume consists of primitive mantle, asthenosphere, and recycled oceanic crust, which become well mixed in the center of the plume. The degree of melting to generate tholeiites may be much higher in the plume than in “typical” mantle because of the high modal abundance of clinopyroxene (from recycled crust). A plume length of 60 km is required for an individual volcano if the degree of plume melting is 20%. Calculated plume ascent rates are very similar to rates of motion for the Pacific plate. Initially, tholeiites erupt almost simultaneously with alkalic lavas (Loihi). As shield building continues, only tholeiitic magmas reach the surface (Kilauea). As the volcanic center moves off the hot spot, either the degree of melting decreases leading to postcaldera or late shield‐building alkalic volcanism (Kohala), or increased mixing of depleted mantle melts with plume‐derived melts occurs (Haleakala). After a long hiatus, resulting from slow segregation rates for very small degrees of melting, nephelinitic volcanism may occur on the flanks of the shield (Haleakala, Kauai).