Chemical and isotopic variations in Mauna Loa tholeiites

Abstract

A suite of basaltic lavas of known eruption age from Mauna Loa, Hawaii, has provided the basis for a detailed investigation of the evolution of the melting process beneath Hawaii. High-precision 238U 230Th 226Ra measurements by mass spectrometry reveal that ( 230Th 238U ) ratios are close to 1.02 in all samples younger than ∼ 7 ka, and that age-corrected ( 226Ra 230Th ) ratios for three samples less than ∼ 2 ka are ∼ 1.15–1.20. These results are indistinguishable from the disequilibria reported previously for young Kilauea lavas and are consistent with dynamic mantle melting within the garnet stability field at high melting rates (> 0.001 kg m −3 a −1) and low porosities (< 0.1%). Somewhat greater ( 230Th 238U ) ratios of up to ∼ 1.16 are found in some older Mauna Loa lavas, but these are likely to have resulted from post-eruptive loss of U by weathering as ( 230Th 238U ) ratios correlate positively with Th U ratios and inversely with K and Rb abundances. Sr and Nd isotopes define an excellent inverse correlation even though the total range in 143Nd 144Nd is only ∼ 1 ϵ Nd. However, changes in the Sr and Nd isotopic compositions over the last ∼ 30 ka have not been progressive but have occurred rapidly and with an apparent cyclicity of ∼ 4–5 ka. Moreover, these isotopic changes are mirrored by cyclical changes in certain trace element abundance ratios. If the melts were produced within a single melting column, then all elements must have travelled at similar velocities because the isotopic and abundance ratios of elements of widely differing compatibilities ( 143Nd 144Nd , 87Sr 86Sr , Ba, Th, La, Yb) vary coherently in the lavas. In this scenario, chromatographic exchange between melt and matrix must have been minimal. However, it is more likely that the lava compositions record the mixing of melts from two or more source regions of different compositions. ( 226Ra 230Th ) ratios in excess of unity in the lavas indicate that the transport of melt was rapid after leaving the melt column and suggest that melt movement through the lithosphere took place in cracks or fractures.