The Paradox of Consistency in the Major Element Compositions of Mauna Loa Magmas Over Time

The vast majority of Mauna Loa lavas plot on well-defined olivine-control trends. When normalized to a constant MgO content (16%) their major element compositions (SiO2 = 48.86 +/- 0.30; Al2O3 = 10.58 +/- 0.17; FeO = 9.83 +/- 0.23; CaO = 8.31 +/- 0.19 (all at 1 sigma)) have remained remarkably constant for over 600 ka. This observation is distinctly at odds with what we think we know about melting processes and source components in the Hawaiian plume. Given a 9-10 cm/yr northwesterly movement of the Pacific plate, Mauna Loa will have traversed about two thirds of the plume melting zone (~ 60 km) over 600 ka. It should, therefore, have sampled magmas of varying compositions produced by melting at different temperatures and depths within the plume. Additionally, in contrast with the major elements, the isotopic ratios of these lavas over 600 ka are highly variable (Sr and Pb 39% and Pb 43% of the entire range for Hawaiian shield lavas). This implies varying contributions of plume (Loihi) and recycled crustal (Koolau) components which should also strongly influence the major element compositions of the magmas. What is the explanation for this paradox? Two ubiquitous processes, one deep the other shallow, are thought to be responsible. First, diverse magma compositions are produced, as anticipated, by melting at varying T and P, and from mixtures of different source components. On ascent, these melts react with overlying depleted harzburgite residue produced by prior melting in the plume. The resulting magma composition will reflect the depth at which it finally segregates from this residue. The silica geobarometer indicates that this depth is relatively shallow (~45 - 60 km), resulting in comparatively uniform high SiO2 and other major element constituents of the parental magmas. Secondly, these parental magmas mix into a magma reservoir at shallow levels within the volcano. This reservoir magma is 'perched' at the end of the olivine-control trend at the pigeonite reaction point. Providing magma-supply is sufficient, this will maintain an almost invariant major element composition (~ 7% MgO). Any major element compositional diversity persisting in the parental magmas will be reduced even further in the resulting mixed magmas. Finally, prior to and during eruption, the mixed magmas entrain an olivine (typically Fo87-89) cumulate mush derived from parental picritic magmas. This results in further reduction of any major element diversity that may have been present in the parental magmas and in lavas with varying olivine content that plot along the characteristic olivine-control trends.