THE ALUMINUM CONUNDRUM IN HAWAIIAN SHIELD-BUILDING LAVAS

In contrast with other major elements (e.g. SiO₂, CaO, TiO₂), isotopic ratios (e.g. Sr, Nd, Pb), and some trace element ratios (e.g. Zr/Nb, Zr/Y, La/Nb), the Al₂O₃ content of Hawaiian shield lavas, at a given MgO content, is remarkably uniform between volcanoes. The overall range at 13 percent MgO is about 0.8% from Loihi (11.0 %) to Koolau (11.8 %). There is no statistically significant difference in the Al₂O₃ content of lavas from the Kea and Loa trend volcanoes. Post-shield lavas are higher in Al₂O₃ at a given MgO content than corresponding shield lavas, consistent with their inferred lower rates of melt production. Taken at face value, this uniformity of Al₂O₃ in shield lavas appears to imply restricted melt production parameters and source components for the shield stage of Hawaiian volcanoes. This observation and inference is completely at odds with what we think we know about melt production in a heterogeneous, thermally zoned, mantle plume. Peridotite melting models and experiments, at varying pressures and temperatures, result in melts with significantly different Al₂O₃ contents at a given MgO content. Appealing to a pyroxenite source, or melts derived from mixed pyroxenite - peridotite sources only exacerbates the problem. The Al₂O₃ problem, of relatively uniform contents at a given MgO, content can be resolved if Hawaiian shield lavas result from melting garnet peridotite, perhaps “fertilized” by melts of entrained crustal lithologies, at depths corresponding to around 3-4 GPa. However, this explanation is at odds with the variable, but relatively high SiO₂ content of most of these lavas, which imply melt production at relatively shallow depths corresponding to less than 2.5 GPa. This apparent paradox can be resolved if low SiO₂ parental magmas, derived from garnet peridotite, react with a depleted harzburgite residue from prior melting as the magma ascends through the upper regions of the plume (e.g. Eggins, Contrib. Mineral. Petrol. 1992; Wagner and Grove, Contrib. Mineral. Petrol. 1997). Further implications of this last resolution are discussed in Rhodes (AGU Monograph, in press).