Mineral Compositions from the Hawaii Scientific Drilling Project (HSDP): Preliminary Results Part III - Olivine

Olivines are the most prevalent phenocrysts in Hawaiian lavas and thus offer a more complete temporal coverage of magmatic processes compared to phenocrysts of more restricted occurrence. As in parts I and II (see accompanying abstracts) we examine olivine compositions from HSDP core samples, which provide a temporal view of the Mauna Kea magma plumbing system. In general, mineral compositions are valuable because they provide an archive of liquid compositions; the compositional record of liquids is always murky due to the facility with which liquids are blended. But minerals, such as olivine, are resistant to re-equilibration and hence provide a much more complete record of the range of liquid compositions that enter a magma plumbing system. Olivine compositions are of particular interest since the Fo content (Mg/(Mg + Fe) ratio) is highly sensitive to T. Olivines that are sampled through the HSDP core thus provide a window into temperature variations across the Hawaiian hot spot. Most olivines are not in Fe-Mg exchange equilibrium with their host whole rock samples. This results is unsurprising and has been noted in numerous studies of olivine phenocrysts at Hawaii. This lack of equilibration limits the precision with which crystallization temperatures can be extracted. But comparisons of Mg/(Mg + Fe) (Fo content) with core depth reveal interesting qualitative results. Most notable are two aspects: (a) Fo contents exhibit nearly constant maximum values throughout the 3000 m of core, near Fo = 90. (b) In contrast, minimum Fo values appear to decrease with decreasing depth. The first observation is counter to what was expected. As Mauna Kea moved over the hot spot, one might expect maximum temperatures to decrease in response to the tapping of cooler mantle material. But near constant maximum values suggest that the hottest parts of the plume are delivered to Hawaiian conduits throughout their eruptive history. In contrast, the decrease in minimum Fo values with time might indeed reflect some T variation across the plume, and is consistent with observations of plagioclase (see part II). We hypothesize that the progressive decrease of Fo contents reflects a gradual decline in eruptive recurrence intervals: as magma supply rates wane, and eruption recurrence intervals increase, the magma conduit may progressively become choked off at various depths, with the result that individual magma batches are stored at a greater range of depths for longer periods of time. But even as supply rates decline, the hottest magmas that are supplied to a conduit during the shield-building stage seem somehow to still be available to the conduit during waning periods of volcanic activity.