PHYSICAL CONSTRAINTS ON THE ORIGIN OF THE MARKER HORIZON IN THE KULANAOKUAIKI TEPHRA, KILAUEA VOLCANO, HAWAII

The Kulanaokuaiki Tephra consists of thin, discontinuously exposed tephra erupted from Kīlauea’s summit between 400-1000 C.E. It is known from sections in the Uwekahuna Bluff and underground at Tree Molds, plus numerous exposures on Kīlauea’s south flank. Correlations between sections depend heavily on locating a marker horizon that contains shards of a distinctive high-TiO₂, high-K₂O glass (the K-2 layer).

On a plot of TiO₂ vs. MgO for samples from the 1959 Kīlauea Iki lava lake, glasses from the high-TiO₂ layer lie at the low end of the olivine-controlled samples, at 6.5-7.0% MgO, falling in the gap between the olivine-controlled compositions and various internal differentiates observed in the lava lake. We infer that the parental magma of the high-TiO₂ layer resembled the 1959 compositions.

We have evaluated Kulanaokuaiki glass compositions using plots of CaO vs. MgO (to determine the extent of crystallization of the melts) and TiO₂ vs. MgO (to establish whether high-TiO₂ parental melts exist in other tephra layers). Glasses with possible parental compositions (with high CaO and TiO₂ plus high K₂O  levels) are present in four layers immediately above the samples that contain the high-TiO₂, high-K₂O glass in the Uwekahuna Bluff section. Similar stratigraphy is observed in the Tree Molds section, where the K-2 horizon is overlain by a single layer containing some glasses like those of the 1959 lavas. The MgO contents of the potential parent melts are 9.2-10.0% MgO, similar to the most magnesian melts erupted in 1959. The lack of intermediate compositions suggests melt was stored at two levels, with the high-TiO₂, high-K₂O source body relatively shallow, and the parental liquids having come from greater depth.

The crystallization behavior of Kīlauea basalts constrains the composition of the K-2 marker horizon in two ways. First, at MgO ~7.0%, the melts shift from 1-phase to 3-phase crystallization, and the increase in heat of crystallization per degree of cooling for the 3-phase assemblage tends to inhibit further evolution of melt compositions at depth. Second, densities of such melts lie near a minimum in the density curve, which may facilitate segregation of these melts into discrete layers or separate bodies consisting almost entirely of minimum-density melts. Both processes acted to produce the K-2 marker horizon.