The magmatic evolution of the Kulanaokuaiki-3 Tephra at Kilauea: Parallels to Hawaiian lava lakes

The scoria-rich central layer (unit 3) of the Kulanaokuaiki Tephra deposit (ca. 900 A.D.) at Hawaii's Kilauea Volcano exhibits systematic changes in chemical composition and mineralogy consistent with those previously observed in Hawaiian lava lakes, such as Kilauea Iki and Makaopuhi (e.g., Helz 2009, Wright & Okamura 1977, Moore & Evans 1967). These recurring patterns, found throughout the thin (6 to 25 cm thick) unit of the widespread deposit produced by the highly explosive eruption (Fiske et al. 2009), include increases and decreases in MgO (varying between >7 and ~14 wt%) as well as in compatible (Ni, Cr, Co) and incompatible (e.g., Nb and Y) elements. Accompanying these changes are subtle shifts in olivine composition (approximately Fo >78 to 82) and abundance, as shown by microprobe analyses of >200 specimens and preliminary point counts. This tephra section, which is the third of five stratigraphic units in the deposit, is interpreted as representing an inverted magma reservoir, consistent with its mineralogical assemblages, with density measurements and with abundances of lithic clasts. Comparisons between XRF analyses of larger (4-8 mm) and smaller (1-2 mm) scoria clasts indicate that the unit has retained its magmatic profile, essentially free from physical effects of post-eruption sedimentary and transport processes. In the earlier studies of Hawaiian lava lakes, MgO has been shown to decrease from the top to a minimum concentration at a relatively shallow depth and then to increase toward a maximum concentration with further increases in depth. This pattern has been associated, in part, with evidence for settling by olivine phenocrysts to yield an olivine-depleted upper region and an olivine-enriched lower region. A similar pattern is evident in the Kulanaokuaiki Tephra unit, which also displays an inflection point above which MgO decreases and below which it increases markedly with depth (when interpreted as representing an inverted magma reservoir). Preliminary data indicate, further, that this pattern may be attributable, in part, to shifts in olivine abundance. In addition to these broad similarities, the tephra unit displays several other features resembling those in lava lakes. For example, vitric pumice and interstitial glass from the tephra deposit show relatively low MgO (>5 to <7 wt%) and relatively high SiO₂ (~51 to >53 wt%). These values are similar to those of segregation veins in lava lakes (e.g., 4.5 to 6 wt% MgO and ~51 to >53 wt% SiO₂ at Kilauea Iki [Helz 1980]). Several unusually olivine-rich specimens, consisting of dense juvenile material within breccias, also have been found in the tephra unit, suggestive of olivine-rich bodies or masses observed in the lava lakes. The chemical and mineralogical similarities between the Kulanaokuaiki Tephra deposit and the lava lakes, along with these other features, suggest that a model derived from extensive research (e.g., Wright & Helz 1987) into passively cooling lava lakes, long regarded as natural laboratories, may provide insights into active volcanic systems, including those from sources deep within Kilauea's edifice.