PETROLOGIC EVOLUTION OF MAGMA ERUPTED AT PU'U O'O, KILAUEA VOLCANO: ARE DECADAL CHANGES IN MAGMA COMPOSITION PRIMARILY DRIVEN BY SHALLOW OR DEEP PETROGENETIC PHENOMENA?

Crustal and mantle processes have played critical roles in the on-going eruption at Pu'u O'o, Kilauea, Hawaii, which initiated in 1983. During the first 20 years, ~2.3 km³ of mostly sparsely olivine phyric magma (~7-9.6 wt.% MgO) erupted, although episodes 1-3 and 54 erupted more evolved sparsely clinopyroxene-plagioclase phyric magmas. From 1983 to ~1989, MgO generally increased to ~9 wt.%, and La/Yb decreased from ~7 to 5 (Thornber 2003). Thornber (2003) hypothesizes that mixing of uniform mafic recharge magma with more evolved pre-1983 summit magma accounts for observed chemical changes, whereas Garcia et al. (1992, 2000) tie compositional diversity to changes in the mantle. To test the efficacy of mixing, we employed the Magma Chamber Simulator (Bohrson et al. 2014), a thermodynamic model that calculates elemental, isotopic, phase, and mass characteristics of magmas undergoing simultaneous recharge/mixing+assimilation+crystallization. Model results support mixing between shallow (1-3 km) evolved resident magma and recharge magma of fixed bulk composition (~9.6 wt.% MgO). For the first 6 years of eruption, mass balance requires progressive recharge of ~4x the resident magma volume. Pietruszka et al. (2015) use Pb isotopes to hypothesize a small (0.1-0.5 km³) melt-rich magma body, and Poland et al. (2015) conclude that the long-term melt supply rate for Kilauea is ~0.1 km³/yr. Thus, over 6 years, recharge/mixing of ~0.1 km³/yr. into a ~0.5 km³ resident magma body explains observed compositional changes. Computational modeling is therefore consistent with observed phase equilibria, recharge rates, and the volume of magma transport systems defined by inflation and seismicity (Shaw 1987; Pietruszka et al. 2015; Poland et al. 2015). But isotopic data require heterogeneous mantle. Additional modeling will test the crustal vs. mantle hypotheses for the first 2 decades of Pu'u O'o eruptions by addressing the mantle partial melting region volume vs. the scale of heterogeneity.