226Ra^230Th^238U disequilibria of historical Kilauea lavas (1790^1982) and the dynamics of mantle melting within the Hawaiian plume

The geochemical variations of Kilauea's historical summit lavas (1790^1982) document a rapid fluctuation in the mantle source and melting history of this volcano. These lavas span nearly the entire known range of source composition for Kilauea in only 200 yr and record a factor of V2 change in the degree of partial melting. In this study, we use high-precision measurements of the U-series isotope abundances of Kilauea's historical summit lavas and two `ingrowth' models (dynamic and equilibrium percolation melting) to focus on the process of melt generation at this volcano. Our results show that the 226Ra^230Th^238U disequilibria of these lavas have remained relatively small and constant with V12 þ 4% excess 226Ra and V2.5 þ 1.6% excess 230Th (both are þ 2c). Model calculations based mostly on subtle variations in the 230Th^238U disequilibria suggest that lavas from the 19th to early 20th centuries formed at significantly higher rates of mantle melting and upwelling (up to a factor of V10) compared to lavas from 1790 and the late 20th century. The shift to higher values for these parameters correlates with a short-term decrease in the size of the melting region sampled by the volcano, which is consistent with fluid dynamical models that predict an exponential increase in the upwelling rate (and, thus, the melting rate) towards the core of the Hawaiian plume. The Pb, Sr, and Nd isotope ratios of lavas derived from the smallest source volumes correspond to the `Kilauea' end member of Hawaiian volcanoes, whereas lavas derived from the largest source volumes overlap isotopically with recent Loihi tholeiitic basalts. This behavior probably arises from the more effective blending of small-scale source heterogeneities as the melting region sampled by Kilauea increases in size. The source that was preferentially tapped during the early 20th century (when the melt fractions were lowest) is more chemically and isotopically depleted than the source of the early 19th and late 20th century lavas (which formed by the highest melt fractions). This inverse relationship between the magnitude of source depletion and melt fraction suggests that source fertility (i.e. lithology) controls the degree of partial melting at Kilauea. Thus, rapid changes in the size of the melting region sampled by the volcano (in the presence of these small-scale heterogeneities) may regulate most of the sourceand melting-related geochemical variations observed at Kilauea over time scales of decades to centuries. ß 2001 Elsevier Science B.V. All rights reserved.