Ra-226-Th-230-U-238 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 t his volcano. These lavas span nearly the entire known range of source compo sition for Kilauea in only 200 yr and record a factor of similar to 2 chang e in the degree of partial melting. In this study, we use high-precision me asurements of the U-series isotope abundances of Kilauea's historical summi t lavas and two 'ingrowth' models (dynamic and equilibrium percolation melt ing) to focus on the process of melt generation at this volcano. Our result s show that the Ra-226-Th-230-U-238 disequilibria of these lavas have remai ned relatively small and constant with similar to 12 +/- 4% excess Ra-226 a nd similar to 2.5 +/- 16% excess Th-230 (both are +/- 2 sigma). Model calcu lations based mostly on subtle variations in the Th-230-U-238 disequilibria suggest that lavas from the 19th to early 20th centuries formed at signifi cantly higher rates of mantle melting and upwelling (up to a factor of simi lar to 10) compared to lavas from 1790 and the late 20th century. The shift to higher values for these parameters correlates with a short-term decreas e in the size of the melting region sampled by the volcano, which is consis tent with fluid dynamical models that predict an exponential increase in th e upwelling rate land, thus, the melting rate) towards the core of the Hawa iian plume. The Pb, Sr, and Nd isotope ratios of lavas derived from the sma llest source volumes correspond to the 'Kilauea' end member of Hawaiian vol canoes, whereas lavas derived from the largest source volumes overlap isoto pically 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 wa s preferentially tapped during the early 20th century (when the melt fracti ons were lowest) is more chemically and isotopically depleted than the sour ce of the early 19th and late 20th century lavas (which formed by the highe st melt fractions). This inverse relationship between the magnitude of sour ce depletion and melt fraction suggests that source fertility (i.e. litholo gy) controls the degree of partial melting at Kilauea. Thus, rapid changes in the size of the melting region sampled by the volcano tin the presence o f these small-scale heterogeneities) may regulate most of the source- and m elting-related geochemical variations observed at Kilauea over time scales of decades to centuries. (C) 2001 Elsevier Science B.V. All rights reserved .