226Ra^230Th^238U disequilibria of historical Kilauea lavas(1790^1982) and the dynamics of mantle melting within the
Aaron J. Pietruszka *, Kenneth H. Rubin, Michael O. GarciaHawaii Center for Volcanology, Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA
Received 13 January 2000; accepted 3 January 2001
The geochemical variations of Kilaueas historical summit lavas (1790^1982) document a rapid fluctuation in themantle source and melting history of this volcano. These lavas span nearly the entire known range of sourcecomposition for Kilauea in only 200 yr and record a factor ofV2 change in the degree of partial melting. In this study,we use high-precision measurements of the U-series isotope abundances of Kilaueas historical summit lavas and twoingrowth models (dynamic and equilibrium percolation melting) to focus on the process of melt generation at thisvolcano. Our results show that the 226Ra^230Th^238U disequilibria of these lavas have remained relatively small andconstant withV12 4% excess 226Ra andV2.5 1.6% excess 230Th (both are 2c). Model calculations based mostlyon subtle variations in the 230Th^238U disequilibria suggest that lavas from the 19th to early 20th centuries formed atsignificantly higher rates of mantle melting and upwelling (up to a factor ofV10) compared to lavas from 1790 and thelate 20th century. The shift to higher values for these parameters correlates with a short-term decrease in the size of themelting region sampled by the volcano, which is consistent with fluid dynamical models that predict an exponentialincrease in the upwelling rate (and, thus, the melting rate) towards the core of the Hawaiian plume. The Pb, Sr, and Ndisotope ratios of lavas derived from the smallest source volumes correspond to the Kilauea end member of Hawaiianvolcanoes, whereas lavas derived from the largest source volumes overlap isotopically with recent Loihi tholeiiticbasalts. This behavior probably arises from the more effective blending of small-scale source heterogeneities as themelting region sampled by Kilauea increases in size. The source that was preferentially tapped during the early 20thcentury (when the melt fractions were lowest) is more chemically and isotopically depleted than the source of the early19th and late 20th century lavas (which formed by the highest melt fractions). This inverse relationship between themagnitude of source depletion and melt fraction suggests that source fertility (i.e. lithology) controls the degree ofpartial melting at Kilauea. Thus, rapid changes in the size of the melting region sampled by the volcano (in the presenceof these small-scale heterogeneities) may regulate most of the source- and melting-related geochemical variationsobserved at Kilauea over time scales of decades to centuries. 2001 Elsevier Science B.V. All rights reserved.
Keywords: Kilauea; volcanoes; Hawaii; lavas; geochemistry; uranium; isotopes; mantle plumes; melting; models
0012-821X / 01 / $ ^ see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 2 3 0 - 8
* Corresponding author. Present address: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 BroadBranch Road, N.W., Washington, DC 20015, USA. Tel. : +1-202-478-8476; E-mail : firstname.lastname@example.org
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Historical lavas from active Hawaiian shieldvolcanoes such as Kilauea and Mauna Loa areideal for investigating mantle melting becausethey display systematic melting-related variationsof incompatible trace element ratios (e.g. La/Ybor Nb/Y) over short periods of time (years todecades [1^4]). Models based on these uctuationsin lava chemistry suggest that the degree of partialmelting has varied by a factor of V2 for Kilaueasince 1790  and V40% relative for Mauna Loasince 1843 , which are signicant portions ofthe overall range inferred for Hawaiian shield vol-canoes (a factor of V2.5 ). The origin of theseshort-term changes in the degree of partial melt-ing is poorly understood, but may be related tothe interplay between mantle melting and small-scale source heterogeneities [2,3]. A similar pat-tern of rapid, source- and melting-related geo-chemical variations has also been observed instratigraphic sections of prehistorical shield lavasfrom Hawaiian volcanoes (e.g. Mauna Loa and Mauna Kea [7,8]). Thus, a detailed geochem-ical sampling of lavas over time scales of decadesto centuries may provide the best clues to under-standing the dynamics of mantle melting withinthe Hawaiian plume.
The U-series isotope abundances of lavas (e.g.230Th and 226Ra) are excellent tracers of mantlemelting because the half-lives of 230Th and 226Ra(V75 kyr and 1600 yr, respectively) are thoughtto be similar to the time scale of magmatic pro-cesses (e.g. ). This unique feature of U-seriesisotopes oers the potential to study both the na-ture and timing of the processes that fractionateincompatible trace elements during magma gene-sis. This possibility derives from two importantproperties of the 238U decay chain. First, priorto melting, the source regions of basaltic lavaswill generally be in a state of radioactive, or sec-ular, equilibrium in which the 238U, 230Th, and226Ra activities (decay rates) are equal. When sec-ular equilibrium prevails, the (230Th/238U) and(226Ra/230Th) ratios (parentheses indicate activ-ities) of the source are known (and equal tounity), in contrast to most other geochemical trac-ers of the melting process (e.g. stable, incompat-
ible trace elements). Second, any process thatfractionates these elements, such as partial melt-ing of the mantle, induces a state of radioactivedisequilibrium in which the activities of 238U,
Fig. 1. Summary of the melting, source, and eruptive historyof Kilauea Volcano. Kilaueas historical lavas record a rapiductuation in the degree of partial melting (inferred from in-compatible trace elements) that correlates with the volcanoseruption rate (and, presumably, its magma supply rate). Thecomposition of the mantle source region tapped by Kilauea(or proportions of the source components) has also changedover time (shown as a % of the early 20th century sourcecomposition, which corresponds to the Kilauea isotopic endmember for Hawaiian volcanoes). The vertical shaded linesmark the explosive summit eruptions of 1790 and 1924. See for the methods and/or references used to calculate eachof these parameters. The average melt fraction and sourcecomposition for the Mauna Ulu rift zone lavas are estimatedusing data from  and unpublished Pb isotope analyses (A.Hofmann, personal communication, 1999). The sample sym-bols are grouped according to eruption date: up triangles(1790), circles (1820^1921), diamonds (1929^1954), squares(1961^1982), left triangles (Mauna Ulu), and right triangles(Puu Oo).
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230Th, and 226Ra are unequal. The magnitude ofthe 226Ra^230Th^238U disequilibria in basalticlavas is believed to depend, in part, upon the ratesof mantle melting and upwelling [10^12].
Early U-series studies of Hawaiian shield lavasused radioactive decay counting techniques toidentify small, but signicant, 230Th and 226Raexcesses relative to their respective parents, 238Uand 230Th, and proposed a magmatic (mantlemelting) origin for these disequilibria [13^15].More recent, high-precision decay counting andmass spectrometric U-series investigations of his-torical Kilauea and Mauna Loa lavas observedsmall 230Th excesses and moderate 226Ra excessesup to 28% [16^20]. Modeling of the small 226Ra^230Th^238U disequilibria of these lavas (comparedto mid-ocean ridge basalts [21,22] and lavas fromother ocean island volcanoes ) has led to theidea that the rates of mantle melting and upwell-ing for tholeiitic basalt production within the Ha-waiian plume are relatively high [16^19,23].
This paper presents new U-series isotope, Srisotope, and trace element abundance data forthe historical summit lavas of Kilauea Volcano(1790^1982). These samples span nearly the entireknown range of source composition for this vol-cano and record a factor of V2 change in thedegree of partial melting . In this study, wefocus on mantle melting at Kilauea using the U-series isotope systematics of its historical lavasand two ingrowth models for producing 226Ra^230Th^238U disequilibria (dynamic melting and equilibrium percolation melting ). Ourgoal is to nd an internally consistent modelthat accounts for the 226Ra^230Th^238U disequilib-ria of Kilauea lavas in the context of other geo-chemical evidence (from incompatible trace ele-ments and Pb, Sr, and Nd isotope ratios ) fora short-term change in the mantle source andmelting history of this volcano (Fig. 1).
High-precision measurements of the Ba, Th, U,Sr, and 226Ra concentrations and (230Th/232Th),(230Th/238U), (226Ra/230Th), (234U/238U), and 87Sr/86Sr ratios of Kilaueas historical summit lavas by
thermal ionization mass spectrometry (TIMS) arereported in Table 1 (for additional sample infor-mation and analytical methods see the EPSL On-line Background Dataset1). In this section, we de-scribe the temporal geochemical variations of thelavas (Fig. 2). All uncertainties are 2c.
2.1. 230Th^238U disequilibria
The 230Th^238U disequilibria of Kilauea lavashave remained nearly constant since 1790(V2.5 1.6%, 2c) with a narrow 1.4^4.2% rangein the amount of excess 230Th (Fig. 2). The 230Th^238U disequilibria from other recent studies of Ki-lauea lavas (except [19,20]) are signicantly morevariable than our results and include some sam-ples with (230Th/238U)6 1 [16,17]. Despite thesmall (230Th/238U) range observed in this study,the samples display a systematic temporal varia-tion. The (230Th/238U) ratios increase subtly fromthe 19th to early 20th centuries (V1%), increaseabruptly after the explosive summit eruption of1924 (V2%), decrease slightly thereafter (V1%),and remain essentially constant during the late20th century. These short-term changes in the230Th^238U disequilibria (V1^2%) are close toour average analytical uncertainty ( 1.1%) basedon repeated Th isotope standard measurements( 1.1%) and our 0.3% reproducibility of Th/U ratios. However, we believe these subtle dier-ences are signicant because (1) they were veriedby duplicate Th isotope ratio measurements formost samples, (2) lavas that erupted during re-stricted periods of time have extremely constantTh/U, (230Th/232Th), and (230Th/238U) ratios (the1917 vs. 1919 lavas or the late 20th century lavas;Fig. 2), and (3) duplicate (230Th/232Th) analyses(and recalculated (230Th/238U) ratios) of severalsamples (1820-1, Kil1919, and six Mauna Uluand Puu Oo lavas; A. Pietruszka, unpublisheddata) using new, high-precision plasma ionizationmulti-collector mass spectrometric (PIMMS) tech-niques  agree withinV0.3% of our TIMS data(Fig. 2). A systematic uctuation in exc