Aaron J. Pietruszka

The size and shape of Kilauea Volcano's summit magma storage reservoir: a geochemical probe

One of the most important components of the magmatic plumbing system of Kilauea Volcano is the shallow (2‐4 km deep) magma storage reservoir that underlies the volcano’s summit region. Nevertheless, the geometry (shape and size) of Kilauea’s summit reservoir is controversial. Two fundamentally different models for the reservoir’s shape have been proposed based on geophysical observations: a plexus of dikes and sills versus a single, ‘spherical’ magma body. Furthermore, the size of the reservoir is poorly constrained with estimates ranging widely from 0.08 to 40 km 3 . In this study, we use the temporal variations of Pb, Sr, and Nd isotope and incompatible trace element (e.g., La=Yb and Nb=Y) ratios of Kilauea’s historical summit lavas (1790‐1982) to probe the geometry of the volcano’s summit reservoir. These lavas preserve a nearly continuous, 200-year record of the changes in the composition of the parental magma supplied to the volcano. The systematic temporal variations in lava chemistry at Kilauea since the early 19th century suggest that the shape of the volcano’s summit reservoir is relatively simple. Residence time analysis of these rapid geochemical fluctuations indicates that the volume of magma in Kilauea’s summit reservoir is only 2‐3 km 3 , which is smaller than most geophysical estimates (2‐40 km 3 ). This discrepancy can be explained if the volume calculated from lava chemistry represents the hotter, molten core of the reservoir in which magma mixing occurs, whereas the volumes estimated from geophysical data also include portions of the reservoir’s outer crystal-mush zone and a hot, ductile region that surrounds the reservoir. Although our volume estimate is small, the amount of magma stored within Kilauea’s summit reservoir since the early 19th century is an order of magnitude larger than the magma body supplying Piton de la Fournaise Volcano, another frequently active ocean-island volcano.

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

Abstract 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 ∼2 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 226 Ra– 230 Th– 238 U disequilibria of these lavas have remained relatively small and constant with ∼12±4% excess 226 Ra and ∼2.5±1.6% excess 230 Th (both are ±2σ). Model calculations based mostly on subtle variations in the 230 Th– 238 U 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 ∼10) 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 source- and melting-related geochemical variations observed at Kilauea over time scales of decades to centuries.

A Review of the Recent Geochemical Evolution of Piton de la Fournaise Volcano (1927–2010)

Between 1927 and 2010, more than one hundred eruptions of Piton de la Fournaise produced ~1 km3 of lava, and the volcano’s summit collapsed twice (in 1931 and 2007). These lavas display, respectively, 20 and 65 % of the Sr–Nd and the Pb isotope ranges reported for La Reunion volcanoes over their known eruptive record (3.8 Ma). Variations in major and trace element concentrations and Sr–Pb isotopes do not define a temporal trend at the scale of the century, but display systematic short-term cyclic fluctuations. The positive correlation between 87Sr/86Sr and ratios of trace elements that are more versus less incompatible during partial melting of the mantle (e.g., Nd/Sm, La/Sm) probably results from the sampling of small-scale heterogeneities within the plume source. Changes in the degree of melting and/or crystallization are debated, but these appear ultimately linked to source properties. Lead isotopes do not co-vary with Sr isotopes, in part because of the partitioning of Pb into dense metallic phases that are preferentially sampled during high-flux eruptions. Taken together, Sr–Nd–Pb–Os–Th isotopes do not support contamination of magma with genetically unrelated components, such as the underlying Indian oceanic crust, mantle lithosphere, seawater, or seawater-altered lavas. Yet, in some rare cases (e.g. the 1998 Hudson eruption), the compositional patterns suggest that the parental magma assimilated older volcanic products within the edifice, such as crystal cumulates and/or interstitial differentiated melts. The geochemical fluctuations over the 1927–2010 time period constrain the residence time of magma in the shallow reservoir to 10–30 years and its size to 0.1–0.3 km3. The magma residence time during the course of the long-lived 1998 eruption is estimated to be an order of magnitude shorter, but the reservoir was probably of similar size. Instead, the shorter magma residence for the 1998 eruption was probably due to a higher magma flux.