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 km3. 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 km3, which is smaller than most geophysical estimates (2-40 km3). 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. |
Hypothetical cross-sections through Kilauea's summit region contrasting models for the shape and size of the volcano's shallow magma storage reservoir (lower panels) and the possible geochemical effects of each reservoir geometry (upper panels). (a) Geometrically simple, small-volume magma reservoir. The dashed line marks the approximate limit of the volcano's aseismic region, which is the maximum possible volume of Kilauea's magma reservoir (~40 km3). (b) Geometrically simple, large-volume magma reservoir. (c) Geometrically complex reservoir. The composition of the input magma delivered to the volcano (represented by any geochemical tracer of parental magma composition such as incompatible trace element or isotope ratios) is assumed to vary systematically over time (shown here as a sinusoidal, solid line). These systematic parental magma changes will be progressively buffered (e.g., a delta'R decrease) and delayed (e.g., a delta't increase) as the volume of the reservoir increases (models a to b), and/or destroyed by transport through a complex reservoir (model c). Thus, temporal variations in the output lava chemistry (red circles connected by dashed lines) may be used to probe the geometry of the volcano's magma reservoir. |
