| 英文摘要: | Averaged over millions of years, the rate of lava production at a seamount or island controls its final shape and size. Along long seamount chains, such as the Hawaiian-Emperor chain, observed seamount and island volumes change episodically indicating changes in the lava production rate responsible for their formation. The source of lavas forming these islands is believed to be the melting of mantle plumes: upwelling, stationary conduits of hot, chemically enriched material that originate from deep within the Earth's mantle and rise continuously to the surface. However, in contrast to observations, a continuously upwelling conduit would produce a nearly constant lava production rate; this project aims to address the processes that interrupt or perturb a continuously upwelling mantle plume, and, thus, the rate of lava production along hotspot island chains. Understanding the processes that control changes in lava production rate through time will provide insights into the fundamental connections between volcanoes at the surface and the dynamics of the deep Earth. There are two locations within the mantle where upwelling plumes are likely to be perturbed: 1) the core-mantle boundary, where anomalously dense material may be incorporated into the plume source and change the upwelling rate, and 2) in the mid-mantle, where abrupt changes in the phase, or mineral structure, of mantle rocks can alter the density and, thus, upwelling rate of plumes passing through these transitions. Changes in the upwelling rates will likely differ between these mechanisms and will result in time-varying changes in lava production rate at the surface that differ depending upon the mechanism responsible. The purpose of the proposed work is to use a combination of laboratory experiments and 3D numerical simulations to quantify the magnitude, length, and time scales over which variations in mantle plume upwelling caused by deep mantle processes will affect lava production and compositions at the Earth's surface, changing the shape and size of island chains.
The chemistry and flux of erupted lavas differ strikingly between hotspot seamount chains; some hotspots have nearly uniform chemical sources and volcanic output (e.g., Kerguelen), others vary episodically over millions of years (e.g., Hawaii), and yet others appear to decrease slowly with time (e.g., Louisville). The processes that control this inter-hotspot variability, as well as intra-hotspot variations, are poorly constrained. The proposed work will quantify the degree to which two deep-mantle mechanisms affect plume upwelling and, in turn, observed surface manifestations of hotspots: 1) entrainment of material from Large Low Shear-wave Velocity Provinces (LLSVP), and 2) interaction with the mantle transition zone. This project will constrain the impact of these two mechanisms through combined laboratory and numerical experiments to quantify how deep mantle dynamics lead to predictable magnitudes, length-, and time-scales of variations in mantle plume upwelling and, consequently, melt production and erupted lava compositions at the Earth's surface. The first objective will be to document the range of surface variability in nature by assembling a global database of excess hotspot magmatism, spacing between volcanoes, and geochemical data (major and trace elements, and radiogenic isotopes) along age-progressive hotspot tracks. Next, the investigators will conduct complementary laboratory and numerical experiments to quantify the physics of each of the above mechanisms individually. Finally, numerical simulations including both mechanisms will assess their impact on surface manifestations of mantle plumes. Laboratory experiments will be conducted in glass-walled tanks filled with glucose syrup as an analogue for the mantle. Using a 3D, finite-difference, marker-in-cell code, two sets of numerical simulations will be run for each process: 1) initial simulations with identical conditions to the laboratory experiments to verify the numerical approach, and 2) extension to more Earth-like conditions. Throughout the proposed work, the compiled database of observations from natural hotspot tracks will constrain the laboratory and numerical results. |