NUCLEI

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NUCLEI

Convection dynamics of hot mantle plume and cold downwelling curtains in Hawaii
A joint electromagnetic and seismic experiment

PI: E. Attias (UTIG)
co-PIs: N. Harmon & C. Rychert (WHOI), A. Haroon (HIGP)
Collaborators: S. Wang (NGU), A. Ray (Geoscience Australia), A. Grayver (University of Cologne),
M. Zhang (Jilin University), H. Turner (Chaminade University)

The Hawaiian hotspot consists of two crucial geological/tectonic components: A plume of molten rock that connects the Hawaiian Islands to the deep mantle, whose location, diameter, and depth extent are still subject to heated debate, and an associated bathymetric rise (swell) whose dynamics and geology also are still subject to debate. While progress has been made using seismic methods to address the geometry of the Hawaiian plume, detecting and quantifying variations in temperature, composition, and melt fraction are still challenging. Apart from the hot plume, geodynamic numerical models (Ballmer et al., 2011) and Ps receiver function data (Agius et al., 2017) suggest the existence of large-scale cold mantle downwelling curtains situated laterally ~150 km from the plume axis.

3D schematic showing plume dynamics within the transition zone and upper mantle beneath Hawaii (Agius et al., 2017). As the plume rises (black arrow), the uppermost mantle melting and flow may result in the downwelling of cold material (blue arrows), sinking to at least 410 km depth.

NUCLEI

The non-invasive MagnetoTelluric (MT) method uses natural time variations in Earth’s magnetic field, primarily driven by solar wind, to measure the electrical conductivity of the crust and mantle. Conductivity is highly sensitive to changes in temperature and melt content. In this project, we will deploy 80 ocean-bottom electromagnetic instruments to study the 3D geometry and prominent features of the Hawaiian plume. Our proposed experiment complements legacy MT/seismic data collected by the SWELL (1997) and the PLUME (2005) experiments. This enables us to test hypotheses relating to the nature of the Hawaiian plume that cannot be unequivocally tested with either MT or seismic data alone. More importantly, NUCLEI will provide new insights into the convection dynamics of a hot plume and adjacent downwelling curtains, improving our understanding of hot plume mechanics.

NUCLEI experiment: Map of archived and proposed MT receivers and ocean bottom differential pressure gauge (DPG) as seismometers.

’Hawaiian’ plume MT simulations: 3D forward modeling inverted in 2D

3D finite-element discretization of Hawaii's plume forward (true) model. The true model represents a conceptual model derived from seismic OBS data (Laske et al., 2011).

MT forward modeling using COMSOL: The finite-element meshing that includes the 3D geometry of the conduit, upwelling hot plume, and downwelling curtains. These COMSOL simulations will help us determine the frequency range that will maximize the skin depth so that the apparent resistivity and phase responses will be sensitive to the resistivity structure of the plume down to the deep conduit.

2D inversion (MARE2DEM) of the 3D true forward model converged to an RMS misfit of 2.0 (error floor of 10%). The inverted triangles mark 11 MT sites. Seismic-driven (Laske et al., 2011) discontinuity (penalty cut) applied to the deep conduit and plume beneath the Island of Hawaii (i.e., Big Island). The model mash combines triangles and quadrilaterals, finely discretized along the conduit/plume structure. This simulation demonstrates that seismically-constrained 2D inversion of long-period MT data can potentially resolve the electrical resistivity structure of the ’Hawaiian’ plume from the lithosphere down to its asthenospheric base.

A seismically constrained 2D synthetic MT inversion model simulates a dynamic convection scenario where Hawaii's hot mantle plume rises from the lower mantle to the lithosphere-asthenosphere boundary (LAB), coupled by large-scale cold curtains (slabs) downwelling at the plume flanks. The upper panel shows the true model based on the geodynamic modeling of Ballmer et al. (2011) and Agius et al.'s Ps receiver function data (2017). The lower panel displays the resulting electrical resistivity inversion model, highlighting the potential spatial and depth sensitivity of seismically constrained MT inversion to both the upwelling plume and downwelling slabs. Inversion RMS misfit = 1.99, error floor = 10%, with enforced discontinuity of 0.1 (cut penalty).

In this study, we will conduct a series of deterministic and Bayesian MT modelling using advanced algorithms (i.e., GoFEM) and an implicit neural network model to compare the resulting models with a 3D finite-element code that we will develop as part of this project (see below). NUCLEI will provide powerful constraints on the 3D subsurface geology, including the position and lateral extent of the region of magma upwelling beneath Hawaii, its depth of origin, the amount of melt within this volcanic system, and the spatiotemporal extent and thermal structure of the presumed downwelling curtains. Our integrated approach will contribute to our understanding of one of the essential magmatic systems on Earth in terms of planetary convection, plate tectonics, and island-building mechanisms.

lava flow to pacific ocean at big island, hawaii.jpg

In collaboration with WHOI, HIGP, and NTNU, the NUCLEI project will enhance the capabilities of a 3D marine MT finite-element inversion algorithm to account for Hawaii's complex plume morphology, fluctuating seafloor topography, and strong coastal effects. The resulting 3D marine MT code will be made publicly available upon completion.
The broader impact component of NUCLEI will be in collaboration with Chaminade University (a native Hawaiian institution that serves underprivileged students), constructing a three-phase plan to integrate Chaminade's students in this project through (1) undergraduate and master students' participation in both of our research cruises, (2) supporting data science research internships for indigenous and underrepresented students to work on the NUCLEI data, and (3) organising culture-science exploration workshops for students, scientists, and the Hawaiian community to share information, findings, and additional knowledge which evolved from NUCLEI.