Upper Mantle Dynamics Expressed in Hotspot Basalt Chemistry Garrett Ito, Todd Bianco, John Mahoney, Janet Becker, & Michael Garcia Dept. of Geology & Geophysics, SOEST, Univ. Hawaii Mantle is heterogeneous on scales smaller than melting zones Melting depth/temperature is composition-dependent Vertical variations in upper mantle flow can influence rates that different materials melt Case 1: Geochemical variations along the Mid-Atlantic Ridge (MAR) near Iceland reflect differences in flow and lithospheric thickness [Ito & Mahoney, 2005] Case 2: Rejuvenated & arch volcanism in Hawaii caused by decompression melting beneath flexural arches. Geochemical differences from flow reflect “plume” versus “arch-driven” flow [Bianco et al., 2005]. Mantle is heterogeneous on scales smaller than melting zones Melting depth/temperature is composition-dependent Vertical variations in upper mantle flow can influence rates that different materials melt Case 1: Geochemical variations along the Mid-Atlantic Ridge (MAR) near Iceland reflect differences in flow and lithospheric thickness [Ito & Mahoney, 2005] Case 2: Rejuvenated & arch volcanism in Hawaii caused by decompression melting beneath flexural arches. Geochemical differences from flow reflect “plume” versus “arch-driven” flow [Bianco et al., 2005].

Case 1: Geochemical between Iceland and Mid-Atlantic Ridge NS

Mantle Flow and Melting Model

Case 1: Geochemical Variations Along the Mid-Atlantic Ridge

Case 2: Origin of Secondary (Rejuvenated & Arch) Hotspot Volcanism [Bianco et al. 2005]

Evidence for Arch Uplift Causing Secondary Volcanism

Arch Versus Shield Isotope Compositions

Conceptual Model: Decompression of hot plume material beneath arch causes secondary magmatism

Magma Fluxes Beneath Arch vs. Shield Example Starting Source Mixture: 90%DM, 9.5%EM, 0.5%PX

Example Model Sources (90% DM, 9.5% EM, 0.05% PX)

Successful solutions for lithosphere thickness and plume potential temperature Rejuvenated & Arch Shield 143 Nd 144 Nd 143 Nd 144 Nd 87 Sr 86 Sr 87 Sr 86 Sr

Successful solutions for lithosphere thickness and plume potential temperature Rejuvenated & Arch Shield 143 Nd 144 Nd 143 Nd 144 Nd 87 Sr 86 Sr 87 Sr 86 Sr

Future Opportunities: Relate isotope/trace-element variations to length scales of flows in upper mantle

Future Opportunities Relate upper mantle dynamics to major element chemistry. Will require implementing thermodynamic models of melting (e.g., MELTS). Lava composition also influenced by melt-solid interaction during magma transport. Requires calculations of magma & solid, chemical, & energy transport. Relate upper mantle dynamics to major element chemistry. Will require implementing thermodynamic models of melting (e.g., MELTS). Lava composition also influenced by melt-solid interaction during magma transport. Requires calculations of magma & solid, chemical, & energy transport. Conclusions Upper mantle processes of solid flow, melting, & magma transport can lead to large variations in magma composition. Geodynamic models will allow us to use magma geochemistry to infer key aspects of the upper mantle dynamics. And to quantitatively relate lava composition to source, & thus to understand the chemical structure & evolution of the mantle. Upper mantle processes of solid flow, melting, & magma transport can lead to large variations in magma composition. Geodynamic models will allow us to use magma geochemistry to infer key aspects of the upper mantle dynamics. And to quantitatively relate lava composition to source, & thus to understand the chemical structure & evolution of the mantle.

Melting Beneath Shield vs. Arch Shield melt flux  (radial flow, V r ) x (fraction extracted,F e ) Arch melt flux  (vertical flow, V z ) x (productivity dF e /dz)