The Source of Oceanic Volcanism – Plumes vs the Low-Velocity Zone

Dean C. Presnall

Department of Geosciences, University of Texas at Dallas, Richardson, TX

presnall@utdallas.edu

This webpage is a summary of: Presnall, D.C. and Gudfinnsson, G.H. (2011). Oceanic volcanism from the low-velocity zone – without mantle plumes, Journal of Petrology, Peter J. Wyllie Volume, in press. (available on-line at Journal of Petrology, Advanced Access).

In a new model for oceanic volcanism, Presnall & Gudfinnsson (2011) argue that oceanic volcanism originates from magmas in the seismic low-velocity zone (LVZ) that ascend through fractures in the seismic lithosphere. This modeling is supported by:

1. basalt glass compositions from Hawaii and mid-ocean ridges;
2. data on the melting and crystallization behavior of natural lherzolite;
3. phase relations of model lherzolite in the systems CaO-MgO-Al₂O₃-SiO₂-CO₂ (CMAS-CO₂) and CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO (CMASNF) at pressures from 1 atm to 6 GPa; and
4. shear-wave velocities and anisotropy for the oceanic upper mantle.

The LVZ, which extends from about 65 to 220 km depth in the ocean basins, was first suggested to be a region of partial melting by Anderson & Sammis (1970). Wyllie & Huang (1975) and Eggler (1976) then suggested that melting in the LVZ is caused by CO₂. This argument has recently been reinforced (Presnall & Gudfinnsson, 2005, 2011; Hirschmann, 2010). A common assumption is that the melt fraction is very small throughout the depth range of the low-velocity zone. However, for a constant amount of CO₂, phase relations along a normal oceanic geotherm for a lherzolite mineralogy in the CMAS-CO₂ system (Gudfinnsson & Presnall, 2005) indicate a large tholeiitic melt fraction at ~ 4-5 GPa and smaller alkalic (low-SiO₂) melt fractions at both lower (~ 3 GPa) and higher (> 6 GPa) pressures along the geotherm (Figure 2b in Presnall & Gudfinnsson, 2011). Temperatures for the model system are adjusted to those for natural lhezolite on the basis of the volatile-free solidus of garnet lherzolite at 5 GPa, 1500°C (Lesher et al., 2003) and the solidus for a natural carbonated lherzolite (Dasgupta & Hirschmann, 2006). The pressure range of Hawaiian tholeiitic melt-extraction at ~ 4-5 GPa (~ 130-150 km depth) is based on the olivine-controlled crystallization trend of Puna Ridge tholeiitic glasses at Kilauea Volcano, Hawaii (Clague et al., 1995) combined with experimental data in the CMAS-CO₂ system (Gudfinnsson & Presnall, 2011, Figure 1).

Phase relations for the systems CaO-MgO-Al₂O₃-SiO₂ and CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO (Presnall & Gudfinnsson, 2011, and references therein) show that MORB magmas, including those at Iceland, are extracted at ~ 1240-1260°C, 1.2–1.5 GPa. Pressures of melt-extraction higher than this would show olivine-controlled crystallization paths like that found at Hawaii. Thus, melt-extraction at mid-ocean ridges occurs at the shallow depth of ~ 65 km, the maximum pressure for melt-extraction without olivine-controlled crystallization on subsequent cooling. This depth is in excellent agreement with seismic data (Nishimura & Forsyth, 1989) showing that the depth range of minimum shear velocity (maximum melt-fraction) at the crest of the East Pacific Rise is ~ 40-70 km. These very low P-T conditions of melt-extraction are within the thermal boundary layer, and indicate a perturbed geotherm that grazes the lherzolite solidus at these conditions (Figure 2, Presnall & Gudfinnsson, 2011). Such a geotherm requires a steepened conductive dT/dP gradient beneath ridges, which is consistent with heat-flow data (e.g., DeLaughter et al., 2005).

Modeling by Conder et al. (2002) indicates eastward and upward flow within the low-velocity zone beneath and just westward of the East Pacific Rise. In agreement with this modeling, a northwest to southeast cross-section of shear-wave anisotropy across the Pacific Basin (Ekström, 2000) shows that the depth of maximum anisotropy is fairly constant at ~130-150 km for the central Pacific but shallows significantly as the East Pacific Rise is approached from the northwest. Similarly, Nishimura & Forsyth (1989) found that the depth of minimum vertical shear velocity decreases from ~150 km to ~ 40-70 km as the East Pacific Rise is approached from the northwest. This essentially perfect agreement of the seismic and phase equilibrium data for the depths of maximum melting and tholeiitic magma production at Hawaii and mid-ocean ridges is powerful evidence that all oceanic volcanism comes from fractures that tap melts in the low-velocity zone, not from plumes.

References

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