Dean C. Presnall

Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C.
&
Department of Geosciences, University of Texas at Dallas, Richardson, TX

presnall@utdallas.edu

Many “hot spots” or “plumes”, such as Iceland, Azores, Ascension, Tristan da Cunha, Bouvet, Easter, and Galapagos, occur on or close to mid-ocean ridge spreading centers. High mantle temperatures beneath these localities would be consistent with the currently dominant model for the origin of mid-ocean ridge basalts (MORBs) of Klein & Langmuir (1987; see also Klein & Langmuir 1989; McKenzie & Bickle, 1988; Langmuir et al., 1992). In this model, higher potential temperatures cause melting to begin at higher pressures, which produces a longer melting column, a larger amount of melt, a thicker crust, and shallower water depths. Also, this model holds that Na₈ (Na₂O content of a lava, normalized to MgO = 8%) will be lower and Fe₈ higher when the potential temperature is higher. Langmuir et al. (1992) argued that these chemical differences cannot be produced by mantle heterogeneity, but they did not claim mantle homogeneity. The variation in potential temperature in this model is about 240°C according to Langmuir et al. (1992) or ~320°C according to McKenzie & Bickle (1988).

In sharp contrast to this model, Presnall et al. (2002) have presented a model for the generation of MORBs that relies on phase relations in the six-component system CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO in the pressure range of the plagioclase/spinel lherzolite transition (about 0.9-1.5 GPa) and on phase relations at 3-7 GPa in the system CaO-MgO-Al₂O₃-CO₂ (Dalton & Presnall, 1998a; 1998b). In this model, melting at 0.9-1.5 GPa controls the major-element composition of MORBs, whereas melts produced at about 2.6-7 GPa in the low-velocity zone are carbonatitic, are produced by very small amounts of melting (< 0.2%), and mix with the more shallow melts without significantly altering their major-element compositions. However, the deeper melts do exercise significant control on trace element signatures. Presnall et al. (2002) argued that the major-element systematics for Na₈ and Fe₈ can be produced by a combination of mantle heterogeneity and very small variations of potential temperature of only about 20°C in the pressure range, 0.9-1.5 GPa (Figure 1). Currently, the model has been developed only in a semi-quantitative way because detailed phase relations for the six-component system at 0.9-1.5 GPa are not yet available.

Figure 1. Solidus curves for three model systems, CMAS (CaO-MgO-Al2O3-SiO2), CMAS-CO2 (CaO-MgO-Al2O3-SiO2-CO2), and CMASNF (CaO-MgO-Al2O3-SiO2-Na2O-FeO), after Presnall et al. (2002). pl, sp, and gt indicate plagioclase-, spinel-, and garnet-lherzolite stability fields in the CMAS system. Filled circles are invariant points.

Additional experimental data planned for the near future will allow quantitative testing of the model of Presnall et al. (2002). It should then be possible to realize significant progress on two issues: the variability of potential temperatures along ridges, and the relative importance of temperature vs heterogeneity in producing the chemical systematics of MORBs. Resolution of these issues would also bear on the viability of hot plumes along and near ridges and the possibility that apparent variations in melt productivity caused by strong temperature variations along ridges may, in fact, commonly represent variations in density and rate of upwelling caused by mantle heterogeneity (Presnall & Helsley, 1982). Such a model could explain the existence of “hot spots” along ridges that do not have significant heat-flow anomalies (Stein & Stein, 2003) or deep roots.

• Dalton, J. A. and Presnall, D. C., Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al₂O₃-SiO₂-CO₂ from 3 to 7 GPa. Contrib. Mineral. Petrol., 131, 123-135, 1998a.
• Dalton, J. A. and Presnall, D. C., The continuum of primary carbonatitic-kimberlitic melt compositions in equilibrium with lherzolite: Data from the system CaO-MgO-Al₂O₃-SiO₂-CO₂ at 6 GPa, J. Petrol., 39, 1953-1964, 1998b.
• Klein, E. M. and Langmuir, C. H., Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness, J. Geophys. Res., 92, 8089-8115, 1987.
• Klein, E. M. and Langmuir, C. H., Local versus global variations in ocean ridge basalt composition: A reply, J. Geophys. Res., 94, 4241-4252, 1989.
• Langmuir, C. H., Klein, E. M., and Plank, T., Petrological systematics of mid-ocean ridge basalts: Constraints on melt generation beneath ocean ridges. In Mantle Flow and Melt Generration at Mid-Ocean Ridges, eds. J. Phipps Morgan, D. K. Blackman and J. M. Sinton, pp. 183-280, Geophys. Mon. 71, Am. Geophys. Union, 1992.
• McKenzie, D. and Bickle, M. J., The volume and composition of melt generated by extension of the lithosphere, J. Petrol., 29, 625-697, 1988.
• Presnall, D. C., Gudfinnsson, G. H., and Walter, M. J., Generation of mid-ocean ridge basalts at pressures from 1 to 7 GPa, Geochim. Cosmochim. Acta, 66, 2073-2090, 2002.
• Presnall, D. C. and Helsley, C. E., Diapirism of depleted peridotite – a model for the origin of hot spots, Phys. Earth Planet. Int., 29, 148-160, 1982.
• Stein, C. and Stein, S., Mantle plumes: heat-flow near Iceland, Astron. & Geophys., 44, 8-10, 2003.