No Plumes Along Mid-Ocean Ridges

Dean C. Presnall

Department of Geosciences, University of Texas at Dallas, Texas, USA

Bayerisches Geoinstitut, Universität Bayreuth, Germany

Geophysical Laboratory, Washington, D. C., USA

dpresnall@ciw.edu

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Hot plumes were considered by Morgan (1971, 1972) to be basic components of a whole-mantle thermal convection regime that guides and helps maintain volcanism along spreading ridges. These plumes have been considered to be the main or even total (Yamamoto et al., 2007) source of thermal energy that drives plates.  They would also provide the energy for enhanced lava production and locally higher temperatures along spreading ridge.  A recent paper (Presnall & Gudfinnsson, 2007) presents a new model for the generation of mid-ocean ridge basalts (MORBs) and oceanic lithosphere formation.  This model addresses the temperatures of melt extraction along mid-ocean ridges, an approach toward mapping of mantle heterogeneity beneath ridges, and the existence of hot plumes at volcanic centers (e.g., Jan Mayen, Iceland, Azores, St. Helena, Tristan, Bouvet, Afar, Easter) on or close to ridges.

An earlier model (Klein & Langmuir, 1987; 1989; Langmuir et al., 1992; hereafter LKP), involves a range of short (cool) to long (hot) melting columns, with aggregate melts produced at the surface that combine melt increments from the entire depth range of a column.  They introduced the parameters Na₈ and Fe₈ (Na₂O and FeO normalized to MgO = 8%), in order to remove the variations of Na₂O and FeO caused by low-pressure fractional crystallization and to reveal the chemical systematics produced by melting.  To explain variations of Na₈ and Fe₈ vs. axial ridge depth, LKP claimed that long melting columns that start at high T and P produce melts that are high in Fe₈, low in Na₈, and characteristic of shallow ridges with a thick crust. In contrast, they proposed that short melting columns that start at low T and P produce melts that are low in Fe₈, high in Na₈, and characteristic of deep ridges with a thin crust.  These were called the global trends and were considered to be characteristic of ridge lengths of hundreds to thousands of km.

In a global reevaluation of the LKP modeling, Presnall & Gudfinnsson (2007) confirmed that when Na₈ and Fe₈ are plotted against each other, rough positive and inverse trends occur, as reported by LKP.  However, no ridge segments with “global” lengths were found that support an increase of Na8 and decrease of Fe8 with increasing axial ridge depth.  This absence of this key correlation requires a different kind of modeling.  In the system CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO (CMASNF), solidus melt compositions in the plagioclase/spinel lherzolite transition (~0.9-1.5 GPa) show an inverse correlation of Na₂O vs. FeO at constant MgO (Presnall et al., 2002; Presnall & Gudfinnsson, 2007).  This matches the global Na₈-Fe₈ correlation of LKP but occurs for solidus melts over a very narrow P-T range  of ~1.2-1.5 GPa and ~1250-1280°C. 

Also, if MgO is relatively uniform but allowed to vary slightly, Na₂O varies positively with FeO during progressive fractional melting of ascending mantle source material (Presnall & Gudfinnsson, 2007).  In this modeling, the inverse variation of Na₈ vs. Fe₈ is caused by mantle heterogeneity rather than temperature variations.  The positive correlation is produced by fractional melting over a narrow pressure range.  For example, Figure 1a shows, for the Carlsberg Ridge, a ridge of “global” length sampled over a distance of ~5,300 km, that neither Na₈ nor Fe₈ correlate with ridge depth. Thus, the LKP modeling fails.  However, the combined Red Sea and Carlsberg data show an inverse correlation in which the Red Sea data are consistently displaced to higher Fe₈ and lower Na₈ values than the Carlsberg Ridge values (Figure 1b).  This is consistent with mantle heterogeneity in which the Red Sea mantle is enriched in FeO/MgO and depleted in Na₂O relative to the mantle beneath the Carlsberg Ridge.  Melt extraction over a large and varying P-T range from a chemically homogeneous mantle is replaced by melt extraction at relatively constant P-T conditions from a heterogeneous mantle.

Figure 1.  Na₈-Fe₈-depth data for Red Sea Rift and Carlsberg Ridge (from Presnall &
Gudfinnsson, 2007).

Presnall & Gudfinnsson (2007) proposed that the very narrow range of P-T conditions for MORB extraction is imposed by the maximum temperature for explosive escape of CO₂ from the mantle.  This maximum temperature is caused by the abrupt temperature drop of the solidus curve for carbonated lherzolite (Wyllie & Huang, 1975; Eggler, 1976; Falloon & Green, 1989; Presnall & Gudfinnsson, 2005) at about 1.8-1.9 GPa (Figure 2).  These P-T conditions are the same as the narrow P-T range for MORB extraction indicated by the CMASNF solidus phase relations at the plagioclase/spinel lherzolite transition (Presnall et al., 2002; Presnall & Gudfinnsson, 2007). 

Figure 2. Pressure-temperature diagram showing solidus curves for carbonated lherzolite and volatile-free lherzolite, after Figure 1 of Presnall & Gudfinnsson (2005); based on the data of Hirschmann (2000), Dalton & Presnall (1998), Gudfinnsson & Presnall (2005) and Falloon & Green (1989). (From Presnall & Gudfinnsson, 2007).

Clague (2007) found that effusive and strombolian (explosive) eruptive features at oceanic ridges always occur together. In the model of Presnall & Gudfinnsson (2007), oceanic ridges are born when stresses cause fracturing through the entire thickness of the oceanic lithosphere and momentarily expose the partly melted seismic low-velocity zone to zero pressure.  This causes explosive escape of CO₂, which carries melt with it to the surface, and a mature spreading ridge gradually develops.  The energy for melting is decompression of rising mantle from the low-velocity zone.  The flashing of CO₂ to vapor merely controls the P-T conditions for escape of the magma. Ridges die when stresses shift, close the fracture, and open a new fracture at a different locality where a new ridge develops. No interaction with the deep mantle is involved.  This model accounts for the absence of thermal disturbances at the 410- km discontinuity beneath the entire length of the East Pacific Rise (Melbourne & Helmberger, 2002) beneath which the presumed Easter plume is supposd to lie. 

References

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• Dalton, J. A. & Presnall, D. C., 1998, Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al₂O₃-SiO₂-CO₂ from 3 to 7 GPa. Contrib. Min. Petrol., 131, 123-135, doi: 10.1007/S004100050383.
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