Temperature Variations Along Spreading Centers
An Issue Critical to the Existence of Hot Plumes
Geophysical Laboratory, Carnegie Institution of Washington, Washington,
Department of Geosciences, University of Texas at Dallas, Richardson,
Many “hot spots” or “plumes”, such as Iceland,
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.
Solidus curves for three model systems, CMAS (CaO-MgO-Al2O3-SiO2),
and CMASNF (CaO-MgO-Al2O3-SiO2-Na2O-FeO),
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
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,
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.
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of ocean ridge basalt chemistry with axial depth and crustal thickness,
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global variations in ocean ridge basalt composition: A reply, J.
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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,
- Presnall, D. C. and Helsley, C. E., Diapirism of depleted peridotite
– a model for the origin of hot spots, Phys. Earth Planet.
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and Stein, S., Mantle plumes: heat-flow near Iceland, Astron.
& Geophys., 44, 8-10, 2003.