A Hot Source for
Department of Geology, University of Leicester
I briefly review the evidence for elevated potential
temperatures in the mantle source of ultramafic liquids. Evidence of the
eruption of ultramafic liquids is documented in several large igneous
provinces (e.g., North Atlantic Igneous Province1, Caribbean
Plateau2 and Etendeka3) and ocean islands (e.g.,
Kilauea4 and Gorgona5). Such evidence exists in
the form of analysed glass, and/or high-Fo olivine phenocrysts from which
equilibrium liquid compositions can be calculated6-8. Inverse
and forward modelling7,8 indicate high MgO contents of parental
and primary melts, and high potential temperatures of the mantle source
regions (typically, 1520-1570°C for primary magmas with 18-20% MgO7,8).
Even higher source temperatures are predicted for Gorgona komatiites5
and Etendeka picrites3. Such high temperatures are not, however,
indicated for the source of mid-ocean ridges9, where primary
magmas are predicted to contain lower amounts of MgO (generally < 12%).
Icelandic Mg-rich basaltic liquids indicate source temperatures intermediate
(~1400-1450°C 8,9) between those of Kilauea (~1550°C7)
and MOR (~1250°C)9.
Given the extreme conditions necessary for high-temperature,
high-density melts to traverse thick lithosphere with a lower melting
point and lower density10, the scarcity of such liquids at
the Earth’s surface is not surprising. The absence of abundant picritic
melts in places such as Iceland may be a result of such physico-chemical
filtering and may not reflect the average composition of the melts crossing
the Moho. Similar filtering of dense, magnesian primary liquids may also
occur beneath MOR. However, estimates11 of the bulk composition
of the ocean crust restrict the MgO content of average Moho-crossing liquids
to less than about 12%, consistent with experimental studies9,
although some fractionation and Mg-loss may occur within the cool upper
mantle below slow-spreading ridges12. Whilst enhanced volatiles
(H2O or CO2) in the source can reduce the mantle
potential temperatures necessary to produce either magnesian liquids or
increased volumes of basaltic melt, near-fractional melting will rapidly
remove the volatiles from the source, reducing their efficacy.
- 1. Larsen, L.M. and Pedersen, A.K., 2000. Processes in high-Mg, high-T
magmas: evidence from olivine, chromite and glass in Palaeogene pricrites
from West Greenland. Journal of Petrology, 41(7):
- Kerr, A.C. et al., 2002. Pervasive mantle plume head heterogeneity:
evidence from the late Cretaceous Caribbean-Colombian oceanic plateau.
Journal of Geophysical Research, 107(B7):
10.1029/2001JB000790; Kerr, A.C. et al., 1996. The geochemistry and
petrogenesis of the late-Cretaceous picrites and basalts of Curaçao,
Netherlands Antilles: a remnant of an oceanic plateau. Contributions
to Mineralogy and Petrology, 124: 29-43.
- Thompson, R.N. and Gibson, S.A., 2000. Transient high temperatures
in mantle plume heads inferred from magnesian olivines in Phanerozoic
picrites. Nature, 407: 502-506.
- Clague, D.A., Weber, W.S. and Dixon, J.E., 1991. Picritic glasses
from Hawaii. Nature, 353: 553-556.
- Kerr, A.C. et al., 1996. The petrogenesis of Gorgona komatiites,
picrites and basalts: new field, petrographic and geochemical constraints.
Lithos, 37: 245-260.
- Nisbet, E.G., Cheadle, M.J., Arndt, N.T. and Bickle, M.J., 1993.
Constraining the potential temperature of the Archaean mantle: a review
of the evidence from komatiites. Lithos, 30:
- Herzberg, C. and O'Hara, M.J., 2002. Plume-associated ultramafic
magmas of Phanerozoic age. Journal of Petrology, 43:
- Herzberg, C. et al., Submitted. Ultramafic igneous rocks: a challenge
for alternatives to the plume hypothesis. Earth and Planetary Science
- Presnall, D.C., Gudfinnsson, G.H. and Walter, M.J., 2002. Generation
of mid-ocean ridge basalts at pressures from 1 to 7 GPa. Geochimica
et Cosmochimica Acta, 66: 2073-2090.
- Stolper, E. and Walker, D., 1980. Melt density and the average composition
of basalt. Contributions to Mineralogy and Petrology, 74:
7-12; Sparks, R.S.J., Meyer, P. and Sigurdsson, H., 1980. Density variations
amongst mid-ocean ridge basalts: implications for magma mixing and the
scarcity of primitive lavas. Earth and Planetary Science Letters,
- Pallister, J.S. 1984. Parent magmas of the Semail ophiolite, Oman.
In: Gass, I.G., Lippard, S.J. and Shelton, A.W. Ophiolites and Oceanic
Lithosphere. Geol. Soc. Lond. Spec. Pub. 13,
Blackwell, 63-70. Coogan, L.A. et al., 2001. Whole-rock geochemistry
of gabbros from the Southwest Indian Ridge: constraints on geochemical
fractionation between the upper and lower oceanic crust and magma chamber
processes at (very) slow-spreading ridges. Chemical Geology,
178: 1-22. Cannat, M., 1993, Emplacement of Mantle
Rocks in the Seafloor at Mid-Ocean Ridges: J. Gephys. Res.,
98, 4163-4172. Cannat, M., 1996, How thick is the magmatic
crust at slow spreading ridges?: J. Gephys. Res., 101,
2847-2857. Dick, H. J. B., J. H. Natland, et al., 2000, A long in situ
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