A Hot Source for
Picritic Melts
Andy Saunders
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.
References
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magmas: evidence from olivine, chromite and glass in Palaeogene pricrites
from West Greenland. Journal of Petrology, 41(7):
1071-1098.
- 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
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