Skip to main content Accessibility help

The generation of small melt-fractions in truncated melt columns: constraints from magmas erupted above slab windows and implications for MORB genesis

  • Malcolm J. Hole (a1) and Andy D. Saunders (a2)


At a number of locations along the Pacific margin of the Americas and west Antarctica, small volumes of alkalic basalts were erupted following successive ridge crest-trench collisions. The basalts were generated as a result of upwelling of asthenosphere through windows in the subducted plate. There is no evidence for local high temperature mantle plumes or significant lithospheric extension associated with these basalts. For the best sampled area, the Antarctic Peninsula, mean values for fractionation-corrected iron content (FeO*) vary from 6.9 to 10.6 wt.%, and for Na2O (Na8.0) from 3.25 to 4.6 wt.%, implying generation of small melt-fractions at variable mean pressures. The results of rare earth element inversion modelling yield a melt generation interval of 100 to 52 km, with a maximum melt fraction of c. 7% generated from a MORB-like source at T p 1300°C. Trace element and isotope systematics are also consistent with the generation of the basalts from a MORB-like source.

Mean pressures of melt generation increase with increasing distance from the original trench, but trace element and Na8.0 data suggest that there is no systematic variation in extent of melting with distance from the trench. The data are consistent with a model whereby a MORB melting column, initially intersecting the peridotite solidus at between 15 and 30 kbar, is truncated by a lithospheric cap which thickens from c. 15–20 km (≈ 5–8 kbar) up to a maximum of c. 50 km (≈ 15 kbar), such that the mean pressures of melting increase with increasing distance from the palaeo-trench. The MORB-like major element geochemistry of basalt samples closest to the ancestral trench are consistent with initial intersection of the peridotite solidus at low pressures (c. 15 kbar). For areas of thickest lithosphere, mean pressures of melt generation are higher, but extents of melting are lower than for a MORB melting column with a similar initial pressure of intersection of the peridotite solidus. All these basalts therefore potentially represent analogues for the small melt-fraction precursors to the generation of MOR tholeiites.

Thermal constraints suggest that these low volume, small melt-fractions, were generated with CO2 on the solidus, because mean pressures of melt generation are greater than the pressure of intersection of the T p 1300°C mantle adiabat and the dry peridotite solidus. Potentially, all MORB may be generated initially with CO2 on the solidus, and if this is correct, it does not require thermal anomalies to generate large extents of melting at high mean pressures.



Hide All
Barker, P.F. (1982) The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crest-trench interactions. J. Geol. Soc. London, 139, 787–801.
Beattie, P. (1993) Uranium-thorium disequilibria and partitioning on melting of garnet lherzolite. Nature, 363, 63–5.
Bevier, M.L. (1983) Implications of chemical and isotopic composition for petrogenesis of Chilcotin Group Basalts, British Columbia. J. Petrol, 24, 207–26.
Brodholt, J.P. and Batiza, R. (1989) Global systematics of unaveraged mid-ocean ridge basalt compositions: Comment on ‘Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness’ by E.M. Klein and C.H. Langmuir. J. Geophys. Res., 94, 4231–9.
Chaffey, D.J., Cliff, R.A. and Wilson, B.M. (1989) Characterization of the St Helena magma source. In Magmatism in the ocean basins.(Saunders, A.D. and Norry, M.J., eds.. Spec. Pub. Geol. Soc. London, 42, 313–45.
Chauvel, C., Hofmann, A.W. and Vidal, P. (1992) HIMU-EM: the French Polynesian connection. Earth Planet. Sci. Lett., 110, 99–119.
Davies, G.R., Norry, M.J., Gerlach, D.C. and Cliff, R.A. (1989) A combined chemical and Pb-Sr-Nd study of the Azores and Cape Verde hotspots: the geodynamic implications. In Magmatism in the ocean basins. (Saunders, A.D. and Norry, M.J., eds.. Spec. Pub. Geol. Soc. London, 42, 231–56.
Dickinson, W.R. and Snyder, W.S. (1979) Geometry of subducted slabs related to the San Andreas transform. J. Geology, 87, 609–27.
Elliot, T.R., Hawkesworth, C.J. and Gronvold, K. (1991) Dynamic melting of the Iceland plume. Nature, 351, 201–6.
Fitton, J.G. and Dunlop, H.M. (1985) The Cameroon line, West Africa, and its bearing on the origin of oceanic and continental alkali basalt. Earth Planet. L .72, 2338.
Forsythe, R.D. and Nelson, E.P. (1985) Geological manifstations of ridge collision: evidence from the Golfo de Penas-Taito Basin, Southern Chile. Tectonics, 4, 477–495.
Gill, J.B. (1981) Orogenic andesites and plate tectonics. Berlin: Springer-Verlag.
Hart, S.R. (1988) Heterogeneous mantle domains: signatures, genesis and mixing chronologies. Earth Planet. Sci. Lett., 90, 273–96.
Hole, M.J. (1988) Post-subduction alkaline volcanism along the Antarctic Peninsula. J. Geol Soc. London, 145, 985–8.
Hole, M.J. (1990) Geochemical evolution of Pliocene- Recent post-subduction alkalic basalts from Seal Nunataks, Antarctic Peninsula. J. Volcanoi Geotherm. Res., 40, 149–67.
Hole, M.J. and Larter, R.D. (1993) Trench proximal volcanism following ridge crest -trench collision along the Antarctic Peninsula. Tectonics, 12, 897–910.
Hole, M.J., Saunders, A.D., Marriner, G.F. and Tarney, J. (1984) Subduction of pelagic sediments: implications for the origin of Ce-anomalous basalts from the Mariana islands. J. Geol. Soc. London, 141, 453—72.
Hole, M.J., Rogers, G., Saunders, A.D. and Storey, M. (1991) The relationship between alkalic volcanism and slab-window formation. Geology, 19, 657–60.
Hole, M.J., Kempton, P.D. and Millar, I.L. (1993) Trace element and isotope characteristics of small degree melts of the asthenosphere; evidence from the alkalic basalts of the Antarctic Peninsula. Chem. Geol., 109, 51–68.
Hole, M.J., Saunders, A.D., Sykes, M.A. and Rogers, G. (1995) The relationship between alkalic magmatism, lithospheric extension and slab window formation along continental destructive plate margins. I. Volcanism associated with extension along consuming plate margins (Smellie, J.L., ed.) Spec. Pub. Geol. Soc. London, 81, 265—85.
Humler, E., Thirot, J-L. and Montagner, J-P. (1993) Global correlations of mid-ocean ridge basalt chemistry with seismic tomography images. Nature, 364, 225–7.
Ito, E White, W. M. and Gopel, C. (1987) The O, Sr, Nd and Pb isotope geochemistry of MORB. Chem. Geol., 62, 157–76.
Johnson, C.M. and O'Neil, J. R. (1984) Triple junction magmatism: a geochemical study of Neogene volcanic rocks in western California. Earth Planet. Sci. Lett., 71, 241–62.
Klein, E.M. and Langmuir, C.H. (1987) Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness. J. Geophys. Res., 92, 8089–115.
Klein, E.M. and Langmuir, C.H. (1989) Local versus global variations in mid-ocean ridge basalt composition: a reply. J. Geophys. Res., 94, 4241–52.
Larter, R.D. and Barker, P.F. (1991) Effects of ridgecrest trench interaction on Antarctic-Phoenix spreading: forces on a young subducting plate. J. Geophys. Res., 96, 9583-607.
LeMasurier, W.E. and Thomson, J.W. (1990) Volcanoes of the Antarctic Plate and Southern Oceans. American Geophysical Union, Antarctic Research Series, 48, 147—256.
McKenzie, D.P. (1984) The generation and compaction of partially molten rock. J. Petrol, 25, 713–65.
McKenzie, D.P. (1985a) The extraction of magma from the crust and mantle. Earth Planet. Sci. Lett., 74, 81–91.
McKenzie, D.P. (1985/7) 230Th-23SU disequlibrium and the melting processes beneath ridge axes. Earth Planet. Sci. Lett., 72, 149–57.
McKenzie, D.P. and Bickle, M.J. (1988) The volume and composition of melt generated by extension of the lithosphere. J. Petrol, 29, 625–79.
McKenzie, D.P. and O'Nions, R.K. (1991) Partial melting distributions from inversion of Rare Earth Element concentrations. J. Petrol., 32, 1021–91.
McKenzie, D.P. and O’Nions, R.K (1995) The source regions of ocean island basalts.. J. Petrol, 36, 133–59.
Morgan, J.P., and Chen, Y.J. (1993) Dependence of ridge-axis morphology on magma supply and spreading rate. Nature, 364, 706–8.
Olafsson, M. and Eggler, D.H. (1983) Phase relations of amphibole, amphibole-carbonate and phlogopite- carbonate peridotite: petrologic constraints on the asthenosphere. Earth. Planet. Sci. Lett., 64, 305–15.
Oxburgh, E.R. (1980) Heat flow and magmagenesis. In Physics of Magmatic Processes (Hargraves, R.B., ed.) New Jersey: Princeton Univ. Press, 161—99.
Palacz, Z.A. and Saunders, A.D. (1986) Coupled trace element isotope enrichment in the Cook-Austral- Samoa islands, southwest Pacific. Earth. Planet. Sci. Lett., 79, 270–80.
Pearce, J.A. (1983) Role of the sub-continental lithosphere in magma genesis at active continental margins. In Continental Basalts and Mantle Xenoliths. (Hawkesworth, C.J. and Norry, M.J., eds.) Nantwich, England: Shiva Publishing, pp. 230—49.
Ringe, M.J. (1991) Volcanism on Brabant Island, Antarctica. In The geological evolution of Antarctica (Thomson, M.R.A., Crame, J.A. and Thomson, J.W., eds.) Cambridge, England: Cambridge University Press, 515—9.
Rogers, G. and Saunders, A.D. (1989) Magnesian Andesites from Mexico, Chile and the Aleutian Islands: implications for magmatism associated with ridge-trench collisions. In Bonninites and related rocks (Crawford, A.J., ed.) London: Unwin-Hyman, 41645.
Saunders, A.D., Rogers, G., Marriner, G.F., Terrell, D.J. and Verma, S.P. (1987) Geochemistry of Cenozoic volcanic rocks, Baja California, Mexico: implications for the petrogenesis of post-subduction magmas. J. Volcanol. Geotherm. Res., 32, 223–45.
Saunders, A.D., Norry, M.J. and Tarney, J. (1988) Origin of MORB and chemically depleted mantle reservoirs: trace element constraints. J. Petrol. Special Lithosphere Issue, 414—45.
Smellie, J.L., Pankhurst, R.J., Hole, M.J.and Thomsom, J.W. (1988) Age, distribution and eruptive conditions of late Cenozoic alkaline volcanism in the Antarctic Peninsula in eastern Ellsworth Land: Review. British Antarctic Survey Bull., 80, 21–49.
Storey, M., Rogers, G., Saunders, A.D. and Terrell, D., (1989) San Quintm volcanic field, Baja California, Mexico: within-plate magmatism following ridge subduction. Terra Nova, 1, 195–202.
S-S., Sun and McDonough, W.F. (1989) Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. I. Magmatism in the ocean basins (Saunders, A.D. and Norry, M.J., eds.) Spec. Pub. Geol. Soc. London, 42, 31345.
Thompson, A.B. (1992) Water in the Earth's mantle. Nature, 358, 295–302.
Thorkelson, D.J. and Taylor, R.P. (1989) Cordilleran slab windows. Geology, 17, 833–6.
Weaver, B.L. (1991) The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett., 382—97.
Wendlandt, R.F. and Mysen, B.O. (1980) Melting phase relations of natural peridotite +CO2 as a function of melting at 15 and 30 kbar. Amer. Mineral., 65, 37–44.
White, R.S., McKenzie, D.P. and O'Nions, R.K. (1992) Oceanic crustal thickness from seismic measurements and rare earth element inversions. J. Geophys. 97, 683—716.
Wood, D.A., Varet, J., Bougault, H., Corre, O., Joron, J-L., Trueil, M., Bizouard, H., Norry, M.J., Hawkesworth, C.J. and Roddick, J.C. (1978) Transition metal and trace element analyses of Leg 49 samples. Init. Rep. Deep Sea Drill. Proj., 49, 897–902.
Wyllie, P.J. (1982) Subduction products according to experimental predictions. Bull Geol. Soc. America, 93, 468–76.
Wyllie, P.J. (1987) Discussion on recent papers on carbonated peridotite, bearing on mantle metasomatism and magmatism. Earth Planet. Sci. Lett., 82, 391–7.


The generation of small melt-fractions in truncated melt columns: constraints from magmas erupted above slab windows and implications for MORB genesis

  • Malcolm J. Hole (a1) and Andy D. Saunders (a2)


Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed