Contributions to Mineralogy and Petrology

, Volume 74, Issue 1, pp 13–27 | Cite as

A phase diagram for mid-ocean ridge basalts: Preliminary results and implications for petrogenesis

  • Edward Stolper


Samples of a primitive mid-ocean ridge basalt (MORB) glass were encapsulated in a mixture of ol (Fo90) and opx (En90) and melted at 10, 15, and 20 kbar. After quenching, the basaltic glass was present as a pool within the ol+opx capsule, but its composition had changed so that it was saturated with ol and opx at the conditions of the experiment. By analyzing the quenched liquid, the location of the ol+opx cotectic in the complex, multicomponent system relevant to MORB genesis was determined.

As pressure increases from 1 atm to 10 kbar, the dry ol+opx cotectic moves from quartz tholeiitic to olivine tholeiitic compositions. With further increases in pressure, the cotectic continues to move toward the ol-di-plag join (i.e., toward alkalic compositions). Between 15 and 20 kbar, ol+opx+di-saturated liquids change from tholeiitic to alkalic in character, although part of the ol+opx cotectic is still in the tholeiitic (i.e, hy-normative) part of composition space. At pressures of 10–15 kbar, tholeiitic liquids may be able to fractionate to alkalic liquids on the ol+di cotectic.

Primitive MORB compositions come close to but do not actually lie on the ol+opx cotectic under any conditions studied. This suggests that not even the most primitive of known MORBs are primary melts of the mantle. The correspondence of most MORBs to the 1 atm ol+di+plag cotectic suggests that low pressure fractionation was involved in their genesis from parent liquids. Picritic liquids that have been proposed as parents to the MORB suite could equilibrate with harzburgite (or Iherzolite) at 15–20 kbar and thus could be primary. Fractionation of ol from these liquids could yield primitive MORB liquids, but other primary liquids or more complex fractionation paths involving others phases in addition to ol cannot be ruled out. The possibility that these picritic liquids could equilibrate with ol+opx at 25–30 kbar cannot be ruled out.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Bond WL (1951) Making small spheres. Rev Sci Instrum 22:344–345Google Scholar
  2. Bender JF, Hodges FN, Bence AE (1978) Petrogenesis of basalts from the Project FAMOUS area: experimental study from 0 to 15 kbars. Earth Planet Sci Lett 41:277–302Google Scholar
  3. Boyd FR, England JL (1960) Apparatus for phase equilibrium experiments at pressures up to 50 kbars and temperatures up to 1,750° C. J Geophys Res 65:741–748Google Scholar
  4. Cawthorn RG, Ford CE, Biggar GM, Bravo MS, Clarke DB (1973) Determination of the liquid composition in experimental samples: discrepancies between microprobe analysis and other methods. Earth Planet Sci Lett 21:1–5Google Scholar
  5. Chodos AA, Albee AL, Gancarz AJ, Laird J (1973) Optimization of computer-controlled quantitative analysis of minerals. Proc 8th Nat Conf on Electron Probe Analysis, New Orleans, LAGoogle Scholar
  6. Clarke DB (1970) Tertiary basalts of Baffin Bay: Possible primary magma from the mantle. Contrib Mineral Petrol 25:203–224Google Scholar
  7. Clarke DB, O'Hara MJ (1979) Nickel, and the existence of high-MgO liquids in nature. Earth Planet Sci Lett 44:153–158Google Scholar
  8. Elthon D (1979) High magnesia liquids as the parental magma for ocean floor basalts. Nature 278:514–518Google Scholar
  9. Elthon D, Ridley WI (1979) Comments on: “The partitioning of nickel between olivine and silicate melt”, by: SR Hart and KE Davis. Earth Planet Sci Lett 44:162–164Google Scholar
  10. Fujii T, Kushiro I (1977) Melting relations and viscosity of an abyssal tholeiite. Carnegie Inst Washington Yearb 76:461–465Google Scholar
  11. Gansser A, Dietrich VJ, Cameron WE (1979) Paleogene komatiites from Gorgona Island. Nature 278:545–546Google Scholar
  12. Green DH, Ringwood AE (1967) The genesis of basaltic magmas. Contrib Mineral Petrol 15:103–190Google Scholar
  13. Green DH, Hibberson WO, Jaques AL (1979) Petrogenesis of mid-ocean ridge basalts. In: MW McElhinney (ed) The Earth: Its Origin, Structure and Evolution. Academic Press, LondonGoogle Scholar
  14. Hart SR, Davis KE (1978) Nickel partitioning between olivine and silicate melt. Earth Planet Sci Lett 40:203–219Google Scholar
  15. Hart SR, Davis KE (1979) Reply to DB Clarke and MJ O'Hara, “Nickel, and the existence of high-MgO liquids in nature”. Earth Planet Sci Lett 44:159–161Google Scholar
  16. Irvine TN (1977) Definition of primitive liquid compositions for basic magmas. Carnegie Inst Washington Yearb 76:454–461Google Scholar
  17. Johannes W, Bell PM, Mao HK, Boettcher AL, Chipman DW, Hays JF, Newton RS, Siefert F (1971) An interlaboratory comparison of piston-cylinder pressure calibration using the albite breakdown reaction. Contrib Mineral Petrol 32:24–38Google Scholar
  18. Kesson S (1975) Mare basalts: Melting experiments and petrogenetic interpretations. Proc Lunar Sci Conf 6th. Geochim Cosmochim Acta, Suppl 6:921–944Google Scholar
  19. Kushiro I, Thompson RN (1972) Origin of some abyssal tholeiites from the Mid-Atlantic Ridge. Carnegie Inst Washington Yearb 71:403–406Google Scholar
  20. Kushiro I (1973) Origin of some magmas in oceanic and circumoceanic regions. Tectonophysics 17:211–222Google Scholar
  21. Langmuir CH, Bender JF, Bence AE, Hanson GN, Taylor SR (1977) Petrogenesis of basalts from the FAMOUS area: Mid-Atlantic Ridge. Earth Planet Sci Lett 36:133–156Google Scholar
  22. Longhi J, Walker D, Hays JF (1978) The distribution of Fe and Mg between olivine and lunar basaltic liquids. Geochim Cosmochim Acta 42:1545–1558Google Scholar
  23. Melson WG, Byerly GR, Nelen JA, O'Hearn T, Wright TL, Vallier T (1977) A Catalog of the major element chemistry of abyssal volcanic glasses. Smithson Contrib Earth Sci 19:31–60Google Scholar
  24. Mysen BO, Kushiro I (1977) Compositional variations of coexisting phases with degree of melting of peridotite in the upper mantle. Am Mineral 62:843–865Google Scholar
  25. O'Hara MJ (1968a) Are any ocean floor basalts primary magma? Nature 220:683–686Google Scholar
  26. O'Hara MJ (1968b) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4:69–133Google Scholar
  27. O'Hara MJ (1977) Geochemical evolution during fractional crystallization of a periodically refilled magma chamber. Nature 266:503–507Google Scholar
  28. Presnall DC, Dixon SA, Dixon JR, O'Donnell TH, Brenner NL, Schrock RL, Dycus DW (1978) Liquidus phase relations on the join diopside-forsterite-anorthite from 1 atm to 20 kbar: their bearing on the generation and crystallization of basaltic magma. Contrib Mineral Petrol 66:203–220Google Scholar
  29. Presnall DC, Dixon JR, O'Donnell TH, Dixon SA (1979) Generation of Mid-ocean Ridge Tholeiite. J Petrol 20:3–35Google Scholar
  30. Roeder PL (1974) Activity of iron and olivine solubility in basaltic liquids. Earth Planet Sci Lett 23:397–410Google Scholar
  31. Stolper E (1980) A phase diagram for mid-ocean ridge basalts. EØS 61:405Google Scholar
  32. Stolper E, Walker D (1980) Melt density and the average composition of basalt. Contrib Mineral Petrol 74:7–12Google Scholar
  33. Takahashi E (1980) Olivine/liquid nickel partitioning at high pressures: experiments with an olivine capsule. EØS 61:397Google Scholar
  34. Walker D, Longhi J, Lasaga AC, Stolper EM, Grove TL, Hays JF (1977) Slowly cooled microgabbro 15555 and 15065. Proc Lunar Sci Conf 8th. Geochim Cosmochim Acta, Suppl 8:1521–1547Google Scholar
  35. Walker D, Shibata T, Delong SE (1979) Abyssal tholeiites from the Oceanographer Fracture Zone II. Phase equilibria and mixing. Contrib Mineral Petrol 70:111–125Google Scholar
  36. Watson EB (1980a) Apatite saturation in basic to intermediate magmas. Geophys Res Lett 12:937–940Google Scholar
  37. Watson EB (1980b) Apatite saturation in magmas at high pressures. EØS 61:396Google Scholar

Copyright information

© Springer-Verlag 1980

Authors and Affiliations

  • Edward Stolper
    • 1
  1. 1.Division of Geological and Planetary SciencesCalifornia Institute of TechnologyPasadenaUSA

Personalised recommendations