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

Abstract

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 lherzolite) 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.

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