High-pressure phase equilibria studies on a high-MgO basalt from the Tortuga ophiolite complex indicate that olivine + orthopyroxene + clinopyroxene + garnet are its liquidus phases at 25 kbars. At 10 to 20 kbars, olivine is the liquidus phase and is followed by spinel, clinopyroxene, and orthopyroxene as temperature decreases. At <10 kbars, the order of crystalization is olivine, spinel, plagioclase, clinopyroxene, and orthopyroxene.

The high-pressure phase equilibria determined in this study, supplemented with the results obtained on other tholeiitic basalt compositions, are used to construct pseudo-liquidus phase diagrams for evaluating and predicting the nature of primary magmas and melting of the mantle at high pressures. These pseudo-liquidus phase diagrams suggest that “primitive” oceanic basalts with >9.5% MgO are derived from primary oceanic basalts generated at 15 to 25 kbars.

Models for the origin of oceanic basalts at low pressures (≤10 kbars) are considered incapable of generating the most “primitive” oceanic basalts that have >9.5% MgO. It is likely that most oceanic basalts with >9.5% MgO are ultimately derived from primary high-MgO basalts with >14% MgO that have been produced by melting at 15 to 25 kbars. High-MgO basalts of this type are found in numerous oceanic or rifting environments (e.g., Baffin Bay, Gorgona Island, Tortuga ophiolite, Betts Cove ophiolite, Lewis Hills ophiolite, Ungava Peninsula), suggesting that these high-MgO basalts are produced on a world-wide scale throughout geologic time. Chemical differentiation processes, however, normally modify these high-MgO basalts into the more common basalts with 7 to 10% MgO. Because most oceanic basalts with <9.5% MgO have equilibrated to low-pressure cotectics, determination of the conditions of origin for the primary magmas from which they are derived is not well constrained. For this reason, melting of the mantle at pressures of ≤lO kbars could produce primary magmas capable of differentiating to form oceanic basalts that are similar to many of the evolved oceanic basalts that have <9.5% MgO. Until crystallization and mixing processes are more fully understood, melting at < 10 kbars cannot be eliminated as a possibility for the origin of some primary oceanic basalts.

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