Abstract
The PT-phase relations of a Hawaiian tholeiite with 18.2% MgO has olivine–orthopyroxene multiple saturation at 20.5 kbar and 1,550°C. This pressure is less than the pressure at the lithosphere/asthenosphere transition, and it is suggested that tholeiites with this and lesser MgO contents are fractionated. Assuming a harzburgitic residuum it is shown that Hawaiian primary tholeiites contain about 23% MgO, and are generated at 36±5 kbar and 1,680±50°C. This pressure is equivalent to a depth of 112 km, which is consistent with the thickness of the lithosphere and thermal plume modeling. The minimal MgO content of primary Hawaiian tholeiites is suggested as 19% MgO.
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