Abstract
Two contrasting views exist regarding the nature and origin of mid-ocean ridge basalts (MORB): first, that these olivine-tholeiites with Mg/(Mg = Fe) = 0.70–0.72 are primary and are generated by partial fusion of plagioclase + spinel lherzolite at around 30-km (≈9 kbar pressure) in the mantle1. Others2–8 suggest that even the most primitive MORBs are not primary but are derived by fractionation of olivine±pyroxene from picritic parental magmas that are generated at depths of 45 km or more. Duncan and Green2,3 argued for the existence of a second primary magma, a high-MgO quartz-normative or olivine-poor tholeiite, which is produced at depths ≤25 km beneath the ocean ridges. This problem, concerning the composition and depth of origin of primary MORBS, has been approached here by experimentally melting a plagioclase+spinel lherzolite xenolith from Hawaii at 9 kbar pressure. The experimental melts are very similar to some mid-ocean ridge basalts, which suggests that the latter are probably primary and are generated at shallow depths in the mantle.
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Sen, G. Composition of basaltic liquids generated from a partially depleted lherzolite at 9 kbar pressure. Nature 299, 336–338 (1982). https://doi.org/10.1038/299336a0
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DOI: https://doi.org/10.1038/299336a0
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