Abstract We investigate relationships between the chemistry of global Mid-Ocean Ridge Basalts (MORBs), the conditions and composition of their source mantle, and the physical properties of the ridges. To accomplish this, major and trace element compositions of primary MORB are estimated by a back-calculation code PRIMACALC3 assisted by a thermodynamically constrained forward fractional crystallisation model COMAGMAT 3.72 with an additional PRIMELT3-P model to ensure mantle equilibrium of the primary melts. We then apply Ocean Basalt Simulator version 1 (OBS1), a thermodynamically constrained forward calculation code for adiabatic melting of a pyroxenite-bearing mantle peridotite, to the trace element compositions of our primary MORBs. Melting conditions and compositions of the MORB source mantle, such as pressure (P), degree of partial melting (F), mantle potential temperature (Tp), fractions of pyroxenite (fPY) and ambient depleted mantle (fDM), and the degree of depletion of the final residual mantle (FPD) are estimated by the two models. The results show that (a) MORBs are derived from mantle with Tp = 1250 − 1520 °C at F = 3 − 31 %, and both are high beneath shallow ridges; (b) pyroxenites in the source mantle peridotite are both recycled MORBs and ocean island basalts (OIBs) in origin and their fractions are estimated to be 0 − 6 % with a high fraction in deep and cold ridges; (c) the fraction of OIB-type pyroxenite is highest at shallow and hot ridges; (d) the degree of depletion of the residual peridotite is highest at shallow and hot ridges, and (e) the isotopic compositions of the OIB-type pyroxenites have both EM1 and HIMU signatures, while that of the MORB-type pyroxenites and the ambient mantle peridotites have depleted compositions. The high-Tp mantle containing recycled OIB-type pyroxenite has a deep plume origin. It forms ridges shallower than 2500 m. b. s. l. due to thermal and chemical buoyancy relative to the ambient upper mantle. In contrast, deeper ridges have a narrower and lower Tp range, and a lesser but variable amount of peridotite depletion. The depth of mid-ocean ridges is controlled by both Tp and relative mantle fertility.
Kimura et al. (Wed,) studied this question.