Melting of peridotite beneath Mid Ocean Ridges: Simplicity and complexity

Chemical and isotopic compositions of mid ocean ridge basalts (MORBs) are relatively homogeneous when compared with the basalts from other tectonic settings, e.g., ocean islands and arcs. Nevertheless, their variations indicate that the source mantle beneath MORs should be heterogeneous and the melt generation and coalescence processes are complex. I here present an overview of such the melting processes of peridotite beneath MORs. The common (and simple) background of MORBs is the (1) melts from the most depleted mantle source (DMM) in the earth with (2) the most depleted isotopic compositions suggesting ancient melt depletion from the source. The estimated DMM (for normal MORB: N-MORB) has lherzorite composition (cpx = ~15%) formed by ~5% melt extraction from primitive mantle (PM). Isotopic depletion of DMM suggests that melt extraction occurred at ~2 Ga by MORB extraction from chondritic PM in order to form the present day DMM. This age almost correlates with the oldest TDM Os model age in the abyssal peridotite. The narrow compositional range of N-MORB can be accounted for by a common adiabatic melting column in the spreading rides with mantle potential temperature Tp = 1300 °C, termination depth of melting at <1 GPa, and degree of melting F = 15% with source H2O = 100 ppm. The residual mantle after melting is almost harzburgite (cpx < 5%) and their rare earth element (REE) compositions corresponds to those of the abyssal peridotite (harzburgite). 

Although MORB and its source mantle appear to be the products of a simple adiabatic melting of DMM, considerable variations are present. These are (1) major element variation, e.g., Na8.0 and Fe8.0, (2) trace element variation between D-MORB and E-MORB, and (3) isotopic compositions varying from DM to FOZO-EM1, but very scarcely to EM2 and HIMU. These are due to the heterogeneous mantle source and a complex melting regime beneath MORs. The isotopic variations occur typically in the plume-influenced MORs, such as Iceland and Galapagos, and the origin is considered to be recycled materials such as igneous ocean plate slab or delaminated lower continental crust. The enriched sources have pyroxenite (eclogite) compositions, so that contribute to Fe- and Si-rich MORBs. Trace element composition is also affected by the pyroxenite source in DMM resulting theoretically in M-REE humped multi element plot pattern or unusually enriched/depleted Nb, Ta, Ba, and Sr, dependent on the recycling source material. 

Even with the various sources, it is still hard to generate enriched (E)-MORB. E-MORB has a flat heavy (H)REE with a steep light (L)REE-rich patterns. This can be generated by interaction between DMM and LREE-rich material, such as deep partial melt reacted instantaneously with molten DMM or melting of a frozen pyroxenite of deep melt origin with metasomatized DMM. Such the melting regime is achievable by a dynamic melting column where upwelling velocity of a peridotite and melt are different. This is physically feasible because wetting angle of mantle olivine is narrow enough to permit melt migration at F < 0.01%. The upwelling velocity of DMM equals to the half spreading rate of MOR (2-5 cm/y). U-Th disequilibrium (Th-excess) in MORBs indicates that magma ascent occurs <25 ky from garnet stability depth (>70 km), indicating >280 cm/y, two orders of magnitude greater upwelling velocity. If melts are channelized, the velocity can be higher. The differential upwelling between the deep melts and DMM promotes reactions forming D-, N-, and E-MORBs and leaving dunite channels, harzburgite matrix. 

As above, combination between the common framework and the variations in the source composition and in the dynamic mantle processes would account for variations in MORBs and residual mantle thus geological phenomena beneath MORs.