THE TRANSITION FROM CRATON LITHOSPHERIC MANTLE TO DEPLETED OCEANIC MANTLE: IMPLICATION FROM CENTRAL LENA TROUGH K-ENRICHED BASALTS

An unusual kind of potassium-enriched mid-ocean ridge basalt (MORB) was first observed in the Southwest Indian Ridge, and then in the central Lena Trough (CLT) in the Arctic Ocean. These MORBs are distinct from the post-collisional ultra-potassic basalts and cratonic mafic ultra-potassic rocks (i.e. kimberlite and lamporite). In contrast to other MORBs and OIBs, the K-enriched MORBs are higher in SiO₂, Al₂O₃, K₂O, K₂O/TiO₂, and lower in FeO. Their trace elements present strong enrichment in Rb, Ba, Nb, Ta; weak positive anomalies in Sr and Ti; negative anomalies in Th and U; as well as the steeper HREE than MORBs. Low [Dy/Yb]_PM and La/Yb ratios concentrate from 1.21 to 1.36 and from 3.77 to 6.60, respectively, indicating a source with no obvious garnet present under low pressure. [Ba/Th]_PM scatters from 2.51 to 5.06 indicating a heterogeneous mantle source. The Low Th/U ratio from 2.97 to 4.89 excludes the possibility of an amphibole-bearing source. The trace element quantitative modeling indicates that 5% degree of partial melting from 0.97% Phl and 0.03% Amp mixing with 99% DM (70% Ol, 10% Cpx, 20% Opx mantle peridotite) is consistent with the incompatible elements pattern of CLT basalt. P-T estimation shows 1198-1212^oC/4-7kbar as the possible melting condition for CLT basalt, which is along the 1200^oC mantle adiabat. Whereas the chemical composition of north Lena Trough (NLT) basalt is similar to N-MORB, and the P-T estimation ranges from 1312-1339^oC/8-12kbar, which corresponds to 1300^oC normal mantle adiabat. The CLT basalt bulk composition is the mixture of 40% of the K-enriched endmember and 60% N-MORB-like endmember. The K-enriched endmember is 3-5% degree of partial melting of a source (0.97% Phl, 0.03% Amp, and 99% DM) along 1200^oC mantle adiabat from 8 to 10 kbar modelled from AlphaMelts. The N-MORB-like endmember is similar to NLT basalt in composition. Therefore the warm N-MORB-like mantle and the cold K-enriched mantle coexist in the CLT region. The central Lena Trough represents the transition from cold craton lithospheric mantle to hot oceanic mantle, which is simultaneous to the Arctic Ocean opening.