Mechanisms of Bimodal Magmatism Generation Beneath Southwest Indian Ridge: Implications for the Variable Composition of MORBs

At mid-ocean ridges, volcanic activity is predominantly marked by the voluminous effusion of tholeiitic basaltic lavas, with sporadic occurrences of mildly alkalic basalts. However, the genetic link between voluminous tholeiitic basalts and small-volume alkali basalts remains enigmatic. We report both alkaline and tholeiitic volcanism at the Marion Rise segment of the ultraslow spreading (14 mm/yr) Southwest Indian Ridge (SWIR). In contrast to the effusive tholeiitic (MORB) volcanism in Dr 27 along the Marion Rise, the Dr 30 samples consist of transitional to alkalic glass and alkaline scoria with E-MORB-like affinity (e.g., with high K/Ti, volatile contents (e.g., CO₂ and H₂O), La/Sm and radiogenic heavy isotopes). Critically, the petrological and geochemical evidence of melt inclusions and their host minerals suggests shallow mixing of a volatile-rich low-viscosity alkaline magma with entrained MORB crystal mush. The MORB mush represents a later volatile-poor viscous melt erupted effusively from beneath the axial valley onto the seafloor. This indicates bimodal magmatism beneath ocean ridges with the generation of early-formed alkaline melts that ascend independently of the far more voluminous tholeiites, which then interact and mix in the melt storage region in the ocean lithosphere. Mixing of small volume alkaline melts with more voluminous tholeiitic melts then explains the local major element uniformity of MORB and its isotopic and trace element diversity. We suggest this may apply globally as at magmatically more robust ridges the role of alkali basalt is likely masked by the far more voluminous MORB.