Glass and Mineral Chemistry of Northern Central Indian Ridge Basalts:Compositional Diversity and Petrogenetic Significance

The glass and mineral chemistry of basalts examined from the northern central Indian ridge (NCIR) provides an insight into magma genesis around the vicinity of two transform faults: Vityaz (VT) and Vema (VM). The studied mid-ocean ridge basalts (MORBs) from the outer ridge flank (VT area) and a near-ridge seamount (VM area) reveal that they are moderately phyric plagioclase basalts composed of plagioclase (phenocryst [An60-90] and groundmass [An35-79]), olivine (Fo81-88), diopside (Wo45-51, En25-37, Fs14-24), and titanomagnetite (FeOt～63.75 wt% and TiO2～22.69 wt%). The whole-rock composition of these basalts has similar Mg# [mole Mg/mole(Mg+Fe2+)] (VT basalt:～0.56-0.58; VM basalt:～0.57), but differ in their total alkali content (VT basalt:～2.65; VM basalt:～3.24). The bulk composition of the magma was gradually depleted in MgO and enriched in FeOt, TiO2, P2O5, and Na2O with progressive fractionation, the basalts were gradually enriched in Y and Zr and depleted in Ni and Cr. In addition, the ΣREE of magma also increased with fractionation, without any change in the (La/ Yb)N value. Glass from the VM seamount shows more fractionated characters (Mg#: 0.56-0.57) compared to the outer ridge flank lava of the VT area (Mg#: 0.63-0.65). This study concludes that present basalts experienced low-pressure crystallization at a relatively shallow depth. The geochemical changes in the NCIR magmas resulted from fractional crystallization at a shallow depth. As a consequence, spinel was the first mineral to crystallize at a pressure >10 kbar, followed by Fe-rich olivine at <10 kbar pressure.