[Mg/Si]–[C/O] diagram for the M-dwarf sample (overlying orange dots) and GK dwarf sample (underlying colourful dots in density). An ODR fit was applied to the M-dwarf sample, with the best fit shown in the solid line and 1σ and 2σ error intervals shown in dashed and dotted lines (in brown), respectively. The regression parameters are also shown on the plot. A Student’s t-test was also applied to test the significance of the slope different from zero, and the resultant p-value is less than 0.001.