def kd(Cs, Cl):
    """
    kd calculates a partition coefficient for a given set of measurements. For 
    igneous petrology, this is commonly the concentration of a trace element in
    the mineral divided by the concentration of the same trace element in the
    melt (e.g. Rollinson 1993 Eq. 4.3)
    
        Inputs:
        Cs = concnetration in the mineral
        Cl = concentration in the melt
        
        Returns:
        kd = partition coefficient for the given input parameters
        
    """
    kd = Cs / Cl
    return kd