def RbSr_rat(Rb,Sr,Srrat):
    """
    Calculate 87Rb/86Sr from Rb and Sr concentrations
    
    Paramters:
        Rb: Rb concentration (ppm)
        Sr: Sr concentration (ppm)
        Srrat: 87Sr/86Sr ratio
    
    Returns:
        rbsr8786: Calculated 87Rb/86Sr ratio
        
    """
    # Fundamental Rb and Sr isotopic parameters - from CIAAW
    Rb85_mass = 84.91178974 # Da
    Rb87_mass = 86.90918053 # Da
    
    Rb85_abund = 0.7217
    Rb87_abund = 0.2783
    

    Sr84_mass = 83.913419
    Sr86_mass = 85.90926073
    Sr87_mass = 86.90887750
    Sr88_mass = 87.90561226
    
    # Sr abundances vary - only used for calculation of ratios that 
    # don't vary
    Sr84_abund = 0.0056
    Sr86_abund = 0.0986
    Sr87_abund = 0.0700 # Not used
    Sr88_abund = 0.8258
    
    Sr_8886 = Sr88_abund/Sr86_abund # 88Sr/86Sr ratio - doesn't vary
    Sr_8486 = Sr84_abund/Sr86_abund # 84Sr/86Sr ratio - doesn't vary
    
    # Calculate true abundances
    Sr86_abund_calc = Srrat/(Srrat+Sr_8886+Sr_8486+1)
    Sr84_abund_calc = Sr86_abund_calc*Sr_8486
    Sr88_abund_calc = Sr86_abund_calc*Sr_8886
    Sr87_abund_calc = Sr86_abund_calc*Srrat
    
    # Total Mass for Rb and Sr
    Rb_mass = Rb85_mass*Rb85_abund + Rb87_mass*Rb87_abund

    Sr_mass = (
        Sr84_mass*Sr84_abund_calc + Sr86_mass*Sr86_abund_calc
        + Sr87_mass*Sr87_abund_calc + Sr88_mass*Sr88_abund_calc
        )
    
    # 87Rb and 86Sr
    Rb87 = Rb*Rb87_abund/Rb_mass # Get mol of Rb87
    Sr86 = Sr*Sr86_abund_calc/Sr_mass # Get mol of Sr86
    rbsr8786 = Rb87/Sr86
    check = (Rb/Sr)*(2.69295 + 0.28304*Srrat)
    print('Check: ',check)
    
    return(rbsr8786)