How to get 60Hz Refresh Rate: MacBook Pro Retina 2012 and Dell Ultra HD 4K Monitor P2415Q 24"

I have a 1st generation 15" MacBook Pro Retina, and just bought a Dell 24" Ultra HD 4K Monitor (P2415Q).

MY SETUP

• The monitor is plugged into my Thunderbird port with the Mini DisplayPort (mDP) to DisplayPort cable provided by Dell.
• I enabled MST on the monitor (Monitor settings > Menu > Display > MST > Primary)
• I set the resolution (OS X Settings > Displays) to Scaled, using the middle option ("Looks like 2304 x 1096")

  I tried other resolutions as well, and I still have the same 30Hz refresh rate

• I do not have any other external monitors connected.

MY ISSUE

The refresh rate on the Dell 4K monitor is only 30Hz, even though the monitor is capable of 60Hz

• I can tell the difference when scrolling.
• I can confirm the difference by looking at the monitor's settings (Monitor settings > Menu displays "Resolution: 3840x2160, 30HZ" at the bottom of the menu)

Can anyone advise on how to get my refresh rate up to 60Hz?