How can I mute my monitor speakers and listen only through headphones only using a Quantum?

How can I mute my monitor speakers and listen only through headphones only using a Quantum? My previous mixer's outs were going through a Presonus Headphone Amp, which had a mute switch. I could leave the speakersj' power tued on but mute them when desired. Is there a mute switch somewhere on the Quantum (I'm jjust getting started with it). Or would I somehow change the outputs in the DAW so they didn't go to the main outs? Reemember, I still need to listen through the Quantum's headphone jack.