Oracle Text

1UU: Target permanent becomes an artifact in addition to its other types. (This effect lasts indefinitely.)

3U: Gain control of target artifact. (This effect lasts indefinitely.)

Card Rulings

12/1/2004 The effects of Memnarch’s abilities don’t end at end of turn, and they don’t end when Memnarch leaves the battlefield. Both effects last until the affected permanent leaves the battlefield.
12/1/2004 You can use the first ability on a nonartifact permanent, wait for the ability to resolve, and then use the second ability on that permanent.