Oil prices tumbled on Monday as Iran and six world powers closed in on a nuclear deal that would end sanctions on the Islamic Republic and let more Iranian oil on to world markets. News of a unanimous agreement by European leaders on a bailout loan for Athens, which should allow Greece to stay in the euro zone, helped pare early losses. Brent crude for August (LCOc1) fell $1.89 to a low of $56.84 a barrel before rallying back to around $57.30 by 0840 GMT.