Seven Marines were killed Monday night after an explosion during a training exercise at the Hawthorne Army Depot in Nevada. A 60 mm mortar exploded during a live-fire training exercise about 10 p.m. as troops from the 2nd Marine Division based at Camp Lejeune, N.C., were planning to fire it. The Marine Corps issued an indefinite moratorium on firing all 60 mm mortars worldwide until an investigation clears the weapon and ammunition as safe. The mortar round exploded in its firing tube, officials said at a press conference Tuesday afternoon. The 60 mm mortar is a weapon that traditionally requires …