HAWTHORNE, Nev. — A mortar shell explosion killed seven Marines and injured a half-dozen more during a training exercise in Nevada’s high desert, prompting the Pentagon to immediately halt the use of the weapon worldwide until an investigation can determine its safety, a military official said Tuesday. The explosion occurred Monday night at the Hawthorne Army Depot, a facility used by troops heading overseas, during an exercise involving the 2nd Marine Expeditionary Force at Camp Lejeune, N.C. Several Marines from the unit were injured in the blast, authorities said. It was not immediately clear whether the 60mm mortar shell exploded …