The major indexes were narrowly mixed at midday Thursday after erasing most of their early gains. The Nasdaq was up 0.1% while the Dow Jones industrial average and the S&P 500 were off less than 0.1% either. Volume was tracking lower. There were no clear indications that would explain a sell-off that started around 10:40 a.m. ET. But the stock market today remains jittery about the Ukraine crisis.