Ms. Mitchem's precise departure date isn't known; no specific reason was given for her departure other than to “pursue opportunities elsewhere,” according to the source.
Frank Porcelli, head of BlackRock's U.S. operations, and Robert Capaldi, head of the company's U.S. institutional business, will handle Ms. Mitchem's duties until a replacement is named, the source said.
Ms. Mitchem could not be reached for comment.
Brian Beades, a BlackRock spokesman, said: “As a matter of firm policy, we do not comment on employee activity.”
Ms. Mitchem, who was managing director and head of Barclays Global Investors' U.S. defined contribution business, was tapped to lead the combined unit after BlackRock acquired BGI last year. The combined DC assets of BGI and BlackRock were $213.3 billion at the end of 2008, the latest data available.