Cabot Corp.’s first-quarter earnings fell more than 50 percent due mostly to expenses related to acquisition and restructuring. The Boston-based company posted net income of $20 million, or 31 cents per share, compared with $46 million, or 71 cents, a year ago. That includes a charge of 35 cents per share for expenses. Last summer, Cabot agreed to pay $1.1 billion for Norit NV, a Dutch company that makes activated carbon which can be used to clean water and other things.