1. Materials Research Division; Carl Zeiss MicroImaging, Inc.; One Zeiss Drive,; Thornwood; New York; 10594; USA
2. Department of Geosciences; University of Massachusetts; 611 North Pleasant Street,; Amherst; Massachusetts; 01003-9297; USA
3. Department of Earth, Atmospheric, and Planetary Sciences; Massachusetts Institute of Technology; 77 Massachusetts Avenue,; Cambridge; Massachusetts; 02215; USA
4. Department of Earth Sciences; Oxford University; Parks Road,; Oxford; OX1 3PR; UK