The “natural” fractionation of oxygen’s three stable isotopes, based on the average in ocean water, is 99.762 percent oxygen-16, 0.038 oxygen-17, and 0.2 percent oxygen-18. The standard ratio of the four stable isotopes of sulfur is 95.02 percent sulfur-32, 0.75 percent sulfur-33, 4.21 percent sulfur-34, and 0.02 percent sulfur-36. The standard comes from a form of pure iron sulfide called troilite (inset) found in Diablo Canyon iron meteorites whose parent, a fragment of which is shown at center, created Meteor Crater in Arizona. Credit: Berkeley Lab (Diablo Canyon troilite photo courtesy of KD Meteorites, www.kdmeteorites.com/)