The used U-Pb dating of zircons. They did not date the fossil directly, instead they dated other rocks in the same stratigraphic sequence (Isua supracrustal bel

The used U-Pb dating of zircons. They did not date the fossil directly, instead they dated other rocks in the same stratigraphic sequence (Isua supracrustal belt). The relevant sentences: The stromatolite discovery locality (Extended Data Fig. 1) is within the hinge of an anticline cored by 3,709 ± 9-Myr-old andesitic metavolcanic rocks with locally preserved pillow structures and a maximum metamorphic temperature of ~550 °C17, 18. The pillowed metavolcanic rocks are overlain by bedded dolomite-rich metasedimentary rocks and in turn, by interlayered quartzites and metamorphosed banded iron formation that contain rare, small, high Th/U oscillatory-zoned volcano-sedimentary zircons with ages of 3,699 ± 12 and 3,691 ± 6 Myr3, 18. That means rocks with zircons dated to 3709 Ma are interpreted to be below (older) and rocks with 3699 Ma zircons are interpreted as being above (younger). Zircon is an excellent mineral for geochronology as it contains U and Th. Two U isotopes, 235U and 238U, plus 238Th all decay to Pb at different rates (half lives). Zircon forms by growing from magma, and does not contain any Pb when grows, as Pb does not fit into the crystal structure, so any Pb you measure in a zircon today is from radiogenic decay of U and Th. If the ratios of the individual U and Th isotopes and their respective Pb daughter products are all consistent with the same age, i.e. you get the same age when you back calculate to zero Pb for all three isotope systems, then you can be very confident in the date, hence the small uncertainties on the reported dates from the paper.