Hourglass for Ancients Sands

The story of the first 4 billion years of Earth's history could make a lot more sense thanks to a new method for dating minerals in sedimentary rocks. The technique, described in today's Science, could reveal more precisely what our planet was like during the origins of life.

Geoscientists can reliably date igneous rocks of any age by measuring what percentage of their radioactive isotopes has decayed since the rock was molten. It has not been possible, however, to accurately date ancient sediments--which hold clues to past environments--unless these layers contain volcanic ash. And while fossils can help give a rough chronology for sedimentary rocks, most sediments from the Precambrian Era--spanning Earth's formation 4.6 billion years ago until 580 million years ago--lack traces of large multicellular life, because organisms did not have skeletons.

For some time, geologists Neal McNaughton, Birger Rasmussen, and Ian Fletcher of the University of Western Australia at Nedlands in Perth have been searching for minerals in sediments that might be radiometrically dated. They found that a common phosphate mineral called xenotime grows as tiny crystals (about 3 micrometers in size) in sediment after a layer is formed. It grows at an early stage (probably a million or so years--not long in geological time) within the well-known sequence of postdepositional changes in sediment.

Using a high-resolution ion microprobe, they analyzed the relative proportions of uranium isotopes and their lead decay products in xenotime crystals in Precambrian sandstones from the Kimberley Basin in northwestern Australia. The team determined that the rocks were deposited about 1.70 billion years ago. This jibes well with a date of 1.78 billion years for underlying igneous rocks.

"If this method is generally accurate," says geologist Roger Buick of the University of Sydney, "it promises to revolutionize the Precambrian calendar, allow us to date the scattered microfossil evidence for the early life, and establish its rates of biological evolution." The accuracy of the method can be checked quickly by further dating a range of sandstones whose age is already well constrained by other means.

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