A piece of pristine Earth?

Like a batch of cookie dough, Earth is constantly beingstirred: Volcanism, tectonic-plate movements, and thermal convection can all transportmaterial between and within the mantle and crust. Even the lower mantle is notimmune to mixing: Studies of seismic waves’ speeds (see PHYSICS TODAY, August 1997, page 17 ), which measurethe temperature profile across Earth’s interior, show slabs of cool, denseoceanic crust sinking nearly to the core--mantle boundary. But now, BostonUniversity’s MatthewJackson and collaborators have found that rock samples from Baffin Islandin northeastern Canada, the products of an enormous volcanic eruption some 60 million years ago, show all the signs of having come from an ancient mantlereservoir, unstirred since not long after Earth’s formation 4.5 billion yearsearlier. The researchers based their conclusion on isotopic measurements ofseveral chemical elements, each with some isotopes produced by radioactivityand others that are purely primordial. If any mixing had occurred, it wouldhave changed the relative amounts of a radioactive parent element and itsdaughter--and thus the daughter’s present-day isotopic composition, which iseasier to measure than the parent--daughter ratio. If the Baffin samples didindeed come from a primitive reservoir, geophysical modelers will be challengedto explain how any part of the mantle could have remained so isolated for solong; one possible solution involves convective eddies that trap and preservethe pristine mantle material. (M. G. Jackson et al., Nature 466, 853, 2010 .)—Johanna Miller