Geoscientist Online

Geochemists think they have discovered lavas that derived directly from a region of primordial mantle, unchanged since the Earth formed. Harriet Jarlett* reports.

Geoscientist Online 2 November 2010

Scientists have long believed that any traces of the Earth’s primordial geochemistry were destroyed long ago by the planet’s many geological processes, which differentiate and recycle Earth materials, washing them clean of all traces of their origins. However, new evidence from rocks in the Canadian Arctic suggests that some samples may still provide clues to the Earth’s earliest history.

Matthew Jackson (Boston University) and co-authors¹ think they have identified clear isotopic evidence that Cenozoic lavas from Baffin Island and West Greenland, Canada, derive from a deep Earth reservoir that has remained inaccessible to the Earth’s washing-machine processes for four and a half billion years. Their work involves the first ever combined analysis of helium, lead, neodymium and hafnium on these lavas, which are mid ocean-ridge basalts (MORB) erupted during the opening of the North Atlantic.

The greatest evidence for the existence of this reservoir of primitive materials is the higher ratio of primordial (³He) to radiogenic helium (⁴He). Radiogenic ⁴He is produced during the decay of unstable isotopes of uranium and thorium. This indeed is the source of all our available helium gas - a precious resource that is, thanks to its artificially low market price, currently being squandered in toy balloons at a much faster rate than it can accumulate. By contrast, ³He levels have been declining continuously since accretion, as this primitive isotope leaked out in volcanic activity. Unusually high levels of ³He seen in these rocks can only mean that they were derived from a mantle source that has never degassed.

But helium is not the only clue. The lavas’ lead isotope compositions also lie very close to the 4.5Ga geochron, suggesting an age of between 4.55 and 4.45 billion years for their source – also dating them to a remote era before planetary differentiation, when the rock cycle began to overwrite the geochemistry of the source materials of our planet.

The lavas also contain a 5% higher ratio of the element samarium (Sm) to neodymium (Nd) than is found in chondrite meteorites. Neodymium is produced during the radioactive decay of samarium, so the higher ratio found in the Baffin Island lavas seems also to point to a primitive source.

Coauthor Richard Carlson (Carnegie Institution of Washington) says that these lavas “set the stage for everything. Primitive mantle, such as we have identified, would have been the ultimate source of all the magmas, and all the different rock types, we see on Earth today."

Reference

1. Jackson M G, et al. Evidence for the survival of the oldest terrestrial mantle reservoir. Nature, 2010; 466 (7308): 853 DOI: 10.1038/nature09287

*Harriet Jarlett is a UCL graduate geologist working as an intern on Geoscientist