Evidence for the survival of the oldest terrestrial mantle reservoir

• Published in: Nature (London) 2010-08, Vol.466 (7308), p.853-856
• Main Authors: Jackson, Matthew G., Carlson, Richard W., Kurz, Mark D., Kempton, Pamela D., Francis, Don, Blusztajn, Jerzy
• Format: Article
• Language: English
• Subjects: 704/2151/210; 704/2151/213; 704/2151/598; Basalt; Cenozoic; Earth; Earth mantle; Earth sciences; Earth, ocean, space; Exact sciences and technology; Helium; Humanities and Social Sciences; Internal geophysics; Isotope geochemistry; Isotope geochemistry. Geochronology; letter; Magma; Mantle; Mineralogy; multidisciplinary; Properties; Ratios; Reservoirs; Science; Science (multidisciplinary); Solid-earth geophysics, tectonophysics, gravimetry; Terrestrial environments
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature09287

High 3 He/ 4 He ratios in some basalts have been interpreted as evidence for ancient reservoirs preserved in the Earth’s mantle; however, such rocks have never been observed to host the primitive lead-isotopic compositions required for an early formation age. These authors show that Baffin Island and West Greenland lavas exhibit primitive lead-isotope ratios consistent with a mantle source age of 4.55–4.45 billion years, and that their source may be the most ancient accessible reservoir in the mantle. Primordial volcanism today Oceanic island lavas with high 3 He/ 4 He ratios are thought by some to sample a primordial terrestrial reservoir preserved in the Earth since it accreted from the solar nebula about 4.5 billion years ago, but these lavas have never been found to exhibit the primitive lead-isotopic compositions associated with such an early formation age. Now Matthew Jackson and colleagues show that Baffin Island and West Greenland lavas, previously found to host the highest terrestrial mantle 3 He/ 4 He ratios, have primitive lead-isotope ratios that are consistent with an ancient mantle source age of 4.55 billion to 4.45 billion years. The combined helium, lead and neodymium isotopic compositions in these lavas suggests that their source is the most ancient accessible reservoir in Earth's mantle — and it may be parental to all mantle reservoirs that give rise to modern volcanism. Helium is a powerful tracer of primitive material in Earth’s mantle. Extremely high 3 He/ 4 He ratios in some ocean-island basalts suggest the presence of relatively undegassed and undifferentiated material preserved in Earth’s mantle. However, terrestrial lavas with high 3 He/ 4 He ratios have never been observed to host the primitive lead-isotopic compositions that are required for an early (roughly 4.5 Gyr ago) formation age 1 , 2 . Here we show that Cenozoic-era Baffin Island and West Greenland lavas, previously found to host the highest terrestrial-mantle 3 He/ 4 He ratios 3 , 4 , 5 , exhibit primitive lead-isotope ratios that are consistent with an ancient mantle source age of 4.55–4.45 Gyr. The Baffin Island and West Greenland lavas also exhibit 143 Nd/ 144 Nd ratios similar to values recently proposed for an early-formed (roughly 4.5 Gyr ago) terrestrial mantle reservoir 6 , 7 . The combined helium-, lead- and Nd-isotopic compositions in Baffin Island and West Greenland lavas therefore suggest that their source is the most ancient accessible reservoir in the Earth