Episodic entrainment of deep primordial mantle material into ocean island basalts

• Published in: Nature communications Vol. 6; no. 1; p. 8937
• Main Authors: Williams, Curtis D., Li, Mingming, McNamara, Allen K., Garnero, Edward J., van Soest, Matthijs C.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 24.11.2015; Nature Publishing Group; Nature Pub. Group
• Subjects: 119/118; 704/2151/213; 704/2151/2809; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms9937

Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth’s mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of 3 He/ 4 He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of 3 He in OIBs has motivated hypotheses that high 3 He/ 4 He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like 3 He/ 4 He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable 3 He/ 4 He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of 3 He/ 4 He ratios similar to that observed in OIBs worldwide and indicate MORB-like 3 He/ 4 He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots. It is unclear why some ocean island basalts at ‘hotspots’ have low 3 He/ 4 He ratios similar to mid-ocean ridge basalts. Here, the authors perform convection calculations and show that these isotopic ratios can be reproduced by the episodic entrainment of deep isolated mantle reservoirs into thermal plumes.