Neon isotopes in mantle rocks from the Red Sea region reveal large-scale plume-lithosphere interaction

Helium and neon isotopes are ideal tracers to quantify contributions of primitive mantle plumes, which are characterized by a higher proportion of primordial solar-type noble gases compared to lithospheric or asthenospheric mantle sources. This property was used to investigate the role of the Afar mantle plume (a high He-3/He-4 plume of up to 20 R-A; 1 R-A = atmospheric composition) during continental breakup of the Red Sea rift. We analyzed ultramafic rocks from Zabargad Island and mantle xenoliths from the Quaternary volcanic fields of Al Birk and Jizan (Saudi Arabia) that are representative of the local subcontinental lithospheric mantle. He-3/He-4 ratios range from 6.1 to 8.3 RA, similar to results of worldwide lithospheric and asthenospheric mantle, and therefore are not indicative of a plume component. In contrast, we observe significant contributions of plume-derived neon, which is characterized by a higher proportion of primordial solar-type neon than typical mid-ocean ridge basalts (MORB). Considering both helium and neon isotope systematics reveals mixing of a deep mantle plume, a pre-rift MORB-like, and a more radiogenic pre-rift lithospheric mantle component. As the deep mantle plume component has a higher Ne/He ratio when compared to the shallow mantle components, it is more prominent in Ne than in He, with up to 57% plume-derived neon and 11% plume-derived helium present in the investigated samples. This further underlines the importance of high-precision neon measurements. It demonstrates that the Afar plume source contributed primordial noble gases to an intrinsically more radiogenic and nucleogenic lithospheric and asthenospheric component up to a distance of > 1800 km (Zabargad), even in the early stages of continental rifting, similar to20 Ma ago. These results present the first unambiguous geochemical evidence suggesting an active role for the Afar mantle plume during the evolution of the Red Sea rift, supporting geochronological and geodynamic evidence. (C) 2003 Elsevier B.V. All rights reserved.