Gareth G Roberts , Patrick Ball

Harrat Rahat is the largest volcanic  eld in Saudi Arabia and has been active from ~10 Ma to the present day. Due to its close proximity to important population centres, Harrat Rahat's recent eruptions, the Medinah lava flows (< 1.7 Ma), have been extensively studied to identify volcanic risk. However, the evolution of Harrat Rahat's most extensive and oldest lava flows, known collectively as the Shawahit Basalt (> 2.5 Ma), is poorly understood. In this study, we collected, dated and geochemically analyzed lava samples from Harrat Rahat, primarily targeting the under-sampled Shawahit unit. We 40Ar/39Ar dated 23 Shawahit samples that erupted between 9.4-2.7 Ma. Over the lifetime of Harrat Rahat, we observe a geochemical transition from predominantly tholeiitic to alkalic eruptions coupled with a counter-intuitive decrease in incompatible element concentrations. We attribute these changes to a decrease in melt productivity and a reduction in contamination by enriched lithospheric melts, respectively. Thermobarometric analysis of Harrat Rahat basalts indicates that these lavas were generated by melting of asthenospheric mantle with a potential temperature of ~1456 +50/-32 oC beneath lithosphere 50-60 km thick. These results indicate that volcanic activity at Harrat Rahat was initiated by the arrival of a mantle plume beneath lithosphere thinned by a combination of Red-Sea rifting and thermal erosion. Furthermore, we believe that this plume, either acting alone or in combination with a number of other plumes, is responsible for the formation of the Arabian swell, as well as much of the Neogene-recent intraplate volcanic activity observed across western Arabia. Our conclusions are consistent with a wide range of geochemical, seismologic, gravimetric, thermochronologic and geomorphologic observations.