THE ORIGIN OF YOUNGER HARRATS IN WESTERN SAUDI ARABIA

The origin of many intraplate volcanic fields, and the relationship between intraplate volcanism and broader mantle flow and dynamics are still poorly constrained. Intraplate fields in Western Saudi Arabia are no exception to this, and here the long history of intraplate volcanism appears broadly related to the arrival and continued activity of the Afar plume, and the opening of the Red Sea as well as other features – such as the possible impact of “fossil” plumes. Post 12 Ma volcanic fields, also known as “harrats”, extend across western Arabia and have generated magmas that are compositionally different than older regional volcanism (> 20 Ma). Most of these younger harrats are concentrated along a N-S structural trend called the Makkah-Madinah-Nufud-line (MMN-line) and show a northward temporal progression of volcanism.

We aim to develop a more complete understanding of the origin of younger harrats volcanism by investigating spatial-temporal variations in mantle source compositions and contributions using precise trace element and Sr-Nd-Pb-He isotopic measurements of volcanic rocks and minerals for five younger harrats: Kura, Khaybar, Ithnayn, Lunayyir, and Hutaymah. Helium measurements show a narrow range of ³He/⁴He values for the selected harrats (6.3 – 8.3 R_A, where R_A is the atmospheric ratio), except for Harrat Kura (~ 5 Ma), where we see elevated ³He/⁴He values up to 12.7 R_A. Harrat Kura also shows the most radiogenic Pb, Nd, and Sr compositions relative to the selected harrats (e.g., ²⁰⁶Pb/²⁰⁴Pb up to 19.531). We suggest that a mixture of multiple mantle sources contributes to the younger magmatism, and the Afar mantle plume is involved in the earlier stages of this phase, especially along the MMN-line.