Geochronology and Geochemistry of Volcanism at Harrat Ithnayn, Western Saudi Arabia.

Volcanic lava fields (30 Ma - Recent), also known as "harrats" cover a large proportion (~ 90,000 sq km) of western Saudi Arabia and are potentially the source of volcanic hazards. Older volcanism is associated with the opening of the Red Sea, but the ultimate cause of younger volcanic activity (< 12 Ma) remains unknown. Younger volcanism is clearly concentrated at the north-south-trending vents system also called Makkah-Madinah-Nufud line (MMN), which is composed of three alkali volcanic fields: Harrat Rahat (~ 10 Ma), Harrat Khaybar (~ 5 Ma), and Harrat Ithnayn. Harrat Ithnayn is the northern-most and the least studied harrat of the MMN line, and it has been suggested that Ithnayn represents the youngest field produced by age progressive volcanism along the MMN line. Harrat Ithnayn is thus a critical piece in the puzzle in determining the causes of the volcanic activity in the MMN line region.

We present new ⁴⁰Ar - ³⁹Ar age determinations for 10 lava flows from Harrat Ithnayn and the northern part of Harrat Khaybar. Our new ⁴⁰Ar - ³⁹Ar ages ranging from 33.4 - 467 Ka, which suggests that Ithnayn lavas are much younger and formed over a shorter time period than previously thought. The new age constraints agree with the hypothesis of south-to-north volcanic progression of volcanism along the MMN line. In addition, Harrat Ithnayn differs from other harrats along the MMN line where it produced less differentiated magmas, indicating a lack of shallower magmatic chambers. Incompatible trace element and REE concentrations imply that Ithnayn magmas were derived from mantle melts that vary in degree of partial melting (2 - 14%), and most magmas show evidence of primitive mantle source (garnet-field-stability) > 60 km. Petrographic evidence and geochemical modelling also suggest that chemical variations are mainly caused by crystal fractionation at a range of upper mantle and crustal pressures, predominantly of olivine (spinel-free and Cr spinel inclusion), clinopyroxene, and plagioclase with some evidence of crustal assimilation. We interpret these results to show that active mantle upwelling, decompression melting and possible asthenospheric flow from the Afar mantle plume have been the source of the volcanic activity at Harrat Ithnayn.