Eruptive history of northern Harrat Rahat—Volume, timing, and composition of volcanism over the past 1.2 million years

Harrat Rahat, one of several large, basalt-dominated volcanic fields in the western part of the Kingdom of Saudi Arabia, is a prime example of continental, intraplate volcanism. Excellent exposure makes this an outstanding site to investigate changing volcanic flux and composition through time. We present 93 ⁴⁰Ar/³⁹Ar ages and 6 ³⁶Cl surface-exposure ages for volcanic deposits throughout northern Harrat Rahat that, integrated with a new geologic map, define 12 eruptive stages. Exposed volcanic deposits in the study area erupted less than 1.2 million years ago (Ma), and 214 of 234 identified eruptions occurred less than 570 thousand years ago (ka). Two eruptions were in the Holocene, including a historically described basaltic eruption in 1256 C.E. and a trachyte eruption newly recognized as Holocene (4.2±5.2 ka). An estimated approximately 82 cubic kilometers (km³; dense rock equivalent) of volcanic products can be documented as having erupted since 1.2 Ma, though this is a lower limit because of concealment of deposits older than 570 ka. Over the last 570 thousand years (k.y.), the average eruption rate was 0.14 cubic kilometers per thousand years (km³/k.y.), but volcanism was episodic with periods alternating between low (0.04–0.06 km³/k.y.) and high (0.1–0.3 km³/k.y.) effusion rates. Before 180 ka, eruptions vented from the volcanic field’s dominant eastern vent axis and from a subsidiary, diffuse, western vent axis. After 180 ka, volcanism focused along the eastern vent axis, and the composition of volcanism varied systematically along its length from basalt dominated in the north to trachyte dominated in the south. We hypothesize that these compositional variations younger than 180 k.y. reflect the growth of a mafic intrusive complex beneath the southern part of the vent axis, which led to the development of evolved magmas. Lastly, these new age data allow for a reassessment of the volcanic recurrence interval at northern Harrat Rahat. Based on available data, volcanism in northern Harrat Rahat over the last 180 k.y. is poorly described using a Poisson distribution with a single recurrence interval. Instead, data for northern Harrat Rahat are better described using a mixed exponential distribution that is applicable for volcanic systems characterized by two different eruptive states, where one state with a longer recurrence interval corresponding to periods of low eruption frequency and one state with a shorter recurrence interval corresponding to periods of high eruption frequency. The preferred model for northern Harrat Rahat over the last 180 k.y. uses a long recurrence interval of 4.0 k.y. and a short recurrence interval of 0.22 k.y.