Isotopic and Geochemical Evidence for the Source of Volcanism at Harrat Rahat, Kingdom of Saudi Arabia

Professional Paper 1862-J
Jointly published with the Saudi Geological Survey [as Saudi Geological Survey Special Report SGS–SP–2021–1]
By: , and 



Pleistocene and Holocene basalts, hawaiites, mugearites, benmoreites, and trachytes from the northern part of the Harrat Rahat volcanic field, Kingdom of Saudi Arabia, were analyzed for Sr, Nd, Hf, and Pb isotopic compositions. Evolved trachytes with Mg number <0.1 (Mg# = Mg/[Mg+Fe2+], molar) have relatively radiogenic Sr isotopic compositions indicating that they were influenced by contamination probably in the upper crust. Volcanic rocks with Mg# >0.1, consisting chiefly of alkali basalts but encompassing hawaiites, mugearites, and benmoreites, show a limited range in Hf, Nd, Sr, and Pb isotopic compositions. Although the total Pb isotope variation is only 1 percent, the Pb isotope values correlate with Mg#, where the least radiogenic Pb is in samples with the lowest Mg#. The trend formed in Pb isotope space points toward an unradiogenic Pb composition that is similar to the Pb isotopic composition of lower crust of the Precambrian Arabian-Nubian Shields, as well as to feldspars and galena in the upper crust of the western Arabian Shield. This trend is interpreted as progressive but overall minor (no more than 5 weight percent) assimilation of shield rocks, or their partial melts, during fractional crystallization.

Isotopic compositions of the least evolved northern Harrat Rahat magmas are most similar among analyzed Arabian harrats to depleted spreading-ridge basalts of the active Red Sea rift, but isotopic values are displaced toward those of spreading-ridge basalts of the Gulf of Aden that are proximal to the site of the Afar mantle plume. The Pb isotopic compositions very near the Northern Hemisphere Reference Line indicate no discernable lithospheric contribution to yield the parental basalts of northern Harrat Rahat, and their isotopic compositions are consistent with derivation predominantly from depleted Northern Hemisphere asthenosphere with a subordinate (20–30 weight percent) component from the Afar mantle plume.

Trace-element variations show that appreciable portions of melting were in the garnet stability field, confirming the sub-lithospheric origin of the magmas, and that melting extents were low, accounting for the alkalic, trace-element-enriched character of the suite. The presence of possible Afar mantle beneath the western part of the Arabian Shield and its absence beneath the Red Sea rift may result from capture and channelized flow along high-relief structures incised into the base of the sub-continental lithosphere, as revealed by geophysical images.

Suggested Citation

Salters, V.J.M., Sachi-Kocher, A., Downs, D.T., Stelten, M.E., and Sisson, T.W., 2023, Isotopic and geochemical evidence for the source of volcanism at Harrat Rahat, Kingdom of Saudi Arabia, chap. J of Sisson, T.W., Calvert, A.T., and Mooney, W.D., eds., Active volcanism on the Arabian Shield—Geology, volcanology, and geophysics of northern Harrat Rahat and vicinity, Kingdom of Saudi Arabia: U.S. Geological Survey Professional Paper 1862 [also released as Saudi Geological Survey Special Report SGS–SP–2021–1], 30 p.,

ISSN: 2330-7102 (online)

ISSN: 1044-9612 (print)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Sample Suite
  • Analytical Techniques
  • Results
  • Discussion
  • Conclusions
  • Acknowledgments
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Isotopic and geochemical evidence for the source of volcanism at Harrat Rahat, Kingdom of Saudi Arabia
Series title Professional Paper
Series number 1862
Chapter J
DOI 10.3133/pp1862J
Year Published 2023
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Volcano Science Center
Description Report: v, 30 p.: Data Release
Country Kingdom of Saudi Arabia
Other Geospatial northern Harrat Rahat
Online Only (Y/N) N
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details