Hidden but Ubiquitous: The Pre-Rift Continental Mantle in the Red Sea Region

Volcanism in the western part of the Arabian plate resulted in one of the largest alkali basalt provinces in the world, where lava fields with sub-alkaline to alkaline affinity are scattered from Syria and the Dead Sea Transform Zone through western Saudi Arabia to Yemen. After the Afar plume emplacement (∼30 Ma), volcanism took place in Yemen and progressively propagated northward due to Red Sea rifting-related lithospheric thinning (initiated ∼27–25 Ma). Few lava fields were emplaced during the Mesozoic, with the oldest 200 Ma volcanic activity recorded in northern Israel. We report results from volcanic pipes in the Marthoum area, east of Harrat Uwayrid, where over a hundred pipes occupy a stratigraphic level in the early Ordovician Saq sandstones. Most of them are circular or elliptical features marked by craters aligned along NW-SE fractures in the sandstone resulting from phreatomagmatic explosions that occurred when rising magma columns came in contact with the water table in the porous sandstone host. These lavas have Sr-Pb-Nd-Hf isotopic compositions far from the Cenozoic Arabian alkaline volcanism field, being considerably more enriched in Nd-Hf and Pb isotopes than any other Arabian Plate lava ever reported. New K-Ar dating constrains their age from Late Cretaceous to Early Eocene, thus anticipating the Afar plume emplacement and the Red Sea rift. Basalt geochemistry indicates that these volcanic eruptions formed from low-degree partial melting of an enriched lithospheric mantle source triggered by local variations in the asthenosphere-lithosphere boundary. This mantle source has a composition similar to the HIMU-like enriched isotopic component reported in the East African Rift and considered to represent the lowermost lithospheric mantle of the Nubian Shield. The generated melt, mixed in different proportions with melt derived from a depleted asthenosphere, produces the HIMU-like character throughout the Cenozoic Arabian alkaline volcanism. Although apparently hidden, this enriched lithospheric component is therefore ubiquitous and widespread in the cratonic roots of the African and Arabian subcontinental mantle.

Hidden but ubiquitous: the pre-rift continental mantle in the Red Sea region

<p>The extensive volcanism in the western part of the Arabian plate forms one of the largest Cenozoic alkali basalt provinces in the world where large lava fields with sub-alkaline to alkaline affinity are scattered from Syria and the Dead Sea Transform Zone through western Saudi Arabia to Yemen (Coleman et al. 1983). Most of volcanism took place after the emplacement of the Afar plume in Yemen (~30 Ma) and progressively propagated northward due to the lithospheric thinning related to the Red Sea rifting starting from 27-25 Ma (Bosworth and Stockli, 2016). However, few lava fields were emplaced during the Mesozoic, with the oldest volcanic activity as old as 200 Ma in the north Israel (Atlit- 1 and Haifa-1 drillholes) (Khon et al., 1993). Here, we report new results from volcanic pipes in the Marthoum area immediately to the east of Harrat Uwayrid where over a hundred pipes are aligned along NW-SE fractures in the Ordovician sandstone of the Saq Formation. The chilled vitric nature of these basalts suggests that the pipes are the result of phreatomagmatic explosions which occurred when the rising magma columns met the water table in the porous sandstone host. These lavas have Sr-Pb-Nd-Hf isotopic compositions that plot out of the field of the Cenozoic Arabian alkaline volcanism, being far more enriched in Nd-Hf and Pb isotopes than any lava ever reported in the Arabian plate. New K-Ar dating limits their age to 80 and 50 Ma, thus predating the emplacement of the Afar plume and the rifting in the Red Sea. Our findings indicate that these volcanic eruptions formed from melts generated by a low-degree partial melting of an enriched lithospheric source triggered by local variations in the asthenosphere-lithospheric boundary. This mantle source has a composition similar to the HIMU-like enriched isotopic component reported in eastern Africa Rift (Rooney et al., 2014) and considered to represent the lowermost lithospheric mantle of the Nubian shield. Although apparently hidden, this enriched deep lithospheric component is therefore ubiquitous and widespread in the cratonic roots of the Arabian and African lithospheric mantle, but variously mixed with melts derived from a depleted asthenosphere to produce a HIMU-like flavour dispersed in the Cenozoic Arabian alkaline volcanism.</p><p>Bosworth, W. and Stockli, D. Early magmatism in the greater Red Sea rift: timing and significance. Can. J. Earth. Sci., 53, 1158–1176, 2016.</p><p>Coleman, R. G., Gregory, R. T., Brown, G. F. Cenozoic volcanic rocks of Saudi Arabia. Saudi Arabian Deputy Minist. Miner. Resour., Open File Report, USGS-OF-03-93, pp. 82, 1983.</p><p>Khon, B. P., Lang, B. and Steinitz, G. <sup>40</sup>Ar/<sup>39</sup>Ar dating of the Atlit-1 volcanic sequence, northern Israel, Israel J. Earth-Sci., 42, 17–28, 1993.</p><p>Rooney, T. O., Nelson, W. R., Dosso, L., Furman, T., Hanan, B. The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates. Geology, 42, 419–422, 2014.</p>

Chapter 1.2 Antarctic volcanism: volcanology and palaeoenvironmental overview

Since Jurassic time (c. 200 Ma), Antarctica has had a greater diversity of volcanism than other southern continents. It includes: (1) voluminous mafic and felsic volcanism associated with the break-up of Gondwana; (2) a long-lived continental margin volcanic arc, including back-arc alkaline volcanism linked to slab rollback; (3) small-volume mafic alkaline volcanism associated with slab-window formation; and (4) one of Earth's major continental rift zones, the West Antarctic Rift System (WARS), with its numerous large alkaline central volcanoes. Several of Antarctica's volcanoes are still active. This chapter is a review of the major volcanic episodes and their principal characteristics, in their tectonic, volcanological and palaeoenvironmental contexts. Jurassic Gondwana break-up was associated with large-scale volcanism that caused global environmental changes and associated mass extinctions. The volcanic arc was a major extensional arc characterized by alternating volcanic flare-ups and lulls. The Neogene rift-related alkaline volcanism is dominated by effusive glaciovolcanic eruptions, overwhelmingly as both pāhoehoe- and ‘a‘ā-sourced lava-fed deltas. The rift is conspicuously poor in pyroclastic rocks due to the advection and removal of tephra erupted during glacial intervals. Volcanological investigations of the Neogene volcanism have also significantly increased our knowledge of the critical parameters and development of the Antarctic Ice Sheet.

Generation of Calc-Alkaline Magmas During Crystallization at High Oxygen Fugacity: An Experimental and Petrologic Study of Tephras from Buldir Volcano, Western Aleutian Arc, Alaska, USA

Despite agreement that calc-alkaline volcanism occurs at subduction zones and is responsible for the genesis of continental landmasses, there is no consensus on the source of the Fe-depleted signature hallmark to calc-alkaline volcanism. In this study, we utilize mafic tephras collected from Buldir Volcano to address the genesis of strongly calc-alkaline volcanic rocks (those with a low Tholeiitic Index; ≤0.7) in a segment of the western Aleutian Arc to determine if the eruptions are plausibly part of a liquid line of descent, if they are mixtures of crustal melts and parental magmas, or if they are mixtures of melts of the mantle and the subducting slab. We conducted a series of H2O-saturated phase equilibrium experiments (1175–1000 °C; 100 MPa) in a rapid-quench cold-seal (MHC) apparatus on the most primitive natural lava from Buldir (9.34 wt% MgO) at oxidizing conditions near the Re-ReO2 buffer. We confirmed that all experiments equilibrated 0.3 ± 0.23 log units above the Re-ReO2 buffer (ΔQFM ∼ +2.8) using X-ray Absorption Near Edge Structure (XANES) spectroscopy. Chromite is the liquidus phase, followed by olivine, then plagioclase, then clinopyroxene, and finally hornblende. Once clinopyroxene saturates, spinel composition shifts to magnetite. We compared our experimental results to the major element geochemistry and petrology of six tephras (51.9–54.8 wt% SiO2) from Buldir collected during the 2015 field season of the GeoPRISMS shared platform. Tephras contain olivine + plagioclase + clinopyroxene + spinel ± hornblende; plagioclase comprises most of the crystalline volume, followed by either olivine or hornblende. Spinel is ubiquitous; with Cr- rich spinel inclusions in olivine and hornblende, and magnetite in the groundmass. Variations in phenocryst assemblages and compositions between samples can be attributed to differences in pre-eruptive temperatures, where hotter samples are devoid of hornblende, and contain Fo-rich olivine and plagioclase with lower An-contents, owing to the position of the mineral-in curves at fluid-saturated conditions. Experimental glasses match the depletion in FeOT observed in the tephra whole rock compositions. The continuous depletion in FeOT is attributable to saturation of spinel as a liquidus phase (initially as chromite) and continuous crystallization through the experimental series (changing to magnetite at colder temperatures). In contrast to the natural samples, the experiments show enrichment in TiO2 with decreasing MgO, suggesting that differentiation did not occur at 100 MPa on Buldir. The TiO2 depletion in volcanic rocks from Buldir can be accounted for if hornblende crystallization occurs close to the liquidus of a parental magma; a condition that is met at higher pressures and hydrous conditions. The emerging picture for Buldir Island is that (1) oxidizing conditions are required to drive the observed depletions in FeOT via crystallization of spinel, and (2) elevated H2O contents and high pressures are required to saturate hornblende close to the liquidus to reproduce the entire suite of major elements. Our study provides a mechanism to generate the calc-alkaline trends observed at Buldir without requiring mixing of slab and mantle melts. We conclude that calc-alkaline volcanic rocks with extremely low Tholeiitic Indices (0.7), like those from Buldir, cannot be generated in absence of high oxygen fugacity, even at high pressure and/or elevated water pressures.