The geochemistry of the Arabian lithospheric mantle - a source for intraplate volcanism?

We present trace element and Sr-Nd-Hf-Pb isotope compositions for clinopyroxenes from anhydrous spinel peridotite and garnet +/- spinel pyroxenite xenoliths of Pan-African lithospheric mantle from Jordan, including the first high-precision double- spike Pb isotope measurements of mantle clinopyroxene. Clinopyroxenes from the peridotites are variably Th-U-LILE-LREE enriched and display prominent negative Nb, Zr and Ti anomalies. MREE-HREE abundances can generally be modelled as partial melting residues of spinel lherzolite with primitive-mantle-like composition after extraction of 5-10% melt, whereas the enrichments in Th-U-LILE-LREE require a Pan-African or later metasomatic event. The large range of Nd, Sr, Pb and Hf isotope ratios in both peridotites and pyroxenites ( e. g. eNd similar to 1.4-17.5; Pb-206/Pb-204 similar to 17.2-20.4; eHf similar to 0.6-164.6) encompasses compositions more radiogenic than mid-ocean ridge basalt ( MORB), and Pb isotopes cover almost the entire range of oceanic basalt values. eHf values are some of the highest ever recorded in mantle samples and are decoupled from eNd in the same samples. Marked correlations between Sr-Nd-Pb isotopes, LILE-LREE enrichments and HFSE depletion suggest that the metasomatizing agent was a carbonatitic- rich melt and isotopic data suggest that metasomatism may have been related to Pan- African subduction. The metasomatic melt permeated depleted upper mantle (< 16 kbar) during PanAfrican subduction at similar to 600-900Ma, and the variably metasomatized material was then incorporated into the Arabian lithospheric mantle. There is no evidence for recent metasomatism ( < 30Ma) related to the Afar plume like that postulated to have affected southern Arabian lithospheric mantle. Hf isotopes in the mantle clinopyroxenes are unaffected by metasomatism, and even some strongly overprinted lithologies record ancient (> 1.2 Ga) pre- metasomatic Lu-Hf signatures of the depleted upper mantle that was the protolith of the Arabian lithospheric mantle. The `resistance' of the Lu-Hf isotopic system to later metasomatic events resulted in the development of extremely heterogeneous Hf isotopic signatures over time that are decoupled from other isotopic systems. No mantle sample in this study exactly matches the chemical and isotopic signature of the source of Jordanian intraplate basalts. However, the xenolith compositions are broadly similar to those of the source of Arabian intraplate basalts, suggesting that the numerous Cenozoic intraplate volcanic fields throughout Arabia may be the product of melting upper mantle wedge material fertilized during Pan-African subduction and incorporated into the Arabian lithospheric mantle. We propose a model whereby the proto- Arabian lithospheric mantle underwent a major melting event in early Proterozoic-late Archean times (at the earliest at similar to 1.2 Ga). Island-arc volcanism and major crust formation occurred during the Pan-African orogeny, which liberated fluids and possibly small-degree melts that migrated through the mantle creating zones of enrichment for certain elements depending upon their compatibility. Immobile elements, such as Nb, were concentrated near the base of the mantle wedge providing the source of the Nb- rich Jordanian volcanic rocks. More mobile elements, such as LILE and LREE, were transported up through the mantle and fertilized the shallow mantle source of the Jordanian xenoliths. Following subduction, the mantle wedge became fossilized and preserved distinct enriched and depleted zones. Lithospheric rifting in the iocene triggered partial melting of spinel-facies mantle in the lower lithosphere, which mixed with deeper asthenospheric garnetfacies melts as rifting evolved. These melts entrained segments of variably carbonatite-metasomatized shallow lithospheric mantle en route to the surface.