Tracing the metasomatic and magmatic evolution of continental mantle roots with Sr, Nd, Hf and and Pb isotopes: A case study of Middle Atlas (Morocco) peridotite xenoliths

Abstract

Abstract We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr–Nd–Hf isotope data. However, we focus in particular on Pb isotope data and 238 U/ 204 Pb and 232 Th/ 204 Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated 238 U/ 204 Pb, 232 Th/ 238 U and 232 Th/ 204 Pb in continental mantle roots and (c) the 238 U/ 204 Pb and 232 Th/ 204 Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM). Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU. U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between 206 Pb/ 204 Pb = 19.93–20.25, 207 Pb/ 204 Pb = 15.63–15.66 and 208 Pb/ 204 Pb = 39.72–40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. 238 U/ 204 Pb and 232 Th/ 204 Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle. The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental mantle root. Instead, the enrichment is thought to be associated with the Quaternary intra-plate volcanism in the Middle Atlas. However, the erupted mafic melts have unradiogenic Pb isotopes and lower 238 U/ 204 Pb, 232 Th/ 204 Pb and 232 Th/ 238 U relative to the clinopyroxene and do not seem to have equilibrated with the clinopyroxenes. The high Th abundances and the high 232 Th/ 238 U also suggest that the metasomatism was due to carbonatitic liquids. When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in that they are similar to the convecting mantle. This observation does not support the existence of sub-continental lithospheric mantle with high 238 U/ 204 Pb and 232 Th/ 204 Pb for long periods of time. Consequently, the narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest metasomatic events are recorded by incompatible elements such as U, Th and Pb. Numerical modelling of putative magmas generated from Middle Atlas SCLM by fractional, non-modal melting calculations yield extremely high 238 U/ 204 Pb and 232 Th/ 204 Pb ratios. For example, pure SCLM magmas generated from 0.5% to 10% melting are anticipated to have 232 Th/ 204 Pb ratios exceeding those known from terrestrial basalts.