Low intra-crystalline closure temperatures of Cr-bearing spinels from the mantle xenoliths of the Middle Atlas Neogene-Quaternary Volcanic Field (Morocco): Mineralogical evidence of a cooler mantle beneath the West African Craton

摘要: The crystal chemistry of nine Cr-spinels from lherzolite and harzburgite xenoliths the Middle Atlas Neogene-Quaternary Volcanic Field Morocco have been studied by means X-ray single-crystal diffraction electron microprobe analyses. Cell edges are usually within range 8.13–8.14 A, but there three samples with longer a value, so that whole analyzed series is 8.1334(4)–8.2021(2) while oxygen positional parameter values very similar ranging between 0.2626(1) 0.2629(2) for all them. cation distribution shows structure ordered almost divalent cations in tetrahedral site trivalent octahedral site. determined intracrystalline temperatures 550–750 °C lowest ever found mantle as these higher than 730 °C. If we consider behavior some geotherms literature, confined depth about 20–40 km. Lithospheric models area indicate lower crust reaches its deepest value 30 40 Consequently, can assume were emplaced at “shallow” 20–30 km, just beneath crust, where they disrupted brought to surface ascending alkaline lavas. This assumption consistent concomitant presence crustal xenoliths. It important notice even case silicates could be heavily altered, one single Cr-spinel study coordinates ( u ), inversion parameters i Cr content, calculated closure used validate thermal history combined approach structural data, intra- inter-crystalline temperatures, literature geophysical data seems an interesting tool assess pre-exhumation