SpinelS RenaiSSance: The paST, pReSenT, and fuTuRe of ThoSe ubiquiTouS mineRalS and maTeRialS 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† davide lenaz 1, *, naSRRddine Youbi 2,3 , angelo de min 1 , moulaY ahmed boumehdi 2 and mohamed ben abbou 4 1 Department of Mathematics and Geosciences, Università degli Studi di Trieste, Via Weiss 8, 34127 Trieste, Italy 2 Geology Department, Faculty of Sciences-Semlalia, Cadi Ayyad University, Prince Moulay Abdellah Boulevard, P.O. Box 2390, 40 000 Marrakech, Morocco 3 Centro de Geologia da Universidade de Lisboa (CeGUL), Faculdade de Ciências (FCUL), Departamento de Geologia (GeoFCUL), Campo Grande C6, 1749-016 Lisboa, Portugal 4 Geology Department, Faculty of Sciences Dhar Al Mahraz, Sidi Mohammed Ben Abdellah University, P.O. Box 1796, Fès-Atlas, 30003 Fès, Morocco abSTRacT The crystal chemistry of nine Cr-spinels from lherzolite and harzburgite xenoliths from the Middle Atlas Neogene-Quaternary Volcanic Field of Morocco have been studied by means of X-ray single- crystal diffraction and electron microprobe analyses. Cell edges are usually within the range 8.13–8.14 Å, but there are three samples with longer a value, so that the whole analyzed series is within the range 8.1334(4)–8.2021(2) Å, while the oxygen positional parameter values are very similar rang- ing between 0.2626(1) and 0.2629(2) for all of them. The cation distribution shows that the crystal structure is ordered with almost all divalent cations in the tetrahedral site and trivalent cations in the octahedral site. The determined intracrystalline temperatures are in the range 550–750 °C that are the lowest values ever found for Cr-spinels from mantle xenoliths as these are usually higher than 730 °C. If we consider the behavior of some geotherms from literature, the determined temperatures are conined in a depth range of about 20–40 km. Lithospheric models for the studied area indicate that the lower crust reaches its deepest value in a range between 30 and 40 km. Consequently, we can assume that the studied xenoliths were emplaced at a “shallow” depth of about 20–30 km, just beneath the lower crust, where they were disrupted and brought to the surface from the ascending alkaline lavas. This assumption is consistent with the concomitant presence of some crustal xenoliths. It is important to notice that even in the case of a mantle xenoliths where all the silicates could be heavily altered, the presence of one single crystal of Cr-spinel and the study of its oxygen coordinates (u), inversion parameters (i), Cr content, and calculated closure temperatures can be used to validate the thermal history of the mantle xenoliths. The combined approach of structural data, intra- and inter-crystalline temperatures, and the literature geophysical data seems to be an interesting tool to assess the pre- exhumation history of the mantle xenoliths. Keywords: Cr-spinels, mantle xenoliths, crystal chemistry, intracrystalline temperature, Morocco inTRoducTion The structures and site occupancies of natural and synthetic Cr-bearing spinels have been extensively studied using single- crystal X-ray diffraction techniques and various spectroscopies in the past 25 yr (Della Giusta et al. 1986, 1996; Princivalle et al. 1989; Carbonin et al. 1996, 1999; Lenaz and Princivalle 1996, 2005; Lenaz et al. 2002, 2004a, 2004b, 2006, 2010, 2013; Carraro 2003; Bosi et al. 2004; Uchida et al. 2005; Nédli et al. 2008; Derbyshire et al. 2013; Lenaz and Lughi 2013; Lenaz and Skogby 2013) because of their importance as petrogenetic indicators, geobarometers, geothermometers and ore minerals for Cr and platinum group elements extraction. The unit cell of the spinel structure can be described as a slightly distorted cubic close-packed (ccp) array of 32 O atoms with 8 cations at tetrahedrally coordinated T sites, and 16 cations at octahedrally coordinated M sites (Hill et al. 1979). The T and M sites lie on special positions with 4 3 and 3 m symmetry, respectively. The only variable geometrical parameters are the unit- cell edge (a) and oxygen coordinate (u,u,u), which is related to the oxygen packing distortion. The ideal ccp structure shows u = 0.25, but it is observed that u > 0.25 for all the Cr-bearing spinels. The observed distortion is a consequence of similar M-O and T-O bond American Mineralogist, Volume 99, pages 267–275, 2014 0003-004X/14/0203–267$05.00/DOI: http://dx.doi.org/10.2138/am.2014.4655 267 * E-mail: lenaz@units.it † This is a special collection, focused on diverse topics, related to the structure, properties, and applications of natural and syn- thetic spinels and spinelloids. The collection aims to document the revival of interest in spinel materials, with emphasis on non-oxygen containing and nanosized structures. The hope is to bring together experimental and theoretical research studies from mineralogists, crystallographers, petrologists, chemists, materials scientists, physicists, and other spinel aficionados. Submission and other info can be on GSW at http://ammin.geoscienceworld. org/site/misc/virtual_special_list.xhtml.