Crystal chemistry of Cr-spinels from a suite of spinel peridotite mantle xenoliths from the Predazzo Area (Dolomites, Northern Italy)

The crystal chemistry of ten Cr-spinels enclosed in protogranular to weakly porphyroclastic spinel peridotite mantle xenoliths from the Predazzo Area (Dolomites, Northern Italy) was studied by single-crystal X-ray diffraction and electron microprobe. Within the suite, unit cell parameter a(0) increases from 8.1358 to 8.2201 Angstrom, with increasing Cr content, from 0.205 to 0.812 a.f.u., whereas the oxygen coordinate u remains virtually constant (u=0.2627-0.2630), therefore giving a quite constant ratio of octahedral (M-O) and tetrahedral (T-O) bond distances. The cation distribution, obtained from structural and chemical data, shows that, with increasing Cr3+ within the suite, the Al (T) content decreases from 0.144 to 0.056 a.f.u. Data from the Predazzo spinels were compared with those of spinel suites from well-defined mantle rocks (Newer Victoria, Australia; NE Brazil; Assab, Ethiopia; Balmuccia, Italy) from different petrological environments. Estimation of the intracrystalline closure temperature for the investigated spinels gives a mean value of 769degreesC. This indicates a cooling rate intermediate between that of a slowly cooled peridotitic massif and that of very fast cooled mantle xenoliths hosted by volcanics, which is consistent with the origin of the Predazzo spinels from mantle xenoliths hosted in subvolcanic dykes.