Hydrothermally-reset magmatic spinels in retrograde serpentinites : formation of ferritchromit rims and chlorite aureoles

Serpentinized harzburgites from southern Tuscany (Italy) host different kinds of spinels: (a) relic, magmatic Al-spinels, (b) hydrothermally altered spinels, occurring as "ferritchromit" rims, (c) syn- and post-serpentinization magnetites. The composition of relic Al-spinels suggests 5-15% partial melting of a fertile, spinel lherzolite. After the main serpentinization, Al-spinels are progressively replaced by ferritchromit rims by a dissolution-recrystallization process. Transmission electron microscopic investigation shows that ferritchromit actually consists of a complex, nanometric association of Cr-magnetite (Mg0.03 Fe0.972+ Al0.11 Cr0.89 Fe3+ 1.00 O4), chlorite and lizardite, with (001)chl/liz always parallel to (111)Cr-mag. Mg and Al, released during the Al-spinel → ferritchromit replacement, interact with mesh-textured serpentine, giving rise to chloritic aureoles (i.e., randomly intergrown chlorite, lizardite and "septechlorite") that overgrow and postdate mesh textures. © Springer-Verlag 2005.