Dear Mr. Ahmed,

although both types of spinel be it Cr-enriched or Al-enriched have been derived from the mantle, the geodynamic setting within which these spinel end-member types evolve are extremely different. The Cr-enriched types are bound to the mobile geodynamic setting and used to be concentrated in podiform ophiolitic Cr deposits, whereas the Al-enriched spinels kimberlite pipes and alkaline basalts indicative of a more stable cratonic setting, where gemstones deposits carrying diamonds and sapphire deposits are the characteristic deposits. Simple aluminum oxides or composite types such as the spinel group need desilicified basic to ultrabasic environments. The speed of opening the conduit to the subcrustal source in the various geodynamic settings plays a crucial part in the development of one or the other spinel type and has also an impact on the Al content of the chromium spinel.

Best regards

H.G.Dill

Thank Dr. Gill for your kind message and useful information. Actually I am having a peculiar magmatic concentrations (micropods) of Al-spinel (pleonaste) in wehrlitic mantle xenoliths within alkali basalt from the Arabian Shield, Saudi Arabia. I tried to find any similar example from any place to interpret its genesis, but I failed to find. The Al-spinel micropods are almost Cr free (MgFeAl2O4), associated with low Mg# olivines (Fo81) and Mg# cpx (79). So, it looks like a mantle cumulate not residues. Your interpretation is logic, but as you know the Arabian plate is not so stable craton, it is almost Neoproterozoic in age, so I am wonder if such setting is suitable for this association.

If you have any reference of such associations kindly provide me. Thank you very much in advance.

Ahmed 

Even for a mantle cumulate, they seem to be very Cr-poor. Perhaps you should have a look at the GeoRoc website (http://georoc.mpch-mainz.gwdg.de/georoc/), where you can find a precompiled file with spinel analyses to see whether you find a match.

Thanks Martina for your message. I will refer to GeoRoc website to see whether I could find a match of this spinel association. Thanks again for help.

Ahmed

Just a petrologist suggestion. Remember that the amount of Cr in the source rock (mantle) plays a role in its concentration in Cr-phases (like Spinel). Mantle is a bit more heterogeneous than commonly thought.  Also, oxygen fugacity , P and T may induce a variation of the partition coefficient of Cr between solid/melt and solid/solid. In Italy, in a locality called Finero there are peridotites and pyroxenites containing large amounts of Cr-rich clinopyroxene and the coexisting spinel has very low Cr (although not as low as in your case).

Dear dr. Ahmed

I observed Al-spinel  (Al2O3) equal 62 or 64 Wt%. I did not remembered percent for Cr2O3. but this types altered at their edges and mixed with low-T minerals. Hence, we think the origin is related to metamorphism and decomposition of chlorites. Some Al-rich spinels were observed in prograde peridodotites or contact metamorphism peridotites

Thanks Dr. Khedr for your answer. Such assemblage you mentioned is a metamorphic origin. I asked about a magmatic Al-spinel, almost free from Cr2O3.

Thanks again.

Ahmed