I think it will be belonging to the density of its.

Hi Mustafa,

if you are dealing with peridotitic rocks you should, first of all, check your thin sections. Petrography can give more information than simple geochemical analyses. Probably these rocks are rich in orthopyroxene and poor in diopside. It is possible that a former garnet reacted with olivine forming an assemblage stable at lower pressures (e.g., during mantle exhumation) made up of opx+sp.

Coupled with this you can have the formation of metasomatic layers made up of orthopyroxenites whose origin could be related to interaction of SiO2-oversaturated melts with forsteritic olivine, leading to the formation of enstatite-rich metasomes. These SiO2-oversaturated melts can be formed as consequence of partial melting of eclogitic/pyroxenitic lithologies very common in the mantle. Their origin is related to portions of subducted slabs or to basaltic melts crystallized at high depths forming eclogitic assemblages (not basaltic, because plagioclase is not stable at P >1 GPa).

I hope this helps,

michele

Dear Hafizullah Rasouli

Thanks for your interest to my question. But I could not realize what you mean by "density of its".

Best regards,

Mustafa

Dear Michele Lustrino

Thank you very much for your replay. But I need to tell you that there is no orthopyroxenite layers, and also 2 samples are harzburgite (20-30% orthopyroxene, not more) 1 sample is dunite (less than 5% orthopyroxene).

According to analyse results, they have high MgO, Cr2O3, Al2O3 and low SiO2 and CaO... So, could I explain this situation by partial melting of eclogitic assemblages?

Best regards,

Mustafa

Dear Mustafá: all before answers are good explanations for your anomaly, but can be added that mantle orthopyroxene contains an appreciable amount of Al, besides spinel and cpx, and also opx is very low in Ca. Regards, Sebastián.

Dear Meysam Akbari

Thanks for answering. I will check again if my samples could lose their CaO contents by serpentinization of peridotitic mantle by slab-dehydrated fluids. Because I havent seen before such a reaction...

Best regards,

Mustafa

PS: Those samples LOI are generally lower than the others.

Dear Sebastián Grande ,

Thank you for your contribution. I agree with you about " mantle orthopyroxene contains an appreciable amount of Al, besides spinel and cpx, and also opx is very low in Ca". if my sample has opx (harzburgite sample has 20-30% Opx), so this scenario is correct. But what should be said for my dunite samples that have no or less than 5% opx?

The reason should be related to the Cr2O3 ratio, in my opinion...

Best regards,

Mustafa

Dear Mustafa: yes, in dunites Al is in solid solution in chromite!!!

Regards, Sebastián

Dear Sebastián Grande

Thanks a lot...

Best regards,

Mustafa

Dear Mustafa Eren Rizeli

As you mentioned, your samples are harzburgite and dunite these rock contain little clinopyroxene which is high CaO beside this may contain chromite and spinel lead to high Cr2O3 and Al2O3

Regards, Mohammed

Dear Mustafá: here's an interesting question for research... Which is the content of CaO and Al2O3 in peridotitic olivine? It is usually admitted than the amount of Al in olivine is very small, if any; Ca, as a limited solid solution could also be present, but probably in small proportions too. But, how small? Regards, Sebastian.

Dear Sebastián Grande

I checked CaO and Al2O3 content of olivine, they are around 0.01-0.09% and 0.01-0.02%, respectively for all samples. There is no big difference between samples.

Regards,

Mustafa

Dear Mustafa Eren Rizeli,

The high Al2O3 and Cr2O3 contents of the three samples can be the result of a lower degree of ‎partial ‎melting. But the lower CaO content generally is the results of late serpentinization or sub-sea ‎alteration ‎processes (e.g. Niu, 1997). The content of LOI, in addition to the amount of volatile, is also depending ‎on the occurrence of some hydrated minerals such as Phlogopite. Therefore a low LOI value does not ‎necessarily indicate a low degree of serpentinization or vice versa. ‎

For testifying the validity of every reason, at first, you need to compare the composition of ‎the ‎samples with some simple melting residue models, like the plotting of some major oxides (e.g. ‎SiO2, ‎FeOt, TiO2, CaO, and Al2O3) vs. MgO variation (Niu, 1997; 2004). Consistence or every ‎deviation ‎from the defined trends can help you to better understand the effects of which process.‎

Best regards,

Hi Mahlequa,

I have some problems to follow your arguments.

1) I do not understand what you mean with "high Al2O3 and Cr2O3 can be the result of a lower degree of partial melting". What is the rationale behind your statement? Moreover, lower with respect to what?

2) The ultimate effect of serpentinization is H2O increase, rather than CaO gain.

3) Phlogopite has

Mustafa,

I think your powder samples contain chromitite. If you took the samples close to chromite ore, your samples may contain dissemine chromitite. You may not see it in thin section.

Hi Samet Saka

Thanks for asnwering. As I mentioned in my question these sample chromium oxide content is around 4-6%wt.. When I checked EPMA of those samples (especially chromites) again I saw these chromites has Al rim also. As you said, those dissemine chromitite must caused this result.

Selamlar :)

Mustafa Eren Rizeli@ You already have been given good suggestions. Let me summarise. Please do careful petrography. If your rocks are not much altered, you would readily find the answer. Hoping that your harzburgite and dunite do not contain garnet and clinopyroxene, I would think that A2O3 is present in orthopyroxene and chromian spinel.

Dear M. Qasim Jan ,

Thanks for responding. I did not see any garnet in thin sections. But there are secondary clinopyroxene (as exsolution lamellas) less than 5%. It was detected under electron micoprobe.

The possible cause of high Al low Ca is high chromite (dissemine chromite) of those sample.

Best Regards,

Mustafa

Dear Dr.Mohammed

all mantle samples(Hrz.+dunite) from Iraq part of Zagros are suffered by different degree of alteration and serpentinization (no fresh CPX), degree of serpentinization will effect the result of your analysis, further more you must check the percent of spinel contents in your samples.

Best wishes

Dear Mustafa, I agree with M. Qasim Jan regarding doing careful petrography and you may also check with the ACME (Canada) and Academia Sinica (Taiwan) to do more analyses for these three samples to be sure whether if there are any lab contamination.

Following

Hello sir,

I am not an expert in this particular field, but high pressure melting (low to moderate degrees) of lherzolites generally produce melts with high Al2O3 as well as Cr2O3 also. Conforming to the previous discussions, I would like to add that there might be high pressure melting associated with further CaO depletion due to possible serpentinization. If the sample is from a fertile mantle, then the situation holds good.

Thank you.