Keegan Carvalho

First I am thankful to your details,

Second the FeO ranges between 20-23 wt.%, and Fa range lies between 21-24.

I read a paper same geochemical compostions from pyroxinite rock addressed with olivine mineral;

Your answer is bit satisfactory for me.

If you are convinced these are magmatic, cumulate olivines, then their composition is going to be primarily a function of magma composition, temperature and pressure of crystallization, and what other phases may be crystallizing at the same time. In general, the more evolved the magma becomes, the more Fe-rich any olivines tend to be. It is therefore important not to look at the composition of one mineral species in isolation, without fully understanding the history of the other phases that are present. Also, are there any other co-magmatic rocks in the suite that might give you a clue as to what magmatic processes might be occurring? What about this rock has convinced you that it is a cumulate? Cumulate phases are often very susceptible to re-equilibration as they come into contact with successive batches of magma, and olivine particularly so. It is therefore possible that they initially crystallized at higher Fo contents, and either re-equilibrated as the parent magma evolved, or if a new batch of more Fe-rich magma was injected into the magma chamber. But this is just one of many possible scenarios here. In my opinion, trying to decipher the mineral composition of olivine in one rock specimen is impossible without looking at the wider picture.

You have nice explanations above, but the information you provide is insufficient. What is the modal composition of the rock? Ultramafic include many rock-types.

How is textural relation between the olivine and the matrix. Is olivine showing any reaction with the surrounding or appears to be in equilibrium with it, is it uniform or zoned, and what are the other minerals in its contact?

I would also suggest, as did Dr Preston, that one sample may not be sufficient to draw safe conclusions.

M. Qasim Jan Jeremy Preston

Thanks for respective responses; comparatively olivine makes two modes in these rocks.

In these ultramafic wherlites the modal composition and texture of rock is defined as: olivine (25-35 vol.%), clinopyroxene (30-40 vol.%), orthopyroxene (10-20 vol.%), plagioclase (20-30 vol.%), hornblende (5-15 %) with minor amount of chlorite (5-10 vol. %), and magnetite/spinel (2-5 vol.%). Olivine grains form subhedral-anhedral habits, typically occurs as phenocrysts in two size modes; larger phenocryst grains of 4-6 mm in length, and smaller phenocryst grains 2-3 mm in length, that mostly embedded by the concave grain boundaries of clinopyroxene/orthopyroxene/hornblende. Larger phenocryst grains often showing undulatory extinction and/or kinking and shows developed fractures.

Inayat Ullah@ Thank you for the information. To start with, I would be hesitant to call your rock as wehrlite. According to Lae Maitre et al. (2002) classification. Wehrlite, lherzolite and harzburgite should have 40 to 90 vol% olivine. How about calling it olivine-plagioclase websterite or olivine gabbro/gabbronorite? Are you sure the olivine has two size-groups of phenocrysts? Because the larger grains show evidence of deformation, is it not possible the smaller olivines are granulated "phenocrysts" rather than another group of phenocrysts? Your rock may be a cumulate, but the presence of up to 10% chlorite and, perhaps, amphibole suggest some alteration (hydration during uplift?). But I have another issue with your initial question. You said that in your olivine analyses SiO2 ranges from 41 to 37 wt% and Fo from 77 to 74 mol%. In common (Fo-Fa) olivine, there is a good positive correlation between Fo and SiO2 contents. Olivine with 41 wt% SiO2 should have Fo content in the neighborhood of 91. With these comments, I am afraid it would be harsh to respond to your question.

M. Qasim Jan

Dear Professor,

I feel confusion about such olivine as I mentioned in my question about Mg# of olivine which is range between 0.76-0.77 with MgO values of 38.71-40.53, but FeO values are greater about 17.28 to 21.58. The calculated Fa in the olivines 21.99 to 23.44. I have not found any such ratios of olivine in Ultramafic cumulates or mafic cumulates of ophiolites.

Inayat Ullah@

I like to have a look at your analyses and their recalculation. You may send them on my email

M. Qasim Jan

Ok Sir

A rock with this composition [olivine (25-35 vol.%), clinopyroxene (30-40 vol.%), orthopyroxene (10-20 vol.%), plagioclase (20-30 vol.%), hornblende (5-15 %) with minor amount of chlorite (5-10 vol. %), and magnetite/spinel (2-5 vol.%) ] is a gabbro (or a cumulitic gabbro, it is the same), not an ultramafic rock.

Ultramafic rocks have

The issue has been settled. I had a look in the nine analyses. One is not olivine; it contains abnormally high amounts of CaO, Al2O3 and Cr2O3, stoichiometrically not permissible in olivine. Once that analysis is dropped, the analysed spots have very uniform composition, and such olivine occurs in mafic rocks. The very minor difference in Fo% and Mg# is because of calculation procedure. If elements other than Mg, Fe2+ and Mn are ignored, there should not be any difference in Fo and Mg#.

M. Qasim Jan Michele Lustrino

Thanks all,

I got and understand about my data.

It has been nice that listen many in this regard.