What mineral is the reaction rim made of, clinopyroxene? It looks like it's the result of a reaction between the magma and the olivine crystal, with the olivine having been mixed into a more silica-rich magma than the one it would be in equilibrium with. I wouldn't want to say much about the rapidity of the reaction, but it's likely that the olivine didn't spend too much time in its new surroundings, otherwise it may have been replaced in its entirety.

The reaction rim looks like a may have multiple alteration products. The plagioclase looks highly altered and somewhat destroyed so the alteration fluids as mentioned above by Elburg Marlina in his response may be Si-rich magma whose fluids may have reacted with the basalt. It appears that the basalt may have been fractured/faulted? Prior to the altering fluids being introduced. Any carbonates dolomite/ankerite? in reaction rim may indicate vein alteration with increased silica matrix.

Please see very similar textures in the

http://www.iux.es/?page=5&serie=2

Dear Uriah,

Good morning 

What  I would  first like to know is that, are they olivine crystals occuring  in  a basaltic magma crystallized in shallow depth system ( sills, or dykes for examples) or exposures ( pyroclastic materials or  flows)?  I red that in basaltic exposures  olivine  crystals (phenocrysts, antecrysts) may  have two main origins from mantle derived ( inherited crystals with ondulatory  extinction) (Helz, 1987). This  olivine is also called  strained olivine.  Because of the  variation of the crystallization condition  when the cooling magma arrives at the Earth surface, this olivine can undergo dissolution an precipitation of new minerals from rim to core (gradually transforming to more stable mineral: pyroxene). The SEM  analysis of the core and rim of those crystals can give information on the mineralogical phase. 

 Olivine  in the contact zone (introsion of mafic  magmatic in a felsic country rock)  can also show sign of rim dissolution and precipitation of new mineral.  Collected samples from this contact rocks often enclose mineral with  dissolution and recrystallization ( mainly due to solution mixture).

Many thanks for your responses. I have gained more. As suggested I will do a microprobe analysis of the core mineral and the rim minerals to be able to classify the minerals appropriately. There are no doubts as to whether there was some fluid activity or not because I found out that some of the cracks in the olivine phenocrysts were infilled will calcite.

Thanks also

The presence of calcite in olivine cracks is significative. It is often known that cracks in olivine phenocrysts  are mainly  composed of serpentine (serpentinization; also as   pseudomorph),  chlorite ( chloritization), oxide and oxi-hydroxide (oxidation). These wheathering activities are related to hydrothermalism or alteration by the action of rain water. It will  not be easy to relate the presence  of your calcite to any of those  weathering processes, as   calcite has been  found in vesicles in some basalts in Cameroon ( currently studied by Professor Nkoumbou et al). So you still have to continue your research?  

Many Thanks Sylvestre!! I will check this publication and some others from the Cameroon line. I am aware a lot has been done there.

Dear Uriah, 

I am working on crystals and grains external and internal texture, geochemistry, and geochronology: application to petrogenetic and provenance studies. So, I really like to react on topic related to  those research field.

I will soon present a PhD research work entitled: Geology of the western Mamfe corundum deposits, SW Cameroon: petrography, geochemistry, geochronology, genesis and origin.  I will  present this thesis  soon. I use petrography, geochemistry, and geochronology as tools for exploration  of deposited mineral occurrences. Those same methods were used for petrogenetic evaluation, and for provenance studies. Three papers were published from my Thesis. The forth has been submitted.  It is entitled: A possible links between the Mamfe Sedimentary Basin and two west African Megastructure (The Benue Trough and Cameroon Volcanic Line): evidence from geochronological correlations'. The other paper  will soon be submitted.

Your research area is quite interesting. I will appreciate if you can send copies of your published papers to me via e-mail :[email protected] whenever they are ready. Best of luck.

This texture looks most likely to be be due to a magmatic reaction between the olivine and a more siliceous magma. However, normally this results in a complete, unbroken corona ring of orthopyroxene (sometimes perhaps clinopyroxene), at least in coarser-grained intrusive rocks.  These images show rather a ring of radially-arranged subhedral prismatic pyroxenes (both cpx and opx?) or perhaps amphiboles (tremolite?). This radial arrangement of prismatic grains is also seen in a lower temperature hydrous alteration often developed between adjacent olivine and plagioclase (or another Ca and Al-bearing material like basaltic glass) but on a much finer scale (radial tremolite prisms that are 10 to 40 microns long and only 1 or 2 microns wide). I have seen this occurring at greenschist facies in an olivine melanorite, and it is usually accompanied by partial serpentinization of the olivine interior, and partial chlorite alteration of adjacent plagioclase - neither of which I can see on your photos. A useful reference to this type of alteration is by Beard and others in Journal of Petrology, vol.50, no.3, p.387-403 (see link). But to me this doesn't look like a hydrous reaction -  I would favour the higher temperature disequilibrium reaction between olivine and the basalt matrix as Marlina suggests. You should look to see if the olivine is only present as coarse phenocrysts, or is there a second generation of smaller olivines which might not have this rim developed? Good luck. 

http://petrology.oxfordjournals.org/content/50/3/387.abstract

I agree with Marlina and Evans. To me, it seems like the result of a high-temperature reaction between olivine and a silica-saturated magma. This suggests also a refinement in terminology, as the olivine crystal seems to be in disequilibrium with the host magma, so it´s probably not a phenocryst and should be better termed as a xenocryst.

Thanks Enrique! Yes a xenocryst!

Dear Professor,

Good morning

Have  a look on those microphotographs from basaltic rocks outcropping  in the southern part of the Mamfe Basin,  SW Cameroon. They show olivine replacement by other minerals.

Dr Evans, I appreciate the article on serpentinization in ol-troctolite. Quite revealing.