I have some grabbro samples, however, when I made EPMS analysis on the minerals in the rock slice, there are much more amphiboles than clinopyroxenes. Then I think the most clinopyroxenes should have altered to amphiboles. How could I distinguish between the two kinds of amphiboles?
Hi Feng Huang,
Basically, you cannot tell the differences of these amphiboles solely based on mineral chemistry.
I think petrography may help in this regard, given the gabbro was not intensively deformed and metamorphosed. If magmatic texture of the rock is well preserved, it is likely that the amphibole - hornblende is primary, i.e., magmatic. Othewise, it could be metamorphic.
Regards,
Xue-Ming Yang
In igneous rocks, especially mafic igneous rocks, primary amphiboles tend to be pargasite or pargasitic hornblende. The problem arises in that at magmatic conditions, there are processes (hydration crystallization) that produce amphibole by reaction between pyroxene and melt. These amphiboles are usually pargasite or hornblende. However, reaction can continue subsolidus yielding low-Al hornblende, actinolite etc. that are generally construed as "deuteric" or metamorphic. Finally, if you have wholesale replacement of pyroxene by green or blue-green hornblende, it is probably metamorphic. The condition of the feldspar can also yield a clue in these situations. Is it altered? Is it more albitic than you would expect for the rock composition?
Hi, Xue-Ming Yang, James S. Beard and Momi Hamenana, thanks for your answers. I would made more petrography observations and choose some rocks slices for electron probe microanalysis. Based on the observations, I think that many amphiboles were altered from pyroxene. The more information would be obtained after the electron probe microanalysis. I would check the compositions of the amphiboles, such as low-Al hornblende, actinolite, tremolite, glaucophane and barroisite, etc. Thank you again for your help.
Best regards!
Pleace pay an attention also to titanium content in amphiboles. It is elevated and stable in igneous amphiboles and lowered and variable in secondary amphiboles. Try also to find rounded amphibole inclusions in clinopyroxene and plagioclase. If they do exist and demonstrate relatively high Ti contents, this amphibole generation is probably of igneous origin.
Best regards!
Hi Feng.
Using Thin Section:
I would pay close attention to both the condition of your mineral grains i. Thin section as well as how the mineral is situated within your slide. i.e how does the long axid (typically c-axis) align in the sample, does it appear to be perfectly in tact or are there finrous like splays on mineral edges indicating rotation.
This also pairs wepl by noticing discoloration on mineral boundaries on both PLL and XPL as a result of damaged lattice structure due to mineral replacement (i.e secondary/alteration mineral at boundary).
Another method if both minerals are still in reasonable condition would be to measure the extinction angle from your long/c-axis. Similar to plagioclase, the extinction angle is based off composition which has been well documented.
Using Scanning Electron Microscope (SEM):
If you have the opportunity to analyse using this method the mineralogical differences will become quickly prominant. Perhaps a mineral will have two distinct shades of grey which when sampled will indicate different electron volt spikes which demonstrate compositional differences.
Hope this helps
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