Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Decrepitation studies of pegmatites

Pegmatites are often important sources of rare elements or rare-earth elements and form an important class of mineral deposits. However it has been difficult to understand how these enigmatic rocks form and why they often contain gigantic (metres across) perfect single crystals of many minerals. Many models have been proposed to explain their formation.

A good overview of pegmatite crystallization processes and models by David London and George B Morgan is here.
        (published in Elements magazine, August 2012 V8 #4)   Not open access, article cost is $5.


Many, but not all, pegmatites formed from fluids with substantial contents of CO2.  In this comparison plot, samples from Bynoe and Tanco have particularly intense low temperature CO2 decrepitation, while samples from Greenbushes and the massif central in France have no CO2 response.
pegmatite quartz decrepitation

Pegmatites also contain large feldspar crystals which should preserve fluid inclusions. However, the feldspars often show no decrepitation at all, and when they do show decrepitation it does not usually correlate with co-existing quartz analyses. Without further study, feldspars are a problematical host mineral for fluid inclusion and decrepitation analyses.

decrepitation of fedspars in pegmatites

Other less common minerals in the pegmatites can also show decrepitation. However, without a large suite of samples of the same mineral it is not possible to interpret the decrepitation plots. In the following plot, note that the pollucite result has been divided by 5 to fit this scale.


decrepitation of other pegmatite minerals

Samples of pegmatites have been collected from 6 different provinces and have been analysed by acoustic decrepitation to try and understand the fluid environment in which these deposits formed. Results from these provinces are compiled and discussed in the following links.

Massif Central, France

Grenville province, Ontario, Canada

Tanco, Manitoba, Canada

Bynoe Harbour, NT, Australia

Greenbushes, WA, Australia

Londonderry, WA, Australia