Abstract
Measuring water contents of magmas is fundamental to resolving a number of geological questions, such as the mechanisms of silicic magma evolution, the triggering of volcanic eruptions, and the formation of porphyry copper deposits. This study focuses on the correlation between apparent deviations from stoichiometry of plagioclase crystals and high water concentration in the magmatic melt from which they grew. We considered this relationship as a potential geo-hygrometer (water activity indicator). To test and potentially calibrate this new technique, a range of natural and experimental plagioclase crystals were analysed, with particular care taken to identify and avoid analytical bias and artefacts. In contrast to recently published material, we found no systematic aluminium excess in plagioclase, irrespective of the water concentration of the silicate melt it crystallised from. This suggests that aluminium excess in plagioclase cannot serve as a geo-hygrometer. The high likelihood of misinterpreting analytical artefacts (due to alkali migration and imprecise standardisation) as small deviations from stoichiometry, also requires its application as a mineral exploration tool to be treated with caution.
Highlights
Determination of pre-eruptive water contents of magmas is key to understanding the evolution of magmas and subsequent implications for the formation of magmatic-hydrothermal ore deposits
A hygrometer based on Al* would potentially bear the advantages of: (a) wide applicability due to the ubiquitous presence of plagioclase in most igneous rocks, (b) ease of measurement with standard-based wavelength dispersive X-ray spectrometry (WDS) attached to an electron-probe micro-analyser (EPMA), (c) good preservation potential due to slow diffusion of Al, Ca, and Na compared to H (e.g.32–35), and (d) potential to resolve changing concentrations recorded in mineral zoning
Natural plagioclase crystals can be found in barren magmatic rocks from both plate margin and intraplate environments, and in some rocks related to porphyry copper deposits
Summary
Determination of pre-eruptive water contents of magmas is key to understanding the evolution of magmas and subsequent implications for the formation of magmatic-hydrothermal ore deposits. H-compounds were measured directly in nominally anhydrous minerals22), including measurements of plagioclase[23,24,25,26,27], and the use of structural-geochemical proxies for H as geo-hygrometers[28]. These methods either rely on experimental studies, require complicated measurement procedures, suffer from restricted applicability, or pertain to analytes that may not preserve their original compositions (e.g. due to H diffusion). In the course of this evaluation, care was taken to minimise potential analytical artefacts that could lead to erroneous interpretations
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