A Cautionary Note on Amphibole Geobarometry

The classical Al-in-hornblende barometer has been very successful in determining the depth of intrusion of metaluminous cordilleran granitoid plutons that bear the buffering assemblage at near solidus conditions: hornblende-biotite-plagioclase-orthoclase-quartz-sphene-two Fe-Ti-oxides (or one Fe-Ti oxide + epidote)-melt-vapor (e.g., [1-3]).

Ridolfi et al. [4] and Ridolfi and Renzulli [5] derived empirical amphibole-only barometric expressions that could be potentially applied to a larger number of phenocrystic assemblages from volcanic rocks. However, Erdmann et al. [6] claimed that these barometers are inaccurate and can give untenable estimates.

A graphical barometer based on the partitioning of Al and Si between amphibole and plagioclase was derived by Fershtater [7] using amphibole-plagioclase compositional pairs of rocks from the Urals. More recently, Molina et al. [8] calibrated an empirical expression based on experimental data that can be applied to igneous and high-grade metamorphic rocks.

In order to compare the reliability of amphibole-only and amphibole-plagioclase barometry, in this work, we test the performance of the expressions of Ridolfi and Renzulli [5] and Molina et al. [8], using an experimental data set compiled from the literature that has been recently published by Molina et al. [9].

In accordance with Erdmann et al.[6], the test reveals unsustainable pressure estimates with the amphibole-only barometric expressions from Ridolfi and Renzulli [5]. By contrast, the amphibole-plagioclase barometer from Molina et al. [8] performs well and yields a precision better than 1.7 kbar for Qz-Amp-Pl and Ol-free-Cpx-Amp-Pl assemblages with amphibole compositions having > 1 apfu (23O; normalisation to 13-CNK) Al, 0.05-0.27 apfu Ti: and < 1.07 apfu Fe³⁺.

References

1. Hammarstrom and Zen, 1986, American Mineralogist 71, 1297–1313.

2 Schmidt, 1992, Contributions to Mineralogy and Petrology 110, 304–310.

3. Anderson and Smith, 1995, American Mineralogist 80, 549-449.

4. Ridolfi et al., 2010, Contributions to Mineralogy and Petrology 160, 45–66.

5. Ridolfi and Renzulli, 2012, Contributions to Mineralogy and Petrology 163, 877–895.

6. Erdmann et al., 2014, Contributions to Mineralogy and Petrology 167, 1016

7. Fershtater, 1990, Geokhimiya 3, 328–335.

8. Molina et al., 2015, Lithos 232 286–305.

9. Molina et al., 2020; American Mineralogist, in press, https://doi.org/10.2138/am-2020-7400