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Published October 2016 | Published
Journal Article Open

An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer

Abstract

We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total aluminium content (Al^(tot)) of the synthetic calcic amphiboles varies systematically with pressure (P), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Al^(tot) and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An_(15–80)) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate P (in kbar) from Al^(tot) (in atoms per formula unit, apfu) according to the expression: P (kbar)=0.5+0.331(8)×Al^(tot) +0.995(4)×(Al^(tot))^2. This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Al^(tot) value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.

Additional Information

© The Author(s) 2016. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Received: 17 March 2016; Accepted: 7 September 2016; Published online: 24 September 2016. This research was supported through funds from BHP Billiton. JB acknowledges receipt of a Wolfson Research Merit Award from the Royal Society and a Moore Scholarship from the California Institute of Technology where this work was written up. We are grateful to S. Kearns and R. Hinton for assistance, respectively, with EPMA and SIMS analyses, and V. Matjuschkin for help in the high-pressure laboratory. We thank M. van Zalinge for providing two of the starting materials, and both M. van Zalinge and R. Perkins for providing unpublished whole rock data. We greatly appreciate the assistance of J. Dilles in the Yerington Batholith, Nevada, where three of the calibration samples were collected. The paper benefited greatly from the constructive reviews of C. F. Miller, J. J. Ague and an anonymous reviewer as well as the careful editorial handling of G. Moore.

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