A new clinopyroxene-liquid barometer, and implications for magma storage pressures under Icelandic rift zones.

Pressure is one of the key intensive variables that controls magmatic phase equilibria, which thus raises the possibily of using mineral and melt compositions to estimate magma storage pressures from erupted products. Such estimations are crucial for addressing geological problems ranging from understanding crustal accretion through to interpreting signals of unrest at active volcanoes.

In this paper with Keith Putirka, I assess the performance of some commonly used barometers that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. We find that many current barometers overestimate the pressure of phase equilibria experiments carried out on H2O-poor basalts at 1–7 kbar by up to 3 bar. Many published magma storage pressure estimates may thus need to be re-evaluated, and revised towards lower pressures.

In order to resolve the of barometer inaccuracy at low pressures, we thus present a newly calibrated Jd-in-clinopyroxene. Our new barometer is suitable for use on hydrous and anhydrous samples that are ultramafic to intermediate in composition. However, we do not recommend using the barometer at temperatures below 1100 °C and at oxygen fugacities above QFM+1 because of reduced accuracy under these conditions. The barometer reproduces its calibration data with a standard error of estimate (SEE) of 1.4 kbar, and tests performed using experiments on bastilc compositons confirm that it is significantly more accurate than previous models.

Calibration and test datasets used to develop and assess a new Jd-in-clinopyroxene barometer for use on ultramafic to intermediate compositions.

We apply our new barometer to a range previously studied eruptions from Iceland’s neovolcanic rift zones. Most eruptions preserve records of mid-crustal crystallisation at 2.6–3.6 kbar; only the highly primtive Borgarhraun eruption recrds crystallisation in the lower crust at 5.7 kbar. While some magma processing takes place immediately beneath Iceland’s central volcanoes, magma evolution under the island’s neovolcanic rift zones is thus strongly mediated by mid-crustal processes.

Spreadsheet for estimating P-T conditions from clinopyroxene-liquid equilibria

Clinopyroxene P-T Dec16

Publication

Neave, D.A. & Putirka, K.D. 2017. A new clinopyroxene-liquid barometer, and implications for magma storage pressures under Icelandic rift zones. American Mineralogist 102.

Note that there is an erratum concerning a typographical error in the barometric equation published in this paper