[Folda] Extra Seminar - Wednesday 3 May 12:30 - David Neave: "A new clinopyroxene-liquid barometer, and implications for magma storage pressures under Icelandic rift zones"

[Maria H. Janebo]

Dear all,

Next week there will be an additional seminar on Wednesday (3 May) at 12:30 in the seminar room on the 3rd floor, with a presentation given by:


David Neave: "A new clinopyroxene-liquid barometer, and implications for magma storage pressures under Icelandic rift zones"
 
Pressure is one of the key variables that controls magmatic phase equilibria. Various barometers have thus been developed over the past two decades that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. These Jd-in-clinopyroxene barometers have been applied to rift zone magmas from Iceland, where published estimates of magma storage depths span the full thickness of the crust, and extend into the mantle. However, tests performed on commonly used clinopyroxene-liquid barometers revealed substantial pressure-dependent inaccuracies, with some models overestimating pressures of experimental products equilibrated at 1 atm by up to 3 kbar.  In order to improve the accuracy of Jd-in-clinopyroxene barometry at pressures relevant to magma storage in the crust, we calibrated a new barometer using experimental data in the 1 atm to 20 kbar range. This new barometer accurately reproduces its calibration data with a standard error of estimate (SEE) of ±1.4 kbar, and is suitable for use on hydrous and anhydrous samples that are ultramafic to intermediate in composition, but should be used with caution below 1100 °C and at oxygen fugacities greater than one log unit above the QFM buffer. Many current estimates of magma storage pressures may therefore need to be reassessed. To this end, the barometer was applied to numerous published clinopyroxene analyses from Icelandic rift zone tholeiites filtered to exclude compositions affected by poor analytical precision or collected from disequilibrium sector zones. Putative equilibrium liquids were selected from a large database of Icelandic glass and whole-rock compositions using an iterative scheme because most clinopyroxene analyses were too primitive to be in equilibrium with their host glasses. High-Mg# clinopyroxenes from the primitive Borgarhraun eruption record a mean storage pressure in the lower crust, while all other eruptions considered record mean pressures in the mid-crust. Thus, while some magma processing takes place in the shallow crust immediately beneath Iceland’s central volcanoes, magma evolution under the island’s neovolcanic rift zones is dominated by mid-crustal processes.




Maren & Maria



_____________________________
Maria H. Janebo
Postdoctoral Research Fellow

Nordic Volcanological Center
Institute of Earth Sciences
University of Iceland, Askja 223
Sturlugata 7, 101 Reykjavik, Iceland

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