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The Petrological Consequences of the Estimated Oxidation State of Primitive MORB Glass

Asimow, Paul D. (2021) The Petrological Consequences of the Estimated Oxidation State of Primitive MORB Glass. In: Magma Redox Geochemistry. Geophysical Monograph Series. No.266. American Geophysical Union , New York, NY, pp. 139-154. ISBN 9781119473206.

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Discrepancies in Fe²⁺/Fe^T in mid-ocean ridge basalt glasses may be an analytical problem with implications principally for the oxidation state of the source mantle, but the Fe²⁺/Fe^T of such glasses also influences their equilibrium with olivine and petrological models of magmatic temperature. Hence Fe²⁺/Fe^T affects the inference of potential temperature and extent of melting in the sub-ridge mantle and models of complementary formation of erupted basalts and cumulate lower crust. Conversely, evaluation of equilibria between olivine phenocrysts and host glasses may provide an independent constraint on oxidation state. This chapter reviews petrological methods for estimating primary basalt composition, potential temperature, and thickness of igneous crust, focusing on the influence of glass redox state. The 1990's consensus around mutually consistent average values of Fe²⁺/Fe^T of MORB glass, thickness of oceanic crust, and mantle potential temperature apparently needs revisiting on several fronts. Values of average MORB glass Fe²⁺/Fe^T < 0.88 are apparently inconsistent with olivine-glass data and require primary liquidus temperatures too low for traditional estimates of the average thickness of oceanic crust or of lower crustal cumulates. Critical assessments of the absolute accuracy of Fe²⁺/Fe^T measurements, of electron microprobe measurements of olivine composition, of olivine/liquid Fe/Mg partitioning models, and of thermodynamic models of peridotite melting are essential to resolving this inconsistency. New melting models, source compositions, crustal thickness estimates, or interpretations of the plutonic-volcanic relationship may be needed to reconcile the bulk properties of the oceanic crust and its layers with the measured Fe²⁺/Fe^T of MORB glass.

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Asimow, Paul D.0000-0001-6025-8925
Additional Information:© 2022 American Geophysical Union. First published: 17 September 2021. The high-precision olivine data in Figure 7.4 are shown courtesy of Claude Herzberg. The author wishes to thank Claude Herzberg, Elizabeth Cottrell, Charles Langmuir, Maryjo Brounce, and Esteban Gazel for discussions during the development of these ideas, but the responsibility for any shortcomings in the final product rests entirely on the author. Formal reviews from an anonymous reviewer and Elizabeth Cottrell were very helpful in developing the final version of this chapter.
Subject Keywords:Oxidation state; mid-ocean ridge basalt; PRIMELT3; pMELTS; potential temperature; crustal thickness
Series Name:Geophysical Monograph Series
Issue or Number:266
Record Number:CaltechAUTHORS:20211008-183531086
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Official Citation:Asimow, P.D. (2021). The Petrological Consequences of the Estimated Oxidation State of Primitive MORB Glass. In Magma Redox Geochemistry (eds R. Moretti and D.R. Neuville).
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:111286
Deposited By: George Porter
Deposited On:08 Oct 2021 20:25
Last Modified:08 Oct 2021 20:25

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