250 Mineralogical Society of America (MSA) Awards Lectures, Presidential Address, and Annual Business Meeting

Tuesday, 24 October 2017
3:00 PM - 5:30 PM
Skagit 4

MSA ROEBLING MEDAL LECTURE: THE EFFECTS OF SOLID-SOLID PHASE EQUILIBRIA AND PARTIAL MELTING ON THE OXYGEN FUGACITY OF THE UPPER MANTLE

Paper number: 250-1
Program: Technical Sessions
Time: 3:00 PM – 3:30 PM

Authors

Abstract

The redox variability of basalts and their mantle sources is of growing interest due to the increasing number of measurements in glasses and minerals of ƒO2–sensitive ratios such as Fe3+/Fe2+. A simple interpretation of such variability is that if ƒO2 and Fe3+/Fe2+ are higher in one basalt than in another, then its source also had a higher ƒO2 and Fe3+/Fe2+. We show, however, that phase equilibria of peridotite can lead to significant variations in the ƒO2 of the system and Fe3+/Fe2+ of individual phases (including melt) even when the peridotite’s bulk composition (i.e., total O content and Fe3+/Fe2+) is constant.

The effects of phase changes on the ƒO2 of lherzolite can be largely understood based on a simple example: Assume subsolidus plagioclase (plag) lherzolite with a small modal abundance of spinel (sp); its ƒO2 will vary positively with the magnetite content (mt) of the sp. As P increases, the peridotite will transform into a sp lherzolite, and the mt content of the sp will decrease due to dilution by the aluminous sp components produced by this transformation; as a result, the ƒO2 of the system drops despite its unchanged bulk Fe3+/Fe2+. This ƒO2 decrease as peridotite transforms from plag to sp lherzolite is ~1.5 log units based on thermodynamic modeling. The ƒO2 then increases by ~1 log unit as P increases further and the peridotite transforms to garnet (gt) lherzolite, due to the reaction of aluminous sp components into pyroxenes and gt, largely reversing the effects of the reaction to sp lherzolite. Quantitative modeling of peridotite ƒO2 at 0–4 GPa confirms these and other effects of continuous and discontinuous phase changes.

Although isobaric melting generally leads to decreasing ƒO2, isentropic decompression melting can result in the ∆FMQ of peridotite increasing by ~1 log unit. This also results from continuous and discontinuous solid–solid phase transitions, with melting itself only introducing a small perturbation on melt-absent trends. Shallower melts on a single isentrope are thus expected to have generally higher ƒO2 than deeper melts.

We conclude that although observed variations in ƒO2 of basalts may reflect subduction of oxidizing fluids, sediments, and altered oceanic crust and/or the opposing effects of subducted organic material, the effects of peridotite phase equilibria must also be considered.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-304520

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.