29 Melt, Emplace, Mix, Erupt! (MEME) Investigating the Dynamics of Magmatic Systems via Microanalysis

Friday, 8 April 2022
8:00 AM - 9:15 AM
North Meeting Room 211

THE EFFECTS OF MELT ADDITION ON MID-OCEAN RIDGE PERIDOTITES

Paper number: 29-1
Program: Theme Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

Peridotites obtained from mid-ocean ridges record significantly greater geochemical variability than do their basaltic counterparts, as well as greater variability in thermodynamic properties such as oxygen fugacity (fO2) [1,2]. Variability in fO2 may reflect modern processes associated with melt extraction and melt addition beneath the ridge axis, or it may reflect long-lived source heterogeneity associated with recycled material or ancient melting events.

To investigate the effects of melting and melt addition on peridotite geochemistry and fO2, we determined mineral chemistry and oxygen fugacity for a suite of gabbro-veined peridotites from a single segment of the Southwest Indian Ridge (SWIR). We combined these new measurements with previously-published fO2 results for a range of SWIR peridotites [3], to which we also added new petrographic analysis and trace element data.

We find that SWIR peridotites with no evidence of melt addition record low spinel Cr# (Cr#=100*Cr/(Cr+Al) < 30), with no change in fO2 during melting. In contrast, gabbro-veined and plagioclase-bearing peridotites record higher spinel Cr# (Cr# = 38-57). We interpret the high Cr# of these spinels as being due to reaction of spinel to form plagioclase during melt addition. Although melt addition modifies spinel Cr#, it does not result in significant modification of fO2. This stands in contrast to observations at other tectonic settings such as subduction zones, ocean islands, and continental cratons. At these other tectonic settings, peridotite is oxidized by petrogenetically unrelated melts/fluids, whereas at mid-ocean ridges, peridotites interact with MORB, which has little to no oxidizing power over its own mantle residues. Thus, while modern ridge processes modify peridotite geochemistry, they do not generate variability in fO2, suggesting that observed variation in ridge peridotite fO2 may instead reflect pre-existing mantle heterogeneity.

[1] Warren et al., 2016

[2] Cottrell et al., 2021

[3] Birner et al., 2018

Geological Society of America Abstracts with Programs. Vol. 54, No. 4

doi: 10.1130/abs/2022NC-374815

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