Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth's upper mantle

Dubrovinsky, L., Harris, J. W., Stachel, T., Kurnosov, A., Kiseeva, E. S., Marquardt, H., Satta, N. and Koemets, I. (2020) Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth's upper mantle. American Mineralogist, 105(7), pp. 984-991. (doi: 10.2138/am-2020-7136CCBY)

Abstract

Majoritic garnet has been predicted to be a major component of peridotite and eclogite in Earth's deep upper mantle (>250 km) and transition zone. The investigation of mineral inclusions in diamond confirms this prediction, but there is reported evidence of other majorite-bearing lithologies, intermediate between peridotitic and eclogitic, present in the mantle transition zone. If these lithologies are derived from olivine-free pyroxenites, then at mantle transition zone pressures majorite may form monomineralic or almost monomineralic garnetite layers. Since majoritic garnet is presumably the seismically fastest major phase in the lowermost upper mantle, the existence of such majorite layers might produce a detectable seismic signature. However, a test of this hypothesis is hampered by the absence of sound wave velocity measurements of majoritic garnets with relevant chemical compositions, since previous measurements have been mostly limited to synthetic majorite samples with relatively simple compositions. In an attempt to evaluate the seismic signature of a pyroxenitic garnet layer, we measured the sound wave velocities of three natural majoritic garnet inclusions in diamond by Brillouin spectroscopy at ambient conditions. The chosen natural garnets derive from depths between 220 and 470 km and are plausible candidates to have formed at the interface between peridotite and carbonated eclogite. They contain elevated amounts (12–30%) of ferric iron, possibly produced during redox reactions that form diamond from carbonate. Based on our data, we model the velocity and seismic impedance contrasts between a possible pyroxenitic garnet layer and the surrounding peridotitic mantle. For a mineral assemblage that would be stable at a depth of 350 km, the median formation depth of our samples, we found velocities in pyroxenite at ambient conditions to be higher by 1.9(6)% for shear waves and 3.3(5)% for compressional waves compared to peridotite (numbers in parentheses refer to uncertainties in the last given digit), and by 1.3(13)% for shear waves and 2.4(10)% for compressional waves compared to eclogite. As a result of increased density in the pyroxenitic layer, expected seismic impedance contrasts across the interface between the monomineralic majorite layer and the adjacent rocks are about 5–6% at the majorite-eclogite-interface and 10–12% at the majoriteperidotite-boundary. Given a large enough thickness of the garnetite layer, velocity and impedance differences of this magnitude could become seismologically detectable.

• Item Type: Articles
• Additional Information: This research was supported through the project “GeoMaX” funded under the Emmy-Noether Program of the German Science Foundation DFG (MA4534/3-1). H.M. acknowledges support from the Bavarian Academy of Sciences. E.S.K. was supported by NERC grant NE/L010828/1. N.S. was supported by the IRTG “Deep Earth Volatile Cycles” grant (GRK 2156/1).
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Harris, Dr Jeff
• Authors: Dubrovinsky, L., Harris, J. W., Stachel, T., Kurnosov, A., Kiseeva, E. S., Marquardt, H., Satta, N., and Koemets, I.
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences
• Journal Name: American Mineralogist
• Publisher: Mineralogical Society of America
• ISSN: 0003-004X
• ISSN (Online): 0003-004X
• Copyright Holders: Copyright © 2020 Mineralogical Society of America
• First Published: First published in American Mineralogist 105(7):984-991
• Publisher Policy: Reproduced under a Creative Commons License