Evidence of majoritic garnet from the mantle transition zone in the Alpe Arami garnet peridotite

ABSTRACT

Ultra-high-pressure (UHP) peridotites found along collisional zones record rare information from deep within the Earth. However, the estimation of depth of origin for these UHP rocks has been controversial. A major controversy remains related to the conjectural proposition of mantle transition zone (410–660 km) origin of the Alpe Arami (AA) garnet peridotite massif in the Swiss Alps. In this contribution, we show micro-textural evidence of precursor majoritic garnet by documenting exsolved rutile, high-Al orthopyroxene, jadeite-rich clinopyroxene and olivine within the AA garnets in this peridotite. We also document an unforeseen texture of olivine with ‘necklace’ like enstatite corona in the kelyphite formed after decomposition of relict garnet. These olivines bear FeTiO₃ and Cr-spinel exsolution needles indicating retrogression from high-pressure Mg₂SiO₄. Thus, the occurrence of retrogressed high-pressure Mg₂SiO₄ with enstatite corona in kelyphite suggests majorite breakdown to precipitate high-pressure Mg₂SiO₄ near mantle transition zone (MTZ) depth. The SiO₂ released during decompression of majoritic garnets reacts with the high-pressure Mg₂SiO₄ to produce the enstatite corona. Our documented micro-textures show high-pressure Mg₂SiO₄ are breakdown product of precursor majoritic garnet, indicating that these micro-textures of the AA peridotite massif are sourced from the mantle transition zone (MTZ).

KEYWORDS:

• Majoritic garnet
• mantle transition zone
• garnet peridotite
• Alpe Arami

Acknowledgments

The present contribution is dedicated to the memory of Professor Harry W. Green II who was an advisor of the second author’s PhD thesis in 1975. The garnet peridotite samples used in this study were collected in the early 1970s by Professor A. Nicolas and Dr F. Boudier, then at the Université de Nantes. The samples were then provided on request to the second author to determine P–T phase equilibria study. The authors are indebted to Larissa F. Dobrzhinetskaya and Ekaterina S. Kiseeva for many discussions. The authors are grateful to the University of Texas for financial support in the form of a STAR grant to A. R. Basu and both authors are to the University of Texas at Arlington for other supports that made this study possible. The authors are thankful to Dr Gelu Costin, Rice University, for his expertise and invaluable help with the Field Emission Electron Probe Micro Analyses (FE-EPMA). The authors acknowledge Editor Robert J. Stern, reviewer Haibo Zou and an anonymous reviewer for constructive comments that improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

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Additional information

Funding

This work was supported by the University of Texas at Arlington [STAR].