Spinel and garnet textures in mantle peridotite xenoliths and lunar dunite: a micro-CT study

Bhanot, Krishan Kumar (2022) Spinel and garnet textures in mantle peridotite xenoliths and lunar dunite: a micro-CT study. PhD thesis, Birkbeck, University of London.

Abstract

This study uses Micro-CT scanning to investigate spinel and garnet textures in terrestrial and extra-terrestrial mantle xenoliths. Terrestrial studied sample are from: (1) Massif Central, France; (2) Lanzarote, Canary Islands; (3) Calatrava, central Spain; (4) Pali Aike, Patagonia; (5) Vitim, Siberia, and (6) Kimberley, South Africa. Xenoliths from the Massif Central showing typical textural types were analysed to provide a record of examples of protogranular, porphyroclastic and equigranular textures. Micro-CT analysis of xenoliths from Massif Central, Lanzarote and Calatrava revealed a range of spinel textures from <2 mm microcrystals which can be either spatially concentrated or distributed more evenly throughout the rock, to large 4-12 mm individual spinel-pyroxene clusters with complex vermicular textures in random orientation. These clusters are the result of a transition of shallow lithospheric mantle from the garnet stability field to the spinel stability field. We predict that such textures would only occur in the mantle beneath regions that show evidence of thinning of the lithospheric mantle. Metasomatic reactions around spinel-pyroxene clusters in some Lanzarote xenoliths indicate that metasomatism post-dated cluster formation. Micro-CT scanning was also used to examine the textures of garnet-spinel clusters and the relationship between garnet and spinel in garnet-spinel peridotite mantle xenoliths from Pali-Aike and Vitim. A xenolith from the Kaapvaal craton provided an example of a typical garnet peridotite and thus provided a reference to compare and contrast 3D structures in garnet and garnet-spinel peridotites from Vitim and Pali-Aike. These xenoliths record a complex tectonic history, in which the original garnets in garnet peridotites were transformed to spinel-pyroxene clusters in spinel peridotites, via lithospheric thinning, but returned to garnet during cooling. Spinel symplectite textures occur in lunar dunite clasts collected during the Apollo 17 mission. Micro-CT analysis confirms that spinel forms complex structures of varying size, shape and texture. Four types of symplectite with different compositions of spinel were identified. Based on texture and spinel mineral chemistry, we propose that the four textural types have different origins. Each type represents a different stage in the history of the lunar dunite. The oldest were formed from decompression of garnet brought up from ~420 km depth by convective overturn, and thus this is a piece of the lunar mantle. Two small symplectite types are related to melts in the lunar mantle. The youngest type has a shallower origin with interaction of olivine and decompression melts forming anorthite-bearing symplectites. This texture most likely formed from impact melts during the Serenitatis event.

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