Ezad, Isra Shehreen; (2019) The kinetics of garnet breakdown and spinel grain growth in the upper mantle. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Mantle xenoliths provide rare insight into the inaccessible interior of Earth, they are the only samples to preserve both, textures and phases present from depth. Many of these mantle xenoliths will undergo metamorphic reactions upon exhumation, and leave behind textural relics of their journey. Garnet bearing mantle xenoliths are no exception, and transform to spinel lherzolite through a textural evolution of kelyphitic corona, a special type of reaction rim. Kelyphitic corona in transitional garnet to spinel lherzolite have been used to infer; uplift rates, reaction kinetics and pressure, temperature conditions of formation. However, the lack of reaction kinetic data and an understanding of the evolving reaction rim texture, with respect to pressure and temperature has limited complete pressure, temperature, time (PTt) pathway estimates. This Ph.D presents, new experimentally determined reaction kinetics of the garnet to spinel transition in mantle peridotites. Successfully reproducing textures such as, kelyphite corona and symplectites following garnet breakdown. Garnet retrogression experiments have also shown a switch in reaction, from an open system to isochemical breakdown. This switch in reaction can be induced through kinetic effects alone. The results from garnet retrogression experiments, provide strong constraints on minimum uplift velocities of garnet bearing mantle in kimberlitic melts. Eventually as the mantle upwells garnet is entirely consumed, whilst spinel nucleates and grows, in the presence of orthopyroxene and or clinopyroxene. This re-equilibration marks a known point in pressure, temperature, and for exhumation purposes resets time on the newly formed spinel peridotite assemblage. This unique starting point allows the final grain size of spinel upon exhumation to provide an estimate of uplift or residency times. To take advantage of this unique PTt space, time-series grain growth experiments were conducted on spinel-pyroxene mixtures. It can be demonstrated normal grain growth occurs within this system, and the growth rates are similar to other upper mantle polyphase mixtures. The experimentally determined grain growth kinetics of spinel have been applied to spinel peridotite from Lanzarote and Hungary. Grain size in an important and underconstrained parameter especially within the deeper parts of Eart, where it is believed grain size dependent creep dominates. The transition zone is thought to deform in a grain size sensitive regime, and as such three phase grain growth of ringwoodite, majorite and stishovite was experimentally investigated. To determine, the grain growth kinetics of the lowermost transition zone, and attempt to constrain the grain size of a realistic mantle assemblage. In order to successfully investigate polyphase grain growth, multiple starting materials and significant experimental development was required. Finally, traditional methods of grain size analysis result in researcher biases, for example, when to divide one grain into two or three. As well as research bias, traditional methods have often used homogenous grain shapes and assumptions for non-spherical grains. In attempts to improve the traditional methods by which grain size is estimated; advanced image processing techniques have been coded and used on an entire grain growth data set. Application of a watershed segmentation scheme has allowed for the separation of touching grains in a quantitative approach without inherent biases of traditional analysis methods.

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