Matthijs Smit

Assistant Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Geochronology of ultrahigh-pressure rocks to interrogate the evolution of the continental crust and mantle (2019)

The formation of cratonic lithosphere and its participation in continental collision are first-order processes in global tectonics. The Western Gneiss Complex (WGC) of southwestern Norway is a fragment of continental crust that uniquely preserves a complete record of its burial and exhumation during collisional orogeny along with rare fragments of sub-continental lithospheric mantle that were entrained into the terrane during its residence in the mantle. Despite the importance of the WGC for characterising processes operating in the deep crust and mantle during continent-continent collision, its rate and style of burial and exhumation have not been comprehensively studied and the protracted evolution of the included peridotite bodies remains unclear. Lu-Hf garnet and micro-analytical U-Pb rutile geochronology are two powerful tools for lithosphere and tectonics research as they can be used to link ages to conditions of equilibration of rock-forming assemblages. Using these techniques applied to eclogites in the WGC, I constrained the burial rate for continental crust during collisional orogeny to ~5 mm yr-¹, developed a quantitative framework for evaluating geodynamic changes during continental collision, and proposed that deeply buried continental crust is exhumed largely as a flat-slab in the mid-crust, possibly due to erosion of a paleo-plateau in the upper plate. Using Lu-Hf garnet geochronology applied to ultrahigh-pressure (UHP) enstatite-bearing eclogites in the WGC, I provided well-constrained empirical evidence for non-lithostatic eclogitisation, a process that explains the localised occurrence of anomalously-high pressures conditions in deeply buried continental crust. When these research outcomes are compared to the lower plate in the India-Asia collision zone, they demonstrate consistency in the rate and depth of burial and the style of exhumation of continental crust during collisional orogeny. Using Lu-Hf garnet geochronology applied to included peridotite bodies in the WGC, I provided the first well-constrained geochronological evidence for the stabilisation of a buoyant cratonic sub-continental lithospheric mantle in the Archean that melted and recrystallised in concert with major supercontinent break-up intervals. The techniques used herein could be applied to other collisional settings and to other mantle peridotite suites to better constrain the emergence and evolution of global plate tectonics cycles.

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Master's Student Supervision (2010 - 2018)
Archean crustal evolution constrained by strontium isotopes in apatite and uranium-lead geochronology and trace element geochemistry of zircon (2018)

Archean continents were the nuclei for crustal growth and large volumes of continental crust appear to have been produced during the Archean. Much of the preserved Archean crust is of tonalite-trondhjemite-granodiorite (TTG) composition and it is argued that this made up the bulk of Earth’s earliest crust. Other models involve a bulk mafic crust that was very different to the modern crust. New data are therefore needed to test and refine these models and determine how continents were first formed. The Rb-Sr isotopic system provides a potentially powerful proxy for crustal composition yet it has thus far been underutilized in studies on early crustal evolution due to its susceptibility to re-equilibration. Overcoming this issue requires new analytical approaches to micro-sample ancient Sr-rich minerals, such as apatite, that may retain primary 87Sr/86Sr signatures. In this thesis study, a novel method in laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) was applied to apatite from TTG complexes of different Archean age. The first area of focus was the Acasta Gneiss Complex, Northwest Territories, which contains the oldest known terrestrial rocks. Apatite inclusions within ca. 3.7 Ga zircon host grains were subjected to Sr isotope analysis by LA-MC-ICPMS. The initial 87Sr/86Sr values of these inclusions are identical within error and are different from values obtained from altered matrix apatite. Combining the 87Sr/86Sr results with information on the protolith and source-extraction age yields estimates for the range of source Rb/Sr and suggests that an evolved Hadean source was involved in the formation of the Acasta Gneiss Complex. The Sr isotope LA-MC-ICPMS method was also applied to matrix apatite from TTG of the ca. 3.6 Ga Bastar Craton, India, and the 3.0-2.8 Ga Kvanefjord Block, Greenland. The radiogenic 87Sr/86Sr signatures from these apatite grains also require a high Rb/Sr crustal source. This suggests that enriched crustal vestiges played a role in the formation of TTG crust.

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