Petrology and tectonics of the Archean

Surprisingly, there is still little consensus on the crust formation mechanisms that operated on the early Earth.   I am continuing to use integrated petrology and isotope geochemistry to work towards a solution to this important problem, as well as working with collaborators on interdisciplinary themes associated with ancient crustal growth, such as continental freeboard and crustal controls on the atmosphere and ocean chemistry.  I have conducted several summers' of fieldwork in remote northern Canada (supported by NSF), focusing on samples from a wide geographic area that have crystallization ages spanning the breadth of the Archean (4.0-2.5 Ga).  I am using advanced chemical signatures such as zircon Hf-isotope (laser ablation split stream methods) and whole-rock 142Nd and Pb-isotope analyses to trace Hadean crustal material throughout the Archean.  I am also working with Brad Foley (Penn State) on integrating geochemical signatures and geodynamical models to investigate various aspects of Archean crust formation mechanism.

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Above: a photo of a poly-metamorphosed ~3.3 Ga tonalitic and amphibolitic gneiss xenolith (to the right of the hammer) included in a ~2.6 Ga granite, which was further metamorphosed ca. 1.9 Ga.  These are obviously complicated rocks, meaning that targeted and cautious analysis is required to extract meaningful primary information from them.  

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Right: a photo of an incredibly well-preserved plutonic complex from the Slave craton.  These rocks are ~3.2 Ga, yet preserve primary magma mingling textures, mafic enclaves, and a wide range of rock types common in the Archean.  Rocks of this age and preservation state are incredibly rare, so detailed mapping, petrology, and isotopic work will greatly improve our understanding of the petrology and tectonics of Archean crust formation. 

Right: a simplified map of the Slave craton, where I have been working for several years.  The craton consists of 4.02-2.85 Ga basement gneisses, which are the oldest rocks exposed in the area.  Recently, I documented significant regional variability in the isotopic composition of the basement gneisses, suggesting that much of the craton formed by gradual addition of new material to the small, but very ancient, protocrustal nucleus on the western edge.  Thus far, I have worked on the rock units colored blue, the basement gneiss complex.  I have ongoing projects looking at the well-exposed granites that intruded the entire craton at 2.6 Ga, colored green.  

The Earth system change at the end of the Archean

There is increasing evidence that the end of the Archean, at 2.5 Ga, may have been marked by the uplift of continents and an increase in freeboard.  However, there are several viable explanations for the underlying process driving this potential Earth system change.  Interestingly, this fundamental change preceded the oxygenation of the atmosphere by a few hundred million years, yet the two processes may be linked.  The petrology of igneous rocks also changed systematically at the end of the Archean.  As such, many of the possible scenarios for the processes governing the fundamental Earth system change at the end of the Archean are testable with igneous petrology. I am currently working on projects focused on using petrology and isotope geochemistry to evaluate the mechanisms of petrologic change at this boundary, and link them to dramatic changes in the Earth system, including the oxygenation of the atmosphere.  

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  These projects include global analysis of detrital zircon datasets, detailed mapping and petrology of late Archean granites, working with sedimentologists to evaluate landform evolution, and integrating crustal age-isotopic signatures with the information available from cratonic mantle roots.   I am also applying newly-developed thermobarometers and geochronologic techniques to determine the mechanisms and timing of emplacement of these important granitoids.  

Magma mingling textures in ca. 2.6 Ga granites from the southern Slave craton