The silicon isotope composition of Archaean continental crust from ~3.8 Ga West Greenland rocks
Description
Included are locality, XRF, age, and Si isotope data for Eoarchaean orthogneisses, metabasalts, and metasediments from southern West Greenland. Samples were originally collected by Stephen Moorbath and given to Paul Savage at Oxford. This dataset is in association with a project conducted by me (Madeleine Murphy) at the University of St Andrews from October 2018 to August 2019 towards fulfillment of the requirements for the degree of MSc Geochemistry under the supervision of Dr. Paul Savage. 08/08/2019 For additonal information about the MSc dissertation, please contact me at memurphy1896@gmail.com. Highlights: Updated Si isotope analysis provides characterization of ancient continental crust δ30Si of Archaean samples show systematic deviations from modern igneous rocks Data suggest Archaean TTGs formed via horizontal tectonics and were heavily weathered Abstract: Studies of the ancient continental crust may illuminate Earth’s differentiation, tectonic regimes through time, and co-evolution of the atmosphere and continents. Though scarce, representative samples include the Archaean tonalite-trondhjemite-granodiorites (TTGs) and metamorphosed lithologies of southern West Greenland. Due to its potential to trace various geologic processes, Si isotope analysis of these rocks could illuminate the mechanisms behind TTG formation, the inputs that contaminated igneous melts, and the extent of ancient continental weathering. Here, we measured the Si isotope composition of ~3.8 Ga TTGs, metabasalts, felsic volcanics, and metasediments from the Itsaq Gneiss Complex, SW Greenland. A large range in Si isotopes was formerly reported for Archaean Greenland rocks to suggest hydrothermal involvement during the earliest crustal genesis. However, Si isotope analytical techniques have improved in the years since these findings, providing the opportunity to re-characterize the δ30Si of Archaean continental crust using an updated alkali fusion and MC-ICP-MS methodology. For Archaean TTGs, we report an average δ30Si =－0.13 ± 0.03‰, isotopically heavier than Phanerozoic analogues, which may have resulted from the introduction of a seawater-derived contaminant to the TTG source melt. The fact that Archaean TTG Si signatures are not vastly different than Phanerozoic igneous rocks suggests they could have formed via magmatic differentiation in an arc setting. Our data for metabasalts and felsic volcanics closely parallel the Si signatures for modern analogues, pointing to their genesis in a subduction environment. For metasediments, we report δ30Si values from －0.19‰ to －1.2‰, much lighter than the range for heavily altered modern pelitic sediments, which implies their origin involved significant chemical weathering of a TTG protolith to form clay minerals. Combined, these interpretations suggest the formation of continental crust in the >3.8 Ga Archaean eon may have involved subduction and, crucially, the influence of an early hydrosphere.
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Institutions
- University of St Andrews School of Earth and Environmental SciencesFife, St Andrews
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Funders
- School of Earth and Environmental Sciences, University of St AndrewsUnited Kingdom