A new paper published by group member Dr. Matthias Williold and colleagues about Tungsten isotopes of the Acasta Gneiss Complex is available online. You can access the paper (for free!) at http://www.sciencedirect.com/science/article/pii/S0012821X15001260
Tungsten isotope composition of the Acasta Gneiss Complex
M. Willbold, S.J. Mojzsis, H.-W. Chen and T. Elliott
Earth and Planetary Science Letters – doi:10.1016/j.epsl.2015.02.040
The formative years of the Earth > 4 billion years ago were crucial for the Earth’s evolution into the planet we know today but largely remain an enigmatic period. This is because the geological record, which is available to us to study these early geological processes, only dates back ~ 4 billion years.
Here we have analysed the radioactive decay product of a so-called short-lived isotope system (the ‘182Hf-182W system’) in some of the oldest rocks on Earth from the 3.9 billion year old Acasta Gneiss Complex (NW Territories, Canada). The new data allow us to bridge this time gap and are consistent with previous data for rocks from the Isua Supracrustal Belt (SW Greenland) of similar age. They confirm our interpretation that meteoritic material was added to the Earth during a late-accretion event sometimes between ~4.5 and 4 billion years ago. This meteoritic material is believed to have brought most of the siderophile elements (amongst which precious metals) that currently reside in the silicate Earth and could be the major source of volatile compounds, such as water.
The new data also add important constraints on how this meteoritic material mixed into the Earth’s mantle. We suggest that the Earth’s mantle remained stratified during the first 1.5 billion years of our planet’s history and only started to fully convect about 3 billion years ago, most likely when modern-style plate tectonics started to emerge.
The study was funded by NERC (UK) and NASA (USA).
Earth & Solar System 