Timing of Earth's first crust
New Paper: Excess hafnium-176 in meteorites and the early Earth zircon record
Using a our new estimate of the Hf isotopic composition of bulk silicate earth based on a basaltic meteorite, we reinterpret the ancient zircon Hf record as showing the emergence of enriched continental crust was not until ca. 4.3 Gyr and that it was, as expected, accompanied by a complimentary depleted mantle record.
The long-lived 176Lu-to-176Hf decay system is a powerful tool to understand chemical fractionation events associated with planetary differentiation. Furthermore, the high concentrations (2%) of Hf in the dateable mineral zircon have led to a series of high-profile papers based on the analyses of ancient zircons from the Jack Hills of Western Australia. These studies consistently suggest a very early (> 4.5 Ga) formation of continental crust, but show no evidence for an expected complementary depleted mantle reservoir. All these studies based on the Lu-Hf system assume knowledge of the Hf isotopic composition of bulk silicate earth (BSE) through time to provide a framework within which to compare the zircon results. The community has relied on a model that back projects present-day Hf isotopic values of chondrite meteorites to establish the solar system initial that we equate with BSE, despite concerns that these proxy samples may contain a component of excess 176Hf that is not present in BSE.
Our new paper reports Lu-Hf results for a pristine, ancient (4564.58±0.14 Myr) basaltic meteorite (angrite SAH99555) that yields the first direct estimate of the Hf isotopic composition for the initial solar system that is approximately 5 ε (parts per 10,000) lower than previously assumed from modern chondrites. Using this new value for BSE, we show that Earth’s oldest zircons instead highlight a record of coexisting continental crust and depleted mantle reservoirs only as early as 4.3 Gyr, with little evidence for contintental crust prior to 4.4 Gyr. This new view is more consistent with continuous crustal growth and recycling throughout the Hadean and Archean eras, perhaps analogous to modern plate tectonics.