Zircon-modeled melts shed light on the formation of Earth's crust from the Hadean to the Archean
Tamara L. CarleyElizabeth A. BellCalvin F. MillerLily L. ClaiborneA. HuntHeather M. KirkpatrickT. Mark Harrison
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Abstract Elucidating the compositions of melts from which Hadean zircons crystallized can provide insight into early crust construction. We calculated model melts using Ti-calibrated zircon/melt partition coefficients and trace element data for zircons from the Hadean, Archean, and possible analogue environments (e.g., rifts, hotspots, arcs) to constrain petrogenetic relationships. Model melts from oceanic settings (mid-ocean ridges, arcs, Iceland) showed higher heavy rare earth element (HREE) contents and shallower middle REE (MREE) to HREE/chondrite (ch) slopes than those from continental arcs and tonalite-trondhjemite-granodiorite suites (TTGs). However, Hadean and Archean model melts were consistently similar to each other and to those from continental arcs, hotspots, and TTGs (and dissimilar to oceanic settings), with depleted HREE contents and slope reversal in heaviest REEch. Despite close similarities that suggest comparable petrogenesis of Hadean and early Archean magmas from which Jack Hills detrital zircons crystallized, subtle variabilities in REEch and Zr/Hf suggest thickening crust and evolving igneous systems through time.Keywords:
Hadean
Early Earth
Petrogenesis
Rare-earth element
Rare-earth element
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New model growth curves for the continental crust based upon Hf isotopes in zircon suggest that large volumes of felsic continental crust were present in the Hadean and early Archean. These models sit uncomfortably with estimates of the volume of ancient crust preserved today and imply that the large volumes of crust that were created early in Earth history are now lost. However, this paper argues that there is no evidence from modern mantle geochemistry that very large volumes of early continental crust have been recycled into the mantle. In contrast, significant volumes of Archean crust may have been reworked into younger crust, although there is no evidence that this process took place in the early Archean and Hadean.
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Valley et al . review the lines of evidence on which we drew to conclude that continental crust formed much earlier than previously thought. Their comment contains some misrepresentations that we correct, but new information they provide appears to bolster our hypothesis. Nothing in their comment refutes the presence of continental crust or plate boundary processes prior to 4 billion years ago.
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The Archean crust contains direct geochemical information of the Earth's early planetary differentiation. A major outstanding question in the Earth sciences is whether the volume of continental crust today represents nearly all that formed over Earth's history or whether its rates of creation and destruction have been approximately balanced since the Archean. Analysis of neodymium isotopic data from the oldest remnants of Archean crust suggests that crustal recycling is important and that preserved continental crust comprises fragments of crust that escaped recycling. Furthermore, the data suggest that the isotopic evolution of Earth's mantle reflects progressive eradication of primordial heterogeneities related to early differentiation.
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Hadean
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Crustal recycling
Planetary differentiation
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