Hadean plate tectonics – fact or fiction? Martin J.Whitehouse Swedish Museum of Natural History, Stockholm, Sweden Penrose, June 2006

Sampling the Hadean Earth Penrose, June 2006 In the absence of Hadean rocks, what can we look at ? 1)Planetary analogues, e.g. Mars, differentiated meteorites 2)Post-Hadean rock record 3)Super-ancient zircon, e.g. Jack Hills (the only direct sample)

Planetary analogues Penrose, June 2006 Mars – evidence for ancient (<3.9 Ga) long-lived crust Moon & differentiated meteorites (eucrites) – evidence for elevated U-Pb ratios of their surfaces (“high µ”) Frey et al Zuber et al. 2000

Post-Hadean rock record Penrose, June 2006 Long-term isolation of isotopic reservoirs: Pb isotopes from SW Greenland Elevated Pb isotope composition of BIF, metasediments and >3.8 Ga TTG’s at Isua require early separation of a high µ reservoir and isolation for > 500 Ma. 3.7 Ga TTG’s reflect typical depleted mantle and subduction geochemistry. Kramers, in press, after Kamber et al. 2003

Post-Hadean rock record Penrose, June 2006 Long-term isolation of isotopic reservoirs 142 Nd anomaly at Isua Isua metasediments preserve a 142 Nd/ 144 Nd ratio significantly higher than bulk Earth. 146 Sm has a half life of only 103 Ma, hence such anomalies require silicate Earth differentiation in the first few 100 Myr of Earth history. Preservation of the positive 142 Nd anomaly in 3.7 Ga sediments requires isolation of a depleted (high Sm/Nd) reservoir from a mixing mantle Caro et al., 2003

Post-Hadean rock record Penrose, June 2006 Paucity of ancient zircon in early Archaean sediments The oldest metasediments do not contain substantially older detrital zircon (Nutman, 2001). Consistent with a predominantly basaltic and/or juvenile ancient crust with no significant old zircon source. Nutman, 2001

Post-Hadean rock record Penrose, June 2006 Solar rare gases in plume magmas Tolstikhin & Hofmann, He/ 3 He & 21 Ne/ 22 Ne ratios in plume magmas are lower than MORB and suggest a pure solar component in the lower mantle (Earth is chondritic not solar). Solution of this paradox lies in accumulation of solar wind exposed regolith on a long-lived basaltic crust eventually tranferred to an isolated (?) deep mantle reservoir.

Hadean detrital zircon Penrose, June 2006 Hadean zircon: U-Pb & O isotopes Nemchin et al., 2005 Oldest Jack Hills zircon indicates SiO 2 rich melts existed at 4.4 Ga. Elevated δ 18 O of some of these grains has been used to suggest interaction with liquid water but evidence remains equivocal.

Hadean detrital zircon Penrose, June 2006 Hadean zircon: Hf isotopes Kramers, in press; data from Amelin (1999, 2000), Harrison et al. (2005) Jack Hills zircon indicates development of an enriched reservoir >4.4 Ga (also seen for Acasta gneisses). Enriched reservoir is no longer evident in early-mid Archean rocks. Spread may reflect differentiation of long-lived crust which disappeared at the end of the Hadean.

Hadean tectonic model Penrose, June 2006 Kamber et al., 2005 Transition from single to multiple plate tectonics Long-term (500 Ma) isolation of isotopic reservoirs Burial by basaltic outpouring puts hydrated crust at depth – crustal melt products yield zircon (e.g. Jack Hills) Instability ultimately dooms crust to destruction by recycling into mantle at onset of multiple plate tectonics