Hadean plate tectonics – fact or fiction?
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Hadean plate tectonics – fact or fiction? Martin J.Whitehouse Swedish Museum of Natural History, Stockholm, Sweden. Penrose, June 2006. In the absence of Hadean rocks, what can we look at ? Planetary analogues, e.g. Mars, differentiated meteorites Post-Hadean rock record

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Hadean plate tectonics – fact or fiction?

Martin J.Whitehouse

Swedish Museum of Natural History,

Stockholm, Sweden

Penrose, June 2006

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Sampling the Hadean Earth

Penrose, June 2006

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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”)

Planetary analogues

Frey et al. 2002

Zuber et al. 2000

Penrose, June 2006

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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.

Post-Hadean rock record

Kramers, in press, after Kamber et al. 2003

Penrose, June 2006

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Long-term isolation of isotopic reservoirs

142Nd anomaly at Isua

Isua metasediments preserve a 142Nd/144Nd ratio significantly higher than bulk Earth.

146Sm 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 142Nd anomaly in 3.7 Ga sediments requires isolation of a depleted (high Sm/Nd) reservoir from a mixing mantle

Post-Hadean rock record

Caro et al., 2003

Penrose, June 2006

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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.

Post-Hadean rock record

Nutman, 2001

Penrose, June 2006

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Solar rare gases in plume magmas

4He/3He & 21Ne/22Ne 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 .

Post-Hadean rock record

Tolstikhin & Hofmann, 2005

Penrose, June 2006

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Hadean zircon: U-Pb & O isotopes

Oldest Jack Hills zircon indicates SiO2 rich melts existed at 4.4 Ga.

Elevated δ18O of some of these grains has been used to suggest interaction with liquid water but evidence remains equivocal.

Hadean detrital zircon

Nemchin et al., 2005

Penrose, June 2006

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Hadean zircon: Hf isotopes

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 detrital zircon

Kramers, in press; data from Amelin (1999, 2000), Harrison et al. (2005)

Penrose, June 2006

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Transition from single to multiple plate tectonics

Kamber et al., 2005

Hadean tectonic model

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

Penrose, June 2006