Hadean plate Hadean plate tectonics – fact or tectonics – fact or

fiction?fiction?

Martin J.WhitehouseMartin J.Whitehouse

Swedish Museum of Natural History,Swedish Museum of Natural History,

Stockholm, SwedenStockholm, Sweden

Penrose, June 2006

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In the absence of Hadean rocks, what In the absence of Hadean rocks, what can we look at ?can we look at ?

1)1) Planetary analogues, e.g. Mars, Planetary analogues, e.g. Mars, differentiated meteoritesdifferentiated meteorites

2)2) Post-Hadean rock recordPost-Hadean rock record

3)3) Super-ancient zircon, e.g. Jack Super-ancient zircon, e.g. Jack Hills (the only Hills (the only directdirect sample) sample)

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Mars – evidence for ancient (<3.9 Ga) Mars – evidence for ancient (<3.9 Ga) long-livedlong-lived crustcrust

Moon & differentiated meteorites (eucrites) – Moon & differentiated meteorites (eucrites) – evidence for elevated U-Pb ratios of their evidence for elevated U-Pb ratios of their surfaces (“high surfaces (“high µµ”)”)

Frey et al. 2002

Zuber et al. 2000

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Long-term isolation of isotopic reservoirs:Long-term isolation of isotopic reservoirs:

Pb isotopes from SW GreenlandPb isotopes from SW Greenland

Elevated Pb isotope Elevated Pb isotope composition of BIF, composition of BIF, metasediments and metasediments and >3.8 Ga TTG’s at >3.8 Ga TTG’s at Isua require Isua require early early separationseparation of a high of a high µµ reservoir and reservoir and isolationisolation for > 500 for > 500 Ma.Ma.

3.7 Ga TTG’s reflect 3.7 Ga TTG’s reflect typical depleted typical depleted mantle and mantle and subduction subduction geochemistry.geochemistry.

Kramers, in press, after Kamber et al. 2003

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Long-term isolation of isotopic reservoirsLong-term isolation of isotopic reservoirs

142142Nd anomaly at IsuaNd anomaly at Isua

Isua metasediments preserve a Isua metasediments preserve a 142142Nd/Nd/144144Nd ratio significantly Nd ratio significantly higher than bulk Earth.higher than bulk Earth.

146146Sm has a half life of only 103 Sm has a half life of only 103 Ma, hence such anomalies Ma, hence such anomalies require silicate Earth require silicate Earth differentiation in the first few differentiation in the first few 100 Myr of Earth history.100 Myr of Earth history.

Preservation of the positive Preservation of the positive 142142Nd anomaly in 3.7 Ga Nd anomaly in 3.7 Ga sediments requires isolation of a sediments requires isolation of a depleted (high Sm/Nd) reservoir depleted (high Sm/Nd) reservoir from a mixing mantlefrom a mixing mantle

Caro et al., 2003Caro et al., 2003

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Paucity of ancient zircon in early Paucity of ancient zircon in early Archaean sedimentsArchaean sediments

The oldest The oldest metasediments do not metasediments do not contain substantially contain substantially older detrital zircon older detrital zircon (Nutman, 2001).(Nutman, 2001).

Consistent with a Consistent with a predominantly basaltic predominantly basaltic and/or juvenile ancient and/or juvenile ancient crust with no significant crust with no significant old zircon source.old zircon source.

Nutman, 2001Nutman, 2001

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Solar rare gases in plume magmasSolar rare gases in plume magmas

Tolstikhin & Hofmann, 2005

44He/He/33He & He & 2121Ne/Ne/2222Ne ratios in Ne ratios in plume magmas are lower plume magmas are lower than MORB and suggest a than MORB and suggest a pure solar component in the pure solar component in the lower mantle (Earth is lower mantle (Earth is chondritic chondritic notnot solar). solar).

Solution of this paradox lies Solution of this paradox lies in accumulation of solar wind in accumulation of solar wind exposed regolith on a long-exposed regolith on a long-lived basaltic crust lived basaltic crust eventually tranferred to an eventually tranferred to an isolatedisolated (?) deep mantle (?) deep mantle reservoir .reservoir .

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Hadean zircon: U-Pb & O isotopesHadean zircon: U-Pb & O isotopes

Nemchin et al., 2005

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.

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Hadean zircon: Hf isotopesHadean zircon: Hf isotopes

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

Jack Hills zircon indicates Jack Hills zircon indicates development of an development of an enriched reservoir >4.4 Ga enriched reservoir >4.4 Ga (also seen for Acasta (also seen for Acasta gneisses).gneisses).

Enriched reservoir is no Enriched reservoir is no longer evident in early-mid longer evident in early-mid Archean rocks.Archean rocks.

Spread may reflect Spread may reflect differentiation of long-lived differentiation of long-lived crust which disappeared at crust which disappeared at the end of the Hadean.the end of the Hadean.

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Kamber et al., 2005

Transition from Transition from singlesingle to to multiplemultiple plate plate tectonicstectonics

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