Transcript Hadean plate tectonics

Hadean plate tectonics –
fact or fiction?
Martin J.Whitehouse
Swedish Museum of Natural History,
Stockholm, Sweden
Penrose, June 2006
Sampling the Hadean Earth
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)
Penrose, June 2006
Planetary analogues
Mars – evidence for ancient (<3.9 Ga) long-lived crust
Frey et al. 2002
Zuber et al. 2000
Moon & differentiated meteorites (eucrites) –
evidence for elevated U-Pb ratios of their
surfaces (“high µ”)
Penrose, June 2006
Post-Hadean rock record
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
Penrose, June 2006
Post-Hadean rock record
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.
Caro et al., 2003
Preservation of the positive 142Nd
anomaly in 3.7 Ga sediments
requires isolation of a depleted
(high Sm/Nd) reservoir from a
mixing mantle
Penrose, June 2006
Post-Hadean rock record
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
Penrose, June 2006
Post-Hadean rock record
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 .
Tolstikhin & Hofmann, 2005
Penrose, June 2006
Hadean zircon: U-Pb & O isotopes
Hadean detrital zircon
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.
Nemchin et al., 2005
Penrose, June 2006
Hadean detrital zircon
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.
Kramers, in press; data from Amelin
(1999, 2000), Harrison et al. (2005)
Spread may reflect
differentiation of long-lived
crust which disappeared at
the end of the Hadean.
Penrose, June 2006
Transition from single to multiple plate tectonics
Hadean tectonic model
Kamber et al., 2005
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