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References
Peck
WH, Valley
JW, Wilde
SA, and Graham
CM (2000) Ion microprobe Evidence for Pre-4.4 Ga Continental Crust
and Low Temperature Water/Rock Interaction. Geol. Soc. Am. Abstr, vol
32, no. 7, p. 376.
Wilde SA, Valley JW, Peck WH and Graham CM (2001) Evidence from Detrital
Zircons for the Existence of Continental Crust and Oceans on the Earth
4.4 Gyr Ago. Nature. 409: 175-178.
Peck WH, Valley JW, Wilde SA, and Graham CM (2001) Oxygen
isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons: Ion microprobe
evidence for high
d18O continental
crust and oceans in the Early Archean.
Geochimica et Cosmochimica Acta, vo. 64, no 22, pp 4215-4229
Valley
JW, Peck WH, King EM, Wilde SA (2002) A Cool Early Earth, Geology. 30:
351-354.
Cavosie
AJ, Wilde SA, Liu D, Valley JW, Weiblen PW (2004) Internal zoning and
U-Th-Pb chemistry of the jack Hills detrital zircons: a mineral record
of early Archean to mesoproterozoic magmatism. Precambrian Research, 135:231-279.
Valley JW (2003) Oxygen
Isotopes in Zircon. Reviews in Mineralogy and Geochemistry: v. 53, 2003,
Chapter 13, pp. 343-385.
Cavosie AJ, Valley JW,
Wilde SA, and EIMF (2005) Magmatic
d18O in 4400-3900 Ma detrital
zircons: A record of the alteration and recycling of crust in the Early
Archean.
Valley JW (2005) A Cool Early Earth? Scientific American, October 2005,
58-65.
Valley JW, Lackey JS, Cavosie AJ, Clechenko CC, Spicuzza MJ, Basei MAS,
Bindeman IN, Ferreira VP, Sial AN, King EM, Peck WH, Sinha AK, Wei CS
(2005) 4.4 billion years of crustal maturation: oxygen isotopes in magmatic
zircon. Contr. Mineral. Petrol. DOI 10.1007/s00410-005-0025-8.
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1.
Outcrop photographs of the Jack Hills, Western Australia showing the outcrops
where the 4.4Ga zircon sample was collected from interlayered red quartzites
and metaconglomerate. (SA Wilde, unpublished photographs) |
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2a. Outcrop of the
oldest known sample of the Earth, a 4.4 Ga detrital zircon (sample W74)
in the Jack Hills metaconglomerate, Eranondoo Hill, Jack Hills, Western
Australia. From l to r: John Valley, University of Wisconsin-Madison;
Aaron Cavosie, University of Wisconsin-Madison; Simon Wilde, Curtin University.
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2b. Dirt track
leading to Eranondoo Hill, site of 4.4 Ga zircon discovery, Jack
Hills, Western Australia. |
2c. Hazards
of fieldwork in the outback. |
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3a. Geologists study
maps in the cookhouse. Mileura Station, Western Australia. |
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3b. Bales of wool,
Mileura Station, Western Australia. |
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4a. Sheep shearer's
quarters, Mileura, Station, Western Australia. |
4b. South of
theJack Hills, Western Australia. |
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4c. Mileura
Station, Western Australia. |
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| 5.
Cathodoluminescence image of the oldest known material from the Earth,
a single crystal of zircon from the Jack Hills metaconglomerate, Western
Australia. Concentric, magmatic growth zoning is shown about the crystal
core. The crystallization age of 4.40Ga was determined by ion microprobe.
A high oxygen isotope ratio of +7.4 was measured in the core of this
grain with a second ion microprobe. (JW Valley, unpbd image) |
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6.
U-Pb Concordia showing age of 4.404 at better than 99% concordance.
(Wilde et al. 2001) |
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8.
Surface 2 of zircon W74/2-36. The U-Pb age of 4.4 billion years was
determined by ion microprobe from the spot shown. |
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| 9.
Cathodoluminescence image of surface 1 of zircon W74/2-36. Concentric,
magmatic growth zoning is shown about the crystal core. The crystallization
age of 4.40Ga was determined by ion microprobe from surface 2. A high
oxygen isotope ratio of +7.4 was measured in the core of this grain
with a second ion microprobe. (Wilde et al. 2001) |
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10.
Cathodoluminescence image of surface 2 of zircon W74/2-36. Concentric,
magmatic growth zoning is shown about the crystal core. Eight ion
microprobe pits are shown. The crystallization age of 4.40Ga was determined
from the leftmost spot. A high oxygen isotope ratio of +7.4 was measured
in the core of this grain with a second ion microprobe. (Wilde et
al. 2001) |
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| 11.
Cathodoluminescence image of surface 2 of zircon W74/2-36. Concentric,
magmatic growth zoning is shown about the crystal core. The crystallization
age of 4.40Ga (4004+-4Ma) was determined by ion microprobe. A high
oxygen isotope ratio of +7.4 was measured in the core of this grain
with a second ion microprobe. (Wilde et al. 2001) |
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12.
(Peck et al. 2000, 2001) |
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14.
U-Pb Concordia for zircons from sample W74 with ages from 3.3 to 4.4
Ga. (Peck et al. 2000, 2001) |
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| 15.
Optical interferometer image of surface 1 of zircon W74/2-36 after
ion microprobe analysis of U-Pb, oxygen isotope ratio and rare earth
elements. Five micron deep pits are from REE and oxygen analysis.
Shallower pits are from U-Pb. (Peck et al. 2000, 2001) |
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16.
Oxygen isotope analyses of zircons from sample W74 showing clustering
of ages and the range of oxygen isotope ratio for each age. Values
of d18O above 6.5 ‰
indicate alteration of protoliths by liquid water at low temperatures.
The standard data for KIM5 shows the expected values of primitive
magmas from the Earth’s mantle. Values higher than this are seen
for each time period. (Peck et al. 2000, 2001) |
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| 17.
Rare earth element content in 3.3 to 3.6 Ga zircons from W74. (Peck
et al. 2000, 2001) |
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18.
Rare earth element content in ~4.0 to 4.15 Ga zircons from W74. (Peck
et al. 2000, 2001) |
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| 19.
Back scattered electron (BSE) and cathodoluminescence (CL) images
of surface 2 of zircon W74/2-36. Three inclusions of SiO2
(black spots, 10-20 microns in diameter) and 8 ion microprobe pits
from U-Pb analysis are seen in BSE. The 4.40 Ga spot is farthest to
the left. The CL image shows concentric, magmatic growth zoning. (Peck
et al. 2000, 2001) |
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| 20.
U-Pb analyses of surfaces 1 and 2 of zircon W74/2-36. (Peck et al.
2000, 2001) |
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21.
Surface 1. (Peck et al. 2000, 2001) |
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| 22.
Surface 2. (Peck et al. 2000, 2001) |
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23.
Order of analysis of ion microprobe analysis of zircon W74/2-36. (Peck
et al. 2000, 2001) |
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| 24.
Surface 1, REE and oxygen isotope analysis. (Peck et al. 2000, 2001) |
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25.
Surface 1, REE and oxygen isotope analysis. (Peck et al. 2000, 2001) |
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| 26.
Oxygen isotope ratios and U-Pb Concordia for zircons from sample W74.
(Peck et al. 2000, 2001) |
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27.
Timeline of the first billion years of Earth history. (Peck et al.
2000, 2001) |
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Timeline of major events in the history of the Earth. No samples of
rock are known from the first 500 million years and only a few crystals
of zircon have been identified from this era. The oldest zircon provides
evidence of continental crust, low surface temperatures, and liquid
water that suggest an early Earth that was more similar to the present
than previously has been thought. (Andree Valley, Madison, Wisconsin)
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29.
Crystallization age (U-Pb) and oxygen isotope ratio (d18O)
for Archean magmatic zircons. Distribution of magmatic d18O
values does not change throughout the Archean. Most magmas had a
primitive d18O value
similar to that in the mantle today ("mantle zircon"),
but some zircon values are as high as 7.5‰. High-d18O
zircons and host magmas resulted from melting of protoliths that
were altered by interaction with liquid water at low temperatures
near surface of Earth (see text). Timeline (inset, lower right)
shows: (1) accretion of the Earth, (2) formation of the Moon and
the Earth’s core, (3) minimum age of liquid water based on
high d18O zircon, (4)
Acasta gneiss, and (5) Isua metasedimentary rocks. (Valley et al.
2002) |
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30.
Histograms
of d18O. A: Olivine from
mantle xenoliths and Hawaiian basalts. B: Zircon xenocrysts from kimberlites
in S. Africa. C: Zircons from igneous rocks of Superior province,
Canada. D: Ion microprobe analyses of single zircons from Jack Hills,
Western Australia. The Jack Hills zircons (D) are higher in d18O
than the mantle. Such high d18O
values indicate that the protolith of granitic magmas experienced
low temperature interaction with liquid water in a near surface environment
(Valley et al. 2002) |
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31.
Estimates of meteorite impact rate for first two billion years of
Earth history. Two hypotheses are shown: exponential decay of impact
rate (dashes, and a cool early Earth/ late heavy bombardment (solid
curve, this study). In either model, spikes occurred owing to isolated
large impacts. Evidence for liquid water comes from high-d18O
zircons (>4.4 Ga to >4.0 Ga) and sedimentary rocks (Isua 3.8-3.6
Ga). The cool early Earth hypothesis (solid curve) suggests that impact
rates had dropped precipitously by 4.4 Ga, consistent with relatively
cool conditions and liquid water on the surface of the Earth. (Valley
et al. 2002) |
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32.
Artist’s rendering of a Cool Early Earth 4.4 billion years
ago. Recent evidence from single crystals of zircon suggests that
surface temperatures were relatively low and that liquid water would
have formed oceans rather than a thick steam-rich atmosphere (Valley
et al. 2002). Such oceans could have promoted the evolution of life.
The hypothesis of a Cool Early Earth contrasts with earlier ideas
that magma covered the Earth, which lead to the first 500 million
years of Earth history being named "Hadean" (hell-like).
(Graphic: Andrée Valley and Mary Diman)
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