Abstract
Detrital zircons from the Jack Hills are the dominant source of Hadean
(pre-4000 Ma) terrestrial material available for study today. Values of δ18O
in many of these zircons (6.0 to 7.5‰ are above the mantle-equilibrated
value. For two decades, these mildly elevated values have been the
primary evidence that protoliths of the zircon-forming magmas interacted
at low temperature with liquid water before burial and melting,
implying that the surface of Earth cooled quickly after core and moon
formation, and that habitable conditions for life existed within 250 Myr
of the formation of Earth, over 800 Myr before the oldest generally
accepted microfossils. These conclusions are based on oxygen isotope
analyses of zircon domains with well-defined growth zoning and nearly
concordant U-Pb ages within zircon grains with low magnetic
susceptibility, which are further inferred to be unaltered by various
tests. However, no studies of Jack Hills zircons have directly
correlated oxygen isotope ratios and radiation damage, which facilitates
alteration in zircon. Several previous studies have selected zircons
that show radiation damaged, discordant and/or hydrous domains, and have
shown that such altered material is not reliable as a record of igneous
composition. In contrast, this study targeted zircons that are
interpreted to pristine and not altered, and demonstrates the importance
of testing zircons for radiation damage and alteration as part of any
geochemical study, regardless of age. This study expands on existing
data, and presents the first comprehensive evaluation of δ18O,
OH/O, CL imaging, U-Pb concordance and radiation-damage state within
Jack Hills zircons. A total of 115 Hadean zircon grains in this study
have water contents similar to nominally anhydrous standard reference
zircons and are interpreted as pristine. In situ Raman data for band
broadening correlated with δ18O analyses document low levels of radiation damage, indicating significant annealing. The present-day effective doses (Deff)
are uniformly less than the first percolation point (dose where damage
domains, that are isolated at lower damage state, overlap to form a
continuous pathway through the crystal, ~2×1015 α-decays/mg, Ewing et al., 2003) and most zircons have Deff<1×1015
α-decays/mg. Modeling of representative alpha-recoil damage and
annealing histories indicates that most zircons in this study have
remained below the Deff of the first percolation point throughout their history. The δ18O
values for these primary zircons include many that are higher than
would be equilibrated with the mantle at magmatic temperatures and
average 6.32 ± 1.3‰ in the Hadean and 6.26 ± 1.6‰ in the Archean. There
is no correlation in our suite of pristine Hadean zircons between δ18O and OH/O, Deff,
age, or U-Pb age-concordance. These carefully documented Hadean-age
zircons possess low amounts of radiation damage in domains sampled by δ18O analysis, are water-poor. The mildly elevated δ18O values are a primary-magmatic geochemical signature. These results strengthen the conclusion that mildly elevated-δ18O magmas existed during the Hadean, supporting the hypothesis that oceans and a habitable Earth existed before 4300 Ma.
Original language | English |
---|---|
Pages (from-to) | 1670-1681 |
Number of pages | 12 |
Journal | American Mineralogist |
Volume | 109 |
Issue number | 10 |
Early online date | 28 Mar 2024 |
DOIs | |
Publication status | Published - 1 Oct 2024 |
Austrian Fields of Science 2012
- 105113 Crystallography
- 105116 Mineralogy
- 104026 Spectroscopy