Evidence for oceans pre-4300 Ma confirmed by preserved igneous compositions in Hadean zircon

Autor(en)

Emilia M. Cameron, Tyler B. Blum, Aaron J. Cavosie, Kouki Kitajima, Lutz Nasdala, Ian J. Orland, Chloe E. Bonamici, John W. Valley

Abstrakt

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 (D_eff)

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 D_eff<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 D_eff 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, D_eff,

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.

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

University of Wisconsin, Madison

Journal

American Mineralogist

ISSN

0003-004X

DOI

https://doi.org/10.2138/am-2023-9180

Publikationsdatum

03-2024

Peer-reviewed

Ja

ÖFOS 2012

105113 Kristallographie, 105116 Mineralogie, 104026 Spektroskopie

Link zum Portal

https://ucris.univie.ac.at/portal/de/publications/evidence-for-oceans-pre4300-ma-confirmed-by-preserved-igneous-compositions-in-hadean-zircon(a73e9ab9-712c-459d-89d3-5c1694b9bca6).html