This Chapter considers triple oxygen isotope variations and their 4 Gyr temporal evolution in bulk siliciclastic sedimentary rocks and in granites. The d18O and D'17O values provide new insights into weathering in the modern and ancient hydrosphere and coeval crustal petrogenesis. We make use of the known geological events and processes that affect the rock cycle: supercontinent assembly and breakup that influence continent-scale and global climate, the fraction of the exposed crust undergoing weathering, and isotopic values of precipitation. New data from a 5000 m Texas drillhole into the Oligocene Frio Formation demonstrate minimal isotopic shifts from mudrocks to shales during diagenesis, mostly related to expulsion of water from smectite-rich loosely cemented sediment and its conversion to illite-rich shale. Inversion of triple oxygen isotope fractionations return isotopic values and temperatures along the hole depth that are more consistent with weathering conditions in the Oligocene and modern North America (d18O = -7 to -15‰, and T of +15 to +45°C) rather than d18O from 8 to 10‰ diagenetic water in the drill hole at 175-195°C. More precise T and d18Owater are obtained where the chemical index of alteration (CIA) based detrital contribution is subtracted from these sediments. Triple oxygen isotopes from suspended sediments in major world rivers record conditions (T and d18Ow) of their watersheds, and not the composition of bedrock because weathering is water-dominated.
In parallel, the Chapter presents new analyses of 100 granites, orthogneisses, migmatites, tonalite-trondhjemite-granodiorite (TTG), and large-volume ignimbrites from around the world that range in age from 4 Ga to modern. Most studied granites are orogenic and anatectic in origin and represent large volume remelting/assimilation of shales and other metasediments; the most crustal and high-d18O of these are thus reflect and record the average composition of evolving continental crust. Granites also develop a significant progressive increase in d18O values from 6-7‰ (4-2.5 Ga) to 10-13‰ (~1.8-1.2 Ga) after which d18O stays constant or even decreases. More importantly, we observe a moderate -0.03‰ step-wise decrease in D'17O between 2.1 and 2.5 Ga, which is about half of the step-wise decrease observed in shales over this time interval. We suggest that granites, as well as shales, record the significant advent and greater volumetric appearance of low-D'17O, high-d18O weathering products (shales) altered by meteoric waters upon rapid emergence of large land masses at ~2.4 Ga, although consider alternative interpretations. These weathering products were incorporated into abundant 2.0-1.8 Ga orogens around the world, where upon remelting, they passed their isotopic signature to the granites. We further observe the dichotomy of high-D'17O Archean shales, and unusually low-D'17O Archean granites. We attribute this to greater contribution from shallow crustal hydrothermal contribution to shales in greenstone belts, while granites in the earliest 3.0-4.0 Ga crust and TTGs require involvement of hydrothermal products with lower-D'17O signatures at moderately high-d18O, which we attribute to secondary silicification of their protoliths before partial melting.
The Chapter further discusses evolution of the shale record through geologic history and discusses the step-wise change in d18O and D'17O values at Archean/Proterozoic transition. Denser coverage for shales in the past 1 billion years permits investigation of the rocks and their weathering in the last supercontinent cycle, with observed lighter d18O values, characteristic for the mid-Phanerozoic at the initiation of Gondwana breakup. The continuing increase in d18O values of the shales since 4 Ga is interpreted to reflect accumulation of weathering products via shale accretion to continents, as low-density and buoyant shales tend to not subduct back into the mantle.
The rock cycle passes triple oxygen isotopic signatures from precipitation to sedimentary, metasedimentary, and finally to anatectic igneous rocks. Continental crust became progressively heavier in d18O, lighter in D'17O due to incremental accumulation of high-d18O sediments in accretionary wedges. Second-order trends in d18O and D'17O are due to supercontinent cycles and glacial episodes.
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Triple oxygen isotope evidence for a hot Archean ocean
Triple oxygen isotope (δ17O and δ18O) values of high- and low-temperature altered oceanic crust and products of basalt alteration experiments were measured to better constrain ocean isotope compositions in deep time. The data define an array of δ18O and Δ′17O (Δ′17O=δ′17O – λRL × δ′18O + γ) values from mantle values toward 1‰ and –0.01‰, respectively, with a λ of ~0.523. The altered oceanic crust data were used to construct a model for estimating δ18O-Δ′17O values of the ancient oceans if the continental weathering flux (FCW) and/or hydrothermal oceanic crust alteration flux (FHT) changed through time. A maximum lowering of 7‰ and 4‰, respectively, is achieved in the most extreme cases. The δ18O value of the ocean cannot be raised by more than 1.1‰. Eclogites from the Roberts Victor kimberlite (South Africa), with a protolith age of 3.1 Ga, have δ18O-Δ′17O values that precisely overlap with those of the modern altered oceanic crust, suggesting that the Archean oceans had similar isotope values as today. Published triple isotope data for Archean cherts show that all samples have been altered to some degree and suggest an Archean ocean surface temperature of ~70–100 °C. An ocean as light as –2‰ is still consistent with our eclogite data and reduce our temperature estimates by 10 °C.
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- Award ID(s):
- 1903852
- NSF-PAR ID:
- 10336013
- Date Published:
- Journal Name:
- Geology
- ISSN:
- 0091-7613
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/G50230.1
