First-principles calculations of equilibrium fractionation of O and Si isotopes in quartz, albite, anorthite, and zircon

Hydrosphere interactions and alteration of the terrestrial crust likely played a critical role in shaping Earth’s surface, and in promoting prebiotic reactions leading to life, before 4.03 Ga (the Hadean Eon). The identity of aqueously altered material strongly depends on lithospheric cycling of abundant and water-soluble elements such as Si and O. However, direct constraints that define the character of Hadean sedimentary material are absent because samples from this earliest eon are limited to detrital zircons (ZrSiO4). Here we show that concurrent measurements of Si and O isotope ratios in Phanerozoic and detrital pre-3.0 Ga zircon constrain the composition of aqueously altered precursors incorporated into their source melts. Phanerozoic zircon from (S)edimentary-type rocks contain heterogeneous δ18O and δ30Si values consistent with assimilation of metapelitic material, distinct from the isotopic character of zircon from (I)gneous- and (A)norogenic-type rocks. The δ18O values of detrital Archean zircons are heterogeneous, although yield Si isotope compositions like mantle-derived zircon. Hadean crystals yield elevated δ18O values (vs. mantle zircon) and δ30Si values span almost the entire range observed for Phanerozoic samples. Coupled Si and O isotope data represent a constraint on Hadean weathering and sedimentary input into felsic melts including remelting of amphibolites possibly of basaltic origin, and fractional addition of chemical sediments, such as cherts and/or banded iron formations (BIFs) into source melts. That such sedimentary deposits were extensive enough to change the chemical signature of intracrustal melts suggests they may have been a suitable niche for (pre)biotic chemistry as early as 4.1 Ga.

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“…This defines Δ 30 Si(WRzircon) = 0.37‰ for this sample (Fig. 2), in good agreement with ab initio Si isotope fractionation calculations (24).…”

Section: Mantle-derived Mud Tank Carbonatite (Australia) Kimberleysupporting

confidence: 87%

Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon

Trail

Boehnke

Savage

et al. 2018

Proc. Natl. Acad. Sci. U.S.A.

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“…Instead, we analyzed the DFT results on the end‐member composition to estimate the changes in compressibility and volume discontinuities and compared our results with the calculated results of several important minerals in the transition zone and lower mantle (Figure a). The volumes calculated by DFT are usually ∼3% smaller than experimental results because of neglecting vibrational contributions (Hu et al, ; Qin et al, ); however, the relative values give good approximations. The calculated bulk modulus from third‐order Birch‐Murnaghan equation of state for the α ‐phase was 128(4) GPa, with

K_{0}^{'} = 4.0 (2)

and V 0 =422.1(6) Å 3 .…”

Section: Resultsmentioning

confidence: 99%

High‐Pressure γ‐CaMgSi₂O₆: Does Penta‐Coordinated Silicon Exist in the Earth's Mantle?

Kiefer

Bina

et al. 2017

Geophysical Research Letters

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In situ X‐ray diffraction experiments with natural Fe‐ and Al‐ bearing diopside single crystals and density functional theory (DFT) calculations on diopside end‐member composition indicate the existence of a new high‐pressure γ‐diopside polymorph with rare penta‐coordinated silicon. On compression α‐diopside transforms to the γ‐phase at ∼50 GPa, which in turn, on decompression is observed to convert to the known β‐phase below 47 GPa. The new γ‐diopside polymorph constitutes another recent example of penta‐coordinated silicon (VSi) in overcompressed metastable crystalline silicates, suggesting that VSi may exist in the transition zone and the uppermost lower mantle in appreciable quantities, not only in silicate glass and melts but also in crystalline phases contained in the coldest parts of subducted stagnant slabs. VSi may have significant influences on buoyancy, wave velocity anomalies, deformation mechanisms, chemical reactivity of silicate rocks, and seismicity within the slab.

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“…Although modeling is not being covered in the review, its importance cannot be underscored as it has been increasingly used to understand the mechanisms controlling Si isotope fractionation (e.g., Qin et al, 2016) and the development of the Si isotope composition as a tracer of biogeochemical silicon cycle in modern and past natural systems (e.g., Gao et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

et al. 2018

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First-principles calculations of equilibrium fractionation of O and Si isotopes in quartz, albite, anorthite, and zircon

Cited by 58 publications

References 69 publications

Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon

Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon

High‐Pressure γ‐CaMgSi₂O₆: Does Penta‐Coordinated Silicon Exist in the Earth's Mantle?

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

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First-principles calculations of equilibrium fractionation of O and Si isotopes in quartz, albite, anorthite, and zircon

Cited by 58 publications

References 69 publications

Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon

Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon

High‐Pressure γ‐CaMgSi2O6: Does Penta‐Coordinated Silicon Exist in the Earth's Mantle?

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

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High‐Pressure γ‐CaMgSi₂O₆: Does Penta‐Coordinated Silicon Exist in the Earth's Mantle?