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

Abstract

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.

The Emergence of Life

“…However, taking into consideration modern sulfate-poor, acidic, ∼400 • C hydrothermal springs, transition element precipitates, such as 'green rust' (∼[Fe 6x S), and greigite (∼ Fe 5 NiS 8 ), may also have precipitated (Génin et al 2008;Russell 2018). An early (∼4.1 Ga) onset of sedimentary basins (Trail et al 2018) fits well with the geological requirements of a putative hydrothermally-derived emergence of life, where the aforementioned minerals could have promoted the synthesis of organics and their complexification by minerally-mediated energy coupling ( Fig. 1).…”

Significant Zr isotope variations in single zircon grains recording magma evolution history

“…It occurs in a large variety of lithologies and is generated primarily in intermediate to felsic igneous rocks ( 1 , 2 ). Its trace element (e.g., Ti and rare-earth element [REE]) and isotopic (e.g., U–Pb, Lu–Hf, Si, and O) abundances have provided major information on the absolute timing, petrochronologic conditions, and tectonic backgrounds of important geological events along the Earth’s history ( 3 – 8 ). They are especially important for deciphering the growth and reworking history of the continental crust ( 9 ), the early differentiation of the Earth ( 4 , 10 ), and the styles of plate tectonics through time ( 11 , 12 ).…”

Formation of Archean continental crust constrained by boron isotopes

“…Zircon Hf and O isotope data suggest that Archean juvenile continental crust formed by melting of enriched mafic crust extracted from a still primitive mantle, and that sediment recycling and crustal thickening occurred since the Mesoarchean (e.g., Kemp et al, 2010;Naeraa et al, 2012;Reimink et al, 2016;Roberts and Santosh, 2018). The same is indicated by Si-O isotope data for Archean detrital zircon (Trail et al, 2018).…”

“…Some Archean TTGs show elevated 30 Si/ 28 Si and 18 O/ 16 O values, which were interpreted to indicate chert and, by inference, subducted sediments in the source (Deng et al, 2019). The integral Si-O isotope record of Hadean zircon and Archean TTGs indicates source contributions from a variety of supra-crustal rocks, including chemical sediments, serpentinised and silicified basalts (Trail et al, 2018;André et al, 2019). Whether anatexis of such material requires deep burial by subduction is unclear.…”

“…Strikingly, the compositions of the samples with highest Sr/Sr* values match those modelled for low degree garnet-stable partial melting of a tholeiitic source that is not contaminated by chert or other components (Deng et al, 2019). Although Si-O data provide a unique opportunity to discern supra-crustal contributions to TTG sources (Trail et al, 2018), their interpretation to discern the geodynamic setting of TTG formation is still controversial.…”