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.

Keywords: Hadean; origin of life; silica cycle; weathering; zircon.

Fig. 1.

Schematic cartoon of Si and O isotope covariation during fluid alteration and precipitation processes. Chemical weathering, hydration, seawater silica precipitation, may have different trajectories in Si–O isotope space. Note that “nonequilibrium silicification” has no specific vector or slope as this process can be highly variable due to the different behavior of these elements under different rock/water ratios and at different temperatures (19). BIFs may be enriched in ¹⁸O and depleted in ³⁰Si (not shown).

Fig. 2.

Our MC-ICP-MS results showing that mantle-derived zircon megacrysts Mud Tank carbonatite (Australia), Kimberley pool (South Africa), and Orapa Kimberlite (Botswana) yield an average δ³⁰Si value of −0.38 ± 0.02‰ (1 SD). (Top) The Si isotopic difference between zircon, quartz, and WR for the LFB I-type Jindabyne tonalite analyzed here is shown; Δ³⁰Si(WR-zircon) is 0.37‰.

Fig. 3.

Histograms showing δ³⁰Si differences of zircons from 10 LFB granitoids and the Duluth Gabbro (SI Appendix, Table S3). The bin sizes are 0.2‰, commensurate with the 1 SE of our ion microprobe measurements. Some S types contain measured δ³⁰Si values down to −1.5‰, while W060, for example, is largely indistinguishable from the δ³⁰Si of our I-type samples. The A type and zircons show broadly restricted ranges in δ³⁰Si, compared with S- and I-type zircons. WR δ³⁰Si for S-, I-, and A-type LFB granitoids (21) can be found in SI Appendix, Fig. S1. The mantle zircon field is −0.38 ± 0.02‰, after Fig. 2. Histograms for zircon δ¹⁸O values from individual hand samples can be found in SI Appendix, Fig. S3.

Fig. 4.

Zircon LFB δ¹⁸O vs. δ³⁰Si, with annotated path trajectories after Fig. 1. The mantle zircon fields are +5.3 ± 0.3‰ and −0.38 ± 0.02‰ for δ¹⁸O (59) and δ³⁰Si (Fig. 2), respectively. Average δ¹⁸O values for S-type zircons are +8.8‰, consistent with a WR value of δ¹⁸O >10‰ (27). The zircon δ¹⁸O values from I-type rocks yield average values +7.5‰, consistent with WR values of <10‰ (SI Appendix, Table S3). Path 2 may also indicate a balance between assimilation/derivation between chert-like, “path 3” and pelite-like “path 1” protoliths, both of which have high δ¹⁸O.

Fig. 5.

(A) Plot of δ³⁰Si vs. δ¹⁸O for single Hadean (≥4.0 Ga) and Archean zircon, with schematic weathering paths from Fig. 1, revealing isotopic heterogeneities in both age suites. Mantle-derived zircon yield values of +5.3 ± 0.3 (59) and −0.38 ± 0.02‰ (Fig. 1) for δ¹⁸O and δ³⁰Si, respectively. (B) Zircon δ³⁰Si plotted against age showing fractionations away from mantle values. Error bars are 1 SE (SI Appendix, Table S4) or the SD of multiple ion microprobe analyses on a single grain, whichever is larger. The mantle zircon field is drawn––and reliant upon––high-precision MC-ICP-MS data zircon results (SI Appendix, Table S2).