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Edited by Norman H. Sleep, Stanford University, Stanford, CA, and approved August 22, 2018 (received for review May 16, 2018)
Significance
The crust or its chemically weathered derivatives likely served as a substrate for the origin of life, which could have occurred by 4.1 Ga. Yet no known bona fide terrestrial rocks from this time remain. Studies have thus turned to geochemical signatures within detrital zircons from this time. While zircons do not directly record low-temperature weathering processes, they inherit isotopic information upon recycling and remelting of sediment. We developed a method to fingerprint the identity of material involved in water–rock interactions >4 Ga, bolstered by a large Si and O isotopic dataset of more modern zircon samples. The data presented here provide evidence for chemical sediments, such as cherts and banded iron formations on Earth >4 Ga.
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.
Footnotes
- ↵1To whom correspondence should be addressed. Email: dtrail{at}ur.rochester.edu.
Author contributions: D.T. and P.S.S. designed research; D.T., P.B., P.S.S., M.L.M., and I.B. performed research; M.-C.L. contributed new reagents/analytic tools; D.T., P.B., P.S.S., M.L.M., and I.B. analyzed data; and D.T., P.B., P.S.S., M.-C.L., and I.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1808335115/-/DCSupplemental.
Published under the PNAS license.
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Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon
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