Evaluating the geochemistry and paired silicon and oxygen isotope record of quartz in siliceous rocks from the ~3 Ga Buhwa Greenstone Belt, Zimbabwe, a critical link to deciphering the Mesoarchean silica cycle

Latisha A. Brengman, Christopher M. Fedo, Martin J. Whitehouse, Iffat Jabeen, Neil R. Banerjee

Research output: Contribution to journalArticlepeer-review

Abstract

Rocks that make up the Mesoarchean (~3Ga) Buhwa greenstone belt (BGB) of the Zimbabwe craton divide into three associations: shelf, basinal, and transitional. Chert and iron formation exist within the three associations, allowing the unique opportunity to compare textural and geochemical attributes of iron- and silica-rich rocks from distinct positions within a single basin. We analyzed major-, trace-, and rare-earth-elements, and silicon and oxygen isotopes of chert and iron formation from the shelf, basinal, and transitional associations within the BGB. Samples possess elemental signatures consistent with formation from mixed seawater and hydrothermal fluids (LaSN/LaSN* > 1; PrSN/YbSN < 1; Y/Ho = 28.2–44.4; EuSN/EuSN* = 1.8–2.8). Oxygen isotope values broadly overlap for samples from different associations (ranging from δ18OV-SMOW = 9.41 to 15.10‰ (2σ = 0.8‰)), with isotopic variation linked to the presence of oxide inclusions in some samples. Silicon isotope values by contrast, vary widely. Iron formations possess 30Si-depleted silicon isotope signatures (δ30SiNBS-28 = −2.5 to −0.5‰ (2σ = 0.05‰)), in line with previous studies. Basinal and transitional cherts possess 30Si-depleted silicon isotope signatures (δ30SiNBS-28 = −1.1 to −0.5‰ (2σ = 0.05‰)) that overlap with values measured from iron formation samples. Within the transitional association, cherts are not 30Si-enriched compared to iron formation, while chert in the basinal association is 30Si-enriched compared to iron formation. Depleted silicon isotope signatures could result from fractionation during adsorption, re-equilibration, post-depositional processes, or possibly microbial reduction processes. Silicon isotope heterogeneity recorded in chert preserved in different assemblages across the BGB may in part reflect a seawater signature. The range of silicon isotope values preserved within broadly co-eval rocks from the same assemblage may indicate that samples preserve local isotopic signatures of a heterogeneous reservoir. Future work focused on identifying basin-position dependent trends may inform interpretations of the compiled Precambrian silicon isotope record.

Original languageEnglish (US)
Article number120300
JournalChemical Geology
Volume577
DOIs
StatePublished - Sep 5 2021

Bibliographical note

Funding Information:
The research presented in this manuscript was done on archived samples from past field work completed under NSF-EAR 910-4876 grant awarded to K.A. Eriksson, PI. The authors recognize and thank Dereck Mudzamba and Gift Nyirenda for their help with past fieldwork. Petrographic and geochemical work was funded by NASA grant EXOB08-0063 awarded to C.M. Fedo, and student research grants awarded to L.A. Brengman from ExxonMobil, the Geological Society of America, the Sedimentary Division of GSA, and the Department of Earth and Planetary Sciences at the University of Tennessee. The University of Minnesota Duluth provided additional funding for L.A. Brengman. N. Banerjee acknowledges research funding from NSERC and CFI. The NordSIMS facility is a joint Swedish-Icelandic infrastructure operated under Swedish Research Council grant 2014-06375, contribution ##676. We thank D.R. Ortega and C.M.J. Brengman for their help with statistical analyses. We thank Editor B. Kamber, Editor R. Sch?nberg, Reviewer P. Savage, and Reviewer 2 (anonymous), for their helpful, insightful, constructive comments and suggestions that greatly improved the manuscript.

Funding Information:
The research presented in this manuscript was done on archived samples from past field work completed under NSF-EAR 910-4876 grant awarded to K.A. Eriksson, PI. The authors recognize and thank Dereck Mudzamba and Gift Nyirenda for their help with past fieldwork. Petrographic and geochemical work was funded by NASA grant EXOB08-0063 awarded to C.M. Fedo, and student research grants awarded to L.A. Brengman from ExxonMobil, the Geological Society of America, the Sedimentary Division of GSA, and the Department of Earth and Planetary Sciences at the University of Tennessee. The University of Minnesota Duluth provided additional funding for L.A. Brengman. N. Banerjee acknowledges research funding from NSERC and CFI . The NordSIMS facility is a joint Swedish-Icelandic infrastructure operated under Swedish Research Council grant 2014-06375 , contribution ##676. We thank D.R. Ortega and C.M.J. Brengman for their help with statistical analyses. We thank Editor B. Kamber, Editor R. Schönberg, Reviewer P. Savage, and Reviewer 2 (anonymous), for their helpful, insightful, constructive comments and suggestions that greatly improved the manuscript.

Publisher Copyright:
© 2021 Elsevier B.V.

Keywords

  • Algoma-type iron formation
  • Chert
  • Geochemistry
  • Mesoarchean silica cycle
  • Oxygen isotopes
  • Silicon isotopes
  • Source-to-sink
  • Superior-type iron formation

Fingerprint

Dive into the research topics of 'Evaluating the geochemistry and paired silicon and oxygen isotope record of quartz in siliceous rocks from the ~3 Ga Buhwa Greenstone Belt, Zimbabwe, a critical link to deciphering the Mesoarchean silica cycle'. Together they form a unique fingerprint.

Cite this