Triple oxygen isotope trend recorded by Precambrian cherts: A perspective from combined bulk and in situ secondary ion probe measurements

Abstract:

There are large uncertainties associated with the current estimates of temperature and oxygen isotope composition of Precambrian oceans, largely due to preservation issues inherited by sedimentary record. The advancing ability to measure triple oxygen isotope ratios at the precision of 0.01 ‰ or better provides an opportunity to distinguish between process that accompany deposition, early diagenesis and late alteration of marine sediments. Here we undertake a triple oxygen isotope study of marine cherts spanning in age between 3.5 and 1.9 Ga using a suite of samples from the Dresser (Australia), Kromberg, Mendon (S. Africa) and Gunflint (Canada) Formations. To disentangle the effects of recrystallization on a case-by-case basis, we combined triple oxygen isotope measurements by bulk laser fluorination with the in situ secondary ion probe (SIMS) measurements, and Raman spectroscopy of organic matter. Within the studied here samples the δ³⁰Si and δ¹⁸O values measured by SIMS span several ‰ over several 10-100 µm informing us of mixing arrays that are expected in triple oxygen isotope space. To draw parallels with the envisioned environments for Archean silica deposition, we use geothermal amorphous silica scale that formed at ~188 °C due to high temperature seawater-basalt reaction and precipitated in the pipelines of seawater-dominated Reykjanes system in Iceland. The triple oxygen isotope signature of our Archean samples with δ³⁰Si <0 is consistent with early crystallization at 150-170 °C. We suggest that at least some Archean cherts formed in equilibrium with fluids that are 1-2 ‰ higher in δ′¹⁸O and ~20 per meg lower in Δ′¹⁷O_0.528 than the contemporaneous seawater due to influence of submarine vent fluids. The 1.9 Ga Gunflint chert contains several generations of quartz, including the high-δ′¹⁸O microquartz (+24-26 ‰) that is thought to reflect crystallization from marine siliceous sediment based on trace element concentrations and silicon isotope measurements. The δ′¹⁸O – Δ′¹⁷O of such quartz are consistent with the temperature of 60-80 °C, given that the pore water fluids had δ′¹⁸O of around -2 ‰. The extrapolation of vent fluids and marine pore water towards pristine seawater does not require a hydrosphere with δ¹⁸O outside of ± 2 ‰ of the modern value.