Importance of micro-scale oxygen isotopic study in Gunflint cherts (1.9 Ga): new constraints on paleo-temperatures reconstructions

Isotopic composition of sedimentary Precambrian cherts contains a record of the Precambrian environment through their oxygen isotopes (δ18O). Indeed, their δ18O values may reflect their temperature of formation, hence the temperature of the ocean provided the seawater δ18O remained constant around 0 ± 3 permil (Holmden and Muehlenbachs, 1993) during these last 3.5 Ga (Knauth and Lowe, 1978; Robert and Chaussidon , 2006). However, this temperature record can be modified by isotopic exchange with hydrothermal or metamorphic fluids. For a given age, the δ18O values of cherts show large variations (Knauth and Lowe, 2003; Perry and Lefticariu, 2003; Robert and Chaussidon, 2006) which complicates the paleo-temperatures reconstructions. To better understand the origin of these local variations, we have analyzed with SIMS, µm-scale δ18O variations in five cherts from the 1.9 Ga old Gunflint iron-formation (Canada) that stands among the least metamorphosed Precambrian cherts. Five chert samples, containing different types of silica (microcrystalline quartz, drusy quartz, megaquartz, and quartz veins), minor carbonates (siderite and ankerite), hematite and pyrite, were analysed (> 100 spots per sample) by multicollector CAMECA ims 1270 ion microprobe (CRPG-CNRS in Nancy). The external reproducibility on quartz standards was of ± 0.17 permil (1 σ, n=65). The Gunflint cherts show (i) detrital quartz with δ18O 10-12 permil lower than microcrystalline quartz, (ii) a typical 3-6 permil, δ18O range for microcrystalline quartz and (iii) quartz veins with δ18O 5-7 permil, lower than microcrystalline quartz. Variations in the proportion of the different types of silica appear to explain the isotopic heterogeneity observed the whole rock sample scale (i.e. sample 4 of 06/30/84; bulk = 22.75 ± 0.3 permil as compared with a mean δ18O calculated from the measured δ18O values of the different phases of silica = 23.18 ± 0.8 permil). Isotopic mapping of quartz veins shows that the oxygen isotopic composition of microquartz was preserved during hydrothermal circulation. Considering microcrystalline quartz only, a δ18O range of 3-6 permil is found; such a variation is likely to reflect diagenetic processes (Kolodny and Epstein, 1976). We develop an isotopic model of diagenetic evolution of quartz and fluid with different steps of precipiation-recrystallization. This model can give access to the temperature of equilibrium with seawater of the precursors of cherts, which may allow to better constrain oceanic paleo-temperatures in the Precambrian. Holmden C. and Muehlenbachs K., (1993) Science 259, 1733-1736. Knauth L.P. and Lowe R.D., (1978) Earth and Planetary Science Letters. 41, 209-222. Knauth L.P. and Lowe D.R., (2003) Geological Society of America Bulletin 115, 566-580. Kolodny Y. and Epstein S., (1976) Geochim. Cosmochim. Acta 40, 1195-1209. Perry E. C. and Lefticariu L., (2003) In: Treatise on Geochemistry, vol 7, pp. 99-113. Robert F. and Chaussidon M., (2006) Nature. 443, 969-971.