Microscale oxygen isotope variations in 1.9 Ga Gunflint cherts: Assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions Johanna Marin a, * , Marc Chaussidon a , Franc ßois Robert b a CRPG-CNRS, Nancy-Universite ´, 15 rue Notre Dame des Pauvres, 54501 Vandoeuvre-le `s-Nancy, France b Muse ´um National d’Histoire Naturelle, Laboratoire de Mine ´ralogie et Cosmochimie du Muse ´um, CNRS, UMR 7202 (LMCM), Case Postale 52, 57 rue Cuvier, 75005 Paris, France Received 20 January 2009; accepted in revised form 17 September 2009; available online 23 September 2009 Abstract Variations in the oxygen isotope composition (d 18 O) of five cherts from the 1.9 Ga Gunflint iron formation (Canada) were studied at the micrometer scale by ion microprobe to try to better understand the processes that control d 18 O values in cherts and to improve seawater paleotemperature reconstructions. Gunflint cherts show clearly different d 18 O values for different types of silica with for instance a difference of 15& between detrital quartz and microquartz. Microquartz in the five samples is characterized by large intra sample variations in d 18 O values, (d 18 O of quartz varies from 4.6& to 6.6& at the 20 lm scale and from 12& to 14& at 2 lm scale). Isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than 200 lm away from the veins has preserved its original d 18 O value. At the micrometer spatial resolution of the ion probe, data reveal that microquartz has preserved a considerable d 18 O het- erogeneity that must be regarded as a signature inherited from its diagenetic history. Modelling of the d 18 O variations pro- duced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature (T sw at which the amorphous silica precipitated) and diagenesis temperature (T diagenesis at which microquartz formed) that reproduce the d 18 O distributions (mean, range and shape) measured at micrometer scale in microquartz. The two critical parameters in this modelling are the d 18 O value and the mass fraction of the diagenetic fluid. Under these assumptions, the most likely ranges for T sw and T diagenesis are from 37 to 52 °C and from 130 to 170 °C, respectively. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Oxygen and silicon isotope compositions (hereafter reported as d 18 O and d 30 Si notations) of Precambrian cherts have been used to reconstruct variations in the temperature of the oceans over time (Knauth and Epstein, 1976; Knauth and Lowe, 1978, 2003; Karhu and Epstein, 1986; Robert and Chaussidon, 2006). Cherts are complex siliceous rocks, finely crystallized with different forms silica (e.g. microquartz, megaquartz, ...), often interpreted as sed- imentary rocks. They show a regular decrease in d 18 O val- ues through geological times from +36& in modern-day cherts to +21.5& 3.5 Ga ago (Knauth and Lowe, 2003) which would be indicative of a 50 °C change in seawater temperature. When available, “recent” samples (<500 Ma old) allow detailed resolution with time that shows depar- tures from this secular isotopic trend. For example, Triassic samples have d 18 O values up to +36& deviating by 6& from the secular trend which reflects strong changes in sea- water temperature. In the case of Precambrian samples two major factors may obscure the relationship between seawa- ter temperature and chert d 18 O: (i) seawater d 18 O may have 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009. 09.016 * Corresponding author. Tel.: +33 3 83 59 42 44. E-mail address: jmarin@crpg.cnrs-nancy.fr (J. Marin). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 116–130