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Micrometer-scale chemical and isotopic criteria (O and Si) on the origin and history of Precambrian cherts: Implications for paleo-temperature reconstructions

Abstract : Oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperature during the Precambrian. These reconstructions have been challenged because cherts can have various origins (hydrothermal, sedimentary, volcanic silicification) and their isotopic compositions might have been reset by metamorphic fluid circulation. Existing criteria used to assess the pristine sedimentary origin of a chert are based on petrography (criterion #1: chert is composed mostly of microquartz); on the bulk oxygen isotopic composition (criterion #2: bulk δ18O has to be close enough to the maximum δ18O value previously measured in other cherts of the same age); and on the presence of a large δ18O range at the micrometer scale (criterion #3: δ18O range of ∼10‰ at ∼2 μm). However, these criteria remain incomplete in determining precisely the origin and degree of preservation of ancient cherts. We report in situ Si and O isotope compositions and trace element concentrations in seven chert samples ranging from 1.88 to 3.5 Ga in age. Correlations between δ30Si and Al2O3 (and K2O, TiO2) reveal that microquartz is of three different origins, i.e. diagenetic, hydrothermal or silicification. Moreover, chert samples composed mostly of diagenetic microquartz show a large range of δ30Si at the micrometer scale (1.7-4.5‰), consistent with the large range of δ18O previously found in the Gunflint diagenetic cherts. We propose two further quantitative criteria to assess the origin, state of preservation and diagenetic history of cherts. Criterion #4 uses trace element concentrations coupled with δ30Si to ascribe the origin of cherts among three possible end-members (diagenetic, hydrothermal, and silicified). Criterion #5 is the presence of a large range of δ30Si in pure diagenetic microquartz. In the seven samples analyzed in this study, only one (from the Gunflint Iron formation at 1.88 Ga) passes all the criteria assessed here and can be used for paleo-temperature reconstructions. The criteria outlined here will help guide to the identification of pristine diagenetic cherts and or to better constrain seawater paleo-temperature reconstructions by taking into account the effect of diagenesis.
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Johanna Marin-Carbonne, Marc Chaussidon, François Robert. Micrometer-scale chemical and isotopic criteria (O and Si) on the origin and history of Precambrian cherts: Implications for paleo-temperature reconstructions. GEOCHIMICA ET COSMOCHIMICA ACTA, 2012, 92, pp.129-147. ⟨10.1016/j.gca.2012.05.040⟩. ⟨insu-03619307⟩



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