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

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 delta O-18 has to be close enough to the maximum delta O-18 value previously measured in other cherts of the same age); and on the presence of a large delta O-18 range at the micrometer scale (criterion #3: delta O-18 range of similar to 10 parts per thousand at similar to 2 mu m). 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 delta Si-30 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 delta Si-30 at the micrometer scale (1.7-4.5 parts per thousand), consistent with the large range of delta O-18 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 delta Si-30 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 delta Si-30 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. (C) 2012 Elsevier Ltd. All rights reserved.